arXiv subjects

Danny Marfatia

Publications and source records attributed to Danny Marfatia.

At least 19 recordsLinked to original sources

arXiv2026

Sterile neutrino dark matter in conformal Majoron models

We study sterile neutrino dark matter (DM) in a classically conformal U(1)' extension of the Standard Model with three right-handed neutrinos and a Majoron-like singlet scalar that generate the observed pattern of active neutrino masses and mixing via the type-I seesaw mechanism. Working in the regime of strongly suppressed active-sterile mixing, we show that the observed DM abundance can be produced through freeze-in from feeble interactions mediated by the heavy Z' and the conformal scalar. We solve the Boltzmann equation for the nonthermal phase-space distribution and confront the scenario with Lyman-$α$ and strong gravitational lensing data by computing the matter power spectrum. For keV-mass sterile neutrinos we identify the viable parameter space consistent with the inferred relic density and structure-formation and X-ray bounds, including regions compatible with a tentative 3.5 keV line. If a second sterile state is long-lived, late decays can realize a two-component setup that alleviates the $S_8$ tension. However, explanations of the 3.5 keV line and the $S_8$ tension are incompatible with the strong-lensing limit from JWST if the neutrinos saturate the relic abundance. In a highly fine-tuned variant of the model, the 220 PeV KM3NeT event can also be explained by invoking the decay of a superheavy sterile neutrino.

hep-ph↗

arXiv2026

Two-peak gravitational wave spectra from supercooled two-field phase transitions

We study the gravitational wave (GW) spectrum generated by supercooled first-order phase transitions in two-field conformal models. We construct three representative toy models that capture the essential features of these models, derive analytical criteria for the existence of two-step transitions, and show how the two-field dynamics generate a genuine two-peak GW spectrum. We identify regions of parameter space in which two-peak GW spectra are observable at LISA and the Einstein Telescope. By way of illustration, we apply our analysis to a realistic conformal dark SU(3) model.

hep-ph↗

arXiv2026

Unfolding the low-energy reactor neutrino flux from CE$ν$NS data with a finite Dirac sum

We present a novel method to analyze coherent elastic neutrino--nucleus scattering (CE$ν$NS) data to extract the reactor antineutrino spectrum below the inverse beta decay threshold of 1.8 MeV, where it remains unmeasured. Adapting halo-independent analysis techniques developed for direct dark matter detection, we show how to obtain a best-fit and a pointwise confidence band for the integrated neutrino flux, without assuming a parametric form or smoothness prior for the spectrum. In our approach, which follows from convex geometry arguments, the differential neutrino rate is written as a linear combination of Dirac delta functions -- a finite Dirac sum (FDS) -- with a maximum number of terms determined by the number of data points. We apply our "FDS method" to mock CE$ν$NS data for a low-threshold Ge detector and compare it with Tikhonov-regularized unfolding.

hep-ph↗

arXiv2026

Impact of cosmic evolution on the gravitational wave spectrum from cosmic strings

In the standard cosmological paradigm, the gravitational wave (GW) spectrum from a cosmic string network is well defined. However, early-Universe dynamics can significantly alter this relic signal. We systematically investigate how different new physics epochs in the radiation era--including extra relativistic degrees of freedom, early matter domination, intermediate vacuum domination, and kination--modify string network evolution and loop production. We find that a kination era enhances the stochastic GW signal, whereas the other scenarios suppress the signal to varying degrees. We also demonstrate that late-time dynamical dark energy has a negligible impact on the GW spectrum from cosmic strings.

hep-ph↗

arXiv2026

Gravitational waves from cosmic superstrings and gauge strings

We perform a phenomenological comparison of the gravitational wave (GW) spectrum expected from cosmic gauge string networks and superstring networks comprised of multiple string types. We show how violations of scaling behavior and the evolution of the number of relativistic degrees of freedom in the early Universe affect the GW spectrum. We derive simple analytical expressions for the GW spectrum from superstrings and gauge strings that are valid for all frequencies relevant to pulsar timing arrays (PTAs) and laser interferometers. We analyze the latest data from PTAs and show that superstring networks are consistent with 32 nHz data from NANOGrav, but are excluded by 3.2 nHz data at $3σ$ unless the string coupling $g_s<0.2$ or the strings evolve in only about 10% of the volume of the higher-dimensional space. We also point out that while gauge string networks are excluded by NANOGrav-15 data at $3σ$, they are completely compatible with EPTA and PPTA data. Finally, we study correlations between GW signals at PTAs and laser interferometers.

