Pythia results and future developments...Pythia (See e.g. C. Bierlich @ SQM 2019) Large systems: pA, AA event generators usually take one of two approaches Extrapolation of pp dynamics

lund un i v er s i ty dept . o f a stronomy and theoret i ca l phy s i c s

Pythia results and future developments

Christine O. RasmussenDept. of Astronomy and Theoretical Physics

Outline:

• Pythia for heavy ions: Angantyr

• Dipsy for Pythia: Mueller dipole formalism

• Conclusion & outlookSeptember 6th 2019 — Slide 1/23


Motivation

The questions we all would like to answer:

• Is a QGP formed in heavy ion collisions?

• What are the signals of a QGP?

• Why are (some of) these signals also seen in pp?

• Is a plasma also formed in pp?

• Can the measurements be explained by non-QGP models?

Focus on HI physics. A lot of new updates related to pp neglected fromthis talk. See e.g. the Update history of the Pythia 8 webpage.

Christine O. Rasmussen — Pythia results and future developments — September 6th 2019 — Slide 2/23


Heavy ions



Event generatorsSmall systems:

• pp event generators (Pythia, Herwig, Sherpa) successfullydescribe almost all observables

• Primary ingredients in Pythia include:• Multiple subcollisions using MPI models• Possible hard first interaction (e.g. gauge bosons, jets etc)• Initial- and final-state evolution of the MPIs• Colour reconnections between final-state partons• Hadronization of partons

• Also includes elastic and (hard and soft) diffractive processes

• Soon coming: Hadronic rescattering w. URQMD and internally inPythia (See e.g. C. Bierlich @ SQM 2019)

Large systems:

• pA, AA event generators usually take one of two approaches• Extrapolation of pp dynamics to pA and AA: HIJING, ANGANTYR• Assume quark-gluon plasma (QGP): AMPT, EPOS



Angantyr for heavy ionsAngantyr: new model for HI in Pythia 8.Several steps are taken in Angantyr:

• Glauber model for number of collisions.

• Wounded nucleons for particle production.

• Possibility for additional subcollisions in an event (similar to MPIs inpp.)

Only tuning to small systems.



Angantyr for heavy ionsFirst step:

• Glauber-Gribov method to calculate number of interacting nucleonsand binary collisions.

• Event-by-event colour fluctuations parametrized from DIPSY.

• Angantyr is first model to include colour fluctuations in bothtarget and projectile.

10.0 7.5 5.0 2.5 0.0 2.5 5.0 7.5 10.0x [fm]

10.0

7.5

5.0

2.5

0.0

2.5

5.0

7.5

10.0

y [fm

]

Lead spectatorsLead participantsProton

0 50 100 150 200

σ [mb]

0.000

0.005

0.010

0.015

0.020

0.025

0.030

Pwinc(σ)

DIPSYGG Ω=0.37GG Log-normal Ω=0.25GG Log-normal Ω=0.33



Angantyr for heavy ionsSecond step:

• Wounded nucleon model by Bia las and Czyz used to create exclusivefinal states.

• Wounded nucleons contribute equally to multiplicity in η.

• Original model F (η) fitted to data. Angantyr uses own model.

• F (η) fitted to reproduce high-energy pp events, but no fitting to HI.
























Angantyr for heavy ions

Third step:

• Method to combine nucleon-nucleon events needed as collision cancontain scatterings that are hard to describe in large systems.

• Simplest system pD-collision with three wounded nucleons:

Angantyr cheat-sheet:

• First scattering modelled as anormal non-diffractive pp event.

• Second is modelled as a singlediffractive event.

• Generalizes to all pA and AAcollisions.



Angantyr for heavy ionsResults from pPb:

• Centrality often described by binning in∑

E⊥.• Angantyr can use experimental centrality definition or internal

centrality measure.• Centrality and multiplicity well reproduced by Angantyr.



Angantyr for heavy ionsResults from PbPb:

• Multiplicity well described by Angantyr.

• Dip around η = 0 caused by Pythia pp not describingp⊥ < 500 MeV (ALICE measures to p⊥ = 0 MeV).



Dipole formulation



Dipole formulation

• Mueller dipole formalism describes evolution of a single dipole inrapidity.

