New results from the ALICE collaboration are providing additional data to test ideas about how particles are produced out of the quark–gluon plasma (QGP) created in heavy-ion collisions at the LHC.
Experiments at Brookhaven’s Relativistic Heavy Ion Collider (RHIC) observed an enhancement in pT-dependent baryon/meson ratios – specifically the p/π and Λ/K0S ratios – for central nucleus–nucleus (AA) collisions in comparison with proton–proton (pp) collisions, where particle production is assumed to be dominated by parton fragmentation. In addition, constituent-quark scaling was observed in the elliptic-flow parameter, v2, measured in AA collisions. To interpret these observations, the coalescence of quarks was suggested as an additional particle-production mechanism. The coalescence (or recombination) model postulates that three quarks must come together to form a baryon, while a quark and an antiquark must coalesce to form a meson. The pT and the v2 of the particle created is the sum of the respective values of the constituent quarks. Therefore, coalescence models generally predict differences between the pT spectra of baryons and mesons, predominantly in the range 2 < pT < 5 GeV/c, where the enhancement in the baryon/meson ratio has been measured.
While a similar enhancement in the p/π and Λ/K0S ratios is observed at the LHC, the mass scaling of v2 is not, calling into question the importance of the coalescence mechanism. The observed-particle pT spectra reflect the dynamics of the expanding QGP created in local thermal equilibrium, conferring to the final-state particles a common radial velocity independent of their mass, but a different momentum (hydrodynamic flow). The resulting blue shift in the pT spectrum therefore scales with particle mass, and is observed as a rise in the p/π and Λ/K0S ratios at low pT (see figure). In such a hydrodynamic description, particles with the same mass have pT spectra with similar shapes, independent of their quark content. The particular shape of the baryon/meson ratio observed in AA collisions therefore reflects the relative importance of hydrodynamic flow, parton fragmentation and quark coalescence. However, for the p/π and Λ/K0S ratios, the particles in the numerator and denominator differ in both mass and (anti)quark content, so coalescence and hydrodynamic effects cannot be disentangled. To test the role of coalescence further, it is instructive to conduct this study using a baryon and a meson that have similar mass.
Fortunately, nature provides two such particles: the proton, a baryon with mass 938 MeV/c2, and the φ meson, which has a mass of 1019 MeV/c2. If protons and φ mesons are produced predominantly through coalescence, their pT spectra will have different shapes. Hydrodynamic models alone would predict pT spectra with similar shapes owing to the small mass-difference (less than 9%), implying a p/φ ratio that is constant with pT.
For peripheral lead–lead collisions, where the small volume of the quark–gluon plasma reduces the influence of collective hydrodynamic motion on the pT spectra, the p/φ ratio has a strong dependence on pT, similar to that observed for pp collisions. In contrast, as the figure shows, in central lead–lead collisions – where the volume of the QGP produced is largest – the p/φratio has a very different pT dependence, and is constant within its uncertainties for pT < 4 GeV/c. The data therefore indicate that hydrodynamics is the leading contribution to particle pT spectra in central lead–lead collisions at LHC energies, and it does not seem necessary to invoke coalescence models.
In the coming year, the ALICE collaboration will measure a larger number of collisions at a higher energy. This will allow a more precise study of both the pT spectra and elliptic-flow parameters of the proton and φ meson, and will allow tighter constraints to be placed on theoretical models of particle production in heavy-ion collisions.