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Di-jets & M T2 for early SUSY discovery

Di-jets & M T2 for early SUSY discovery. Alan Barr (Oxford) Claire Gwenlan (UCL). Why is MT2 any good as a discriminating variable?. Is the M T2 dijets analysis useful for discovery in early data?. Do we have the appropriate Monte Carlo to study this channel?. Topologies of interest.

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Di-jets & M T2 for early SUSY discovery

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  1. Di-jets & MT2 for early SUSY discovery Alan Barr (Oxford)Claire Gwenlan (UCL) Why is MT2 any good as a discriminating variable? Is the MT2 dijets analysis useful for discovery in early data? Do we have the appropriate Monte Carlo to study this channel? Barr & Gwenlan

  2. Topologies of interest LSP • Concentrate on small number of high-pT jets • Large signal cross-section • Large control statistics • Relatively well known SM backgrounds • Relatively model independent • Does not rely on leptonic cascades • Does not rely on hadronic cascades • Use kinematics rather than “business of event” to pick out SUSY q squark q _ q _ BACKGROUND topology (QCD) q (and similar) LSP SIGNAL topology Barr & Gwenlan

  3. J.Phys.G29:2343-2363,2003 Phys.Lett.B463:99-103,1999 Reminder of MT2 • Variable has a good history… • MT2 first used for sleptons (Cambridge) • Squark masses from dijets in DC1 (Milano) • Sleptonic channel further investigated… • Masses (Pavia) • Spins (UCL) • Di-jets analysis further studied for CSC (Belgrade) Used in events where two identical particles decay semi-invisibly HardScatter “Try all possible directions for the neutralinosand find the minimum heavy sparticle mass” Barr & Gwenlan

  4. MT2 for discovery? • It is a property of the variable that MT2(χ=0) → 0 if: • ET→ 0 • ET parallel to either jet • Either jet ET → 0 • Expect small MT2 for backgrounds from • decays of “light” semi-invisible particles • events with small ET • mis-measurement of a single jet energy • This includes: WW, ttbar, QCD fakes, neutrinos in jets, … • No a-priori reason to expect MT2 to be small for e.g. MET coming from Z￫ + associated ISR • Expect this to dominate at larger MT2 Very nice features!This is why MT2 is a useful discriminator for discovery Barr & Gwenlan

  5. Go for “simple approach” • Cuts • At least two jets with: • ETJet1,2 > 150,100 GeV • |Jet1,2| < 2.5 (simple two-jet cuts only) plot MT2 Simple Analysis • MT2 already “does the job” of traditional cuts: • -> Dππ (δφ) cut • -> MET cut • -> Transverse sphericity cut • (small if MET parallel to either jet) • (small if MET → 0) • (small if 2-jets back-to-back) NB: We don’t claim you’d just plot MT2 and publish(!) but a simple selection can be easier to e.g. calculate systematics, and so speeds up the whole process Barr & Gwenlan

  6. Dijet cuts only 1 fb-1 Two-Jet Dijet inclusive: - No lepton veto - No b-jet veto - No multi-jet veto Barr & Gwenlan

  7. Dijet cuts only Two-Jet low mass region (SU4) Dijet inclusive: - No lepton veto - No b-jet veto - No multi-jet veto bulk region (SU3) coannihilation (SU1) Barr & Gwenlan

  8. Dijet cuts only Two-Jet low mass region (SU4) 1fb-1 bulk region (SU3) bulk region (SU3) bulk region (SU3) coannihilation (SU3) coannihilation (SU3) coannihilation (SU3) Barr & Gwenlan

  9. Di-jet analysis without MT2? • More traditionally (since TDR), Meff distribution used as a discriminator • Cuts • at least two jets with: • ETJet1,2 > 150,100 GeV • |Jet1,2| < 2.5 Meff = Jets pTi + MET Barr & Gwenlan

  10. No MT2Dijet cuts only Meff alone gives large QCD BD Barr & Gwenlan

  11. Di-jet analysis without MT2? • More traditionally (since TDR), Meff distribution used as a discriminator • Cuts • at least two jets with: • ETJet1,2 > 150,100 GeV • |Jet1,2| < 2.5 Meff = Jets pTi + MET • But with addition of some other cuts…  MET > 100 GeV  cuts based on i = |(Jet,i)-(MET)|): • R1 = (22+(-1)2) > 0.5 rad • R2 = (12+ (-2)2) > 0.5 rad • no jet with i < 0.5 rad MT2 basically “does this job” implicitly Barr & Gwenlan

  12. No MT2Dijet cuts + MET +  Two-Jet Barr & Gwenlan

  13. No MT2Dijet cuts + MET +  Two-Jet 1fb-1 Barr & Gwenlan

  14. Need: Some understanding of ET and energy scale Degree needs to be determined Some lepton ID Estimate Z→νν from Z→μμ Some idea of ttbar background “Do not need”: B-tagging Only if needed to measure ttbar background Detailed understanding of jet resolution tails In the limit where only one jet per event fluctuates Missing ET tails from multi-jets Useful for early discovery? • Need to validate the above statements: • Need 2-parton Alpgen to validate against 2->2 MC • QCD and Drell Yan backgrounds • Study effect of extra jet mis-calibration/resolution Barr & Gwenlan

