Cold-nuclear-matter effects on heavy-quark production at forward and backward rapidity in d+Au collisions at sqrt(s_NN)=200 GeV

The PHENIX experiment has measured open heavy-flavor production via semileptonic decay muons over the transverse momentum range 1<pT<6 GeV/c at forward and backward rapidity (1.4<|y|<2.0) in d+Au and p+p collisions at ?sNN = 200 GeV. In central d+Au collisions an enhancement (suppression) of heavy-flavor muon production is observed at backward (forward) rapidity relative to the yield in p+p collisions scaled by the number of binary collisions. Modification of the gluon density distribution in the Au nucleus contributes in terms of anti-shadowing enhancement and shadowing suppression; however, the enhancement seen at backward rapidity exceeds expectations from this effect alone. These results, implying an important role for additional cold nuclear matter effects, serves as a key baseline for heavy-quark measurements in A+A collisions and in constraining the magnitude of charmonia breakup effects at the Relativistic Heavy Ion Collider and the Large Hadron Collider.

A. Taketani, 55, 56 R. Tanabe, 64 Y. Tanaka, 47 S. Taneja, 61 K. Tanida, 34,55,56,59  The PHENIX experiment has measured open heavy-flavor production via semileptonic decay over the transverse momentum range 1 < pT < 6 GeV/c at forward and backward rapidity (1.4 < |y| < 2.0) in d+Au and p+p collisions at √ s N N = 200 GeV. In central d+Au collisions an enhancement of heavy-flavor muon production is observed at backward rapidity, whereas suppression is seen at forward rapidity relative to the yield in p+p collisions scaled by the number of binary collisions. The difference observed between forward and backward rapidity exceeds predictions based on a model of initial parton density modification. These results can be used to probe predicted cold nuclear matter effects, which may significantly affect heavy-quark production at the Relativistic Heavy Ion Collider and the Large Hadron Collider, in addition to helping constrain the magnitude of charmonia breakup effects in nuclear matter. Heavy quarks are essential probes of the evolution of the medium created in heavy-ion collisions, because they are produced in the early stages of nuclear collisions. Heavy-quark production has been measured via semileptonic decay electrons and muons, as well as fully reconstructed D mesons, at RHIC and the LHC [1,2]. In p+p collisions, heavy-quark production tests perturbative quantum chromodynamics and provides a baseline for heavy-ion collisions [3][4][5]. In central Au+Au collisions at √ s N N = 200 GeV, strong suppression of high transverse momentum (p T ) electrons from semileptonic decay of open heavy-flavor hadrons has been observed at midrapidity [6,7]. At forward rapidity, a similar level of suppression has been measured for the production of heavyflavor muons in central Cu+Cu collisions [8]. Although suppression of high p T particles was predicted as an effect of partonic energy loss in the dense medium created in heavy-ion collisions [9][10][11], it is difficult to account for this comparable suppression solely with hot nuclear matter effects [8,12]. To interpret such measurements, it is essential to probe underlying cold-nuclear-matter (CNM) effects, which may also be present.
Control experiments with d+Au collisions allow us to probe those CNM effects, including modifications of the parton distribution function (PDF) and k T broadening, with minimal impact from the hot nuclear medium. Because heavy quarks are produced primarily by gluon fusion at RHIC, modification of the gluon density in the nucleus can be observed in the charm and bottom production rates [13,14]. Based on pythia [15] calculations, the average parton momentum fraction x in the Au nucleus leading to heavy-flavor muons with 1 < p µ T < 6 GeV/c at backward (−2.0 < y < −1.4, Au-going direction) and forward (1.4 < y < 2.0, d-going direction) rapidity is ≈ 8 × 10 −2 and ≈ 5 × 10 −3 for the antishadowing and shadowing regions, respectively. Parton energy loss and multiple scattering in the nucleus can change the resulting heavy-flavor hadron momentum spectrum [16]. Previous results in d+Au collisions at midrapidity show a significant enhancement of heavy-flavor electrons at moderate p T [17]. In this Letter, we present measurements of the p T spectra and the nuclear modification factor (R dA ) of negatively charged muons from open heavy flavor at forward and backward rapidity in d+Au collisions at √ s N N = 200 GeV.
