arXiv subjects
Publications and source records attributed to A. Pich.
The total $τ$ hadronic width can be accurately calculated using analyticity and the operator product expansion. The theoretical analysis of this observable is updated to include all available perturbative and non-perturbative corrections. The experimental determination of $α_s(M_τ^2)$ and its actual uncertainties are discussed.
The Standard Model requires the three known leptonic families to have identical couplings to the gauge bosons. The present experimental tests on lepton universality are reviewed, both for the charged and neutral current sectors. Our knowledge about the Lorentz structure of the $l^-\toν_l l'^-\barν_{l'}$ transition amplitudes is also discussed.
The pure leptonic or semileptonic character of tau decays makes them a good laboratory to test the structure of the weak currents and the universality of their couplings to the gauge bosons. The hadronic tau decay modes constitute an ideal tool for studying low-energy effects of the strong interactions in very clean conditions; a well-known example is the precise determination of the QCD coupling from tau-decay data. New physics phenomena, such as a non-zero tau-neutrino mass or violations of (flavour / CP) conservation laws can also be searched for with tau decays.
Non-decoupling effects related to a large $m_t$ affecting non-oblique radiative corrections in vertices ($Z\bar{b}b$) and boxes ($B$-$\bar{B}$ mixing and $ε_K$) are very sensitive to the particular mechanism of spontaneous symmetry breaking. We analyze these corrections in the framework of a chiral electroweak standard model and find that there is only one operator in the effective lagrangian which modifies the longitudinal part of the $W^+$ boson without touching the oblique corrections. The inclusion of this operator affects the $Z\bar{b}b$ vertex, the $B$-$\bar{B}$ mixing and the CP-violating parameter $ε_K$, generating interesting correlations among the hard $m_t^4 \log m_t^2$ corrections to these observables, for example, the maximum vertex $Z b\bar{b}$ correction allowed by low energy physics is about one percent.
Rare K decays are an important testing ground of the electroweak flavour theory. They can provide new signals of CP-violation phenomena and, perhaps, a window into physics beyond the Standard Model. The interplay of long-distance QCD effects in strangeness-changing transitions can be analyzed with Chiral Perturbation Theory techniques. Some theoretical predictions obtained within this framework for radiative kaon decays are reviewed, together with the present experimental status.
These lectures provide an introductory overview of the dynamics of flavour-changing transitions. The main emphasis is put on present tests of the quark-mixing matrix structure and the phenomenological determination of its parameters. The interplay of strong interactions in weak decays and the important role of flavour symmetries for controlling the size of QCD corrections to some semileptonic transitions are discussed.
It is shown that there exist symmetry constraints for non--leptonic weak amplitudes which emerge when the $1/N_c$--expansion restricted to the leading and next--to--leading approximations only is systematically combined with $χ$PT limited to the lowest non--trivial order. We discuss these constraints for the couplings $\mbox{\bf g}_8$ and $\mbox{\bf g}_{27}$ of $ΔS=1$ transitions and the $\mbox{\bf B}_{K}$--parameter of $K^0-{\bar K}^0$ mixing.
$τ$ decays provide a powerful tool to test the structure of the weak currents and the universality of their couplings to the $W$ boson. The constraints implied by present data and the possible improvements at the $τ$-charm Factory are analyzed.
The recent measurements of the Michel parameters in tau decays enable, for the first time, a thorough analysis of the leptonic sector. In general, in models beyond the Standard Model, these parameters will be altered through changes in the W and Z couplings, and/or through interactions mediated by new gauge bosons. We perform a complete, model independent analysis of the constraints imposed by the present data on such boson-mediated interactions, and point out the existence of useful relations among the couplings.
These lectures provide an overview of Quantum Chromodynamics (QCD), the $SU(3)_C$ gauge theory of the strong interactions. After briefly reviewing the empirical considerations which lead to the introduction of {\it colour}, the QCD Lagrangian is discussed. The running of the strong coupling and the associated property of {\it Asymptotic Freedom} are analyzed. Some selected experimental tests and the present knowledge of $α_s$ are summarized. A short description of the QCD flavour symmetries and the {\it dynamical breaking of chiral symmetry} is also given.
The real part of the $K^{+}$ selfenergy for the interaction of the $K^{+}$ with the pion nuclear cloud is evaluated in lowest order of chiral perturbation theory and is found to be exactly zero in symmetric nuclear matter. This removes uncertainties in that quantity found in former phenomenological analyses and is supported by present experimental data on $K^{+}$ nucleus scattering.
An introduction to the basic ideas and methods of Chiral Perturbation Theory is presented. Several phenomenological applications of the effective Lagrangian technique to strong, electromagnetic and weak interactions are discussed.
What follows is an updated version of the lectures given at the CERN Academic Training (November 1993) and at the Jaca Winter Meeting (February 1994). The aim is to provide a pedagogical introduction to the Standard Model of electroweak interactions. After briefly reviewing the empirical considerations which lead to the construction of the Standard Model Lagrangian, the particle content, structure and symmetries of the theory are discussed. Special emphasis is given to the many phenomenological tests (universality, flavour-changing neutral currents, precision measurements, quark mixing, etc.) which have established this theoretical framework as the Standard Theory of electroweak interactions.
The total $τ$ hadronic width can be accurately calculated using analyticity and the operator product expansion. The theoretical analysis of this observable is updated to include all available perturbative and non-perturbative corrections. The experimental determination of $α_s(M_τ^2)$ and its actual uncertainties are discussed.
The interplay of QCD in $ΔS=1,2$ non-leptonic weak transitions can be rigorously analyzed, at the inclusive level, by studying the 2--point functions associated with the corresponding $ΔS=1,2$ effective Hamiltonians. The next-to-leading order calculation of these correlators shows a huge ($\gsim 100\%$) gluonic enhancement of the $|ΔI|=1/2$ channel, providing a qualitative understanding of the $|ΔI|=1/2$ rule within QCD.
We summarize the present knowledge on the non-perturbative hadronic inputs needed in the analysis of $B^0$-$\bar B^0$ mixing and the CP-violating parameter $\varepsilon$ of the $K^0$-$\bar K^0$ system. Using this information, together with the recently determined value of the top-quark mass, we update the phenomenological constraints on the unitarity triangle.
An overview of the phenomenology of CP violation is presented. The Standard Model mechanism of CP violation and its main experimental tests, both in the kaon and bottom systems, are discussed. (Lectures given at the 1993 Trieste Summer School and at the Escuela Latinoamericana de Fisica, ELAF'93, Argentina).
The Tau-Charm Factory combines the optimum conditions to perform a high-precision investigation of the $τ$ and $ν_τ$ leptons, the $c$ quark, and light-quark, glueball and hybrid spectroscopy. An overview of the broad physics program that this facility will address is presented.