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Scientific Report 2007-2009
Particle physics
P9. Study of B-mixing and CP Violation with the CDF experiment
The accurate determination of charge-conjugationparity
(CP) violation in meson systems has been one of
the goals of particle physics since the effect was first discovered
in neutral kaon decays in 1964. Standard model
CP-violating effects are described through the Cabibbo-
Kobayashi-Maskawa mechanism, which has proved to be
extremely successful in describing the phenomenology of
CP violation in B 0 and B ± decays in the past decade.
However, comparable experimental knowledge of Bs 0 decays
has been lacking. In the Bs 0 system, the mass eigenstates
are admixtures of the flavor eigenstates. This
causes oscillations between the flavor states with a frequency
proportional to the mass difference of the mass
eigenstates (∆m s ). In the standard model this effect is
explained in terms of second-order weak processes that
provide a transition amplitude between the Bs 0 and ¯B s
0
states. The magnitude of this mixing amplitude is proportional
to the oscillation frequency, while its phase is
responsible for CP violation in Bs 0 → J/ψϕ decays. The
presence of physics beyond the standard model could
significantly modify the magnitude or the phase of the
mixing amplitude. We have preformed the first time
dependent analysis of the Bs 0 → J/ψϕ decay, separating
the time evolution of mesons produced as Bs 0 or ¯B s 0 using
flavor tagging tecniques. By relating this time development
with the CP eigenvalue of the final states, which is
accessible through the angular distributions of the J/ψ
and ϕ mesons, we obtain direct sensitivity to the CP violating
phase. With 1.35 fb −1 of data collected by the
CDF experiment, we obtained the confidence bounds on
the CP violation parameter 2β s and the width difference
∆Γ, shown in Figure 1. Assuming the standard model,
the probability of a deviation as large as the level of the
observed data is 15%, which corresponds to 1.5 Gaussian
standard deviations.
states. Such behavior is referred to as “mixing”, as
first explained in 1955 for the K 0 meson in terms of
quantum-mechanical mixed states. Mixing was next
observed for Bd 0 mesons in 1987. The years 2006
and 2007 have seen landmark new results on mixing:
observation of Bs 0 mixing, and evidence for the D 0
mixing. The latter comes from two different types of
measurements: direct evidence for a longer and shorter
lived D 0 meson, from the decay time distributions for
D 0 decays to the CP-eigenstates K + K − and π + π −
compared to that for the CP-mixed state K − π + ,
evidence for D 0 mixing in the difference in decay time
distribution for D 0 → K + π − compared to that for the
Cabibbo-favored (CF) decay D 0 → K − π + . Such a
difference depends on the combined effects of differences
in the masses and lifetimes of the D 0 meson weak
eigenstates. We have presented a new measurement
based on this latter tecnique, performed for the first
time at a hadron collider machine. We use a signal of
12.7 × 10 3 D 0 → K + π − decays recorded with the CDF
detector at the Fermilab Tevatron, which corresponds
to an integrated luminosity of 1.5 fb −1 for p¯p collisions
at √ s = 1.96 TeV. We search for D 0 − ¯D 0 mixing
and measure the mixing parameters, finding that the
data are inconsistent with the no-mixing hypothesis
with a probability equivalent to 3.8 Gaussian standard
deviations (see Figure 2), confirming the evidence
obtained at the B factory experiments.
R
0.01
0.008
0.006
0.004
)
-1
∆Γ (ps
0.6
0.4
0.2
0.0
SM prediction
68% C.L.
95% C.L.
0.002
0 2 4 6 8 10
t/τ
Figure 2: Ratio of prompt D ∗ “wrong-sign” to “right sign”
decays as a function of normalized proper decay time. The
dashed curve is from a least-squares parabolic fit. The dotted
line is the fit assuming no mixing.
-0.2
-0.4
-0.6
0 2 4
2β (rad)
s
Figure 1: Feldman-Cousins confidence region in the 2β s −∆Γ
plane, where the standard model favored point is shown with
error bars.
References
1. T. Aaltonen et al., Phys. Rev. Lett. 100, 161802 (2008).
2. T. Aaltonen et al., Phys. Rev. Lett. 100, 121802 (2008).
3. T. Aaltonen et al., Phys. Rev. Lett. 100, 121803 (2008).
Authors
S. De Cecco 1 , C. Dionisi, S. Giagu, M. Iori, C. Luci, P.
Mastrandrea 1 , M. Rescigno 1 , L. Zanello
http://www.roma1.infn.it/exp/cdf/
Since the discovery of the charm quark in 1974,
physicists have been searching for the oscillation of
neutral charm mesons between particle and anti-particle
Sapienza Università di Roma 116 Dipartimento di Fisica