Intellectual Development Statement

Ashutosh V. Kotwal

Duke University

April 26, 2009

I joined the Physics Department at Duke University in January 1999 as an AssistantProfessor in the field of experimental particle physics. I submitted my tenure dossier inOctober 2004, and was promoted to Associate Professor with tenure in July 2005. Inthis statement I will describe the research I have conducted and and leadership respon-sibilities that I have held in my field, since the submission of my tenure dossier.

1 Introduction to Experimental Particle Physics

Particle physics involves the study of fundamental building blocks of matter andtheir interactions. Experimental particle physicists seek to make measurements thatwill reveal the physical properties of nature at its most elementary level.

Experimental physicists have successfully used the technique of scattering particlesoff a target to study the structure of the target. The theory of Quantum Mechanicsdictates that studying physical properties at smaller distances requires probe particlesof increasing energy. A hundred years ago, Rutherford studied the pattern of α particles(later understood to be the nuclei of Helium atoms) scattering off gold atoms to concludethat atoms consisted of a very small, dense, charged nucleus, surrounded by a cloud ofelectrons. Using higher energy collisions of nuclei, the nucleus was found to be a tightlybound state of protons and neutrons. Yet higher energy collisions in the 1970’s led tothe discovery that protons, neutrons and eventually a host of other similar particles werebound states of more fundamental particles called quarks.

At the same time, a deeper understanding of the forces acting on matter was gained.In addition to the electric and magnetic forces and gravity, two new interactions were

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discovered. The “weak” interaction, initially hypothesized to be responsible for radioac-tivity, and has since manifested itself in many other phenomena. The “strong” interac-tion is responsible for binding protons and neutrons in nuclei and for binding quarks inprotons and neutrons. The theories of quarks, electrons and other electron-like particles,and their non-gravitational interactions have merged into an elegant theory called theStandard Model (SM). The Standard Model provides a coherent explanation for a largenumber of measurements of the behavior of elementary particles.

While the Standard Model has been very successful, it leaves some important questionsunanswered:

1. Is there an underlying structure or symmetry to what we consider today to be thefundamental building blocks of matter, such as quarks and electrons?

2. Are the four known forces different manifestations of a single unified interaction?

3. What is the origin of the mass of particles?

4. What is the structure of space and time? Are there additional dimensions at smalldistances?

Answering these questions is one of the highest priorities of the field. In my researchas an experimental particle physicist, I have chosen to pursue measurements that couldprovide evidence of new physical phenomena which cannot be explained by the StandardModel. The questions mentioned above have inspired alternate theories, and by testingtheir predictions, I hope to shed light on some of these mysteries of nature.

Rutherford’s pioneering technique of scattering particles to understand their propertiesis still the mainstay of the particle physics. The particle accelerator with the highestenergy in the world is called the Tevatron, located at the Fermi National AcceleratorLaboratory (Fermilab) near Chicago, Illinois. The energy of the particle collisions at theTevatron is available to create new forms of matter that may interact via new forces,due to the equivalence of matter and energy given by Einstein’s equation E = mc2. Twolarge detectors, called CDF and DØ respectively, have been built by teams of particlephysicists to record electronically the properties of the outgoing particles produced inthese collisions.

I obtained my Ph.D degree in experimental particle physics, making precise measure-ments of the internal structure of the proton and neutron in the highest-energy muonscattering experiment at Fermilab. Since then, I have worked over the last 14 years onthe design and construction of the CDF and DØ experiments, and on the computer pro-grams used to analyze the electronic data and produce physical interpretations. Due to

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the enormous complexity of these modern experiments at the frontier of research, eachof the CDF and DØ collaborations consist of ≈ 700 physicists from U.S. universities,national laboratories such as Fermilab, and a large number of international institutions.Together they include about half of the U.S. particle physics community and aboutone-fifth of the world-wide community.

The questions posed above, strongly motivate new experiments at higher energies thanthe Tevatron can provide. The Large Hadron Collider (LHC) has been built at CERN,Switzerland (the European laboratory for particle physics) to generate particle collisionswith seven times more energy than the Tevatron. The LHC will start collecting data inthe near future, and it is anticipated that it will run for 10-20 years. My future researchprogram will focus on physics at the LHC.

2 Executive Summary of Research Program

Possible extensions of the Standard Model include theories of compositeness (i.e.substructure) and additional symmetries relating the known particles to hypothetical,new particles. I have chosen to test these theories using two complementary strategies:precision measurements to test for deviations from the Standard Model predictions, anddirect searches for new particle production.

The mass of the W boson, a mediator of the weak force, is one of the most preciseand important measurements at the Tevatron. It is influenced by the existence of newparticles via quantum mechanical corrections, making it a sensitive observable to probenew physics. I have therefore chosen to measure the W boson mass using the CDF andthe DØ experimental data.

I strategized the current CDF W mass analysis from its conception, created the soft-ware infrastructure for it, and made precise determinations of the leptonic and hadronicdetector response. Under my leadership, we published the first measurement of theW boson mass from Run II of the Tevatron, using the data from the ungraded CDFdetector. At the time of publication, this measurement was the single most precisemeasurement of this quantity in the world, with an uncertainty of 48 MeV. The impactof this measurement, via the precision electroweak fits, was to lower the inferred valueof the SM Higgs boson mass by 6 GeV. The best-fit value of the (undiscovered) Higgsmass is lower than the direct exclusion lower bound from the European large electronpositron (LEP) experiments by 38 GeV, hinting (though not conclusively due to compa-rable uncertainties) with an incompatibility with the SM and thus pointing to possiblenew physics beyond the SM. This result has been published in Physical Review Let-

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ters (CV # 5, TT # 1) and Physical Review D (CV # 4, TT # 7), and is stillthe best published measurement of the W boson mass.

I have co-authored a comprehensive review article describing the status of current Wboson mass measurements, and prospects and challenges for further improvements, inan invited article for the Annual Reviews of Nuclear and Particle Science (CV# 3, TT # 9).

