Search for high-mass dilepton resonances with the ATLAS detector - - PowerPoint PPT Presentation

Search for high-mass dilepton resonances with the ATLAS detector

Sarah Heim, Michigan State University

Experimental Particle Physics Seminar University of Pennsylvania, 02.28.2012 1

Overview

Sarah Heim 2

• 1. Why are we looking for high-mass dilepton resonances?
• 2. 1fb-1 analysis (Phys. Rev. Lett. 107, 272002 (2011))
• event selection
• backgrounds
• high energy electrons and muons
• signal search and limit setting
• 3. 5 fb-1 analysis (in preparation)
• updates

Why are we looking for Physics beyond the SM?

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The Standard Model of Particle Physics is a very successful theory, but cannot be the end of the story... For example, it doesn't

• have a dark matter candidate
• explain, why gravity is so weak

compared to the other fundamental forces Also: Before we find the Higgs, we cannot be sure of how electroweak symmetry is broken

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Search for dilepton resonances (ee/μμ)

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? q/g q/g l+ l-

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Search for dilepton resonances

5 Here be dragons

Dilepton resonances have been the window to a better understanding of elementary particles and forces before...

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Search for dilepton resonances

Dilepton resonances could be a signature of

• new heavy gauge boson in the E6 model (Grand Unified Theory model)
• ---> spin-1
• excited Kaluza-Klein mode of the Randall Sundrum graviton
• ---> spin-2

...and many others (resonance search is fairly model independent)

• Benchmark model: Sequential Standard Model Z' (same couplings as

Z boson), not theoretically motivated

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Additional symmetries

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Dilepton resonances could be a signature of

• new heavy gauge boson in the E6 model (Grand Unified Theory model)

(Phys. Rev. D 34 (1986), arXiv:0801.1345v3) GUT theories: Unification of electroweak and strong forces at high energies

• ---> 1 overall symmetry, which breaks down at lower energies

2 additional U(1) groups lead to Z' Several motivated choices of θE6

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Randall Sundrum graviton

8 Dilepton resonances could be a signature of

• excited Kaluza-Klein mode of the Randall Sundrum graviton

(arXiv:hep-ph/9905221v1) 1 finite warped extra dimension, 2 branes Only gravitons propagate to extra brane

• ---> wave functions are suppressed away from extra brane
• ---> gravity is weak!

Finite extra dimension

• ---> excitation like in harmonic oscillator possible
• ---> Kaluza-Klein tower of massive graviton states

Planck brane Standard brane extra dimension G(x) 2 parameters:

• m1 (first excitation)
• k (curvature)

Sarah Heim 1 0.1

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1 fb-1 analysis

Event Selection – electrons

10 Main background: Acceptance (Z', 1.5 TeV): 67%

• ATLAS data quality (stable beam, functioning subdetectors etc.)
• pick two standard electrons, central (η < 2.47), ET > 25 GeV
• require track and EM shower shape cuts (“Medium”)
• require hit in innermost layer of detector
• leading electron should be isolated (calorimeter based)
• --> energy in a cone of 0.2 around the electron < 7 GeV
• form invariant mass, require mass > 70 GeV

Z/γ* Sarah Heim

Event Selection – muons

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• ATLAS data quality (stable beam, functioning subdetectors etc.)
• pick two standard combined muons, pT > 25 GeV
• require 3 hits in 3 muon spectrometer layers,

no overlap barrel-endcap, veto misaligned chambers

• distance from primary vertex needs to be small
• require tracks to be isolated
• muons must have opposite charge
• form invariant mass, require mass > 70 GeV

Main background: Acceptance (Z', 1.5 TeV): 42% Z/γ*

Backgrounds

12 Backgrounds with two prompt electrons/muons:

• Drell Yan
• WW,WZ,ZZ
• ttbar (dileptonic decay)

Backgrounds with QCD jets, which can fake prompt leptons

• W+jets
• QCD multijet production
• ---> what is the fake rate at high energies?

Drell Yan is dominant background by far, all background except for QCD multijet taken from simulated samples Sarah Heim

QCD multijet background electrons

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Jets can fake electrons. How large is the fake rate?

