Charged particle Multiplicities at BRAHMS INPC2001 July 30-Aug 3, 2001 Berkeley

Charged particle Multiplicities at Charged particle Multiplicities at BRAHMS BRAHMS INPC2001

July 30-Aug 3, 2001July 30-Aug 3, 2001BerkeleyBerkeley

Ramiro Debbe

Physics Department

Brookhaven National Laboratory

COLLABORATION

BNL 8

University of Bucharest 7

Jagellonian University 5

Johns Hopkins University 2

New York University 2

Niels Bohr Institute 10

Texas A & M University 5

Fysisk Institutt Bergen, NORWAY 3

University of Kansas 2

University of Lund 2

University of Oslo 3

Overview of presentationOverview of presentation

• How much can we learn from charged particle multiplicity densities.

• Description of our detectors.

• Descriptions of data analysis.

• Our data and comparison to some models.

• Summary.

WHAT CAN WE LEARN ?

In the context of highly transparent interactions our measurement opens a window to the “blob” at CM and possibly the rapidity shifted barions (~4) ybeam=5

At the strong force scale a very long time has elapsed between interaction and detection, the system has evolved through many stages.

Multiplicity densities can be related to entropy in an statistical approach to this problem. If the expansion of the “blob” leaves entropy unchanged, our measurements provide a limit to the initial entropy production.

By comparison to models the shapes can give hints about late stages.

Perspective View of Spectrometer

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Detectors used to extract the multiplicity density.

TPM1

BBC

TMA

SiMa

TPM1 Time Projection Chamber

SiMa Silicon strips

TMA Scintillator tiles + PMT

BBC Čerenkov radiator + PMT

BBC

BEAM - BEAM detector

Array of 79 UV transmitting Lucite radiators coupled to PMTs

(Čerenkov detectors)

Coverage: 2.1 < η < 4.7

These detector have good resolution, self calibrated to count charged particles

Each array is located 219 cm away from the nominal IP

TDC resolution

Vertex reconstruction

Z BBC - Z TPM1

Zero Degree Calorimeter

– Used as one of our least biased triggers.

– Has good resolution to count neutrons.

Single neutron peak

TMA Tile multiplicity array

38 scintillator tiles read with wls fibers and PMTs

12 x 12 x 0.5 cm

Nominal coverage:

2.2 < η < 2.2

Placed 14 cm from beam axis

25 Si strip detectors

4 x 6 cm x 300 μm each subdivided in 7 strips. Same nominal coverage as TMA.

Located 5.3 cm from beam axis.

Multiplicity measured with a TPC

background

Multiplicities are extracted by counting tracks that point to IP.

That number is corrected for angular acceptance and tracking efficiencies

Typical event

Y from tracking Vs BBCx - TPCx

ENERGY CALIBRATION OF TMA AND SiMA

Both detectors were calibrated with the 1 MIP peak extracted from peripheral data.

MULTIPLICITY DENSITIES

θΔη

• Find vertex with TPC or BBC or ZDC

• Define η and Δη

• Translate ADC into number of MIP equivalent with MC that includes secondaries

• Average over sample of events

• Correct for Φ acceptance

CENTRALITY DEFINITION

TMA and SiMA used a minimum-biased multiplicity; centrality as fraction of it

To extend the coverage of BBC we used cuts along ridge of ZDC vs BBC multiplicity

BBC and SiMA + TMA is well correlated

RESULTS

0 - 5 % 5 -10%

10-20% 20-30%

30-40% 40-50%

TPM1

BBCSiMA

TMA

Multiplicity densities for different centrality samples. Statistical errors shown if bigger than symbols size.

SYSTEMATIC ERRORS

We assign the following systematic errors arising from energy calibration and secondary interactions:

• SiMA: 8% for |<1.5 and for 2.5

•TMA: for and for

•BBC: (mainly secondary interactions)

•TPM1: in central events and for the most peripheral.

MODEL COMPARISON

These distributions are the average of all different detectors and positive and negative .

Error are statistical + systematic.

dN/dη per Participant pair

PHENIXFRITIOF

HIJINGEIKONAL

EIKONAL GLAUBER

MC GLAUBER

EKRT

SUMMARY

After the first run of RHIC all four experiments have collected data that open interesting puzzles,but did not match the most optimistic predictions.

BRAHMS has measured charged particle multiplicity density in a quite wide pseudo-rapidity range.

The agreement with the other RHIC experiments is good

The yield of charged particles in the most central collisions turned out to be lower than expected.

The shape of the measured distributions points to interactions and possibly to a thermalized system.

With our resolution, we cannot resolve any hint of rapidity shifted baryons.

Spectrometer System

Front and Back Forward Spectrometers

All magnets, detectors and control systems arein place.We started commissioning the detectors close toIR