Radiative return at NLO and the measurement of the hadronic cross-section in electron–positron annihilation

Electron–positron annihilation into hadrons plus an energetic photon from initial state radiation allows the hadronic cross-section to be measured over a wide range of energies. The full next-to-leading order QED corrections for the cross-section for annihilation into a real tagged photon and a virtual photon converting into hadrons are calculated where the tagged photon is radiated off the initial electron or positron. This includes virtual and soft photon corrections to the process and the emission of two real hard photons: . A Monte Carlo generator has been constructed, which incorporates these corrections and simulates the production of two charged pions or muons plus one or two photons. Predictions are presented for centre-of-mass energies between 1 and 10 GeV, corresponding to the energies of DANE, CLEO-C and B-meson factories. Received: 14 December 2001 / Published online: 5 April 2002     

The radiative return at $\phi$- and B-factories: small-angle photon emission at next-to-leading order

The radiative return offers the unique possibility for a measurement of the cross section of electron-positron annihilation into hadrons over a wide range of energies. The large luminosity of present - and B-factories easily compensates for the additional factor of due to the emission of a hard photon. Final states with photons at large angles can be easily identified. The rate for events with collinear photons, however, is enhanced by a large logarithm and allows, in particular at lower energies, for a complementary measurement. The Monte Carlo generator PHOKHARA, which includes next-to-leading order corrections from virtual and real photon emission, has been extended from large photon angles into the collinear region, using recent results for the virtual corrections. In addition, the present version includes final state radiation for muon and pion pair production and final states with four pions. Implications for the experimental analysis at three typical energies, 1.02, 4 and 10.6 GeV, are presented: the magnitude of these new corrections is studied, possibilities for the separation of initial and final state radiation are proposed, and the differences with respect to the previous treatment based on structure functions are investigated. Received: 16 December 2002 / Published online: 26 February 2003 RID="a" ID="a" e-mail: czyz@us.edu.pl RID="b" ID="b" e-mail: grzel@joy.phys.us.edu.pl, Supported in part by "Marie Curie Training Site" at Karlsruhe University RID="c" ID="c" e-mail: jk@particle.uni-karlsruhe.de RID="d" ID="d" e-mail: german.rodrigo@cern.ch, Supported in part by E.U. TMR grant HPMF-CT-2000-00989     

Photoproduction of positive pions from hydrogen at large angles in the energy range 0.3–2.0 GeV

The differential cross-section for the reaction γp↦π ⁺ n was measured using the bremsstrahlung beam of the Bonn 2.5 GeV electron synchrotron. The pions were detected and momentum analysed in a multichannel magnet spectrometer. Data reduction resulted in 1278 measured cross-sections which are presented as energy distributions at six laboratory angles between 180° and 95°. The range of laboratory photon energies extended from 0.3 to 2.1 GeV. The statistical accuracy is better than 3 percent, the systematic error is about 5 percent. The data are compared with other experimental results and predictions of a phenomenological analysis. These cross-sections are the result of a measurement program which was started in the seventies. Further results will be reported in forthcoming publications. Received: 2 February 2000 / Accepted: 14 August 2001     

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     

Measurement of the target asymmetry for the reaction γp↑ → π+n at photon energies between 0.7 and 2.2 GeV and a pion CM-angle of 65°

At the Bonn 2.5.GeV electron synchrotron the target asymmetry for the photoproduction of positive pions has been measured. Data were taken at photon energies between 0.7 and 2.2 GeV and a pion CM-angle of 65°.     

Photoproduction of ϱ′(1.2) and ϱ′(1.6) in the final states π+ π− π+ π− and π+ π− π0 π0

The elastic photoproduction of four pions has been studied at incident photon energies between 2.8 and 4.8 GeV. Production cross-sections are presented and an analysis of the angular decay correlations is also described, indicating a large 1^? contribution in both final states, π⁺ π^? π⁺ π^? and π⁺ π^? π⁰ π⁰. A quantitative understanding of these and other available 4π photoproduction data in terms of the ρ′(～1.2GeV) and the ρ′(～1.6) is presented.     

