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Register a new userExclusive diffractive final states in electron-proton collisions
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S. Fazio
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The exclusive diffractive production of vector mesons and real photons in ep collisions has been studied at HERA in a wide kinematic range. The most recent experimental results are presented.
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In this paper we describe a formalism for generating exclusive final states in diffractive excitation, based on the optical analogy where diffraction is fully determined by the absorption into inelastic channels. The formalism is based on the Good--Walker formalism for diffractive excitation, and it is assumed that the virtual parton cascades represent the diffractive eigenstates defined by a definite absorption amplitude. We emphasize that, although diffractive excitation is basically a quantum-mechanical phenomenon with strong interference effects, it is possible to calculate the different interfering components to the amplitude in an event generator, add them and thus calculate the reaction cross section for exclusive diffractive final states. The formalism is implemented in the DIPSY event generator, introducing no tunable parameters beyond what has been determined previously in studies of non-diffractive events. Some early results from DIS and proton-proton collisions are presented, and compared to experimental data.
A model for exclusive diffractive resonance production in proton-proton collisions at high energies is presented. This model is able to predict double differential distributions with respect to the mass and the transverse momentum of the produced resonance in the mass region $sqrt{M^2}le$5 GeV. The model is based on convoluting the Pomeron distribution in the proton with the Pomeron-Pomeron-meson total cross section. The Pomeron-Pomeron-meson cross section is saturated by direct-channel contributions from the Pomeron as well as from two different $f$ trajectories, accompanied by the isolated f$_0(500)$ resonance dominating the $sqrt{M^{2}} leq $ GeV region. A slowly varying background is taken into account.
A model for exclusive diffractive resonance production in proton-proton collisions at LHC energies is presented. This model is based on the convolution of the Donnachie-Landshoff parameterisation of Pomeron flux in the proton with the Pomeron cross section for resonance production. The hadronic cross section for f$_{0}$(980) and f$_{2}$(1270) production at midrapidity is given differentially in mass and transverse momentum of the resonance. The proton fractional longitudinal momentum loss is presented.
This paper reports results from a study of the reaction pp->pK0Sigma+ at beam momenta of p_{beam} = 2950, 3059, and 3200 MeV/c (excess energies of epsilon= 126, 161, and 206 MeV). Total cross sections were determined for all energies; a set of differential cross sections (Dalitz plots; invariant mass spectra of all two-body subsystems; angular distributions of all final state particles; distributions in helicity and Jackson frames) are presented for epsilon= 161 MeV. The total cross sections are proportional to the volume of available three-body phase-space indicating that the transition matrix element does not change significantly in this range of excess energies. It is concluded from the differential data that the reaction proceeds dominantly via the N(1710)P_{11} and/or N(1720)P_{13} resonance(s); N(1650)S_{11} and Delta(1600)P_{33} could also contribute.
Using samples of 102 million $Upsilon(1S)$ and 158 million $Upsilon(2S)$ events collected with the Belle detector, we study exclusive hadronic decays of these two bottomonium resonances to $ks K^+ pi^-$ and charge-conjugate (c.c.) states, $pi^+ pi^- pi^0 pi^0$, and $pi^+ pi^- pi^0$, and to the two-body Vector-Pseudoscalar ($K^{ast}(892)^0bar{K}^0+ {rm c.c.}$, $K^{ast}(892)^-K^+ + {rm c.c.}$, $omegapi^0$, and $rhopi$) final states. For the first time, signals are observed in the modes $Upsilon(1S) to ks K^+ pi^- + {rm c.c.}$, $pi^+ pi^- pi^0 pi^0$, and $Upsilon(2S) to pi^+ pi^- pi^0 pi^0$, and evidence is found for the modes $Upsilon(1S)to pi^+ pi^- pi^0$, $K^{ast}(892)^0 bar{K}^0+ {rm c.c.}$, and $Upsilon(2S) to ks K^+ pi^- + {rm c.c.}$ Branching fractions are measured for all the processes, while 90% confidence level upper limits on the branching fractions are also set for the modes with a statistical significance of less than $3sigma$. The ratios of the branching fractions of $Upsilon(2S)$ and $Upsilon(1S)$ decays into the same final state are used to test a perturbative QCD prediction for OZI-suppressed bottomonium decays.
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