Measuring the top quark Yukawa coupling at the ILC at s = 500 GeV

Transcription

1 Measuring the top quark Yukawa coupling at the ILC at s = GeV Phys. Rev. D 84, 1433 (11) Katsumasa Ikematsu (Uni Siegen) for Ryo Yonamine (Sokendai/KEK), Tomohiko Tanae (Univ. of Tokyo), Keisuke Fujii (KEK), Yuichiro Kiyo (Tohoku Univ.), Yukinari Sumino (Tohoku Univ.), Hiroshi Yokoya (CERN/National Taiwan Univ.) Top quark physics at lepton colliders (1 May, Paris (Amphithéâtre Chaudron) 1

2 Main motivation Verification of EWSB & mass generation is a critical task which must e done efore BSM physics can e estalished ILC is an ideal proe for measuring the couplings related to the Higgs (self-coupling, Yukawa coupling) SUSY XD LHT DH m f = g f v Gauge Principle Symmetry Breaking & Mass Generation Estalished y precision EW studies Untested! e+e- -> ZH -> ZHH -> TTH -> HH

3 Motivation at s = GeV Well-known energy thresholds for ILC: (m Higgs = 1 GeV) - GeV: ZH - 3 GeV: ttar - GeV: ZHH & tth tth measurement is possile due to QCD ound-state effects which enhance the cross section (f / GeV) d /dm tt..1.. Based on Farrell & Hoang, PRD74, 148 (6) with 1S Peak tt ound-state effects without ound-state effects (GeV) tt s m t = GeV = 17 GeV m tt P e ± = cross section (f) cross section (f) ZHH concurrent measurements at s = GeV without ISR/eam with ISR/eam with ISR/eam +NRQCD Higgs from Z Note: this also enhances ttz while ttg* is not enhanced ecause tt in this case is not a color singlet tth 3

4 Indirect vs direct measurement Indirect measurement of top quark Yukawa coupling is possile at the ttar threshold and also at the LHC via gluon fusion to ttar (ut the jet ackground makes it challenging) - if an anomaly is found in the production rate, one cannot distinguish (1) the coupling anomaly or () the presence of a new particle in the loop Need direct measurement; feasiility already shown for s = 7-8 GeV LC => we show this for s = GeV - direct measurement at LHC using H->ττ has een proposed ut it can only measure σ x BR(H->ττ) () X? g t? e + e γ/z g t t t W H W + 4

5 (c) Evaluate accuracy of g t e + γ/z t W = Accuracy of e + e - -> tth cross-section - E cm = GeV, m Higgs = 1 GeV, H -> (68%) g t g t = σ tth σ tth Estimate the statistical uncertainty e e + Z/γ g t t t e W H W + t H f f f f W + σtth σ tth = S + B S + Bsyst S + L L + Statistical Uncertainty Background Shape Systematic Uncertainty of Integrated Luminosity Uncertainty of Event Reco.

6 Signal & ackground Signal: - tth -> WW (.4 f for 6-jet + lepton & 8-jet modes) Main ackground: - ttg* (g*->, ~1 f), ttz -> tt (~. f), tw (~7 f) 1 3 cross section (f) tth (with tth tt ound-state effects) (without tt ound-state effects) tth e - /e + pol. = (.,.) tw s (GeV) tt ttz * ttg tt (Higgs radiated off Z) 1 e e e e A t t graph 19 W t ve W graph 18 W W W

7 tw events scaled to xsec tw em-1. ep. tt em-1. ep. tw em+1. ep. tt em+1. ep. scaled to xsec tw em-1. ep. tt em-1. ep. tw em+1. ep. tt em+1. ep. 1-1 mass of W system 1-1 (parton-level) W+ mass W- mass Jet cominatorial effects & misidentification of -jets result in significant ackground - tail of distriution is important! WW* enhances the tw tails for e - Le + R 1 e e W t ve W graph 18 W

8 Analysis framework Event generation: physsim - helicity amplitude y HELAS; phase space integration; BASES - ISR & eamstrahlung included - ttar threshold correction to tth & ttz - dedicated ttg & tw generator Parton shower & hadronization: Pythia Fast detector simulation: JSFQuickSim process cross-section (f) generated numer of events equivalent luminosity (a -1 ) tth 1.4, 4.3 ttz 4.4, 1.4 ttg* -> tt 1.93,.9 tw 1633.,, 6.1 tth.4, 9.6 ttz 1.34, 37.8 ttg* -> tt.89, 8. tw 7.,, semi-realistic PFA reconstruction axis. Detector Resolution Vertex detector σ =7. (./p sin 3/ θ) µm Drift chamer σ pt /p T =1.1 4 p T.1% ECAL σ E /E =1%/ E 1% HCAL σ E /E =4%/ E % e - /e + pol. = (-1., +1.) e - /e + pol. = (+1., -1.) (target integrated luminosity = 1 a -1 ) 8

