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Study on search of a SM Higgs (120GeV) produced via VBF and decaying in two hadronic taus

Study on search of a SM Higgs (120GeV) produced via VBF and decaying in two hadronic taus. V.Cavasinni, F.Sarri, I.Vivarelli. OUTLINE. Motivation Signal and backgrounds Preliminary results of analysis Trigger issues. Cross sections and branching ratios for the Higgs boson.

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Study on search of a SM Higgs (120GeV) produced via VBF and decaying in two hadronic taus

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  1. Study on search of a SM Higgs (120GeV) produced via VBF and decaying in two hadronic taus V.Cavasinni, F.Sarri, I.Vivarelli

  2. OUTLINE • Motivation • Signal and backgrounds • Preliminary results of analysis • Trigger issues

  3. Cross sections and branching ratios for the Higgs boson σVBFH120= 4.35 pb (value from VV2H with CTEQ5L for PDF ) BR(H 120tt) = 0.0679 (value from Hdecay)

  4. Tau channel for low Higgs mass Previous study (hep-ph/0402254): tt  lnln , tt ln hn . In 30 fb-1 about 32 signal events and 22 background events

  5. mean =128 GeV sigma = 12 GeV Mass reconstruction in collinear approximation For tthad had, if x1,2 are the visible energy fraction of the two taus :

  6. VBF signature • 2 high pT forward jets • - depleted jet activity in the central region Rapidity distribution for the tagged forward jets for signal and for ttbar background.

  7. HVBF tt hhnn Tau decays 65% of times in hadrons The BR(tthh) is 42.25 % • Main features of this event : • 4 jets : • 2 forward jets • 2 central tau jets • Missing transverse energy due to neutrinos Main background: QCD Other backgrounds : ttbar, Z/γ+jets

  8. Treatment of QCD backgrounds • Due to the high cross section, impossible to generate all the needed events for the QCD, even with ATLFAST. • After all the analysis cuts, very poor background statistics. • To “solve” the problem : when requiring tau jets, keep all the background events and weight them with the rejection factor for tau jets. • The performance of the tau identification algorithm in FULLSIM depending on pT and h is parameterized and put directly into ATLFAST. (M.Heldmann work)

  9. Tau id in full simulation - tauRec • Reconstruct tau-candidate: • Start from Calorimetric Clusters (default). • Associate tracks to the candidate. • Calibrate candidate. • Build the set of pT dependent variables for tau-identification (REM, FISO, Ntrack, Strip Width, Nstrip, Charge, Impact parameter, ET/pT(1sttrack)) and then calculates Likelihood. • Apply a set of the basic cuts for tau-identification. M.Heldmann L > 4

  10. Typical cuts of the analysis (hep-ph/0402254) • at least 4 jets in the event; • 2 tau tagged jets with pTs over given thresholds; • 2 candidate forward jets with pTs above given thresholds; • negative eta product for the forwards and Dh > 4; • taus between the 2 forwards ; • 0 < x1(2) < 1 and taus not back to back; • invariant mass for the forwards > 700 GeV; • missing transverse energy above a threshold ; • no jet with pT > 20 GeV between the 2 forward, apart from the taus; • cut on mass window. Two different sets of cuts used for the analysis

  11. at least 4 jets; 2 tau tagged jets with pT above 35** and 30GeV; missing transverse energy > 45 GeV**; 2 forward candidates with pT above 50 and 30 GeV; negative eta product for the forwards and Dh > 4; taus between the forwards ; 0 < x1(2) < 1 and taus not back to back (Df < 3); cut on mass window : 104 GeV < mtt < 150 GeV. ** dedicated trigger menu Soft cuts

  12. Soft cuts using the trigger thresholds for low luminosity period

  13. Soft cuts

  14. Hard cuts • at least 4 jets; • 2 tagged tau jets with pt over 45 and 35 GeV; • missing transverse energy over 45 GeV; • 2 forward candidates with pt above 60 and 40 GeV; • negative product of forward jet eta and Dh > 4; • taus between 2 forward ; • 0 < x1(2) < 1 and taus not back to back (Df < 3); • forward jet invariant mass > 700 GeV; • no jet with pt > 30 GeV between the forwards, apart from the taus; • cut on mass window : 104 GeV < mtt < 150 GeV.

  15. Hard cuts using the trigger thresholds for low luminosity period

  16. Hard cuts

  17. ttbar background Both Ws dacay in tn : ~ 1% In 30 fb-1 about 17*10^6 events. Almost 40 *10 ^6 generated events. On ttbar events the rejection from parameterization is worse by about a factor 2.

