The perspectives for the observation of the rare decay H -> mumu decay and
the determination of the muon Yukawa coupling at a TeV-class and at a multi-TeV
e+e- linear colliders are discussed. The signal for the decay can be obtained
at 0.8 TeV and a first estimate of the coupling derived. A linear collider
operating at 3 TeV, with high luminosity, is able to improve the accuracy on
this couplings to 4% to 11% for 120 GeV < M_H < 150 GeV.Comment: 3 pages, 2 figures, to appear on the proceedings of the Snowmass 2001
  Summer Study, Snowmass CO, June-July 200

Battaglia, M.

De Roeck, A.

English

arXiv

The perspectives for the observation of the rare decay H -> mumu decay and the determination of the muon Yukawa coupling at a TeV-class and at a multi-TeV e+e- linear colliders are discussed. The signal for the decay can be obtained at 0.8 TeV and a first estimate of the coupling derived. A linear collider operating at 3 TeV, with high luminosity, is able to improve the accuracy on this couplings to 4% to 11% for 120 GeV  mumu decay and the determination of the muon Yukawa coupling at a TeV-class and at a multi-TeV e+e- linear colliders are discussed. The signal for the decay can be obtained at 0.8 TeV and a first estimate of the coupling derived. A linear collider operating at 3 TeV, with high luminosity, is able to improve the accuracy on this couplings to 4% to 11% for 120 GeV < M_H < 150 GeV

