Recent highlights in the light-flavour sector from ALICE

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1 Recent highlights in the light-flavour sector from ALICE Enrico Fragiacomo INFN - Trieste MIAMI 2016 Lago Mar Resort, Fort Lauderdale, Florida December 2016

2 Ultra-Relativistic Heavy-Ion collisions Quark-Gluon Plasma (QGP) created in ultra-relativistic heavy-ion collisions [J. D. Bjorken, Phys. Rev. D 27 (1983) 140] After the collision, QGP fireball expands, develops collective flow and cools down Phase transition to hadron gas at a temperature T critical Chemical compositions frozen at T chem Final-state interactions in the late hadron gas phase Kinetic freeze-out at T kin once elastic collisions stop 2

3 What we learn from light-flavour particle production in HI collisions Bulk production of particles Statistical models predict the yields at chemical freeze-out Collective behaviour Radial flow modifies particle spectra Strangeness enhancement Late hadronic phase after chemical freeze-out Resonance reconstruction is affected 3

4 What we learn from light-flavour particle production in HI collisions Bulk production of particles Statistical models predict the yields at chemical freeze-out Collective behaviour Radial flow modifies particle spectra Strangeness enhancement Late hadronic phase after chemical freeze-out Resonance reconstruction is affected Smaller systems (pp, p-pb) Benchmark for Pb-Pb Interesting per sé to study possible transition to QGP 4

5 The ALICE experiment Size: 16 x 26 meters Weight: 10,000 tons Tracking down to p T ~0.1 GeV/c High granularity for high multiplicity Unique particle identification capabilities 5

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7 Inner Tracking System (ITS) Six layers of different-technology silicon detectors Full azimuthal coverage at mid-rapidity ( η <0.9) Trigger, tracking, vertex and PID via de/dx ALICE Coll., Int. J. Mod. Phys. A29 (2014)

8 Inner Tracking System (ITS) Six layers of different-technology silicon detectors Full azimuthal coverage at mid-rapidity ( η <0.9) Trigger, tracking, vertex and PID via de/dx ITS layers ALICE Coll., Int. J. Mod. Phys. A29 (2014)

9 Time Projection Chamber (TPC) 90 m 3 drift volume filled with Ar-CO 2 Full azimuthal coverage at mid-rapidity ( η <0.9) Tracking, vertex, PID via truncated-mean de/dx ALICE Coll., Int. J. Mod. Phys. A29 (2014)

10 Time-Of-Flight (TOF) detector Multigap Resistive Plate Chamber (MRPC) technology PID at intermediate momenta via time-of-flight Resolution of 80 ps for pions with p T = 1 GeV/c ALICE Coll., Int. J. Mod. Phys. A29 (2014)

11 High-Momentum Particle ID detector Ring-imaging Cherenkov detector Liquid C 6 F 14 radiator and CsI photo-cathode Charged-hadron PID at intermediate momenta ALICE Coll., Int. J. Mod. Phys. A29 (2014)

12 Separation power of hadron ID Δ pk (Δ Kp ) = distance between peaks of pion and kaon (kaon and proton) s p (s K ) = standard deviation of pion (kaon) response ALICE Coll., Int. J. Mod. Phys. A29 (2014)

13 Particle multiplicity and centrality Multiplicity is defined as the number of primary charged particles per event It is related to the collision centrality in Pb Pb collisions (i.e. impact parameter) 2.8 < h LAB < 5.1 and -3.7 < h LAB < -1.7 Pb-Pb In ALICE, event activity is measured at forward rapidity with the V0 detector Centrality classes defined as percentiles of the V0 signal distribution and related to observables via Glauber model

14 What we learn from light-flavour particle production in HI collisions Bulk production of particles Statistical models predict the yields at chemical freeze-out Collective behaviour Radial flow modifies particle spectra Strangeness enhancement Late hadronic phase after chemical freeze-out Resonance reconstruction is affected 14

