HOT QUARKS'04

Taos, NM - July 24th, 2004

David d’Enterria

Nevis Labs, Columbia University, NY

Relevance of baseline
hard p+p spectra for A+A
and p+A physics at high pT

[ or ... how to bring (part of) the RHIC high pT excitement to SPS energies ]

[ or ... how to make M.J.Tannenbaum happy ]

[ or ... how not to make friends in BRAHMS (or CGC “community”) :-) ]

[ or ... how to make life difficult to PHENIX “photoners” ]

High pT p+p baseline spectra: Overview

Case I: p+p → π+X at √s ≈ 20 GeV

Is high pT A+A hadroproduction at SPS enhanced or suppressed ?

Case II: p+p → π,h± +X at √s = 62.4 GeV

How accurate are our current ISR-averaged p+p references ?

Case III: p+p → h± +X at √s = 200 GeV at forward y vs pQCD:

Is there truly suppresssion in d+Au collisions at η=3.2 ?

Case IV: p+p → g +X at √s = 200 GeV

Is there a way to disentangle suppressed prompt photons (from quenched

jet fragmentation) from enhanced (thermal) photons in Au+Au at RHIC ?

Case I:
p+p high pT reference at √s ≈ 20 GeV

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

Wish list (1): Run RHIC Au+Au, p+p @ √sNN≈ 20 – 30 GeV (onset of suppr.)

0-8% central

Peripheral

1% top central

0-8% central

Case II:
p+p high pT reference at √s = 62.4 GeV

p+p high pT data @ 62.4 GeV



Au+Au @ 62.4 GeV measured at RHIC in Run-4. But no concurrent p+p ...

p+p @ 62–63 GeV measured at ISR: pions: π0 (8), π± (4); charged hadrons (1)

How consistent are the p+p → π +X spectra @ 62.4 GeV ?



Discrepancies as large as a

factor of ~3 between data sets ...

Unsubstracted π 0 “contaminations” at ISR (1)

All but one measurement at ISR didn't substract the η and direct-

“World average” η/π0 ~0.45 ratio at high pT in hadronic colls.

18% η contribution needs to be substracted from “unresolved” π0 spectra.

BRη →  • Rη/π0 = 0.39 • 0.45 ≈ 0.18

Unsubstracted π 0 “contaminations” at ISR (2)

All but one measurement at ISR didn't substract the η and direct-

/π0 ratio at high pT in p+p at 62 GeV (data compared to NLO pQCD):

Prompt  are a significant source of e.m. clusters above pT ~6 GeV/c

that needs to be substracted too

Final corrected p+p spectra @ 62.4 GeV



2 data sets (outliers) excluded.

Final corrected data consistent

among each other within ±25%

Uncertainties of final p+p high pT refs. @ √s = 62.4 GeV



Neutral pions: ± 25%	Charged hadrons: ± 35%

High pT suppression in Au+Au @ 62.4 GeV

Using p+p  π0 X parametrization

Using [p+p  h± X parametrization] / 1.6

Parton energy loss calculations

with dNg/dy = 650 – 800

I. Vitev nucl-th/0404052

Propagated uncertainties in the RAA for π0:

pT dependent, mimic physical (e.g. “Cronin”-like) effects

Wish list (2): Run RHIC p+p @ √sNN= 62.4 GeV (detailed quantitative

study of √s-dependence of high pT suppression)

PHENIX

preliminary

Ratio p/h at high pT

Case III:
p+p high pT reference at √s = 200 GeV
at forward rapidities

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(*)

High pT p+p reference spectra vs pQCD (mid-rapidity)

Good agreement with NLO pQCD

η = 0	η = 1

[calculations by W. Vogelsang]

High pT p+p reference spectra vs pQCD (forward η)

Good agreement with NLO pQCD

η = 2.2

η = 3.8

[calculations by W. Vogelsang]

STAR Collaboration

PRL92 171801 (2004)

High pT p+p reference spectrum vs pQCD (η=3.2)

Two highest pT points (there where the RdAu “suppression” appears) are “enhanced” w.r.t. NLO (which describes all other rapidities, even y = 3.8 !).

Let's be provocative ... Is there d+Au suppression or (“genuine” ?)	p+p “enhancement” ?

η = 3.2

Case IV:
p+p high pT photon reference at √s = 200 GeV

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 p+p baseline spectra: Summary

Case I: p+p → π+X at √s ≈ 20 GeV

Fact: High pT A+A hadroproduction @ SPS is (slightly) suppressed.

Wish: Measuring p+p, Au+Au at RHIC at √s = 20 – 30 GeV (Run-6 ?)

Case II: p+p → π,h± +X at √s = 62.4 GeV

Fact: Current ISR-averaged p+p refs. have uncertainties of order ~30%

precluding detailed quantitative study of √s-dependence of high pT A+A suppr.

Wish: Measuring p+p at RHIC at √s = 62.4 GeV (Run-5 ?)

Case III: p+p → h± +X at √s = 200 GeV at forward y

Fact: High pT spectra at η=0,1,2.2,(3.2),3.8 are (not) well reproduced by pQCD

putting (perhaps) into question the claimed d+Au suppression.

Wish: Independent confirmation of p+p h- spectrum at η=3.2

Case IV: p+p → g +X at √s = 200 GeV

Fact: Depleted prompt g (from quenched jet fragmentation) can hide the	thermal photon signal in Au+Au at RHIC.

Wish: Measure isolated and non-isolated g in p+p at √s = 200 GeV

hot quarks cooking guide (I)

Nuclear modification factor with a incorrect depleted reference:

=

p+p =

A+A =

“cold” quarks

Nuclear modification factor with an incorrect enhanced reference:

=

A+A =

p+p =

“burnt” quarks

Nuclear modification factor with a incorrect depleted reference:

hot quarks cooking guide (II)

Nuclear modification factor with a correct baseline:

=

p+p =

A+A =

Enjoy the hot quarks !

“because we can” © ...

©Richard Witt

backup slides ...

BRAHMS Rcp vs RdAu

Charged hadron over pion ratio at high pT

.

Thermal photons from other hydros

.

Final p+p → π +X reference @ √s = 62.4 GeV



Parametrization:	Good agreement with NLO:

Calculations by

Werner Vogelsang

Nuclear modification factors below RHIC energies

High pT π0 production in ~0 ─10% central A+A at SPS and ISR energies:

David d'Enterria

2004-07-18T02:48:03