arXiv subjects

Mariana Frank

Publications and source records attributed to Mariana Frank.

At least 19 recordsLinked to original sources

arXiv2026

Multiboson Signatures of Doubly Charged Scalars at a Same-Sign Muon Collider

We investigate the sensitivity of a same-sign $\mu^+\mu^+$ collider to doubly charged scalars in the Type-II seesaw framework, focusing on the regime in which the doubly charged scalar decays dominantly into same-sign $W$-boson pairs. Motivated by the $\mu$TRISTAN proposal, for a benchmark we consider a degenerate triplet spectrum at a center-of-mass energy of $2~{\rm TeV}$ and an integrated luminosity of $1~{\rm ab}^{-1}$. The signal process is studied in the fully hadronic $W$-decay mode, leading to a characteristic $\mu^+\mu^+ + 8j$ final state. We develop a cut-based analysis based on high jet multiplicity and global hadronic $W$ reconstruction, and then improve its sensitivity with a multivariate strategy exploiting reconstructed $W$ observables, global event kinematics, multi-boson variables and spectator-muon information. The best-performing setup reaches a $2\sigma$ sensitivity level for doubly charged scalar masses up to $425-430~{\rm GeV}$, extending the cut-based reach by a few tens of GeV. This indicates an improvement over the current LHC coverage, while also providing an independent probe based on a qualitatively different production mode and collider environment.

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arXiv2025

Vector Boson Fusion Signatures of Superheavy Majorana Neutrinos at Muon Colliders

We investigate the sensitivity of future high-energy muon colliders to heavy Majorana neutrinos, considering both opposite-sign ($\mu^+\mu^-$) and same-sign ($\mu^+\mu^+$) collision modes. We focus on $\mu^+\mu^-$ colliders operating at centre-of-mass energies of 1, 3 and 10 TeV, as well as the proposed $\mu$TRISTAN facility at 2 TeV, and we analyse the production of heavy neutrinos via vector boson fusion in the $t$-channel, a mechanism that becomes dominant in the multi-TeV regime. We evaluate its exclusion potential in terms of the heavy neutrino mass and the mixing of the heavy neutrino with its Standard Model counterparts, using both cut-based selections and boosted decision trees trained to exploit the distinct kinematic signatures of heavy Majorana neutrino exchanges. Our results demonstrate the complementarity between collider configurations, and show that active-sterile mixing angles as small as 0.001 could be probed for neutrino masses up to 100 TeV, an experimentally inaccessible region of the parameter space at current facilities. Altogether, this work establishes the discovery potential of muon colliders for testing super-heavy Majorana neutrinos, complementary to conventional probes, and provides compelling motivation for the next generation of high-energy lepton colliders.

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arXiv2025

Exploring two component doublet dark matter

We propose a two-component dark matter (DM) scenario by extending the Standard Model with two additional $SU(2)_L$ doublets, one scalar, and another fermion. To ensure the stability of the DM components, we impose a global $Z_2 \times Z_2^\prime$ symmetry. The lightest neutral states for both the scalar and fermion, which are non-trivially transformed under the extended symmetry, behave as stable two-component DM candidates. While single components are under-abundant due to their gauge interactions, in a mass region between $m_W$ and $525$ GeV for the scalar and a mass below $1200$ GeV for the fermion, and the fermion DM conflicts with direct detection limits over the whole parameter space, having two components helps to saturate relic density in the regions with under-abundance. Compliance with direct detection constraints leads to two options, either introducing dim-5 effective operators, or embedding the scenarios into a complete UV theory, which reproduces a type II seesaw model, thus naturally including neutrino masses. We analyze the consequences of this scenario at the LHC.

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arXiv2024

Leptonic probes of Alternative Left-Right Symmetric Models

We explore constraints on the parameter space of the alternative left-right model originating from the leptonic sector. Our analyses focuses on both lepton-flavour-conserving observables, particularly the anomalous magnetic moment of the muon, and lepton-flavour-violating processes like $\mu \to e \gamma $ decay and $\mu-e$ conversions in nuclei. While contributions to the anomalous magnetic moment fall below the measured values at 2$\sigma$, current and future experimental sensitivities to flavour-violating branching rations of the Standard Model leptons are expected to impose lower bounds on the mass of the peculiar $SU(2)_R$ gauge boson of the model. This provides complementary constraints relative to existing limits, which are indirect and derived from collider bounds on the mass of the associated neutral gauge boson $Z^\prime$.

