Elliptic Flow Of Thermal Photons Research Articles

Photon production from Pb + Pb collisions at sNN=5.02 TeV at the CERN Large Hadron Collider and at sNN=39 TeV at the proposed Future Circular Collider facility

We calculate the production of prompt and thermal photons from Pb+Pb collisions at 5.02A TeV at the Large Hadron Collider (LHC) and at 39A TeV at the proposed Future Circular Collider (FCC) facility. The photon spectra and anisotropic flow at these energies are compared with the results obtained from 2.76A TeV Pb+Pb collisions at the LHC for three different centrality bins. The prompt photons originating from initial hard scatterings are found to increase by a factor of 1.5 to 2 at 5.02A TeV in the $p_T$ region 2 to 15 GeV and the enhancement is found to be about 5 to 15 times at FCC energy compared to 2.76A TeV in the same $p_T$ region. The evolution of the Quark-Gluon Plasma (QGP) formed in Pb+Pb collisions at LHC and FCC energies are studied using a hydrodynamical model and the $p_T$ spectra and elliptic flow of thermal photons are calculated using state-of-the-art photon rates. The relative enhancement in the production of thermal photons is found to be more compared to prompt photons at FCC than at the LHC energies. Although the production of direct (prompt+thermal) photons is found to enhance significantly with increase in beam energy, the photon elliptic flow increases only marginally and does not show strong sensitivity to the collision energy.

Effects of initial-state nucleon shadowing on the elliptic flow of thermal photons

Recently the effect of nucleon shadowing on the Monte-Carlo Glauber initial condition was studied and its role on the centrality dependence of elliptic flow ($v_2$) and fluctuations in initial eccentricity for different colliding nuclei were explored. It was found that the results with shadowing effects are closer to the QCD based dynamical model as well as to the experimental data. Inspired by this outcome, in this work we study the transverse momentum ($p_T$) spectra and elliptic flow of thermal photons for Au+Au collisions at RHIC and Pb+Pb collisions at LHC by incorporating the shadowing effects in deducing the initial energy density profile required to solve the relativistic hydrodynamical equations. We find that the thermal photon spectra remain almost unaltered, however, the elliptic flow of photon is found to be enhanced significantly due to shadowing effects.

Spectra and elliptic flow of thermal photons from full-overlap U+U collisions at energies available at the BNL Relativistic Heavy Ion Collider

We calculate ${p}_{T}$ spectra and elliptic flow for tip-tip and body-body configurations of full-overlap uranium-uranium $(\text{U}+\text{U})$ collisions by using a hydrodynamic model with smooth initial density distribution and compare the results with those obtained from $\text{Au}+\text{Au}$ collisions at the BNL Relativistic Heavy Ion Collider (RHIC). Production of thermal photons is seen to be significantly larger for tip-tip collisions compared with body-body collisions of uranium nuclei in the region ${p}_{T}>1$ GeV. The difference in the results for the two configurations of $\text{U}+\text{U}$ collisions depends on the initial energy deposition which is yet to be constrained precisely from hadronic measurements. The thermal photon spectrum from body-body collisions is found to be close to the spectrum from most-central $\text{Au}+\text{Au}$ collisions at RHIC. The elliptic-flow parameter calculated for body-body collisions is found to be large and comparable to the ${v}_{2}({p}_{T})$ for mid-central collisions of Au nuclei. On the other hand, as expected, ${v}_{2}({p}_{T})$ is close to zero for tip-tip collisions. The qualitative nature of the photon spectra and elliptic flow for the two different orientations of uranium nuclei is found to be independent of the initial parameters of the model calculation. We show that the photon results from fully overlapping $\text{U}+\text{U}$ collisions are complementary to the results from $\text{Au}+\text{Au}$ collisions at RHIC.

Pseudo-critical enhancement of thermal photons in relativistic heavy-ion collisions?

