Photon-photon Collisions Research Articles

Single charged Higgs boson production in polarized photon collisions and the probe of new physics

We study single charged Higgs boson production in photon-photon collision as a probe of the new dynamics of Higgs interactions. This is particularly important when the mass ($M_{H^\pm}$) of charged Higgs bosons ($H^{\pm}$) is relatively heavy and above the kinematic limit of the pair production ($M_{H^\pm} > \sqrt{s}/2$). We analyze the cross sections of single charged Higgs boson production from the photon-photon fusion processes, $\gamma\gamma\to \tau \bar \nu H^+$ and $\gamma\gamma\to b \bar c H^+$, as motivated by the minimal supersymmetric standard model and the dynamical Topcolor model. We find that the cross sections at such a $\gamma\gamma$ collider can be sufficiently large even for $M_{H^\pm} > \sqrt{s}/2$, and is typically one to two orders of magnitude higher than that at its parent $e^-e^+$ collider. We further demonstrate that the polarized photon beams can provide an important means to determine the chirality structure of Higgs Yukawa interactions with the fermions.

Multi-electron production at high transverse momenta in ep collisions at HERA

Multi-electron production is studied at high electron transverse momentum in positron- and electron-proton collisions using the H1 detector at HERA. The data correspond to an integrated luminosity of 115 pb(-1). Di-electron and tri-electron event yields are measured. Cross sections are derived in a restricted phase space region dominated by photon-photon collisions. In general good agreement is found with the Standard Model predictions. However, for electron pair invariant masses above 100 GeV, three di-electron events and three tri-electron events are observed, compared to Standard Model expectations of 0.30 +/- 0.04 and 0.23 +/- 0.04, respectively.

Charmonium production in polarized high-energy collisions

We investigate the inclusive production of prompt $J/\ensuremath{\psi}$ mesons in polarized hadron-hadron, photon-hadron, and photon-photon collisions in the factorization formalism of nonrelativistic quantum chromodynamics (NRQCD) providing all contributing partonic cross sections in analytic form. In the case of photoproduction, we also include the resolved-photon contributions. We present numerical results appropriate for BNL RHIC-Spin, the approved SLAC fixed-target experiment E161, and the ${e}^{+}{e}^{\ensuremath{-}}$ and $\ensuremath{\gamma}\ensuremath{\gamma}$ modes of DESY TESLA. Specifically, we assess the feasibility to access the spin-dependent parton distributions in the polarized proton and photon. We also point out that preliminary data on $J/\ensuremath{\psi}$ inclusive production taken by the PHENIX Collaboration in unpolarized proton-proton collisions at RHIC tend to favor the NRQCD factorization hypothesis, while they significantly overshoot the theoretical prediction of the color-singlet model at large values of transverse momentum.

Physics Opportunities at a Photon–Photon Collider

The advent of back-scattered laser beams for e±e-colliders will allow detailed studies of a large array of high energy γγ and γe collision processes with polarized beams. These include tests of electroweak theory in photon-photon annihilation such as γγ → W+W-, γγ → Higgs bosons, and higher-order loop processes, such as γγ → γγ, Zγ, H0Z0and ZZ. Methods for measuring the anomalous magnetic and quadrupole moments of the W and Z gauge bosons to high precision in polarized electron-photon and photon-photon collisions are discussed. Since each photon can be resolved into a W+W-pair, high energy photon-photon collisions can also provide a remarkably background-free laboratory for studying WW collisions and annihilation. I also review high energy γγ and eγ tests of quantum chromodynamics, including the production of two gluon jets in photon-photon collisions, deeply virtual Compton scattering on a photon target, and leading-twist single-spin asymmetries for a photon polarized normal to a production plane. Exclusive hadron production processes in photon-photon collisions provide important tests of QCD at the amplitude level, particularly as measures of hadron distribution amplitudes which are also important for the analysis of exclusive semi-leptonic and two-body hadronic B-decays.

Isolated prompt photon production in photon-photon collisions at √ See = 183–209 GeV

An OPAL measurement of isolated prompt photon production in photon-photon collisions at center-of-mass energies between 183 and 209 GeV is presented. The total cross-section for isolated prompt photons with transverse momenta p T of more than 3 GeV and with pseudorapidities | η| < 1 is measured to be σ = 0.26 ± 0.04(stat.) ± 0.03(syst.) pb. Differential cross-sections as functions of p T and |η| are compared to the predictions of an NLO QCD calculation and of the PYTHIA Monte Carlo program.