  15. Simulation and Trigger? • Monte Carlo • Currently have to use CSC Pythia/Herwig MC • Existing SUSY Alpgen has 4-jet truth filter • Probably not ideal for these types of cuts • Z+2 jets, W+2 jets not well modelled Would be good to cross-check against 2 ￫ n parton MC • Trigger? • “SUSY” 4-jet trigger no good • Existing jet/MET triggers probably sufficient • Including pre-scales for lower threshold jets • Needs to be confirmed Barr & Gwenlan

  16. No MT2Tri-jet cuts + MET +  3-jet also sensitive Three-Jet Hundreds of signal events rapidly Barr & Gwenlan

  17. No MT2Tri-jet cuts + MET +  3-jet also sensitive Three-Jet 1fb-1 Hundreds of signal events rapidly Barr & Gwenlan

  18. Summary • Dijets and trijets + MET have good SUSY sensitivity • Large cross-section • High S/√B than multi-jet at low lumi • Goods stats signal & control regions • Should be part of early search strategy • MT2‘does the job’ of several traditional cuts • Expected from its properties • Combination of {MET, sphericity, } • Reduce # cuts  simplfy analysis? • Needed: • Monte Carlo to better cover this region • Geant and ATLFAST • Same true for lepton(s) + 2/3 jets + MET channels? Not just “4 or more jets”! Barr & Gwenlan

  19. Backups Barr & Gwenlan

  20. V12 BACKGROUND SAMPLES USED (SO FAR) Barr & Gwenlan

  21. SUSY POINTS Barr & Gwenlan

  22. Dijet cuts only MET distribution Barr & Gwenlan

  23. 4-jets + MET + dφ Four-Jet Do not believe the backgrounds! Barr & Gwenlan

  24. 4-jets + MET + dφ Four-Jet 1fb-1 Do not believe the backgrounds! Barr & Gwenlan

  25. Dijet MEff plot at least two jets with: • ETJet1,2 > 150,100 GeV • |Jet1,2| < 2.5  MET > 100 GeV • MT2 plot at least two jets with: • ETJet1,2 > 150,100 GeV • |Jet1,2| < 2.5  cuts based on i = |(Jet,i)-(MET)|): • R1 = (22+(-1)2) > 0.5 rad • R2 = (12+ (-2)2) > 0.5 rad • no jet with i < 0.5 rad Cuts • 3-jet plot at leastthree jets with: • ETJet1,2,3 > 150,150,100 GeV • |Jet1,2,3| < 2.5  MET > 100 GeV  cuts based on i = |(Jet,i)-(MET)|): • R1 = (22+(-1)2) > 0.5 rad • R2 = (12+ (-2)2) > 0.5 rad • no jet with i < 0.5 rad  |Jet1| < 1.5 Barr & Gwenlan

  26. 100 fb-1Plots and significances Barr & Gwenlan

  27. Dijet cuts only Two-Jet Barr & Gwenlan

  28. Dijet cuts only Two-Jet 100 pb-1 Barr & Gwenlan

  29. dijet + MET + dφ Two-Jet Barr & Gwenlan

  30. dijet + MET + dφ Two-Jet 100 pb-1 Barr & Gwenlan

  31. Some MC Issues/Questions • 1. Different samples used for Z->nunu backgrounds in v11 and v12, giving rather different results at low MT2. Comparing JO’s: •  v11, sample used: • csc11.005183.PythiaZnunu.recon.AOD.v11004206* • - has lower pTZ cut of 50 GeV • (also has *no* line which turns off FSR: ‘pydat1 parj 90 20000’ – hopefully dosen’t make a difference?) •  v12, sample used: trig1_misal1_mc12.008192.pythia_Znunu_qg_Nj2.recon.AOD.v12000601* • - no lower pTZ cut • - requires 2 jets at truth level (Cone4) with pT1 > 80 GeV, pT2 > 40 GeV • (I am weighting this sample by a “cross section x filter efficiency” of 58 pb, which I got from: http://jarguin.home.cern.ch/jarguin/dc3requests_sm.html). However, I can’t find any other information anywhere else (e.g. on AMI) to help me verify that this is the correct number for PYTHIA, and haven’t yet had time to check it ourselves “by hand”. • NOTES on Jets Samples used: So far, have just used the PYTHIA samples. In the v12 plot the J1-J6 samples have the have 30 micron step length while J7 suffers from 1mm bug (as no samples available with 30 micron yet) – this should be a small effect presumably. Also in the v12 plot, the J8 sample has not been included as couldn’t get any samples, which had MET_RefFinal (which is what I used for J1-J7) – however, inclusion or not of J8 sample in the plot gives only small differences . • ALPGEN samples study is now in progress but it is difficult to perform full study with current available samples e.g. there are not yet any 2-Jet ALPGEN QCD samples, and not sure that there are all the necessary Drell-Yan samples either e.g. WmunuNp2, ZnunuNp2, etc.) • Other samples that might be useful: MC@NLO Znunu? (already using MC@NLO for Zee, Zmumu) Barr & Gwenlan

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