Physics, P-DO
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Muon Production with the PHENIX Muon Spectrometers and Color Glass Condensate
M.X. Liu, J.G. Boissevain, M.L. Brooks, P. Constantin, G.J. Kunde, D.M. Lee, M.J. Leitch, P.L. McGaughey, B. Norman, A.K. Purwar, W.E. Sondheim, H.W. van Hecke (P-25), J.P. Sullivan (ISR-1) IntroductionUnderstanding the substructure of nuclei is fundamentally important in nuclear physics. In modern theory, the strong nuclear interaction observed between nucleons (protons and neutrons) inside the nucleus is only a van-der-Waals-type residual force of a more fundamental interaction between the nucleon's constituents. This interaction involving quarks and gluons, collectively called partons, is referred to as quantum chromodynamics (QCD). Studying the parton distribution inside nuclei can shed light on why and how quarks and gluons are confined inside hadrons.
In a naïve picture, the nucleus-parton distribution is simply the sum of the nucleon's PDFs inside the nucleus. However, the subject of this study is to see if internucleon interaction will eventually modify such distributions. In a DIS scattering to the first order, an incoming lepton only couples with the charged quark or antiquark, not with a neutral-charged gluon. Using measured quark and antiquark distributions from DIS, one can calculate the gluon distribution employing QCD-based parton evolution equations. Figure 2 One could expect that such saturation effects are particularly important for heavy nuclei where small-x gluons from different nucleons have a high probability to overlap in space (Figure 3). The RHIC at BNL and the PHENIX ExperimentThe Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) collides two ion beams of various types, ranging from protons and deuterons to gold nuclei types, at energies of 100 GeV per nucleon. In such high-energy collisions, quarks and gluons interact directly, thus allowing us to directly probe the nucleus' parton distributions. The PHENIX experiment at RHIC consists of a general-purpose detector that possesses unique attributes among the five RHIC experiments, including two spectrometers designed to measure high-energy muons over the forward and backward pseudorapidity range 1.2 < |η| < 2.4. They are also the largest subsystems in the PHENIX experiment. The Subatomic Physics (P?]25) PHENIX Team has led the design, construction, and operation of the muon spectrometers, and is now leading efforts to analyze the muon data. Figure 4 shows an artist's view of the PHENIX muon spectrometers. PHENIX also has the ability to measure the centrality of individual ion-ion collisions, where centrality is a measure of the degree of overlap between the two colliding ions. One can compare particle production per nucleon-nucleon reaction in the head-on collisions with these in glancing ones. If there are no nuclear medium effects, i.e., the nucleus-parton distribution is simply the sum of the nucleons and there is no initial- and final-state interaction with nuclear medium, the particle yield per nucleon-nucleon collision will be independent of the collision’s centrality. Muon production in asymmetric deuteron-gold collisions at RHICIn 2003, RHIC collided deuteron and gold nuclei at a center of mass energy of 200 GeV per nucleon pair. At this energy, most hadrons with a transverse momentum of Pt > 2 GeV arise from parton-parton interactions and can be used to probe the nuclear parton structure. Particle production in the deuteron direction (forward) is sensitive to small?]x parton distribution in the gold nuclei, whereas particle production in the gold direction (backward) is sensitive to the large-x parton distributions. It has been predicted that the gluon saturation at small-x in gold will suppress the particle production yield in the forward direction.1 Very recently, other hadron-production mechanisms, such as quark recombination and coherent multiple-scattering models, could explain the observed suppression in the forward-rapidity region.2 The spectrometers were originally designed to measure muons, however, we recently developed new methods to expand the capability to include hadron measurement. In the following, we briefly discuss how to use muon spectrometers to measure hadrons in deuteron-gold collisions. Due to the finite distance from the collision vertex to the hadron absorber in front of the muon tracking system, charged pions and kaons have a small probability to decay into a muon before the absorber, through decay modes such as π ± → μ ± + υ, with the decay probability given by,
