strangeness production in high multiplicity pp collisions€¦ · •n part-scaling does not hold...

Strangeness production in high multiplicity pp collisions

Marek Bombara on behalf of the ALICE Collaboration(Pavol Jozef Šafárik University, Košice, Slovakia)

Critical Point and Onset of Deconfinement 2017 Stony Brook University, USA

7-11 August 2017

1

• Historically, an enhanced production of strangeness was proposed as a signature of QGP in nucleus-nucleus collisions [J. Rafelski, B. Müller, Phys. Rev. Lett. 48 (1982) 1066–1069]

• However, the measured effect can be qualitatively explained as a strangeness suppression in reference sample (pp and p−A collisions)

Strangeness enhancement

Marek Bombara, Critical Point and Onset of Deconfinement 2017, Stony Brook, August 7, 2017

⟩part

N⟨1 10 210

re

lativ

e t

o p

p⟩

pa

rtN⟨

Yie

ld /

1

10

= 2.76 TeVNNsPb-Pb at

+Ω+

-Ω

-ΞΛ

⟩part

N⟨1 10 210

re

lativ

e t

o p

p/p

-Be

⟩p

art

N⟨Y

ield

/

1

10

+Ω+

-Ω

-ΞΛ

NA57 Pb-Pb, p-Pb at 17.2 GeV

STAR Au-Au at 200 GeV

ALI−DER−80680

2

ALICE, Phys. Lett. B 728 (2014) 216

• Npart-scaling does not hold at LHC energies, ratios to pions (as the most abundant particle) are studied

• hyperon-to-pion ratios for pp higher at LHC than at RHIC energies

• for A−A - no obvious collision energy dependence


⟩part

N⟨1 10 210

Hyp

ero

n-t

o-p

ion

ra

tio

-410

-310

π/Ξ

π/Ω

ALICE Pb-Pb at 2.76 TeV

ALICE pp at 7 TeV

ALICE pp at 900 GeV

STAR Au-Au, pp at 200 GeV

ALICE Pb-Pb at 2.76 TeV

ALICE pp at 7 TeV

STAR Au-Au, pp at 200 GeV

(c)

ALI−PUB−78357

3

LHC offers not only unprecedented energies, but also unprecedented collision statistics in small systems ⇒ allows extensive multiplicity study of the reference samples (pp and p−Pb).

ALICE, Phys. Lett. B 728 (2014) 216

Strangeness enhancement

Multiplicity studies at the LHC


η∆-4-2

02

4

φ∆ 02

4

φ∆

dη∆d

pair

N2 d tri

gN1 1.6

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110≥ trkoffline = 5.02 TeV, NNNsCMS pPb

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p 1 <

= 5.02 TeVNN

sp-Pb

(0-20%) - (60-100%)

ALI−PUB−46246

η∆-4

-20

24

φ∆0

2

4

)φ

∆,η

∆R

( -202

>0.1GeV/c T

(a) CMS MinBias, p

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φ∆0

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• structures previously connected to collectivity in Pb−Pb collisions

• many other observables in high multiplicity pp and p−Pb resemble those in A−A

4

CMS, JHEP 09 (2010) 091

CMS, PLB 718 (2013) 795

ALICE, PLB 719 (2013) 29

It all started with ridges:



η∆-4-2

02

4

φ∆ 02

4

φ∆

dη∆d

pair

N2 d tri

gN1 1.6

1.71.8


< 3 GeV/cT

1 < p(b)

(rad)

ϕ∆

-1

0

1

2

3

4

η∆

-2

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0.80

0.85

c < 4 GeV/T,trig

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p 1 <

= 5.02 TeVNN

sp-Pb

(0-20%) - (60-100%)

ALI−PUB−46246

η∆-4

-20

24

φ∆0

2

4

)φ

∆,η

∆R

( -202

>0.1GeV/c T

(a) CMS MinBias, p

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R(

-101

<3.0GeV/cT


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-20

24

φ∆0

2

4

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∆,η

∆R

(

-4-202

>0.1GeV/c T

110, p≥(c) CMS N

η∆-4

-20

24

φ∆0

2

4

)φ

∆,η

∆R

( -2-101

<3.0GeV/cT


5

CMS, JHEP 09 (2010) 091

CMS, PLB 718 (2013) 795

ALICE, PLB 719 (2013) 29




Are there any unusual trends also in strangeness production in high multiplicity pp collisions?