  18. Soft cuts using the trigger thresholds for low luminosity period

  19. Hard cuts using the trigger thresholds for low luminosity period

  20. Important background g*/Z + jets with g*/Z  tt has a s = 1742 pb: 736 pb with two true hadronic taus. UNDER STUDY g*/Z +jets background

  21. Hadronic Tau Trigger (I) (ATL-COM-DAQ-2003-030) LVL1 trigger: look at 4X4 matrix of calorimetrictowers(DhDf= 0.1 x 0.1 each). ET threshold for the central core (EM+Had) and isolation thresholds between core and 12 external towers for e.m. and had. calorimeters. second layer of EM calorimeter LVL2 trigger: look at the shower shape in the 2nd layer of e.m. calorimeter and at the track multiplicity inside the RoI defined at LVL1. Cut on the ratio between ET contained in a 3x7 cell cluster and ET in a 7x7 cell cluster and on track multiplicity h + track multiplicity in the RoI f

  22. Hadronic Tau Trigger (II) (ATL-COM-DAQ-2003-030) • LVL3 (Event Filter) : • look at the complete event. • By now the variables of the offline algorithms are used as an approximation LVL3 trigger five variables: • number of reconstructed tracks, within DR = 0.3 of the candidate calorimeter cluster, between 1 and 3; • cut on isolation fraction, defined as the difference between the ET contained in a cone size of DR=0.2 and 0.1 normalized to the total jet ET; • cut on EM jet radius, an energy weighted radius calculated only in the e.m. calorimeter ; • cut on EM energy fraction, defined as the fraction of the total jet energy in the e.m. calorimeter; • threshold on the pT of the highest pT track.

  23. LVL1 Trigger Rates Illustrative menu M.Bosman talk@ATLAS Physics Workshop June 2005

  24. Inclusive High Level Trigger Event Selection No safety factors - large uncertainties ! Current global understanding of trigger rates M.Bosman talk@ATLAS Physics Workshop June 2005

  25. Trigger efficiency While the trigger rate depends on the QCD backgrounds, the trigger efficiencydepends on the physics channel under study. By now, only studies on Ztt and A/Htthh. The trigger impact on the VBF H  tt  hh has to be studied.

  26. Conclusions The channel VBF Higgs  tt hh appears to be promising to improve the statistical significance in the critical low Higgs mass region. Fast simulation shows that QCD and ttbar backgrounds may be rejected at the desired level. Z+jet background still under study. Efficiency of the trigger has to be studied for this channel. Future work : redo the analysis with full simulated events and with ME QCD generators.

  27. BACKUP SLIDES

  28. Builds set of variables for t-identification – they are pT dependant Calculate Likelihood from: REM, FISO, Ntrack, Strip Width, Nstrip, Charge, Impact parameter, ET/pT(1sttrack) M.Heldmann t id in full simulation - tauRec REM FISO Charge Ntrack Nstrip Impact parameter Strip Width ET/pT(1sttrack) SignalA→tt, background QCD, 0< pT<44 full line and 134< pT< 334 dashed line

  29. From M.Heldmann tid in fast Simulation - Parameterization Rejection vs. h (jet) Rejection vs. pT (jet)

  30. From M.Heldmann Parametrisation (R vs. pT) 20% 25% 30% 35% 45% 40% 50% 55% 60% R falls again with high pT Rmax and slope depend on e Rmax~300GeV R grows with pT

  31. d Parametrisation (R vs.η) From M.Heldmann 20% 25% 30% 35% 45% 40% 50% 55% 60% a lot of structure in R vs. h, but taken to be flat and than falling like a gauss R falls for h > 1.5 (endcap, higher granularity, lower ID resolution) At h1.5 crack region should be excluded

  32. Previous studies (ATL-DAQ-98-127) used: ETCore(em+h) > 50 fr(Core)= ETCore(em) / ETRoI(em) > 0.85 1 ≤ Ntrk ≤ 3 Now using infrastructure of LVL2 calo algorithm (TrigT2Calo): Adding AlgTools for Tau calculations at LVL2. Variables considered: ETCore(em), ETCore(h), ETRoI(em), ETRoI(h) in regions set in the jobOptions. Evaluating offline variables: em radius of the cluster, width in energy deposition, isolation fraction. Procedure in developing HLT code: Use offline athena for programming Test the code in the multithreading environment with athenaMT. Testbed integration. Hadronic Tau Trigger Pilar Casado, Martine Bosman & Carlos Osuna

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