Emergence of Hot and Dense QCD in Small Systems

Emergence of Hot and Dense QCD in Small Systems

Naghmeh Mohammadi (CERN)

On behalf of the WG5 Small Systems group YR WG5 meeting

30/10/2018

Naghmeh Mohammadi @WG5 HI meeting 1

30/10/2018

2 30/10/2018

Contributors

Naghmeh Mohammadi @WG5 HI meeting

3 30/10/2018

Emergence of Hot and Dense QCD in Small Systems

❖ Chapter outline:

❖ Introduction

❖ Overview of Experimental Results and Critical Assessment

❖ Open Questions and Addressing them at HL-LHC

❖ Proton–Proton Collisions at Extreme Multiplicities

❖ Global-Event Properties

❖ Particle Correlations

❖ Strangeness Enhancement

❖ Energy Loss

❖ Thermal Radiation

❖ Potential of O–O Collisions

❖ Summary

Naghmeh Mohammadi @WG5 HI meeting

4 30/10/2018

Emergence of Hot and Dense QCD in Small Systems

❖ Initially a reference for the effects observed in Pb-Pb collisions

❖ Observations in high multiplicity pp collisions:

❖ Azimuthal correlations of final state hadrons

❖ Enhanced production of multi-strange hadrons

JHEP09(2010)091

Nature Physics 13, 535–539 (2017)

Phys. Lett. B 765 (2017) 193

Naghmeh Mohammadi @WG5 HI meeting

Have not been measured in small systems

5

30/10/2018 Naghmeh Mohammadi @WG5 HI meeting

Overview of Experimental Results

Open questions

6

30/10/2018 Naghmeh Mohammadi @WG5 HI meeting

❖ For details check Aleksi Kurkela’s slides in HL-/HE-LHC Workshop on 19/06/18

❖ Link to the workshop:

❖ https://indico.cern.ch/event/686494/timetable/#42-emergence-of-hot- and-dense

❖ Is there a unified theory to describe small and large systems simultaneously?

❖ Is there a smooth transition from pp to PbPb collisions?

❖ Is there a perfect fluid in pp collisions?

❖ Is the physical origin of collectivity the same in small and large systems?

❖ To what extent the signs we have taken as sign of perfect fluidity are unique to perfect fluid?

❖ To tackle these questions: Higher luminosity needed for more detailed studies

Proton-proton collisions at extreme multiplicities

7 30/10/2018

❖ Multiplicity distribution extrapolation based on the current ALICE and ATLAS data

❖ Parametrization with single negative binomial distribution for various center of mass energies

❖ Extrapolated to 14 TeV pp collisions at ALICE and ATLAS

❖ Predict no. of events at a given multiplicity using smaller phase space (|η|<1.5)

❖ Extrapolate up to |η|<2.5 using flat η distribution

❖ Use PYTHIA to go to |η|<4.0 for run 4 0 20 40 60 80 100 120 140 160 N _ch (| η | < 1.5)

) ch P(N

10 -7

10 -6

10 -5

10 -4

10 -3

10 -2

10 -1 0.9 TeV

2.76 TeV 7 TeV 8 TeV

ALICE (|η|<1.5)

Naghmeh Mohammadi @WG5 HI meeting

Proton-proton collisions at extreme multiplicities

8 30/10/2018

| < 1.5) (| η

N ch

0 20 40 60 80 100 120 140 160

) ch P(N

10 -7

10 -6

10 -5

10 -4

10 -3

10 -2

10 -1 0.9 TeV

2.76 TeV 7 TeV 8 TeV

ALICE (|η|<1.5)

Naghmeh Mohammadi @WG5 HI meeting

❖ Multiplicity distribution extrapolation based on the current ALICE and ATLAS data

❖ Parametrization with single negative binomial distribution for various center of mass energies

❖ Extrapolated to 14 TeV pp collisions at ALICE and ATLAS

❖ Predict no. of events at a given multiplicity using smaller phase space (|η|<1.5)

❖ Extrapolate up to |η|<2.5 using flat η distribution

❖ Use PYTHIA to go to |η|<4.0 for run 4

Proton-proton collisions at extreme multiplicities

9 30/10/2018

| < 1.5) (| η

N ch

0 20 40 60 80 100 120 140 160

) ch P(N

10 -7

10 -6

10 -5

10 -4

10 -3

10 -2

10 -1 0.9 TeV

2.76 TeV 7 TeV 8 TeV

ALICE (|η|<1.5)

Naghmeh Mohammadi @WG5 HI meeting

❖ Multiplicity distribution extrapolation based on the current ALICE and ATLAS data