η∆-4-2

02

4

φ∆ 02

4

φ∆

dη∆d

pair

N2 d tri

gN1 1.6

1.71.8


< 3 GeV/cT

1 < p(b)

(rad)

ϕ∆

-1

0

1

2

3

4

η∆

-2

-1

0

1

2

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(ra

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N1

0.75

0.80

0.85

c < 4 GeV/T,trig

p2 <

c < 2 GeV/T,assoc

p 1 <

= 5.02 TeVNN

sp-Pb

(0-20%) - (60-100%)

ALI−PUB−46246

η∆-4

-20

24

φ∆0

2

4

)φ

∆,η

∆R

( -202

>0.1GeV/c T

(a) CMS MinBias, p

η∆-4

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∆,η

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( -2-101

<3.0GeV/cT


6

CMS, JHEP 09 (2010) 091

CMS, PLB 718 (2013) 795

ALICE, PLB 719 (2013) 29




Are there any unusual trends also in strangeness production in high multiplicity pp collisions?

A Large Ion Collider Experiment

Marek Bombara, Critical Point and Onset of Deconfinement 2017, Stony Brook, August 7, 2017 7

Data: - pp √s = 7 TeV, 100 M 2010 - pp √s = 13 TeV, 50 M 2015

ITS (Inner Tracking System): primary vertex reconstruction, tracking, particle identification

V0: triggering, background suppression, event multiplicity determination

TPC (Time Projection Chamber): main tracking detector, particle identification

Multiplicity definition


(h/)

(h/)ppINEL>0

= 1 + aSb log

hdNch/di

hdNch/dippINEL>0

Class name I II III IV V

/INEL>0 0 0.95% 0.95 4.7% 4.7 9.5% 9.5 14% 14 19%

hdNch/di 21.3± 0.6 16.5± 0.5 13.5± 0.4% 11.5± 0.3 10.1± 0.3

Class name VI VII VIII IX X

/INEL>0 19 28% 28 38% 38 48% 48 68% 68 100%

hdNch/di 8.45± 0.25 6.72± 0.21 5.40± 0.17% 3.90± 0.14 2.26± 0.12

1

• analysed events have at least one charged particle in |η|<1 (INEL>0)

• event sample divided into 10 classes according to ionisation energy (~ proportional to the charged particle multiplicity) deposited in V0 detectors, covering the regions 2.8 < η < 5.1 and −3.7 < η < −1.7

• ⟨dNch/dη⟩ - average pseudorapidity density of primary charged particles in |η|< 0.5

Multiplicity classes used in pp@7TeV analysis:

8

(Multi-)Strange particle identification


)2) (GeV/cπ, Λ(invM

1.28 1.3 1.32 1.34 1.36

)2

Counts

/(M

eV

/c

0

5

10

15

20

25

310×

> 0.6 GeV/cT

p

)±Ξ(PDGM

(a)

-Ξ

+Ξ

)2, K) (GeV/cΛ(invM

1.64 1.66 1.68 1.7

)2

Counts

/(M

eV

/c

0

0.2

0.4

0.6

0.8

1

1.2

1.43

10×

> 0.8 GeV/cT

p

)±Ω(PDGM

(b)

-Ω

+Ω

ALI−PUB−14756

Ξ− Ω−

ΛΛ

π−

π−

K−

π−

p

p

⊗ B

• the (multi-)strange particle candidates are identified in the ALICE detector via decay topology

