We report the measurement of $K^{*0}$ meson at midrapidity ($|y|<$ 1.0) in Au+Au collisions at $\sqrt{s_{\rm NN}}$~=~7.7, 11.5, 14.5, 19.6, 27 and 39 GeV collected by the STAR experiment during the RHIC beam energy scan (BES) program. The transverse momentum spectra, yield, and average transverse momentum of $K^{*0}$ are presented as functions of collision centrality and beam energy. The $K^{*0}/K$ yield ratios are presented for different collision centrality intervals and beam energies. The $K^{*0}/K$ ratio in heavy-ion collisions are observed to be smaller than that in small system collisions (e+e and p+p). The $K^{*0}/K$ ratio follows a similar centrality dependence to that observed in previous RHIC and LHC measurements. The data favor the scenario of the dominance of hadronic re-scattering over regeneration for $K^{*0}$ production in the hadronic phase of the medium.
$p_{\mathrm T}$-differential yield of $\mathrm{K^{*0}} + \bar{\mathrm{K^{*0}}}$ in AuAu collisions at $\sqrt{s_{\mathrm{NN}}}~=~$7.7 GeV (Multiplicity class 0-20%).
$p_{\mathrm T}$-differential yield of $\mathrm{K^{*0}} + \bar{\mathrm{K^{*0}}}$ in AuAu collisions at $\sqrt{s_{\mathrm{NN}}}~=~$7.7 GeV (Multiplicity class 20-40%).
$p_{\mathrm T}$-differential yield of $\mathrm{K^{*0}} + \bar{\mathrm{K^{*0}}}$ in AuAu collisions at $\sqrt{s_{\mathrm{NN}}}~=~$7.7 GeV (Multiplicity class 40-60%).
Vector mesons may be photoproduced in relativistic heavy-ion collisions when a virtual photon emitted by one nucleus scatters from the other nucleus, emerging as a vector meson. The STAR Collaboration has previously presented measurements of coherent $\rho^0$ photoproduction at center of mass energies of 130 GeV and 200 GeV in AuAu collisions. Here, we present a measurement of the cross section at 62.4 GeV; we find that the cross section for coherent $\rho^0$ photoproduction with nuclear breakup is $10.5\pm1.5\pm 1.6$ mb at 62.4 GeV. The cross-section ratio between 200 GeV and 62.4 GeV is $2.8\pm0.6$, less than is predicted by most theoretical models. It is, however, proportionally much larger than the previously observed $15\pm 55$% increase between 130 GeV and 200 GeV.
Acceptance corrected invariant mass distributions for the coherently produced $\rho^0$ candidates collected with trigger A (left) and B (right). The fit function (solid) encompasses the Breit-Wigner (dashed), the mass independent contribution from direct $\pi^+\pi^-$ production (dash-dotted), and the interference term (dotted). The hatched area is the contribution from the combinatorial background. The statistical errors are shown.
Acceptance corrected invariant mass distributions for the coherently produced $\rho^0$ candidates collected with trigger A (left) and B (right). The fit function (solid) encompasses the Breit-Wigner (dashed), the mass independent contribution from direct $\pi^+\pi^-$ production (dash-dotted), and the interference term (dotted). The hatched area is the contribution from the combinatorial background. The statistical errors are shown.
Transverse momentum distribution of the $\rho^0$ candidates (open distribution) overlaid by the combinatorial background estimated with like-sign pairs (not corrected to the acceptance and reconstruction efficiency) and scaled to match in the high transverse momentum region, $p_T$ ≥ 250 MeV/$c$ (hatched distribution). The plot is based on the dataset collected with trigger B.
We have measured direct photon production in pp collisions at the CERN intersecting storage rings for c.m. energies 31 < √ s < 63 GeV and transverse momenta up to 9GeV/ c , using segmented lead/liquid-argon calorimeters. The ratio of direct photon to π 0 production is significantly larger than zero, starting at p T ≈ 4 GeV/ c and increasing to values of about 0.4 at 9GeV/ c . No significant √s dependence is seen.
No description provided.
