Neutron angular distributions from the charge-exchange (π0n) and inelastic modes (π0π0n,π+π−n) of the π−−p interaction have been investigated at 313 and 371 MeV incident-pion kinetic energy. The data were obtained with an electronic counter system. Elastic and inelastic neutrons were separated in the all-neutral final states by time of flight. At both energies the charge-exchange differential cross section at the forward neutron angles differs from that determined by Caris et al. from measurements of the π0-decay gamma distributions, but generally agrees with the phase-shift-analysis calculations of Roper. The distribution of inelastic neutrons from both modes shows a strong preference for low center-of-mass neutron energies. The distribution of these neutrons does not correspond to that expected from the I=0, π−π interaction (ABC effect) suggested to account for the anomaly in p−d collisions observed by Abashian et al. Finally, all available charge-exchange differential-cross-section data from this and other experiments were combined by at least-squares fit to a Legendre expansion of the form dσdΩ*(cosθπ0*)=Σl=0NalPl(cosθπ0*) with the following results (in mb/sr):
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Measurements of the ratio (R) of positron-proton and electron-proton elastic-scattering cross sections have been made, with the square of the four-momentum transfer (q2) equal to 0.20, 0.69, 0.73, 1.54, 2.44, 3.27, 3.79, and 5.00 (GeV/c)2. The measurements, after radiative corrections, are consistent with R=1, with standard errors ranging from ±0.016 to ±0.123. The results give limits for the size of the two-photon effects.
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Results are presented of an untagged e + e − → e + e − + π + π − experiment performed at PEP with the DELCO detector. In the invariant-mass range 0.7 ⩽ W ππ < 2.0 GeV/ c 2 , the QED e + e − background is identified and eliminated, and both the π + π − predictions and the μ + μ − and K + K − background substractions are normalized to the measurement of the e e + e − events. The results agree with a simple model of superposition and interference of the f 0 (1270) resonance, produced with helicity 2, with a Born-term continuum. From a fit of the model to the data, the radiative width of the f 0 is determined to be Γ f 0 → γγ = 2.70 ± 0.21 keV.
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We have studied inclusive D*± production using the DELCO detector at PEP. Our technique involved kaon identification in the momentum range above 3.2 GeV/c using a threshold gas Čerenkov counter. This leads to a model-independent upper limit on D0−D¯0 mixing of 8.1% (90% confidence level). We also have measured the charm fragmentation function, which peaks at x≡PD*(Ebeam2−MD*2)12 of 0.56±0.06(stat.), and the total cross section for D* production, σ(D*±)=0.140±0.021(stat.)±0.032(syst.) nb (x>0.3, with radiative correction).
SYSTEMATIC ERROR DOES NOT INCLUDE THE UNCERTAINTY ON THE BRANCHING RATIOS USED.
Electron-proton elastic-scattering cross sections have been measured at the Stanford Linear Accelerator Center for four-momentum transfers squared q 2 from 1.0 to 25.0 (GeVc)2. The electric (GEp) and magnetic (GMp) form factors of the proton were not separated, since angular distributions were not measured at each q 2. However, values for GMp were derived assuming various relations between GEp and GMp. Several theoretical models for the behavior of the proton magnetic form factor at high values of q 2 are compared with the data.
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Electron-proton elastic scattering cross sections have been measured at the Stanford Linear Accelerator Center at four-momentum transfers squared (q 2 ) of 1.0, 1.5, 2.0, 2.5and 3.75 (GeV/ c ) 2 . The angular distributions at q 2 = 2.5 and 3.75 (GeV/ c ) 2 are sufficient to provide values of the ratio G E / G M independent of the results from other laboratories. Our results are compatible with scaling, G E (q 2 ) = G M (q 2 )/ μ , within the experimental errors.
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We present a measurement of inclusive $J/\psi$ production at mid-rapidity ($|y|<1$) in $p+p$ collisions at a center-of-mass energy of $\sqrt{s}$ = 200 GeV with the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The differential production cross section for $J/\psi$ as a function of transverse momentum ($p_T$) for $0
Top$:$ J/$\psi$ cross section times branching ratio as a function of pT in p+p collisions at $\sqrt{s_{NN}}$ = 200 GeV. Solid circles are from this analysis for |y| < 1; open circles and blue squares are the published results for |y| < 1 from STAR; triangles are the published results for |y| < 0.35 from PHENIX. Bars and boxes are statistical and systematic uncertainties, respectively. The curves are CEM (green), NLO NRQCD A (orange) [4], CGC + NRQCD (blue) , and NLO NRQCD B (magenta) theoretical calculations, respectively. Bottom$:$ ratios of these results with respect to the central value from this analysis.
The corrected $n_{ch}$ distributions at mid-rapidity (|$\eta$| < 1) for MB events (open circles) and J/$\psi$ events with J/$\psi$ $p_{T}$ greater than 0 (purple circles), 1.5 (blue squares), and 4 GeV/c (red triangles) in p+p collisions at $\sqrt{s}$ = 200 GeV. The fit function is a negative binomial function. Bars and boxes are statistical and systematic uncertainties, respectively.
