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Arcadi Santamaria

Publications and source records attributed to Arcadi Santamaria.

At least 37 records · Page 2Linked to original sources
arXiv2011

Neutrino masses from new generations

We reconsider the possibility that Majorana masses for the three known neutrinos are generated radiatively by the presence of a fourth generation and one right-handed neutrino with Yukawa couplings and a Majorana mass term. We find that the observed light neutrino mass hierarchy is not compatible with low energy universality bounds in this minimal scenario, but all present data can be accommodated with five generations and two right-handed neutrinos. Within this framework, we explore the parameter space regions which are currently allowed and could lead to observable effects in neutrinoless double beta decay, $μ- e$ conversion in nuclei and $μ\rightarrow e γ$ experiments. We also discuss the detection prospects at LHC.

hep-ph↗
arXiv2009

A model for right-handed neutrino magnetic moments

A simple extension of the Standard Model providing Majorana magnetic moments to right-handed neutrinos is presented. The model contains, in addition to the Standard Model particles and right-handed neutrinos, just a singly charged scalar and a vector-like charged fermion. The phenomenology of the model is analysed and its implications in cosmology, astrophysics and lepton flavour violating processes are extracted. If light enough, the charged particles responsible for the right-handed neutrino magnetic moments could copiously be produced at the LHC.

hep-ph↗
arXiv2009

Right-handed neutrino magnetic moments

We discuss the phenomenology of the most general effective Lagrangian, up to operators of dimension 5, build with standard model fields and interactions including right-handed neutrinos. In particular we find there is a dimension 5 electroweak moment operator of right-handed neutrinos, not discussed previously in the literature, which could have interesting phenomenological consequences.

hep-ph↗
arXiv2008

Prospects for the Zee-Babu Model at the LHC and low energy experiments

We analyze the viability of the Zee-Babu model as an explanation of observed neutrino masses and mixings and the possibility that the model is confirmed or discarded in experiments planned for the very close future. The allowed parameter space is studied analytically by using some approximations and partial data. Then, a complete scanning of all parameters and constraints is performed numerically by using Monte Carlo methods. The cleanest signal of the model will be the detection of the doubly charged scalar at the LHC and its correlation with measurements of the branching ratio of $μ\to eγ$ at the MEG experiment. In addition, the model offers interesting predictions for $τ^{-}\toμ^{+}μ^{-}μ^{-}$ experiments, lepton-hadron universality tests, the $θ_{13}$ mixing in neutrino oscillations and the $ _{ee}$ parameter of neutrinoless double beta decay.

hep-ph↗
arXiv2005

From transition magnetic moments to majorana neutrino masses

It is well known that a majorana mass induces a (small) transition magnetic moment. The converse is also true; in this paper we estimate the loop contribution of transition magnetic moments $[μ]_{αβ}$ to the neutrino mass matrix $[m]_{αβ}$. We show that for hierarchical neutrino masses, the contribution of $[μ]_{e τ}$ to $[m]_{e τ}$ can exceed the experimental value of $[m]_{e τ}$.

hep-ph↗
arXiv2001

Neutrino masses from operator mixing

We show that in theories that reduce, at the Fermi scale, to an extension of the standard model with two doublets, there can be additional dimension five operators giving rise to neutrino masses. In particular there exists a singlet operator which can not generate neutrino masses at tree level but generates them through operator mixing. Under the assumption that only this operator appears at tree level we calculate the neutrino mass matrix. It has the Zee mass matrix structure and leads naturally to bimaximal mixing. However, the maximal mixing prediction for solar neutrinos is very sharp even when higher order corrections are considered. To allow for deviations from maximal mixing a fine tuning is needed in the neutrino mass matrix parameters. However, this fine tuning relates the departure from maximal mixing in solar neutrino oscillations with the neutrinoless double beta decay rate.

hep-ph↗
arXiv2001

Bounds on the Tau Magnetic Moments: Standard Model and Beyond

We obtain new bounds for the magnetic dipole moments of the tau lepton. These limits on the magnetic couplings of the tau to the electroweak gauge bosons (gamma, W, Z) are set in a model independent way using the most general effective Lagrangian with the SU(2)_L x U(1)_Y symmetry. Comparison with data from the most precise experiments at high energies shows that the present limits are more stringent than the previous published ones. For the anomalous magnetic moment the bounds are, for the first time, within one order of magnitude of the standard model prediction.