• in order to distinguish between signal and background, many geometrical selection criteria were applied on the candidates

• the yield is extracted by analysing invariant mass distributions of the decay products

Example of multi-strange decay topology

9

Physics Letters B Volume 712, 12 June 2012, Page 309

Identified particle pT spectra at 7 TeV


-1 )c

(G

eV

/T

p/d

yd

N2

devt

N1

/

7−10

6−10

5−10

4−10

3−10

2−10

1−10

1

10

210

310

410 -π++π

7

6

5

4

3

2

1

1

10

2

3

4 -+K

+K

0 5 10 15 20

8−10

7−10

6−10

5−10

4−10

3−10

2−10

1−10

1

10

210

310 pp+

7

6

5

4

3

2

1

1

10

2

3

4

S0

K

0 5 10 15 20

8

7

6

5

4

3

2

1

1

10

2

3

Λ+Λ

0 2 4 6 8 10 12

8

7

6

5

4

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2

1

1

10

2

3 +Ξ+

-Ξ

)c (GeV/T

p0 2 4 6 8 10

8

7

6

5

4

3

2

1

1

10

2

3

ALICE Preliminary

=7 TeVspp at

V0M Mult. Evt. Classes

+Ω+

-Ω

7

6

5

4

3

2

1

1

10

2

3

4

*KK*+p, p+

-+K

+, K-

π++π :

+Ξ+

-Ξ, Λ+Λ, S

0 K

)0

X (x 2)1IX (x 2

)2VIII (x 2)

3VII (x 2

)4VI (x 2)

5V (x 2

)6

IV (x 2)7III (x 2

)8

II (x 2)

9I (x 2

:+

Ω+-

Ω )

0IX + X (x 2

)1VII + VIII (x 2)2V + VI (x 2)

3III + IV (x 2

)4I + II (x 2

*:K K*+)

0X (x 2

)1IX (x 2)2VIII (x 2)

3VII (x 2

)4VI (x 2)

5IV + V (x 2

)6

III (x 2)7II (x 2)

8I (x 2

ALI−PREL−111017

• pT spectra get harder as the multiplicity increases

10

Baryon to meson ratios


ALI-PREL-110561

11

• shift + increase of the peak in most central Pb−Pb collisions ⇒ radial flow + quark recombination

• B/M qualitatively similar in all three colliding systems: depletion at low pT, enhancement in mid pT and no change in high pT

• more-or-less smooth evolution across the systems in low and mid pT region, production is driven by final multiplicity

• high pT independent on multiplicity

Baryon to meson ratios


ALI-PREL-110561

ALI-PREL-110566

12

• shift + increase of the peak in most central Pb−Pb collisions ⇒ radial flow + quark recombination

• B/M qualitatively similar in all three colliding systems: depletion at low pT, enhancement in mid pT and no change in high pT

• more-or-less smooth evolution across the systems in low and mid pT region, production is driven by final multiplicity

• high pT independent on multiplicity

• enhancement of strange particle w.r.t. non-strange yield clearly visible for high multiplicity pp collisions

• very nice overlap with p−Pb and Pb−Pb results

Strangeness enhancement in pp


ALI-PREL-134502

13

Enhancement observed as a function of ⟨dNch/dη⟩ is independent of collision type!

• remarkable overlap of p−Pb and peripheral Pb−Pb with Cu−Cu and Au−Au - even for 25 times smaller energy the strangeness production is similar!

• need more data for pp from RHIC (either multiplicity dependence study or from Beam Energy Scan programme)

Is the enhancement dependent on collision energy?


ALI-PREL-135175

14

ALI-PUB-106882

• The ratios of particle yields with a big mass difference do not show enhancement as a function of multiplicity

• no model* is able to reproduce the increase of the hyperon-to-pion ratios and the “flatness” baryon-to-meson ratios

Is the enhancement mass related?


