arXiv subjects

A. Santamaria

Publications and source records attributed to A. Santamaria.

13 recordsLinked to original sources

arXiv2013

Neutrinoless double $β$ decay with small neutrino masses

If the only contribution to neutrinoless double beta decay (NLDBD) are neutrino masses its discovery may be very difficult, especially if neutrinos present a normal hierarchy spectrum. However, this is not the only possibility; NLDBD can in general produce electrons of either chirality, in contrast with the neutrino induced NLDBD which predicts two left-handed electrons. Using an effective Lagrangian approach we classify the lepton number violating (LNV) effective operators with two leptons of either chirality but no quarks, ordered according to the magnitude of their contribution to NLDBD. We point out that, for each of the three chirality assignments, $e_{L}e_{L},e_{L}e_{R}$ and $e_{R}e_{R}$, there is only one LNV operator of the corresponding type to lowest order, and these have dimensions 5, 7 and 9, respectively. Neutrino masses are always induced by these extra operators but can be delayed to one (dimension 7) or two loops (dimension 9). Under the assumption that NLDBD is dominated by the operators of dimension 7 or 9 we find that the scale of new physics should be relatively low ($\lesssim 30$ TeV). We also list the SM additions generating these operators upon integration of the heavy modes, and discuss simple realistic examples of renormalizable theories for each case. The phenomenology of a model giving rise to the dimension 9 operator has been analyzed with some detail: if NLDBD is going to be seen in the next round of experiments, the doubly charged scalars of the model could be seen at the LHC and lepton flavour violating (LFV) rates could be at the reach of foreseen experiments. Moreover neutrino masses, which arise at two loops, display a very particular structure and are strongly constrained, in fact, $\sin^{2}θ_{13}\gtrsim0.008$, when $μ\rightarrow eee$ is required to lie below its present experimental limit.

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arXiv2011

Implications of new generations on neutrino masses

We explore the possible implications that new families, that are being searched for at the LHC, would have on neutrino masses. In particular, we have explored the possibility that the smallness of the observed neutrino masses is naturally understood in a modified version of the Standard Model (SM) with complete extra generations of fermions, i.e., that have right-handed neutrinos, in which neutrino masses are generated at two loops. With one extra family it is not possible to fit the observed spectrum of masses and mixings. However, the radiative mass generated provides an important constraint in these kind of models, so the neutrino masses do not exceed their cosmological bound. Within the context of two extra families, we analyse the allowed parameter space and the possible phenomenological signals. Contribution to NUFACT 11, XIIIth International Workshop on Neutrino Factories, Super beams and Beta beams, 1-6 August 2011, CERN and University of Geneva (Submitted to IOP conference series).

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arXiv2011

Can new generations explain neutrino masses?

In this talk we explore the possibility that the smallness of the observed neutrino masses is naturally understood in a modified version of the standard model with N extra generations of fermions and N right-handed neutrinos, in which light neutrino masses are generated at two loops. We find that with N = 1 it is not possible to fit the observed spectrum of masses and mixings while with N = 2 it is. Within this extension, we analyse the parameters which are allowed and the possible phenomenological signals of the model in future experiments. Contribution to the proceedings of Les Rencontres de Moriond EW 2011, Young Scientist Forum.

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arXiv2009

Present and Future Bounds on Non-Standard Neutrino Interactions

We consider Non-Standard neutrino Interactions (NSI), described by four-fermion operators of the form $(\barν_α γν_β) (\bar{f} γf)$, where $f$ is an electron or first generation quark. We assume these operators are generated at dimension $\geq 8$, so the related vertices involving charged leptons, obtained by an SU(2) transformation $ν_δ \to e_δ$, do not appear at tree level. These related vertices necessarily arise at one loop, via $W$ exchange. We catalogue current constraints from $\sin^2 θ_W$ measurements in neutrino scattering, from atmospheric neutrino observations, from LEP, and from bounds on the related charged lepton operators. We estimate future bounds from comparing KamLAND and solar neutrino data, and from measuring $\sin^2 θ_W$ at the near detector of a neutrino factory. Operators constructed with $ν_μ$ and $ν_e$ should not confuse the determination of oscillation parameters at a $ν$factory, because the processes we consider are more sensitive than oscillations at the far detector. For operators involving $ν_τ$, we estimate similar sensitivities at the near and far detector.

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arXiv2003

Power corrections in models with extra dimensions

We critically revisit the issue of power-law running in models with extra dimensions. The general conclusion is that, in the absence of any additional physical principle, the power-corrections tend to depend strongly on the details of the underlying theory.

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arXiv2003

Bounds on models with one latticized extra dimension

We study an extension of the standard model with one latticized extra dimension accessible to all fields. The model is characterized by the size of the extra dimension and the number of sites, and contains a tower of massive particles. At energies lower than the mass of the new particles there are no tree-level effects. Therefore, bounds on the scale of new physics can only be set from one-loop processes. We calculate several observables sensitive to loop-effects, such as the $ρ$ parameter, $b\to s γ$, $Z\to b\bar b$, and the $B^0\rightleftharpoons\bar{B}^0$ mixing, and use them to set limits on the lightest new particles for different number of sites. It turns out that the continuous result is rapidly reached when the extra dimension is discretized in about 10 to 20 sites only. For small number of sites the bounds placed on the usual continuous scenario can be reduced by roughly a factor of 10%--25%, which means that the new particles can be as light as $320 {GeV}$. Finally, we briefly discuss an alternative model in which fermions do not have additional modes.

