In this paper, we review the role of a multi-TeV $e^+e^-$ linear collider to
complete the mapping of the Higgs boson profile and studying heavier bosons in
extended scenarios with more than one Higgs doublet.Comment: 8 pages, 4 figures, to appear on the Proceedings of the Int. Workshop
  on Linear Colliders, LCWS2002, Jeju Island (Korea

Battaglia, Marco

De Roeck, Albert

English

arXiv

In this paper, we review the role of a multi-TeV $e^+e^-$ linear collider to complete the mapping of the Higgs boson profile and studying heavier bosons in extended scenarios with more than one Higgs doublet.In this paper, we review the role of a multi-TeV $e^+e^-$ linear collider to complete the mapping of the Higgs boson profile and studying heavier bosons in extended scenarios with more than one Higgs doublet

CERN Document Server

Studying the Higgs Sectorat the CLIC Multi-TeV e+e− ColliderM. Battaglia and A. De Roeck yCERN, Geneva, SwitzerlandAbstractIn this paper, we review the role of a multi-TeV e+e− linear collider tocomplete the mapping of the Higgs boson prole and studying heavier bosonsin extended scenarios with more than one Higgs doublet.1 IntroductionUnderstanding the origin of electro-weak symmetry breaking and mass generationstands as a central theme of the research programme in physics in the comingdecades. The Standard Model (SM), successfully tested to an unprecedented levelof accuracy by the LEP and SLC experiments, and now also by the B-factories,addresses this question with the Higgs mechanism [1]. The rst manifestation ofthe Higgs mechanism through the Higgs sector is the existence of at least one Higgsboson, H0. The observation of a new spin-0 particle would represent a rst signthat the Higgs mechanism of mass generation is realised in Nature. Present data in-dicates that it is heavier than 114 GeV and possibly lighter than about 195 GeV [2].We expect the Higgs boson to be discovered at the Tevatron or at the Lhc, theCERN hadron collider, which will determine its mass and perform a rst surveye-mail address: Marco.Battaglia@cern.chye-mail address: Albert.De.Roeck@cern.ch12 International Workshop on Linear Collidersof its basic properties. A TeV-class linear collider, operating at centre-of-mass en-ergies 350 GeV<ps  1 TeV, will bring the accuracy needed to further validatethis picture and to probe the SM or extended nature of the Higgs sector. It willperform crucial measurements in a model-independent way and it will also com-plement the Lhc in the search for heavy Higgs bosons. Their observation wouldprovide direct evidence that nature has chosen a route dierent than the minimalHiggs sector of the SM. But neither the precision study of the Higgs prole northe search for additional Higgs bosons will be completed at energies below 1 TeV.There are measurements which will be limited in accuracy or may be not feasibleat all, due to limitations in both the available statistics and centre-of-mass energy.The two-beam acceleration scheme, presently developed within the Clic study [3]at CERN, aims at collisions atps=3-5 TeV, representing a unique opportunity toextend e+e− physics to constituent energies of the order of the Lhc energy frontier,and beyond, with very high luminosity. It is therefore important to assess the po-tential of a multi-TeV LC in complementing information that the Lhc, and possiblya lower energy LC, will obtain.2 Completing the Light Higgs Boson ProfileThe TeV-class LC will perform highly accurate determinations of the Higgs prole.However, even at the high design luminosity of Tesla and the Nlc there are prop-erties which cannot be tested exhaustively. In fact, it is essential to ensure thatMH (GeV)σ ZH+Hυυ (fb)1010 2200 300 400 500 600 700Figure 1: Inclusive Higgs production cross section as a function of the Higgs massMH for three values of the e+e− centre-of-mass