Direct CP Violation Research Articles

Measurement ofD0–D¯0Mixing Parameters and Search forCPViolation UsingD0→K+π−Decays

Measurements of charm mixing parameters from the decay-time-dependent ratio of D0->K+pi- to D0->K-pi+ rates and the charge-conjugate ratio are reported. The analysis uses data, corresponding to 3 fb^{-1} of integrated luminosity, from proton-proton collisions at 7 and 8 TeV center-of-mass energies recorded by the LHCb experiment. In the limit of charge-parity (CP) symmetry, the mixing parameters are determined to be x'^2=(5.5 +- 4.9)x10^{-5}, y'= (4.8 +- 1.0)x10^{-3}, and R_D=(3.568 +- 0.066)x10^{-3}. Allowing for CP violation, the mixing parameters are determined separately for D0 and D0bar mesons yielding A_D = (-0.7 +- 1.9)%, for the direct CP-violating asymmetry, and 0.75 < |q/p|< 1.24 at the 68.3% confidence level, where q and p are parameters that describe the mass eigenstates of the neutral charm mesons in terms of the flavor eigenstates. This is the most precise determination of these parameters from a single experiment and shows no evidence for CP violation.

Branching fractions and directCPviolation in charmless three-body decays ofBmesons

Charmless three-body decays of B mesons is studied using a simple model based on the framework of the factorization approach. Hadronic three-body decays receive both resonant and nonresonant contributions. Dominant nonresonant contributions to tree-dominated three-body decays arise from the $b\to u$ tree transition which can be evaluated using heavy meson chiral perturbation theory valid in the soft meson limit. For penguin-dominated decays, nonresonant signals come mainly from the penguin amplitude governed by the matrix elements of scalar densities $<M_1M_2|\bar q_1 q_2|0>$. We use the measurements of $\bar B^0\to K_SK_SK_S$ to constrain the nonresonant component of $<K\bar K|\bar ss|0>$. The intermediate vector meson contributions to three-body decays are identified through the vector current, while the scalar meson resonances are mainly associated with the scalar density. While the calculated direct CP violation in $B^-\to K^+K^-K^-$ and $B^-\to \pi^+\pi^-\pi^-$ decays agrees well with experiment in both magnitude and sign, the predicted CP asymmetries in $B^-\to \pi^- K^+K^-$ and $B^-\to K^-\pi^+\pi^-$ are wrong in signs when confronted with experiment. It has been conjectured recently that a possible resolution to this CP puzzle may rely on final-state rescattering of $\pi^+\pi^-$ and $K^+K^-$. Assuming a large strong phase associated with the matrix element $<K\pi|\bar sq|0>$ arising from some sort of power corrections, we fit it to the data of $K^-\pi^+\pi^-$ and find a correct sign for $\pi^- K^+K^-$.

CPviolation inDmeson decays: Would it be a sign of new physics?

Ascribing the large SU(3) violations in the Cabibbo forbidden decays of neutral D mesons to the final state interactions, one gets large strong phase differences, necessary for substantial direct CP violation. While the absolute value of the CP violating asymmetries depend on the uncertain strength of the penguin contribution, we predict an asymmetry for the decays into charged pions more than twice as large and having opposite sign with respect to that for charged kaons.

Studies of the decaysB+→pp¯h+and observation ofB+→Λ¯(1520)p

Dynamics and direct CP violation in three-body charmless decays of charged B mesons to a proton, an antiproton and a light meson (pion or kaon) are studied using data, corresponding to an integrated luminosity of 1.0 fb−1, collected by the LHCb experiment in pp collisions at a center-of-mass energy of 7 TeV. Production spectra are determined as a function of Dalitz-plot and helicity variables. The forward-backward asymmetry of the light meson in the pp¯ rest frame is measured. No significant CP asymmetry in B+→pp¯K+ decay is found in any region of the Dalitz plane. We present the first observation of the decay B+→Λ¯(1520)(→K+p¯)p near the K+p¯ threshold and measure B(B+→Λ¯(1520)p)=(3.9−0.9+1.0(stat)±0.1(syst)±0.3(BF))×10−7, where BF denotes the uncertainty on secondary branching fractions.3 MoreReceived 23 July 2013DOI:https://doi.org/10.1103/PhysRevD.88.052015This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.© 2013 CERN, for the LHCb Collaboration

