In Search of Lonely Top Quarks at the Tevatron

New (BSM) Physics Searches with Single Jets: Jet Substructure and Jet GroomingLPC @ Fermilab 11.18.10Steve Ellis

(See Also Jesse Thaler last week)

1Background/Assumptions:LHC is intended to find new stuff (BSM physics)

Hadronic stuff will be organized into jets

At 14 (7) TeV many interesting particles (t, W, Z, Susy, ..) will be boosted enough to be in a single jet

Want to use single jets in search for BSM physics

Want to distinguish jets with decays from QCD jets

Want to use jet substructure for this purposeLPC @ Fermilab S.D. Ellis 11/18/102

Outline & IssuesBrief review of (QCD) jets

- defined by algorithms (no intrinsic definition)

- jets have substructure, including masses (not just 1 parton, 1 jet)Typically use Recombination (kT) jets

natural substructure, but also

algorithm systematics (shaping of distributions) contributions from (uncorrelated) ISR, FSR, UE and Pile-up

But can use Cone or Anti-kT as jet finder

LPC @ Fermilab S.D. Ellis 11/18/103

Warning!Outline & Issues (contd)Search for BSM physics in SINGLE jets at the LHC by separating from QCD; want generic techniques, which are robust under systematic effects Consider a variety of jet substructure measures:

1) masses (simply look for bumps) 2) angularities (single jet version of thrust-like) distribution

3) subjets found with jet algorithm with smaller size parameter, e.g., taggers top q, Higgs, or groomers that delete all but the leadingsubjets

LPC @ Fermilab S.D. Ellis 11/18/104Outline & Issues (contd)QCD yields a large but Smooth (limited structure) QCD background

But substructure (mass bumps) in signal can degraded by

algorithm systematics

uncorrelated UE and Pile-Up contributions

Need to clean-up or Groom the jets, e.g., PRUNING

- remove large angle, soft branchings

Validate with studies of boosted top qs, Ws, Zs at Tevatron and LHC

LPC @ Fermilab S.D. Ellis 11/18/105Jet Substructure a new tool for the LHC!

Two general approaches

1) Taggers that use specific decay properties, e.g., top quarks, W/Zs, Higgs, to predict jet substructure (subjets)

2) More generic jet grooming to let the underlying structure, e.g., mass, be manifest (even when dont know what it is, i.e., searches) and reduce impact of UE, PU and algorithm details

See the overview in the (immediately) forthcoming Proceedings of Boost 2010!LPC @ Fermilab S.D. Ellis 11/18/106LPC @ Fermilab S.D. Ellis 11/18/10Defining Jets Map the observed (hadronic) final states onto the (short-distance) partons by summing up all the approximately collinear stuff, ideally on an event-by-event basis. Need rules for summing jet algorithmStart with list of particles/towersEnd with list of jets (and stuff not in jets)E.g.,

Cone Algorithms, based on fixed geometry focus on core of jet

Simple, well suited to hadron colliders with Underlying Events (UE),but found jets can/do overlap

Recombination (or kT) Algorithm, based on pairwise merging to undo shower

Tends to vacuum up soft particles, well suited to e+e- colliders 77LPC @ Fermilab S.D. Ellis 11/18/10Recombination Algorithm focus on undoing the shower pairwise, Natural definition of substructureMerge partons, particles or towers pairwise based on closeness defined by minimum value of kT, i.e. make list of metric values(rapidity y and azimuth , pT transverse to beam)

If kT,(ij) is the minimum, merge pair (add 4-vectors), replace pair with sum in list and redo list;

If kT,i is the minimum i is a jet! (no more merging for i, it is isolated by D),

1 angular size parameter D (NLO, equals Cone for D = R, Rsep = 1), plus

= 1, ordinary kT, recombine soft stuff first

= 0, Cambridge/Aachen (CA), controlled by angles only

= -1, Anti-kT, just recombine stuff around hard guys cone-like (with seeds)

88LPC @ Fermilab S.D. Ellis 11/18/10Recombination Algorithm the good and bad newsJet identification is unique no merge/split stage as in Cone

Everything in a jet, no Dark Towers as in Cone

Resulting jets are more amorphous, energy calibration difficult (need area for subtraction for UE?), Impact of UE and pile-up not so well understood, especially at LHC but less of an issue for Anti-kT

Analysis can be very computer intensive (time grows like N3, recalculate list after each merge)

New version (Cacciari, Salam & Soyez) goes like N ln N (only recalculate nearest neighbors) , plus scheme for finding area and doing UE correction

