Cross sections of the e+e- annihilation into one photon plus two vector mesons, such as J/psi rho, J/psi phi, and rho phi at the center of mass energy \sqrt{s}=10.58GeV are calculated. These shed light on the measurement of the e+e- annihilation cross section via radiative return in B-factories. At another center of mass energy \sqrt{s}=3.097GeV, namely the J/psi resonance peak, the processes e+e- --> gamma VV and e+e- --> gamma gamma V (with V being rho, omega, or phi) are calculated with similar method. These calculations give estimation of the background levels in the study of radiative or double radiative decays of J/psi.
Recent results on total cross-section measurements in gamma-gamma and gamma*-gamma* collisions at LEP are reported. Phenomenological fits to the data are presented.
We perform a numerical study of various improvements of the LO BFKL prediction for the gamma^* gamma^* total cross section in the kinematical region of the L3 detector at LEP. In particular, we study the effects of a massive charm quark and of different photon polarizations. The variation of the BFKL prediction under changes of alpha_s is investigated.
A saturation model for the total gamma-gamma and gamma*-gamma* cross-sections and for the real photon structure function F_2^gamma(x,Q^2) is described. The model is based on a QCD dipole picture of high energy scattering. The two-dipole cross-section is assumed to satisfy the saturation property with the saturation radius taken from the GBW analysis of the gamma*-p interaction at HERA. The model is combined with the QPM and non-pomeron reggeon contributions an it gives a very good description of the data on the gamma-gamma total cross-section, on the photon structure function F_2^gamma(x,Q^2) …
We present a numerical estimate of the $γ^* γ^*$ total cross section at the designed 500 GeV $e^+e^-$ Linear Collider, based upon the BFKL Pomeron. We find that the event rate is substantial provided electrons scattered under small angles can be detected, and a measurement of this cross section provides an excellent test of the BFKL Pomeron.
We perform a detailed analysis of the total inelastic cross-section for gamma*gamma* collisions. Different contributions coming from the quark box diagram, reggeons, the soft and hard pomeron are included. The QCD pomeron contribution contains a dominant part of subleading effects which reduces its intercept and delays the onset of the asymptotic pomeron to high energies. Estimates of the cross-section for doubly-tagged (e+e- to e+e- hadrons) events are presented and compared with the existing LEP data. Good agreement between the theoretical results and the experimental data is found. We also …
Recent results from NLO QCD calculations for inclusive jet cross sections in gamma^*-gamma scattering at e+e- colliders, especially for LEP, are reported. The virtuality Q^2 of the virtual photon is non-zero and can be unlimited large. The virtuality of the second photon is zero and the spectrum is calculated with the Weizs"acker-Williams approximation. Four components of the cross sections have to be distinguished, involving direct and resolved real and virtual photon contributions. Since Q^2 is non-zero, the virtual photon structure function is needed to calculate the contributions involving…
Low Q2 photon-proton cross sections are analysed using a simple, QCD-motivated parametrisation $σ_{γ^\star p}\propto 1/(Q^2+Q_0^2)$, which gives a good description of the data. The Q2 dependence of the gamma* p cross section is discussed in terms of the partonic transverse momenta of the hadronic state the photon fluctuates into.
We study in the BFKL approach the total hadronic cross section for the collision of two virtual photons for energies in the range of LEP2 and in the range of future linear colliders. The BFKL resummation is done at the next-to-leading order in the BFKL Green's function; photon impact factors are taken instead at the leading order, but with the inclusion of the subleading terms required by invariance under changes of the renormalization scale and of the BFKL scale s_0. We compare our results with previous estimates based on a similar kind of approximation.
A recently suggested high-energy Born-form representation of the one-loop helicity amplitudes for e^+e^- to W^+W^- is supplemented by including W^+- decay and hard-photon radiation. Results for the differential and the total cross section for e^+e^- to W^+W^- to 4 fermions + gamma are given for the high-energy region of sqrt(s) > 500 GeV.
