Home  |  Past Issues  |  Contact Us  | RHIC
 

First Collisions of Polarized Protons at a Center-of-mass Energy of 500GeV at the STAR Experiment: Status, Expectations and Future Projections

Bernd SurrowBy Bernd Surrow, Massachusetts Institute of Technology

The STAR experiment has recently completed a program, collecting data during the first collisions of polarized proton beams at a beam energy of 250GeV, giving rise to a center-of-mass energy of 500GeV. This marks the beginning of a multi-year program studying the polarization of antiquarks inside the proton using the production of W± bosons which are produced only at an appreciable rate above a center-of-mass energy of 500 GeV for the colliding proton beams. This research effort has long been stressed as a key element of the RHIC Spin program [1, 2]. The first observation of W± bosons together with the Z0 boson as a the force carrier of weak interactions goes back to 1983 at the SPS at CERN and was awarded in 1984 with a Nobel prize in physics to Carlo Rubbia and Simon van der Meer. Weak interactions, in contrast to strong and electromagnetic interactions violate parity, which is a fundamental ingredient in the description of the underlying quantum theory within the Standard Model of particle physics.

In recent decades, numerous experimental results have contributed  to the testing and exploration of Quantum Chromodynamics (QCD), the quantum field theory that describes quarks and gluons, also known as the strong interaction. This forms the theoretical basis for the high-energy polarized proton-proton program at RHIC. It is now known that quark masses make little contribution to the mass of the proton. This mass originates mostly from the interactions mediated by the massless gluons. The strong force that confines quarks inside the proton leads to the creation of abundant gluons and quark-antiquark pairs. These ‘silent partners’ make the dominant contribution to the mass of the proton. It is not yet completely known how the silent partners contribute to the spin of the proton.

One of the great early successes of the quark model was its apparently simple account of the spin of the proton from the spin alignment of three constituent quarks. At higher energies, however, such a simple description fails. Polarized deep-inelastic scattering (DIS) experiments, have demonstrated that the spins of all quarks and antiquarks combined account for only about 25% of the proton spin. This finding became known as the proton spin crisis and initiated several theoretical and experimental efforts world wide to deepen our understanding of the proton spin structure. The proton's missing spin can arise from spin alignment of gluons and the orbital motion of quarks and gluons. Solving the puzzle of the proton's missing spin is essential to understand how the constituent quarks of the naive quark model are related to the actual quarks and gluons probed in high-energy  experiments through high-momentum transfer probes.

Figure 1

Figure 1: Difference of polarized u-antiquark and polarized d-antiquark distribution functions as a function of x for different global fit results, DSSV, DNS and GRSV (val) [4]. The result obtained by the DSSV fit is shown together with uncertainty bands for Delta-Chi2=1 and Delta-Chi2/Chi2=2%. Also shown is a model prediction within the chiral quark soliton mode.. The difference of unpolarized antiquark distribution functions as obtained by the CTEQ global analysis is shown for comparison

A dedicated program by the PHENIX and STAR collaborations is devoted to the understanding of the gluon polarization. Results obtained by both collaborations have been included now in a first global analysis together with data from polarized deep-inelastic scattering (DIS) experiments constraining the degree to which gluons are polarized [3].  The net contribution of the gluon spin to the proton spin for momentum fraction of the gluon between 5% and 20% is very small. This finding is dominated by data collected at RHIC by the PHENIX and STAR collaborations in polarized proton-proton collisions at a center-of-mass energy of 200GeV.

In spite of the large body of DIS data, significant uncertainties remain in the polarized antiquark distributions. Hence the interest in high-energy polarized proton-proton collisions which could offer new insight, complementary to DIS using parity violating processes in polarized proton-proton collisions.

Parity violation was first discovered in beta decays. The dominance of electromagnetic and strong interactions results in a rather small signal in most observed measurements in the quark sector. The collision of polarized proton beams above ~500GeV in center-of-mass energy allow to observe pure weak interactions through W± and Z0 boson exchange and generate large parity violating single-spin asymmetries. The production of W-(+) bosons provides an ideal tool to study the spin-flavor structure of the proton. W-(+) bosons are produced in u-bar + d and d-bar + u collisions and can be detected through their leptonic decays, e- + anti-ve or e+ + ve' where only the respective charged lepton (electron/positron in the case of STAR) is measured.

Figure 2

Figure 2: Monte Carlo predictions for QCD background (QCD) and W signal (W) events before cuts (top left) and after cuts (right) as a function of the STAR Barrel Electromagnetic Calorimeter (BEMC) transverse cluster energy ET including detector effects from a full detector GEANT simulation.  All distributions are scaled to a sampled luminosity of 10pb-1. Two results are shown on the QCD background / W signal discrimination, without the Barrel Shower Maximum Detector (BSMD) (top left) and with the BSMD (bottom right). A clear Jacobian peak can be obtained even without the BSMD. The QCD background distribution is expected to be negligible at low ET including the BSMD.