hep-ph↗
arXiv2001

Chiral fermions and gauge-fixing in five-dimensional theories

We study in detail the issue of gauge-fixing in theories with one universal extra dimension, i.e. theories where both bosons and fermions display Kaluza-Klein (KK) excitations. The extra dimension is compactified using the standard orbifold construction for a massless chiral fermion. We carry out the gauge-fixing procedure at the level of the five-dimensional theory and determine the tree-level propagators and interaction vertices needed for performing perturbative calculations with the effective four-dimensional theory resulting after the compactification. The gauge-independence of the tree-level S-matrix involving massive KK modes is verified using specific examples. In order to obtain massive fermionic zero modes one has to enlarge the theory by introducing a set of mirror fermions, a construction which is carried out in detail. Finally, the gauge-independence of the tree-level S-matrix involving the resulting new mass-eigenstates is proved by resorting to generalized current conservation equations.

hep-ph↗
arXiv2000

Extra dimensions at the one loop level: Z -> b,b(bar) and B-B(bar) mixing

We study, at the one loop level, the dominant new physics contributions from extra dimensions to Z -> b,b(bar), as well as B-B(bar) and K-K(bar) mixing. We use a model with one extra dimension containing fermions which live in four dimensions, and gauge bosons and one scalar doublet propagating in five dimensions. We find that the effect of the infinite tower of Kaluza-Klein modes in Z-> b,b(bar) is finite and gives a negative correction to R_b=Gamma_b/Gamma_h, which is used to set a lower bound of 1 TeV on the compactification scale M_c. On the other hand, we show that the box diagrams contributing to B-B(bar) and K-K(bar) mixing are divergent and, after proper regularization, we find that they increase the value of the function S(x_t) which governs this mixing. The obtained value is perfectly compatible with available data.

hep-ph↗
arXiv2000

Model independent bounds on the tau lepton electromagnetic and weak magnetic moments

Using LEP1, SLD and LEP2 data, for tau lepton production, and data from CDF, D0 and LEP2, for W decays into tau leptons, we set model independent limits on non-standard electromagnetic and weak magnetic moments of the tau lepton. The most general effective Lagrangian giving rise to tau moments is used without further assumptions. Precise bounds ($2σ$) on the non-standard model contributions to tau electromagnetic ($-0.007<a_γ< 0.005$), tau Z-magnetic ($-0.0024 <a_Z< 0.0025$) and tau W-magnetic ($-0.003 < κ^W < 0.004$) dipole moments are set from the analysis.

hep-ph↗
arXiv1999

"Secret" neutrino interactions

We review the information about a potentially strong non-standard four-neutrino interaction that can be obtained from available experimental data. By using LEP results and nucleosynthesis data we find that a contact four-fermion neutrino interaction that involve only left-handed neutrinos or both left-handed and right-handed neutrinos cannot be stronger than the standard weak interactions. A much stronger interaction involving only right-handed neutrinos is still allowed.

hep-ph↗
arXiv1999

$m_b(m_Z)$ from jet production at the $Z$ peak in the Cambridge algorithm

We consider the production of heavy quark jets at the $Z$-pole at the next-to-leading order (NLO) using the {\it Cambridge jet-algorithm}. We study the effects of the quark mass in two- and three-jet observables and the uncertainty due to unknown higher order corrections as well as due to fragmentation. We found that the three-jet observable has remarkably small NLO corrections, which are stable with respect to the change of the renormalization scale, when expressed in terms of the {\it running quark mass} at the $m_Z$-scale. The size of the hadronization uncertainty for this observable remains reasonably small and is very stable with respect to changes in the jet resolution parameter $y_c$.

hep-ph↗
arXiv1999

Quark-mass effects for jet production in e^+ e^- collisions at the next-to-leading order: results and applications

We present a detailed description of our calculation of next-to-leading order QCD corrections to heavy quark production in e^+ e^- collisions including mass effects. In particular, we study the observables $R_3^{b\ql}$ and $D_2^{b\ql}$ in the E, EM, JADE and DURHAM jet-clustering algorithms and show how one can use these observables to obtain $m_b(m_Z)$ from data at the $Z$ peak.

hep-ph↗
arXiv1999

Improved Determination of the b-Quark Mass at the Z Peak

Next-to-leading order calculations of heavy quark three-jet production in e^+ e^- annihilation are reviewed. Their application for the measurement of the b-quark mass at LEP/SLC and to the test of the flavour independence of the strong coupling constant are discussed. Prospects for future improvements are studied.

hep-ph↗
arXiv1998

Do quark masses run?

The importance of measuring the b-quark mass at different scales is emphasized. The recent next-to-leading order calculation of three jet heavy quark production at LEP and its use to measure $m_b(m_Z)$ is discussed.

hep-ph↗
arXiv1997

The running of the b-quark mass from LEP data

Next-to-leading order QCD corrections to three jet heavy quark production in $e^+ e^-$ collisions, including quark mass effects, are presented. The extraction of the b-quark mass form LEP data is considered and the first experimental evidence for the running of a quark mass is discussed.

hep-ph↗