Phenomenology of heavy-ion collisions

How can one characterize what is created in a heavy-ion collision?

Focus on “collective phenomena” present in nucleus-nucleus collisions, but absent in pp collisions (“condensed matter physics of QCD”)

Establish a reference, in which collective effects are absent.

Quantify the deviation from these benchmarks in nucleus-nucleus collisions.

Analyze the origin of these deviations.

First measurement: multiplicity

number Nch of charged particles ∝

PHOBOS Collaboration, Phys. Rev. Lett. 85 (2000) 3100

First measurement: multiplicity

number Nch of charged particles ∝

PHOBOS Collaboration, Phys. Rev. Lett. 85 (2000) 3100

Nucleon-nucleon cross-section

taken from Miller, Reygers, Sanders & Steinberg, Ann. Rev. Nucl. Part. Sci. 57 (2007) 205

Multiplicity distribution

Vary the equivalent number of nucleon-nucleon collisions between and :

Probability P (n,b) to find a multiplicity n in a particular A-B collision at impact parameter b:

Gaussian around , with some dispersion; given by a Monte-Carlo simulation.

event-multiplicity distribution:

probability that an inelastic process occur

N̄AB(b) =(

1− x

2N̄AB

part(b) + xN̄ABcoll (b)

)N̄NN

N̄ABpart(b) N̄AB

coll (b)

N̄AB(b)

dNevts

dn=

∫dbP (n,b)

{1−

[1− σinel

NNTAB(b)]AB

}

taken from Miller, Reygers, Sanders & Steinberg, Ann. Rev. Nucl. Part. Sci. 57 (2007) 205

Multiplicity distribution

Multiplicity distributiondNevts

dn=

∫dbP (n,b)

{1−

[1− σinel

NNTAB(b)]AB

}

figure from Kharzeev & Nardi, Phys. Lett. B 507 (2001) 121

Multiplicity vs. geometry

taken from Miller, Reygers, Sanders & Steinberg, Ann. Rev. Nucl. Part. Sci. 57 (2007) 205

Multiplicity vs. geometry

taken from Miller, Reygers, Sanders & Steinberg, Ann. Rev. Nucl. Part. Sci. 57 (2007) 205

Cross-checking Glauber theoryMultiplicity at projectile rapidity vs. at midrapidity

data from PHOBOS Collaboration Phys. Rev. C 74 (2006) 021901(R)

Pseudorapidity distributions

figure taken from Miller, Reygers, Sanders & Steinberg, Ann. Rev. Nucl. Part. Sci. 57 (2007) 205

Collision centralities: 0-6%, 6-15%, 15-25%, 25-35%, 35-45%, 45-55% (missing / not shown at the lower two energies)

taken from BRAHMS Collaboration, Phys. Rev. Lett. 94 (2005) 162301

Rapidity distributions

Multiplicity at mid-rapidityBeware: in fact, at η=0, not y=0!

taken from PHOBOS Collaboration Phys. Rev. C 74 (2006) 021901(R)

data from PHOBOS Collaboration Phys. Rev. C 74 (2006) 021901(R)

Charged hadron multiplicity

data from PHOBOS Collaboration Phys. Rev. C 74 (2006) 021901(R)

We boost everything to the rest frame of one nucleus (“projectile”)

universal

“limiting fragmentation”

Charged hadron multiplicity

data from PHOBOS Collaboration Phys. Rev. C 74 (2006) 021901(R)

We boost everything to the rest frame of one nucleus (“projectile”)

universal

“limiting fragmentation”

ln√

sNN

growslike

Charged hadron multiplicity

We boost everything to the rest frame of one nucleus (“projectile”)

universal

“limiting fragmentation”

Charged hadron multiplicity

We boost everything to the rest frame of one nucleus (“projectile”)

universal

“limiting fragmentation”

−ybeam @ LHC

Charged hadron multiplicity

We boost everything to the rest frame of one nucleus (“projectile”)

universal

“limiting fragmentation”

ln√

sNN

growslike

−ybeam @ LHC

Busza 2004; N.B. & Wiedemann 2008

Charged hadron multiplicity

The naive extrapolation of RHIC data yields at -increase, in opposition to conventional power-law rise

dN ch

dη≈ 1100 η = 0

ln√

sNN

Charged hadron multiplicity

The naive extrapolation of RHIC data yields at -increase, in opposition to conventional power-law rise

dN ch

dη≈ 1100 η = 0

ln√

sNN

organized byN.Armesto, N.B., S.Jeon & U.A.Wiedemann

Hijing + baryon junctions: 3500 EPOS (multiple scattering): 2500 pQCD minijets + saturation

(EKRT) of produced gluons: 2570 AMPT (Hijing+ZPC): ≈2500 Percolating strings:

DMPJET III: ≈1900 Pajares et al.: 1500-1600

2-component + shadowing: ≈1700

“Geometric scaling” (Armesto, Salgado, Wiedemann): 1700-1900 Gluon saturation (Kharzeev,

Levin, Nardi 2000-05): 1800-2100 B-K eq.+ running coupling

(Albacete, Kovchegov): ≈1400 “CGC” (Gelis, Stasto, Venugopalan):

1000-1400 ALCOR (quark-antiquark plasma

+ recombination): 1250-1830 = dN ch

dy

Charged hadron multiplicity

taken from BRAHMS Collaboration, Phys. Rev. Lett. 93 (2004) 102301

Net baryon-number density

bulk: “soft particles”

high-pT particles

Transverse-momentum spectrum

∝ N̄ABcoll (b)∝ N̄AB

part(b)