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Register a new userTransverse-energy distributions at midrapidity in $p$$+$$p$, $d$$+$Au, and Au$+$Au collisions at $sqrt{s_{_{NN}}}=62.4$--200~GeV and implications for particle-production models
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Measurements of the midrapidity transverse energy distribution, $dEt/deta$, are presented for $p$$+$$p$, $d$$+$Au, and Au$+$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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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.
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