The total hadronic gamma*gamma* cross sections at high energy are calculated as a function of energy and photon virtuality in a model combining Regge exchange, the quark box diagram (a fixed pole in Regge language) and soft and hard pomeron exchanges evaluated in the context of dipole-dipole scattering. Good agreement is obtained with the data for the real gamma-gamma cross section and for the real photon structure function. However the model prediction for the gamma* gamma* cross section is too small. This is attributed to an incorrect extrapolation of the Q^2 dependence of the hard pomeron a…
We formulate and analyse a saturation model for the total gamma gamma and gamma* gamma* cross-sections and for the real photon structure function F_2^gamma(x,Q^2). The model is based on a picture in which the gamma* gamma* total cross-section for arbitrary photon virtualities is driven by the interaction of colour dipoles, into which the virtual photons fluctuate. The cross-section describing this interaction is assumed to satisfy the saturation property with the saturation radius taken from the Golec-Biernat and Wusthoff analysis of the gamma* p interaction at HERA. The model is supplemented …
We present one- and two-jet inclusive cross sections for gamma*gamma scattering and virtual photoproduction in ep collisions. The hard cross sections are calculated in next-to-leading order QCD. Soft and collinear singularities are extracted using the phase-space-slicing method. The initial state singularity of the virtual photon depends logarithmically its' virtuality. This logarithm is large and has to be absorbed into the parton distribution function of the virtual photon. We define for this purpose an MS-bar factorization scheme similar to the real photon case. We numerically study the dep…
We discuss exclusive electromagnetic production of two neutral $ρ$ mesons in coherent photon-photon processes in ultrarelativistic heavy-ion collisions. The cross section is calculated in the equivalent photon approximation (EPA). Both uncertainties of the flux factors and photon-photon cross sections are discussed in details. We show that inclusion of precise charge densities in nuclei is essential for realistic evaluations of the nuclear photon-photon cross sections. We find that the cross section, especially with realistic flux factors, is sensitive to low energy in the subsystem $γγ\to ρ^0…
We present the calculation of gamma gamma -> 2e+e- process cross section. The construction are perfomed using both helicity amplitude method and method of precision covariant calculation. The magnitude of cross section is obtained by the Monte-Carlo method of numerical integration. Different energies, polarization states and kinematics cuts are considered.
The purpose of this note is to show that the cross section factorization relation $σ_{nn}(s)/σ_{γp}(s) = σ_{γp}(s)/ σ_{γγ}(s)$ is satisfied experimentally in the energy domain $8\le\sqrt s\le 2000$ GeV, where the $σ$'s are total cross sections and $nn$ denotes the even portion of the $pp$ and $\pbar p$ total cross section. A convenient phenomenological paramaterization for a global simultaneous fit to the $pp$, $\pbar p$, $γp$ and $γγ$ total cross section data together with the $ρ$-value data for $pp$ and $\pbar p$ is provided by using real analytic amplitudes. Within experimental errors, we s…
The production cross sections of $e^+e^- $ to a heavy quarkonium with $C$-parity even $S-$wave and $P-$wave associated with a photon are analyzed in the framework of non-relativistic Quantum Chromodynamics(NRQCD) factorization formalism. The short-distance coefficients are determined up to the next-to-leading order(NLO) QCD radiative corrections for $S-$ and $P-$wave and NLO relativistic corrections for $S-$wave in analytical forms. Applying the results to estimate the cross sections we find that both the radiative and relativistic corrections are considerable.
I discuss the minijet contribution to total photoproduction and photon--photon cross sections. While minijets with $p_T$ around 2 GeV have recently been observed directly in \gamgam\ experiments, the total \gamp\ cross section measured at HERA is in excellent agreement with predictions based on purely soft physics. Due to the large number of free parameters, predictions for the minijet contribution to total cross sections can be brought into agreement with these seemingly paradoxical observations. However, the currently used eikonalization procedure may not be applicable at all to a large part…
We introduce a saturation model for photon-photon interactions, based on a QCD dipole picture of high energy scattering. The two-dipole cross-section is assumed to satisfy the saturation property. This pomeron-like contribution is supplemented with QPM and non-pomeron reggeon contributions. The model gives a very good description of the data on the gamma-gamma total cross-section, on the photon structure function F_2^gamma(x,Q^2) at low x and on the gamma*-gamma* cross-section.
