Recent results on light flavor hadron productionin the ALICE experiment at the LHC
Victor Riabov for the ALICE Collaboration

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1Introduction

 In heavy-ion collisions, the interacting system evolves through different stages and thencools down. Light flavor hadrons are the most copiously produced particles in the final state Measurements of light flavor hadrons are used to probe the collective evolution of thesystem, test the phase transition, properties of the system at chemical and kinetic freeze-out,
the particle production mechanisms and reaction dynamics.
 ALICE studies the system size and collision energy dependence of particle production withthe same apparatus using different data samples accumulated in Run-1 and Run-2
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Particle spectra in heavy-ion collisions
Phys.Rev.C 101 (2020) 4, 044907

arXiv:2106.13113

 A variety of hadrons measured in wide pT ranges at different centralities
 Spectra become harder with increasing multiplicity (flatten at low pT), most pronounced for heavier
particles  expected from collective hydrodynamic expansion (radial flow)
 Similar hardening of spectra has been also observed in high-multiplicity pp and p-Pb collisions
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Mean transverse momenta

Mass-dependent hardening of spectra manifests itself in <pT> increasing with multiplicity
Steeper increase of <pT> in smaller collision systems
Modest increase of <pT> with collision energy
In heavy-ion collisions <pT> is independent of the size of colliding nuclei (Xe-Xe vs. Pb-Pb)
In central heavy-ion collisions particles with similar masses have similar values of <pT> as expectedfrom hydrodynamic flow The mass ordering of <pT> is violated in peripheral heavy-ion collisions as well as in pp and p-Pb
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Blast-Wave model fits to π/K/p spectraBoltzmann-Gibbs Blast-Wave fits
are used to determine parameters of
the radial flow: Tkin – kinetic freeze-out temperature <T> - transverse velocity
 n – velocity profile

Fit parameters are extracted froma simultaneous fits to π, K, pspectra, results are sensitive to thefitting range



Kinetic freeze-out temperature decreases, transverse flow velocity increases with multiplicity
Consistent results for Pb-Pb and Xe-Xe at similar multiplicities, Tkin ~ 100 MeV < Tchpp and p-Pb are also consistent but with larger values of <T> at similar multiplicities

Tkin stays constant in pp and slightly decreases in p-Pb, Tkin ~ 160 MeV ~ Tch  earlier
decoupling compared to heavy-ion collisions

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Baryon-to-meson ratios in heavy-ion collisions
Phys. Rev. C 101, 044907

Enhanced baryon-to-meson ratios (p/,/K) in central heavy-ion collisions atintermediate pT  quark coalescence +flowing mediumEnhancement is similar at different
energies, peak is shifted to the right at
higher energyp/ ratio is almost constant vs. pT atintermediate momenta in Pb-Pb and
Xe-Xe collisions  spectral shapes
are driven by particle masses:
 consistent with hydrodynamics
 recombination models are not
ruled-out (V. Greco et al, PRC 92
054904 (2015))

Eur.Phys.J.C 81 (2021) 7, 584

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Baryon-to-meson ratios in small systems
Phys. Rev. C99 (2019) no.2, 024906

pp@7



p-Pb@5

Pb-Pb@2.76

Qualitatively similar behavior for three systems, from peripheral to central collisions: depletion at low pT enhancement at intermediate pT
 consistent at high pTSmooth evolution with multiplicity between the collision systems at given pT
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Baryon-to-meson ratios in jets
arXiv:2105.04890



Baryon-to-meson enhancement in p-Pb andpp collisions: not seen in jets driven by soft processes (bulk effect)

 PYTHIA 8 is consistent with ratios in jets butdoes not reproduce the inclusive ratios atintermediate pT Input for model tuning  contributions of thefragmentation, coalescence and flow

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8Strangeness production: pp, p-Pb, Xe-Xe and Pb-Pb
Nature Phys. 13 (2017) 535

Strangeness enhancement increases with strangeness content and particle multiplicity
Ratios saturate in central A-A at values predicted by statistical hadronization models
Smooth evolution vs. multiplicity in pp, p-Pb, Xe-Xe, Pb-Pb collisions at various sNN
STAR measurements in Cu-Cu, Au-Au at sNN = 200 GeV are in agreement with ALICE p-Pb
results at similar <dNch/d>. Results from pp collision at s = 200 GeV are consistent within
the large uncertainties with ALICE results Origin of the strangeness enhancement in small/large systems is still under debate
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Effective energy analysis in pp

Energy available for particle productionin the initial stateismeasured with ZDC at || > 8.8

Relative  production has been shown to depend on the final state multiplicity in theprevious slide

Relative  production does not show dependence on

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(initial state)

