download report - Sapienza

Scientific Report 2007-2009
Particle physics
Tier2 site is a national facility, used by either local or remote users of several physics groups of
the experiments. The ATLAS and CMS facilities are also using their resources to compute the
calibrations of the detector. The calibration data coming from CERN are collected in the Tier2
site and either analyzed in offline mode or automatically processed by the calibration agents, to
provide prompt calibration constants back to CERN. The remote calibration infrastructure has
been designed and realized in INFN Roma for the first time.
1.1.2 The Intensity Frontier
Detailed studies of known physics systems lead on one side to better understand their properties
and interactions and on the other side to investigate deviations from the standard model which
would represent an alternative path to new phenomena. This field is also known as ”flavour”
physics because its interest arises from the existence of three replicas of the leptons (e, µ, and τ
and the corresponding neutrinos) and the quarks (u, c, and t, and d, s, and b). The open questions
in this field are the level of mixing between the different replicas and the amount of violation of
the Charge-Parity symmetry (CP). The latter is particularly relevant because on one side it is
a key ingredient in understanding the current difference in abundance between matter and antimatter,
and on the other side one of the first effects of physics beyond the standard model would
be alterations of the amount of CP violation in processes mediated by new particles. It is to be
noted that the mixing between quarks is regulated by the so called ”CKM” matrix, where ”C” is
the initial of Prof. Cabibbo, eminent member of this Department, as he was the first to introduce
the concept of mixing in the early ’60. The understanding of the generation of CP violation in
the quark-mixing was awarded the 2009 Nobel Prize.
Concerning mixing measurements,
groups of the Department have been
recently involved in the first observation
of the mixing between B s [P9] and
D 0 mesons [P16]. Mixing is a basic
quantum-mechanics process, which was
expected in these systems, but it has
never been observed because of the extremely
low expected rates. It is mediated
by two virtual particles and therefore
measurements of mixing parameters
are critical because they could be
affected if the particles exchanged are
among those that are yet to be seen. Indeed,
after the first low resolution measurement
of the B s mixing phase at
Tevatron, a high precision determination
of this phase is part of the mission
of the LHCb experiment at LHC [P7] as
a test of the Standard Model.
Mixing between lepton families would
η
1
0.5
0
-0.5
-1
ε K
β
V ub
V cb
-1 -0.5 0 0.5 1
γ
∆m d
∆m s
sin(2β+γ)
Figure 3: 68% C.L. contours of all the measurement of the apex
of the Unitarity Triangle and the combined fit (black ellipses).
instead directly signal new physics: the MEG experiment, of which an INFN group is hosted in
the Department, searches for µ → eγ decays at a level of accuracy such that most models of new
physics predict a signal. Searches sensitive to similar effects which can occur only if mediated by
yet unseen particles have been performed in rare decays of the B mesons in BaBar [P15] and CDF
[P11]. The future plans include searching for these effects in rare decays of the Kaon mesons with
the NA62 experiment [P17], K → πν¯ν.
The number of experiments contributing to the understanding of CP violation is very large since
α
∆md
ρ
Sapienza Università di Roma 104 Dipartimento di Fisica