Photon-photon Collisions Research Articles

Measurement of charm production in two-photon processes using inclusive lepton events at TRISTAN

Measurements of open charm production in photon-photon collisions made with the AMY detector at TRISTAN are reported. Charmed hadrons were identified by detecting the high momentum muons or electrons from their semileptonic decays. The data sample corresponds to an integrated luminosity of 275 pb −1 at an average center of mass energy of 58 GeV. Results are presented in the form of cross sections of inclusive leptons from charm for both muons and electrons. The measured cross section is 1.8 standard deviations higher than theoretical predictions based on the direct and photon-gluon fusion process, where the mass of charm quark is assumed to be 1.6 GeV.

Probing the top-quark electric dipole moment at a photon linear collider

We probe the top-quark electric dipole moment (EDM) in top-quark pair production via photon-photon fusion at a photon linear collider. We show how linearly-polarized photon beams can be used to extract information on the top-quark EDM without the use of complicated angular correlations of top-quark decay products. If the luminosity of the laser back-scattered photon-photon collisions is comparable to that of the e + e − collisions, then the measurement of the top-quark EDM obtained by counting top-quark-production events in photon fusion can be as accurate as the measurement obtained by studying the t t decay correlations in e + e − collisions using a perfect detector.

Measurement of the D ∗± cross section in two photon collisions at LEP

The inclusive production of D ∗± mesons in photon-photon collisions has been measured by the Aleph experiment at LEP with a beam energy of 45 GeV. The D ∗+ are detected in their decay to D 0 π + with the D 0 observed in three separate decay modes: (1) K − π +, (2) K − π + π 0 and (3) K − π + π − π +, and analagously for the D ∗− modes. A total of 33 events was observed from an integrated luminosity of 73 pb −1 which corresponds to a cross section for Σ( e +e − → e +e −D ∗±X ) of 155 ± 33 ± 21 pb. This result is compatible with both the direct production γγ → c c in the Born approximation and with a more complete calculation which includes both radiative QCD corrections and contributions in which one of the photons is first resolved into its quark and gluon constituents. The shapes of distributions for events containing a D ∗+ are found to be better described by the latter.

Extracting the intermediate-mass Higgs resonance at photon-photon colliders.

We address the problem of how to extract the signal of a Higgs boson within the intermediate mass range at a photon-photon collider that has a wide energy spectrum. All backgrounds from two-jet production are included: direct, so-called resolved and twice-resolved, as well as single Z and W production. Uncertainties in the evaluation of the QCD-initiated processes such as the choice of structure function and the issue of radiative corrections are discussed. We consider various combinations of the polarizations and invariant mass resolutions as well as jet-tagging strategies with different efficiencies. The analysis is based on an automatized technique that, given a specific detector and machine configuration, returns the optimal set of cuts corresponding to the best significance one may hope to achieve for each particular Higgs boson mass. We find that at a photon machine obtained from a 500 GeV ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ linear collider with FscrL=10 ${\mathrm{fb}}^{\mathrm{\ensuremath{-}}1}$ it will be possible to extract a Higgs signal in the range 110--140 GeV, while with the same luminosity, a 350 GeV option not only extends the discovery limit down to 90 GeV but gives much better significance levels.

Gamma-ray-loud blazars and beaming

We study the entire sample of quasars detected above 100 MeV (by the EGRET instrument), on board CGRO. We find that the γ-ray luminosity correlates better with the radio luminosity than with the optical or X-ray luminosity, even if the different variability behaviour indicates different emission regions. A strong correlation is also found between the broad-band spectral indices αro, connecting the radio and the optical fluxes, and αoγ, connecting the optical and the γ-ray fluxes. The γ-ray luminosity in BLLac objects is on average much less dominant than in the other sources. With the requirement that the γ-rays do not interact with X-rays (assumed cospatial) through photon-photon collisions, we calculate the lower limit on the Doppler or beaming factor of the γ-ray emission region for a large fraction of the sources of the sample, finding that relativistic motion is required in all cases.

Production mechanisms for Bc mesons in photon-photon collisions

Using photon-photon collisions as a particularly transparent study case we investigate the production mechanisms for B c mesons. In the nonrelativistic approximation and to O( a 2 a s 2) it is shown that recombination of b- and c-quarks dominates by far over b and c fragmentation. This dominance persists up to the highest accessible transverse momenta and leads to distributions in energy which differ completely from the spectra expected on the basis of the fragmentation functions. For processes in which a b b- pair is radiated from a primary c-quark, the fragmentation description is found to be inadequate. We anticipate important implications of these results for hadronic production of heavy quark resonances. Using realistic photon spectra we predict two-photon production rates for B c and B c ∗ at present and future e + e −-machines.

