arXiv subjects

Amartya Sengupta

Publications and source records attributed to Amartya Sengupta.

9 recordsLinked to original sources

arXiv2026

A Multi-Axion Ladder Across Cosmic History: From Inflation, BBN, and Early Dark Energy to Late-Time Accelerated Expansion

What if dark energy is recurrent throughout cosmic history? In this picture, episodes of scalar-field dark energy become less surprising and more natural features of cosmic evolution. We construct a homogeneous multi-axion cosmology with a transient contribution during Big Bang nucleosynthesis, two early dark energy components before recombination, and a thawing field that supplies the present dark-energy density. As Hubble friction weakens, the fields begin to roll at successive epochs set by their potential-curvature scales. Their initial displacements affect the rolling delays and peak energy fractions. The transient fields have third-power cosine potentials, whose sextic minima allow faster-than-radiation dilution during rapid, small-amplitude oscillations. All four fields are evolved in a common Friedmann background, with their first roll and subsequent dynamics resolved numerically. With reference matter and radiation densities taken from Planck 2018, the benchmark has a nucleosynthesis-era peak fraction of approximately $0.99\%$ near $z=10^9$. The two early dark energy fields peak near $z=7.9\times10^3$ and $2.4\times10^3$, with individual fractions of $8.6\%$ and $7.8\%$; their combined fraction reaches $9.7\%$. The late-time field is normalized to supply a present fraction of approximately $0.685$, while the three transients leave a combined fraction of approximately $6.0\times10^{-7}$. An aligned two-axion example illustrates the enhanced field range available for an inflationary extension. We also outline how searches for transient contributions to the expansion rate at other epochs could constrain additional axion scales.

astro-ph.CO↗

arXiv2026

The flavour of SU(15) composite quarks and leptons

We study the flavour structure of an $SU(15)_p$ confining chiral gauge theory in which the Standard Model (SM) quarks, leptons, and Higgs emerge as composite bound states. The couplings of two scalar fields in the conjugate antisymmetric ($\overline{\mathbf{105}}$) and conjugate symmetric ($\overline{\mathbf{120}}$) representations of $SU(15)_p$ provide two $SU(4)_F$ flavour-breaking spurions that generate both the SM Yukawa couplings and the flavour-changing processes. The up and down Yukawa matrices are tightly-correlated due to a "right-handed isospin" symmetry, which predicts a trivial CKM matrix in the absence of spontaneous symmetry breaking. The lepton Yukawas are correlated with the quarks due to a common source of flavour spurions. With a judicious Froggatt-Nielsen-like texture for the two flavour spurions we find that a benchmark fit with $\mathcal{O}(1)$ non-perturbative coefficients reproduces all six quark masses, three charged lepton masses, and the CKM matrix. The same spurions mediate charged lepton flavour violation, neutral meson mixing, rare kaon decays, and induce electric dipole moments. We compare the reach on the compositeness scale $\Lambda_{\rm pre}$ across these observables in the numerical benchmark and find that the electron EDM and $K^0$-$\bar{K}^0$ mixing provide the strongest sensitivity, reaching $\mathcal{O}(10^4)$ TeV, the lower end of the range probed by proton decay, while $\mu\to e\gamma$, $D^0$-$\bar{D}^0$ mixing, and $\mu$-$e$ conversion give complementary reach at $10^2$-$10^3$ TeV. The projected electron EDM sensitivity extends this to $\mathcal{O}(10^6)$ TeV, beyond the reach of planned proton decay searches.

hep-ph↗

arXiv2026

Minimal dark $SU(2)$ origin of a massless Dirac neutrino

We propose a gauge-symmetry origin of a rank-two Dirac neutrino mass matrix that enforces one exactly massless neutrino, while being consistent with the oscillation data, as well as cosmological constraints. The mechanism relies on a minimal dark $SU(2)_D$ gauge symmetry under which one right-handed-neutrino-like Weyl fermion is charged, thereby forbidding its Standard Model Yukawa coupling. Quantum consistency then fixes the minimal dark-sector completion: Cancellation of the Witten anomaly requires a second fermionic $SU(2)_D$ doublet, while a discrete $Z_4$ symmetry that forbids Majorana masses allows the two dark doublets to form a vectorlike pair. This anomaly-free completion gives rise to a secluded, confining dark sector with a viable dark matter candidate, linking the protected neutrino texture to dark infrared dynamics.

