Study of VBF H->tt->lep+had Junichi Tanaka ICEPP, Univ. of TOKYO Discovery of Higgs and Supersymmetry to Pioneer Particle Physics in the 21th Century 24th Nov. 2005

Outline Introduction Study of 3rd jet by ME event generator Study of fake BGs Summary Prospects : Discovery Potential of SM Higgs 24th Nov. 2005

SM Higgs Production & Decay @ LHC Excluded by LEP Production Decay t+t- VBF LEPEWWG2005S 114.4 < mH < 219 GeV/c2 @95% C.L. VBF H->tt at low-mass region (mH = ~120GeV) Large production cross section and branching ratio: sxBR ~ 300fb 24th Nov. 2005

Vector Boson Fusion Process Rapidity gap f h PRD59(1999)14037 hep-ph/9908378 Characteristics High Pt jets in forward region Rapidity gap Right fig : h*3 = h of 3rd jet – average of h of 2 forward jets Signal … a dip in the center BG … a peak in the center -> jet activities depend on structure of color flow. (EW) Treatment of 3rd jet is important in this mode! 24th Nov. 2005

Study @ ATLAS ATLAS Scientific note: Eur.Phys.J.C32S2:19-54,2004 BG by Comphep VBF H->tt VBF H->tt is one of promising channels near the LEP limit. (left fig) Reconstruct M(tt) by using missing Et and observe a peak (right fig) on the right side of the Z mass peak. “VBF H->tt” is one of discovery channels at the low mass region. 24th Nov. 2005

Motivation Improvement and progress of our tools since the previous studies (Eur.Phys.J.C32S2,19,2004, the pervious slide) 1) Events with multi jets can be generated by the recent ME event generators. 3rd jet was evaluated by Parton Shower (PS) model in the previous studies. We evaluate 3rd jet by the ME event generator. 2) Environment of reconstruction and full simulation, which will be used in 2007, is available. Fake rate can be estimated from the full simulation, which is based on Geant4. Large QCD backgrounds, which have not been considered in the previous studies, are checked by using results of the full simulation. 24th Nov. 2005

Type of Simulation Full simulation Fast simulation Based on Geant4 Initial layout Used for detail studies Development of reconstruction tools, for example, tracking, reconstruction of missing Et, Particle ID tools (e, m, tau, b, g). Fake rates, crack effect for missing Et and so on Take much time, typically, ~15mins/event with 3GHz CPU. Fast simulation Based on parameterized performance of full simulation results Used for the BG study, where we need high statistics. Most of today’s results are obtained by the fast simulation. 24th Nov. 2005

Study of 3rd jet by ME event generator 24th Nov. 2005

ME ALPGEN + PS Alpgen (M.L.Mangano) is one of the ME event generators. Studies using ME event generators becomes popular. ME has an advantage for producing high Pt jets. To remove collinear and soft divergence, Pt and Rjj are required at parton level. Soft and collinear kinematical regions are covered by PS. We produced exclusive samples for Z+njets(n=2,3,4) with modified ALPGEN ver1 (small weight events can be produced). “Z+njets” is a dominant BG. MLM method is used to avoid double counting. The similar method is officially introduced from ver2. 24th Nov. 2005

To Avoid Double Counting We use the following method (“our MLM method”) to avoid the double counting of events: (not CKKW method) Veto events having jets (Pt>20GeV) at the outside of cone (dR=0.4), whose center is defined by the direction of a ME-parton. ME-parton Rec-jet Good Bad See T.Sasaki’s talk later 24th Nov. 2005

Pt and h Distributions 3rd jets with high Pt increase. Alpgen sample(ver1), after lepton cut, normalized to 1fb-1. Pt of 3rd jet eta of 3rd jet preliminary “Z2j” “Z2j”+”Z3j” “Z2j”+”Z3j”+”Z4j” with MLM method to avoid double counting preliminary log10Pt 25GeV 100GeV 3rd jets with high Pt increase. 3rd jets become more central. It has a large effect on Central Jet Veto. Pt and h of 3rd jet depend on the number of jets. Cannot ignore them. 24th Nov. 2005

