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Scientific Report 2007-2009
Particle physics
P16. Properties of the Charmed Particles Studied at BaBar
Members of the Physics Department of the Rome University
“La Sapienza” are involved in the study of the
properties of the charmed particles. These analyses have
been carried out inside the B Collaboration, which used
the BaBar detector for measuring events generated by
e + e − interactions at the PEP-II asymmetric collider of
the Stanford Linear Accelerator Center. The results obtained
in the years 2007-2009 by the BaBar Collaboration
in the field of the charmed particles are discussed
below.
The D 0 and ¯D 0 mesons are generated as flavor eigenstates
(containing a charm quark) and decays as mixture
of two opposite CP eigenstates. Because of that,
a pure beam of D 0 ( ¯D 0 ) evolves in time becoming a
mixture of both particles. This well known quantummechanical
phenomena called flavor mixing is typical of
self-conjugate pairs of neutral mesons, and has been previously
observed in the K 0 , B 0 , and Bs 0 neutral-meson
systems. It is expected to show a very tiny effect in
the case if the neutral D meson, making its observation
very difficult. Nevertheless, the BaBar collaboration
produced the first evidence of the D 0 − ¯D 0 mixing by
studying the D 0 and ¯D 0 decays into K + π − and K − π +
[1]. Subsequent BaBar analyses have confirmed the evidence
of the mixing. Overall the effect is extablished
also if none channel has found an evidence of the mixing
higher than 5 standard deviations.
As regard as the direct CP violation for D 0 mesons,
hitherto the BaBar collaboration has not found any evidence
for such violation.
The BaBar Collaboration has studied the properties
of several charmed baryons. The most important results
has been the first observation of the decay Λ c (2880) + →
D 0 p and the discovery of the Λ c (2940) + baryon [2].
The BaBar Collaboration has studied the initial state
radiation (ISR) production of heavy mesons decaying
into pairs of D mesons. The study of the reactions
e + e − → γ ISR D (∗) ¯D(∗) has shown evidence for several
ψ excited states.
The BaBar Collaboration discovered in 2003 a narrow
meson decaying to D s + π 0 at the mass of 2.32 Gev/c 2
[Phys. Rev. Lett. 90, 242001 (2003)]. This discovery
opened a new field in particle elementary physics: the
study of the excited D mesons. The Collaboration has
continued to study these excited mesons
The BaBar Collaboration has studied several D meson
decays. The study of the D s
+ → µ + ν µ decay has allowed
to obtain the most precise evaluation for the D s
+ decay
constant: f Ds = 283 ± 23 MeV [3].
Lastly, the BaBar collaboration has carried out
several Dalitz plot analyses. An interesting result has
been observed by the study of the D 0 → π + π − π 0
decay [4]. Its Dalitz plot, shown in the figure, has the
typical structure of a π + π − π 0 state with isospin zero.
A phenomenological analysis, to which has partecipated
one of the members of the Rome group of BaBar, has
)
4
/c
2
(GeV
s +
3
2
1
0
1 2 3
s -
(GeV
2
/c
Figure 1: The Dalitz-plot of the D 0 → π + π − π 0 decay. s +
and s − are respectively m 2 (π + π 0 ) and m 2 (π − π 0 ). The fine
diagonal line at low π + π − mass corresponds to the events
removed by the cut 489 < M(π + π − ) < 508 MeV/c 2 . The
Dalitz-plot shows three diagonals with low density, indicating
the dominance of the isospin zero.
proved that the I = 0 amount in the final state is higher
than 90%. This result is unexpected because the isospin
is not a good quantum number for the weak interactions.
It is also interesting because the final state has the exotic
quantum numbers I G J P C = 0 − 0 −− . Furthermore, this
result tell us that the decay D 0 → π + π − π 0 is dominated
by the CP = +1 eigenstate.
References
1. B. Aubert et al. Phys. Rev. Lett. 98, 211802 (2007).
2. B. Aubert et al. Phys. Rev. Lett. 98, 012001 (2007).
3. B. Aubert et al. Phys. Rev. Lett. 98, 141801 (2007).
4. B. Aubert et al. Phys. Rev. Lett. 99, 251801 (2007).
Authors
E. Baracchini, F. Anulli 1 , 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, S. Rahatlou, F. Renga,
and C. Voena 1 .
http://babar.roma1.infn.it/
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Sapienza Università di Roma 123 Dipartimento di Fisica