hep-ph↗

arXiv2026

Prominence: A discriminator of gravitational wave signals

The concept of prominence is familiar to signal engineers, topographers and mountaineers. We introduce Prominence $\cal P$ as a discriminator of gravitational wave (GW) signals. We treat black hole and neutron star binaries as astrophysical background sources, and show how $\cal P$ can be used to distinguish between GW spectra produced by first-order phase transitions, domain walls and cosmic strings, and combinations thereof. Prominence can also be used to discriminate between these and off-piste sources of GWs. The uncertainty in the measured energy density in GWs at Pulsar Timing Arrays needs to be at the sub-percent to percent level for $\cal{P}$ to achieve discrimination at 3$σ$. LISA and ET data are expected to have sufficiently small uncertainties that Prominence can play a central role in their analysis. We define and apply a $χ^2$ statistic based on an inner product in signal space to show that sources indistinguishable at $2σ$ using the signal-to-noise ratio can be distinguished using $\mathcal{P}$ with significances above and in some cases far above $3σ$.

hep-ph↗

arXiv2026

Massive right-handed neutrinos in $\bar{B} \to D^* τ\bar X$ decay

We explore signatures of a massive right-handed neutrino (RHN) in angular distributions of $\bar{B} \to D^* (\to D π) τ(\to πν_τ) \bar X$ decays, where $X$ is an invisible state. We assume the new physics is described by the standard model effective field theory extended with an RHN in the MeV-GeV mass range. We calculate for the first time the full differential distributions in terms of the visible final states, including the decay of the $τ$ lepton. We evaluate the sensitivity of various distributions to the new physics operators.

hep-ph↗

arXiv2026

Primordial black holes from Q-balls produced in a first-order phase transition

We consider the formation of Q-balls in false vacuum remnants during a cosmological first-order phase transition. We find that under certain circumstances Q-balls can collapse to form primordial black holes. This scenario can produce multimessenger signals that may be observed at upcoming experiments, including 1-100 nHz gravitational waves from the phase transition, and gamma-rays emitted from primordial black holes as Hawking radiation and as superradiance. These signals are quite distinctive, and differ markedly from signals expected from Fermi-balls. The reheating of the dark sector from the phase transition may address the Hubble tension.

hep-ph↗

arXiv2026

Axion-like particles and sterile neutrinos solve the $B\to Kν\barν$ and $B\to πK$ puzzles

The recent measurement of the branching ratio of $B^+ \to K^+ + \mathrm{inv}$ (where ``inv'' denotes invisible states) by the Belle II collaboration is enhanced relative to the standard model expectation by 2.7$σ$. An older puzzle persists in measurements of the branching ratios and CP asymmetries of $B \to πK $ decays. We address these two anomalies in flavor-changing neutral current $B$ decays, with a short-lived axion-like particle (ALP) with mass close to that of the $π^0$. In the model with the minimum number of new couplings, the ALP has couplings to the photon, top quark and a heavy sterile neutrino. The ALP contributes to the $B \to π^0 K $ decays by mixing with the $π^0$. It contributes to $B^+ \to K^+ + \mathrm{inv}$ by its off-shell coupling to sterile neutrino pairs. The model can explain the excess in the total rate, but not the observed distribution of signal events. We make predictions for all $B \to K^{(*)} + \mathrm{inv}$ modes and for the rare kaon decays, $K^+ \to π^+ + \mathrm{inv}$ and $K_L \to π^0 + \mathrm{inv}$. We find an appreciable contribution to the magnetic moment of the muon, and negligible contributions to the magnetic moment of the electron and $b \to s e^+ e^-$.