Searching for the origin of CP violation in Cabibbo-suppressed D-meson decays

The recent evidence of large direct CP violation in D0 decay at LHCb suggests that such CP studies may become an important avenue for understanding CP. In this paper, we make several suggestions to try to clarify the role of new physics in these results. We propose that the enhancement needed in the Standard Model to attribute the observed CP violation in D to two pseudoscalar modes may not operate for inclusive final states where asymmetries will likely approximate the quark level expectation. Experimentally testing this principle requires a search for CP asymmetries in final states containing K and K-bar with a total energy of less than M_D. We also propose that CP asymmetries may be enhanced in modes where the tree is color suppressed. In particular, the final state rho0 rho0 is of special interest; similarly Ds-> rho0 K+ and rho0 K*+ likewise appear interesting. We discuss how isospin symmetry yields observables sensitive to certain classes of new physics and suppressed in the Standard Model. Some modes considered in this context are D-> pi pi, rho pi, rho rho as well as Ds-> K* pi. We also consider how such analysis may eventually be supplemented by information about the weak phases in D0 decay. In order to obtain this information experimentally, we consider various methods for preparing an initial state which is a quantum mechanical mixture of D0 and D0-bar. This may be done through the use of natural D0 D0-bar oscillations; observing D0-mesons which arise from Bd or Bs mesons which themselves are oscillating or from quantum correlations in D0 pairs which arise from either psi" decay or B-meson decay. Observing CP violation in the magnitudes of decay amplitudes should be within the capability of experiments in the near future however obtaining the weak phases through the methods we discuss will likely require future generations of machines.

The form factors of τ → K π (η) ν and the predictions for CP violation beyond the standard model

We study the hadronic form factors of $\tau$ lepton decays $\tau \to K \pi(\eta) \nu$. We compute one loop corrections to the form factors using the chiral Lagrangian including vector mesons. The counterterms which subtract the divergence of the one-loop amplitudes are determined by using background field method. In the vector form factor, $K^\ast$ resonance behavior is reproduced because the diagram with a vector meson propagator is included. We fit the data of the hadronic invariant mass spectrum measured by Belle by determining some of the counterterms of the Lagrangian. Besides the hadronic invariant mass spectrum, the forward-backward asymmetry is predicted. We also study the effect of CP violation of a two Higgs doublet model. In the model, CP violation of the neutral Higgs sector generates the mixing of CP even Higgs and CP odd Higgs. We show how the mixing leads to the direct CP violation of the $\tau$ decays and predict the CP violation of the forward-backward asymmetry.

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.

Direct CP violation in charm and flavor mixing beyond the SM

We discuss the impact of physics beyond the Standard Model on direct CP violation in charm. On general grounds, models in which the primary source of flavor violation is linked to the breaking of chiral symmetry (left-right flavor mixing) are natural candidates to explain this effect, via enhanced chromomagnetic operators. In the case of super-symmetric models, we identify two motivated scenarios: disoriented A-terms and split families. Non-supersymmetric models with Z-mediated and scalar-mediated FCNC are also discussed.

CP violation in $ B_s^0\to {K^{-}}{\pi^{+}} $ , B 0 → K +π− decays and tests for SU(3) flavor symmetry predictions

The LHCb collaboration has recently measured the first direct CP violation in $B^0_s$ decays with a rate asymmetry $A_{CP}(B^0_s\to K^- \pi^+)$ given by $0.27 \pm 0.04(stat)\pm 0.01(syst)$. At the same time they also made the most precise measurement for $A_{CP}(B^0 \to K^+ \pi^-) = -0.080\pm 0.007(stat)\pm 0.003(syst)$. These data confirm the predicted relation, $A_{CP}(B^0 \to K^+ \pi^-)/A_{CP}(B^0_s \to K^- \pi^+) = - Br(B^0_s\to K^- \pi^+)\tau_{B^0}/Br(B^0 \to K^+ \pi^-)\tau_{B^0_s}$, in the standard model with SU(3) flavor symmetry. We discuss possible modifications due to SU(3) breaking effects to this relation. There are several other similar relations in B decays. Using current available data we study whether relevant relations hold in $B^0$ and $B^0_s$ to $PP$ and $PV$ decays. Here $P$ and $V$ indicate pesudoscalar and vector mesons in the flavor octet representations.

