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Guido Altarelli

Publications and source records attributed to Guido Altarelli.

At least 73 records · Page 4Linked to original sources
arXiv2003

An anomalous dimension for small x evolution

We construct an anomalous dimension for small x evolution which goes beyond standard fixed order perturbative evolution by including resummed small x logarithms deduced from the leading order BFKL equation with running coupling. Surprisingly, we find that once running coupling effects are properly taken into account, the leading approximation is very close to standard perturbative evolution in the range of x accessible at HERA, in overall agreement with the data, with no need for phenomenological parameters to summarise subleading effects. We also show that further corrections due to subleading small x logarithms derived from the Fadin-Lipatov kernel can be kept under control, but that they involve substantial resummation ambiguities which limit their practical usefulness.

hep-ph↗
arXiv2003

Phenomenology 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.

hep-ph↗
arXiv2002

Theoretical Models of Neutrino Masses and Mixings

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

hep-ph↗
arXiv2002

A QCD Primer

1. Introduction 2. Massless QCD and Scale Invariance 3. The Renormalisation Group and Asymptotic Freedom 4. More on the Running Coupling 5. Application to Hard Processes 5.1 $R_{e^+e^-}$ and Related Processes 5.2 The Final State in $e^+e^-$ Annihilation 5.3 Deep Inelastic Scattering 5.4 Factorisation and the QCD Improved Parton Model 6. Measurements of $α_s$ 6.1 $α_s$ from $e^+e^-$ Colliders 6.2 $α_s$ from Deep Inelastic Scattering 7. Conclusion 8. Appendix: The Formalism of Gauge Theories

hep-ph↗
arXiv2001

Factorization and Resummation of Small x Scaling Violations with Running Coupling

We discuss the inclusion of running coupling effects in perturbative small x evolution equations. We show that a running coupling BFKL-like x-evolution equation is fully compatible, up to higher twist corrections, with the standard factorized perturbative evolution of parton distributions. We then use this result, combined with the well-known Airy asymptotics, to prove that the oscillations which are present in the running-coupling BFKL solution do not affect the associated splitting functions, which instead remain smooth in the small x limit. This allows us to give a prescription to include running-coupling corrections in the small-x resummation of scaling violations. We show that these corrections are small in the HERA region.

hep-ph↗
arXiv2001

Models of neutrino masses and mixings

We briefly review models of neutrino masses and mixings. In view of the existing experimental ambiguities many possibilities are still open. After an overview of the main alternative options we focus on the most constrained class of models based on three widely split light neutrinos within SUSY Grand Unification.

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arXiv2001

Extrapolating Structure Functions to Very Small x

We review small $x$ contributions to perturbative evolution equations for parton distributions, and their resummation. We emphasize in particular the resummation technique recently developed in order to deal with the apparent instability of naive small $x$ evolution kernels and understand the empirical sucess of fixed--order perturbation theory. We give predictions for the gluon distribution and the structure functions F_2(x,Q^2) and F_L(x,Q^2) in an extended kinematic region, such as would be relevant for THERA or LEP+LHC ep colliders.

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arXiv2001

Small-x Resummation and HERA Structure Function Data

We apply our systematic NLO small x resummation of singlet splitting functions to the scaling violations of structure functions and compare the results with data. We develop various theoretical tools which are needed in order to relate resummed parton distributions to measurable structure functions, and we present results from a variety of fits to HERA data for the structure functions F_2 and F_L using the resummation. The behaviour of the singlet splitting functions at small x and fixed Q^2 is effectively parametrized as x^{-lambda}. We find that, for lambda small or negative, the resummed description of scaling violations may be phenomenologically as good as or even better than the standard next-to-leading order treatment. However, the best fit gluon density and value of alpha_s can be significantly modified by the resummation.

hep-ph↗
arXiv2001

SU(5) Grand Unification in Extra Dimensions and Proton Decay

We analyse proton decay in the context of simple supersymmetric SU(5) grand unified models with an extra compact spatial dimension described by the orbifold S^1/(Z_2 x Z_2'). Gauge and Higgs degrees of freedom live in the bulk, while matter fields can only live at the fixed point branes. We present an extended discussion of matter interactions on the brane. We show that proton decay is naturally suppressed or even forbidden by suitable implementations of the parity symmetries on the brane. The corresponding mechanism does not affect the SU(5) description of fermion masses also including the neutrino sector, where Majorana mass terms remain allowed.

hep-ph↗
arXiv2000

From Minimal to Realistic Supersymmetric SU(5) Grand Unification

We construct and discuss a "realistic" example of SUSY SU(5) GUT model, with an additional U(1) flavour symmetry, that is not plagued by the need of large fine tunings, like those associated with doublet-triplet splitting in the minimal model, and that leads to an acceptable phenomenology. This includes coupling unification with a value of alpha_s(m_Z) in much better agreement with the data than in the minimal version, an acceptable hierarchical pattern for fermion masses and mixing angles, also including neutrino masses and mixings, and a proton decay rate compatible with present limits (but the discovery of proton decay should be within reach of the next generation of experiments). In the neutrino sector the preferred solution is one with nearly maximal mixing both for atmospheric and solar neutrinos.

hep-ph↗
arXiv2000

Resummation of Singlet Parton Evolution at Small x

We propose an improvement of the splitting functions at small x which overcomes the apparent problems encountered by the BFKL approach. We obtain a stable expansion for the x-evolution function chi(M) near M=0 by including in it a sequence of terms derived from the one- and two-loop anomalous dimension gamma. The requirement of momentum conservation is always satisfied. The residual ambiguity on the splitting functions is effectively parameterized in terms of the value of lambda, which fixes the small x asymptotic behaviour x^-lambda of the singlet parton distributions. We derive from this improved evolution function an expansion of the splitting function which leads to good apparent convergence, and to a description of scaling violations valid both at large and small x.

hep-ph↗
arXiv1999

Large Neutrino Mixing from Small Quark and Lepton Mixings

We discuss the special class of models where nearly maximal neutrino mixing is produced through the see-saw mechanism, starting from only nearly diagonal matrices for charged leptons, Dirac neutrinos and Majorana right-handed neutrinos.

hep-ph↗
arXiv1999

A Simple Grand Unification View of Neutrino Mixing and Fermion Mass Matrices

Assuming three light neutrinos and the see-saw mechanism we present a semiquantitative model of fermion masses based on (SUSY) SU(5) and abelian horizontal charges. A good description of the observed pattern of quark and lepton masses is obtained. For neutrinos we naturally obtain widely split masses and large atmospheric neutrino mixing as a consequence of SU(5)-related asymmetric mass matrices for d quarks and charged leptons.

hep-ph↗
arXiv1998

The Standard Electroweak Theory and Beyond

This set of lectures provides an elementary introduction to the standard electroweak theory, followed by a detailed discussion of its experimental tests. We then consider the conceptual limitations of the Standard Model and briefly review the existing phenomenological indications of new physics. We summarize the case for a supersymmetric extension of the Standard Model. Finally we describe the present and planned searches for the Higgs and for new physics.

hep-ph↗
arXiv1998

Models of Neutrino Masses from Oscillations with Maximal Mixing

We discuss models of neutrino masses that lead to a large mixing angle for atmospheric neutrino oscillations. In particular we study a mechanism where a simple texture for the Dirac matrix leads to the observed pattern of mixings for a continuous range of Majorana matrices in a reasonably natural way. Both nearly maximal and small mixing solutions for solar neutrinos are compatible with this model. Possible dynamical realizations are discussed and a detailed example in terms of horizontal U(1) charges is presented.

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