Quark-antiquark-gluon jets in dis diffractive dissociation

We study the diffractive production of jets with large transverse momenta in the region of large diffractive masses (small ). Cross sections for transverse and longitudinal photons are obtained in the leading log and log approximation, keeping all powers in log . We perform a numerical study and illustrate the angular distribution of the three jets. We also estimate the integrated diffractive three jet cross section and compare with the dijet cross section obtained before. Received: 11 March 1999 / Published online: 14 October 1999     

Physics with tagged forward protons at the LHC

We emphasize the importance of tagging the outgoing forward protons to sharpen the predictions for New Physics at the LHC (such as the diffractive production of a light Higgs boson). The rescattering effects lead to a rich distinctive structure of the cross section as a function of the transverse momenta of the protons. We show that a study of the correlations between the proton transverse momenta for double-diffractive production of central dijets will provide a detailed check of the whole diffractive formalism. Adopting a perturbative two-gluon structure of the Pomeron, we emphasize that quarkonium production, via Pomeron-Pomeron fusion, is strongly suppressed. This offers a favourable production mechanism for non- states, such as glueballs. Received: 15 March 2002 / Published online: 21 June 2002     

Validity of double scaling analysis in semi-inclusive processes –J/\psi production at HERA

In this paper we check the validity of the ideas of double scaling as given by Ball and Forte in a semi inclusive process like production at HERA, in different kinematical regions, for low values of the Bjorken variable . In particular, we study production in the inelastic and diffractive (elastic) regimes using the double scaling form of the gluon distribution functions. We compare these predictions with data (wherever available) and with other standard parameterisations. We find that double scaling holds in the inelastic regime over a larger kinematic region than that given by the analysis of the proton structure function . However, in the diffractive region, double scaling seems to suggest an admixture of hard pomeron boundary conditions for the gluon distribution, while predicting a steeper rise in the cross section than suggested by present data. Received: 17 December 1997 / Published online: 10 March 1998     

Low Q^2 jet production at HERA in next-to-leading order QCD

We present next-to-leading order calculations of one- and two-jet production in eP collisions at HERA for photon virtualities in the range GeV. Soft and collinear singularities are extracted using the phase space slicing method. Numerical results are presented for HERA conditions with the Snowmass jet definition. The transition between photoproduction and deep-inelastic scattering is investigated in detail. We compare two approaches, the usual deep-inelastic theory, where the virtual photon couples only directly to quarks and antiquarks, and the photoproduction approach, where the photon couples either in the direct way or in the resolved way via the parton constituents of the virtual photon with the proton constituents. Finally we compare with recent H1 data of the dijet rate obtained for various photon virtualities with special attention to the region, in which two jets have equal transverse momenta. Received: 23 April 1998 / Published online: 26 August 1998     

J/\psi +{\mathrm{jet}} diffractive production in the direct photon process at HERA

We present a study of the diffractive production in direct photon processes at HERA based on the factorization theorem for lepton-induced hard diffractive scattering and the factorization formalism of the non-relativistic QCD (NRQCD) for quarkonia production. Using the diffractive gluon distribution function extracted from HERA data on diffractive deep inelastic scattering and diffractive dijet photon production, we demonstrate that this process is measurable at DESY HERA. The experimental study of this process can give valuable insight in the diffractive production mechanism and test the color-octet mechanism for heavy quarkonia production in a new environment. Received: 13 September 1999 / Revised version: 31 December 1999 / Published online: 6 April 2000     

Central exclusive production of dijets at hadronic colliders

In view of the recent diffractive dijet data from CDF run II, we critically re-evaluate the standard approach to the calculation of central production of dijets in quasi-elastic hadronic collisions. We find that the process is dominated by the non-perturbative region, and that even perturbative ingredients, such as the Sudakov form factor, are not under theoretical control. Comparison with data allows us to fix some of the uncertainties. Although we focus on dijets, our arguments apply to other high-mass central systems, such as the Higgs boson. Permanent address: Marianopolis College, 4873 Westmount Ave., Montréal, QC, H3Y 1X9, Canada.     

Photon polarisation in diffractive deep inelastic scattering

The distribution of a suitably defined azimuthal angle in diffractive deep inelastic scattering contains information on the polarisation of the exchanged photon. In particular it allows one to constrain the longitudinal diffractive structure function. We investigate the potential of such bounds in general and for particular diffractive final states. Received: 17 September 1997 / Published online: 23 February 1998     

Observation of diffractive b-quark production at the fermilab tevatron

We report a measurement of the fraction of b quarks produced diffractively in &pmacr;p collisions at sqrt[s] = 1.8 TeV. Diffraction is identified by the absence of particles in a forward pseudorapidity region. From events with an electron of transverse momentum 9.5相似文献   

Can one discriminate the thermal dilepton signal against the open charm and bottom decay background in ultrarelativistic heavy-ion collisions?

