CP Violation In D-meson Research Articles

CP violation in D-meson decays

Since most probably Standard Model cannot explain large value of CP asymmetries recently observed in D -meson decays the fourth quark-lepton generation explanation of it is proposed. As a byproduct weakly mixed leptons of the fourth generation enable to save the baryon number of the Universe from erasure by sphalerons.

D-meson lifetimes within the heavy quark expansion

Even if new data indicate that direct CP violation in D-meson decays is compatible with the standard model expectation, it has triggered a lot of interest, and charm phenomenology will remain an essential part of new physics searches due to its unique role as a probe for flavor-changing neutral currents among up-type quarks. Charm physics poses considerable theoretical challenges, because the charm mass is neither light nor truly heavy. The heavy quark expansion (HQE) provides a perturbative expansion in the inverse heavy quark mass for inclusive rates. It has proved to be very successful in the B sector, yet its validity for charm decays has often been questioned. We present results of a HQE study of D-meson lifetimes including NLO QCD and subleading 1/m_c corrections. We find good agreement with experimental data, but with huge hadronic uncertainties due to missing lattice input for hadronic matrix elements.

Gauge mediation beyond minimal flavor violation

Abstract We study a minimal modification of Gauge Mediation in which the messenger sector couples directly to the MSSM matter fields. These couplings are controlled by the same dynamics that explain the flavor hierarchies, and therefore are parametrically as small as the Yukawas. This setup gives rise to an interesting SUSY spectrum that is calculable in terms of a single new parameter. Due to large A-terms, the model can easily accommodate a 126 GeV Higgs with a relatively light SUSY spectrum. The flavor structure depends on the particular underlying flavor model, but flavor-violating effects arise dominantly in the up-sector and are strongly suppressed in ΔF = 2 observables. This strong suppression is reminiscent of what happens in the case of wave function renormalization or Partial Compositeness, despite the underlying flavor model can be a simple U(1) flavor model (which in the context of Gravity Mediation suffers from strong ΔF = 2 constraints). This structure allows to account for the recent observation of direct CP violation in D-meson decays.

Repercussions of flavour symmetry breaking on CP violation in D-meson decays

We investigate to what extent the recently measured value for a non-vanishing direct CP asymmetry in D0 -> K+ K- and D0 -> pi+ pi- decays can be accommodated in the Standard Model (SM) or extensions with a constrained flavour sector, for instance from a sequential 4th generation of quarks (4G). From the comparison with D0 -> K- pi+ branching ratios, we establish large U-spin symmetry (d <-> s) breaking effects with large strong phases between different interfering amplitudes. On the basis of conservative estimates on amplitude ratios - which are supported by an analysis of the breaking of a (c <-> u) symmetry in non-leptonic B0 decays - we find that, in the SM, direct CP asymmetries in the pi+ pi- or K+ K- modes (or in their difference) of the order of several per mille are still plausible. Due to the constraints on the new CP phases in the 4G model, only moderate effects compared to the SM estimates are possible. We suggest CP studies at LHCb as well as at (Super)B-factories of several distinctive modes, such as D+ -> Kbar(*)0 pi+, phi pi+ and Ds -> K(*)0 pi+, phi pi+ (K+) etc., which should shed more light on the short- and long-distance issues underlying CP violation in non-leptonic D-meson decays.

CP violation in D-meson decays and the fourth generation

The LHCb collaboration measured CPV at the level of one percent in the difference of asymmetries in \( D^0 (\mathop {D^0 }\limits^ \leftharpoondown ) \) → π+π−, K+K− decays. If confirmed on a larger statistics and final systematics this would mean New Physics manifestation. The fourth quark-lepton generation can be responsible for the observed effect.