15 Thermal models of particle production Production of (most) lightflavour hadrons (including nuclei) in Pb-Pb is well described by thermal models with a common freeze-out temperature GSI-Heidelberg: Andronic et al., Phys. Lett. B673 (2009) THERMUS: S. Wheaton et al., Comp. Phys. Com. 180 (2009) SHARE: Petran et al., arxiv: Good fits with few parameters: temperature T, baryochemical potential μ B, and volume V. Assuming thermal and chemical equilibrium Deviations for protons: incomplete hadron spectrum,... K* 0 resonance: final-state effects in the late hadronic phase? 15

16 What we learn from light-flavour particle production in HI collisions Bulk production of particles Statistical models predict the yields at chemical freeze-out Collective behaviour Radial flow modifies particle spectra Strangeness enhancement Late hadronic phase after chemical freeze-out Resonance reconstruction is affected 16

17 Collective behaviours in Pb Pb Mass dependence of the spectral shape is indicative of radial flow ALICE Coll., B. Adelev et al., Centrality dependence of π, K, and p production in Pb-Pb collisions at snn = 2.76 TeV, Phys. Rev. C88 (2013) Blast Wave simplified hydrodynamic model: describes spectral evolution well with three parameters: m T, β T and T kin (β T radial velocity, T kin kinetic freeze-out temperature) E. Schnedermann et al., Thermal phenomenology of hadrons from 200A Gev S+S collisions, Phys. Rev. C48 (1993) 2462 Used for a combined fit to p, K and p spectra gives β T and T kin for each centrality class 17

18 Collective behaviours in Pb Pb Clear trend is observed indicating a faster expansion for more central collisions, with lower kinetic freeze-out temperature 18

19 Collectivity in small systems Same study repeated for p-pb and pp collisions p-pb: ALICE Coll., Phys. Lett B760 (2016) 720 pp: ALICE preliminary Highest multiplicity pp (Class I+II in Fig.) Simultaneous fit to p, K and p spectra to extract the parameters Blast Wave model with p, K and p fit parameters predicts spectra of K S0, L, X -, W -, K *0 well 19

20 Collectivity in small systems Fit parameters in pp and p-pb collisions show a trend qualitatively resembling the trend in Pb-Pb collisions Hints of collective effects in smaller systems?

21 What we learn from light-flavour particle production in HI collisions Bulk production of particles Statistical models predict the yields at chemical freeze-out Collective behaviour Radial flow modifies particle spectra Strangeness enhancement Late hadronic phase after chemical freeze-out Resonance reconstruction is affected

22 Strangeness enhancement Strangeness enhancement was one of the first signatures proposed for QGP formation J. Rafelski and B. Müller, Strangeness production in the quark-gluon plasma, Phys. Rev. Lett. 48, 16 (1982) 1066 [Erratum: Phys. Rev. Lett. 56 (1986) 2334] Clear increase of strangeness production from pp to Pb-Pb GSI-Heidelberg: Andronic et al., Phys. Lett. B673 (2009) THERMUS: S. Wheaton et al., Comp. Phys. Com. 180 (2009) Pb-Pb: ALICE Coll., B. Abelev et al., Phys. Lett. B728 (2014) [Erratum: Phys. Lett. B734 (2014) 409] pp: ALICE Coll., B. Abelev et al., Phys. Lett. B712 (2012)

23 Strangeness production in smaller systems Results for p-pb collisions are consistent with pp at low multiplicities and with central Pb-Pb at high multiplicities p-pb: ALICE Coll., J. Adam et al., Phys. Lett. B758 (2016) 389 K. Redlich and A. Tounsi, Strangeness enhancement and energy dependence in heavy ion collisions, Eur. Phys. J. C24 (2002) A canonical suppression mechanism describes the multiplicity decrease (with respect to Pb-Pb) for smaller systems, indicating a shrinking of the production volume 23

