arXiv subjects
Publications and source records attributed to Gino Isidori.
We present an overview of rare K, D and B decays. Particular attention is devoted to those flavour-changing neutral-current processes of K and B mesons that offer the possibility of new significant tests of the Standard Model. The sensitivity of these modes to physics beyond the Standard Model and the status of their experimental study are also discussed.
If the Higgs mass m_H is as low as suggested by present experimental information, the Standard Model ground state might not be absolutely stable. We present a detailed analysis of the lower bounds on m_H imposed by the requirement that the electroweak vacuum be sufficiently long-lived. We perform a complete one-loop calculation of the tunnelling probability at zero temperature, and we improve it by means of two-loop renormalization-group equations. We find that, for m_H=115 GeV, the Higgs potential develops an instability below the Planck scale for m_t>(166\pm 2) GeV, but the electroweak vacuum is sufficiently long-lived for m_t < (175\pm 2) \GeV.
These lectures provide an elementary introduction to Chiral Perturbation Theory, focused on the sector of pseudoscalar meson interactions. Basic concepts and technical methods of this approach are discussed on general grounds and with the help of a few specific examples.
Rare flavour-changing neutral-current transitions of the type $s \to d \ell^+\ell^-(ν\barν)$ and $b \to s \ell^+\ell^-(ν\barν)$ are analysed in supersymmetric extensions of the Standard Model with generic flavour couplings. It is shown that these processes are particularly sensitive to the left--right mixing of the squarks and that, in the presence of non-universal $A$ terms, they could lead to unambiguous signatures of new physics in exclusive $K$ and $B$ meson decays.
A short overview of recent progress in describing kaon and pion decays within Chiral Perturbation Theory is presented. Particular attention is devoted to the issues of final-state interactions and isospin breaking in $K \to 2 π$, as well as to the estimate of long-distance contributions in $K \to \ell^+\ell^- (π)$.
Rare B decays of the type $b \to s~\ell^+\ell^-(ν\barν)$ are analyzed in a generic scenario where New Physics effects enter predominantly via $Z$ penguin contributions. We show that this possibility is both phenomenologically allowed and well motivated on theoretical grounds. The important role played in this context by the lepton forward-backward asymmetry in $B\to K^*\ell^+\ell^-$ is emphasized.
We present a brief overview of rare K decays, emphasizing the different role of Standard Model and possible New Physics contributions in various channels.
We present a detailed estimate of the transverse muon polarization (P_T) due to electromagnetic final-state interactions in the decay $K^+ \to μ^+ νγ$. This CP-conserving effect represents the dominant contribution to P_T within the Standard Model. As a result of an explicit calculation, we find that the CP-conserving contribution to P_T is quite small, typically of $\cO(10^{-4})$, essentially due to the suppression factor $α/4π^2$. This enforces the sensitivity of P_T in probing extensions of the Standard Model with new sources of time-reversal violations. A brief discussion about possible CP-violating contributions to P_T in the framework of supersymmetric models with unbroken R-parity is also presented.
A short overview of FCNC and CP-violating observables in K decays is presented. Particular attention is paid to the possibility of performing precision tests of flavour dynamics and to the search for New Physics.
We analyze the contributions to rare kaon decays mediated by flavor-changing Z-penguin diagrams in a generic low-energy supersymmetric extension of the Standard Model. In order to perform a model-independent analysis we expand the squark mass matrices around the diagonal, following the so called mass-insertion approximation. We argue that in the present case it is necessary to go up to the second order in this expansion to take into account all possible large effects. The current bounds on such second-order term, which was neglected in previous analyses, are discussed in detail and the corresponding upper bounds for the rare kaon decay rates are derived. As a result, we show that supersymmetric effects could lead to large enhancements of K -> pi nu{\bar nu} and K_L -> pi^0 e^+ e^- branching ratios.
The rare decay $K_L\toπ^0ν\barν$ is dominated by direct CP violation and can be computed with extraordinarily high precision. In principle also a CP conserving contribution to this process can arise within the Standard Model. We clarify the structure of the CP conserving mechanism, analyzing both its short-distance and long-distance components. It is pointed out that the calculation of the CP conserving amplitude, although sensitive in part to nonperturbative physics, is rather well under control. The resulting CP conserving contribution to the rate for $K_L\toπ^0ν\barν$ turns out to be very strongly suppressed due to several factors, which we discuss in detail.
We reconsider the calculation of ${\cal O}(Λ^2_{QCD}/m^2_b)$ nonperturbative corrections to $\bar B\to X_sl^+l^-$ decay. Our analysis confirms the results of Ali et al. for the dilepton invariant mass spectrum, which were in disagreement with an earlier publication, and for the lepton forward-backward asymmetry. We also give expressions for the ${\cal O}(Λ^2_{QCD}/m^2_b)$ corrections to the left-right asymmetry. In addition we discuss the breakdown of the heavy quark expansion near the point of maximal dilepton invariant mass $q^2$ and consider a model independent approach to this region using heavy hadron chiral perturbation theory. The modes $\bar B\to\bar Kl^+l^-$ and $\bar B\to\bar Kπl^+l^-$, which determine the endpoint region of the inclusive decay, are analyzed within this framework. An interpolation is suggested between the region of moderately high $q^2$, where the heavy quark expansion is still valid, and the vicinity of the endpoint described by chiral perturbation theory. We also comment on further nonperturbative effects in $\bar B\to X_sl^+l^-$.
The difference in the time dependent CP asymmetries between the modes $B --> psi K_S$ and $B --> phi K_S$ is a clean signal for physics beyond the Standard Model. This interpretation could fail if there is a large enhancement of the matrix element of the $b --> u ubar s$ operator between the $B_d$ initial state and the $phi K_S$ final state. We argue against this possibility and propose some experimental tests that could shed light on the situation.
A short overview of theoretical predictions for various kaon decays is presented. Particular attention is devoted to pure and radiative nonleptonic decays in the framework of Chiral Perturbation Theory. The relevance of KLOE's future results to improve our knowledge of kaon physics and more generally of the Standard Model at low energy is also emphasized.
We analyze $K \to ππγ$ decays in the framework of Chiral Perturbation Theory. We study the different Dalitz plot distributions, trying to find regions where o($p^6$) contributions could be more easily detected. To fulfill this program we compute all the the o($p^4$) loop and counterterm contributions, finding a substantial agreement with the existing calculations and adding some small missing terms in $K_S \to ππγ$.