Penetrating and puzzling: photons shed light on heavy-ion collisions

Résumé

Les émissions de photons éclairent les collisions d’ions lourds

La lumière émise dans des collisions à haute énergie de noyaux lourds révèle des informations uniques sur le plasma quark-gluon.

Quark–gluon plasma (QGP) is a thermalised state of matter at extreme temperatures consisting of deconfined quarks and gluons. Because the mean free path length of energetic photons is much larger than the size of the hot nuclear medium in which they are produced, they provide unscathed and direct information on the thermodynamic state of the QGP. In contrast to photons, hadrons that are produced after the QGP has cooled off to a temperature of about 1.8 × 10¹² K (k_BT = 155 MeV) mostly reflect the properties of the hadronic phase and carry only indirect information about the preceding QGP phase.

The role of direct photons

Photons are emitted over the entire duration of a heavy-ion collision via various production mechanisms. First, direct photons are distinguished from photons originating from the decay of neutral mesons, which constitute the background in the direct-photon measurement. Prompt direct photons are produced in initial hard-parton scatterings, prior to the formation of a QGP, and dominate the photon spectrum at large values of the transverse momentum (p_T), beyond 4 GeV/c. Because photons do not interact with the medium, their yield, well described by perturbative quantum chromodynamics (pQCD), directly reflects the rate of initial hard-scattering processes. By contrast, the yield of high-p_T hadrons is suppressed, an observation interpreted as the result of the energy lost in the QGP by quarks and gluons produced in hard-scattering processes. The interpretation of this effect, known as “jet quenching”, strongly relies on the observation that direct photons at high p_T are not suppressed.

Thermal direct photons are produced in the QGP and in the subsequent hadron gas. They are expected to give a significant contribution at low p_T (1 < p_T < 3 GeV/c), convey information about the QGP temperature, and provide a test for models of the space–time evolution of a heavy-ion collision. For a given temperature, the spectrum of thermal photons falls off exponentially with transverse momentum, so that the temperature of the photon source can be read off from the slope. This is similar to the determination of the temperature of a red-hot heating element, or the surface of the Sun, based on the emitted thermal photon radiation. Note, however, that unlike in these examples, the photons from the QGP and the hadron gas are not in thermal equilibrium with the surrounding medium. In heavy-ion collisions, the thermal photon spectrum is an effective average over the different volume elements in the QGP and the hadron gas at different temperatures. However, during the latter stages of the collision, volume elements move with considerable velocity towards the detector. The resulting blue-shift of the spectrum leads to an apparent temperature that can be as large as twice the actual temperature of the source. Determining the initial QGP temperature therefore relies on comparison with models such as the hydrodynamic description of the evolution of heavy-ion collisions, which has proved to be very successful for hadrons. Being produced at all stages of the collisions, low-p_T direct photons therefore provide an independent constraint for these models.

The ALICE experiment and experiments at RHIC have measured two distinct features of direct photons: their yield and their azimuthal anisotropy as a function of p_T. ALICE has measured the direct-photon spectrum in central-to-peripheral Pb–Pb collisions at √s_NN = 2.76 TeV (see figure opposite). For p_T > 4 GeV/c, the spectrum agrees with the expectation for direct-photon production in initial hard-parton scatterings, as calculated by pQCD. At lower p_T, most notably in central Pb–Pb collisions, there appears to be an additional source of (most likely) thermal photons from the QGP and the hadron gas. State-of-the-art hydrodynamic models agree with the measured direct-photon spectra within uncertainties; however, they tend to under-predict the central values of the measurement. For the direct-photon spectrum measured in central Au–Au collisions at √s_NN = 200 GeV at RHIC, the differences between measurements and predictions become more prominent.

Anisotropies as puzzling as yields

Even more surprising, azimuthal anisotropies of low-p_T direct photons measured in heavy-ion collisions were found to be much larger than predicted by hydrodynamic models. The anisotropies are a consequence of the approximately almond-shaped overlap zone of the two nuclei in non-central collisions. This gives rise to a variation of pressure gradients as a function of the azimuthal angle. As a consequence, azimuthally dependent collective flow velocities develop as the system expands, and give rise to the experimentally observed elliptic flow, an azimuthal variation of hadron yields. This is quantified by a Fourier coefficient, v₂(p_T). Taking into account the fact that collective flow fields take time to build up, and that photon production is dominated by photons from the early hot QGP phase, the azimuthal anisotropy v₂ of direct photons at low p_T was expected to be smaller than that of hadrons. The PHENIX experiment at RHIC, however, measured v₂ values similar to the values of hadrons. ALICE seems to confirm this observation. These measurements could indicate that thermal photons from the late hadron gas phase outshine photons from the early QGP.

The observation that models under-predict both the thermal photon spectrum and the v₂ is puzzling, and is one of the most pressing challenges for our understanding of heavy-ion collisions. The puzzle is currently more apparent in Au–Au collisions at RHIC, with much less of a tension in Pb–Pb collisions at the LHC. Its solution could be related to the hydrodynamic modelling of the space–time evolution of heavy-ion collisions, or to the theoretical description of photon production. It could also point to so-far-unknown photon production mechanisms. Many new theoretical ideas have recently been put forward in this direction. Until this puzzle is solved, the question about the role of thermal photons remains open: are they messengers of the QGP, or are thermal photons from the QGP only a small contribution buried under photons produced much later in the hadron gas? An answer is eagerly awaited.

• For further details, see arxiv.org/abs/1509.07324 and arxiv.org/abs/1212.3995, ALICE Collaboration; arxiv.org/abs/1405.3940 and arxiv.org/abs/1509.07758, PHENIX Collaboration.