The IUCF STAR Group Page

Our group from IUCF is a member of the STAR collaboration at the Relativistic Heavy Ion Collider (RHIC). RHIC was built to search for the quark-gluon plasma, a new state of matter predicted by quantum chromodynamics, in very high energy collisions of gold nuclei.

RHIC also accelerates polarized protons up to 250 GeV to collide at a center of mass energy of 500 GeV. In collisions of the polarized protons we are studying the spin structure of the proton by looking at collisions of two protons where the quarks and gluons in the protons interact violently. Analysis of these reactions are being used to learn about how the spin of the proton is built up of the spins and orbital angular momentum of its constituent quarks and gluons.

The violent collisions create high energy photons and jets of particles such as pions. The STAR Detector is designed to detect electrons, photons and jets of particles over a large solid angle. We have built an endcap electromagnetic calorimeter which covers the crucial regions of solid angle needed to study the spin structure of the proton and in particular make the best measurement of the gluon's contribution to the proton's spin.

You can learn more about the spin of the proton and our project with the following links:

"Proposal to Construct an Endcap Electromagnetic Calorimeter for Spin Physics at STAR" reviewed at the 8-9 October 1998 meeting of the HENP program advisory committee at Brookhaven National Laboratory.

A description of and documentation for the endcap calorimeter is here.

Current group projects

Constraints on DG thru Inclusive Jets

As described in the proposal one of our main goals is to determine the contribution of the spin of gluons to that of the proton denoted by DG. As RHIC has ramped up it has been advantageous to make use of large cross section channels such as the detection of inclusive jets. With the spins of the protons aligned along or opposite their momentum we measure the difference in rate of inclusive jet production for like vs. opposite helicity, a quantity called A_LL. Data has been taken in each of the 2003, 2004, 2005 and 2006 runs with increasing statistics and reach in transverse momentum each year. The 2003/4 data has been published, and the 2005 data just recently submitted for publication (see below). Preliminary results from the 2006 data were recently released and are shown below. Positive contributions to the proton's spin from the gluons with .03 < x_Bjorken < 0.3 seem to be excluded by our data, although this needs to be checked against other models of the parton distribution functions.

Above at the left is our preliminary A_LL data for inclusive jets from 2006. The curves are all from GRSV, ie one model of the parton distribution functions, with different constraints on DG. The red curve is for all the gluons aligned along the proton helicity (at the input scale) while the green curve is for all the gluons aligned opposite to the proton helicity. The blue line is for no gluon polarization at the input scale and the black line is the best fit to DIS data as of 2001. Above and to the right is a plot of the confidence level calculated by comparing 2005 and 2006 data to a whole family of curves (one curve per plotted point) from the same authors spanning the range of (and including) the 4 curves in previous plot. We see now that the GRSV fit to DIS data is strongly disfavored. It has a DG of 0.24 at the input scale. We anxiously await A_LL calculations with other distribution functions and the inclusion of our data in global fits. References can be found in the publications below.

Di-jet Sivers

The Sivers effect involves correlations of the transverse motion of the partons correlated with the spin of the proton. It is related to the orbital angular momentum of the partons. This effect is measured, unlike the other physics described here, with the protons polarized perpendicular to the proton's momentum. The transverse motion of the partons involved in scattering to make dijets has the effect of moving the jets away from the expectation that they come out 180 deg opposite in phi, or are back to back as shown in the figure. The Sivers effect says that the movement away from back to back will be in one direction for one spin of the proton vs. the other. The measurements have been made and published. We found a spin dependence consistent with zero. It appears now that the measurement is explained by the fact that, to observe an effect, the transverse motion of the partons has to couple to an initial state interaction or final state interaction. With the pp system both are present and they apparently have opposite signs and thus cancel. This is consistent with measurements at Hermes that gives a non-zero effect, since only final state interactions are present in those measurements.

Constraints on DG thru Di-jet A_LL

Our inclusive jet data averages over large ranges of Bjorken x as shown for two pT bins in the figure below.