They have been used and understood at the Tevatron Using Anti-kT at LHC, so well understood soon, but it does not provide useful substructure, but could find jets with Anti-kT and substructure with CA/kT

99LPC @ Fermilab S.D. Ellis 11/18/10Recombination Algorithm in action, here CA algorithm on QCD jet 10

Think of starting with calorimeter cells, recombine closest pair at each step leading to larger pT

low pT to high pT

For CA close in quantity (0.05 x 0.05) Cells with E > 1 GeV10Note: the details of the substructure (at each step) depend on the algorithmLPC @ Fermilab S.D. Ellis 11/18/1011

CA (just angle) kT (pT and angle)Jet Masses in QCD: A Brief ReviewIn NLO PertThy LPC @ Fermilab S.D. Ellis 11/18/10

Phase space from pdfs, f ~ 1 & constDimensionsJet Size, D = R ~ , determined by jet algorithm

Useful QCD Rule-of-Thumb12

Peaked at low mass(log(m)/m behavior), cuts off for (M/P)2 > 0.25 ~ D2/4 (M/P > 0.5) large mass cant fit in fixed size jet, QCD suppressed for M/P > 0.3 (~ < 3)

Want heavy particle boosted enough to be in a jet (use large-ish D ~1), but not so much to be QCD like (~ 2 < < 5)

Soft Collinear pole versionJet Mass in PYTHIA (matched set)D = 1, 500 GeV/c < pT < 700 GeV/c

LPC @ Fermilab S.D. Ellis 11/18/1013Turns overAlgorithm mattersTails depends (somewhat) on being matched set

Jet Mass CDF Data (CDF/PUB/JET/PUBLIC/10199 7/19/10)LPC @ Fermilab S.D. Ellis 11/18/1014

At least qualitatively the expected shape masses slightly larger than MC need the true hard emissions (as in matched sets)Large mass tail grows, as expected, with jet size parameter in the algorithm -You find what you look for!Angularity a jet shape measure(introduced by G. Sterman and collaborators)(Full) Event shapes in e+e- - (from early days)First define Thrust direction and value via

wrt the Thrust direction define angularity with parameter a

Note for a = 0 just (1-T). For massless particlesLPC @ Fermilab S.D. Ellis 11/18/1015

Angularity Apply to single jetFind jet components with algorithm, jet direction replaces Thrust directionSingle Jet Angularity some choice involved, e.g., in CDF note Public/10199 (following Sterman, et al.)

where this is applied to jets binned in both pT,J (EJ) and mJThis results in a limited range for general a

LPC @ Fermilab S.D. Ellis 11/18/1016

16CDF data

LPC @ Fermilab S.D. Ellis 11/18/1017

Cone, R = 0.4Cone, R = 0.7Theory = minimal double pole expressionAngularity Apply to single jet - IIFind jet components with algorithm, jet direction replaces Thrust directionSingle Jet Angularity a different choice, Q 2E (SDE, Hornig, Lee, Vermilion & Walsh, 1001.0014, applied with SCET in e+e-)

now applied to jets binned only in pT,J (EJ) and we have

ie., factor of m/2E compared to earlier. AlsoLPC @ Fermilab S.D. Ellis 11/18/1018

Recall earlierA narrow jetSCET Results NLO + NLL (in global logs) no hadronization

LPC @ Fermilab S.D. Ellis 11/18/1019Distributions: Broader as R (~D) increases

Broader as a increases

Broader for gluons General features: Damped at small (Sudakov)

Peak and then falls off Shoulder at large

More SCET: Compare to MC, look at peak

LPC @ Fermilab S.D. Ellis 11/18/1020Peak moves with R and aDistribution qualitatively like MC, but hadronization is relevant (broader)The Future for AngularitiesData plot for other definition either without mass binning or in multiple mass bins (even with top contamination) test for systematics noted above

Theory evaluate the double distribution LPC @ Fermilab S.D. Ellis 11/18/1021

-30Pythia VersionWith a = -3

Finding Heavy Particles with Jets - Issues

ttbarQCD dijet QCD multijet production rate >> production rate for heavy particles In the jet mass spectrum, production of non-QCD jets may appear as local excesses (bumps!) but must be enhanced using analyses Use jet substructure as defined by recombination algorithms to refine jets Algorithm will systematically shape distributionsUse top quark as surrogate new particle.ttbar 10-3jjarb. unitsarb. unitsfalling, no intrinsic large mass scaleshaped by the jet algorithm22LPC @ Fermilab S.D. Ellis 11/18/10mJ (GeV/c2)mJ (GeV/c2)interpret last recombinationsas a heavy particle or QCD jetThe algorithm metricaffects the substructure- introduces biasReconstruction of Jet Substructure QCD vs Heavy ParticleWant to identify a heavy particle reconstructed in a single jet via substructureNeed correct ordering in the substructure and accurate reconstruction (to obtain masses accurately)Need to understand how decays and QCD differ in their expected substructure, e.g., distributions at branchings.

