Mixing and CP Violation in Charm Meson Decays Page: 3 of 7
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34th International Conference on High Energy Physics, Philadelphia, 2008
The strong phase 3Ki was poorly known, so this measurement was only able to determine values for y' (9.7
4.4 3.1) x 10-3, X2 (-0.22 0.30 0.21) x 10-3 and R = (0.303 0.019)%, where
X' Xc OK + y sin (, ; y'= y cos K, - x sin (K
and RD is the ratio of DCS ro CF decay rates. The BABAR result was found to be inconsistent with a no mixing
hypothesis, with a significance estimated at 3.9o. These results assumed no CPV. Separate fits to D0 and to D
showed no significant difference. It was concluded that no CPV was observable at the current precision.
Using a similar technique, but with data from 1.96 TeV pp collisions, the CDF experiment has confirmed this result
with a 3.8o significance . The plot of WS/RS ratio vs. t is shown in Fig. 1(b). Assuming no CPV, they obtained
values x'2 (-0.12 0.35) x 10-3, y' (8.5 7.6) x 10-3 and R = (0.304 0.055)%, in excellent agreement with
those from BABAR.
2.2. Lifetimes for Decays to CP-even Final States
Deviations from exponential decay, introduced by mixing, alters the effective lifetimes T for D0 decays in ways
that depend upon the CP symmetry of the final state [20, 21]. Decays to K--r+ (mixed CP) and to K-K+ or 7--r+
(CP-even) have been used in searches for mixing by measuring the quantities
w(D0-> K-r+) _ ( + (4)
ycP = -1 ;A_ = 27K--r+
r(D-h-h+) 1 T 2T- + T +Tra
Both are zero in the absence of mixing or CPV. Clear evidence for mixing (yP $ 0) has now been seen in this way
by both the Belle  and BABAR  collaborations with significances of 3.2o and 3.0o, respectively These results,
with those of earlier, unsuccessful searches for mixing, are summarized in Fig. 1.
No evidence for CPV (AT $ 0) is yet observed, however. Belle measured AT (0.010 0.300 0.150)% and BABAR
obtained 2 AT (0.260 0.360 0.080)% giving an average AT (0.123 0.248)%.
2.3. Mixing in D0 -> K+7ru-r0 Decays
The BABAR experiment has just reported further evidence for mixing in WS D0 decays to K+--. . For
this three-body channel, each decay is represented as a point in a Dalitz plot, defined by the squared invariant
masses (so, s+) of the K+u-- and K+ro systems, respectively. The final state f and the value for the strong phase
6f =K, + Arg (A/A f) in Eq. (3) is then unique at each such point. The CF and DCS decay amplitudes Af and
Af, respectively, can be determined from fits to their Dalitz plot densities, but the constant phase 3Kn. cannot.
The BABAR collaboration identified a sample of ~ 3, 000 WS and ~ 660, 000 RS D0 -> K7r0 events in a tight
signal region with flavours identified by their origin from D*+ decays. The RS sample was very clean, though the
WS sample had a 50% background, mostly from "mis-tagged" RS decays.
An isobar model for the amplitude A1 was determined from a time-integrated fit to the distribution of events on
the RS (mostly CF) Dalitz plot. A second, time-dependent fit to the WS Dalitz plot then determined parameters
for the DCS amplitude Af and x", y" (x and y rotated by the unknown phase 3Kn) summarized in Table I.
Table I: Mixing parameters x" = X cos 6K-n + y sin 6K,, and y" y cos - x sin 6K=n from the BABAR analysis of
Do - K+7-rh decays. These are related to x and y by an undetermined strong phase rotation 6K-r.
2BABAR measured the asymmetry in AY = (y+ P - CP Py-f 2TOA/(wm + ).
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Meadows, B & U., /Cincinnati. Mixing and CP Violation in Charm Meson Decays, article, August 26, 2010; [California]. (digital.library.unt.edu/ark:/67531/metadc1015050/m1/3/: accessed November 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.