arXiv subjects

Sanjoy Mandal

Publications and source records attributed to Sanjoy Mandal.

At least 19 recordsLinked to original sources

arXiv2026

Probing lepton number violation at FCC-ee

We propose high-multiplicity final-state signatures, such as $e^+e^-\to N\overline{N}\to \ell^+\ell^+ 4j$ with $\ell$ denoting $e,~μ$, $τ$, as probes of lepton number violation (LNV) at FCC-ee, featuring negligible Standard Model background. In contrast to conventional searches such as $pp\to \ell^+ N \to \ell^+ \ell^+ jj$ or the process $e^+e^-\toνN$, which are suppressed by the small neutrino masses in conventional seesaw scenarios, the minimal linear seesaw picture avoids this suppression. This enables a direct LNV probe from final-state topology, with over $\mathcal{O}(10^3)$ events expected at FCC-ee. Besides probing the Majorana nature of neutrinos, this offers a novel avenue to test the neutrino mass ordering established by oscillation experiments in a high-energy collider setting.

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arXiv2026

Heavy neutral leptons from light scalar in fixed target and forward search experiments

The observation of neutrino masses strongly motivates $U(1)_{B-L}$ extensions of the Standard Model, in which heavy neutral leptons acquire Majorana masses through spontaneous $U(1)_{B-L}$ symmetry breaking and generate light neutrino masses via the seesaw mechanism. In this framework, the singlet scalar responsible for symmetry breaking mixes with the SM Higgs boson, allowing it to be produced in rare meson decays. We investigate a scenario in which this light scalar promptly decays into a pair of long-lived heavy neutrinos that subsequently decay into visible charged leptons and hadrons through light-heavy neutrino mixing inside the proposed Forward Physics Facility (FPF) at the FCC-hh and the SHiP beam-dump experiment. Taking into account realistic detector geometries, decay probabilities, and visible branching fractions, we estimate the projected sensitivities to the scalar-Higgs mixing angle as a function of the scalar mass and to the light-heavy neutrino mixing as a function of the heavy neutrino mass. We find that FPF and SHiP can significantly extend the discovery reach for both light scalars and long-lived heavy neutrinos beyond existing experimental limits, providing powerful and complementary probes of neutrino-mass generation and hidden-sector physics.

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arXiv2026

Quantum spin correlations in $Z^\prime$-mediated $t\bar{t}$ production at future lepton colliders

We study quantum spin correlations in top-quark pair production at future lepton colliders in the presence of a neutral gauge boson from anomaly-free general $U(1)$ extensions of the Standard Model. The process $\ell^+\ell^-\to t\bar t$, with $\ell=e,μ$, is analyzed through the spin-density matrix including $γ$, $Z$ and $Z^\prime$ exchange and their interference. We focus on quantum-information observables such as the sufficient entanglement marker $\mathcal{D}_{\min}$, concurrence, purity and the maximal Clauser-Horne-Shimony-Holt (CHSH) parameter, and compare their behavior with conventional rate information. Within the $U(1)_X$ framework, we consider several representative charge assignments to investigate how different chiral structures influence these observables, with particular emphasis on the $Z^\prime$ resonance region and polarized $e^-e^+$ collisions, where the two allowed initial-state helicity configurations can be selectively enhanced. We show that electron-beam polarization provides a direct handle on the left- and right-handed lepton charges of various $U(1)_X$ scenarios. These results demonstrate that quantum spin observables provide information complementary to cross sections and angular distributions in searches for chiral neutral gauge interactions.

