15 research outputs found
Thermal Dileptons from Coarse-Grained Transport as Fireball Probes at SIS Energies
Utilizing a coarse-graining method to convert hadronic transport simulations
of Au+Au collisions at SIS energies into local temperature, baryon and pion
densities, we compute the pertinent radiation of thermal dileptons based on an
in-medium spectral function that describes available spectra at
ultrarelativistic collision energies. In particular, we analyze how far the
resulting yields and slopes of the invariant-mass spectra can probe the
lifetime and temperatures of the fireball. We find that dilepton radiation sets
in after the initial overlap phase of the colliding nuclei of about 7 fm/c, and
lasts for about 13 fm/c. This duration closely coincides with the development
of the transverse collectivity of the baryons, thus establishing a direct
correlation between hadronic collective effects and thermal EM radiation, and
supporting a near local equilibration of the system. This fireball "lifetime"
is substantially smaller than the typical 20-30 fm/c that naive considerations
of the density evolution alone would suggest. We furthermore find that the
total dilepton yield radiated into the invariant-mass window of
GeV/, normalized to the number of charged pions, follows a relation to
the lifetime found earlier in the (ultra-) relativistic regime of heavy-ion
collisions, and thus corroborates the versatility of this tool. The spectral
slopes of the invariant-mass spectra above the mass provide a
thermometer of the hottest phases of the collision, and agree well with the
maximal temperatures extracted from the coarse-grained hadron spectra.Comment: 9 pages, 6 figures; v2: extended discussion, matches the version
accepted for publicatio
Polarization of Thermal Dilepton Radiation
The spectra of dileptons radiated from the fireballs formed in high-energy
heavy-ion collisions have been successfully used to investigate key properties
of hot and dense QCD matter. In this paper we study polarization observables
which have thus far received little attention. Microscopic calculations of
in-medium electromagnetic spectral functions have thus far mostly focused on
integrated yields which are proportional to the sum of the longitudinal and
transverse components of the virtual photon's selfenergy. Photon polarization
results from the difference of these components, which in general does not
vanish for lepton pairs at finite three-momentum relative to the heat bath (and
is maximal for fully transverse real photons). Using a model that successfully
describes dilepton spectra in heavy-ion collisions, with hadronic emission via
medium-modified vector mesons and quark-antiquark annihilation constrained by
lattice QCD, we compute polarization observables in different dilepton mass
bins and confront them with data of the HADES and NA60 experiments.Comment: 5 pages, 3 figure
Strong Association of a Common Dihydropyrimidine Dehydrogenase Gene Polymorphism with Fluoropyrimidine-Related Toxicity in Cancer Patients
variations associated with enhanced drug toxicity. = 0.001; the attributable risk was 56.9%. Comparing tumor-type matched sets of samples, correlation of c.496A>G with toxicity was particularly present in patients with gastroesophageal and breast cancer, but did not reach significance in patients with colorectal malignancies. polymorphism strongly contributes to the occurrence of fluoropyrimidine-related drug adverse effects. Carriers of this variant could benefit from individual dose adjustment of the fluoropyrimidine drug or alternate therapies
Measurements of dielectron production in Au+Au collisions at \sqrt {s_{NN}} 27, 54.4 and 200 GeV with the STAR experiment
Global Λ-hyperon polarization in Au+Au collisions at √sNN = 3 GeV
Global hyperon polarization, ¯PH, in Au+Au collisions over a large range of collision energy, √sNN, was recently measured and successfully reproduced by hydrodynamic and transport models with intense fluid vorticity of the quark-gluon plasma. While naïve extrapolation of data trends suggests a large ¯PH as the collision energy is reduced, the behavior of ¯PH at small √sNN<7.7 GeV is unknown. Operating the STAR experiment in fixed-target mode, we measured the polarization of Λ hyperons along the direction of global angular momentum in Au+Au collisions at √sNN=3 GeV. The observation of substantial polarization of 4.91±0.81(stat.)±0.15(syst.)% in these collisions may require a reexamination of the viscosity of any fluid created in the collision, of the thermalization timescale of rotational modes, and of hadronic mechanisms to produce global polarization
Measurements of Proton High Order Cumulants in √sNN = 3 GeV Au+Au Collisions and Implications for the QCD Critical Point
We report cumulants of the proton multiplicity distribution from dedicated fixed-target Au+Au collisions at √sNN=3.0 GeV, measured by the STAR experiment in the kinematic acceptance of rapidity (y) and transverse momentum (pT) within −0.5<y<0 and 0.4<pT<2.0 GeV/c. In the most central 0%–5% collisions, a proton cumulant ratio is measured to be C4/C2=−0.85±0.09 (stat)±0.82 (syst), which is 2σ below the Poisson baseline with respect to both the statistical and systematic uncertainties. The hadronic transport UrQMD model reproduces our C4/C2 in the measured acceptance. Compared to higher energy results and the transport model calculations, the suppression in C4/C2 is consistent with fluctuations driven by baryon number conservation and indicates an energy regime dominated by hadronic interactions. These data imply that the QCD critical region, if created in heavy-ion collisions, could only exist at energies higher than 3 GeV
Dilepton signature of a first-order phase transition
The search for a first-order phase transition in strongly interacting matter is one of the major objectives in the exploration of the phase diagram of quantum chromodynamics (QCD). In the present work we investigate dilepton radiation from the hot and dense fireballs created in Au-Au collisions at projectile energies of 1–2 A GeV for potential signatures of a first-order transition. Toward this end, we employ a hydrodynamic simulation with two different equations of state, with and without a phase transition. The latter is constrained by susceptibilities at vanishing chemical potential from lattice-QCD as well as neutron star properties, while the former is implemented via modification of the mean fields in the quark phase. We find that the latent heat involved in the first-order transition leads to a substantial increase in the low-mass thermal emission signal by about a factor of two above the crossover scenario