The mass of the heaviest quark, the top quark, is another very important parameterin the SM. The quantum mechanical radiative corrections to the W boson mass, as wellas to another precision parameter called sin2θW (which is measured at LEP and theStanford Linear Collider), receive contributions from the top quark. In order to extractthe most sensitivity to Higgs and other new physics, the top quark contribution hasto be subtracted. For this calculation the top quark mass is needed. I have publishedin Physical Review D, Rapid Communications (CV # 7, TT # 8) the mostprecise measurement of the top quark mass in the dilepton channel, using multivariatematrix element technique to extract the most information per data event. Following thispublication, the technique has been augmented with a new method of event selection,based on evolving neural networks (modelled on genetic evolution in biology). This novelmethod, applied for the first time in high energy physics, is used to find a neural net-work that minimizes the top quark mass uncertainty directly, rather than classificationaccuracy or other secondary criteria. Also doubling the size of the data set, the newtechnique and result has been published in Physical Review Letters (CV # 1, TT# 2).

The SM fit to the values of the W boson mass, top quark mass and sin2θW stronglyfavor a relatively light Higgs boson, with the preferred value of its mass below 200GeV. Building on the techniques I am using for the top quark mass measurement inthe dilepton channel (with the final state containing two leptons and two b quarks), Ihave initiated a direct search for the Higgs boson in this preferred mass range. In theHiggsstrahlung process, a Higgs boson is radiated off a Z boson, with the Higgs decayingpreferably to two b quarks if it is light. This search for associated Higgs production,ZH → ll̄bb̄, is based on a per-event likelihood estimator using multivariate matrixelements. The result will be submitted for publication in summer 2009.

My other research on direct new particle production has centered on testing modelsof compositeness and extended symmetries. These models explore different mechanismsof explaining the structure of the SM and creating a more unified description of nature.My work in these areas has also been at the research frontier.

Based on my research on compositeness, I have produced new results on excited elec-tron states (CV # 11, TT # 6) and excited muon states (CV # 8, TT # 4),

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leading to two publications in Physical Review Letters. The excited electron searchwas the first of its kind at the Tevatron.

Pursuing the possibility of additional symmetries, I have tested the extension of theleft-handed weak interaction to the Left-Right Symmetric model [6], which is motivatedby recent results on neutrino oscillations. I have pursued the extended Higgs sectorpredicted by the Left-Right model, choosing the doubly-charged Higgs boson as theideal testing ground. Following on a previous publication of a doubly-charged Higgsboson into electrons and muons, I have also obtained the most stringent direct limit onthe production of a long-lived doubly-charged Higgs boson, which has been published inPhysical Review Letters (CV # 10, TT # 5).

Many grand unified theories attempting to unify the different particles and forces athigh energies using a larger symmetry group, predict that at lower energies the brokensymmetry group contains an additional, heavy neutral boson. This particle would appearas a narrow resonance decaying to two leptons, and is often called the Z ′ boson. Narrowdilepton resonances are also predicted in theories of warped extra dimensions, whereinthe extremely weak force of gravity is predicted to become strong at the energy scaleaccessed by current particle colliers. I have completed an extensive search for thesenarrow resonances decaying in the muon channel, and the results have been publishedin Physical Review Letters (CV # 2, TT # 3). These are the most stringent masslimits on supersymmetric neutrinos, Z ′ bosons and Randall-Sundrum [1] gravitons forcertain values of model parameters.

I have led large teams of particle physicists on several projects. I served as co-leaderof the CDF Offline Software and Computing Project for two and half years. I supervisedall the analysis software and computing activities of the experiment. This is one of theseven most important positions in the collaboration. I was responsible for all recon-struction and simulation software, the processing of all data and simulation events, thefinal detector calibrations, as well as all computing infrastructure, budget and personnelissues. I conceived of and implemented a rapid data processing scheme, and deployed im-proved reconstuction software and distributed (GRID) computing infrastructure. Undermy leadership, these developments resulted in a substantial increase in the experiment’sphysics productivity. Some of the new methodology, processes and technical designs aredescribed in two publications in Nuclear Instrumentation and Methods (CV #6) and IEEE Transactions on Nuclear Science (CV # 9).

I have given nine major presentations in international conferences and workshops inthe last four years, all upon direct invitation and in the plenary session. I have alsogiven thirteen invited seminars at research universities, international institutes and lab-oratories.

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I was elected as Chairperson of the Fermilab Users Executive Committee. Currentlyserving in this role, I am the primary contact person between the Fermilab user commu-nity and the Fermilab management, and the U.S. Congress. I have also served on theinternational advisory committee of the Hadron Collider Physics Symposium, the jointNSF/DoE review panel of the US LHC project, the DoE Outstanding Junior InvestigatorAward selection committee.

I chaired and organized the Hadron Collider Physics conference on the Duke campus in2006. This is an international conference, and the special theme of this conference was tobring the Fermilab and Large Hadron Collider (LHC at CERN in Europe) communitiestogether. The conference was highly successful and well-appreciated by the participants.

I have been elected Fellow of the American Physical Society in 2008, with the citationto my W boson mass measurements.

I have supervised four post-doctoral research associates, six graduate students andthree undergraduate students. I have been successful in securing funding to fully supportmy research group. Since receiving tenure I have supervised two Ph.D. theses, twoMasters theses and one undergraduate Honors thesis.

3 Research Accomplishments

In this section I will describe my research achievements in the area of precisionelectroweak measurements, followed by direct searches for new particle production.

Tevatron experiments have collected data during 1992–1995 and again since 2001. Inthe following, I refer to the former as “Run I” data and the latter as “Run II” data.

I am a co-author of over 200 publications from the CDF, DØ and the E665 Collab-orations at Fermilab. The convention in these collaborations is that all collaborationmembers are listed as authors on every publication. I describe below my own researchsince receiving tenure.

3.1 Precision Electroweak Measurements

In the Standard Model, the gauge symmetry that predicts massless W and Z bosonsas mediators of the weak force is broken to impart masses to these particles. This

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“electroweak symmetry breaking” is induced through the “Higgs” mechanism where ahypothetical Higgs field acquires a non-zero vacuum expectation value. The coupling ofthe Higgs condensate to other particles imparts them their mass.

