Ultimate parameters of the photon collider at the ILC

At linear colliders, the e+e- luminosity is limited by beam-collision effects, which determine the required emittances of beams in damping rings (DRs). While in gamma-gamma collisions at the photon collider, these effects are absent, and so smaller emittances are desirable. In present damping rings designs, nominal DR parameters correspond to those required for e+e- collisions. In this note, I would like to stress once again that as soon as we plan the photon-collider mode of ILC operation, the damping-ring emittances are dictated by the photon-collider requirements--namely, they should be as small as possible. This can be achieved by adding more wigglers to the DRs; the incremental cost is easily justified by a considerable potential improvement of the gamma-gamma luminosity. No expert analysis exists as of yet, but it seems realistic to obtain a factor five increase of the gamma-gamma luminosity compared to the ``nominal'' DR design.Comment: Talk at LCWS06, Bangalore, India, March 2006, to be published in Indian Journal of Physics, 5 pp, Latex, 1 .eps figur

Relativistic mask method for electron momentum distributions after ionization of hydrogen-like ions in strong laser fields

Wavefunction-splitting or mask method, widely used in the non-relativistic calculations of the photoelectron angular distributions, is extended to the relativistic domain within the dipole approximation. Since the closed-form expressions for the relativistic Volkov states are not available within the dipole approximation, we build such states numerically solving a single second-order differential equation. We calculate the photoelectron energy spectra and angular distributions for highly charged ions under different ionization regimes with both the direct and the relativistic mask methods. We show that the relativistic mask method works very well and reproduces the electron energy and angular distributions calculated by the direct method in the energy range where both methods can be used. On the other hand, the relativistic mask method can be applied for longer laser pulses and/or higher photoelectron energies where the direct method may have difficulties

Restriction on the energy and luminosity of e+e- storage rings due to beamstrahlung

The role of beamstrahlung in high-energy e+e- storage-ring colliders (SRCs) is examined. Particle loss due to the emission of single energetic beamstrahlung photons is shown to impose a fundamental limit on SRC luminosities at energies 2E_0 >~ 140 GeV for head-on collisions and 2E_0 >~ 40 GeV for crab-waist collisions. With beamstrahlung taken into account, we explore the viability of SRCs in the E_0=240-500 GeV range, which is of interest in the precision study of the Higgs boson. At 2E_0=240 GeV, SRCs are found to be competitive with linear colliders; however, at 2E_0=400-500 GeV, the attainable SRC luminosity would be a factor 15-25 smaller than desired.Comment: Latex, 5 pages. v2 differs only by minor changes is abstract and introduction, one reference is added. v3 corresponds to the paper published in PR

Full one-loop QCD and electroweak corrections to sfermion pair production in γγ\gamma \gamma collisions

We have calculated the full one-loop electroweak (EW) and QCD corrections to the third generation scalar-fermion pair production processes $e^+e^- \to \gamma \gamma \to \tilde{f_i}\bar{\tilde{f_i}} (f=t,b,\tau)$ at an electron-positron linear collider(LC) in the minimal supersymmetric standard model (MSSM). We analyze the dependence of the radiative corrections on the parameters such as the colliding energy $\sqrt{\hat s}$ and the SUSY fundamental parameters $A_f$, $\tan \beta$, $\mu$, $M_{SUSY}$ and so forth. The numerical results show that the EW corrections to the squark-, stau-pair production processes and QCD corrections to the squark-pair production processes give substantial contributions in some parameter space. The EW relative corrections to squark-pair production processes can be comparable with QCD corrections at high energies. Therefore, these EW and QCD corrections cannot be neglected in precise measurement of sfermion pair productions via $\gamma\gamma$ collision at future linear colliders.Comment: to be appeared in Phys. Rev.

High energy behaviour of gamma gamma to f f(bar) processes in SM and MSSM

We compute the leading logarithms electroweak contributions to gamma gamma to f f(bar) processes in SM and MSSM. Several interesting properties are pointed out, such as the importance of the angular dependent terms, of the Yukawa terms, and especially of the $\tan^2\beta$ dependence in the SUSY contributions. These properties are complementary to those found in e+e- to f f(bar). These radiative correction effects should be largely observable at future high energy gamma gamma colliders. Polarized beams would bring interesting checks of the structure of the one loop corrections. We finally discuss the need for two-loop calculations and resummation.Comment: 22 pages and 12 figures. e-mail: [email protected]

Electron (positron) beam polarization by Compton scattering on circularly polarized laser photons

In a number of papers an attractive method of laser polarization of electrons (positrons) at storage rings or linear colliders has been proposed. We show that these suggestions are incorrect and based on errors in simulation of multiple Compton scattering and in calculation of the Compton spin-flip cross sections. We argue that the equilibrium polarization in this method is zero.Comment: 11 pages, LaTeX, talk at 9-th Intern. Workshop on Linear Colliders (LC02), Feb. 4-8, 2002, SLAC, Stanford, US

Backward scattering of low-energy antiprotons by highly charged and neutral uranium: Coulomb glory

Collisions of antiprotons with He-, Ne-, Ni-like, bare, and neutral uranium are studied theoretically for scattering angles close to 180$^{\circ}$ and antiproton energies with the interval 100 eV -- 10 keV. We investigate the Coulomb glory effect which is caused by a screening of the Coulomb potential of the nucleus and results in a prominent maximum of the differential cross section in the backward direction at some energies of the incident particle. We found that for larger numbers of electrons in the ion the effect becomes more pronounced and shifts to higher energies of the antiproton. On the other hand, a maximum of the differential cross section in the backward direction can also be found in the scattering of antiprotons on a bare uranium nucleus. The latter case can be regarded as a manifestation of the screening property of the vacuum-polarization potential in non-relativistic collisions of heavy particles.Comment: 14 pages, 5 figure