Physics Commissioning and Initial Background Estimation...

11Dan ToveyDan Tovey University of SheffieldUniversity of Sheffield

Physics Commissioning and Initial Background Estimation for SUSY

Dan ToveySUSY Working Group

Physics Commissioning and Initial Background Estimation for SUSY

Dan ToveySUSY Working Group


IntroductionIntroduction• Preparations needed to ensure efficient/reliable searches

for/measurements of SUSY particles in timely manner:– Initial calibrations (energy scales, resolutions, efficiencies etc.);– Minimisation of poorly estimated SM backgrounds;– Estimation of remaining SM backgrounds;– Development of useful tools.

• NB This is not the Tevatron (no previous σ measurements at same √s) !• Many issues will be common with other WG, esp:

– Standard Model (W ( lν) + n jet, Z( ll) + n jet) from Z( l+l-) + n jet);– Top (full reconstruction of semi-leptonic ttbar events);– Higgs (Estimation of high ET

miss backgrounds)– Jet/ET

miss (Estimation of fake ETmiss QCD backgrounds, jet energy scale

etc.);– Combined Performance groups (calibration of energy scales, resolutions

and efficiencies).• Should work together to develop common tools and analysis strategies

wherever possible …


StrategyStrategy• R-Parity conserving SUSY search

channels:– Large ET

miss;– Large jet multiplicity;– Large ET

sum.• Will need convincing estimates of

backgrounds with as little data as possible.

• Background estimation techniques will change depending on integrated lumi.

• Ditto optimum search channels & cuts.• Aim to use combination of

– Fast/’brisk’-sim;– Full-sim;– Estimations from data.

• Use comparison between different techniques to validate estimates and build confidence in (blind) analysis.

ATLAS

5σ


StrategyStrategy• Main backgrounds:

– Z( ll) + n jets– W ( lν) + n jets– ttbar– QCD

• Generic approach :– Select low ET

miss background calibration samples;

– Extrapolate into high ETmiss signal region.

Jets + ETmiss + 0 leptons

ATLAS

10 fb-1

• Used by CDF / D0• Extrapolation non-trivial.

– Must find variables uncorrelated with ET

miss

• Several approaches developed.

QCDW+jetZ+jetttbar

• Also:– Single top– WW/WZ/ZZ

ATLAS


W/Z + n JetsW/Z + n Jets• Comes from Z νν + n jets, W lν + n jets, W τν + (n-1) jets (τ fakes jet)• Estimate from Z l+l- + n jets• Tag leptonic Z• EITHER : Discard one or both leptons and use data sample • OR : Use simulation normalised to Z l+l- + >1 jet data (good stats - CDF)• Scale by inclusive N/N+1 jets factor (below) + appropriate σ.BR ratio• Correct for lepton identification efficiencies• Also appropriate for WW/WZ/ZZ

CDF

M. SpiropuluThesis


Top BackgroundsTop Backgrounds• Estimation using simulation possible (normalised to data ttbar

selection) - cross-check with data ?• Standard (TDR) semileptonic top cuts look rather like SUSY cuts with

looser ETmiss requirement!

• If harden ETmiss cuts top sample contaminated with SUSY signal …

• Possible approach (probably extremely difficult? - combinatorics):– Select semi-leptonic candidates (standard cuts – what btag available?);– Fully reconstruct top and W momenta;– Replace hadronic W with leptonic decay (appropriate boosted 2-body

decay distribution) high ETmiss events.

– Worry about correlations between selection cuts and ETmiss distribution.

ATLASPhysics TDR


QCD and Fake Missing ETQCD and Fake Missing ET

• Caused by lack of detector hermeticity, dead channels, non-gaussiantails to jet energy distributions (high tail from pile-up, low tail from dead material, punch-through etc.)

• Hardest background to estimate.– Simulations require detailed understanding of detector performance (not

easy with little data).– Would require vast full simulation effort.

• Strategy: 1) Initially choose channels which minimise contribution until well

understood (e.g. jets + ETmiss + n leptons).

2) Choose hard cuts which minimise contribution to background.3) Estimate background using data and/or calibrated fast MC.


