Precision Measurement of Run05 Proton Beam Polarization
by Itaru Nakagawa/RIKEN
RHIC spin program has been studying the decomposition of nucleon spin in terms of the contributions from its constituents - quarks and gluons - using various observed asymmetries through polarized proton-proton collision process. All these asymmetry studies requires the knowledge of the beam polarization in order to interpret observed asymmetry with the spin structure of nucleon. Therefore the measurement of the beam polarization is a critical path. Here I explain how and how well we measure the beam polarization at RHIC.
Fig.1 Polarized Hydrogen Jet target and
proton-Carbon polarimeter setups.
The polarization of the proton beam at RHIC is measured through an elastic scattering from Carbon. We insert a Carbon ribbon target to the center of the proton beam and detect recoil Carbon ions by silicon detectors mounted 90 degrees left and right hand sides of the beam. The magnitude of left-right asymmetry is governed by so called "analyzing power" which has a dependence on the impact of the collision (momentum transfer). The analyzing power predicts the asymmetry should appear between the number of recoil carbons in the left and the right detectors when the beam is 100% polarized. From observed actual asymmetry, we know how many fraction of protons in the beams are polarized following the relation : polarization = asymmetry / analyzing power.
Fig.2 An image of elastic proton-carbon
scattering. The recoil Carbon ions are detected by silicon
detectors mounted at 90 degrees with respect to the incident
proton beam.
In the past, the major source of the systematic error for the carbon polarimeter has been the energy correction for the detected recoil carbon. Due to the existence of insensitive regions in the silicon detector near a surface area, some portion of the energy loss in the detector cannot be measured. This unmeasured energy needs to be corrected using the correlation between the deposited energy information and a flight time from the target. Kinetic energy of the recoil carbon is proportional to the momentum transfer, which is necessary to know in order to refer to the adequate analyzing power event by event basis. Due to the improvement of the grounding design of the silicon detectors used for for Run05, the contamination of noise to a true signal has been greatly reduced. As a consequence, controlling unmeasured energy became more reliable.
Fig.3 Side view of the silicon detector. A recoil carbon
looses non-negligible fraction of energy in the insensitive
surface region.
In reality, we also don't know well about the absolute scale of the analyzing power of the elastic proton carbon scattering. In order to determine the scale, we also operate the polarized proton gas jet target. Using the same technique as carbon case, we evaluate the left/right asymmetries not only for the beam polarization, but also for the target polarization simultaneously. Since we have the independence measurement of the target polarization by a Breit-Rabi polarimeter within 2% accuracy, the analyzing power of the proton-proton elastic scattering can be measured as well. Thus the gas jet polarimeter measures beam polarization in an absolute scale. By synchronizing the operation of these two polarimeters over the extended period which is statistically sufficient enough for the jet polarimieter, we can normalize the scale of the analyzing power of carbon, asking both polarimeters to measure the same polarization averaged over the period. Another major achievement of Run05 was improvement of statistical accuracy of gas jet by factor of 4 compared to previous runs.
On the other hand, the accuracy of the determination of Run05 polarization was suffered from the new source of uncertainty, i.e. polarization profile appeared strongly in the yellow beam, which was not the case for the previous years. The polarization profile is the spacial distribution of the polarization. Since we were measuring some "local spot" of the polarization in the finite size of the beam using ultra-thin carbon wire target, we ended up with assigning relatively large error (about 5%) in order to convert a "peak" polarization to the "average" polarization, which is seen by the jet and experiments.
Finally we achieved to measure the Run05 beam polarization within 6% precision, which is about factor of 2 to 3 improvement from previous years, 12 - 18%. For more details, please visit the Polarimeter group's web page.
The Relativistic Heavy Ion Collider at Brookhaven National
Laboratory is a world-class scientific research facility primarily
funded by the U.S. Department of Energy Office of Science. Hundreds
of physicists from around the world use RHIC to study what the
universe may have looked like in the first few moments after its
creation. What physicists learn from these collisions may help us
understand more about why the physical world works the way it does,
from the smallest subatomic particles, to the largest stars.