Hadronic Rescattering in Pythia

(1)

Hadronic Rescattering in Pythia

Marius Utheim, Torbj¨orn Sj¨ostrand

Department of Astronomy and Theoretical Physics Lund University

ALICE working group presentation, 1 October

(2)

Outline

Introduction

The rescattering framework

Results for pp collisions

Preliminary results for AA collisions

(3)

2/28 Introduction

The rescattering framework Results for pp collisions Preliminary results for AA collisions

Outline

Introduction

(4)

Introduction The rescattering framework Results for pp collisions Preliminary results for AA collisions

Heavy ion research in Lund

I Several projects in Lund are trying to explore heavy ion physics without a QGP, to see how well other effects can explain experimental data.

I The rescattering framework was released in Pythia 8.303, and we are now working on integrating with Angantyr.

(5)

3/28 Introduction

Heavy ion research in Lund

I Rescattering is one such effect. Other effects include string shoving and rope formation.

(6)

Heavy ion research in Lund

I Rescattering is one such effect. Other effects include string shoving and rope formation.

(7)

4/28 Introduction

Why rescattering in Pythia?

Other frameworks for hadronic transport already exist (UrQMD, SMASH, ...), so why implement rescattering in Pythia?

I Our framework is fully integrated and is trivial to interface with other parts of Pythia

I Leverages other features of Pythia, such as the event record I Some new physics features, such as interactions involving

charm and bottom, and open for further extensions

(8)

Why rescattering in Pythia?

(9)

4/28 Introduction

Why rescattering in Pythia?

I Leverages other features of Pythia, such as the event record

I Some new physics features, such as interactions involving charm and bottom, and open for further extensions

(10)

Why rescattering in Pythia?

I Leverages other features of Pythia, such as the event record I Some new physics features, such as interactions involving

(11)

5/28 Introduction

The Lund string model

space time

quark antiquark pair creation

(12)

The Lund string model

space time

quark antiquark pair creation

(13)

6/28 Introduction

Spacetime picture of the Lund string model

String tension κ ∼ 1 GeV/fm

(Ferreres-Sol´e & Sj¨ostrand, arXiv:1808.04619)

(14)

Outline

Introduction

(15)

7/28 Introduction

Rescattering overview

(16)

Rescattering overview

(17)

7/28 Introduction

Rescattering overview

(18)

Rescattering overview

(19)

8/28 Introduction

The collision criterion

The probability of an interaction depends on the cross section σ and the impact parameter b

The characteristic range of the interaction is b_crit=^pσ/π The cross section σ depends on the particle types and the center-of-mass energy.

(20)

Low-energy interactions

Elastic

Diffractive

Non-diffractive

Annihilation

Resonant

(21)

10/28 Introduction

Cross sections

2.0 2.5 3.0 3.5 4.0 4.5 5.0

s (GeV) 0

20 40 60 80 100 120 140

(mb)

pp

Total (Pythia) Elastic (Pythia) Total (PDG) Elastic (PDG)

Based on PDG data and HP R1R2

parameterization

(DOI: 10.1103/PhysRevD.98.030001)

2.0 2.5 3.0 3.5 4.0 4.5 5.0

s (GeV) 0

20 40 60 80 100 120 140

(mb)

pp

Total (Pythia) Elastic (Pythia) Total (PDG) Elastic (PDG)

Based on UrQMD (arXiv:nucl-th/9803035) and CERN/HERA parameterization

(DOI 10.1103/PhysRevD.50.1173)

(22)

Cross sections

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

s (GeV) 0

20 40 60 80 100 120 140

(mb)

+

f0(500) 0 f0(980) f2

0(1700) Other Total

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

s (GeV) 0

20 40 60 80 100 120 140

(mb)

0 0

f0(500) f0(980) f2 Other Total

Based on work by Pelaez, Rodas, Ruiz de Elvira et al.

(arXiv:1102.2183, arXiv:1907.13162, arXiv:1602.08404)

(23)

12/28 Introduction

Tuning

In our framework, we consider rescattering between only two particles at a time. This means that we can have two-to-many interactions, but not many-to-two. Therefore, rescattering increases charged multiplicity.