Fake Missing ET MinimisationFake Missing ET Minimisation• Ensure careful pre-calibration of

calorimeters• Inter-calibration precision most important• Reject events where fake ET

miss likely :– Reject beam-gas and machine background

with event ‘cleaning’.– Require primary vertex in central region – Reject events with hot cells– Reject CR muons etc.– Reject events where ET

miss vector points in (opposite) direction of (to) jets (jet fluctuations)

– Reject events with jets pointing at regions of poor response (barrel-extended barrel, barrel-endcap, endcap-FCAL, FCAL (for high pT)).

– Cut on Missing ET Significance

CDF

M. Spiropulu Thesis

R1

R2Dππ

Collinear dijets?δφ1

δφ2δφ1(2) = φ1(2)-φETmissR1(2)=sqrt(δφ2(1)

2+(π-δφ1(2))2)Dππ=sqrt((π-δφ1)2 + (π-δφ2)2)


Missing ET SignificanceMissing ET Significance• Used at Tevatron.• Useful variable for identifying ‘real’

ETmiss.

• Several definitions:– ET

miss/√ETsum;

– Likelihood-type quantity calculated with MC smearing of energies, primary VX position etc.

• ATLAS needs similar tools.

D0 Note 3629

W+jets

D0 Note 3629

QCD

ATLAS


Fake Missing ET EstimationFake Missing ET Estimation• Fake ET

miss rejection partially successful. • Estimate remaining contribution from data or MC.• Possible approaches?• Find variables uncorrelated with ET

miss (e.g. Dππ) which reject SUSY and measure background in sidebands.

• Use γ+jet / pT balance in low ETmiss collinear di-jets

to estimate jet energy distributions (inc. tails).– Feed results into dedicated fast simulation / smear

low ETmiss multijets using bootstrap technique.

– May require dedicated calibration run with prescaledlow pT jet trigger (a la CDF etc.)

• Use ‘brisk’ simulation with real geometry.• Set upper limits to background using fullsim?

CDF

CDF

D0D0

Data MC SUSY QCD


Fake Missing ETFake Missing ET• Can poorer (resolution) ET

miss

measures be found for which the QCD tails can be estimated more accurately?

• Example: ETmiss based on

reconstructed physics objects– Easier to use bootstrap/fast

simulation to estimate;– Easier to calculate Missing

ET Significance;– ~40% increase in gaussian

resolution;– Could also add in

unclustered energy .• At what point does it become

worth using these for searches (as opposed to measurements)?

ATLAS

For illustrative purposes only!


Action PlanAction PlanSUSY Group Plans:• Study optimum search strategies for low mass scale models at limit of

statistical sensitivity as function of integrated lumi.• Develop fake ET

miss rejection methods and tools.• Develop background estimation methods (QCD, ttbar, Z/W+n jets).• Determine required pre-scaled trigger thresholds and stats for QCD jet

calibration samples.Requests:• Vital to have detailed plan of expected calibration uncertainties in

energy scales, resolutions, efficiencies etc. of physics objects as a function of integrated luminosity benchmarks (e.g. 0 pb-1, 10 pb-1, 100 pb-1, 1 fb-1, 10 fb-1 or finer), also inter-calibration precision.

• ‘Brisk’ simulation tools (possibly integrated with ATLFAST).• Tools for rejection of hot/dead cells, beam related background, beam-

gas, cosmic rays (inc. inside events), out of time events etc.• Large fully simulated data samples matching pre-scaled trigger run.


‘Brisk’ Simulation‘Brisk’ Simulation• For ET

miss estimation useful to have intermediate (‘brisk’) simulation between fast and full.– Detailed calorimeter model (simplified geometry

database?);– Access to conditions database (dead channels,

calibrations etc.);– Fast 3D shower simulation (a la GFLASH /

energy spotting) – also e.g. Conversions.– Goes beyond FastShower (3D, higher

granularity).• Already have something similar in CMS

(CMSJET Famos).• Build on existing work:

– LAr: Barberio and Straessner (SW June 2003)– Tile: Sutiak, Tokar, Zenis and Kulchitsky (SW

March 2003)

longitudinal profile

d(mm)radial profile

r/rM

ATLAS

ATLAS


Tevatron ExperienceTevatron Experience

CDF

CDF

CDF


Tevatron ExperienceTevatron ExperienceCDF

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