Photon Physics @ RHIC:
Highlights

AGS/RHIC Users Meeting 2005

BNL, June 24th, 2005

David d’Enterria

Nevis Labs, Columbia University, NY

Agenda Photon Workshop (R6)

http://www.phenix.bnl.gov/~enterria/ags_rhic_photons_05/

Why direct photons ?

[Definition: direct photons = photons not coming from hadronic decays]

Advantages:

- Weakly interacting. Travel unaffected to detectors after production.

- Direct coupling to (point-like) scatterings.

- Good theoretical understanding: γ – quark coupling precisely known (QED)

- “Clean” probes in many QCD environments (pp, pA, AA).

pp collisions:

- Access to (polarized) gluon PDF (via qg Compton).

- Testing ground of pQCD. Baseline reference for AA collisions.

pA collisions:

- Access to nuclear modifications of gluon PDF

- Initial-state multiple scattering (Cronin)

- Cold nuclear-matter effects

AA collisions:

t<~0.2 fm/c: perturbative photons unaffected by QCD medium (monitoring jet quenching: yields and correlations).

t<~0.6 fm/c: secondary (“cascading”) photons – thermalization mechanism

t<~5 fm/c: thermal photons – access to medium T and EoS (!)

g-g correlations: space-time size of radiating source

p + p  g + X @ RHIC:
Testing ground for pQCD

Preliminary p+p  g +X @ √s = 200 GeV

Good agreement w/ NLO pQCD

Isolated ~ Inclusive spectra

photon

jet

photon

jet

+

photon

jet

Hisa Torii

[W.Vogelsang]

Direct photon components

Monique Werlen

“Direct” γ

Fragmentation

Only the sum σ(D) + σ(F) is a physical observable

O(ααs)

O(ααs2)

(Compton, Annihilation)

(NLO and higher order)

g “world data” vs. state-of-the-art NLO/NLL

Monique Werlen

PHENIX preliminary data in very good agreement w/ NLO QCD predictions (like most of ISR & fixed target data).

Enhanced pBe (E-706) production: non-perturbative kT + Cronin ? (partially) accounted for by latest NLL gluon resummation calculations ?

Calculations

by Annecy Group

Aurenche, Fontannaz,

Guillet, Kniehl, Pilon,

M.Werlen EPJC9,107 (1999)

23 GeV ≤√s≤ 63 GeV

INCNLO

Theoretical uncertainties at RHIC

Monique Werlen

Relatively small:

Scales uncertainty

Fragmentation BFGI/BFGII

~20%

<10%

Isolated versus non-isolated photons

Monique Werlen

Handle on the g fragmentation component via “isolation cut” measurement (important also for AA studies of possible (?) g energy loss):

=0.5

Isolated/Non-isolated

JETPHOX NLO

CTEQ6M BFG II M=MF=μ=pT

K.Odaka hep-ex/0501066

in a cone around photon with:

ET (parton) ≤ ET max =0.1Eγ

Isolated spectrum: yield depends on applied isolation criteria.

Inclusive ≈ isolated (for R=0.5): Small non-isolated production in data and theory @ RHIC

R=

~85%

d + Au  g + X @ RHIC:
Probing cold nuclear-matter medium

Preliminary d+Au  g +X @ √sNN = 200 GeV

Hisa Torii

The analysis method is similar to p+p

NLO pQCD Calculation

p+p collisions [W.Vogelsang]

CTEQ6M + GRV

Scale(renormalization and factorization scale) 0.5,1.0,2.0pT

Averaged number of collisions (8.42) from the Glauber model was multiplied to the calculation.

Result consistent with the binary –scaled NLO-pQCD calculation

Nuclear effects in d+Au @ √sNN = 200 GeV

Hisa Torii

Nuclear Modification Factor:

Consistent with ~1. No modification within errors

Small or null Cronin as measured in 0

A + A  g + X:
Probing hot & dense QCD medium

Photons in AA @ RHIC: different contributions

Paul Stankus

Photons = sum of emission from the entire history of

nuclear collision:

Turbide, Rapp, Gale

E

Rate

Hadron Gas Thermal Tf

QGP Thermal Ti

“Pre-equilibrium”

(“secondary” or “cascading”

Jet Re-interaction √(Tix√s)

pQCD Prompt x√s

Emission time

Photons in AA: the pre-RHIC (fixed-target) era

Paul Stankus

Summary:

The early days had more enthusiasm than rigor.

In S+Au upper limits on thermal photons were used to set limits on initial temperatures; weak evidence for high # d.o.f.

Direct photon spectrum (ie upper and lower limits) observed in heavier Pb+Pb collisions.

