possible non-prompt photons in pp collisions and their ...€¦ · possible non-prompt photons in...

Possible non-prompt photons in pp collisions and their effects in AA collisions

Akihiko Monnai (KEK, Japan)

Hard Probes 2018

3rd October 2018, Aix-les-Bains, France

Akihiko Monnai (KEK), Hard Probes 2018, October 3rd 2018

Introduction

◼ Relativistic nuclear colliders: a gateway to quark-gluon plasma

Transverse momentum spectraRelativistic Heavy Ion Collider (RHIC)@BNL, √sNN = 5.5-200 GeV (2000-)

2 / 24


Introduction



Large Hadron Collider (LHC)@CERN, √sNN = 2.76-5.44 TeV (2010-)

2 / 24


Introduction



Large Hadron Collider (LHC)@CERN, √sNN = 2.76-5.44 TeV (2010-)

FAIR@GSI, NICA@JINR, SPS@CERN, J-PARC@JAEA/KEK … ?

2 / 24


Introduction

◼ Relativistic nuclear collisions: QCD point of view

Transverse momentum spectra

Hydrodynamic stage (~1-10 fm/c)

Glasma (~0-1 fm/c)

Hadronic transport (> 10 fm/c)

QGP fluid

Hadronic fluid

Hadrons

Freeze-out

Graphics by AM

z

t

Colliding nuclei

Freeze-out

Equilibration

Color glass condensate (< 0 fm/c)

Little bang

3 / 24


Introduction

◼ Relativistic nuclear collisions: QCD point of view



Glasma (~0-1 fm/c)


QGP fluid

Hadronic fluid

Hadrons

Freeze-out

Graphics by AM

z

t

Colliding nuclei

Freeze-out

Equilibration


Little bang

Color opaque

Most information before freeze-out is lost in “thermal hadrons”

3 / 24


Introduction

◼ Relativistic nuclear collisions: Electroweak point of view


QGP fluid

Hadronic fluid

Hadrons

Freeze-out

Graphics by AM

z

t

Colliding nuclei


Glasma (~0-1 fm/c)


Freeze-out

Equilibration


Little bang

Color opaque


Electroweak transparent

Photons retain information of the space-time evolution of the system

4 / 24


Introduction



QGP fluid

Hadronic fluid

Hadrons

Freeze-out

Graphics by AM

zColliding nuclei

Color opaque



t


4 / 24


Introduction



Prompt photonsQGP fluid

Hadronic fluid

Hadrons

Freeze-out

- from hard processesGraphics by AM

zColliding nuclei

Color opaque




t

4 / 24


Introduction




Hadronic fluid

Hadrons

Freeze-out

Pre-equilibrium photons


zColliding nuclei

Color opaque




t

4 / 24


Introduction



Thermal photons


Hadronic fluid

Hadrons

Freeze-out


- from soft radiation


zColliding nuclei

Color opaque




t

4 / 24


Introduction



Thermal photons


Hadronic fluid

Hadrons

Freeze-out

Decay photons


- from hadronic decay



zColliding nuclei

Color opaque




t

4 / 24


Introduction



Thermal photons


Hadronic fluid

Hadrons

Freeze-out

Decay photons




- from hard processes

Direct p

ho

ton

s

Graphics by AM

zColliding nuclei

Color opaque




t

4 / 24


Introduction



Thermal photons


Hadronic fluid

Hadrons

Freeze-out

Decay photons




- from hard processes

Direct p

ho

ton

s

Graphics by AM

zColliding nuclei

Color opaque




t

(Jet photons, etc)

4 / 24


Introduction

◼ “Collectivity” in small systems

Weller and Romatschke, PLB 774,351 (2017)

Hydrodynamic and non-hydrodynamic models both work

?

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Thermalized QGP in pp is still controversial; but there is a possibility in most central collisions

Mace et. al., PRL 121, 052301 (2018)


Introduction

◼ Direct photon ingredients

AA collisions vs. pp collisions

pp×Ncoll

ThermalPrompt

AA

pp ~ pQCD

6 / 24


Introduction



pp×Ncoll ?

ThermalPrompt

AA

pp ~ pQCD?

