A Search For Technicolor with the ATLAS Detector Jeremy Love

A Search For Technicolor with the ATLAS Detector

Jeremy Love

Outline

PreambleTitle slide, Outline

TheoryStandard ModelTechnicolorLSTC

Search StrategyExperimental Apparatus

ATLASMuon Spectrometer Transition ChambersPerformance

Dimuon mass resolution

Experimental TechniquesDatasets

Data, MCSelection criteria

Event displayInvariant Mass SpectrumSystematicsStatistical Methods

Signal Eff Comparison

Results1-D LSTC Limit

Combined and single lepton2-D combined LSTC Limit

Conclusions

4/26/12 Jeremy Love - ANL ATLAS Group 2

Motivation

Though investigated for many decades the Standard Model mechanism of Electroweak Symmetry Breaking has not yet been observed The Standard Model provides an accurate description of all experimental data

to date

To directly test the Standard Model at the TeV scale must produce interactions at that energy

In the past dilepton final states have uncovered unexpected physics, and led to early discoveries at new accelerators Famous examples include the J/ψ, Υ, and Z


Standard Model

Describes the interactions of matter fermions and force carrying bosons

Fermions grouped in two categories with three generations

Leptons – ElectroweakQuarks – Electroweak and Quantum Chromo Dynamics

BosonsConfirmed– γ, W±, Z, gluonsUnconfirmed – Higgs

Mechanism for Electroweak symmetry breaking (EWSB) has not been observed

4/26/12 4Jeremy Love - ANL ATLAS Group

Standard Model

In the Standard Model the coupling of W± and Z to the scalar Higgs give them masses which break Electroweak Symmetry

Fermions get masses through the same coupling to the Higgs field

Using experimental measurements to fit for the Higgs mass gives a preferred mass of 89 GeV

Ruled out by direct searchWhat is at 125 GeV?


Technicolor Theories

Technicolor models predict a new strong QCD like force responsible for EWSB

Techniquarks and technigluons form colorless technihadrons in analogy with the QCD spectrum

The lightest are the scalar πT0,±

and the vector ωT0 and ρT

0,±

The πT now give masses to the W and Z breaking EWS

With no Higgs boson the π of QCD breaks EWS

This correctly predicts the ratio of MW/MZ

Mass of MW and MZ low by 103

Gives EWSB with no fundamental scalar

What if the scale of QCD was 1000 GeV instead of 1 GeV?


Technicolor Phenomenology

The lightest states can be produced at colliders with sufficient energy

Produced through quark anti-quark annihilation

The vector mesons decay into πT[γ,W±,Z], and fermion pairs such as μμ and ee

Dominant background Drell-Yan processTechnihadrons do not directly couple to SM fermions


Low-Scale Technicolor

LSTC is a baseline technicolor model which describes the phenomenology of the light technihadrons

Implemented in PYTHIA at Leading OrderPreviously tested by D0, CDF, CMS

Techni-isospin symmetry is valid making ρT/ωT resonances degenerate in mass, they have an intrinsic width of order 1 GeV

Observed line shape is dominated by detector resolution

The ρT/ωT preferentially decay to multiple πT and πT plus SM gauge bosons if allowed

The difference of ρT/ωT to πT mass changes the available decay modesm(πT) = m(ρT/ωT) – 90 GeV allows for decays to πT/[W,Z]

In LSTC nothing keeps m(πT) light so it is expected to be greater than half the m(ρT/ωT ) For the benchmark parameter choice we take m(πT) = m(ρT/ωT ) – 100 GeV to allow for ρT/ωT to decay to πT/SM gauge boson


LSTC Cross Sections

Cross section times branching fraction of ρT/ωT to dimuons Also shown is the cross section times branching fraction dependence of ρT/ωT

on πT massIn LSTC m(πT) is expected to be close to m(ρT/ωT)

Jeremy Love - ANL ATLAS Group 94/26/12

MC normalized to number of data events in the Z peak

Search for new resonance every 40 GeV above 130 GeV

Search StrategySearch for new narrow resonances in the dilepton invariant mass spectrum

Using the ee and μμ final state

Combine measurements for increased sensitivity

Look for bump in smoothly falling spectrum

If no resonance observed set limits on cross section and mass of ρT/ωT

Most interesting region m(ρT/ωT ) = 200 – 600 GeV

Similar to SSM Z’ searchQuantify differences


ATLAS


Tracking Detectors – reconstruct particle momentum by measuring deflection in a magnetic field Muon Spetrometer – enclosed in toroidal field with ~4Tm bending power

Precision chambers measure curvature of track to determine pT

Fast chambers provide trigger and aid in reconstruction Inner Tracker – in a 2T solenoid field

Orthogonal momentum measurement to MS

Close to beam pipe good vertex information

Track based isolation

Calorimeters – measure energy of showeringparticles Measure e, γ, hadrons

Minimum ionizing particle

The ATLAS Muon Spectrometer uses four distinct detector technologies to provide the performance required Designed to achieve a resolution of 10% on 1TeV pT muon track Arranged in three stations each with a cylindrical barrel portion and two disk

shaped end caps Precision technologies Monitored Drift Tubes and Cathode Strip Chambers Fast response chambers Restive Plate Chambers and Thin Gap Chambers

Muon Spectrometer


Transition Region MDTsMDTs in the transition region are necessary to increase acceptance and measure point of inflection for tracks with low B dl or where three stations not otherwise crossed

Passing inside coils and then outside the returnMDT BEE chambers mounted on End Cap Toroid present unique challenges

Grounding and shielding issues, coherent noise, magnetic field dependent noise, long services, no optical alignment…

