Nuclear collisions recreate conditions in the universe microseconds after the Big Bang. Only a very small fraction of the emitted fragments are light nuclei, but these states are of fundamental interest. We report the observation of antihypertritons
- composed of an antiproton, antineutron, and antilambda hyperon - produced by colliding gold nuclei at high energy. Our analysis yields 70 +- 17 antihypertritons and 157 +- 30 hypertritons. The measured yields of hypertriton (antihypertriton) and helium3 (antihelium3) are similar, suggesting an equilibrium in coordinate and momentum space populations of up, down, and strange quarks and antiquarks, unlike the pattern observed at lower collision energies. The production and properties of antinuclei, and nuclei containing strange quarks, have implications spanning nuclear/particle physics, astrophysics, and cosmology.
The STAR Collaboration at RHIC presents a systematic study of high transverse momentum charged di-hadron correlations at small azimuthal pair separation dphino, in d+Au and central Au+Au collisions at $rts = 200$ GeV. Significant correlated yield for
pairs with large longitudinal separation deta is observed in central Au+Au, in contrast to d+Au collisions. The associated yield distribution in detano$times$dphi can be decomposed into a narrow jet-like peak at small angular separation which has a similar shape to that found in d+Au collisions, and a component which is narrow in dphi and textcolor{black}{depends only weakly on} $deta$, the ridge. Using two systematically independent analyses, textcolor{black}{finite ridge yield} is found to persist for trigger $pt > 6$ GeVc, indicating that it is correlated with jet production. The transverse momentum spectrum of hadrons comprising the ridge is found to be similar to that of bulk particle production in the measured range ($2 < pt < 4 GeVc$).
We present results for the measurement of $phi$ meson production via its charged kaon decay channel $phi to K^+K^-$ in Au+Au collisions at $sqrt{s_{_{NN}}}=62.4$, 130, and 200 GeV, and in $p+p$ and $d$+Au collisions at $sqrt{s_{_{NN}}}=200$ GeV from
the STAR experiment at the BNL Relativistic Heavy Ion Collider (RHIC). The midrapidity ($|y|<0.5$) $phi$ meson transverse momentum ($p_{T}$) spectra in central Au+Au collisions are found to be well described by a single exponential distribution. On the other hand, the $p_{T}$ spectra from $p+p$, $d$+Au and peripheral Au+Au collisions show power-law tails at intermediate and high $p_{T}$ and are described better by Levy distributions. The constant $phi/K^-$ yield ratio vs beam species, collision centrality and colliding energy is in contradiction with expectations from models having kaon coalescence as the dominant mechanism for $phi$ production at RHIC. The $Omega/phi$ yield ratio as a function of $p_{T}$ is consistent with a model based on the recombination of thermal $s$ quarks up to $p_{T}sim 4$ GeV/$c$, but disagrees at higher transverse momenta. The measured nuclear modification factor, $R_{dAu}$, for the $phi$ meson increases above unity at intermediate $p_{T}$, similar to that for pions and protons, while $R_{AA}$ is suppressed due to the energy loss effect in central Au+Au collisions. Number of constituent quark scaling of both $R_{cp}$ and $v_{2}$ for the $phi$ meson with respect to other hadrons in Au+Au collisions at $sqrt{s_{_{NN}}}$=200 GeV at intermediate $p_{T}$ is observed. These observations support quark coalescence as being the dominant mechanism of hadronization in the intermediate $p_{T}$ region at RHIC.
We present a systematic analysis of two-pion interferometry in Au+Au collisions at $sqrt{s_{rm{NN}}}$ = 62.4 GeV and Cu+Cu collisions at $sqrt{s_{rm{NN}}}$ = 62.4 and 200 GeV using the STAR detector at RHIC. The multiplicity and transverse momentum d
ependences of the extracted correlation lengths (radii) are studied. The scaling with charged particle multiplicity of the apparent system volume at final interaction is studied for the RHIC energy domain. The multiplicity scaling of the measured correlation radii is found to be independent of colliding system and collision energy.
We report the measurement of charged $D^*$ mesons in inclusive jets produced in proton-proton collisions at a center of mass energy $sqrt{s}$ = 200 GeV with the STAR experiment at RHIC. For $D^{*}$ mesons with fractional momenta $0.2 < z < 0.5$ in in
clusive jets with 11.5 GeV mean transverse energy, the production rate is found to be $N(D^{*+}+D^{*-})/N(mathrm{jet}) = 0.015 pm 0.008 (mathrm{stat}) pm 0.007 (mathrm{sys})$. This rate is consistent with perturbative QCD evaluation of gluon splitting into a pair of charm quarks and subsequent hadronization.