Multiplicity of charged and neutral pions in deep-inelastic scattering of 27.5 GeV positrons on hydrogen

Measurements of the individual multiplicities of and produced in the deep-inelastic scattering of 27.5 GeV positrons on hydrogen are presented. The average charged pion multiplicity is the same as for neutral pions, up to , where z is the fraction of the energy transferred in the scattering process carried by the pion. This result (below ) is consistent with isospin invariance. The total energy fraction associated with charged and neutral pions is (syst.) and , respectively. For fixed z, the measured multiplicities depend on both the negative squared four momentum transfer and the Bjorken variable x. The observed dependence on agrees qualitatively with the expected behaviour based on NLO-QCD evolution, while the dependence on x is consistent with that of previous data after corrections have been made for the expected -dependence. Received: 2 April 2001 / Revised version: 27 June 2001 / Published online: 7 September     

Measurement of the low-x behaviour of the photon structure function F_2^\gamma

The photon structure function has been measured using data taken by the OPAL detector at centre-of-mass energies of 91 GeV, 183 GeVand 189 \GeV, in ranges of 1.5–30.0 GeV (LEP1), and 7.0–30.0 GeV (LEP2), probing lower values of x than ever before. Since previous OPAL analyses, new Monte Carlo models and new methods, such as multi-variable unfolding, have been introduced, reducing significantly the model dependent systematic errors in the measurement. The results do not conclusively prove, but are completely consistent with, the presence of a rise in at low-x as expected from QCD. Received: 9 June 2000 / Published online: 13 November 2000     

Hadron formation in deep-inelastic positron scattering in a nuclear environment

The influence of the nuclear medium on the production of charged hadrons in semi-inclusive deep-inelastic scattering has been studied by the HERMES experiment at DESY using a 27.5 GeV positron beam. The differential multiplicity of charged hadrons and identified charged pions from nitrogen relative to that from deuterium has been measured as a function of the virtual photon energy and the fraction z of this energy transferred to the hadron. There are observed substantial reductions of the multiplicity ratio at low and at high z, both of which are well described by a gluon-bremsstrahlung model of hadronization. A significant difference of the -dependence of is found between positive and negative hadrons. This is interpreted in terms of a difference between the formation times of protons and pions, using a phenomenological model to describe the - and z-dependence of . Received: 20 February 2001 / Revised version: 3 May 2001 / Published online: 8 June 2001     

Photoproduction of negative pions on a polarized neutron target in the resonance region

At the Bonn 2.5 GeV electron synchrotron the first measurements of the target asymmetry for the reaction γ + n ↑ → π^? + p have been performed. The negative pions were detected in a magnetic spectrometer at a constant pion c.m. angle of 40° and photon energies between 0.45 GeV and 2.0 GeV. Deuterated butanol was used as target material. The polarization of the deuterons was about 16%. The results show a significant difference from the previously measured π⁺ asymmetry.     

Pions in primary cosmic rays of ultrahigh energies

In the framework of the Coleman-Glashow hypothesis of an extremely weak violation of Lorentz invariance, neutral and charged pions can be stable for energies above 10¹⁹ eV and enter into the composition of primary cosmic rays of ultrahigh energies. The kinematic exclusion of reactions of pions with relic photons is particularly important, because it allows the Greisen-Zatsepin-Kuzmin paradox to be resolved. The parameters of extensive air showers induced by primary pions calculated within the model of quark-gluon strings with allowance for the Landau-Pomeranchuk-Migdal effect and interactions of neutral pions of ultrahigh energies are not contradictory to the available data of observations. It has been shown that observations of production heights of muons with energies above 10GeV will make it possible to distinguish between primary nuclei, protons, and pions; to verify Lorentz invariance for energies above 10²⁰ eV; and to obtain a new limit on the difference between the maximum possible velocities of muons and pions (c_µ?c_π)<4×10^?26.     

MO analysis of the high statistics Belle results on γ γ→π + π −, π 0 π 0 with chiral constraints

We reconsider Muskhelishvili–Omnès (MO) dispersive representations of photon–photon scattering to two pions, motivated by the very high statistics results recently released by the Belle collaboration for charged as well as neutral pion pairs and also by recent progress in the determination of the low-energy π π scattering amplitude. Applicability of this formalism is extended beyond 1 GeV by taking into account inelasticity due to  $K\bar{K}$ . A modified MO representation is derived which has the advantage that all polynomial ambiguities are collected into the subtraction constants and have simple relations to pion polarizabilities. It is obtained by treating differently the exactly known QED Born term and the other components of the left-hand cut. These components are approximated by a sum over resonances. All resonances up to spin two and masses up to ?1.3 GeV are included. The tensor contributions to the left-hand cut are found to be numerically important. We perform fits to the data imposing chiral constraints, in particular, using a model independent sum-rule result on the p ⁶ chiral coupling c ₃₄. Such theoretical constraints are necessary because the experimental errors are dominantly systematic. Results on further p ⁶ couplings and pion dipole and quadrupole polarizabilities are then derived from the fit. The relevance of the new data for distinguishing between two possible scenarios of isospin breaking in the f ₀(980) region is discussed.     