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arXiv2024

Stability of the Standard Model Vacuum with Vector-Like Leptons: A Critical Examination

The stability of the Standard Model (SM) Higgs vacuum is a long-standing issue in particle physics. The SM Higgs quartic coupling parameter is expected to become negative at high scales, potentially generating an unstable vacuum well before reaching the Planck scale. We investigate whether the introduction of vector-like leptons (VLLs) in six distinct gauge anomaly-independent representations can stabilize the SM vacuum without introducing additional scalar fields. By analyzing the mass spectrum and mixing angles of these VLLs with the SM leptons, we identify conditions under which the Higgs potential remains stable. We demonstrate that there exists a set of allowed but narrowed VLL spectra that can indeed stabilize the SM vacuum. We also study the effect of these VLLs on electroweak precision observables, particularly the oblique parameters S and T to ensure fit within global experimental constraints. Finally, we perform a comprehensive analysis of the parameter space allowed by electroweak precision data and the parameter space required for stability, highlighting cases that accommodate both. This study opens up new avenues for understanding the role of vector-like leptons in extending the validity of the Standard Model up to the Planck scale.

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arXiv2024

Interplay of inert doublet and vector-like lepton triplet with displaced vertices at the LHC/FCC and MATHUSLA

We study the interaction between the inert Higgs doublet (IDM) dark matter and a vector-like $SU(2)$ triplet lepton (VLL), both of which are $Z_2$-odd. The vector current of the VLL with the $Z$-boson rules out a fermionic or two-component dark matter scenario. However, a compressed mass spectrum and a sufficiently small Yukawa coupling allows co-annihilation and late decay of the VLL into the IDM sector, affecting the relic density of the pseudoscalar dark matter. The same two factors enable displaced decay of the VLL states, providing novel signatures involving hadronically quiet displaced multi-lepton final states. Such signatures to probe the model are studied at the 14 and 27 TeV LHC, as well as the 100 TeV FCC-hh. In addition to being detectable at the CMS/ATLAS experiments, if the new particles have sub-100 GeV masses, signals can also be seen at the proposed MATHUSLA detector.

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arXiv2024

Collider imprint of vector-like leptons in light of anomalous magnetic moment and neutrino data

We investigate the impact of incorporating vector-like leptons into the Standard Model, aiming to address longstanding puzzles related to the anomalous magnetic moments of the muon and electron while maintaining consistency with neutrino masses and mixings. We find that among the various representations of vector-like leptons permitted by the Standard Model gauge symmetry, only weak doublets and singlets offer satisfactory solutions, all associated with a significantly constrained parameter space. Our analysis delves into the associated parameter space, identifying representative benchmark scenarios suitable for collider studies. These setups yield a distinctive six-lepton signature whose associated signals can easily be distinguished from the Standard Model background, providing a clear signal indicative of new physics models featuring vector-like leptons. Our work hence sheds light on the potential implications of vector-like leptons in resolving discrepancies inherent to the Standard Model, while also offering insights into experimental avenues for further exploration.

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arXiv2024

Flavour-changing top quark decays in the alternative left-right model

We examine flavour-changing neutral-current decays of the top quark, $t\to q γ$, $t \to qZ$, $t \to q H$, and $ t\to q g$ (with $q=u, c$), in the Alternative Left-Right Model, a left right-symmetric model featuring exotic quarks and light bosons. These decays have a very small probability of occurring within the Standard Model, but they can be enhanced in this model through the presence of the exotic states. While associated signals may be detected directly at the LHC, rare decays have the advantage of offering means to probe new particles indirectly, through loop-contributions. We perform a comprehensive analysis of the model's parameter space to demonstrate the possible existence of enhancements in the corresponding branching ratios, of $10^6$ for the branching ratios $\mathcal{B}(t\to uZ)$ and $\mathcal{B}(t \to uH)$, and in the range of $10^{2} - 10^{4}$ for the other decays, relative to the Standard Model. We subsequently determine the preferred parameter space regions of the model in terms of potential of being reached in the near future.

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arXiv2024

A novel search strategy for right-handed charged gauge bosons at the Large Hadron Collider

We explore the potential of the Large Hadron Collider (LHC) in detecting a signal originating from the production of a heavy $SU(2)_R$ charged gauge boson that then decays into a top-bottom quark pair via the mediation of a right-handed neutrino, $p p \to W_R \to N_R \ell \to (\ell' t b)\ell$. Such a channel, that we study in the context of the minimal Left-Right Symmetric Model, contrasts with conventional smoking-gun signatures targeted experimentally and phenomenologically in which only light quarks are involved. We propose a selection strategy aimed at extracting such a top-bottom signal and we estimate the resulting sensitivity of the LHC to the model. Our results demonstrate the potential impact of such a search and we therefore urge the experimental collaborations to carry out a similar analysis in the light of present and future data.

