Search for W' tb in the hadronic final state at ATLAS Ho Ling Li The University of Chicago January 5, 2015 1
Introduction What is W'? A charged, heavy, Standard-Model-like W gauge boson Analysis channels W' lν, l = e, μ W' tb First result on W' in the decay channel of tb qqbb Available on http://arxiv.org/abs/1408.0886 Why this analysis? Why W'? Why W' tb? Why W' tb qqbb? You may also ask: Why hasn't it been done before? 2
Why W' and W' tb? Why W'? Consequence of many BSM theories Extra Dimension model, e.g. Kaluza-Klein tower of W A new gauge sector, e.g. Little Higgs theories Left-right symmetry: W' counterpart for SM WL R Why W' tb? Search for W' without assuming the existence and mass of νr R Couple to the 3rd generation Current mass limit: W' lν (3.35 TeV, CMS) > W' tb (2.03 TeV, CMS), but SM couplings assumed Weakly coupled to leptonic channel, or even leptophobic, while strongly coupled to quarks? 3
Why W' tb qqbb? leptonic top decay channel leptonic W' tb hadronic top decay channel hadronic W' tb t e/μ + ν ~ 2/9 t qq ~ 2/3 Before August 2014, only W' tb lνbb published Current observed limits at ATLAS ~ 1.8 TeV Adv.: Existence of l significantly lowers background from light quarks and gluons (QCD) Disadv.: Lose sensitivity when m > 1.75 TeV W' 4 7
Why W' tb qqbb? leptonic top decay channel leptonic W' tb hadronic top decay channel hadronic W' tb t e/μ + ν ~ 2/9 t qq ~ 2/3 This is the first experimental result on W' tb qqbb Advantages Br(t qqb) ~ 3 x Br(t lνb), l = e or μ Able to reconstruct a sharp W' mass peak Sensitivity maintained for high m W' Disadvantage Enormous background from QCD 5
W' tb model Use a model independent effective field theory approach, keep this analysis as generic search Independent of other phenomena in the models, only concern W' g' MC signal sample: SM W to fermion couplings (gsm) as benchmark Both W' and W' generated from 1 3.5 TeV in 250 GeV L R increments Set W' mass limit as a function of g'/gsm 6
The ATLAS detector z-direction 7
Jet reconstruction Signal involves top quarks and b quarks; QCD as main background Top, b, QCD are identified as jets Additional criteria to distinguish top jets and b jets from QCD 8
b-tagging: the method to identify b jets B hadron has a very long lifetime (~10-12 s) millimeters 9
Hadronic top reconstruction Want to differentiate jets from hadronic top decays from QCD Top jet: high mass (173 GeV) and contains 3 showers (q, q, b) Search for W' with mass greater than 1.5 TeV Top has high P decay products tend to merge T Reconstruct top decay products by large-r (radius para. R=1.0) jets ATLAS standard jets have radius parameter R=0.4 Use jet substructure information to distinguish top jets from light jets low PT top e.g. 100 GeV high PT top e.g. 800 GeV 10
Jet substructure variables for top-tagging Splitting scale d12, ratios of n-subjettiness τ21 and τ32 are used to distinguish top jets and light jets Splitting scale d 12 d12 = min(pt(1), PT(2)) x ΔR(1,2) 11
Jet substructure variables for top-tagging Ratio of n-subjettiness τ32 and τ21 Ratio related to number of constituents inside the jet τ32 0 top-like τ32 after applying d12 τ32 1 QCD-like τ21 0 QCD-like τ21 1 top-like τ21 after applying d12 and τ32 12
W' top-tagger performance Initially named Love-Li top-tagger due to its simplicity and lovely performance Performance compatible with more complex algorithms 2 TeV WL' tb fake rate QCD sample (2012 data) 46% 6% 13
Analysis strategy Looking for tagged dijet events b-tagged small-r jet with PT > 350 GeV (always required) R(large-R, small-r) > 2.0 top-tagged large-r jet with PT > 350 GeV (always required) jet jet b 45% b-tagging efficiency if identified, 2 b-tag category; otherwise, 1 b-tag category Electron and muon veto applied Still, leptonic W' tb contributes 3.5 10% of signal yields Search for a bump in mtb spectrum Start the search at W' mass > 1.5 TeV 14
