STAR Physics with a TOF Patch
Ahmad, Bonner, Crosetto, Llope, Mutchler, Platner, Roberts, Themann, Wright, Yepes
(The Rice University STAR Group)
Introduction
This document provides a very brief report of estimates of the size of TOF patch needed to
achieve the physics goals of STAR. Much greater detail is given in the TOF Proposal submitted to
the STAR Collaboration in August, 1994. For the present estimates, we made the reasonable
working assumption that small statistical errors must be achieved with one million central AuAu
events. This corresponds to a month of running with 50% efficiency at the design luminosity.
Inclusive Distributions
The transverse mass (mt) distributions for different species contain essential information
about the dynamics of the collision. The temperature achieved in the interaction and the transverse
flow of the products can be extracted from the mt distributions for different species. Since various
factors influence these distributions, a high precision measurement is required to extract the
interesting physics. In order to make a careful study of mt distributions, it is essential to measure
mt over a large range, since the slope of the distribution changes with mt. Moreover, the high mt
tail of the distributions is of great interest for studies of jet quenching. Naturally, high mt requires
TOF for particle identification.
Fig. 1 shows the mt distributions for pi's, K's, and p's in Au+Au interactions, normalized to
one event. A statistical error of 1% for the largest mt where particle separation is possible was
required. Patch sizes of 1/3 (1/20) TOF for p's (K's) are required to achieve such precision with
106 central Au+Au events.
Fig. 1. Invariant mt spectra for π, K and p in
central AuAu events for dE/dx and a 1/3 patch.
Phi-Meson
The Phi-meson has been proposed as a probe to understand the nature of the hadronic matter
produced in ultrarelativistic heavy ion collisions. It is important to study all possible kinematic
variables of this particle. STAR can detect phi's through the two Kaon decay channel, since
BR(phi->K+K-) ~ 50%. Fig. 2 shows the opening angle at the TOF radius in the plane perpendicular to the beam,
for Kaons from phi decay. From the figure, it is clear that a TOF patch with at least a coverage of 60
degrees (~1/6) in azimuthal angle is needed for efficient phi reconstruction. In the case of symmetric
beams, the particle distributions should be identical for negative and positive rapidities. For
asymmetric beams this property no longer holds and a large eta coverage is required. The ~1/6
coverage, arranged as delta-eta=2, delta-phi=60 degrees, allows this measurement to be made.
Fig. 2. Opening angle in the plane perpendicular to the beam between K's at TOF radius from f decay.
Only single phi's were generated for these calculations.
In Fig. 3 we plot the pt distribution of phi-mesons identifiable with TPC dE/dx, and compare this
with the distribution measured including a 1/6 TOF patch. The error bars reflect the correction for
geometrical acceptance and detection efficiency. It is obvious that without a TOF patch, there will
be very limited information about the pt distribution. Consider a 10 bin distribution in the range 0-3
GeV. With the 1/6 patch the precision would be 9% for the 2.4-2.7 GeV bin, and on the order of
1% for bins below 1.8 GeV.
Fig. 3. The phi meson pt spectrum using only dE/dx from the TPC (solid points), and including a
one-sixth patch of the TOF system (open points). The distributions have been corrected for the
detection efficiencies and the geometrical acceptance, and the error bars reflect this fact. The
number of phi's detected scales roughly with TOF patch size.
Event-by-Event
A complete TOF system is necessary for STAR to realize its full potential for event-by-
event physics, especially for variables involving kaons. This is illustrated in Table 1, where the
number of identified kaons with and without TOF are given for two different Monte Carlo models.
| Model | Without TOF | With Full TOF |
|---|
| Hijing | 13 | 35 |
|---|
| Fritiof | 10 | 29 |
|---|
TABLE 1. Number of Kaons identified in a central Au+Au event.
The inclusion of the full TOF system triples the number of identified kaons; the number
identified with patch fraction, t, scales approximately as follows: NK = Nw/o + t ( NFT - Nw/o ),
where Nw/o and NFT are taken from the table. Full coverage would have an enormous impact on
event-by-event measurements of the K/pi ratio, for example. However, a patch smaller than ~1/3
will not have a strong impact on event-by-event Kaon-related measurements.
The inclusion of a TOF system has a larger impact on measurements of Kaon temperatures
(or the mean pt). Without a TOF system only low pt Kaons are identified by the TPC-SVT
combination; hence the lever arm for the measurement of the average Pt is small. Based on these results, we
conclude that at least 1/3 of the TOF system is necessary to do an adequate job on event-by-event
Kaon physics.
Conclusions
We conclude that 1/6 of the TOF system will provide sufficient information to support
inclusive and phi-specific measurements. A larger patch would reduce statistical errors and make the
results easier to interpret. A minimum of 1/3 of the TOF system is necessary for STAR to do
significant event-by-event physics with Kaons.
References
1. Proposal for a STAR TOF System, B. E. Bonner, et al., August, 1994, available on WWW in STAR TOF.
2. See, for example: E. Schnedermann, et al., Phys. Rev. C48 (1993) 2462.
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