There can be thermal photons in pp Cf. C. Shen, J.-F. Paquet, G. S. Denicol, S. Jeon and C. Gale, PRC 95, 014906 (2017)

7 / 24


Introduction



ThermalPrompt

AA

pp

Pre-equilibrium

Pre-equilibrium photons can be importantCf. Early hydro vs. glasma: J. Berges et. Al., PRC 95, 054904 (2017),

mBU-Glasma: V. Khachatryan et. al., NPA 978, 123 (2018)

pp×Ncoll ?

~ pQCD

8 / 24


Introduction



ThermalPrompt

AA

pp

Pre-equilibrium

pp×Ncoll -(thermal + pre-eq.)?

- It may pose limit to the amount of QGP in pp collisions

- Pre-eq. photons may affect the analyses of AA collisions

Aim of this study: Non-prompt photons in both AA and pp (glasma production more likely at LHC)

~ pQCD?

9 / 24


Introduction

◼ Photon puzzle

Observed azimuthal momentum anisotropy (v2) is large

Systematic tension not removed yet; clarification from experiments is essential

10 / 24

PHENIX, PRL 109, 122302 (2012)


Introduction

◼ Photon puzzle

Observed azimuthal momentum anisotropy (v2) is large

Systematic tension not removed yet; clarification from experiments is essential

10 / 24

PHENIX, PRL 109, 122302 (2012)

Non-prompt photons in pp may improve the situation; pre-equilibrium photons in AA collisions may do the opposite

Momentum anisotropy in AA system

Prompt Pre-eq. Thermal


Timeline

◼ Photons from different stages of HIC

Prompt Pre-equilibrium Thermal

τ ~ 0.1 fm/c τ ~ 1 fm/c

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Timeline

◼ Thermal photons

Prompt Thermal

τ ~ 0.1 fm/c τ ~ 1 fm/c

Pre-equilibrium

Estimated using a (2+1)-D hydrodynamic model

τth: tuned to 1 fm/c

Equation of state: lattice QCD

Initial condition: Glauber model (event-averaged)

Collision energy: 2.76 TeV

12 / 24


Timeline

◼ Thermal photons

Prompt Thermal

τ ~ 0.1 fm/c τ ~ 1 fm/c

Pre-equilibrium

Thermal photon emission rate

Arnold, Moore and Yaffe, JHEP 0112, 009Turbide, Rapp and Gale, PRC 69, 014903

where and

13 / 24


Timeline

◼ Pre-equilibrium photons

Bottom-up / turbulent thermalization approach to glasma

Prompt Thermal

τ ~ 0.1 fm/c τ ~ 1 fm/c

Pre-equilibrium

R. Baier, A. H. Mueller, D. Schiff, and D. T. Son, PLB 502, 51 (2001)

(i)

(ii)

(iii)

are introduced to “squeeze” the processes into the pre-hydro phase

J. Berges et. al., PRC 95, 054904 (2017)

14 / 24


Timeline


Emission rate: Berges-Reygers-Tanji-Venugopalan model

Prompt Thermal

τ ~ 0.1 fm/c τ ~ 1 fm/c

Pre-equilibrium

J. Berges et. al., PRC 95, 054904 (2017); N. Tanji and R. Venugopalan PRD 95, 094009 (2017)

Stages (i), (ii)

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: Self-similar scaling distribution

(Exponentially cut off for pT > Qs; normalized as nq = nq

th at τth)


Timeline


Emission rate: Berges-Reygers-Tanji-Venugopalan model

Prompt Thermal

τ ~ 0.1 fm/c τ ~ 1 fm/c

Pre-equilibrium

J. Berges et. al., PRC 95, 054904 (2017); N. Tanji and R. Venugopalan PRD 95, 094009 (2017)

Stage (iii)

with effective

Note: The model is parametrically extended to fit into the pre-hydro time; quantitative discussion should be made carefully

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Timeline

◼ Prompt photons

pp direct photons - (other photons) scaled by Ncoll

Thermal

τ ~ 0.1 fm/c τ ~ 1 fm/c

Pre-equilibriumPrompt

Alternatively one may perform pQCD analyses; it should be noted that yet other sources of photons can be hidden

Turbide, Rapp and Gale, PRC 69, 014903

17 / 24


Photons in AA collisions

◼ Pb-Pb 2.76 TeV, Glauber, 0-5%, Qs = 3 GeV

Semi-hard reflecting the saturation momentum scale Qs

Absolute value is dependent on parametrization; they could be comparable to thermal photons


