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Scientific Report 2007-2009<br />
Theoretical physics<br />
that experimental observations in different final states that were attributed to different exotic<br />
states by different research groups turn out indeed, when studied in a consistent frame, to be the<br />
same state. QCD calculations are used for understanding B decay spectra in [T2]. In order to<br />
measure the strength of the weak coupling of a beauty quark to an up quark (the CKM matrix<br />
element) one has to compute for example the lepton energy spectrum in the semileptonic decay<br />
B → X u lν l , where X u is a hadron state coming from the fragmentation of the u quark. The only<br />
analytic tool available at present to compute QCD effects is perturbation theory, but a fixed-order<br />
expansion is made unreliable by many-body effects related to infrared divergences. A modelization<br />
of non-perturbative effects has allowed to use experimental data from B fragmentation to derive<br />
resummed B spectra. A value for the CKM matrix element has been obtained, and it turns out<br />
to be smaller than the one obtained by other groups and in good agreement with estimates from<br />
lattice QCD.<br />
The study of Flavor and of CP violation is at the root of [T3]: most of the processes relevant<br />
to the evaluation of CP violations at low energies have been computed in all known extensions<br />
of the Standard Model. The Rome group has given major contributions in these directions,<br />
discussing possible “New Physics” contributions to flavor and CP violations beyond the leading<br />
order in QCD. Among other results the ∆F = 2 hadronic matrix element in Lattice QCD has<br />
been evaluated, including the computation of the next to leading order anomalous dimensions<br />
for the most general ∆F = 2 operator basis. Here the role of the APE experiment [APE], a<br />
remarkable achievement of our Department, has been paramount. The group has also pioneered<br />
the phenomenology of non-leptonic decays, devising several strategies to estimate the Standard<br />
Model uncertainty in a reliable way. “The origin of Electroweak Symmetry Breaking” has been<br />
investigated by the same researchers of our Department in [T4], also looking for possible “New<br />
Physics at the Electroweak scale”. Despite the abundance of experimental information we do<br />
not know much about the dynamics responsible for the spontaneous breaking of the electroweak<br />
symmetry. The group has worked on the formulation of realistic composite Higgs theories and on<br />
the investigation of their phenomenology. Recent progresses hint to a connection between gravity<br />
in higher dimensional curved spacetimes and strongly coupled gauge theories: this suggests that<br />
the dynamics that generates a light Higgs could be realized by the bulk of an extra dimension.<br />
The group has proposed a realistic five dimensional composite Higgs model, where the potential is<br />
calculable and predicted in terms of a few parameters. Particular care has been devoted to derive<br />
constraints implied by Flavor Changing Neutral Current effects and to analyze the best strategies<br />
to observe the new particles at the LHC. A further important field of study in this domain has<br />
been the analysis of the “properties of hadron collisions at high energy” [T5]. It is of large<br />
interest to study the evolution with energy of the cross-sections in hadron-hadron collisions and<br />
the properties of multiparticle production in these interactions. The researchers of the group have<br />
tried to determine the effects of fluctuations of the partonic configurations in colliding hadrons,<br />
and the relation between these fluctuations and the abundance of inelastic diffractive events: they<br />
have suggested that to describe the fluctuations in the number of elementary interactions at a<br />
given impact parameter in terms of a simple function, for which a parametrization was given.<br />
We can summarize: this is an exciting period for particle physics, since LHC is proudly entering<br />
its full blossom period. It looks clear from what we have described that our Theory Group is<br />
ready to give an important contribution to the understanding of the new physics picture that will<br />
emerge out from a huge amount of data.<br />
A different, but also crucial part of this research investigates “Theory and phenomenology of<br />
quantum-spacetime symmetries” [T6]. We go back here to the crucial role of symmetries, that<br />
has already played an important role in the work we have described up to here. Researchers of<br />
our group have been among the first advocates of centering on symmetry analysis the study of<br />
noncommutative spacetimes: one looks here both for a suitable frame for the problem and for<br />
tools to make the research program phenomenological in nature. For example the cases where<br />
the symmetries of a noncommutative spacetime are described by a Hopf algebra are of particular<br />
<strong>Sapienza</strong> Università di Roma 20 Dipartimento di Fisica