CJV Survival Probability Central Jet Veto (CJV) : Veto events having additional jets with a “given” Pt in the rapidity gap. preliminary No additional jet in the rapidity gap jet t t jet f Rapidity gap h (additional jets in the rapidity gap) Survival probability of “Z+2j” is higher than “Z+3j” and “Z+4j” due to smaller jet activity in the rapidity gap. ~40% of Z+njets (n=2,3,4) remains in total in case of Pt=20GeV. 24th Nov. 2005

M(tt) Distribution After all the selection criteria L=30fb-1 After all the selection criteria A amount of remaining Z+2j is large due to large cross section and high CJV survival probability. Z+3j and Z+4j still remains. They are not negligible. Since resolution becomes worse as the number of jets increases, their tails can contribute to BGs. Z+2j Z+3j Z+4j preliminary Signal region 24th Nov. 2005

Study of fake BGs 24th Nov. 2005

Origin of Fake Lepton Since cross section (x-sec) of events with QCD jets is much larger than that of the signal (x-sec~300fb), we suffer from these events, which have fake leptons from jets. W j j j -> W t j j : x-sec ~ >10nb j j j j -> e/m t j j : x-sec ~ >10mb Fake Tau 1 or 3 p+- with p0, which have most of energy of a jet Fake Electron Pair production of gamma (from p0) Dalitz decay p+- and p0 with E/p~1 B/D semileptonic decay Fake Muon pion punch through pion decay in flight 24th Nov. 2005

Tau Reconstruction and ID Hadronic decay: 1prong ~50% and 3prongs ~15% We need to identify tau-jets from many QCD-jets. Charged tracks : 1 or 3 Photons from p0 Reconstruction : two ways at ATLAS 1) Cell based : start from cells, which is the similar as cone jet algo. 2) Track based : start from charged track(s) Properties of tau-jet : for identification 1 or 3 charged pion tracks Narrowness and Isolation 24th Nov. 2005

Tau Rec/ID Performance Cell based method “Fake rate of 10-2” means we misidentify one QCD jet as a tau in 100 QCD jets. Depend on Pt and type of jet. Gluon fake rate is better than quark fake rate due to difference in narrowness. Light Quark Gluon fake rate preliminary 20<Pt<40GeV 40<Pt<60GeV 60<Pt<100GeV 100<Pt<140GeV efficiency This result is parameterized and then is used in fast simulation. At least we need ~50% eff. with these fake rates for the discovery with L=10fb-1. 24th Nov. 2005

e/m Fake Rates Fake rates of electron and muon from jets are obtained from QCD di-jet samples using the full simulation. See O.Jinnouchi’s talk later. Typical performance Jet -> e : ~10-3 Jet -> m : ~10-4 These results are taken into account in the fast simulation to evaluate BG contributions. 24th Nov. 2005

W+3jets & 4jets W j j j -> W t j j j j j j -> e t j j BG Corresponds to L ~ 70fb-1 Corresponds to L ~ 10fb-1 W+3jets 4jets BG x-sec(fb) W+3jets 0.26 4jets 0.30 Z+njets 0.13 preliminary fake(e->jet) : 10-3 preliminary (Signal region:110-135GeV) W j j j -> W t j j j j j j -> e t j j W+3j and 4jets remain with the same order of Z+njets. Improvement of tauID for both cases and decrease of fake electron for 4jets are critical to decrease these BGs. 24th Nov. 2005

M(tt) and Significance Signal (m=120) very preliminary Z+nj(QCD) M(tt) and Significance Z/ttjj(EW) L=30fb-1 tt, WWjj A a kind of BGs is the same as our paper(page5). B +”W+3jets” C +”jjjj” 10 sBG=0.18fb sBG=0.43fb sBG=0.73fb 10 10 sSIG=0.45fb S/B S/B0.5 L=10fb-1 L=30fb-1 A 2.5 3.3 5.8 B 1.0 2.1 3.7 C 0.61 1.6 2.9 We assume that lepton(e,m) efficiency is 85%. Significance becomes worse by a factor of 2 due to the new backgrounds. We need to improve tauID/fake electron and to optimize cuts. Signal region:110-135GeV 24th Nov. 2005