(GeV)T

p0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

)-2

dy (

Ge

VT

dp

T)

dN

/pp

(1/2

-810

-710

-610

-510

-410

-310

-210

-110

1

10

210

310

Pb-Pb 2.76 GeV

pre-eq

thermal

thermal+pre-eq

Preliminary

18 / 24







(GeV)T

p0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

)-2

dy (

Ge

VT

dp

T)

dN

/pp

(1/2

-810

-710

-610

-510

-410

-310

-210

-110

1

10

210

310

Pb-Pb 2.76 GeV

pre-eq

thermal

thermal+pre-eq

Preliminary

(GeV)T

p0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

)-2

dy (

Ge

VT

dp

T)

dN

/pp

(1/2

-810

-710

-610

-510

-410

-310

-210

-110

1

10

210

310

410

Pb-Pb 2.76 GeV

scaled pppre-eq

thermal

direct

Preliminary

Prompt photons if pp is not modified by thermal or pre-eq. photons

Ncoll scaled pp

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(GeV)T

p0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

)-2

dy (

Ge

VT

dp

T)

dN

/pp

(1/2

-810

-710

-610

-510

-410

-310

-210

-110

1

10

210

310

Pb-Pb 2.76 GeV

pre-eq

thermal

thermal+pre-eq

Preliminary

(GeV)T

p0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

)-2

dy (

Ge

VT

dp

T)

dN

/pp

(1/2

-810

-710

-610

-510

-410

-310

-210

-110

1

10

210

310

410

Pb-Pb 2.76 GeV

scaled pppre-eq

thermal

direct

Preliminary

Prompt photons if pp is not modified by thermal or pre-eq. photons

Ncoll scaled pp

Typical momentum scale

19 / 24

thermal pre-eq prompt


Photons in pp collisions

◼ p-p 2.76 TeV, Glauber, Qs = 0.9 GeV

They can be com-parable to direct photons in most central events

Thermal photonsCf: C. Shen, Hard Probes ‘16

They can be comparable to direct photon near Qs


Modified prompt background?

(GeV)T

p0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

)-2

dy (

Ge

VT

dp

T)

dN

/pp

(1/2

-910

-810

-710

-610

-510

-410

-310

-210

-110

1

10

p-p 2.76 GeV

direct

pre-eq

prompt

Preliminary

20 / 24

No thermal QGP is assumed for the moment to be “conservative”




Reduced at pT ~ Qs(p) and enhanced at pT ~ Qs(Pb)

Prompt & pre-eq photons

(GeV)T

p0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

)-2

dy (

Ge

VT

dp

T)

dN

/pp

(1/2

-810

-710

-610

-510

-410

-310

-210

-110

1

10

210

310

410

Pb-Pb 2.76 GeV

promptpre-eq

thermal

direct

Preliminary

Modified prompt photons do not have much effect on spectra as thermal photons are dominant at pT ~ 1 GeV

Results are dependent on the value of Qs; v2 may be affected more

21 / 24


Summary and outlook

◼ We have studied non-prompt photons in PbPb and pp collisions

22 / 24

Pre-equilibrium photons can provide visible semi-hard contribution to pT spectra

pp direct photons may not be pure prompt photons; the baseline for AA analyses may well be modified by glasma creation

One may probe and constrain the pre-equilibrium physics (Qs etc.) through direct photon analyses


Summary and outlook

◼ We have studied non-prompt photons in PbPb and pp collisions

◼ Future prospects include:

Pre-equilibrium photons can provide visible semi-hard contribution to pT spectra

pp direct photons may not be pure prompt photons; the baseline for AA analyses may well be modified by glasma creation

22 / 24

One may probe and constrain the pre-equilibrium physics (Qs etc.) through direct photon analyses

Analyses of the effects on elliptic flow v2Full pp thermal photon analyses can be used to test if the QGP is produced in pp collisions

Implementation of chemically equilibrating QGP in hydrodynamic models Cf: AM, PRC 90, 021901(R) (2014)


Fin

◼ Merci de votre attention!

possible non-prompt photons in pp collisions and their ...€¦ · possible non-prompt photons in...

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