Trends in high energy nuclear collisions

Trends in high energy nuclear collisions

Discovery of QGP at RHIC

Energy frontier Various collision

energies Small colliding

systems

QGP created even in small size?

 Collectivity in small colliding systems

RHIC beam energy scan (BES) program

 Onset of the QGP formation?

 Investigation of super- dense matter

QGP at the highest

collision energy at LHC

 Precision

measurements and analyses

Discovery stage Precision study, new findings,…

~2005

Physics of the QGP

Fukushima and Sasaki (2013)

Investigation of matter under extreme conditions

• Order of phase transition

• Location of critical point and 1^st order phase transition line

• Equation of state

• Transport coefficients

• Structure of “vacuum”

• …

High-energy nuclear collisions: Unique approach to create

matter under extreme conditions on the Earth

Bottom-up approach

3-D event display from STAR

• Momentum distribution

• Particle species

• Correlation

• …

Phenomenological approach

Physics properties of the QGP

• Equation of state

• Transport coefficients

• Stopping power

• Phase structure

Top-down approach Results from lattice QCD, …

Standard picture of dynamics in high- energy nuclear collisions

0

collision axis

time

Color glass condensate Dilute parton gas (Mini-)

jets

Fragmentation

Glasma QGP

fluid Interaction

Hadron gas

Recombination

Hadronic observables

Soft Hard

Energy frontier

Anisotropic flow and

precision QGP physics

Lessons from observational cosmology

http://www.esa.int/spaceinimages/ Images/2013/04/ Planck_CMB_black_background

Cosmic Microwave Background Fluctuations of temperature

(Planck) 𝑙 ≈ 180°/𝜃

Energy budget and lifetime of the Universe, inflation, …

One can reach eras before decoupling through these analyses.

E.Komatsu, talk at IPMU(2013)

Precision measurements and analysis

power spectrum

Response to initial fluctuations of geometry

𝜀_𝑛

Initial deformation Response 𝑣_𝑛

CMS Collaboration (2013) TH et al. (2013)

How does the system respond to initial deformation?

 Contain information about transport properties of the QGP

𝑛 = 2 (quadrupole) Elliptic flow

𝑛 = 3 (hexapole) Triangular flow 𝑛 = 4 (octapole) Quadrangular flow

Alver, Roland (2010) Ollitrault (1993)

Kolb (2003)

Entropy density distribution

Precision QGP physics using Bayesian parameter estimation

Experimental data  Posterior probability of parameters

Comparison with results from lattice QCD

Bernhard et al. (2016) Pratt et al.(2015)

(Shear viscosity)/(Entropy density) Sound velocity vs. Temperature

Correlation of initial conditions along collision axis

Heavy ion collision as a chromoelectric capacitor

 Formation of color flux tubes ~Approximate boost invariance

 Correlation of initial conditions in rapidity space

𝑥 𝑦

𝜂_𝑠

𝑥 𝑦

(De-)Correlation of elliptic flow along rapidity

• Ideal and viscous hydro

 Hard to break up correlations

• Random force from thermal (hydrodynamic) fluctuations in QGP

 break up correlations

• New channel to constrain transport coefficients

3.0 < 𝜂^𝑏 < 4.0

A. Sakai* (QM2017)

*Winner of Nuclear Physics A Young Scientist Awards

Energy frontier

Medium response

and hard probes

Di-jet asymmetric event

d’Enterria (2009)

𝐸~200 GeV jet dragged by medium with 𝑇~300 MeV in a few femtometer

Where the lost energy goes?

Change of jet structure as a function of 𝑟?

CMS Collaboration (Quark Matter 2011)

𝑟 = Δ𝜂² + Δ𝜙²

Large angle emission of soft particles

Y.Tachibana et al. (2017)

Mach-cone like medium

response at large angle from jet axis

Jet structure at large 𝑟: A new channel to constrain transport properties of QGP?