In ultrarelativistic heavy-ion collisions at 20 (120) AGeV a copious production of charm (bottom) production sets in which, via correlated semileptonic () decays, gives rise to a dilepton yield at invariant mass 2–3 GeV in excess of the Drell-Yan yield and the thermal dilepton signal from deconfined matter as well. We show that appropriate single-electron transverse momentum cuts (suitable for ALICE at LHC) cause a threshold like behavior of the dilepton spectra from heavy-quark meson decays and the Drell-Yan process and can allow to observe a thermal dilepton signal from hot deconfined matter. Received: 28 September 1998 / Revised version: 27 November 1998 / Published online: 22 March 1999     

A QCD analysis of diffractive deep-inelastic scattering data

We perform a novel type of analysis of diffractive deep-inelastic scattering data, in which the input parton distributions of the pomeron are parameterised using the perturbative QCD expressions. In particular, we treat individually the components of the pomeron of different size. We are able to describe simultaneously both the recent ZEUS and H1 diffractive data. In addition to the usual two-gluon model for the perturbative pomeron, we allow for the possibility that it may be made from two sea quarks.Received: 12 July 2004, Revised: 3 September 2004, Published online: 24 September 2004     

Hunting a light <Emphasis Type="Italic">CP</Emphasis>-violating Higgs boson via diffraction at the LHC

We study the central diffractive production of the (three neutral) Higgs bosons, with a rapidity gap on either side, in an MSSM scenario with CP-violation. We consider the and decay for the light H₁ boson and the four b-jet final state for the heavy H₂ and H₃ bosons, and discuss the corresponding backgrounds. A direct indication of the existence of CP-violation can come from the observation of either an azimuthal asymmetry in the angular distribution of the tagged forward protons (for the exclusive process) or of a contribution in the azimuthal correlation between the transverse energy flows in the proton fragmentation regions for the process with the diffractive dissociation of both incoming protons ( ). We emphasise the advantage of reactions with the rapidity gaps (that is, production by pomeron-pomeron fusion) to probe CP-parity and to determine the quantum numbers of the produced central object.Received: 16 January 2004, Published online: 31 March 2004     

Perturbative thresholds in the physical region

We consider unstable-particle scattering in the context of 3-body processes. We show that all partial-wave cross-sections are finite and positive, and the total cross-section is proportional to the transverse size of space in the region of on-shell particle exchange. We comment on the role of loop corrections. Received: 24 October 1997 / Published online: 26 February 1998     

Evidence for factorization breaking in diffractive low-Q2 dijet production

We calculate diffractive dijet production in deep-inelastic scattering at next-to-leading order of perturbative QCD, including contributions from direct and resolved photons, and compare our predictions to preliminary data from the H1 Collaboration at DESY HERA. In contrast to recent experimental claims, evidence for factorization breaking is found only for resolved, and not direct, photon contributions. No evidence is found for large normalization uncertainties in diffractive parton densities. The results confirm theoretical expectations for the (non)cancellation of soft singularities in diffractive scattering as well as previous results for (almost) real photoproduction.     

Self-consistent diffraction in the multiperipheral model (II)

We present a self-consistent solution to a multi-peripheral model in which the triple pomeron vertex vanishes. This solution possesses all of the good features of our previous model, having naturally diffractive and multiperipheral terms in the total cross section and in particle production. In addition, its region of validity has been increased to include the normal Regge region and a smooth transition from Regge-like behavior to the diffractive limit is obtained.     

Spin effects in diffractive J/\Psi leptoproduction and the structure of Pomeron coupling

We calculate the cross section and asymmetry of the diffractive J/ leptoproduction for a simple model of a Pomeron coupling with a proton, which contains the term and a spin-dependent part. It is shown that the asymmetry caused by the Pomeron coupling does not vanish for nonzero . The sensitivity of the polarized diffractive J/ production to the spin-dependent part of the Pomeron coupling is found to be rather weak. As a result, it is difficult to study the structure of the Pomeron coupling with the proton in future polarized diffractive experiments on the J/ production. Received: 27 April 1999 / Revised version: 29 June 1999 / Published online: 14 October 1999     

Direct measurement of the pion valence-quark momentum distribution, the pion light-cone wave function squared

We present the first direct measurements of the pion valence-quark momentum distribution which is related to the square of the pion light-cone wave function. The measurements were carried out using data on diffractive dissociation of 500 GeV/c pi(-) into dijets from a platinum target at Fermilab experiment E791. The results show that the /q&q> light-cone asymptotic wave function describes the data well for Q2 approximately 10 (GeV/c)(2) or more. We also measured the transverse momentum distribution of the diffractive dijets.     

The system of four reggeized gluons and the three-pomeron vertex in the high colour limit

The amplitude for 4 interacting reggeized gluons is studied in the high-colour limit. The leading order amplitude is explicitly shown to reduce to a pair of reggeons, i.e to a single BFKL pomeron. The next-to-leading order diffractive amplitude is found to split into a double pomeron exchange and triple pomeron contribution. The obtained three-pomeron vertex is different from the originally proposed in [2]. Received: 26 June 1997 / Revised version: 28 February 1998 / Published online: 3 September 1998     

Microdeposition of metal and oxide structures using ultrashort laser pulses

2 O₃ on glass and silicon substrates is performed. The superior quality of the results allows the direct, one-step fabrication of binary-amplitude and multilevel optical diffractive structures. Received: 4 February 1998/Accepted: 9 February 1998     

Observation of events with an isolated high energy lepton and missing transverse momentum at HERA

A search for events with an imbalance in transverse momentum and with isolated high energy leptons has been carried out at the positron-proton collider HERA. One event with an and five events with a are found together with evidence for undetected particles carrying transverse momentum. Within the Standard Model the dominant origin of events with this kind of topology is the production of W bosons with subsequent leptonic decay. Three of the six events are within measurement errors found in a region of phase space likely to be populated by this process, while the remaining events show kinematic properties which are atypical of all Standard Model processes considered. Received: 9 June 1998 / Published online: 26 August 1998