But jet substructure affected by the systematics of the algorithm, and by kinematics when jet masses/subjet masses are fixed.23LPC @ Fermilab S.D. Ellis 11/18/10Systematics of the Jet Algorithm Consider generic recombination step: i,j p

Useful variables: (Lab frame)Merging metrics:

Daughter masses (scaled by jet mass) :

In terms of z, R, the algorithms will give different kinematic distributions:

CA orders only in R : z is unconstrained

kT orders in z R : z and R are both regulated

The metrics of kT and CA will shape the jet substructure.

24LPC @ Fermilab S.D. Ellis 11/18/10

Phase Space for 12 The allowed phase space in R, z for a fixed (mJ and pT) is nearly one-dimensional

QCD and decays will weight the phase space differentlyCutoffs on variables set by the kinematics, not the dynamicsSample of phase space slices for different subjet masses,a1 = 0, a2 = 0a1 = 0.3, a2 = 0.1a1 = 0.9, a2 = 0a1 = 0.46, a2 = 0

25LPC @ Fermilab S.D. Ellis 11/18/102 is softer1 is softer1 never softer

Goal is to identify jets reconstructing a heavy particle and separate them from QCD jetsConsider a 12 decay (J 1,2) reconstructed in a jet, massless daughters (a1 = a2 = 0) for nowRequirement to be in a jet: R12 < D - algorithm independentLook at the decay in terms of the algorithm variables12 Decay in a JetLarge D is needed to reconstruct jets with a lower boost - use D = 1.0,sweet spot ~ 3, pT ~3 mJ, xJ ~ 0.126LPC @ Fermilab S.D. Ellis 11/18/10Recall for QCD jet pT ~5

12 Decay in a Jet (unpolarized)No enhancement at the lower limit in - unlike QCD

Enhancement at the lower limit for - like QCD

Decays not reconstructed: small , large

211beamdirectionJ (boost to the lab)J2lab frameJ rest frame27LPC @ Fermilab S.D. Ellis 11/18/10

Cutoffs are set by the kinematics- same between QCD and decaywith fixed

a1 = 0, a2 = 0

12 Decay in a JetNo enhancement at the lower limit in - unlike QCD

Enhancement at the lower limit for - like QCD

28LPC @ Fermilab S.D. Ellis 11/18/10

QCD SplittingsTake a leading-log approximation of QCD:For small angles - good approximation for a splitting in a jet:This lets us fix xJ (or ). Distribution in xJ :

29LPC @ Fermilab S.D. Ellis 11/18/10

a1 = 0, a2 = 0QCD Splittings: R12 and z

Enhancement at the lower limit in z - unlike decaysEnhancement at the lower limit in - like decays

Fix (xJ ), find distributions in R12 and zLimits set by the kinematicsQCD will have many more soft (small z) splittings than decays do - QCDsplittings are small z, small xJ enhanced30LPC @ Fermilab S.D. Ellis 11/18/10Summary of Dynamics of QCD Vs Decays:Distributions in R very similar (for fixed boost)QCD enhanced at small z, xJ Will these be represented in the last recombinations of a jet?31LPC @ Fermilab S.D. Ellis 11/18/10Recombination metrics:Recombinations are almost always monotonic in the metricThe algorithm cuts out phase space in (z, Rij) as it proceeds

Certain decays will be reconstructed earlier in the algorithm, or not at all

Effects of the Jet Algorithm Algorithm BiasCAkTlateearlyintermediatelateearlyintermediatepTp dependentboundaries32LPC @ Fermilab S.D. Ellis 11/18/10

