Search

The production cross section of prompt +c charm baryons was measured with the ALICE detector at the LHC at midrapidity in proton-proton (pp) and proton- lead (p–Pb) collisions at a center-of-mass energy per nucleon pair of √sNN = 5.02 TeV. The +c and −c baryons were reconstructed in the hadronic decay channels +c → pK−π+ and + c → pK0S and respective charge conjugates. The measured differential cross sections as a function of transverse momentum (pT ) and the pT -integrated +c production cross section in pp and in p–Pb collisions are presented. The +c nuclear modification factor (RpPb), calculated from the cross sections in pp and in p–Pb collisions, is presented and compared with the RpPb of D mesons. The +c/D0 ratio is also presented and compared with the light-flavor baryon-to-meson ratios p/π and /K0S, and measurements from other LHC experiments. The results are compared to predictions from model calculations and Monte Carlo event generators.

In this Letter, the ALICE Collaboration presents the first measurements of the charged- particle multiplicity density, dNch/dη , and total charged-particle multiplicity, Nchtot , in Xe–Xe collisions at a centre-of-mass energy per nucleon–nucleon pair of sNN=5.44TeV . The measurements are performed as a function of collision centrality over a wide pseudorapidity range of −3.5

Pion production in nuclear collisions at the Super Proton Synchrotron (SPS) is investigated with the aim to search, in a restricted domain of the phase diagram, for power laws in the behavior of correlations that are compatible with critical QCD. We analyzed interactions of nuclei of different sizes (p + p, C + C, Si + Si, Pb + Pb) at 158A GeV adopting, as appropriate observables, scaled factorial moments in a search for intermittent fluctuations in transverse dimensions. The analysis is performed for π +π− pairs with an invariant mass very close to the two-pion threshold. In this sector one may capture critical fluctuations of the sigma component in a hadronic medium, even if the σ meson has no well-defined vacuum state. It turns out that for the Pb + Pb system the proposed analysis technique cannot be applied without entering the invariant mass region with strong Coulomb correlations. As a result the treatment becomes inconclusive in this case. Our results for the other systems indicate the presence of power-law fluctuations in the freeze-out state of Si + Si approaching in size the prediction of critical QCD.

Tri raspršenja su eksperimentalno dostupna u sustavu piona i nukleona: dva elastična raspršenja te reakcija izmjene naboja. Da bi se precizno opisao sustav piona i nukleona potrebni su kvalitetni eksperimentalni podaci za sva tri raspršenja i to u svim područjima energija od interesa. Reakcija izmjene naboja na niskim energijama (ispod 100 MeV kinetičke energije pionskog snopa) ne zadovoljava taj uvjet: prethodnih mjerenja nema puno, a dio ih je i upitne kvalitete. S druge strane upravo je područje niskih energija vrlo zanimljivo jer su tu najizraženiji efekti narušenja izospinske simetrije. Drugi razlog je taj što su mjerenja na niskim energijama presudna za određivanje vrlo važne veličine: pionskonukleonskog sigma člana. Prikazani su diferencijalni udarni presjeci za reakciju pionsko-nukleonske izmjene naboja za sva područja kutova pri sedam različitih energija pionskog snopa u rasponu od 34 MeV do 87 MeV. Mjerenja su napravljena s Crystal Ball detektorom na Nacionalnom laboratoriju Brookhaven u Sjedinjenim Američkim Državama. Reakcija izmjene naboja prepoznata je mjerenjem energija i smjerova dvaju fotona nastalih iz raspada neutralnog piona. Statistički značaj prikazanih rezultata usporediv je sa svim prethodnim eksperimentima ispod 100 MeV zajedno. Rezultati su uspoređeni s predviđanjima analize parcijalnih valova sa Sveučilišta George Washington zasnovane na prethodnim mjerenjima. Statistički značajna odstupanja su uočena za sve energije osim najviše.