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Register a new userPHENIX measurements of low momentum direct photon radiation from large and small systems in (ultra)relativistic heavy ion collisions: direct photon scaling
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Authors
Vladimir Khachatryan
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The PHENIX collaboration has measured low momentum direct photon radiation in Au+Au collisions at 200 GeV, 62.4 GeV and 39 GeV, in Cu+Cu at 200 GeV as well as in p+p, p+Au and d+Au at $sqrt{s_{NN}} =$ 200 GeV. In these measurements PHENIX has discovered a large excess over the scaled p+p yield of direct photons in A+A collisions, and a non-zero excess, observed within systematic uncertainties, over the scaled p+p yield in central p+A collisions. Another finding is that at low-$p_{T}$ the integrated yield of direct photons, $dN_{gamma}/dy$, from large systems shows a behavior of universal scaling as a function of the charged-particle multiplicity, $(dN_{ch}/deta)^{alpha}$, with $alpha = 1.25$, which means that the photon production yield increases faster than the charged-particle multiplicity.
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The versatility of RHIC allowed the PHENIX collaboration to measure low momentum direct photons from small systems, such as p+p, p+A, d+Au at $sqrt{s_{NN}} = $200 GeV as well as from large A+A systems, such as Au+Au and Cu+Cu at 200 GeV and Au+Au at 62.4 GeV and 39 GeV. In these measurements PHENIX has discovered a large excess over the scaled p+p yield of direct photons in A+A collisions, and a non-zero excess over the scaled p+p yield in central p+A collisions. Another PHENIX discovery is that at low-$p_{T}$ the integrated yield of direct photons, $dN_{gamma}/dy$, from large systems follows a universal scaling as a function of the charged-particle multiplicity, $(dN_{ch}/deta)^{alpha}$, with $alpha = 1.25$. The observed scaling properties of direct photons from these systems show that the photon production yield increases faster than the charged-particle multiplicity.
A recent analysis from the PHENIX collaboration of available direct photon measurement results in collisions of various systems such as Au+Au, Cu+Cu, and Pb+Pb, at different beam energies ranging from 39 to 2760 GeV, has shown a universal, within experimental uncertainties, $multiplicity$ scaling, in which direct photon $p_{T}$-spectra for transverse momenta up to 2 GeV/$c$ are scaled with charged hadron pseudorapidity density at midrapidity raised to power $alpha=1.25$. On the other hand, those direct photon $p_{T}$-spectra also exhibit $geometrical$ scaling in the similar $p_{T}$ range. Assuming power-law dependence of the scaled photon spectra for both scaling laws, we formulate two independent conditions for the power $alpha$, which overshoot experimental data by $sim 10%$ on average. We discuss possible sources that might improve this estimate.
The PHENIX collaboration presents first measurements of low-momentum ($0.4<p_T<3$ GeV/$c$) direct-photon yields from Au$+$Au collisions at $sqrt{s_{_{NN}}}$=39 and 62.4 GeV. For both beam energies the direct-photon yields are substantially enhanced with respect to expectations from prompt processes, similar to the yields observed in Au$+$Au collisions at $sqrt{s_{_{NN}}}$=200. Analyzing the photon yield as a function of the experimental observable $dN_{rm ch}/deta$ reveals that the low-momentum ($>$1,GeV/$c$) direct-photon yield $dN_{gamma}^{rm dir}/deta$ is a smooth function of $dN_{rm ch}/deta$ and can be well described as proportional to $(dN_{rm ch}/deta)^alpha$ with $alpha{approx}1.25$. This scaling behavior holds for a wide range of beam energies at the Relativistic Heavy Ion Collider and the Large Hadron Collider, for centrality selected samples, as well as for different, $A$$+$$A$ collision systems. At a given beam energy the scaling also holds for high $p_T$ ($>5$,GeV/$c$) but when results from different collision energies are compared, an additional $sqrt{s_{_{NN}}}$-dependent multiplicative factor is needed to describe the integrated-direct-photon yield.
We investigate the behavior of low energy photons radiated by the deceleration processes of two colliding nuclei in relativistic heavy ion collisions using the Wigner function approach for electromagnetic radiation fields. The angular distribution reveals the information of the initial geometric configurations. Such a property is reflected in the anisotropic parameter $v_{2}$, showing an increasing $v_{2}$ as energy decreases, which is a behavior qualitatively different from $v_{2}$ from hadrons produced in the collisions.
A review on experimental results for direct photon production in heavy ion reactions is given. A brief survey of early direct photon limits from SPS experiments is presented. The first measurement of direct photons in heavy ion reactions from the WA98 collaboration is discussed and compared to theoretical calculations. An outlook on the perspective of photon measurements at RHIC is given.
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