hep-ph↗

arXiv2026

Phenomenology of bubble size distributions in a first-order phase transition

In a cosmological first-order phase transition (FOPT), the true and false vacuum bubble radius distributions are not expected to be monochromatic, as is usually assumed. Consequently, Fermi balls (FBs) and primordial black holes (PBHs) produced in a dark FOPT will have extended mass distributions. We show how gravitational wave (GW), microlensing and Hawking evaporation signals for extended bubble radius/mass distributions deviate from the case of monochromatic distributions. The peak of the GW spectrum is shifted to lower frequencies, and the spectrum is broadened at frequencies below the peak frequency. Thus, the radius distribution of true vacuum bubbles introduces another uncertainty in the evaluation of the GW spectrum from a FOPT. The extragalactic gamma-ray signal at AMEGO-X/e-ASTROGAM from PBH evaporation may evince a break in the power-law spectrum between 5 MeV and 10 MeV for an extended PBH mass distribution. Optical microlensing surveys may observe PBH mass distributions with average masses below $10^{-10} M_\odot$, which is not possible for monochromatic mass distributions. This expands the FOPT parameter space that can be explored with microlensing.

hep-ph↗

arXiv2026

Primordial black holes and magnetic fields in conformal neutrino mass models

Sufficiently strong and long-lasting first-order phase transitions can produce primordial black holes (PBHs) that contribute substantially to the dark matter abundance of the Universe, and can produce large-scale primordial magnetic fields. We study these mechanisms in a generic class of conformal $\mathrm{U(1)}^\prime$ models that also explain active neutrino oscillation data via the type-I seesaw mechanism. We find that phase transitions that occur at seesaw scales between $10^4$ GeV and $10^{11}$ GeV produce gravitational wave signals (from the dynamics of the phase transition and from the decay of cosmic string loops) at LISA/ET that can be correlated with microlensing signals of PBHs at the Roman Space Telescope, while scales near $10^{11}$ GeV can be correlated with Hawking evaporation signals at future gamma-ray telescopes. LISA can probe the entire range of PBH masses between $1\times 10^{-16}M_\odot$ and $8\times 10^{-11}M_\odot$ if PBHs fully account for the dark matter abundance. For Z' masses between 5 TeV and 100 TeV, and $\sim 3$ TeV right-handed neutrinos, helical magnetic fields can be produced with magnitudes $\sim 10^{-16}$-$10^{-13}$ G and coherence lengths $\sim 10^{-4}$-$10^{-2}$ Mpc, above current blazar lower bounds.

hep-ph↗

arXiv2026

Angular momentum of vacuum bubbles in a first-order phase transition

The formation of primordial black holes (PBHs) during a first-order phase transition (FOPT) in a dark sector has been of recent interest. A quantity that characterizes a black hole is its spin. We carry out the first step towards determining the spin of such PBHs, by calculating the spin of spherical false vacuum bubbles induced by cosmological perturbations. The angular momentum is given by the product of density and velocity perturbations. We carefully track the evolution of background quantities and calculate the transfer functions during the FOPT. We find that the dimensionless spin parameter $s = J/(G_{\rm N} M^2)$ of false vacuum bubbles of mass $M$ and angular momentum $J$, take a wide range of values from ${\cal{O}}(10^{-5})$ to ${\cal{O}}(10)$ for FOPTs between 10 keV and 100 GeV and a dark sector that is 0.1 to 0.4 times cooler than the visible sector. We also find a scaling relation between the root-mean-square value of the spin, the FOPT time scale, the bubble wall velocity, and the dark sector-to-visible sector temperature ratio.

hep-ph↗

arXiv2026

Massive neutrinos and interacting dark matter look alike through the lens of lensing

We demonstrate that the suppression in the lensing power spectrum of the cosmic microwave background (CMB) caused by massive neutrinos can be mimicked by dark matter-baryon interactions at the precision of next-generation CMB experiments. Thus, a determination of neutrino masses from the CMB lensing power spectrum may be compromised. We illustrate the degeneracy for a dark matter-proton cross section $\propto v^{-4}$, which arises in the $t$-channel exchange of an ultralight mediator in the nonrelativistic limit.

