Large Extra Dimensions in Rare Decays

The prospect of experimental verification of the large extra dimension scenario in rare decays is discussed. The case of J/\Psi and \Upsilon to photon + missing energy is calculated in detail, and it is shown that the limit on the compactification scale M_S lies at present in the ten GeV range. In contrast to the quarkonium systems, signals in Kaon and Pion decays will be small.Comment: 11 pages, 3 figures, LaTeX, minor stylistic changes, version to be published in JHE

Center-Vortex Solutions of the Yang-Mills Effective Action in Three and Four Dimensions

We calculate the one-loop effective action of the SU(2) Yang-Mills theory for center-vortex configurations, both in 3d and 4d. We find that in both cases there are minima of the effective action, corresponding to vortices of the transverse size approximately 4/g_3^2 and 1.7/Lambda_MS, respectively. The values of the effective actions at the minima are negative, suggesting that the Euclidian vacuum may be unstable with respect to creation of vortices.Comment: 18 pages, 5 figures, RevTeX, some discussion added, minor stylistic changes, none of the results changed, version to appear in Phys Rev.

Integral equation for spin dependent unintegrated parton distributions incorporating double ln^2(1/x) effects at low x

In this paper we derive an integral equation for the evolution of unintegrated (longitudinally) polarized quark and gluon parton distributions. The conventional CCFM framework is extended at small x in order to incorporate the QCD expectations concerning the double ln^2(1/x) resummation at low x for the integrated distributions. Complete Altarelli-Parisi splitting functions are included, that makes the formalism compatible with the LO Altarelli-Parisi evolution at large and moderately small values of x. The obtained modified polarized CCFM equation is shown to be partially diagonalized by the Fourier-Bessel transform. Results of the numerical solution for this modified CCFM equation for the non-singlet quark distributions are presented.Comment: 24 pages, 3 figures, LaTeX, some discussion added, none of the results changed, version to appear in Phys. Rev.

Semi-Exclusive Production of Photons at HERA

We study the feasibility of measuring semi-exclusive photon production \gamma + p \to \gamma + Y at HERA. The cross section of photons produced at large transverse momenta, recoiling off an inclusive system Y of limited mass, can without photon isolation cuts be simply expressed in terms of hard pQCD subprocesses and standard target parton distributions. With the help of event generators we identify the kinematic region where quark and gluon fragmentation processes can be neglected. The cross section in this semi-exclusive region is large enough to be measured with an upgraded HERA luminosity of L = 100/pb. The subprocesses of lowest order in \alpha_s are suppressed at low recoil masses M_Y, compared to higher order gluon exchange, i.e. BFKL contributions. The distinct M_Y-dependence makes it possible to determine experimentally the kinematic range where the higher order processes dominate.Comment: 11 pages, 7 figures, submitted to Eur. Phys. J. C, minor stylistic changes, references update

Prospects for parity-nonconservation experiments with highly charged heavy ions

We discuss the prospects for parity-nonconservation experiments with highly charged heavy ions. Energy levels and parity mixing for heavy ions with 2–5 electrons are calculated. We investigate two-photon transitions and the possibility of observing interference effects between weak-matrix elements and Stark matrix elements for periodic electric field configurations

On Statistical Mechanics of Instantons in the CP^{N_c-1} Model

We introduce an explicit form of the multi-instanton weight including also instanton--anti-instanton interactions for arbitrary $N_c$ in the two-dimensional $CP^{N_c-1}$ model. To that end, we use the parametrization of multi-instantons in terms of instanton `constituents' which we call `zindons' for short. We study the statistical mechanics of zindons analytically (by means of the Debye-H\"uckel approximation) and numerically (running a Metropolis algorithm). Though the zindon parametrization allows for a complete `melting' of instantons we find that, through a combination of dynamical and purely geometric factors, a dominant portion of topological charge is residing in well-separated instantons and anti-instantons.Comment: 38 pages, LaTe