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Register a new userSearch for dark photons from neutral meson decays in $p$$+$$p$ and $d$$+$Au collisions at $sqrt{s_{_{NN}}}$=200 GeV
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The standard model (SM) of particle physics is spectacularly successful, yet the measured value of the muon anomalous magnetic moment $(g-2)_mu$ deviates from SM calculations by 3.6$sigma$. Several theoretical models attribute this to the existence of a dark photon, an additional U(1) gauge boson, which is weakly coupled to ordinary photons. The PHENIX experiment at the Relativistic Heavy Ion Collider has searched for a dark photon, $U$, in $pi^0,eta rightarrow gamma e^+e^-$ decays and obtained upper limits of $mathcal{O}(2times10^{-6})$ on $U$-$gamma$ mixing at 90% CL for the mass range $30<m_U<90$ MeV/$c^2$. Combined with other experimental limits, the remaining region in the $U$-$gamma$ mixing parameter space that can explain the $(g-2)_mu$ deviation from its SM value is nearly completely excluded at the 90% confidence level, with only a small region of $29<m_U<32$ MeV/$c^2$ remaining.
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The PHENIX experiment has measured $phi$ meson production in $d$$+$Au collisions at $sqrt{s_{_{NN}}}=200$ GeV using the dimuon and dielectron decay channels. The $phi$ meson is measured in the forward (backward) $d$-going (Au-going) direction, $1.2<y<2.2$ ($-2.2<y<-1.2$) in the transverse-momentum ($p_T$) range from 1--7 GeV/$c$, and at midrapidity $|y|<0.35$ in the $p_T$ range below 7 GeV/$c$. The $phi$ meson invariant yields and nuclear-modification factors as a function of $p_T$, rapidity, and centrality are reported. An enhancement of $phi$ meson production is observed in the Au-going direction, while suppression is seen in the $d$-going direction, and no modification is observed at midrapidity relative to the yield in $p$$+$$p$ collisions scaled by the number of binary collisions. Similar behavior was previously observed for inclusive charged hadrons and open heavy flavor indicating similar cold-nuclear-matter effects.
We report $e^pm-mu^mp$ pair yield from charm decay measured between midrapidity electrons ($|eta|<0.35$ and $p_T>0.5$ GeV/$c$) and forward rapidity muons ($1.4<eta<2.1$ and $p_T>1.0$ GeV/$c$) as a function of $Deltaphi$ in both $p$$+$$p$ and in $d$+Au collisions at $sqrt{s_{_{NN}}}=200$ GeV. Comparing the $p$$+$$p$ results with several different models, we find the results are consistent with a total charm cross section $sigma_{cbar{c}} =$ 538 $pm$ 46 (stat) $pm$ 197 (data syst) $pm$ 174 (model syst) $mu$b. These generators also indicate that the back-to-back peak at $Deltaphi = pi$ is dominantly from the leading order contributions (gluon fusion), while higher order processes (flavor excitation and gluon splitting) contribute to the yield at all $Deltaphi$. We observe a suppression in the pair yield per collision in $d$+Au. We find the pair yield suppression factor for $2.7<Deltaphi<3.2$ rad is $J_{dA}$ = 0.433 $pm$ 0.087 (stat) $pm$ 0.135 (syst), indicating cold nuclear matter modification of $cbar{c}$ pairs.
The PHENIX experiment has measured direct photons at $sqrt{s_{NN}}$ = 200 GeV in $p+p$, $d$+Au and Au+Au collisions. For $p_{T}$ $<$ 4 GeV/$c$, the internal conversion into $e^{+}e^{-}$ pairs has been used to measure the direct photons in Au+Au.
Azimuthal angular correlations of charged hadrons with respect to the axis of a reconstructed (trigger) jet in Au+Au and p+p collisions at $sqrt{s_{text{NN}}} = 200 text{GeV}$ in STAR are presented. The trigger jet population in Au+Au collisions is biased towards jets that have not interacted with the medium, allowing easier matching of jet energies between Au+Au and p+p collisions while enhancing medium effects on the recoil jet. The associated hadron yield of the recoil jet is significantly suppressed at high transverse momentum ($p_{text{T}}^{text{assoc}}$) and enhanced at low $p_{text{T}}^{text{assoc}}$ in 0-20% central Au+Au collisions compared to p+p collisions, which is indicative of medium-induced parton energy loss in ultrarelativistic heavy-ion collisions.
We report the measurements of $Sigma (1385)$ and $Lambda (1520)$ production in $p+p$ and $Au+Au$ collisions at $sqrt{s_{NN}} = 200$ GeV from the STAR collaboration. The yields and the $p_{T}$ spectra are presented and discussed in terms of chemical and thermal freeze-out conditions and compared to model predictions. Thermal and microscopic models do not adequately describe the yields of all the resonances produced in central $Au+Au$ collisions. Our results indicate that there may be a time-span between chemical and thermal freeze-out during which elastic hadronic interactions occur.
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