arXiv subjects

M. Fabbrichesi

Publications and source records attributed to M. Fabbrichesi.

62 records · Page 4Linked to original sources

arXiv1994

The Relevance of the Dipole Penguin Operators in $ε'/ε$

The standard model contribution to $ε'/ε$ of the magnetic- and electric-dipole penguin operators $Q_{11} = \frac{g_s}{16π^2} \, m_s \: \bar{s}\, σ_{μν} t^a G^{μν}_a (1-γ_5 )\, d $ and $Q_{12} = \frac{eQ_d}{16π^2}\, m_s \: \bar{s}\, σ_{μν} F^{μν} (1-γ_5 )\, d $ is discussed. While the electromagnetic penguin $Q_{12}$ seems to have a vanishingly small matrix element, we find that the gluonic dipole operator $Q_{11}$ may give a contribution to $ε'/ε$ comparable to that of other operators so far considered, and should therefore be consistently included in the analysis.

hep-ph↗

arXiv1993

Planckian Energy Scattering and Surface Terms in the Gravitational Action

This is a revised version of our previous paper by the same name and preprint number. It contains various changes, two figures and new results in sect.5. We propose a new approach to four-dimensional Planckian-energy scattering in which the phase of the ${\cal S}$-matrix is written---to leading order in $\hbar$ and to all orders in $R/b =Gs/J$---in terms of the surface term of the gravity action and of a boundary term for the colliding quanta. The proposal is checked at the leading order in $R/b$ and also against some known examples of scattering in strong gravitational fields.

hep-th↗

arXiv1993

The Rate Difference between the Weak Decays of $t$ and $\bar{t}$ in Supersymmetry

We find that the $CP$-violating asymmetry $$ \frac{Γ\left(t \rightarrow bW^+ \right) - Γ\left(\bar{t} \rightarrow \bar{b}W^- \right)} {Γ\left(t \rightarrow bW^+ \right) + Γ\left(\bar{t} \rightarrow \bar{b}W^- \right)} $$ at the one-loop order within the minimal supersymmetric extension of the standard model is of the order of few per cent for maximal $CP$ violation. It could be measured by considering the rate difference in the one-lepton events.

hep-ph↗

arXiv1993

Testing Supersymmetry by Means of Time Reversal Invariance

The minimal supersymmetric extension of the standard model allows for some of the coupling strengths to be complex parameters. The presence of such imaginary phases can lead to violations of time reversal invariance, which can be tested if correlations in products of an odd number of polarizations and momenta are measured and found to be different from zero. As an example, we consider the triple product $\mbox{\bf J} \cdot \left(\mbox{\bf p}_1 \times \mbox{\bf p}_ {2} \right)$ in the $β$-decay of the neutron, of the $Σ^-$, and in the decay $K^{+}_{3μ}$. For these low-energy decays, we find that the present experimental precision is not enough to provide useful bounds on combinations of such phases and the masses of the supersymmetric particles. At higher energies, the same time reversal violating correlation in the semileptonic decay of the $t$ quark is of the order of $α_s/π$, made bigger by the large mass of the decaying quark.

hep-ph↗

arXiv1993

T Violation Induced by Supersymmetry in t\bar{t} and W^+W^- Physics

T-odd correlations of polarizations and momenta provide a promising testing ground for new physics beyond the standard model. We estimate the contribution of the minimal supersymmetric extension of the standard model to two such observables: in the production of $t\bar{t}$, we look for a term proportional to \mbox{${\bf J}_t \cdot ({\bf p}_q \times {\bf p}_{t})$}-where {\bf J}$_t$ is the polarization of the $t$ quark and {\bf p} $_{q,t}$ are the momenta of the initial and final particles-and find that it is of the order of $10^{-1} \times (α_s/π)$. In the production of $W^+W^-$, we look for a term proportional to \mbox{${\bf E}_W \cdot ({\bf p}_q \times {\bf p}_{W})({\bf p}_q \cdot {\bf E}_W)$}-where {\bf E}$_W$ is the transverse polarization of W- to find that it can be as large as $10^{-1} \times (α_w/π)$.

hep-ph↗

arXiv1992

The Gravitational Field of String Matter When the Dilaton is Massive

We study numerically the gravitational field of a star made of massive and neutral string states for the case in which the dilaton is massive. The solution exhibits very simple scaling properties in the dilaton mass. There is no horizon and the singularity is surrounded by a halo (the physical size of which is inversely proportional to the dilaton mass) where the scalar curvature is very large and proportional to the square of the dilaton mass.

hep-th↗

arXiv1992

Strong Antigravity. Life in the Shock Wave

Strong anti-gravity is the vanishing to all orders in Newton's constant of the net force between two massive particles at rest. We study this phenomenon and show that it occurs in any effective theory of gravity which is obtained from a higher-dimensional model by compactification on a manifold with flat directions. We find the exact solution of the Einstein equations in the presence of a point-like source of strong anti-gravity by dimensional reduction of what is a shock-wave solution in the higher-dimensional model. (Latex file, no macros, figures not included)

hep-th↗

arXiv1992

The Gravitational Field of String Matter

We study the scattering of a massless and neutral test particle in the gravitational field of a body (the string star) made of a large number of scalar states of the superstring. We consider two cases, the one in which these states are neutral string excitations massive already in ten dimensions and the one in which their masses (and charges) originate in the process of compactification on tori. A perturbative calculation based on superstring amplitudes gives us the deflection angle up to the second order in Newton's constant. A comparison with field theory explicitly shows which among the various massless fields of the superstring give a contribution to the scattering process. In both cases, the deflection angle is smaller than the one computed in general relativity. The perturbative series can be resummed by finding the exact solution to the classical equations of motion of the corresponding low-energy action. The space-time metric of our two examples of string stars has no horizon.

hep-th↗