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Probing the Type-II Seesaw Mechanism Through the Production of Higgs Bosons at a Lepton Collider

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Probing the Type-II Seesaw Mechanism Through the Production of Higgs Bosons at a Lepton Collider PHYSICAL REVIEW D 98, 015024 (2018) Probing the type-II seesaw mechanism through the production of Higgs bosons at a lepton collider † ‡ ∥ Pankaj Agrawal,1,2,* Manimala Mitra,1,2, Saurabh Niyogi,3, Sujay Shil,1,2,§ and Michael Spannowsky4, 1Institute of Physics, Sachivalaya Marg, Bhubaneswar, Odisha 751005, India 2Homi Bhabha National Institute, Training School Complex, Anushakti Nagar, Mumbai 400085, India 3Gokhale Memorial Girls’ College, Harish Mukherjee Road, Kolkata 700020, India 4Institute for Particle Physics Phenomenology, Durham University, Durham DH1 3LE, United Kingdom (Received 23 March 2018; published 16 July 2018) We investigate the production and decays of doubly-chargedp Higgsffiffiffi bosons for the Type-II seesaw mechanism at an eþe− collider with two center of mass energies, s ¼ 380 GeV and 3 TeV, and analyze the fully hadronic final states in detail. Lower mass ranges can be probed during the 380 GeV run of the collider, while highpffiffiffi mass ranges, which are beyond the 13 TeV Large Hadron Collider discovery reach, can be probed with s ¼ 3 TeV. For such a heavy Higgs boson, the final decay products are collimated, resulting in fat-jets. We perform a substructure analysis to reduce the background and find that a doubly- charged Higgs boson in the mass range 800–1120 GeV can be discovered during the 3 TeV run, with integrated luminosity L ∼ 95 fb−1 of data. For 380 GeV center of mass energy, we find that for the doubly- charged Higgs boson in the range 160–172 GeV, a 5σ significance can be achieved with only integrated luminosity L ∼ 24 fb−1. Therefore, a light Higgs boson can be discovered immediately during the run of a future eþe− collider. DOI: 10.1103/PhysRevD.98.015024 I. INTRODUCTION number violating (LNV) d ¼ 5 operator LLHH=Λ [1,2]. Such operator is not forbidden as the lepton number is only With the discovery of the Higgs boson at the Large a classical symmetry of the SM, violated by quantum Hadron Collider (LHC), we start to develop an under- effects. standing of how the standard model (SM) fermion and There are three proposed categories, commonly known gauge boson masses are generated in terms of the Brout- as, Type-I, -II, and -III seesaw mechanisms in which the Englert-Higgs (BEH) mechanism. However, one of the SM is extended by a SUð2Þ singlet fermion [3–9], main puzzles that still remains unclear is the origin of light L SUð2Þ triplet scalar boson [10–13],andSUð2Þ triplet neutrino masses and mixings. The same BEH mechanism L L fermion [14], respectively. In particular, the second can, in principle be employed to generate Dirac mass of SM possibility, i.e., where a triplet scalar field with the neutrinos by extending the SM to include right-handed hypercharge Y ¼þ2 is added to the SM, is the simplest neutrinos. However, the required large hierarchy of the model with an extended Higgs sector. The neutral com- Yukawa couplings raises uncomfortable questions. A ponent of the triplet acquires a vacuum expectation value completely different ansatz is that neutrinos are their (vev) vΔ, and generates neutrino masses through the own antiparticles and hence, their masses have a different Yukawa interactions. Perhaps, the most appealing feature origin than the other SM fermions. A tiny eV Majorana neutrino mass can be generated by the seesaw mechanism, of this model is its minimality. The same Yukawa where light neutrinos acquire their masses from a lepton interaction between the lepton doublet and the triplet scalar field generates Majorana masses for the neutrinos, and also dictates the phenomenology of the charged Higgs *[email protected] † bosons. [email protected] ‡ [email protected] A number of detailed studies have already been §[email protected] performed for the Hadron colliders like, Tevatron [15] ∥ [email protected] and LHC [15–29] to search for the triplet Higgs scenario. One attractive feature of this model is the presence of the Published by the American Physical Society under the terms of doubly-charged Higgs boson, and its distinguishing decay the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to modes. Depending on the triplet vev, the doubly-charged the author(s) and the published article’s title, journal citation, Higgs boson can decay into same-sign dilepton, same- and DOI. Funded by SCOAP3. sign gauge bosons, or even via a cascade decay [16–18]. 