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

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

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

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  1. Charged particle Multiplicities at BRAHMS INPC2001 July 30-Aug 3, 2001Berkeley Ramiro Debbe Physics Department Brookhaven National Laboratory

  2. 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

  3. Overview 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.

  4. 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.

  5. Perspective View of Spectrometer

  6. Detectors used to extract the multiplicity density. BBC TPM1 SiMa TMA BBC TPM1 Time Projection Chamber SiMa Silicon strips TMA Scintillator tiles + PMT BBC Čerenkov radiator + PMT

  7. 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

  8. TDC resolution Vertex reconstruction ZBBC -ZTPM1

  9. Zero Degree Calorimeter • Used as one of our least biased triggers. • Has good resolution to count neutrons. Single neutron peak

  10. 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.

  11. Multiplicity measured with a TPC Typical event Y from tracking Vs BBCx - TPCx Multiplicities are extracted by counting tracks that point to IP. That number is corrected for angular acceptance and tracking efficiencies background

  12. ENERGY CALIBRATION OF TMA AND SiMA Both detectors were calibrated with the 1 MIP peak extracted from peripheral data.

  13. MULTIPLICITY DENSITIES Δη θ • 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

  14. 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

  15. BBC and SiMA + TMA is well correlated

  16. RESULTS 0 - 5 % 5 -10% TPM1 SiMA BBC 10-20% 20-30% 30-40% 40-50% TMA

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

  18. 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.

  19. MODEL COMPARISON These distributions are the average of all different detectors and positive and negative . Error are statistical + systematic.

  20. dN/dη per Participant pair EKRT FRITIOF PHENIX HIJING EIKONAL EIKONAL GLAUBER MC GLAUBER

  21. 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.

  22. Spectrometer System Front and Back Forward Spectrometers All magnets, detectors and control systems are in place. We started commissioning the detectors close to IR

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