energyps.LCWS(2002), Jeju, Korea 3µ+µ− Mass (GeV)Evts. / 5 ab-1050100150200250110 120 130 140 150 160bb Mass (GeV)Evts/1 ab-102004006008001000120014001600140 160 180 200 220 240 260 280 300Figure 2: Reconstructed signals for e+e− ! H0 ! +− (left) and e+e− ! H0 !bb (right) for MH = 120 GeV and 200 GeV respectively atps=3 TeV.the fundamental test of the scaling of Higgs couplings to fermions with their massesgHXXgHY Y/ MXMY can be performed with signicant accuracy over a wide range of Higgsboson masses and for all particle species. Atps = 350 − 500 GeV, the LC willtest the couplings to gauge boson, and those to quarks if the Higgs boson is light.To complete this program for leptons and intermediate masses of the Higgs and tostudy the Higgs boson self-couplings, it is necessary to study rare processes, whichneed Higgs samples in excess to 105 events. As the cross section for e+e− ! H0production increases with energy as log sM2H, it dominates at Clic energies. Theresulting large Higgs production rate atps  3 TeV and the expected luminosity,L = 1035 cm−2s−1, yield samples of order of 0.5-1 106 decays of SM-like Higgsboson in 1-2 years (see Figure 1).Measuring the muon Yukawa coupling by the determination of the H0 ! +−branching fraction completes the test of the coupling scaling for gauge bosons, quarksand leptons separately. This ensures that the observed Higgs boson is indeed re-sponsible for the mass generation of all elementary particles. Atps=3 TeV, theH ! +− signal can be easily observed and the gHµµ coupling measured to 4-11%accuracy for Higgs masses in the range 120 GeV < MH < 150 GeV, with 3 ab−1.The sensitivity on the Higgs couplings to fermions also needs to be tested for heavierHiggs bosons. Beyond the H ! WW threshold, the branching fractions H ! f ffall rapidly with increasing MH values. e+e− ! H ! bb atps  1 TeV oersa favourable signal-to-background ratio to probe gHbb for these intermediate-massHiggs bosons. The H0 ! bb decay can be measured for masses up to about 240 GeVand the accuracies are summarised in Table 1.4 International Workshop on Linear CollidersTable 1: Signal significance and relative accuracy for the determination of the bYukawa coupling for different values of the MH mass atps=3 TeV.MH (GeV) S=pB  gHbb = gHbb180 40.5 0.016200 25.0 0.025220 18.0 0.034Table 2: Relative accuracy for the determination of the triple Higgs coupling gHHHfor different values of the MH mass atps=3 TeV, assuming unpolarised beams.MH (GeV) Counting Fit120  0.131 (stat)  0.093 (stat)180  0.191 (stat)  0.115 (stat)Another fundamental test of the Higgs sector with a light Higgs boson, whichsignicantly benets from multi-TeV data, is the study of the Higgs self-couplingsand the reconstruction of the Higgs potential. The triple Higgs coupling gHHH canbe accessed at a TeV-class LC in the double Higgs production processes e+e− !HHZ [4]. This measurement is made dicult by the tiny production cross sectionand by the dilution due to diagrams leading to double Higgs production, but notsensitive to the triple Higgs vertex. A LC operating atps = 500 GeV can measurethe HHZ production cross section to about 15% accuracy if the Higgs boson mass is120 GeV, corresponding to a fractional accuracy of 23% on gHHH [5]. Improvementscan be obtained by performing the analysis at multi-TeV energies, through theprocess e+e− ! HH and by introducing observables sensitive to the presence ofthe triple Higgs vertex (see Table 2) [6]. On the contrary, the quartic Higgs couplingremains elusive, due to the smallness of the relevant triple Higgs production crosssections.Precision electro-weak data indicate that the Higgs boson must be lighter thanabout 195 GeV. However, this limit can be evaded if New Physics exists to cancelthe eect of the heavy Higgs boson mass. In these scenarios, it is interesting tosearch for an heavier