Subthreshold and near threshold K+ meson photoproduction\newline on nuclei

The inclusive K⁺ meson production in photon–induced reactions in the near threshold and subthreshold energy regimes is analyzed with respect to the one–step (γN→K ⁺ Y, Y=Λ,Σ) incoherent production processes on the basis of an appropriate new folding model, which takes properly into account the struck target nucleon removal energy and internal momentum distribution (nucleon spectral function), extracted from recent quasielastic electron scattering experiments and from many–body calculations with realistic models of the NN interaction. Simple parametrizations for the total and differential cross sections of the K⁺ production in photon–nucleon collisions are presented. Comparison of the model calculations of the K⁺ differential cross sections for the reaction γ+C¹² in the threshold region with the existing experimental data is given, that displays the contributions to the K⁺ production at considered incident energies coming from the use of the single–particle part as well as high momentum and high removal energy part of the nucleon spectral function. Detailed predictions for the K⁺ total and differential cross sections from γH², γC¹² and γPb²⁰⁸ reactions at subthreshold and near threshold energies are provided. The influence of the uncertainties in the elementary K⁺ production cross sections on the K⁺ yield is explored. Received: 12 April 1999 / Revised version: 11 September 1999     

Exclusive leptoproduction of \rho^0 mesons from hydrogen at intermediate virtual photon energies

Measurements of the cross section for exclusive virtual-photoproduction of mesons from hydrogen are reported. The data were collected by the HERMES experiment using 27.5 GeV positrons incident on a hydrogen gas target in the HERA storage ring. The invariant mass W of the photon-nucleon system ranges from 4.0 to 6.0 GeV, while the negative squared four-momentum of the virtual photon varies from 0.7 to 5.0 GeV. The present data together with most of the previous data in the intermediate W-domain are well described by a model that infers the W-dependence of the cross section from the dependence on the Bjorken scaling variable x of the unpolarized structure function for deep-inelastic scattering. In addition, a model calculation based on Off-Forward Parton Distributions gives a fairly good account of the longitudinal component of the production cross section for GeV. Received: 25 April 2000 / Revised version: 3 July 2000 / Published online: 25 September 2000     

J/\psi-photoproduction and the gluon structure of the nucleon

Using short distance QCD methods based on the operator product expansion, we calculate the photoproduction cross section in terms of the gluon distribution function of the nucleon. Comparing the result with data, we show that experimental behaviour of the cross section correctly reflects the x-dependence of the gluon distribution obtained from deep inelastic scattering. Received: 3 March 1999 / Published online: 28 May 1999     

Suppression of charmonium production in PbPb collisions at 158 GeV/c per nucleon and quark-gluon deconfinement

An anomalous suppression of the charmonium yield in central collisions was observed in studying charmonium production in collisions of Pb nuclei accelerated to a momentum of 158 GeV/c per nucleon with Pb target nuclei. It is shown that, in peripheral collisions, the ratio of the cross section for J/ψ production to the cross section for the Drell-Yan process decreases exponentially (as in the case of collisions of lighter nuclei) owing to the ordinary absorption of J/gy in nuclear matter. The observed threshold effect of the anomalous suppression of charmonium production agrees well with the predictions based on the assumption of Debye color screening in the formation of quark-gluon plasma.     

Measurement of the $b\overline{b}$ production cross section in 920 GeV fixed-target proton-nucleus collisions

Using the HERA - B detector, the production cross section has been measured in 920 GeV proton collisions on carbon and titanium targets. The production was tagged via inclusive bottom quark decays into by exploiting the longitudinal separation of decay vertices from the primary proton-nucleus interaction. Both e ⁺ e ^- and channels have been reconstructed and the combined analysis yields the cross section . Received: 6 June 2002 / Revised version: 2 September 2002 / Published online: 8 November 2002     

Double-spin asymmetries in the cross section of \rho^0 and \phi production at intermediate energies

Double-spin asymmetries in the cross section of electroproduction of and mesons on the proton and deuteron are measured at the HERMES experiment. The photoabsorption asymmetry in exclusive electroproduction on the proton exhibits a positive tendency. This is consistent with theoretical predictions that the exchange of an object with unnatural parity contributes to exclusive electroproduction by transverse photons. The photoabsorption asymmetry on the deuteron is found to be consistent with zero. Double-spin asymmetries in and meson electroproduction by quasi-real photons were also found to be consistent with zero; the asymmetry in the case of the meson is compatible with a theoretical prediction which involves knockout from the nucleon. Received: 11 February 2003 / Published online: 11 June 2003     

ψψ production and limits on beauty meson production from 400 GeV/c protons

From a study of multimuon events obtained in a high luminosity proton-platinum experiment at 400 GeV/c, we find 5 ± 4ψψ events, which correspond to a production cross section σ(ψψ = 27 ± 10 pb. The observed production is compared to the ψψ events previously found in the same apparatus from incoming π^?; a comparison with QCD predictions is performed, giving a good agreement with expectations from gluon-gluon fusion. Finally, using like-sign dimuon, trimuon and quadrimuon events, we give model-dependent upper limits on beauty meson production: 2 nb/nucleon for central models, 20 nb/nucleon for diffractive production.     