hep-ph↗

arXiv2026

Signals of Doomsday III: Cosmological signatures of the late time $U(1)_{EM}$ symmetry breaking

Of the universe's original gauge symmetries, only $SU(3)_c$ (quantum chromodynamics) and $U(1)_{\rm EM}$ (electromagnetism) remain unbroken today. There is, however, no reason to assume that these symmetries are permanent. This paper explores the potential astrophysical observational signatures of a late-time breaking of $U(1)_{\rm EM}$. We present a model with a new massive scalar field whose potential supports a first-order phase transition through the nucleation of true-vacuum bubbles. If the propagation of the bubble walls slows down due to interactions with the surrounding matter and radiation, these signals can reach us before the bubble wall itself arrives. Using the vacuum-mismatch method, we calculate the spectrum of particles produced by such a bubble until the terminal velocity is reached. In addition, we show that frictional dissipation at terminal wall velocity generates a large population of thermally produced scalars and massive photons, which continues even after the mismatch channel shuts off. We then use event generators to simulate the decays of the new scalar and the massive photon into Standard Model particles and obtain, as the final result, the energy spectra of photons and neutrinos. Since the dominant final decay products after hadronization and the decay of unstable particles are photons and neutrinos, they act as long-range signatures of the transition. We also estimate the possible lead time of these photon and neutrino signals relative to the arrival of the bubble wall itself, showing that even a modest subluminal wall velocity can in principle provide an observable precursor. For the conservative set of parameters used here, the thermal channel produces a macroscopically large burst of high-energy photons and neutrinos, which could in principle be detectable from sufficiently nearby bubbles with present or future multi-messenger facilities.

hep-ph↗

arXiv2025

Residual Symmetries and Scalar Multiplet Vacuum Alignment in Non-Abelian Flavour Models

We demonstrate that, upon minimizing a renormalizable, single-scalar potential invariant under a non-Abelian symmetry, special orientations in the associated vacuum alignment of the scalar multiplet correspond to the preservation of a discrete residual flavour symmetry in the broken phase of the theory. Conversely, we show that these special scalar alignments are perturbed when additional Lagrangian operators (e.g. renormalizable, multi-flavon operators and/or effective, higher-dimensional operators) are present that break said residual symmetry, leading to a vacuum reorientation and phenomenological consequences. We therefore construct a one-to-one correspondence principle between broken residual symmetries and vacuum alignment corrections, providing a mechanism to identify (and correct) a subtle but persistent form of phenomenologically relevant fine-tuning embedded in -- but often ignored by -- most successful non-Abelian flavour models. We first establish this correspondence in a set of toy models based on the S4 permutation symmetry, and then apply the lessons learned to the more realistic A4 Altarelli-Feruglio and $\Delta(27)$ Universal Texture Zero models.

hep-ph↗

arXiv2025

The signals of doomsday II: Cosmological signatures of late time $SU(3)_c$ symmetry breaking

Only two gauge symmetries remain unbroken today: $SU(3)_c$ and $U(1)_{\rm EM}$. Both are crucial to our universe as it is and to our form of life. Unless we are special observers at the end of the cosmological symmetry-breaking chain, there is no reason they must remain unbroken forever. We investigate the cosmological signatures of late-time $SU(3)_c$ breaking in our model with a new colored scalar whose potential supports a first-order phase transition through true-vacuum bubble nucleation. We analyze three physically distinct production mechanisms of the expanding bubble wall. The first is direct nonthermal production from vacuum mismatch across the accelerating wall in the scalar and massive-gluon sectors. The second arises from frictional dissipation into a shocked layer. Including the finite-temperature effective potential, we show that our benchmark critical temperature $T_c$ is below the massive-gluon and scalar masses; an equilibrated relativistic broken-phase thermal bath would therefore restore $SU(3)_c$. We formulate a subcritical thermal freeze-out estimate below $T_c$, where massive broken-phase particles can still be produced but only with Boltzmann suppression. The third is a non-equilibrium wall--matter transition-radiation channel: ambient baryonic matter crossing the relativistic wall can radiate broken-phase massive gluons and scalar excitations. This requires no thermal bath above $T_c$ and can also continue after the wall reaches terminal velocity. We study the decays of the physical color-octet scalar and massive gluons, using \texttt{Pythia} to hadronize their decay products and determine the resulting photon and neutrino spectra. If the wall reaches a subluminal terminal velocity, these particles can arrive before the wall. The resulting high-energy spectra are a long-range observational signature that, if observed, could signal cosmic doomsday.