Z

-jet correlations as a new probe

𝑞𝑔 → 𝑞𝑍 and ത𝑞𝑔 → ത𝑞𝑍 less background than

𝑞𝑔 → 𝑞𝛾 or ത𝑞𝑔 → ത𝑞𝛾 𝑥_𝑗𝑍~1  Balance btw. jet and Z Peak shifted to lower 𝑥_𝑗𝑍

 New probe for jet tomography

CMS Collaboration (2017)

Discovery of top quarks in p+Pb collisions

e.g.) 𝑔𝑔 → 𝑡 ҧ𝑡 → 𝑊⁺𝑏𝑊⁻𝑏ത

• Constraint on nPDFs

5 ∙ 10⁻³ < 𝑥 < 0.05 𝑄²~3 ∙ 10⁴ GeV²

• b-quark energy loss in heavy ion collision case

𝑐𝜏 of top quarks~0.15 fm

<< Dimension of the medium ~ several fm

 New channel to probe the QGP

d’Enterria et al. (2015) CMS Collaboration (2017)

Small colliding systems

New challenge to models

To be QGP or not to be?

p+Au d+Au ³He+Au p+Pb

RHIC LHC

2003~2010: Control experiment  Initial state effects such as Cronin effect, (anti-)shadowing and saturation

2010~today: Discussion of possibility to create QGP in small colliding systems

* “Collectivity” = Correlated particle emission ≠ flow

That is THE question!

p+p

Everything starts from CMS findings

CMS Collabortion(2010)

What is “Ridge”?

Correlation of two particle emission with the same azimuthal angle but large rapidity gap (Δ𝜂~2-4)

Ridge in heavy ion collisions

Interpreted as collective flow

First ridge observation in high-multiplicity pp collisions at 𝑠 = 7 TeV !

Collectivity in pp and pPb collisions at LHC

pp p+Pb Pb+Pb

Collectivity in p,d,He+Au collisions at

RHIC

Large elliptic flow measured at RHIC

• Mass ordering

• Consistent with hydrodynamic calculations

𝜂

𝑠 = 0.08 The same hydro models reproduce experimental

results in both large and small systems at RHIC.

Strangeness enhancement in pp

ALICE, Nature Physics (2017)

ℎ_𝑆Τ𝜋 increase with multiplicity

Multi-strange hadrons increase more rapidly

 Commonly seen in heavy ion data from SPS to LHC

Violation of “jet universality”?

QGP formation (EPOS, 2015)

Rope hadronization (DIPSY, 2015, 2016)

Thermodynamical string model

(Fischer,Sjöstrand, 2017)

 Need more studies in final stage

Initial or Initial + Final?

Schlichting, Tribedy(2016)

Large system:

Final state effect Small system:

Initial or Initial + Final state effect

 Necessity for sophisticated modeling in small systems

 Thermalization,

hydrodynamization, …

Short summary of small colliding systems

Experimental data in pp and pA:

Collectivity (ridge, finite 𝑣

,…) Strangeness enhancement

How small can the QGP be?

Collectivity or fluidity?

Interpretation not settled:

Final state effects: QGP fluid, rope + shove,

themodynamical string frag, color reconnection,…

Initial state effects: Color glass condensate

Various collision energies

RHIC-Beam Energy Scan program

and beyond

Scanning phase diagram

Centrality dependence of 𝜇_𝐵 at low energies Baryon stopping

Control baryon density and initial energy density

Scan broad regions of phase diagram

Chemical freezeout parameters from particle yields in Au+Au

collisions at various energies

STAR Collaboration (2017) Strange hadron 𝑦 < 0.5

𝜋, 𝐾, 𝑝 in 𝑦 < 0.1

Collision energy evolution of third harmonics

Au+Au or Pb+Pb Response of the system

 Minimum at 𝑠_𝑁𝑁~20 GeV (mostly seen in semi-central collisions)

 Indication of softest point (minimum sound velocity) in equation of state?