Battaglia, Marco

De Roeck, Albert

CERN Document Server

arXiv:hep-ph/0111307v1  23 Nov 2001Determination of the muon Yukawa couplingat high energy e+e− linear collidersMarco Battaglia∗ and Albert De Roeck†CERN, CH-1211 Geneva 23, Switzerland(Dated: December 30, 2013)The perspectives for the observation of the rare decay H0 → µ+µ− decay and thedetermination of the muon Yukawa coupling at a TeV-class and at a multi-TeV e+e−linear colliders are discussed. The signal for the decay can be obtained at√s=0.8 TeV anda first estimate of the coupling derived. A linear collider operating at 3 TeV, with highluminosity, is able to improve the accuracy on this couplings to 4% to 11% for 120 GeV<MH <150 GeV.I. INTRODUCTIONUnderstanding the mechanism of mass generation is one of the main quests for scientific research, today andin the coming decades. In the Standard Model (SM) the Higgs mechanism is held responsible for breaking theelectro-weak symmetry and providing particles with their mass through their interactions with the Higgs field.Beyond the observation of a Higgs boson, the fundamental test that the Higgs couplings to particles scale astheir masses needs to be verified experimentally, for gauge bosons and fermions separately. The potential of aTeV-class high luminosity e+e− linear collider (LC) has been already established for W and Z gauge bosons, t,b and c quarks and for the τ lepton [1]. We now discuss the possibility of measuring the muon Yukawa couplingby the determination of the branching fraction of the rare Higgs decay H0 → µ+µ−. This measurement wouldenable to complete the test of the coupling scaling for gauge bosons, quarks and leptons separately and thusensuring that the observed Higgs boson is indeed responsible for the mass generation of all elementary particles.We present results for a TeV-class LC operating at a centre-of-mass energy√s=0.8 TeV and for a multi-TeV LCwith√s=3.0 TeV. We considered data sets of 1 ab−1, corresponding to about two years of data taking at thenominal Tesla luminosity, and 5 ab−1, corresponding to five years at the Clic project luminosity, respectively.In the SM, the branching fraction BR(H0 → µ+µ−) is of the order of only 10−4, for Higgs masses MHbelow the WW threshold and even lower above it. But the signal can be efficiently reconstructed from theinvariant mass of two oppositely charged muons both in the Higgs-strahlung, e+e− → H0Z0, and in theWW fusion, e+e− → H0νeν¯e, processes and isolated from the backgrounds. The signal reconstruction andbackground rejections have been studied using the Simdet parametric detector simulation program [2], tunedto the expected responses for detectors at TeV and multi-TeV LCs. In particular, a muon momentum resolutionδp/p2 = 5×10−5 GeV−1 has been assumed. The signal and background cross sections have been obtained usingthe CompHEP program [3], while the SM BR(H0 → µ+µ−) has been computed with HDecay [4]. The signalevents have been generated using the Pythia-6 Monte Carlo [5] while parton level generation of the backgroundprocesses has been performed with CompHEP and the events have then been hadronised using Pythia [6].II. H0 → µ+µ− AT A TEV-CLASS LCWhile most of the profile of a light Higgs boson is best studied with lower energy operation of a LC, 300 GeV<√s <500 GeV, there are advantages in performing some measurements at energies around 1 TeV. The totalHiggs production cross section is within a factor of two from that at the peak for the Higgs-strahlung processand this is more than compensated by the increase in the achievable luminosity at these larger energies. Furtherthe ZZ∗ background is significantly decreased. We have considered√s=0.8 TeV with an integrated luminosityL=1 ab−1, corresponding to 1.75×105 Higgs bosons produced in the e+e− → Hνν¯ channel and about 40H → µ+µ− decays, for MH=120 GeV.The analysis starts from events with two oppositely charged, identified muons and significant missing energyand assumes an accurate knowledge of the Higgs mass, obtained at the Lhc and lower energy LC operation.∗Marco.Battaglia@cern.ch†Albert.De.Roeck@cern.ch2Higgs Mass (GeV)BR (H → mm)10-710-610-510-4100 120 140 160 180 200 220 240m+m- Mass (GeV)Evts. / 1 ab-10510152025115 120 125 130 135 140 145 150 155FIG. 1: Left: SM prediction for BR(H → µ+µ−) as a function of the Higgs boson mass. Right: distribution of thedi-muon invariant mass Mµµ for MH=120 GeV. The points with error bars represent 1 ab−1 of data at√s=0.8 TeV andthe continuous line the result of the fit used to extract the number of signal events.The main background from WW → µνµν is reduced by cuts on the di-muon recoil mass M recoilµµ and energyEµµ. The total background has been estimated including also the ZZνν¯, WWνν¯ and the inclusive µµνν¯processes, evaluated without the Higgs contribution. The resulting di-muon invariant mass is shown in Figure 1.The H → µµ signal can be extracted from the underlying background with more than 5σ significance, forMH ≃ 120 GeV. From the event rate, the product of production cross section and µµ decay branching fractionσ(e+e− → Hνν)× BR(H → µµ) is measured with ≃ 30 % accuracy.III. H0 → µ+µ− AT A MULTI-TEV LCMulti-TeV e+e− collisions provide