The underlying dynamics causing the formation of the Higgs condensate is not known.A precise measurement of the W boson mass helps to uncover the mechanism of elec-troweak symmetry breaking. Radiative corrections to the W mass due to quantum loopsin the W boson propagator depend on the spectrum of the particles in the loops, includ-ing the Higgs boson. A measurement of the W mass implies a measurement of theseradiative corrections, which can be converted into a Higgs mass constraint.

Various extensions of the Standard Model predict new particles coupling to the W bo-son, such as Supersymmetric (SUSY) particles. The theory of Supersymmetry predicts aduality between particles of matter and particles that mediate interactions. In the Mini-mal Supersymmetric extension of the Standard Model (MSSM), for example, additionalcorrections can increase the predicted W boson mass by up to 250 MeV [3]. Thereforea precise measurement of the W mass provides a sensitive search of new physics beyondthe SM. It is complementary to direct searches and is sensitive to new particles that areheavier than the mass reach of direct searches.

3.1.1 CDF W Mass Analysis

I conceived all aspects of the Run II W mass measurement at CDF. I developed theanalysis strategy to perform the electron and muon channel analyses in a commonframework. I developed a fast detector simulation for generating W , Z, J/ψ, Υ andE/p spectrum lineshapes, and the template fitter for performing maximum-likelihoodfits to data. The detector simulation is based on a hit-level simulation of the COTdrift chamber, in which I incorporated a detailed simulation of multiple scattering, ion-ization energy loss, bremsstrahlung, photon conversion, electromagnetic and hadroniccalorimetry, and electron and muon acceptance.

An important success of my analysis is the determination of a consistent detectorenergy and momentum calibration using the experimental data, with a precision of0.03%. The momentum scale of the tracker is set using the precisely known masses ofthe J/ψ → µµ and Υ → µµ resonances. The muon momentum dependence of the J/ψmass is used to tune the passive material map in terms of the ionization energy loss,such that the momentum dependence is eliminated. The Υ → µµ mass fit provided aconsistent measurement of the momentum scale, as well as confirming the correctness ofthe beam-constraining procedure.

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The tracker momentum scale is transferred to the electromagnetic calorimeter for theelectron channel measurement, by performing a template fit to the Ecal/ptrack spectrumof electrons from W → eν decays. The position of the peak in the Ecal/ptrack spectrumis sensitive to the calorimeter energy scale, and also sensitive to the bremsstrahlungspectrum (and at second order to the conversion of the radiated photons). The fractionof events in the tail of the Ecal/ptrack spectrum is fitted as a function of a passive materialtune factor, which is then propagated into the peak fit for extracting the energy scale.The non-linearity of the calorimeter response is measured by repeating the energy scalefit in bins of ET .

A key success and confirmation of these calibration procedures was that the measuredZ boson masses in both the electron and muon channels were consistent with the pre-cisely known world average of the Z boson mass. This validation was a challenge for anearlier, Run I analysis of the W boson mass.

The Z → µµ mass fit is used to set the tracker momentum resolution, by tuning the hitresolution and hit efficiency in the simulation. The calorimeter resolution is measuredfrom the observed width of the E/p peak, and from the observed width of the Z → eemass peak.

I developed and tuned the parametric model for the calorimeter response to hadronicparticles accompanying W and Z boson production. The model includes the contribu-tions from the spectator interaction, instantaneous luminosity-dependent additional pp̄interactions, and the hard recoil. The model is tuned using pT -balance in Z → ee, µµevents.

As a result of my methods to use all available data for the various calibrations, mostof the systematic uncertainty was determined from the statistics of the control datasets.This had a number of positive consequences. First, the measurement was more precisethan what one would expect extrapolating from the Run I statistics. Second, I estab-lished the methods to be used for future, even more precise measurements of the Wboson mass, using larger datasets.

Examples of maximum-likelihood template fits to the W transverse mass and leptonpT fits are shown in Figs. 1-2. The simulation gives a good description of the data in allthe lineshapes. A total of six separate fits for the W boson mass were performed: thedistributions of the transverse mass, charged-lepton transverse momentum and neutrinotransverse momentum, for the electron and muon channels respectively. They produceconsistent measurements and cross-check each other since they have different systematicuncertainties, and they are combined to obtain the final result:

MW = 80.413 ± 48 MeV,

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the most precise published single measurement of theW boson mass in the world. Table 1summarizes the uncertainties in the various W mass fits, along with the correlationsbetween the electron and muon channels. The analysis was performed using a “blind”technique, so that the value was unknown until the analysis was final, and no changeswere made after the value was unmasked. The result was published in Physical ReviewLetters (CV # 5, TT # 1) and Physical Review D (CV # 4, TT # 7).

In addition to the elements of the W mass analysis, my work on the drift chambertracker software, alignment and calibration were cornerstones of this success.

My collaborators in this work were the following colleagues from CDF: Prof. WilliamTrischuk (University of Toronto), Oliver Stelzer-Chilton and Ian Vollrath, both graduatestudents from University of Toronto, my post-doc Christopher Hays, and Larry Nodul-man (Argonne National Laboratory). I continued to work with Oliver Stelzer-Chiltonwhen he became a post-doc at University of Oxford, as well as with Christopher Hayswhen he became a Lecturer at Oxford.

3.1.2 Review Article on W Mass Measurement

The W boson mass measurement is reaching the same level of precision as the mea-surement of sin2θW from the LEP and SLC experiments, in the ability to provide astringent test of the SM, including the ability to constrain the Higgs boson mass andother new physics.

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Systematic W → eν W → µν Common

pT (W ) model 3 3 3QED radiation 11 12 11

Parton distributions 11 11 11Lepton energy scale 30 17 17

Lepton energy resolution 9 3 0Recoil energy scale 9 9 9

Recoil energy resolution 7 7 7u|| efficiency 3 1 0

Lepton removal 8 5 5Backgrounds 8 9 0

Total systematic 39 27 26Total uncertainty 62 60 26

Table 1: Systematic and total uncertainties in MeV/c2 for the transverse mass fits, whichare the most precise. The last column shows the correlated uncertainties.

