arXiv subjects

F. J. Botella

Publications and source records attributed to F. J. Botella.

At least 37 records · Page 2Linked to original sources

arXiv2005

Can one detect new physics in I=0 and/or I=2 contributions to the decays B --> pi pi?

We study the effects of new-physics contributions to B --> pi pi decays, which can be parametrized as four new complex quantities. A simple analysis is provided by utilizing the reparametrization invariance of the decay amplitudes. We find that six quantities can be reabsorbed into the definitions of Standard Model-like parameters. As a result, the usual isospin analysis provides only two constraints on new physics which are independent of estimates for the Standard Model contributions. In particular, we show that one is not sensitive to new physics affecting the I=0 amplitudes. On the other hand, I=2 new physics can be detected, and its parameters can be measured by using independent determinations of the weak phases. We obtain constraints on these new-physics parameters through a fit to the current experimental data.

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arXiv2005

Reparametrization invariance of B decay amplitudes and implications for new physics searches in B decays

When studying B decays within the Standard Model, it is customary to use the unitarity of the CKM matrix in order to write the decay amplitudes in terms of only two of the three weak phases which appear in the various diagrams. Occasionally, it is mentioned that those two weak phases can be used in order to describe any decay amplitude, even beyond the Standard Model. Here we point out that, when describing a generic decay amplitude, the two weak phases can be chosen completely at will, and we study the behavior of the decay amplitudes under changes in the two weak phases chosen as a basis. Of course, physical observables cannot depend on such reparametrizations. This has an impact in discussions of the SM and in attempts to parametrize new physics effects in the decay amplitudes. We illustrate these issues by looking at B --> psi K_S and the isospin analysis in B --> pi pi.

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arXiv2004

The size of chi = arg (-Vts Vtb* Vcs* Vcb) and physics beyond the Standard Model

We analyse the allowed range of values of chi, both in the Standard Model and in models with New Physics, pointing out that a relatively large value of chi, e.g. of order lambda, is only possible in models where the unitarity of the 3x3 Cabibbo-Kobayashi-Maskawa matrix is violated through the introduction of extra Q=2/3 quarks. We study the interesting case where the extra quark is an isosinglet, determining the allowed range for chi and the effect of a large chi on various low-energy observables, such as CP asymmetries in B meson decays. We also discuss the correlated effects which would be observable at high energy colliders, like decays t -> cZ, modifications of the cross section and forward-backward asymmetry in e+ e- -> ttbar and the direct production of a new quark.

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arXiv2004

Bounds on gamma from CP violation measurements in B -> pi+ pi- and B -> psi K_S

We study the determination of gamma from CP-violating observables in B -> pi+ pi- and B -> psi K_S. This determination requires theoretical input to one combination of hadronic parameters. We show that a mild assumption about this quantity may allow bounds to be placed on gamma, but we stress the pernicious effects that an eightfold discrete ambiguity has on such an analysis. The bounds are discussed as a function of the direct (C) and interference (S) CP-violating observables obtained from time-dependent B -> pi+ pi- decays, and their behavior in the presence of new physics effects in B-Bbar mixing is studied. (V2: Misprints corrected. Slightly improved discussion.)

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arXiv2002

Unitarity Triangles and the Search for New Physics

Assuming that the Kobayashi-Maskawa mechanism gives the dominant contribution to CP violation at low energies, we propose a novel way of testing the flavour sector of the Standard Model which has the potencial for discovering New Physics. Using 3 x 3 unitarity of the V_{CKM} matrix and choosing a complete set of rephasing invariant phases, we derive a set of exact relations in terms of measurable quantities, namely moduli of V_{CKM} and arguments of rephasing invariant quartets. These tests complement the usual analysis in the ρ, ηplane and, if there is New Physics, may reveal its source.

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arXiv2001

Constraining models with vector-like fermions from FCNC in K and B physics

In this work, we update the constraints on tree level FCNC couplings in the framework of a theory with n isosinglet vector-like down quarks. In this context, we emphasize the sensitivity of the B -> J/psi K_S CP asymmetry to the presence of new vector-like down quarks. This CP asymmetry, together with the rare decays B -> X_{s,d} l bar{l} and K -> pi nu bar{nu} are the best options to further constrain the FCNC tree level couplings or even to point out, in the near future, the possible presence of vector-like quarks in the low energy spectrum, as suggested by GUT theories or models of large extra dimensions at the TeV scale.