• Defined by the dipole splitting probability,

dP

d2r3dy=

3αS(Q2)

2π2r212

r213r223

,

1

2

r12 →r12

2

1r13

r23

3→ → → · · ·

• After evolution the two chains of dipoles are allowed to interact.



Dipole formulation• Interaction given by dipole-dipole scattering probability,

fij =α2S(Q2)

2log2

[r14r23r24r13

],

1r12

2 3r344 →

1

2 3

4r14

r23

• Measurable quantities obtained from unitarized dipole-dipolescattering amplitude plus Good-Walker formalism,

T (b) = 1− exp

− NA∑i=1

NB∑j=1

fij

σtot =

∫d2b2 〈T (b)〉, σel =

∫d2b〈T (b)〉2



Dipole formulation

Initial state not from first principles:

−2 −1 0 1 2x [fm]

−2

−1

0

1

2

y[f

m]

r0

Initial photon

−2 −1 0 1 2x [fm]

−2

−1

0

1

2

y[f

m]

r0

Initial proton



Dipole formulation

After a full evolution:

−2 −1 0 1 2x [fm]

−2

−1

0

1

2

y[f

m]

r0

Evolved photon

−2 −1 0 1 2x [fm]

−2

−1

0

1

2

y[f

m]

r0

Evolved proton



Dipole formulationModel tuned to γp and pp cross sections.

102 103 104 105

100

200

300

400

σto

t[m

b]

ABMST

SaS+DL

pp 7 TeV

PY8 dipoles unconfined

PY8 dipoles confined

Data

102 103 104 105√s [GeV]

0.5

1.0

1.5

MC

/Dat

a 102 103

10

20

30

40

50

σel

[mb

]

ABMST

SaS+DL

pp 7 TeV



Data

102 103√s [GeV]

0.5

1.0

1.5

MC

/Dat

a

103 104 105

W 2 [GeV2]

10−1

100

101

102

103

σγ∗ p

tot

[µb

]

Q2 = 1.5 GeV2

Q2 = 8.5 GeV2

Q2 = 35 GeV2

Q2 = 60 GeV2

×0.3

Q2 = 120 GeV2

×0.1



Data 103 104 105

1.01.5

Q2 = 1.5 GeV2

103 104 105

1.01.5

Q2 = 8.5 GeV2

103 104 105

1.01.5

MC

/dat

aQ2 = 35 GeV2

103 104 105

1.01.5

Q2 = 60 GeV2

103 104 105

W 2 [GeV2]

1.01.5

Q2 = 120 GeV2



Dipole formulation

• Study effects of asymmetry in partonic eccentricities εn andnormalised symmetric cumulants in pp,pA,AA

• Linear response function often assumed in AA: vn = f (εn) ≈ aεn

εn =

√〈rn cos(nφ)〉2 + 〈rn sin(nφ)〉2

〈rn〉

NSC (n,m) =〈v2n v2m〉 − 〈v2n 〉〈v2m〉

〈v2n 〉〈v2m〉≈ 〈ε

2nε

2m〉 − 〈ε2n〉〈ε2m〉〈ε2n〉〈ε2m〉

Space-time information used as input for Pythia 8 MPI model:

• Glauber: MPIs moved to nucleon position

• Gaussian model: x , y chosen from gaussian with rp = 0.7 andwr = 0.1

• Dipole model: x , y chosen w.r.t. dipole-dipole interaction strength fijChristine O. Rasmussen — Pythia results and future developments — September 6th 2019 — Slide 15/23


Dipole formulation

20 40 60 80〈dNch/dη〉||η|<0.8

0.0

0.2

0.4

0.6

0.8

1.0

1.2

ε 22

pp 7 TeV

white

2D Gaussian

PY8 dipoles

pPb 5.02 TeV

Glauber

2D Gaussian

PY8 dipoles

PbPb 5.02 TeV

Glauber

2D Gaussian

PY8 dipoles

15 20 25 30 35 40〈dNch/dη〉||η|<0.8

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

ε 22

rati

o

2D Gaussian

PY8 dipoles

pPb/pp (ALICE data)