Thermal radiation from boosted Hadron Gas may dominate thermal radiation from cooler QGP.

Ambiguity between pQCD sources with intrinsic plus nuclear kT effects, and hotter thermal sources. More definitive pQCD calculations would be a great help.

Limiting initial temperatures in Pb+Pb possible, not yet done.

AuAu   (p0) +X @ √sNN = 130, 62.4 GeV (STAR)

Marcia Moura

Measurement obtained from  conversion in TPC

e+ and e- are selected through dE/dx

Centrality dependence

Error bars: statistical only

Systematic uncertainty: 20%

Combinatorial background has been subtracted

Other contributions, such as  decays, were verified to be negligible

Efficiency

 ~ 2%

0 ~ 0.04%

Goes lower in pT than PHENIX meas.

(important ! see later)

PRC 70 (2004) 044902

AuAu  p0 +X @ √sNN = 62.4 GeV (STAR)

Marcia Moura

Direct photons accessible (hopefully soon) after subtraction of measured p0 (and other meson) decays:

Each point is the gaussian fit of the 2 invariant mass distribution for a given pT

~10 MeV width, depending on pT

Systematic uncertainty of 30%

Comparison of 0 to + and - from STAR TPC dE/dx and TOFr shows good agreement.

Au+Au  g +X @ √sNN = 200 GeV (PHENIX)

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

p+p incoherent scattering (“Ncoll-scaled” pQCD) predictions:

To appear in PRL

nucl-ex/0503003

Direct photon production in Au+Au

unmodified by QCD medium.

Hisa Torii

Au+Au  g +X @ 200 GeV. Theory vs data: prompt + thermal

Dmitri Peressounko

PHENIX 0-10% AuAu photons compared to 5 hydro (+ pQCD) calculations:

Calculations with similar

initial time (temperature)

result in similar spectra.

Dependence on used

emission rates & details

of description of evolution

is modest.

All calculations predict

considerable thermal

contribution for pT < 3 GeV

All calculations agree with

current (upper limit) data.

Theory vs data: prompt + (cascade) + thermal

Steffen Bass

Photon yield very sensitive to parton-parton rescattering

relevant processes:

Compton: q g q γ

annihilation: q qbar  g γ

bremsstrahlung: q* q γ

Short emission time in the PCM, 90% of photons before 0.3 fm/c

Hydrodynamic calculation with τ0=0.3 fm/c allows for a smooth continuation of emission rate

Theory vs data: prompt + (cascade) + thermal

Steffen Bass

LPM destructive interference important !

PCM w/o LPM:

Large overprediction of g yield

PCM with LPM:

g yield for pt < 6 GeV strongly reduced

strong pt dependence of LPM suppression

good agreement with data

below ~3 GeV/c, need to take thermal contributions into account

Note this is the old result

(should be scaled down by factor ~7)

Theory vs data: extra jet-photon contribution

Steffen Bass

Latest calculations by Duke group:

calibrate pQCD calculation of direct and Bremsstrahlung photons via p+p data:

Reduced effect compared to previous calculations (consistent K-factors for g and jets now).

For pt<6 GeV, FMS photons give still significant contribution to photon spectrum: 50% @ 4 GeV

[Fries, Mueller & Srivastava, ms in preparation]

plasma mediates

a jet-photon conversion:

jet passing through the medium:

large energy loss: jet quenching.

electromagnetic radiation (real & virtual photons) from jet-medium interactions.

can escape without rescattering

use as probe of energy loss?

Fries, Muller, Srivastava

PRL 90 132301 (2003)

Back to the “thermal region” (pT ~ 1 – 4 GeV/c)

Current upper limits in pT = 1 – 4 GeV/c consistent w/ possible thermal g comp.

Caveat 1: Upper limits only as of now (data could be lower).

Caveat 2: Prompt g reference used is NOT real p+p data but NLO pQCD:

Large uncertainties below pT ~ 4 GeV/c (unknown contribution of dominant jet bremsstralung component). Need direct p+p measurement for pT< 4 GeV/c

D.d'E. & D.Peressounko

(in preparation)

Dmitri Peressounko

QGP temperature from thermal g

Good correlation between exponential photon slope and initial temperature:

Teff dominated by hottest phase

Dmitri Peressounko

EoS with

phase transition

EoS without

phase transition

Small smearing of effective temperature

due to:

Final thickness of matter

Temperature gradients T(r)

Collective velocity

D.d'Enterria and D.Peressounko,
nucl-th/0503054

QGP “EoS” from thermal g & hadron multiplicities (I)

Access EoS correlating thermal g slopes (temperature) & hadron multiplicities (dNch/dη ∝entropy in isentropic expansion) measured in diff AuAu centralities.

