Observation of the muon inner bremsstrahlung at?LEP1

Muon bremsstrahlung photons converted in front of the DELPHI main tracker (TPC) in dimuon events at LEP1 were studied in two photon kinematic ranges: 0.2<E _γ ≤1 GeV and transverse momentum with respect to the parent muon p _T <40 MeV/c, and 1<E _γ ≤10 GeV and p _T <80 MeV/c. A good agreement of the observed photon rate with predictions from QED for the muon inner bremsstrahlung was found, contrary to the anomalous soft photon excess that has been observed recently in hadronic Z ⁰ decays. The obtained ratios of the observed signal to the predicted level of the muon bremsstrahlung are 1.06±0.12±0.07 in the photon energy range 0.2<E _γ ≤1 GeV and 1.04±0.09±0.12 in the photon energy range 1<E _γ ≤10 GeV. The bremsstrahlung dead cone is observed for the first time in the direct photon production at LEP. Deceased.     

Measurement of isolated photon production in deep-inelastic scattering at HERA

The production of isolated photons in deep-inelastic scattering ep→eγX is measured with the H1 detector at HERA. The measurement is performed in the kinematic range of negative four-momentum transfer squared 4<Q²<150 GeV² and a mass of the hadronic system W_X>50 GeV. The analysis is based on a total integrated luminosity of 227 pb^-1. The production cross section of isolated photons with a transverse energy in the range 3<E_T ^γ<10 GeV and pseudorapidity range -1.2<η^γ<1.8 is measured as a function of E_T ^γ, η^γ and Q². Isolated photon cross sections are also measured for events with no jets or at least one hadronic jet. The measurements are compared with predictions from Monte Carlo generators modelling the photon radiation from the quark and the electron lines, as well as with calculations at leading and next to leading order in the strong coupling. The predictions significantly underestimate the measured cross sections.     

Measurement of prompt photons with associated jets in photoproduction at HERA

The photoproduction of prompt photons, together with an accompanying jet, has been studied in ep collisions at a centre-of-mass energy of 318 GeV with the ZEUS detector at HERA using an integrated luminosity of 77 pb^-1. Cross sections were measured for the transverse energy of the photon and the jet larger than 5 and 6 GeV, respectively. The differential γ+jet cross sections were reconstructed as functions of the transverse energy, pseudorapidity and x_γ ^obs, the fraction of the incoming photon momentum taken by the photon-jet system. Predictions based on leading-logarithm parton-shower Monte Carlo models and next-to-leading-order (NLO) QCD generally underestimate the cross sections for the transverse energies of prompt photons below 7 GeV, while the k_T-factorisation QCD calculation agrees with the data better. When the minimum transverse energy of prompt photons is increased to 7 GeV, both NLO QCD and the k_T-factorisation calculations are in good agreement with the data.     

Inclusive D*± meson cross sections and D*±-jet correlations in photoproduction at HERA

Differential photoproduction cross sections are measured for events containing D^*± mesons. The data were taken with the H1 detector at the ep collider HERA and correspond to an integrated luminosity of 51.1 pb^-1. The kinematic region covers small photon virtualities Q² < 0.01 GeV² and photon–proton centre-of-mass energies of 171 < W_γ p < 256 GeV. The details of the heavy quark production process are further investigated in events with one or two jets in addition to the D^*± meson. Differential cross sections for D^*+jet production are determined and the correlations between the D^*± meson and the jet(s) are studied. The results are compared with perturbative QCD predictions applying collinear- or k_t-factorisation.     

Measuring the photon fragmentation function at HERA

The production of final state photons in deep inelastic scattering originates from photon radiation off leptons or quarks involved in the scattering process. Photon radiation off quarks involves a contribution from the quark-to-photon fragmentation function, corresponding to the non-perturbative transition of a hadronic jet into a single, highly energetic photon accompanied by some limited hadronic activity. Up to now, this fragmentation function was measured only in electron–positron annihilation at LEP. We demonstrate by a dedicated parton-level calculation that a competitive measurement of the quark-to-photon fragmentation function can be obtained in deep inelastic scattering at HERA. Such a measurement can be obtained by studying the photon energy spectra in γ+(0+1)-jet events, where γ denotes a hadronic jet containing a highly energetic photon (the photon jet). Isolated photons are then defined from the photon jet by imposing a minimal photon energy fraction. For this so-called democratic clustering approach, we study the cross sections for isolated γ+(0+1)-jet and γ+(1+1)-jet production as well as for the inclusive isolated photon production in deep inelastic scattering.     

