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