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arXiv2026

Forward Searches for Heavy Neutrinos and $Z'$ Bosons at FCC-hh

The discovery of neutrino masses strongly motivates extensions of the Standard Model containing heavy neutral leptons and additional gauge interactions. We investigate the prospects for probing these states at the proposed Forward Physics Facility (FPF) of the 100 TeV Future Circular Collider (FCC-hh) within a broad class of anomaly-free chiral $U(1)$ gauge extensions. These models predict a new neutral gauge boson, $Z'$, together with right-handed neutrinos responsible for generating light neutrino masses through the seesaw mechanism. We study long-lived particle signatures arising from both heavy neutrinos and the $Z'$ boson produced in the far-forward region. In particular, we analyze heavy neutrino production from meson decays, visible decays of long-lived $Z'$ bosons produced through meson decays and proton bremsstrahlung, long-lived $Z'$ bosons decaying into heavy-neutrino pairs, and prompt $Z'$ decays yielding long-lived heavy neutrinos. The expected event rates are evaluated for the proposed FPF detector configurations, taking into account realistic detector geometry, decay probabilities, and visible final states. We derive projected sensitivities to the heavy neutrino mass and active-sterile mixing as well as to the $Z'$ mass and gauge coupling for several representative $U(1)$ charge assignments. Our results demonstrate that the FPF at FCC-hh can substantially extend the discovery reach for light long-lived heavy neutrinos and light $Z'$ bosons beyond existing and proposed experiments, providing a powerful and complementary probe of neutrino-mass models and hidden gauge sectors. https://github.com/SouvikPhD/RHN-Detection-with-FASER-2-

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arXiv2026

Particle and Gravitational Wave Probes of Minimal Seesaw Neutrinos

Observable gravitational waves (GWs) from first-order phase transitions (FOPTs) can coexist with distinct particle physics signatures. These include same-sign dilepton plus four~jet events at colliders, such as $e^+ e^-/μ^+μ^- \to \ell^\pm \ell^\pm 4j$, neutrinoless double beta decay, as well as charged lepton flavor violating (cLFV) processes such as $μ\to e γ$. We explore this synergy within the minimal low-scale linear seesaw model. This framework successfully reproduces neutrino oscillation data, providing a direct avenue to probe the neutrino mass ordering and Majorana nature at colliders. Crucially, the FOPT responsible for the GW background is driven by a leptophilic Higgs doublet, establishing a direct link between early-universe cosmology and terrestrial laboratory experiments.

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arXiv2026

Probing Inelastic Dark Matter via Cosmic-Ray Upscattering in NGC 1068

We study constraints on sub-GeV inelastic dark matter (iDM) from cosmic-ray (CR) cooling in the active galactic nucleus (AGN) NGC 1068. In dense dark matter (DM) spikes surrounding supermassive black holes, high-energy CR protons can efficiently lose energy through scatterings with dark matter particles. We consider a minimal vector-portal iDM framework and consistently include both elastic and deep inelastic scattering (DIS) contributions to the CR energy-loss rate. We find that DIS processes dominate at high momentum transfer and substantially enhance the DM-induced cooling effect. By requiring the resulting cooling timescale to remain compatible with the observed Standard Model cooling in NGC 1068, we derive constraints on the iDM parameter space. Our results demonstrate that AGN cosmic-ray cooling probes previously unexplored regions of sub-GeV iDM parameter space inaccessible to current direct-detection experiments.

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arXiv2026

Dark matter as the source of neutrino mass: theory overview and experimental prospects

We review theoretical frameworks in which small neutrino masses arise radiatively through interactions with a dark sector that also accounts for cosmological dark matter (DM). A prototype is provided by scotogenic schemes, that extend the inert Higgs doublet model to include dark fermions. We outline their key features and limitations, discussing the advantages of the revamped scotogenic extension. The phenomenological signatures of fermionic and bosonic scotogenic dark matter are discussed, along with scoto-seesaw models that merge scotogenic and seesaw mechanisms. We also consider scenarios where the dark sector seeds a low-scale seesaw. These frameworks can accommodate dark matter as Weakly or Feebly Interacting Massive Particles (WIMPs or FIMPs). While hidden dark sector models are inherently difficult to exclude, visible dark sector schemes should be confirmed--or ruled out--by forthcoming dark matter, collider, and lepton flavor violation studies.

