Di-jets & MT2 for early SUSY discovery

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Presentation transcript:

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? 6 June 2007 Barr & Gwenlan

Topologies of interest LSP q squark q _ q _ q BACKGROUND topology (QCD) (and similar) 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 SIGNAL topology 6 June 2007 Barr & Gwenlan

Reminder of MT2 Used in events where two identical particles decay semi-invisibly Hard Scatter J.Phys.G29:2343-2363,2003 Phys.Lett.B463:99-103,1999 “Try all possible directions for the neutralinos and find the minimum heavy sparticle mass” 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) 6 June 2007 Barr & Gwenlan

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 6 June 2007 Barr & Gwenlan

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) 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 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 6 June 2007 Barr & Gwenlan

Dijet cuts only 1 fb-1 Two-Jet Dijet inclusive: - No lepton veto - No b-jet veto - No multi-jet veto 6 June 2007 Barr & Gwenlan

Dijet cuts only Two-Jet Dijet inclusive: - No lepton veto low mass region (SU4) Dijet inclusive: - No lepton veto - No b-jet veto - No multi-jet veto bulk region (SU3) coannihilation (SU1) 6 June 2007 Barr & Gwenlan

1fb-1 mT2 > 200 GeV mT2 > 400 GeV Dijet cuts only Two-Jet low mass region (SU4) 1fb-1 mT2 > 200 GeV mT2 > 400 GeV Model S/sqrt(B) S/sqrt(S+B) SU1 24 23 80 36 SU2 1.3 1.9 1.8 SU3 52 45 110 44 SU4 180 120 64 32 SU6 15 54 28 SU8.1 22 20 74 34 bulk region (SU3) bulk region (SU3) bulk region (SU3) coannihilation (SU3) coannihilation (SU3) coannihilation (SU3) 6 June 2007 Barr & Gwenlan

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 6 June 2007 Barr & Gwenlan

No MT2 Dijet cuts only Meff alone gives large QCD BD Barr & Gwenlan 6 June 2007 Barr & Gwenlan

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 6 June 2007 Barr & Gwenlan

No MT2 Dijet cuts + MET +  Two-Jet 6 June 2007 Barr & Gwenlan

1fb-1 meff > 350 GeV meff > 700 GeV No MT2 Dijet cuts + MET +  Two-Jet 1fb-1 meff > 350 GeV meff > 700 GeV Model S/sqrt(B) S/sqrt(S+B) SU1 27 24 55 37 SU2 0.8 1.4 SU3 58 47 110 61 SU4 320 160 400 130 SU6 14 13 29 23 SU8.1 22 20 46 33 6 June 2007 Barr & Gwenlan

Useful for early discovery? 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 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 6 June 2007 Barr & Gwenlan

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 6 June 2007 Barr & Gwenlan

3-jet also sensitive No MT2 Tri-jet cuts + MET +  Three-Jet Hundreds of signal events rapidly 6 June 2007 Barr & Gwenlan

3-jet also sensitive 1fb-1 meff > 350 GeV meff > 700 GeV No MT2 Tri-jet cuts + MET +  3-jet also sensitive Three-Jet 1fb-1 meff > 350 GeV meff > 700 GeV Model S/sqrt(B) S/sqrt(S+B) SU1 30 21 40 23 SU2 1.6 1.5 2.0 1.9 SU3 61 34 80 36 SU4 200 69 250 68 SU6 18 14 24 16 SU8.1 27 19 35 Hundreds of signal events rapidly 6 June 2007 Barr & Gwenlan

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”! 6 June 2007 Barr & Gwenlan