boson through the ZZ fusion process e+e− ! H0e+e− !Xe+e−, at high energies. Similarly to the associate HZ production in the Higgs-strahlung process at lower energies, this channel allows a model-independent searchof Higgs boson, through the tag of the two forward electrons and the reconstructionLCWS(2002), Jeju, Korea 5of their recoil mass [7]. This analysis needs to identify electrons, and measure theirenergy and direction down to ’ 100 mrad, close to bulk of the γγ ! hadrons andpair backgrounds, making it a challenge for the forward tracking and calorimetricresponse of the detector. Preliminary results show that a clean Higgs signal can beextracted atps=3 TeV for MH  900 GeV.3 Testing New Physics in the Higgs SectorIf heavy Higgs bosons exist above pair-production threshold, they are accessibleat the LC through the e+e− ! H0 A0 ! bbbb, tttt and e+e− ! H+H− ! tbtbprocesses, resulting in very distinctive, yet challenging, multi-jet nal states withmultiple b-quark jets, which must be eciently identied and reconstructed. TheMH- (GeV)σ(e+e- → H+H-) (fb)10-310-210-11400 600 800 1000 1200 140002468101214161820222426400 600 800 1000 1200 1400S+B after kinematical fitmrec(H) = 893.7 ± 9.5 GeVmrec(H) in GeVN / 20 GeV400 600 800 1000 12000.0.020.040.060.080.10.120.14Figure 3: Charged Higgs analysis at Clic: e+e− ! H+H− production cross sectionforps=3 TeV as function of the charged Higgs mass (left). Signal reconstructedin the H+ ! tb decay mode with the irreducible background overlayed (centre).Magnitude of the CP asymmetry jCP j estimated as function of the charged Higgsmass (right).sizeable production cross sections provides sensitivity for masses up to 1 TeV andbeyond for all values of tan , thus extending the LHC reach. A detailed analysis hasbeen performed for the reconstruction of e+e− ! H+H− ! tbtb with MH=880 GeV,corresponding to the CMSSM benchmark point J of reference [9], by identicationof the WbbWbb nal state, applying a mass constrained t [8]. The irreducibleSM e+e− ! tbtb background and the overlay of accelerator-induced γγ ! hadronsevents have been included. By reconstructing either both Hs, or only one, the6 International Workshop on Linear Collidersanalysis can be optimised in terms of eciency and resolution for mass measurement,or for an unbiased study of H decays, respectively. Fractional accuracies of 1% onthe heavy boson mass and of 5-10% on the product of production cross section andtb decay branching fraction are expected, with L=3 ab−1 at ps = 3 TeV.Extensions of the SM may introduce new sources of CP violation, through ad-ditional physical phases whose eects can be searched for in the Higgs sector. Su-persymmetric one-loop contributions can lead to dierences in the decay rates ofH+ ! tb and H− ! tb, in the MSSM with complex parameters [10]. This CPasymmetry is expressed as: jCP j = jΓ(H−!bt)−Γ(H+!tb)jΓ(H−!bt)+Γ(H+!tb) and it can amount to upto ’ 15% as shown in Figure 3. The leading contributions, from loops with ~t, ~b and~g, CP , are sensitive to the masses of these sparticles. With the expected statis-tics of e+e− ! H+H− ! tbtb at ps=3 TeV and assuming realistic charge taggingperformances, a 3  eect would be observed with L=5 ab−1, for an asymmetryjCP j=0.10.Figure 4: Left: Display of a e+e− ! W+W− event at 3 TeV reconstructed in amulti-layered Si Tracker. Right: Distribution of the reconstructed WW mass in aSSB model with a 2 TeV resonance. The background is overlayed.4 EWSB without the Higgs BosonIf the breaking of the electro-weak symmetry is not due to the Higgs mechanism,then the e+e− ! W+W− and Z0Z0 processes may reveal new dynamics ofgauge boson interactions [11]. In fact when no elementary Higgs boson exists withLCWS(2002), Jeju, Korea 7MH  700 GeV, we expect W and Z0 bosons to develop strong interactions at ascale ’ 1 TeV. The experimental signatures are