Probing Radiative Neutrino Mass Models Using Trilepton Channel at the LHC
Dounia Cherigui, Chahrazed Guella, Amine Ahriche, Salah Nasri
PProbing radiative neutrino mass models using trilepton channel at the LHC
Dounia Cherigui, ∗ Chahrazed Guella, † Amine Ahriche,
2, 3, 4, ‡ and Salah Nasri
5, 6, § Facult´e de Physique, D´epartement de G´enie Physique,Universit´e des sciences et de la technologie, BP 1505, Oran, El M’Naouer, Algerie Department of Physics, University of Jijel, PB 98 Ouled Aissa, DZ-18000 Jijel, Algeria The Abdus Salam International Centre for Theoretical Physics, Strada Costiera 11, I-34014, Trieste, Italy. Department of Physics and Center for Theoretical Sciences,National Taiwan University, Taipei 106, Taiwan. Department of physics, United Arab Emirates University,P.O. Box 15551, Al-Ain, United Arab Emirates. Laboratoire de Physique Th´eorique, DZ-31000, Es-Senia University. Oran, Algeria.
In this work, we probe a class of neutrino mass models through the lepton flavor violating inter-actions of a singlet charged scalar, S ± at the LHC proton-proton collisions with 8 TeV and 14 TeVenergies. This scalar couples to the leptons and induces many processes such as pp → (cid:96) ± (cid:96) ± (cid:96) ∓ + /E T .In our analysis we discuss the opposite sign same flavor leptons signal, as well as the backgroundfree channel with the tau contribution which can enhance the signal/background ratio for center ofmass energies √ s = 8 TeV and √ s = 14 TeV. Keywords : Trilepton events, charged scalar, missing energy, LHC.
PACS : 04.50.Cd, 98.80.Cq, 11.30.Fs.
I. INTRODUCTION
There are number of motivations why the standard model (SM) of particle physics needs to be extended with newdegrees of freedom. This includes the observation of neutrino oscillations for which the data can not be explainedby massless neutrinos, the nature of dark matter (DM), and the origin of the matter-antimatter asymmetry of theuniverse.One of the most popular mechanisms that generates small neutrino mass is the seesaw mechanism which comes indifferent types: type-I [1], the type-II [2, 3] and type-III [4]. This mechanism introduces new particles many orders ofmagnitude heavier than the electroweak scale that give rise to tiny neutrino mass after being integrated out from thelow energy theory. To avoid fine tuning of the SM couplings, the mass scale of the new particles needs to be of order10 GeV which makes the high scale see-saw mechanism impossible to be test at laboratory experiments. In addition,for such superheavy mass scale the electroweak vacuum can be destabilized [5]. Other alternative realizations invoking‘low-scale mechanisms’ were proposed in [6].Another attractive way to induce naturally small neutrino mass is the radiative neutrino mass generation, whereneutrino mass are generated at loop level [7–11]. Moreover, the scale of new physics is much smaller than in theconventional see-saw and can be of the same order as the electroweak scale for the three-loop radiative neutrino massmodels. For instance, the KNT model proposed in [9] extends the SM with two singlet charged scalars, S , , andone singlet fermion, N , all having masses around the TeV scale, making it testable at collider experiments. Differentphenomenological aspects of this model, such as the DM relic density, were investigated in [12]. However, in orderto match the neutrino mass and mixing with the experimental data without being in conflict with the bound on theprocess µ → e + γ , three generations of singlet fermions are required [13]. Generalization of the KNT model wasproposed in [14] by promoting S and N to multiplets of the SU (2) L gauge symmetry. In these models, the use of a ∗ Electronic address: [email protected] † Electronic address: [email protected] ‡ Electronic address: [email