Status of QGP search at RHIC
- A PHENIX perspective(*) -

IKF Seminar

Inst. für Kernphysik, Frankfurt, September 30, 2004

David d’Enterria

Nevis Labs, Columbia University, NY

(*) Based upon “PHENIX White Paper”: http://www.phenix.bnl.gov/phenix/WWW/info/comment/

Overview/Summary

1. Introduction:

Goal: Study Quantum Chromo (many-body) Dynamics (QGP, CGC) in high-energy A+A collisions [by comparing to p+A (“cold” QCD medium), p+p (“no medium”)].

2. Energy densities:

Maximum dET/dη ~ 600 GeV at midrapidity consistent w/ initial ε > 5 GeV/fm3

3. Elliptic flow:

Strong elliptic flow v2 consistent w/ short thermalization times t0 ~ 1 fm/c

4. Soft particle spectra:

Shapes & yields consistent w/ hydrodynam. (thermal+coll. velocity) source emission

Particles ratios consistent w/ chemical equilibrium before hadronization

5. Hard particle spectra:

Strong high pT suppression in central A+A (compared to p+p, p+A & pQCD)	consistent w/ final-state partonic energy loss in dense system: dNg/dy~1100

6. Intermediate pT spectra:

Enhanced baryon yields & v2 (compared to meson) consistent w/ quark	recombination mechanisms in dense thermal system

7. Summary & open questions: “QGP” ? Observations @ lower √s ?

Future measurements.

High-energy heavy-ion physics program (in 4 plots)

1. Learn about 2 basic properties of	strong interaction: (de)confinement, chiral symm. breaking (restoration)

as(Q2) ~1/ln(Q2/L2), L~200 MeV

3. Probe quark-hadron

phase transition

of the primordial

Universe (few μsec

after the Big Bang)

2. Study the phase diagram of QCD

matter: esp. produce & study the QGP

4. Study the regime of non-linear (high density)

many-body parton dynamics at small-x (CGC)

(as=g2/4p)

<qq>

_

The "Little Bang" in the lab.

High-energy nucleus-nucleus collisions: fixed-target reactions (√s~17 GeV, SPS) or colliders (√s~200 GeV, RHIC. √s~5.5 TeV, LHC)

QGP expected to be formed in a tiny region (~10-14 m) and to last very short times (~10-23 s).

Collision dynamics: Diff. observables sensitive to diff. react. stages

Time

Penetrating

probes

t~0.1 fm/c

t ~ 10 fm/c

t ~ 107 fm/c

Penetrating probes

Final state probes

Relativistic Heavy-Ion Collider (RHIC) @ BNL

STAR

PHOBOS

PHENIX

Specifications:

3.83 km circumference

2 independent rings:

120 bunches/ring

106 ns crossing time

A + A collisions @ √sNN = 200 GeV

Luminosity: 2·1026 cm-2 s-1 (~1.4 kHz)

p+p collisions @ √smax= 500 GeV

p+A collisions @ √smax= 200 GeV

4 experiments:

BRAHMS, PHENIX, PHOBOS, STAR

Runs 1 - 4 (2000 – 2004):

Au+Au @ 200, 130, 62.4 GeV

p+p @ 200 GeV

d+Au @ 200 GeV

BRAHMS

The 4 RHIC experiments

BRAHMS detector

RHIC Au+Au luminosities

Luminosities

maximum
projection

physics target

minimum
projection

Au+Au collisions @ 200 GeV

~ 700 charged particles per unit rapidity at midrapidity (top 10% central)

Energy densities at RHIC

Energy density (Au+Au @ 200 GeV, y=0)

Bjorken ~5.0 GeV/fm3

pR2

t0

~ 1 fm/c

Au+Au @ 200 GeV

Bjorken estimate:

(longitudinally expanding plasma)

dET/dη at mid-rapidity measured by calorimetry (using PHENIX EMCal as

hadronic calorimeter: EThad = (1.17±0.05) ETEMCal)

<dET/dh> ~ 600 GeV (top 5% central)

(~70% larger than at SPS)

> QCD critical density (~1 GeV/fm3)

1 fm/c thermalization time ?

