Strangeness Production
and Nuclear Modification
at LHC Energies
Oliver Busch
University of Tsukuba
Heidelberg University

for the ALICE collaboration

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Outline• Introduction• Nuclear modification of identified hadrons in Pb-Pb collisions• System size dependence of multi-strange particle
production in pp, p-Pb and PbPb collisions

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Strangeness with ALICE
forward detectors:• trigger, timing,• multiplicity, centrality

Inner Tracking System• vertex reconstruction• low-pT tracking

central barrel:• tracking |η |< 0.9• PID
Oliver Busch – Hard Probes 2016

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Charged particle identification• practically all known particle identification techniques are used in ALICE• identification of light flavour hadrons with pT from 0.1 to 20 GeV/c4

(Multi-)strange hadron reconstruction• charged tracks reconstructedin ITS and TPC• TPC PID: identify decay daughters• decay topology: combine reconstructed
tracks to particle candidates

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(Multi-)strange hadron reconstruction• charged tracks reconstructedin ITS and TPC• TPC PID: identify decay daughters• decay topology: combine reconstructed
tracks to particle candidates• yield extraction via invariant mass
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π, K, p in pp and Pb-Pb at 2.76 TeV• low pT (< 3 GeV/c): bulk particle production, collectivity• high pT (> 10 GeV/c): fragmentation, parton energy loss
• intermediate pT: interplay soft-hard, hadronization via recombination ?

Phys. Rev. C93 (2016) 034914

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Nuclear modification factor• RAA < 1: at high pT, strong, centrality dependent suppression• no species dependence at high pT

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(Multi-)strange hadron spectra• particle and anti-particle spectra compatible• spectra at low pT harder for more central collisions and higher particle mass9

Ξ nuclear modification factor• RAA < 1: at high pT, strong suppression: jet quenching• no species dependence at high pT• RAA of Ξ consistent with p• vacuum fragmentation in jet core ?
• fragmentation bias of hadronic
observable ?
Strangeness production in jets:
Y. Zhang, Session III
09/24 15:20

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Ω nuclear modification factor

R AA• RAA < 1: at high pT, strong suppression: jet quenching• no species dependence at high pT• RAA of Ξ consistent with p, Ω RAA > Ξ RAA
Pb-Pb/pp s NN = 2.76 TeV±K [|y|<0.8 for p T>3 GeV/c]
p+p [|y|<0.8 for p T>3 GeV/c]
-+-+

Ξ +Ξ

• mass ordering at mid-pT• mp = 0.94 GeV/c2• mΞ = 1.32 GeV/c2• mΩ = 1.67 GeV/c2

ALICE preliminary

π± [|y|<0.8 for p T>2 GeV/c]00-10%|y| < 0.5

uncertainties: stat. (bars)
uncertainties: sys. (empty boxes)Ω +Ωuncertainties: norm. (gray box)

21012345678p (GeV/ c )ALI−PREL−86198

T11

Nuclear modification factor

R AA• RAA < 1: at high pT, strong suppression, jet quenching• no species dependence at high pT• RAA of Ξ consistent with p, Ω RAA > Ξ RAA
Pb-Pb/pp s NN = 2.76 TeV±

K [|y|<0.8 for p T>3 GeV/c]
p+p [|y|<0.8 for p T>3 GeV/c]-+-+Ξ +Ξ• mass ordering at mid-pT• mp = 0.94 GeV/c2• mΞ = 1.32 GeV/c2• mΩ = 1.67 GeV/c2• peripheral collisions: RAA closer to

ALICE preliminary

π± [|y|<0.8 for p T>2 GeV/c]

60-80%|y| < 0.5

uncertainties: stat. (bars)
uncertainties: sys. (empty boxes)Ω +Ω

uncertainties: norm. (gray box)

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unity for all species• how about system size dependence(strangeness enhancement) ?