Small  Initial energy density  Large

Collision energy evolution of jet quenching

Yield at high 𝑝_𝑇 is suppressed at the top RHIC energy as an

evidence for QGP formation

Monotonic change with 𝑠_𝑁𝑁

 Null results on onset of QGP formation?

Hard to disentangle jet quenching from Cronin effect (random

transverse kicks in the initial collision)

STAR Collaboration (2017)

Ratio of central to peripheral

Higher order fluctuations of conserved

quantity

𝜅𝜎² = 𝜒₄ 𝜒₂ 𝜒_𝑛 = 𝜕^𝑛 Ƹ𝑝

𝜕 ො𝜇^𝑛 Ƹ𝑝 = 𝑝

𝑇⁴ , ො𝜇 = 𝜇 𝑇 Non-monotonic behavior expected around critical point

X.F.Luo, talk at Fudan (2017)

Collision energy dependence of 𝜅𝜎 ²

𝜅𝜎² = 𝛿𝑁_𝐵 ⁴

𝛿𝑁_𝐵 ² = 𝜒₄ 𝜒₂

Esha for STAR (2017)

Expected non-monotonic behavior seen in experimental data

Signature of critical point!?

*In actual experimental data, not net baryon, but net proton

Future study of

Super-dense nuclear/quark matter

tps://physics.aps.org/articles/v10/114

Binary neutron star merger

M. Shibata, talk at QM2015 http://j-parc.jp/researcher/Hadron/

en/pac_1607/pdf/LoI_2016-16.pdf

Outlook (instead of Summary)

• Construct robust models against precision data

• Correlation measurement and its analysis

• New (hard) probes

• Interplay between soft and hard

• Need much more studies even in pp collisions!

• Initial state: Particle production, thermalization?

• Final state: hydro? Interacting color fields? Novel fragmentation?

Final question: Everything flows?

𝜋𝛼𝜈𝜏𝛼 𝜌𝜖𝜄! Everything flows!

Figures taken from M.A.Fardin, On the rheology of cats, Rheology Bulletin, 83(2) July 2014

Even cats flow!

The 2017 Ig Nobel Prize in Physics:

M.A. Fardin for using fluid dynamics to probe the question "Can a Cat Be Both a Solid and a Liquid?“

(https://www.improbable.com/ig) Spontaneous rotation

Correlation of elliptic flow parameter between different rapidity

−𝜂

+𝜂

3.0 < 𝜂

< 4.0: Reference flow

Same quadrupole emission pattern across rapidity?

Rope + shove model

Ridge appears in central pp events shoving model ~ hydro?

Bierlich et al.(2014, 2016)

Strings overlapping in transverse plane

”Rope” formation (with larger string tension)

𝑃 ∝ exp − 𝜋𝑚_𝑞² 𝜅

𝜅 → 𝜅^′(> 𝜅) expected to enhance yields of strange hadrons

Schwinger mechanism

Lönnblad(2017)

QGP as the most vortical fluid

Z.T.Liang, X.N.Wang (2005), Voloshin (2004, unpublished),Betz, Gyulassy, Torrieri (2007)

𝝎~ 1

2 𝛁 × 𝒗

𝑑 𝑣_𝑧⁺ 𝑣_𝑧⁻

𝑣_𝑧⁺ − 𝑣_𝑧⁻ ~0.1𝑐 𝑑~10fm

𝝎 ~10²²s⁻¹ 𝑃_Λ + 𝑃_Λഥ = ℏ𝜔 𝑘_𝐵𝑇

Beccatini et al. (2017)

Protons from Λ carry

information about polarization

9 ± 1 × 10²¹𝑠⁻¹ 𝜔 =

STAR Collaboration (2017)

Contents

• Introduction

• Energy frontier

• Anisotropic flow and precision QGP physics

• Medium response and hard probes

• Small colliding systems

• Various collision energies

• RHIC-Beam Energy Scan program and beyond

• Summary