arXiv subjects

Alejandro Ayala

Publications and source records attributed to Alejandro Ayala.

At least 127 records · Page 7Linked to original sources

arXiv2008

Feynman parametrization and Mellin summation at finite temperature

We show that the Mellin summation technique (MST) is a well defined and useful tool to compute loop integrals at finite temperature in the imaginary-time formulation of thermal field theory, especially when interested in the infrared limit of such integrals. The method makes use of the Feynman parametrization which has been claimed to have problems when the analytical continuation from discrete to arbitrary complex values of the Matsubara frequency is performed. We show that without the use of the MST, such problems are not intrinsic to the Feynman parametrization but instead, they arise as a result of (a) not implementing the periodicity brought about by the possible values taken by the discrete Matsubara frequencies before the analytical continuation is made and (b) to the changing of the original domain of the Feynman parameter integration, which seemingly simplifies the expression but in practice introduces a spurious endpoint singularity. Using the MST, there are no problems related to the implementation of the periodicity but instead, care has to be taken when the sum of denominators of the original amplitude vanishes. We apply the method to the computation of loop integrals appearing when the effects of external weak magnetic fields on the propagation of scalar particles is considered.

hep-ph↗

arXiv2008

Minimal Standard Model self-energies at finite temperature in the presence of weak magnetic fields: towards a full symmetry restoration study

The study of the universe's primordial plasma at high temperature plays an important role when tackling different questions in cosmology, such as the origin of the matter-antimatter asymmetry. In the Minimal Standard Model (MSM) neither the amount of CP violation nor the strength of the phase transition are enough to produce and preserve baryon number during the Electroweak Phase Transition (EWPT), which are two of the three ingredients needed to develop baryon asymmetry. In this talk we present the first part of the analysis done within a scenario where it is viable to have improvements to the aforementioned situation: we work with the degrees of freedom in the broken symmetry phase of the MSM and analyze the development of the EWPT in the presence of a weak magnetic field. More specifically, we calculate the particle self-energies that include the effects of the weak magnetic field, needed for the MSM effective potential up to ring diagrams.

hep-ph↗

arXiv2008

Dynamical mass generation in QED with magnetic fields: arbitrary field strength and coupling constant

We study the dynamical generation of masses for fundamental fermions in quenched quantum electrodynamics, in the presence of magnetic fields of arbitrary strength, by solving the Schwinger-Dyson equation (SDE) for the fermion self-energy in the rainbow approximation. We employ the Ritus eigenfunction formalism which provides a neat solution to the technical problem of summing over all Landau levels. It is well known that magnetic fields catalyze the generation of fermion mass m for arbitrarily small values of electromagnetic coupling α. For intense fields it is also well known that m \propto \sqrt eB. Our approach allows us to span all regimes of parameters αand eB. We find that m \propto \sqrt eB provided αis small. However, when αincreases beyond the critical value α_c which marks the onslaught of dynamical fermion masses in vacuum, we find m \propto Λ, the cut-off required to regularize the ultraviolet divergences. Our method permits us to verify the results available in literature for the limiting cases of eB and α. We also point out the relevance of our work for possible physical applications.

hep-ph↗

arXiv2008

Collisional parton energy loss in a finite size QCD medium revisited: Off mass-shell effects

We study the collisional energy loss mechanism for particles produced off mass-shell in a finite size QCD medium. The off mass-shell effects introduced are to consider particles produced in wave packets instead of plane waves and the length scale associated to an in-medium particles' life-time. We show that these effects reduce the energy loss as compared to the case when the particles are described as freely propagating from the source. The reduction of the energy loss is stronger as this scale becomes of the order or smaller than the medium size. We discuss possible consequences of the result on the description of the energy loss process in the parton recombination scenario.

hep-ph↗

arXiv2008

Proton to pion ratio at RHIC from dynamical quark recombination

We propose an scenario to study, from a dynamical point of view, the thermal recombination of quarks in the midsts of a relativistic heavy-ion collision. We coin the term dynamical quark recombination to refer to the process of quark-antiquark and three-quark clustering, to form mesons and baryons, respectively, as a function of energy density. Using the string-flip model we show that the probabilities to form such clusters differ. We apply these ideas to the calculation of the proton and pion spectra in a Bjorken-like scenario that incorporates the evolution of these probabilities with proper time and compute the proton to pion ratio, comparing to recent RHIC data at the highest energy. We show that for a standard choice of parameters, this ratio reaches one, though the maximum is very sensitive to the initial evolution proper time.

hep-ph↗

arXiv2008

Dynamical quark recombination in ultrarelativistic heavy-ion collisions and the proton to pion ratio

