The ALICE Collaboration has observed that the ratio of strange to non-strange hadron yields increases strongly with multiplicity in small collision systems at LHC energies. The origin of this effect ...is still not fully understood. Models need to incorporate final-state interactions to accommodate the new ALICE results, and require new measurements to discriminate between the various phenomenological descriptions.
Two different ALICE studies, aiming to further elucidate the underlying mechanisms of strangeness production in high-multiplicity proton–proton (pp) collisions at √S = 13 TeV, are presented in this proceeding. First, the production of
η
and
π
0
are studied as a function of multiplicity, combining several analysis techniques, allowing a measurement for the neutral meson production over a vast transverse momentum (
p
T
) range. This is followed by a measurement of
π
,
K
,
ϕ
, and
Ξ
as a function of the unweighed transverse spherocity
S
O
PT=1
, an event-shape observable which allows us to explore particle production in azimuthal topologies dominated by either hard or soft QCD processes. The results are compared to predictions from PYTHIA and EPOS-LHC.
The ALICE Collaboration has observed that the relative fraction of strange hadrons grows strongly with multiplicity in small collision systems at LHC energies, in particular for multi-strange ...baryons. The origin of this effect is still under debate, and models need significant final-state interactions to accommodate the new ALICE results, requiring new tools to discriminate experimentally between the various phenomenological ideas.
In these proceedings we present a multi-differential study of identified particle spectra with respect to the event multiplicity and two different event shape observables, the unweighted transverse spherocity S
O
pT=1
, and the self-normalized, transverse charged particle density,
R
T
, which allows us to explore particle production associated with hard and soft QCD processes. ALICE has measured π,
K
, ϕ, and Ξ production at mid-rapidity (|η| < 0.8) as a function of S
O
pT=1
and
R
T
in pp collisions at √
S
= 13 TeV. This work reports on how these multi-differential measurements compare with predictions from PYTHIA and EPOSLHC.
ALICE has observed that the relative fraction of strange hadrons grows strongly with multiplicity in small collision systems (proton-proton and proton-lead collisions) at LHC energies, in particular ...for multi-strange baryons. This implies that a proton-proton collision cannot be described as an incoherent sum of parton-parton collisions, an idea that has been central in most proton-proton generators, for example PYTHIA. To accommodate the new ALICE results, models have to introduce significant final-state interactions. These final-state effects can have very different phenomenological origin. To be able to eventually discriminate experimentally between different final-state models, new experimental tools are required and in this paper different multi-differential observables will be tested with this goal in mind. Transverse Spherocity (SO) is an observable that allows a topological selection of events that are "isotropic" (dominated by multiple soft processes) and "jetty" (where a single hard process is responsible for a significant part of the multiplicity). The underlying event activity is another observable that can be used to vary the amount of soft processes. It can be estimated by measuring the charged-particle multiplicity in the Transverse region (RT). Using these new observables, ALICE has obtained results for π, K, φ, p, and Ξ production at mid-rapidity (|η|<0.8) as a function of event shape and underlying event activity in proton-proton collisions at (SNN)=13TeV. Finally, this contribution will report on how these new multi-differential measurements compare to predictions from PYTHIA and EPOS-LHC.
Resonance particles, such as the K*(892) meson, are reconstructed from the invariant mass (M i n v ) distribution of possible particle pairs. To extract the yield with the highest precision, the ...combinatorial background must be determined as precisely as possible. An event-mixed M i n v distribution is often used to describe the combinatorial background. However, this distribution will not contain the mini-jet-like structures present inside an event. This analysis introduces a new re-weighing scheme, where two M i n v distributions of like-sign particles in the same-event and in mixed-events are used to correct the mixed-event background estimate for the mini-jet-like structure. Using PYTHIA 8.2 generated proton-proton collisions at s NN = 5.02 TeV, it is shown that the new method can be used to more accurately describe the combinatorial background.
The global polarization of the Λ and Λ hyperons is measured for Pb-Pb collisions at sNN=2.76 and 5.02 TeV recorded with the ALICE at the Large Hadron Collider (LHC). The results are reported ...differentially as a function of collision centrality and hyperon's transverse momentum (pT) for the range of centrality 5-50%, 0.5<pT<5 GeV/c, and rapidity |y|<0.5. The hyperon global polarization averaged for Pb-Pb collisions at sNN=2.76 and 5.02 TeV is found to be consistent with zero, (PH)(%)≈0.01±0.06(stat.)±0.03(syst.) in the collision centrality range 15-50%, where the largest signal is expected. The results are compatible with expectations based on an extrapolation from measurements at lower collision energies at the Relativistic Heavy Ion Collider, hydrodynamical model calculations, and empirical estimates based on collision energy dependence of directed flow, all of which predict the global polarization values at LHC energies of the order of 0.01%.
Cross sections for the coherent photoproduction of ρ0 vector mesons in ultra-peripheral Pb-Pb collisions at sNN−−−√ = 5.02 TeV are reported. The measurements, which rely on the π+π− decay channel, ...are presented in three regions of rapidity covering the range |y| < 0.8. For each rapidity interval, cross sections are shown for different nuclear-breakup classes defined according to the presence of neutrons measured in the zero-degree calorimeters. The results are compared with predictions based on different models of nuclear shadowing. Finally, the observation of a coherently produced resonance-like structure with a mass around 1.7 GeV/c2 and a width of about 140 MeV/c2 is reported and compared with similar observations from other experiments.
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