Nonleptonic weak decays of charmed mesons

New physics searches at the BESIII experiment

The standard model (SM) of particle physics, comprised of the unified electroweak and quantum chromodynamic theories, accurately explains almost all experimental results related to the micro-world, and has made a number of predictions for previously unseen particles, most notably the Higgs scalar boson, that were subsequently discovered. As a result, the SM is currently universally accepted as the theory of the fundamental particles and their interactions. However, in spite of its numerous successes, the SM has a number of apparent shortcomings, including: many free parameters that must be supplied by experimental measurements; no mechanism to produce the dominance of matter over antimatter in the universe; and no explanations for gravity, the dark matter in the universe, neutrino masses, the number of particle generations, etc. Because of these shortcomings, there is considerable incentive to search for evidence for new, non-SM physics phenomena that might provide important clues about what a new, beyond the SM theory (BSM) might look like. Although the center-of-mass energies that BESIII can access are far below the energy frontier, searches for new, BSM physics are an important component of its research program. This report reviews some of the highlights from BESIII's searches for signs of new, BSM physics by: measuring rates for processes that the SM predicts to be forbidden or very rare; searching for non-SM particles such as dark photons; performing precision tests of SM predictions; and looking for violations of the discrete symmetries C and CP in processes for which the SM expectations are immeasurably small.

A Review of CP Violation Measurements in Charm at LHCb

The LHCb experiment has been able to collect the largest sample ever produced of charm-hadron decays, performing a number of measurements of observables related to CP violation in the charm sector. In this document, the most recent results from LHCb on the search of direct CP violation in D0→Ks0Ks0, D(s)+→h+π0 and D(s)+→h+η decays are summarised, in addition to the most precise measurement of time-dependent CP asymmetry in D0→h+h− decays and the first observation of mass difference between neutral charm-meson eigenstates.

Observation of CP Violation in Charm Decays

A search for charge-parity (CP) violation in D^{0}→K^{-}K^{+} and D^{0}→π^{-}π^{+} decays is reported, using pp collision data corresponding to an integrated luminosity of 5.9  fb^{-1} collected at a center-of-mass energy of 13 TeV with the LHCb detector. The flavor of the charm meson is inferred from the charge of the pion in D^{*}(2010)^{+}→D^{0}π^{+} decays or from the charge of the muon in B[over ¯]→D^{0}μ^{-}ν[over ¯]_{μ}X decays. The difference between the CP asymmetries in D^{0}→K^{-}K^{+} and D^{0}→π^{-}π^{+} decays is measured to be ΔA_{CP}=[-18.2±3.2(stat)±0.9(syst)]×10^{-4} for π-tagged and ΔA_{CP}=[-9±8(stat)±5(syst)]×10^{-4} for μ-tagged D^{0} mesons. Combining these with previous LHCb results leads to ΔA_{CP}=(-15.4±2.9)×10^{-4}, where the uncertainty includes both statistical and systematic contributions. The measured value differs from zero by more than 5 standard deviations. This is the first observation of CP violation in the decay of charm hadrons.

Search for CP Violation in D_{s}^{+}→K_{S}^{0}π^{+}, D^{+}→K_{S}^{0}K^{+}, and D^{+}→ϕπ^{+} Decays

A search for charge-parity (CP) violation in Cabibbo-suppressed D_{s}^{+}→K_{S}^{0}π^{+}, D^{+}→K_{S}^{0}K^{+}, and D^{+}→ϕπ^{+} decays is reported using proton-proton collision data, corresponding to an integrated luminosity of 3.8  fb^{-1}, collected at a center-of-mass energy of 13 TeV with the LHCb detector. High-yield samples of kinematically and topologically similar Cabibbo-favored D_{(s)}^{+} decays are analyzed to subtract nuisance asymmetries due to production and detection effects, including those induced by CP violation in the neutral kaon system. The results areA_{CP}(D_{s}^{+}→K_{S}^{0}π^{+})=(1.3±1.9±0.5)×10^{-3},A_{CP}(D^{+}→K_{S}^{0}K^{+})=(-0.09±0.65±0.48)×10^{-3},A_{CP}(D^{+}→ϕπ^{+})=(0.05±0.42±0.29)×10^{-3},where the first uncertainties are statistical and the second systematic. They are the most precise measurements of these quantities to date, and are consistent with CP symmetry. A combination with previous LHCb measurements, based on data collected at 7 and 8 TeV, is also reported.