Not unrealistic at RHIC... (for the 1st time: ttherm > tcross = 2R/g = 0.13 fm/c)

Energy density time evolution in long. expanding system: e ~1/t

tcross=2R/g = 0.13 fm/c

ttherm~ 0.6 - 1 fm/c

tsecondaries formation = hbar/E ~ hbar/<mT>

= 0.35 fm/c

<mT> = (dET/dh)/(dN/dh) ~

(dET/dh)/(2/3*dNch/dh)

= 0.57 GeV

(from hydrodynamics, see next)

Elliptic flow at RHIC

Elliptic flow

Elliptic flow = v2

(2nd Fourier coefficient)

Initial anisotropy in x-space in non-central collisions (overlap) translates into final azimuthal asymmetry in p-space (transverse to react. plane)

1. Truly collective effect (absent in p+p collisions).

2. Early-state phenomenon:

develops only in 1st instants of reaction,

strongly self-quenches after t~2 fm/c

Time evolution of ellipsoid eccentricity:

Based on hydro calculations

Kolb, Sollfrank, Heinz

PRC62, 054909 (200)

Elliptic flow at RHIC

Large v2 signal at RHIC:

Exhausts hydro limit for pT<1.5 GeV/c

 Strong (collective) pressure grads.

 Large & fast parton rescattering:

early thermalization.

But(!) no significant increase

between 62 and 200 GeV

√s-dependence of v2:

~50% increase from SPS

Apparent saturation within 62-200 GeV

Mass dependence of v2

consistent w/ hydrodynamics too:

Soft particle production at RHIC

Soft particle spectra

Bulk p±, p(pbar) spectra ~reproduced by hydro w/ QGP EOS at t0=0.6 fm/c

pions:	protons:

Solid lines: QGP+HG

Dashed lines: HG

Radial collective flow

built-up at freeze-out:

<bT> » 0.55

Ratios of particle yields

Ratios of hadron yields consistent w/ system at chemical equilibrium (strangeness saturation factor gs=1) before hadronization (Tchem.freeze-out ~ Tcrit) :

PBM, Redlich, Stachel

nucl-th/0304013

Kaneta, Xu nucl-th/0405068

Hard QCD production at RHIC

Hard QCD probes (I)

Hard probes: High-pT, jets, direct , heavy-quarks (D, B), ...

1. Early production ( ~ 1/pT< 0.1 fm/c) in parton-parton scatterings with large Q2:

Closest experimental probes to underlying QCD (q,g) degrees of freedom.

2. Direct probes of partonic phase(s)  Sensitive to QCD medium properties:

q: fast color triplet

g: fast color octet

Q: slow color triplet

QQbar: slow color

singlet/octet

Virtual photon: colorless

Real photon: colorless

Unknown QCD Medium

Induced

gluon

radiation ?

Energy

Loss ?

Dissociation ?

Controls

QCD probe in

QCD medium

(possible quark-gluon plasma)

Modification?

QCD probe out

Hard QCD probes (II)

3. Production yields theoretically calculable via perturbative-QCD:

A

B

AB

dAB → hard = A·B·dpp → hard

dNAB → hard (b) = TAB(b)·dpp → hard

Nuclear Modification Factor:

AB

At impact parameter b:

geom. nuclear overlap at b

production is “shadowed”

TAB ~ # NN collisions (“Ncoll scaling”)

“Factorization theorem”:

Independent scattering of “free” partons:

A+B = “simple superposition of p+p collisions”

High pT p+p baseline data well described by pQCD

Good theoretical (NLO pQCD) description:

KKP FF

Kretzer FF

(PDF: CTEQ6M)

PHENIX Collab.

PRL91, 241803

hep-ex/0304038

Well calibrated (experimentally &

theoretically) p+p references at hand

p+p  p0 X	p+p  h± X (non singly diffractive)

“ g (direct+decay) / g decay ”

“Ncoll scaling” in Au+Au @ 200 GeV: Direct Photons

Direct photon production in Au+Au (all centralities) consistent w/

Ncoll-scaled p+p pQCD predictions:

[ J.Frantz QM'04 ]

Preliminary

“Ncoll scaling” in Au+Au @ 200 GeV: Total charm

Open-charm indirect measurement via semi-leptonic channel: D e± +X

Single e± Au+Au spectra & total cross-section consistent w/ Ncoll -scaled p+p charm production:

pQCD parton scattering holds for hard processes in Au+Au (all centralities).

