A Study of the Higgs Boson Production in the Dimuon Channelat 14 TeV M.S.El-Nagdy 1, A.A.Abdelalim 1,2, A.Mahrous 1, G.Pugliese 3 and S.Aly 1 (1) Physics department, Faculty of Science, Helwan University, Cairo, Egypt (2) CFP, Zewail City of Science and Technology, Cairo, Egypt (3) Universita e INFN, Sezione di Bari, Via Orabona 4, IT-70126 Bari, Italy Received: 15/8/2015 Accepted: 20/10/215 ABSTRACT This paper describes the Higgs boson decay to dimuon using simulation tools MadGraph5 and DELPHES. The simulated data is scaled assuming data size of 300 fb 1 integrated luminosity collected with an LHC experiment such as ATLAS or CMS experiments in pp collisions at s =14 TeV.Theexpected events rate was predicted for both Higgs signal and other Standard Model backgrounds. Keywords: Higgsboson, LHC, CMS, Madgraph5, DELPHES. 1. INTRODUCTION The Standard Model of Particle Physics (SM) is a theory that describes a wide range of phenomena to a very high precision degree. In the SM, a mechanism to spontaneously break the Electroweak (EW) symmetry was introduced in 1964 by Brout Englert Higgs, also known as BEH mechanism. The BEH mechanism breaks the EW gauge symmetry by generating masses to the weak force mediators; W ± and Z gauge bosons, while leaving the Electromagnetic force mediator (the photon) massless. Moreover the BEH mechanism gives masses for the charged fermions via Yukawa couplings (1-6). In searches for the Higgs boson predicted by the BEH mechanism, both of ATLAS and CMS collaborations at the Large Hadron Collider (LHC) discovered the Higgs boson, via decays into gauge bosons (7, 8), with a mass of approximately 125.5 GeV and measured properties consistent with those predicted by the SM (9-11). Higgs boson decays to bb, τ+ τ and μ+ μ can be measured at the LHC with their SM branching ratios proportional to the squares of the fermion masses. The SM branching ratio for the H μ+μ decay is 21.9 10 5 for a Higgs boson mass (m H ) of 125 GeV (12, 13). The H μ+μ decay has a clean final state signature that allows a measurement of the Higgs boson coupling to secondgeneration fermions. The dominant irreducible background is the so-called Drell-Yan process (Z/γ* μ+μ ) which has an approximately three orders of magnitude higher production rate compared to that of the expected signal from the SM Higgs boson. In this paper, we study the SM Higgs boson decay to μ+μ. 2. Monte-Carlo Samples and Detector Simulation The Monte-Carlo samples for signal and all SM background was generated with MadGraph5 (14),which is a Matrix-Element generator at parton level. Then the partons need to be hadronize because of quark confinement. After hadronization, the initial state and final state radiation (ISR and FSR) from both QCD and Electroweak processes need to be simulated. The hadronzation, ISR and FSR processes were simulated with the general-purpose event generator Pythia (15). The detector and electronics response was simulated with the fast detector simulator DELPHES (16) assuming detector configuration like that of the CMS at the LHC. The CMS has a superconducting solenoid up 4 T, which is large enough to saturate a 1.5 m iron plates in the return yoke and provide a large bending power for tracking measurements. The configuration considered for the CMS is that it has a cylindrical shape with different layers of detectors measure the different 11
particles in order to build up a picture of events at the heart of the collision. These layers are a silicon micro-strip tracker combined with the strong solenoidal field to provide good momentum resolution, three layers of silicon pixel detector in the barrel region complemented by two forward disk at each end. The Electromagnetic calorimeter provides coverage up to pseudo-rapidity η =3 surrounded by a hadron calorimeter also with coverage up to η = 3,an iron/quartz-fiber calorimeter with coverage from η =3 to η =5 and a muon tracking system consists of four muon stations with also with coverage up to η =2.5. Throughout this paper, the signal MC sample refers to the Higgs to dimuons samples while thebackground refers to Drell-Yan (DY),dibosons(WW, WZ, and ZZ) and top-anti-top (tt ) SM processes. Table 1 shows the cross-section in pb for signal and background samples. The branching ratio in the dimuon channel is also shown. Table (1): Cross sections and branching ratios values Signal Z/γ* Dibosons W + W - W ± Z ZZ tt Cross section (pb) 3.26x10-05 5.6x10-08 900.2 ± 2 0.47± 0.00052 0.048 ±3.22 x10-05 0.01 ± 4.6e-06 5.35 ± 0.0061 Branching ratios 2.87x10-04 5001. x10-08 37.7 x10-04 17.04 x10-04 2.75 x10-04 62.5 x10-04 3. Event Selection: H μ+μ candidate events are selected by requiring each event to have at least two muons, the- second requirement is that the two muons have to be oppositely charged. Moreover to suppress backgrounds from tt production and diboson