hep-ph↗

arXiv2025

The Signals of Doomsday I: False Higgs vacuum decay signatures

The measured standard model parameters indicate that we might live in a false Higgs vacuum, though with a very long lifetime. However, small black holes can serve as catalysers and significantly speed up the phase transition. In fact, bubbles of true vacuum might already exist in our universe. If the propagation of the bubble walls slows down due to interaction with the surrounding matter and plasma, these signals can reach us before the bubble wall hits us. Using the vacuum mismatch method, we calculate the spectrum of the Higgs particles produced by such a bubble until the terminal velocity is reached. In addition, we show that frictional dissipation at the terminal wall velocity generates a large population of thermally produced Higgs particles, which continues even after the mismatch channel shuts off. Since the Higgs is neutral, a good part of the final decay products (after hadronization, annihilation and decay of unstable particles) will be photons and neutrinos, which will then act as a long-range signature. For the conservative set of parameters used here, the thermal channel produces a macroscopically large burst of high energy neutrinos and photons from Higgs decays, which could be detectable from sufficiently nearby bubbles with current or upcoming multi messenger facilities.

hep-ph↗

arXiv2024

Constraining flavoured leptoquarks with LHC and LFV

We consider the framework of flavoured leptoquarks, in models with scalar or vector leptoquarks, and look for constraints to the parameter space of these models. Using primarily direct searches at the Large Hadron Collider (LHC) and precision processes that probe Lepton Flavour Violation (LFV), we present lower bounds for the masses of the leptoquarks in different flavoured scenarios. We classify the models according to the specific leptoquark, distinguished with respect to their couplings to charged leptons (lepton isolation, two-columned patterns) and in scenarios with specific hierarchies in the couplings (hierarchical, 2-3 democratic, and flipped).

hep-ph↗

arXiv2023

Status of leptoquark models after LHC Run-2 and discovery prospects at future colliders

We study limits from dilepton searches on leptoquark completions to the Standard Model in the parameter space motivated by anomalies in the $b \rightarrow s$ sector. After a full Run-2 analysis by LHCb, the disparity in lepton flavour violation has disappeared. However, the mismatch in angular distributions as well as in $B_s \rightarrow μ^+ μ^-$ partial width is still unresolved and still implies a possible new physics contribution. We probe three models of leptoquarks -- scalar models $S_3$ and $R_2$ as well as vector leptoquark model $U_1$ using non-resonant dilepton searches to place limit on both the mass and couplings to SM fermions. The exclusions of leptoquarks coupling either non-uniformly to different lepton flavours or uniformly is examined. Interestingly, if leptoquark couplings to electrons and muons are indeed universal, then the $U_1$ model parameter space that corresponds to the anomalous contribution should already accessible with Run-2 data in the non-resonant $eμ$ channel. In the non-universal case, there is a significant exclusion in couplings, but not enough to reach regions that explain observed anomalies. We, therefore, examine the prospective sensitivity at the HL-LHC as well as of a 3 TeV future muon collider. For the vector leptoquark model, we find that a muon collider can probe all of the relevant parameter space at 95% confidence with just 1 fb$^{-1}$ data whereas $R_2$ and $S_3$ models can be excluded at 95% with 5 fb$^{-1}$ and 6.5 fb$^{-1}$ luminosity respectively.

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