Absolute Calibration of the RHIC CNI
Polarimeters Using 125 GeV/A C Ions
George Igo* and Isao Tanihata1^
^Department of Physics and Astronomy, University of California, Los Angeles, Los Angeles, CA 90095
f RIKEN, 2-1 Hirosawa, Wako, Saitama 351-01, Japan
ABSTRACT. A polarized hydrogen jet target will be installed at an intersection point in RHIC in the
next few years to make an absolute measurement of the proton beam polarizations possible. We discuss
here a procedure to measure the p-C analyzing power Ap.c in the Coulomb-nuclear interference region
at beam energies between injection energy and 125 GeV. A carbon ion beam (23 - 125 GeV/A) and the
polarized hydrogen jet target would be used in the measurements.
INTRODUCTION
The experimental program at RHIC involving the collision of polarized proton
beams began in 2001. It has already begun to provide new and exciting physics. As an
example, preliminary measurements1 of the asymmetry of forward pi-zero mesons
produced in the collisions of transversally polarized protons at the highest CM energy
ever measured,-^ = 200 GeV, show unexpectedly large asymmetries. Unique
opportunities will soon be available to study spin effects in hard processes2 at high
luminosities including the measurement of the gluon spin structure function and quark
and anti-quark spin structure functions by flavor, the latter in parity violation
processes in W and Z production3. These measurements require knowledge of the
absolute beam polarization at the 5% level. To implement these measurements a
polarized jet target is being prepared which will be located at an intersection point in
the RHIC rings. Measurements of the p-p asymmetry in the Coulomb-nuclear
interference (CNI) angular region will be then possible with unpolarized (polarized)
proton beams in both the blue and yellow rings on the polarized (unpolarized) jet
target. These measurements taken together can determine the absolute polarizations of
the blue and yellow proton beams. Simultaneous measurements of the asymmetries
made with the RHIC CNI polarimeters will determine their analyzing power, Ap.c.
p-p and p-C Methods
Figures 1 and 2 illustrate the absolute calibration procedures using the accepted p-p
method referred to in the Introduction and the p- C method which is being proposed
as an alternate to the p-p method in this paper. In the p-p method, (see Figure 1) first
CP675, Spin 2002:15th Int'l. Spin Physics Symposium and Workshop on Polarized Electron
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an unpolarized proton beam is incident on the above mentioned polarized jet target in
order to measure the left - right (L-R) asymmetry at a particular energy E and four
momentum transfer -t in the CNI range 8Tp_p(E,-t) = Ap_p(E,-t) \ PT \ where Ap.p is
the p-p analyzing power. In a second set of measurements, polarized proton beams
circulating in the blue and yellow rings are incident on the jet target run in an
unpolarized mode to measure the asymmetry £p .
Measure L-E ssyui, |?*w f E, -t),
in the CNI-t range.
(p »*}
14, (E,-t) = A S==,
%]
alysing pewer
t rp I _ « P
i
/ A
n | -£pp /
/Vpp
liFph{4/4)iPT
FIGURE 1. Absolute Calibration of the RHIC CNI Polarimeters by the p-p Method
These measurements allow the extraction of the proton beam polarization
\Pp\=€pp_p/App
also
expressible
in
terms
of
the
asymmetries
as
\ P p \ = ( e p p _ p / / p _ p ) \ P r \ . During this latter set of measurements, the RHIC CNI
polarimeters must be inserted into the blue and yellow beams sufficiently often to
monitor the behavior of the asymmetry £CM with time as a check on the constancy of
the beam polarization during the fill, and finally to determine the analyzing power
Ap_c =£CNI/\PT | where £ CM is the asymmetry measured with the RHIC CNI
polarimeter.
Absolute calibration by the p-C method is illustrated in Figure 2. A carbon ion (C)
beam is stored in one of the rings and the asymmetry £p_c is measured with the jet
target polarized. The analyzing power Ap.c can then be extracted directly from the
—>
expression Ap_c = £p_cl \ PT \.
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rv - ci , IV
f Pa
s
L»
tfa$ CMI -t range to decermine
ite analyzing power, A c>
where £tm li the asymmetry iii§iinrecl
with I!M RHIC CNT polaiiwstef.