Four weak gauge boson production at photon linear collider and heavy Higgs signal

We study the signals and backgrounds for a heavy Higgs boson in the processes γγ → WWWW, γ → WWZZ at the proton linear collider. The results are based on the complete tree-level SM calculation for these reactions. We show that the invariant mass spectrum of central WW, ZZ pairs is sensitive to the signal from Higgs boson with a mass up to 1 TeV linear collider for integrated luminosity of 300 fb −1. At 1.5 TeV PLC Higgs boson with a mass up to 700 GeV can be studied. The nonresonant longitudinal gauge boson scattering ( m H = ∞) can be detected in photon-photon collisions at e + e − center-of-mass energy of 3 TeV.

Transverse polarization of top quarks produced at a photon-photon collider.

At future \ensuremath{\gamma}\ensuremath{\gamma} colliders copious production of t t\ifmmode\bar\else\textasciimacron\fi{} pairs is possible. This would allow for a detailed investigation of the interactions involving the top quark. We propose some correlations which are sensitive to t t\ifmmode\bar\else\textasciimacron\fi{} final state interactions and we compute the QCD and standard model Higgs boson contributions to these correlations. A correlation resulting from the QCD induced transverse polarization of top quarks is found to be sizable and measurable at a high energy ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ collider, which is operated as a photon collider through backscattering of laser photons, at an integrated luminosity of 10 ${\mathrm{fb}}^{\mathrm{\ensuremath{-}}1}$.

Hot accretion disks with nonthermal pairs

view Abstract Citations (13) References (35) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hot Accretion Disks with Nonthermal Pairs Kusunose, Masaaki ; Mineshige, Shin Abstract The structure of hot accretion disks in active galactic nuclei (AGNs) is investigated, including soft photon injection, nonthermal relativistic electron injection, and pair cascades. A part of gravitational energy is used to accelerate nonthermal electrons, which produce γ-rays by Compton scattering of soft photons. Those γ-rays then create e^+/-^ pairs by photon-photon collisions. Pairs are thermalized before annihilation. Thermalized pairs contribute to thermal Compton scattering together with ionized electrons of accreting matter. The structure of a disk in the (surface density, accretion rate) plane is qualitatively the same as that of pure thermal disks, where no injection of nonthermal electrons occurs. There are three branches of solutions for a given surface density: the lower branch with a very low pair density and dominated by gas pressure, the middle branch dominated by pairs and gas pressure, and the higher branch dominated by pairs and radiation pressure. Critical rates related to pair production and radiation pressure, known to exist for pure thermal disks, are not found for the parameter sets used. The emission spectrum is dominated mainly by thermal and nonthermal Compton scattering of soft photons, which produces the power-law X-rays observed from AGNs. When the luminosity is high, e.g., more than 30% of Eddington luminosity if 30% of gravitational energy is directed to nonthermal injection, the feature of pair annihilation radiation appears around 511 keV. Our results show that sources of nonthermal radiation can exist self-consistently in a hot accretion disk near the central black hole. The validity of this model might be tested by future observations of pair annihilation lines. Publication: The Astrophysical Journal Pub Date: February 1995 DOI: 10.1086/175251 Bibcode: 1995ApJ...440..100K Keywords: Accretion Disks; Active Galactic Nuclei; Astronomical Models; Black Holes (Astronomy); Elementary Particle Interactions; Galactic Structure; Gravitation Theory; Photons; Relativistic Electron Beams; Annihilation Reactions; Compton Effect; Electron Acceleration; Gamma Rays; Pair Production; Stellar Luminosity; X Ray Astronomy; Astrophysics; ACCRETION; ACCRETION DISKS; BLACK HOLE PHYSICS; ELEMENTARY PARTICLES; GALAXIES: ACTIVE; RADIATION MECHANISMS: NONTHERMAL full text sources ADS | data products SIMBAD (2)

QCD corrections to [formula omitted] pair production in polarized γγ collisions and the intermediate mass Higgs signal

We present production rates of the two- and three-jet final states for the processes of massive c c b b quark production in circularly polarized photon-photon collisions, including QCD radiative corrections. Lowest-order cross section, one-loop virtual correction, and gluon emission correction are shown to be of the same order of magnitude for b b quark production at S γγ ∼ GeV . It is shown that the signal from an intermediate mass Higgs boson is observable at a photon-photon collider, though the statistical significance is substantially reduced with respect to the tree-level calculation.