Measurement of photons via conversion pairs in $\sqrt{s_{NN}}$ =200 GeV Au+Au collisions with the PHENIX experiment at RHIC

Thermal photons can provide information on the temperature of the new state of matter created at RHIC. In the p_T region of 1–3 GeV/c thermal photons are expected to be the dominant direct photon source. Therefore, a possible excess compared to a pure decay photon signal due to a thermal photon contribution should be seen in the double ratio (γ/γ(π⁰))_Measured/(γ/γ(π⁰))_Simulated, if sufficient accuracy can be reached. We present a method to reconstruct direct photons by measuring e⁺e^--pairs from external photon conversions.     

Bremsstrahlung photon polarization for ee ± → ( e γ) e ± and ep → ( e γ) p high-energy collisions

The polarization of a bremsstrahlung photon during the processes ee ^± → (eγ)e ^± and ep → (eγ)p is calculated for peripheral kinematics in the high-energy limit, where the cross section does not decrease with the incident energy. When the initial electron is unpolarized (longitudinally polarized), the final photon can be linearly (circularly) polarized. The Stokes parameters of the photon polarization are calculated as a function of the kinematical variables of the process: the energy of the recoil particle, the energy fraction of the scattered electron, and the polar and azimuthal angles of the photon. The numerical results are given in the form of tables for typical values of the relevant kinematic variables. The text was submitted by the authors in English.     

Search for solar axions emitted in the M1-transition of 7Li* with Borexino CTF

Results of background measurements with a prototype of the Borexino detector were used to search for 478 keV solar axions emitted in the M1-transitions of ⁷Li^*. The Compton conversion of axion to a photon A+e→e+γ, axioelectric effect A+e+Z→e+Z, decay of axion in two photons A→2γ and Primakoff conversion on nuclei A+Z→γ+Z are considered. The upper limit on constants of interaction of axion with electrons, photons and nucleons – g_Aeg_AN≤(1.0–2.4)×10^-10 at m_A≤450 keV and g_Aγg_AN≤5×10^-9 GeV^-1 at m_A≤10 keV are obtained (90%c.l.). For heavy axions with mass at 100<m_A<400 keV the limits g_Ae<(0.7–2.0)×10^-8 and g_Aγ<10^-9–10^-8 are obtained in assumption that g_AN depends on m_A as for KSVZ axion model. These limits are stronger than obtained in previous laboratory-based experiments using nuclear reactor and artificial radioactive sources. PACS 14.80.Mz; 29.40.Mc; 26.65.+t     

The equivalent photon approximation for coherent processes at colliders

We consider coherent electromagnetic processes for colliders with short bunches, in particular the coherent bremsstrahlung (CBS). CBS is the radiation of one bunch particles in the collective field of the oncoming bunch. It can be a potential tool for optimizing collisions and for measuring beam parameters. A new simple and transparent method to calculate CBS is presented based on the equivalent photon approximation for this collective field. The results are applied to theφ-factoryDAΦNE. For this collider about 5·10¹⁴ d E _γ/E _γ photons per second are expected in the photon energyE _γ range from the visible light up to 25 eV.     

Soft photon production in 450 GeV/cp-Be collisions

We have measured the inclusivep _T spectra of soft photons produced at central and backward rapidities in 450 GeV/cp-Be collisions down to 1 MeV/c in transverse momentum. In the region 1<p _T <20 MeV/c an excess of photons over those expected from hadronic decays is observed. This excess is comparable, within systematic errors, with estimates of direct photons produced via hadronic bremsstrahlung. An upper limit is derived on the presence of additional sources of direct photons at small transverse momentum.     