BEE commissioning able to reduce noise rate by ~103 and achieve high efficiency

Track based alignment hasimproved

End cap orientation have optical alignment and are stillbeing installed

Currently 36 out of 62


Dimuon Mass Resolution

Use resolution function to smear MC muons

Fitted smearing values from Z peak region, using alignment constraint

Barrel, Transition, End Cap

Dominant term is S2 the intrinsic curvature resolutionS0 is negligible

Smeared MC shows good agreement with data

Used in all ATLAS muon analyses


Impact on resolution estimated by shifting parameters

Impact on 1.5 TeV SSM Z’ sensitivity is 5%

Dataset and MC Samples

Data from 2011 periods B-IUse standard E/γ and Muon Good Runs Lists

Electrons – 1.08 fb-1

Muons – 1.21 fb-1

Background SamplesDrell-Yan

Pythia with LO* PDFsDiboson (WW, WZ, ZZ)

Herwig with LO* PDFsW+jets

ALPGEN with LO* PDFsTop

MC@NLO with NLO PDFs

Technicolor ρTC/ωTC SignalPythia, with LO* PDFs

K-factor corrected to NNLO Drell-Yan both EW and QCDTechnicolor signal to NLO

Same as SSM Z’4/26/12 15Jeremy Love - ANL ATLAS Group

Electron ChannelEvent Selection

Medium Electron Trigger 20 GeV thresholdE/gamma Good Runs List Primary Vertex with 3 tracks

Electron Object Selection|η| < 2.47 & ET > 25 GeVMedium electronIf expected 1 Blayer hitEtcone 20 < 7 GeV

Final event selectionTotal efficiency of 67%

Normalize between 70 GeV < Mee < 110 GeVSearch region Mee > 130 GeV


Dielectron Event Display


mee = 993 GeV

Mee Spectrum


Muon Selection CriteriaEvent selection

22 GeV Muon triggerPrimary vertex with 3 tracks

Muon object selectionMS and ID combined trackMuon pT > 25 GeVHit requirements for IDMS require hits in 3 stations with no transition or overlap hitsImpact parameter selectionIsolationOpposite charge

Final Event selectionTotal efficiency 42%

Normalize70 GeV < Mμμ < 110 GeVSearch Mμμ > 130 GeV


Dimuon Event Display


mμμ = 959 GeV

Dimuon Invariant Mass Distribution


Signal Comparison

Compare generator level distributions to determine difference in acceptance

Show good level of agreement in regions of interest

For fully simulated signalsFit the LSTC efficiency with the SSM Z’ efficiency function plus a constant

Fit gives good agreement and efficiencies are consistent within uncertainties


Systematic Uncertainties

Normalize sum of MC backgrounds to the Z region 70–110 GeV Removes mass independent systematics such as luminosity

Dominant systematic uncertainty comes from the PDF For SSM Z’ and ρT/ωT it was shown that differences in acceptance

are within the 1.5% and 4.5% efficiency systematics Same limits can be used for both models


Statistical Methods

Search invariant mass spectrum above 130 GeV using signal templates

SSM Z’ every 40 GeVA scan of mass versus cross section is performed

The most probable signal is determined

By means of a likelihood

Then the consistency of this signal with the background only hypothesis is determined

Dimuon – 24%Dielectron – 54%

Using a Bayesian approach 95% Confidence Level limits are set

Limits on signal cross section times branching ratio normalized to Z cross sectionSystematics are taken as nuisance parameters and marginalized

To combine channels the likelihood function is multiplied bin by bin

Dielectron and Dimuon


Excluded ranges of ρT/ωT mass at 95% CL from the dielectron and dimuon channels

Dielectron & Dimuon – 95% CL Limits


Dilepton – 95% CL Limits

Excluded ranges of ρT/ωT mass at 95% CL from the dilepton combined channel


Combined 2D Exclusion

Interpreting the 1D 95% CL on ρT/ωT vs πT cross section plane Simulated cross section at 833 points in plane with less than 25 GeV spacing For each ρT/ωT mass determine the πT mass where the production cross

section intersects the 95% CL excluded cross section using a linear interpolation

LSTC ρT/ωT masses are excluded between130 – 480 GeV For m(πT) between

50 – 480 GeV


Status of ATLAS Exotics Searches


This Analysis

Conclusions

Using over 1 fb-1 of 7 TeV proton proton collisions taken with the ATLAS detector we exclude m(ρT/ωT) between 130 – 480 GeV for m(πT) between 50 – 480 GeV at 95% CL This represented the worlds best limit on the Low-scale technicolor model

For the parameter choice of m(πT) = m(ρT/ωT) – 100 GeV masses of the ρT/ωT are excluded below 470 GeV at 95% CL In the dimuon channel masses of ρT/ωT are excluded below 280 GeV and

between 304 and 376 GeV at 95% CL In the dielectron channel masses of ρT/ωT are excluded below 323 GeV and

between 386 and 445 GeV at 95% CL

Analysis of the full 2011 run with 5 fb-1 nearing completion Updated muon object selection Minimal Walking Technicolor as well as Low-scale Technicolor

Including technicolor axial vector in addition to the ρT/ωT

Dedicated technicolor templates in limit setting framework

Thank you.


Additional Material

Electron QCD Estimation

Reverse identificationLoose 2 γ trigger – 20 GeVRequire 2 loose electrons

Failing strip hit requirementLead electron isolated

Fit spectrum with dijet function:

Fit to data with function and sum of MC backgroundsGood agreement

Cross checksIsolation Fit MethodFake Rate Method


Dielectron Event Yields Per Mass Bin


Dimuon Event Yields Per Mass Bin


Electron 2-D Posterior Probability


Muon 2-D Posterior Probability


Documents

A Search For Technicolor with the ATLAS Detector Jeremy Love