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arXiv2026

$h \to Υγ$ Decay: Smoking Gun Signature of Wrong-Sign $hb\bar{b}$ Coupling

We perform a model-independent study of new physics effects in the Higgs decay $h \to Υγ$, focusing on scenarios that spoil the accidental cancellation between the direct and indirect amplitudes. After imposing all existing constraints from Higgs production and decay measurements, we find that a wrong-sign $h b\bar b$ coupling is the only viable scenario capable of enhancing the $h \to Υγ$ decay width by nearly two orders of magnitude. Therefore, an observation of a significantly enhanced $h \to Υγ$ rate at the LHC or future colliders would provide unambiguous evidence for a wrong-sign $h b\bar b$ coupling, directly pointing to the presence of an extended Higgs sector.

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arXiv2025

Right handed neutrino production from $Z^\prime$ interactions in forward search experiments

We study two general $U(1)$ extensions of the Standard Model (SM) those generate tiny neutrino masses via the seesaw mechanism after general $U(1)$ breaking. These models predict a new neutral gauge boson ($Z'$) and right-handed neutrinos (RHNs), the latter introduced for anomaly cancellation and neutrino mass generation. In both scenarios, left- and right-handed fermions couple differently to the $Z'$, and RHNs mix with light neutrinos, enabling variety of decay modes. Focusing on the high-luminosity LHC (HL-LHC) and the future FASER2 experiment, we explore RHN pair production from $Z'$ decays in two cases: (i) long-lived $Z'$ decays to visible modes and long-lived RHNs, and (ii) short-lived $Z'$ decays to long-lived RHNs, which further decay visibly inside FASER2. We estimate projected limits on the general $U(1)$ gauge coupling, $Z'$ mass, RHN mass, and light-heavy neutrino mixing for various $U(1)$ charge assignments, and compare them with current experimental bounds.

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arXiv2025

$B-L$ model in light of the CDF II result

Recent CDF II collaboration's result on $W$ mass measurements contradict Standard Model prediction, requiring new physics to explain this anomaly. Such new physics may manifest through tree-level or loop-level corrections to the mass of the $W$ boson. In this work, we investigate the possibility that the CDF-II result is indicative of new physics not directly changing the $W$ boson mass but rather the $Z$ boson mass. Since the $Z$ boson mass goes as an input into the Standard Model prediction for $W$ boson mass, this change in $Z$ mass ultimately leads to the discrepancy between the CDF-II measurement and the Standard Model expectation. We demonstrate this idea through one of the simplest and most studied $U(1)$ gauge extensions of the Standard Model, namely the gauged $U(1)_{B-L}$ extension. We demonstrate that $B-L$ extended models can explain the revised best-fit values for $S$, $T$, and $U$ following the CDF II results. We studied the parameter space of models with and without mixing between neutral gauge bosons. We also reviewed the dark matter constraints and demonstrated that there is parameter space that is compatible with the current $W$ boson mass, relic abundance, and direct detection experiments.

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arXiv2025

Phenomenology of Dirac neutrino EFTs up to dimension six

The gauge-singlet right-handed neutrinos would be essential to explain the tiny masses of active neutrinos. We consider the effective field theory of the Standard Model extended with these fields under the assumption that neutrinos are Dirac particles. In this framework, we provide a comprehensive study for the phenomenological consequences of various dimension six interactions employing various high and low energy observables. These include the neutrino mass itself, constraints from electroweak precision test and collider searches for lepton or jet plus missing energy, coherent neutrino-nucleus scattering, beta decays, as well as decays of proton, meson, tau, and top. We also study their astrophysical and cosmological implications for stellar cooling and relativistic degrees of freedom.