Diffractive \eta_c photo-and electroproduction with the perturbative QCD Odderon

Using a set of new odderon states, we calculate their contribution to the diffractive photo- and electroproduction process. Compared to previous simple 3-gluon exchange calculations we find an enhancement of about one order of magnitude in the cross section. The t-dependence of the cross section exhibits a dip structure in the small t region. Received: 18 March 2001 / Published online: 11 May 2001     

K+ nucleon elastic scattering at 180° between 1.0 and 1.5 GeV/c incident momentum

We have measured the cross section at 180° for K⁺p and K⁺n elastic scattering in the momentum range 1.0 to 1.5 GeV/c. The K⁺n cross section was measured on deuterium and the K⁺p on hydrogen and deuterium. We were thus able to measure directly the difference between free nucleon (proton) scattering and bound nucleon (proton) scattering at large angles. This difference was found to be small and within our experimental accuracy the K⁺p(n) cross section should be equal to the K⁺p (free) cross section at 180°. We found no evidence for an s-channel resonance Z¹ in either the K⁺p or K⁺n system. A comparison of our data and those of other groups with theoretical predictions is given.     

Coulomb interference in high-energy pp and $\bar p p$ scattering

An analysis of the Coulombic amplitude and its interference with the nuclear amplitude which is driven by the three-component pomeron is presented. It is shown that different approaches towards the evaluation of the Coulomb phase give approximately uniform results at all energies and the differences are negligible at RHIC and LHC energies. We show that the use of the amplitude which was fitted to accommodate the nucleon data only (in the region (GeV²)), combined with the Coulomb amplitude, reproduces the existing data in the Coulomb interference domain quite accurately without any adjustment of the parameters. As a consequence, we predict the differential cross section in the region of the Coulomb nucleon interference for both RHIC and LHC energies. Received: 3 June 2002 / Revised version: 27 February 2003 / Published online: 5 May 2003 RID="a" ID="a" e-mail: petrov@mx.ihep.su RID="b" ID="b" e-mail: predazzi@to.infn.it RID="c" ID="c" e-mail: prokudin@to.infn.it     

Production of scalar Higgs and pseudoscalar Higgs in multi-Higgs doublet models at \gamma\gamma colliders

We present the effects of heavy CP-even (H) and CP-odd (A) Higgs bosons on the production cross section of the process at the energy around the mass poles of the Higgs bosons. It is found that interference between H and A with small mass gap, as well as the ones between Higgs bosons and continuum, contributes to the cross section, if the photon beams are polarized and if we observe the helicity of the top quarks. It is demonstrated in the framework of the minimal supersymmetric extension of the standard model that the H and A contributions can be sizable at future colliders for small values of . The methods to measure the CP-parity of the Higgs boson are also presented. The statistical significance of detecting the Higgs signals and measuring the Higgs CP-parity is evaluated. Received: 16 December 1999 / Revised version: 30 January 2000 / Published online: 6 April 2000     

Production cross sections of multiply charged fragments in heavy ion interactions at 150–200 MeV/nucleon

A total of 1720 ¹⁶O-emulsion nucleus interactions at 150–200 MeV/nucleon have been investigated. Production cross sections of multiply charged projectile fragments are given. It is found that the cross section for the production of N, C, B and Be projectile fragments in ¹⁶O-nucleus interactions is similar at 0.2 and 2 GeV/nucleon. The fragmentation cross sections for Li and He are larger at 0.2 than at 2 GeV/nucleon.     

Diffractive $\gamma \gamma$ production at hadron colliders

We compute the cross section for exclusive double-diffractive production at the Tevatron, , and the LHC. We evaluate both the gg and -channel exchange contributions to the process. The observation of exclusive production at the Tevatron will provide a check on the model predictions and offer an opportunity to confirm the expectations for exclusive double-diffractive Higgs production at the LHC.Received: 10 September 2004, Revised: 8 October 2004, Published online: 1 December 2004     

Inclusive D production in $\gamma \gamma$ collisions: including the single-resolved contribution with massive quarks

We have calculated the next-to-leading order cross section for the inclusive production of charm quarks as a function of the transverse momentum p_T and the rapidity in two approaches using massive or massless charm quarks. For the single-resolved cross section we have derived the massless limit from the massive theory. We find that this limit differs from the genuine massless version with factorization by finite corrections. By adjusting subtraction terms we establish a massive theory with subtraction which approaches the massless theory very fast with increasing transverse momentum. With these results and including the equivalent results for the direct cross section obtained previously as well as double-resolved contributions, we calculate the inclusive cross section in collisions using realistic evolved non-perturbative fragmentation functions and compare with recent data from the LEP collaborations ALEPH, L3 and OPAL. We find good agreement. Received: 14 February 2003 / Published online: 5 May 2003     