Typical RecombinationsLate recombinations are set by the available phase spaceFor CA, R must be near D, and the phase space tends to create small z recombinationsFor kT, z R will be larger, with a pT dependent cutThe soft (small z) radiation is recombined earlier in kT, meaning it is harder to identify - leads to poorer mass resolutionMatched QCD sample (2, 3, 4 partons) from MadGraph/Pythia, jet pT between 500-700 GeV

last recombinationlast recombination33LPC @ Fermilab S.D. Ellis 11/18/10Comparing CA and kT:Final recombinations for CA not QCD-like- No enhancement at small RFinal recombinations for kT more QCD-like- Enhanced at small z and RkT has poorer mass resolution- Soft objects recombined early in algorithm - more merged

tt sample from MadGraph/Pythiajet pT between 500-700 GeV-34LPC @ Fermilab S.D. Ellis 11/18/10

Matched QCD, jet pT between 500-700 GeVSummary: Identifying Reconstructed Decays in JetsReconstruction of a decay can be hidden in the substructure Small z recombination unlikely to accurately give decay Small z recombinations also arise from UE and pile-up The jet algorithm significantly shapes the jet substructure less so for kT but has poorer mass resolution Proposing a method to deal with these issues: modify the jet substructure to reduce algorithm effects and improve mass resolution, background rejection, and heavy particle identification - pruning35LPC @ Fermilab S.D. Ellis 11/18/10Pruning :Procedure:Start with the objects (e.g. towers) forming a jet found with a recombination algorithm (kT, CA, Anti-kT)

Rerun with kT or CA algorithm, but at each recombination test whether soft large angle:z < zcut and Rij > Dcut

If true (a soft, large angle recombination), prune the softer branch by NOT doing the recombination and discarding the softer branch (bottom up approach)

Proceed with the algorithm

The resulting jet is the pruned jet36LPC @ Fermilab S.D. Ellis 11/18/10

SDE, Jon Walsh and Chris Vermilion, 0903.5081, 0912.0033

- go to tinyurl.com/jetpruningChoices of the pruning parameters

no pruningno pruningover-pruningover-pruning

shower pruning

optimalpruningCA: zcut = 0.1 and Dcut = mJ/PT,JkT: zcut = 0.15 and Dcut = mJ/PT,J mJ/PT,J is IR safe measureof opening angle of found jet and adaptive to the specific jet37LPC @ Fermilab S.D. Ellis 11/18/10After studies we choose:Prune Fixed Order Result

LPC @ Fermilab S.D. Ellis 11/18/1038Move small masses to zero mass(asymmetric splitting)Large mass tail unchanged(symmetric splitting)

Pruning in ActionRed is higher pTBlue is lower pTGreen X is a pruningStart with cells withenergy > 1 GeVPruning of a QCD jetnear the top masswith the CA algorithmpT: 600 590 GeVmass: 170 160 GeVa typical jet (see above)

zR39LPC @ Fermilab S.D. Ellis 11/18/10Prune

Pruning in ActionRed is higher pTBlue is lower pTGreen X is a pruningStart with cells withenergy > 1 GeVPruning of a QCD jetnear the top masswith the CA algorithmpT: 600 550 GeVmass: 180 30 GeVatypical jet

zR40LPC @ Fermilab S.D. Ellis 11/18/10Prune

Impact of Pruning qualitatively just what we want!Top jetsQCD jetsCAkTThe mass resolution of pruned top jets is narrower Pruned QCD jets have lower mass, sometimes much lower41LPC @ Fermilab S.D. Ellis 11/18/10500 < pT < 700 GeV

WinWinTest Pruning in more detail:Study of top reconstruction:Hadronic top decay as a surrogate for a massive particle produced at the LHCUse a QCD multijet background based on matched samples from 2, 3, and 4 hard parton MEsME from MadGraph, showered and hadronized in Pythia, jets found with FastJet

Look at several quantities before/after pruning:

Mass resolution of reconstructed tops (width of bump),small width means smaller background contribution

pT dependence of pruning effect (bin in pT)Dependence on choice of jet algorithm and angular parameter DUE dependence42LPC @ Fermilab S.D. Ellis 11/18/10

Defining Reconstructed Tops Search ModeA jet reconstructing a top will have a mass within the top mass window, and a primary subjet mass within the W mass window - call these jets top jets

Defining the top, W mass windows:Fit the observed jet mass and subjet mass distributions with (asymmetric) Breit-Wigner plus continuum widths of the peaks