FIGURE 2. Absolute Calibration of the RHIC CNI Polarimeters by the p-C Method
The p-C method, although more straightforward, is limited by the maximum energy C
beam that RHIC can produce. By scaling the ratio of beam momentum to the ion
charge, the maximum value of PC/AC for a C beam can be seen to be approximately
125 GeV/A. Figure 3 illustrates that a carbon beam, with a certain PC/AC, incident on
the polarized jet target, will allow the measurement of Ap.c for a proton beam
momentum Pp = PC . The p-C method therefore will be useful for beam energies
from injection energy up to 125 GeV.
PC
25.8 Ge
125 Ge
25 J Ge¥
Transform from CM fame to rest fame of proton:
'
PC
25,8 Ge^/
fix,
25,8 GeV
125 GeV/A
PC/AC = 0,99Pp
FIGURE 3. The relation between the momentum of a proton beam [Pp + Ctgt, CNI pol] and a C beam
[ PC I AC + pol jet tgt ] at the same V S .
A limitation, inherent in the p-C method, is that there is no possibility from kinematics
to separate elastic events in the CNI region from events at the same -t when the 12C ion
( projectile ) is left in an excited state since the recoil protons from the polarized jet
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target cannot be distinguished in the two cases. In principle, inelastic events could
contaminate the desired measurement of Ap.c in the CNI region. Fortunately the
elastic cross section is expected to be very large compared to inelastic scattering in the
CNI region. Further, it is possible to learn about the relative magnitude of inelastic
events from the existing RHIC CNI polarimeter data since elastic events can be
distinguished kinematically. No evidence4'5 of events where the recoil C nucleus was
produced in its first excited state at 4.44 MeV appears in the RHIC CNI polarimeter
spectra. It should be noted in passing that the extra broadening of the distribution
when the carbon nucleus is left in this state due to recoil from the emission of the 4.44
MeV gamma ray is comparable to broadening caused by multiple Coulomb scattering
in the thin carbon target of the CNI polarimeter. Most higher lying states in the carbon
inelastic spectrum have broad alpha particle decay widths and will decay therefore
mainly by alpha particle emission ( which will be followed by breakup of the residual
Be nuclei into two alpha particles).
Considerable care will be necessary to relate the (E, -t) coverage of the RHIC CNI
polarimeters to the coverage obtained from measurements using the p-C method.
This has not been studied carefully by us at the present time.
SUMMARY
The p-C method described in this paper is a procedure which allows a measurement
of the absolute polarization Ap.c directly. The beam energy range is from injection
energy to 125 GeV. The p-C method requires, in addition to the polarized jet target,
the production of C beams in RHIC ( a costly use of beam time ). Inelastic
contributions, i.e. excitation of states in C in the CNI region may be a 'show stopper'.
However events in the kinematical region of the first excited state (4.44 MeV, 2") are
not visible in CNI polarimeter spectra at 23 GeV and 100 GeV beam energies. Studies
of excitation of higher lying states in carbon-12 may be investigated by moving the
RHIC CNI target with respect to the detector array to change the kinematic region of
the detected recoils.
Finally, the virtue of having two essentially independent methods to measure the
absolute beam polarization cannot be underestimated.
ACKNOWLEDGMENTS
The authors would like to thank G. Bunce, A. Bravar, O. Jinnouchi and N. H.
Buttimore for helpful discussions about the p-C method. Thanks are also due to J.
Kiryluk for help in preparation of the figures in this paper.. N. H. Buttimore has
independently proposed a similar scheme using a carbon analyzer in about the same
time frame.
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REFERENCES
1. G. Rackness et al, "Forward p + p ^ 7 r + X Production at STAR," in
Abstracts, Spin 2002, Brookhaven National Laboratory, September 9-14, 2002.
2. D. Underwood et al., Part. World 3, 1-15 ( 1992 ).
3. C. Bourrely and J. Soffer, Phys. Lett. B314. 132 ( 1993 ).
4. J. Tojo et al, Phys. Rev. Lett. 89, 52302-52305 ( 2002 ).
5. I. G. Alekseev et al., "RHIC pC CM Polarimeter: Status and Performance
from the First Collider Run," in Abstracts, Spin 2002, Brookhaven National
Laboratory, September 9-14, 2002.
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