We compute the spectra and elliptic flow of thermal photons emitted in ultrarelativistic heavy-ion collisions (URHICs) at RHIC and LHC. The thermal emission rates are taken from complete leading-order rates for the QGP and hadronic many-body calculations including baryons and antibaryons, as well as meson-exchange reactions (including Bremsstrahlung). We first update previous thermal fireball calculations by implementing a lattice-QCD based equation of state and extend them to compare to recent LHC data. We then scrutinize the space–time evolution of Au–Au collisions at RHIC by employing an ideal hydrodynamic model constrained by bulk- and multistrange-hadron spectra and elliptic flow, including a non-vanishing initial flow. We systematically compare the evolutions of temperature, radial flow, azimuthal anisotropy and four-volume, and exhibit the temperature profile of thermal photon radiation. Based on these insights, we put forward a scenario with a “pseudo-critical enhancement” of thermal emission rates, and investigate its impact on RHIC and LHC direct photon data.

Elliptic flow of thermal photons from an event-by-event hydrodynamic model

Elliptic flow of direct photons in relativistic heavy ion collisions is believed to be dominated by contribution from thermal radiation of quark gluon plasma up to $p_T$ $\sim 5$ GeV/$c$, although other sources start outshining the thermal contribution at already smaller values of $p_T$ in the direct photon spectrum. The elliptic flow of thermal photons from ideal hydrodynamics considering a smooth initial density distribution under-predicts the PHENIX direct photon data from 200A GeV Au+Au collisions at RHIC by a large margin in the range $1 < p_T < 5$ GeV/$c$. However, a significant enhancement of thermal photon production due to fluctuations in the initial QCD matter density distributions is expected. We show that such fluctuations result in substantially larger photon elliptic flow for $p_T > 2.5 $ GeV/$c$ compared to a smooth initial-state-averaged density profile. The results from event-by-event hydrodynamics are found to be sensitive to the fluctuation size parameter. However, the effects of initial state fluctuations are insufficient to account for the discrepancy to the PHENIX data for direct photon elliptic flow. Furthermore, the photon $v_2$ is reduced even more when we include the NLO pQCD prompt photon component. We also calculate the spectra and elliptic flow of thermal photons for 2.76A TeV Pb+Pb collisions at LHC and for the 0--40% centrality bin. Thermal photons from event-by-event hydrodynamics along with prompt photons from NLO pQCD calculations explain the ALICE preliminary direct photon data well in the region $p_T \ge 2.5$ GeV/c. Similar to RHIC, the elliptic flow results at LHC are again found to be much smaller than the ALICE preliminary $v_2$ data.

Influence of initial state fluctuations on the production of thermal photons

Inhomogeneities in the initial QCD matter density distribution increase the production of thermal photons significantly compared to a smooth initial-state-averaged profile in the region $p_T > 1$ GeV/$c$ in an ideal hydrodynamic calculation. This relative enhancement is more pronounced for peripheral collisions, for smaller size systems as well as for lower beam energies. A suitably normalized ratio of central-to-peripheral yield of thermal photons reduce the uncertainties in the hydrodynamical initial conditions and can be a useful parameter to study the density fluctuations and their size. The fluctuations in the initial density distribution also lead to a larger elliptic flow of thermal photons for $p_T >$ 2.0 GeV/$c$ compared to the flow from a smooth profile.

Elliptic flow of thermal photons in heavy-ion collisions at energies available at the BNL Relativistic Heavy Ion Collider and at the CERN Large Hadron Collider

We calculate the thermal photon transverse momentum spectra and elliptic flow in $\sqrt{s_{NN}} = 200$ GeV Au+Au collisions at RHIC and in $\sqrt{s_{NN}} = 2.76$ TeV Pb+Pb collisions at the LHC, using an ideal-hydrodynamical framework which is constrained by the measured hadron spectra at RHIC and LHC. The sensitivity of the results to the QCD-matter equation of state and to the photon emission rates is studied, and the photon $v_2$ is discussed in the light of the photonic $p_T$ spectrum measured by the PHENIX Collaboration. In particular, we make a prediction for the thermal photon $p_T$ spectra and elliptic flow for the current LHC Pb+Pb collisions.

A Reanalysis of Single Photon Data at CERN SPS

We reanalyze the WA98 single photon data [M. M. Aggarwal et al. [WA98 Collaboration], Phys. Rev. Lett. 85, 3595 (2000)] at CERN SPS by incorporating several recent developments in the study of prompt and thermal photon production from relativistic heavy ion collisions [R. Chatterjee, D. K. Srivastava, and S. Jeon, Phys. Rev. C 79, 034906 (2009)]. Isospin and shadowing corrected NLO pQCD, along with an optimized scale for factorization, fragmentation and renormalization are considered for prompt photon production. Photons from thermal medium are estimated by considering a boost invariant azimuthally anisotropic hydrodynamic expansion of the plasma along with a well tested equation of state and initial conditions. A quantitative explanation of the data is obtained by combining κ× prompt with thermal photons, where κ is an overall scaling factor. We show that, elliptic flow of thermal photons can play a crucial role to distinguish between the ‘with’ and ‘without’ phase transition scenarios at SPS energy.

