download report - Sapienza

Scientific Report 2007-2009
Particle physics
P15. Study of B meson rare decays and implications for new Physics
The Standard Model (SM) of particle physics has been
succefully tested in many experiments in the last 40
years. However solid arguments exist that it cannot be
the final theory and great efforts for New Physics (NP)
search are on-going. The study of rare B decays plays
a unique role in such searches. B meson processes mediated
by flavour-changing neutral-currents (FCNC) are
forbidden at the leading order and NP contributions can
be of the same order of magnitude of the SM contribution.
Complementary information can be obtained from
the study of the purely leptonic B decays which are often
unaccessible with the present experiments, unless NP effects
enhance the rate up to the current experimental
sensitivity. For some of these decays, just the observation
by itself would provide an unambiguous evidence of
NP. We were part of the BaBar international collaboration
that built and ran a detector at the Stanford Linear
Accelerator Center where a copious amount of BB meson
pairs was produced at the PEP-II e + e − collider. We
were involved in data analysis with a special focus on
the rare B decays. The study of these processes represent
a big challenge for experiments due to the necessity
of extraction of a small signal from a huge background
(> 1000 times bigger). We developed a novel technique
in which one of the two B (B tag ) is reconstructed in
a frequent mode, while the signal signature is searched
for in the rest of the event (the recoil), composed by
all tracks and neutral particles not associated to the
B tag . The method provides a pure sample of BB events
and a clean environment to look for rare decays. This
technique was applied to the measurement of the FCNC
B → X s γ branching ratio where more than 1000 fully
hadronic B → DX decays for the B tag were used [1].
This decay is an ideal framework for the study of flavour
physics. The shape of the photon energy spectrum shape
is used to infer theoretical parameters fundamental for
the determination of the Cabibbo-Kobasyashi-Maskawa
matrix element V ub in B → X u lν decays. The power
of the recoil approach has been exploited for the search
of the very rare FCNC B → K ∗ νν decays where the
two neutrinos escape detection. Using both semileptonic
B → D (∗) lν and hadronic B → DX decays for the B tag
reconstruction, upper limits at 90% confidence level (CL)
on the branching ratio have been set [2]:
B(B 0 → K ∗0 νν) < 12 × 10 −5 , (1)
B(B + → K ∗+ νν) < 8 × 10 −5 . (2)
Though they are a factor 10 above the SM expected values,
those limits are used to constrain several NP scenarios.
Complementary information can be obtained looking
for purely leptonic B processes. According to the SM,
they occur through annihilation diagrams and hence are
highly suppressed. The only among them that is accessible
at the present experiments is the B + → τ + ν decay.
In the search for B + → τ + ν the recoil technique is
adopted and both leptonic and hadronic τ decay modes
are used. The measured branching ratio is [3,4]:
B(B + → τ + ν) = (0.9 ± 0.6 ± 0.1) × 10 −4 , (3)
B(B + → τ + ν) = (1.8 +0.9
−0.8 ± 0.4) × 10−4 , (4)
for the semileptonic and hadronic B tag reconstruction,
respectively, in agreement with the expected SM value.
B + → l + ν decays have been also studied by looking for
one mono-energetic lepton and requiring the rest of event
to be consistent with the decay of the other B meson.
Upper limits are set at 90% CL :
B(B + → µ + ν) < 1.0 × 10 −6 , (5)
B(B + → e + ν) < 1.9 × 10 −6 . (6)
No evidence of deviations from the SM in the rare
processes has been found up to now. However these
mesuraments are very important in order to discriminate
among different New Physics scenarios. A typical
example of exclusion “plot” is shwon in Fig.1. Future
tan(β)
80
60
40
20
95% CL from K→µν/π→µν
95% CL from B→τν
lavi
net Kaon WG
100 200 300 400 500
charged Higgs mass (GeV/c 2 )
Figure 1: Excluded Region in Charged Higgs Boson models
by the B → τ + ν decay
experiments, with larger B meson data sample will
allow to test the SM at a deeper level.
References
1. B.Aubert et al., Phys. Rev. D 77, 011107 (2008).
2. B.Aubert et al., Phys. Rev. D78, 072007 (2008).
3. B.Aubert et al., Phys. Rev. D 76, 052002 (2007).
4. B.Aubert et al., Phys. Rev. D 77, 051103 (2008).
Authors
E. Baracchini, F. Bellini, G. Cavoto 1 , D. del Re, E. Di
Marco, R. Faccini, F. Ferrarotto 1 , F.Ferroni, M. Gaspero, P.
D. Jackson 1 , L. Li Gioi, M. A. Mazzoni 1 , S. Morganti 1 , G.
Piredda 1 , F. Polci, F. Renga 1 , C. Voena 1
http://babar.roma1.infn.it/roma
Sapienza Università di Roma 122 Dipartimento di Fisica