ALICE, Nature Physics 13 (2017) 535

* There is a very recent improvement of the DIPSY prediction for p/π ratio, see the talk of C. Bierlich at SQM’17

• can be fitted by empirical function with the number of valence quarks as a fixed parameter (S=1,2,3):


ALI-PUB-106886

(h/)

(h/)ppINEL>0

= 1 + aSb log

hdNch/di

hdNch/dippINEL>0

16

Strangeness enhancement in pp


Hierarchy of the enhancement determined by the hadron strangeness!


0 2 4 6 8 10 12

]-1 )

c)

[(G

eV

/yd

Tp

/(d

N2 d

ev

N1

/

8−10

7−10

6−10

5−10

4−10

3−10

2−10

1−10

1

10

210

310

V0M multiplicity event classes)

9I (x 2 )

8II (x 2

)7III (x 2 )6

IV (x 2)

5V (x 2 )4VI (x 2

)3

VII (x 2 )2VIII (x 2IX (x 2) X

Lévy-Tsallis fit

= 13 TeVsALICE Preliminary - pp

S0K

|<0.5y|

)c (GeV/T

p

0 2 4 6 8 10 12

Ra

tio t

o I

NE

L>

0

1−10

1

10

ALI-PREL-116278

0 1 2 3 4 5 6 7 8

]-1 )

c)

[(G

eV

/yd

Tp

/(d

N2 d

ev

N1

/

8−10

7−10

6−10

5−10

4−10

3−10

2−10

1−10

1

10

210


9I (x 2 )

8II (x 2

)7III (x 2 )6

IV (x 2)

5V (x 2 )4VI (x 2

)3


Lévy-Tsallis fit

= 13 TeVsALICE Preliminary - pp

Λ+Λ|<0.5y|

)c (GeV/T

p

0 1 2 3 4 5 6 7 8

Ra

tio t

o I

NE

L>

0

1−10

1

10

ALI-PREL-116282

17

• hardening of the spectra with increasing multiplicity observed in the pT-region:| pT |<4 GeV/c

Strange particle pT spectra at 13 TeV


0 1 2 3 4 5

]-1 )

c)

[(G

eV

/yd

Tp

/(d

N2 d

ev

N1

/

7−10

6−10

5−10

4−10

3−10

2−10

1−10

1

V0M multiplicity event classes)4I+II (x 2 )

3III+IV (x 2

)2V+VI (x 2 VII+VIII (x 2)

IX+X

Lévy-Tsallis fit

= 13 TeVsALICE Preliminary - pp +Ω+

-Ω

|<0.5y|

)c (GeV/T

p

0 1 2 3 4 5

Ratio

to IN

EL>

0

1−10

1

10

ALI-PREL-116290

0 1 2 3 4 5 6

]-1 )

c)

[(G

eV

/yd

Tp

/(d

N2 d

ev

N1

/

7−10

6−10

5−10

4−10

3−10

2−10

1−10

1

10

210


9I (x 2 )

8II (x 2

)7III (x 2 )6

IV (x 2)

5V (x 2 )4VI (x 2

)3


Lévy-Tsallis fit

= 13 TeVsALICE Preliminary - pp +Ξ+

-Ξ

|<0.5y|

)c (GeV/T

p

0 1 2 3 4 5 6

Ratio

to IN

EL>

0

1−10

1

10

ALI-PREL-116286

18

• hardening of the spectra seems to be species dependent

Multi-strange particle pT spectra at 13 TeV

⟨pT⟩ as a function of multiplicity


ALI-PREL-116330 ALI-PREL-116336


19

• spectra harder in 13 TeV than in 7 TeV for the same multiplicity

Yield as a function of multiplicity




20

• linear increase of the yields with multiplicity, yields are invariant with respect to collision energy for a given event multiplicity!