To compensate for this, we set

MultipartonInteractions:pT0Ref = 2.345 when doing our analyses (default is 2.28).

We have verified that this is not responsible for the results I am about to discuss.

(24)

Outline

Introduction

(25)

13/28 Introduction

Rescattering rates

0 50 100 150 200

n(|y|<2.5) 0

20 40 60 80 100 120

nrescatter® (n)

Rescattering dependence on multiplicity simple fit

nrescatter

®(nprimary)

n_rescatter^®(n_charged)

nondiffractive events at 13 TeV, simple fit ∝ n^1.3

(26)

Rescattering rates

Mean number of interactions per nondiffractive event at 13 TeV incoming rate incoming rate

π + π 12.63 K + N 0.39 π + ρ 4.59 ρ + ρ 0.38 π + K 3.84 ρ + N 0.36 π + N 3.44 ρ + ω/φ 0.34 π + ω/φ 2.08 ρ + η/η⁰ 0.30 π + η/η⁰ 1.80 π + f₀(500) 0.29 π + K^∗ 1.33 K + ω/φ 0.27 π + ∆ 1.10 K + K 0.26 ρ + K 0.54 π + Λ 0.25

process rate

resonant 17.80

elastic 14.08

nondiffractive 6.92 annihilation 0.49 diffractive 0.05

(27)

15/28 Introduction

Rescattering invariant mass

0 1 2 3 4 5

m (GeV) 10^-3

10^-2 10^-1 10⁰ 10¹ 10²

dN/dm

Rescattering mass by particle kind

ππother meson-meson meson-baryon baryon-baryon sum

(28)

p

_⊥

spectra

b b b b b bbbbbbbbbbbbbbbbb b b b b b b b b b b b b b b b b b b b

b Data

Rescattering off Rescattering on

10⁻¹ 1 10¹

π⁺+ π⁻yield in INEL pp collisions at√s = 7 TeV in |y| < 0.5.

1Nineld2NdpTdy(c/GeV)

b b b b b bbbbbbbbbbbbbbbbb b b b b b b b b b b b b b b b b b b b

0.5 1 1.5 2 2.5 3

0.50.6 0.70.8 0.91 1.11.2 1.31.4

p_T(GeV/c)

MC/Data

b b bb b b b b b bb b bb bbbbbbbbbbbbbbbbbbbbb b b b b b b b b b b b

b Data

Rescattering off Rescattering on

10⁻⁴ 10⁻³ 10⁻² 10⁻¹

p + ¯p yield in INEL pp collisions at√s = 7 TeV in |y| < 0.5.

1Nineld2NdpTdy(c/GeV)

b b b b b b b b b b b b b b bbbbbbbbbbbbbbbbbbbbb b b b b b b b b b b b

1 2 3 4 5 6

0.50.6 0.70.8 0.91 1.11.2 1.31.4

p_T(GeV/c)

MC/Data

(29)

17/28 Introduction

Mean p

_⊥

b b b b b b b b b

b Data

Rescattering off Rescattering on 1

Mean pTvs mass in INEL pp collisions at√s = 7 TeV in |y| < 0.5.

hpTi(GeV/c) b b b b b b b b b

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

0.7 0.8 0.9 1 1.1 1.2

m (GeV/c²)

MC/Data

(Data from ALICE, arXiv:1504.00024, arXiv:1406.3206)

(30)

η spectra

bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb

bData

1 2 3 4 5 6 7 8

Charged particle η, pT>100 MeV, |η| < 2.5, τ > 30 ps

1/NevdNch/dη bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb

-2 -1 0 1 2

0.50.6 0.70.8 0.91 1.11.2 1.31.4

η

MC/Data bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb

b Data

0.5 1 1.5 2 2.5 3 3.5

Charged particle η, p_⊥>500 MeV, |η| < 2.5, τ > 30 ps

1/NevdNch/dη bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb

-2 -1 0 1 2

0.50.6 0.70.8 0.91 1.11.2 1.31.4

η

MC/Data

(31)

19/28 Introduction

Flow

We introduced an artificially strong anisotropy in the x-direction to see if rescattering can produce flow

4 2 0 2 4

fm 0.0

0.5 1.0 1.5 2.0 2.5

dN/dx

x and y production vertices of primary hadrons xProd yProd

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

0.630 0.632 0.634 0.636 0.638 0.640 0.642

d/d

spectra relative to event plane Primary

ncharged< 75 75 ncharged< 150 150 ncharged< 250 250 ncharged

Under more realistic conditions, we saw no clear signs of flow

⇒ Rescattering can cause flow in principle, but is not the main source of flow in pp collisions!