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arXiv2024

Collider Signatures of $W_R$ boson in the Alternative Left-Right Model

Alternative Left-Right Models offer an attractive option to left-right models. Emerging from $E_6$ grand unification, these models are consistent with light scalars which do not induce flavour-changing neutral currents due to the presence of exotic quarks. Here we investigate the signature at the LHC collider of the charged $W_R$ boson, which can be lighter than in left-right models. We include constraints from collider data and show that $W_R$ can be produced in pairs, or in conjunction with a light charged Higgs boson. The final decay products involve leptons or jets. We explore all production and decay possibilities and indicate which ones are most promising to be observed at the colliders. Our analysis shows that signals of $W_R$ bosons can be observed at the LHC at 27 TeV, some for lower luminosity, and under most favourable conditions, even at 13 TeV.

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arXiv2023

Vacuum Stability and Electroweak Precision in the Two Higgs Doublet Model with Vector-Like Quarks

We present a comprehensive analysis of the vacuum stability of the Two-Higgs Doublet Model, for both Type-I and Type-II, augmented by vector-like quarks in either singlet, doublet or triplet representations. We review the model briefly before introducing the extra fermionic states and their interactions, and impose restrictions on the parameters coming both from theoretical considerations and experimental bounds. We then study the renormalization group equation evolution of the parameters of the model in order to isolate the parameter regions that satisfy vacuum stability requirements. We then add the electroweak precision observables to insure that the resulting parameter space is consistent with the data. We include complete expressions for the renormalization group equations and the S and T parameters used. Finally we summarize the effects of various vector-like quark representations on the parameter space. We indicate the regions constrained, highlighting the differences between representations in Type-I and Type-II, and pinpoint the effects of the interplay between the extended model and the additional fermions.

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arXiv2023

Reconciling collider signals, dark matter, and the muon anomalous magnetic moment in the supersymmetric $ U(1)_{R}\times U(1)_{B-L}$ model

We study the low-scale predictions of the supersymmetric model extended by $U(1)_R \times U(1)_{B-L}$ symmetry, obtained by breaking $SO(10)$ symmetry at GUT scale via a left-right supersymmetric model. Two new singlet Higgs fields ($χ_R$, $\barχ_R$) are responsible for the $U(1)_R \times U(1)_{B-L}$ symmetry breaking to the standard model gauge group. We explore the phenomenology of this model by assuming universal and non-universal boundary conditions at the GUT scale and their effects in obtaining consistency among low-energy observables, dark matter experiments, muon magnetic moment measurements, and $Z^{\prime}$ phenomenology. We examine different scenarios with both the lightest neutralino and sneutrino mass eigenstates as the dark matter candidates that satisfy all the experimental constraints. We explore the collider signals of various scenarios including different benchmarks and their significance versus standard model background. To complement our analysis, we perform recasting of several LHC analyses to verify the credibility of different benchmarks. We find that relaxing the universality conditions at $M_{\rm GUT}$ can significantly improve the agreement of the model against the experimental bounds. While the muon anomalous magnetic moment is found to be the most challenging observable to fit with the model, we find points in the parameter space consistent within $2 σ$ from the average measured value, employing non-universality at the GUT scale.

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arXiv2023

A nearly Dirichlet Higgs for lower-scale warped extra dimensions

We consider a minimal extension of the Standard Model in warped extra dimensions, with fields propagating in the bulk including a bulk SM-like Higgs doublet. We show that the Higgs can acquire a non-trivial oscillatory VEV, strongly localized towards the TeV brane, but such that its value at that brane could be highly suppressed due to its oscillatory behaviour. Within the minimal Randall-Sundrum metric background, this oscillatory VEV can alleviate the bounds coming from oblique precision electroweak parameters, such that the KK gluon mass can be around 3 TeV (instead of about 8 TeV for the usual non-oscillatory bulk Higgs). We also discuss the stability of the configuration as well as the naturalness of the model parameters.

