Electroproduction of neutral pions at energies above the resonance region

The reaction e p→e'pπ⁰ has been measured at W=2.55 GeV a fixed electron scattering angle of 10.3°. Two magnetic spectrometers and a lead glass hodoscope were used to detect all four final state particles. Electroproduction cross sections in the t range ?0.15 to ?1.4 (GeV/c)² at q² = ?0.22, ?0.55 and ?0.85 (GeV/c)² are presented. Above |t|=0.6 (GeV/c)² the cross sections are considerably smaller than those for photoproduction.     

Production of high transverse momentum prompt photons and neutral pions in proton-proton interactions at 280 GeV/c

The inclusive cross sections for production of prompt photons and π⁰ s by 280 GeV/c protons incident on a liquid hydrogen target, have been measured forp _T in the range 4.0 to 6.5 GeV/c and for |x _F |<0.45. A quantitative comparison of the prompt photon cross section with next-to-leading order QCD predictions using Duke and Owens structure functions is performed. Phenomenological fits to the π⁰ and prompt photon cross sections are given.     

Photoproduction of neutral pions on hydrogen at photon energies between 200 and 440 MeV

At the Bonn 500 MeV electron synchrotron the differential cross section of the reaction γ+p → π⁰+p has been measured detecting the recoil proton by a magnetic spectrometer. Angular distributions between pion c.m. angles of 50 ° and 160 ° were taken at photon energies between 220 and 420 MeV.     

Hard photons and neutral pions as probes of hot and dense nuclear matter

The dynamics of heavy-ion collisions is studied in an energy domain in the vicinity of the Fermi energy. The early history of the collision is analyzed from the theoretical and experimental point of view in which the message conveyed by bremsstrahlung photons and neutral pions is exploited. The Boltzmann-Uehling-Uhlenbeck model and the Dubna Cascade Model, both based on similar principles but each adopting different computation technics, are briefly described and their respective predictions are discussed. In particular the emission pattern of bremsstrahlung photons is discussed. The photon production has been measured in the systems ⁸⁶Kr+⁵⁸Ni at 60A MeV, ¹⁸¹Ta+¹⁹⁷Au at 40A MeV and ²⁰⁸Pb+¹⁹⁷Au at 30A MeV and energy spectra, angular distributions and two-photon correlations have been analyzed. We find that bremsstrahlung photons are emitted from two distinct sources that can be correlated with nuclear-matter density oscillations. The properties of photon emission are discussed in terms of collective properties of nuclear matter. The high energy tail of the photon spectrum is interpreted by π⁰ and Δ decay but predominantly by radiative capture of pions. The π⁰ absorption in the nuclear medium is further analyzed by examining their emission pattern.     

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.     

Results from neutral kaon regeneration at high energies

The experimental neutral kaon regenerations results at Serpukhov energies up to 50 GeV are presented, including the coherent regeneration on hydrogen, deuterium and carbon regenerators and elastic regeneration on deuterium and preliminary carbon regenerators.Presented at the Symposium on Hadron-Hadron Scattering at High Energies, Liblice, Czechoslovakia, June 16–21, 1975.I wish to thank all my collegues who participated in the regeneration experiments.     

Polarization entangled photons at x-ray energies

We show that polarization entangled photons at x-ray energies can be generated via spontaneous parametric down-conversion. Each of the four Bell states can be generated by choosing the angle of incidence and polarization of the pumping beam.     

Fluctuations of the number of neutral pions at high multiplicity in pp interactions at 50 GeV

Results obtained by measuring fluctuations of the number of neutral pions in the SERP-E-190 Experiment (Thermalization Project) upon irradiating a liquid-hydrogen target of the SVD-2 setup with a beam of 50-GeV protons are presented. A simulation of the detection of photons from the decay of neutral pions with the aid of an electromagnetic calorimeter revealed a linear relation between the number of detected photons and the mean number of neutral pions in an event. After the introduction of corrections for the loss of charged tracks because of a limited acceptance of the setup, trigger operation, and the efficiency of the data-treatment system, distributions of the number of neutral pions, N ₀, were obtained for each value of the total number of particles in an event, N _tot = N _ch + N ₀. The fluctuation parameter ?? = D/??N ₀?? was measured. In the region N _tot > 22, fluctuations of the number of neutral pions increase, which, within statistical models (GCE, CE, MCE), indicates that the system involving a large number of pions approaches the pion-condensate state. This effect was observed for the first time.     

Direct photons in nuclear collisions at fair energies

Using the extrapolation of existing data, estimations of prompt-photon production at FAIR energies have been made. At y = y _c.m. the rapidity density of prompt photons with p _t > 1.5 GeV/c per central Au + Au event at 25 A GeV is estimated as ∼10⁻⁴. With the planned beam intensity 10⁹ per second and 1% interaction probability, for 10% of most central events one can expect the prompt-photon rate ∼10² photons per second. Direct photons from the hadron scenario of ion collisions generated by the Hadron-String-Dynamics (HSD) transport approach with implemented meson scatterings πρ → πγ, ππ → ργ have been analyzed. Photons from short-living resonances (e.g., ω → π ⁰ γ) decaying during the dense phase of the collision should be considered as direct photons. They contribute significantly in the directphoton spectrum at p _t = 0.5–1 GeV/c. At the FAIR energy 25 A GeV in Au + Au central collisions the HSD generator predicts, as a lower estimate, γ _direct/ ≃ 0.5% in the region p _t = 0.5–1 GeV/c. At p _t = 1.5–2 GeV/c γ _prompt/ ≃ 2%. Thermal direct photons have been evaluated with the Bjorken Hydro-Dynamics (BHD) model. The BHD spectra differ strongly from the HSD predictions. The direct-photon spectrumis very sensitive to the initial temperature parameter T ₀ of the model. The 10-MeV increase in the T ₀ value leads to ∼2 times higher photon yield. The text was submitted by the authors in English.     

