arXiv subjects

Sebastián Mendizabal

Publications and source records attributed to Sebastián Mendizabal.

4 recordsLinked to original sources

arXiv2026

Nonequilibrium coherent effects at finite chemical potential

We study a nonequilibrium coherent effect generated by a finite chemical potential in a complex scalar field with a conserved $U(1)$ charge. The scalar excitation is treated as a probe coupled to an equilibrium thermal reservoir, so the self-energy is an equilibrium kernel and there is no backreaction on the bath. Solving the Schwinger-Keldysh-Kadanoff-Baym equations in the normal phase, when the chemical potential is smaller than the dispersion relation, we keep the particle and antiparticle quasiparticle poles separate. The source-driven inhomogeneous statistical propagator is fixed by the reservoir and relaxes to the usual decoherent equilibrium form. By contrast, the homogeneous solution carries initial-condition memory; finite chemical potential turns this memory into a transient particle-antiparticle interference pattern by splitting the two charge-sector phases. The effect is not a new equilibrium mode, but a phase-sensitive remnant of the initial data that is erased by damping as $t\to\infty$. We define a normalized interference contrast extracted from the mixed charge-sector terms, illustrate the relaxation using the plasmon damping rate of hot scalar $ϕ^4$ theory, and show that the same normal-phase solution displays the infrared enhancement that precedes Bose-Einstein condensation.

hep-ph↗

arXiv2026

Which Neutron Stars Reach the Stiffening Regime? Multimessenger Constraints on Core Sound Speed and Stellar-Mass Thresholds

We present a concise multimessenger inference of the neutron-star core sound-speed profile using GW170817 and three \textit{NICER} mass--radius posteriors (PSR J0030$+$0451, PSR J0740$+$6620, and PSR J0437$-$4715). The main result is not only a preference for intermediate-density stiffening within smooth equation-of-state families, but a translation of that inference into the stellar masses that access the relevant density regime. In the baseline smooth peaked family, the posterior probability that $c_s^2 > 1/3$ at $3.5\,n_{\rm sat}$ is $85.4\,\%$, while equal-prior averaging over peaked, monotonic, piecewise, and transition-capable families gives a more conservative $79.0\,\%$. Posterior-resampled exact Tolman--Oppenheimer--Volkoff solutions show that the onset density of the inferred stiffening is typically reached near $1.6\,M_\odot$, whereas the peak region is accessed only near $2.1\,M_\odot$. A J0740-like $2.07\,M_\odot$ pulsar reaches the onset in $91\,\%$ of posterior draws but the peak in only $46\,\%$, showing that current data mainly constrain whether massive stars have entered the stiffening regime rather than traversed its full peak.

astro-ph.HE↗

arXiv2025

Refining the Greisen Profile for Low-Energy Cosmic Gamma-Rays: Quantifying Deviations Across Altitudes and Zenith Angles

This study refines the Greisen formalism by comparing the classical Greisen profile and a modified Greisen profile, which incorporates an empirical correction to the shower age parameter with zenith-angle dependence, aiming to better describe low-energy cascades against CORSIKA simulations of cosmic gamma-ray showers (20-800 GeV). Fittings across altitudes of 5000-5900 m and zenith angles from 0 to 40 degrees quantify deviations in particle numbers, showing that the modified profile yields deviations below 4.7%, compared to up to 12.5% for the classical profile. These improvements address low-energy ionization losses, atmospheric density variations, and zenith-angle effects, enhancing accuracy for high-altitude observatories like HAWC and the proposed CONDOR array. The modified profile offers a computationally efficient alternative, providing precise particle number predictions to advance gamma-ray astrophysics and cosmic-ray research.

astro-ph.HE↗

arXiv2025

Revisiting the Longitudinal Development of Electromagnetic Air Showers: Analytical Improvements to the Greisen Formalism with Zenith Angle Dependence

We present a new analytical approach to the longitudinal development of electromagnetic air showers, offering improvements to the classical Greisen formalism. We introduce a novel profile for the slope function $λ_1(s)$ that achieves an agreement less than $0.75\%$ with the original $λ_1$ for shower age parameter $s$ between $0.3 < s < 1.4$, where $s$ represents the stage of shower development. Our new formalism provides an improved representation of shower evolution, particularly near and beyond the shower maximum. In addition, we derive a complete expression for the number of particles $N(t)$. Our implementation includes the zenithal angle dependence on the number of particles at the detector level at high altitudes, making it particularly useful for high-altitude observatories. This expression is suitable for implementing air shower simulation tool fitting procedures over a wide range of energies and geometries. Our analysis suggests that the proposed new formalism may provide better agreement with the expected evolution of particle numbers compared to the traditional Greisen formulation.

astro-ph.HE↗