I was invited to co-author a comprehensive review article on the W boson mass mea-surement at the Tevatron. In this paper we discuss the general techniques, the finalTevatron Run I measurements and their uncertainties, my Run II measurement fromCDF data and the DØ experiment’s Run II analysis (in progress at the time), as wellas what we have learnt about the Higgs. Finally, we discuss how the W boson massmeasurement may be further improved at the Tevatron. The article was published inAnnual Reviews of Nuclear and Particle Science (CV # 3, TT # 9).

My collaborator on this paper was my co-author, Dr. Jan Stark (Universite JosephFourier Grenoble).

3.1.3 Top Quark Mass Measurement in the Dilepton Channel

The top quark mass mt is another key parameter of the Standard Model. The massvalue is needed to calculate various radiative corrections in the SM, in order to comparevalues of precision observables to predictions and hence test the theory.

In the dilepton decay channel, top quark pair production is followed by the quarksdecaying into W bosons and b quarks, and both W bosons decay into leptons (electronsor muons): tt̄ → Wb + Wb̄ → lν̄b + l̄′ν ′b̄. Due to the presence of two neutrinos in thefinal state, the mass fit is under-constrained on an event by event basis. In spite of

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this challenge, it is possible to perform an event-by-event analysis by assigning a mt-dependent probability to each event, and obtaining a best-fit mt value from the ensembleof event probabilities.

I have worked with my student Ravi Shekhar, post-doc Bodhitha Jayatilaka and CDFcollaborator Prof. Daniel Whiteson (UC Irvine, previously postdoc at University ofPennsylvania) to extract the top quark mass from dilepton events using such a per-eventtechnique. More information is extracted from each event, in the form of a posteriorprobability curve P(mt), as compared to the use of a single number to characterize theevent. We use a multivariate method to extract all the kinematic information in theevent in order to construct P(mt). This method uses the full vector of kinematics ~xfrom the event, ie. the momentum 3-vectors of the two leptons and the two b-quark jets,and information on the remaining transverse energy flow in the event, as input to the SMmatrix element M(~x;mt) for top quark pair production. We then use Bayes Theoremto obtain the posterior probability P(mt), from the SM prediction of the probability forproducing an event with kinematics ~x.

This technique has the benefits that the full event information is combined with thecomplete SM prediction of top quark production and decay, in order to constrain themeasured top quark mass. We published in Physical Review D Rapid Communi-cations (CV # 7, TT # 8) the world’s then best measurement of the top quark massin the dilepton channel:

mt = 164.5 ± 3.9(stat) ± 3.9(syst.) GeV,

using 1 fb−1 of CDF Run II data.

Following this publication, we have augmented this multivariate “matrix-elements”technique with a novel method of optimizing the event selection. The matrix-elementstechnique extracts the most information about the measured parameter for a givensample of candidate events. However the event selection criteria are not defined by themethod. Ideally, one wants to select the sample (containing an admixture of signaland background events with a kinematics-dependent signal efficiency and backgroundcontamination), that maximizes the sensitivity to the measured parameter. We solve thisdifficult heuristic problem by using neural networks in a novel application for high energyphysics. In a method modelled on biological evolution, we create a set of randomly-generated neural networks, whose inputs are the event kinematics. Each network’soutput is used to provide a binary cut to select/reject a candidate event. The optimumnetwork is defined as the one whose selected candidate sample would provide the smalleststatistical uncertainty on the mt measurement. This optimum network is created asfollows: all networks in the initial set are tested on simulated samples of signal andbackground events (pseudo-experiments). A subset of networks that predict the smallestmt uncertainty is “bred”, ie. copied and randomly modified to create a new full set of

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networks. In this way the “fittest” networks are used to derive the next generationof networks, and this “breeding of the fittest” process is iterated until the generationsasymptote to an optimal performance. The final event selection on the data sample isperformed using the “best” network from all generations, which is typically from thefinal generation.

Our analysis of 2 fb−1 of CDF Run II data, using the above amalgamation of matrix-element and evolutionary neural network techniques, has been published in PhysicalReview Letters (CV # 1, TT # 2). The use of the evolutionary neural networktechnique for event selection resulted in a 20% improvement in the statistical uncertainty,over the use of likelihood fitting using matrix elements alone. The result is

mt = 171.2 ± 2.7(stat) ± 2.9(syst) GeV,

again the world’s best measurement of the top quark mass in the dilepton channel.

3.2 Higgs Search in the ZH Associated Production Mode

In addition to precise measurements of the W boson and top quark masses, thedirect search for the Higgs boson is one of the highest priorities of the CDF experiment.The electroweak measurements, via SM fits, prefer a low value of the SM Higgs mass,making the search for a low-mass Higgs boson a very interesting topic. In this massrange (mH < 130 GeV), the principle modes of sensitivity for the Tevatron are theassociated production modes of Higgs boson along with a W or Z boson, where theHiggs is radiated off the latter. The Higgs then decays predominantly into a pair of bquarks.

One of the final states of the process Z + H → ll̄bb̄ leads to a very similar finalstate as the top mass analysis discussed above, i.e. two charged leptons and two bquarks. Exploiting this similarity, I am pursuing with my student Ravi Shekhar, post-docBodhitha Jayatilaka and collaborator Daniel Whiteson, a search for the SM Higgs bosonin this mode. This is Ravi Shekhar’s Masters thesis topic. To obtain as high a sensitivityas possible, we are using the per-event likelihood technique, where the likelihood isconstructed from SM matrix elements for the signal and background processes. Wechoose the measurement parameter to be the fractional ZH content of the data, for agiven value of the Higgs boson mass. As with the top mass measurement, we exploitthe full kinematic information in the data events, including all momentum and angularcorrelations such as those due to the Higgs being a scalar particle.

