search for w'→tb in the hadronic final state at atlas · 2015. 1. 15. · 10 hadronic top...
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Search for W'→tb in the hadronic final state at ATLAS
Ho Ling LiThe University of Chicago
January 5, 2015
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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?
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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'
R counterpart for SM W
L
● Why W'→tb?● Search for W'
R without assuming the existence and mass of νR
● 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?
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Why W'→tb→qqbb?
● t → e/μ + ν ~ 2/9● t → qq ~ 2/3
leptonic top decay channelleptonic W' → tb
hadronic top decay channelhadronic W' → tb
● 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 mW'
> 1.75 TeV
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Why W'→tb→qqbb?
● 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
leptonic top decay channelleptonic W' → tb
hadronic top decay channelhadronic W' → tb
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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'
● MC signal sample: SM W to fermion couplings (gSM
) as benchmark● Both W'
L and W'
R generated from 1 – 3.5 TeV in 250 GeV
increments● Set W' mass limit as a function of g'/g
SM
g'
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The ATLAS detector
z-direction
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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
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b-tagging: the method to identify b jets
● B hadron has a very long lifetime (~10-12 s)
millimeters
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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
T → decay products tend to merge
● 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
high PT top
e.g. 800 GeV
low PT top
e.g. 100 GeV
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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
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√d12
= min(PT(1), P
T(2)) x ΔR(1,2)
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Jet substructure variables for top-tagging
● Ratio of n-subjettiness τ32 and τ21● Ratio related to number of constituents inside the jet
τ32
after applying √d
12
τ21
after applying √d
12
and τ32
τ32
→ 0 τ32
→ 1 τ21
→ 0 τ21
→ 1
top-like QCD-like top-likeQCD-like
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W' top-tagger performance
2 TeV WL' → tb
QCD sample (2012 data)
● Initially named Love-Li top-tagger due to its simplicity and lovely performance● Performance compatible with more complex algorithms
46%
6%fa
ke r
ate
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Analysis strategy● Looking for tagged dijet events
● 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
jetjet
45% b-tagging efficiencyif identified, 2 b-tag category; otherwise, 1 b-tag category
top-tagged large-R jet with P
T > 350 GeV
(always required)
b-tagged small-R jet with P
T > 350 GeV
(always required)
b
∆R(large-R, small-R) > 2.0
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Selection efficiency● Normalized to 20.3 fb-1
● Leptonic W' → tb analysis has ~2% efficiency
total
2 b-tag category
1 b-tag category
QCD background ttbar background 2 TeV W'L signal 2 TeV W'
R signal
1 b-tag 16100 1900 58 85
2 b-tag 2600 300 45 74
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Statistical strategy
● Parametrize signal distributions by analytic function● Fit background from data with analytic function● Unbinned likelihood fit to m
tb 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 category2 TeV W'
L
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Signal parametrization● Hadronic signal MC parametrized by Skew Normal + Gaussian
● Fit parameters can be parametrized into nice functions● Interpolate parameters between generated m
W' points
● Leptonic W' → tb signal MC parametrized by double Gaussian
central value of Skew NormalW'
L, 2 b-tag
2.5 TeV W'L
3 TeV W'L
2.5 TeV W'L
3 TeV W'L
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Data-driven method for QCD background estimation
not b-tagged b-tagged
not top-tagged NA
NB
top-tagged NC
Nsignal
Extrapolate into signal region:● If N
B/N
A = N
signal/N
C → good
● Compare extrapolation with N
signal, example by using QCD
MC
b?
b? if yes, 2 b-tag category
top?
