Quarkonia Measurements with STAR
We report results on quarkonium production from the STAR experiment at the Relativistic Heavy-Ion Collider (RHIC). spectra in + and Cu+Cu collisions at = 200 GeV with transverse momenta in the range of 0.5-14 GeV/ and 5-8 GeV/, respectively, are presented. We find that for GeV/ yields in + collisions are consistent with those in minimum-bias Cu+Cu collisions scaled with the respective number of binary nucleon-nucleon collisions. In this range the nuclear modification factor, , is measured to be . For the first time at RHIC, high- -hadron correlations were studied in + collisions. Implications from our measurements on production mechanisms, constraints on open bottom yields, and dissociation mechanisms at high- are discussed. In addition, we give a brief status of measurements of production in + and Au+Au collisions and present projections of future quarkonia measurements based on an upgrades to the STAR detector and increased luminosity achieved through stochastic cooling of RHIC.
pacs:25.75.Cj and 25.75.Nq and 12.38.Mh and 14.40.Gx
The dissociation of quarkonia due to color-screening of their constituent quarks in a Quark-Gluon Plasma (QGP) is a classic signature for deconfinement in relativistic heavy-ion collisions colorscreen . The suppression of the various and states is determined by their binding energy and the temperature in the plasma. Results from the PHENIX experiment at RHIC show that the suppression of as a function of centrality (the number of participants) is similar to that observed by NA50 and NA60 at the CERN-SPS, even though the temperature and energy density reached in these collisions is significantly lower than at RHIC colorscreen ; RHICIIQuarkonia . This indicates that at RHIC energies additional mechanisms countering the suppression, such as recombination of charm quarks in the later stage of the collision, may play an important role; they will need to be studied systematically before conclusions from the observed suppression pattern can be drawn. In most theoretical models, the dissociation of quarkonia states is computed at rest relative to the QGP colorscreen . Recently, techniques based on the AdS/CFT duality have been utilized to study the dissociation of quark-antiquark pairs with large velocities relative to the strongly coupled QGP. Calculations in this framework show that bound states of heavy fermion pairs (an analog of quarkonium in QCD) have an effective dissociation temperature that decreases with as adscft , implying an increasing suppression, i.e. decreasing , with . To test this conjecture measurements of above GeV/ are needed where the effective dissociation temperature is expected to be lower than the temperatures reached in RHIC collisions (1.5 ) adscft ; colorscreen ; rhicwhitepapers .
Understanding production requires knowing what what fraction of s are produced from (i) gluon and heavy-quark fragmentation, from (ii) decay feed-down of B mesons and states, and (iii) what fraction originates from direct production either through a color-octet or color-singlet state. Our understanding of production mechanisms has gone through several cycles in recent decades CDF ; jpsippoverview ; belle . There is no convincing model explaining all the major features of the existing data in annihilation and hadron-hadron collisions. The color-singlet model (CSM) CDF ; jpsippoverview underpredicts the spectra by an order of magnitude in collisions at = 1.8 TeV CDF . The color octet (COM) and color evaporation model (CEM) were proposed to explain the production yields, but fail to explain the recent measurements of large spin alignment (polarization) from the same experiment CDF ; RHICIIQuarkonia . In addition, is found to be dominantly associated with open-charm pair production in annihilation at = 10.6 GeV belle . These later findings suggest that yields calculated from leading-order pQCD may not be the dominant contribution to the production. UA1 and D0 have analyzed the -hadron correlations in collisions to separate sources at high UA1 from decay without a near-side correlation and from hadron decay with a strong near-side correlation UA1 .
In this paper we report on the measurement of production at midrapidity with the STAR experiment in + and Cu+Cu collisions at = 200 GeV with transverse momenta in the range of 0.5-14 GeV/ and 5-8 GeV/, respectively. In addition, -hadron correlations, originally proposed and studied by UA1 UA1 are presented. The technique used is analog to that deployed in hadron-hadron correlations studies in STAR fuqiang . We will also briefly report on the status of measurements of the in + and Au+Au collisions and present projections of yields that will become available after completion of the STAR detector upgrades and the increase of RHIC luminosity that will be achieved once stochastic cooling for ion beams is implemented.