protected] § Electronic address: [email protected] a r X i v : . [ h e p - ph ] N ov discrete symmetry that precludes the tree-level mass term for neutrinos allows the existence of a DM candidate whichplays a role in the radiative neutrino mass generation and could also trigger the electroweak symmetry breaking [15].Most of the neutrino mass motivated models, based either on radiative or seesaw mechanisms, contain chargedscalar(s) whose interactions induce lepton flavor violating (LFV) processes, and thus their couplings are subject tosevere experimental constraints [16, 17]. Probing these interactions is of great importance to identify through whichmechanism neutrino mass is generated, and whether it is a Dirac or Majorana particle.There has been many attempts to investigate different consequences of the new interactions in models motivatedby neutrino mass at future colliders [18]. Ref. [19] investigated the possibility of testing the KNT model through theprocess e + + e − → e − µ + + /E T at the ILC, where it was shown that it could be probed at ILC at center of massenergies 500 GeV and 1 TeV with and without the use of polarized beams. Similar study has been carried out for theprocesses pp → e − e + ( µ − µ + , e − µ + ) + /E T through the production of the charged scalar S ± via the Drell-Yan processand their decay modes which can give a detectable signal with two charged leptons and missing energy in the finalstate. The observation of an electron (positron) and anti-muon (muon) (the latter presents the most favorite channel),give us an indication for the signature of this class of model, where it has been shown that the LHC@14 TeV with100 fb − luminosity can test this model [20].In this work, without refering to a specific model of radiatively induced neutrino mass, we investigate the effect ofthe charged scalar S ± on the trilepton final state ( (cid:96) ± (cid:96) ± (cid:96) ∓ ) at the LHC, where the background consists of processesmediated by the gauge bosons W Z ( W γ ∗ ) [21]. Then we will propose sets of benchmark points for different chargedscalar masses and couplings which are consistent with LFV constraints and investigate the signal feasibility withinthe CMS analysis [22].This paper is organized as follows. In section II, we describe the model and different experimental constraints.Then in section III, we use the 8 TeV LHC RUN-I data to put constraints on this class of models and probe the modelat 14 TeV. In section IV, we consider two benchmark points and perform detailed analysis. Possible test of this classof models through a LFV background free process is investigated. Finally, we give our summary. II. MODEL & SPACE PARAMETER
In this work, we consider a class of models that contain the following term in the Lagrangian [7, 9, 14, 15, 23]
L ⊃ f αβ L Tα C(cid:15)L β S + − m S S + S − + h . c ., (1)where L α is the left-handed lepton doublet, C is the charge conjugation operator, (cid:15) is the anti-symmetric tensor, f αβ are Yukawa couplings which are antisymmetric in the generation indices α and β , and S ± is an SU (2) L -singletcharged scalar field. The interactions above induce LFV processes such as µ → e + γ and τ → µ + γ , with branchingfractions B ( µ → e + γ ) (cid:39) α em υ π | f ∗ τe f µτ | m S , (2) B ( τ → µ + γ ) (cid:39) α em υ π | f ∗ τe f µe | m S , (3)where α em is the fine structure constant, and υ = 246 GeV is the vacuum expectation value of the neutral componentin the SM scalar doublet field. These two branching ratios must satisfy the experimental bounds B ( µ → e + γ ) < . × − [16] and B ( τ → µ + γ ) < . × − [17]. Moreover, a new contribution to the muon’s anomalous magneticmoment is induced at one-loop, given by δa µ ∼ m µ π | f eµ | + | f µτ | m S . (4)The constraints on the LFV processes (2), (3) and (4), implies that | f αβ | (cid:46) ςm S , with ς is a dimensionful constantthat depends on the experimental bounds. This means that the couplings f are suppressed for small values of thecharged scalar mass.Here, we consider the charged scalar mass in the range 100 GeV < m S < f αβ takerandom values that respect the above mentioned constraints (2), (3) and (4). These values are illustrated in Fig. 1,where we show the allowed space parameter for the charged scalar mass and couplings. It is worth mentioning thatthe couplings f αβ shown in Fig. 1 could match the observed neutrino oscillation values, and their values depend onthe details of the models [13–15, 23] . Since our analysis is not restricted to a particular radiatively induced neutrinomass model, we present a scatter plot in Fig. 1-right for the combination | f αρ f βρ | which enter the expressions ofthe LFV observables. The large overlap between the region populated by the blue points in Fig. 1-left plot with thegreen is due to the fact that they get a common tau contribution in the expressions of the branching ratios in (2) and(3), whereas the red points correspond to larger values of f eµ as compared to the two other combinations. Fig. 1-right shows the parameter space region for which upper experimental bounds of B ( µ → e + γ ) and B ( τ → µ + γ ) aresatisfied along the identified range of mass noting that this LFV bounds processes prompt m S to large values oncethe corresponding coupling product f αρ f βρ becomes important. -18 -16 -14 -12 -10 -8 -6 -4 -2
0 500 1000 1500 2000 | f α β | m S (GeV) αβ =e µαβ = µταβ =e τ -14 -12 -10 -8 -6 -4 -2
0 500 1000 1500 2000 | f α β f βρ | m S (GeV) |f τ e f µτ ||f τ e f µ e | FIG. 1: The magnitude of f ’s versus m S (left) and combination of the couplings versus m S (right) with the experimentalbounds µ → e + γ and τ → µ + γ are represented by dashed lines. III. CURRENT CONSTRAINTS ON TRILEPTON SIGNAL AT THE LHC
At the LHC, it is possible to produce a singly charged scalar associated with different sign different flavor chargedleptons through W-boson exchange which at the parton level read as q ¯ q (cid:48) → W ± → (cid:96) ± (cid:96) ± S ∗∓ → (cid:96) ± (cid:96) ± (cid:96) ∓ + /E T , (5)where the charged scalar S ± decays into charged lepton and neutrino giving rise to three leptons plus missing energyin the final state as shown in Fig. 2-a.According to the diagram presented in Fig. 2-a, we have 7 contributions to this trilepton signal: (cid:96)(cid:96)(cid:96) ≡ eeµ, eµµ, eeτ, eτ τ, µµτ, µτ τ, eµτ, (6)Here, the process that maximally violates the lepton flavor ( eµτ ) has a small background, while the other six areaccompanied by a large SM background. Such process with maximal LFV ( eµτ ) can be a direct probe to the For example, the benchmark points values shown in Table I correspond to the model studied in [13], where the other model parameters(the couplings g iα and the mass of the other charged scalar mass) are chosen in a way to match neutrino oscillation data, DM relicdensity and LFV constraints. For the models proposed in [14, 15], one can adjust the parameters so that most of the benchmark pointsshown in Fig. 1 fulfill the aforementioned constraints. q ¯ q ′ q ¯ q ′ W ± W ± ℓ ± α ℓ ± β νℓ ∓ γ ν S ∗∓ ℓ ± α ℓ ∓ α νℓ ± β ( a ) ( b ) q ¯ q ′ W ± Z/γ ∗ ℓ ± β νℓ ± α ℓ ∓ α ( c ) W ∓ ν FIG. 2: Diagrams corresponding to the trilepton signal (a) and SM background (b,c). interactions in (1). However, this process involves purely S ± mediated diagrams, and therefore has a very small crosssection due to the smallness of the couplings f αβ and the heaviness of the charged scalars as dictated by the LFVconstraints (2), (3) and (4). For the processes with the large SM background, such as pp → e ± e ∓ µ ± + /E T , thetransverse missing energy receives