Deep inelastic scattering with tagged photons at HERA

We calculate cross-sections for neutral current deep inelastic scattering at HERA with photon tagging. Both, the exact lowest-order cross-section and a leading logarithmic approximation of next-order corrections are calculated. The latter amounts to less than 20% in a large kinematical range. The integrations are performed numerically but without relying on Monte Carlo methods.     

Search for fermiophobic Higgs bosons in final states with photons at LEP 2

Higgs boson production with subsequent decay to photons was searched for in the data collected by the DELPHI detector at centre-of-mass energies between 183 GeV and 209 GeV, corresponding to a total integrated luminosity of nearly 650 pb^-1. No evidence for a signal was found, and limits were set on h ⁰ Z ⁰ and h ⁰ A ⁰ production with h ⁰ decay to photons. These results were used to exclude regions in the parameter space of fermiophobic scenarios of Two Higgs Doublet Models.     

On MSSM charged Higgs-boson production in association with an electroweak W boson at electron–positron colliders

We present a calculation of the cross section for the process e⁺e^-→W^±H^∓ in the minimal supersymmetric standard model (MSSM) and the two Higgs doublet model (THDM). We study the basic features of the MSSM prediction for some distinctive parameter scenarios. We find large effects from virtual third-generation squarks for scenarios with large mixing, which can lead to a cross section vastly different from a THDM with identical Higgs sector parameters. We investigate this interesting behaviour in more detail by thoroughly scanning the MSSM parameter space for regions of large cross section. For a charged Higgs boson too heavy to be pair produced at a future high-energy electron–positron collider it turns out that a large MSSM cross section with a good chance of observation is linked to a squark mass scale below 600 GeV and a considerable amount of mixing in either the stop and sbottom sector.     

Visible cascade Higgs decays to four photons at hadron colliders

The presence of a new singlet scalar particle a can open up new decay channels for the Higgs boson, through cascades of the form h --> 2a --> X, possibly making discovery through standard model channels impossible. If a is CP odd, its decays are particularly sensitive to new physics. Quantum effects from heavy fields can naturally make h --> 4 g the dominant decay which is difficult to observe at hadron colliders, and is allowed by CERN LEP for m(h) > 82 GeV. However, there are usually associated decays, either h --> 2g2gamma or h --> 4gamma, which are more promising. The decay h-->4gamma is a clean channel that can discover both a and h. At the CERN LHC with 300 fb(-1) of luminosity, a branching ratio of order 10(-4) is sufficient for discovery for a large range of Higgs boson masses. With total luminosity of approximately 8 fb(-1), discovery at the Fermilab Tevatron requires more than 5 x 10(-3) in branching ratio.     

Absorption of light at electron and positron states in quasi-zero-dimensional systems

A theory of interaction of the electromagnetic field with one-particle electron and positron Coulomb states emerging in nanopores of semiconductors is developed. It is established using the dipole approximation that the oscillator strengths of the transitions and the dipole moments of one-particle electron and positron states in nanopores assume giant values considerably exceeding the typical values of the corresponding quantities for semiconductors.     

Single pion production by linearly polarized photons at high energies

There exists a one-to-one correspondence between the dependence of the single-pion-photoproduction amplitude on the linear polarization of the photons at high energies and small momentum transfer in the direct channelγ+N→π+N on one hand and the parity of the exchanged particle-system in the crossed channelγ+π→N+¯N on the other.     

Mass determination of new states at hadron colliders

We propose an improved method for hadron-collider mass determination of new states that decay to a massive, long-lived state like the LSP in the MSSM. We focus on pair-produced new states which undergo three-body decay to a pair of visible particles and the new invisible long-lived state. Our approach is to construct a kinematic quantity which enforces all known physical constraints on the system. The distribution of this quantity calculated for the observed events has an endpoint that determines the mass of the new states. However we find it much more efficient to determine the masses by fitting to the entire distribution and not just the end point. We consider the application of the method at the LHC for various models and demonstrate that the method can determine the masses within about 6 GeV using only 250 events. This implies the method is viable even for relatively rare processes at the LHC such as neutralino pair production.     

Inclusive charged pion photoproduction on12C using tagged photons in the energy range (200–390) MeV

The photoproduction of charged pions on¹²C has been investigated using the tagged photon beam facility at the Bonn 500 MeV synchrotron. Pions were detected in a broad range magnetic spectrometer. Double differential cross sections for pion emission at fixed photon energies are presented. The integration of the measured distributions yields the total cross section for inclusive pion production. The data are compared to the predictions of a simple model that takes the quasifree production of pions and their subsequent scattering and reabsorption into account.     

Coalescence of three photons at an electron. I

In the Born approximation of perturbation theory, two particular cases of the coal-escence of three parallel photons at an electron at rest are considered: 1) when two of the incident quanta are less hard than the third; 2) when one of the incident quanta is less hard than two identical photons.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 11–14, August, 1979.     

