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Scientific Report 2007-2009<br />
Particle physics<br />
P12. Study of CP violation with the measurement of time-dependent<br />
CP asymmetries in B meson decays<br />
As far as one can see, our Universe is made of matter<br />
and no primordial anti-matter is evident. Whether<br />
this imbalance is a chance occurrence during the birth of<br />
the Universe or due to some fundamental difference between<br />
the behavior of matter and anti-matter under the<br />
charge-parity (CP) symmetry remains to be understood,<br />
and represents one of the biggest puzzles in Cosmology<br />
and Particle Physics. An elegant explanation of CP violation,<br />
within the framework of the Standard Model<br />
of Particle Physics, was proposed in 1973 by Kobayashi<br />
and Maskawa, and predicted large CP violation in B 0<br />
mesons.<br />
We have participated since 1993 in the design, detector<br />
commissioning and operation, and analysis of the<br />
data collected with the BABAR detector at the electronpositron<br />
collider PEP-II, located at the Stanford Linear<br />
Accelerator Center in California, USA, and operating at<br />
a center-of-mass energy of 10.580 GeV. In many of these<br />
e − e<br />
+<br />
B tag<br />
t tag<br />
∆ t =<br />
t rec<br />
−<br />
+<br />
l<br />
t tag<br />
B rec<br />
+<br />
K<br />
Figure 1: Production of a B 0 –B 0 pair where one B decays to<br />
a CP eigenstate, B rec , and the other B to a flavor eigenstate,<br />
B tag .<br />
collisions, a pair of B 0 (particle) and B 0 (anti-particle)<br />
mesons is produced in a quantum-entagled state: conservation<br />
of the quantum number known as beauty implies<br />
there is one particle and one anti-particle at any given<br />
time after their production, until one of the two mesons<br />
(B tag ) decays in a final state which allow us to determine<br />
whether it was a particle (B 0 tag) or an anti-particle<br />
(B 0 tag). The other meson is then free to propagate and<br />
decay in a CP eigenstate (B rec ) accessible to both B 0<br />
and B 0 mesons. Thanks to the excellent performance<br />
of the silicon detector, we are able to identify the decay<br />
vertices of the two mesons and measure the distance<br />
between them that, in average, is ∼ 250µm, with a precision<br />
of ∼ 150µm. The knowledge of the relativistic boost<br />
of these particles allows us to determine the time interval<br />
∆t between the two decays, and finally count the number<br />
of observed B 0 → B rec and B 0 → B rec decays as a<br />
function of the time interval ∆t and the time-dependent<br />
asymmetry between them as illustrated in Fig. 2 for the<br />
final states B rec = J/ψ KS, 0 ψ(2S)KS, 0 η c KS, 0 χ c1 KS, 0 and<br />
J/ψ K 0 L<br />
[1,2]. The striking asymmetry between the particles<br />
and anti-particles is due to the excellent signal purity<br />
t rec<br />
K S<br />
0<br />
J/ψ<br />
z<br />
in these final states.<br />
Events / ( 0.4 ps )<br />
Raw Asymmetry<br />
Events / ( 0.4 ps )<br />
Raw Asymmetry<br />
0<br />
400 B tags<br />
200<br />
0.2<br />
0<br />
-0.2<br />
-0.4<br />
300<br />
0<br />
B<br />
tags<br />
η f =-1<br />
0.4 (b)<br />
200<br />
100<br />
0.2<br />
0<br />
-0.2<br />
-0.4<br />
0<br />
B<br />
0<br />
B<br />
tags<br />
tags<br />
η f =+1<br />
0.4 (d)<br />
-5 0 5<br />
(a)<br />
(c)<br />
∆t (ps)<br />
Figure 2: a) Number of J/ψ KS, 0 ψ(2S)KS, 0 η c KS, 0 χ c1 K 0 S<br />
candidates in the signal region with a B 0 tag (N B 0) and<br />
with a B 0 tag (N B 0), and b) the CP asymmetry, (N B 0 −<br />
N B 0)/(N B 0 + N B 0), as functions of ∆t; c) and d) are the<br />
corresponding distributions for the J/ψ K 0 L candidates. The<br />
solid (dashed) curves in (a) and (c) represent the fit projections<br />
in ∆t for B 0 (N B 0) tags. The shaded regions represent<br />
the estimated background contributions to (a) and (c).<br />
We have also studied these asymmetries in the more<br />
challenging final states η ′ KS, 0 ΦKS, 0 and J/ψ π 0 [3,4].<br />
The expected probability of neutral B mesons decaying<br />
to these final states is rather small in the Standard<br />
Model. Hence the interest in these decays is twofold.<br />
Firstly, deviations of the measured decay probability<br />
could be a hint of New Physics beyond the Standard<br />
Model. Secondly, the measured time-dependent<br />
asymmetries should agree, within the experimental<br />
uncertainties, with the the (cc)K 0 S<br />
channels, or else<br />
it could be another hint of New Physics phenomena.<br />
The measured asymmetry between the behavior of B 0<br />
and B 0 mesons contributed to the establishment of the<br />
theoretical model proposed by Kobayashi and Maskawa<br />
who were assigned the 2008 Nobel Prize in Physics.<br />
References<br />
1. B. Aubert et al., Phys. Rev., D79, 072009 (2009).<br />
2. B. Aubert et al., Phys. Rev. Lett. 99, 171803 (2007).<br />
3. B. Aubert et al., Phys. Rev. Lett. 101, 021801 (2008).<br />
4. B. Aubert et al., Phys. Rev. Lett. 99, 081801 (2007).<br />
Authors<br />
F. Anulli 1 , F. Bellini, G. Cavoto 1 , D. del Re, E. di Marco,<br />
R. Faccini, F. Ferrarotto 1 , F. Ferroni, M. Gaspero, M.<br />
A. Mazzoni 1 , S. Morganti 1 , G. Piredda 1 , S. Rahatlou, F.<br />
Renga, C. Voena 1 .<br />
http://babar.roma1.infn.it/roma<br />
<strong>Sapienza</strong> Università di Roma 119 Dipartimento di Fisica