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The transverse mass mt distributions for deuterons and protons are measured in Pb+Pb reactions near midrapidity and in the range 0<mt–m<1.0 (1.5) GeV/c2 for minimum bias collisions at 158A GeV and for central collisions at 40 and 80 A GeV beam energies. The rapidity density dn/dy, inverse slope parameter T and mean transverse mass <mt> derived from mt distributions as well as the coalescence parameter B2 are studied as a function of the incident energy and the collision centrality. The deuteron mt spectra are significantly harder than those of protons, especially in central collisions. The coalescence factor B2 shows three systematic trends. First, it decreases strongly with increasing centrality reflecting an enlargement of the deuteron coalescence volume in central Pb+Pb collisions. Second, it increases with mt. Finally, B2 shows an increase with decreasing incident beam energy even within the SPS energy range. The results are discussed and compared to the predictions of models that include the collective expansion of the source created in Pb+Pb collisions.
The energy dependence of multiplicity fluctuations was studied for the most central Pb+Pb collisions at 20A, 30A, 40A, 80A and 158A GeV by the NA49 experiment at the CERN SPS. The multiplicity distribution for negatively and positively charged hadrons is significantly narrower than Poisson one for all energies. No significant structure in energy dependence of the scaled variance of multiplicity fluctuations is observed. The measured scaled variance is lower than the one predicted by the grand-canonical formulation of the hadron-resonance gas model. The results for scaled variance are in approximate agreement with the string-hadronic model UrQMD.
Measurements of charged pion and kaon production in central Pb+Pb collisions at 40, 80 and 158 AGeV are presented. These are compared with data at lower and higher energies as well as with results from p+p interactions. The mean pion multiplicity per wounded nucleon increases approximately linearly with s_NN^1/4 with a change of slope starting in the region 15-40 AGeV. The change from pion suppression with respect to p+p interactions, as observed at low collision energies, to pion enhancement at high energies occurs at about 40 AGeV. A non-monotonic energy dependence of the ratio of K^+ to pi^+ yields is observed, with a maximum close to 40 AGeV and an indication of a nearly constant value at higher energies.The measured dependences may be related to an increase of the entropy production and a decrease of the strangeness to entropy ratio in central Pb+Pb collisions in the low SPS energy range, which is consistent with the hypothesis that a transient state of deconfined matter is created above these energies. Other interpretations of the data are also discussed.
Rapidity distributions for Lambda and anti-Lambda hyperons in central Pb-Pb collisions at 40, 80 and 158 AGeV and for K 0 s mesons at 158 AGeV are presented. The lambda multiplicities are studied as a function of collision energy together with AGS and RHIC measurements and compared to model predictions. A different energy dependence of the Lambda/pi and anti-Lambda/pi is observed. The anti-Lambda/Lambda ratio shows a steep increase with collision energy. Evidence for a anti-Lambda/anti-p ratio greater than 1 is found at 40 AGeV.
Rapidity distributions for $\Lambda$ and $\bar{\Lambda}$ hyperons in central Pb-Pb collisions at 40, 80 and 158 A$\cdot$GeV and for ${\rm K}_{s}^{0}$ mesons at 158 A$\cdot$GeV are presented. The lambda multiplicities are studied as a function of collision energy together with AGS and RHIC measurements and compared to model predictions. A different energy dependence of the $\Lambda/\pi$ and $\bar{\Lambda}/\pi$ is observed. The $\bar{\Lambda}/\Lambda$ ratio shows a steep increase with collision energy. Evidence for a $\bar{\Lambda}/\bar{\rm p}$ ratio greater than 1 is found at 40 A$\cdot$GeV.
Event-by-event fluctuations of particle ratios in central Pb + Pb collisions at 20 to 158 AGeV
(2004)
In the vicinity of the QCD phase transition, critical fluctuations have been predicted to lead to non-statistical fluctuations of particle ratios, depending on the nature of the phase transition. Recent results of the NA49 energy scan program show a sharp maximum of the ratio of K+ to Pi+ yields in central Pb+Pb collisions at beam energies of 20-30 AGeV. This observation has been interpreted as an indication of a phase transition at low SPS energies. We present first results on event-by-event fluctuations of the kaon to pion and proton to pion ratios at beam energies close to this maximum.
We present the first measurement of fluctuations from event to event in the production of strange particles in collisions of heavy nuclei. The ratio of charged kaons to charged pions is determined for individual central Pb+Pb collisions. After accounting for the fluctuations due to detector resolution and finite number statistics we derive an upper limit on genuine non-statistical fluctuations, perhaps related to a first or second order QCD phase transition. Such fluctuations are shown to be very small.
Evidence for an exotic S=-2, Q=-2 baryon resonance in proton-proton collisions at the CERN SPS
(2004)
Results of resonance searches in the Xi - pi -, Xi - pi +, Xi -bar+ pi -, and Xi -bar+ pi + invariant mass spectra in proton-proton collisions at sqrt[s]=17.2 GeV are presented. Evidence is shown for the existence of a narrow Xi - pi - baryon resonance with mass of 1.862±0.002 GeV/c2 and width below the detector resolution of about 0.018 GeV/c2. The significance is estimated to be above 4.2 sigma . This state is a candidate for the hypothetical exotic Xi --3/2 baryon with S=-2, I=3 / 2, and a quark content of (dsdsu-bar). At the same mass, a peak is observed in the Xi - pi + spectrum which is a candidate for the Xi 03/2 member of this isospin quartet with a quark content of (dsusd-bar). The corresponding antibaryon spectra also show enhancements at the same invariant mass.
Freeze-out radii extracted from three-pion cumulants in pp, p–Pb and Pb–Pb collisions at the LHC
(2014)
In high-energy collisions, the spatio-temporal size of the particle production region can be measured using the Bose–Einstein correlations of identical bosons at low relative momentum. The source radii are typically extracted using two-pion correlations, and characterize the system at the last stage of interaction, called kinetic freeze-out. In low-multiplicity collisions, unlike in high-multiplicity collisions, two-pion correlations are substantially altered by background correlations, e.g. mini-jets. Such correlations can be suppressed using three-pion cumulant correlations. We present the first measurements of the size of the system at freeze-out extracted from three-pion cumulant correlations in pp, p–Pb and Pb–Pb collisions at the LHC with ALICE. At similar multiplicity, the invariant radii extracted in p–Pb collisions are found to be 5–15% larger than those in pp, while those in Pb–Pb are 35–55% larger than those in p–Pb. Our measurements disfavor models which incorporate substantially stronger collective expansion in p–Pb as compared to pp collisions at similar multiplicity.