• 1. Baseline Method

►Reverse Identification

• dijet shape from reverted electron identification cuts
• extrapolation to high invariant

masses by fitting with empirical function

• normalization by 2-component template fit
• 2. Cross-check and systematic uncertainties

►Isolation fit method

• use calorimeter isolation distributions
• fit signal/background templates

from data for 1st and 2nd electron

• system of equations

to avoid double counting ►Fake rate estimate

• measure probability for jet-like
• bjects to pass Z' selection (η, ET)
• apply fake rate on normalization

sample (Z' selection on leading, jet selection on second electron)

QCD multijet background muons

14 QCD multijet background much smaller for muons

• 1. Shape:

Anti-track-isolated data (0.1 - 1.0)

• 2. Normalization:

Ratio of isolated (0.00-0.05) /anti-isolated (0.1-1.0) dimuon events in QCD (heavy flavor) simulated samples QCD from simulation Sarah Heim

High pT leptons

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Looking for resonances at high invariant masses

• --> need to understand properties of highly energetic objects in ATLAS

Very small control sample, handles: calibration runs, cosmics, Tag-and-Probe around Z pole ---> extrapolation, simulation

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High pT leptons – resolution

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Electrons (Resolution 1.1 – 1.8% at 1 TeV):

• energy measurement from electromagnetic calorimeter
• resolution at high energies dominated by constant term
• improvement at high energies of

linearity and resolution shown in calibration runs Muons (Resolution > 15% at 1 TeV):

• pt measurement from hits in inner detector and muon spectrometer
• require stringent cuts on number of hits, veto misaligned areas
• measured (as a function of pt) using cosmics, magnet off runs, overlap

regions, inner detector vs. muon spectrometer comparisons, Z peak

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Efficiencies and scale factors

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Determine trigger, reconstruction and identification efficiencies in data with Tag-and-Probe

• allows to get relatively unbiased control sample by applying

strict cuts on “Tag” and test efficiency on “Probe” (p.ex. Z ---> l+l-)

• electrons: need to subtract QCD jet background
• leptons from W/Z decay: no estimate above ~200 GeV
• extrapolation by observing of trends, simulation

Electrons: No decrease of selection efficiency expected at high energies (careful with isolation cut) Muons: highly energetic muons occasionally radiate so much bremsstrahlung, that their tracks might be too distorted for reconstruction

Sarah Heim l+ l- Z

Invariant mass distributions

18 The MC and QCD estimate are normalized to data in the mass range 70-110 GeV (Normalization factor: 99% for both electron and muon channel.) Electrons Muons Sarah Heim

Systematic Uncertainties

19 Uncertainties on yield at invariant mass of 1.5 TeV:

• only mass-dependent uncertainties are considered
• no theoretical uncertainties on signal (by convention)

(except for 5% Z boson cross-section uncertainty, which replaces the luminosity uncertainty)

• uncertainties below 2% negligible

(Pileup, energy calibration, momentum/energy resolution, electron trigger, reconstruction, identification efficiency, QCD multijet estimate) Sarah Heim

Signal templates

20 Two templates for every tested signal mass: Z' (spin-1)

• limits will be set on Z' (SSM) and motivated Z' (E6)
• use shape of Sequential Standard Model Z'
• neglect interference with DY
• reweight flat sample (with Breit-Wigner, Parton Luminosity)

G* (spin-2):

• limits will be set for different couplings (0.01-0.1)
• use shape with largest width (0.1)
• fully simulated (in 5fb-1 analysis: flat sample)

Sarah Heim Phys.Rev. D63 (2001) 075004 1 0.1

Statistical Method – Likelihoods

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• compare invariant mass distribution in data with SM background

and signal templates (for invariant masses above 130 GeV)

• binned Poisson likelihood (invariant mass bins k):
• Systematic uncertainties are considered through nuisance parameters

(for which we assume Gaussian probability functions)

• Reduced likelihood: Integral over all nuisance parameters
• Convert Nsig to cross-section

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Search for a signal

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• use 2D maximum likelihood fit to find most probable MZ' and σZ'
• get p-value by comparing to background-only pseudo-experiments
• --> what is the percentage of pseudo experiments,

which show an excess at least as significant as the one seen in data

• electrons: p = 54%, muons: p = 24% (evidence: 0.1%, discovery 0.00003%)
• --> No significant excess found
• --> Setting limits

Statistical Method – Setting limits

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• Bayes theorem:
• no prior knowledge on cross-section:
• 95% confidence level (C.L.) limits can be set by finding upper edge of integral,

such that:

• ---> with 95% C.L. we say, that σsigB is below (σsigB)95

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Limits Spin 1

24 Combined limits on E6 models [TeV]: Limits on SSM Z' [TeV]: Previous limits on SSM Z' [TeV]: Tevatron (1.071), ATLAS (1.042), CMS (1.140) LEP (indirect, 1.79) CMS 1 fb-1 [TeV]: 1.94

• Sequential Standard Model Z' as baseline model

(same couplings as the Z boson)

• different E6 models

Observed Expected ee 1.70 1.70 mumu 1.61 1.61 combined 1.83 1.83 Z' (ψ) Z' (N) Z' (η) Z' (I) Z' (S) Z' (χ) 1.49 1.52 1.54 1.56 1.60 1.64 Sarah Heim