Detecting and studying Higgs bosons at a photon-photon collider

We examine the potential for detecting and studying Higgs bosons at a photon-photon collider facility associated with a future linear collider. Our study incorporates realistic $\gam\gam$ luminosity spectra based on the most probable available laser technology. Results include detector simulations. We study the cases of: a) a SM-like Higgs boson; b) the heavy MSSM Higgs bosons; c) a Higgs boson with no $WW/ZZ$ couplings from a general two Higgs doublet model.

The QCD pomeron in ultraperipheral heavy ion collisions: I. The double $J/\Psi$ production

The contribution of the QCD Pomeron to the process AA -> AA J/Psi J/Psi is discussed. We focus on the photon-photon collision, with the quasi-real photon coming from the Weizsacker - Williams spectrum of the nuclei. We calculate the cross section for this process considering the solution of the LLA BFKL equation at zero momentum transfer using a small-t approximation for the differential cross section of the subprocess. Furthermore, the impact of non-leading corrections to the BFKL equation is also analyzed. In both cases the cross section is found to increase with the energy, predicting considerable values for the LHC energies. Moreover, we compare our results with the Born two-gluon approximation, which is energy independent at the photon level. Our results indicate that the experimental analyzes of this process can be useful to discriminate the QCD dynamics at high energies.

Radiative production of invisible charginos in photon-photon collisions

If in a supersymmetric model, the lightest chargino is nearly degenerate with the lightest neutralino, the former can decay into the latter alongwith a soft pion (or a lepton-neutrino pair). Near degeneracy of the chargino and neutralino masses can cause the other decay products (the pion or the lepton) to be almost invisible. Photon-photon colliders offer a possibility of clean detection of such an event through a hard photon tag.

Chargino production and decay in photon-photon collisions

We discuss the pair production of charginos in collisions of polarized photons $\gamma\gamma \to \tilde{\chi}_i^+ \tilde{\chi}_i^-$, ($i=1,2$) and the subsequent leptonic decay of the lighter chargino $\tilde{\chi}_1^+ \to \tilde{\chi}_1^0 e^+ \nu_e$ including the complete spin correlations. Analytical formulae are given for the polarization and the spin-spin correlations of the charginos. Since the production is a pure QED process the decay dynamics can be studied separately. For high energy photons from Compton backscattering of polarized laser pulses off polarized electron beams numerical results are presented for the cross section, the angular distribution and the forward-backward asymmetry of the decay positron. Finally we study the dependence on the gaugino mass parameter $M_1$ and on the sneutrino mass for a gaugino-like MSSM scenario.

High-energy QCD asymptotic behavior of photon-photon collisions

The high-energy behavior of the total cross section for highly virtual photons, as predicted by the BFKL equation at next-to-leading order (NLO) in QCD, is discussed. The NLO BFKL predictions, improved by the BLM optimal scale setting, are in good agreement with recent OPAL and L3 data at CERN LEP2. NLO BFKL predictions for future linear colliders are presented.

Heavy quark production in photon-nucleon and photon-photon collisions

We discuss several mechanisms of heavy quark production in (real) photon-nucleon and (real) photon - (real) photon collisions. In particular we focuse on application of the Saturation Model. We discuss how to generalize the formula from virtual photon - proton scattering and analyze threshold effects. We discuss a possibility to measure the cross section for $\gamma \gamma \to 2 c 2 \bar c$. In addition to the main dipole-dipole contribution included in a recent analysis, we propose how to calculate within the same formalism the hadronic single-resolved contribution to heavy quark production. At high energies this yields a sizeable correction of about 30-40 % for inclusive charm production and 15-20 % for bottom production. We consider a subasymptotic component to the dipole-dipole approach. We get a good description of recently measured $\sigma(\gamma \gamma \to c \bar c X)$. Adding all possible contributions to $e^+ e^- \to b \bar b X$ together removes a huge deficit observed in earlier works but does not solve the problem totally. Whenever possible, we compare the present approach to the standard collinear one. We propose how to distinguish different mechanisms by measuring heavy quark-antiquark correlations.

Single Chargino Production with R-Parity Lepton Number Violation in Photon-Photon Collisions**The project supported in part by National Natural Science Foundation of China (19675033, 10005009), the Education Ministry of China and the State Commission of Science and Technology of China

We examine the process at photon-photon collider in the minimal supersymmetric standard model with R-parity violation, where all the one-loop diagrams are considered. We mainly discuss the effects of bilinear breaking terms, and conclude that their contributions may be important compared with trilinear terms. Our results show that the events of this process could be detectable at photon-photon colliders, if the values of the parameters are favorable.