processes, events are required to have transverse missing energye T miss < 70 GeV. 4. RESULTS Table 2 shows the number of weighted events before any cut and after each cut in our event selection discussed above. From the number of events left after our event selection, only 5 signal events can be observed in the dimoun channel above the other SM backgrounds. This analysis need carful choice of the- events as the signal is a very rare process and need more statistics to be sensitive to new physics above the SM expectation (the predicted 5 events by the SM only). Any access of events above this number is a good probe the Beyond Standard Model (BSM) physics. Table (2): Number of weighted events before and after cuts. Signal Z/γ* Dibosons W + W - W ± Z ZZ tt Before cuts 9.792 2.7 x10 +08 140850 13338 2.04x10 +08 1.6x10 +06 Selection of 2 muons 5.64803 1.27 x10 +08 72742 10912.22 1.92 x10 +08 803478 Selection of only two oppositely charged muons 5.64705 1.27 x10 +08 72742 7504.75 1.16 x10 +08 802996 E T miss < 70 GeV 5.07911 1.14 x10 +08 49263.7 4612.41 1.02 x10 +08 347592 12
The Higgs Invariant Mass: Figure 1 shows the dimuon invariant mass distributions after each cut. This is the key plot as the-higgs signal can easily distinguished from other SM background by the Higgs mass (125 GeV). Fig. (1): the dimuon invariant mass distribution after each cut. 13
The Dimuon Transverse Momentum pt: Figure 2 shows the pt distributions after each cut. The Higgs pt events are accumulated in the- low pt (less than 200 GeV) region which is dominated by SM backgrounds and this distribution will distinguish the BSM events coming from Higgs boson from other BSM predicted particles like Z`. Fig. (2): the dimuonp T distributions after each cut. 14
5. CONCLUSION The SM decaying to dimuon at LHC assuming 300 fb -1 data sample at 14 TeV has been studied. The- number of weighted events before any cut and after each cut in our event selection is given. This number, as well as the expected number of other SM processes in the dimuon channelis important to be well predicted as any access above this predictions is a good probe for BSM physics that is the main target of LHC after the Higgs discovery. From the number of events left after our event selection, only 5 signal events can be observed in the dimoun channel above the other SM backgrounds that is predicted as well. This analysis needs a carful choice of the events as the signal is a very rare process and needs more statistics to be sensitive to new physics above the SM expectation (the predicted 5 events by the SM only). The kinematical distributions after each cut and after our full event selection for both signal and background are shown that can be used to distinguish dimuon coming from Higgs decay from that coming from other BSM predicted particles like Z` and graviton. 6. REFERENCES (1) F.Englert and R.Brout, Broken symmetry and the mass of gauge vector mesons, Phys. Rev. Lett. 13 (1964)321 323. (2) P.W.Higgs, Broken symmetry and the mass of gauge bosons, Phys. Rev. Lett. 13 (1964) 508 509. (3) G.Guralnik, C.Hagen, and T.Kibble, Global conservation laws and massless particles, Phys. Rev. Lett. 13 (1964) 585 587. (4) S.Weinberg,AModelofLeptons,Phys.Rev.Lett.19(1967) 1264 1266. (5) S.L.Glashow, Partial Symmetries of Weak Interactions, Nucl. Phys. 22 (1961) 579 588. (6) A.Salam, Weak and Electromagnetic Interactions,Conf. Proc. C 680519 (1968) 367 377. (7) ATLAS Collaboration, Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC, Phys. Lett. B 716 (2012) 1 29, arxiv:1207.7214. (8) CMS Collaboration, Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC, Phys. Lett. B 716 (2012) 30 61, arxiv:1207.7235. (9) ATLAS Collaboration, Measurements of Higgs boson production and couplings in diboson final states with the ATLAS detector at the LHC, Phys. Lett. B 726 (2013) 88 119, arxiv:1307.1427. (10) ATLAS Collaboration, Evidence for the spin-0nature of the Higgs boson using ATLAS data, Phys. Lett. B 726 (2013) 120 144, arxiv:1307.1432. (11) CMS Collaboration, Evidence for the direct decay of the 125 GeV Higgs boson to fermions, arxiv:1401.6527. (12) LHC Higgs Cross Section Working Group, S.Dittmaier, C. Mariotti, G. Passarino, and R. Tanaka (Eds.), Handbook of LHC Higgs Cross Sections: 1. Inclusive Observables, CERN- 2011-002 (CERN, Geneva, 2011), arxiv:1101.0593. (13) LHC Higgs Cross Section Working Group, S. Dittmaier, C. Mariotti, G. Passarino, and R. Tanaka (Eds.), Handbook of LHC Higgs Cross Sections: 2. Differential Distributions, CERN-2012-002 (CERN, Geneva, 2012), arxiv:1201.3084. (14) The automated computation of tree-level and next-to-leading order differential cross sections, and their matching to parton shower simulations,arxiv:1405.0301 [hep-ph]. (15) http://home.thep.lu.se/~torbjorn/pythia.html. (16) HEP 02 (2014) 057, arxiv:1307.6346 [hep-ex]. 15