The pseudoscalar hadronic channel contribution of the light-by-light process to the muon (<Emphasis Type="Italic">g</Emphasis>−2)<Subscript><Emphasis Type="Italic">μ</Emphasis></Subscript> within the nonlocal chiral quark model

The light-by-light contribution to the anomalous magnetic moment of muon (g−2) _μ from the hadronic exchanges in the neutral pseudoscalar meson channel is calculated in the nonlocal chiral quark model. The full kinematic dependence of the meson two-photon vertices from the virtualities of the mesons and photons is taken into account. It is demonstrated that the effect of the full kinematic dependence in the meson–photon vertices is to reduce the contribution of pseudoscalar exchanges comparing with the most of previous estimates and the result is a_m^PS,LbL=(5.85±0.87)·10^-10a_{\mu}^{\mathrm{PS,LbL}}=(5.85\pm0.87)\cdot10^{-10}. The status of various phenomenological and QCD short-distance constraints is discussed and the comparison with the predictions of other models is performed.     

Unparticle searches through gamma–gamma scattering

We investigate the effects of unparticles on γγ→γγ scattering for the photon collider mode of the future multi-TeV e⁺e^- linear collider. We show the effects of unparticles on the differential, and total scattering cross sections for different polarization configurations. Considering 1-loop standard model background contributions from the charged fermions and W^± bosons to the cross section, we calculate the upper limits on the unparticle couplings λ₀ to the photons for various values of the scaling dimension d (1<d<2) at = 0.5–5 TeV.     

Observation of the radiative kaon decay K − → µ−π0γν

Using data collected with the ISTRA+ spectrometer during the 2001 run of the U-70 proton synchrotron in Protvino, the first observation of the radiative kaon decay K ⁻ → μ⁻π⁰γν is reported. The ratio Br(K _μ3γ, 5 < E*_γ < 30 MeV)/Br(K _μ3) is found to be [0.270 ± 0.029(stat.) ± 0.026(syst.)]% and the ratio Br(K _μ3γ, 30 < E*_γ < 60 MeV)/Br(K _μ3) = [0.0448 ± 0.0068(stat.) ± 0.0099(syst.)]%. These ratios are consistent with the theoretical predictions 0.21 and 0.047%, respectively. The measured angular distribution asymmetry for the region 5 < E*_γ < 30 MeV, A(cos ϑ*_μγ) = 0.093 ± 0.141, is two standard deviations away from the theoretical prediction of 0.354. The measured asymmetry in the T-odd variable ξ = p_γ · (p_μ × p_π)/m _K ³ is −0.03 ± 0.13. The text was submitted by the authors in English.     

Spontaneous interference bremsstrahlung effect in the scattering of a relativistic electron by a nucleus in the field of two light waves

This paper presents, for the general relativistic case, a theoretical study of nonresonance spontaneous bremsstrahlung by an electron scattered by a nucleus in the field of two elliptically polarized light waves propagating in the same direction. We show that there are two significantly different kinematic regions: the noninterference region where the main multiphoton parameters are the Bunkin-Fedorov quantum parameters γ _1,2, and the interference region where interference effects play an important role and where the quantum interference parameters α(±) act as multiphoton parameters. We encounter the spontaneous interference bremsstrahlung effect in two cases: in the special case of the same linear polarization of both waves, and in the general case of elliptical polarization of the waves. The effect manifests itself in the interference region and is due to stimulated, correlated emission and absorption of photons of both waves. For moderately strong fields, we find the cross sections of spontaneous bremsstrahlung by an electron scattered by a nucleus in the given kinematic regions. Finally, we show that the differential cross section in the interference region with correlated emission (absorption) of equal numbers of photons of both waves can be much greater than the corresponding cross section in any other geometry. Zh. éksp. Teor. Fiz. 116, 1210–1240 (October 1999)     

Study of inclusive strange-baryon production and search for pentaquarks in two-photon collisions at LEP

Measurements of inclusive production of the Λ, Ξ^- and Ξ^*(1530) baryons in two-photon collisions with the L3 detector at LEP are presented. The inclusive differential cross sections for Λ and Ξ^- are measured as a function of the baryon transverse momentum, p_t, and pseudo-rapidity, η. The mean number of Λ, Ξ^- and Ξ^*(1530) baryons per hadronic two-photon event is determined in the kinematic range 0.4 GeV<p_t<2.5 GeV, |η|<1.2. Overall agreement with the theoretical models and Monte Carlo predictions is observed. A search for inclusive production of the pentaquark θ⁺(1540) in two-photon collisions through the decay θ⁺→pK⁰ _S is also presented. No evidence for production of this state is found.     