two contributions /E T ≡ ν τ , ν µ . The process with /E T ≡ ν τ occurs only throughpurely S -mediated diagrams, and therefore has a suppressed cross section. However, the second process occurs through S -mediated and W/Z/ γ -diagrams, and hence the cross section can be written as σ M = σ SM + σ S + σ interference .Therefore, the expected excess of events number could be either σ S and/or σ interference , where the former couldbe significant only when the charged scalar is on-shell. However we found that σ S /σ interference < O (10 − ) for thebenchmark points considered in our analysis. This leads us to confirm that the event number excess comes mainlyfrom the interference contribution term.Due to the difficulty in identifying the tau lepton at the LHC, we consider in our detailed analysis only the finalstate leptons (cid:96) = e, µ , where the missing energy /E T can be any neutrino or antineutrino. The main process thatcontributes to the SM background for trilepton production is the irreducible background q ¯ q (cid:48) → W ± → (cid:96) ± (cid:96) ∓ W ± → (cid:96) ± (cid:96) ± (cid:96) ∓ + /E T , q ¯ q (cid:48) → ZW ± ( γ ∗ W ± ) → (cid:96) ± (cid:96) ± (cid:96) ∓ + /E T , (7)as shown in Fig. 2-b and -c. We use CalcHEP [24] to generate both the SM background events as well as the eventsfrom processes due to the extra interactions in (1) for CM energies √ s = 8 TeV and 14 TeV. Here, the consideredvalues of the f αβ Yukawa couplings and the charged scalar mass ( m S ) make the branching ratios B ( µ → e + γ ) and B ( τ → µ + γ ) just below the experimental bounds.In our analysis, we look for the event number difference N ex = N M − N BG , where N M is the expected numberof events number coming from both the new interactions and the SM processes, while N BG is the background eventnumber. Thus, with integrated luminosity L int , the excess of events is N ex = L int ( σ M − σ BG ), and N BG = L int σ BG ,with σ BG and σ M are the total cross sections due to interactions of the SM interactions and the one in Eq. (1),respectively, after imposing the selection cuts. Therefore the signal significance is given by S = N ex √ N ex + N BG = N ex √ N M . (8)One has to mention that the largest source of the SM background is the multi-jets events which can be misidentifiedas leptons in the detector. Among the dominant sources that give rise to these fake leptons we have the semileptonicdecays of the charm and the bottom quark; and the photons conversion [25]. In order to reduce the contaminationin the signal region, we require the electron events to have p T >
15 GeV and | η | < .
5, whereas all the muoncandidates are required to have p T > | η | < .
4. The hadronic decay of the tau charged lepton τ had can bediscriminated with p T >
15 GeV and | η | < . − at √ s = 8 TeV LHC.The analysis is based on the following criteria: • The presence of at least three isolated leptons (muon, electron). • The transverse momentum of muon and electron must satisfy p (cid:96)T >
10 GeV. • The pseudo-rapidity of leptons | η (cid:96) | < . • The missing transverse energy /E T <
50 GeV. • In order to remove the low-mass Drell-Yan processes as well as the ’Below-Z’ and ’Above-Z’ regions coming frombackground, the invariant mass of each opposite sign same flavor lepton pair must be in the range 75 GeV < M (cid:96) + (cid:96) − <
105 GeV.Using these cuts, it has been found that a bound on the heavy-light neutrino mixing parameter ( | B lN | ) for heavyneutrino masses up to 500 GeV can be established. For instance, | B lN | < × − has been derived for m N ∼ σ M at the parton level for the first two processes in (6) as afunction of the charged scalar mass for the benchmark points that are consistent with experimental bound on theLFV processes discussed in the previous section.
