Collider Implications of Models with Extra Dimensions

We discuss the collider signals of large extra dimensions in which gravity as well as the SM particles propagate into the extra dimensions. These signals arise either from the production of Kaluza-Klein excitations of the SM particles and their subsequent decay, or from their off-shell exchanges. Depending on the scenario, the dominant signals are two high p_T jets + missing energy, two high p_T photons + missing energy and soft leptons, or a combination of photon + jet and missing energy. For the scenario in which only the gauge bosons propagate into the extra dimensions, Tevatron Run II (LHC) can observe such signals up to a compactification scale of about 2 TeV (7 TeV), while for the case of universal extra dimensions, the corresponding limits are about 600 GeV (3 TeV) respectively.Comment: 3 pages; invited talk presented by S. Nandi at the ICHEP 2002, Amsterdam, The Netherlands, July 24-31, 200

Origins of the baryon spectrum

I begin with a key problem of light and strange baryon spectroscopy which suggests a clue for our understanding of underlying dynamics. Then I discuss spontaneous breaking of chiral symmetry in QCD, which implies that at low momenta there must be quasiparticles - constituent quarks with dynamical mass, which should be coupled to other quasiparticles - Goldstone bosons. Then it is natural to assume that in the low-energy regime the underlying dynamics in baryons is due to Goldstone boson exchange (GBE) between constituent quarks. Using as a prototype of the microscopical quark-gluon degrees of freedom the instanton-induced 't Hooft interaction I show why the GBE is so important. When the 't Hooft interaction is iterated in the qq t-channel it inevitably leads to a pole which corresponds to GBE. This is a typical antiscreening behavior: the interaction is represented by a bare vertex at large momenta, but it blows up at small momenta in the channel with GBE quantum numbers, explaining thus a distinguished role of the latter interaction in the low-energy regime. I show how the explicitly flavour-dependent short-range part of the GBE interaction between quarks, perhaps in combination with the vector-meson exchange interaction, solves a key problem of baryon spectroscopy and present spectra obtained in a simple analytical calculation as well as in exact semirelativistic three-body approach.Comment: Plenary talk given at PANIC 99 (XV Particles and Nuclei International Conference, 10 - 16 June 1999, Uppsala

On the analytical approximation to the GLAP evolution at small x and moderate Q^2

Comparing the numerically evaluated solution to the leading order GLAP equations with its analytical small-x approximation we have found that in the domain covered by a large fraction of the HERA data the analytic approximation has to be augmented by the formally non-leading term which has been usually neglected. The corrected formula fits the data much better and provides a natural explanation of some of the deviations from the $\sigma$ scaling observed in the HERA kinematical range.Comment: LaTeX, 13 pages including 6 figures, figure 5 replaced, to appear in Phys. Lett.

Baryon Spectrum and Chiral Dynamics

New results on baryon structure and spectrum developed in collaboration with Dan Riska [1-4] are reported. The main idea is that beyond the chiral symmetry spontaneous breaking scale light and strange baryons should be considered as systems of three constituent quarks with an effective confining interaction and a chiral interaction that is mediated by the octet of Goldstone bosons (pseudoscalar mesons) between the constituent quarks.Comment: 12 pages + 1 fig., LaTeX, fig. is available from author, to appear in Proceedings of the Int. School of Nucl. Physics: Quarks in Hadrons and Nuclei (Erice, 19-27 September, 1995) - Progr. Part. Nucl. Phys., v. 36 (1996

Light Baryons in a Constituent Quark Model with Chiral Dynamics

It is shown from rigorous three-body Faddeev calculations that the masses of all 14 lowest states in the $N$ and $\Delta$ spectra can be described within a constituent quark model with a Goldstone-boson-exchange interaction plus linear confinement between the constituent quarks.Comment: 12 pages, submitted to Phys. Lett.

Low-Lying Nucleons from Chirally Improved Fermions

We report on our preliminary results on the low-lying excited nucleon spectra which we obtain through a variational basis formed with three different interpolators.Comment: Contributed to Lattice 2003(spectrum), Tsukub

Quark spin coupling in baryons - revisited

A direct connection can be made between mixing angles in negative parity baryons and the spin coupling of constituent quarks. The mixing angles do not depend on spectral data. These angles are recalculated for gluon exchange and pion exchange between quarks. For pion exchange the results of Glozman and Riska are corrected. The experimental data on mixing are very similar to those derived from gluon exchange but substantially different from the values obtained for pion exchange.Comment: 10 pages, RevTex; a sign error is corrected, spin-orbit results are include

The QCD Coupling Constant

This paper presents a summary of the current status of determinations of the strong coupling constant alpha_s. A detailed description of the definition, scale dependence and inherent theoretical ambiguities is given. The various physical processes that can be used to determine alpha_s are reviewed and attention is given to the uncertainties, both theoretical and experimental.Comment: 56 page

New Signal for Universal Extra Dimensions

In the universal extra dimensions (UED) scenario, the tree level masses of the first level Kaluza-Klein (KK) excitations of Standard Model particles are essentially degenerate. Radiative corrections will, however, lift this degeneracy, allowing the first level excitations to decay to the lightest KK particle (LKP), which is the gamma^*. KK number conservation implies that the LKP is stable. Then, since the SM particles radiated during these decays are rather soft, the observation of KK excitations production and decay in collider experiments will be quite difficult. We propose to add to this model KK number violating interactions mediated by gravity, which allow the gamma^* to decay to a photon and a KK graviton. For a variety a models and a large range of parameters, these decays will occur within the detector. Thus, pair production of KK excitations will give rise to a striking collider signal, consisting of two hard photons plus large missing energy (due to escaping gravitons). We evaluate the cross-section for these signals at the Tevatron and LHC, and derive the reach of these colliders in the search for universal extra dimensions.Comment: 11 pages, 6 eps figures. One reference and minor comments adde

Some New Results on the H Dibaryon in the Quark Cluster Model

The H dibaryon channel, (I=0,J=0,S=-2), is revisited in the non-relativistic quark cluster model (NRQCM) using a basis extended beyond the usual set of baryon cluster pairs to include an explicit spatially symmetric 6q state, analogous in structure to the MIT bag model H. We find that the binding predicted using the two-baryon basis alone is significantly deepened by the addition of the additional 6q configuration. The NRQCM thus appears, contrary to earlier findings, to be incompatible with the experimental information available for this channel.Comment: 11 pages, REVTE