Excited Muon Discovery Potential in s = 13 TeV proton-proton Collision with CMS Experiment

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1 Excited Muon Discovery Potential in s = 13 TeV proton-proton Collision with CMS Experiment W. Esmail 1, A. A. Abdelalim 1, Y. Assran 2, and M.N.Yasein 1 (1) Physics Department, Faculty of Science, Helwan University (2) Science and Mathematics Department, Faculty of Pet. &Min,Suze University Received: 20/8/2015 Accepted: 26/10/2015 ABSTRACT In this investigation, a phenomenological study of excited muons μ at the center of mass energy s = 13 TeV and integrated luminosity Ldt = 100 fb -1. These excited muons are produced via contact interactions in conjunction with a Standard Model muon. The decay modes considered in this study is μ μz, with the Z boson decay into muon-antimuon pair producing a final state of four muons. The mass of the excited muon is a free parameter; a mass range of TeV with a step of 0.5 TeV is chosen. Selection criteria are applied to both the signal and background to enhance the signal-to-background ratio. 1. INTRODUCTION Although the Standard Model of particle physics (SM) is a very successful theory, it has a number of shortcomings; in particular it provides no answer for the three generations of fermions. Compositeness models (1, 2) attempt to solve this problem by postulating that quarks and leptons might be composite objects of fundamental particles called preons, a direct consequence of this proposition is that below a certain characteristic energy scale Λ excited states of SM particles may be found. Excited leptons are supposed to produced via four-fermions contact (CI) interaction, this CI is described by an effective Lagrangian: L contact = 2π Λ 2 jμ j μ j μ = f L γμ f L +f Lγ μ f L + f Lγ μ f L + h.c. where Λ is the compositeness scale, and j μ is the fermion current for the ground state f and excited states f*, "h.c." stands for Hermitian conjugate, and the subscripts L refer to left- handed fermions, right handed fermions are set to zero for simplicity (3). Gauge-mediated transitions between SM and excited fermions are described by an effective Lagrangian: L GM = 1 2Λ f R σ μυ λ a [g s f s 2 G μυ a + gf τ 2 W μυ + g f Y 2 B μυ ] f L + h. c. where f and f* are lepton and excited lepton fields, G a μυ, W μυ, and B μυ are strength tensors of the gluon, SU(2) L and U(1) Y gauge field, and g s, g, and g are the corresponding coupling constants. In this paper we focus on the muon compositeness in s = 13 TeV proton-proton collision and integrated luminosity of 100 fb -1. With the CMS experiment at the Large Hadron Collider, single production qq μμ is considered here, the excited muon has different modes of decay as shown in fig,[1.a]. The decay mode considered here is the neutral-current decay μ μz μμμ, so the final state will be four muons as shown in fig.[1.b] and this will be the signature that we are looking for at

2 the detector. Although the branching ratio of this decay mode is much smaller than γ-mediating process, the four muon final state gives out less background than μμγ. Previous searches at LEP (4-7), HERA (8, 9) and the Tevatron (10-13) have found no evidence for excited leptons. For the special case where M μ = Λ, μ masses below 2.2 TeV are excluded by ATLAS experiment using s = 8 TeV data (14). (a) (b) Fig 1: (a) Branching ratio of excited muon as a function of its mass, (b) Feynman diagrams of the production and final state of the signal. 2. Event Simulation and Methodology: Signal samples are generated at leading order (LO) using Pythia 8.2 (15) with a mass range 0.5 to 4 TeV with a step of 0.5 TeV, and a compositeness scale Λ = 10 TeV using CTEQ6L1 parton distribution function (PDF). The signal events are then passed to Delphes fast simulation tool (16) to simulate the CMS detector response and reconstruct the final state particles. The different mass points with its corresponding LO cross sections are listed in table 1. Table (1): Summary of simulated signal mass points and its cross section. M μ [GeV] Cross Section [fb] Number of Events x x x x x x x x Standard Model background is generated also at LO accuracy using Madgraph (17) event generator interfaced with pythia for haronization and fragmentation, and then passed to Delphes for detector simulation. The dominate background for our decay mode is the SM ZZ production from quark anti-quark annihilation as illustrated in the first row of table2, it is irreducible and

3 overwhelming contributes about 90% of the total background expectations. The different backgrounds with its corresponding cross section are listed in table 2. Table (2): Summary of SM background simulated samples with its cross section. Process Cross Section [fb] Number of Events qq ZZ 4μ gg ZZ 4μ qq ZZ 2μ2τ tt Z ZZZ WWZ First of all the event of interest must contain at least four muons, each muon candidate must pass kinematic criteria, and we use the "High Ptmuon ID" of the muon POG (18) : 1. All four muons should have P T > 25 GeV and η < 2.4. For a first look between the signal and SM background, we look at after applying selectio1 where figure (2) shows the basic kinematic variables of the candidate muons without any further cuts. Fig. (2): (Top) Transverse momentum of muons, (left) Pseudorapidity distribution of muons, (Right) Angular distribution of muons.