a large sample of Higgs bosons, produced in the WW fusion process,becoming the dominant Higgs production mode since σHνν¯ ∝ log sM2H. However, beyond ∼ 3 TeV the furthergain in production cross section is largely lost due to the reduced acceptance, due to the forward Higgs bosonproduction. A data set of 5 ab−1 at√s=3 TeV corresponds to a sample of 2.7×106 Higgs bosons and 650H → µµ decays, for MH=120 GeV.The analysis is again based on events with two identified muons and the background is suppressed in partby selection cuts on the di-muon recoil mass and energy. The H → µµ signal is clearly visible in the di-muoninvariant mass distribution above the background. A signal significance of more than 5σ is obtained up toMH ≃155 GeV. The number of signal events is extracted from a fit to the di-muon invariant mass where the signalis modelled by a Gaussian distribution peaked at the nominal Higgs mass and the background by a polynomialcurve fitted on the peak side bands (see Figure 2). The accuracies on the product of production cross sectionand µµ decay branching fraction derived from the fitted number of signal events, are summarised in Table I fordifferent values of MH .IV. DISCUSSION AND CONCLUSIONSThe gHµµ coupling can be extracted from the measured branching fractions with an independent measurementof the production cross section, obtained using the dominant bb¯ and/or WW ∗ Higgs boson decay modes. Anuncertainty of 2% is assumed for the production cross section, extrapolating results from lower energies. Theresulting accuracies on the muon Yukawa coupling are given in Table I.A muon collider (FMC), running at energies around the H mass, can determine the product σ(µµ → H) ×BR(H → bb¯) to a statistical accuracy of 3.5% to 0.2% forMH ≃ 120 GeV, depending on the assumed luminosityand beam energy spread [7, 8]. By taking the BR(H → bb¯) measured at the LC to 2.5%, the combination of theFMC+NLC data allows to extract the gHµµ coupling with an accuracy of ≃ 2.2− 1.3%. To this accuracy, thebb¯ branching fraction contributes with comparable or dominant uncertainty depending on the FMC luminosityassumption.The decay H → µ+µ− may also be investigated at hadron colliders. However, a first study [9] has shownthat the Lhc could get a 5σ signal of this decay only with an integrated luminosity in excess of 2 ab−1.3m+m- Mass (GeV)Evts. / 5 ab-1050100150200250110 120 130 140 150 160m+m- Mass (GeV)Evts. / 5 ab-1020406080100120140120 125 130 135 140 145 150 155 160 165FIG. 2: Distribution of the di-muon invariant mass Mµµ for two data sets with MH=120 GeV (left) and MH=140 GeV(right). The points with error bars represent 5 ab−1 of data at√s=3 TeV and the continuous lines the results of the fitsused to extract the numbers of signal events.TABLE I: Statistical accuracy on the product σ(e+e− → Hνν¯) × BR(H → µµ) and the gHµµ coupling expected withan integrated luminosity of 5 ab−1 at√s=3 TeV, for different values of the Higgs boson mass MH .MH 120 GeV 140 GeV 150 GeVδ(σHνν¯×BR)/(σHνν¯×BR) 0.072 0.120 0.210δgHµµ/gHµµ 0.040 0.062 0.106Therefore, it may become relevant only for a high luminosity upgrade. At higher energies, a VLhc would beable to get a 5σ evidence with about 0.25 ab−1 and a gHµµ coupling determination with 10-14% accuracy for120 GeV< MH <140 GeV at√s=200 TeV.The measurements of the muon Yukawa coupling allow to extend the test of the Higgs mechanism of massgeneration to the lepton sector by verifying thatgHµµgHττ=MµMτ. The gHττ coupling can be determined at theLC with an accuracy of 3% to 5% for 120 GeV< MH <140 GeV, already at√s=300 GeV-500 GeV [1]. Theaccuracies of the determination of the two couplings being comparable, the test of the mass scaling can beobtained with a precision of 5.0% to 8.0% for 120 GeV< MH <140 GeV for the case of a LC and of 3.3% to3.7% for MH=120 GeV at a FMC, depending on the luminosity.In summary, we discussed the measurement of the muon Yukawa coupling by the determination of thebranching fraction of the rare Higgs decay H0 → µ+µ− at √s = 0.8 TeV and 3.0 TeV. The H → µ+µ− signalcan be observed at the lower energy for MH = 120 GeV and the gHµµ coupling measured to ≃15%. Thisaccuracy can be to pushed to 4-11% for the wider Higgs mass range 120 GeV < MH < 150 GeV with e+e−collisions at√s=3 TeV. The anticipated accuracy of a multi-TeV LC in the determination of gHµµ is comparableto that achievable at a muon collider operating at the Higgs mass peak and more than two times better thanat a 200 TeV VLHC hadron collider.REFERENCES[1] M. Battaglia and K. Desch (2000), hep-ph/0101165.[2] M. Pohl and H. J. Schreiber (1999), DESY-99-030.[3] A. Pukhov et al. (1999), hep-ph/9908288.[4] A. Djouadi, J. Kalinowski, and M. Spira, Comput. Phys. Commun. 108, 56 (1998), hep-ph/9704448.[5] T. Sjostrand, L. Lonnblad, and S. Mrenna (2001), hep-ph/0108264.[6] A. S. Belyaev et al. (2000), hep-ph/0101232.[7] J. F. Gunion (1997), hep-ph/9703203.[8] B. Autin, A. Blondel, and J. R. Ellis (1999), CERN-99-02.[9] T. Plehn and D. Rainwater, Phys. Lett. B520, 108 (2001), hep-ph/0107180.

Determination of the muon Yukawa coupling at high energy e+e- linear colliders

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