and charm at STAR

Nuclear Physics A 854 (2011) 212–221

www.elsevier.com/locate/nuclphysa

J/ψ and charm at STAR

Chris Perkins, for the STAR Collaboration

UC Berkeley/Space Sciences Laboratory, Berkeley, CA 94720, USA

Received 30 August 2010; accepted 4 January 2011

Available online 8 January 2011

Abstract

Gluon saturation effects and Color Glass Condensate models become important at a scale Q2S(x). If

the saturation scale can be raised to above the charm-quark mass, saturation effects can influence charmproduction and it is expected, based on CGC models, that charm particle production will follow similarpatterns to those found in light quarks. This scale can potentially be reached in the charm sector by goingto forward rapidities at RHIC and using nuclear “targets”.

The large acceptance and pseudorapidity coverage (2.5 < η < 4) of the Forward Meson Spectrometer(FMS) at STAR provide excellent geometric efficiency to measure high-xF J/ψ production in p+p andd+Au collisions. A significant observation of high-xF J/ψ in p+p collisions has been obtained. Measure-ments of forward open charm production also look to be feasible based on initial simulation studies. Bycomparing d+Au measurements to p+p measurements in both charmonium and open charm productions,the FMS at STAR has the potential to test several predictions of CGC/Saturation models in the charm sector.© 2011 Elsevier B.V. All rights reserved.

Keywords: Color Glass Condensate; Forward Meson Spectrometer; J/ψ ; Charm

1. Introduction

It has been widely reported that gluon densities in the proton rise rapidly for lower values ofproton momentum fraction, x. This density, however, cannot grow forever, as x decreases, dueto unitarity constraints. At some point, gluon recombination becomes important and non-linearevolution contributions must be included in the gluon evolution. The Color Glass Condensate(CGC) is a semi-classical effective field theory that allows one to compute low-x gluons in nuclei.

E-mail address: [email protected].

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STAR Collaboration / Nuclear Physics A 854 (2011) 212–221 213

Fig. 1. (Left) The CGC model expects a suppression of forward hadron production in p(d)+A collisions compared top+p collisions that increases with centrality and more forward rapidity [3]. (Right) RdAu for forward neutral pions (fromSTAR [4]) and charged hadrons (from BRAHMS [5,6]) as a function of pT .

The relevant parameter for the onset of this regime is the saturation scale, defined by [1,2]:

Q2S(x) ∼ A1/3(ey

√s)λ (1)

As can be seen from this relation, saturation becomes relevant for low-x, large√

s, large rapidities(y), or large nuclei (A).

The CGC model makes several predictions for the effect of gluon saturation on the productionof light-quark mesons [3,7] of which I will highlight one that will be relevant to later discussions.The CGC model expects a suppression of forward hadron production in p(d)+A collisions com-

pared to p+p collisions, indicated in Fig. 1 by a suppression of the ratio Rp(d)A = dNp(d)A/dyNcoll dNpp/dy

.This suppression is expected to become stronger with increasing centrality and more forwardrapidity. The effectively lower gluon density due to gluon saturation qualitatively explains thissuppression.

Qualitative agreement with these predictions has been seen in the production of forward π0

at STAR [4] in which sizable suppression has been observed (see Fig. 1). The suppression offorward π0 is also expected by a combination of pQCD + shadowing but suppression beyondthis expectation is seen. The CGC model presently gives the best description of the pT dependentsuppression. The rapidity dependence of charged hadrons observed at BRAHMS is also quali-tatively consistent with the expectations from the CGC framework, namely that these particlesbecome increasingly suppressed at more forward rapidities [4,8].

2. J/ψ production

Although it is commonly believed that the main production mechanism for J/ψ in p+p colli-sions is through gluon fusion, there is still much debate on how exactly the J/ψ is produced. Ithas not yet been resolved whether the c–c̄ pair is produced in a color-singlet or color-octet state[9,10]. There is also not yet a consensus on how the c–c̄ pair coalesces to form a color-neutral

214 STAR Collaboration / Nuclear Physics A 854 (2011) 212–221

J/ψ . Regardless of the exact J/ψ production mechanism, however, it must be the case that itdepends on the gluon distribution because of the initial and final state gluon interactions. If thegluon distribution becomes saturated, this will have implications on the production of J/ψ andother heavy-flavor mesons.

In addition to J/ψ production in p+p collisions, it is important to consider production incollisions involving heavy nuclei since colliding with heavier nuclear targets, A, can increase thesaturation scale. When J/ψ is produced in dilute systems, i.e. not in the saturation regime, it hasbeen suggested that the c–c̄ pair scatters coherently off only a few nucleons [2]. When saturationis reached, however, the CGC framework suggests that the c–c̄ pair scatters coherently off all thenucleons along its trajectory [2].

