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Guido Altarelli

Publications and source records attributed to Guido Altarelli.

At least 55 records · Page 3Linked to original sources
arXiv2006

An Update on Models of Neutrino Masses and Mixings

At the School I gave three lectures on neutrino masses and mixings. Much of the material covered in my first two lectures is written down in a review on the subject that I published not long ago with F. Feruglio \cite{rev}. Here, I make a relatively short summary (with updates) of the content of my first two lectures, referring to our review for a more detailed presentation, and then I expand on the content of the third lecture which was dedicated to recent work on A4 models of tri-bimaximal neutrino mixing which were not covered in the review.

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arXiv2006

Introduction to the Terascale

We briefly review the status of the electroweak theory, in the Standard Model and beyond, on the brink of the LHC start.

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arXiv2006

Tri-bimaximal Neutrino Mixing from Orbifolding

We show that the A4 discrete symmetry that naturally leads to tri-bimaximal neutrino mixing can be simply obtained as a result of an orbifolding starting from a model in 6 dimensions. This particular orbifolding has four fixed points where 4 dimensional branes are located and the tetrahedral symmetry of A4 connects these branes. In this approach A4 appears after the reduction from six to four dimensions as a remnant of the 6D space-time symmetry. A previously discussed supersymmetric version of A4 is reinterpreted along these lines.

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arXiv2006

Particle Physics: a Progress Report

We present a concise review of where we stand in particle physics today. First we discuss QCD, then the electroweak sector and finally the motivations and the avenues for new physics beyond the Standard Model.

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arXiv2006

How Resummation Depresses the Gluon at Small x

We summarize recent progress in the resummation of perturbative evolution at small x. We show that the problem of incorporating BFKL small x logs in GLAP evolution is now completely solved, and that the main effect of small x resummation is to reduce the growth of the gluon at small x in the HERA and LHC regions.

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arXiv2006

Perturbatively Stable Resummed Small x Evolution Kernels

We present a small x resummation for the GLAP anomalous dimension and its corresponding dual BFKL kernel, which includes all the available perturbative information and nonperturbative constraints. Specifically, it includes all the information coming from next-to-leading order GLAP anomalous dimensions and BFKL kernels, from the constraints of momentum conservation, from renormalization-group improvement of the running coupling and from gluon interchange symmetry. The ensuing evolution kernel has a uniformly stable perturbative expansion. It is very close to the unresummed NLO GLAP kernel in most of the HERA kinematic region, the small x BFKL behaviour being softened by momentum conservation and the running of the coupling. Next-to-leading corrections are small thanks to the constraint of gluon interchange symmetry. This result subsumes all previous resummations in that it combines optimally all the information contained in them.

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arXiv2006

Neutrino Masses with Inverse Hierarchy from Broken L_e-L_μ-L_τ: a Reappraisal

We discuss a class of models of neutrino masses and mixings with inverse hierarchy based on a broken U(1)_F flavour symmetry with charge L_e-L_μ-L_τfor lepton doublets and arbitrary right-handed charges. The symmetry breaking sector receives separate contributions from flavon vev breaking terms and from soft mass breaking in the right handed Majorana sector. The model is able to reproduce in a natural way all observed features of the charged lepton mass spectrum and of neutrino masses and mixings (even with arbitrarily small θ_{13}), with the exception of a moderate fine tuning which is needed to accomodate the observed small value of r = Delta m^2_{sol} / Delta m^2_{atm}.

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arXiv2006

Tri-Bimaximal Neutrino Mixing, A4 and the Modular Symmetry

We formulate and discuss a 4-dimensional SUSY version of an A4 model for tri-bimaximal neutrino mixing which is completely natural. We also study the next-to-the-leading corrections and show that they are small, once the ratios of A4 breaking VEVs to the cutoff are fixed in a specified interval. We also point out an interesting way of presenting the A4 group starting from the modular group. In this approach, which could be interesting in itself as an indication on a possible origin of A4, the lagrangian basis where the symmetry is formulated coincides with the basis where the charged leptons are diagonal. If the same classification structure in A4 is extended from leptons to quarks, the CKM matrix coincides with the unit matrix in leading order and a study of non leading corrections shows that the departures from unity of the CKM matrix are far too small to accomodate the observed mixing angles.

