The spectra for different collision

The spectra for different collision centralities are multiplied by the scaling coefficients: 102 (1), 101 (2), 100 (3), 10–1(4), 10–2 (5), 10–3(6). Vertical bars mark the statistical errors, boxes mark the systematic measurement errors. The nuclear modification factors for φ-mesons in Pb+Pb collisions at = 2.76 TeV with different centralities were extracted based on the obtained results. The nuclear modification factor is determined as: Where is the differential particle yield in A+B interactions and is the production cross section in (p+p) interactions; are the nuclear overlap functions [21] calculated in the Glauber model. Results of nuclear modification factors for φ-mesons in (Pb+Pb) collisions are presented in Fig. 3. In central Pb+Pb collisions at high transverse momentum the φ-meson production is strongly suppressed with respect to p+p interactions. The suppression is similar to that of other identified hadrons. Nuclear modification factors become close to unity going from the most central to peripheral collisions. At intermediate transverse momentum nuclear modification factors for φ-mesons are between the factors for protons and kaons. In the most central Pb+Pb collisions these factors are closer to these of light mesons (kaons, pions), while in peripheral collisions they closer to those measured for protons. In Fig. 4 the p/φ ratios for different centralities of Pb+Pb collisions at = 2.76 TeV are presented as a function of transverse momentum. Results obtained in peripheral Pb+Pb collisions are in agreement within uncertainties with the results in p+p collisions. In the most central Pb+Pb collisions the p/φ ration does not depend on in the transverse momentum range < 4 GeV/c.
Conclusion The obtained results show that in (semi)central Pb+Pb collisions φ-meson production is significantly suppressed at high transverse momentum. The level of suppression is consistent within uncertainties with the suppression observed for other light hadrons (π, K, p). The observed suppression cannot be explained by cold nuclear p2y receptor effects and may be a signal of quark-gluon plasma formation. A weak dependence of the p/φ ratio on transverse momentum in the most central Pb+Pb collisions at < 4 GeV/c (Fig. 4) indicates that shapes of the hadron production spectra are determined by particle masses. This observation is in agreement with hydrodynamic model predictions and does not require the introduction of recombination models. The difference between nuclear modification factors for φ-mesons and protons in the most central Pb+Pb collisions can be explained by the difference in the reference production spectra in p+p interactions.
Introduction Quantum chromodynamics (QCD) predicts the phase transition from ordinary nuclear matter to the state of free quarks and gluons, the so-called quark–gluon plasma (QGP) [1-3]. In laboratory conditions such a transition can occur in relativistic heavy ion collisions. The detailed study of the phase transition and the properties of the new state of matter is the main goal of experiments at the RHIC (Relativistic Heavy Ion Collider) [4] and ALICE experiment at the LHC (Large Hadron Collider) [5]. It is not possible to interpret the results obtained in heavy ion collisions without fully understanding the smaller collision systems like proton–proton or proton–nuclei. The comparison of results from interactions of light and heavy systems can help to separate the effects related to the formation of the cold nuclear matter or hot and dense matter associated with quark–gluon plasma. The experimental observation of hadron production suppression at high transverse momentum () in central heavy nuclei collisions was one of the most important results obtained at the RHIC [6, 7]. The suppression could not be explained by the cold nuclear matter effects. In previous studies performed in p + A and A + A collisions at lower energies the production of hadrons at high transverse momentum ( > 2 GeV/c) was enhanced [8]. The enhancement called the Cronin effect [9] was explained by multiple soft rescattering of partons in nuclear matter before hard interaction.