Photon Final States Research Articles

Searches for new physics in events with photon final states at CDF at the TeVatron

The first CDF Run II analyses based on about 80 pb–1 on searches for events containing photons in the final state are presented. No deviations from the Standard Model predictions are found and limits on some scenarios of physics beyond the Standard Model are presented. PACS: 01.30.Cc Conference proceedings – 12.60.-i Models beyond the standard model

Searches for new physics in photonic final states at LEP

A brief review of searches for physics beyond the Standard Model in photonic final states at LEP is given here. These include searches for supersymmetry, large extra dimensions and contact interactions. Recent results from all four LEP experiments are presented, including improved limits on the new scale of gravity for models with large extra dimensions and the most precise direct measurement of the number of light neutrino species. Status and prospects of the LEP combined searches are also discussed.

Spontaneous emission rates of a single impurity molecule in a uniaxial host crystal

A theory of the dependence of the spontaneous emission rate of a single impurity molecule on the orientation of the transition dipole moment vector in a birefringent host crystal is developed. The dependences of the density of the emitted photon final states (expressed through the phase and the ray velocities of the photons) and the local and the average electric field strengths on the orientation of the wave vector of the photon in the host crystal and on the principal refractive indices are taken into account. In a uniaxial crystal we have calculated how the spontaneous emission rate depends on the ordinary and the extraordinary principal refractive indices and on the angle between the transition dipole moment vector and the optical axis.

Theory of deeply virtual Compton scattering on the nucleon

We compute the cross section for leptoproduction of the real photon off the nucleon, which is sensitive to the deeply virtual Compton scattering amplitude with power accuracy. Our considerations go beyond the leading twist and involve the complete analysis in the twist-three approximation. We discuss consequences of the target and lepton beam polarizations for accessing the generalized parton distributions from experimental measurements of the azimuthal angular dependence of the final state photon or nucleon. We introduce several sets of asymmetries, defined as Fourier moments with respect to the azimuthal angle, which allow for a clear separation of the twist-two and -three sectors. Relying on a simple ansatz for the generalized parton distributions, we give quantitative estimates for azimuthal and spin asymmetries, discuss the uncertainties of these predictions brought in by radiative corrections, and compare them with experimental data as well as other theoretical expectations. Furthermore, we derive a general parametrization of the DVCS amplitudes in the region of small Bjorken variable.

Factorization in deeply virtual Compton scattering: local OPE formalism and structure functions

We give a complete treatment of factorization of deeply virtual Compton scattering (DVCS) in the generalized Bjorken limit, using the local operator product expansion. The method allows a straightforward proof that, at leading twist, the DVCS amplitude factorizes into an integral over coefficient functions and skewed parton distribution functions. The integral is well defined for on-shell final state photons if the Wilson coefficients satisfy a certain factorization condition, which we derive. We also show that it enables a simple proof that soft singularities either cancel out or, in the case where the final state photon is on shell, are integrable. This confirms the argument of Collins and Freund. Further, we repeat the tree-level calculation of twist-three contributions to DVCS off a scalar target, where factorization was found to be violated. We propose a new definition of the structure functions and calculate the coefficient functions, which are such that factorization works.

Two-photon final states in peripheral heavy ion collisions

We discuss processes leading to two photon final states in peripheral heavy ion collisions at RHIC. Due to the large photon luminosity we show that the continuum subprocess $\gamma \gamma \to \gamma \gamma$ can be observed with a large number of events. We study this reaction when it is intermediated by a resonance made of quarks or gluons and discuss its interplay with the continuum process, verifying that in several cases the resonant process ovewhelms the continuum one. It is also investigated the possibility of observing a scalar resonance (the $\sigma$ meson) in this process. Assuming for the $\sigma$ the mass and total decay width values recently reported by the E791 Collaboration we show that RHIC may detect this particle in its two photon decay mode if its partial photonic decay width is of the order of the ones discussed in the literature.