“Ncoll scaling” in A+A @ 17, 31 GeV: High pT hadrons

High pT π0 production in (0-10%) central A+A at SPS (and a+a @ ISR) energies: consistent w/ “Ncoll-scaling” (or Cronin enhancement):

D.d'E. PLB 596, 32 (2004)

“Ncoll scaling” in d+Au @ 200 GeV: High pT hadrons

d+Au @ √sNN = 200 GeV	p+A @ √sNN = 20 - 40 GeV

Enhanced high pT production in d+Au (RdAu > 1) also found in p+A at lower Ös (“Cronin enhancement”): pT broadening due to initial-state soft & semihard scattering.

Expected pQCD behaviour (RpA,dA~1) recovered for pT > 8 GeV/c

Suppressed high pT hadroproduction in Au+Au @ RHIC !

Au+Au p0 X (peripheral) Au+Au p0 X (central)

Peripheral data agree well with Strong suppression in

p+p (data&pQCD) plus Ncoll scaling central Au+Au collisions

D.d'E. nucl-ex/0309015

Suppressed high pT hadroproduction @ RHIC : RAA (po)

Discovery of

high pT suppression

(one of most significant

results @ RHIC so far)

PHENIX Collab.

PRL 91, 072303 (2003)

Ncoll scaling

(“hard” production)

x4-5 suppression

Npart scaling

(“soft” production)

RAA << 1: well below pQCD (collinear

factorization) expectations for

hard scattering cross-sections

Suppressed high pT hadroproduction @ RHIC: RAA (h± vs po)

Inclusive charged hadrons suppressed a factor ~ 4 – 5 at pT> 5 GeV/c

Universal (PID-wise) suppression above pT = 5 GeV/c

... less suppression than π0

within pT = 2 – 5 GeV/c (see later)

“Jet quenching” predictions

Multiple final-state gluon radiation off the produced

hard parton induced by the traversed dense

colored medium.

Mean parton energy loss µmedium properties:

DEloss ~ gluon (gluon density)

DEloss ~ DL(2) (medium length)

Energy is carried away by gluonstrahlung

outside jet cone: dE/dx ~ s k2T

Formalisms: BDMPS (thick plasma), GLV (thin plasma),

Correction for expanding plasma (1-D):

E1-D =(20/RA) · Estatic ~ 15·Estatic (0=0.2 fm/c, RA=6 fm)

Expected result: Suppression of high pT leading hadrons due to non-Abelian final-state gluon radiation.

g

Jet

High- pT hadron

Gluon

bremsstrahlung

QGP

“Jet quenching” model vs. data (I)

Dense medium properties according to final-state parton energy loss models:

Initial gluon densities:

dNg/dy ~ 1100 [Vitev & Gyulassy]

Opacities:

<n> = L/≈ 3 – 4 [Levai et al.]

Transport coefficients:

<q 0> ~ 3.5 GeV/fm2 [BDMPS, F.Arleo ]

Plasma temperatures:

T ~ 0.4 GeV [G. Moore]

Medium-induced radiative

energy losses:

dE/dx ≈ 0.25 GeV/fm (expanding)

dE/dx|eff ≈ 14 GeV/fm (static source) [X.N.Wang]

Large opacities imply fast thermalization.

All these values imply energy densities well above ecrit QCD in thermalized syst.

“Jet quenching” model vs. data (II)

I. Vitev nucl-th/0404052

sqrt(s) dependence of high pT suppression:

RAA ~ 1 for s~20 GeV (dNg/dy~400)

RAA ~ 0.3 for s~62 GeV (dNg/dy~650)

RAA ~ 0.2 for s~200 GeV (dNg/dy~1100)

Au+Au @ 62.4 GeV

Energy loss in a dense hadronic medium ?

Hadronic transport models (HSD, UrQMD) or DPM-based models do not produce enough suppression. Additional pre-hadronic energy loss needed.