The top and W windows are defined separately for pruned and not pruned - test whether pruning is narrowing the mass distribution

prunedunprunedsample mass fit43LPC @ Fermilab S.D. Ellis 11/18/10

Defining Reconstructed Topsfit mass windows to identify a reconstructed top quarkfit top jet masspeak width jet2jetpeak function: skewed Breit-Wigner

plus continuum background distribution

44LPC @ Fermilab S.D. Ellis 11/18/10

Defining Reconstructed Topsfit mass windows to identify a reconstructed top quarkcut on masses of jet (top mass) and subjet (W mass)fit W subjet massfit top jet masspeak width jet2jet2145LPC @ Fermilab S.D. Ellis 11/18/10

Defining Reconstructed Topsfit mass windows to identify a reconstructed top quarkcut on masses of jet (top mass) and subjet (W mass)window widths for pruned (pX) and unpruned jetsfit top jet massfit W subjet mass46LPC @ Fermilab S.D. Ellis 11/18/10Mass Windows and Pruning - SummaryFit the top and W mass peaks, look at window widths for unpruned and pruned (pX) cases in (200 - 300 GeV wide) pT bins

Pruned windows narrower, meaning better mass bump resolution - better heavy particle ID

Pruned window widths fairly consistent between algorithms (not true of unpruned), over the full range in pT47LPC @ Fermilab S.D. Ellis 11/18/10

Statistical Measures:Count top jets in signal and background samples in fitted bins

Have compared pruned and unpruned samples with 3 measures: , R, S - efficiency, Sig/Bkg, and Sig/Bkg1/2

Here focus on S

48LPC @ Fermilab S.D. Ellis 11/18/10S > 1 (improved likelihood to see bump if prune), all pT, all bkgs, both algorithms

D = 1Heavy Particle Decays and D

Heavy particle ID with the unpruned algorithm is improved when D is matched to the expected average decay angle

Rule of thumb (as above): R = 2m/pT

Two cases:RDD > R lets in extra radiationQCD jet masses largerRDD < Rparticle will not be reconstructed49LPC @ Fermilab S.D. Ellis 11/18/10See also Krohn, Thaler & Wang (0903.0392)Improvements in PruningOptimize D for each pT bin: D = min(2m/pTmin, 1.0) (1.0,0.7,0.5,0.4) for our pT bins

Pruning still shows improvements

How does pruning compare between fixed D = 1.0 and D optimized for each pT bin SD = SD opt/SD=1? Little further improvement obtained by varying D

SD = 1 in first bin

Pruning with Fixed D does most of the work50LPC @ Fermilab S.D. Ellis 11/18/10

Underlying Event Rejection with Pruningtop study

no pruningpruningCAkT

The mass resolution of pruned jets is (essentially) unchanged with or without the underlying event51LPC @ Fermilab S.D. Ellis 11/18/10500 < pT < 700 GeV

Underlying Event Rejection with PruningQCD studyno pruningpruningCAkTThe jet mass distribution for QCD jets is significantly suppressed for pruned jets (essentially) independent of the underlying event52LPC @ Fermilab S.D. Ellis 11/18/10

500 < pT < 700 GeVPruning reveals for hidden truth - LPC @ Fermilab S.D. Ellis 11/18/1053

Other Groomers (Boost 2010 defn) applied to found jets (Anti-kT, R = 1.0)Trimming Re-cluster into Anti-kT subjets with Rsub = 0.35, discard if pTsub < 0.03 pTjet

Krohn, Thaler & Wang (0912.1342)

Filtering - Re-cluster into C/A subjets with Rfilt = 0.35, kept only 3 hardest subjets

Butterworth, Davison, Rubin & Salam (0802.2470) Seymour (Z. Phys. C62 (1994) 127) LPC @ Fermilab S.D. Ellis 11/18/1054Results from Boost 2010mass distribution in pT bins

LPC @ Fermilab S.D. Ellis 11/18/1055Narrower peaksSuppressed background300-400500-600Compare top taggers

LPC @ Fermilab S.D. Ellis 11/18/1056Similar exponential correlation for all optimized taggers (lowest mistag rate for given efficiency)ATLAS, T/W use similar kT reclustering and no groomingCMS, Hopkins use similar C/A reclustering and filteringPruning as top tagger -

LPC @ Fermilab S.D. Ellis 11/18/1057Eff = 20% (use late)Eff = 50% (use early)zcut = 0.1, Dcut = 0.4 m/pT 68 < MW/GeV < 88, 150 < Mtop/GeV < 190Zcut = 0.05, Dcut = 0.2 m/pT 28 < MW/GeV < 128, 120 < Mtop/GeV < 228By choice not optimizing for Signal/BGK, i.e., broad mass binsSummary - PruningPruning narrows peaks in jet and subjet mass distributions of reconstructed top quarks

Pruning improves both signal purity (R) and signal-to-noise (S) in top quark reconstruction using a QCD multijet background

The D dependence of the jet algorithm is reduced by pruning - the improvements in R and S using an optimized D exhibit only small improvement over using a constant D = 1.0 with pruning

A generic pruning procedure based on D = 1.0 CA (or kT) jets can Enhance likelihood of success of heavy particle searchesReduce systematic effects of the jet algorithm, the UE and PUCannot be THE answer, but part of the answer, e.g., use with b-tagging, require correlations with other jets/leptons (pair production), use more than one groomer, etc.