❖ Parametrization with single negative binomial distribution for various center of mass energies

❖ Extrapolated to 14 TeV pp collisions at ALICE and ATLAS

❖ Predict no. of events at a given multiplicity using smaller phase space (|η|<1.5)

❖ Extrapolate up to |η|<2.5 using flat η distribution

❖ Use PYTHIA to go to |η|<4.0 for run 4

❖ Number of events with equivalent multiplicity ranges

in pPb and Pb-Pb collisions

ch /dy

0 20 40 60 80 100 dN

) 3 (GeV/fm ε

1 10

10 2 Central Pb-Pb = 0.2 fm/c

τ

pp p-Pb Pb-Pb

IP Glasma Glauber MC

❖ Energy density:

❖ An estimate for pp, pPb and Pb-Pb collisions based on

❖ IP-Glasma

❖ Glauber MC (for pPb and PbPb) + Bjorken estimate

❖ Dependent on the system at fixed multiplicity

❖ It can reach large values in pp and pPb collisions, of the order of central Pb-Pb collisions

❖ One way of calculating the energy density

Naghmeh Mohammadi @WG5 HI meeting 10 30/10/2018

Energy density in different collision systems

Same multiplicity does not mean same energy density

Global-event properties

Naghmeh Mohammadi @WG5 HI meeting 11 30/10/2018

❖ Shape of the multiplicity distribution

❖ Mechanisms producing very high multiplicity events not clear

❖ Mean p T increases with multiplicity

❖ Measurements exist only up to dN ch /dη ~55

❖ HL-LHC will provide twice this value

❖ High multiplicity collisions originate from MPI within the same pp collision

❖ Understanding particle production in high energy pp collisions

❖ Saturation at large multiplicity

❖ Number of low momentum transfer parton interactions increase with multiplicity

❖ Maximal number of Parton interactions in a pp collision??

❖ Proton substructure

Particle correlations: multi-particle cumulants

Naghmeh Mohammadi @WG5 HI meeting 12 30/10/2018

❖ Particle correlations:

❖ In high multiplicity pp to compare with pPb and PbPb collisions

❖ In low multiplicity regions to investigate the onset of the collective-like effects

❖ Suppresses correlations from jets and dijets

❖ Measured in pp and pPb with Run 1 & 2 using 3 subevent method

❖ c3{4} lacks statistics in pp and mostly consistent with zero

❖ c3{4} negative non zero magnitude in PbPb collisions

❖ Is c 3 {4} negative in pp collisions?

c

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_n {4} = hhe ^{in( 1 + 2 3 4 )} ii 2 hhe ^{in( 1 2 )} ii ²

❖ 4 particle cumulants (c n {4})

N ch

50 100 150 200 250

{4} 3 c

− 1.5

− 1

− 0.5 0 0.5 1 1.5

− 6

× 10

prescaled HMT unprescaled HMT

Internal ATLAS

=13 TeV s

pp

<3.0 GeV

|<2.5 0.3<p T

| η

Run 2, 0.9 pb -1

Projected, 200 pb -1

3 {4}

1.5% v

3 {4}

2.0% v

❖ Symmetric cumulants

❖ Measure correlations between different flow harmonics

❖ Sensitive to

❖ Initial conditions

❖ Hydrodynamic evolution

❖ In small systems: better description of the initial condition and proton substructure

Particle correlations: symmetric cumulants

ch 〉

〈 N

100 150 200 250 300

SC(2,3)

− 0.4

− 0.2 0 0.2 0.4 0.6 0.8 1

− 6

× 10 CMS Internal

= 13 TeV s

p+p

< 3 GeV/c 0.3 < p T

| < 2.4

| η

Run 1+2, 2 pb -1

no-sub

Projected, 200 pb -1

no-sub 2-sub 3-sub 4-sub

ch 〉

〈 N

100 150 200 250 300 350 400 450 500

SC(2,3)

− 1

− 0.8

− 0.6

− 0.4

− 0.2 0 0.2 0.4 0.6

− 6

× 10 CMS Internal

= 5.02 TeV s NN

p+Pb

< 3 GeV/c 0.3 < p T

| < 2.4

| η

Run 1+2, 35 nb -1

no-sub

Projected, 1000 nb -1

no-sub 2-sub 3-sub 4-sub

Naghmeh Mohammadi @WG5 HI meeting 13 30/10/2018

❖ Current measurements -> large uncertainties

❖ Projections for SC(3,2) for HL pp and pPb collisions

❖ Projections for no-sub: uncertainties invisible but largely contaminated with non-flow

