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G. Altarelli

Publications and source records attributed to G. Altarelli.

At least 19 recordsLinked to original sources
arXiv2013

Concluding talk: fundamental lessons and challenges from neutrinos

We present a concise review of the experimental developments on neutrino mixing and their theoretical implications as presented and discussed at this Conference. The recent data disfavour many models but the surviving ones still span a wide range going from Anarchy (no structure, no symmetry in the neutrino sector) to a maximum of symmetry, as for the models based on discrete non-abelian flavour groups which can be improved following the indications from the recent data.

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arXiv2012

An Overview of Neutrino Mixing

We present a concise review of the recent important experimental developments on neutrino mixing (hints for sterile neutrinos, large $θ_{13}$, possible non maximal $θ_{23}$, approaching sensitivity on $δ_{CP}$) and their implications on models of neutrino mixing. The new data disfavour many models but the surviving ones still span a wide range going from Anarchy (no structure, no symmetry in the lepton sector) to a maximum of symmetry, as for the models based on discrete non-abelian flavour groups.

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arXiv2011

Concluding Talk: A Theorist Overview on Particle Physics

This is a Concluding Talk, not a Summary of the FPCP 2011 Conference. I will first make some comments on the status and the prospects of particle physics and then review some of the highlights that particularly impressed me at this Conference (a subjective choice).

hep-ph↗
arXiv2009

Revisiting Bimaximal Neutrino Mixing in a Model with S4 Discrete Symmetry

In view of the fact that the data on neutrino mixing are still compatible with a situation where Bimaximal mixing is valid in first approximation and it is then corrected by terms of order of the Cabibbo angle, arising from the diagonalization of the charged lepton masses, we construct a model based on the discrete group S4 where those properties are naturally realized. The model is supersymmetric in 4-dimensions and the complete flavour group is S4 x Z4 x U(1)_FN, which also allows to reproduce the hierarchy of the charged lepton spectrum. The only fine tuning needed in the model is to reproduce the small observed value of r, the ratio between the neutrino mass squared differences. Once the relevant parameters are set to accommodate r then the spectrum of light neutrinos shows a moderate normal hierarchy and is compatible, within large ambiguities, with the constraints from leptogenesis as an explanation of the baryon asymmetry in the Universe.

hep-ph↗
arXiv2009

Parton Distributions

We provide an assessment of the state of the art in various issues related to experimental measurements, phenomenological methods and theoretical results relevant for the determination of parton distribution functions (PDFs) and their uncertainties, with the specific aim of providing benchmarks of different existing approaches and results in view of their application to physics at the LHC. We discuss higher order corrections, we review and compare different approaches to small x resummation, and we assess the possible relevance of parton saturation in the determination of PDFS at HERA and its possible study in LHC processes. We provide various benchmarks of PDF fits, with the specific aim of studying issues of error propagation, non-gaussian uncertainties, choice of functional forms of PDFs, and combination of data from different experiments and different processes. We study the impact of combined HERA (ZEUS-H1) structure function data, their impact on PDF uncertainties, and their implications for the computation of standard candle processes, and we review the recent F_L determination at HERA. Finally, we compare and assess methods for luminosity measurements at the LHC and the impact of PDFs on them.

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arXiv2008

Status of the Standard Model at the LHC Start

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

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arXiv2007

HERA and the LHC - A workshop on the implications of HERA for LHC physics: Proceedings - Part B

The HERA electron--proton collider has collected 100 pb$^{-1}$ of data since its start-up in 1992, and recently moved into a high-luminosity operation mode, with upgraded detectors, aiming to increase the total integrated luminosity per experiment to more than 500 pb$^{-1}$. HERA has been a machine of excellence for the study of QCD and the structure of the proton. The Large Hadron Collider (LHC), which will collide protons with a centre-of-mass energy of 14 TeV, will be completed at CERN in 2007. The main mission of the LHC is to discover and study the mechanisms of electroweak symmetry breaking, possibly via the discovery of the Higgs particle, and search for new physics in the TeV energy scale, such as supersymmetry or extra dimensions. Besides these goals, the LHC will also make a substantial number of precision measurements and will offer a new regime to study the strong force via perturbative QCD processes and diffraction. For the full LHC physics programme a good understanding of QCD phenomena and the structure function of the proton is essential. Therefore, in March 2004, a one-year-long workshop started to study the implications of HERA on LHC physics. This included proposing new measurements to be made at HERA, extracting the maximum information from the available data, and developing/improving the theoretical and experimental tools. This report summarizes the results achieved during this workshop.

