Higgs Boson Precision Studies at a Linear Collider

This report summarizes the progress in the study of Higgs physics at a future linear electron positron collider at center-of-mass energies up to about 1000 GeV and high luminosity. After the publication of the TESLA Technical Design Report [1], an extended ECFA/DESY study on linear collider physics and detectors was performed. The paper summarizes the status of the studies with main emphasis on recent results obtained in the course of the workshop. OBJECTIVES OF THE STUDY Elucidating the mechanism responsible for electro-weak symmetry breaking is one of the most important tasks of future collider based particle physics. Experimental and theoretical indications of a light Higgs boson make the precision study of the properties of Higgs bosons one of the major physics motivations of a linear collider (LC). Both the Higgs boson of the Standard Model (SM) and those of extended models will be copiously produced in ee collisions in various production mechanisms. A large variety of different decay modes can be observed with low backgrounds and high efficiency. These measurements allow us Most of the work reported in this talk was done by members of the Higgs working group of the Extended ECFA/DESY Study: V. Barger , M. Battaglia , M. Beccaria , E. Boos , J.C. Brient , S.Y. Choi , D. Choudhury , A. Datta , S. Dawson, S. DeCurtis , G. Degrassi , A. Denner , A. DeRoeck , N.G. Deshpande , S. Dittmaier , A. Djouadi , D. Dominici , M. Dubinin, H. Eberl , J. Ellis, A. Ferrari , M. Frank, E. Gabrielli , A. Gay, I.F. Ginzburg , D.K. Ghosh, E. Gross , J. Guasch , J.F. Gunion , T. Hahn , T. Han ,S. Heinemeyer , W. Hollik , K. Huitu , A. Imhof , J. Jiang , A. Kiiskinen , T. Klimkovich , B.A. Kniehl , M. Krawczyk , T. Kuhl P. Langacker , F. Madricardo z , W. Majerotto , T. Maki , B. McElrath , B. Mele , N. Meyer , D.J. Miller , S. Moretti , M. Mühlleitner , K. Olive , P. Osland , S. Peñaranda , A. Pilaftsis , A. Raspereza , F.M. Renard , M. Ronan , M. Roth , H.J. Schreiber , M. Schumacher , P. Slavich , A. Sopczak , V.C. Spanos , M. Steinhauser , S. Trimarchi , C. Verzegnassi , A. Vologdin , Z. Was , M.M. Weber, G. Weiglein , M. Worek M. Yao , P.M. Zerwas , a University of Wisconsin, b CERN, c INFN, University di Lecce, d Moscow State University, e LPNHE Ecole Polytechnique , f Chonbuk National University, g Helsinki Instiute of Physics , h BNL Brookhaven National Laboratory, i INFN, Firenze, j INFN, Roma, k University La Sapienza, Roma, l PSI Villigen, m University of Oregon n MPI München, o Universite Montpellier, p University of Florence, q Inst.f.Hochenergiephysik Oesterr.Akademie d.Wissenschaften, Wien, r Uppsala University, s University Karlsruhe, t IRES Strasbourg, u NSC Novosibirsk, v Weizmann Institute, w University of California, Davis, x LMU München, y DESY Hamburg, z University of Hamburg, aa ANL Argonne, bb Warsaw University, cc University of Pennsylvania, dd University of Minnesota, ee University of Bergen, ff Manchester University, gg LBNL Berkeley, hh University of Bonn, ii Lancaster University, jj INFN, University Trieste, kk INP Cracow, ll University of Durham, mm University of Silesia, Katowice. to extract the fundamental parameters of the Higgs sector with high precision. The series of ECFA/DESY workshops aims at a comprehensive study of the physics case, a determination of the achievable precisions on Higgs observables as well as on a fruitful cross-talk between theory, physics simulations and detector layout. A future linear collider offers also the option of photonphoton collisions from back-scattered laser light. The physics potential and progress in Higgs physics at a photon collider is discussed elsewhere in these proceedings [2]. STANDARD MODEL HIGGS BOSON Theoretical Predictions In ee collisions, the SM Higgs boson is predominantly produced through the Higgs-strahlung process, ee → HZ [3] and through the vector boson fusion processes ee → νeν̄e(ee)H [4]. The SM production cross-sections are precisely known including full electroweak corrections at the one-loop level. For a recent review of the theoretical calculations see e.g. [5]. Recently the full one-loop corrections to the WW-fusion process have been calculated [6, 7]. The radiatively corrected cross-sections for Higgs-strahlung and WW-fusion are shown in Fig. 1. For Higgs-strahlung the corrections are positive for small Higgs masses and negative for large Higgs masses and are of O(10%). For WW-fusion the corrections are of similar size but always negative. With the Higgs boson being responsible for mass generation its couplings to massive SM particles are proportional to their masses: gffH = mf/v, gV V H = 2M V /v. Thus Higgs bosons decay preferentially into the heaviest kinematically possible final states. State-of-the-art branching ratio calculations including electro-weak and QCD corrections [8] are coded in the program HDECAY [9] for the SM and its minimal supersymmetric extension, the MSSM. Branching ratios of the neutral Higgs bosons in the MSSM can be also calculated with program FeynHiggsDecay [10]. The SM Higgs branching ratios in the mass range relevant to a LC are shown in Fig. 2. Tools for Simulation A variety of leading-order Monte Carlo generators exist which are commonly used for Higgs studies in ee collisions. They are PYTHIA [11], HERWIG [12], HZHA [13], CompHep [14], and WHiZard [15]. CompHep and WHiZard offer the possibility of generating the complete 2 → 4 and (in the case of WHiZard) also 2 → 6 processes including their interference with SM backgrounds. tree full ZH produ tion ps = 500GeV total MH [GeV℄ [fb℄ 450 400 350 300 250 200 150 100 100 101 0:1 tree full WW fusion ps = 500GeV total MH [GeV℄ [fb℄ 450 400 350 300 250 200 150 100 100 101 0:1 WW ZH total ps = 500GeV MH [GeV℄ d d tree 1 [%℄ 450 400 350 300 250 200 150 100 20 100 10 20 30 40 WW ZH total ps = 500GeV MH [GeV℄ d d IBA 1 [%℄ 450 400 350 300 250 200 150 100 14 12 86420 2 4 6 Figure 1: Upper plots: cross-section for the processes ee → ZH and ee → νeν̄eH including complete one-loop electro-weak corrections for √ s = 500 GeV. Lower plots: Relative amount of one-loop corrections relative to Born level result (left) and relative to an improved Born approximation (IBA) (from [7]).