boosted hh 4b search selection and data-driven background ... · data-driven background estimation...

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


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


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


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


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


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


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


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


Data-driven Background Estimation

Background estimation in sideband (left) and control (right)


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


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


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


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


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


Ex: Leading Large-R Jet pT > 450 GeV

Higher pT cuts result in worsening fit between 2b and 4b data


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!


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/


Extra Slides


Significance Parts

Large-R jet pT cuts:

Track jet pT cuts:


Num Events - track jet > 7GeV


Num Events - 350GeV


Num Events - 400GeV


Num Events - 450GeV


Num Events - 500GeV


boosted hh 4b search selection and data-driven background ... · data-driven background estimation...

Documents