Top-Higgs Yukawa coupling measurements at the LC and LHC

1. Top-Higgs Yukawa coupling measurements at the LC and LHC ALCP04@SLAC, Jan 6-10 2004 A. Juste • OUTLINE • Motivation • Measurements at the LHC • Measurements at the LC • Summary and conclusions

2. Motivation • Precise measurements of the Higgs couplings to gauge bosons and fermions are instrumental in order to try to probe the nature of a discovered Higgs boson, in particular in the case of production cross-section and branching ratios rather SM-like. • The top-Higgs Yukawa coupling is the largest coupling of the Higgs boson to fermions (gttH ~ 0.7 vs gbbH ~ 0.02). Precise measurement very important since the top quark is the only “natural” fermion from the EWSB standpoint. • The goal is to understand what precision can be achieved in the measurement of the top-Higgs Yukawa coupling by each machine (LHC and LC) separately, how available measurements at one machine can benefit the other and what is the ultimate reach in probing this coupling from a combination of measurements at both machines. Indeed, aim at the most model-independent possible measurement of the top-Higgs Yukawa coupling. DISCLAIMER: Results shown here derived from the following contribution to the (unreleased yet) LHC/LC Study Working Group document: S.Dawson, A. Juste, L. Reina and D. Wackeroth, “Associated tth production at the LC and LHC” and references therein.

3. The Nature of the Measurements at the LHC • A number of production and decay channels are expected to be available at the LHC for detailed measurements of the Higgs boson properties: • Each cross-section measurement is proportional to the product of squares of Yukawa couplings: • To determine the top-Higgs Yukawa coupling with good accuracy: • Need to be able to minimize systematic uncertainties (both experimental and theoretical) that plague cross-section estimates at hadron colliders:  experimental systematics: luminosity, reconstruction efficiencies, etc  availability of theoretical predictions at higher order in QCD  PDF uncertainties Ratio of cross-sections (large cancellations of systematics)ratios of Yukawa couplings (already extremely useful to start probing the nature of the Higgs boson) • For an absolute measurement, need to have a measurement of the Yukawa couplings in each decay mode considered: gbbh, gtth, gWWh . Two possibilities:  internal (derived from other LHC measurements)  external (from high precision measurements at the LC). e.g. stth,hbb g2tth g2bbh

4. Combination of LHC Measurements • Express all available measurements in terms of combinations of partial widths: • Perform ratios of cross-sections within a given set and a global fit (a-la HFITTER) to all available ratios from all sets. • Assumptions required for an absolute determination of Yukawa couplings (in particular gtth) : 1) W-Z universality: (can be tested at 15-20% level for mh>130 GeV using ggh(hWW,ZZ) ) 2) Higgs boson width mostly saturated by observable decay channels: Other (a priori avoidable): b-t universality, only top contributes to ggh vertex,… Set 1 (3 measurements) Set 2 (3 measurements) Set 3 (4 measurements) D. Zeppenfeld et al, PRD 62 (2000) 013009 D. Zeppenfeld, hep-ph/0203123 A. Belayev and L. Reina, JHEP 0208 (2002) 041

5. + + … Associated Higgs Production at the LHC Associated Higgs Production at the LHC • LO prediction: • NLO QCD corrections • gg-fusion is the dominant contribution (~90%), although other subprocesses (e.g. qq-annihilation) are relevant and cannot be neglected. • stth gtth2 • stth ~ 500 fb (mh=120 GeV, m=mt+mh/2) but scale-dependence is significant (~20-30%)  higher order QCD corrections needed • Increase rate: KNLO~1.2-1.6 (depending on m and PDF choicemainly coming from LO!). • Much reduced scale-dependence. • Estimated overall theoretical uncertainty (residual m-dependence, Dmt, DPDF) is ~15-20% • stth ~ 700 fb (mh=120 GeV, m=mt+mh/2)  ~7(70)k events/year at low(high) luminosity W. Beenakker et al, PRL 87 (2001) 201805,\; Nucl. Phys. B653 (2003) S. Dawson et al, PRD 67 (2003) 071503; PRD 68 (2003) 034022

6. ATLAS (30 fb-1, no K-factors) ATLAS(CMS) L=30 fb-1, no K-factors mh (GeV) = 120 130 140 S/B 2.8(3.5) 1.9(2.8) 1.0 Dgtth/gtth (%) 19(18) 29(21) 50 No systematics J. Cammin et al, ATL-PHYS-2003-024 Direct gtth Measurement at the LHC: hbb • Final state signature: • one high pT isolated lepton (used to trigger the event) and high MET • 6 jets out of which 4 b-tagged jets (requires high performance b-tagging: eb=60%, Rc=10, Rl=100) • Main background is tt+jets: S/B~1‰ After the 4 b-tags requirement, the dominant contribution is ttbb (QCD): S/B~few% • Recent analyses use improved background simulation and sophisticated likelihood techniques to reduce combinatorial background in top reconstruction. • The signal is extracted as a bump in the mbb distribution over a large background. J. Cammin et al, ATL-PHYS-2003-024 V. Drollinger et al, hep-ph/0111312 • Significant discrepancy between ATLAS and CMS in the ttbb (QCD) background prediction  LO cross-sections used are very sensitive to factorization scale, PDF and parton level cuts choices.

