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Probing a Light Sterile Neutrino Through Heavy Charged Higgs Boson Decays at the LHC

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Probing a Light Sterile Neutrino Through Heavy Charged Higgs Boson Decays at the LHC PHYSICAL REVIEW D 98, 035043 (2018) Probing a light sterile neutrino through heavy charged Higgs boson decays at the LHC Yi-Lei Tang* School of Physics, KIAS, 85 Hoegiro, Seoul 02455, Republic of Korea (Received 23 April 2018; published 29 August 2018) We present a 13 TeV proton-proton collider simulation in a ν-two-Higgs-doublet model. The heavy charged Higgs bosons are produced in pairs through the electroweak processes and decay to the light sterile neutrinos (lighter than the W=Z boson masses). The light sterile neutrino further decays into a jet-like object with a muon in it. This helps us discriminate the signal from the backgrounds composed of the standard model jets. DOI: 10.1103/PhysRevD.98.035043 I. INTRODUCTION well [40,41]. However, as known, there is a gap in the interesting parameter space where m ≪ m Æ and The seesaw mechanisms [1–5] introduce some right- N H m ≲ m . In this area, each of the largely boosted sterile handed or sterile neutrinos with extremely heavy Majorana N Z=W masses (∼109–1012 GeV) to create the light neutrino neutrinos decays into a single jet-like collimated object, masses (≲0.1 eV according to the oscillation data). and thus the usual method of analyzing separated objects However, this sort of model is far beyond the reach of a loses its effectiveness. (For some works on neutrino jets, m <m practical collider. If we reduce the masses down to about see Refs. [42,43].) Sometimes, if N W=Z, the three- ’ 1 TeV, the Yukawa coupling among the sterile neutrinos, body suppression on the sterile neutrino s decay width will left-handed neutrinos, and the Higgs doublet are predicted reveal a secondary vertex for us to discriminate the signal to be so small that it is nearly impossible to produce a (such as that addressed in Ref. [40]). However, the decay practical signal at a collider. In the literature, there are some length is very sensitive to the model parameters, and it can alternative models within which we can obtain a TeV-scale possibly be well below the 1-.cm scale. Therefore, for a sterile neutrino as well as a relatively stronger connection conservative discussion, we neglect all of the secondary with the standard model (SM) sectors. One group of such vertex information and treat the signal/SM backgrounds in models utilizes the pseudo-Dirac sterile neutrinos (e.g., the usual way. Unlike most SM jets, the sterile neutrino jets Refs. [6–8]) with a relatively larger Yukawa coupling y, usually contain leptons. This feature has been applied in, yet collider searches still involve hunting within an e.g., Refs. [16,17] to search for the lepton jets decayed from the sterile neutrinos. However, we mainly consider had- extremely small fraction of ZðÞ=WðÞ-decay products. ronic decay and other new physics sectors that participate in (See Refs. [9–11] for some experimental results, and the processes in this paper. We also apply this feature to Refs. [12–27] for some theoretical approaches.) Another efficiently eliminate the background. approach at colliders is to associate the seesaw mechanisms with other new physics models [28–35]. We can therefore II. MODEL DESCRIPTION investigate sterile neutrinos with the aid of other new physics particles. Here, we rely on a standard ν-THDM model. The i−1 In this paper, we consider a situation where the charged THDM with Φi → ð−1Þ Φi Z2 symmetry including Higgs boson mainly decays to a sterile neutrino plus a the soft-breaking terms is characterized by the effective charged lepton. This scenario can be found in some ν-two- potential [44] Higgs-doublet models (ν-THDMs) [36–39]. Collider phe- 2 † 2 † 2 † † nomenology has also been studied in the literature as V ¼ m1Φ1Φ1 þ m2Φ2Φ2 − m12ðΦ1Φ2 þ Φ2Φ1Þ λ λ þ 1 ðΦ†Φ Þ2 þ 2 ðΦ†Φ Þ2 þ λ ðΦ†Φ ÞðΦ†Φ Þ *[email protected] 2 1 1 2 2 2 3 1 1 2 2 † † λ5 † 2 † 2 Published by the American Physical Society under the terms of þ λ4ðΦ1Φ2ÞðΦ2Φ1Þþ ½ðΦ1Φ2Þ þðΦ2Φ1Þ ; ð1Þ the Creative Commons Attribution 4.0 International license. 