We discuss a common feature of all known reactions on nuclear targets - a significant suppression at large
x. Simple interpretation of this effect is based on energy conservation restrictions in ...initial state parton rescatterings. Using the light-cone dipole approach this mechanism is shown to control variety of processes on nuclear targets: high-
p
T
particle production at different rapidities as well as direct and virtual (Drell-Yan) photon production. We demonstrate universality and wide applicability of this mechanism allowing to describe large-
x effects also at SPS and FNAL energies too low for the onset of coherent effects or shadowing.
QCD factorization at forward rapidities Čepila, J; Nemchik, J; Šumbera, M
Journal of physics. Conference series,
09/2011, Letnik:
312, Številka:
1
Journal Article
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We analyze particle production in several reactions on nuclear targets at forward rapidities and different energies. The forward kinematic region at high energies allows to access the smallest ...Bjorken x. Nuclear effects are then usually interpreted as a result of the coherence effects associated with shadowing or the Color Glass Condensate. QCD factorization of soft and hard interactions requires the nucleus to be an universal filter for different Fock components of the projectile hadron. We demonstrate, however, that this is not the case in the vicinity of the kinematic limit, x → 1, where sharing of energy between the projectile constituents becomes an issue. The rise of suppression of particle production with x is confirmed by the E772 and E886 data on Drell-Yan and heavy quarkonia. We show that this effect can also be treated alternatively as an effective energy loss proportional to initial energy. This leads to a nuclear suppression at any energy, and predicts Feynman xF scaling of the suppression. We demonstrate how the kinematic limit influences the high-pT particle production at mid-rapidity where the Cronin enhancement at medium-high pT switches to a suppression at larger pT violating thus QCD factorization. Such an expectation seems to be confirmed by RHIC data for pion and direct photon production. We show that this effect as an additional large-pT suppression significantly revises calculations for jet quenching in heavy ion collisons at RHIC.
Using recent high-statistics STAR data from Au+Au and Cu+Cu collisions at full RHIC energy I discuss strong and Coulomb-induced final state interaction effects on identical (pi-pi) and non-identical ...(pi-xi) particle correlations. Analysis of pi-xi correlations reveals the strong and Coulomb-induced FSI effects, allowing for the first time to estimate spatial extension of pi and xi sources and the average shift between them. Source imaging techniques provide clean separation of details of the source function and are applied to the one-dimensional relative momentum correlation function of identical pions. For low momentum pions, and/or non-central collisions, a large departure from a single-Gaussian shape is observed.
The extreme energy densities generated by ultra-relativistic collisions between heavy atomic nuclei produce a state of matter that behaves surprisingly like a fluid, with exceptionally high ...temperature and low viscosity. Non-central collisions have angular momenta of the order of 1,000ћ, and the resulting fluid may have a strong vortical structure that must be understood to describe the fluid properly. The vortical structure is also of particular interest because the restoration of fundamental symmetries of quantum chromodynamics is expected to produce novel physical effects in the presence of strong vorticity. However, no experimental indications of fluid vorticity in heavy ion collisions have yet been found. Since vorticity represents a local rotational structure of the fluid, spin-orbit coupling can lead to preferential orientation of particle spins along the direction of rotation. Here we present measurements of an alignment between the global angular momentum of a non-central collision and the spin of emitted particles (in this case the collision occurs between gold nuclei and produces Λ baryons), revealing that the fluid produced in heavy ion collisions is the most vortical system so far observed. (At high energies, this fluid is a quark-gluon plasma.) We find that Λ and hyperons show a positive polarization of the order of a few per cent, consistent with some hydrodynamic predictions. (A hyperon is a particle composed of three quarks, at least one of which is a strange quark; the remainder are up and down quarks, found in protons and neutrons.) A previous measurement that reported a null result, that is, zero polarization, at higher collision energies is seen to be consistent with the trend of our observations, though with larger statistical uncertainties. These data provide experimental access to the vortical structure of the nearly ideal liquid created in a heavy ion collision and should prove valuable in the development of hydrodynamic models that quantitatively connect observations to the theory of the strong force.
The Λ (Λ¯) hyperon polarization along the beam direction has been measured in Au+Au collisions at sNN=200 GeV, for the first time in heavy-ion collisions. The polarization dependence on the ...hyperons' emission angle relative to the elliptic flow plane exhibits a second harmonic sine modulation, indicating a quadrupole pattern of the vorticity component along the beam direction, expected due to elliptic flow. The polarization is found to increase in more peripheral collisions, and shows no strong transverse momentum (pT) dependence at pT greater than 1 GeV/c. The magnitude of the signal is about 5 times smaller than those predicted by hydrodynamic and multiphase transport models; the observed phase of the emission angle dependence is also opposite to these model predictions. In contrast, the kinematic vorticity calculations in the blast-wave model tuned to reproduce particle spectra, elliptic flow, and the azimuthal dependence of the Gaussian source radii measured with the Hanbury Brown–Twiss intensity interferometry technique reproduce well the modulation phase measured in the data and capture the centrality and transverse momentum dependence of the polarization signal.
Global polarization of Λ hyperons has been measured to be of the order of a few tenths of a percent in Au+Au collisions at √SNN = 200 GeV, with no significant difference between Λ and Λ¯. These new ...results reveal the collision energy dependence of the global polarization together with the results previously observed √SNN = 7.7 – 62.4 GeV and indicate noticeable vorticity of the medium created in non-central heavy-ion collisions at the highest RHIC collision energy.
We report the first measurement of rapidity-odd directed flow (v1) for D0 and D0¯ mesons at midrapidity (|y|<0.8) in Au+Au collisions at sNN=200 GeV using the STAR detector at the Relativistic Heavy ...Ion Collider. In 10–80% Au+Au collisions, the slope of the v1 rapidity dependence (dv1/dy), averaged over D0 and D0¯ mesons, is −0.080±0.017(stat)±0.016(syst) for transverse momentum pT above 1.5 GeV/c. The absolute value of D0 meson dv1/dy is about 25 times larger than that for charged kaons, with 3.4σ significance. These data give a unique insight into the initial tilt of the produced matter, and offer constraints on the geometric and transport parameters of the hot QCD medium created in relativistic heavy-ion collisions.
We report on first measurements of low-mass electron-positron pairs in Pb-Au collisions at the CERN SPS beam energy of 40 AGeV. The observed pair yield integrated over the range of invariant masses ...0.2<m</=1 GeV/c(2) is enhanced over the expectation from neutral meson decays by a factor of 5.9+/-1.5(stat)+/-1.2(syst data)+/-1.8(syst meson decays), somewhat larger than previously observed at the higher energy of 158 AGeV. The results are discussed with reference to model calculations based on pi(+)pi(-)-->e(+)e(-) annihilation with a modified rho propagator. They may be linked to chiral symmetry restoration and support the notion that the in-medium modifications of the rho are more driven by baryon density than by temperature.
Here, we report the energy dependence of mid-rapidity (anti-)deuteron production in Au+Au collisions at $ \sqrt{s_{NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4, and 200 GeV, measured by the STAR ...experiment at RHIC. The yield of deuterons is found to be well described by the thermal model. The collision energy, centrality, and transverse momentum dependence of the coalescence parameter B2 are discussed. We find that the values of B2 for antideuterons are systematically lower than those for deuterons, indicating that the correlation volume of antibaryons is larger than that of baryons at $ \sqrt{s_{NN}}$ from 19.6 to 39 GeV. In addition, values of B2 are found to vary with collision energy and show a broad minimum around $ \sqrt{s_{NN}}$ = 20 to 40 GeV, which might imply a change of the equation of state of the medium in these collisions.
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