Towards a measurement of the two-photon decay width of the Standard Model Higgs boson at a Photon Collider

0 West University of Timisoara , 300223 Timisoara, Romania 1 DESY, 15738 Zeuthen, Germany A study of the measurement of the two photon decay width times the branching ratio of the Standard Model Higgs boson with a mass of 120 GeV in photon-photon collisions is presented, assuming a integrated luminosity of 80 fb1 in the high energy part of the spectrum. The analysis is based on the reconstruction of the Higgs events produced in the H process, followed by the decay of the Higgs into a bb pair. A statistical error of the measurement of the two-photon width, (H ), times the branching ratio of the Higgs boson, BR(H bb) is found to be 2.1% for one year of data taking. 1 Introduction The central challenge for particle physics nowadays is the origin of mass. In the Standard Model the masses of both fermions and gauge boson are generated through interactions with the same scalar field which gives rise to the Higgs boson H. While it can only be produced in association with other particles at an e+e collider, the Higgs boson can be produced singly in the s-channel of the colliding photons at a Photon Collider. If it exists, the Higgs boson will certainly be discovered by the time such a facility will be constructed. The Photon Collider as an upgrade option at the ILC [1] will then permit a high precision measurement of the H partial width, which is a quantity sensitive to the existence of new charged particles. For this reason such a measurement is significantly important. If we find a deviation of the two photon width from the Standard Model prediction it means that an additional contribution from unknown particles is present, and so it is a signature of physics beyond the Standard Model. For example, the minimal extension of the Standard Model predicts the ratio of the two photon width (H , MSSM)/ (H , SM) < 1.2 [2] for a Higgs boson with a mass of 120 GeV,1 assuming a Supersymmetry scale of 1 TeV and the chargino mass parameters M and of 300 and 100 GeV, respectively. At a Photon Collider one can measure the product (H ) BR(H X). To obtain the two-photon partial width independent of the branching ratio one has to combine the above measurement with an accurate measurement of the BR(H X) from another machine. This study investigates the capability of an ILC detector to measure the two photon decay width times the branching ratio for a Standard Model Higgs boson with the mass of 120 GeV, the preferred mass region by recent electroweak data [3]. The simulation of the signal and background processes is described in Sect. 2. Event selection is described in Sect. 3. Results are summarised in Sect. 4. The feasibility of the measurement of the two photon decay width of the Standard Model Higgs boson in this mass region has also been reported in [47]. In the previous studies however, the background events were generated using a tree level matrix element program in which the NLO corrections of Jikia and Tkabladze [811] had been implemented, while the PS algorithm was not considered for the qq background production. Lund string fragmentation model was used afterwards for the next stages. It is known from LEP that this procedure is not able to get the fragmentation properties right, which is important for an accurate b-tagging [12]. In our simulation study we used a recently available generator SHERPA [13], which takes care properly for matching the matrix elements with the PS, as described in Sect. 2. Our analysis presents for the first time a simulation of the background processes that takes the special gluon radiation pattern of the events produced with J = 0 into account. 1In this paper we use = c = 1. 2 Simulation of the signal and background processes The cross section for the Higgs boson formation is given by a Breit-Wigner approximation where MH is the Higgs boson mass, (H ) and tot are the two photon and total decay width of the Higgs boson, 1 and 2 are the initial photon helicities and s is the centre-of-mass energy. The initial photons should have equal helicities, so that Jz = 0, in order to make a spin-0 resonance as it is the case of the Higgs boson. If polarised photon beams are used, the signal cross section is increased up to a factor of 2. The experimentally observed cross section is obtained by folding this basic cross section with the collider luminosity distribution. For MH = 120 GeV the Higgs decays dominantly into bb. The event rate is thus given by: This rate depends strongly on the value of the differential luminosity at the Higgs mass, ddLs |MH . High energy photon beams can be produced at a high rate in Compton backscattering of laser photons off high energy electrons [14]. The beam spectra with see = 210 GeV are simulated using CompAZ [15], a fast parametrisation which includes multiple interactions and non-linear effects. The shape of the luminosity distribution depends on the electron and laser beam parameters. The electron and laser beam energy co (...truncated)