Enhanced CP violation in D 0→ρ 0(ω)ρ 0(ω)→π + π − π + π −

In the factorization approach we study the direct CP violation in D0→ρ0(ω)ρ0(ω)→π+π−π+π− (with unpolarized ρ0(ω)) via the ρ−ω mixing mechanism. We find that the CP violation can be enhanced due to a large strong phase difference when the masses of the π+π− pairs are in the vicinity of the ω resonance. The CP violation parameter depends on the effective parameter Nc which includes nonfactorizable effects and is determined by the experimental data. Taking into account double ρ−ω mixing contributions, we find the CP violating asymmetry is larger than these in the cases where no ρ−ω mixing or single ρ−ω mixing is taken into account. We also discuss the possibility to observe the predicted CP violation at BEPCII.

Charm mixing and CP violation at LHCb

In pp collisions at the LHC the LHCb experiment has collected the world's largest sample of charmed hadrons. Using data of an integrated luminosity of 1.0 fb−1 recorded in 2011 a measurement of direct and indirect CP violation in the charm sector and of D0 mixing parameters was performed. Results from several decay modes are presented with complementary time-dependent and time-integrated analyses.

CharmlessBu,d,s→VTdecays in perturbative QCD approach

Motivated by the experimental data, we study charmless $B_{u,d,s}\to VT$ ($V$ and $T$ denote light vector and tensor mesons respectively) decays in the perturbative QCD approach. The predictions of branching ratios, polarization fractions and direct CP violations are given in detail. Specifically, within this approach we have calculated the polarization fractions and the branching ratios of $B\to \phi(K_2^{*-}, \bar K_2^{*0})$ which agree well with the observed experimental data, however the branching ratios of $B\to \omega(K_2^{*-}, \bar K_2^{*0})$ are hard to be explained, where the polarization fractions are well accommodated. The tree dominated channels with a vector meson emitted have longitudinal polarization fraction of 90%, while the penguin dominating ones have subtle polarization fractions. Fortunately, most branching ratios of $B_{u,d}$ decays are of the order $10^{-6}$, which would be straight forward for experimental observations. For the $B_s$ decays the branching ratios can reach the order of $10^{-6}$ in tree dominated decays, while in penguin dominated decays those are of order of $10^{-7}$ which require more experimental data to be observed.

Measurement ofCP-violating asymmetries inB0→(ρπ)0decays using a time-dependent Dalitz plot analysis

We present results for a time-dependent Dalitz plot measurement of CP-violating asymmetries in the mode B^0 -> pi^+ pi^- pi^0. The dataset is derived from the complete sample of 471 x 10^6 BBbar meson pairs collected with the BABAR detector at the PEP-II asymmetric-energy e^+e^- collider at the SLAC National Accelerator Laboratory operating on the Upsilon(4S) resonance. We extract parameters describing the time-dependent B^0 -> rho pi decay probabilities and CP asymmetries, including C = 0.016\pm0.059\pm0.036, DeltaC = 0.234\pm0.061\pm0.048, S = 0.053\pm0.081\pm0.034, and DeltaS = 0.054\pm0.082\pm0.039, where the uncertainties are statistical and systematic, respectively. We perform a two-dimensional likelihood scan of the direct CP-violation asymmetry parameters for B^0 -> rho^\pm pi^\mp decays, finding the change in chi^2 between the minimum and the origin (corresponding to no direct CP violation) to be Delta(chi^2)=6.42. We present information on the CP-violating parameter alpha in a likelihood scan that incorporates B^\pm -> rho pi measurements.

Up sector of minimal flavor violation: top quark properties and direct D meson CP violation

Abstract Minimal Flavor Violation in the up-type quark sector leads to particularly interesting phenomenology due to the interplay of flavor physics in the charm sector and collider physics from flavor changing processes in the top sector. We study the most general operators that can affect top quark properties and D meson decays in this scenario, concentrating on two CP violating operators for detailed studies. The consequences of these effective operators on charm and top flavor changing processes are generically small, but can be enhanced if there exists a light flavor mediator that is a Standard Model gauge singlet scalar and transforms under the flavor symmetry group. This flavor mediator can satisfy the current experimental bounds with a mass as low as tens of GeV and explain observed D-meson direct CP violation. Additionally, the model predicts a non-trivial branching fraction for a top quark decay that would mimic a dijet resonance.