❖ 2,3 and 4-sub event methods possible: uncertainties of the order of a few 10 ^-7

SC(3, 2) =

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hv ₂ ² v ₃ ² i hv ₂ ² ihv ₃ ² i

❖ Particle correlations:

❖ In high multiplicity pp to compare with pPb and PbPb collisions

❖ In low multiplicity regions to investigate the onset of the collective-like effects

) c (GeV/

p T

0 1 2 3 4 5 6 7 8

sub 2

v

0.00 0.05 0.10 0.15 0.20

HL-LHC Projection p-Pb

0-20%

= 500 nb -1

L int

ALICE

< 0.04 y cms

in -1.26 <

→ e (c,b)

< 3.53 y cms

< -2.96 or 2.03 <

y cms

in -4.46 <

Inclusive J/ ψ

< 250

offline

N trk

185 <

= 2 pb -1

L int

CMS

< 0.54 y cms

in -1.46 <

Prompt D

< 1.94 y cms

< -1.86 or 0.94 <

y cms

in -2.86 <

Prompt J/ ψ

Naghmeh Mohammadi @WG5 HI meeting 14 30/10/2018

Particle correlations: heavy flavor in small systems

❖ v 2 for heavy flavor objects feasible in pPb collisions with HL-LHC:

❖ Inclusive J/ψ with ALICE, Prompt J/ψ and D by CMS

❖ Minor uncertainties expected

❖ Heavy flavor hadrons originate from heavy quarks that experienced all stages of the system evolution

❖ heavy flavor flow measurements:

❖ Low p T : test if heavy flavor quarks participate in the collective expansion dynamics

❖ Intermediate p T : sensitive to the heavy-quark hadronization

mechanism/recombination

v 2

0.1 0.2

) 2 v p(

− 4

10

− 3

10

− 2

10

− 1

10 1 10 10 2

ATLAS, JHEP 11 (2013) 183

= 2.76 TeV s NN

60-65% Pb-Pb, at

= 200 pb -1

= 14 TeV, L int

s Projection for pp at

HL-LHC Projection pp < 137

/d η N ch

98 < d

Naghmeh Mohammadi @WG5 HI meeting 15 30/10/2018

❖ Probability distribution of event-by-event v 2 (p(v 2 )) in an approximate level:

❖ Sensitive to

❖ Initial conditions and final state dynamics of the medium

❖ Not measured in small systems so far

❖ Expected to have a narrower width

❖ Feasible in small systems in HL-LHC

❖ Projections for pp at 14 TeV, L int = 200 pb ^-1 :

❖ Based on 60-65% Pb-Pb collisions at 2.76 TeV

Particle correlations: probability distribution of event-by-event v n

Strangeness enhancement

|< 0.5 η

〉 |

η

ch /d d N

〈

10 10 ² 10 ³

) - π + + π / ( + Ω + - Ω

10 -4

10 -3

= 7 TeV Nat. Phys. 13 (2017) 535 pp, s

Projection for 14 TeV pp, 200 pb -1

= 5.02 TeV PLB 728 (2014) 25 s NN

p-Pb,

= 5.02 TeV s NN

Preliminary Pb-Pb,

PYTHIA8 DIPSY

EPOS LHC ALICE Upgrade projection

Multiplicity slicing with mid-rapidity estimator

ALI−SIMUL−160917

Naghmeh Mohammadi @WG5 HI meeting 16 30/10/2018

❖ Key observable in run 2 pp physics:

❖ Smooth increase in strange-particle production as a function of system size

❖ pp collisions up to dN ch /dη = 17

❖ Most peripheral PbPb collisions down to dN ch /dη = 96

❖ Projection of the reach with pp collisions in HL-LHC

❖ Strangeness enhancement scaling with the energy density of the system

❖ continuous increase

❖ saturation at PbPb value (thermal limit)

