A spectrometer for study of high mass objects created in relativistic heavy ion reactions

Abstract

Experiment E864 at the Brookhaven AGS accelerator uses a high sensitivity, large acceptance spectrometer, designed to search for strangelets and other novel forms of matter produced in high-energy heavy ion collisions. The spectrometer has excellent acceptance and rate capabilities for measuring the production properties of known particles and nuclei such as p̄, d̄ and ${}^6\text{He}$. The experiment uses a magnetic spectrometer and employs redundant time of flight and position detectors and a hadronic calorimeter. In this paper we describe the design and performance of the spectrometer.

Introduction

Experiment 864 is an open geometry, two-dipole magnetic spectrometer designed to search for Strange Quark Matter (SQM) in Au+heavy nucleus collisions at 11.5 A GeV/c. Due to the nature of the design of the rare particle search, the spectrometer is also well suited for detecting nuclear isotopes ${}^6\text{He}$, ${}^7\text{Li}$, etc.) and anti-matter (anti-protons $\text{p}\text{̄}$ and anti-deuterons $\text{d}\text{̄}$). The spectrometer identifies particles via their mass M and charge Z. A perspective view of the spectrometer is shown in Fig. 1. The main components are beam defining counters, a target system (usually Pb or Pt), a multiplicity counter for triggering on the centrality of the event, two analysis dipole magnets, three stations of hodoscopes for Time of Flight (TOF) and tracking, two stations of straw tubes for tracking, a hadronic calorimeter and a large vacuum tank not shown in this figure. The experiment also utilizes a high-speed data acquisition system (2.7 Mbytes per second) and a flexible second-level trigger (Late Energy Trigger) based on TOF and energy as measured by the calorimeter. In this paper we will describe the design considerations, their implementation and the performance of the various components and the integrated system.

The paper is organized as follows. The introduction contains a discussion of the design, an overview of the apparatus and a discussion of the expected backgrounds. The later sections give detailed descriptions of each subsystem and their performance:

Unless otherwise noted, the description will be that of the configuration during the 1996/1997 AGS heavy ion run. The experiment was also operated during the 1994 heavy ion run with a partial implementation of the calorimeter and S3 tracking station and during the 1995 run with the complete calorimeter and S3. The full LET was implemented in the 1995 run. The final run was in 1998.

The primary goal of the experiment is to search for the specific class of charged and neutral strangelets [1], [2] which are predicted to have the following characteristics:

These characteristics suggest a strategy of looking for mid-rapidity, massive objects with an unusual Z/A ratio in an apparatus with a high rate capability and redundancy for background rejection. The E864 apparatus shown in Fig. 1 implements this strategy by measuring the momentum, charge, energy and TOF of particles. Charged particles are tracked and their TOF determined using fast scintillator hodoscopes. In addition straw tube planes are used for precision tracking, thus the system has considerable redundancy. The search for neutral strangelets requires a hadronic calorimeter with good energy and TOF resolution. The calorimeter also adds an independent mass measurement for charged tracks. The spectrometer design uses an open geometry so that the acceptance is not overly model dependent. This also facilitates a high sensitivity and allows for correlation studies and growth potential.

The experiment is located in the A3 beam line of the Alternating Gradient Synchrotron (AGS) accelerator at Brookhaven National Laboratory. The incoming Au ion is fully stripped to its charge of +79 and has a momentum of 11.5 A GeV/c. The ions are incident on a target whose composition and thickness depend on the particular physics topic. The thickness is typically between 5% and 60% of a Au interaction length. The nucleon–nucleon center of mass energy is 4.6 GeV and its rapidity y is 1.6.