Limits Spin 2

25 Limits on RS graviton (k/mPl = 0.1): Combined limits for additional couplings: Previous limits on Gravitons (k/mPl = 0.1) [TeV]: CMS, CDF, D0 (all below 1.08) Combination with diphoton channel (2.2fb-1): 1.95 TeV (see next slide)

• Randall-Sundrum Graviton (different couplings)

Observed Expected ee 1.51 1.50 mumu 1.45 1.44 combined 1.63 1.63 Coupling 0.01 0.03 0.05 0.71 1.03 1.33 Sarah Heim

Limits G* - Combination with diphoton channel26

• diphoton branching fraction ~2 times dielectron/dimuon
• limits on Gravitons (k/mPl = 0.1) [TeV]: 1.85 (diphoton), 1.95 (diphoton+dilepton)

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Technicolor search (ATLAS-CONF-2011-125) 27

Alternative mechanism of electroweak symmetry breaking (no scalar Higgs) Low Scale Technicolor model (Lane, Eichten): techni-isospin good symmetry QCD-like technihadron spectrum: near-degenerate ρT, ωT narrow spin-1 resonance, same acceptance as Z'

• --> same cross-section limits as

for the Z' analysis Low Scale technicolor interpretation of CDF Wjj excess

(Phys. Rev. Lett. 106 (2011) 171801)

Non-resonant search: Contact Interactions

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arXiv:1112.4462v2 (same event selection as for Z' search)

• looking for broad excess at high dilepton masses
• models: quark/lepton compositeness, exchange particles at

masses inaccessible to LHC energies

• cross-section: XS ~ XS(DY) – η FI/Λ2+ FC/Λ4
• --> prior chosen flat in 1/Λ2 and alternatively 1/Λ4

p-values in signal search > 5%

• --> set limits

η= +/- 1 (pos./neg.

interference)

Λ: scale

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5 fb-1 analysis (full 2011 dataset)

Status of the analysis

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Two parts:

• --> conference result showing limits on Z', G* for Moriond Electroweak

(approved by collaboration last week, awaiting final sign off)

• --> more extensive paper in a couple of weeks

Improvements:

• --> better background estimates
• --> 2 station muons (already in conference note)
• --> limits on more models (will be in paper)

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2-station muons

31 Low acceptance in muon channel due to strict cuts on the hits in the muon spectrometer (inner, middle, outer layer)

• ---> decent momentum resolution
• ---> but three layers not available for every region of the detector

Inclusion of 2-station muons

• 1 leg 3 stations, 1 leg 2 stations
• combined muons (inner detector and spectrometer)
• require 5 hits in each of the inner and outer layer
• veto regions with misalignments
• significance cut (ID-MS): 3σ (instead of 5 like for 3-station muons)
• --> acceptance increase by ~4% (absolute)
• --> resolution ~25% (dominated by inner detector)

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Models

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• setting limits on more models:

Z', G*, Z*, Technicolor (MWTC, LSTC), Torsion, Kaluza-Klein Z'/γ*

• model independent limits: Minimal Z' models
• template production from DY for many models, using matrix

elements or generator level distributions

• limits on couplings instead of cross-sections, including

interference effects (for Kaluza-Klein, Minimal models)

Sarah Heim arXiv:1004.2432v2

5 fb-1 analysis: Minimal Z' models

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• class of models with same coupling structure, parametrized

by y' and cos(θ) (arXiv:0909.1320v2)

• known models in this parametrization:
• make templates by reweighting DY

with matrix element

• --> includes interference and width
• set limit on coupling y'

illustration Sarah Heim

Conclusion

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• search for high-mass dielectron and dimuon

resonances with the ATLAS detector

• no significant excess found in 1 fb-1
• lower mass limits set on Z' models, RS graviton,

technimesons in Low Scale Technicolor

• 5 fb-1 analysis with improvements is underway

Sarah Heim

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BACKUP

Event yield tables

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Tight muon selection

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Tight-loose muon selection

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Event Display (mee = 993 GeV )

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Event Display (mμμ = 959 GeV )

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Branching fractions graviton decay

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arXiv:hep-ph/0211205v1

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Minimal Z' Models (arXiv:0909.1320v2)

uL,dL uR dR νL,eL νR eR 1/6 gY+1/3 gBL 2/3 gY+1/3gBL

• 1/3 gY+1/3 gBL
• 1/2 gY- gBL
• gBL
• gY-gBL

Class of models with following couplings: gY = γ' cos θ, gBL = γ' sin θ

• ---> two parameters (γ', θ)

Known models covered by this parametrization:

Sarah Heim