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arXiv2001

Tree-level FCNC in the B system: from CP asymmetries to rare decays

Tree-level Flavor-Changing Neutral Currents (FCNC) are characteristic of models with extra vector-like quarks. These new couplings can strongly modify the B^0 CP asymmetries without conflicting with low--energy constraints. In the light of a low CP asymmetry in B --> J/ψK_{S}, we discuss the implications of these contributions. We find that even these low values can be easily accommodated in these models. Furthermore, we show that the new data from B factories tend to favor an O(20) enhancement of the b --> d l \bar{l} transition over the SM expectation.

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arXiv1998

D^0-D^0(bar) mixing in the weak-gauged U(4)_L x U(4)_R Chiral Lagrangian Model

The D^0-D^0(bar) mixing is analyzed in a weak gauged U(4)_L x U(4)_R chiral lagrangian model where the electroweak interaction is introduced as a gauge theory over the meson degrees of freedom. This model allows a particular realization of the G.I.M. mechanism and then could be useful in the study of processes where G.I.M. suppression is effective. As a test of the model we have also analyzed the K^0-K^0(bar) mixing. We find Δm_K in good agreement with the experimental result and we show that the D^0-D^0(bar) mixing is very much suppressed in agreement with previous estimates in the Heavy Quark expansion framework.

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arXiv1997

P-odd observables at the $Υ$ peak

We study the $γ$-Z interference in the process $e^+ e^-\to Υ\to τ^+ τ^-$ as a means to measure the neutral current coupling of the b-quark. The helicity amplitudes are calculated from resonant and background diagrams and the spin density matrix of the final state is discussed. The spin analyzer of the $τ$'s is illustrated with the decays $πν$ and $ρν\to (ππ) ν$. With $10^8 Υ$ a sensitivity to $g_V^b$ of a few per cent could be reachable.

hep-ph↗

arXiv1997

How sensitive to FCNC can $B^0$ CP asymmetries be?

We show that the study of CP asymmetries in neutral B-meson decays provides a very sensitive probe of flavour-changing neutral currents (FCNC). We introduce two new angles, $α_{SM}$ and $β_{SM}$, whose main feature is that they can be readily obtained from the measurement of the CP asymmetries $a_{J/ψK_s}$, $a_{π^+ π^-}$ and the ratio $R_u \equiv|V_{ud}V_{ub}^*|/|V_{cd}V_{cb}^*|$, providing a quantitative test of the presence of new physics in a model-independent way. Assuming that new physics is due to the presence of an isosinglet down-type quark, we indicate how to reconstruct the unitarity quadrangles and point out that the measurements of the above asymmetries, within the expected experimental errors, may detect FCNC effects, even for values of $|\sum_{i=1}^3 V_{id} V_{ib}^* / (V_{td} V_{tb}^*)| $ at the level of a few times $10^{-2}$.

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arXiv1997

$ΔF = 2$ Effective Lagrangian in theories with vector like fermions

In this work we analyze a new piece present in the $ΔF = 2$ effective Lagrangian in models with extra vector-like quarks. This piece, which was not taken into account previously, is required in order to preserve gauge invariance once the unitarity of the CKM matrix is lost. We illustrate the effects of this new piece in both, CP conserving and CP violating processes.

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arXiv1996

$γ$ - Z interferometry at a $Φ$-factory

We analyze the possibilities that the proposed $Φ$-factories offer to measure $γ-Z$ interference. In the unpolarized beam case, we study different signatures in the $ρπ$ channel, taking advantage of the presence of the near-by $a_1$ resonance. We build a C-odd forward-backward asymmetry, estimated to be around $10^{-5}$, and (P-even, T-even) and (P-odd, T-odd) alignments of the $ρ$, to be seen from the angular distribution of its $ππ$ decay products. With polarized electrons a left-right asymmetry around $2\times 10^{-4}$ is present in all channels. At leading order this asymmetry is independent of hadronic matrix elements and is sensitive to the $Z^0-s\bar{s}$ vector coupling. In the $ρπ$ channel, a combined left-right forward-backward asymmetry is considered.