The d+Au and p+p data presented here were recorded with the PHENIX detector during the 2008 and 2009 RHIC running periods, respectively. The minimum-bias collision is selected by using the beam-beam counter (BBC) [18], and this selection covers 88 ± 4% (55 ± 5%) of the total d+Au (p+p) inelastic cross section [19]. The integrated luminosity, sampled using single muon triggers [8] in coincidence with the minimum-bias trigger, used for this analysis of d+Au (p+p) collisions is 50 nb −1 (10 pb −1 ). The d+Au collisions are categorized into five centrality classes: 0%-20%, 20%-40%, 40%-60%, 60%-88%, and 0%-100%, where 0%-20% represents the 20% highest multiplicity events, as determined by the amount of total charge deposited in the BBC on the Au-going side. For each centrality class, the average number of binary nucleon-nucleon collisions N coll is calculated from the BBC charge in a Glauber model [20]. Correction for the underlying event correlation and the efficiency of the BBC trigger to 100% is applied as in [21,22]. The values of N coll for the five d+Au centrality classes specified above are 15.1 ± 1.0, 10.2 ± 0.7, 6.6 ± 0.4, 3.2 ± 0.2, and 7.6 ± 0.4 respectively.
Two muon spectrometers [23] provide full azimuthal coverage in the pseudorapidity range −2.2 < η < −1.2 (backward rapidity) and 1.2 < η < 2.4 (forward rapidity). Each muon arm, located behind copper (19 cm) and iron (60 cm) absorbers, is composed of a muon tracker (MuTr) followed by a muon identifier (MuID). The MuTr comprises three stations of cathode strip chambers surrounded by a radial magnetic field, and the MuID comprises five interleaved layers of steel absorber and Iarocci tube planes. The MuTr provides the momentum measurement for charged tracks in the magnetic field. The momentum information for each charged track is then combined with its penetration depth reported by the MuID to provide effective discrimination between muons and hadrons (pion rejection rate: ∼ 10 −3 ) [24].
Despite the large hadron rejection power of the muon arms and strict selection criteria, most of the tracks reaching the last MuID layer are not heavy-flavor muons. For p T < 3 GeV/c the majority of these background tracks originate from the decays of light-flavor mesons (mostly π ± and K ± ) into muons before reaching the absorber material. Another source of background, called "punch-through hadrons," are the hadrons produced at the collision vertex, which penetrate all MuID layers. These become the dominant background at p T > 3 GeV/c. Other, less significant sources of background include muons from hadrons that decay inside the MuTr which are misreconstructed with erroneously high p T , muons from heavy-flavor resonances (χ c , J/ψ, ψ ′ , and Υ), and muons from light vector mesons (ρ, φ, and ω). The backgrounds are subtracted as follows.
For each data set, we measure the double differential heavy-flavor muon invariant yield, defined as (1) where ∆p T and ∆y are the bin widths in p T and y; N I is the number of inclusive muon candidates; N C is the number of decay and punch-through hadron background tracks determined using a hadron cocktail method (described below); N F is the estimated number of fake tracks that pass the selection criteria; N J/ψ is the number of muons from J/ψ decays; N evt is the number of sampled events; Aǫ is the detector acceptance and efficiency correction; and c is the BBC bias correction factor for the trigger efficiency and centrality determination of events containing a heavy-flavor muon. The contribution from remaining background sources is less than 5% [8,25]. Only negative muons are used, because the signal-to-background ratio is better than for positive muons [8]. The typical signal-to-background ratio, N µ /(N C + N F + N J/ψ ), increases from 0.3 at p T = 1 GeV/c to 0.6 at p T = 6 GeV/c. The hadron cocktail method estimates the overall background owing to light hadron sources using a fully data-driven geant simulation based on measured p T spectra. Details on background estimation procedure and associated systematic uncertainty are described in [3,8,25]. Figure 1 shows the invariant yield of heavy-flavor muons in d+Au collisions at backward and forward rapidity along with the invariant yield in p+p collisions. The vertical bars represent statistical uncertainties, while boxes are systematic uncertainties in the acceptance and efficiency correction, background estimate, and trigger bias correction for each centrality class. The main source of the systematic uncertainties is the background estimate including initial hadron production (∼10%) and hadron simulation (∼10%). All components of the systematic uncertainty are added in quadrature. Solid lines show a modified Kaplan function A 1 + (p T /8.3 (GeV/c)) 2 −3.9 [26], fit to the p T spectrum in p+p collisions, and then scaled by N coll for each d+Au centrality class. The p+p results are consistent with previous PHENIX measurements [8].
To quantify nuclear effects in d+Au collisions, we calculate the ratio of heavy-flavor muon yields in d+Au to p+p collisions scaled by the average number of binary collisions for a given centrality bin, backward rapidity. This enhancement shows a p T dependence consistent with p T broadening and gluon antishadowing. A suppression is observed at forward rapidity in the most central collisions. At forward rapidity, p T broadening is indicated by the slope of R dA , combined with a suppression that could be caused by gluon shadowing and/or partonic energy loss in CNM.