Backups 6 June 2007 Barr & Gwenlan

V12 BACKGROUND SAMPLES USED (SO FAR) J1 trig1_misal1_csc11.005010.J1_pythia_jetjet.recon.AOD.v12000601* J2 trig1_misal1_csc11.005011.J2_pythia_jetjet.recon.AOD.v12000601* J3 trig1_misal1_csc11.005012.J3_pythia_jetjet.recon.AOD.v12000603* J4 trig1_misal1_csc11.005013.J4_pythia_jetjet.recon.AOD.v12000601* J5 trig1_misal1_csc11_V1.005014.J5_pythia_jetjet.recon.AOD.v12000601* J6 trig1_misal1_csc11.005015.J6_pythia_jetjet.recon.AOD.v12000603* J7 trig1_misal1_csc11.005016.J7_pythia_jetjet.recon.AOD.v12000601* J8 - T1 trig1_misal1_mc12.005200.T1_McAtNlo_Jimmy.recon.AOD.v12000603* TTbar trig1_misal1_mc12.005204.TTbar_FullHad_McAtNlo_Jimmy.recon.AOD.v12000601* Zee trig1_misal1_mc12.005152.McAtNloZee.recon.AOD.v12000601* Zmumu trig1_misal1_mc12.005151.McAtNloZmumu.recon.AOD.v12000601* Ztautau trig1_misal1_csc11.005146.PythiaZtautau.recon.AOD.v12000601* Znunu trig1_misal1_mc12.008192.pythia_Znunu_qg_Nj2.recon.AOD.v12000601* Wenu trig1_misal1_csc11.005100.JimmyWenu.recon.AOD.v12000601* Wmunu trig1_misal1_csc11.005101.JimmyWmunu.recon.AOD.v12000601* Wtaunu trig1_misal1_csc11.008275.pythia_Wtaunu_qg_Nj2.recon.AOD.v12000601* 6 June 2007 Barr & Gwenlan

SUSY POINTS Point m_0 (GeV) m_1/2 (GeV) A0 (GeV) tan() sign()  (pb) Coannihilation (SU1) 70 350 10 + 7.43 Focus Point (SU2) 3550 300 4.86 Bulk (SU3) 100 -300 6 18.59 Low Mass (SU4) 200 160 -400 262 Funnel (SU6) 320 375 50 4.48 Coannihilation (SU8.1) 210 360 40 6.44 Coannihilation (SU8.2) 215 6.40 Coannihilation (SU8.3) 225 6.32 6 June 2007 Barr & Gwenlan

Dijet cuts only MET distribution 6 June 2007 Barr & Gwenlan

Do not believe the backgrounds! 4-jets + MET + dφ Four-Jet Do not believe the backgrounds! 6 June 2007 Barr & Gwenlan

1fb-1 meff > 350 GeV meff > 700 GeV 4-jets + MET + dφ Four-Jet Model S/sqrt(B) S/sqrt(S+B) SU1 19 12 SU2 2.5 2.3 SU3 40 18 SU4 120 34 110 SU6 15 10 SU8.1 11 Do not believe the backgrounds! 6 June 2007 Barr & Gwenlan

Cuts MT2 plot  at least two jets with: ETJet1,2 > 150,100 GeV 3-jet plot  at least three jets with: ETJet1,2,3 > 150,150,100 GeV |Jet1,2,3| < 2.5  MET > 100 GeV Dijet MEff plot  at least two jets with: ETJet1,2 > 150,100 GeV |Jet1,2| < 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  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 6 June 2007 Barr & Gwenlan

100 fb-1 Plots and significances 6 June 2007 Barr & Gwenlan

Dijet cuts only Two-Jet 6 June 2007 Barr & Gwenlan

100 pb-1 mT2 > 200 GeV mT2 > 400 GeV Dijet cuts only Two-Jet Model S/sqrt(B) S/sqrt(S+B) SU1 7.7 7.1 25 11 SU2 0.42 0.59 0.57 SU3 16 14 35 SU4 57 39 20 10 SU6 4.8 4.6 17 8.9 SU8.1 6.9 6.5 23 6 June 2007 Barr & Gwenlan

dijet + MET + dφ Two-Jet 6 June 2007 Barr & Gwenlan

100 pb-1 meff > 350 GeV meff > 700 GeV dijet + MET + dφ Two-Jet Model S/sqrt(B) S/sqrt(S+B) SU1 8.6 7.6 17 12 SU2 0.25 0.43 SU3 18 15 35 19 SU4 100 49 130 42 SU6 4.4 4.2 9.2 7.2 SU8.1 7.1 6.4 14 10 6 June 2007 Barr & Gwenlan

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) 6 June 2007 Barr & Gwenlan