Coherent exclusive exponentiation for precision Monte Carlo calculations

In this contribution we give a short overview of the new Coherent Exclusive Exponentiation (CEEX) which is implemented in the new ϰϰMC event generator for the process e +e − → f f + nγ, f = μ, τ, d, u, s, c, b with validity for center of mass energies from τ lepton threshold to 1TeV, that is for LEP1, LEP2, SLC, future Linear Colliders, b, c, τ-factories etc. In CEEX effects due to photon emission from initial beams and outgoing fermions are calculated in QED up to second-order, including all interference effects. Electroweak corrections are included in first-order, at the amplitude level. Beams can be polarized longitudinally and transversely, and all spin correlations are incorporated in an exact manner. We describe briefly the essence of CEEX as compared with older exclusive variants of the exponentiation (EEX) and present samples of the numerical results, concentrating on the question of the technical and physical precision for the total cross section and for the charge asymmetry. New results on the interference between initial and final state photon emission at LEP2 energies are also shown.

Photoinduced reactions on the nucleon at ELSA

With a tunable electron energy of up to 3.5 GeV the Electron Stretcher Accelerator ELSA at the University of Bonn covers the domain of nucleon resonances and exceeds the thresholds of the lowest lying nonets of pseudoscalar and vector mesons. ELSA delivers a beam current of I = 80 nA in the stretcher mode and up to I = 20 nA in the postaccelerator mode. The duty factor of up to 90 % allows to perform coincidence experiments in electron scattering and photoproduction with tagged photon beams. The electron scattering facility ELAN had been used to study elastic and transition form factors for the proton as well as for the neutron. Results on the deformation of N Δ are reported elsewhere in these proceedings ([1]). This experiment position is now used by a set-up for the measurement of the Gerasimov-Drell-Hearn sumrule. The work at SAPHIR focussed on the production of associated strangeness in KΛ, KΣ as well as vectormeson-production and nucleon resonances. In its place the Crystal Barrel detector with its excellent capability for the detection and spectroscopy of photonic final states has been set up and is presently in the commissioning phase. The main characteristics of these experiments, their aims and results are described on the following pages.

Final state photon production at LEP

We present a detailed study of photon production in hadronic events in electron-positron annihilation at LEP energies. We show that estimates of the inclusive photon spectrum using the quark-to-photon fragmentation function determined using the ALEPH ‘photon’ + 1 jet data agree well with the observations of the OPAL collaboration. This agreement shows that the photon fragmentation function determined in this way can be used for inclusive observables. We also compare next-to-leading order and beyond leading logarithm predictions obtained using the numerically resummed solutions of the fragmentation function evolution equation of Bourhis, Fontannaz and Guillet and Glück, Reya and Vogt with the data. Moreover, in order to check the general behaviour of the fragmentation function, we consider an analytic series expansion in the strong coupling. We find that the parameterizations are inaccurate at large x values. While the OPAL data is in broad agreement with estimates based on any of these approaches, the ALEPH data prefers the resummed BFG parameterization. Finally, there is some ambiguity as to whether the fragmentation function is treated as ${\cal O}(\alpha)$ or ${\cal O}(\alpha/\alpha_s)$ . We show that at present this ambiguity affects mainly the prediction for the ‘photon’ + 1 jet rate at large z.

Final state photon production at LEP

We present a detailed study of photon production in hadronic events in electron-positron annihilation at LEP energies. We show that estimates of the inclusive photon spectrum using the quark-to-photon fragmentation function determined using the ALEPH ‘photon’ + 1 jet data agree well with the observations of the OPAL collaboration. This agreement shows that the photon fragmentation function determined in this way can be used for inclusive observables. We also compare next-to-leading order and beyond leading logarithm predictions obtained using the numerically resummed solutions of the fragmentation function evolution equation of Bourhis, Fontannaz and Guillet and Glück, Reya and Vogt with the data. Moreover, in order to check the general behaviour of the fragmentation function, we consider an analytic series expansion in the strong coupling. We find that the parameterizations are inaccurate at large x values. While the OPAL data is in broad agreement with estimates based on any of these approaches, the ALEPH data prefers the resummed BFG parameterization. Finally, there is some ambiguity as to whether the fragmentation function is treated as ${\cal O}(\alpha)$ or ${\cal O}(\alpha/\alpha_s)$ . We show that at present this ambiguity affects mainly the prediction for the ‘photon’ + 1 jet rate at large z.