In the analyses reported here, the and are reconstructed through their decays into electron pairs, . The large acceptance of the tracking system in the STAR Time Projection Chamber (TPC) stardetector and the electron trigger capability from the Barrel Electromagnetic Calorimeter (BEMC) stardetector with are very well suited for such analyses. At STAR, both the TPC and BEMC can provide electron identification stardetector . At high , the BEMC is a very powerful tool for electron identification and can be used for online triggering to enrich the electron sample. At moderate , the TPC provides sufficient energy-loss () resolution to identify electrons efficiently.
Various online trigger schemes were deployed to maximize the recorded yield. The low- trigger is based on two spatially separated BEMC towers with GeV. A higher level trigger makes a final selection based on the approximated invariant mass of the pair. However, this trigger can only be efficiently used in + collisions; in A+A collisions this trigger does not provide sufficient discrimination power due to the high hit occupancy in the BEMC. The high- trigger requires only one tower above a given high-energy threshold. For the data presented here the thresholds were: GeV (+ run 5), GeV (Cu+Cu run 5), and GeV (pp run 6), respectively. The highly efficient trigger is based on a single high-tower signal ( GeV) and a subsequent pair selection with invariant mass cut performed in an online high-level trigger system using the full BEMC tower data.
For the measurement the sampled luminosities were 9 pb in + collisions and 12 pb (+ equivalent) in Au+Au collisions. The low- is from a dataset with 0.4 pb of + luminosity. The data used for the high- analysis was recorded during the + and Cu+Cu runs in 2005 and the + run in 2006 using the trigger setup described above in coincidence with a minimum bias trigger which required a coincidence between the two Zero Degree Calorimeters (ZDCs). The integrated luminosity was 2.8 (11.3) pb for + collisions collected in year 2005 (2006), and 860 b (3 pb + equivalent) for Cu+Cu collisions. In Cu+Cu data, the most central 0-60% of the total hadronic cross-section was selected by using the uncorrected charged particle multiplicity at mid-rapidity () stardetector ; rhicwhitepapers .
For the high- analysis, we required one high- electron identified with the combination of BEMC tower energy, shower shape (from shower-max detectors embedded in the BEMC), and measured in the TPC. The cut for the second electron was substantially lower and only in the TPC was used for identification.
Figure 1 shows the invariant mass spectra for the high- dielectron sample in + collisions from runs 5 and 6 (2005 and 2006) and in Cu+Cu collisions (run 5). The combinatorial background, derived from like-sign pairs, is depicted by dashed lines. The signal is extracted in the mass window GeV/. Due to the high- cuts used in this study the signal-to-background ratio (S/B) is exceptionally large. In + collisions we obtain S/B = 22/2 (40/14) for run 5 (6) and 17/23 in Cu+Cu collisions. The coverage in + and Cu+Cu collisions taken in run 5 is GeV/. For + collisions in run 6 the larger integrated luminosity, the larger BEMC coverage, and optimized trigger thresholds allow us to extend the reach to 14 GeV/. Figure 2 shows the fully corrected invariant cross-section as a function of in + collisions for all three datasets. Our spectrum is in good agreement with results from the PHENIX experiment, which is plotted for comparison phenixpp .
Figure 3 shows the nuclear modification factors as a function of in 0-20% and 0-60% Cu+Cu collisions calculated from PHENIX phenixcucu and STAR measurements. The data suggest that is rising towards unity for GeV/, although the large errors currently preclude strong conclusions. A combined fit to all high data points above 5 GeV/ gives , consistent with unity and higher than that at low- (). This result is in contradiction with expectations from AdS/CFT-based adscft and Two-Component-Approach tca models that predict a decrease in with increasing . A similar trend was also observed by NA60 Collaboration in In+In collisions at GeV na60 . There, however, reaches unity at considerably smaller than at RHIC, suggesting that the effects are most likely of a different physics origin. Our results could indicate that other production mechanisms that counter the suppression such as recombination and formation time effects csmraa play an increasingly dominant role at higher . The small suppression of the at high- stands in contrast to a substantial suppression of open charm production at similar charmsuppression .
