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