Properties of photon plus two-jet events inp¯pcollisions ats=1.8TeV

We present the first general measurements (invariant-mass, transverse-energy, and angular distributions) of the process, {bar p}p{r_arrow}{gamma}+2 jets+X, using data collected by the CDF at Fermilab. We compare the data with predictions from a tree-level QCD calculation and the PYTHIA shower Monte Carlo program. Our data sample is particularly sensitive to contributions from initial- and final-state radiation of photons and jets. Using the PYTHIA Monte Carlo program, we contrast the kinematical distributions for direct photon production with those for initial- and final-state photon radiation (bremsstrahlung). Based on the angular distributions, we find that our data favor a mixture of bremsstrahlung and direct photon production, as predicted, over either process alone. {copyright} {ital 1997} {ital The American Physical Society}

Observation of multiple hard photon final states at [formula omitted] GeV at LEP

We have studied the process e+e− → nγ (n ≥ 2) at an average center-of-mass energy of 133 GeV using the L3 detector at LEP. For an integrated luminosity of 4.95 pb−1 we find one γγγγ(γ) final state with only hard photons. The rates of both γγγ and γγ events are consistent with QED expectations. The cross section of the reaction e+e− → γγ(γ) in the polar range 16° < θγ < 164° is measured to be 22.6 ± 2.2 pb. Decays into photons of narrow scalar resonances with masses between 90 and 130 GeV are not observed. The observation of the event with four energetic photons is consistent with QED although the kinematic configuration of the photons is atypical.

Study of prompt photon production in hadronicZ 0 decays

From a sample of 1.5 millionZ 0→qq decays collected by the DELPHI detector during 1991, 1992 and 1993, the production cross section for isolated final state photons is measured and is compared with theO(α, α s) matrix element calculations implemented in the EEPRAD and GNJETS Monte Carlo generators. The observed photon yield is used to derive the electroweak couplings of charge 2/3 (u-type) and charge 1/3 (d-type) quarks to theZ 0 boson. The measured valuesc 2/3=0.91 −0.36 +0.25 andc 1/3=1.62 −0.17 +0.24 are compatible with the Standard Model prediction,c 2/3=1.145 andc 1/3=1.477.

Study of prompt photon production in hadronic $Z^0$ decays

From a sample of 1.5 millionZ 0→qq decays collected by the DELPHI detector during 1991, 1992 and 1993, the production cross section for isolated final state photons is measured and is compared with theO(α, α s) matrix element calculations implemented in the EEPRAD and GNJETS Monte Carlo generators. The observed photon yield is used to derive the electroweak couplings of charge 2/3 (u-type) and charge 1/3 (d-type) quarks to theZ 0 boson. The measured valuesc 2/3=0.91 −0.36 +0.25 andc 1/3=1.62 −0.17 +0.24 are compatible with the Standard Model prediction,c 2/3=1.145 andc 1/3=1.477.

Comparisons of the properties of final state photons in hadronic Z0 decays with predictions from matrix element calculations

The properties of hadronic Z0 decays with final state photons, measured with OPAL at LEP, have been compared with predictions from two different matrix element calculations ofO(αα s ). Two calculations, GNJETS and EEPRAD, have been investigated which use different schemes to restrict the phase space around the poles of the cross section. Assuming the E0-JADE jet definition, both calculations describe the data well in large regions of phase space fory cut values around 0.06. For very large and very small jet-photon masses some deviations from the predictions have been found, indicating the importance of higher order corrections. Significant differences between the calculations are only apparent in the predicted rate of 1-jet plus photon events. The rate is higher in GNJETS which reproduces the data better than EEPRAD.

Deep inelastic compton-scattering with polarized beam and target

The helicity difference of the photon photoproduction process γp→γx is calculated using four different polarized proton structure functions. Also discussed are the contributions originating from the Compton cross-section and the subprocesses of $$\mathcal{O}(\alpha \alpha _s )$$ induced by photon structure and fragmentation. Up-quark and gluondominance can be separated by choosing different regions of pseudorapidity η and normalized transverse momentumX′ T of the final-state photon. We show, that it is possible to distinguish between the two extreme models of proton spin: polarized gluon vice polarized strange-quark.