We have calculated the next-to-leading order cross section for the inclusive production of charm quarks as a function of the transverse momentum p_T and the rapidity in two approaches using massive or massless charm quarks. For the single-resolved cross section we have derived the massless limit from the massive theory. We find that this limit differs from the genuine massless version with MS-bar factorization by finite corrections. By adjusting subtraction terms we establish a massive theory with MS-bar subtraction which approaches the massless theory very fast with increasing transverse mome…
In this note we address the issue of theoretical estimates of the hadronic cross-sections for \gamgam processes. We compare the predictions of the minijet model with data as well as other models, highlighting the band of uncertainties in the theoretical predictions as well as those in the final values of the $σ(\gamgam \go {\rm hadrons})$ extracted from the data. We find that the rise of $σ_{γγ}^{\rm tot}$ with energy shown in the latest \gamgam data is in tune with the faster rise expected in the Eikonal Minijet Models (EMM). We present an estimate of the accuracy with which this cross-sectio…
We have constructed a Monte Carlo generator for lowest-order predictions for the processes gamma gamma -> 4f and gamma gamma -> 4f+gamma in the Standard Model and extensions thereof by an effective gamma gamma Higgs coupling as well as anomalous triple and quartic gauge-boson couplings. Polarization is fully supported, and a realistic photon beam spectrum can be taken into account. For the processes gamma gamma -> 4f all helicity amplitudes are explicitly given in a compact form. The presented numerical results contain, in particular, a survey of cross sections for representative final states …
We show that t-channel unitarity constraints make it possible to obtain the photon-photon cross sections from the photon-proton and proton-proton cross sections, for Q2<150 GeV2. In order to do so, one must postulate the existence of double-pole or triple-pole singularities in the complex j plane.
We observe that the saturation model of deep inelastic scattering, which successfully describes inclusive and diffractive data at small x, predicts a geometric scaling of the total gamma^* p cross section in the region of small Bjorken variable x. The geometric scaling in this case means that the cross section is a function of only one dimensionless variable tau = Q^2 R_0^2(x), where the function R_0(x) (called saturation radius) decreases with decreasing x. We show that the experimental data from HERA in the region x<0.01 confirm the expectations of this scaling over a very broad region of Q^…
The reactions gamma-gamma to eta_c eta_c and gamma gamma to eta_c + X are discussed within the three gluon exchange model. We give predictions for the differential cross-sections and discuss feasibility of measuring these processes at LEP2 and TESLA. The total cross-sections were estimated to be approximately equal to 40 fb and 120 fb for gamma-gamma to eta_c eta_c and gamma-gamma to eta_c + X respectively assuming exchange of elementary gluons that corresponds to the odderon with intercept equal to unity. These values can be enhanced by a factor equal to 1.9 and 2.1 for LEP2 and TESLA energie…
In order to understand the initial partonic state in proton-nucleus and electron-nucleus collisions, we investigate the total, inelastic, and (quasi-)elastic cross sections in pA and gamma-A collisions, as these observables are insensitive to possible collective effects in the final state interactions. We used as a tool the DIPSY dipole model, which is based on BFKL dynamics including non-leading effects, saturation, and colour interference, which we have extended to describe collisions of protons and virtual photons with nuclei. We present results for collisions with O, Cu, and Pb nuclei, and…
We analyse the consequences of t-channel unitarity for the photon hadronic cross sections. We show that gamma(*)p and gamma(*) gamma(*) elastic amplitudes must include the complex-j plane singularities associated with hadronic elastic amplitudes, and give the characteristics of possible new singularities. We also show that several different models of hadronic total cross sections can be used to predict the LEP data from the HERA measurements.
A model for total cross sections with virtual photons is presented, in particular gamma-p and gamma-gamma cross sections are considered. Our approach is based on an existing model for photoproduction, which subdivides the total cross section into three distinct event classes: direct, VMD and anomalous. In the region of large photon virtualities, the Deep Inelastic Scattering processes (up to O(alpha_s) corrections) are obtained. Hence, the model provides a smooth transition between the two regions. By the breakdown into different event classes, a complete picture of all event properties is the…
We investigate the effects of unparticles on gamma gamma--> gamma gamma scattering for 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 lambda_0 to the photons for various values of the scaling dimension d(1<d<2) at sqrt{s}=0.5-5 TeV.
