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Register a new user$rm{J}/psi$ production at low transverse momentum in p+p and d+Au collisions at $sqrt{s_{NN}}$ = 200 GeV
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We report on the measurement of $rm{J}/psi$ production in the dielectron channel at mid-rapidity (|y|<1) in p+p and d+Au collisions at $sqrt{s_{NN}}$ = 200 GeV from the STAR experiment at the Relativistic Heavy Ion Collider. The transverse momentum $p_{T}$ spectra in p+p for $p_{T}$ < 4 GeV/c and d+Au collisions for $p_{T}$ < 3 GeV/c are presented. These measurements extend the STAR coverage for $rm{J}/psi$ production in p+p collisions to low $p_{T}$. The $<p_{T}^{2}>$ from the measured $rm{J}/psi$ invariant cross section in p+p and d+Au collisions are evaluated and compared to similar measurements at other collision energies. The nuclear modification factor for $rm{J}/psi$ is extracted as a function of $p_{T}$ and collision centrality in d+Au and compared to model calculations using the modified nuclear Parton Distribution Function and a final-state $rm{J}/psi$ nuclear absorption cross section.
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We report $J/psi$ spectra for transverse momenta $p_T$> 5 GeV/$c$ at mid-rapidity in p+p and Au+Au collisions at sqrt(s_{NN}) = 200 GeV.The inclusive $J/psi$ spectrum and the extracted $B$-hadron feed-down are compared to models incorporating different production mechanisms. We observe significant suppression of the $J/psi$ yields for $p_T$> 5 GeV/$c$ in 0-30% Au+Au collisions relative to the p+p yield scaled by the number of binary nucleon-nucleon collisions in Au+Au collisions. In 30-60% collisions, no such suppression is observed.The level of suppression is consistently less than that of high-$p_T$ $pi^{pm}$ and low-$p_T$ $J/psi$.
Multiparticle production at high energies is described in terms of color strings stretched between the projectile and target. As string density increases, overlap among the strings leads to cluster formation. At some critical density a macroscopic cluster appears, spanning the entire system. This marks the percolation phase transition. Data from p+p, d+Au and Au+Au collisions at 200 GeV has been analyzed using the STAR detector to obtain the percolation density parameter, $eta$. For 200 GeV Au+Au collisions, the value of $eta$ is found to lie above the critical percolation threshold, while for 200 GeV d+Au collisions it is below the critical value. This supports the idea of string percolation, which at high enough string density is a possible mechanism to explore the hadronic phase transition to a quark-gluon plasma.
We report a new measurement of $D^0$-meson production at mid-rapidity ($|y|$,$<$,1) in Au+Au collisions at ${sqrt{s_{rm NN}} = rm{200,GeV}}$ utilizing the Heavy Flavor Tracker, a high resolution silicon detector at the STAR experiment. Invariant yields of $D^0$-mesons with transverse momentum $p_{T}$ $lesssim 9$,GeV/$c$ are reported in various centrality bins (0--10%, 10--20%, 20--40%, 40--60% and 60--80%). Blast-Wave thermal models are used to fit the $D^0$-meson $p_{T}$ spectra to study $D^0$ hadron kinetic freeze-out properties. The average radial flow velocity extracted from the fit is considerably smaller than that of light hadrons ($pi,K$ and $p$), but comparable to that of hadrons containing multiple strange quarks ($phi,Xi^-$), indicating that $D^0$ mesons kinetically decouple from the system earlier than light hadrons. The calculated $D^0$ nuclear modification factors re-affirm that charm quarks suffer large amount of energy loss in the medium, similar to those of light quarks for $p_{T}$,$>$,4,GeV/$c$ in central 0--10% Au+Au collisions. At low $p_{T}$, the nuclear modification factors show a characteristic structure qualitatively consistent with the expectation from model predictions that charm quarks gain sizable collective motion during the medium evolution. The improved measurements are expected to offer new constraints to model calculations and help gain further insights into the hot and dense medium created in these collisions.
The $jpsi$ $pt$ spectrum and nuclear modification factor ($raa$) are reported for $pt < 5 gevc$ and $|y|<1$ from 0% to 60% central Au+Au and Cu+Cu collisions at $snn = 200 gev$ at STAR. A significant suppression of $pt$-integrated $jpsi$ production is observed in central Au+Au events. The Cu+Cu data are consistent with no suppression, although the precision is limited by the available statistics. $raa$ in Au+Au collisions exhibits a strong suppression at low transverse momentum and gradually increases with $pt$. The data are compared to high-$pt$ STAR results and previously published BNL Relativistic Heavy Ion Collider results. Comparing with model calculations, it is found that the invariant yields at low $pt$ are significantly above hydrodynamic flow predictions but are consistent with models that include color screening and regeneration.
Measurements of the midrapidity transverse energy distribution, $dEt/deta$, are presented for $p$$+$$p$, $d$$+$Au, and Au$+$Au collisions at $sqrt{s_{_{NN}}}=200$ GeV and additionally for Au$+$Au collisions at $sqrt{s_{_{NN}}}=62.4$ and 130 GeV. The $dEt/deta$ distributions are first compared with the number of nucleon participants $N_{rm part}$, number of binary collisions $N_{rm coll}$, and number of constituent-quark participants $N_{qp}$ calculated from a Glauber model based on the nuclear geometry. For Au$+$Au, $mean{dEt/deta}/N_{rm part}$ increases with $N_{rm part}$, while $mean{dEt/deta}/N_{qp}$ is approximately constant for all three energies. This indicates that the two component ansatz, $dE_{T}/deta propto (1-x) N_{rm part}/2 + x N_{rm coll}$, which has been used to represent $E_T$ distributions, is simply a proxy for $N_{qp}$, and that the $N_{rm coll}$ term does not represent a hard-scattering component in $E_T$ distributions. The $dE_{T}/deta$ distributions of Au$+$Au and $d$$+$Au are then calculated from the measured $p$$+$$p$ $E_T$ distribution using two models that both reproduce the Au$+$Au data. However, while the number-of-constituent-quark-participant model agrees well with the $d$$+$Au data, the additive-quark model does not.
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