The large S/B ratio of the high- data in + collisions allows us, for the first time at RHIC, to study -hadron correlations, which can potentially provide important constraints on the underlying production mechanisms. Figure 4 shows the azimuthal angle correlations per between s with GeV/ and charged hadrons with GeV/. No significant yield for near-side correlations () is observed. This is in stark contrast to the case of hadron-hadron correlations fuqiang .
Monte-Carlo simulations (PYTHIA 6.3.19) PYTHIA show a strong near-side correlation dominantly due to the feed-down of from -meson decays UA1 , . Thus, the comparison of the measured near-side yields with PYTHIA simulations allows one to infer the fraction of s originating from meson decays that contribute to the observed yield, albeit in a model dependent way. Assuming no other contribution to the near-side, we obtain an upper limit of of the -meson feed-down to the inclusive cross-section at GeV/. The shape on the near-side is not matched well. A detailed comparison of the spectral shapes is not currently possible due to the limited statistics. This will requires data samples which we hope to collect in the upcoming RHIC run.
It is worth noting that the direct comparison of our inclusive high- spectra with the feed-down spectra from pQCD calculations provides less constraints than the method described above. Using NLO predictions for meson production bcx_pQCD and decay parameter from measurements by the CLEO collaboration BDecay_CLEO we obtain an upper limit for the meson feed-down fraction of for GeV/. This is in large parts due to mass and scale uncertainties in the NLO calculations RHICIIQuarkonia . Further studies of -hadron correlation and cross-section with higher statistics will allow us to constrain the cross-section substantially in the future.
Figure 5 shows a projection of for an integrated luminosity of 5 nb for Au+Au collisions and 200 pb for + collisions with an upgraded STAR detector. The key upgrades are: (i) the full barrel time-of-flight detector to improve the electron identification in the low- region ( c) and (ii) the upgrade of the TPC readout electronics in conjunction with an upgrade of the data acquisition system which will increase our data-taking rate tenfold and eliminate dead-time losses for rare triggers. This will allow STAR to make full use of the increased luminosity of RHIC once the stochastic beam cooling is in place.
In addition, with the improved electron identification capabilities and higher data-taking rate the measurement of elliptic flow becomes feasible. Simulations show that with the statistics of minimum bias Au+Au events we will be able to measure with statistical error. This will provide stringent tests of quark coalescence and charm flow RHICIIQuarkonia .
Many of the difficulties that plague the interpretation of the observed charmonium suppression can be avoided when studying bottomonia. Due to the small production cross-section, recombination effects become negligible and absorption by hadronic co-moving matter is unimportant Lin:2000ke . The small cross-section, however, also makes bottomonium states extremely difficult to measure. While the is predicted to be not suppressed at RHIC and even LHC energies, the (2S) ((3S)) state is assumed to dissociate at temperatures similar to that of the (). The separation of the three states, while possible in STAR, requires substantial statistics which will only become available with the the RHIC luminosity upgrades. Detailed simulation and projections can be found in RHICIIQuarkonia .
The STAR experiment already reported on the first RHIC measurement of the (1S+2S+3S) cross-section at mid-rapidity in + collisions at GeV. We find = 91 28 (stat.) 22 (syst.) pb, which is consistent with the world data and NLO pQCD calculations in the CEM Das:2008nr . The first ever measurements of mesons in Au+Au collisions at GeV are underway. We observe a stable signal that will allow us to get first information on the nuclear modification factor of the . This will be complemented by measurements in d+Au collisions taken in 2008.
In summary, we report measurements of spectra from 200 GeV + up to transverse momenta of 14 GeV/ and from minimum bias Cu+Cu collisions at high ( GeV/) at mid-rapidity. The nuclear modification factor in Cu+Cu at 5 GeV/ is and is about above the values at low measured by PHENIX phenixcucu . The study of -hadron azimuthal correlations show an absence of near-side correlations. Using PYTHIA simulations we derive an upper limit for the fraction of feed-down of mesons from meson decays of at GeV/. Near future upgrades to the STAR detector and the RHIC machine will allow us to make substantial contributions to the understanding of quarkonia production and provide detailed information of their interaction with the hot dense matter created at RHIC.
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