Energy loss: hadron-jet correlations

) c (GeV/

T,jet

p ch

0 20 40 60 80 100 120 140 160 180

Ratio High EA / Low EA

0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

= 1.48e+06 N trig

High EA: percentile = 0.001,

= 7.39e+08 N trig

Low EA: percentile = 0.500,

c 90 percent CL, spectrum shift = -0.175 GeV/

ALICE Upgrade simulation = 14 TeV, 200 pb -1

s pp,

c < 20 GeV/

p T

Trigger: 15 <

= 0.4 R

T , k Jets: charged-only, anti-

) c (GeV/

T,jet

p ch

0 50 100 150 200

Ratio High EA / Low EA

0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

= 8.44e+06 N trig

High EA: percentile = 0.050,

= 8.44e+07 N trig

Low EA: percentile = 0.500,

c 90 percent CL, spectrum shift = -0.069 GeV/

ALICE Upgrade simulation = 5 TeV, 0.5 pb -1

s NN

− Pb, p

c < 20 GeV/

p T

Trigger: 15 <

= 0.4 R

T , k Jets: charged-only, anti-

Naghmeh Mohammadi @WG5 HI meeting 17 30/10/2018

❖ Absence of jet quenching in p-Pb collisions in run 1 & 2

❖ If final state interactions explain observed collective phenomena

❖ energy loss should be measurable OR put stringent limit

❖ Potential to identify small energy loss effects in small systems with jet recoil against other objects

❖ Projections for the modification of jet recoil yields extracted from hadron-jet correlations in run 3 and 4 for

pp and pPb collision: based on a PYTHIA simulation for pp collisions

Energy loss: γ and Z + jet correlations

T

/p γ T

= p jet j γ

x

0 0.5 1 1.5 2

γj dx γj

dN _γ

N 1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

ch 〉

〈 N

< 5

ch 〉

〈 N 5 - 7

ch 〉

〈 N 7 - 10

CMS ^Projection

> 60 GeV/c

γ

p T

< 1.44 η γ

8 π > 7

j γ

φ Δ

jet R = 0.3 anti-k T

> 30 GeV/c

jet

p T

< 1.6 η jet

pp 200 pb -1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Z

/p T jet

=p T

x jZ

0 0.5 1 1.5

2 2.5 3

/dx dN 1/N jZ jZ Z 3.5

Simulation Internal ATLAS

+ jet

→ ll Z

=8.8 TeV, 2 pb -1

s NN

+Pb, p

Powheg+Pythia8 0-100% centrality

>30 GeV

jet

p T

>60 GeV,

Z

p T

8 π

| > 7 φ

| Δ

|<2.5 η l

>20 GeV, |

l

p T

< -2

CM

η jet

-4.5 <

| < 1

CM

η jet

|

< 4.5

CM

η jet

2 <

Naghmeh Mohammadi @WG5 HI meeting 18 30/10/2018

❖ Absence of jet quenching in p-Pb collisions in run 1 & 2

❖ If final state interactions explain observed collective phenomena

❖ energy loss should be measurable OR put stringent limit

❖ Potential to identify small energy loss effects in small systems with jet recoil against other objects

❖ Projections for the correlations between jet, γ and Z in run 3 and 4 for pp and pPb collision

❖ γ and Z unmodified by final state interactions

Thermal radiation

❖ Search for thermal dilepton signal in pp and pPb

❖ QGP thermal radiation detection in pPb

❖ Extract a medium temperature

❖ Measurements in pPb collisions

❖ smoking gun for QGP

❖ Statistical uncertainty of 10% on the temperature

❖ If predictions accurate -> L int = 50 nb ^-1 sufficient for the measurement

❖ If signal 50% smaller -> 4-5 times the statistics is needed

❖ See Michael Weber’s talk on dileptons and thermal radiation

Naghmeh Mohammadi @WG5 HI meeting 19

30/10/2018

20 30/10/2018

Potential of O-O collisions

Naghmeh Mohammadi @WG5 HI meeting

❖ An opportunity to study

❖ The emergence of collective phenomena

❖ Possible energy loss

Highest multiplicities in pPb in the tail of the distribution Similar multiplicities reached in O-O collisions

❖ Study properties of low multiplicity (peripheral) Pb-Pb collisions

❖ O-O collision multiplicities similar to p-Pb collisions

❖ Collision geometry well defined

Summary and outlook

Naghmeh Mohammadi @WG5 HI meeting 21 30/10/2018

|< 0.5 η

〉 |

η

ch /d d N

〈

10 10 ² 10 ³

) - π + + π / ( + Ω + - Ω

10 -4

10 -3

= 7 TeV Nat. Phys. 13 (2017) 535 pp, s

Projection for 14 TeV pp, 200 pb -1

= 5.02 TeV PLB 728 (2014) 25 s NN

p-Pb,

= 5.02 TeV s NN

Preliminary Pb-Pb,

PYTHIA8 DIPSY EPOS LHC ALICE Upgrade projection

Multiplicity slicing with mid-rapidity estimator

ALI−SIMUL−160917

ch 〉

〈 N

100 150 200 250 300

SC(2,3)

