SSingle Particle Probes of d + Au Collisions in PHENIX
Zvi Citron for the PHENIX collaboration
Stony Brook University, Department of Physics and Astronomy, Stony Brook, NY, 11794, USA
Abstract
Deuteron-gold collisions provide insights into the nuclear structure function and a valuable base-line for Au + Au collisions. Measurement of the nuclear modification factor, R dAu , in d + Au inthe PHENIX central arms for hadrons and photons allows us to disentangle cold nuclear mattere ff ects from the hot medium e ff ects that are important in Au + Au collisions. In addition, thed + Au system can yield important insights into the gluonic structure of the Au nucleus. RHICexperiments have previously measured suppression of forward rapidity particle production rela-tive to p + p scaled by the number of binary N-N collisions, but a definitive explanation of thesedata is thus far elusive. Correlations between hadrons with a large rapidity gap are a particularlysensitive probe of gluon saturation. We will discuss probing this physics via particle productionin events tagged with high momentum particles at di ff erent rapidities, along with R dAu in newforward rapidity regions.The 2008 RHIC run of deutreron-gold collisions, with ≈
30 times increase in statistics withrespect to the 2003 dataset, provided for important measurements of the properties of cold nu-clear matter. PHENIX has measured the mid-rapidity charged hadron production to calculate thenuclear modification factor, R dAu , and has made use of new forward rapidity detectors to gainsensitivity to di ff erent regions in x .The nuclear modification factor is defined as: R dAu ( p T ) = (1 / N d + Auevt ) d N d + Au / d p T d η (cid:104) N coll (cid:105) (1 / N p + pevt ) d N p + p / d p T d η . (1)It is calculated separately for di ff erent centrality bins as categorized by the PHENIX Beam-BeamCounters (BBC), with each centrality bin yield appropriately corrected due to BBC e ff ects[1].This quantity is a useful baseline to gauge the impact of cold nuclear matter e ff ects as comparedto the analogous quantity R AuAu which shows suppression due to plasma e ff ects [2]. It is alsoimportant for understanding other e ff ects which are di ffi cult to isolate in a heavy ion environment.Figure 1 shows the mid-rapidity PHENIX measurements from the 2003 RHIC run [1] as well asthe new preliminary data from the higher statistics 2008 run.Besides the nuclear modification factor measured at mid-rapidity, RHIC experiments havepreviously measured the nuclear modification factor in d + Au collisions at forward, d going side,and backward, Au going side, rapidities and found suppression in the forward direction [3, 4]. Adefinitive explanation of this observation is thus far elusive, but there are several models whichseek to explain it. To further study these phenomena PHENIX installed forward (North) andbackward (South) electromagnetic calorimeters, named the Muon Piston Calorimeters (MPC)for their location in PHENIX, covering 3.1 to 3.9 and -3.1 to -3.7 in η respectively. Several of Preprint submitted to Nuclear Physics A November 20, 2018 a r X i v : . [ nu c l - e x ] O c t igure 1: Mid-rapidity inclusive charged hadron R dAu measured by PHENIX in the 2003 and 2008 RHIC runs. The solidrectangle indicates global systematic uncertainty, this uncertainty may not fully cancel between the two data sets. the models are sensitive to correlations between particles at forward rapidity and those at mid-rapidity [5, 6]. To study this we can trigger on particles in the forward (and backward) MPC toaccess relatively lower (and higher) x and measure the inclusive mid-rapidity charged particleyield. To trigger in the MPC a π is reconstructed from two electromagnetic calorimeter clusters.At energy greater than ≈
17 GeV a π can not be reliably distinguished from a single γ due tocluster overlap, so to reach higher energy a separate data set is kept in which the triggers are sim-ply inclusive electromagnetic clusters (the preponderance of γ s are from π decays). Althoughthe two MPCs do not have identical acceptances to enable a direct comparison of the forwardrapidity triggered to backward rapidity triggered data sets the acceptances are “symmetrized”.An occupancy correction is applied to the measured trigger energy which is ≈ %10 for triggersin the MPC on the Au going side in d + Au collisions.With the triggers thus defined, the mid-rapidity inclusive charged hadron per trigger yieldcan be measured for forward and backward rapidity triggers. A cartoon schematic of the mea-surement is shown in figure 2. In the symmetric p + p system the labels of forward and backward Figure 2: A cartoon of the measurement. Both forward (N) and backward (S) rapidity triggered type events are illustrated. FB is defined: R FB ( p T ) = (1 / N . <η< . ) d N + η trig | η | < . / d p T d η (1 / N − . >η> − . ) d N − η trig | η | < . / d p T d η . (2)R FB of p + p collisions for three di ff erent trigger energies is plotted in figure 3. It is consistentwith unity for all triggers, as expected, demonstrating that there is no lurking artificial asymmetrydue to detector e ff ects. Figure 3: The ratios of the forward MPC triggered mid-rapidity h ± to the backward triggered mid-rapidity h ± for di ff erenttrigger energies in p + p collisions. In the left plot the MPC trigger is an inclusive EM cluster, and on the right the MPCtrigger is a reconstructed π . Of more interest than the p + p system is the d + Au system, in which forward and backwardrapidity are not arbitrary but refer to the d going and Au going side, respectively. In the d + Ausystem shadowing, saturation, or other e ff ects may lead to R FB (cid:44)
1. R FB is calculated for fourcentrality bins to assess the significance of the nuclear volume at play in the collision. In themost peripheral collisions ( < N coll > = + p as theyare shown to be in the top panel of figure 4. However, in more central collisions the d goingside triggered mid-rapidity inclusive charged hadron spectra are suppressed compared to the Augoing side triggered sample. This is seen in the bottom panel of figure 4 where R FB ( p T ) < ff erent triggers in di ff erent centrality bins, by showing R FB integrated over 1.0 < p T < . / c as a function of the trigger energy.Further study is necessary to contextualize this study in terms of an understanding of thephysics underlying the observations, in particular, whether these data support shadowing, anti-shadowing, or saturation in the nucleus. See also a complementary analysis of the same data setin these proceedings [7]. 3 igure 4: The ratio of the d going side π MPC triggered mid-rapidity h ± to the Au going side π triggered mid-rapidityh ± in the most peripheral collisions ( < N coll > = < N coll > = π trigger energies of 5.5 < E < < E < < E < References [1] S. S. Adler et al. [PHENIX Collaboration], Phys. Rev. C , 014905 (2008)[2] S. S. Adler et al. [PHENIX Collaboration], Phys. Rev. C , 034909 (2004)[3] S. S. Adler et al. [PHENIX Collaboration], Phys. Rev. Lett. , 082302 (2005)[4] I. Arsene et al. [BRAHMS Collaboration], Phys. Rev. Lett. , 242303 (2004)[5] J. w. Qiu and I. Vitev, Phys. Lett. B , 507 (2006)[6] D. Kharzeev, E. Levin and L. McLerran, Nucl. Phys. A , 627 (2005)[7] B. Meredith (for the PHENIX Collaboration) this proceedings, 627 (2005)[7] B. Meredith (for the PHENIX Collaboration) this proceedings