The production cross section of inclusive J/$\psi$ pairs in pp collisions at a centre-of-mass energy $\sqrt{s} = 13$ TeV is measured with ALICE. The measurement is performed for J/$\psi$ in the rapidity interval $2.5 < y < 4.0$ and for transverse momentum $p_{\rm T} > 0$. The production cross section of inclusive J/$\psi$ pairs is reported to be $10.3 \pm 2.3 {\rm (stat.)} \pm 1.3 {\rm (syst.)}$ nb in this kinematic interval. The contribution from non-prompt J/$\psi$ (i.e. originated from beauty-hadron decays) to the inclusive sample is evaluated. The results are discussed and compared with data.
The cross section for K + meson production in collisions of 36 Ar ions on a 48 Ti target has been measured at an incident energy of 92 MeV per nucleon. A description of the experimental set-up is given. Twelve events attributed to monoenergetic muons following the decay of stopped kaons have been identified. From these events, one infers a production cross section of 240 pb. Data are briefly discussed.
The TPC/Two-Gamma Collaboration has measured the inclusive cross section for production of charmed D ∗± mesons in photon-photon collisions. The reaction utilized was e + e - →e + e - D ∗± X, with D ∗± →D O π +- , D O →K -+ π ± , and either zero or one outgoing e ± detected. The result, σ(e + e - → e + e - D ∗± X) = 74±26±19 pb , is in agreement with the quark parton mo del prediction for e + e - → e + e - c c , combined with a Lund model for the hadronization of the charmed quarks.
Data are presented on the reaction pp → pX in the range of four-momentum transfer squared 0.04< − t <0.80 GeV 2 and of c.m. energy squared 550 < s < 3880 GeV 2 . Invariant cross sections are given as a function of M 2 / s , where M is the mass of the missing system X, and of t . The cross sections are shown to scale in the variable M 2 / s , for M 2 / s > 0.01. The total diffractive cross section integrated over t and M 2 / s up to M 2 / s =0.05 rises by approximately 15% from σ dif =6.5±0.2 mb at 550 GeV 2 to σ dif =7.5±0.3 mb at 3880 GeV 2 .
A sample of Lambda's produced in 2 A*GeV Ni + Cu collisions has been obtained with the EOS Time Projection Chamber at the Bevalac. Low background in the invariant mass distribution allows for the unambiguous demonstration of Lambda directed flow. The transverse mass spectrum at mid-rapidity has the characteristic shoulder-arm shape of particles undergoing radial transverse expansion. A linear dependence of Lambda multiplicity on impact parameter is observed, from which a total Lambda + Sigma^0 production cross section of $112 +/- 24 mb is deduced. Detailed comparisons with the ARC and RVUU models are made.
The production of neutrons carrying at least 20% of the proton beam energy ($\xl > 0.2$) in $e^+p$ collisions has been studied with the ZEUS detector at HERA for a wide range of $Q^2$, the photon virtuality, from photoproduction to deep inelastic scattering. The neutron-tagged cross section, $e p\to e' X n$, is measured relative to the inclusive cross section, $e p\to e' X$, thereby reducing the systematic uncertainties. For $\xl >$ 0.3, the rate of neutrons in photoproduction is about half of that measured in hadroproduction, which constitutes a clear breaking of factorisation. There is about a 20% rise in the neutron rate between photoproduction and deep inelastic scattering, which may be attributed to absorptive rescattering in the $\gamma p$ system. For $0.64 < \xl < 0.82$, the rate of neutrons is almost independent of the Bjorken scaling variable $x$ and $Q^2$. However, at lower and higher $\xl$ values, there is a clear but weak dependence on these variables, thus demonstrating the breaking of limiting fragmentation. The neutron-tagged structure function, ${{F}^{\rm\tiny LN(3)}_2}(x,Q^2,\xl)$, rises at low values of $x$ in a way similar to that of the inclusive \ff of the proton. The total $\gamma \pi$ cross section and the structure function of the pion, $F^{\pi}_2(x_\pi,Q^2)$ where $x_\pi = x/(1-\xl)$, have been determined using a one-pion-exchange model, up to uncertainties in the normalisation due to the poorly understood pion flux. At fixed $Q^2$, $F^{\pi}_2$ has approximately the same $x$ dependence as $F_2$ of the proton.
The cross section of charm production in γγ collisions σ(e + e − →e + e − c c ̄ X) is measured at LEP with the L3 detector at centre-of-mass energies from 91 GeV to 183 GeV. Charmed hadrons are identified by electrons and muons from semileptonic decays. The direct process γγ→c c ̄ is found to be insufficient to describe the data. The measured cross section values and event distributions require contributions from resolved processes, which are sensitive to the gluon density in the photon.
Inclusive D^{*+-} production in two-photon collisions is studied with the L3 detector at LEP, using 683 pb^{-1} of data collected at centre-of-mass energies from 183 to 208 GeV. Differential cross sections are determined as functions of the transverse momentum and pseudorapidity of the D^{*+-} mesons in the kinematic region 1 GeV < P_T < 12 GeV and |eta| < 1.4. The cross sections sigma(e^+e^- -> e^+e^-D^{*+-}X) in this kinematical region is measured and the sigma(e^+e^- -> e^+e^- cc{bar}X) cross section is derived. The measurements are compared with next-to-leading order perturbative QCD calculations.
Cross sections for the production of two isolated muons up to high di-muon masses are measured in ep collisions at HERA with the H1 detector in a data sample corresponding to an integrated luminosity of 71 pb^-1 at a centre of mass energy of sqrt{s} = 319 GeV. The results are in good agreement with Standard Model predictions, the dominant process being photon-photon interactions. Additional muons or electrons are searched for in events with two high transverse momentum muons using the full data sample corresponding to 114 pb^-1, where data at sqrt{s} = 301 GeV and sqrt{s} = 319 GeV are combined. Both the di-lepton sample and the tri-lepton sample agree well with the predictions.
Results from the HELIOS External Spectrometer on kaon production in 200 GeV/ A S + W and p + W collisions are presented. The K π ratios are compared with results from a lower beam energy and are found to be remarkably similar. Evidence for secondary production of K + by meson-baryon rescattering is reviewed. Our results at y = 1.0–1.5 are compared with neutral strange particle results at midrapidity.
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