• Charged-particle pseudorapidity density at mid-rapidity in p–Pb collisions at √sNN = 8.16 TeV

      Acharya, S.; Acosta, F.-T.; Adamová, D.; Adhya, S. P.; Adler, A.; Adolfsson, J.; Aggarwal, M. M.; Rinella, G. Aglieri; Agnello, M.; Ahammed, Z.; et al. (Springer Science, 2019-04-04)
      The pseudorapidity density of charged particles, dNch/dη, in p–Pb collisions has been measured at a centreof-mass energy per nucleon–nucleon pair of √sNN = 8.16 TeV at mid-seudorapidity for non-single-diffractive events. The results cover 3.6 units of pseudorapidity, |η| < 1.8. The dNch/dη value is 19.1 ± 0.7 at |η| < 0.5. This quantity divided by Npart /2 is 4.73±0.20, where Npart is the average number of participating nucleons, is 9.5% higher than the corresponding value for p–Pb collisions at √sNN = 5.02 TeV. Measurements are compared with models based on different mechanisms for particle production. All models agree within uncertainties with data in the Pb-going side, while HIJING overestimates, showing a symmetric behaviour, and EPOS underestimates the p-going side of the dNch/dη distribution. Saturation-based models reproduce the distributions well for η > −1.3. The dNch/dη is also measured for different centrality estimators, based both on the chargedparticle multiplicity and on the energy deposited in the ZeroDegree Calorimeters. A study of the implications of the large multiplicity fluctuations due to the small number of participants for systems like p–Pb in the centrality calculation for multiplicity-based estimators is discussed, demonstrating the advantages of determining the centrality with energy deposited near beam rapidity.
    • First observation of an attractive interaction between a proton and a cascade baryon

      Acharya, S.; Adamová, D.; Adhya, S. P.; Adler, A.; Adolfsson, J.; Aggarwal, M. M.; Aglieri Rinella, G.; Agnello, M.; Agrawal, N.; Ahammed, Z.; et al. (American Physical Society (APS), 2019-09-13)
      This Letter presents the first experimental observation of the attractive strong interaction between a proton and a multistrange baryon (hyperon) Ξ−. The result is extracted from two-particle correlations of combined p−Ξ−⊕¯p−¯Ξ+ pairs measured in p−Pb collisions at √sNN=5.02 TeV at the LHC with ALICE. The measured correlation function is compared with the prediction obtained assuming only an attractive Coulomb interaction and a standard deviation in the range [3.6, 5.3] is found. Since the measured p−Ξ−⊕¯p−¯Ξ+ correlation is significantly enhanced with respect to the Coulomb prediction, the presence of an additional, strong, attractive interaction is evident. The data are compatible with recent lattice calculations by the HAL-QCD Collaboration, with a standard deviation in the range [1.8, 3.7]. The lattice potential predicts a shallow repulsive Ξ− interaction within pure neutron matter and this implies stiffer equations of state for neutron-rich matter including hyperons. Implications of the strong interaction for the modeling of neutron stars are discussed.
    • Measuring K0sK± interactions using pp collisions at √s=7 TeV

      Boca, G.; Bock, F.; Bogdanov, A.; Boldizsár, L.; Bolozdynya, A.; Bombara, M.; Bonomi, G.; Bonora, M.; Borel, H.; Borissov, A.; et al. (Elsevier, 2019-03-10)
      We present the first measurements of femtoscopic correlations between the KS0 and K± particles in pp collisions at √s=7 TeV measured by the ALICE experiment. The observed femtoscopic correlations are consistent with final-state interactions proceeding solely via the a0(980) resonance. The extracted kaon source radius and correlation strength parameters for KS0K− are found to be equal within the experimental uncertainties to those for KS0K+. Results of the present study are compared with those from identical-kaon femtoscopic studies also performed with pp collisions at √s=7 TeV by ALICE and with a KS0K± measurement in Pb–Pb collisions at √sNN=2.76 TeV. Combined with the Pb–Pb results, our pp analysis is found to be compatible with the interpretation of the a0(980) having a tetraquark structure instead of that of a diquark.
    • Search for a common baryon source in high-multiplicity pp collisions at the LHC

      Acharya, S.; Adamová, D.; Adler, A.; Adolfsson, J.; Aggarwal, M.M.; Aglieri Rinella, G.; Agnello, M.; Agrawal, N.; Ahammed, Z.; Ahmad, S.; et al. (Elsevier BV, 2020-10-08)
      We report on the measurement of the size of the particle-emitting source from two-baryon correlations with ALICE in high-multiplicity pp collisions at √s = 13 TeV. The source radius is studied with low relative momentum p–p, pbar-pbar, p–Λ , and pbar-Λbar pairs as a function of the pair transverse mass m_T considering for the first time in a quantitative way the effect of strong resonance decays. After correcting for this effect, the radii extracted for pairs of different particle species agree. This indicates that protons, antiprotons, Λ s, and Λbar s originate from the same source. Within the measured m_T range (1.1–2.2) GeV/c^2 the invariant radius of this common source varies between 1.3 and 0.85 fm. These results provide a precise reference for studies of the strong hadron–hadron interactions and for the investigation of collective properties in small colliding systems.