astro-ph.CO↗

arXiv2026

The generic basis and flavour non-universal SMEFT

Whenever an anomaly in the flavour sector appears, analyses are performed examining whether it can be explained by adding a small number of carefully-chosen flavour non-universal four-fermion SMEFT operators. These analyses are typically carried out in the down or the up basis, i.e., it is assumed that the weak and mass eigenstates are the same for the left-handed down-type or up-type quarks. In these bases, there is no dependence on the matrices that transform from the weak to the mass basis, and which are unmeasurable in the Standard Model. In this paper, we argue that it is better to use a generic weak basis, in which no assumptions about the alignment of weak and mass eigenstates are made. The analysis now directly includes elements of the transformation matrices. By doing a fit to the data, it is possible to both determine if the flavour anomaly can be explained and extract the transformation matrices. In principle, this can be extended to a complete reconstruction of the Yukawa matrices.

hep-ph↗

arXiv2025

Boosted dark matter from primordial black holes produced in a first-order phase transition

During a cosmological first-order phase transition in a dark sector, fermion dark matter particles $χ$ can form macroscopic Fermi balls that collapse to primordial black holes (PBHs) under certain conditions. The evaporation of the PBHs produces a boosted $χ$ flux, which may be detectable if $χ$ couples to visible matter. We consider the interaction of $χ$ with electrons, and calculate signals of the dark matter flux in the XENON1T, XENONnT, Super-Kamiokande and Hyper-Kamiokande experiments. A correlated gravitational wave signal from the phase transition can be observed at THEIA and $μ$Ares. An amount of dark radiation measurable by CMB-S4 is an epiphenomenon of the phase transition.

hep-ph↗

arXiv2025

Supercooled phase transitions in conformal dark sectors explain NANOGrav data

According to recent lore, it is difficult to explain the evidence for a stochastic gravitational wave background obtained by pulsar timing arrays with supercooled first-order phase transitions (FOPTs). We demonstrate that supercooled FOPTs in dark U(1)' models with a conformal dark sector easily explain the nHz signal at NANOGrav.

hep-ph↗

arXiv2025

Distinguishing between Dirac and Majorana neutrinos at FASER

Some of the simplest models for the origin of neutrino mass involve right-handed neutrinos (RHNs), which could be either Dirac or Majorana particles - a distinction that has profound implications for lepton number conservation and the fundamental nature of neutrinos. We investigate the potential of the FASER experiment to distinguish between these two possibilities using signatures predicted by the Standard Model Neutrino Effective Field Theory (SMNEFT), where RHNs interact with Standard Model particles through higher-dimensional operators. We focus on RHNs produced via $B$, $D$, $K$, and $π$ meson decays at the Large Hadron Collider and their subsequent three-body decays within the FASER detector. The kinematic and angular distributions of the decay products in the RHN rest frame differ significantly for Dirac and Majorana RHNs, and these differences manifest as distinct spatial distributions of electron-positron pairs at FASER. Using Monte Carlo simulations and a $χ^2$ analysis, we demonstrate that these spatial observables provide a robust experimental probe for determining the Dirac or Majorana nature of RHNs. For select production and decay operator combinations and RHN masses around 0.1 GeV, FASER can achieve discrimination at the $3σ$ level.

hep-ph↗

arXiv2025

Right-handed Dirac and Majorana neutrinos at Belle II

We assess the ability of the Belle II experiment to probe the Dirac or Majorana nature of a massive right-handed neutrino (RHN) $N$ in the MeV to GeV mass range. We consider the production and decay of RHNs to proceed via new interactions described by the standard model effective field theory (SMEFT) extended with right-handed neutrino fields (SMNEFT), and not via mass mixing with active neutrinos. We find that Belle II has the potential to discover $N$ if kinematically accessible. We perform detailed simulations of the angular distributions of lepton pairs from the decay of $N$ produced in two-body and three-body decays of $B$ mesons. We show that for $m_N$ above 100 MeV, Belle II can distinguish between Dirac and Majorana neutrinos at more than the 5$σ$ CL for most operators, and the combination of the production and decay operators can be identified from the subsequent decay of the heavy neutrino. Also, the production operators can be identified using three-body $B$ meson decay for any $m_N$ if the $B\to D\ell N$ and $B\to D^*\ell N$ events can be well separated.

hep-ph↗