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