where L ~ 40 cm is the distance from the collision's vertex to the absorber and p, M, and τ are the momentum, mass, and proper lifetime of the particle. Collisions that occurred far from the absorber will have a higher probability to contain muons that originated from light meson decays than those that occurred close to the absorber. Figure 5 shows the normalized collision's vertex distribution for events with forward (positive z-direction) muons in deuteron-gold collisions. The large slope indicates a significant fraction of the muons are from pion and kaon decays. In addition to muons from light meson decays, about 1% of hadrons from the collisions can also punch through the absorber and get into the muon spectrometer. These hadrons are identified by the muon identification (MuID) system. Most of the punch-through hadrons interact strongly and stop within the first a few layers of the MuID absorbers while most muons will likely sail through all of the MuID absorbers. ResultsWe studied the hadron production as a function of centrality and rapidity. Figure 6 We observed suppression in charged hadron yield at forward rapidity and enhancement at backward rapidity in the ratio between central and peripheral deuteron-gold collisions. The forward suppression is consistent with the expectation of gluon shadowing or saturation in the small-x region in large nuclei. For a typical hadron of transverse momentum Pt ~ 1.5 GeV, the x-value probed in gold nuclei at the very forward rapidity y ≈ −2 is estimated to be 2 × 10-3, and at the very backward direction y ≈ −2, the x-value is close to 1 × 10-1. Further detailed comparisons with various theoretical approaches are necessary in order to discriminate between different models, such as CGC and parton recombination models. Currently, we have no sound theoretical understanding for the enhancement in the backward rapidity. Antishadowing (enhancement) in parton distribution at large-x or final-state multiple scattering could lead to such effects. ConclusionWe have studied high-energy particle production at the forward and backward directions in asymmetric deuteron-gold collisions at RHIC. The preliminary results are consistent with parton shadowing or CGC saturation models at small-x, and antishadowing or Cronin effects at large-x inside gold nuclei. In the near future, we expect to measure heavy flavor production in a similar kinematic region. Such measurements are particularly important since the final-state multiple scattering effect is expected to be minimal for heavy quarks, thus they could provide an unambiguous experimental test of various particle production models. References:
AcknowledgmentWe thank the staff of the Collider-Accelerator and Physics Departments at BNL for their vital contributions. We acknowledge support from the Department of Energy and NSF (U.S.A.), MEXT and JSPS (Japan), CNPq and FAPESP (Brazil), NSFC (China), IN2P3/CNRS, CEA, and ARMINES (France), BMBF, DAAD, and AvH (Germany), OTKA (Hungary), DAE and DST (India), ISF (Israel), KRF and CHEP (Korea), RMIST, RAS, and RMAE (Russia), VR and KAW (Sweden), U.S. CRDF for the FSU, US-Hungarian NSF-OTKA-MTA, and US-Israel BSF. For further information, contact Ming Liu, 505-667-7125, mliu@lanl.gov. |
Our knowledge about parton distributions is mainly from deep inelastic lepton-nucleus scattering (DIS) experiments. In parton models, high-energy electron-nucleus scattering does not affect the nucleon as a whole, but just one of its constituents (Figure 1). Each constituent carries a fraction x of the nucleon's momentum with the probability density f(x), also known as the parton density function (PDF).
shows the gluon distribution inside the proton at a various probing energy scale represented as Q2. It is interesting to note that the rapid rise in the small-x gluon PDF predicted by QCD calculation will eventually violate unitarity and lead to a breakdown in the parton model picture of scattering off independent partons. At a sufficiently high density, it becomes possible for a second gluon to overlap in space with the first, leading to gluon fusion, thus limiting the achievable gluon density at small-x. This saturation is sometimes described as the formation of a color glass condensate (CGC).1
shows the particle yield ratios per nucleon-nucleon collisions versus rapidity of the most central collisions (0%–20%) to peripheral collisions (60%–88%). Without nuclear medium effects, this ratio should be unity.