Outlook

• a special high-multiplicity trigger in pp at 13TeV was used during the data taking in 2016

• enough statistics to study 0−0.1% and 0−0.01% multiplicity samples


ALI-PERF-131160

Summary

• intriguing trends observed in strangeness production in high multiplicity pp collisions resembling the trends from Pb−Pb, like e.g. hardening of pT spectra or similarities in B/M ratios

• enhancement of strange particle production w.r.t. pion production as a function of multiplicity seen in pp collisions

• for the same multiplicity: the strangeness production does not depend on collision system and (at least for LHC) also on collision energy

• production of particles with higher strangeness content is more enhanced in comparison with inclusive production

• stay tuned for the results obtained using data collected with a high-multiplicity trigger in pp collisions at 13 TeV


Backup slides

23

(dNch/dη)/(⟨Npart⟩/2) as a function of the number of participants


⟩part

N ⟨0 100 200 300 400

/2)

⟩p

art

N ⟨)/

(η

/dc

hN

(d

4

5

6

7

8

9

DPMJET III [10]HIJING 2.0 (sg=0.23) [11]HIJING 2.0 (sg=0.20) [11]Armesto et al. [12]Kharzeev et al. [13]Albacete et al. [14]

Pb-Pb 2.76 TeV ALICE

ALI−PUB−8816

ALICE, Phys. Rev. Lett. 106 (2011) 032301


ALI-PREL-108831

• the hardening is more pronounced for heavier particles

• similar behaviour observed also in p−Pb and Pb−Pb collisions where it can be interpreted as a particle emission from a collectively expanding thermal source (radial flow)

• do we see a radial flow in pp as well?

25

pT spectra at 7 TeV

Blast-Wave fit


-1 )c

(G

eV

/T

p/d

ydN

2 d

evt

N1/

-710

-610

-510

-410

-310

-210

-110

1

10

210

310

410 = 7 TeVsALICE Preliminary, pp

V0M Class I+II

100× π

50×K p

10× S0K

0.1× Λ

0.05× Ξ

0.05× Ω-5 10×K*

, K, pπBlast-Wave fit to

Blast-Wave prediction

0 1 2 3 4 5 6 7 8 9

11.5

π cfit range: 0.5-1.0 GeV/

0 1 2 3 4 5 6 7 8 91

1.5 Kc0.2-1.5 GeV/

0 1 2 3 4 5 6 7 8 91

1.5 pc0.3-3.0 GeV/

0 1 2 3 4 5 6 7 8 9

11.5

S0

K

0 1 2 3 4 5 6 7 8 91

1.5Λ

0 1 2 3 4 5 6 7 8 91

1.5Ξ

0 1 2 3 4 5 6 7 8 9

11.5

Ω

)c (GeV/T

p0 1 2 3 4 5 6 7 8 9

11.5

K*

ALI−PREL−109116

5

10

15

20

25

30

35

40

5 10 15 20 25 30 35 40 450

1

2

3

4

5

6

7

8

9

10

|< 0

.5η|

⟩η

/dch

Nd⟨

⟩T

β⟨

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

(G

eV

)ki

nT

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

)c) , p (0.3-3.0 GeV/c) , K (0.2-1.5 GeV/c (0.5-1 GeV/πGlobal Blast-Wave fit to

= 7 TeVsALICE Preliminary, pp,

= 5.02 TeVsALICE, p-Pb,

= 2.76 TeVNNsALICE, Pb-Pb,

= 5.02 TeVNNsALICE Preliminary, Pb-Pb,

ALI-PREL-122516

26

• for the similar multiplicity: the kinetic freeze-out temperature and the average transverse velocity are higher for pp than for Pb−Pb

• Spectra are harder for higher multiplicity event classes. Similar effect seen in p−Pb can be explained by collectivity.

• in HI such behaviour could be explained by models based on relativistic hydrodynamics

• blast-wave fit to all species for class I points to thermal source at kinetic freeze-out temperature 163 MeV and common transverse velocity 0.49

pT spectra at 7 TeV


ALI-PUB-106874

27


strangeness production in high multiplicity pp collisions€¦ · •n part-scaling does not hold...

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