Development of a trigger algorithm for the measurement of rare probes in the CBM Experiment at FAIR Partha Pratim Bhaduri∗1 , Subhasis Chattopadhyay1 , and Premomoy Ghosh1 1
Introduction
Variable Energy Cyclotron Centre, 1/AF Bidhan Nagar, Kolkata-700 064, India
The planned Compressed Baryonic Matter (CBM) experiment [1] at the upcoming FAIR center at GSI Darmstadt is aiming at the investigation of baryonic matter at high density produced in relativistic heavy-ion collisions. The proposed key observables include the measurement of charmonia, which can be measured via their decay into the dimuon channel [2]. But the multiplicity of charmonium is extremely small at FAIR energies (EL = 10 - 40 AGeV) which requires an extreme interaction rate (up to 107 events per second) to have a good statistics. But the existing Data Acquisition systems (DAQ) can record the events at a rate 25 KHz. In this report we describe the development of an algorithm for online event selection trigger to pick up the interesting physics events (events containing the rare particles) and reject the junk.
Algorithm The idea is to select the events online containing muon pairs coming from the charmoium decay. Since our muon detection system is placed outside the magnetic field so the high momentum muons coming from the decay of J/ψ will go approximately in straight line reaching upto the last station. The present algorithm is based on the following steps : 1. Search for a hit-triplet in the last three muon stations. The members of a triplet should belong to at the same straight line. The angular (both polar and azimuthal) information is being used for the formation of a triplet. 2. The selected event should have atleast two hit-triplet, since we are looking for di-muons 3. Projecting the triplets up to the target region (vertex) using the information of the non bending plane. Uptill now we have implemented the first two steps. Our focus is to suppress the background events as much as possible without losing the signal reconstruction efficiency. A schematic diagram depicting the different steps of the algorithm is given in figure [1]
Preliminary results The first results of our trigger study is shown in the Table 1. For estimation of background suppression factors we have considered 1k of each central and minimum bias UrQMD [5] events. Signal reconstruction efficiencies have been calculated employing PLUTO [4] generator (generat∗
[email protected]
(0,0.0) B field 16
17
18
Figure 1: Schematic of the trigger logic for selection of rare events ing phase space decay of J/ψ) embedded in a background generated by UrQMD. Table 1: Results for background suppression Collision Events suppression mbias 1000 250 Central 1000 50
Summary Muons are a potential candidate for the measurement of observables like low-mass vector mesons and charmonia. But due to their extremely low production rate at CBM energies, very high event rates are required for their measurement with a well defined statistics. An algorithm to filter out the rare events is under development to suppress the background events as much as possible without reducing the signal reconstruction efficiency. The algorithm is being optimized to achieve the sought suppression rate (400) .
References [1] P. Senger J.Phys.G: Nucl. PArt. Phys. 31 (2005) S1111S1114 [2] A. Kiseleva, P. P. Bhaduri et. al. Quark Matter Proceedings (2008) to be published in Indian Journal of Physics. [3] S. Chattopadhyay Quark Matter Proceedings (2008) to be published in J. Phys. G. [4] http://www−hades.gsi.de/computing/pluto [5] S. A. Bass et al. , Prog. Part. Nucl. Phys. 41 (1998) 255.