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arXiv2005

Normal and Special Models of Neutrino Masses and Mixings

One can make a distinction between "normal" and "special" models. For normal models $θ_{23}$ is not too close to maximal and $θ_{13}$ is not too small, typically a small power of the self-suggesting order parameter $\sqrt{r}$, with $r=Δm_{sol}^2/Δm_{atm}^2 \sim 1/35$. Special models are those where some symmetry or dynamical feature assures in a natural way the near vanishing of $θ_{13}$ and/or of $θ_{23}- π/4$. Normal models are conceptually more economical and much simpler to construct. Here we focus on special models, in particular a recent one based on A4 discrete symmetry and extra dimensions that leads in a natural way to a Harrison-Perkins-Scott mixing matrix.

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arXiv2005

Tri-Bimaximal Neutrino Mixing from Discrete Symmetry in Extra Dimensions

We discuss a particularly symmetric model of neutrino mixings where, with good accuracy, the atmospheric mixing angle theta_{23} is maximal, theta_{13}=0 and the solar angle satisfies sin^2(theta_{12})=1/3 (Harrison-Perkins-Scott (HPS) matrix). The discrete symmetry A_4 is a suitable symmetry group for the realization of this type of model. We construct a model where the HPS matrix is exactly obtained in a first approximation without imposing ad hoc relations among parameters. The crucial issue of the required VEV alignment in the scalar sector is discussed and we present a natural solution of this problem based on a formulation with extra dimensions. We study the corrections from higher dimensionality operators allowed by the symmetries of the model and discuss the conditions on the cut-off scales and the VEVs in order for these corrections to be completely under control. Finally, the observed hierarchy of charged lepton masses is obtained by assuming a larger flavour symmetry. We also show that, under general conditions, a maximal theta_{23} can never arise from an exact flavour symmetry.

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arXiv2004

Neutrino 2004: Concluding Talk

We review the highlights of Neutrino 2004 and recall the main lessons from neutrinos in recent years that had a great impact on particle physics and cosmology.

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arXiv2004

Models of Neutrino Masses and Mixings

We review theoretical ideas, problems and implications of neutrino masses and mixing angles. We give a general discussion of schemes with three light neutrinos. Several specific examples are analyzed in some detail, particularly those that can be embedded into grand unified theories.

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arXiv2004

Precision Electroweak Tests of the Standard Model

The study of electron-positron collisions at LEP, together with additional measurements from other experiments, in particular those at SLC and at the Tevatron, has allowed for tests of the electroweak Standard Model with unprecedented accuracy. We review the results of the electroweak precision tests and their implications on the determination of the Standard Model parameters, in particular of the Higgs boson mass, and comment on the constraints for possible new physics effects.

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arXiv2004

Can Neutrino Mixings Arise from the Charged Lepton Sector?

The neutrino mixing matrix U is in general of the form U=U_e^+ U_nu, where U_e arises from the diagonalization of charged leptons and U_nu is from the neutrino sector. We discuss the possibility that U_nu is nearly diagonal (in the lagrangian basis) and the observed mixing arises with good accuracy from U_e. We find that the fact that, in addition to the nearly maximal atmospheric mixing angle theta_{23}, the solar angle theta_{12} is definitely also large while at the same time the third mixing angle theta_{13} is small, makes the construction of a natural model of this sort considerably more complicated. We present an example of a natural model of this class. We also find that the case that U_nu is exactly of the bimixing type is severely constrained by the bound on theta_{13} but not excluded. We show that planned experimental searches for theta_{13} could have a strong impact on bimixing models.

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arXiv2003

Models of Neutrino Masses: Anarchy versus Hierarchy

We present a quantitative study of the ability of models with different levels of hierarchy to reproduce the solar neutrino solutions, in particular the LA solution. As a flexible testing ground we consider models based on SU(5)xU(1)_F. In this context, we have made statistical simulations of models with different patterns from anarchy to various types of hierachy: normal hierarchical models with and without automatic suppression of the 23 (sub)determinant and inverse hierarchy models. We find that, not only for the LOW or VO solutions, but even in the LA case, the hierarchical models have a significantly better success rate than those based on anarchy. The normal hierachy and the inverse hierarchy models have comparable performances in models with see-saw dominance, while the inverse hierarchy models are particularly good in the no see-saw versions. As a possible distinction between these categories of models, the inverse hierarchy models favour a maximal solar mixing angle and their rate of success drops dramatically as the mixing angle decreases, while normal hierarchy models are far more stable in this respect.

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arXiv2003

An Improved Splitting Function for Small x Evolution

We summarize our recent result for a splitting function for small x evolution which includes resummed small x logarithms deduced from the leading order BFKL equation with the inclusion of running coupling effects. We compare this improved splitting function with alternative approaches.

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