CP Asymmetries in Charm Decays into Neutral Kaons

We find a new CP-violation effect in charm decays into neutral kaons, which results from the interference between two tree (Cabibbo-favored and doubly Cabibbo-suppressed) amplitudes with the mixing of final-state mesons. This effect, estimated to be of an order of 10^{-3}, is much larger than the direct CP asymmetries in these decays, but missed in the literature. It can be revealed by measuring the difference of the time-dependent CP asymmetries in the D^{+}→π^{+}K_{S}^{0} and D_{s}^{+}→K^{+}K_{S}^{0} modes, which are accessible at the LHCb and Belle II experiments. If confirmed, the new effect has to be taken into account, as the above direct CP asymmetries are used to search for new physics.

Sum Rules of Charm CP Asymmetries beyond the SU(3)_{F} Limit

We find new sum rules between direct CP asymmetries in D meson decays with coefficients that can be determined from a global fit to branching ratio data. Our sum rules eliminate the penguin topologies P and PA, which cannot be determined from branching ratios. In this way, we can make predictions about direct CP asymmetries in the standard model without ad hoc assumptions on the sizes of penguin diagrams. We consistently include first-order SU(3)_{F} breaking in the topological amplitudes extracted from the branching ratios. By confronting our sum rules with future precise data from LHCb and Belle II, one will identify or constrain new-physics contributions to P or PA. The first sum rule correlates the CP asymmetries a_{CP}^{dir} in D^{0}→K^{+}K^{-}, D^{0}→π^{+}π^{-}, and D^{0}→π^{0}π^{0}. We study the region of the a_{CP}^{dir}(D^{0}→π^{+}π^{-})-a_{CP}^{dir}(D^{0}→π^{0}π^{0}) plane allowed by current data and find that our sum rule excludes more than half of the allowed region at 95% C.L. Our second sum rule correlates the direct CP asymmetries in D^{+}→K[over ¯]^{0}K^{+}, D_{s}^{+}→K^{0}π^{+}, and D_{s}^{+}→K^{+}π^{0}.

Measurement of the difference in CP-violating asymmetries in D(0)→K(+)K(-) and D(0)→π(+)π(-) decays at CDF

We report a measurement of the difference (ΔA(CP)) between time-integrated CP-violating asymmetries in D(0)→K(+)K(-) and D(0)→π(+)π(-) decays reconstructed in the full data set of proton-antiproton collisions collected by the Collider Detector at Fermilab, corresponding to 9.7  fb(-1) of integrated luminosity. The strong decay D(*+)→D(0)π(+) is used to identify the charm meson at production as D(0) or D[over ¯](0). We measure ΔA(CP)=[-0.62±0.21(stat)±0.10(syst)]%, which differs from zero by 2.7 Gaussian standard deviations. This result supports similar evidence for CP violation in charm-quark decays obtained in proton-proton collisions.

Evidence for CP violation in the decay D+ → K(S)(0)π+

We observe evidence for CP violation in the decay D+ → K(S)(0)π+ using a data sample with an integrated luminosity of 977  fb(-1) collected by the Belle detector at the KEKB e+ e- asymmetric-energy collider. The CP asymmetry in the decay is measured to be (-0.363±0.094±0.067)%, which is 3.2 standard deviations away from zero, and is consistent with the expected CP violation due to the neutral kaon in the final state.

Search for CP violation in the decays D(0)→K(S)(0)P(0)

We have searched for CP violation in the decays D(0)→K(S)(0)P(0) where P(0) denotes a neutral pseudoscalar meson that is either a π(0), η, or η' using KEKB asymmetric-energy e(+)e(-) collision data corresponding to an integrated luminosity of 791 fb(-1) collected with the Belle detector. No evidence of significant CP violation is observed. We report the most precise CP asymmetry measurement in the decay D(0)→K(S)(0)π(0) to date: A(CP)(D(0)→K(S)(0)π(0))=(-0.28±0.19±0.10)%. We also report the first measurements of CP asymmetries in the decays D(0)→K(S)(0)η and D(0)→K(S)(0)η': A(CP)(D(0)→K(S)(0)η)=(+0.54±0.51±0.16)% and A(CP)(D(0)→K(S)(0)η')=(+0.98±0.67±0.14)%, respectively.