The paper describing this analysis is being reviewed for publication in CDF, and willbe submitted for publication in summer 2009.

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3.3 Searches for New Phenomena

The CDF detector sits at the energy frontier and has sensitivity to many of the newphysics phenomena that potentially underlie the Standard Model. The direct discoveryof new physics through the identification of its signature(s) in particle production anddecay is a primary goal of research in particle physics.

3.3.1 Exotic and Excited Leptons

The Standard Model of particle physics describes the non-gravitational interactionsusing the SU(3)C × SU(2)L × U(1)Y gauge group. The particle content of the model isgiven by three generations of quarks and leptons, each containing an SU(2)L doublet.This fermion multiplicity motivates a description in terms of underlying substructure, inwhich all quarks and leptons consist of fewer, more elementary particles bound by a newstrong interaction [5]. In this compositeness (CI) model, quark-antiquark annihilationsmay result in the production of excited lepton states, such as the excited electron, e∗

and the excited muon µ∗. The SM gauge group may be embedded in larger gauge groupssuch as SO(10) or E(6), motivated by grand unified theories or string theory. Theseembeddings also predict additional, exotic fermions such as the e∗ and the µ∗, which canbe produced via their gauge interactions [5] (GM model). I have published two analysesusing the CDF Run II data, searching for the e∗ and the µ∗ respectively.

The Ph.D. thesis topic of my graduate student Heather Gerberich was the search forassociated ee∗ production followed by the radiative decay e∗ → eγ. This mode yields thedistinctive eeγ final state, which is fully reconstructable with high efficiency and goodmass resolution, and has small backgrounds. The evidence for e∗ production would bethe observation of a narrow resonance in the eγ invariant mass distribution.

Since no signal above background predictions was observed, limits were set on the e∗

production cross section and mass. Figure 3 shows the limits in the parameter spaceof f/Λ vs Me∗ for the GM model, and Me∗/Λ vs Me∗ for the CI model. In the gauge-mediated model, we exclude Me∗ < 430 GeV for f/Λ ≈ 0.01 GeV−1 at the 95% con-fidence level (C.L.), extending the exclusion region well beyond other limits. We havealso presented the first e∗ limits in the CI model as a function of Me∗ and Λ, excluding100 < Me∗ < 906 GeV for Me∗ = Λ. These search results for excited and exotic elec-trons were the first at a hadron collider. The results have been published in PhysicalReview Letters (CV # 11, TT # 6).

We performed a similar search for excited and exotic muon production, using a sig-

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Figure 3: The 2-D parameter space regions excluded by this analysis for the GM model(left) and the CI model (right), along with other limits.

nificantly larger dataset. The search was significantly more sensitive than LEP for highmass values, and HERA has no sensitivity for µ∗ production, making this search quiteunique. This was the first hadron-collider search in the context of the GM model, andextended previous mass limits in both the GM and CI models. In the GM model, weexclude Mµ∗ < 400 GeV/c2 for 10−3 GeV−1 < f/Λ < 10−1 GeV−1 at the 95% C.L., wellbeyond previous limits. We have also presented the first µ∗ limits in the CI model asa function of Mµ∗ and Λ, excluding Mµ∗ < 853 GeV/c2 for Λ = Mµ∗ . These results,shown in Fig. 4, have been published in Physical Review Letters (CV # 8, TT #4).

I supervised the senior honors thesis of Edward Daverman, an undergraduate at Duke,on this topic. I collaborated with Heather Gerberich on this paper (then post-doc atUniversity of Illinois, Urbana-Champaign).

3.3.2 Doubly-Charged Higgs Bosons

In the standard Model, the Higgs field is postulated to break the SU(2)L × U(1)Yelectroweak gauge symmetry to U(1)EM . The Higgs boson is eagerly sought after buthas not yet been observed. Extensions of the SM predict larger Higgs sectors. For in-

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Figure 4: The 2-D parameter space regions excluded by this excited/exotic muon analysisfor (a) the GM model, along with other limits, and (b) the CI model.

stance, the introduction of a Higgs triplet containing neutral, singly, and doubly-chargedmembers is required in the Left-Right Symmetric Model, which is well-motivated by therecent discovery of neutrino oscillations. The Supersymmetric Left-Right Model [6] fur-ther implies a relatively light doubly-charged Higgs boson (H±±), motivating its searchat the Tevatron.

The observation of any Higgs particle would be an important step toward understand-ing the physics of the electroweak scale. The H±± boson is particularly fascinatingbecause of its simultanous implications for two theories beyond the Standard Model,in addition to exposing one of the untested foundations of the Standard Model. Fur-thermore, the experimental signature is clean with high efficiency and low background,making the search for the (H±±) an ideal hunting ground for new physics.

By charge conservation, the doubly-charged Higgs boson can only decay to like-signleptons, W bosons and W±H±. For light H±± bosons, the branching ratio to W±W±

and W±H± is small. Hence either leptonic decays will prevail or the boson may have asufficiently long life-time to be detected as a doubly-charged particle.

The dominant H±± production mode at a hadron collider is in pairs via Z/γ exchange.The production cross section is O(0.1 pb) for H±± mass around 100 GeV. We exploitedthe quadrupled ionization of the H±± in this search, which was the Masters thesis topic

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of my graduate student Joshua Tuttle. We use an a-priori “blind-to-data” strategy todefine two separate sets of ionization cuts. The loose cuts, to be used exclusively forsetting a mass limit, seek to maximize efficiency. The tight cuts, to be used only toquote observation of H±± signal, further suppress the mis-identification backgrounds.The tight cuts were optimized for single-event sensitivity by using ionization informationfrom the calorimeters and the drift chamber. Loose cuts use drift chamber ionizationonly.

My cosmic-ray finder was particularly useful in obtaining a clean sample of cosmic-raysfor efficiency and background studies. Experimental backgrounds from muons, electrons,hadronic τ ’s, and jets were estimated using fake rates from either the data or appropriatesimulation samples and were shown to be extremely small, giving this search single-eventsensitivity.

