The Muon Piston Calorimeter
By Mickey Chiu
One of the pleasures of collaborating on the PHENIX
experiment is the chance to build an actual piece of the
detector, particularly if you enjoy tinkering with advanced
technology and coming up with ways to push the state of the art
with detectors in order to enable interesting physics
measurements. While PHENIX has already been incredibly
successful, with over 45 published Physical Review Letters to
date, the upcoming luminosity increases with RHIC-II place an
increased demand to improve the detector in order to optimize
the physics output. For PHENIX, which was designed as a
high-rate detector with very good particle identification but
limited acceptance, one of the natural imperatives for
improvement is to increase the acceptance, particularly in
kinematic regions which were previously inaccessible. We have
recently been able to do this with the installation of the Muon
Piston Calorimeter (MPC), a small lead-tungstate (PbWO4) based
electromagnetic calorimeter with Avalanche Photodiode (APD)
readout about 5° from the beam direction covering a
pseudorapidity range -3.7<η<-3.1 and 3.1<η<3.9 in the south and
north sides of PHENIX (figure 1).
Figure 1
The idea of installing a calorimeter at forward rapidities came between discussions with Matthias Grosse-Perdekamp and myself when I was a post-doc at the University of Illinois (UIUC). Any upgrades in PHENIX require careful consideration since much of the available space is already taken up by existing detectors. However, we noticed a small cylindrical hold with a radius of 22.5 cm and only 40 cm deep, in the front face of the muon piston just behind the beam-beam counters. The hole was hardly big enough for a mouse let alone a detector, but Matthias realized that lead-tungstate crystals would be compact enough to fit in there. After a discussion with Terry Awes, we realized that we could possibly build on the extensive research and development done by the Kurchatov and Hiroshima groups for the PHOS calorimeter, which is a compact PbW04 crystal calorimeter with APD readout for the Alice detector at the LHC. This is an almost ideal solution since PbWO4 is one of the densest scintillators available, and APDs would be able to withstand the large magnetic fields in the piston hole without generating much heat in the confined space. In particular, we were able to enlist the collaboration of the Kurchatov Institute and Hiroshima groups to provide the crystals and APD/preamps, thus saving dramatically on both cost and time, leaving the electronics and mechanical support to be developed by the University of Illinois and BNL. Besides Kurchatov, Hiroshima, UIUC, BNL, and ORNL, there were important contributions from UC Riverside, Stony Brook, Colorado, RIKEN, and UMass-Amherst. At last count there were 7 PhD theses and 1 Masters thesis which should come from work on the MPC. Special thanks go to graduate students John Koster, Nathan Means, and Andrey Kazantsev who toiled for long hours preparing the crystals, testing electronics, and taking data during the test beam.
Figure 2
The MPC was installed in stages, with the south side installed first in 2006, in time for the 62.4 GeV run and the 200 GeV longitudinal run, but not in time for the transverse run during that year. Figure 2 shows the first pi0's from the 62.4 GeV transverse run. The north MPC was installed the following year so that both sides were installed for the 2007 Au+Au run. It's often thought that in the forward region, the particle densities are too high to be practical in heavy ion collisions, but within a few weeks of Au+Au collisions the MPC was producing a measurement of the reaction plane consistent with that seen by the beam-beam counters.
The benefit of putting a detector very forward is that one increases both the kinematic reach to low momentum fraction and to high momentum fraction in both the nucleon or nucleus. In a leading order analysis of a hard scattering between two partons, one gets the following simple relations between the momentum fractions x1 and x2 of the two partons involved in the scattering and the rapidities y3 and y4 of the outgoing jets:
x1 = pT/√s(e(y3)+e(y4))
x2 = pT/√s(e−(y3)+e−(y4))
From these simple relations one can deduce that larger values of y3 or y4 lead to both lower and higher x values.
Some of the most interesting things are expected to occur in nuclei at low x. At low enough x, the gluons must start to overlap and coalesce, so that this gluon cloud forms into a wall. This effect is even stronger for a large nucleus since the overlaps occur between gluons in different nucleons. Some theorists believe that the densities become so high that the gluons reach a saturation level, and finding where and how this occurs will tell us much about the behavior of gluons in a nucleus, and thus about how the mass of a nucleus is acquired through the dynamics of QCD [1]. The MPC helps in this study since one can measure forward pi-zeros, which gives one sensitivity to partons at low x. One can also look at correlations between the pi-zero in the MPC and particles at other rapidities, to see if there are any modifications to the standard QCD description of dijet production. Some theorists have proposed that such modifications might come from gluon saturation effects. With the recent Run08 d+Au data, we will be able to contribute to addressing this important issue in nuclear structure.
Another area that the MPC has an unique ability to contribute in PHENIX is the study of single spin asymmetries at very high xF in transversely polarized proton collisions. With the first viable MPC data-set taken during the 62 GeV transverse run, we were able to measure very large asymmetries of high xF pi-zeros (see figure 3) [2], qualititatively consistent with what had been seen by the STAR collaboration [3] and BRAHMS [4]. The source of these asymmetries is not well understood, and while there is still much theoretical and experimental work to be done, these large asymmetries are exciting because they hint at new avenues for studying how the quarks and gluons contribute to the spin and angular momentum structure of the proton. This might lead to a more complete understanding of proton structure and perhaps even allow us to determine the proton wavefunction.
Figure 3
What I hope to have shown is that there is still much left to
be discovered and understood at RHIC, and the coming upgrades
will be essential to fully exploit the increased luminosity and
running time of the RHIC-II program. This program includes under
its purview a quantitative understanding of the properties of
the strongly-interacting Quark Gluon Plasma, the behavior of
gluons at low-x in the nucleus, and a determination of the spin
structure of the proton. These myriad topics highlight the
incredible flexibility and unique capabilities of RHIC for
studying many aspects of QCD. The Muon Piston Calorimeter is
just one of a coming wave of upgrades in PHENIX which should be
installed in time for RHIC-II and which are important for
teaching the next generation of students new detector
technologies. Some of the other PHENIX upgrades to expect in the
future are the VTX and FVTX, silicon tracking upgrades for
measuring leptons from heavy flavor decays. Another forward
electromagnetic calorimeter covering 1.0<η<3.0, the FOCAL, is
under study and would add another order of magnitude in
acceptance, thus providing yet another dramatic improvement to
PHENIX.
[1] "The Origin of Mass and the Feebleness of Gravity," Frank Wilczek, MITWorld Lecture, http://mitworld.mit.edu/video/204/
[2] "Transversely Polarized Proton Spin Measurements in Polarized ed p+p Collisions in PHENIX," M. Chiu for the PHENIX Collaboration, Proc. International Nuclear Physics Conference, Tokyo 2007 Vol. 2 pp 117.
[3] “Forward Neutral Pion Transverse Single Spin Asymmetries in p+p Collisions at s**(1/2) = 200 GeV,” Abelev et al., PRL 101:222001,2008; “Cross-sections and transverse single spin asymmetries in forward neutral pion production from proton collisions at s**(1/2)=200 GeV,” Adams et al., PRL 92:171801,2004
[4] “Single Transverse Spin Asymmetries of Identified Charged Hadrons in Polarized p+p Collisions at s**(1/2)=62.4 GeV,” Arsene et al., PRL 101:042001,2008.
The Relativistic Heavy Ion Collider at Brookhaven National
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from the smallest subatomic particles, to the largest stars.