1

0

1

2345

678p (GeV/ c )ALI−PREL−86214T12

System Size Dependence

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Strangeness enhancement: A-AHyperon-to-pion ratio• one of the earliest proposed QGP signatures(c)• pp: increase withΞ/π• A-A: enhancement over pp• saturation for Npart > 150

10-3

at value expected from statistical
grand-canonical modelsΩ/πALICE Pb-Pb at 2.76 TeV
ALICE pp at 7 TeV
ALICE pp at 900 GeV
STAR Au-Au, pp at 200 GeV

10-4

ALICE Pb-Pb at 2.76 TeV

GSI-Heidelberg Tch = 164 MeV
Thermus Tch = 170 MeV

ALICE pp at 7 TeVSTAR Au-Au, pp at 200 GeV

110Phys. Lett. B 728 (2014) 216

102〈N part〉A. Andronic et al., PLB 673 (2009) 142
J. Cleymans et al., PRC 74 (2006) 034903

ALI−PUB−78357

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Multiplicity dependence: p-Pb
• in p-Pb, both Ξ / π and Ω / π exhibit
strong rise• Ξ / π reaches Pb-Pb saturation value• Ω / π up to 60-80%• Ω stronger multiplicity dependence→ strangeness dependent effectPhys. Lett. B 758 (2016) 389
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Multiplicity dependence: pp• multiplicity classes based on summed amplitude in V0 detectors( 2.8 < η < 5.1, -3.7 < η < -1.7)• p / π and Λ / K0S: compatible between pp and p-Pb
• no significant multiplicity dependence• enhancement in p-Pb governedby strangeness content→ how about hyperons in pp ?

arXiv: 1606.07424 [nucl-ex]
16Multiplicity dependence: pp• Ξ and Ω in pp: strong multiplicity
dependence !

• pp similar to p-Pb
• pp models: color ropes

needed to produce multiplicity dependence• no model describes both Ξ and Ω

arXiv: 1606.07424 [nucl-ex]
17Multiplicity dependence: pp• Ξ and Ω in pp: strong multiplicity
dependence !

• pp similar to p-Pb
• pp models: color ropes

needed to produce multiplicity dependence• no model describes both Ξ and Ω… and Λ and p !

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Multiplicity dependence: pp• yield ratios normalised to pp INEL > 0: reduced uncertainties• p / π consistent with unity

arXiv: 1606.07424 [nucl-ex]

ALI-PUB-106925

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Multiplicity dependence: pp• yield ratios normalised to pp INEL > 0: reduced uncertainties• p / π consistent with unity• dependence on strangeness
content well described bywith b = 1.67

arXiv: 1606.07424 [nucl-ex]

ALI-PUB-106886

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Summary• identified charged π, K, p RAA for 0.1 < pT < 20 GeV/c• no significant species dependence observed• Ω RAA > Ξ RAA in measured range (pT < 7 GeV/c)• first observation of a multiplicity dependence of strangenessproduction in pp collisions• per-pion yields consistent between pp and p-Pb
• effect driven by strangeness content

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backup

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Multiplicity dependence in pp• INEL > 0 event class: 1 charged particle in |η|< 1• lines: Levy-Tsallis fits
• blast wave OK within ~5%, Tfo = 163 +/- 10 MeV, β = 0.49

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(Multi-)strange hadron spectra in p-Pb
• V0A multiplicity classes• low-pT spectral shape: similar trends as Pb-Pb

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p-Pb nuclear modification factor• π, K, p RpPb at high-pT consistent with unity→ suppression in Pb-Pb a final state effect• p, Ξ, Ω show enhancement
at intermediate pT• mass orderingOliver Busch – Hard Probes 201625

Nuclear modification factor• RAA < 1: at high pT, strong suppression, jet quenching

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Multiplicity dependence: p-Pb
• in p-Pb, both Ξ / π and Ω / π exhibit
strong rise• Ξ / π reaches PbPb saturation value• Ω / π up to 60-80%• Ω stronger multiplicity dependence• Ξ (15300): strangeness content as Ξ• mΞ < mΞ* < mΩ
• increase of Ξ* / π similar to Ξ / π→ multiplicity dependence driven bystrangeness, not mass

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Outlook: hyperons in pp• first measurement of hyperon production in pp minimum bias
collisions at 13 TeV• hint for an increase of hyperon-to-pion ratio with

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