Thermal emission of pions in antiproton-proton annihilation at 12 GeV/c

Using data of \(\bar pp\) annihilation at 12 GeV/c we have found that centrally-emitted pions follow a thermal energy spectrum with an inclusive temperature of 111±2 MeV, the same as has been found for pions produced in πp collisions at 16 GeV/c. An analysis of individual high-multiplicity events to search for fluctuations in the temperature parameter on an event-byevent basis has been made, using two different methods. We obtained the value 0.18±0.02 for the relative dispersion of the temperature fluctuations.     

Interferometrical signature of mixed phase in A+A collisions

We have calculated the pion rapidity distribution, the transversal momentum spectrum and the pion correlator in outward, sideward and longitudinal directions for S+S collisions at high energy. We used a two-temperature model that allows us to explain the abundance of pions at smallp _T and highp _T . The first source, which is at high temperature, is in a quark-gluon-hadron mixed phase radiating pions due to a deflagration shock wave on the background of longitudinal expansion. The second source consists of the unburnt part of the matter in hadronic phase. The interferometrical analysis at differentp _T gives a signature for two expanding sources separated in time and allows the measurement of the lifetime and other parameters of each of them.     

Freeze-out conditions in ultrarelativistic heavy-ion collisions

We present recent results on single particle transverse momentum distributions of pions, kaons and protons, measured in CERN Experiment NA44, of 200 AGeV/c S+S and 158 AGeV/c Pb+Pb central collisions. By comparing these data with thermal and transport models, freeze-out parameters like the temperatureT _fo and the chemical potentials (μ _q ,μ _s ) are extracted and discussed.     

Nuclear effects on open charm production in p+A reactions

We calculate and resum the nuclear enhanced power corrections from the final state partonic scattering in nuclear matter to open charm production and correlations. In p+A reactions, we find that single and double inclusive D mesons can be suppressed as much as the neutral pions from the dynamical high twist shadowing. Effects of initial state energy loss in p+A collisions are also investigated and may lead to significantly weaker transverse momentum dependence of the nuclear suppression.     

Abundance of Δ resonances in 58Ni + 58Ni collisions between 1 and 2 AGeV

Charged pion spectra measured in ⁵⁸Ni⁵⁸Ni collisions at 1.06, 1.45 and 1.93 AGeV are interpreted in terms of a thermal model including the decay of Δ resonances. The transverse momentum spectra of pions are well reproduced by adding the pions originating from the Δ -resonance decay to the component of thermal pions, deduced from the high transverse momentum part of the pion spectra. About 10 and 18% of the nucleons are excited to Δ states at freeze-out for beam energies of 1 and 2 AGeV, respectively.     

Δ<Subscript>33</Subscript> -medium mass modification and pion spectra

We study the π^± spectra obtained in 2, 4, 6 and 8A GeV Au - Au collisions within the thermal model. We find that the main features of the data can be well described after we include the pions from the decay of the Δ -resonance with medium mass modification.     

Momentum dependence of the pion cloud for ϱ-mesons in nuclear matter

We extend hadronic models for ϱ-meson propagation in cold nuclear matter via coupling to in-medium pions to include finite three-momentum. Special care is taken to preserve gauge invariance. Consequences for photoabsorption on the proton and on nuclei as well as for the dilepton production in relativistic heavy-ion collisions are discussed.     

Negative pion production by 800 MeV/A C,Ne, and Ar beams

Energy and angular distributions of π's produced in relativistic heavy-ion collisions have been measured at 800 MeV/A incident energy. Cross sections for low-energy pions (E_cm ≈ 100–200 MeV) are forward and backward peaked in the nucleon-nucleon c.m. frame. Such anisotropy becomes weaker for higher energy pions. The shape of the energy spectra at θ_cm = 90° is approximately exponential. Their slopes are steeper than those observed for protons. The data are discussed in terms of several models.     

Associated π0 production in events with a particle of high transverse momentum at\sqrt s=63 Gev

Events with a single highp _T charged particle were recorded with the Split-Field-Magnet Detector in proton-proton collisions at the CERN-ISR. In the jet opposite to the trigger region the densities of photons and reconstructed neutral pions were measured with a liquid argon shower counter. Scaled momentum distributions of these particles are given and compared with those of charged pions. The spectra of charged and neutral pions coincide. The production cross-section of neutrals in the away jet shows no dependence on the flavour of the trigger particle.     

On spectral temperatures of negative pions produced in d 12C, 4He12C, and 12C12C collisions at 4.2 A GeV/c

The experimental transverse momentum distributions of negative pions produced in d ¹²C, ⁴He¹²C, and ¹²C¹²C collisions at 4.2 A GeV/c were analyzed in the framework of Hagedorn Thermodynamic Model. The spectral temperatures of π ^? mesons as well as their relative contributions to the total multiplicity of π ^? mesons were extracted from fitting the p _t spectra by two-temperature Hagedorn function. The results were compared systematically with the earlier results obtained from analysis of non-invariant center-of-mass energy spectra of negative pions produced in the analyzed collisions.     

Possible evidence of disoriented chiral condensates from the anomaly in Ω. and -Ω abundances at the super proton synchrotron

No conventional picture of nucleus-nucleus collisions has yet been able to explain the abundance of Ω and -Ω. in central collisions between Pb nuclei at 158 A GeV at the CERN SPS. We argue that such a deviation from predictions of statistical thermal models and numerical simulations is the evidence that they are produced as topological defects in the form of skyrmions arising from the formation of disoriented chiral condensates. The estimated domain size falls in the right range to be consistent with the so far non-observation of disoriented chiral condensate (DCC) from the distribution of neutral pions.     

