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Boosted HH → 4b Search Selection and Data-drivenBackground Estimation
Alison MarshSupervisor: Dr. Emily Thompson
Host Institution: Columbia UniversityHome Institution: University of Alabama
July 29, 2014
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 1 / 18
1 IntroductionThe ATLAS DetectorCalorimeter Jets and Jet GroomingTrack JetsB-tagging
2 AnalysisOverview and MotivationData-driven Background Estimation
3 SelectionsTrack Jet pT CutLarge-R Jet pT Cut
4 Conclusions
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 2 / 18
The ATLAS Detector
46 meters long, 25 meters in diameter, 7000 ton, general-purposedetector
Four major parts
Inner detector - tracks particle trajectories, measures particle momentaElectromagnetic Calorimeter - measures the energy of electrons andphotonsHadronic Calorimeter - measures the energy of hadronsMuon Spectrometer - identifies and measures the momenta of muons
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 3 / 18
Calorimeter Jets and Jet Grooming
Jets are a spray of particles and/or energy
This analysis utilizes jets with distance parameters of R = 1.0(“large-R”) and R = 0.4 (“small-R”)
High luminosity at the LHC causes pile-up contaminated jets anddifficulties reconstructing particles from hard interaction
Necessitates jet grooming; ATLAS utilizes trimming
Trimming starts with a large-R jet; uses kt algorithm to create subjets
of size R = 0.3; removes subjets withpiTpjetT
< fcut
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 4 / 18
Track Jets
Track jets are jets that contain the track of a charged particle
Track quality selections ensure a good track originating from primarypp collision; include pT > 500 MeV and various hit criteria andimpact parameters
This analysis uses track jets that are formed from at least two tracksfrom the primary collision
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 5 / 18
B-tagging
B-tagging consists of looking for a secondary vertex within each trackjet; secondary vertex formed by decay of b hadronThis analysis makes use of the MV1 b-tagging algorithm, using alltracks within ∆R < 0.3 of the track jet axis as inputsBenefits of b-tagged track jets:
Track jets chosen explicitly to originate from primary vertexOnly use inner detector informationBetter angular resolution
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 6 / 18
Analysis Overview
Boosted X → HH → bbbb
Motivation:
to study the primary decay mode of the Higgs (2b final state) that haspreviously not been observed due to high backgroundto commission the use of b-tagged track jets before the start of Run2
Utilizes large-R trimmed jets of R = 1.0 to capture boosted Higgsdecay products and two b-tagged R = 0.3 track jets within eachlarge-R jet
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 7 / 18
Analysis Baselines
MV1 > 0.70
c = 2.0
20 GeV pT cut on the track jets
350 GeV pT cut on leading large-R jet
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 8 / 18
Data-driven Background Estimation
Majority of background comes from QCD process of gluon splitting to2b, with some from tt and Z jetsLow statistics prevent an analytic fit to predict background so weutilize a data-driven background estimationScale 2b to 4b in sideband and test normalization factor (µQCD) inthe controlControl region defined as a box in the 2-D mass plane from 95-160GeV for the leading jet and 85-155 GeV for the subleading jetSideband region defined as all mass points outside the control
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 9 / 18
Data-driven Background Estimation
Background estimation in sideband (left) and control (right)
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 10 / 18
Track Jet pT Cut Optimization
Current cut is > 20 GeV; studied lower cut of > 7 GeV
Significance quantizes how well we cut out background and keepsignal; based on spreading mass windows
Lower track jet pT cut only has slight effect on significance levels,especially at higher potential resonance masses
More performance work is necessary before a lower track jet pT cutwill be helpful
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 11 / 18
Large-R Jet pT Cut
One way in which to cut out background and therefore enhance oursensitivity to the signal is to make pT cuts on the two large-Rcalorimeter jets
Angular separation of the 2-body decay products of a heavy particle:
∆R = 2mpT
,
where ∆R is angular distance between the two decay products andpT and m are transverse momentum and mass, respectively
This leads to a 250 GeV pT cut on both large-R jets to keep all decayproducts within ∆R = 1
We also currently employ a 350 GeV pT cut on the leading large-Rjet in order to keep the top quark background out of our signal region
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 12 / 18
Large-R Jet pT Cut Optimization
To optimize the pT cut on the leading large-R jet, the maximum pT
cut that could be made for each potential resonance mass while stillmaintaining 90% of the signal events was computed
This led us to study pT cuts between our threshold of 350 GeV to ourmaximum cut of 500 GeV
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 13 / 18
Significance of Large-R Jet pT Cuts
Significance quantizes how well we cut out background and whilemaintaining signal efficiency as much as possible
Above a resonance mass of 1 TeV, a higher leading large-R jet pT cutwould result in higher significance
Note: widening of signal results in larger windows in high resonancemasses
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 14 / 18
Benefits and Issues with Large-R Jet pT Optimization
Higher pT cuts cut out more background than signal, leading to agreater signal significance
Unfortunately, at high pT cuts we lose statistics in the control andsignal region
Without enough events, signal becomes more difficult to pick out anddata-driven background estimation begins to break down
Table: Events in control for resonance mass 1 TeV
pT cut 350 GeV 400 GeV 450 GeV 500 GeV
signal 3.53 3.06 2.36 1.46background 90 46 20 11
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 15 / 18
Ex: Leading Large-R Jet pT > 450 GeV
Higher pT cuts result in worsening fit between 2b and 4b data
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 16 / 18
Conclusions
Studied potential selections such as varying track jet and large-R jetpT cuts to optimize signal over background
Track jet pT cuts lower than 20 GeV may increase significance aftermore performance work is done to understand them
Large-R jet pT cuts higher than current threshold of 350 GeV showpromise of a higher significance
Recommend mass-dependent pT cuts starting with those outlined in90% efficiency plot for future analyses
I would like to thank the Columbia REU Program for this opportunity,Emily for her mentorship, and Lei for his help with the code!
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 17 / 18
References
The ATLAS Collaboration, Performance of jet substructuretechniques for large-R jets in proton-proton collisions at sqrt(s) = 7TeV using the ATLAS detector, CERN-PH-EP-2013-069
M. Bellomo, M. Kagan, E.N. Thompson, L. Zhou, Search for aresonance in the boosted di-Higgs to 4b final state, Internal note
Images
cern.chatlas.chhttp://profmattstrassler.com/articles-and-posts/particle-physics-basics/the-known-apparently-elementary-particles/jets-the-manifestation-of-quarks-and-gluons/b-tagging-identifying-jets-from-bottom-quarks/
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 18 / 18
Extra Slides
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 19 / 18
Significance Parts
Large-R jet pT cuts:
Track jet pT cuts:
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 20 / 18
Num Events - track jet > 7GeV
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 21 / 18
Num Events - 350GeV
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 22 / 18
Num Events - 400GeV
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 23 / 18
Num Events - 450GeV
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 24 / 18
Num Events - 500GeV
Alison Marsh (Columbia REU) Boosted HH → 4b Search Selection July 29, 2014 25 / 18