We study quark thermal recombination as a function of energy density during the evolution of a heavy-ion collision in a numerical model that reproduces aspects of QCD phenomenology. We show that starting with a set of free quarks (or quarks and antiquarks) the probability to form colorless clusters of three quarks differs from that to form colorless clusters of quark-antiquark and that the former has a sharp jump at a critical energy density whereas the latter transits smoothly from the low to the high energy density domains. We interpret this as a quantitative difference in the production of baryons and mesons with energy density. We use this approach to compute the proton and pion spectra in a Bjorken scenario that incorporates the evolution of these probabilities with energy density, and therefore with proper time. From the spectra, we compute the proton to pion ratio and compare to data at the highest RHIC energies. We show that for a standard choice of parameters, this ratio reaches one, though the maximum is very sensitive to the initial evolution proper time.

hep-ph↗

arXiv2006

Effective potential at finite temperature in a constant hypermagnetic field: Ring diagrams in the Standard Model

We study the symmetry breaking phenomenon in the standard model during the electroweak phase transition in the presence of a constant hypermagnetic field. We compute the finite temperature effective potential up to the contribution of ring diagrams in the weak field, high temperature limit and show that under these conditions, the phase transition becomes stronger first order.

hep-th↗

arXiv2006

Proton and pion transverse spectra at RHIC from radial flow and finite size effects

We show that the proton and pion transverse momentum distributions measured at RHIC for all collision centralities for pions and most of the collision centralities for protons, can be simultaneously described in terms of a thermal model with common values for the radial flow and temperature, when accounting for the finite size of the interaction region at the time of decoupling. We show that this description is obtained in terms of a simple scaling law of the size of the interaction region with the number of participants in the collision. The behavior of the proton to pion ratio at mid-rapidity can also be understood as a consequence of the strength of the radial flow and system size reached at RHIC energies.

nucl-th↗

arXiv2006

Dynamical mass generation in strongly coupled Quantum Electrodynamics with weak magnetic fields

We study the dynamical generation of masses for fundamental fermions in quenched quantum electrodynamics in the presence of weak magnetic fields using Schwinger-Dyson equations. Contrary to the case where the magnetic field is strong, in the weak field limit the coupling should exceed certain critical value in order for the generation of masses to take place, just as in the case where no magnetic field is present. The weak field limit is defined as eB << m(0)^2, where m(0) is the value of the dynamically generated mass in the absence of the field. We carry out a numerical analysis to study the magnetic field dependence of the mass function above critical coupling and show that in this regime the dynamically generated mass and the chiral condensate for the lowest Landau level increase proportionally to (eB)^2.

hep-ph↗

arXiv2006

Finite size effects on pion spectra in relativistic heavy-ion collisions

We compute the pion inclusive transverse momentum distribution assuming thermal equilibrium together with transverse flow and accounting for finite size effects and energy loss at the time of decoupling. We compare to data on mid-rapidity pions produced in central collisions in RHIC at sqrt{s_{NN}}=200 GeV. We find that a finite size for the system of emitting particles results in a power-like fall-off of the spectra that follows the data up to larger p_t values, as compared to a simple thermal model.

hep-ph↗

arXiv2005

Exact solution for charged-particle propagation during a first-order electroweak phase transition with hypermagnetic fields

We obtain the exact solution of the Klein-Gordon equation describing the propagation of a particle in two regions of different constant magnetic field, separated by an infinite plane wall. The continuity of the wave function and of its derivative at the interface is satisfied when including evanescent-wave terms. We analyze solutions on truncated spaces and compare them with previously obtained approximate solutions. The findings of this work have applications in the problem of the propagation of particles in the presence of a bubble wall in the midsts of an electroweak phase transition, where the two regions separated by the wall are influenced by different (hyper)magnetic field strengths.

hep-ph↗

arXiv2005

Effective potential at finite temperature in a constant magnetic field I: Ring diagrams in a scalar theory

We study symmetry restoration at finite temperature in the theory of a charged scalar field interacting with a constant, external magnetic field. We compute the finite temperature effective potential including the contribution from ring diagrams. We show that in the weak field case, the presence of the field produces a stronger first order phase transition and that the temperature for the onset of the transition is lower, as compared to the case without magnetic field.

hep-ph↗

arXiv2004

rho^0 mass in a hot hadron gas

We study the behavior of the rho^0 vector mass during the hadronic phase of an ultra-relativistic heavy-ion collision at finite temperature. We show that scattering with the most abundant particles during this stage, namely, pions, kaons and nucleons, leads to an overall temperature dependent, decrease of the rho^0 intrinsic mass at rest, compared to its value in vacuum. The main contributions arise from s-channel scattering with pions through the formation of a_1 resonances as well as with nucleons through the formation of even parity, spin 3/2 [N(1720)] and 5/2 [$Δ$(1905)] nucleon resonances. We show that it is possible to achieve a shift in the intrinsic rho^0 mass of order - 40 MeV, when including the contributions of all the relevant mesons and baryons that take part in the scattering, for temperatures between chemical and kinetic freeze-out.

