The inclusive π0 production cross-section and the η/π0 ratio have been measured inpp collisions at\(\sqrt s= 63 GeV\) at the CERN Intersecting Storage Rings in the rapidity range 2.00<y<2.75. The π0 cross-section exhibits a strongy-dependence and falls more steeply as a function ofpT, compared with the cross-section measured aty∼0. We find a value of 0.46±0.07 for the η/π0 ratio with no significantpT dependence over the range 2.0<pT<4.0 GeV/c.
We have determined the double inclusive cross-section for opposite-side high-pT photons and away-side jets withθγ≈θjet≈90° produced inpp collisions at the CERN Intersecting Storage Rings at\(\sqrt s= 63\) GeV. Under the assumption that these events arise predominantly from the QCD gluon Compton process we have calculated the gluon structure function in the range 0.15≦x≦0.30 at an average square of the four-momentum transfer of 40 GeV2/c2. The data favour a soft gluon distribution in the proton.
We report on the production ofe± μ∓ pairs in 450 GeV/c pBe collisions at the CERN SPS. Theeμ signal, which has average missing energy of 21 GeV, is shown to be consistent with expectations from charm decay, and implies a σ ×B for\(c\bar c\) production in p-nucleon collisions of 0.63 ± 0.35μb. Alternatively, using an estimate of charm production from other experiments, the data imply a 95% confidence level upper limit of 1.16μb on any new physics process which producese±μ∓.
We have measured the inclusive cross-section as a function of missing energy, due to the production of neutrinos or new weakly interacting neutral particles in 450 GeV/c proton-nucleus collisions, using calorimetric measurements of visible event energy. Upper limits are placed on the production of new particles as a function of their energy. These upper limits are typically an order
Measurements are presented of single and double-differential dijet cross sections in diffractive photoproduction based on a data sample with an integrated luminosity of 47 pb^-1. The events are of the type ep -> eXY, where the hadronic system X contains at least two jets and is separated by a large rapidity gap from the system Y, which consists of a leading proton or low-mass proton excitation. The dijet cross sections are compared with QCD calculations at next-to-leading order and with a Monte Carlo model based on leading order matrix elements with parton showers. The measured cross sections are smaller than those obtained from the next-to-leading order calculations by a factor of about 0.6. This suppression factor has no significant dependence on the fraction x_gamma of the photon four-momentum entering the hard subprocess. Ratios of the diffractive to the inclusive dijet cross sections are measured for the first time and are compared with Monte Carlo models.
Using a secondary pion beam from the Argonne Zero Gradient Synchrotron we have studied the process π−p→φn in the region of the cross-section enhancement near kinematic threshold. For incident momenta between 1.6 and 2 GeV/c, we have determined production and decay angular distributions and extrapolated total cross sections from a sample of about 160 φ's above background. The production and decay distributions are consistent with isotropy over this entire incident-momentum range. The extrapolated total cross section varies between 19 and 25 μb.
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.
We have studied several features of the production of charged-hardon pairs by γγ collisions. We have measured the f0 partial width Γf0→γγ(Q2) for Q2 in the range 0<Q2<1.4 GeV2/c2, and obtained Γf0→γγ=2.52±0.13±0.38 keV at Q2≈0. The measured Q2 dependence is in agreement with the generalized vector-dominance model. The cross section for γγ→(π+π−+K+K−) in the mass region 1.6≤Mππ≤2.5 GeV/c2 has also been measured and the result compared with that expected from the QCD continuum.
We report measurements of the two-photon processes e+e−→e+e−π+π− and e+e−→e+e−K+K−, at an e+e− center-of-mass energy of 29 GeV. In the π+π− data a high-statistics analysis of the f(1270) results in a γγ width Γ(γγ→f)=3.2±0.4 keV. The π+π− continuum below the f mass is well described by a QED Born approximation, whereas above the f mass it is consistent with a QCD-model calculation if a large contribution from the f is assumed. For the K+K− data we find agreement of the high-mass continuum with the QCD prediction; limits on f′(1520) and θ(1720) formation are presented.
We report a measurement of the cross section for the process γγ → π + π − at invariant masses 500 MeV/ c 2 < m ππ <2000 MeV/ c 2 . A value for the radiative width of the f(1270) tensor meson Γ f→ γγ = 3.6 ± 0.3 ± 0.5 KeV (helicity λ = 2) has been obtained from a fit to the observed ππ mass spectrum.
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