Charm production at a linear e+e− and photon collider and its sensitivity to the gluon content of the photon

Abstract

A high energy linear e + e − collider (LC) can also be used as a Photon Collider (PC), using Compton scattering of laser photons on the e + /e − beams. The leading order cross-section for the production of heavy quarks, e + e − → e + e − Q(Q)X, at high transverse momenta is calculated for both LC and PC modes. The sensitivity of this process to the parton distribution parametrizations of real photons, especially the gluon content, is tested for both modes.For the study of a future electron-positron Linear Collider (LC) it is important to examine the physics potential for its main and possible derived options. The so called Photon (or Compton) Collider (PC) is an option in which high energy real photons can be obtained by backscattering photons from a laser beam on the electron or positron beam [1,2]. This way an excellent tool for the study of γγ collisions at high energies can be constructed.In high energy e + e − collisions the hadronic final state is predominantly produced in γ ⋆ γ ⋆ interactions where the virtual photons are almost on mass shell. These processes can be described by an effective (real) photon energy spectrum, i.e. using the Weizsäcker-Williams (WW) approximation. A Photon Collider based on Compton scattering, however, provides beams of real photons, which can be produced in a definite polarization state and with high monochromaticity. Moreover, the resulting photon spectrum (denoted as LASER) is much harder then the WW one. The comparison of the photon spectra used in this analysis (see Appendix) is presented in Fig. 1.The main goal of this work is to compare the LC and PC opportunities for probing the gluon distribution in the photon, without making use of polarization. Heavy quark 1

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

“…This must be better understood in the future because usually the charm production in photon-photon collisions is considered as a good tool to extract the gluon distribution in the photon (see for instance [2]). …”

“…The heavy quark production is interesting also in the context of a deficit of standard theoretical QCD predictions relative to the experimental data as observed recently for bottom quark production in proton-antiproton, photonproton and photon-photon scattering. This must be better understood in the future because usually the charm production in photon-photon collisions is considered as a good tool to extract the gluon distribution in the photon (see for instance [2]).…”

The Photon Collider at Tesla

“…Charm production [151], which is sensitive to the mechanism γg → cc Both these processes, which are at least in certain kinematical regions dominated by the photon-gluon fusion mechanisms, are sensitive to the gluon distribution in the photon. The detailed discussion of these processes have been presented in [152,153].…”