Diffractive photoproduction of D*±(2010) at HERA

Diffractive photoproduction of D^*±(2010) mesons was measured with the ZEUS detector at the ep collider HERA, using an integrated luminosity of 78.6 pb^-1. The D^* mesons were reconstructed in the kinematic range: transverse momentum p_T(D^*) > 1.9 GeV and pseudorapidity |η(D^*)|<1.6, using the decay D^*+→D⁰π⁺ _s followed by D⁰→K^-π⁺(+c.c.). Diffractive events were identified by a large gap in pseudorapidity between the produced hadronic state and the outgoing proton. Cross sections are reported for photon–proton centre-of-mass energies in the range 130 < W < 300 GeV and for photon virtualities Q² < 1 GeV², in two ranges of the Pomeron fractional momentum x_IP<0.035 and x_IP<0.01. The relative contribution of diffractive events to the inclusive D^*±(2010) photoproduction cross section is about 6%. The data are in agreement with perturbative QCD calculations based on various parameterisations of diffractive parton distribution functions. The results are consistent with diffractive QCD factorisation.     

Inclusive production of neutral pions in hadronic Z decays

A measurement of the inclusive production of π⁰ mesons in hadronic Z decays is presented and compared to Monte Carlo model predictions. The analysis is based on approximately 2 million hadronic events recorded with the ALEPH detector at LEP at a centre-of-mass energy of ?s = 91.2 GeV. Neutral pions are reconstructed using photons measured in the electromagnetic calorimeter and photons from conversion pairs. The inclusive π⁰ momentum spectrum is measured in the range 0.025 < x _p = p/p _beam < 1. In this range the number of π⁰ per hadronic Z is found to be 4.80 ± 0.07(stat) ± 0.31(sys). The differential inclusive π⁰ cross section is also measured as a function of transverse momentum with respect to the event plane (p _T ⁱⁿ and p _T ^out ).     

A BaF2 crystal array for high energy γ-ray measurements

We shall discuss about the scientific motivation and construction of a 7×7 BaF₂ crystal array at Variable Energy Cyclotron Centre, Calcutta. This detector would be used to measure high energy γ-ray photons from GDR decay and proton-neutron bremsstrahlung reactions at the present 88″ cyclotron and upcoming superconducting cyclotron at VECC, Calcutta. This detector can also be used to measure photons from quark-gluon plasma at the relativistic heavy ion collider (RHIC) in USA.     

Limits on the production of large transverse momentum direct photons deduced from the measurement of low-mass electron pairs

The hadronic production of electron pairs with masses between 200 and 500 MeV and large transverse momentum has been measured at the CERN Intersecting Storage Rings (ISR). The expected relation between low-mass electron pairs and real photons is used to determine the direct hadronic production of photons. Contrary to indications from some previous experiments, the observed spectrum is consistent with expectations from the decay of known mesons, and leads to a value for the ratio of direct photons to π⁰ of γ/π⁰ = (0.55 ± 0.92)% for 2 < p_T < 3 GeV and 〈√s〉 = 55 GeV.     

Large contribution of virtual Delbrück scattering to the emission of photons by relativistic nuclei in nucleus–nucleus and electron–nucleus collisions

Delbrück scattering is the elastic scattering of a photon in the Coulomb field of a nucleus via a virtual electron loop. The contribution of this virtual subprocess to the emission of a photon in the collision of ultra-relativistic nuclei, Z₁Z₂→Z₁Z₂γ, is considered. We identify the incoming virtual photon as being generated by one of the relativistic nuclei involved in the binary collision and the scattered photon as being emitted in the process. The energy and angular distributions of the photons are calculated. The discussed process has no infrared divergence. The total cross section obtained is 14 barn for Au–Au collisions at the RHIC collider and 50 barn for Pb–Pb collisions at the LHC collider. These cross sections are considerably larger than those for ordinary tree-level nuclear bremsstrahlung in the considered photon energy range, m_e⪡E_γ⪡m_eγ, where γ is the Lorentz factor of the nucleus. Finally, photon emission in electron–nucleus collisions, eZ→eZγ, is discussed in the context of the eRHIC option.