Upon unblinding the signal data sample, we found no H±± candidates in either cate-gory. Figure 5 shows the theoretical and 95% C.L. cross section limits for our doubly-charged Higgs search. We set a lower mass limit of 133 GeV/c2 (109 GeV/c2) for thequasi-stable H±±

L (H±±R ) boson, which is much more stringent than the previous best

limit of 97.3 GeV/c2 from the DELPHI collaboration. The paper describing this resulthas been published in Physical Review Letters (CV # 10, TT # 5). I collaboratedwith my post-doc Christopher Hays on this publication.

) 2 Mass (GeV/c±±H90 100 110 120 130 140 150 160

Cro

ss S

ecti

on

(p

b)

0

0.1

0.2

L±±H

R±±H

R±± & HL

±±degenerate H

experimental limit(95% C.L.)

Figure 5: The theoretical and experimental H±± cross section limits for the loose ion-ization cuts.

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3.3.3 Z ′ Boson, Graviton and Heavy Dimuon Resonance

The Standard Model (SM) is usually viewed as an effective theory, expected to bemodified at higher energies. Larger symmetry groups, eg. those motivated above, mayundergo spontaneous symmetry breaking such that a broken U(1) gauge symmetry mayappear. Associated with it would appear a new, neutral heavy boson, called the Z ′

boson. Like the SM Z boson, the decay Z ′ → ll̄ provides an excellent experimentalsignature, due to the excellent efficiency and momentum resolution of the leptons.

Additional spatial dimensions are a possible explanation for the gap between the elec-troweak symmetry-breaking scale and the gravitational energy scale MPlanck [1, 2]. Inthe Randall-Sundrum (RS) scenario [1], the space-time metric varies exponentially in afourth spatial dimension, corresponding to the ground-state wave function of the gravi-ton which is localized on another brane in this dimension. The wave function overlapwith the SM brane is exponentially suppressed, thus explaining the apparent weakness ofgravity and the large value of MPlanck. This model predicts excited Kaluza Klein modesof the graviton, which are localized on the SM brane and therefore couple with SMparticles with electroweak strength. Such Randall-Sundrum gravitons G∗ would appearas spin-2 resonances in the process qq̄ → G∗ → µµ̄, with a narrow intrinsic width whenk/MPlanck < 0.1, where k2 is the spacetime curvature in the extra dimension. Finally,spin-0 resonances such as sneutrinos, qq̄ → ν̃ → µµ̄ are predicted by supersymmetrictheories with R-parity violation, in addition to the scalar Higgs bosons in the SM andits extensions.

I have designed and performed an analysis in the dimuon channel, to search 2.3 fb−1

of CDF Run II data for evidence of the production and decay process B → µµ̄, where Bdenotes a boson with spin-0, 1 or 2. I have performed a precise alignment and calibrationof the CDF drift chamber using cosmic rays, for this dataset, in order to achieve thebest momentum resolution possible. This work results in the narrowest possible dimuonmass peak, giving the best search sensitivity. Through my work on the W boson massmeasurement, I have developed a deep understanding of muon tracking, which is thekey aspect of this search. The cosmic ray tagger I developed has achieved very goodperformance, making the cosmic ray background negligible. The improvements I devel-oped in the drift chamber track reconstruction and fitting have allowed us to suppress“ghost” muons from π,K decays in flight to a very low level. I have also developed thestatistical methods to search the data for all possible mass values of a heavy dimuonresonance, and to quantify the significance of a potential signal. The binning of the datawas optimized using the momentum-dependent resolution, and full simulated lineshapeswere used to extract the most information from the data.

The search was designed as a “blind” analysis, i.e. the entire procedure was developed

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without access to the collider data. The resulting publication in Physical ReviewLetters (CV # 2, TT # 3) provides some of the world’s most stringent mass limitson supersymmetric neutrinos, Z ′ bosons and RS Gravitons, as shown in Table 2.

Z ′ Z ′ RS graviton graviton ν̃ ν̃model mass limit k/MPlanck mass limit λ2 · BR mass limitZ ′I 789 0.01 293 0.0001 397

Z ′sec 821 0.015 409 0.0002 441Z ′N 861 0.025 493 0.0005 541Z ′ψ 878 0.035 651 0.001 662

Z ′χ 892 0.05 746 0.002 731

Z ′η 904 0.07 824 0.005 810

Z ′SM 1030 0.1 921 0.01 866

Table 2: 95% C.L. lower limits on Z ′, graviton, and sneutrino masses (in GeV) forvarious model parameters [1, 7]. For the R-parity-violating sneutrino model, λ is thedd̄ν̃ coupling and BR denotes the ν̃, ˜̄ν → µµ̄ branching ratio.

I collaborated with Christopher Hays and Oliver Stelzer-Chilton on this paper.

4 Experimental Project Leadership

I served as the co-leader of the CDF Offline Analysis Project for a period of twoand half years, from July 2004 through December 2006. This is one of the seven mostimportant positions in the 700-member collaboration.

The Offline Analysis group is responsible for all CDF activity related to reconstructionand simulation software, offline operations related to software releases, calibrations, pro-cessing of data and simulation, and computing resources. The goal is to ensure that thephysics potential of the experiment is realized, by providing core analysis software andresources to the physicists. My strong background in physics analyses and convenership,stood me in good stead when leading the Offline Analysis Project.

I was responsible for management and coordination of ongoing operations, and forstrategizing future development of the offline analysis infrastructure. The data recordedby CDF grew by a factor of four during my term. Further increases by a factor of 4-5

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were anticipated in future running of the experiment. It was my responsibility to (a)understand and predict the physics analysis, software and computing needs of the ex-periment, (b) provide the best possible software for data reconstruction and simulation,(c) manage the day-to-day operations of the experiment related to detector calibrations,databases, timely data reconstruction and all the computing, (d) strategize, develop anddeploy new computing technology and infrastructure to meet the ever growing needs,and (5) develop and defend the computing budget of about $1.5 million per year.

In addition to these duties, I implemented some new initiatives of my own. I presentsome highlights and achievements during my term as Offline Project co-leader.