2470-0010=2018=98(1)=015024(13) 015024-1 Published by the American Physical Society AGRAWAL, MITRA, NIYOGI, SHIL, and SPANNOWSKY PHYS. REV. D 98, 015024 (2018) The details of the Higgs spectrum have been discussed in CLIC with 95 fb−1 of data. For lower masses, the range [30–32]. For the branching ratios and collider signatures, 160–172 GeV can be covered with only L ∼ 24 fb−1 of see [16–20]. The CMS and ATLAS collaborations have luminosity. For the earlier discussions on Higgs triplet searched for the same-sign dilepton final states for all model at a linear collider, see [49–52]. For the other SM flavors, and constrained the mass of the doubly-charged and BSM searches at CLIC and other linear colliders, see ÆÆ 820 Higgs as MH > , 870 GeV at 95% C.L. [33,34]. [47,53–67] for Higgs physics and effective field theory, However, this is only relevant for a very tiny vev [68–72] for different BSM scenarios, and [73–80] for −4 vΔ < 10 GeV, where the doubly-charged Higgs boson seesaw and radiative neutrino mass model searches. For decays into the same-sign dilepton with 100% branching the discussion on probing dark-sector at eþe− collider, ratio. For larger triplet vev such as vΔ ≳ 0.01 GeV, this see [81,82]. branching ratio is negligibly small. Therefore, a direct Our paper is organized as follows: we briefly review the bound on the mass of the HÆÆ from the same-sign basics of the Type-II seesaw model in Sec. II. In Sec. III,we dileptonic final state cannot be obtained. An alternative discuss existing experimental constraints. In the subsequent search where the HÆÆ is produced in association with two subsections, Secs. IVA and IV B, we analyze in detail the jets (vector boson fusion channel) gives relaxed con- production cross-sections and the discovery potential of the −4 þ − straints [35,36].ForvΔ ≥ 10 GeV, the doubly-charged multi-jet final states at the e e collider. Finally, we present Higgs boson predominantly decays into same-sign dibo- our conclusions in Sec. V. son. The collider signatures and the discovery prospect of this scenario have been discussed in [37–39],and[40,41] II. MODEL DESCRIPTION (see [42] for the discussion on the composite Higgs model In addition to the SM Higgs field Φ, the type-II seesaw and [43] for discussion on flavor violating τ decays). model [10–13] contains an additional SUð2Þ triplet Higgs Previous searches for HÆÆ in the pair-production channel L field and their subsequent decays into same-sign leptons ÆÆ 97 3 þ ! at LEP-II has put a constraint MH > . GeV at Δpffiffi Δþþ 95% C.L. [44]. Δ ¼ 2 ∼ ð1 3 2Þ ð Þ þ ; ; : 2:1 While a number of searches at the LHC are ongoing to Δ0 − Δpffiffi 2 experimentally verify the presence of the doubly-charged Higgs boson, in this work we perform a detailed collider We denote the neutral components of the SM doublet analysis to explore the discovery prospects at a future and triplet Higgs fields as Φ0 ¼ p1ffiffi ðϕ0 þ iχ0Þ and Δ0 ¼ lepton collider. For a large mass of the doubly-charged 2 p1ffiffi ðδ0 þ η0Þ ϕ0 δ0 Higgs boson, the pair-production cross-section at the LHC 2 i , respectively. The real scalars and 2 2 2 becomes small. Furthermore, the presence of numerous acquire vevs denoted as vΦ and vΔ with v ¼ vΦ þ vΔ ¼ backgrounds weakens its discovery prospects. Therefore, a ð246 GeVÞ2. The light neutrino mass is proportional to the lepton collider with a much cleaner environment will be triplet vev vΔ. The new scalar field Δ, being a triplet under more suitable to search the high mass regime of the doubly- SUð2Þ, interacts with the SM gauge bosons. The relevant charged Higgs boson. In addition, we also exhaust the low kinetic term has the form mass regime, yet unconstrained by the LHC, and by LEP-II L ðΔÞ¼ ½ð ΔÞ†ð μΔÞ ð Þ measurements. kin Tr Dμ D ; 2:2 We consider the pair-production of the doubly-charged g a a Higgs boson at a lepton collider and its subsequent decays with the covariant derivative DμΔ ¼ ∂μΔ þ i 2 ½τ Wμ; Δþ 0 into same-sign gauge bosons. We focus on the hadronic ig BμΔ. The Yukawa interactions of Δ with the lepton decays of the produced gauge bosons and analyze the fields are multijet final states in detail. As a prototype example, we consider the future eþe− collider Compact Linear Collider L ðΦ ΔÞ¼ c τ Δ þ ð Þ Y ; YΔLLi 2 LL H:c: 2:3 (CLIC) [45pffiffiffi–48], that will operate with the center of mass energies s ¼ 380 GeV, 1.4 and 3 TeV. We first analyze In the above, YΔ is a 3 × 3 matrix and c denotes charge the discovery reach of the doubly-charged Higgs boson at conjugation. The triplet field Δ carries lepton number þ2 380 GeV center of mass energy. Subsequently, we carry out and hence the Yukawa term conserves lepton number. The a detailed simulation for the very heavy doubly-charged scalar potential of the Higgs fields Φ and Δ is Higgs boson with a mass around and beyond one TeV.
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