Cassing, Gallmeister, Bratkovskaya,

Greiner, Stoecker, nucl-th/0312049

Capella, Ferreiro, Kaidalov, Sousa

hep-ph/0403081

Hadron production at intermediate pT

Unsuppressed baryon production

Rc p (ratio central/peripheral) at intermediate pT = 2 – 4 GeV/c:

Particle composition inconsistent with known (universal) fragmentation functions.

Additional production mechanism for baryons in the intermediate pT range

STAR Collab.

subm. to PRL,

nucl-ex/0306007

PHENIX Collab.

PRL91:172301(2003)

_ _

Mesons: p0, k0s, η,

equally suppressed.

Baryons: p, p, Λ, Λ NOT (or much less)

suppressed in central Au+Au.

Unsuppressed baryon production: not a mass effect !

Rc p (ratio central/peripheral) at intermediate pT = 2 – 4 GeV/c:

Particle composition inconsistent with known (universal) fragmentation functions.

Additional production mechanism for baryons in the intermediate pT range

STAR Collab.

subm. to PRL,

nucl-ex/0306007

PHENIX Collab.

PRL91:172301(2003)

_ _

Heavy f as suppressed

as other mesons (p0, k0s, η)

Baryons: p, p, Λ, Λ NOT (or much less)

suppressed in central Au+Au.

Enhanced baryon/meson ratio

Central Au+Au: p/π ~ 0.8 (at pT = 2 - 4 GeV/c) at variance with perturbative production mechanisms (favour lightest mesons).

Periph. Au+Au: p/π ~ 0.2 as found in p+p (ISR,FNAL) & e+e- jet fragmentation

“Anomalous” particle composition: hadron/meson ratio

p+p collisions: hadron/meson ~ 1.6

RHIC

Au+Au (central): hadron/meson ~ 2.5

at pT = 1 - 4 GeV/c (inconsistent w/

known fragmentation functions).

Baryon enhancement limited to

pT < 5 GeV/c (h±/ π ~ 1.6, perturb. ratio):

h±, π0 equally suppressed

ISR

PHENIX Collab.

PRL91:172301(2003)

Enhanced baryonic elliptic flow

Particle species

hierarchy of flow values:

v2meson > v2baryon at low pT

v2meson ≈ v2baryon at pT≈ 2 GeV/c

v2meson < v2baryon at higher pT

Simple v2 scaling behaviour

when v2 and pT normalized by

number of constituent quarks:

n = 2 mesons

n = 3 baryons

PHENIX Collab.

to appear in PRL

nucl-ex/0305013

“Quark recombination” models vs. data

Quark recombination (coalescence) mechanisms provide a simple explanation of anomalous baryon enhancement & v2 at pT= 2-5 GeV/c:

Via quark momenta addition, recombination

dominates for pT ~ 1- 4 GeV/c:

pT(baryons)> pT(mesons)> pT(quarks)

Fragmentation dominates for pT > 5 GeV/c:

pT(hadrons)= z pT(partons) , with z<1

Constituent-quark scaling of v2 naturally

explained

However ... pure thermal + thermal parton

recombination inconsistent w/ jet-like

(baryon and meson) near-side azimuthal

correlations. Simple recomb. does not work.

Summary

1. Energy densities:

Maximum dET/dη ~ 600 GeV at midrapidity consistent w/ initial ε > 5 GeV/fm3 > εcrit

2. Elliptic flow:

Strong elliptic flow v2 consistent w/ short thermalization times t0 ~ 1 fm/c

3. Soft particle spectra:

Shapes & yields consistent w/ hydrodyn. (thermal+coll. velocity) source emission

Particles ratios consistent w/ chemically equilibrated system before hadronization

4. Hard particle spectra:

Strong high pT suppression in central A+A (compared to p+p, p+A & pQCD)	consistent w/ final-state partonic energy loss in dense system: dNg/dy~1100

5. Intermediate pT spectra:

Enhanced baryon yields & v2 (compared to meson) consistent w/ quark	recombination mechanisms in a thermal and dense system

All observations consistent with formation of thermalized

dense partonic matter in central Au+Au collisions

OK ... but didn't we hear the same at SPS in 2000 ?