Resummation of large QCD logarithms inH→γγ

We study the strong corrections to the Higgs coupling to two photons. This coupling is the dominant mechanism for Higgs production in photon-photon collisions. In addition, the two photon decay mode of the Higgs is an important and relatively background free channel of relevance at the LHC and the Tevatron. We develop a method for the resummation of large QCD corrections in the form of Sudakov-like logarithms of the type \alpha_s^n\ln^{2n}(m/m_H) and \alpha_s^n\ln^{2n-1}(m/m_H) (where m is the light quark mass) which can contribute to this process in certain models (for example, the MSSM for large tan \beta) up to next-to-leading-logarithmic (NLL) accuracy. The NLL correction is moderate, the substantial part of which comes from terms not related to running coupling effects.

QCD MEASUREMENTS IN PHOTON-PHOTON COLLISIONS AT LEP

An overview of the latest results of the LEP collaborations on QCD measurements in photon-photon collisions is presented, including measurements of the total hadronic cross-section, the production of heavy quarks and dijets and the structure functions of real and virtual photons.

Inclusive $D^*$ production in photon-photon collisions at next-to-leading order QCD}

The next-to-leading order cross section for the inclusive production of charm quarks in gamma-gamma collisions is calculated as a function of the transverse momentum pT and the rapidity y in approaches using massive or massless charm quarks. For the direct cross section we derive the massless limit from the massive theory with the result that this limit differs from the massless version with MSbar factorization by finite corrections. Subtracting or adding these corrections allows us to compare the two approaches on equal footing. We establish massless and massive versions with 3 and 4 initial flavours which are shown to approach the massless approximations very fast with increasing pT. With these results we calculate the inclusive D* cross section in gamma-gamma collisions using realistic evolved fragmentation functions with appropriate factorization scales and compare with recent data for dsigma/dpT from three LEP collaborations after single- and double-resolved contributions have been added.

DoubleJ/ψproduction at photon colliders

The double J/\psi(DJ) production in direct photon-photon collision is investigated. It is found that the J/\psi production rate in this process is of the same order of magnitude as those of previously discussed ones, which hints the dominant J/\psi inclusive production process in the direct photon-photon collision is still not being touched. As DJ production process has a clear distinction from the single J/\psi inclusive processes in the final states, experimental study on it would be helpful to clarify the validity of color-singlet model.

Loop inducedW±H∓associated production via photon-photon collisions in the two-Higgs-doublet model and the minimal supersymmetric standard model

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Charm and beauty production in photon-photon collisions

Inclusive heavy flavour production in γγ collisions has been studied at the LEP collider by ALEPH, L3, and OPAL. Preliminary results on charm and beauty production at ▪ = 189 GeV are presented. Hadronic final states containing charm particles were identified either by detecting electrons or muons from their semi-leptonic decays or by reconstruction of charged D * meson decays. The bb̄ cross section, the charm structure function F 2, c γ and the high energy behaviour of σ(γγ → cc̄) cross section as function of γγ centre-of-mass energy are measured for the first time in two-photon collisions.

Heavy quark pair production in polarized photon-photon collisions

We present the next-to-leading-order cross sections of the heavy quark-antiquark pair production in polarized photon-photon collision for the general case of photon polarizations. The numerical results for top-antitop production cross sections together with production asymmetries are obtained for linearly polarized photon-photon collisions, including one-loop QCD radiative corrections.

Study of dimuon production in photon-photon collisions and measurement of QED photon structure functions at LEP

Muon pair production in the process $e^+e^-\to e^+e^-\mu^+\mu^-$ is studied using the data taken at LEP1 ( $\sqrt{s}\simeq m_Z$ ) with the DELPHI detector during the years 1992-1995. The corresponding integrated luminosity is 138.5 pb $^{-1}$ . The QED predictions have been tested over the whole $Q^2$ range accessible at LEP1 (from several GeV $^2/c^4$ to several hundred GeV $^2/c^4$ ) by comparing experimental distributions with distributions resulting from Monte Carlo simulations using various generators. Selected events are used to extract the leptonic photon structure function $F_2^\gamma$ . Azimuthal correlations are used to obtain information on additional structure functions, $F_A^\gamma$ and $F_B^\gamma$ , which originate from interference terms of the scattering amplitudes. The measured ratios $F_A^\gamma/F_2^\gamma$ and $F_B^\gamma/F_2^\gamma$ are significantly different from zero and consistent with QED predictions.