The gamma + D -> Lambda + Theta+ and gamma + D -> Sigma + Theta+ reactions can be used to determine the width of Theta+ almost model-independently. We calculate the differential cross sections of the gamma +D -> Lambda + Theta+, gamma + D -> Sigma + Theta+ and relevant background reactions in the photon energy range 1.2 < E_γ < 2.6 GeV. We determine the most favorable kinematic conditions and observables for the experimental studies of Theta+ in the considered processes. We argue that a comparison of the gamma + D -> Lambda + Theta+ and gamma + D -> Sigma + Theta+ cross sections should unambig…
A model for total cross sections with virtual photons is presented. In particular gamma*-p and gamma*-gamma* cross sections are considered. Our approach extends on a model for photoproduction, where the total cross section is subdivided into three distinct event classes: direct, VMD and anomalous. With increasing photon virtuality, the latter two decrease in importance. Instead Deep Inelastic Scattering dominates, with the direct class being the O(alpha_s) correction thereof. Hence, the model provides a smooth transition between the two regions. By the breakdown into different event classes, o…
Strong interactions between pairs of the K+ K- and K0 K0bar mesons can be studied in radiative decays of phi(1020) mesons. We present a theoretical model of the reactions e+ e- --> phi --> K+ K- gamma and e+ e- --> phi --> K0 K0bar gamma. The K+ K- and K0 K0bar effective mass dependence of the differential cross sections is derived. The total cross sections and the branching fractions for the two radiative phi decays are calculated.
We discuss predictions for the total inelastic gamma-gamma cross-section and their model dependence on the input parameters. We compare results from a simple extension of the Regge Pomeron exchange model as well as predictions from the eikonalized mini-jet model with recent LEP data.
A model for total cross sections with virtual photons is presented. In particular gamma*-p and gamma*-gamma* cross sections are considered. Our approach extends on a model for photoproduction, where the total cross section is subdivided into three distinct event classes: direct, VMD and anomalous. With increasing photon virtuality, the latter two decrease in importance. Instead Deep Inelastic Scattering dominates, with the direct class being the O(alpha_s) correction thereof. Hence, the model provides a smooth transition between the two regions. By the breakdown into different event classes, o…
We calculate the cross-section for the diffractive exclusive process gamma*_L (Q_1^2) gamma*_L(Q_2^2) -> rho^0_L rho^0_L, in view of its study in the future high energy e+e- linear collider. The Born order approximation of the amplitude is completely calculable in the hard region Q_1, Q_2 >> Lambda_QCD. The resulting cross-section is large enough for this process to be measurable with foreseen luminosity and energy, for Q_1 and Q_2 in the range of a few GeV.
We make a QCD-inspired parameterization of all accelerator data on forward proton-proton and antiproton-proton scattering amplitudes. Using vector dominance and the additive quark model, we show that the same parameters also fit gamma p and gamma gamma interactions. Using the high energy predictions of our model, along with Glauber theory, we calculate proton-air cross sections at energies near sqrt s approx 30 TeV. The comparisons of p-air cosmic ray measurements with our QCD model predictions provide a strong constraint on the inclusive particle production cross section.
In this contribution, we discuss a total cross-section model which can be applied to both photon and purely hadronic processes. We find that the model can reproduce photo-production cross-sections, as well as extrapolations of gamma* p processes to gamma p using vector meson dominance models, with minimal modifications from the proton case.
We present the production cross-section of gamma-rays based on data of p-p collisions at LHC, revising the previous semi-empirical formula mainly for 1) the inelastic cross-section in p-p collisions, $σ_{pp}(E_0)$, and 2) the inclusive gamma-ray spectrum in the forward region, $σ_{pp \rightarrow γ}(E_0, E_γ)$. We find that the previous cross-section gives a significantly softer spectrum than found in the data of LHC. In this paper, we focus our interest mainly upon the LHC forward (LHCf) experiment, giving gamma-ray spectra in the very forward region with the pseudo-rapidity $η^*$ > 8.8 in the…
A quark model calculation of the processes gamma gamma -> pi+ pi- and gamma gamma -> pi0 pi0 is performed. At tree level, only charged pions couple to the initial state photons and neutral pions are not expected in the final state. However, a small but significant gamma gamma -> pi0 pi0 cross section is observed. We demonstrate that this may be accounted for by a rotation in isospin space induced by final state interactions. The resulting pi+ pi- cross section is in good agreement with experiment while the pi0 pi0 cross section is in qualitative agreement with the data.