Selection efficiency Normalized to 20.3 fb-1 QCD background ttbar background 1 b-tag 2 b-tag 16100 2600 1900 300 2 TeV W'L signal 2 TeV W'R signal 58 45 85 74 Leptonic W' tb analysis has ~2% efficiency total 1 b-tag category 2 b-tag category 15
Statistical strategy Parametrize signal distributions by analytic function Fit background from data with analytic function Unbinned likelihood fit to mtb distribution Determine excess from probability for signal+background hypothesis If no excess, set 95% Confidence Level limits Systematics taken into account as nuisance parameters 2 b-tag category 2 TeV W'L 16
Signal parametrization Hadronic signal MC parametrized by Skew Normal + Gaussian Fit parameters can be parametrized into nice functions Interpolate parameters between generated m points W' central value of Skew Normal W'L, 2 b-tag Leptonic W' tb signal MC parametrized by double Gaussian 2.5 TeV W'L 2.5 TeV W'L 3 TeV W'L 3 TeV W'L 17
Data-driven method for QCD background estimation not b-tagged b-tagged not top-tagged NA NB top-tagged NC Nsignal If found b-tagged small-r jet inside large-r jet 2 b-tag category extrapolation 1 b-tag channel b? top? b? if yes, 2 b-tag category Extrapolate into signal region: If N /N = N /NC good B A signal Compare extrapolation with Nsignal, example by using QCD MC 18
Background parametrization Use data-driven QCD background sample + ttbar from MC to test functional form 2 b-tag category parametrized by ExpPoly2: a0*exp(a1*m+a2*m2) 1 b-tag category parametrized by ExpPoly4 19
Background systematics spurious signal Spurious signal: bias from choice of background function Fit background with signal+background hypothesis to extract number of signal events as spurious signal Use data-driven QCD sample + ttbar from MC ExpPoly2, 2 b-tag, W'L choose simplest functions with low spurious signals 20
Systematic evaluations Percent change of systematic uncertainties on event yield of 2 TeV W' systematics b-tagging top-tagging luminosity jet energy scale jet energy resolution W'L 1 btag +13/-20 +11.0/-12.5 ±2.8 +1.0/-1.3-0.0 2 btag +45/-37 +8.8/-10.0 ±2.8 +1.5/-1.9-0.2 W'R 1 btag +15/-21 +10.3/-11.0 ±2.8 +0.6/-0.8-0.1 2 btag +40/-34 +8.0/ -8.8 ±2.8 +1.4 / -1.9-0.5 b-tagging is the largest systematics in 2 b-tag category Jet energy scale (JES) and jet energy resolution (JER) are small 21
Systematic example: b-tagging systematics Example from b-tagging systematics for W'L in 2 b-tag category Change in event yields 22
Systematic example: jet energy scale (JES) Example from JES for W'L in 2 b-tag category Shift in the width of Gaussian Shift in the width of skewnormal 23
Now we open the box 24
Data Background obtained from fitting data with functional forms 25
probability probability probability Data Data consistent with SM 26
Observed (expected) 95% CL limits Observed (expected) limits on cross section x Br assuming gsm mass of W' > 1.68 (1.63) TeV L mass of W' > 1.76 (1.85) TeV R 27
Limits in terms of coupling strength g' Set limits on g'/gsm up to 2 as a function of mw' At g'/g = 2, mass limit is 2.18 (2.29) for W'L (W'R) SM At m = 1.5 TeV, g'/gsm < 0.70 (0.55) W' 28
Leptonic and hadronic W' tb Currently, comparable expected limits Hadronic: flat sensitivity up to mw' ~ 3 TeV hadronic W' tb leptonic W' tb hadronic W' tb leptonic W' tb 29