The multiplicity dependence of J/$\psi$ production in p+p collisions at $\sqrt{s}$ = 200 GeV. Purple circles, blue squares, and red triangles represent the results for J/$\psi$ with $p_{T}$ greater than 0, 1.5, and 4 GeV/c, respectively. Bars and open boxes are statistical and systematic uncertainties, respectively. The ALICE result is shown in the left panel. The purple, blue and red bands in the middle panel are generated from PYTHIA8 for J/$\psi$ with $p_{T}$ greater than 0, 1.5, and 4 GeV/c, respectively. The blue and red bands in the right panel are from EPOS3 model calculations for D$^{0}$ with 2 < $p_{T}$ < 4 and 4 < $p_{T}$ < 8 GeV/c, respectively, while the green curve is from the Percolation model for J/$\psi$ with $p_{T}$ > 0 GeV/c.
We report precision measurements of hypernuclei ${}^3_\Lambda \rm{H}$ and ${}^4_\Lambda \rm{H}$ lifetimes obtained from Au+Au collisions at \snn = 3.0 GeV and 7.2 GeV collected by the STAR experiment at RHIC, and the first measurement of ${}^3_\Lambda \rm{H}$ and ${}^4_\Lambda \rm{H}$ mid-rapidity yields in Au+Au collisions at \snn = 3.0 GeV. ${}^3_\Lambda \rm{H}$ and ${}^4_\Lambda \rm{H}$, being the two simplest bound states composed of hyperons and nucleons, are cornerstones in the field of hypernuclear physics. Their lifetimes are measured to be $221\pm15(\rm stat.)\pm19(\rm syst.)$ ps for ${}^3_\Lambda \rm{H}$ and $218\pm6(\rm stat.)\pm13(\rm syst.)$ ps for ${}^4_\Lambda \rm{H}$. The $p_T$-integrated yields of ${}^3_\Lambda \rm{H}$ and ${}^4_\Lambda \rm{H}$ are presented in different centrality and rapidity intervals. It is observed that the shape of the rapidity distribution of ${}^4_\Lambda \rm{H}$ is different for 0--10% and 10--50% centrality collisions. Thermal model calculations, using the canonical ensemble for strangeness, describes the ${}^3_\Lambda \rm{H}$ yield well, while underestimating the ${}^4_\Lambda \rm{H}$ yield. Transport models, combining baryonic mean-field and coalescence (JAM) or utilizing dynamical cluster formation via baryonic interactions (PHQMD) for light nuclei and hypernuclei production, approximately describe the measured ${}^3_\Lambda \rm{H}$ and ${}^4_\Lambda \rm{H}$ yields. Our measurements provide means to precisely assess our understanding of the fundamental baryonic interactions with strange quarks, which can impact our understanding of more complicated systems involving hyperons, such as the interior of neutron stars or exotic hypernuclei.
The measured $^{3}_{\Lambda}$H and $^{4}_{\Lambda}$H lifetimes from STAR (2021)
B.R. times dN/dy of $^{3}_{\Lambda}$H vs y in 3 GeV 0-10% Au+Au collisions
B.R. times dN/dy of $^{4}_{\Lambda}$H vs y in 3 GeV 0-10% Au+Au collisions
The two-particle angular correlation functions, $R_2$, of pions, kaons, and protons in Au+Au collisions at $\sqrt{s_{NN}}=$ 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4, and 200 GeV were measured by the STAR experiment at RHIC. These correlations were measured for both like-sign and unlike-sign charge combinations and versus the centrality. The correlations of pions and kaons show the expected near-side ({\it i.e.}, at small relative angles) peak resulting from short-range mechanisms. The amplitudes of these short-range correlations decrease with increasing beam energy. However, the proton correlation functions exhibit strong anticorrelations in the near-side region. This behavior is observed for the first time in an A+A collision system. The observed anticorrelation is $p_{T}$-independent and decreases with increasing beam energy and centrality. The experimental results are also compared to the Monte Carlo models UrQMD, Hijing, and AMPT.
Angular correlation function R2(∆y,∆φ) of like-sign pions in Au+Au collisions at mid centrality 30%-40% and 0.2 < pT < 2.0 GeV/c at 7.7 GeV
Angular correlation function R2(∆y,∆φ) of like-sign pions in Au+Au collisions at mid centrality 30%-40% and 0.2 < pT < 2.0 GeV/c at 11.5 GeV
Angular correlation function R2(∆y,∆φ) of like-sign pions in Au+Au collisions at mid centrality 30%-40% and 0.2 < pT < 2.0 GeV/c at 14.5 GeV
We report new STAR measurements of the single-spin asymmetries $A_L$ for $W^+$ and $W^-$ bosons produced in polarized proton--proton collisions at $\sqrt{s}$ = 510 GeV as a function of the decay-positron and decay-electron pseudorapidity. The data were obtained in 2013 and correspond to an integrated luminosity of 250 pb$^{-1}$. The results are combined with previous results obtained with 86 pb$^{-1}$. A comparison with theoretical expectations based on polarized lepton-nucleon deep-inelastic scattering and prior polarized proton--proton data suggests a difference between the $\bar{u}$ and $\bar{d}$ quark helicity distributions for $0.05 < x < 0.25$. In addition, we report new results for the double-spin asymmetries $A_{LL}$ for $W^\pm$, as well as $A_L$ for $Z/\gamma^*$ production and subsequent decay into electron--positron pairs.
Distribution of the product of Q, the TPC recon-structed charge-sign, and $E_T/p_T$ in the BEMC region.
Distribution of the product of Q, the TPC recon-structed charge-sign, and $E_T/p_T$ in the EEMC region.
$E_T^e$ distributions of electron candidate events, background contributions, and sum of backgrounds and $W \rightarrow e\nu$ MC signal in the BEMC region for $-1.1 < \eta_e < -0.5$.
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