Elliptic flow of thermal photons at midrapidity in Au+Au collisions at [formula omitted

The elliptic flow v 2 of thermal photons at midrapidity in Au+Au collisions at S N N = 200 GeV is predicted, based on three-dimensional ideal hydrodynamics. Because of the interplay between the asymmetry and the strength of the transverse flow, the thermal photon v 2 reaches a maximum at p T ∼ 2 GeV/ c and the p T -integrated v 2 reaches a maximum at about 50% centrality. The p T -integrated v 2 is very sensitive to the lower limit of the integral but not sensitive to the upper limit due to the rapid decrease in the spectrum of the transverse momentum.

Elliptic flow of thermal photons inAu+Aucollisions atsNN=200GeV

The transverse momentum $({p}_{t})$ dependence, the centrality dependence, and the rapidity dependence of the elliptic flow of thermal photons in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{\mathit{NN}}}=200$ GeV are predicted on the basis of a three-dimensional ideal hydrodynamic description of the hot and dense matter. The elliptic flow parameter ${v}_{2}$, i.e., the second Fourier coefficient of azimuthal distribution, of thermal photons first increases with ${p}_{t}$ and then decreases for ${p}_{t}&gt;2$ GeV/$c$, because of the weak transverse flow at the early stage. The ${p}_{t}$-integrated ${v}_{2}$ first increases with centrality, reaches a maximum at about 50% centrality, and then decreases. The rapidity dependence of the elliptic flow ${v}_{2}(y)$ of direct photons (mainly thermal photons) is very sensitive to the initial energy density distribution along the longitudinal direction, which provides a useful tool to extract the realistic initial condition from measurements.

Elliptic flow of thermal photons and formation time of quark gluon plasma at energies available at the BNL Relativistic Heavy Ion Collider (RHIC)

We calculate the elliptic flow of thermal photons from $\mathrm{Au}+\mathrm{Au}$ collisions at RHIC energies for a range of values for the formation time ${\ensuremath{\tau}}_{0}$ but a fixed entropy (or particle rapidity density). The results are found to be quite sensitive to ${\ensuremath{\tau}}_{0}$. The value of ${v}_{2}$ for photons decreases as ${\ensuremath{\tau}}_{0}$ decreases and admits a larger contribution from the quark gluon plasma phase, which has a smaller ${v}_{2}$. The elliptic flow coefficient for hadrons, however, is only marginally dependent on ${\ensuremath{\tau}}_{0}$.

Elliptic Flow of Thermal Photons/Dileptons

In this talk we describe the recently discovered rich phenomenology of elliptic flow of electromagnetic probes of the hot matter created in relativistic heavy-ion collisions. Using a hydrodynamic model for the space-time dynamics of the collision fireball created in Au+Au collisions at RHIC, we compute the transverse momentum spectra and elliptic flow of thermal photons and dileptons. These observables are shown to provide differential windows into various stages of the fireball expansion.

Elliptic Flow of Thermal Photons in Relativistic Nuclear Collisions

We predict the transverse momentum (p(T)) dependence of elliptic flow of thermal photons for Au + Au collisions at the BNL Relativistic Heavy Ion Collider. We model the system hydrodynamically, with a thermalized quark-gluon plasma at early times followed by hadronization and decoupling. Photons are emitted throughout the expansion history. Contrary to hadron elliptic flow, which increases monotonically with p(T), the elliptic flow nu2(p(T)) of thermal photons is predicted to first rise and then fall again. Photon elliptic flow at high p(T) reflects the quark momentum anisotropy at early times when it is small, while at low p(T) it mirrors the large pion momentum anisotropy during the late hadronic emission stage. An interesting structure is predicted at intermediate p(T) approximately 0.4 GeV/c, where photon elliptic flow reflects the momenta and the (compared to pions) reduced nu2 of heavy vector mesons in the late hadronic phase.