Physics Commissioning and Initial Background Estimation for SUSY

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1 Physics Comioning and Initial Background Estimation for SUSY SUSY Working Group 1

2 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

3 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

4 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

5 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

6 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

7 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

8 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

9 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

10 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

11 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

12 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

13 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

14 evatron Experience 14

15 evatron Experience 15