9 Analysis Strategy Analysis mode: - (i) 8-jet mode: 4% - (ii) 6-jet + lepton mode (e or μ): 9% - (iii) 4-jet + -lepton mode (ee, eμ, or μμ): % (we don t reconstruct) Higgs reconstructed in mode Higgs = 1GeV) Cut-and-count analysis Event selection ased on: - identification/rejection of isolated leptons - event shape variales - -tagging of jets - comination of jets into top & Higgs candidates - invariant mass: m3j(top) and mj(higgs) 9

10 Lepton selection / rejection Lepton identification y MC information (assumes % efficiency & purity for E > 1 GeV leptons) Distinguish isolated leptons & leptons from jets y using the energy sum of the particles around the lepton candidate (cone energy) versus the lepton energy -D selection (rejection) of isolated leptons for the 6-jet + lepton analysis (8-jet analysis) Cone Energy (GeV) cos θ <.98 Lepton Energy (GeV)

11 Event shape Use the thrust variale to discriminate signal from ackground (peaks at 1 for di-jet events) T = max ˆn =1 i ˆn p i i p i Jet clustering Use the Durham jet clustering algorithm to force the event into the 6 or 8 jet topology (after removing the isolated leptons) Y ij = max(e i,e j )(1 cos θ ij) E CM 11

12 -tagging Identify -jets via their large impact parameter significance (IPS) of secondary tracks -tagging criteria: - tight (6J+L): require 4 tracks with IPS >. - tight (8J): require tracks with IPS > 3. efficiency: 47%, fake rate: c-jet 3.%, uds-jet.1% - loose: require tracks with IPS >. efficiency: 8%, fake rate: c-jet 4%, uds-jet.% Event selection: - tight + loose for Higgs candidate - tight for at least one top, loose for the other top efficiency & fake rate estimated on Z->qq 91. GeV -jet 1

13 Jet comination Choose the jet comination which is most consistent with the tth mass hypothesis is chosen y minimizing the following chisquared value: χ = (m j M H ) σh + (m j M W1 ) σw + (m 3j M t1 ) 1 σt 1 (mj M W ) + σw + (m 3j M t ) σt 8j σ H, σ W, σ t correspond to the mass resolutions in the case of perfect jet clustering and jet cominations (σ H = 17.7 GeV, σ W = 9,3 GeV, σ t = 14.3 GeV) 13

14 Summary of cuts cut 6-jet + lepton 8-jet numer of isolated lepton 1 thrust <.77 <.7 jet clustering Y ->4 >. Y 8->7 >.8 -tagging 4x -jets 4x -jets top mass (GeV) 14 < m t < 14 < m t < 1 higgs mass (GeV) 9 < m h < 1 8 < m h < 1 14

15 6-jet + lepton analysis Events 1 (a) Events 1 () * ttg ttz tw tt tth (8j/4j) tth (6j) Thrust Y 4 Events / ( GeV) 1 (c) Events / ( GeV) (d) Top candidate mass (GeV) Higgs candidate mass (GeV) Scaled to 1 a -1 Beam polarization ( Pol(e - ), Pol(e + ) ) = (-.8, +.3) All other cuts applied 1

16 6-jet + lepton cut flow L = 1 a -1, polarized eams cut \ sample tth (6J) tth (8J/4J) tt ttz ttg * -> tt significa nce no cuts # isolated lepton = 1 thrust <.77 Y ->4 > x tag mass cuts

17 8-jet analysis Events (a) Events () * ttg tt ttz tw tth (6j/4j) tth (8j) Thrust Y 8 7 Events 3 (c) Events (d) Top candidate mass (GeV) Higgs candidate mass (GeV) Scaled to 1 a -1 Beam polarization ( Pol(e - ), Pol(e + ) ) = (-.8, +.3) All other cuts applied 17

18 8-jet cut flow L = 1 a -1, polarized eams cut \ sample tth (8J) tth (6J/4J) tt ttz ttg * -> tt significa nce no cuts # isolated lepton = thrust < Y 8->7 > x tag mass cuts

19 Results eam pol. (e -, e + ) 6 jet + lepton 8 jet (.,.).9.8 (-.8, +.3) eam pol. (e -, e + ) comined significance comined Δg t / g t (.,.) 4. 1% (-.8, +.3). 9.6% 19

20 Conclusions Our fast simulation study shows that, with - m Higgs = 1 GeV - s = GeV ILC - nominal eam polarizations (-.8, +.3) - Integrated Lumi = 1 a -1 the top quark Yukawa coupling can e measured with % statistical accuracy for increased confidence in this results, a study with full detector simulation is on-going also trying to meet the demands of the DBD enchmark which asks for the same analysis at s = 1 TeV for the ILD detector

21 Backup 1

22 Mass dist. (ILD Full Simulation) 6-jet + lepton mode Events 1 WW ttz Events 1 tt tth 1 Top candidate mass (GeV) 1 Higgs candidate mass (GeV) 8-jet mode Preliminary Events 1 WW ttz Events tt 1 tth 1 Top candidate mass (GeV) 1 Higgs candidate mass (GeV)