Analysis of Monte Carlo Events: Multiplicity

The total multiplicity (N_total) and the detected multiplicity (N_meas) are related via (N_meas) = F(N_total), or N_total N_meas/F, where F and N_total are given in table 2. The uncertainty on N_total due to statistical fluctuations⁸ in the number of particles in the detector is:

Using the numbers in table 2 with eq. 1 allows the value of _total/N_total to be estimated as 1%, 13%, and 19% for Au+Au (central), p+Au (min-bias), and p+p, respectively. These fluctuations in the fraction of the particles detected set limits on the performance of the detector as an event-by-event multiplicity detector.

The performance of the detector will introduce further uncertainties in the multiplicity measurement. The important factors included in the simulations which affect the multiplicity measurement are the efficiency of the individual strips (see fig. 2), noise, the number of strips "hit" by a single particle, and multiple hits on a single strip. However, corrections for all of these effects can be made.

When⁶ P_noise = 10^-3, the average number of hits due to noise in the parallel strips will be 38.4 per barrel. This is not an important correction for central Au+Au collisions, but is significant for p+p and p+Au. In these cases, this (pessimistic) noise level is frequently greater than the number of real hits in the inner barrel. Therefore, it will be important to both minimize and understand the noise in the detector.

Eq. 1 gives an estimate of the loss in performance from a reduction in the coverage of the vertex detector. Given the large multiplicity in a central Au+Au collision, a sightly smaller detector would give a good measurement of the multiplicity. However, the reduced coverage of the vertex detector described in the Tales/Sparhc letter of intent⁷ would further degrade the already marginal accuracy of the p+p and p+Au multiplicity measurements. Although the solid angle coverage of such a detector would be sufficient to measure the multiplicity for central Au+Au collisions, its use of parallel alone strips could complicate the measurement.