11 research outputs found
Centrality dependence of photon yield and elliptic flow from gluon fusion and splitting induced by magnetic fields in relativistic heavy-ion collisions
We compute the photon yield and elliptic flow coefficient in relativistic
heavy-ion collisions from gluon fusion and splitting processes induced by a
magnetic field for different centralities. The calculation accounts for the
intense magnetic field and the high gluon occupation number at early times. The
photon production induced by these process represents an excess contribution
over calculations without magnetic field effects. We compare this excess to the
difference between PHENIX data and recent hydrodynamic calculations for the
photon transverse momentum distribution and elliptic flow coefficient .
The time evolution of the field strength and reaction volume is computed using
UrQMD. We show that with reasonable values for the saturation scale, the
calculation helps to better describe the experimental results obtained at RHIC
energies for the lowest part of the transverse photon momentum at different
centralities.Comment: Expanded discussion. Version to appear in The European Physical
Journal
Prompt photon yield and coefficient from gluon fusion induced by magnetic field in heavy-ion collision
We compute the production of prompt photons and the harmonic
coefficient in relativistic heavy-ion collisions induced by gluon fusion in the
presence of an intense magnetic field, during the early stages of the reaction.
The calculations take into account several parameters which are relevant to the
description of the experimental transverse momentum distribution, and elliptic
flow for RHIC and LHC energies. The main imput is the strength of the magnetic
field which varies in magnitude from 1 to 3 times the pion mass squared, and
allows the gluon fusion that otherwise is forbidden in the absence of the
field. The high gluon occupation number and the value of the saturation scale
also play an important role in our calculation, as well as a flow velocity and
geometrical factors. Our results support the idea that the origin of at least
some of the photon excess observed in heavy-ion experiments may arise from
magnetic field induced processes, and gives a good description of the
experimental data.Comment: 6 pages, 2 figures, conference paper from ISMD 201
Anisotropic photon emission from gluon fusion and splitting in a strong magnetic background I: The two-gluon one-photon vertex
Having in mind the pre-equilibrium stage in peripheral heavy-ion collisions
as a possible scenario for the production of electromagnetic radiation, we
compute the two-gluon one-photon vertex in the presence of an intense magnetic
field at one-loop order. The quarks in the loop are taken such that two of them
occupy the lowest Landau level, with the third one occupying the first exited
Landau level. When the field strength is the largest of the energy (squared)
scales, the tensor basis describing this vertex corresponds to two of the three
vector particles polarized in the longitudinal direction whereas the third one
is polarized in the transverse direction. However, when the photon energy is of
order or larger than the field strength, the explicit one-loop computation
contains extra tensor structures that spoil the properties of the basis,
compared to the case when the field strength is the largest of the energy
scales, which signals that the calculation is incomplete. Nevertheless, by
projecting the result onto the would-be basis, we show that the squared
amplitude for processes involving two gluons and one photon exhibits the
expected properties such as a preferred in-plane photon emission and a slightly
decreasing strength for an increasing magnetic field strength. We comment on
possible venues to improve the one-loop calculation that include accounting for
progressive occupation of the three quarks of the lowest and first excited
Landau levels such that, still working in the large field limit, a more
complete description can be achieved when the photon energy increases.Comment: 12 pages, 4 figures. Part
Relaxation time for the alignment between quark spin and angular velocity in a rotating QCD medium
We compute the relaxation times for massive quarks and anti-quarks to align
their spins with the angular velocity in a rigidly rotating medium at finite
temperature and baryon density. The rotation effects are implemented using a
fermion propagator immersed in a cylindrical rotating environment. The
relaxation time is computed as the inverse of the interaction rate to produce
an asymmetry between the quark (anti-quark) spin components along and opposite
to the angular velocity. For conditions resembling heavy-ion collisions, the
relaxation times for quarks are smaller than for anti-quarks. For semi-central
collisions the relaxation time is within the possible life-time of the QGP for
all collision energies. However, for anti-quarks this happens only for
collision energies GeV. The results are quantified in
terms of the intrinsic quark and anti-quark polarizations, namely, the
probability to build the spin asymmetry as a function of time. Our results show
that these intrinsic polarizations tend to 1 with time at different rates given
by the relaxation times with quarks reaching a sizable asymmetry at a faster
pace. These are key results to further elucidate the mechanisms of hyperon
polarization in relativistic heavy-ion collisions.Comment: 9 pages, 10 figure
Promp photon yield and Ï…2 coefficent from gluon fusion induced by magnetic field in heavy-ion collisions
We compute the production of prompt photons and the Ï…2 harmonic coefficient in relativistic heavy-ion collisions induced by gluon fusion in the presence of an intense magnetic field, during the early stages of the reaction. The calculations take into account several parameters which are relevant to the description of the experimental transverse momentum distribution, and elliptic flow for RHIC and LHC energies. The main imput is the strenght of the magnetic field which varies in magnitude from 1 to 3 times the pion mass squared, and allows the gluon fusion that otherwise is forbidden in the absence of the field. The high gluon occupation number and the value of the saturation scale also play an important role in our calculation, as well as a flow velocity and geometrical factors. Our results support the idea that the origin of at least some of the photon excess observed in heavy-ion experiments may arise from magnetic field induced processes, and gives a good description of the experimental data
Collision energy dependence of the critical end point from baryon number fluctuations in the Linear Sigma Model with quarks
We show that the Linear Sigma Model with quarks produces an effective description of the QCD phase diagram and of the system’s equilibrium distribution properties that deviate from those of the Hadron Resonance Gas Model. The deviation is due to the inclusion of plasma screening properties, encoded in the contribution of the ring diagrams and thus to the introduction of a key feature of plasmas near phase transitions, namely, long-range correlations. After fixing the model parameters using input from LQCD for the crossover transition at vanishing chemical potential, we study the location of the Critical End Point in the effective QCD phase diagram. We use the model to study baryon number fluctuations and show that in heavy-ion collisions, the CEP can be located for collision energies GeV, namely, in the lowest NICA or within the HADES energy domain