ε22 as a function of average central multiplicity

• Asymmetry gives rise to more eccentricity• Initial state fluctuations important in AA at low multiplicity• If response function for pA equals that of pp at same average

multiplicity, then• Eccentricity ratio should be comparable to flow ratios measured in

data• Dipole model predicts flat pA/pp ratio also seen in data



Dipole formulation

20 40 60 80〈dNch/dη〉||η|<0.8

0.0

0.2

0.4

0.6

0.8

1.0

1.2

ε 32

pp 7 TeV

white

2D Gaussian

PY8 dipoles

pPb 5.02 TeV

Glauber

2D Gaussian

PY8 dipoles

PbPb 5.02 TeV

Glauber

2D Gaussian

PY8 dipoles

20 40 60 80〈dNch/dη〉||η|<0.8

−0.4

−0.3

−0.2

−0.1

0.0

0.1

0.2

0.3

0.4

NSC

(2,3

)

pp 7 TeV

white

2D Gaussian

PY8 dipoles

pPb 5.02 TeV

Glauber

2D Gaussian

PY8 dipoles

PbPb 5.02 TeV

Glauber

2D Gaussian

PY8 dipoles

ε32, NSC (3, 2) as a function of average central multiplicity

• ε3 related to initial geometry

• Intial geometry not distinguishable in NSC (3, 2) for pp

• Very different behaviour in pA for symmetric and asymmetric initialstates



Dipole formulationRatios of higher order eccentricities: CMS

√sNN = 8.16 TeV.

50 100 150 200 250 300Nch (|η| < 2.4, p⊥ > 0.1 GeV)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

v 24/v 22

pPb√snn = 8.16 TeV

2D Gaussian ε24/ε22Dipole evolution ε24/ε22CMS data

50 100 150 200 250 300Nch (|η| < 2.4, p⊥ > 0.1 GeV)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

v 34/v 32


2D Gaussian ε34/ε32Dipole evolution ε34/ε32CMS data

0.60 0.65 0.70 0.75 0.80 0.85 0.90v24/vsub2 2(|∆η| > 2)

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

v 26/v 24


Dipole evolution

CMS data

0.60 0.65 0.70 0.75 0.80 0.85 0.90v24/vsub2 2(|∆η| > 2)

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

v 28/v 26


Dipole evolution

CMS data



Coming soon



• Use dipole formulation in Angantyr instead of parametrization

0 50 100 150 200σtot [mb]

0.000

0.002

0.004

0.006

0.008

0.010

0.012

P(σ

tot)

[mb−

1]

GG Log-normal fit

GG fit

Dipole

pp colour fluctuations.

100 101 102

σtot [µb]

0.00

0.05

0.10

0.15

0.20

0.25

0.30

P(σ

tot)

[µb−

1]

Q2 = 2 GeV2

Q2 = 5 GeV2

Q2 = 20 GeV2

γp colour fluctuations.

• Define response function using the shoving model

• Opens up for predictions for vnm



• UPCs using photon fluxes

• Full eA collisions with dipole model + Angantyr

10−4 10−3 10−2 10−1 100

x

10−2

10−1

100

101

102

103

104

105

xfγ/

b(x)

π/α e

m

leptonsprotonsPb nuclei

Pythia photon fluxes.

1 2 3 4 5 6 7 8 9 10 11 12 13 14Nabsw

10−4

10−3

10−2

10−1

100

P(N

abs

w)

Frozen wave function Q2 = 2 GeV2




Black disk σtot = 35µb (Q2 = 2 GeV2)

No. wounded nucleons in γAcollision.



Conclusion and outlook

• Full generation of pA and AA events with Angantyr[arXiv:1806.10820[hep-ph]].

• Good description of many HI observables.

• Recent implementation of Mueller dipole model[arXiv:1907.12871[hep-ph]].

• Dipole model allows for asymmetric initial states.

• Eccentricities with dipole model in agreement with data.

• Merging of dipole model with Angantyr opens up for collisionswith photons.

• UPCs, eA next to come.



Thank you!What do you want from us?



Thank you!What do you want from us?


Documents

Pythia results and future developments...Pythia (See e.g. C. Bierlich @ SQM 2019) Large systems: pA, AA event generators usually take one of two approaches Extrapolation of pp dynamics