The entire spectrometer is shown in the elevation and plan views in Fig. 2. The global coordinate system used throughout this paper has the plan view in the (x–z) plane and the elevation view in the (y–z) plane (using a right handed coordinate system). The large gray box in the elevation view is an aluminum and steel vacuum chamber which surrounds the target and extends over the entire length of the apparatus. The vacuum chamber allows non-interacting Au ions and forward going beam fragments to continue in vacuum over the remainder of the sensitive detectors, thus avoiding interactions with air which would shower secondary particles throughout the detector. Particles of interest (within the acceptance) exit the vacuum chamber just after the second dipole magnet approximately 8.5 m downstream of the target through a thin vacuum window made of layers of Mylar and Kevlar. These particles then proceed through the downstream portion of the spectrometer, which resides just below the vacuum chamber. Thus, the spectrometer acceptance does not extend to zero degrees, but the detectors are as close to the vacuum chamber as possible. The acceptance is defined by a collimator residing inside the first dipole magnet M1. The various angular restrictions are shown in Fig. 3. The beam transport, plug and collimator designs are extremely important in reducing backgrounds due to secondary interactions. Details can be found in Ref. [3]. The effectiveness of the design is demonstrated in the low occupancies in the downstream detectors in agreement with the Monte Carlo (MC) simulations (see Section 9).

The downstream spectrometer consists of three TOF hodoscope planes (H1–H3), two straw tube tracking stations (S2, S3), and a hadronic calorimeter (CAL), whose positions and sizes are shown in Table 1. The parameters in the table are measured with respect to the target except for the horizontal and vertical axis positions which are measured with respect to the neutral line. The hodoscopes provide three measurements of spatial position, time and charge. The velocity of particles is determined from the timing information. Track candidates, found using the hodoscopes only, are projected to the straw planes and straw tube hits are added for better track spatial resolution. Then, the downstream track segments are projected into the magnetic fields. The tracks are assumed to come from the target, thus determining their rigidities (R=p/Z). The masses are calculated using

$$\text{m=}\text{R×Z}\text{γβ}\text{.}$$

Particles are thus uniquely identified by their mass and charge state. The tracks are then projected to the calorimeter where the energies of the particles are determined. Using the measured energies and the particle velocities from the downstream hodoscopes, one obtains a second measurement of the particle masses:

$$\text{m=}\text{E}\text{γ−1}\text{baryons}$$

$$\text{m=}\text{E}\text{γ+0.6}\text{anti-baryons}\text{.}$$

(The formula for anti-baryons takes into account the fact that approximately 80% of the annihilation energy is detected in the calorimeter.) Neutral particles can be identified on the basis of the calorimeter time and energy alone, using the tracking detectors to reject charged particles which deposit energy in the calorimeter.

The experiment operates with the two dipole magnets (M1 and M2). Their field strengths and polarities can be varied in order to optimize the acceptance for various particle species and to sweep out uninteresting lighter particles. Table 2 shows a list of the standard field settings used in the experiment. The two magnets M1 and M2 are set to the same polarity and field strength for any given setting. The sign convention for the setting indicates the charged species which is bent into the acceptance of the detector. It should be noted that neutral particle searches could be done at any of the field settings, but are best done at +1.5 T because this field is the most effective at sweeping charged particles out of the spectrometer.

The trigger for the experiment is based on centrality as measured by pulse height in a scintillator annulus just downstream of the target. In order to achieve the desired sensitivities a second-level trigger (Late Energy Trigger) based on time and energy information from the calorimeter has been implemented.

Given the layout just described, the main backgrounds are expected to be those which produce real tracks with the same directions and velocities as the tracks of interest. Sources of such tracks are:

The first class of backgrounds has been minimized with veto counters and the detection of multiple beam tracks in the trigger counters. The second class of background can be minimized by requiring that the momentum as measured by the tracking chambers agree with the energy as measured in the calorimeter. The rationale for the two magnet design plus the straw tube station S1 near the target was to detect this second class of background. Due to cost and construction problems S1 was not implemented. Λ's and $\text{Λ}\text{̄}$'s decays are not an important source of background because most of them decay upstream of the first magnet. The p or p̄ carries most of the momentum of the parent Λ or $\text{Λ}\text{̄}$ and is reconstructed as if it came from the target. Background problems associated with searches for neutral particles are more severe. In addition to all the background associated with charged particle searches additional backgrounds are present due to the inability to track neutral particles.