Prospects for a B-L Higgs boson discovery at LHC: wait for high luminosity Run 6 (2035 or 3000 fb-1)!

Let's call it the B(rout)-(Eng)L(ert) scalar...

It is well known that the SM cannot be the final theory of nature. The successful explanation of the hierarchy problem requires some new physics (NP) near the TeV scale. In addition, the observation of small neutrino masses and their very particular mixing indicates the presence of physics beyond the standard model (BSM)... neither ATLAS nor CMS have yet conclusively discovered any particle that serves as proof for BSM physics. Now, with the discovery of the Higgs boson, effects of new physics can be searched for in its coupling measurements [4–37]. In this paper, we consider the simplest manifestation of a BSM extension through an extra singlet scalar. As a first step, we would like to see how the addition of just an additional neutral Higgs boson fares with the discovery prospects at the high-luminosity run at LHC (HL-LHC) with a final integrated luminosity of 3000 fb^-1. 

The presence of a heavy Higgs-like neutral scalar is innate in various models, such as, the minimal supersymmetric standard model (MSSM), two Higgs doublet models (2HDMs), models with extra spatial dimensions, etc. However, the simplest among these models is the SM augmented with a gauge singlet. This can originate very naturally from a U(1)_B-L model with an extra U(1) local gauge symmetry, where B and L represents the baryon number and lepton number respectively. In particular, we focus on a TeV scale B−L model, that can further be embedded in a TeV scale Left-Right symmetric model [38–41]. The B−L symmetry group is a part of a Grand Unified Theory (GUT) as described by a SO(10) group [42]. Besides, the B−L symmetry breaking scale is related to the masses of the heavy right-handed Majorana neutrinos, which participate in the celebrated seesaw mechanism [43–46] and generate the light neutrino masses. 

Another important theoretical motivation of this model is that the right handed neutrinos, that are an essential ingredient of this model participate in generating the baryon asymmetry of the universe via leptogenesis [47]. Hence, the B−L breaking scale is strongly linked to leptogenesis via sphaleron interactions that preserve B−L. It is important to note that in the U(1)_B-L model, the symmetry breaking can take place at scales much lower than that of any GUT scale, e.g. the electroweak (EW) scale or TeV scale. Because the B+L symmetry is broken due to sphaleron interactions, baryogenesis or leptogenesis cannot occur above the B−L breaking scale. Hence, the B−L breaking around the TeV scale naturally implies TeV scale baryogenesis. 

The presence of heavy neutrinos, a TeV scale extra neutral gauge boson and an additional heavy neutral Higgs, makes the model phenomenologically rich, testable at the LHC as well as future e +e − colliders [48, 49,50,51,52,53,54,55,56]. The Majorana nature of the heavy neutrinos can be probed for example through same-sign dileptonic signatures at the LHC [57]. On the other hand, the extra gauge boson Z 0 in this model interacts with SM leptons and quarks. Non-observation of an excess in dilepton and di-jet signatures by ATLAS and CMS have placed stringent constraints on the Z' mass [58–63]. 

In this work, we examine in detail the discovery prospects of the second Higgs at the HL-LHC for a TeV scale U(1)B−L model. The vacuum expectation value (vev) of the gauge singlet Higgs breaks the U(1)_B-L symmetry and generates the masses of the right handed neutrinos. We consider the B−L breaking scale to be of the order of a few TeVs, for which the right handed neutrino masses can naturally be in the TeV range. The physical second Higgs state mixes with the SM Higgs boson with a mixing angle θ, constrained by electroweak precision measurements from LEP [64, 65, 66], as well as from Higgs coupling measurements at LHC [67, 68]. The second Higgs is dominantly produced by gluon fusion with subsequent decay into heavy particles. The largest branching ratios are into W, Z and Higgs bosons. We discuss in detail the different channels through which the second Higgs state can be probed at the HL-LHC... 

We studied the discovery prospect of a heavy Higgs H₂ in the 4l, 2l2j and the ljjE_T channels at the LHC (with ∫Ldt=100 fb^-1) and HL-LHC ( ∫Ldt=3000 fb^-1), where we employed a boosted decision tree to separate signal from background... 

The channel with four leptons was found to be the cleanest. The signal and background cross-sections for these processes are σ_S≃0.1 fb and σ_B≃42 pb, respectively. Using the cuts on the i) invariant mass of 4l and on the reconstructed Z bosons, ii) the pT cuts on the momenta of four leptons, as well as, the reconstructed Z bosons, we found that for a mass M_H2≤ 500 GeV, the H₂ can be discovered with a significance of ∼5σ at HL-LHC with 3000 fb^-1.

Searching for a Heavy Higgs boson in a Higgs-portal B-L Model

Shankha Banerjee, Manimala Mitra, Michael Spannowsky

(Submitted on 21 Jun 2015)

A LHC time schedule

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