7. No systematics * * * * * * * * * Parton level study F. Maltoni et al, PRD 66 (2002) 034033 * * J. Leveque et al, ATL/PHYS-2002-19 Direct gtth Measurement at the LHC: hWW* • Very important decay channel to increase sensitivity to gtth up to higher values of mh: BR(hWW*)BR(hbb) for mh=135 GeV, dominant decay channel for higher mh • Combining pptth, hWW* with ggh, hWW*, the ratio gtth/gggh can be measured (~15-20%) in a model-independent way, thus probing loop effects from non-SM colored degrees of freedom. • Final state signatures: • Require double b-tagging • Backgrounds: • Very large background from ttl+jets (ttll+jets) in the 2l(3l) channel unless semileptonic b-decays are vetoed. • After full selection, the main backgrounds are: tt, ttZ, ttW and tttt. • Large systematic uncertainties on ttZ, ttW and tttt background predictions.

8. L=300 fb-1 mh (GeV) = 115 120 125 130 140 S/B (4.0) 12.5(3.4) (2.6) 8 4.4 Dgtth/gtth (%) (16) 7(19) (25) 9.5 15 No systematics Direct gtth Measurement at the LHC: ht+t- • For mh<130-135 GeV, this channel allows to for a more model independent measurement of the Higgs couplings: in particular, together with tth(hbb), allows to measure the ratio gbbh/gtth, avoiding having to impose “b-t universality”. • Final states: semileptonic: 2 same sign leptons: tri-lepton: • After double b-tagging and hadronic W+top reconstruction (when applicable), the main background is ttZ/g(tt). • Results: A. Belyaev and L. Reina, JHEP 0208 (2002) 041 L. Zivkovic et al, A. Ito et al, R. Tanaka et al (ATLAS Higgs working group) Semileptonic A. Belyaev and L. Reina, JHEP 0208 (2002) 041 2 same sign leptons+tri-leptonR. Tanaka et al (ATLAS Higgs working group)

9. Direct gtth Measurement at the LHC: Overview • Combined fit to the following set of measurements (no external input from LC): Assumed luminosity is: • L=300 fb-1: pptth, hWW and qqWh, hbb • L=30 fb-1 : ggh, hWW • L=200 fb-1: rest of the channels • Only systematics considered are theoretical (ggh: 20%, pptth: 10%, qqqqh: 5%) • Assumptions made: • W-Z universality • Saturation of Higgs total width to observable decays • Similar studies: From: V. Drollinger et al, hep-ph/0111312 A. Belyaev and L. Reina, JHEP 0208 (2002) 041 D. Zeppenfeld, hep-ph/0203123 M,. Duhrssen, ATL/PHYS-2003-030

10. The Nature of the Measurements at the LC • Measurements of couplings at the LC can be very precise (~few % level) and model independent. In particular, as compared to the LHC, there is no need to assume: and powerful input for the LHC. • gZZh: model independent determination from sZh measurement using recoil method • gWWh: model independent determination through cross-section ratio: • G: determined from GW (via gWWH) and BR(hWW*) (using Zh,hWW*ln+2j or 4j) For mh200 GeV, G>2 GeV and directly resolvable. • Couplings to fermions are determined from measurements of sZH, Hnn x BR(Hff) and using the estimated total Higgs width. L=500 fb-1, s = 350, 500, 800 GeV

11. mh = 120 GeV stth (fb) Associated Higgs Production at the LC • LO prediction: • g-exchange is the dominant contribution. • Radiation off the Z boson ~0.5% contribution stth gtth2 • Rather small cross-section: stth ~ 0.4(2.7) fb at s = 500(750) GeV (mh=120 GeV)  ~750 events/year (mh=120 GeV) at s = 800 GeV, L=3x1034 cm-2s-1  High luminosity required (1 ab-1) for a precise measurement. • QED corrections in the initial state (bremsstrahlung+beamstrahlung) Significantly distort the tth lineshape: • Maximum of cross-section shifted towards higher s. • stth reduced by a factor of 2 ats=500 GeV • But not full O(a) calculation! A. Juste et al, hep-ph/9910301

12. Associated Higgs Production at the LC • NLO QCD corrections • Dominant effect is from rescattering diagrams generated by coulombic gluons exchange between the top quarks near the tth threshold. • Estimated systematic uncertainty: ~10% (from residual m –dependence at NLO) KNLO~1.5 (0.9) at s = 0.5(1) TeV (mh=120 GeV) S. Dawson and L. Reina, PRD 59 (1998) 054012 S. Dittmaier et al, PL B441(1998) 383 • EW corrections (full O(a)) • Partial cancellation between photonic (negative) and weak (positive) corrections. • Total EW correction is negative and partly cancels positive contribution from QCD corrections. • Important to take into account in view of anticipated experimental accuracy in gtth. G. Belanger et al, PL B571 (2003) 163 Y. Yu et al, PL B571 (2003) 85 A. Denner et al, hep-ph/0307193