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arXiv2006

HERA and the LHC - A workshop on the implications of HERA for LHC physics: Proceedings - Part A

The HERA electron--proton collider has collected 100 pb$^{-1}$ of data since its start-up in 1992, and recently moved into a high-luminosity operation mode, with upgraded detectors, aiming to increase the total integrated luminosity per experiment to more than 500 pb$^{-1}$. HERA has been a machine of excellence for the study of QCD and the structure of the proton. The Large Hadron Collider (LHC), which will collide protons with a centre-of-mass energy of 14 TeV, will be completed at CERN in 2007. The main mission of the LHC is to discover and study the mechanisms of electroweak symmetry breaking, possibly via the discovery of the Higgs particle, and search for new physics in the TeV energy scale, such as supersymmetry or extra dimensions. Besides these goals, the LHC will also make a substantial number of precision measurements and will offer a new regime to study the strong force via perturbative QCD processes and diffraction. For the full LHC physics programme a good understanding of QCD phenomena and the structure function of the proton is essential. Therefore, in March 2004, a one-year-long workshop started to study the implications of HERA on LHC physics. This included proposing new measurements to be made at HERA, extracting the maximum information from the available data, and developing/improving the theoretical and experimental tools. This report summarizes the results achieved during this workshop.

hep-ph↗
arXiv2005

Parton Distributions: Summary Report

We provide an assessment of the impact of parton distributions on the determination of LHC processes, and of the accuracy with which parton distributions (PDFs) can be extracted from data, in particular from current and forthcoming HERA experiments. We give an overview of reference LHC processes and their associated PDF uncertainties, and study in detail W and Z production at the LHC. We discuss the precision which may be obtained from the analysis of existing HERA data, tests of consistency of HERA data from different experiments, and the combination of these data. We determine further improvements on PDFs which may be obtained from future HERA data (including measurements of $F_L$), and from combining present and future HERA data with present and future hadron collider data. We review the current status of knowledge of higher (NNLO) QCD corrections to perturbative evolution and deep-inelastic scattering, and provide reference results for their impact on parton evolution, and we briefly examine non-perturbative models for parton distributions. We discuss the state-of-the art in global parton fits, we assess the impact on them of various kinds of data and of theoretical corrections, by providing benchmarks of Alekhin and MRST parton distributions and a CTEQ analysis of parton fit stability, and we briefly presents proposals for alternative approaches to parton fitting. We summarize the status of large and small x resummation, by providing estimates of the impact of large x resummation on parton fits, and a comparison of different approaches to small x resummation, for which we also discuss numerical techniques.

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arXiv2001

Indication for Light Sneutrinos and Gauginos from Precision Electroweak Data

The present Standard Model fit of precision data has a low confidence level, and is characterized by a few inconsistencies. We look for supersymmetric effects that could improve the agreement among the electroweak precision measurements and with the direct lower bound on the Higgs mass. We find that this is the case particularly if the 3.6 sigma discrepancy between sin^2 theta_eff from leptonic and hadronic asymmetries is finally settled more on the side of the leptonic ones. After the inclusion of all experimental constraints, our analysis selects light sneutrinos, with masses in the range 55-80 GeV, and charged sleptons with masses just above their experimental limit, possibly with additional effects from light gauginos. The phenomenological implications of this scenario are discussed.

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arXiv2001

The t->WZb decay in the Standard Model: A Critical Reanalysis

We compute the t->WZb decay rate, in the Standard Model, at the leading order in perturbation theory, with special attention to the effects of the finite widths of the W and Z bosons. These effects are extremely important, since the t->WZb decay occurs near its kinematical threshold. They increase the value of the decay rate by orders of magnitude near threshold or allow it below the nominal threshold. We discuss a procedure to take into account the finite-width effects and compare the results with previous studies of this decay. Within the Standard Model, for a top quark mass in the range between 170 and 180 GeV, we find BR(t->WZb) ~ 2 x 10^{-6}, which makes the observation at the LHC very difficult if at all possible.

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arXiv2000

A Summary from the Theorist's Point of View

1. Introduction - 2. Astrophysics and Cosmology - 3. Neutrino Oscillations - 4. Higgs and New Physics Searches - 5. Flavour Physics and CP Violation - 6. QCD - 7. Heavy Ion Collisions - 8. Outlook

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arXiv2000

The Standard Electroweak Theory and Beyond

1. Introduction, 2. Gauge Theories, 3. The Standard Model of Electroweak Interactions, 4. The Higgs Mechanism, 5. The CKM Matrix, 6. Renormalisation and Higher Order Corrections, 7. Why we do Believe in the Standard Model: Precision Tests, 7.1.Precision Electroweak Data and the Standard Model, 7.2.A More General Analysis of Electroweak Data, 8. Why we do not Believe in the Standard Model, 8.1.Conceptual Problems, 8.2.Hints from Experiment, -8.2.1 Unification of Couplings, -8.2.2 Dark Matter, -8.2.3 Neutrino Masses, -8.2.4 Baryogenesis, 9. Status of the Search for the Higgs and for New Physics, 10.Conclusion

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arXiv1999

The crucial problem: the electroweak symmetry breaking

1. Why we do Believe in the Standard Model 2. Why we do not Believe in the Standard Model 2.1 Conceptual Problems 2.2 Hints from Experiment 2.2.1 Unification of Couplings 2.2.2 Dark Matter 2.2.3 Neutrino Masses 2.2.4 Baryogenesis 3. Status of the Search for the Higgs and for New Physics 4. Conclusion

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arXiv1998

Theoretical Analysis of Polarized Structure Functions

We review the analysis of polarized structure function data using perturbative QCD at next-to-leading order. We use the most recent experimental data to obtain updated results for polarized parton distributions, first moments and the strong coupling. We also discuss several theoretical issues involved in this analysis and in the interpretation of its results. Finally, we compare our results with other similar analyses in the recent literature.

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