Summary (2)

1. Energy densities:

Maximum dET/dη ~ 600 GeV at midrapidity consistent w/ initial ε > 5 GeV/fm3 > εcrit

Seen at SPS ? Yes. ε > 3.9 GeV/fm3 > εcrit (Note, however tcross=1.6 fm/c >t0 = 1 fm/c)

2. Elliptic flow:

Strong elliptic flow v2 consistent w/ short thermalization times t0 ~ 1 fm/c

Seen at SPS ? No. Not as large v2. Described by hydro only with too short freezeout.

3. Soft particle spectra:

Shapes & yields consistent w/ hydrodynam. (thermal+coll. velocity) source emission

Particles ratios consistent w/ chemically equilibrated system before hadronization

Seen at SPS ? Yes.

4. Hard particle spectra:

Strong high pT suppression in central A+A (compared to p+p, p+A & pQCD)	consistent w/ final-state partonic energy loss in dense system: dNg/dy~1100

Seen at SPS ? Probably. Suppression (not that strong) w.r.t. peripheral.

5. Intermediate pT spectra:

Enhanced baryon yields & v2 (compared to meson) consistent w/ quark	recombination mechanisms in a thermal and dense system

Seen at SPS ? No.

We need a few additional observables to claim “QGP” at RHIC ...

thermal photons (temperature of system), J/Ψ (deconfined system)

Summary (2)

1. Energy densities:

Maximum dET/dη ~ 600 GeV at midrapidity consistent w/ initial ε > 5 GeV/fm3 > εcrit

Seen at SPS ? Yes. ε > 3.9 GeV/fm3 > εcrit (Note, however tcross=1.6 fm/c >t0 = 1 fm/c)

2. Elliptic flow:

Strong elliptic flow v2 consistent w/ short thermalization times t0 ~ 1 fm/c

Seen at SPS ? No. Not as large v2. Described by hydro only with too short freezeout.

3. Soft particle spectra:

Shapes & yields consistent w/ hydrodynam. (thermal+coll. velocity) source emission

Particles ratios consistent w/ chemically equilibrated system before hadronization

Seen at SPS ? Yes.

4. Hard particle spectra:

Strong high pT suppression in central A+A (compared to p+p, p+A & pQCD)	consistent w/ final-state partonic energy loss in dense system: dNg/dy~1100

Seen at SPS ? Probably. Suppression (not that strong) w.r.t. peripheral.

5. Intermediate pT spectra:

Enhanced baryon yields & v2 (compared to meson) consistent w/ quark	recombination mechanisms in a thermal and dense system

Seen at SPS ? No.

We need a few additional observables to claim “QGP” at RHIC ...

thermal photons (temperature of system), J/Ψ (deconfined system)

Stay tuned !

backup slides ...

Thermal photons at RHIC

Thermal photon production in Au+Au @ √sNN = 200 GeV



Thermal (real&virtual) g are the most direct (the only ?) probe sensitive to

the thermodynamical state (EOS) of underlying matter.

Au+Au 200 GeV

0-10% central

2+1 hydro predictions:

Initial conditions:

t0 = 0.15 fm/c

T0 = 580 MeV

(reproduces bulk p, K, p dN/dpT)

EOS:

QGP (2.5 flavors) + hadron res. gas +

1st order phase transition (DE=0.8 GeV)

D. Peressounko, D.d'E.

(in preparation)

Thermal (QGP+HG) component

NLO pQCD x TAB scaling

Thermal production (only)

apparent within

pT ≈ 1 – 2.5 GeV/c

Medium effects in Au+Au→ g + X @ √s = 200 GeV



However, (part of the) prompt photons can be distorted by the dense

QCD medium (esp. in the region pT< 4 GeV/c).