The discrimination of u-bar + d (d-bar + u) quark combinations requires distinguishing between high pT e-(+) through their opposite charge sign, which in turn requires precise tracking information. At mid rapidity, STAR will rely at first on the existing Time Projection Chamber (TPC) augmented in the future by high precision inner silicon detectors. At forward rapidity, new tracking capabilities will be provided by a Forward GEM Tracker (FGT) consisting of six triple-GEM detectors currently under construction. Figure 1 provides an overview of the current understanding of polarized antiquark distribution functions, quantified as the difference of the respective anti-u (x Delta u-bar) and anti-d (x Delta d-bar) helicity distribution functions [4]. The DSSV global fit results in a difference of x(Delta-u-bar - Delta-d-bar) with a tendency to be larger than zero. A model calculation within the chiral quark soliton model also shows a positive difference. It has long been expected that non-perturbatice QCD mechanisms play an important role to account for observed asymmetries in the production of anti-u and anti-d quarks. Various model calculations suggest an even larger difference for polarized anti-u and anti-d quarks.

The yield of W boson signal events (weak interactions) over QCD background (strong interactions) is expected to be substantially smaller. The suppression of QCD background over W boson signal events by several orders of magnitude is accomplished by using the highly segmented STAR Electromagnetic Calorimeter (EMC), requiring an isolation criteria suppressing jet events, and vetoing di-jet events based on the measured away side energy. Establishing a first W signal over QCD background is an important milestone and was a physics goal of the recently completed 500GeV data taking period.

Figure 3

Figure 3: Raw transverse single-spin asymmetry for forward neutron production measured in the STAR EAST / Yellow Forward direction (left) and STAR West / Blue Forward direction (right) as a function of the azimuthal angle.

One of the major goals for the first 500GeV data taking period by the STAR collaboration was the commissioning of the local polarimetry system during the initial period of transverse polarized colliding proton beams. The STAR collaboration has so far successfully employed the STAR Beam-Beam counter (BBC) for this purpose during the past 200GeV RHIC runs. The STAR collaboration has upgraded the STAR ZDC calorimeter to provide an independent means for a local polarimeter system besides the STAR BBC. Two groups, LBNL and UCLA, and ANL and Valparaiso University, have now independently analyzed data during the initial period of transverse polarized colliding proton beams at 500GeV and found a large analyzing power at the level of ~8%. The observed analyzing power using the online measured beam polarization is shown in Figure 3 as function of the reconstructed azimuthal angle employing the STAR ZDC SMD. A future upgrade effort is foreseen to include the STAR ZDC SMD into a scaler system to provide a higher yield output resulting in the ability for rapid feedback over a much shorter timescale. Besides the commissioning of a local polarimeter system, the STAR collaboration commissioned various new detector systems during the initial 500GeV running period.

As previously mentioned, the main physics goal of the recent 500GeV RHIC run is the observation of a W signal over QCD background. Here, the main emphasis is placed on the observation of the so-called Jacobian peak characteristic for the production of W bosons. A clear Jacobian peak ~ MW/2 can be obtained based on simulation results using the full STAR simulation and reconstruction framework assuming a data sample of 10pb-1. The QCD background distribution is expected to be negligible at low transverse energies (ET). A sampled luminosity of 10pb-1 was thus the target for the STAR collaboration. In addition, projections were performed on the observation of a parity violating single-spin asymmetry, AL, based on 10pb-1 and 50% beam polarization resulting in a Figure-of-Merit of 2.5pb-1 characteristic for the uncertainty of a single-beam measurement.

The actual mean polarization of the STAR recorded data sample amounts to approximately 35%. More work is clearly necessary to increase the beam polarization at a beam energy of 250GeV, keeping in mind the fact that there are three additional strong depolarizing resonances between 100GeV and 250GeV. Figure 4 shows the Figure-of-Merit (FOM = P2 ∙ STAR sampled luminosity) separately for Yellow and Blue RHIC beams (left) and the STAR Sampled luminosity (right). It is shown that the luminosity target of 10pb-1 was already reached in mid-April. A dedicated analysis effort within the STAR collaboration is now underway to aim for a fast production of 500GeV data.

Figure 4

Figure 4: Figure-of-Merit (FOM = P2 ∙ STAR sampled luminosity) separately shown for Yellow and Blue RHIC beams (left) and STAR Sampled luminosity (right).

The 500GeV run at STAR was only possible due to the dedication of many colleagues within the Collider-Accelerator Department at BNL, the polarimeter group and the STAR Collaboration. I would like to express my sincere thanks to all who have worked very hard to make the 500GeV run come true. More commissioning time is clearly needed to reach a better performance of the collider at higher polarization and luminosity. Also the installation of the STAR Forward GEM Tracker will be critical for the future STAR W physics program.  But it goes without saying that an exciting time is ahead of us!

References

[1] G. Bunce ET al., Ann. Rev. Nucl. Part. Sci. 50, 525 (2000).

[2] G. Bunce ET al., ‘Plans for the RHIC Spin Physics Program’, BNL Internal Report submitted to DOE Nuclear Physics, June 2009.

[3] D. de Florian ET al., Phys. Rev. Lett. 101 072001 (2008).

[4] D. de Florian ET al., ‘Extraction of Spin-Dependent Parton Densities and Their Uncertainties’, hep-ph 0904.3821v1.