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