We evaluate the cross-section for gamma production by neutrinos through a meson exchange effect which derives from the concept of axial-vector mixing. The resulting cross-section leads to some increase of the gamma production cross-section by neutrinos, especially at low neutrino energies, which may influence the understanding of the low energy excess of electron-like events seen in the MiniBooNE experiment.
The differential cross section for gamma-deuteron Compton scattering from a tensor polarized deuteron is computed in an effective field theory. The first non-vanishing contributions to this differential cross section are the interference terms between the leading electric coupling diagrams and the subleading single potential pion exchange diagrams or the subleading magnetic moment coupling diagrams. At 90 degree photon scattering angle, only the pion term contributes at this order to the tensor polarized differential cross section. This provides a clean way to study the photon pion dynamics in…
The construction of a computer code to calculate the cross sections for the spin-polarized processes e-gamma=>e-gamma,e-gamma-gamma,e-e+e- to order-alpha**3 is described. The code calculates cross sections for circularly-polarized initial-state photons and arbitrarily polarized initial-state electrons. The application of the code to the SLD Compton polarimeter indicates that the order-alpha**3 corrections produce a fractional shift in the SLC polarization scale of -0.1% which is too small and of the wrong sign to account for the discrepancy in the Z-pole asymmetries measured by the SLD Collabo…
Motivated by the interpretation of the recent results on the TeV gamma radiation from the Galactic center, including the new 2004 HESS data, as a by-product of dark matter particles annihilations, we address the question of the largest possible neutralino masses and pair annihilation cross sections in supersymmetric models. Extending the parameter space of minimal models, such as the mSUGRA and the mAMSB scenarios, to general soft SUSY breaking Higgs masses gives access to the largest possible pair annihilation rates, corresponding to resonantly annihilating neutralinos with maximal gaugino-hi…
Recent claims of a line in the Fermi-LAT photon spectrum at 130 GeV are suggestive of dark matter annihilation in the galactic center and other dark matter-dominated regions. If the Fermi feature is indeed due to dark matter annihilation, the best-fit line cross-section, together with the lack of any corresponding excess in continuum photons, poses an interesting puzzle for models of thermal dark matter: the line cross-section is too large to be generated radiatively from open Standard Model annihilation modes, and too small to provide efficient dark matter annihilation in the early universe. …
Because the production cross sections of gamma-rays, electrons, and positrons made in p-p collisions, $σ_{pp\rightarrow γ}$ and $σ_{pp\rightarrow {e}^\pm}$, respectively, are kinematically equivalent with respect to the parent pion-production cross section $σ_{pp\rightarrow π}$, we obtain $σ_{pp\rightarrow {e}^\pm}$ directly from the machine data on $σ_{pp\rightarrow γ}$. In Sato et al. (2012), we give explicitly $σ_{pp\rightarrow γ}$, reproducing quite well the accelerator data with LHC, namely $σ_{pp\rightarrow {e}^\pm}$ is applicable enough over the wide energy range from GeV to 20\,PeV for…
Recently, Dicus and Repko computed nu gamma --> nu gamma gamma for energies below the threshold for e^{+}e^{-} pair production. They found across section on the order of 10^{-52} omega ^{gamma} with gamma =10, where omega is the CMS energy of one of the initial particles in MeV. Cross sections for the crossed processes are the same to factors of order one. This note investigates the extent to which these processes could, if their result extrapolates past 1 MeV: affect supernova dynamics; cut off the energy distribution of very high energy cosmic photons and neutrinos; and possibly give rise to…
The differential cross section for gamma-deuteron Compton scattering is computed to next-to-leading order (NLO) in an effective field theory that describes nucleon-nucleon interactions below the pion production threshold. Contributions at NLO include the nucleon isoscalar electric polarizability from its 1/m_pi behavior in the chiral limit. The parameter free prediction of the gamma-deuteron differential cross section at NLO is in good agreement with data.