Double- π0 photoproduction from the deuteron

The photoproduction of two neutral pions from the deuteron has been studied for incident photon energies from 200 MeV to 820 MeV with the TAPS spectrometer at MAMI (Mainz Microtron). The total cross-section was determined and used to deduce the cross-section from the neutron. Due to the good statistical quality of the data Dalitz plots for the three particles in the exit channel (π⁰π⁰ N) could be constructed. The invariant mass distributions derived from them are presented in this paper. They indicate that the important reaction mechanism in the second resonance region is a sequential decay pattern involving the population of the Δ(1232)-resonance as an intermediate state. Received: 5 June 2000 / Accepted: 4 September 2000     

Measurement of \Sigma beam asymmetry in \pi^{0}_{} photoproduction off the neutron in the second and third resonances region⋆

The $ \Sigma$ beam asymmetry in the photoproduction of neutral pions from quasi-free nucleons in a deuteron target was measured for the first time between 0.60 and 1.50GeV, with the GRAAL polarized and tagged photon beam. The asymmetry values from the quasi-free proton were found equal to the ones extracted from a pure proton target. The asymmetries from quasi-free proton and quasi-free neutron were found equal up to 0.82GeV and substantially different at higher energies. The results are compared with recent partial-wave analyses.     

Charged particle identification with PHOS and central tracking of ALICE

The ALICE Photon Spectrometer (PHOS) has been designed to measure the photon spectrum produced in pp and AA collisions at LHC over a broad transverse momentum range from 0.5–100 GeV/c. We present a study on the matching of charged tracks, detected by the ALICE central tracking detectors, with clusters of hit PHOS elements. Matching efficiency and contamination due to false matches have been deduced for charged pions, muons and electrons. The study indicates also that material from other detectors in front of PHOS disturbs the photon detection.     

Constraints for the nuclear sea quark distributions from the Drell–Yan process at the SPS

Nuclear modifications to the Drell–Yan dilepton production cross sections in and collisions in the leading twist approximation are caused by nuclear effects in the parton distributions of bound nucleons. For non-isoscalar nuclei, isospin corrections must also be considered. We calculate these effects for and Pb+Pb collisions at CERN SPS energies. Our goal is to place constraints on nuclear effects in sea quark distributions in the region . We show that the net nuclear corrections remain small for collisions at GeV. However, in collisions at GeV, effects of are predicted at large M. The data collected by the NA50 collaboration could thus be used to constrain the nuclear effects in the sea quark distributions in the region of the EMC effect, . Received: 6 October 2000 / Revised version: 2 July 2001 / Published online: 19 September 2001     

Multiplicity and pseudorapidity distributions of photons in Au+Au collisions at square root of (S(NN)) = 62.4 GeV

We present the first measurement of pseudorapidity distribution of photons in the region 2.3 < or = eta < or = 3.7 for different centralities in Au+Au collisions at square root of (S(NN)) = 62.4 GeV. We find that the photon yield scales with the number of participating nucleons at all collision centralities studied. The pseudorapidity distribution of photons, dominated by pi0 decays, has been compared to those of charged pions, photons, and inclusive charged particles from heavy-ion and nucleon-nucleon collisions at various energies. The photon production has been shown to be consistent with the energy and centrality independent limiting fragmentation scenario.     

Dilepton and vector meson production in heavy-ion reactions

We present a nonperturbative dynamical study of dilepton and vector meson production in heavy-ion collisions from 1 to 2 GeV/A bombarding energies incorporating all known sources relevant in this energy range. The dynamical evolution of the nucleus-nucleus collision is described by a transport equation of the Boltzmann-Uehling-Uhlenbeck type evolving phase-space distribution functions for nucleons, baryon resonances, pions,η’s,ρ’s andσ’s with their isospin degrees of freedom. We investigate in particular the sensitivity of the calculated yields to predicted changes of theρ andω mesons in dense matter.     

Observation of the two photon cascade 3.7 → 3.1 + γγ via an intermediate state Pc

The two photon cascade decay of the 3.7 GeV resonance into the 3.1 GeV resonance has been observed in two nearly independent experiments. The clustering of the photon energies around 160 MeV and 420 MeV observed in the channel 3.7 → (3.1 → μ⁺μ^?) + γγ indicates the existence of at least one intermediate state with even charge conjugation at a mass around 3.52 GeV or 3.26 GeV.     

The Spectra of Cosmic Pions and Nucleons at Low and High Geomagnetic Latitudes at the Top of the Atmosphere

The spectra of cosmic pions and nucleons at an atmospheric depth of 100 g cm^?2 have been derived from sea level muon spectra at low and high latitudes. The derived spectra of pions and nucleons at low and high latitudes follow the relations N(Eπ) dEπ = 0.23 E_π^?2.68dEπ and N(E_p) dE_p = 2.98 E dE_p per cm² sec str GeV in the energy range 15–1200 GeV. Below 15 GeV it is found that the exponent of the energy spectrum is very much dependent upon the incident energy and latitude of the location. The major difference between low and high latitude pion spectra can be explained in terms of geomagnetic effects.