24 Strangeness production in smaller systems First observation of enhanced production of strange particles in high-multiplicity pp collisions ALICE Coll., J. Adam et al., Multiplicity-dependent enhancement of strange and multi-strange hadron production in protonproton collisions at s = 7 TeV, arxiv: Common Monte Carlo models used at LHC (e.g. PYTHIA8) fail to describe the data PYTHIA8: T. Sjostrand, S. Mrenna, and P.Z. Skands, Comp. Phys. Comm. 178 (2008) 852 DIPSY: C. Bierlich and J.R. Chrstiansen, Phys. Rev. C92 (2015) EPOS: T. Pierog et al., Phys. Rev. C 92 (2015)

25 Strangeness production in smaller systems First observation of enhanced production of strange particles in high-multiplicity pp collisions ALICE Coll., J. Adam et al., Multiplicity-dependent enhancement of strange and multi-strange hadron production in protonproton collisions at s = 7 TeV, arxiv: Common Monte Carlo models used at LHC (e.g. PYTHIA8) fail to describe the data Strength of enhancement depends on strangeness content 25

26 What we learn from light-flavour particle production in HI collisions Bulk production of particles Statistical models predict the yields at chemical freeze-out Collective behaviour Radial flow modifies particle spectra Strangeness enhancement Late hadronic phase after chemical freeze-out Resonance reconstruction is affected 26

27 QGP Resonances and the hadronic phase K - f K *0 Re-scattering K - p + K *0 Regeneration p + K + Phys. Rev. C 91 (2015) Hadronic phase K - Chemical freeze-out Kinetic freeze-out ( Particle yields) ( Spectral shapes) t (fm/c) 27

28 Resonances and the hadronic phase Measured resonance yields depend on: Initial yields at chemical freeze-out Duration of hadronic phase between chemical and kinetic freeze-out Resonance lifetimes Compare several resonances with different lifetime G. Torrieri and J. Rafelski, Phys. Lett. B509 (2001) Cross sections for re-scattering and regeneration processes UrQMD model: S.A. Bass et al., Prog. Part. Nucl. Phys. 41 (1998) 255 S.V. Vogel and M. Bleicher, Proc. Nucl. Phys. Winter Meeting (2005) 28

29 Comparing K* and f K* 0 /K - decreases going from pp to most central Pb-Pb collisions f/k - shows a flat behaviour consistent with the value measured in pp collisions pp: ALICE Coll., B. Abelev et al., Eur. Phys. J. C72 (2012) 2183 p-pb: ALICE Coll., J. Adam et al., Eur. Phys. J. C76 (2016) 245 Pb-Pb: ALICE Coll., B. Abelev et al., Phys. Rev. C91 (2015)

30 Comparing K* and f GC Thermal model: Andronic et al., Phys. Lett. B673 (2009) K* 0 /K - decreases going from pp to most central Pb-Pb collisions f/k - shows a flat behaviour consistent with the value measured in pp collisions K* 0 /K - is suppressed in central Pb-Pb collisions relative to thermal model predictions Hints of re-scattering effects in the hadronic phase Decrease is described by EPOS3 with UrQMD f/k - consistent with thermal models EPOS3 with UrQMD: A.G. Knospe et al., Phys. Rev. C 93 (2016)

31 Adding r to the scene t (fm/c) r 1.3 K* 0 4 f 45 r/p decreases going from pp to most central Pb-Pb collisions Similar to K* 0 /K - decrease Trend is predicted by EPOS3 with UrQMD THERMUS: S. Wheaton et al., Comp. Phys. Com. 180 (2009) EPOS3 with UrQMD: A.G. Knospe et al., Phys. Rev. C 93 (2016)

32 Other resonances: S* ± and X* 0 Yield ratios of excited to ground-state hyperons with the same strangeness content show no dependence on charged-particle multiplicity Behaviour unexpected considering K* 0 /K - and similar lifetimes of S* and K* 0 Regeneration compensates for re-scattering? t (fm/c) r 1.3 K* 0 4 S* 5.5 X* 0 22 f 45 32

33 Conclusions Thermal models describe hadron production assuming thermal equilibrium Effects of radial flow in p-pb and pp Effects of suppression of strangeness enhancement in pp collisions Resonances probe the late hadronic phase of the expanding fireball Thank you for your attention!