Photon Physics in Heavy Ion Collisions at the LHC

Abstract

Various pion and photon production mechanisms in high-energy nuclear collisions at RHIC and LHC are discussed. Comparison with RHIC data is done whenever possible. The prospect of using electromagnetic probes to characterize quark-gluon plasma formation is assessed.

Figures (11)

• Figure 1: PHENIX results from 130 GeV Au+Au collisions (Run-1). The yields per event at mid-rapidity for ( left) charged hadrons and ( right) neutral pions are shown as a function of $p_{_T}$ for 60--80% ( lower) and 0--10% ( upper) event samples, with the $\pi^{0}$ results from the PbSc and PbGl analyses plotted separately. The error bars indicate the statistical errors on the yield; the surrounding brackets indicate the systematic errors. Shown for reference are the yields per collision in N+N collisions, of charged hadrons and neutral pions respectively, each scaled up by $\langle N_{coll} \rangle$ for the class. The bands indicate both the uncertainty in the N+N reference and in the determination of $\langle N_{coll} \rangle$.
• Figure 2: PHENIX results from 200 GeV Au+Au collisions (Run-2). Invariant yields of $\pi^0$ at mid-rapidity are plotted as a function of $p_{_T}$ for minimum bias and 9 different centrality selections (0-10% is the most central, 80-92% is the most peripheral). The yields are scaled for clarity.
• Figure 3: PHENIX results from 130 GeV Au+Au collisions (Run-1). Left panel: the ratio $R_{AA}$ for charged hadrons ($(h^{+}+h^{-})/2$) and neutral pions in central Au+Au collisions. The error bars indicate the statistical errors, the surrounding bands (shaded for $\pi^{0}$'s, brackets for $(h^{+}+h^{-})/2$) indicate the combined statistical and systematic errors on the ratio, including the uncertainty in the $p p$ data and the uncertainty in $\langle N_{coll} \rangle$. Also shown for reference are $R_{AA}$ for $\alpha+\alpha$ ($\sqrt{s}=31$ GeV) and for central Pb+Pb collisions ($\sqrt{s}=17.3$ GeV) measured at the CERN-SPS. Right panel: Ratio of central to peripheral $p_{_T}$ spectra (both normalized with the calculated $N_{coll}$) for charged hadrons and $\pi^0$.
• Figure 4: PHENIX results from $\sqrt{s}$ = 200 GeV. Nuclear modification factor $R_{AA}(p_{_T})$ for $\pi^0$ in central (closed circles) and peripheral (open circles) Au+Au collisions. The error bars include all point-to-point experimental ($p p$, Au+Au) errors. The shaded bands represent the fractional uncertainties in $\langle T_{AuAu} \rangle$ and in the $\pi^0$ yields normalization added in quadrature, which can move all the points up or down together (in the central case the shaded band shown is the fractional error for the first point).
• Figure 5: Top: Nuclear modification factor $R_{\rm{dA}}$ for $(h^+ + h^-)/2$ in minimum bias D+Au compared to $R_{\rm{AA}}$ in the 10% most central Au+Au collisions. Inner bands show systematic errors which can vary with $p_{_T}$, and outer bands include also the normalization uncertainty. Bottom: Comparison of $R_{\rm{DA}}$ for $(h^+ + h^-)/2$ and the average of the $\pi^0$ measurements in D+Au. The bar at the left indicates the systematic uncertainty in common for the charged and $\pi^0$ measurements.