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arXiv2025

Testing tree level TeV scale seesaw scenarios in $μ$TRISTAN

We investigate TeV scale seesaw scenarios at $μ^+ e^-$ and $μ^+ μ^+$ colliders in the $μ$TRISTAN experiment. In minimal type-I seesaw scenario we consider two generations of Standard Model (SM) singlet heavy Majorana type Right Handed Neutrinos (RHNs) which couples with SM gauge bosons through light-heavy neutrino mixing. We discuss the prospects of probing heavy neutrinos via the processes such as $μ^+e^-\to νN_i\to e^+ j jν$ or $μ^- j jν$ for $\sqrt{s}=346$~GeV and $1\text{ ab}^{-1}$ luminosity. Studying these process, we estimate limits on the light-heavy neutrino mixing angles as a function of heavy neutrino mass, which could be two orders of magnitude stronger than electroweak precision data. Further, we study the effect of doubly charged scalar boson $(H^{++})$ from the type-II seesaw scenario in $μ^+ μ^+$ collision at $\sqrt{s}=2$ TeV. In this case we consider $μ^+ μ^+ \to \ell_i^+ \ell_j^+$ and $μ^+ μ^+ \to H^{++} Z/ γ$ processes followed by the same sign dilepton decay of $H^{++}$. We find that events involving $e^+ e^+$ among these final states can probe the neutrino mass ordering in $μ$TRISTAN experiment at 5$σ$ significance. In addition to that we study the production of positively charged triplet fermion in $μ$TRISTAN following $μ^+ μ^+ \to μ^+ Σ^+$ process where $Σ^+$ decays into $μ^+ jj$ mode through $Z$ boson exchange. Considering a triplet at 1 TeV and studying SM backgrounds we estimate the discovery potential of $μ^+ μ^+ jj$ signal at $μ$TRISTAN with respect to projected luminosity.

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arXiv2024

Large lepton number violation at colliders: predictions from the minimal linear seesaw mechanism

Small neutrino masses can be sourced by a tiny vacuum expectation value of a leptophilic Higgs doublet, and mediated by Quasi-Dirac heavy neutrinos. In such simplest linear seesaw picture the neutrino mass mediators can be accessible to colliders. We describe novel charged Higgs and heavy neutrino production mechanisms that can be sizeable at $e^+ e^-$, $e^- γ$, $pp$, or muon colliders and discuss some of the associated signatures. The oscillation length of the heavy neutrino mediators is directly related to the light neutrino mass ordering. Moreover, lepton number violation can be large despite the smallness of neutrino masses, and may shed light on the Majorana nature of neutrinos and the significance of basic symmetries in weak interaction.

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arXiv2024

Dark matter-electron scattering and freeze-in scenarios in the light of $Z^\prime$ mediation

We investigate dark matter (DM)-electron scattering in a minimal $U(1)_X$ extension of the Standard Model (SM), where the DM can appear as a Majorana fermion, a complex singlet scalar or a Dirac fermion. To study bounds on the $U(1)_X$ gauge coupling $(g_X)$ and new gauge boson mass $(M_{Z^\prime})$, from DM-electron scattering, we consider several direct search experiments like CDMS, DAMIC, SENSEI, PandaX-II, DarkSide-50 and XENON1T-S2 for different $U(1)_X$ charges. In this set-up we consider DM production via freeze-in both in radiation dominated and modified cosmological background to project sensitivities on $g_X-M_{Z^\prime}$ plane satisfying observed relic abundance. DM-electron scattering could provide comparable, or even stronger bounds than those obtained from the electron/ muon $(g-2)$, low energy scattering and intensity frontier experiments within 0.01 GeV $\lesssim M_{Z^\prime} \lesssim$ 0.1 GeV. Constrains from freeze-in could provide stronger sensitivities for $M_{Z^\prime}\gtrsim \mathcal{O}(1)$ GeV, however, these limits are comparable to those obtained from LHCb, LEP experiments for $\mathcal{O}(10)$ GeV $\lesssim M_{Z^\prime} \lesssim 150$ GeV. In future, electron-muon scattering (MUonE), proton (FASER, DUNE) and electron/positron (ILC) beam dump experiments could probe these parameters.