The shape of the gluon polarized distribution function Dg(x_Bjorken) is poorly constrained. Fits to DIS data allow positive solutions, negative solutions and solutions that have a zero crossing. By detecting two jets in coincidence and using two body kinematics we can determine the initial state kinematics, in particular determining x_Bjorken of the two partons. Initial data was taken in 2006 and is being analyzed. This channel has the advantage that it has a large cross section. However the pT of the jets is not as precisely measured as a direct photon's pT can be. Thus the resolution in x_Bjorken is somewhat poor. Nevertheless, with the large coverage of STAR with the Barrel and Endcap EM Calorimeters as well as the new FMS we will be able to map out the x dependence of Dg(x_Bjorken) over a wide range. For example if both jets are in the barrel calorimeter we cover a range in x_Bjorken of 0.2-0.08. Coincidences involving the endcap calorimeter give information down to x_Bjorken ~0.02. Including coincidences between the FMS and other calorimeters can get down close to a few 10^-3.

Dijets will be a major focus of the 2008 run. The cross section is so large that with the expected luminosities we have had to be selective in what events we will take. We have decided to set thresholds by angle region so that we will emphasize interactions were the larger x_Bjorken of the partons interacting is > 0.2. This emphasizes the region where the quarks have their largest polarization and begin to dominate in number over gluons. This will be a major focus of the 2008 STAR measurements.

Constraints on DG thru photon jet coincidences

The generation of direct photons in coincidence with away side jets by quark-gluon Compton scattering is the cleanest channel for investigating DG with STAR as discussed in our original proposal. It offers nearly complete determination of the initial kinematics event by event with accurate measurement of the gamma energy. The quark-gluon Compton process dominates the gamma-jet production cross section at the 90% level. The disadvantage is that the cross section is rather low and there is a large background form pi0s in di-jet events. A significant sample of these events were collected in the 2006 run and are serving as the basis for refining techniques to select the gamma-jet events from the data set. Collecting enough events to make a significant measurement of the gluon polarization Dg(x_Bjorken) with gamma-jets over the next two years of RHIC running is a major goal for STAR.

Measuring the Sea Quark Polarization with W Production at 500 GeV

The role of non-perturbative processes in generating the sea is sensitive to the polarization difference between the sea quarks ubars and dbars. The polarization of these anti-quarks can be measured by detecting parity violating asymmetries in W boson production. This requires running at a collision energy of 500 GeV with RHIC. It is expected that this program will begin in the next few years and runs to test polarization transmission to this energy have already occurred. An important part of the kinematic regime for these measurements occurs in the endcap calorimeter where the TPC tracking fails. We have been working with collaborators to prepare a proposal to build GEM tracking chambers to install in STAR to provide critical charge separation between the electrons and positrons from W- and W+ production. Our role is to work on simulations, software infrastructure and readout electronics for the project. The goal is to be ready for the first significant 500 GeV running in 2010.

STAR Spin Publications

Longitudinal double-spin asymmetry for inclusive jet production in $\vec{p} + \vec{p}$ collisions at sqrt(s)=200 GeV
Submitted October 12, 2007
e-Print Archives (arXiv:0710.2048): Abstract | PS | PDF

Measurement of Transverse Single-Spin Asymmetries for Di-Jet Production in Proton-Proton Collisions at sqrt(s) = 200 GeV
Submitted May 31, 2007, published October 2, 2007
Phys. Rev. Lett. 99 (2007) 142003
e-Print Archives (0705.4629): Abstract | PS | PDF
SLAC-Spires HEP: Entry | Cited by | Citebase
Journal article: Phys. Rev. Lett. server
Data and figures: click here

Longitudinal Double-Spin Asymmetry and Cross Section for Inclusive Jet Production in Polarized Proton Collisions at sqrt(s) = 200 GeV
Submitted August 9, 2006, published December 18, 2006
Phys. Rev. Lett. 97 (2006) 252001
e-Print Archives (hep-ex/0608030): Abstract | PS | PDF
SLAC-Spires HEP: Entry | Cited by | Citebase
Journal article: Phys. Rev. Lett. server
Data and figures: click here

Cross Sections and Transverse Single-Spin Asymmetries in Forward Neutral Pion Production from Proton Collisions at sqrt(s) = 200 GeV
Submitted October 29, 2003, published April 29, 2004
Phys. Rev. Lett. 92 (2004) 171801
e-Print Archives (hep-ex/0310058): Abstract | PS | PDF
SLAC-Spires HEP: Entry | Cited by | Citebase
Journal article: Phys. Rev. Lett. server
Data and figures: click here