58LPC @ Fermilab S.D. Ellis 11/18/10Summary general:Systematics of the jet algorithm are important in studying jet substructureThe jet substructure we expect from the kT and CA algorithms are quite differentShaping can make it difficult to determine the physics of a jet

Should certify Groomers and Taggers in data by finding tops, Ws and Zs in single jets in early LHC running and with Tevatron data

Should study other shape variables, e.g., angularities, in data

59LPC @ Fermilab S.D. Ellis 11/18/10But only 10s of well boosted tops in 100 pb-1 at 7 TeV (pT > 500 GeV/c)Summary general:Much left to understand about jet substructure (here?), e.g.,

How does the detector affect jet substructure and the systematics of the algorithm? How does it affect techniques like pruning? What are experimental jet mass uncertainties?

How can jet substructure fit into an overall analysis? How orthogonal is the information provided by jet substructure to other data from the event?Which tools work best when used together?Better theory tools for multiple scales, pJ and mJ SCET

60LPC @ Fermilab S.D. Ellis 11/18/10LPC @ Fermilab S.D. Ellis 11/18/10Extra Detail Slides6161Jets a brief history at Hadron CollidersJETS I Cone jets applied to data at the ISR, SpbarpS, and Run I at the Tevatron to map final state hadrons onto LO (or NLO) hard scattering, essentially 1 jet 1 parton (test QCD)

Little attention paid to masses of jets or the internal structure, except for energy distribution within a jet studied at leading order or with MC

JETS II Run II & LHC, starting to look at structure of jets: masses and internal structure a jet renaissance

LPC @ Fermilab S.D. Ellis 11/18/1062

(Some) Other Jet Grooming techniques:LPC @ Fermilab S.D. Ellis 11/18/1063Specific tagging, e.g., top or Higgs tagging

MassDrop/Filtering - Butterworth, Davison, Rubin & Salam (0802.2470) - reprocess jets (top down) to find (fixed number) of subjets (2 or 3)

Top Tagging - Kaplan, Rehermann, Schwartz & Tweedie (0806.0848) look for specific substructure of tops (3 or 4) Thaler & Wang (0806.0023)

Generic Grooming

Trimming Krohn, Thaler & Wang (0912.1342) reprocess to find primary subjets (pT > pTcut, any number of subjets)

Trimming & Pruning, Soper & Spannowsky (1005.0417)

64LPC @ Fermilab S.D. Ellis 11/18/10

65LPC @ Fermilab S.D. Ellis 11/18/10Note: Top jet with CALPC @ Fermilab S.D. Ellis 11/18/1066PruningNo Pruning

Consider impact of (Gaussian1) smearingLPC @ Fermilab S.D. Ellis 11/18/1067

Smear energies in calorimeter cells with Gaussian width (300 GeV/c < pT < 500 GeV/c)1 From P. Loch

Pruning still helps (pruned peaks are more narrow), but impact is degraded by detector smearingQCDStatistical Measures:LPC @ Fermilab S.D. Ellis 11/18/1068RSNoSmearingpCA/CA0.902.251.42pkT/kT0.683.011.44ReasonableSmearingpCA/CA0.981.751.31pkT/kT0.722.201.26WorstSmearingpCA/CA1.001.591.26pkT/kt0.742.001.22

Smearing degrades but does not eliminate the value of pruning

Simulated data plots (Peskin plots)Include signal (tops) and bkg (QCD) with correct ratio and simulated statistical uncertainties and fluctuations, corresponding to 1 fb-1 (300 GeV/c < pT < 500 GeV/c)LPC @ Fermilab S.D. Ellis 11/18/1069 Find (small) mass bump and cut on itFind daughter mass bump and cut on itNow a clear signal in jet massPruning enhances the signal, but its still tough in a real search

For known top quark, pruning + 100 pb-1 may be enough (especially with b tags)