13 13.5 14 14.5 15 15.5 16 16.5 17 0 500 1000 1500 2000 σ ( f b ) m S (GeV) √ s = 8 TeVpp -> e ± µ ± µ + − +E miss BG
24 26 28 30 32 0 500 1000 1500 2000 σ ( f b ) m S (GeV) √ s = 14 TeVpp -> e ± µ ± µ + − +E miss BG
15 15.5 16 16.5 17 17.5 18 0 500 1000 1500 2000 σ ( f b ) m S (GeV) √ s = 8 TeVpp -> e ± µ ± e + − +E miss BG
24 26 28 30 32 0 500 1000 1500 2000 σ ( f b ) m S (GeV) √ s = 14 TeVpp -> e ± µ ± e + − +E miss BG FIG. 3: The production cross section for the processes pp → e ± µ ± µ ∓ + /E T (top), pp → e ± µ ± e ∓ + /E T (bottom) at √ s = 8 TeV(left) and √ s = 14 TeV (right) as function of charged scalar mass. The red lines correspond to the background cross sectionvalues. We see that σ M is larger than the one of the SM background σ BG within the cuts used by the CMS collaboration,and increases with CM energy whereas it is essentially independent of the charged scalar mass. To see how importantis the signal, we compute the significance taking into account the previous CMS cuts, for the two first processes in(6) for the set of benchmark points that fulfill the constraints on the LFV processes (2), (3) and (4) that are usedpreviously in Fig. 1. After applying of the selection criteria quoted above, we show in Fig. 4 the significance for thetwo considered channels at both 8 TeV and 14 TeV CM energy. S i gn i f i c an c e m S (GeV) √ s = 8 TeVpp -> e ± µ ± e + − +E miss pp -> e ± µ ± µ + − +E miss S i gn i f i c an c e m S (GeV) √ s = 14 TeVpp -> e ± µ ± e + − +E miss pp -> e ± µ ± µ + − +E miss FIG. 4: The significance for the process pp → (cid:96) ± (cid:96) ± (cid:96) ∓ + /E T at 8 TeV (left) and 14 TeV (right) versus the charged scalar massfor the integrated luminosity values 20.3 fb − and 100 fb − , respectively. The horizontal blue line indicates the significancevalue S = 5. These results are consistent with searches for new phenomena in events with multilepton final states, they have notshown any significant deviation from SM expectations at 8 TeV CM energy. However, after imposing the same cutsat 14 TeV with 300 fb − of integrated luminosity, one shows that it is possible to get at least a 4 sigma excess for anybenchmark point defined in Sec. II. Hence, we carry this study by searching a significant trilepton signal within thisclass of models at √ s = 8 TeV and 14 TeV by choosing two benchmark points and look for different cuts where thesignificance could be larger. IV. BENCHMARK ANALYSIS
In this section, we consider two benchmark points, denoted by B and B , with the charged scalar masses 472 GeVand 1428 GeV (see Tab. I). Here, we first analyze the trilepton production with missing energy involving e and µ decay modes of the heavy charged scalar S ± with √ s = 8 TeV and 14 TeV. Then, we discuss possibility of observingthe maximally LFV process signal (cid:96) ± (cid:96) ± (cid:96) ∓ ≡ e ± µ ± τ ∓ .A critical part in the analysis of signal events associated with new physics is the accurate estimation of the SMbackground. For this purpose, we study the event distributions for the SM background as well as the backgroundplus the trilepton signal, and impose the cuts on the relevant observables as shown in Tab. II. Point m S (GeV) f eµ f eτ f µτ B
472 -(9 .
863 + i . × − -(6 .
354 + i . × − (0 .
78 + i . × − B .
646 + i . × − -(2 .
265 + i . × − -(0 . − i . × − TABLE I: Two benchmark points selected from the allowed parameter space of the model.