Combining data on unpolarized and polarized inclusive proton structure functions, we perform the first detailed study of quark-hadron duality in individual helicity-1/2 and 3/2 virtual photoproduction cross sections. We find that duality is realized more clearly in the helicity-1/2 channel, with duality violating corrections < 10% over the entire nucleon resonance region, while larger, < 20% corrections are found in the helicity-3/2 sector. The results are in general agreement with quark model expectations, and suggest that data above the Delta resonance region may be used to constrain both sp…
The High Altitude Water Cherenkov (HAWC) gamma-ray observatory is a wide field-of-view observatory sensitive to 100 GeV - 100 TeV gamma rays and cosmic rays. The HAWC observatory is also sensitive to diverse indirect searches for dark matter annihilation, including annihilation from extended dark matter sources, the diffuse gamma-ray emission from dark matter annihilation, and gamma-ray emission from non-luminous dark matter subhalos. Among the most promising classes of objects for the indirect detection of dark matter are dwarf spheroidal galaxies. These objects are expected to have few astro…
We report a new extraction of nucleon resonance couplings using pi- photoproduction cross sections on the neutron. The world database for the process gamma n --> pi- p above 1 GeV has quadrupled with the addition of new differential cross sections from the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab in Hall B. Differential cross sections from CLAS have been improved with a new final-state interaction determination using a diagramatic technique taking into account the NN and piN final-state interaction amplitudes. Resonance couplings have been extracted and compared to previous …
We provide conservative bounds on the dark matter cross-section and lifetime from final state radiation produced by annihilation or decay into charged leptons, either directly or via an intermediate particle $ϕ$. Our analysis utilizes the experimental gamma-ray flux upper limits from four Milky Way dwarf satellites: HESS observations of Sagittarius and VERITAS observations of Draco, Ursa Minor, and Willman 1. Using 90% confidence level lower limits on the integrals over the dark matter distributions, we find that these constraints are largely unable to rule out dark matter annihilations or dec…
We explore for the first time the possibilities to measure an intermediate-mass (mH = 115-140 GeV/c^2) Standard-Model Higgs boson in electromagnetic proton-lead (pPb) interactions at the CERN Large Hadron Collider (LHC) via its b-bbar decay. Using equivalent Weizsaecker-Williams photon fluxes and Higgs effective field theory for the coupling gamma-gamma --> H, we obtain a leading-order cross section of the order of 0.3 pb for exclusive Higgs production in elastic (pPb --> gamma-gamma --> p H Pb) and semielastic (pPb --> gamma-gamma --> Pb H X) processes at sqrt(s) = 8.8 TeV. After applying var…
We study the implications of the flavour SU(3) symmetry for various interpretations of the neutron anomaly in the $γN\toηN$ cross section. We show that the explanation of the neutron anomaly due to interference of known N(1535) and N(1650) resonances implies that N(1650) resonance should have a huge coupling to $ϕ$-meson -- at least 5 times larger than the corresponding $ρ^0$ coupling. In terms of quark degrees of freedom this means that the well-known N(1650) resonance must be a "cryptoexotic pentaquark"-- its wave function should contain predominantly an $s\bar s$ component. It turns out tha…
Preliminary differential cross-sections and the $ρ^0_{MM'}$ spin density matrix elements (SDME) for the reaction $γp \to ϕp$ for both charged- ($ϕ\to K^+ K^-$) and neutral-mode ($ϕ\to K^0_L K^0_S$) topologies obtained from CLAS are presented. Our kinematic coverage is from near production threshold ($\sqrt{s} \sim 1.97$ GeV) to $\sqrt{s} = 2.84$ GeV, with a wide coverage in the production angle. As seen in previous LEPS results, the differential cross-sections show a localized "bump" between $\sqrt{s} \sim 2$ and 2.2 GeV that is not expected from a simple Pomeron exchange picture. Comparisons …
The dark matter halo of the Milky Way is predicted to contain a very large number of smaller subhalos. As a result of the dark matter annihilations taking place within such objects, the most nearby and massive subhalos could appear as point-like or spatially extended gamma-ray sources, without observable counterparts at other wavelengths. In this paper, we use the results of the Aquarius simulation to predict the distribution of nearby subhalos, and compare this to the characteristics of the unidentified gamma-ray sources observed by the Fermi Gamma-Ray Space Telescope. Focusing on the brighte…
