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Scientific Report 2007-2009
Particle physics
P13. Observation of direct CP violation in B meson decays.
Matter and antimatter were produced in equal amount
according to the Big Bang theories on the origin of the
Universe, but our experience tells us that the Universe is
clearly matter-dominated. According to A.D. Sakharov
one of the key element to understand the disappearance
of antimatter is the nonconservation of the charge-parity
(CP ) symmetry in the fundamental forces governing the
interactions of particles.
So far, two types of CP violation have been observed
in the neutral K meson (K 0 ) and B meson (B 0 ) systems:
CP violation involving the flavour mixing between the
meson and its antiparticle ( B 0 and ¯B 0 ) and direct CP
violation in the decays of each meson. Direct CP violation
effects are explained by the quantum interference
of (at least) two competiting decay amplitudes. This interference
has opposite sign for B 0 with respect to the
¯B 0 decays, resulting in a non-null difference of the decay
rate for the B 0 and the ¯B 0 .
Events / 30 MeV
400
200
0
-0.1 0 0.1
∆E (GeV)
Figure 2: ∆E observable (equivalent to the invariant mass
of the decay products) for the decays B 0 → K + π − (blue)
and for ¯B 0 → K − π + (red). The direct CP asymmetry is
given by the clear unbalance in the number of events.
Figure 1: Quartz bar of the detector for internally reflected
Cerenkov light of the BaBar experiment
In the last years the BaBar group of Roma was part
of a large international collaboration that operated a detector
at the Stanford Linear Accelerator Center (California)
where a copious source of B meson was available
at the PEP-II e + e − collider. The group was involved
in data analysis with a special focus to rare decays of
the B meson into light mesons. Combinations of rates
measurements [1] of such decays are very sensitive to the
CP parameters of the Standard Model (SM) of particle
interaction.
One interesting channel is the B decay into the twobody
final state K ± π ∓ [2]. The identification of the
mass of the final state particles was possible through an
innovative apparatus able to detect the Cerenkov light
emitted by the two particles crossing a quartz bar (shown
in Fig.1). By measuring the ultraviolet light emission angles,
kaon and pion were identified with very high precision.
A careful analysis of the collected data led the
Roma group to publish the first evidence of direct CP
violation in the B meson system. This was in fact possible
by counting the B 0 → K + π − decays versus the
¯B 0 → K − π + decays and finding a clear difference between
the two samples (as shown in Fig.2).
The observed CP violation effects are large in the
B 0 meson system and they are consistent with the SM
prediction, which has a unique source of CP violation.
The theoretical mechanism generating such effect is
known as the Cabibbo-Kobayashi-Maskawa model:
the 2008 Nobel Prize in Physics was awarded to M.
Kobayashi and T. Maskawa for this theory. Nevertheless
the SM CP violation is too small to account for the
matter-dominated Universe. Further investigations of
such effects in several decays channels are therefore
necessary to explain such dominance of matter. They
have been started by the Roma BaBar group [3,4] but
they may become conclusive only in future experiments
with larger B meson data sets.
References
1 B. Aubert et al., Phys. Rev. D 76, 091102 (2007)
2 B. Aubert et al., Phys. Rev. Lett. 99, 021603 (2007).
3 B. Aubert et al., Phys. Rev. Lett. 99, 161802 (2007).
4 B. Aubert et al., Phys. Rev. D 78, 012004 (2008).
Authors
F. Bellini, G. Cavoto 1 , D. del Re, E. Di Marco, R. Faccini,
F. Ferrarotto 1 , F.Ferroni, M. Gaspero, L. Li Gioi, M. A.
Mazzoni 1 , S. Morganti 1 , G. Piredda 1 , F. Renga 1 , C. Voena 1
http://babar.roma1.infn.it/roma
Sapienza Università di Roma 120 Dipartimento di Fisica