We note that the imposed cut values on the kinematic variables are different than those provided by CMS, exceptfor the range of the invariant mass of two charged leptons M (cid:96) + (cid:96) − , and the pseudo rapidity η (cid:96) which still relevant fordiscriminating the signal from background. Moreover, we attempt to introduce supplementary criteria by applyingcuts on the invariant masses M e + µ + and M (cid:96),ν of the fermion pairs ( e + µ + ) and ( (cid:96), ν ), respectively. These extra cutsallowed us to optimize the total cross section for the signal at √ s = 8 TeV and 14 TeV. This is illustrated in Fig. 5where we present the angular distribution between pairs of leptons, the energy distribution of lepton, and the invariantmass distribution of the three leptons at √ s = 14 TeV. e ± µ ± e ∓ + (cid:54) E T @ 8 TeV e ± µ ± e ∓ + (cid:54) E T @ 14 TeV e ± µ ± µ ∓ + (cid:54) E T @ 8 TeV e ± µ ± µ ∓ + (cid:54) E T @ 14 TeV70 < M e − e + <
110 70 < M e − e + <
110 80 < M µ − µ + <
100 80 < M µ − µ + < M e + µ + < M e + µ + < M e + µ + < M e + µ + < M e − ν < M e − ν < M µ − ν < M µ − ν < < p (cid:96)T <
100 10 < p (cid:96)T <
90 10 < p (cid:96)T <
100 10 < p (cid:96)T < (cid:12)(cid:12) η (cid:96) (cid:12)(cid:12) < (cid:12)(cid:12) η (cid:96) (cid:12)(cid:12) < (cid:12)(cid:12) η (cid:96) (cid:12)(cid:12) < (cid:12)(cid:12) η (cid:96) (cid:12)(cid:12) < (cid:54) E T < (cid:54) E T < (cid:54) E T < (cid:54) E T < M (cid:96)(cid:96) (invariant mass), p (cid:96)T (charged lepton transverse momentum), (cid:54) E T (transverse missing energy), and η (cid:96) (pseudo-rapidity). The energy dimension variables are in GeV unit. d N / d E E e - (GeV) pp -> e ± µ ± e + − +E miss SignalBG d N / d M M e + µ + e - (GeV) pp -> e ± µ ± e + − +E miss SignalBG d N / d θ θ ll ( ° ) pp -> e ± µ ± e + − +E miss SignalBG d N / d E E µ + (GeV) pp -> e ± µ ± µ + − +E miss SignalBG d N / d M M e + µ + µ - (GeV) pp -> e ± µ ± µ + − +E miss SignalBG d N / d θ θ ll ( ° ) pp -> e ± µ ± µ + − +E miss SignalBG
FIG. 5: Number of events of the energy distribution E (cid:96) , the invariant mass distribution of the three leptons M (cid:96)(cid:96)(cid:96) , and theangular distribution between pairs of leptons θ (cid:96)(cid:96) at √ s = 14 TeV and ´ L dt = 300 fb − . The kinematical distributions in Fig. 5 show a significant excess of events which is an indication of a trileptonsignal. Clearly, there is a larger excess in the channel eµµ than in the eeµ channel for this benchmark. Accordingto the cross section values in Fig. 3, we expect the same difference for other benchmarks. The overall shape of thedistributions for the signal and the background looks very similar due to two reasons: (1) the source of the event excessis the interference contribution σ interference , and (2) the cuts are chosen such that the difference d ( σ M − σ BG ) /dX isstrictly positive, where X represents the kinematic variables in Tab. II. In Tab. III, we present the cross section valuesof the signal and background after imposing the cuts for the CM energies 8 TeV and 14 TeV. The correspondingsignificance for each benchmark point is shown in Tab. IV. Process B @8 T eV B @8 T eV B @14 T eV B @14 T eVσ BG (cid:0) e ± µ ± e ∓ + (cid:54) E T (cid:1) σ BG (cid:0) e ± µ ± µ ∓ + (cid:54) E T (cid:1) σ EX (cid:0) e ± µ ± e ∓ + (cid:54) E T (cid:1) .
12 28 .
06 49 .
70 48 . σ EX (cid:0) e ± µ ± µ ∓ + (cid:54) E T (cid:1) .
13 26 .
06 57 .
