# Presentation: 787850

## Systematics of Charged Particle Production with the PHOBOS detector

**Systematics of ****Charged Particle Production ****with the PHOBOS detector**

- Peter Steinberg
- Brookhaven National Laboratory
- ISMD2001, China

## PHOBOS Detector @ RHIC

- (not to scale!)

- (2 out of 6 shown)

## Multiplicity Measurements in 4

**Multiplicity Measurements in 4******

- -5.4

- +5.4

- Single-event display

- Vertex “tracklets” – 3 point tracks

## Systematic Measurements

- Wish to understand if AA collisions show collective behavior
- How does the energy density scale with energy?
- Do particle production scale simply with participants or collisions?
- Is there a central plateau?
- Does the final state carry memory of the initial geometry?

- pp collisions

- pA collisions

- We study this with systematics of charged particle production:
**Energy, Rapidity, Centrality, Azimuthal angle**

## Centrality Selection

- HIJING shows particle production in paddles is monotonically related to Npart

- Spectator matter measured in ZDC *anti*-correlates
- Expected if

- Cut on *fractions* of total cross section to estimate Npart

- Central 6%

## Energy Dependence at =0

- Errors are dominated by systematics

- AGS/SPS points extracted by measured dN/dy and <mT>

- New data at 200 GeV shows a continuous logarithmic rise at midrapidity

- fpp(s) =

## Ratio of dN/d at 200 & 130 GeV 

- 90% Confidence Level

## Centrality Dependence at =0

- Measurement using vertex and spectrometer tracklets
- Systematic errors, 4.5% in spectrometer, 7% in vertex detector
- Participants estimated using Glauber model in HIJING

- PHOBOS 130 GeV

- EKRT: saturation model
- approx. constant in Npart

- Kharzeev/Nardi:Saturation and modifiedwounded nucleon model
- give same results.

- HIJING:
- Soft/hard scaling

## Pseudo-rapidity Distributions

- Using Octagon and Ring subdetectors
- Measure out to ||<5.4
- Corrections
- Acceptance
- Occupancy
- Backgrounds (from MC)
- Systematic errors
- 10% near =0
- Higher near rings

- 

- Background Corr.

- HIJING Simulation

- PRL 87 (2001) forthcoming

## Centrality Dependence vs. 

- Total charged multiplicity is about 4200 ± 420 for central events
- At high 3-4 multiplicity starts to *decrease* as a function of Npart
- Similar feature seen in *pA* collisions

- PRL 87 (2001) forthcoming

## Comparison to pp and models

- Peripheral

- Central

- Scaled UA5 data

- HIJING

- AMPT(rescattering)

- Ybeam

-  (Y130/Y200)
- dN/d = fpp(s)

- PRL 87 (2001) forthcoming

- DeMarzo et al, 1984

- Systematic error not shown

## Change in dN/d with energy

- UA5 looked for ‘limiting fragmentation’ by plotting dN/d with  - Ybeam
- We can do the same thing with the PHOBOS data
- agreement for AA in the fragmentation region

**200 GeV**

**130 GeV**

- UA5 200 GeV
- (NSD)

- UA5, Z.Phys.C33, 1 (1986)

## Saturation model fits to 130 Data

*m**2**=2Q**s**m***, *p**T**=Q**s*, ~.3 extracted from HERA F2 data

- Kharzeev & Levin, nucl-th/0108006, input from Golec-Biernat & Wüsthoff (1999)

## Azimuthal Asymmetry at =0

**Azimuthal Asymmetry at ********=0**

- 130 GeV data from PHOBOS
- Correct for occupancy, resolution of reaction plane estimate
- Good agreement with hydro calcuations for central events (and STAR...)

- X = cos(ni), Y = sin(ni)
- n = atan(Y/X)
-  v2 = <cos 2(2)>
- At midrapidity, using “SymOct”
- Account for detector response using “weighting matrix”.

**Pads****in ******

**Pads in z**

## Asymmetry vs. Multiplicity

- Compare
- Flow for mid-central events
- Multiplicity per participant pair for central events (only 10% variation down to Npart=100)
- Flow (for more peripheral events) seems to scale with particle density
- 200 GeV data should be interesting!
- Saturation or scaling?

**PRELIMINARY**

## Asymmetry vs. Rapidity

**Systematic Error ~ .007**

- Preliminary Results – final results coming soon!

- P. Kolb, Utah proc.

## Conclusions

- Systematics of charged particle production have been explored by the PHOBOS experiment
**Energy** – multiplicity rises approximately logarithmically
**Centrality** – data shows simple interpolation between pp and central AA
**Rapidity** – scaling with Npart changes in fragmentation region
**Azimuthal** – elliptic flow appears to scale with multiplicity
**Particle ratios** are reaching zero net baryon number, small B
- Broad features of particle production are consistent with soft nature of strong interactions
***pp*** and ***pA*** collisions are very instructive
- Theoretical models are assimilating new data
- None simultaneously describe *full* set of systematics!

## Results on particle ratios

- Measured near midrapidity
- 0 < y <1 (species-dependent)
- Final results submitted to PRL (Apr. 2001)
- Consistent within systematic errors with results presented at QM2001 (Jan. 2001)
- Smaller systematic errors (10%  6%)

## Glauber Calculations

- Optical Approximation:

- Full Calculation:

- We use full calculation by means of Monte Carlo approach in HIJING

- Correct Npart(b), Ncoll(b) – incorrect extraction!

## Energy dependence of v2

- Logarithmic rise