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Using 13.8 fb^{-1} of data collected at or just below the Upsilon(4S) with the CLEO detector, we report the result of a search for the flavor changing neutral current process D^{0} to gammagamma. We observe no significant signal for this decay mode and determine 90% confidence level upper limits on the branching fractions Br(D^{0}to gammagamma)/Br(D^{0}topi^{0}pi^{0}) < 0.033 and Br(D^{0}togammagamma) < 2.9 times 10^{-5}.
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We report the first search for the $K_L to pi^0 gamma$ decay, which is forbidden by Lorentz invariance, using the data from 2016 to 2018 at the J-PARC KOTO experiment. With a single event sensitivity of $(7.1pm 0.3_{rm stat.} pm 1.6_{rm syst.})times 10^{-8}$, no candidate event was observed in the signal region. The upper limit on the branching fraction was set to be $1.7times 10^{-7}$ at the 90% confidence level.
Processes involving flavor changing neutral currents (FCNC) provide excellent signatures with which to search for evidence of new physics. They have very small branching fractions in the Standard Model since they are highly suppressed by Glashow-Iliopoulos-Maiani (GIM) mechanism. They occur only through higher order diagrams, and new particles contributions can provide a significant enhancements, which would be an uniquevocal signs of physics beyond the Standard Model. In this paper we present the most recent measurements on FCNC processes performed by CDF and D0 Collaborations, while last section is devote to the charm physics at CDF.
The KTeV E799 experiment has conducted a search for the rare decay $K_{L}topi^{0}pi^{0}gamma$ via the topology $K_{L}topi^{0}pi^{0}_Dgamma$ (where $pi^0_Dtogamma e^+e^-$). Due to Bose statistics of the $pi^0$ pair and the real nature of the photon, the $K_{L}topi^{0}pi^{0}gamma$ decay is restricted to proceed at lowest order by the CP conserving direct emission (DE) of an E2 electric quadrupole photon. The rate of this decay is interesting theoretically since chiral perturbation theory predicts that this process vanishes at level $O(p^4)$. Therefore, this mode probes chiral perturbation theory at $O(p^6)$. In this paper we report a determination of an upper limit of $2.43times 10^{-7}$ (90% CL) for $K_{L}topi^{0}pi^{0}gamma$. This is approximately a factor of 20 lower than previous results.
Top quark flavor changing neutral current (FCNC) interactions are highly suppressed in the Standard Model. Therefore, any large signal of FCNCs will indicate the existence of new interactions. In this paper, searches for FCNC interactions in top quark production and decay at the Tevatron and LHC are presented. FCNC searches in $trightarrow qZ$ and $trightarrow Hq$ decays, and in top quark production in $pprightarrow t+j$, $pprightarrow t+Z$ are summarized. Effect of top quark FCNCs on single top quark cross-section, and the searches for same-sign top quark pair production through FCNCs are also described. None of the searches yielded positive results and exclusion limits on branching rations, coupling strengths and cross-sections are obtained. Future prospects of FCNC searches are also briefly discussed.
Using an electron-positron collision data sample of 2.93 fb$^{-1}$ collected at a center-of-mass energy of $sqrt{s}=3.773$ GeV with the BESIII detector, we present the first search for the radiative leptonic decay $D^{+} rightarrow gamma e^{+}{ u}_{e}$. The analysis is performed with a double tag method. We do not observe a significant $D^{+} rightarrow gamma e^{+}{ u}_{e}$ signal, and obtain an upper limit on the branching fraction of $D^{+} rightarrow gamma e^{+}{ u}_{e}$ decay with the energy of radiative photon larger than 10 MeV of $3.0times10^{-5}$ at the 90% confidence level.
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