Measuring the FSR-inclusive $\pi^+\pi^-$ cross section

Final state radiation (FSR) in pion pair production cannot be calculated reliably because of the composite structure of the pions. However, FSR corrections have to be taken into account for a precise evaluation of the hadronic contribution to g-2 of the muon. The role of FSR in both energy scan and radiative return experiments is discussed. It is shown how FSR influences the pion form factor extraction from experimental data and, as a consequence, the evaluation of . In fact the FSR corrections should be included to reach the precision we are aiming at. We argue that for an extraction of the desired FSR-inclusive cross section a photon-inclusive scan measurement of the ”” cross section is needed. For exclusive scan and radiative return measurements in contrast we have to rely on ad hoc FSR models if we want to obtain either or the FSR-exclusive cross section . We thus advocate to consider seriously precise photon-inclusive energy scan measurements at present and future low energy e ⁺ e ^- -facilities. Then together with radiative return measurements from DANE and BABAR and forthcoming scan measurements at VEPP-2000 we have a good chance to substantially improve the evaluation of in the future. Received: 2 January 2003 / Published online: 24 March 2003     

Cross sections for exclusive channels in\bar pp reactions at 12 GeV/c

The data obtained in an exposure of BEBC to an r.f. separated beam of antiprotons at 12 GeV/c are used to derive exclusive cross sections for reactions having one or more neutral strange particles in the final state. Particular attention is given to the estimation of the background. An upper limit of the cross section for the inclusive production of the charmed mesonD ^* is also given.     

Forward elastic scattering of light on light, \gamma+\gamma\rightarrow\gamma+\gamma

The forward elastic scattering of light on light, i.e., the reaction in the forward direction, is analyzed utilizing real analytic amplitudes. We calculate , the ratio of the real to the imaginary portion of the forward scattering amplitude, by fitting the total cross section data in the high energy region 5 GeV GeV, assuming a cross section that rises asymptotically as ln² s. We then compare , the ratio of the even portions of the pp and forward scattering amplitudes, as well as to [1], the value for Compton scattering. Within errors, we find that the three -values in the c.m.s. energy region 5 GeV 130 GeV are the same, as predicted by a factorization theorem of Block and Kaidalov [2]. Received: 18 June 2002 / Published online: 9 August 2002     

Deep-inelastic inclusive ep scattering at low x and a determination of \alpha_s

A precise measurement of the inclusive deep-inelastic scattering cross section is reported in the kinematic range GeV and . The data were recorded with the H1 detector at HERA in 1996 and 1997, and correspond to an integrated luminosity of 20 pb. The double differential cross section, from which the proton structure function and the longitudinal structure function are extracted, is measured with typically 1% statistical and 3% systematic uncertainties. The measured derivative is observed to rise continuously towards small x for fixed . The cross section data are combined with published H1 measurements at high for a next-to-leading order DGLAP QCD analysis. The H1 data determine the gluon momentum distribution in the range to within an experimental accuracy of about 3% for GeV. A fit of the H1 measurements and the data of the BCDMS collaboration allows the strong coupling constant and the gluon distribution to be simultaneously determined. A value of is obtained in NLO, with an additional theoretical uncertainty of about , mainly due to the uncertainty of the renormalisation scale. Received: 21 December 2000 / Revised version: 10 May 2001 / Published online: 6 July 2001     

Rescattering mechanisms for\bar pd \to pX reactions

Inclusive proton spectra for the reactions¯pd→3πp, ¯pd→5πp, and¯p d →K¯Kmπp are calculated. In addition to nucleon exchange, the rescattering of pions and kaons, and the production and absorption of heavy mesons is evaluated. In the proton spectator momentum regime of 200 to 600 MeV/c, the role of heavy mesons is found to be negligible. The proper counting of the topological variants of pion and kaon rescattering can explain the cross sections with the exception of the¯pd→3πp channel.     

\gamma^* \gamma^* reactions at high energies

The total hadronic cross sections at high energy are calculated as a function of energy and photon virtuality in a model combining Reggeon exchange, the quark box diagram (a fixed pole in Regge language) and soft and hard pomeron exchanges evaluated in the context of dipole-dipole scattering. Good agreement is obtained with the data for the real cross section and for the real photon structure function . However the model prediction for the cross section is too small. This is attributed to an incorrect extrapolation of the dependence of the hard pomeron adopted here. Parametrising it independently shows that the hard part of the cross section can be well represented by a simple Regge pole with intercept . Received: 19 August 1999 / Published online: 3 February 2000