Effect of direct CP violation in charm on γ extraction from B ±→DK ±, $D\to K^{0}_{S}\pi^{+}\pi^{-}$ Dalitz plot analysis

A possible effect of direct CP violation in $D\to K^{0}_{S}\pi^{+}\pi^{-}$ decay on the γ measurement from B ±→DK ±, $D\to K^{0}_{S}\pi^{+}\pi^{-}$ Dalitz plot analysis is considered. Systematic uncertainty of γ coming from the current limits on direct CP violation in $D\to K^{0}_{S}\pi ^{+}\pi^{-}$ is estimated, and a modified model-independent procedure of B ±→DK ±, $D\to K^{0}_{S}\pi^{+}\pi^{-}$ Dalitz plot analysis is proposed that gives an unbiased γ measurement even in presence of direct CP violation in charm decays. The technique is applicable to other three-body D decays such as $D^{0}\to K_{S}^{0}K^{+}K^{-}$ , D 0→π + π − π 0, etc.

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.

Search for direct CP violation in [formula omitted] modes using semileptonic B decays

A search for direct CP violation in D0→h−h+ (where h=K or π) is presented using data corresponding to an integrated luminosity of 1.0 fb−1 collected in 2011 by LHCb in pp collisions at a centre-of-mass energy of 7 TeV. The analysis uses D0 mesons produced in inclusive semileptonic b-hadron decays to the D0μX final state, where the charge of the accompanying muon is used to tag the flavour of the D0 meson. The difference in the CP-violating asymmetries between the two decay channels is measured to beΔACP=ACP(K−K+)−ACP(π−π+)=(0.49±0.30(stat)±0.14(syst))%. This result does not confirm the evidence for direct CP violation in the charm sector reported in other analyses.

The form factors of -&gt; K ( ) and the predictions for CP violation beyond the standard model

We study the hadronic form factors of the τ lepton decays |$\tau \rightarrow K \pi (\eta) \nu $|⁠. We compute one-loop corrections to the form factors using the chiral Lagrangian including vector mesons. The counterterms that subtract the divergence of the one-loop amplitudes are determined by using the background field method. In the vector form factor, K* resonance behavior is reproduced because a diagram with a vector meson propagator is included. We fit the data of the hadronic invariant mass spectrum measured by Belle by determining some of the counterterms of the Lagrangian. Besides the hadronic invariant mass spectrum, forward–backward asymmetry is predicted. We also study the effect of CP violation of a two Higgs doublet model. In the model, CP violation of the neutral Higgs sector generates the mixing of CP-even Higgs and CP-odd Higgs. We show how the mixing leads to direct CP violation of the τ decays and predict the CP violation of the forward–backward asymmetry.

Standard Model prediction and new physics tests for $ {D^0}\to h_1^{+}h_2^{-}{\ell^{+}}{\ell^{-}}\left( {h=\pi, K;\ell =e,\mu } \right) $

Motivated by the recent evidence for direct CP-violation in D0 -> h+h- decays, we provide an exhaustive study of both Cabibbo-favored and Cabibbo-suppressed (singly and doubly) D0 -> h1+h2-l+l- decays. In particular, we study the Dalitz plot for the long-distance contributions in the (m_{ll}^2,m_{hh}^2) parameter space. We find that near-resonant effects, i.e., D0 -> V(h1+h2-)l+l- with V=\rho,K*,\phi, are sizeable and even dominant (over Bremsstrahlung) for the \mu+\mu- decay modes, bringing the branching ratios close to the LHCb reach. We also provide a detailed study of the angular asymmetries for such decays and identify signatures for new physics detection. In particular, new physics signals can be neatly isolated in asymmetries involving the semileptonic operator Q_{10}, where for typical new physics scenarios the effects can be as sizeable as O(1%) for the doubly Cabibbo-suppressed modes.

B±→DK±with directCPviolation in charm

We investigate the implications of direct $CP$ violation (CPV) on the determination of the unitarity triangle angle $\ensuremath{\gamma}$ from $B\ensuremath{\rightarrow}DK$ decays. We show that $\ensuremath{\gamma}$ can still be extracted even with the inclusion of direct CPV in charm if (i) at least one of the $D$ decays has negligible $CP$ violation; and (ii) data from a charm factory at threshold are used. If approximate expressions without direct $CP$ violation in charm are used, this can result in a shift in $\ensuremath{\gamma}$ that is $\mathcal{O}({r}_{D}/{r}_{B})$. It is modest for $B\ensuremath{\rightarrow}DK$ but can be $\mathcal{O}(1)$ for $B\ensuremath{\rightarrow}D\ensuremath{\pi}$. We illustrate the size of the shift using an example of the Gronau-London-Wyler method.