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arXiv2024

Collider imprints of right handed neutrino magnetic moment operator

We consider most general effective Lagrangian up to dimension five, built with Standard Model~(SM) fields and right-handed neutrinos~(RHNs) $N_i$. Assuming that the RHNs are present near the electroweak scale, we study the phenomenology of the RHNs and highlight the differences that arise due to the inclusion of dimension five operators. We specifically focus on the production process $e^+e^-/pp\to N_i N_j$ which comes from the dimension five magnetic moment operator. We find that this production process followed by the decay chains such as $N_i\to N_jγ$, $N_i\toν_jγ$ and $N_i\to\ell^\pm j j$ leads to striking collider signatures which might help to probe the Majorana nature of neutrinos. We discuss the current collider constraints on this operator, as well as projected limit at future colliders. In addition, we discuss the stellar-cooling bounds applicable to the RHN mass below 0.1 GeV.

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arXiv2023

CDF-II $W$ Boson Mass Anomaly in the Canonical Scotogenic Neutrino-Dark Matter Model

The CDF-II collaboration's recent high-precision measurement of $W$ boson mass indicates new physics contribution(s) beyond the Standard Model. We investigate the possibility of the well-known canonical Scotogenic model to explain the CDF-II measurement. The Scotogenic model is a popular scenario beyond the Standard Model that induces neutrino masses at the 1-loop level and includes a viable dark matter candidate, either scalar or fermionic. For both scalar and fermionic dark matter possibilities, we simultaneously examine the constraints coming from (a) neutrino mass, oscillation, neutrinoless double beta decay and lepton flavour violation experiments, (b) from LEP and LHC (c) from dark matter relic density and direct detection experiments (d) from the oblique $S,T,U$ parameter values consistent with CDF-II $W$ boson measurement. We demonstrate that the new CDF-II measurement rules out the feasible parameter space of the scalar dark matter in the high mass regions ($m_{η_{R}} \gtrsim 500~\text{GeV}$), while still allowing the intermediate mass regions $54~\text{GeV} \lesssim m_{η_{R}} \lesssim 76~\text{GeV}$. We also showed that the fermionic dark matter candidate in the canonical Scotogenic model, in the range $M_{N_{1}} \lesssim 500~\text{GeV} $ , can simultaneously explain all the aforementioned issues. Furthermore, we investigated how the recent findings from ATLAS 2023 impact this study.

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arXiv2023

Fermionic Dark Matter in Dynamical Scotogenic Model

In the Dynamical Scotogenic Model, the global $B-L$ symmetry is supposed to be broken spontaneously resulting in a massless Goldstone boson called majoron, and massive right handed neutrinos which participate in the generation of light neutrino massses at one-loop. One of them being the lightest stable particle can be a thermal dark matter candidate. We discuss how the dark matter phenomenology differs from the original Scotogenic model, taking into account all the constraints coming from the observed neutrino masses and mixing, lepton flavor violations such as $μ\to eγ, μ\to e J$, astrophysical and cosmological observations of stellar cooling and $N_{eff}$, as well as collider signatures such as Higgs invisible decays. We find that the dark matter annihilation to majorons plays an important role to produce the right relic abundance.

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arXiv2023

Phenomenology of the simplest linear seesaw mechanism

The linear seesaw mechanism provides a simple way to generate neutrino masses. In addition to Standard Model particles, it includes quasi-Dirac leptons as neutrino mass mediators, and a leptophilic scalar doublet seeding small neutrino masses. Here we review its associated physics, including restrictions from theory and phenomenology. The model yields potentially detectable $μ\to eγ$ rates as well as distinctive signatures in the production and decay of heavy neutrinos ($N_i$) and the charged Higgs boson ($H^\pm$) arising from the second scalar doublet. We have found that production processes such as $e^+e^-\to NN$, $e^-γ\to NH^-$ and $e^+ e^-\to H^+ H^-$ followed by the decay chain $H^\pm\to\ell_i^\pm N$, $N\to \ell_j^{\pm}W^\mp$ leads to striking lepton number violation signatures at high energies which may probe the Majorana nature of neutrinos.

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