A Scenario for Contact Interactions at HERA

The four fermion contact interactions, required to explain the anomalous HERA result, could come from the exchange of new heavy (probably composite) resonances. Depending on their charges and quantum numbers, one gets different scenarios and finds that many of these configurations are unsuitable. For example, new neutral resonances seems to be disfavored by the data coming from the TEVATRON, LEP 2 and atomic parity violation. These experiments allow only few helicity combinations that cannot arise from neutral currents in a natural way. On the contrary, a global large symmetry SU(8) * SU(8) (which is contained in SU(16)) embeds some lepto-quarks of spin 1 that could give suitable four fermion interactions (compatible with all other experiments) if these resonances are the lightest new (probably composite) states with a mass comparable to the scale of the contact interactions.Comment: 8 pages, LaTeX, no figur

Electroweak Precision Tests: A Concise Review

1. Introduction 2. Status of the Data 3. Precision Electroweak Data and the Standard Model 4. A More General Analysis of Electroweak Data 4.1 Basic Definitions and Results 4.2 Experimental Determination of the Epsilon Variables 4.3 Comparing the Data with the Minimal Supersymmetric Standard Model 5. Theoretical Limits on the Higgs Mass 6. ConclusionComment: Submitted to Int. Journal of Modern Physics

Indication for Light Sneutrinos and Gauginos from Precision Electroweak Data

The present Standard Model fit of precision data has a low confidence level, and is characterized by a few inconsistencies. We look for supersymmetric effects that could improve the agreement among the electroweak precision measurements and with the direct lower bound on the Higgs mass. We find that this is the case particularly if the 3.6 sigma discrepancy between sin^2 theta_eff from leptonic and hadronic asymmetries is finally settled more on the side of the leptonic ones. After the inclusion of all experimental constraints, our analysis selects light sneutrinos, with masses in the range 55-80 GeV, and charged sleptons with masses just above their experimental limit, possibly with additional effects from light gauginos. The phenomenological implications of this scenario are discussed.Comment: 17 pages LaTex, 9 figures, uses epsfi

How to Integrate Divergent Integrals: a Pure Numerical Approach to Complex Loop Calculations

Loop calculations involve the evaluation of divergent integrals. Usually [1] one computes them in a number of dimensions different than four where the integral is convergent and then one performs the analytical continuation and considers the Laurent expansion in powers of epsilon =n-4. In this paper we discuss a method to extract directly all coefficients of this expansion by means of concrete and well defined integrals in a five dimensional space. We by-pass the formal and symbolic procedure of analytic continuation; instead we can numerically compute the integrals to extract directly both the coefficient of the pole 1/epsilon and the finite part.Comment: 13 pages, 1 Postscript figur

Determination of the CKM unitarity triangle parameters by end 1999

Within the Standard Model, a review of the current determination of the CKM unitarity triangle parameters is presented, using experimental constraints from the measurements of |epsilon_K|, V_ub/V_cb, Delta m_d and from the limit on Delta m_s, available by end 1999.Comment: Talk given by A. Stocchi at the Third International Conference on B physics and CP violation, National Taiwan Univ., Taipei, Tawain, December 3-7, 199

Top-quark physics in six-quark final states at the Next Linear Collider

The processes of six-quark production with one $b\bar b$ pair are studied by means of a complete tree-level electroweak calculation. The top-quark signal is examined: the importance of electroweak backgrounds, of the order of 10% above the $t\bar t$ threshold and of about 30% of the purely electroweak signal at threshold, is further stressed by studying the dependence of the cross-section at threshold on the Higgs mass in the range between 100 GeV and 185 GeV, and finding variations of the order of 10%. In the study of some event-shape variables, a strong effect of initial-state radiation is found, in particular for the thrust distribution, which is studied for several centre-of-mass energies at the TeV scale. The effectiveness of cuts on the thrust for isolating QCD backgrounds, as pointed out by some authors, is confirmed also in the presence of electroweak backgrounds and initial-state radiation.Comment: LaTeX (using elsart.sty), 17 pages, 9 figures include