represented both by deviations ofthe e+e− ! WLWL cross section from its SM expectation and also by the possibleformation of vector resonances at masses beyond 1 TeV (see Figure 4). Clean nalstates can be reconstructed using W ! qq0 and the resonant components identiedalso when accounting for backgrounds and detector response [12].5 ConclusionsA Multi-TeV e+e− LC, such as Clic, has the potential to complete the study of theHiggs boson and to investigate an extended Higgs sector over a wide range of modelparameters. Preserving the LC signature properties of clean events, with well denedkinematics, at a Multi-TeV LC will require a substantial eort of machine parameteroptimisation, detector design and data analysis techniques. However, exploratorystudies, accounting for realistic experimental conditions, conrm that Clic willperform precision measurements and push its sensitivity up to the kinematical limits.While the main motivation for experimentation at a Multi-TeV LC arise from theTable 3: Comparison of expected accuracies on Higgs couplings at Clic and at theVLhc hadron colliders [13]CLIC VLHC-I VLHC-IIps 3 TeV 40 TeV 200 TeV∫ L 5 ab−1 300 fb−1 300 fb−1gHtt=gHtt 0.05-0.10 (?) 0.05-0.10 0.01-0.02gHbb=gHbb 0.01-0.03 - -gHµµ=gHµµ 0.03-0.10 0.15-0.25 0.10-0.13gHHH=gHHH 0.07-0.09 ??? 0.10-0.30 (?)gHHHH 6= 0 (?) - -search of new phenomena, its role in studying the Higgs sector will also be crucialin completing the mapping of the H0 Boson prole, studying heavy Higgs bosonsin extended scenarios or explore otherwise the origin of symmetry breaking, if noelementary Higgs boson is observed. In this way, the Clic multi-TeV LC projectensures a continue competitive e+e− physics program through two generation ofprojects and of physics questions.We are grateful to K. Desch, S. Kraml and A. Ferrari for contributions.8 International Workshop on Linear CollidersReferences[1] P.W. Higgs, Phys. Rev. Lett. 12 (1964) 132; idem, Phys. Rev. 145 (1966)1156; F. Englert and R. Brout, Phys. Rev. Lett. 13 (1964) 321; G.S. Gural-nik, C.R. Hagen and T.W. Kibble, Phys. Rev. Lett. 13 (1964) 585.[2] M. Gru¨newald, to appear in the Proc. of the 31st Int. Conf. on High EnergyPhysics, Amsterdam, July 2002 and [arXiv:hep-ex/0210003].[3] A 3 TeV e+e− Linear Collider Based on CLIC Technology, The CLIC StudyTeam, G. Guignard (editor), CERN-2000-008.[4] A. Djouadi, W. Kilian, M. Muhlleitner and P. M. Zerwas, Eur. Phys. J. C 10(1999) 27 [arXiv:hep-ph/9903229].[5] C. Castanier, P. Gay, P. Lutz and J. Orlo, [arXiv:hep-ex/0101028].[6] M. Battaglia, E. Boos and W. M. Yao, in Proc. of the APS/DPF/DPB Sum-mer Study on the Future of Particle Physics (Snowmass 2001) ed. N. Graf,[arXiv:hep-ph/0111276].[7] M. Battaglia, CERN-CLIC-NOTE-474 and [arXiv:hep-ph/0103338].[8] M. Battaglia A. Ferrari and A. Kiiskinen, in Proc. of the APS/DPF/DPB Sum-mer Study on the Future of Particle Physics (Snowmass 2001) ed. N. Graf,[arXiv:hep-ph/0111307].[9] M. Battaglia et al., Eur. Phys. J. C22 (2001) 535.[10] E. Christova, H. Eberl, W. Majerotto and S. Kraml, Nucl. Phys. B 639 (2002)263 [arXiv:hep-ph/0205227].[11] T. L. Barklow, in Proc. of the APS/DPF/DPB Summer Study on the Futureof Particle Physics (Snowmass 2001) ed. N. Graf, [arXiv:hep-ph/0112286].[12] T. L. Barklow and A. De Roeck, in Proc. of the APS/DPF/DPB Summer Studyon the Future of Particle Physics (Snowmass 2001) ed. N. Graf, [arXiv:hep-ph/0112313] .[13] U. Baur et al., in Proc. of the APS/DPF/DPB Summer Study on the Future ofParticle Physics (Snowmass 2001) ed. N. Graf, [arXiv:hep-ph/0201227].

Studying the Higgs Sector at the CLIC Multi-TeV $e^{+}e^{-}$ Collider

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Studying the Higgs Sector at the CLIC Multi-TeV $e^+e^-$ Collider

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Studying the Higgs Sector at the CLIC Multi-TeV $e^+e^-$ Collider