STAR Forward π 0 Detector Upgrade
Akio Ogawa
for the STAR collaboration 1
Brookhaven National Laboratory, Department of Physics
Upton, NY, 11973-5000, USA
Abstract.
Forward rapidity in hadron-hadron collisions is an interesting place to look because one can
access high x quarks, which may be highly polarized, and also low x gluons. The STAR experiment
at RHIC had a prototype Forward π 0 Detector (FPD) composed of electromagnetic calorimeters in
the forward region during the January 2002 run with the first polarized proton-proton collisions. In
this paper, I will discuss some aspects of the data, such as the correlation measurement between
the FPD and mid-rapidity detectors and measurements in the negative x F region. These data may
provide clues to the origin of the single spin asymmetries A N which were measured at FNAL-E704
[1]. At the end, I will discuss the plan for the FPD upgrade for next RHIC run.
1. PHYSICS OF THE FORWARD π 0 DETECTOR
The STAR Forward Pion Detector (FPD) covers forward pseudo-rapidity (3 η 4),
high energy (20 GeV E), high Feynman xF (02 xF ) and moderate transverse
momentum (1 pT 4 GeV). The FPD measures primarily electromagnetic energy and
is capable of π 0 reconstruction up to an energy of more than 60 GeV. By supplementing
the capabilities of the STAR detector to identify particles at large rapidity and moderate
transverse momentum, the FPD allows quantitative measurements related to several
interesting phenomena both in polarized proton-proton collisions and in deuteron-gold
collisions.
1.1. Physics of Polarized Proton-Proton Collisions
In polarized proton-proton collisions, the transverse single spin asymmetry (A N ) for
the p p π 0 +X reaction was measured at FNAL-E704 [1] at s 20 GeV and found
to be large. After more than a decade, this asymmetry still remains a mystery.
In perturbative QCD at leading twist with co-linear factorization, this A N has to be
zero. If one takes a step beyond this simple scheme and allows particles to have k T ,
there are 3 terms which can contribute to the asymmetry. The first is the Sivers effect
[2, 3], which is an initial state correlation between k T , the internal transverse momentum
of the quark, and the transverse spin of the nucleon. The second is nucleon transversity
1
For the full author list and acknowledgements,see the appendix to the proceedings.
CP675, Spin 2002: 15th Int'l. Spin Physics Symposium and Workshop on Polarized Electron
Sources and Polarimeters, edited by Y. I. Makdisi, A. U. Luccio, and W. W. MacKay
© 2003 American Institute of Physics 0-7354-0136-5/03/$20.00
407
and the Collins effect [4, 5] in the fragmentation process. The third involves a correlation
between kT and quark spin within the unpolarized nucleon. This is believed to be very
small [6]. There are also calculations of twist-3 contributions [8, 7] which have structures
similar to models in the k T picture but are not identical.
There are experiments either underway or planned that will challenge this mystery.
Semi-inclusive polarized DIS experiments have reported [9, 10, 11] azimuthal asymmetries which may have the same origin as the E704 asymmetry. Azimuthal correlations in
di-jet events at e e colliders will be sensitive to the Collins-Heppelmann fragmentation function [13, 12, 15]. The RHIC spin program will measure azimuthal asymmetries
within a jet at mid-rapidity. Such measurements are expected to be sensitive to transversity, if e e collider experiments find these chiral-odd fragmentation functions to be
non-zero [12, 14].
STAR has just reported [16] AN for π 0 production at forward rapidity at an order
of magnitude higher s compared to the E704 experiment. In the coming years, it is
important for STAR to continue these measurements to further explore this phenomenon.
The polarization in the next RHIC run is expected to be increased by a factor of 2
(from 0.2 to 0.4) and the luminosity will be increased by a factor of 10 (from 10 30
to 1031 cm2 sec). Thus the figure of merit (P2 L) should increase by a factor of 40. A
few hours (or a single RHIC fill) will have the same statistical significance as last year’s
entire data set. There may also be a measurement at s 500 GeV. These improvements
allow us to map the asymmetry as a function of s, xF and transverse momentum pT .
The models described above predict the pT and s dependence of AN . Measurements at
STAR will provide important tests of these models.
The Sivers effect is in the initial state, and is based on the incident quark having k T
relative to the beam axis. On the other hand, the Collins effect is in the fragmentation
process, and depends on hadronic kT relative to the final state quark or the jet. One reason
why these different effects are presently indistinguishable is because there are only
inclusive measurements. With the addition of hadron calorimetry at forward rapidity,
we should be able to distinguish which of these effects are contributing.
