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Claudio Rebbi

Publications and source records attributed to Claudio Rebbi.

52 records · Page 3Linked to original sources
arXiv2006

K^0-\bar K^0 mixing beyond the standard model and CP-violating electroweak penguins in quenched QCD with exact chiral symmetry

We present results for the ΔS=2 matrix elements which are required to study neutral kaon mixing in the standard model (SM) and beyond (BSM). We also provide leading chiral order results for the matrix elements of the electroweak penguin operators which give the dominant ΔI=3/2 contribution to direct CP violation in K->ππdecays. Our calculations were performed with Neuberger fermions on two sets of quenched Wilson gauge configurations at inverse lattice spacings of approximately 2.2 GeV and 1.5 GeV. All renormalizations were implemented non-perturbatively in the RI/MOM scheme, where we accounted for sub-leading operator product expansion corrections and discretization errors. We find ratios of non-SM to SM matrix elements which are roughly twice as large as in the only other dedicated lattice study of these amplitudes. On the other hand, our results for the electroweak penguin matrix elements are in good agreement with two recent domain-wall fermion calculations. As a by-product of our study, we determine the strange quark mass. Our main results are summarized and discussed in Sec. VII. Within our statistics, we find no evidence for scaling violations.

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arXiv2005

Light hadron spectroscopy in quenched QCD with overlap fermions

A simulation of quenched QCD with the overlap Dirac operator has been carried out using 100 Wilson gauge configurations at beta = 6 on an 18^3 x 64 lattice and at beta = 5.85 on a 14^3 x 48 lattice. Here we present results for meson masses, meson final state "wave functions," decay constants, and other observables, as well as details on our algorithmic and data analysis techniques. We also summarize results for baryon masses and quark and diquark propagators in the Landau gauge.

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arXiv2004

Electroweak penguins and SUSY K^0-\bar K^0 mixing with Neuberger quarks

We present results for Delta I=3/2 and Delta S=2 matrix elements relevant for CP violation in K->Pi Pi decays and for the K_S-K_L mass difference in the standard model and beyond. They were obtained with Neuberger fermions on quenched gauge configurations generated with the Wilson plaquette action at beta=6.0 on an 18^3x64 lattice.

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arXiv2003

B_K from quenched QCD with exact chiral symmetry

We present a calculation of the standard model ΔS=2 matrix element relevant to indirect CP violation in K->ππdecays which uses Neuberger's chiral formulation of lattice fermions. The computation is performed in the quenched approximation on a 16^3x32 lattice that has a lattice spacing a\sim 0.1 fm. The resulting bare matrix element is renormalized non-perturbatively. Our main result is B_K^{RGI}=0.87(8)^{+2+14}_{-1-14}, where the first error is statistical, the second is systematic and the third is an estimate of the uncertainty associated with the quenched approximation and with the fact that our kaons are composed of degenerate s and d quarks with masses \sim m_s/2.

hep-ph↗
arXiv2001

Schwinger Model with the Overlap-Dirac Operator: exact results versus a physics motivated approximation

We propose new techniques for the numerical implementation of the overlap-Dirac operator, which exploit the physical properties of the underlying theory to avoid nested algorithms. We test these procedures in the two-dimensional Schwinger model and the results are very promising. These techniques can be directly applied to QCD simulations. We also present a detailed computation of the spectrum and the chiral properties of the Schwinger model in the overlap lattice formulation.

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arXiv1997

Numerical Approaches to High Energy Electroweak Baryon Number Violation Above and Below the Sphaleron Barrier

We review some promising numerical techniques for calculating high energy baryon number violating cross sections in the standard model. As these lectures are designed to be self- contained, we present in some detail the formalism of Rubakov, Son, and Tinyakov, which provides a means of bounding the two-particle cross sections in a semi-classical manner. The saddle-point solutions required by this method must be found computationally and are of two basic types, corresponding to tunneling events between adjacent topological sectors on the one hand, and classically allowed evolution over the sphaleron barrier on the other. In both cases one looks for topology changing solutions of small incident particle number. In the classically allowed regime we have developed a Monte Carlo technique that systematically lowers the particle number while still ensuring that a change in topology takes place. We also make progress towards a numerical method amenable for the more computationally challenging problem of finding the complexified tunneling solutions, and we present some of our numerical findings, both above and below the sphaleron barrier.

hep-ph↗
arXiv1997

Lattice Formulation of the Standard Model

Combining the Kaplan surface mode approach for chiral fermions with added terms motivated by Eichten and Preskill suggests the possibility for a lattice regularization of the standard model which is finite, exactly gauge invariant, and only has physically desired states in its low energy spectrum. The conjectured scheme manifestly requires anomaly cancelation and explicitly contains baryon and lepton number violating terms.