13. Direct gtth Measurement at the LC: hbb • Final state signature: Semileptonic: 6 jets (4 b-jets), lepton + MET Fully hadronic: 8 jets (4 b-jets) • Interfering (QCD and EW ttbb processes) and non-interfering (tt+jets,WW,ZZ,qq) backgrounds. Most dangerous ones are tt+X. • Realistic detector effects (fastsim) and reconstruction procedures (including b-tagging). Low S/B requires highly efficient preselections and multivariate selection procedures. • For mh=120 GeV and L=1 ab-1 (no K-factors): s=500 GeV: Dgtth/gtth ~ 33% (semilept only) s=800 GeV: Dgtth/gtth ~ 5.5% A. Juste, Chicago LCWS02 Cuts: ŝ > 100 GeV for all processes Ejlab>5 GeV, mjj>5 GeV for ttX H. Baer et al, PRD 51 (2000) 013002 A. Juste et al, hep-ph/9910301; Chicago LCWS02 A. Gay, 2nd-6th ECFA/DESY Workshop

14. Direct gtth Measurement at the LC: hWW* • Final state signature: fully determined by W decay channels Expected to have low backgrounds. A simple topological selection sufficient. Much larger backgrounds. Sophisticated analysis as in tth(hbb) required. • Preliminary estimate of 6-fermion backgrounds indicates small effect. • Assuming s=800 GeV and L=1 ab-1 and both channels combined: Dgtth/gtth  15% for 140 GeV  mh  200 GeV A. Gay, 2nd-6th ECFA/DESY Workshop

15. * Hbb semilep+hadro (A. Juste et al) * Direct gtth Measurement at the LC: Overview • Require high energy (s=800 GeV) and luminosity (1 ab-1). For 120 GeV  mh  190 GeV: Some open questions: • How much can other decay channels: help? • What is the actual uncertainty that can be achieved at s=500 GeV? Preliminary result for hbb, mh=120 GeV: But: • No explicit use of kinematic information • No K-factors • Only hbb considered. 5.5%  Dgtth/gtth  10% Dgtth/gtth ~ 33% (semilept only) (Expect 23% from semilept+had) A. Juste, Chicago LCWS02 A. Gay, 6th ECFA/DESY Workshop (Montpellier)

16. Indirect gtth Measurement at the LC: Top Threshold • Significant s-dependence of threshold observables in the e+e-tt process: • Total cross-section • Peak of top-quark momentum distribution • Top-quark forward-backward asymmetry provides sensitivity to fundamental top-quark parameters (mt, Gt, as, gtth). As far as gtth is concerned, the main observable is the total cross-section (Higgs exchange between the top quarks affects the interquark potential near threshold). Recent theoretical progress ((Dstt/stt )syst~ 3%) crucial. • Experimental analysis: • mh = 120 GeV • 300 fb-1 in a 10 point scan (9 points symmetrically distributed around mt(1S) in intervals of 1 GeV, one point below threshold to measure background) • (Dstt/stt )syst~ 1% • Simultaneous 4-parameter fit (mt, Gt, as, gtth ), with an external constraint on as (e.g. from GigaZ), to the three threshold observables • Not competitive with direct measurement at s=500 GeV. • Must assume there is no New Physics in the g-t-t and Z-t-t vertices (till direct measurements of those couplings become available). M. Martinez and R. Miquel, EPJ C27 (2003) 49 • mt = 31 MeV; s=0.001 (external constraint) • t = 34 MeV; gtth/gtth= +0.35 –0.65 (correlation with mt ~ 83%)

17. Summary and Conclusions • By properly combining ratios of cross-sections and performing a global fit to all existing measurements, the LHC can make Higgs couplings determinations in a quasi-model independent way. Some assumptions still have to be made: • W-Z universality • Saturation of Higgs width to all observable decay channels In this global analysis the top-Higgs Yukawa coupling is among the best measured couplings at the LHC: (*) existing (unknown to me) correlations among couplings need to be kept in mind • A LC can perform better, but high energy (s=800 GeV) and luminosity (1 ab-1) are required (ongoing study trying to assess performance at 500 GeV): • On the other hand, the real strength of the LC is the precise and model independent measurement of all couplings to gauge bosons and fermions (accuracies tipically  few%). • The LHC global fit (and a priori the top-Higgs Yukawa coupling determination) can greatly benefit from the external output provided by the LC: can drop remaining model-dependent assumptions and feed precise measurements. Actual gain still to be quantified. pptth, hbb, tt (200 fb-1) WW* (300 fb-1)  Dgtth/gtth 7-14% for mh=110-140 GeV e+e-tth, hbb, WW* (1 ab-1)  Dgtth/gtth 6-10% for mh=120-190 GeV