Search for CP violation in the decays D(s)+ --> KS0pi+ and D(s)+ --> KS0K+

We have searched for CP violation in the charmed meson decays D((s))(+) --> K(S)(0)pi(+) and D((s))(+) --> K(S)(0)K(+) using 673 fb(-1) of data collected with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider. No evidence for CP violation is observed. We report the most sensitive CP asymmetry measurements to date for these decays: A(CP)(D(+)-->K(S)(0)pi(+)) = (-0.71 +/- 0.19 +/- 0.20)%, A(CP)(D(s)(+) --> K(S)(0)pi(+)) = (+5.45 +/- 2.50 +/- 0.33)%, A(CP)(D(+) --> K(S)(0)K(+)) = (-0.16 +/- 0.58 +/- 0.25)%, and A(CP)(D(s)(+) --> K(S)(0)K(+)) = (+0.12 +/- 0.36 +/- 0.22)%, where the first uncertainties are statistical and the second are systematic.

Search for CP violation in the decays D0 --> K- K+ and D0 --> pi- pi+

We measure time-integrated CP-violating asymmetries of neutral charmed mesons in the modes D0 --> K(-) K(+) and D0 --> pi(-) pi(+) with the highest precision to date by using D0 --> K(-) pi(+) decays to correct detector asymmetries. An analysis of 385.8 fb(-1) of data collected with the BABAR detector yields values of a(CP)(KK)=(0.00+/-0.34(stat)+/-0.13(syst))% and a(CP)(pipi)=(-0.24+/-0.52(stat)+/-0.22(syst))%, which agree with standard model predictions.

New physics contributions to the lifetime difference in D0-D0 mixing

We present the first general analysis of New Physics contributions to the D0-D[over ](0) lifetime difference (equivalently DeltaGamma(D)). We argue that New Physics (NP) contributions to |DeltaC|=1 processes can dominate the lifetime difference in the flavor SU(3) limit. We provide several specific examples of models that produce sizable effects in DeltaGamma(D) for realistic values of light quark masses, even if such NP contributions are undetectable in the current round of D0 decay experiments. This makes DeltaGamma(D) a viable observable for studies of indirect effects of New Physics.

Measurement of partial widths and search for direct CP violation in D0 meson decays to K-K+ and pi-pi+

We present a measurement of relative partial widths and decay rate CP asymmetries in K-K+ and pi(-)pi(+) decays of D0 mesons produced in pp collisions at sqrt[s]=1.96 TeV. We use a sample of 2x10(5) D(*+)-->D0pi(+) (and charge conjugate) decays with the D0 decaying to K-pi(+), K-K+, and pi(-)pi(+), corresponding to 123 pb(-1) of data collected by the Collider Detector at Fermilab II experiment at the Fermilab Tevatron collider. No significant direct CP violation is observed. We measure Gamma(D0-->K-K+)/Gamma(D0-->K-pi(+))=0.0992+/-0.0011+/-0.0012, Gamma(D0-->pi(-)pi(+))/Gamma(D0-->K-pi(+))=0.035 94+/-0.000 54+/-0.000 40, A(CP)(K-K+)=(2.0+/-1.2+/-0.6)%, and A(CP)(pi(-)pi(+))=(1.0+/-1.3+/-0.6)%, where, in all cases, the first uncertainty is statistical and the second is systematic.

Search for CP violation in the decays D+--> K(S)pi+ and D+-->K(S)K+

A high-statistics sample of photoproduced charm from the FOCUS experiment has been used to search for direct CP violation in the decay rates for D+-->K(S)pi+ and D+-->K(S)K+. We have measured the following asymmetry parameters relative to D+-->K-pi+pi+: A(CP)(K(S)pi+) = (-1.6+/-1.5+/-0.9)%, A(CP)(K(S)K+) = (+6.9+/-6.0+/-1.5)%, and A(CP)(K(S)K+) = (+7.1+/-6.1+/-1.2)% relative to D+-->K(S)pi+. We have also measured the relative branching ratios and found Gamma(D+-->K(0)pi+)/Gamma(D+-->K-pi+pi+) = (30.60+/-0.46+/-0.32)%, Gamma(D+-->K(0)K+)/Gamma(D+-->K-pi+pi+) = (6.04+/-0.35+/-0.30)%, and Gamma(D+-->K(0)K+)/Gamma(D+-->K(0)pi+) = (19.96+/-1.19+/-0.96)%.