• Single-pass data reconstruction plan: Before I started my term, the prevailingmode of data processing was such there is a delay of about six months betweenrecording the raw data and availability of analysis-quality data. This caused a longdelay in the publication of results from the data, hurting the physics productivity ofthe experiment. The reason for the delay was the “double-processing”, where datacollected over 6-8 months were processed, then calibrated, and finally reprocessed.

I had the idea that this delay could be significantly reduced by moving to a newscheme of “single-pass” data processing. I developed the details of the new scheme,which I presented to the CDF collaboration and built a consensus to implement it.In this scheme, this latency was reduced to about 6 weeks. The idea was to pre-process small, well-defined calibration datasets, from which detector calibrationscould be speedily extracted and verified. These calibrations would be applied inthe reconstruction of collider data, segmented in small time periods. The wholeprocess would be repeated for sequential data-taking periods. This scheme in-creases the time available for analysis and also requires less effort and computingfor processing the data. The scheme was very successful and has been in use eversince I implemented it. The time to publication for CDF physicists has reduceddramatically as a result. Furthermore, the process of detector calibration was au-tomated significantly, increasing the reliability and effectiveness of the calibrationsand hence the data quality.

• New reconstruction software package: Many detector upgrades were per-formed in the time leading up to my term as Offline Project co-leader. Theseupgrades required new software to process the data, incorporating the detectorchanges. The Offline Project produced a new software package on schedule. Thesoftware also incorporated significant improvements to reconstruction algorithms,which had a positive impact on the physics capability of the experiment. Finally,a number of technical improvements in software and computing technology werealso incorporated into the package. This package has been used by the experimentfor the last four years, since it was released under my leadership. Improvements todrift chamber track reconstruction that resulted in increased acceptance at large

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rapidity, which were developed by my research group, are described in a publicationin Nuclear Instrumentation and Methods ( CV # 12, TT # 10).

• Upgrade of data processing platform: Motivated by the growing computingneeds for timely data processing, I initiated a project to create a new comput-ing platform for data processing. The goal was to create a scalable and moremaintainable platform which would also require less human effort to operate. Wewere successful in creating this platform, which is still in use after four years sincedeployment. This infrastructure has frequently set new records in demonstratedprocessing power. This project is described in two publications in Nuclear In-strumentation and Methods (CV # 6) and IEEE Transactions on Nu-clear Science (CV # 9).

• Distributed computing: The computing needs of a large collider experiment likeCDF are so large and growing so rapidly, that it cannot be satisfied by the resourcesat Fermilab. It became vital to use offsite computing facilities, including facilitiesin Canada, Europe and Asia, for CDF needs. This created a whole new challengewith technical, organizational and budgetary challenges. During my term, we dealtwith all three challenges. We developed and deployed a new distributed computingtechnology, which makes the remote computing facility appear dynamically (ie atrun-time) as if it is an extension of a local facility. Users are then able to run theirprograms on the remote facility in a transparent fashion, as if they are running ona local facility. This technology was therefore extremely convenient for the CDFphysicists, minimizing their time and effort spent on computing issues and allowingthem to focus on physics. Also, the dynamic nature of this technology, called“condor glide-in” technology, allows the computing facility to be shared easillybetween users, with efficient resource allocation. As a result, I was able to negotiatethe organizational and budgetary issues between collaborating institutions andinternational funding agencies.

Due to CDF’s success in this arena under my leadership, CDF has become aleading player in the multi-disciplinary “Open Science Grid” project of the USgovernment, in which distributed computing has been identified as a major toolfor scientific research. Distributed computing technologies such as the “glide-in”technology and network-accessible virtual file-servers with global namespaces thatI pioneered at CDF have since become integral components of the Open ScienceGrid.

I have organized several reviews of the CDF Offline Project, including annual and bi-annual reviews of progress, plans and budgets by Fermilab and international fundingagencies.

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5 Grants and Funding

Prior to receiving tenure, I received the Outstanding Junior Investigator (OJI) Awardfrom the Department of Energy, with an annual grant of $60,000. This OJI award wasindependent of the HEP group umbrella grant, and is funded in perpetuity.

I was selected to lead the Software and Computing Project of the CDF experimentstarting June 2004. My deputation to Fermilab for 21

2years as CDF project leader

generated revenue for Duke University. My academic salary during this period wasprovided by Fermilab, releasing equivalent funds for Duke University.

My leadership of this CDF project opened a new area of experimental activity for theDuke HEP group. I got Douglas Benjamin, a senior scientist supported by the DOEgrant, involved in the CDF Software and Computing Project in 2005. This was a newarea of activity for him, but under my supervision he gained valuable computing skillsand became a valued member of the CDF Software and Computing Project. He has heldnumerous positions of responsibility in this project, including head of the data-handlinggroup and head of the distributed computing group in CDF. As a result, we have beenable to justify and obtain funding to support Douglas Benjamin as a senior scientist.

Secondly, I negotiated partial salary support for Dr. Benjamin from Fermilab, ascompensation for his work on CDF computing. This support from Fermilab releasedfunds from the DOE grant for use towards other research efforts.

Dr. Benjamin has now transitioned his computing support work from CDF to ATLAS,which is welcomed by the ATLAS management due to the experience he gained onCDF with me. The Duke group’s involvement that I initiated, has thus proved quitefruitful for the long term. It continues to generate new funding opportunities: we nowreceive partial salary support for Dr. Benjamin from Brookhaven National Laboratory,in compensation of his work on ATLAS computing.

I am co-principal investigator of the Duke HEP grant from the Department of Energy.This umbrella grant funds the research of Duke professors Al Goshaw, Mark Kruse, SeogOh, Kate Scholberg, Chris Walter and myself on the CDF and ATLAS experiments atthe energy frontier, and neutrino physics. I will be the principal investigator/programdirector of this grant starting June 2009. This grant is renewable every three years.

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6 Future Research Plans

The CDF experiment will continue to record data for the next two years, bringing asubstantial increase in the precision of key measurements.