1 b-tag channel
● If found b-tagged small-R jet inside large-R jet → 2 b-tag category
extrapolation
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Background parametrization
● Use data-driven QCD background sample + ttbar from MC to test functional form
2 b-tag category
parametrized by ExpPoly2: a
0*exp(a
1*m+a
2*m2)
1 b-tag category
parametrized by ExpPoly4
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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
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● 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
Systematic evaluations
systematics W'L
W'R
1 btag 2 btag 1 btag 2 btagb-tagging +13/-20 +45/-37 +15/-21 +40/-34
top-tagging +11.0/-12.5 +8.8/-10.0 +10.3/-11.0 +8.0/ -8.8luminosity ±2.8 ±2.8 ±2.8 ±2.8
jet energy scale +1.0/-1.3 +1.5/-1.9 +0.6/-0.8 +1.4 / -1.9jet energy resolution -0.0 -0.2 -0.1 -0.5
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Systematic example: b-tagging systematics
Change in event yields
● Example from b-tagging systematics for W'L in 2 b-tag category
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Systematic example: jet energy scale (JES)
● Example from JES for W'L in 2 b-tag category
Shift in the width of skewnormal
Shift in the width of Gaussian
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Now we open the box
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Data● Background obtained from fitting data with functional forms
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Data● Data consistent with SM
prob
abili
ty
prob
abili
ty prob
abili
ty
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Observed (expected) 95% CL limits
● Observed (expected) limits on cross section x Br assuming gSM
● mass of W'L > 1.68 (1.63) TeV
● mass of W'R > 1.76 (1.85) TeV
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Limits in terms of coupling strength g'
● Set limits on g'/gSM
up to 2 as a function of mW'
● At g'/gSM
= 2, mass limit is 2.18 (2.29) for W'L (W'
R)
● At mW'
= 1.5 TeV, g'/gSM
< 0.70 (0.55)
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Leptonic and hadronic W' → tb
● Currently, comparable expected limits● Hadronic: flat sensitivity up to m
W' ~ 3 TeV
hadronic W' → tb
leptonic W' → tb
hadronic W' → tb
leptonic W' → tb
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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'
L > 1.68 TeV
● Mass of W'R > 1.76 TeV
● Limits on g'/gSM
as a function of mW'
● At g'/gSM
= 2, mass limit is 2.18 (2.29) for W'L (W'
R)
● Outlook● Sensitivity up to W' mass ~ 3 TeV● More important as LHC increases to 13 - 14 TeV for Run II (2015 –
2018)
● Thank you!
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backup
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Trimming
● Remove parton contamination in fat jets
● Remove subjets with a cone size R that have PT < P
Tjet * f
cut
● In our case, R is 0.3 and fcut
is 0.05
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Splitting scale
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N-subjettiness
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W' top-tagger performance
● Shown compatible performance in ATLAS top tagger performance note
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Trigger and HT performance
● Use HT trigger EF_j170_a4tchad_ht700
● Study the HT trigger plateau
● Offline HT calculated by summing over all AntiKt4 jets with P
T > 25
GeV and |η|<2.5
● Events used in this study are selected with the following cuts● Trimmed AntiKt10 jet with P
T > 350 GeV and |η| < 2.0
● AntiKt4 jet with PT > 350 GeV and |η| < 2.5
● 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
T > 280 GeV
● EF_j280_a4tchad● H
T trigger with H
T > 600 GeV
● EF_j170_a4tchad_ht600
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Trigger and HT performance
● Trigger efficiency in data from period A● 96% at offline H
T = 800 GeV
● 99% at offline HT = 840 GeV
● 100% at offline HT = 900 GeV
● 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
trigger efficiency with respect to single jet trigger with P
T > 280 GeV
in data from period A
trigger efficiency with respect to H
T trigger with H
T > 600 GeV in
data from period A
ATLAS internal
ATLAS internal
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Selection efficiency: 2 TeV W'
● Absolute and relative selection efficiencies for 2 TeV W'L (W'
R)
● b-tagging requirement on high PT small-R jet is the tightest
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Selection efficiency: data
● 8 TeV data taken in 2012● b-tagging requirement on high P
T small-R jet is the tightest
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Acceptance interpolation
● Cross sections interpolated● Acceptance interpolated● Allows to set limits on cross
sections for all masses between 1.5 TeV and 3 TeV
acceptance x efficiency2 b-tag
acceptance x efficnecy1 b-tag
left-handed hadronic W' → tb → qqbbcross section with g' = g
SM
cros
s se
ctio
n (p
b)
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● 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
Systematic evaluations
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Top-tagging systematics
● Example from top-tagging systematics for W'L in 2 b-tag category
Change in event yields
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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
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Treatment of systematics
● Uncertainties treated correlated in the combination of two channels● Spurious signal uncorrelated between channels
source yield asymmetric uncertainties
shape constraint
top-tagging efficiency yes yes no log-normalb-tagging efficiency yes yes no log-normal
c- and τ-tagging yes no no log-normal
mis-tagging yes no no log-normal
JER no no yes log-normal
JES yes no yes log-normalspurious signal yes no no Gaussian
luminosity yes no no log-normal
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p-value of signal region fits
● Largest excess 1.4 σ local significance● Consistent with SM
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Effect of including 1 btag channel
statistics onlyW'
L
statistics onlyW'
R
statistics + systematicsW'
L
statistics + systematicsW'
R
● Clear gain from adding 1 btag at high mW'
when systematics included
statistics onlyW'
L