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arXiv1996

Computational Advances in the Study of Baryon Number Violation in High Energy Electroweak Collisions

We present some recent advances in the computational study of baryon number violation in high energy electroweak collisions. We examine classically allowed processes above the sphaleron barrier and, using a stochastic search procedure, we explore the topology changing region in the energy and particle number plane. Finding topology changing classical solutions with small incident particle number would be an indication that baryon number violation becomes unsuppressed in high energy collisions. Starting with a topology changing solution of approximately 50 incoming particles, our Monte-Carlo procedure has produced other topology changing solutions with 40% lower incident particle numbers, with energies up to one and a half times the sphaleron energy. While these solutions still involve a rather large number of incident particles, we have nonetheless demonstrated that our search procedure is effective in reducing the particle number while ensuring topology change. Taking advantage of more powerful computational resources, we plan to extend the search to still higher energies.

hep-ph↗
arXiv1996

Computational Study of Baryon Number Violation in High Energy Electroweak Collisions

We use semiclassical methods to study processes which give rise to change of topology and therefore to baryon number violation in the standard model. We consider classically allowed processes, i.e.~energies above the sphaleron barrier. We develop a computational procedure that allows us to solve the Yang Mills equations of motion for spherically symmetric configurations and to identify the particle numbers of the in- and out-states. A stochastic sampling technique is then used to map the region spanned by the topology changing solutions in the energy versus incoming particle number plane and, in particular, to determine its lower boundary. A lower boundary which approaches small particle number would be a strong indication that baryon number violation would occur in high energy collisions, whereas a lower asymptote at large particle number would be evidence of the contrary. With our method and the computational resources we have had at our disposal, we have been able to determine the lower boundary up to energies approximately equal to one and a half time times the sphaleron energy and observed a 40\% decrease in particle number with no sign of the particle number leveling off. However encouraging this may be, the decrease in incoming particle number is only from particles 50 down to approximately 30. Nevertheless, the formalism we have established will make it possible to extend the scope of this investigation and also to study processes in the classically forbidden region, which we plan to do in the future.

hep-ph↗
arXiv1994

Phase structure and monopoles in U(1) gauge theory

We investigate the phase structure of pure compact U(1) lattice gauge theory in 4 dimensions with the Wilson action supplemented by a monopole term. To overcome the suppression of transitions between the phases in the simulations we make the monopole coupling a dynamical variable. We determine the phase diagram and find that the strength of the first order transition decreases with increasing weight of the monopole term, the transition thus ultimately getting of second order. After outlining the appropriate topological characterization of networks of currents lines, we present an analysis of the occurring monopole currents which shows that the phases are related to topological properties.

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arXiv1994

Chiral Perturbation Theory on the Lattice; Strong Coupling Expansion

We evaluate the coefficients of the effective chiral Lagrangian to $O(p^4)$ in the strong coupling, large-N expansion. In this limit we explicitly perform the functional integral over fundamental degrees of freedom and obtain the effective chiral Lagrangian. We perform this calculation on the Body Centered Hypercubical lattice which preserves Euclidean invariance to order $p^4$. We further discuss how the coefficients could be obtained numerically, out of the strong-coupling domain.

hep-lat↗
arXiv1994

Lattice QCD and Chiral Lagrangians

After a very brief review of the formalism of lattice gauge theories we show how one can calculate the parameters of the continuum chiral Lagrangians proceeding through the derivation of an effective lattice chiral Lagrangian as an intermediary step. The derivation is done in the strong coupling limit. We also discuss how the derivation could be carried out in the intermediate coupling domain by numerical simulation techniques. Based on a lecture given by C. Rebbi at the 1993 ICTP Summer School in High Energy Physics and Cosmology, to be published in the proceedings of the School (World Scientific Publishing - Singapore - 1994.)

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