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Rapporteur II: Global & Flow Observables

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Rapporteur II:Global & Flow Observables

Peter Steinberg

Brookhaven National Laboratory

Global Flow

Peter Steinberg

BNL

Global Variables

Event shape

dN/dh

Centrality dependence

dN/dh

dET/dh

pT

Initial Energy density

“Flow”

Event shape

dN/df

Centrality dependence

dN/df

Species

v1,v2,

Initial Pressure

- In principle, we are looking at two important pieces of the equation of state…

The collision geometry (i.e. the impact parameter) determines the number of nucleons that participate in the collision

“Spectators”

Only ZDCs measure Npart

Zero-degreeCalorimeter

“Participants”

“Spectators”

- Many things scale with Npart:
- Transverse Energy
- Particle Multiplicity
- Particle Spectra

Produced Particles

Fluctuations modify the response

less central events fluctuate to central bins

The final “measurement” of Npart is the best attempt to factor out the facts of life!

In principle, we could work with % of cross section

Final measurement of Npart is best attempt to correct for facts of life

Npart

Multiplicity in 3<|h|<4.5

- Clearly, fluctuations affect your centrality estimator

- Very difficult to compare experimental results without serious estimate of Npart
- Must incorporate fluctuations in the measurement of the centrality estimators
- OK, Glauber implementation is a real uncertainty
- Even if you don’t “like” participants, the exercise is critical for inter-experiment comparisons

ZDC

BBC

Percentile

- Only shared detector
- Rates: luminosity via well-known reference process
- Timing: substantial background rejection
- Pulse height: measures centrality

- Directly confirms monotonic relationship between participants with multiplicity

Reference: szdc =10.7+/-0.5 b

Measurement: (geo / tot)exp =

(Nbbc/ Ntot)exp/ bbc= (0.668 0.022)

Theory: geo / tot = (0.673 <0.034)

(measured)

(from Glauber)

- Can the models get the “big picture” right?
- However, let’s not ignore the details…

- Magnitude
- Integral over energy density, stopping, shadowing, quenching, flow

- Centrality dependence
- Study effect of system size (onset of interesting effects above critical volume)
- Interplay between Npart and Ncoll

- Shape
- Stopping, Final state interactions

PHENIX

STAR prelim. 10%

PHOBOS

BRAHMS prelim.

with quenching

no quenching

LEXUS

- AMPT, LEXUS, DSM, HIJING, EKRT
- Please be careful about scaling y to h
- Not boost invariant!
- Not .9, .95 etc.
- Jacobian depends on velocity: dy = b dh
- Depends on species and mean pT!

- Still not sure who gets the champagne…wait for 200 GeV

AMPT

dN/dh / .5Npart

Npart

Measurement sensitive to trigger bias

“Minimum-bias” still has bias

Affects most peripheral events

% Error on Npart

This measurement

Npart

- Estimating 96% when really 90% overestimates Npart
- Creates “pivot point” at central events
- Hard to rule out EKRT…

dN/dh / .5Npart

Npart

Octagon

Rings

dNch/dh

45-55%

35-45%

25-35%

dNch/dh

15-25%

6-15%

0-6%

h

h

h

%s

Mean Npart

0-3

Data

HIJING

354

15-20

216

35-40

102

dNch/dh

dNch/dh

Data

HIJING

(dNch/dh)/(½Npart)

(dNch/dh)/(½Npart)

h

h

Systematic error ±(10%-20%)

PHOBOS Prelim.

Solid lines: HIJING

Symbols:

Errors are systematic

|h|

<1

2-2.4

(dNch/dh)/(½Npart)

3-3.4

4-4.4

5-5.4

Npart

PHOBOS Prelim.

Nch

HIJING

Npart

- EKRT, HIJING disfavored by both PHENIX & PHOBOS
- Initial state saturation looks like modified Glauber
- No way to resolve using Nch alone

- What about ET?
- Hydro does p dV work during longitudinal expansion, decreases dET/dh
- Eskola: “ET will be more efficient model killer”…

- So far, few papers predicting ET, but surely on the way
- PHOBOS got 9 in two months after the first paper…

PHENIX submitted

PHENIX Preliminary

- ET and charged particles appear to vary in lockstep
- Fits are a modified WNM, possibly allow extraction of fraction of hard production (NB. ambiguities persist…)

Independent of centrality

Appears to be same as WA98 (@SPS)

Energy dependence

Possible 20% discrepancy betw. NA49/WA98

Where is the increased <pT> seen by STAR/PHENIX?

PHENIX Preliminary

PHENIX Preliminary

Sorry, I won’t tell you…

Implication of PHENIX

Constant ET/charged particle

Energy density (via Bj formula) simply scales with multiplicity!

(Even PHOBOS can do it!)

~50% higher than SPS…

Ambiguities persist

Formation time might be substantially less

Radial flow

Not seen in angular distributions

Use HBT, spectra (T = To + m<b2> - Nu Xu)

Directed flow

Forward rapidities

Not measured yet

Sensitivity estimated at PHOBOS/STAR

Interesting predictions for phase transition…

Elliptic flow

Early time push, hydrodynamic evolution

Strongest at midrapidity

Method used by PHENIX

Similar information content as Fourier method

OK for partial acceptance

Sensitive to other correlations

Jets (at 180o) , HBT (at 0o)

But is that bad?

CERES data

0-5%

Df

5-15%

15-30%

- Boxes show “initial spatial anisotropy”e scaled by 0.19-0.25

PRL 86, (2001) 402

|| < 1.3

0.1 < pt < 2.0

midrapidity : |h| < 1.0

V2

Hydrodynamic model

Preliminary

SPS

AGS

Normalized Paddle Signal

- Excitation function
- CERES 40 GeV fits in with existing energy systematics

- Back-to-Back correlations
- Extraction of v2 is substantially higher than normal event plane analysis

- Hydro calculations: P. Huovinen, P. Kolb and U. Heinz

PHENIX Preliminary

- Hydro fails at large transverse momentum
- Possible interpretations suggested by jet quenching (wait for A. Drees talk)
- However, perhaps composition is a critical part of this effect…

v2

PHENIX (pT>500 MeV)

nch/nmax

v2

v2

- NA49 (y), PHOBOS(h) (mainly pions)
- Different shape at midrapidity
- PHOBOS shape similar to dN/dh!
- Low-density limit?? v2 ~ e dN/dy

- However, v2 appears to fall faster than multiplicity

PHOBOS Preliminary

y

h

PHOBOS Preliminary

dN/dh

h

ÖsNN dependence

- Assumptions:
in Lab in C.M.

- Energy density (Bjorken):
- From SPS to RHIC
- ~50% increase in dNch/dy
- ~50% increase in dEt/dy
- at least 50% increase in e

- Pions
steep rise and plateau

- Protons
gradual rise and higher <pt>

- Non-Flow Effects
- Momentum conservation
- HBT, Coulomb (final state)
- Resonance decays
- Jets