High energy photon-photon collisions

The collisions of high energy photons produced at an electron-positron collider provide a comprehensive laboratory for testing QCD, electroweak interactions, and extensions of the standard model. The luminosity and energy of the colliding photons produced by backscattering laser beams is expected to be comparable to that of the primary e +e − collisions. In this overview, we shall focus on tests of electroweak theory in photon-photon annihilation, particularly γγ → W +W −, γγ → Higgs bosons, and higher-order loop processes, such as γγ → γγ, Zγ and ZZ. 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. We also review high energy γγ tests of quantum chromodynamics, such as the scaling of the photon structure function, t t production, mini-jet processes, and diffractive reactions.

Production of four-weak-bosons and heavy Higgs signals in TeV photon-photon collisions

We have studied the signals for a heavy Higgs boson in the processes γγ → WWWW, and γγ → WWZZ at a photon linear collider. The results are based on the first complete tree-level calculation for these reactions. We show that, with a forward “spectator” W tag, and a central “spectator” W veto to suppress backgrounds from transverse W, Z production, the invariant mass spectrum of central WW, ZZ pairs is sensitive to Higgs bosons with a mass up to 1 TeV in a 2-TeV linear collider.

Conventional detectors for a photon-photon collider

Detectors for a photon-photon collider are envisaged using as guide-lines the physics goals and the interaction point environment. Production of SUSY Higgs scalar and pseudo-scalar is emphasized. Some aspects of the interaction point environment are discussed.

Models of pair annihilation in 1E 1740.7-2942 and the HEAO 1 A-4 annihilation source

We study possible models of two Galactic sources of transient pair annihilation radiation, 1E 1740.7-2942 and a source observed by High Energy Astronomy Observatory (HEAO) 1 A-4. We fit the observed spectral features by thermal annihilation spectra and find that the redshifts obtained by us are much larger than those obtained from fitting Caussian lines centered on 511 keV. This effect, which is due to the net blueshift (with respect to 511 keV) of the annihilation spectrum due to the thermal energies of pairs, puts strong constraints on models of sources. We consider those constraints first without considering the mechanism of positron production. From the shape of the observed spectra, we are able to rule out both spherical clouds and layers above cold matter as possible source geometries. The observed spectra are compatible with two source geometries: (1) a nearly face-on disk in the Kerr metric and (2) a jet close to a black hole. We consider, then, the origin of the pairs. Theories of both thermal and nonthermal pair equilibria predict that photon-pair production is unable to produce annihilation features that contain as much as half of the bolometric luminosity, which is observed. A possible solution to this problem is obscuration of a nonthermal source (in which pairs are produced by photon-photon collisions) and an outflow of pairs to an unobscured region. This makes annihilation in a jet the most likely model of the considered sources.

CP violation in photon-photon collisions

The effective lagrangian parametrization is used to determine the CP violating effects in γγ collissions. for the processes studied the effects are found to be very small, the one exception being scalar production.

CP violation in photon-photon collisions

The effective lagrangian parametrization is used to determine the CP violating effects in γγ collissions. for the processes studied the effects are found to be very small, the one exception being scalar production.

Jets in Photon-Photon Collisions: From TRISTAN to J/N-LC

particle spectrum, has been measured at theMARK II [3] detector. Theoretical work on this subject started long ago[4],[5]. Here for the first time, we discuss the QCD predictions, at the next-to-leading logarithm accuracy, of the production of jets in γ − γ collisions.This study seems appropriate at this time since the above data are ratherprecise and, furthermore, data at higher energies are coming from LEP [6]. Itis well known that the rate of γ −γ events increases with energy in contrastto the rate of e

Low-energy photon-photon collisions to two-loop order

We evaluate the amplitude for γγ → π 0 π 0 to two loops in chiral perturbation theory. The three new counterterms which enter at this order in the low-energy expansion are estimated with resonance saturation. We find that the cross section agrees rather well with the available data and with dispersion theoretic calculations even substantially above threshold. Numerical results for the Compton cross section and for the neutral pion polarizabilities are also given to two-loop accuracy.

Testing the Higgs system at a photon-photon collider

Testing the Higgs system at a photon-photon collider

Higgs boson interference in γγ→ W+W−

We study interference effects between resonant and nonresonant amplitudes for the γγ → W + W − process at a backscattered photon-photon collider. We show that a Higgs boson with M H ⪆ 200 GeV is manifest as a resonant dip in the W + W − invariant mass spectrum and we investigate its statistical significance.