The dotted line in Fig. 2(c) is a prediction of R dA for muons from D and B mesons at forward rapidity, y = 1.7 [16,27]. This prediction, including CNM effects such as shadowing, initial-state energy loss, and k T broadening, is consistent with the data at forward rapidity for the 0%-100% centrality class. The same model, with additional energy loss in deconfined hot nuclear  [14]. The theoretical calculation shown in (c) is for forward rapidity [16].
matter, also describes the forward heavy-flavor muon results in central Cu+Cu collisions within uncertainties [8]. This agreement and the suppression at forward rapidity in central d+Au collisions suggest that CNM effects may be important for the interpretation of the suppression of heavy-flavor muon production at forward rapidity at RHIC [8] and the Large Hadron Collider [28]. We use the EPS09s leading-order (LO) nuclear PDF (nPDF) set [14] to calculate R dA for muons from D mesons at backward (solid lines) and forward (dashed lines) rapidity as described in [29]. The EPS09s nPDF further incorporates a spatial dependence within the nucleus to the nPDF. The modification of nPDF is determined based on the input parameters x, momentum transfer (Q 2 ) of charm production generated by pythia [15], and transverse radial positions of binary collisions in the nucleus for each centrality class. The un- certainty bands are calculated as described in [13]. From this calculation, we can take solely the initial parton density modification into account. In central collisions, shown in Fig. 2(b), the EPS09s nPDF based calculation does not reproduce the data at backward rapidity, particularly in the moderate p T region; the difference is ∼ 2σ near p T = 2 GeV/c. At forward rapidity, R dA calculated with the EPS09s nPDF is consistent with the data over the entire p T range within the systematic uncertainties of the data and calculation. The presence of other CNM effects is suggested, because the difference between forward and backward rapidity is significantly larger in the data than in the EPS09 nPDF calculation. Figure 3 shows the heavy-flavor muon R dA as a function of N coll for (a) 1.0 < p T [GeV/c] < 3.0 and (b) 3.0 < p T [GeV/c] < 5.0, compared to the heavyflavor electron measurement at midrapidity [17]. Bars (boxes) around the data points represent the statistical (systematic) uncertainties determined as the quadratic sum of statistical (systematic) uncertainties on R dA for each centrality class. In both p T ranges midrapidity and backward rapidity results agree within systematic uncer- tainties, showing a large enhancement for more central collisions. At forward rapidity the low-p T bin shows suppression increasing with centrality, whereas the high-p T bin shows little or no centrality dependence. The EPS09s nPDF based calculations are consistent with the data at forward rapidity within uncertainties.
Quarkonia and open heavy-flavor hadrons are sensitive to the same effects on heavy-quark production. However, quarkonium states are additionally influenced by breakup in nuclear matter. Therefore, open heavy-flavor production can provide a baseline for interpreting the nuclear breakup of quarkonia. Previous measurements suggest that nuclear breakup has a significant effect on quarkonia production in nuclear collisions [21,[29][30][31][32][33][34]. Figure 4 shows a comparison of R dA between heavyflavor muons and J/ψ [21] for central collisions. A similar behavior across the entire p T range is observed at forward rapidity, within the systematic uncertainties, whereas a distinct difference is seen at backward rapidity, particularly for p T < 2.5 GeV/c where charm contributions dominate over those from bottom [35]. The larger difference of the R dA between J/ψ and open charm at backward rapidity compared to forward rapidity could be related to the longer time this cc state requires to traverse the nuclear matter or the larger density of comoving particles after the initial collision at backward rapidity [36]. This comparison suggests that an additional CNM effect, nuclear breakup, significantly affects J/ψ production at mid-and backward rapidity. This measurement provides a key additional constraint on theoretical models attempting to describe quarkonia yields in nuclear collisions.
We have presented a measurement of negatively charged heavy-flavor muons produced at for-ward and backward rapidity in d+Au collisions at √ s N N = 200 GeV, for several centrality classes. We observe no significant modification in the most peripheral d+Au collisions. However, in central d+Au collisions, suppression (enhancement) of heavy-flavor muons is observed at forward (backward) rapidity. The large difference between forward and backward rapidity, which is not reproduced by pythia calculations with the EPS09s nPDF sets, suggests that various CNM effects combine to produce the observed modifications. A comparison between the measured nuclear modification factors for J/ψ and open heavy-flavor production provides strong indication that nuclear breakup significantly affects quarkonia production.