28 56 . B and B . Process Benchmark N . S . N S pp → e ± µ ± e ∓ + /E T B B pp → e ± µ ± µ ∓ + /E T B B L int = 20.3 (300) fb − at 8 TeV (14 TeV) forthe benchmark points B and B . In order to see how does the significance change with large charged scalar mass values, we consider the benchmarkpoint B given in Tab. I, and increase m S at both CM energies 8 TeV and 14 TeV for the integrated luminosity 20.3 f b − and 300 f b − , respectively. We first keep the couplings f αβ to be constant and therefore the LFV constraintsget relaxed with larger m S values. In the second case, we vary m S values while keeping LFV observables, such as B ( (cid:96) α → (cid:96) β + γ ), constant. The two cases are shown in Fig. 6 with dashed and solid lines, respectively. Thus, whateverthe values of charged scalar mass or the LFV branching ratios, the significance should lie in between these two curves.We can see from the figure that the significance can reach 3 σ for any charged scalar S ± mass under 2 TeV, and 5 σ isensured until m S = 3 TeV in the case where √ s = 14 TeV. S i gn i f i c an c e m S (TeV) √ s = 8 TeVpp -> e ± µ ± e + − +E miss pp -> µ ± e ± e + − +E miss S i gn i f i c an c e m S (TeV) √ s = 14 TeV pp -> e ± µ ± e + − +E miss pp -> e ± µ ± µ + − +E miss FIG. 6: Significance for the relevant process pp → (cid:96) ± (cid:96) ± (cid:96) ∓ + /E T at √ s = 8 TeV (left) and √ s = 14 TeV (right) within the newcuts. The black dashed horizontal lines represent the significance value S = 3, 5, respectively. solid and the dashed lines areexplained in the text. We remark here that the Feynman diagrams that mediate the processes pp → (cid:96) ± (cid:96) ± (cid:96) ∓ + /E T can be classified asSM and non-SM diagrams with amplitudes M SM and M S , respectively. Therefore, the event number difference N ex = N M − N BG is proportional to the combination σ interference ∝ Re (cid:16) M † SM M S (cid:17) , since σ S is negligible asmentioned previously. In other words, the significance shown in Fig. 6 is directly proportional to the couplingscombination | f αρ f βρ | that appears in the expressions of the branching ratios of the processes µ → e + γ and τ → µ + γ . This means that there is a direct correlation between the discovery of the LFV processes and the signals.In our analysis at √ s = 14 TeV, we have presented the points which can be discovered with an integrated luminosityof 300 f b − . However, another way to probe the interaction (1) is to extend our analysis by considering a maximallyLFV process like the process pp → e ± µ ± τ ∓ + /E T , where the tau lepton can be identified through its hadronicdecay [29] rather than its leptonic one in order to avoid an additional source of missing energy. In addition to thecase of √ s = 14 TeV, we consider also the very high energy such as at the HL-LHC √ s = 100 TeV. Then, the eventnumber here is given by N eµτ = L × σ ( pp → e ± µ ± τ ∓ + /E T ) B ( τ → hadrons ) , (9)where the corresponding background event number is given by N BG = L × σ ( pp → W W W ) B ( W → eν ) B ( W → µν ) B ( W → τ ν ) B ( τ → hadrons ) . (10)We find that the significance is so small at both √ s =14 TeV and 100 TeV for luminosity values of the order O (ab − ). Detailed investigation is required to reach final conclusion about the possibility of detecting the maximallyLFV in this model (1). V. SUMMARY
In this paper, we investigate the effect of a singlet charged scalar at the LHC by performing a detailed analysisof three isolated leptons in the final state. First we applied the same cuts used by the CMS collaboration at 8 TeVon a large number of benchmarks that are consistent with LFV bounds and we found no significant deviation fromthe SM. Whereas, within the same cuts we expect significant deviation at 14 TeV. So to enhance the signal over thebackground, we applied new cuts for both 8 TeV and 14 TeV. We have chosen two benchmark points B and B withdifferent values of m S in order to probe the effect of this charged scalar in the tripleton channel and we found that adeviation from the SM can be seen using 8 TeV data and expect that a discovery is potentially possible at 14 TeV.Using our analysis of 8 TeV (14 TeV), we can exclude charged scalar masses m S < m S < B ( (cid:96) α → (cid:96) β + γ ), and hence there is a direct correlation between theLFV discovery and our signal.Another way to search for the trilepton signal is via the maximally LFV processes such e ± µ ± τ ∓ , where the taulepton is identified through its hadronic decay. However, even at √ s = 100 TeV the significance is too small forluminosity values of the order O ( ab − ). Acknowledgments
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