BRAHMS Beam-Beam Counters. Fall'2000

BRAHMS Beam-Beam Counters in First Collisions at RHIC.

Oral Presentation at Annual Fall Meeting of the Division of Nuclear Physics.
Session DC -- Mini-Symposium: First Results from RHIC I.

Yury Blyakhman, Burton Budick
New York University, Physics Department.
BRAHMS Collaboration.

October 2000

The Broad RAnge Hadron Magnetic Spectrometer experiment (BRAHMS) is designed to measure charged hadrons over a wide range of rapidity and transverse momentum for all available beams and energies in experiments at RHIC (Relativistic Heavy Ion Collider), Brookhaven National Laboratory. Figure 1 shows a perspective view of BRAHMS detector system. One set of these detectors, called Beam--Beam Counters is employed to characterize collisions from a global perspective. As one can see on this picture, they are located some distance backwards and forwards of the assumed collision vertex. Beam-beam Counters are designed to provide an initial trigger, permit a rapid estimate for the collision point (vertex location) and serve as a first probe of charged multiplicity for a wide range of pseudorapidity.

First, detectors were designed and tested using computer simulations with the help of the CERN code GEANT. FRITIOFF702, Venus and HIJING models were used as event generators, in order to study detectors response with respect to different parameters. Later detector's performance was tested in the Test Beam of Summer'99. First real results were achieved during the Commissioning and Data Run of this summer. All these studies can be summed up as follows.

Figure 2 shows the actual view of the two Beam--Beam Counters arrays. Each detector consists of Cerenkov radiators, glued to the photomultiplier tube. Phototubes diameters are 19mm or 51mm with 4cm long or 3cm long radiators respectively. As it can be seen on the bottom picture, the Right Array has reduced azimuthal coverage because of the magnet D1. It is partially compensated by higher spatial density of modules in the Right Array. Also, several counters in both arrays are located at the same distance from the beam-pipe, which provides better precision and might help in case of any malfunction diagnostics.

Basic experiment logic and outline are given on Figure 3. A module will trigger when a Coincidence Unit receives three signals at the same time: from two OR units, related to each of the two arrays (Left and Right); and a signal from the RHIC Clock, saying that beam entered the Interaction Region. The OR unit can be set to click if some desired number of tubes in the Left and in the Right Array are hit within the desired Gate. This number depends on the kind of events one wants to detect.

Figure 4 describes trigger configurations, which were employing Beam-beam Counters. These trigger conditions were derived from the computer simulations, using different kind of collisions, and real data analysis from the Test beam. All of the Beam-Beam triggers were using precisely timed big tubes, as the most fastest ones in the arrays. Following trigger configuration were used:

Any 2 Left and 2 Right Big tubes in coincidence within a 5nsec Gate
Any 3 Left and 3 Right Big tubes in coincidence within a 14nsec Gate
Perfectly matched and timed 2 Left and 2 Right Big tubes in coincidence within a 5nsec gate -- Best collision determination

On Figure 4 you can also see a graph of the famous first collision, detected at BRAHMS. It was detected with the condition of Time Differences in Beam-Beam and ZDC's to be equal to 0.

Figure 5 describes beam-Beam Timing and vertex determination. As for the good timing of an interesting high multiplicity event, it will be produced if at least one charged particle with beta=1 strikes the active area of the module in every array and arrives within 30 ps of the arrival time expected for it. Tubes alignment and an intrinsic time resolution for each tube of 46ps, will make possible fast coincidence between arrays which will serve to identify the reaction vertex to +- 9mm precision. Known time of the collision makes possible to construct beam's vertex distribution. It was found to be distributed with sigma=73.0+-0.1cm. By the comparison with the vertex found by the TPC's, Beam-Beam Z-resolution was accepted to be Delta Z = +- 4.2cm.

Conditions for the event to be considered as collision are shown on Figure 6

Two Big tubes coincidence in the BBC within a narrow gate;
ZDC trigger, to include most peripheral collisions
Collision vertex location, determined by the BBC to be within 50cm of the physical vertex to ensure proper BBC acceptance, since they are pointing to the physical center of the detector.

Events, satisfying all these conditions were considered as MinBias events. ADC spectrum of our tubes allowed us to measure up to 5-6 clear hits in the detector for them.

`` Multiplicity in the Detector'' for these events, as measured by Beam-Beam Counters is showed on Figure 7. Cuts on BBC multiplicity were used to determine centrality of the collision events. Multiplicity was calculated as ADC value without a pedestal, divided by the gain of the tube. Events, contributing to the top 6% of the multiplicity, were considered Central. Here I show central events ADC spectra as well as top 32% ADC spectra, as something in between the MinBias and Central events.

Charged multiplicity estimations for large dynamic range at different values of pseudorpidity can be called as one of the most important results, obtained by the BBC during this year run. Geometrical location of Beam-Beam Counters gives us coverage of 3.0 < | eta | < 4.0. Let me show some theoretical estimations first, before coming to the real data. Figure 8 shows dN/dEta distribution for the FRITIOFF702 event generator with the conditions on the impact parameter as 4fm< b< 6fm. This event generator was chosen because it most closely corresponds to the real data in the midrapidity region. Marks on the graph represent real modules locations. One can easily see, that detector's response for the primary particles (in red) follows really close the theoretical curve. GEANT allows us to make some rough background estimation, as shown on the example of the Left and Right Module. Correction, extracted from this two lines will be used later for the background correction of the real data.

Figure 9 shows VERY PRELIMINARY graphs of dN/dEta extracted from the real beam data. Here are the facts:

dN/dEta values were extracted on event-per-event basis;
Only Big tubes were used for this analysis;
Event selection was done through the detector's multiplicity distribution;
Vertex was defined by the Beam-Beam Counters for most of the events and by ZDC's in the rest of the cases;
Wide dynamic range was achieved due to the events happening within 45cm of the physical vertex;
Results variation due to the Vertex precision of 4.2cm was found to be less than 2%;
Background corrections were done using Fritioff702 simulations and the agree with the real data estimations by more than 95%;
Errors on eta estimations were defined by the method and, thus are constant and equal to .05 units of eta.
Statistical Error on dN/dEta is very small (under 5%);
Systematic Error is consider to be at least 10% for now, due to the very rough background estimation mechanism;

Thus, for the conclusion we can say, that from the performance during the Commissioning and Data Run of this Summer it is clear, that Beam--Beam Counters show a great performance in serving as a Level0 trigger for BRAHMS, to identify collisions for RHIC and also to provide dN/dEta information for wide range of pseudorapidity values.

to the Beam-Beam Counters Public Page

Prof. Burton Budick, Yury Blyakhman.