− 0.4

− 0.2 0 0.2 0.4 0.6 0.8 1

− 6

× 10 CMS Internal

= 13 TeV s

p+p

< 3 GeV/c 0.3 < p T

| < 2.4

| η

Run 1+2, 2 pb -1

no-sub

Projected, 200 pb -1

no-sub 2-sub 3-sub 4-sub

ch 〉

〈 N

100 150 200 250 300 350 400 450 500

SC(2,3)

− 1

− 0.8

− 0.6

− 0.4

− 0.2 0 0.2 0.4 0.6

− 6

× 10 CMS Internal

= 5.02 TeV s NN

p+Pb

< 3 GeV/c 0.3 < p T

| < 2.4

| η

Run 1+2, 35 nb -1

no-sub

Projected, 1000 nb -1

no-sub 2-sub 3-sub 4-sub

T

/p γ T

= p jet j γ

x

0 0.5 1 1.5 2

γj dx γj

dN _γ

N 1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

ch 〉

〈 N

< 5

ch 〉

〈 N 5 - 7

ch 〉

〈 N 7 - 10

CMS Projection

> 60 GeV/c

γ

p T

< 1.44 η γ

8 π > 7

j γ

φ Δ

jet R = 0.3 anti-k T

> 30 GeV/c

jet

p T

< 1.6 η jet

pp 200 pb -1

❖ Discoveries in recent years caused a paradigm shift in modelling:

❖ Heavy ion collisions

❖ Underlying events in pp collisions

❖ Multi-particle correlations present also in small systems

❖ No evidence for other features related to final state interactions, e.g. energy loss

❖ HL-LHC provides the data required for understanding the remaining open question in small systems

❖ Higher order correlations

❖ Measurement of strange-particle yields

❖ Thermal radiation

❖ Energy loss signals

❖ …

❖ Understanding of non-perturbative QCD

❖ Universal description of small to large collision systems

Back-up

Naghmeh Mohammadi @WG5 HI meeting 22

30/10/2018

Particle correlations: multi-particle cumulants

Naghmeh Mohammadi @WG5 HI meeting 23 30/10/2018

❖ Particle correlations:

❖ In high multiplicity pp to compare with pPb and PbPb collisions

❖ In low multiplicity regions to investigate the onset of the collective-like effects

❖ pp: 1.5% v 3 {4} accessible for N ch > 170

❖ pPb: 1.5% v 3 {4} accessible for 100 < N ch < 500

❖ Larger tracker acceptance in run 4 ATLAS &

CMS -> 1% v3{4} accessible

c

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_n {4} = hhe ^{in( 1 + 2 3 4 )} ii 2 hhe ^{in( 1 2 )} ii ²

❖ 4 particle cumulants (c n {4})

N ch

100 200 300 400 500

{4} 3 c

− 0.6

− 0.4

− 0.2 0 0.2 0.4 0.6 0.8 1

− 6

× 10

Internal ATLAS

=5.02 TeV s NN

p+Pb

<3.0 GeV

|<2.5 0.3<p T

| η

Run 1 and 2, 28 nb -1

Projected, 1000 nb -1

3 {4}

1.5% v

3 {4}

2.0% v

N ch

50 100 150 200 250

{4} 3 c

− 1.5

− 1

− 0.5 0 0.5 1 1.5

− 6

× 10

prescaled HMT unprescaled HMT

Internal ATLAS

=13 TeV s

pp

<3.0 GeV

|<2.5 0.3<p T

| η

Run 2, 0.9 pb -1

Projected, 200 pb -1

3 {4}

1.5% v

3 {4}

2.0% v

|<2.5) (| η

N ch

50 100 150 200 250

{4} 3 c

− 0.3

− 0.2

− 0.1 0 0.1 0.2 0.3

− 6

× 10

prescaled HMT unprescaled HMT

Projected 200 pb -1

Internal ATLAS

=13 TeV s

pp

<3.0 GeV 0.3<p T

|<2.5

| η

|<4.0

| η

3 {4}

1.0% v

3 {4}

1.5% v

3 {4}

2.0% v

Particle correlations: multi-particle cumulants

Naghmeh Mohammadi @WG5 HI meeting 24 30/10/2018

❖ Particle correlations:

❖ In high multiplicity pp to compare with pPb and PbPb collisions

❖ In low multiplicity regions to investigate the onset of the collective-like effects

c

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_n {4} = hhe ^{in( 1 + 2 3 4 )} ii 2 hhe ^{in( 1 2 )} ii ²

❖ 4 particle cumulants (c n {4})

c3{4} in pPb collisions