On the comparison of matrix element calculations ofO(ααs) with the measurement of photon emission in hadronicZ 0 decays

The uncertainties in interpreting photon bremsstrahlung in the process\(e^ + e^ - \to Z^0 \to q\bar q\) with matrix element calculations ofO(ααs) are discussed. We address the stability of the calculations with respect to the emission of collinear photons and to higher-order QCD corrections and discuss the bias due to experimental photon isolation cuts. We analyze the resulting uncertainties for various procedures to define an event with a final state photon. Of particular interest are (i) a two-step procedure where first jets are reconstructed from hadrons alone and in a second step the photon is required to be isolated from these jets, and (ii) a ‘democratic’ procedure where the photon is inculded in the jet reconstruction but a certain maximum hadronic energy is allowed in the photon jet. In both cases we estimate that the uncertainties of the theoretical predictions, hadronization effects and the experimental photon isolation are of the order of 4%. To obtain this level of accuracy, however, the democratic procedure requires very hard cuts that reduce the event samples significantly.

Evidence for spin-one resonance production in the reaction gamma gamma *--> pi + pi - pi 0 pi 0.

Using data from the TPC/Two-Gamma experiment at the SLAC e+e− storage ring PEP, a C=+1 resonance has been observed in the π+π−π0γ final state resulting from the fusion of one nearly real and one quite virtual photon. The actual decay channel is probably π+π−π0π0, where one final-state photon is not detected, and the mass of the fully reconstructed state would be approximately 1525 MeV. A four-pion decay mode in turn implies that the resonance has even isospin. The nonobservation of this R(1525) when both initial-state photons are nearly real suggests a spin-1 assignment. Since the large measured value of the product of the branching ratio into π+π−π0π0 and the γγ coupling makes it unlikely that this state is the mostly s¯s f1(1510), its interpretation may lie outside of conventional meson spectroscopy. There is a second, less-significant enhancement observed in the same reaction at a four-pion mass centered around 2020 MeV.Received 15 March 1993DOI:https://doi.org/10.1103/PhysRevD.48.3976©1993 American Physical Society

Studies of strong and electroweak interactions using final state photon emission in hadronicZ 0 decays

The cross section and jet rates ofZ0 decays into photons and quarks are compared with matrix element Monte Carlos ofO(ααs). Good agreement is found between data and theoretical predictions. From the ratio of events with three jets plus a photon to those with two jets plus a photon, αs in first order is determined to be 0.176±0.010. Combining the cross section of final state photon events with the LEP average hadronic partial decay width of theZ0, the widths of theZ0 into up and down type quarks can be calculated. The results $$\begin{gathered} \Gamma _{u - type} = 242 \pm 46MeV; \hfill \Gamma _{d - type} = 419 \pm 30MeV \hfill \end{gathered} $$ are in good agreement with the standard model expectation. A comparison of the measured cross section as a function ofycut with predictions of QCD shower models shows that, at the current level of accuracy ARIADNE and, to a lesser extent, HERWIG and JETSET can reproduce the measurement.

Study of final state photons in hadronicZ 0 decay and limits on new phenomena

The differential cross section for final state radiation from primary quarks is obtained from a study of isolated energetic photons produced in the reactione+e−→Z0→hadrons+γ, as measured in the DELPHI detector at the CERN LEP collider. When combined with the measurement of the total hadronic width of theZ0, the observed rate determines the electroweak coupling constants of up and down type quarks, i.e., $$v_{{1 \mathord{\left/ {\vphantom {1 3}} \right. \kern-\nulldelimiterspace} 3}}^2 + a_{{1 \mathord{\left/ {\vphantom {1 3}} \right. \kern-\nulldelimiterspace} 3}}^2 = 1.13 \pm 0.29 and v_{{2 \mathord{\left/ {\vphantom {2 3}} \right. \kern-\nulldelimiterspace} 3}}^2 + a_{{2 \mathord{\left/ {\vphantom {2 3}} \right. \kern-\nulldelimiterspace} 3}}^2 = 1.65 \pm 043.$$ No evidence is seen for additional photon production from anomalous decays of theZ0 or from decays of new particles. This measurement leads to upper limits on the production cross section times branching fraction of (a) the Higgs boson in the reactione+e−→Z0→H +γ,H→hadrons, (b) an excited quark,q*→q+γ, and (c) the contribution of an anomalous decay of theZ0 into a photon and hadrons. These limits, all at the 95% confidence level, vary from 3 to 10pb as the mass of the intermediate state (H,q* orZ*) varies from 10 GeV/c2 to 80 GeV/c2.