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arXiv1995

Parameter free calculation of non-resonant three body decays of charmed mesons in a weak gauged U(4)_L x U(4)_R Chiral Lagrangian Model

Non--resonant decays of charmed mesons into three pseudoscalars are analyzed in a weak gauged $U(4)_L \otimes U(4)_R$ chiral lagrangian model. The calculation is free of unknown parameters and only requires the masses of pseudoscalar mesons as hadronic inputs. When comparison with experimental data is possible we find that in some processes we have good agreement and in others we are an order of magnitude below. This may be due to the absence in our calculation of final state interactions, presumably important here, and the manifest difficulty in extracting the non--resonant contribution from an experimental point of view.

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arXiv1994

Jarlskog-like invariants for theories with scalars and fermions

Within the framework of theories where both scalars and fermions are present, we develop a systematic prescription for the construction of CP-violating quantities that are invariant under basis transformations of those matter fields. In theories with Spontaneous Symmetry Breaking, the analysis involves the vevs' transformation properties under a scalar basis change, with a considerable simplification of the study of CP violation in the scalar sector. These techniques are then applied in detail to the two Higgs-doublet model with quarks. It is shown that there are new invariants involving scalar-fermion interactions, besides those already derived in previous analyses for the fermion-gauge and scalar-gauge sectors.

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arXiv1994

Neutrino Mass and Magnetic Moment from Neutrino-Electron Scattering

We study both the elastic ($νe \rightarrow νe$) and the radiative process ($νe \rightarrow νe γ$) and discuss how these processes can shed light on some current topics in neutrino physics such as a neutrino magnetic moment and neutrino oscillations. The radiative process allows to reach low values of $Q^2$ without the need to operate at very small energies of recoil electrons, a favourable scenario to search for a neutrino magnetic moment. The elastic cross section contains a dynamical zero at $E_ν=m/(4 sin^2θ_{W})$ and forward electrons for the electron antineutrino channel, which is reachable at reactor facilities and accessible after the convolution with the antineutrino spectrum. The implication for lepton flavour changing transitions in that energy region searched for in neutrino oscillation experiments, which combine disappearance and appearance rates, is discussed.

hep-ph↗

arXiv1994

A Novel Kind of Neutrino Oscillation Experiment

A novel method to look for neutrino oscillations is proposed based on the elastic scattering process $\barν_{i} e^{-}\rightarrow \barν_{i} e^{-}$, taking advantage of the dynamical zero present in the differential cross section for $\barν_{e} e^{-}\rightarrow \barν_{e} e^{-}$. An effective tunable experiment between the "appearance" and "disappearance" limits is made possible. Prospects to exclude the allowed region for atmospheric neutrino oscillations are given.

hep-ph↗

arXiv1994

Neutrino Magnetic Moment and the Process $νe \rightarrow νe γ$

The contribution of a neutrino magnetic moment $μ_ν$ to the cross section of the process $νe^{-}\rightarrow νe^{-} γ$ has been calculated and compared with the Standard Electroweak one. The radiative process allows to reach low enough values of $Q^2$ without the need to operate at very small energies of recoil electrons. Regions in the phase space which are more favourable to set bounds on $μ_ν$ are suggested.

hep-ph↗

arXiv1993

Dynamical zeros in neutrino-electron elastic scattering at leading order

We show the existence of dynamical zeros in the helicity amplitudes for neutrino-electron elastic scattering at lowest order in the standard theory. In particular, the $λ=1/2$ non-flip electron helicity amplitude in the electron antineutrino process vanishes for an incident neutrino energy $E_ν=m_{e}/(4sin^{2}θ_{W})$ and forward electrons (maximum recoil energy). The rest of helicity amplitudes show kinematical zeros in this configuration and therefore the cross section vanishes. Prospects to search for neutrino magnetic moment are discussed.

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