On the partial-wave analysis of mesonic resonances decaying to multiparticle final states produced by polarized photons

Meson spectroscopy is going through a revival with the advent of high statistics experiments and new advances in the theoretical predictions. The Constituent Quark Model (CQM) is finally being expanded considering more basic principles of field theory and using discrete calculations of Quantum Chromodynamics (lattice QCD). These new calculations are approaching predictive power for the spectrum of hadronic resonances and decay modes. It will be the task of the new experiments to extract the meson spectrum from the data and compare with those predictions. The goal of this report is to describe one particular technique for extracting resonance information from multiparticle final states. The technique described here, partial wave analysis based on the helicity formalism, has been used at Brookhaven National Laboratory (BNL) using pion beams, and Jefferson Laboratory (JLab) using photon beams. In particular this report broadens this technique to include production experiments using linearly polarized real photons or quasi-real photons. This article is of a didactical nature. We describe the process of analysis, detailing assumptions and formalisms, and is directed towards people interested in starting partial wave analysis.     

Momentum analysis of kaon and pion pairs produced from time-like photons at 1.6 GeV energy

We present results for the total cross section of e⁺e^? annihilation into two hadrons at 1.6 GeV: σ_ππ = σ_KK = (1.8 ± 1.1) × 10^-33 cm².From these values we obtain the time-like electromagnetic form factors these mesons: |F_π|² = 0.24 ± 0.14 and |F_K|² = 0.46 ± 0.26.     

Toward the theory of electron and positron states in dielectric clusters

Analytical expressions for the binding energy of electrons and positrons in dielectric clusters, analyzed in this work, neglect the elastic effects. Therefore, we present the density-functional theory for neutral liquid clusters that experience the spontaneous deformation. Using the 1/R-expansion, R being the cluster radius, the exact analytical expressions for the size corrections to the chemical potential, surface tension, and atomic density are derived from the condition of mechanical equilibrium. The problem of calculating these corrections is reduced to calculating the quantities for a liquid with a flat surface. The size compression and tension of density occur in the 1/R and 1/R ² orders respectively. The sizes of charged rigid and elastic critical clusters, for which the electron or positron binding energy is close to zero, are calculated for Xe _N ^? , Kr _N ^? , Ar _N ^? , Ne _N ⁺ , He _N ⁺ . The calculations show significant contribution of self-compression to the binding energy of the excess electron in contrast to the positron.     

Role of final state correlations in electron and positron impact ionization of H and He

Single ionization of H and double ionization of He are investigated using a multiple scattering approach and a correlated final state wavefunction, which satisfies the correct asymptotic condition for the electron and positron impact ionization of atoms. Analyses presented here show the influence of the Coulomb correlations in the final state on the three- and fivefold differential cross sections. Situations are discussed in which consideration of every pair of electron-electron and/or electron-positron correlation in the final state becomes important for physically satisfactory results.     

Prospects for observation of double parton scattering with four-muon final states at LHCb

We study the prospects for observing double parton scattering through four-muon final states, forming two opposite-sign muon pairs, in the LHCb experiment in pp collisions at 14 TeV centre of mass energy. We consider two special cases, namely double Drell–Yan and J/ψ-pair production. The kinematic properties and prospects for observing these processes are discussed. We find that the production rate depends strongly on the origin of the four muons, while many kinematic properties can be used to help identify the presence of double parton scattering events.     

Cross sections for multi-particle final states at a linear collider

In this paper total cross sections for signals and backgrounds of top- and Higgs-production channels in e ⁺ e^- collisions at a future linear collider are presented. All channels considered are characterized by the emergence of six-particle final states. The calculation takes into account the full set of tree-level amplitudes in each process. Two multi-purpose parton level generators, HELAC/PHEGAS and AMEGIC + + , are used, and their results are found to be in perfect agreement.Received: 26 November 2003, Revised: 15 January 2004, Published online: 3 March 2004     

MSSM Higgs searches in multi-b-jet final states at the LHC

This paper discusses the possibility to observe a signal from MSSM Higgs boson decays into final states containing four b-jets. Two specific channels are considered: bb?H and bb?A production with H,A → bb?, for large values of m_H, m_A and tan β, and H → hh → bb?bb? decays for 150 GeV < m_H < 2m_t and for low values of tan β. Both channels are difficult to extract because of the very large reducible and irreducible QCD backgrounds. Even with an ultimate integrated luminosity, expected per LHC experiment, of 3 · 10⁵ pb^-1, the region of the MSSM parameter space covered by these channels does not extend the reach beyond that accessible to other channels that were studied in the past. Nevertheless, their observation would help in constraining the couplings and branching ratios of the MSSM Higgs bosons.