At RHIC, the colliding beams are both polarized, with the Yellow beam heading
toward the prototype FPD and the Blue beam heading away from it. Just by sorting the
data in a different way, i.e. averaging over the Yellow beam polarization and calculating
the spin asymmetry based on the Blue beam polarization, we should be able to determine
AN at negative Feynman x. All three models described above predict that A N has to be
zero at negative xF . There are some speculations [17] that we may see a non-zero value
arising from three-gluon correlations. Unfortunately, only a limited amount of data from
the prototype FPD can be sorted on the Blue beam polarization. Results at negative x F
from the next RHIC run may give new insights to the A N puzzle.
All of the above will provide more hints about the physics behind the E704 mystery.
, from which
If the Sivers picture turns out to be most important, then A N measures f1T
we may learn something about orbital angular momentum of the quarks in the nucleon.
If the Collins effect is the dominant contribution, then A N measures the transversity
distribution which is the last unmeasured quark distribution at leading twist. If A N is
described by either of these perturbative QCD models based on two-to-two parton hard
scattering, then measuring the second final state parton will constrain the kinematics and
x region of the spin-dependent distribution function. STAR has measured the azimuthal
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FIGURE 1. The left figure shows STAR data for the azimuthal angle difference between the π 0 detected
with the FPD and the leading charged particle detected at mid rapidity. The right figure shows the same
distribution from a PYTHIA simulation.
angle correlation between the π 0 at the FPD and the leading charged particle at midrapidity. PYTHIA simulations show fair agreement with the data (Fig. 1) and indicate
that the FPD is sensitive to high Bjorken-x quarks (0.3 and above) and low x gluons
(down to 0.001). Over the next 3 years, the STAR barrel and endcap electromagnetic
calorimeters will increase their acceptance to be azimuthally complete for 1 η 2.
Correlations between the FPD and midrapidity detectors will be studied.
1.2. Tuning Spin Rotators
One of the main goals for the STAR spin physics program is to measure longitudinal
double-spin asymmetries to determine the gluon polarization in a nucleon. In the RHIC
ring, the stable polarization direction is vertical. To get longitudinal polarization, spin
rotators are being installed before and after the STAR and PHENIX interaction points
(IP). Due to steering magnets between the spin rotators and the STAR IP, the horizontal
polarization precession is more than a full turn when the beam energy is 250 GeV.
Therefore an in situ polarimeter is required to tune the STAR spin rotators.
A left-right and up-down symmetric FPD will serve as a local polarimeter at STAR,
providing a continuous monitor of unwanted vertical and radial polarization components
during runs that aim to measure longitudinal two-spin asymmetries. Given the measured
large AN for forward rapidity π 0 production and with expected luminosity and polarization improvements in RHIC, the FPD will be able to give feedback within a typical
RHIC fill of a few hours length.
409
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∆φ
1.3. Physics in Deuteron-Gold Collisions
To reliably interpret data from relativistic heavy ion collisions at RHIC, it is critical to
know the initial state gluon density in the colliding heavy ions. Recent models suggest
that the color fields of the colliding ions are so dense as to reach saturation [18]. d-Au
collisions at RHIC may provide a key test of the gluon saturation model [19]. At forward
rapidity in the deuteron direction, high x quarks will be scattered by low x gluons in
gold nuclei. The FPD can measure the ratio of energetic π 0 ’s produced in the incident
deuteron direction to the corresponding yield in the direction of the incident gold ion
beam. That ratio is expected to be sensitive to the gluon density in a heavy nucleus.
2. UPGRADE PLAN
With these motivations, we are proposing to construct electromagnetic calorimeters
similar to what is shown in Fig. 2, with the left and right calorimeters each consisting
of:
a 7 7 matrix of lead-glass counters which were built by the IHEP, Protvino group
for the E-704 experiment at FNAL.
• two finely segmented scintillator-strip shower maximum detectors.
• a 7-element lead-glass preshower detectors with a lead radiator in front.
•
For top and bottom, we will use a simpler 5 5 matrix of lead-glass counters due to
the limitation in the available space.
These detectors will be read out with other STAR detectors for full event reconstruction, as well as without the slow detectors to accumulate greater statistics. The data from
the upgraded FPDs will be included in bunch-sorted scalers operating at the 9 MHz
RHIC bunch crossing frequency. This readout mode will provide STAR with an in situ
polarimeter.
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425m m
Side View
Scale: 1: 5
775m m
875 mm
Top View
FIGURE 2. Schematic of a calorimeter module proposed for the upgrade of the STAR Forward π 0
Detector. The module consists of a main calorimeter built from an 77 matrix of Pb-glass detectors,
two 48-strip scintillator shower maximum detectors, and a 7-element Pb-glass preshower detector. A
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