Summary and outlook First result on W' tb in the decay channel of t qqb Dijets events with one jet top-tagged and the other one b-tagged Developed new and simple top-tagger with high performance Limits on cross section x Br (g' = gsm) Mass of W' > 1.68 TeV L Mass of W' > 1.76 TeV R Limits on g'/gsm as a function of mw' At g'/g = 2, mass limit is 2.18 (2.29) for W'L (W'R) SM Outlook Sensitivity up to W' mass ~ 3 TeV More important as LHC increases to 13-14 TeV for Run II (2015 2018) Thank you! 30
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Trimming Remove parton contamination in fat jets Remove subjets with a cone size R that have PT < PTjet * fcut In our case, R is 0.3 and fcut is 0.05 32
Splitting scale 33
N-subjettiness 34
W' top-tagger performance Shown compatible performance in ATLAS top tagger performance note 35
Trigger and HT performance Use HT trigger EF_j170_a4tchad_ht700 Study the HT trigger plateau Offline H calculated by summing over all AntiKt4 jets with P > 25 T T GeV and η <2.5 Events used in this study are selected with the following cuts Trimmed AntiKt10 jet with P > 350 GeV and η < 2.0 T AntiKt4 jet with P > 350 GeV and η < 2.5 T ΔR(AntiKt10 jet, AntiKt4 jet) > 2.0 Trigger efficiency measured in data from period A with respect to a looser pre-scaled trigger Single jet trigger with P > 280 GeV T EF_j280_a4tchad H trigger with H > 600 GeV T T EF_j170_a4tchad_ht600 36
Trigger and HT performance Trigger efficiency in data from period A 96% at offline H = 800 GeV T 99% at offline H = 840 GeV T 100% at offline H = 900 GeV T Trigger efficiency is at least 97% at HT = 850 GeV for 1.5 TeV or heavier left- and right-handed W' Require HT at 850 GeV ATLAS internal ATLAS internal trigger efficiency with respect to single jet trigger with PT > 280 GeV in data from period A trigger efficiency with respect to HT trigger with HT > 600 GeV in data from period A 37
Selection efficiency: 2 TeV W' Absolute and relative selection efficiencies for 2 TeV W'L (W'R) b-tagging requirement on high P small-r jet is the tightest T 38
Selection efficiency: data 8 TeV data taken in 2012 b-tagging requirement on high P small-r jet is the tightest T 39
Cross sections interpolated Acceptance interpolated Allows to set limits on cross sections for all masses between 1.5 TeV and 3 TeV acceptance x efficiency 2 b-tag cross section (pb) Acceptance interpolation left-handed hadronic W' tb qqbb cross section with g' = gsm acceptance x efficnecy 1 b-tag 40
Systematic evaluations Percent change of systematic uncertainties on event yield of 2 TeV W' b-tagging is the largest systematics in 2 b-tag category Jet energy scale (JES) and jet energy resolution (JER) are small 41
Top-tagging systematics Example from top-tagging systematics for W'L in 2 b-tag category Change in event yields 42
JER systematics Rerun the analysis with 1 sigma shift in variable and obtain the invariant mass spectrum Parametrize the shifted spectrum Compare the nominal spectrum with the shifted one to determine nuisance parameters Example from JER for W'L in 2 b-tag category Shift in the width of Gaussian 43
Treatment of systematics Uncertainties treated correlated in the combination of two channels Spurious signal uncorrelated between channels source yield shape constraint yes yes yes asymmetric uncertainties yes yes no top-tagging efficiency b-tagging efficiency c- and τ-tagging mis-tagging JER JES spurious signal luminosity no no no log-normal log-normal log-normal yes no yes yes yes no no no no no no yes yes no no log-normal log-normal log-normal Gaussian log-normal 44
p-value of signal region fits Largest excess 1.4 σ local significance Consistent with SM 45
Effect of including 1 btag channel Clear gain from adding 1 btag at high mw' when systematics included statistics only W'L statistics only W'R statistics + systematics W'R statistics + systematics W'L 46