We evaluate differential distributions for the four-body pp ! pp þ À (and p " p ! p " p þ À ) reaction which constitutes an irreducible background to three-body processes pp ! ppM, where M are a broad resonances in the þ À channel, e.g., M ¼ , 0 , f 0 ð980Þ, f 2 ð1275Þ, f 0 ð1500Þ. We include both double-diffractive contribution (both Pomeron and Reggeon exchanges) as well as pion-pion rescattering contribution. The first process dominates at higher energies and small pion-pion invariant masses while the second becomes important at lower energies and higher pion-pion invariant masses. The amplitude(s) is(are) calculated in the Regge approach. We compare our results with measured cross sections for the Intersecting Storage Ring and Fermi National Accelerator Laboratory experiments. We make predictions for future experiments at the anti-Proton ANnihilation at DArmstadt (PANDA), Relativistic Heavy Ion Collider, Tevatron, and LHC energies. Differential distributions in invariant two-pion mass, pion rapidities and transverse momenta of pions are presented. The two-dimensional distribution in ðy þ ; y À Þ is particularly interesting. The higher the incident energy, the higher preference for the same-hemisphere emission of pions. The processes considered constitute a sizeable contribution to the total nucleon-nucleon cross section as well as to pion inclusive cross section.

We discuss consequences of the models of "tensorial pomeron" and "vectorial pomeron" for exclusive diffractive production of scalar and pseudoscalar mesons in proton-proton collisions. Diffractive production of f 0 (980), f 0 (1370), f 0 (1500), η, and η ′ (958) mesons is discussed. Different pomeronpomeron-meson tensorial coupling structures are possible in general. In most cases two lowest orbital angular momentum -spin couplings are necessary to describe experimental differential distributions. For f 0 (980) and η production reggeon-pomeron, pomeron-reggeon, and reggeon-reggeon exchanges are included in addition, which seems to be necessary at relatively low energies. The theoretical results are compared with the WA102 experimental data. Correlations in azimuthal angle between outgoing protons, distributions in rapidities and transverse momenta of outgoing protons and mesons, in a special "glueball filter variable", as well as some two-dimensional distributions are presented. We discuss differences between results of the vectorial and tensorial pomeron models. We show that high-energy central production, in particular of pseudoscalar mesons, could provide crucial information on the spin structure of the soft pomeron.

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We consider central exclusive diffractive dipion production in the reactions pp → ppπ + π − and pp → ppπ + π − at high energies. We include the dipion continuum, the dominant scalar f 0 (500), f 0 (980), and tensor f 2 (1270) resonances decaying into the π + π − pairs. The calculation is based on a tensor pomeron model and the amplitudes for the processes are formulated in terms of vertices respecting the standard crossing and charge-conjugation relations of Quantum Field Theory. The formulae for the dipion continuum and tensor meson production are given here for the first time.The theoretical results are compared with existing STAR, CDF, CMS experimental data and predictions for planned or being carried out experiments (ALICE, ATLAS) are presented. We show the influence of the experimental cuts on the integrated cross section and on various differential distributions for outgoing particles. Distributions in rapidities and transverse momenta of outgoing protons and pions as well as correlations in azimuthal angle between them are presented. We find that the relative contribution of resonant f 2 (1270) and dipion continuum strongly depends on the cut on proton transverse momenta or four-momentum transfer squared t 1,2 which may explain some controversial observations made by different ISR experiments in the past. The cuts may play then the role of a ππ resonance filter. We suggest some experimental analyses to fix model parameters related to the pomeron-pomeron-f 2 coupling.

We calculate cross sections for diphoton production in (semi)exclusive PbPb collisions, relevant for the LHC. The calculation is based on equivalent photon approximation in the impact parameter space. The cross sections for elementary γγ → γγ subprocess are calculated including two different mechanisms. We take into account box diagrams with leptons and quarks in the loops. In addition, we consider a vector-meson dominance (VDM-Regge) contribution with virtual intermediate hadronic (vector-like) excitations of the photons. We get much higher cross sections in PbPb collisions than in earlier calculation from the literature. This opens a possibility to study the γγ → γγ (quasi)elastic scattering at the LHC. We present many interesting differential distributions which could be measured by the ALICE, CMS or ATLAS Collaborations at the LHC. We study whether a separation or identification of different components (boxes, VDM-Regge) is possible. We find that the cross section for elastic γγ scattering could be measured in the heavy-ion collisions for subprocess energies smaller than W γγ ≈ 15 − 20 GeV.