Physics Comioning and Initial Background Estimation for SUSY SUSY Working Group 1
Introduction 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 his is not the evatron (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); op (full reconstruction of semi-leptonic ttbar events); Higgs (Estimation of high E backgrounds) Jet/E (Estimation of fake E 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 2
Strategy R-Parity conserving SUSY search channels: Large E ; Large jet multiplicity; Large E 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. 5σ ALAS 3
Strategy Main backgrounds: Z( ll) + n jets W ( lν) + n jets ttbar QCD Generic approach : Also: Single top WW/WZ/ZZ Select low E background calibration samples; Extrapolate into high E signal region. Jets + E + 0 leptons ALAS 10 fb -1 QCD W+jet Z+jet ttbar ALAS Used by / D0 Extrapolation non-trivial. Must find variables uncorrelated with E Several approaches developed. 4
W/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 ag leptonic Z EIHER : Discard one or both leptons and use data sample OR : Use simulation normalised to Z l + l - + >1 jet data (good stats - ) Scale by inclusive N/N+1 jets factor (below) + appropriate σ.br ratio Correct for lepton identification efficiencies Also appropriate for WW/WZ/ZZ M. Spiropulu hesis 5
op Backgrounds Estimation using simulation possible (normalised to data ttbar selection) - cross-check with data? Standard (DR) semileptonic top cuts look rather like SUSY cuts with looser E requirement! ALAS Physics DR If harden E 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 E events. Worry about correlations between selection cuts and E distribution. 6
QCD and Fake Missing E Caused by lack of detector hermeticity, dead channels, non-gaussian tails 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 + E + n leptons). 2) Choose hard cuts which minimise contribution to background. 3) Estimate background using data and/or calibrated fast MC. 7
Fake Missing E Minimisation Ensure careful pre-calibration of calorimeters Inter-calibration precision most important Reject events where fake E 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 E 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 p )). Cut on Missing E Significance δφ 1(2) = φ 1(2) -φ E R 1(2) =sqrt(δφ 2(1) 2 +(π-δφ 1(2) ) 2 ) D ππ =sqrt((π-δφ 1 ) 2 + (π-δφ 2 ) 2 ) 8 M. Spiropulu hesis R 2 Collinear dijets? δφ 1 δφ 2 D ππ R 1
Missing E Significance Used at evatron. Useful variable for identifying real E. Several definitions: E / E sum ; Likelihood-type quantity calculated with MC smearing of energies, primary VX position etc. ALAS needs similar tools. ALAS D0 Note 3629 D0 Note 3629 W+jets QCD 9
Fake Missing E Estimation Fake E rejection partially successful. Estimate remaining contribution from data or MC. Possible approaches? Find variables uncorrelated with E (e.g. D ππ ) which reject SUSY and measure background in sidebands. Use γ+jet / p balance in low E collinear di-jets to estimate jet energy distributions (inc. tails). Feed results into dedicated fast simulation / smear low E multijets using bootstrap technique. May require dedicated calibration run with prescaled low p jet trigger (a la etc.) Use brisk simulation with real geometry. Set upper limits to background using fullsim? Data D0 MC SUSY QCD D0 10
Fake Missing E Can poorer (resolution) E measures be found for which the QCD tails can be estimated more accurately? Example: E based on reconstructed physics objects Easier to use bootstrap/fast simulation to estimate; Easier to calculate Missing E 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)? ALAS For illustrative purposes only! 11
SUSY Group Plans: Action Plan Study optimum search strategies for low mass scale models at limit of statistical sensitivity as function of integrated lumi. Develop fake E 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 ALFAS). ools for rejection of hot/dead cells, beam related background, beamgas, cosmic rays (inc. inside events), out of time events etc. Large fully simulated data samples matching pre-scaled trigger run. 12
Brisk Simulation For E 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 (CMSJE Famos). Build on existing work: LAr: Barberio and Straessner (SW June 2003) ile: Sutiak, okar, Zenis and Kulchitsky (SW March 2003) longitudinal profile ALAS d(mm) radial profile ALAS r/r M 13
evatron Experience 14
evatron Experience 15