PHENIX Reaction Plane Detector
By Xie Wei
The large hadron elliptic flow (v2) provides key evidence of
the hot and dense matter formation in Au+Au collisions at RHIC.
The measurements in rare probes such as electrons from heavy
flavor decays, direct photons, and high pT particles provide
even richer information on the properties of the matter. The
accuracy of these measurements is not only limited by
luminosities but also rely on accurate reaction plane
measurement. During the 2007 RHIC run, PHENIX successfully
installed and commissioned a new reaction plane detector (RxNP)
which significantly improves the measured reaction plane
resolution.
Elliptic flow is represented as the second Fourier coefficient of particle azimuthal angle distribution relative to the reaction plane described in Fig.1.
Figure 1: illustration of reaction plane definition.
In PHENIX, we have been using the Beam-Beam Counter (BBC) to determine the reaction plane. The statistical power of the v2 measurement is reduced by a factor of 6 to 100 compared with the ideal case in Au+Au collisions due to the poor reaction plane resolution. This has been a major limiting factor of our rare signals v2 measurements. Between late 2005 and early 2007, a group of physicists including many students and postdocs from Brookhaven National Lab, Columbia University, University of Colorado, Iowa State University, University of Maryland, Oak Ridge National Lab, Riken-BNL Research Center, Stony Brook and Tskuba University proposed and designed RxNP which improves the statistical power of v2 measurements in PHENIX by almost a factor of four comparing to the BBC measurements. It is worth mentioning that students and postdocs played a critical role in the detector development.
The RxNP is a scintillator paddle detector located in front of the PHENIX nosecone as shown in Fig.2.
Figure 2: (a) Structure details for one quadrant. (b) One quadrant being assembled. (c) One fully assembled quadrant before the installation of lead converter. (d) One arm of the detector installed in PHENIX central area.
The active area is made of plastic ntillators with a lead converter in front to increase the detector performance. The detector covers a pseudorapidity range of 1.0<|h|<2.8 which was chosen to balance the competing effects of maximum acceptance and jet induced autocorrelations with the central arm. The detector is segmented into 12 sectors in azimuthal direction to minimize the impact from dead channels. Each sector is further divided into two radial sub-sectors. The inner sub-sector extends from a radius of 5cm to 18cm and the outer sub-sector continues from 18cm to a radius of 33cm. This effectively divides the detector into two rapidity regions, 1.0<h<1.5 and 1.5<h<2.8, to allow for the possibility to study the effect of jet induced autocorrelations on the reaction plane determination.
Due to the strong magnetic field in the region where RxNP is positioned, Hamamatsu H6155 fine mesh photomultiplier (PMTs) are used. Embedded fiber light guides made of BCF92 are used to connect the plastic scintillators to the PMTs. The use of fiber light guides has several advantages for the design including allowing for flexibility in the final positioning the PMTs. This is important due to the sensitivity of the PMTs response to the magnetic field.
A lead-antimony composite converter is installed directly in front of the scintillator. The addition of 2-3% of antimony hardens the converter material and improves the mechanical stability of converter when it is mounted. An LED calibration system is also installed for monitoring purpose.
The installation and commissioning of RxNP was successfully completed shortly after RUN7 started. The performance of the detector has reached the ultimate design goal and is being fully functional through out the run. It will provide the significantly improved reaction plane measurements for all the RUN7 PHENIX analysis. Beside that, the detector is configured to be able to trigger on collisions. It has the potential to serve as an efficient minimum bias trigger in low energy Au+Au runs due to its much larger rapidity coverage compared to BBC which is the current minimum bias trigger in PHENIX.
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.