The inclusive cross section for larger p T π 0 production near 90° in p-p collisions at the CERN ISR is presented for centre-of-mass energies 30.7, 53.1 and 62.4 GeV. The data are inconsistent with scaling of the form p T − n F ( x T ), with constant n or with n allowed to depend on x T = 2p T / s . For s = 53.1 and 62.4 GeV , the value of n found for 3.5 < p T < 7.0 GeV/ c is n = 8.0 ± 0.5, in agreement with previous experiments. However, for 7.5 < p T < 14.0 GeV/ c the value becomes n = 5.1 ± 0.4.
No description provided.
No description provided.
No description provided.
An apparatus consisting of a superconducting solenoid magnet, cylindrical drift-chambers, and two arrays of lead-glass Čerenkov counters has been used at the CERN ISR to study the production of e + e − pairs of invariant mass above 6.5 GeV/ c 2 . Cross sections for the continuum and the ϒ family of resonances are presented, as well as the mean transverse momentum 〈 p T 〉 of the electron-positron pairs in the continuum and resonance region.
No description provided.
We present a measurement of the total cross section σ t in proton-proton collisions at the CERN ISR. The method involves determination of the total interaction rate and machine luminosity. A two-arm scintillation hodoscope observes ∼ 90% of the total interaction rate, while a streamer chamber is employed for event topologies missed by the main trigger. An increase of about 10% in σ t is observed in the energy range √ s = 23.6 to √ s = 62.8 GeV/ c in agreement with previous experiments.
VAN DER MEER METHOD.
The inclusive production of π and η mesons at 90° from proton-proton collisions has been measured at the CERN ISR at centre-of-mass energies between 23.5 and 62.4 GeV. The momentum correlation of charged particles emitted together with a large transverse momentum pion has also been studied, using two magnetic spectrometers each centred at 90°.
No description provided.
No description provided.
No description provided.
The inclusive cross-section for π0 production near 90° inpp collisions at the CERN Intersecting Storage Rings has been studied for thepT range 3<pT<16GeV/c at four different centre-of-mass energies (\(\sqrt s = 30.6\), 44.8, 52.7, and 62.8 GeV). In this experiment the two photons from the π0→yy decay were resolved and measured separately forpT values up to 10 GeV/c. Results indicate an agreement with thepT−8 behaviour for the lower values ofpT and a slower decrease of the cross-section for the higher values ofpT. The high-pT data deviate from the scaling expressionpT−nF(xT), which holds for the lowerpT values (pT<8GeV/c).
USING RETRACTED GEOMETRY.
USING SUPER-RETRACTED GEOMETRY.
USING SUPER-RETRACTED GEOMETRY.
The helicity structure of the diffractive electroproduction of rho mesons, e + p -> e + rho + Y, is studied in a previously unexplored region of large four-momentum transfer squared at the proton vertex, t: 0 < t' < 3 GeV^2, where t' = |t| - |t|_min. The data used are collected with the H1 detector at HERA in the kinematic domain 2.5 < Q^2 < 60 GeV^2, 40 < W < 120 GeV. No t dependence of the r^04_00 spin density matrix element is found. A significant t dependent helicity non-conservation from the virtual photon to the rho meson is observed for the spin density matrix element combinations r^5_00+2r^5_11 and r^1_00+2r^1_11. These t dependences are consistently described by a perturbative QCD model based on the exchange of two gluons.
Measurements of the combinations of spin density matrices as a function of TP = |T| - |Tmin|, where |Tmin| is the minimal values of |T| kinematically required for the vector meson and the system X to aquire their effective mass through longitudinal momentum transfer.
Infrared and collinear safe event shape distributions and their mean values are determined in e+e- collisions at centre-of-mass energies between 45 and 202 GeV. A phenomenological analysis based on power correction models including hadron mass effects for both differential distributions and mean values is presented. Using power corrections, alpha_s is extracted from the mean values and shapes. In an alternative approach, renormalisation group invariance (RGI) is used as an explicit constraint, leading to a consistent description of mean values without the need for sizeable power corrections. The QCD beta-function is precisely measured using this approach. From the DELPHI data on Thrust, including data from low energy experiments, one finds beta_0 = 7.86 +/- 0.32 for the one loop coefficient of the beta-function or, assuming QCD, n_f = 4.75 +/- 0.44 for the number of active flavours. These values agree well with the QCD expectation of beta_0=7.67 and n_f=5. A direct measurement of the full logarithmic energy slope excludes light gluinos with a mass below 5 GeV.
1-THRUST distribution.
THRUST-MAJOR distribution.
THRUST-MINOR distribution.
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