10Strangeness enhancement for (1020)
Phys. Lett. B807 135501(2020)  with hidden strangeness is a key
probe to study strangenessenhancement / increases with multiplicity in
pp/p-Pb  not expected forcanonical suppression / saturates in Pb-Pb and isconsistent with thermal model
predictions



Ratios /K and / show weak dependence on multiplicity  has an effective strangeness of 1 or 2
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Short-lived resonances

Final state yields of resonances depend on:resonance yields at chemical freeze-out
lifetime of the resonance and the hadronic phasetype and scattering cross sections of daughter particles
Lomonosov conference - 202112/, K*0/K, */, */, *0/  and /K ratios
PLB 802 (2020) 135225



Smooth evolution of the ratios with multiplicity from pp to Pb-Pb; suppression for short-lived 0, K*(892)0,(1385) and (1520)  dominance of rescattering over regeneration; no significant effects for longer lived
(1530)0 and (1020)

EPOS + UrQMD qualitatively describe the trends; consistency between RHIC and the LHCLower limit on the hadronic phase lifetime, had.phase ~ 5-7 fm/c in central Pb-Pb



Hint of the finite lifetime of hadronic phase in high multiplicity pp/p-Pb
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Polarization of vector mesons
Non-central heavy-ion collisions: Light quarks can be polarized byand If vector mesons are produced via
recombination their spin may align

Quantization axis:
 normal to the production plane(momentum of the vector meson andthe beam axis)

 large angular momentum due to medium rotations(PRC 77 (2008) 024906, Beccattini et al.) very high magnetic field (~ 1014 T) formed for a shortperiod of time (NPA 803 (2008), Kharzeev et al.) normal to the event plane (impactparameter and beam axis)

 Measure as anisotropies in angular
distributions:𝑑𝑁= 𝑁 1 − 𝜌 , + 𝑐𝑜𝑠 𝜃 3𝜌 , − 1𝑑𝑐𝑜𝑠𝜃𝜌 , is a probability for vector meson to bein spin state = 0  𝜌 , = 1/3 corresponds
to no spin alignment

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Measure using K*(892)0 and (1020)
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Results for K*0 and  vs. pT
PRL 125, 012301 (2020) 00 ~ 1/3 for:


pT(K*0) > 2 GeV/c and pT() > 0.8 GeV/c
𝐾 with zero spin
K*0 and  in pp collisions
K*0 and  with random plane in Pb-Pb@2.76

 00 < 1/3 for K*0 and  at low pT in semi-central PbPb@2.76 collisions
 Confirmed with new measurements in Pb-Pb@5.02

 Results for pT and centrality dependence of 00 are
qualitatively consistent with quark recombination ina polarized mediumLomonosov conference - 2021

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Summary Light-flavor hadron spectra become harder with increasing multiplicity, mean transverse
momenta show mass ordering in central A-A collisions, the ordering is broken in peripheral
collisions as well as in p-Pb and pp
 Blast-Wave fits allow to extract common expansion velocity and kinetic freeze-out
temperature Enhancement of baryon to meson ratios at intermediate pT is a bulk effect driven by flow
and quark coalescence Relative particle abundances are driven by the final state multiplicities rather than bycollision system size or energy
 Strangeness enhancement smoothly evolves with final state multiplicity, increasing /
ratio does not support the canonical suppression scenario
 Short-lived resonance yields are suppressed, the hadronic phase lifetime > 5-7 fm/c, a hintof finite hadronic phase lifetime is observed in small systems
 Spin alignment for vector mesons is qualitatively consistent with quark recombination in apolarized medium, more theoretical efforts are required for understanding of the data

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BACKUP

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Particle identification, ALICE
TPC

TOF

Int. J. Mod. Phys. A 29 1430044 (2014)

ITS

HMPID

Secondary vertex
reconstruction andtopology selections

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Baryon-to-meson ratios: pp
Phys. Rev. C99 (2019) no.2, 024906

No unique explanation for baryon-to-meson ratios in small systems



Pythia8 with color reconnection and DIPSY with color ropes qualitatively describe pp dataEPOS-LHC over-predicts effect by collective radial expansion
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Particle yields in pp and p-Pb vs. dNch/d
PLB 807 (2020) 135501

 Hadron yields increase ~ linearly with multiplicity, consistently for pp and p-Pb collisions
at different energies
 Hadrochemistry is driven by event activity rather than by collision energy or size of thecollision system Qualitative description by models

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p/π and K/π ratios
PLB 760 (2016) 720-735

At similar multiplicities, particle ratios are consistent for different collision systems at different sNN
 driven by event activity rather than by type of colliding nuclei and/or collision energy p/π shows a modest decrease with centrality at the LHC, consistent with antibaryon-baryon annihilationin the hadronic phase, which is more important in dense systems (Phys. Rev. Lett. 110, 042501)
 Increasing K/π ratio is consistent with strangeness enhancement

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