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We explore the discovery prospect of the doubly-charged component of an

The observations of flavor-changing neutrinos in neutrino oscillation experiments have conclusively established that neutrinos have nonzero masses and mixing. Specifically, the solar and atmospheric mass square differences are measured to be

In this paper, we focus on the LNV signatures of seesaw at future

Feynman diagrams for the single production of doubly-charged Higgs boson at the

For large diagonal Yukawa couplings, the off diagonal entries of

The direct search constraints on the doubly-charged scalar from the LHC put a lower bound on

The rest of the paper is organized as follows. In Sec.

We consider a hybrid

Particle content and their corresponding charges under the SM gauge group. The

The complete Lagrangian with the additional triplet scalar and rh neutrino is given by

The Yukawa term in Eq.

The scalar potential in Eq.

We expand the doublet and triplet scalar fields around their VEVs to obtain ten real-valued field components,

The mass matrix of the singly-charged Higgs boson in the basis (

Among the four neutral scalars, two of them are

In this section, we analyze the discovery prospects of a doubly-charged Higgs boson in the future

Thus, in the small VEV limit, our analysis equally applies to both

It is worthwhile to note that the production of a doubly-charged scalar at FCC-eh has a major difference when compared with its production at the LHC. At the LHC, one can never exploit the large Yukawa

It is also worth noting that we always produce the negatively-charged state at

Note that signal I with a trilepton final state comes from the subprocesses listed as

The production cross sections for

Note that the neutrinoless double beta decay limits are not relevant here, because the amplitude of the

By choosing the diagonal elements

Allowed off diagonal Yukawa couplings

To simulate the signal events, we implement the model described in Sec.

We show the variation of the single production cross section of the doubly-charged Higgs boson as a function of its mass in Fig.

Parton-level production cross sections (normalized to

In Table

Representative benchmark points used in our analysis for studying the doubly-charged scalar production at the FCC-eh.

In what follows, we perform a detailed cut-based analysis for the signal and background for both signal I and signal II identified above.

Since we consider the on shell production of the doubly-charged scalar, and its width/mass is less than 1% (see Table

The trilepton signal containing at least a pair of same-sign, same-flavor charged leptons can come from

The most dominant SM background for the above final state, at an

A subdominant contribution can also come from the reducible, on shell

A relatively weaker background contribution could also come from an

These numbers from Ref.

One should also account for the possibility of contributions to the SM background, where a jet may fake a charged lepton. Notwithstanding the fact that the fake-rate for such events would be significantly smaller, one begs the question whether a relatively sizable cross section for jet-enriched final state leaves an imprint on the signal under consideration. For the estimation of fake-rate induced background, we have considered the process

Cut-flow table of the cross section for the relevant SM background channels for the cuts

To begin with, all the above channels for the background contribute substantially with basic acceptance cuts for the subprocess under consideration. However, the contributions to the actual signal topology starts showing the relative importance of the backgrounds that would eventually contribute to the final analysis. We begin the event generation by demanding:

In Table

Cut-flow table of cross section for signal for the cuts

In Fig.

The normalized distribution of

We now consider the multilepton channel where we have at least two charged leptons in the final state and both need to be of the same sign and flavor. This would correspond to the inclusive search of the signal for a singly-produced doubly-charged Higgs boson coming from both

The irreducible background coming from

To add to the fake-rate contribution, we must consider the process

In addition to fake rates, we note that charge mismeasurement can also lead to potential backgrounds which mimic the signal. We evaluate

We also estimate the background that arises due to the jet faking a lepton which as before is assumed to be 0.1%.

As before,

Similar to the previous analysis, we again implement a few similar cuts on the events to suppress the background and improve signal sensitivity.

In Tables

Cut-flow table of the cross section for the relevant SM background channels for the cuts

Cut-flow table of cross section for signal for the cuts

An important point which we should highlight here is the fact that so far we have only considered the electron flavor for the same-sign leptons in the decay of the doubly-charged Higgs boson, while in principle, it could decay to any of the lepton flavors depending on the Yukawa structure

Signal cross section after all the cuts for

Note that the leading contribution for the SM background in the

Therefore, the cross sections for the signal events given in Table

In determining the statistical significance of the signal we have used the following general expression

Required integrated luminosity for achieving a

To highlight the sensitivity of the FCC-eh to a doubly-charged Higgs mass at a given integrated luminosity, we should know what values of the Yukawa coupling

The

The more robust and striking channel of discovery would however be the

The sensitivity reach of

We have analyzed the discovery prospect of a doubly-charged Higgs boson at the newly proposed FCC-eh electron-proton collider, operating with beam energies

We have studied two types of final states, namely,

B. D., M. M., and S. K. R. thank the organizers of WHEPP-XIV at IIT, Kanpur where this work was initiated, and the organizers of SUSY 2017 at TIFR, Mumbai where part of this work was done, for the local hospitality. S. K. would like to thank Subhadeep Mondal for help with numerical aspects at the initial stage of the work. The work of B. D. is supported by the US Department of Energy under Grant No. DE-SC0017987. M. M. acknowledges the support of DST-INSPIRE research Grant No. IFA14-PH-99. The work of S. K. R. was partially supported by funding available from the Department of Atomic Energy, Government of India, for the Regional Centre for Accelerator-based Particle Physics (RECAPP), Harish-Chandra Research Institute. S. K. would like to acknowledge support from the Department of Atomic Energy (DAE) Neutrino Project under the plan project of Harish-Chandra Research Institute and the European Union’s Horizon 2020 research and innovation programme Invisibles Plus RISE under the Marie Sklodowska-Curie Grant No. 690575.