CHARGED PARTICLE MULTIPLICITIES AT BRAHMS. Page: 3 of 6
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a 0. 0.a 1.2 1.6 2 0 0.4 06 1.2 1.6 2t 0 2 4 6 8 10 12 14 16 18 20
ES: (M-V) Eas (M-V) Nch
FIGURE 2. Energy deposited in one of the silicon detectors versus vertex position. A band related to a single minimum-ionizing
particle is clearly seen in panel a. Bottom panels show the matching of data and GEANT simulated single MIP peaks used for
energy calibration of the detector. Similar work is done to calibrate the TMA. Panel d shows the ADC distribution of one of the
BBC detectors. The pedestal of the distribution has been subtracted and up to three MIPs can clearly be seen.
.The centrality of the collision is directly related to its charged particle multiplicity, BRAHMS measures the centrality
of each collision as a fraction of a minimum-biased multiplicity distribution.
The first stage in the analysis of each event is the extraction of the charged particle multiplicity. That multiplicity
can be extracted from the MA or the BBC detectors depending on the location of the interaction vertex. If the vertex
is confined to the physical size of the TMA (30 cm from the center of the array), the multiplicity of the event will be
the average of the values measured with the TMA and SiMA detectors.
The minimum-biased multiplicity distribution extracted from the MA counters requires that there was a time
coincidence between the two ZDC detectors, the vertex of the collision be within the limits mentioned above, and
a multiplicity of at least four in the TMA array. Simulations based in the event generator HIJING and a GEANT
package that includes all elements of the experimental setup, indicate that this minimum-biased distribution represents
95% of the total nuclear cross section.
If the vertex of the collision lies beyond the physical size of the MA counter, (vertices within 120 cm of the MA
center) the multiplicity of the event is extracted from the number of charged particles that impinged on the BBC
detectors. This in turn, extends the coverage of the BBC detectors up to q = 4.6 Figure 1 shows the minimum-biased
multiplicity distribution extracted with the MA
The upper right insert in figure 1 shows the good correlation between multiplicities extracted from the MA as well
as the BBC counters. In the angular region where both system have overlapping coverage; 3.0 < a < 4.2 the BBC data
was analyzed with both centrality selections and the results came up within 1% of each other.
The procedure to extract multiplicities from both of the MA systems is the same, the vertex of the collision is
measured with the TPC, BBC or ZDC, an average angle of incidence to tiles or strips is calculated from that point and
the surveyed position of each detector element. Tiles and strips are for the remainder of the analysis grouped in rings
of equal q. A factor to translate the amount of energy deposited in each element to the number of primary charged
particles is extracted from a GEANT simulation. A sample of the 6% most central events from an event generator is
used as input, the mean value of the number of primary charged particles hitting each element is extracted, as well
as the mean energy deposited. The energy deposited includes secondary interactions and different angles of incidence
as well as different particle mixtures. This simulation is done for different vertex positions and a correlation between
number of primary particles and energy deposited is obtained. The amount of charge collected in each element of
the array is calibrated by matching the data and the Monte-Carlo simulations in very peripheral events where a peak
assigned to single minimum-ionizing-particle (MIP) is clearly visible. Figure 2 a shows the band of 1 MIP in the
silicon array and its correlation to the GEANT simulated distributions appears in panels b and c.
To extract the pseudo-rapidity distributions at very forward angle we use the information from the Beam-Beam
detectors, as can be seen in figure 2 d, these detectors are self calibrated and have few charged particles going through
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DEBBE, R., FOR THE BRAHMS COLLABORATION. CHARGED PARTICLE MULTIPLICITIES AT BRAHMS., article, July 30, 2001; Upton, New York. (digital.library.unt.edu/ark:/67531/metadc723474/m1/3/: accessed October 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.