The traditional scale associated with J/ψ and other heavy-flavor meson production is the massof the heavy quark involved. When the mass of the heavy quark exceeds ΛQCD, it is expectedthat production proceeds perturbatively. In the CGC/Saturation picture, however, the more rele-vant scale is the saturation scale previously described. When the saturation scale is larger thanthe mass of the heavy quark, QS > mQ, saturation may affect the heavy flavor production. Inthis regime, the heavy-quark mesons should follow similar patterns to those specified for light-quark mesons in the CGC/Saturation model. This applies to both charmonium and open charmproductions.

At mid-rapidity at STAR, the saturation scale in d–A collisions is on the order of or below themass of the charm quark and it is expected that the saturation scale cannot be reached for charmedmesons. For this reason, I will not focus on the wide range of Open and Closed Charm resultsthat STAR is capable of measuring at mid-rapidity other than to mention that some theoriesexpect a value of RdAu = 1 at mid-rapidity with moderate Ncoll scaling in the CGC framework[2]. STAR’s low-pT J/ψ measurement from d–Au collisions in Run 8 is seen to be consistentwith this expectation [11].

By looking at forward rapidities at STAR, however, we are able to probe asymmetric partoniccollisions in which high-x valence quarks interact with low-x gluons. With the current apparatusat STAR, these gluons can reach values of x on the order of 10−3. When looking at forwardrapidities at STAR and using heavy nuclear targets, we have the potential to reach the regimewhere the saturation scale possibly becomes larger than the charm-quark mass thereby makingcharm-quark production subject to the CGC/Saturation model.

Several expectations for charm production in the CGC framework have been put forward [12](see Fig. 2). First, the charmed meson yield is expected to scale by

√Npart in d–A collisions

at forward rapidities compared to the Ncoll scaling expected at mid-rapidities. In A–A colli-sions, this scaling becomes Npart scaling in the CGC framework. Also, a harder pT spectrum isexpected because of the harder gluon spectrum. This is shown for D-meson production at mid-rapidity in Fig. 2. As a consequence of this, the total transverse momentum is not expected tovanish and is expected to yield a value of 〈p2

T 〉 ≈ Q2s max at forward rapidities at RHIC. In the

saturation regime, the CGC model also expects a suppression of RdAu and a disappearance of theCronin enhancement at more forward rapidities. This suppression should also be stronger withcentrality because of the

√Npart scaling.

It should be noted, however, that other non-CGC models exist that give similar expectationsfor the nuclear dependence at forward rapidities. The nuclear dependence is often characterizedby α, defined by σA = Aασp . The intrinsic heavy flavor model, for example, has similar expec-tations for α at forward rapidities [13–15].

Intrinsic heavy flavor may also play a significant role in future CGC measurements in anotherway. A letter of intent has been submitted to perform a low-mass, high-xF Drell–Yan measure-


Fig. 2. Expectations for charm production in the CGC model [12]. (Left) A modified scaling is expected in A–A andp(d)–A collisions at forward rapidities as compared to mid-rapidities. (Right) A harder pT spectrum is expected. Thisplot shows a pT spectrum for D-mesons at mid-rapidity [12].

ment at RHIC which would probe gluon x to values even lower than can currently be reached atRHIC. The particularly simple color structure of the Drell–Yan interaction raises the possibilityto measure more universal quantities in low-x physics. The letter of intent proposes to measuredielectron pairs in the mass range 2 GeV/c2 < M < 4 GeV/c2. Current event generators do notmodel intrinsic heavy flavor which is expected to be most prominent in the high-xF range probedby this Drell–Yan measurement. Consequently, the only way to differentiate between true Drell–Yan dilepton pairs and dilepton pairs coming from decays of mesons that arise from intrinsicheavy flavor is to measure open charm production in this mass and rapidity range and then usethis information to model the background.

3. Experimental setup

A top view of the Solenoidal Tracker at RHIC (STAR) detector as used in the RHIC Run 8d+Au configuration is shown in Fig. 3. While the Barrel Electromagnetic Calorimeter and TimeProjection Chamber provide excellent coverage for measuring J/ψ and Charm at mid-rapidity,only forward detectors were used in this analysis, namely the Forward Meson Spectrometer(FMS). The Beam–Beam Counters (BBC) were also used offline for additional event selection.