Evolution of the effective # of degrees of freedom, g(s,T), with centrality:

ghydro(s,T) ~ 42 (QGP) for all centralities: AuAu-200 GeV (medium too “hot”, even periph.)

ideal-gas QGP “plateau” in # d.o.f. should be observable in the data.

Dmitri Peressounko

D.d'E. & D.Peressounko

nucl-th/0503054

QGP “EoS” from thermal g & hadron multiplicities (II)

D.d'E. & D.Peressounko

(in preparation)

AuAu @ 200 GeV produces “too hot” medium (QGP for all centralities).

Insensitive to any centrality-dependent change due to QCD phase transition.

Preliminary hydro calculations for AuAu @ 62 GeV :

Apparent phase transition change in geff(dNch/dη,Teff) for centrality 50-60%

Should show up in more central collisions for lighter / lower-√s:

AuAu, CuCu @ 62 GeV. AuAu, p+p @ √s = 40 GeV (RHIC-II) ?

Dmitri Peressounko

i=4R/= 0.45 fm/c

Reducing systematics at low pT !

Isolating the thermal component requires:

(i) small systematic uncertainties in AA and pp !

(ii) within pT ~ 1 – 4 GeV/c !

?

“Tips” from the expert (I): Dominating systematics

Justin Frantz

EM Calorimeter “Base Method”: Count Cal Hits

PHENIX Calorimeters (seg. ) PbSc/Gl MidRap

STAR Calorim. (BC seg.  MidRap, ECC ForwardRap seg smaller )

Systematic errors for photon 10-15%,  14-18%

High pT: dominated by Energy Scale and Efficiency

Low pT : large hadron contamination

Limitations at low pT :

Hadronic showers (50% @ pt < 1GeV)

Resolution (E/E = A/√E+B )

Cluster splitting effects

Decay bckgd: Acceptance for 0 and h

“Tips” from the expert (II): Going lower in pT

Justin Frantz

Use the conversion measurement ! (e+- ID runs out @ 5 Gev/c)

Energy Resolution has opposite behavior:

At low pT < ~3 GeV systematics smaller. Total systematics 13% (STAR)

Factor of 102-4 loss in statistics won’t hurt in Run-4.

In the region of overlap with EMCal measurements, reduce  energy scale uncertainty by “combining” rate normalization.

Extend p+p -->  measurement to low pT (need baseline in “interesting” region !)

Result: Constrain thermal rates below 4 GeV and confirm/deny jet-medium enhancement

PHENIX Emcal

PHENIX Tracking

E/E Resolution (%)

pT (GeV/c)

PHENIX

Ex.: photon yields @ 62.4 GeV

Arbitrary Units

Many exciting topics in photon physics

Paul Stankus

Justin Frantz

Summary:

-h correlations: calibrated away-side energy. separation angular jet shape differences btw Bremss., Compton  bkg. Also difference in flow contribution of -h angular correlation shape.

Reaction plane dependent direct g analysis. Measure direct  flow directly (may be 0). Constrain path dependencies.

Direct photon- (and Z-) tagged jet fragmentation

Jet+medium-induced direct photons

Investigate the approach to thermal equilibrium (parton cascade)

Beam-stopping collective bremstrahlung

Source size via gg HBT correlations

...

The central interest in thermal direct photons continues in RHIC and LHC nuclear collisions. But photon production, as well as W and Z production, touches on a wide range of physics topics beyond QCD thermodynamics:

Backup slides

Energy loss in Au+Au→ g + X @ √s = 200 GeV ?

(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]

+

( )

( )

( )

“Tips” from the expert (II): photon energy loss ?

Justin Frantz

Any (bremsstrahlung) photon energy loss ?

p+p Preliminary Comparison between isolation/non-iso method: null Brems?

Plenty of room in those systematics

Make real RAA (with p+p it’s all there!) More precise also look for nuclear effects (kT, Cronin)?

pQCD Brems ~ 20%

Fra

DdE. Hot-Quarks'04

“RAA”~0.8

Theoretical models

Monique Werlen

ll

Guillet, DIS04

Disentangling “thermal” g from quenched prompt g

Current best p+p g data:

Current best Au+Au g data:

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.

Photons from quark jets in the medium ?

Duke group predictions for Compton & annih. of fast quark in medium

LO for photons (& not most recent thermal photon rates)

But NLO (K = 2.5) for jets, no energy loss taken into account ...

Effect probably overestimated

Fries, Muller, Srivastava

PRL 90 132301 (2003)

David d'Enterria

2004-01-14T06:23:12