2 Further distribution of this work must maintain attribution to Φ the author(s) and the published article’s title, journal citation, where 1;2 are the two-Higgs doublets with hypercharge 3 ¼ 1 λ 2 and DOI. Funded by SCOAP . Y 2, 1−5 are the coupling constants, and m1;2;12 are the 2470-0010=2018=98(3)=035043(7) 035043-1 Published by the American Physical Society YI-LEI TANG PHYS. REV. D 98, 035043 (2018) mass parameters. The ν-THDM is based on the type-I Therefore, we do not consider the possibility of the THDM in which all the SM particles QL, uR, dR, LL, and secondary vertex cases. eR couple with the Φ2 field, III. SIMULATION DETAILS AND RESULTS SM ¯ ˜ ¯ L ¼ −Y Q Φ2u − Y Q Φ2d Yukawa uij Li Rj dij Li Rj In this paper, we concentrate on the decay pp → ZÃ= − ¯ Φ þ ð Þ Ã þ − Æ Æ Æ ∓Ã Æ YlijLLi 2lRj H:c: 2 γ → H H , with the H → μ N, N → μ W → μ qq¯ decay chains as shown in Fig. 1. We have chosen the The sterile neutrino together with the left-handed lepton Φ hadronic decay channel of the sterile neutrino because it doublets couple with the 1 field. In this paper, without has the largest branching ratio and it is more convenient loss of generality, we consider only one sterile neutrino N. Æ for reconstructing the H masses. The muon appearing in Therefore, the corresponding Lagrangian is given by the decay products (clustered inside a jet together with the ν ¯ ¯ ˜ other elements) can help us tag the jets decayed from the L ¼ −m NN − ðY L Φ1N þ H:c:Þ; ð3Þ Yukawa N i Li sterile neutrinos. where mN is the mass of the sterile neutrino and Yi, i ¼ 1, The muons appearing in the decay products can also be 2, 3 are the Yukawa coupling constants corresponding to replaced by electrons or taus. Here we have chosen the the e, μ, and τ lepton doublets. muon channels for the collider’s distinctive ability to In the following discussions, we do not need to care identify a muon, especially a muon inside of a jet. about the details of the electroweak symmetry breaking, nor As for the electron cases, we can estimate the correspond- do we need to discuss the neutral Higgs bosons. After the ing SM background from the result in this paper due to the electroweak symmetry breaking, we acquire the coupling lepton universality in the SM. However, one should be aware of the greater difficulties in identifying an electron þ¯ inside a jet than a muon when attempting to apply our result L ⊃−Yi sin βH liPRN þ H:c:; ð4Þ to the other lepton cases. v2 where tan β ¼ is the ratio of the Φ1 2 vacuum expectation The missing energy is predicted to be small in our v1 ; Æ expected signal. The reducible backgrounds with neutrinos values, and H is the charged Higgs with the mass mHÆ . In the large tan β ≫ 1 case, Eq. (4) becomes the most should be eliminated by applying the transverse missing significant coupling for HÆ decay, and therefore HÆ → energy (MET) cuts. However, the pileup effects at the lÆN will become the dominant decay channel when future high-luminosity colliders might seriously smear the MET distributions in both the signal and background mN <mHÆ . At a proton-proton collider, the HþH− pairs can be events, making it difficult to separate them. There are à à three main sources of reducible backgrounds with neutri- produced via either the s-channel γ and Z , or the off-shell þ − à à nos: from the τ τ ’s leptonic decay, from the gauge bosons Higgs particles h and H . The off-shell Higgs particle þ − → þν −ν¯ → ννμ¯ þμ− channel is usually negligible due to the rather small light W W l l decay and the ZZ decay ¯ ¯ → ¯ þ − quark–Higgs coupling constants, except in some special associated with the bb, and the tt bbW W decay. The τþτ−’s leptonic decay is suppressed by the relatively cases when λ3 or λ4 is very large. Therefore, in this paper we only consider the qq¯ → ZÃ=γà → HþH− processes. small branching ratio, and these less energetic muons have Another important thing that we should mention is the a much smaller chance of passing the kinematic cuts. decay length of the sterile neutrino N. In fact, the Gauge boson decay channels are further reduced by the higher order of the coupling constants. The only significant smallness of the left-handed neutrino masses constrains ¯ þ − −6 reducible channel is pp → tt¯ → bbμ μ νν¯. Therefore, in Yi cos β < 10 , which in turn amplifies the decay length to ≳1 this paper the only reducible background that we consider m. This will destroy most of our results discussed in ¯ this paper. However, the model we use in this paper is only is the tt channel. DELPHES with its CMS card shows that the a simplified model. In reality, this can be quite different in MET distribution of the signal sample is also rather the case of the existence of the other sterile neutrino significant even in the case without pileup. This sort of singlets. For example, in Ref. [45] the appearance of the pseudo-Dirac sterile neutrinos allowed a much larger −3 2 3 Yi cos β ∼ 10 in a naturally smaller tan β ∼ 10 –10 case, compared with the simplest ν-THDM in which tan β ≳ 104.
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