In the near term, I have planned an improved measurement of the W boson mass,increasing the precision by another factor of two beyond my published analysis. In thecontext of the Standard Model, the Higgs boson mass will be constrained to 30%, or aninconsistency with direct search limits will point to new physics.

The full Tevatron dataset offers the opportunity to either exclude or see 3σ evidenceof the SM Higgs boson if its mass is less than 200 GeV, the range preferred by theprecision electroweak fit. Certain extensions of the SM also motivate the mass of thelightest Higgs boson in this range. I plan to continue the SM Higgs boson search bybuilding on the tools I have recently developed.

I am developing my research program at the Large Hadron Collider (LHC), the newaccelerator being built in Europe which will produce particle collisions with 7× theenergy of the Tevatron, or 14 TeV. The energy scale of electroweak symmetry breakingis O(1 TeV), within the reach of the LHC. It is likely that a new and rich dynamics atthis energy will be revealed. Research at the ATLAS experiment is going to be my focusfor the forseeable future.

I plan to continue searches for new phenomena using novel methods to maximize thediscovery potential of the LHC. This research will build on my work on excited/exoticleptons, extended Higgs sectors and gauge symmetries at the Tevatron, as well as myexperience with advanced analysis methods that I have developed and used for the Wmass and top mass measurements and the SM Higgs search.

The task of reconstructing and analysing the LHC data will be extremely challenging,due to the large number of detector elements and the high luminosity environment. MyTevatron experience with software, algorithms and computing will help me make rapidprogress at the LHC, on the ATLAS experiment.

I am developing the tools to pursue two analyses of ATLAS data. A powerful signaturefor new physics is the presence of same-sign leptons in the event, for which the SMbackgrounds are small. As charge misidentification is an issue with tracking, I will workon tracking software at ATLAS in order to gain an in-depth understanding of trackingperformance and momentum/charge measurement at high momentum. Second, the topquark mass measurement has excellent potential for improvement at ATLAS, given thelarge top pair production cross section and luminosity and hence very high statistics.

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The dilepton channel has the advantage that only the two b quark jets need calibration.I propose to use the SM process ZZ → ll̄bb̄ for b quark jet calibration: the reconstructionof the Z → bb̄ decay can provide a precise in − situ calibration, in a very similar finalstate as top dilepton decays. Thus, a very precise top quark mass measurement shouldbe possible.

The LHC will be upgraded for higher luminosity around 2017, and this will requireupgrades to the ATLAS experiment to handle the increased data rate. The replace-ment being considered for the charged particle tracking detector is a detector comprisedcompletely of silicon sensors. I have a strong interest in this ATLAS upgrade project,because the tracking performance is the key to the success of the ATLAS physics pro-gram in the 2020’s. I have experience in electronics, and I plan to build the necessaryinfrastructure using the clean-room facilities in the Fitzpatrick Center at Duke.

7 Summary

My research focusses on physics beyond the Standard Model through the precisionmeasurement of the W boson mass and direct searches for new particle production. MyCDFW mass measurement was published as the most precise single measurement. I haveco-authored a comprehensive review article on W mass measurement at the Tevatron. Ihave also published the most precise measurements of the top quark mass in the dileptonchannel. My search for associated Z+Higgs production will be submitted for publicationin summer 2009.

My research on new particle production has centered on testing models of compos-iteness and extended symmetries. I have published the best limits on excited electronsand excited muon states. I have also published the best limits on the doubly-chargedHiggs boson in the long-lived mode. My search for heavy dimuon resonances has beenpublished, with some of the most stringent mass limits.

Since receiving tenure, I have six publications in Physical Review Letters, one eachin Physical Review D and Physical Review D Rapid Communications, one in AnnualReviews of Nuclear and Particle Science, two in Nuclear Instrumentation and Methods,and one in IEEE Transactions on Nuclear Science.

I have held several high-level leadership positions on the CDF experiment at Fermilab,including co-leadership of the CDF Software and Computing Project. I have recentlygiven nine invited plenary talks, and thirteen invited seminars at research universitiesand international institutes and laboratories. I am currently chairing the Fermilab Users

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Organization.

I have supervised four post-doctoral research associates, six graduate students andthree undergraduate students. I have supervised two Ph.D. theses, two Masters thesesand one undergraduate Honors thesis.

My work on the W boson mass measurements was cited for my election as Fellow ofthe American Physical Society.

References

[1] L. Randall and R. Sundrum, Phys. Rev. Lett. 83, 4690 (1999); L. Randall and R.Sundrum, Phys. Rev. Lett. 83, 3370 (1999).

[2] N. Arkani-Hamed, S. Dimopoulos, and G. Dvali, Phys. Lett. B 429, 263 (1998).

[3] P. Chankowski et al., Nucl. Phys. B417, 101 (1994); D. Garcia and J. Sola, Mod.Phys. Lett. A 9, 211 (1994); A. Dabelstein, W. Hollik and W. Mosle, in Perspec-

tives for Electroweak Interactions in e+e− Collisions, ed. by B. A. Kniehl (WorldScientific, Singapore, 1995) p. 345; D. Pierce et al., Nucl. Phys. B491, 3 (1997).

[4] G. Degrassi et al. Phys. Lett. B 418, 209 (1998); G. Degrassi, P. Gambino, andA. Sirlin, Phys. Lett. B 394, 188 (1997).

[5] U. Baur, M. Spira and P. M. Zerwas, Phys. Rev. D 42, 815 (1990), and referencestherein; E. Boos et al., Phys. Rev. D 66, 013011 (2002), and references therein.

[6] R. N. Mohapatra, Unification and Supersymmetry (Springer, New York, 1992), andreferences therein.

[7] D. Choudhury, S. Majhi, and V. Ravindran, Nucl. Phys. B 660, 343 (2003). Weassume the ν̃ and ˜̄ν have equal masses and couplings and contribute equally to apotential signal, as in W. Shao-Ming, H. Liang, M. Wen-Gan, Z. Ren-You and J. Yi,Phys. Rev. D 74, 057902 (2006).

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