Photon production in p+p @ 200 GeV:

Below pT ≈ 4. GeV/c dominated by

g from collinear q,g fragmentation

NLO pQCD [W. Vogelsang]

+

( )

( )

( )

“Nuclear modif. factor” Au+Au→ g + X @ √s = 200 GeV



Back-of-the-envelope ansatz for g suppression: RAA(g frag.) = RAA(q,g) ≈ 0.25

RAA ≈ Ratio of g(tot – 0.75*frag)/g(tot):

~50% depleted prompt photon yield could mask the expected (enhanced)

thermal emission around pT = 2 GeV/c

Fra

NLO pQCD [W. Vogelsang]

“RAA”~0.5	“RAA”~0.8

Results confirmed by more

involved calculations:

F.Arleo: hep-ph/0406291

(No shadowing corrs.

included)

3 different scales

Disentangling “thermal” g from quenched prompt g



Step 1: Measure p+p → g(isolated) + X

down to pT = 1 GeV/c

with uncertainties ~10%

Handle on g from qg-Compton, qqbar annihilation

Step 2: Measure p+p → g(total) + X

down to pT = 1 GeV/c

with uncertainties ~10%

Handle on fragmentation g production

Step 3: Measure Au+Au → g(total) + X

down to pT = 1 GeV/c

with uncertainties ~10%

Step 4: (AuAu gtotal) – TAB•(pp gisolated)

Upper limit on thermal spectrum.

Step 5: (AuAu gtotal) – TAB•(pp gtotal)

Lower limit on thermal spectrum.

Daunting tasks ! ...

Current best p+p g data:

Current best Au+Au g data:

High pT suppression at SPS ?

Enhanced high pT production in Pb+Pb @ CERN-SPS ?

NO p+p  π0 X baseline measurement at SPS Pb+Pb energy (√s = 17.3 GeV)

RAA for central Pb+Pb constructed with 2 different parametrizations:

WA98 Collab.

EPJ C23 (93)225

Huge Cronin

enhancement

Wang&Wang

PRC 64 (01) 034901

Cronin

enhancement

But suppressed Rcp at

the same time (?!)

p+p → π +X references @ √s ≈ 20 GeV

p+p  π0 X parametrizations confronted to data @ √s = 16 – 20 GeV:

D.d'E. nucl-ex/0403055

PLB 596 (2004) 32

WA98 parametrization

undershoots the data

by up to a factor of ~3

X.N.Wang parametriz.

undershoots the data

by up to a factor of ~2

New p+p → π +X reference @ √s ≈ 20 GeV

D.d'E. nucl-ex/0403055

PLB 596 (2004) 32

Much better agreement

in shape and magnitude

(within exp. uncertainties)

New parametrization [Blattnig00] versus p+p data √s = 16 – 20 GeV:

New WA98, WA80 & CERES nuclear modification factors:

RAA at SPS are not

enhanced but consistent

with “collision scaling”.

Agreement with parton

energy loss calculations

[I.Vitev nucl-th/0404052]

in moderately dense

system (dNg/dy ~ 400-600)

Indications of high pT suppression @ SPS

Centrality evolution of high pT π0 production at SPS:

“Collision scaling” in 0-8% central collisions (RAA ~ 1).

“Cronin” enhancement in peripheral ... and suppression in 1% most central

Look for onset of suppression at RHIC Au+Au, p+p @ √sNN≈ 20 GeV ?

0-8% central

Peripheral

1% top central

0-8% central

Nuclear modification factor (p0): √sNN dependence

A.L.S.Angelis, PLB 185, 213 (1987)

WA98, EPJ C 23, 225 (2002)

(*) Reanalysis: D.d'E. PLB 596, 32 (2004)

PHENIX, PRL 88 022301 (2002)

PHENIX, PRL 91, 072303 (2003)

CERN-SPS: Pb+Pb central (√sNN = 17.3 GeV): no suppression(*) (within errors)

CERN-ISR: a+a (√sNN = 31 GeV): Cronin enhancement.

RHIC: Au+Au (√sNN = 62, 130, 200 GeV): x3 - x5 suppression.

RAA ~ 0.4

RAA (p0) compilation in nucleus-nucleus collisions:

RAA ~ 1.0

RAA~ 1.5

RAA~0.2

High pT suppression at 62.4 GeV

Au+Au @ 62.4 GeV (central): suppression predictions

I. Vitev nucl-th/0404052

X.N. Wang nucl-th/0405029

Adil & Gyulassy nucl-th/0405036

RAA (π0) ~ 0.5 - 0.3

Au+Au @ 62.4 GeV (RAA central): π0 suppression !