hep-ph↗

arXiv2004

Mass of the rho^0 meson in ultra-relativistic heavy-ion collision

We study the behavior of the rho vector mass in the context of the almost baryon-free environment of an ultra-relativistic heavy-ion collision. We show that rho scattering within the hadronic phase of the collision leads to a temperature dependent, decrease of its intrinsic mass at rest, compared to the value in vacuum. The main contributions arise from s-channel scattering with pions through the formation of a1 resonances as well as with nucleons through the formation of even parity, spin 3/2 [N(1720)] and 5/2 [Delta(1905)] nucleon resonances. We show that it is possible to achieve a shift in the intrinsic rho^0 mass of order - 40 MeV, when including the contributions of all the relevant mesons and baryons that take part in the scattering, for temperatures between chemical and kinetic freeze-out.

hep-ph↗

arXiv2004

Chiral fermion mass and dispersion relations at finite temperature in the presence of hypermagnetic fields

We study the modifications to the real part of the thermal self-energy for chiral fermions in the presence of a constant external hypermagnetic field. We compute the dispersion relation for fermions occupying a given Landau level to first order in g'^2, g^2 and g_phi^2 and to all orders in g'B, where g' and g are the U(1)_Y and SU(2)_L couplings of the standard model, respectively, g_phi is the fermion Yukawa coupling, and B is the hypermagnetic field strength. We show that in the limit where the temperature is large compared to sqrt{g'B}, left- and right-handed modes acquire finite and different B-dependent masses due to the chiral nature of their coupling with the external field. Given the current bounds on the strength of primordial magnetic fields, we argue that the above is the relevant scenario to study the effects of magnetic fields on the propagation of fermions prior and during the electroweak phase transition.

hep-ph↗

arXiv2003

rho propagation and dilepton production at finite pion density and temperature

We study the propagation properties of the rho vector in a dense and hot pion medium. We introduce a finite value of the chemical potential associated to a conserved pion number and argue that such description is valid during the hadronic phase of a relativistic heavy-ion collision, between chemical and thermal freeze-out, where the strong interaction drives pion number to a fixed value. By invoking vector dominance and rho saturation, we also study the finite pion density effects into the low mass dilepton production rate. We find that the distribution moderately widens and the position of the peak shifts toward larger values of the pair invariant mass, at the same time that the height of the peak decreases when the value of the chemical potential grows. We conclude by arguing that for the description of the dilepton spectra at ultra-relativistic energies, such as those of RHIC and LHC, the proper treatment of the large pion density might be a more important effect to consider than the influence of a finite baryon density.

hep-ph↗

arXiv2003

In Medium modifications to rho from finite pion density effects and dilepton spectrum

The behavior of the pion dispersion relation in a pion medium is strongly modified by the introduction of a finite chemical potential associated to the finite pion number density. Such behavior is particularly important during the hadronic phase of a relativistic heavy-ion collision, between chemical and thermal freeze-out, where the pion number changing processes, driven by the strong interaction, can be considered to be frozen. We make use of an effective Lagrangian that explicitly respects chiral symmetry through the enforcement of the chiral Ward identities. The pion dispersion relation is computed through the computation of the pion self-energy in a non-perturbative fashion by giving an approximate solution to the Schwinger-Dyson equation for this self-energy. Given the strong coupling between rho vectors and pions, we argue that the modification of the pion mass due to finite pion density effects has to be taken into account self-consistently for the description of the in-medium modifications of rho's. We finally study some possible consequences of finite pion density effects for the low-mass dilepton spectrum produced in relativistic heavy-ion collisions.

hep-ph↗

arXiv2003

Dynamical mass generation for fermions in quenched Quantum Electrodynamics at finite temperature

We study the dynamical generation of masses for fundamental fermions in quenched quantum electrodynamics (qQED) at finite temperature in the bare vertex approximation, using Schwinger-Dyson equations (SDE). Motivated by perturbation theory, a further simplification is introduced by taking the wave function renormalization to be unity. In the zeroth mode approximation, the SDE for the fermion propagator resembles QED in 2+1 dimensions (QED3) at zero temperature with an effective dimensionful coupling alpha'=alpha T. For a fixed temperature, mass is dynamically generated above a certain critical value of this coupling. As expected, raising the temperature restores chiral symmetry and fermions become massless again. We also argue that by summing over the frequency modes and under suitable simplifications, qualitative aspects of the result do not undergo significant changes.

hep-ph↗