4 If we look at the transverse momentum of muon in figure (2), we will observe that the SM background tends to be in low momentum while the signal tends to be at high momentum which is a unique feature of our signal. 2. We search for the muon pair with opposite charged leptons which is closest to the mass of Z. We apply invariant mass cut M Z1,Z 2 < 60 GeV. 3. For the other two muons we apply a Z-veto cut, we use the same cut as RPV SUSY multilepton search (M > 106 GeV), with this cut applied to the muon pairs that are no from the Z we try to reduce the main background ZZ 4μ. 4. Final Selection: to reconstruct the mass of the excited muons, Z and one of the two remaining muons needed to do this task, but the two muons are indistinguishable. So, we have two possible 3μ 3μ invariant masses, they are called M min, Mmax in increasing order of mass, both minimum and maximum invariant masses form two dimensional plot that has the form of an inverted "L" around the mass of the excited muon, this plot is called "L-shape plot". The background in L-shape plot tends to be at low invariant masses, so to discriminate signal over background we put a mass search window around the excited muon mass, and this will our final selection which is called "L-shape cut". The mass resolution of muons decreases with increasing energy, so the mass window should be broader at higher masses so the mass window is chosen individually for different mass points that by optimizing the signal efficiency. For a further comparison between signal and background figure (3) shows the invariant mass of the reconstructed Z before and after invariant mass cut of selection 2, and it is clear that large number of SM background event was effectively reduced by this cut. From figure (4) we can see that the main background ZZ 4μ is rejected at low invariant mass after the veto-z selection as compared to the invariant mass of veto-z before the invariant mass cut as described above. Additionally figure (5) shows the reconstructed mass of the hypothetical muon as minimum and maximum invariant masses. It is observed that the excited muon invariant mass has a peak at its mass. For the final selection 4, figure (6) shows the "L-shape" plot after the "L-shape cut". As we can see a potential signal of excited muon has an inverted L shape around its mass, while the background tends to be at low invariant masses, such a cut efficiently discriminates the signal against background. This cut will be used to discover the excited muon if it exists. Fig. (3): (Left) Invariant mass of Z before invariant mass cut, (Right) Invariant mass of Z after invariant mass cut.

5 Fig. (4): (left) Invariant mass of veto-z before invariant mass cut, (Right) Invariant mass of veto-z after invariant mass cut. Fig (5): (left) minimum invariant mass distribution, (Right) Maximum invariant mass distribution. Fig (6): L-shape for background and different mass points of the signal. 02

6 3. RESULTS For a numerical comparison between signal and background table [3] and [4] lists event yields of the different mass points of the signal, and the background yield for the given cut flow. It appears from table [4] that L-shape cut efficiently reduces the main background fromzz 4μ. It is apparent also that the efficiency of selection of signal against background is sufficiently high. Table (3): Summarizing event yields for different mass points of the excited muon for the given cut flow. M μ [GeV] Event yield after Z Event yield after Event yield after L- mass cut vetoz mass cut shape cut e e e-05 Table (4): Summarizing event yields for the SM background of the given cut flow. Process Event yield after Z mass cut Event yield after vetoz mass cut Event yield after L- shape cut qq ZZ 4μ gg ZZ 4μ qq ZZ 2μ2τ /- 0 tt Z 0 +/ / /- 0 ZZZ WWZ / /- 0 Finally, acceptance x efficiency as a function of the excited muon mass is shown in figure (6), the large values of efficiency is due to the good efficiency of the muon ID. Fig (7): Acceptance x efficiency as a function of excited muon mass. 06

7 CONCLUSION Up until now no evidence for muon or lepton compositeness, we reported a model for a search of excited muon in a data of size 100 fb -1 at CMS detector with a four muon signature, we have optimized a selection and cut flow for the excited muon and discriminated the signal against background. In the next data taking of CMS detector, excited lepton may be accessible with a potential signal that has a form of inverted L-shape, excited lepton should be discovered if it exist and its mass is lower than 4000 GeV at compositeness scale of 10 TeV. REFERNCEES (1) Pati J C and Salam A 1974 Phys. Rev. D (2) Eichten E J, Lane K D and Peskin M E 1983 Phys. Rev. Lett (3) Baur U, Spira M and Zerwas P M 1990 Phys. Rev. D (4) ALEPH Collaboration, Buskulic D et al 1996 Phys. Lett. B (5) OPAL Collaboration, Abbiendi G et al 2000 Eur. Phys. J. C (6) L3 Collaboration, Achard P et al 2003 Phys. Lett. B (7) DELPHI Collaboration, Abdallah J et al 2006 Eur. Phys. J. C (8) ZEUS Collaboration, Chekanov S et al 2002 Phys. Lett. B (9) H1 Collaboration, Aaron F D et al 2008 Phys. Lett. B (10) CDF Collaboration, Acosta D et al 2005 Phys. Rev. Lett (11) CDF Collaboration, Abulencia A et al 2006 Phys. Rev. Lett (12) D0 Collaboration, Abazov V M et al 2006 Phys. Rev. D (13) D0 Collaboration, Abazov V M et al 2008 Phys. Rev. D (14) ATLAS Collaboration 2013 CERN-PH-EP New J. Phys. (15) T Sjostrand, S. Mrenna, and P. Z. Skands, 2008,Comput.Phys. Commun (16) J. de Favereau, C. Delaere, P. Demin, A. Giammanco, V. Lematre,et al., 2013 DELPHES 3. (17) J. Alwall et al., 2007: The New Web Generation, JHEP (18) E.M.Group, HighpTMuons, New_HighPT_Version_recommended. 00