Pions more suppressed than h+/- at intermediate pT (also found at 200 GeV):

RAA ~ 0.6 at pT~2 GeV/c

“Universal” (PID) and constant suppr. at high pT: RAA ~ 0.3 for pT > 6 GeV/c

Neutral pions

Charged hadrons vs. pions

p+p  h± X ref. in denominator

p+p  πo X ref. in denominator

Gluon saturation at RHIC ?

Initial-state CGC effects

Initial conditions at RHIC: high-energies + large nuclei

Values of small-x: xBj= 2pT/√s <<1

Large gluon densities

Colliding nuclei described via a colored

highly saturated gluonic wave-function

(“Color Glass Condensate”).

"Classical" approach valid around “sat. scale”: Qs~1.5 GeV/c

Particle production via glue-glue collisions:

Extension to pT > Qs (“geometric scaling”)

via quantum evolution.

Expected result: gluon fusion at low x leads to an effective depletion of the number of partonic scattering centers in the initial state.

RHIC ~ HERA x A1/ 3

Particles density at central rapidity in Au+Au

dNch/dy constraints mechanisms of initial multi-particle production:

dNch/dy (per participant pair) increases faster than linearly with Npart :

Particle density at y=0 well described by pQCD- & CGC- based models alike:

 "Soft + hard" (string + pQCD “minijet”): increased hard contribution (Ncoll )

 Initial-state gluon saturation (CGC): dNch/dy ~ dNgluon/dy ~ 1/as ~ Npartln(Npart )

Au+Au 200 GeV

N.Armesto et al.

Int.J.Mod.Phys. A15, 2019

dNch/dy

centrality dependence:

Unquenched d+Au production at high pT

Suppression in central Au+Au not due to initial-state effects

The quest for gluon saturation effects @ RHIC ...

RHIC kinematical regime:

High pT @ midrapidity:

y = 0, Q2 = 1-100 GeV2/c2

- pQCD collinear factorization

- DGLAP evolution (g splitting)

- small nuclear effects in PDFs

(LT shadowing).

Moderate pT, rapidities:

y ≈ 1-3, Q2 ≈ 10 GeV2/c2

- kT factorization

- linear BFKL evolution (g splitt.)

- “moderate” nuclear effects (LT

shadowing).

Low pT @ large rapidities:

y > 3, Q2 < Qs2 ≈ 5 GeV2/c2

- pQCD factorization breakdown

- non-linear evolution (g fusion)

- strong nuclear effects in the

initial-state

xT = pT /√s (e-y+ey)

(2 → 2)

x small: Look forward in rapidity !

d+Au nuclear modification factor (η = 3.2)

First time a large “shadowing” is seen at small-x and high pT in nuclear syst.

So far unexplored(*) perturbative region of nuclear (x,Q2) plane.

(*) Caveat: The “back-of-the-envelope” estimates (x~pTe-y)

of the small-x values probed at forw. rapidities

at RHIC provide usually too low values.

Effective <x2> values are at least ~10 times larger.

Factor ~2 suppression

in hadron production:

pT = 1-3 GeV/c, η = 3.2

x2 ~ 10-4 in Au(*)

Is this “standard” nuclear shadowing ?

Take the predictions of your favourite leading-twist approach ...

Maximum gluon shadowing at x~10-4 (indirectly) constrained by available DIS data on nuclear targets is ~0.8

IF indeed RdAu(pT~2 GeV/c) ≈ 0.5 º RG(x=10-4) ≈ 0.5 this could be an evidence of extra higher-twist effects at small-x (breakdown of QCD factorization). [BUT, soft physics effects can still be playing a role here ...]

D. de Florian & R.Sassot

hep-ph/0311227

Q2=10 GeV2

RG(x)

NLO DGLAP global analysis

of nuclear PDFs

Au+Au @ 200 GeV (central): baryons > mesons !

Baryon/meson ratios: ~1 !

Rcp baryons vs. mesons: very heavy mesons ( ,k*) =! baryons

Additional production mechanism for baryons in the intermediate pT range

David d'Enterria

2004-01-14T06:23:12