(32)

Flow

We introduced an artificially strong anisotropy in the x-direction to see if rescattering can produce flow

4 2 0 2 4

fm 0.0

0.5 1.0 1.5 2.0 2.5

dN/dx

x and y production vertices of primary hadrons xProd yProd

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

0.630 0.632 0.634 0.636 0.638 0.640 0.642

d/d

spectra relative to event plane Primary

ncharged< 75 75 ncharged< 150 150 ncharged< 250 250 ncharged

Under more realistic conditions, we saw no clear signs of flow

(33)

19/28 Introduction

Outline

Introduction

(34)

Angantyr

(35)

21/28 Introduction

Rescattering rates

0 5000 10000 15000 20000 25000

ncharged

0.0 0.2 0.4 0.6 0.8 1.0 1.2

nrescatter

1e5 Rescattering dependence on multiplicity nrescatter(ncharged)

simple fit

PbPb events at 3.140 TeV, simple fit ∝ n^1.4

(36)

Generation time

10² 10³ 10⁴ 10⁵ 10⁶ 10⁷

tgen (ms)

Generation time per event

rescattering on rescattering off

(37)

23/28 Introduction

Rescattering rates

process rate fraction resonant 17.80 45.2 % elastic 14.08 35.8 % nondiff. 6.92 17.6 %

ann. 0.49 1.2 %

diff. 0.05 0.1 %

Total 39.34

Nondiffractive pp at 13 TeV

process rate fraction resonant 8351.9 51.7 % elastic 5721.2 35.4 % nondiff. 1999.1 12.4 %

ann. 71.3 0.4 %

diff. 15.2 0.1 %

Total 16158.8 PbPb at 3.140 TeV

(38)

Rescattering rates

0 1 2 3 4 5

m (GeV) 10^-3

10^-2 10^-1 10⁰ 10¹ 10²

dN/dm

Rescattering mass by particle kind ππother meson-meson meson-baryon baryon-baryon sum

Nondiffractive pp at 13 TeV

0 1 2 3 4 5

m (GeV) 10⁰

10¹ 10² 10³ 10⁴ 10⁵

d/dm

Rescattering mass by particle kind other meson-meson meson-baryon baryon-baryon sum

PbPb at 3.140 TeV

(39)

25/28 Introduction

pT spectra

0 1 2 3 4 5

p (GeV) 10^-2

10^-1 10⁰ 10¹ 10² 10³ 10⁴

dN/dp

Transverse momentum, rescattered or not π, rescattering on π, rescattering off K, rescattering on K, rescattering off N, rescattering on N, rescattering off D, rescattering on D, rescattering off

(40)

pT spectra

0.5 1.0 1.5 2.0 2.5

d/dp rescatter on/off

Transverse momentum ratios, rescattered or not , rescattering on/off K, rescattering on/off N, rescattering on/off D, rescattering on/off

(41)

27/28 Introduction

Longitudinal production time

0 5 10 15 20 25 30 35

(fm) 0.0

0.2 0.4 0.6 0.8 1.0 1.2

d/d

1e3 Longitudinal production time, rescattered or not rescattered not rescattered sum both rescattering off

(τ =√ t²− z²)

(42)

Outlook

I We have started doing analyses in Angantyr. One of the next things we want to look at is flow.

I When these individual components are done, we will start putting it all together.

(43)

28/28 Introduction

Outlook

I The future of Angantyr will also involve shoving, ropes, and other effects.

(44)

Outlook

I The future of Angantyr will also involve shoving, ropes, and other effects.