The most forward detectors at STAR began with small, modular electromagnetic calorimeters.Starting in Run 8, the full FMS was implemented, providing ∼20× more acceptance than the pre-vious forward detectors and full azimuthal coverage over the pseudorapidity range 2.5 < η < 4.The FMS is an electromagnetic calorimeter located approximately 7.5 meters to the west of theSTAR interaction point and consists of an array of ∼1200 lead-glass cells. A schematic drawingof this detector is shown in Fig. 3. Fig. 4 shows the combined coverage over three RHIC runs inthe xF –pT plane for inclusive neutral pions and also shows the same coverage in RHIC Run 8using the full FMS. The increased acceptance of the FMS not only gives increased pion yieldsand kinematic range but also gives much higher geometric efficiency for high-xF J/ψ .

4. Forward J/ψ at STAR

The J/ψ analysis reported here used a dataset collected during the 2008 RHIC Run withp+p collisions using a high-tower trigger on the FMS cells with a BBC minimum bias condi-tion imposed offline (sampled luminosity ∼6 pb−1). Clusters were formed which were fit with


Fig. 3. (Left) Top view of the STAR detector in the Run 8 configuration. Note that the transverse scale is stretchedcompared to the longitudinal scale. (Right) Front view (x–y plane) of the FMS.

Fig. 4. (Left) pT vs. xF of neutral pions for three combined RHIC runs using the first configuration of forward calorime-try at STAR (FPD) [16]. (Right) pT vs. xF of neutral pions for RHIC Run 8 using the full FMS.

shower-shapes calculated from test-beam data to reconstruct incident electrons, positrons andphotons. Two analyses were performed to reconstruct J/ψ in the FMS: a cluster-pair analysisand a three-cluster analysis that observed J/ψ through its feed-down from χC . The aim of thethree-cluster analysis, described further below, was not to determine the relative contributioncoming from feed-down but to provide an additional arm with which to eliminate background.

The reconstructed 2-cluster mass is shown in Fig. 6. This plot requires that the pair energy is

greater than 60.0 GeV, the energy sharing, Z = Ee+−Ee− , to be less than 0.7, and that clusters are
Ee++Ee−


Fig. 5. Reconstructed invariant mass of the minimum bias simulation [17]. The data used in this plot include only onedata-taking run from the entire sample. The reconstructed simulation includes both an uncorrected reconstruction and areconstruction that includes resolution smearing to match realistic detector resolutions.

isolated by R = √(η)2 + (φ)2 > 0.5. The isolation cut suppresses combinatorial background

from lower mass particles. The energy sharing cut focuses the analysis on relatively symmetricdecays to ensure that most of the J/ψ decay products will both fall within the acceptance of theFMS. The signal was fitted with a gaussian plus a linear polynomial, resulting in a fit with asignificance of 2.1σ . The background simulation was normalized to the integral of the data inthis mass region.

A further cut was applied to the pT of individual clusters to filter those clusters coming fromthe large background near the beampipe (pT,cluster > 1.0 GeV/c). The reconstructed 2-clusterinvariant mass with this new cut and a slightly lower isolation radius cut is shown in Fig. 6.This data was fitted with a gaussian plus an offset, resulting in a fit with a significance of 4.5σ .The mean of the gaussian fit is very close to the expected J/ψ mass. Incomplete field effectsimulations and energy dependent gain calibrations can account for a fit width that is lower thanexpectations from simulations.

We also performed a 3-cluster analysis that observes J/ψ through its feeddown from χc (χc →J/ψ + γ → e+ + e− + γ ). This technique was motivated by an analysis of ω → π0 + γ asshown in [17] and was also used by the COMPASS experiment in looking at D → K + π inD∗ resonance decays [18]. For each group of 3 clusters within an event, we associate the pairwith reconstructed mass closest to the accepted J/ψ mass (3.097 GeV/c2) with the J/ψ and theremaining cluster with the γ . Mass plots of the pair chosen to be the J/ψ compared with massplots of the other cluster combinations indicate that we are correctly identifying the J/ψ .

The reconstructed invariant mass of the pair associated with the J/ψ from the 3-cluster anal-ysis is shown in Fig. 7. The background and signal were fitted with separate gaussians resultingin a fit with a significance of 2.9σ . We found that the significance of the fit depends highly onthe background model used. A realistic simulation of this background is still in progress but thereported significance of 2.9σ is a conservative estimate. The mean and sigma of the gaussian


Fig. 6. (Left) Reconstructed invariant mass at forward rapidity in p+p collisions with original cuts for 2-cluster analysis[11]. Errors bars on data points are statistical only. Blue curve shows fitted background. Red curve shows fitted J/ψsignal. Gray bands show simulated background. (Right) Reconstructed invariant mass at forward rapidity for 2-clusteranalysis with additional cuts on cluster pT [11]. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)

Fig. 7. Reconstructed invariant mass of pair associated with J/ψ in 3-cluster analysis [11].

fit are also lower than expected, as was found in the 2-cluster analysis. This observation couldsuffer from the same field effects and energy calibration issues described previously.