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arXiv2003

Can power corrections be reliably computed in models with extra dimensions?

We critically revisit the issue of power-law running in models with extra dimensions. The analysis is carried out in the context of a higher-dimensional extension of QED, with the extra dimensions compactified on a torus. It is shown that a naive $β$ function, which simply counts the number of modes, depends crucially on the way the thresholds of the Kaluza-Klein modes are crossed. To solve these ambiguities we turn to the vacuum polarization, which, due to its special unitarity properties, guarantees the physical decoupling of the heavy modes. This latter quantity, calculated in the context of dimensional regularization, is used for connecting the low energy gauge coupling with the coupling of the $D$-dimensional effective field theory. We find that the resulting relation contains only logarithms of the relevant scales, and no power corrections. If, instead, hard cutoffs are used to regularize the theory, one finds power corrections, which could be interpreted as an additional matching between the effective higher-dimensional model and some unknown, more complete theory. The possibility of estimating this matching is examined in the context of a toy model. The general conclusion is that, in the absence of any additional physical principle, the power corrections depend strongly on the details of the underlying theory. Possible consequences of this analysis for gauge coupling unification in theories with extra dimensions are briefly discussed.

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arXiv2002

Universal extra dimensions and Z->b bar-b

We study, at the one loop level, the dominant contributions from a single universal extra dimension to the process (Z\to b\bar{b}). By resorting to the gaugeless limit of the theory we explain why the result is expected to display a strong dependence on the mass of the top-quark, not identified in the early literature. A detailed calculation corroborates this expectation, giving rise to a lower bound for the compactification scale which is comparable to that obtained from the $ρ$ parameter. An estimate of the subleading corrections is furnished, together with a qualitative discussion on the difference between the present results and those derived previously for the non-universal case.

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arXiv2002

B Physics and Extra Dimensions

We compute the dominant new physics contributions to the processes Z -> b b and B - B in the context of two representative models with extra dimensions. The main thrust of the calculations focuses on how to control the effects of the infinite tower of Kaluza-Klein modes inside the relevant one-loop diagrams. By comparing the results with the existing experimental data, most importantly those for Rb, we show that one may derive interesting lower bounds on the size of the compactification scale Mc.

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arXiv1999

An effective field theory approach to the electroweak corrections at LEP energies

In the framework of the effective field theory (EFT) we discuss the electroweak (EW) corrections at LEP energies. We obtain the effective Lagrangian in the large m_t limit, and reproduce analytically the dominant EW corrections to the LEP2 processes e+ e- --> gamma Z and e+ e- --> Z Z. To include effects of finite top-quark and Higgs masses, we use the effective Lagrangian at tree level and fit LEP1/SLD observables with four arbitrary parameters, plus alpha_s(m_Z). The EFT approach works remarkably well. Using the effective couplings determined from the fit, and tree-level EFT formulae, we predict the cross sections for e+ e- --> Z Z, gamma Z at a level better than 1%.

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arXiv1998

μ-e conversion in nuclei versus μ-->e γ: an effective field theory point of view

Using an effective lagrangian description we analyze possible new physics contributions to the most relevant muon number violating processes: $μ\to e γ$ and $μ$--$e$ conversion in nuclei. We identify a general class of models in which those processes are generated at one loop level and in which $μ$--$e$ conversion is enhanced with respect to $μ\to e γ$ by a large $\ln(m^2_μ/Λ^2),$ where $Λ$ is the scale responsible for the new physics. For this wide class of models bounds on $μ$--$e$ conversion constrain the scale of new physics more stringently than $μ\to e γ$ already at present and, with the expected improvements in $μ$--$e$ conversion experiments, will push it upwards by about one order of magnitude more. To illustrate this general result we give an explicit model containing a doubly charged scalar and derive new bounds on its couplings to the leptons.

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arXiv1997

New constraints on R-parity violation from μ-e conversion in nuclei

We derive new constraints on the products of explicitly R-parity violating couplings $λ$ and $λ'$ in MSSM from searches for μ-e conversion in nuclei. We concentrate on the loop induced photonic coherent conversion mode. For the combinations $|λλ|$ which in μ-e conversion can be probed only at loop level our constraints are in many cases more stringent than the previous ones due to the enhancement of the process by large $\ln(m^2_f/m^2_{\tilde f}).$ For the combinations of $|λ'λ'|$ the tree-level μ-e conversion constraints are usually more restrictive than the loop ones except for two cases which involve the third generation. With the expected improvements in the experimental sensitivity, the μ-e conversion will become the most stringent test for all the involved combinations of couplings.

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arXiv1996

Hard $m_t$ Corrections as a Probe of the Symmetry Breaking Sector

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.

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