Bremsstrahlung photons from cosmic ray muons of high and ultrahigh energies

The results from calculations for fluxes of bremsstrahlung photons generated at different depths in the atmosphere by cosmic ray muons with energies of up to 10¹⁰ GeV are presented. It is shown that the generation of charmed particles in the atmosphere, along with the generation of J/ψ mesons, must be taken into account in interpretations of experimental data on studying EASes at high and ultrahigh energies.     

Inclusive photoproduction of neutral pions in the photon hemisphere at HERA

The inclusive cross section for the photoproduction of neutral pions has been measured as a function of the transverse momentum, rapidity, and Feynman x of the mesons at an average photon–proton centre-of-mass energy of 208 GeV and for photon virtualities below GeV. The measurement extends the range covered by previous charged particle measurements at HERA by two units of rapidity in the photon direction down to a value of in the centre-of-mass frame. The transverse momentum distribution is well described over the whole measured range by a power law ansatz, while an exponential fit falls below the data at transverse momentum values above 1.5 GeV/c. Good agreement with the predictions of the Monte Carlo models PYTHIA and PHOJET is found. In the context of the PYTHIA model the data are inconsistent with large intrinsic transverse momentum values in the photon. Received: 14 June 2000 / Published online: 27 November 2000     

Thermodynamics at high energies

A new thermodynamics, applicable to cosmic ray showers and high-energy physics, is developed. Although all density expressions are unaltered, their global forms are modified due to the new dependence between the volume and the temperature. This occurs in bound systems where the number of particles, instead of being an increasing function of the temperature, is a decreasing one. That certain global expressions for the entropy turn out to be convex functions of the energy necessitates their reinterpretation as the reduction in entropy caused by the volume-temperature constraint. The continuous distribution for the production of hadrons with energies greater than a given amount is shown to correspond to the fact that discrete particle fluctuations follow Poisson's law.     

Disintegration of the deuteron by linearly polarized photons at low energies

The cross section and the Σ-asymmetry of the reaction d(γ^→,p) were measured in the energy interval E_γ = 5–10 MeV. Cross sections dσ and dσ_⊥ were derived from the measured quantities dσ₀ and Σ-asymmetry. The results are compared with theoretical calculations available for this energy region.     

On the measurement of correlations of neutral pions in a high multiplicity environment

The role of the combinatorial background of photon pairs in the analysis of correlations of neutral pions is investigated. The influence of four-particle effects in the Bose-Einstein enhancement is studied within a simulation. The HBT effect can be observed even in a high multiplicity environment, if the source size is not too large. A new method for neutral pion interferometry is introduced, which would allow also the measurement of large sources in high statistics experiments.     

Transverse momentum distributions of neutral pions from nuclear collisions at 200 AGeV

New results on transverse mass spectra of neutral pions measured at central rapidity are presented for impact parameter selected 200 GeV S + S and S + Au collisions. The distributions cover more than 8 orders of magnitude in cross section over the range . Detailed comparisons to results from pp collisions are made. The spectra from all systems show a clear power-law like shape with similar curvature. Collisions of S + Au exhibit a larger mean transverse momentum than pp increasing with centrality. Predictions of string models and by hydrodynamic approaches including collective expansion and decays of short lived resonances are compared to the data and the implications are discussed. Received: 10 March 1998 / Revised version: 22 May 1998 / Published online: 12 August 1998     

Elastic photoproduction of neutral pions from helium

Zeitschrift für Physik A Hadrons and nuclei - Differential cross sections are calculated for the elastic photoproduction of neutral pions from He4 under the impulse approximation. We use a...     

Pomeron and odderon at high energies

One of the crucial questions which high energy elastic data ought to be able to answer is: is the odderon contribution really necessary? Using a model with mildly energy dependent form factors as input and performing full eikonalization before we compare with the data, the answer that we find from a careful examination of the various options is that the odderon is indeed necessary if: 1) we trust the ISRpp data to be precise enough that their comparison with the collider data onpc \(\bar p\) is meaningful, 2) we demandquantitative (i.e. not justqualitative) agreement between theory and experiment and 3) we account for the large |t| data.     

Gamma-ray bursts at high and very high energies

Gamma-Ray Bursts (GRBs) are extra-galactic and extremely energetic transient emissions of gamma rays, which are thought to be associated with the death of massive stars or the merger of compact objects in binary systems. Their huge luminosities involve the presence of a newborn stellar-mass black hole emitting a relativistic collimated outflow, which accelerates particles and produces non-thermal emissions from the radio domain to the highest energies. In this article, I review recent progresses in the understanding of GRB jet physics above 100 MeV, based on Fermi observations of bright GRBs. I discuss the physical implications of these observations and their impact on GRB modeling, and I present some prospects for GRB observation at very high energies in the near future.     

Intermittency regions at high energies

We investigate the non-statistical multiplicity fluctuations (intermittency) of hadrons produced in high-energy collisions. Within the framework of the two-mechanism model, we determine the limits of the intermittent regions. We observe that these regions decrease in extent with increasing total energy $\sqrt s$ . In addition, we determine the value of the exponent from the universal scaling law (discovered recently by R. Hwa et al.). The value of this exponent is close to corresponding experimental data.