Light-front analysis of π− mesons produced in MgMg collisions at 4.3A GeV/c

Light-front analysis of π⁻ mesons in MgMg collisions is carried out. The phase space of secondary pions is naturally divided into two parts in one of which the thermal equilibration assumption seems to be in good agreement with the data. Corresponding temperatures are extracted and compared to the results of other experiments. The experimental results have been compared with the predictions of the Quark-Gluon String Model (QGSM) and satisfactory agreement between the experimental data and the model has been found.     

Probing nuclear expansion dynamics with π−/π+-spectra

We study the dynamics of charged pions in the nuclear medium via the ratio of differential π^? - and π⁺-spectra in a coupled-channel BUU (CBUU) approach. The relative energy shift of the charged pions is found to correlate with the pion freeze-out time in nucleus-nucleus collisions as well as with the impact parameter of the heavy-ion reaction. Furthermore, the long-range Coulomb force provides valuable information on the expansion dynamics of the hot nuclear system. Detailed comparisons with experimental data for Au + Au at 1 AGeV and Ni + Ni at 2.0 AGeV are presented.     

Universal pion freeze-out in heavy-ion collisions

Based on an evaluation of data on pion interferometry and on particle yields at midrapidity, we propose a universal condition for thermal freeze-out of pions in heavy-ion collisions. We show that freeze-out occurs when the mean free path of pions lambda(f) reaches a value of about 1 fm, which is much smaller than the spatial extent of the system at freeze-out. This critical mean free path is independent of the centrality of the collision and beam energy from the Alternating Gradient Synchrotron to the Relativistic Heavy Ion Collider.     

A study of deep inelastic hadron-hadron collisions with a large acceptance calorimeter trigger

Large transverse energy cross sections of 300 GeV/c pions and protons on hydrogen have been measured with a segmented calorimeter covering the central rapidity region ?0.88 < y < 0.67 and 2π in azimuth. The selected events show large multiplicities and no jet-like event structure. Processes more complicated than the scattering of two constituents appear to dominate these inelastic collisions.     

Pomeron universality from identical pion correlations at the LHC

Bose-Einstein correlations of identical pions produced in high-energy pp collisions at the LHC allow a probe of the Pomeron exchange mechanism. The size of the domain which emits the pions depends on the multiplicity of events, but not on the collider energy. This confirms the universal structure of Pomeron exchange. The data at relatively low multiplicities indicate that the size of the source created by one-Pomeron exchange is much less than the size of the proton.     

Can recent CERN experiments with 200 GeV/N O16 ions be explained by the independent NN collisions?

We argue that high energy nuclear collisions cannot be described by the “independent NN collision” models. In particular, secondary pions definitely should have multiple rescattering, and only by taking them into account, may one understand such observations as large “pion source” radius and the “cool component”, seen in the pion spectra at small p_⊥.     

Pion production in mass- symmetric heavy ion collisions at 0.8–1.8 AGeV

Double differential cross sections of positively charged pions and protons have been measured in nuclear collisions of mass-symmetric systems (Ne+NaF, Ni+Ni, Au+Au, Bi+Pb) at incident energies between 0.8 and 1.8 AGeV as a function of the centrality of the reaction. Using a magnetic spectrometer pions and protons were detected with laboratory angles between 40 and 48 degrees, and with momenta up to about 1400 MeV/c. This setting allows for the study of pions and protons emitted close to midrapidity. The center-of-mass pion spectra deviate from a Boltzmann distribution. The inverse slope parameters of the high-energetic pions are smaller than those of the proton spectra and they exhibit a weaker centrality dependence. A scenario is presented where the shape of the pion spectra reflects the decay kinematics of nucleonic resonances embedded in the thermal and the collective motion of the nucleons in the reaction zone. The number of emitted pions per participating nucleon is higher for light than for heavy mass systems. For a given mass system, the total pion multiplicity increases linearly with the number of participating nucleons, whereas the multiplicity of high-energy pions increases more than linearly. This result is consistent with a scenario where the high-energy pions are produced in multiple energetic baryon-baryon collisions occurring in the high-density phase of the collision.     

Search for Hawking radiation in heavy ion collisions

The creation of “white holes” that decay by Hawking radiation has been proposed as one way to achieve the very early thermalization observed in heavy ion collisions at RHIC. The charartistic temperature of the radiations depends only on the ratio of the baryon number to the transverse energy. The yields of pions, kaons, protons and antiprotons measured by BRAHMS in central Au+Au collisions can be described within a thermal model where T drops with rapidity, and beam energy. We find that the chemical freeze-out temperature drops as the ratio of baryon number to energy increases but much more rapidly than predicted by the model.     

Non-equilibrium processes in heavy-ion collisions at relativistic energies

We use transport theory to describe the inclusive cross sections for protons and pions produced in collisions between two identical heavy ions at an energy of 800 MeV per particle. In addition to the nucleonic we take the Δ-degree of freedom into account. Thus we consider a two-component system whose distributions in transverse momentum and rapidity we describe by two coupled Fokker-Planck equations. These transport equations contain the one-nucleon knock-out process as initial condition. In the limit of large interaction times they lead to thermal equilibrium (fireball) distributions. For light nuclei the interaction time is not large enough for equilibrium to be reached. A recent experiment for two colliding carbon nuclei at 800 MeV per nucleon shows evidence of nonequilibrium effects. We compare our calculations with experimental data for ¹²C on ¹²C and Ne on NaF at 800 MeV/N.