Because there is no tracking detector that overlaps with the FMS, we cannot distinguish thecharge sign of the J/ψ decay electrons and must therefore rely on simulations for the backgroundshape in the region of the J/ψ mass. In addition to performing a minimum bias simulation to ver-


ify our reconstruction procedure and resolutions, we performed a high-mass filtered PYTHIA +GEANT simulation to generate the background shape. It can be seen from the reconstructed in-variant mass of the minimum bias simulation in Fig. 5 that sufficient simulation statistics aredifficult to generate. The additional sample generated with a high-mass PYTHIA filter containedsignificantly more statistics than the minimum bias simulation but still not enough to tune finalcuts or perform an accurate background subtraction. An association analysis of the high-massPYTHIA filtered simulation determined that hadron–hadron pairs make only a minor contribu-tion to the background once preliminary cuts are applied. As a result, a new PYTHIA filter wasdesigned and analysis of this new sample is still ongoing. Once an accurate background studyhas been completed it will be used to tune final cuts and perform background subtraction.

5. Forward open charm production at STAR

Initial studies have also been performed to assess the feasibility of measuring forward opencharm production through the decay channel D → K0

S + π0 → 3π0 → 6γ . This analysis posesthe difficult task of reconstructing the displaced vertex of the K0

S using only the bare calorimeterresponse of the FMS. To assess the feasibility of this analysis, a sample of D’s was generatedusing PYTHIA plus a fast simulator with realistic resolution smearing. Because one of the neutralpions comes directly from the D decay, one pair of the six clusters found in the FMS should havea reconstructed mass fairly close to the PDG value for the π0 mass. The other two cluster pairscome from the displaced decay vertex of the K0

S and should have similar decay lengths. Theimpact point of the K0

S in the FMS x–y plane is also known from the energy weighted positionof the remaining four clusters. To determine the K0

S decay vertex, the line connecting the primaryvertex to the K0

S impact point is traced and a quality factor is calculated at each point along theline. The quality factor at each point includes how well the K0

S mass and the two π0 masses arereconstructed as calculated from the candidate displaced vertex. The point along the K0

S pathwhere the quality factor is minimized gives the most likely K0

S decay vertex.It can be seen in Fig. 8 that the displaced K0

S decay vertex can be reconstructed quite wellfrom the calorimeter response alone using the method outlined above. From this analysis itwas determined that the K0

S decay length resolution is dominated by the primary vertex reso-lution necessitating a cut at 2σvertex on the reconstructed K0

S decay length. The primary vertexin this analysis would be measured using timing information from the Beam–Beam Counters(BBC) which has a resolution of ∼30 cm. Some signal is lost in this cut but it is known fromprevious simulations that a cut on the displaced vertex can highly suppress backgrounds. Thereconstructed D mass is shown in Fig. 9 and the study of this simulation sample indicates thatD’s can be reconstructed in the FMS with high efficiency. The PYTHIA sample generated indi-cates that we could have ∼1650 reconstructible D events in our p+p data sample, up to triggereffects. PYTHIA is known to get forward charm production wrong but we should still have asizable number in our data. The FMS high-tower trigger that was used in Run 8 also may not beso efficient for triggering on D mesons via this decay. The Cluster trigger used in Run 9 may bemore efficient but further studies of trigger efficiencies for open charm are needed.

6. Conclusion and outlook

It is possible that the CGC/Saturation regime could be accessible to the charm sector at RHICby going to forward rapidities and using d+Au collisions. CGC models expect qualitatively


Fig. 8. (Left) K0S

decay vertex-simulated vs. reconstructed. (Right) Reconstructed proper K0S

decay length from simula-tion.

Fig. 9. Reconstructed D mass from simulation.

similar trends to light-quark mesons for charm production in the saturation regime. It should benoted, however, that other theories may also contribute to any suppression or enhancement seenat this rapidity and should be considered.

The FMS detector at STAR is positioned such that it has the possibility to measure both char-monium and open charm productions at forward rapidities. A significant signal of forward J/ψin p+p collisions has already been reported and work is ongoing to report forward cross sections


and comparisons to d+Au collisions. Preliminary studies of the feasibility of reconstructing opencharm look promising and further analysis of p+p and d+Au data should begin soon.

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and charm at STAR