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 BR(H 120   ) = (value from Hdecay) σ VBF H 120 = 4.35 pb (value from VV2H with CTEQ5L for PDF )

Tau channel for low Higgs mass Previous study (hep-ph/ ) :    l l,   l h. In 30 fb -1 about 32 signal events and 22 background events

Mass reconstruction in collinear approximation For   had had, if x 1,2 are the visible energy fraction of the two taus : mean =128 GeV sigma = 12 GeV

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

H VBF    hh Main features of this event : -4 jets : 2 forward jets 2 central tau jets -Missing transverse energy due to neutrinos Main background: QCD Tau decays 65% of times in hadrons The BR(   hh) is % Other backgrounds : ttbar, Z/γ+jets

 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 p T and  is parameterized and put directly into ATLFAST. (M.Heldmann work) Treatment of QCD backgrounds

Reconstruct tau-candidate:  Start from Calorimetric Clusters (default).  Associate tracks to the candidate.  Calibrate candidate.  Build the set of p T dependent variables for tau- identification (R EM, F ISO, N track, Strip Width, N strip, Charge, Impact parameter, E T /p T(1sttrack) ) and then calculates Likelihood.  Apply a set of the basic cuts for tau-identification. Tau id in full simulation - tauRec L > 4 M.Heldmann

Typical cuts of the analysis (hep-ph/ ) at least 4 jets in the event; 2 tau tagged jets with p T s over given thresholds; 2 candidate forward jets with p T s above given thresholds; negative eta product for the forwards and  > 4; taus between the 2 forwards ; 0 < x 1(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

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  > 4; taus between the forwards ; 0 < x1(2) < 1 and taus not back to back (  < 3); cut on mass window : 104 GeV < m  < 150 GeV. ** dedicated trigger menu Soft cuts

cutSignal efficiency Events 30 fb -1 (300 fb -1 ) QCD rejection Events 30 fb -1 (300 fb -1 ) 4jet (2 tau), E t miss > 45 GeV 4.4 % P t tau2.5 % P t forward1.9 %  forward0.94 %  cuts0.65 % 24 (241) (1551) Mass window0.55 %21 (206) (231) Soft cuts using the trigger thresholds for low luminosity period

Soft cuts

at least 4 jets; 2 tagged tau jets with p t over 45 and 35 GeV; missing transverse energy over 45 GeV; 2 forward candidates with p t above 60 and 40 GeV; negative product of forward jet eta and  > 4; taus between 2 forward ; 0 < x 1(2) < 1 and taus not back to back (  < 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 < m  < 150 GeV. Hard cuts

cutSignal efficiency Events 30 fb -1 (300 fb -1 ) QCD rejection Events 30 fb -1 (300 fb -1 ) 4jet (2 tau), E t miss > 45 GeV 4.4 % Pt tau1.6 % Pt forward0.97 %  forward0.49%  cuts0.33 % M jj 0.3% Jet veto0.26% 10 (99) (144) Mass window0.24%9 (89) (15) Hard cuts using the trigger thresholds for low luminosity period

Hard cuts

ttbar background Both Ws dacay in  ~  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.

cutSignal efficiency Events 30 fb -1 (300 fb -1 ) ttbar rejection Events 30 fb -1 (300 fb -1 ) 4jet (2 tau), E t miss > 45 GeV 4.4 % (7.3/2) P t tau2.5 % (1.78/2) P t forward1.9 % (1.46/2)  forward0.94 % (5.09/2)  cuts0.65 % 24 (241) (1.02/2) (338) Mass window0.55 %21 (206) (5.07/2) (32) Soft cuts using the trigger thresholds for low luminosity period

cutSignal efficiency Events 30 fb -1 (300 fb -1 ) ttbar rejection Events 30 fb -1 (300 fb -1 ) 4jet (2 tau), E t miss > 45 GeV 4.4 % (7.3/2) Pt tau1.6 % (1.1/2) Pt forward0.97 % (6.8/2)  forward0.49% (2.4/2)  cuts0.33 % (0.56/2) M jj 0.3% (4.05/2) Jet veto0.26% 10 (99) (1.77/2) (58) Mass window0.24%9 (89) 0 Hard cuts using the trigger thresholds for low luminosity period

Important background  */Z + jets with  */Z   has a  = 1742 pb: 736 pb with two true hadronic taus. UNDER STUDY  */Z +jets background

Hadronic Tau Trigger (I) (ATL-COM-DAQ )   LVL1 trigger: look at 4X4 matrix of calorimetric towers (  = 0.1 x 0.1 each). E T threshold for the central core (EM+Had) and isolation thresholds between core and 12 external towers for e.m. and had. calorimeters. + track multiplicity in the RoI 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 E T contained in a 3x7 cell cluster and E T in a 7x7 cell cluster and on track multiplicity

Hadronic Tau Trigger (II) (ATL-COM-DAQ ) 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  R = 0.3 of the candidate calorimeter cluster, between 1 and 3;  cut on isolation fraction, defined as the difference between the E T contained in a cone size of  R=0.2 and 0.1 normalized to the total jet E T ;  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 p T of the highest p T track.

LVL1 Trigger Rates Illustrative menu M.Bosman Physics Workshop June 2005

Inclusive High Level Trigger Event Selection Selection2x10 33 cm -2 s -1 Rates (Hz) Electrone25i, 2e15i ~40 Photong60i, 2g20i ~40 Muonm20i, 2m10 ~40 Jetsj400, 3j165, 4j110 ~25 Jet & E T miss j70 + xE70 ~20 tau & E T miss t35 + xE45 ~5 B-physics2m6 with m B /m J/y ~10 Otherspre-scales, calibration, … ~20 Total~200 Current global understanding of trigger rates No safety factors - large uncertainties ! M.Bosman Physics Workshop June 2005

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

Conclusions The channel VBF Higgs    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.

BACKUP SLIDES

 id in full simulation - tauRec Builds set of variables for  -identification – they are p T dependant Calculate Likelihood from: R EM, F ISO, N track, Strip Width, N strip, Charge, Impact parameter, E T /p T (1 st track) R EM F ISO N track Strip Width N strip Charge Impact parameter E T /p T (1 st track) M.Heldmann Signal A →  , background QCD, 0< p T <44 full line and 134< p T < 334 dashed line

Rejection vs. p T (jet) Rejection vs.  (jet) From M.Heldmann  id in fast Simulation - Parameterization

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

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

Previous studies (ATL-DAQ ) 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. Pilar Casado, Martine Bosman & Carlos Osuna Hadronic Tau Trigger