Summary VBF H->tt->lep+had is important for the SM Higgs discovery in the low mass region (mH~120GeV). We evaluated 3rd jet by the ME event generator. There are differences in Pt and h of 3rd jet by the number of jets. Central jet veto survival probability and tail of M(tt) depend on the number of jets. We should use ME generators for multi-jets studies as much as possible. We studied fake BGs. W+3jets and 4jets become large BGs for the lep+had channel. We need to improve our tauID and other PID performance. We must understand QCD BGs for any LHC physics since their x-sec is very large in the LHC. (maybe beyond our expectation…) 24th Nov. 2005

Discovery potential of SM Higgs ATLAS-PHYS-2004-005 preliminary NB: not include our latest results. 10fb-1 ATLAS Today L=10fb-1: VBF tt(ll+lh) ~ 2.6 gg ~ 5.0 @ mH=120GeV 200 120GeV Two promising channels at mH=~120GeV! We are preparing for this discovery! 24th Nov. 2005

Backup 24th Nov. 2005

ATLAS (A Toroidal LHC ApparatuS) E,P resolution (P ~ 100GeV) : m ~ 2%　 e, g ~ 1.5% Jets ~ 8% 40MHz beam crossing Readout channel = 160M channels Raw data = 320Mbyte/sec (1TB/hour) Inner tracking system : 2T solenoid magnet Liq. Ar EM calorimeter Muon spectrometer : air-core troidal magnet 24th Nov. 2005

Summary of SM Higgs Studies Production Decay Mass region and purpose Gluon Fusion H -> gg 110-140GeV Mass H -> ZZ-> 4 l 140-1000GeV Discovery, Mass, Spin, Coupling H -> WW 130-170GeV Discovery Vector Boson Fusion H -> tt Discovery, Mass, Coupling 130-200GeV Discovery, W coupling Discovery, Mass H -> bb Yb coupling ttH 110-130GeV Yt coupling 130-180GeV WH 140-170GeV ATLAS 24th Nov. 2005

Hadronic tau decay mode Branching ratio of H->tt : BR(lep-lep) = ~12% BR(lep-had) = ~46% “Lep-had” mode plays an important role for the Higgs discovery. factor of ~4 Mtt Distributions of VBF H->tt ＭＨ=120GeV ATLAS (em only) ATLAS 24th Nov. 2005

Fake rates Pt(GeV) Jet -> e Jet -> m BJet->e BJet->m very preliminary Pt(GeV) Jet -> e Jet -> m BJet->e BJet->m BJet->t 15-35 6.7+-0.8 2.4+-0.5 19+-8 < 3.2 25+-9 35-50 6.0+-1.3 0.6+-0.4 10+-7 < 5.1 50-80 3.9+-0.9 2.3+-0.7 3.7+-3.7 7.5+-5.3 80-130 3.8+-1.0 0.3+-0.3 < 4.2 130-200 3.4+-1.0 < 5.2 5.2+-5.2 200- 1.4+-0.3 0.2+-0.1 < 2.4 in 10-4 24th Nov. 2005

Simulated data W(->ln)+3jets 4jets Alpgen ver1 Jet condition : Pt>15GeV, |eta|<5, Rjj>0.7 Max(J1,J2)>30GeV, M(J1,J2)>200GeV, deltaEta(J1,J2)>2 x-sec = 1.4nb … very large! 4jets Jet condition : Rjj>0.7 Forward region : Pt>30GeV, eta1xeta2<0, |eta1-eta2|>2.5 Mjj>350GeV, |eta|<5.0 Central region : Pt>20GeV, |eta|<2.7, Mjj>80GeV x-sec = 2mb … very large!!! 24th Nov. 2005

qqH -> qqtt -> qq lep had tau->e FWJet1 FWJet2 tau->h (1prong+pi0) Missing Et We’ll look this type of event in the real data !!! 24th Nov. 2005

ME+PS Z2j sample Z3j sample Z4j sample z2j without MLM method z2j with MLM method + z3j without MLM method Z4j sample + z3j with MLM method + z4j without MLM method 24th Nov. 2005