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arXiv2022

Dark Matter in the Alternative Left Right Model

The Alternative Left-Right Model is an attractive variation of the usual Left-Right Symmetric Model because it avoids flavour-changing neutral currents, thus allowing the additional Higgs bosons in the model to be light. We show here that the model predicts several dark matter candidates naturally, through introduction of an $R$-parity similar to the one in supersymmetry, under which some of the new particles are odd, while all the SM particles are even. Dark matter candidates can be fermionic or bosonic. We present a comprehensive investigation of all possibilities. We analyze and restrict the parameter space where relic density, direct and indirect detection bounds are satisfied, and investigate the possibility of observing fermionic and bosonic dark matter signals at the LHC. Both the bosonic and fermionic candidates provide promising signals, the first in LHC at 300 fb$^{-1}$, the second at higher luminosity, 3000 fb$^{-1}$. Signals from bosonic candidates are indicative of the presence of exotic $d^\prime$ quarks, while fermionic candidates imply the existence of charged Higgs bosons, all with masses in the TeV region.

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arXiv2022

Additional Higgs bosons: Supersymmetry or warped extra dimensions?

We investigate and compare additional CP-even, CP-odd and charged scalar states appearing in two popular Beyond the Standard Model scenarios. We focus on the simplest possible Higgs sector within warped extra-dimensions and supersymmetry, with the aim to differentiate between them. In each case, we analyze the couplings of the new Higgs states, looking for distinguishing signatures. We show that the couplings of the Standard Model gauge bosons to the first Kaluza-Klein Higgs states of the extra-dimensional setup (CP-even, CP-odd and charged) are very similar to those of the heavy Higgs states of the MSSM in the decoupling region. We also find that the Yukawa couplings in the extra-dimensional scenario can mimic the different types of Yukawa couplings of general Two-Higgs Doublet Models, in particular the so-called Type-II couplings, which are similar to those in the MSSM.

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arXiv2022

Restricting the parameter space of Type-II Two Higgs Doublet Models with CP violation

We explore the parameter space of the type-II two-Higgs-doublet model with softly broken Z2 symmetry, allowing for CP violation in the scalar potential. Imposing theory-motivated and experimentally-driven constraints, we show that as the CP-violating phases are increased, only small regions of parameter space survive, including regions slightly away from the alignment limit. Electroweak oblique parameters and electric dipole moments emerge as most restrictive constraints. We show that imposing these constraints (as well as theoretical bounds such as perturbativity) the masses of the charged and heavy neutral Higgs, unlike in the CP conserving case, are bound from above and below. In particular, the heavy neutral Higgs masses are almost degenerate. In this parameter space region we highlight the relevant decay signals of the heavy neutral Higgs, involving both Zh and WW=ZZ, indicative of CP violation in the model.

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arXiv2022

Vacuum stability in the Standard Model with vector-like fermions

The discovery of Standard-Model like Higgs at 125 GeV may raise more questions than the answers it provides. In particular, the hierarchy problem remains unsolved, and the Standard Model Higgs quartic self-coupling becomes negative below the Planck scale, necessitating new physics beyond the Standard Model. In this work we investigate a popular scenario, extensions of the Standard Model with vector-like fermion fields, such as the ones present in models with extra dimensions or in Higgs composite models, using a model independent approach. Since fermions decrease the Higgs quartic coupling at high energies, only exacerbating the self-coupling problem, we introduce first an additional scalar, which by itself is enough to overcome the vacuum stability limit, and then explore the effects of vector-like fermions in singlet, doublet and triplet representations. For each model, we identify the allowed fermion masses and mixing angles with the third family fermions required to satisfy the vacuum stability condition, and compare different representations. Allowed fermion masses emerge at around 1 TeV, raising hope that these will be found at the LHC. We also examine corrections to oblique parameters S and T from additional scalar and vector-like quarks which also impose constraints on mixing and mass splitting of both sectors., but these restrictions are relatively weak compared to the vacuum stability.

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arXiv2022

Vacuum Structure of Alternative Left-Right Model

We investigate an Alternative Left-Right Model (ALRM) with $SU(2)_L$ as well as $SU(2)_R$ gauge groups, but unlike the traditional left-right symmetric models (LRSM) is not symmetric under the exchange of the fermion content. Interestingly, it can be embedded in $E_6$, while its low energy Higgs structure resembles the LRSM, involving Higgs doublets $χ_{L,R}$ and one Higgs bidoublet $Φ$. We analyze the scalar potential and the vacuum structure of the theory analytically to ensure the stability of scalar potential via bounded from below (BFB) and copositivity criteria, accompanied by a numerical study. We establish the necessary criteria for electric charge preserving vacua, yielding constraints on various coupling parameters of the theory. Finally we obtain constraints on the parameters of the model from collider data on the masses of the Higgs scalars.

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