CP-violating Asymmetries Research Articles

STATUS OF ASYMMETRY MEASUREMENT AND RARE DECAYS AT NA48/2 EXPERIMENT

A high statistics study of charged kaon decays, using a novel design for simultaneous K+/K- beams and an upgraded NA48 setup, is in progress. The main goal is to measure the CP-violating asymmetry in the matrix element linear slope of the K±→π+π-π± decays with an accuracy of 2.2×10-4. In addition, the analogous CP-violating asymmetry is measured in K±→π0π0π± decays. More than 106 Ke4 decays allow to measure the scattering length parameter [Formula: see text] with an accuracy better than 0.01, and some other rare decays of charged kaons are studied as well.

Final state interactions in hadronicBdecays

There exist many experimental indications that final-state interactions (FSIs) may play a prominent role not only in charmful B decays but also in charmless B ones. We examine the final-state rescattering effects on the hadronic B decay rates and their impact on direct CP violation. The color-suppressed neutral modes such as $B^0\to D^0\pi^0,\pi^0\pi^0,\rho^0\pi^0,K^0\pi^0$ can be substantially enhanced by long-distance rescattering effects. The direct CP-violating partial rate asymmetries in charmless B decays to $\pi\pi/\pi K$ and $\rho\pi$ are significantly affected by final-state rescattering and their signs are generally different from that predicted by the short-distance approach. For example, direct CP asymmetry in $B^0\to\rho^0\pi^0$ is increased to around 60% due to final state rescattering effects whereas the short-distance picture gives about 1%. Evidence of direct CP violation in the decay $\bar B^0\to K^-\pi^+$ is now established, while the combined BaBar and Belle measurements of $\bar B^0\to\rho^\pm\pi^\mp$ imply a $3.6\sigma$ direct CP asymmetry in the $\rho^+\pi^-$ mode. Our predictions for CP violation agree with experiment in both magnitude and sign, whereas the QCD factorization predictions seem to have some difficulty with the data. Current data on $\pi K$ modes seem to violate the isospin sum rule relation, suggesting the presence of electroweak penguin contributions. We have also investigated whether a large transverse polarization in $B\to \phi K^*$ can arise from the final-state rescattering of $D^{(*)}\bar D_s^{(*)}$ into $\phi K^*$.

Long baseline neutrino oscillations and leptonic CP violation

The status of 3-generation lepton mixing is reviewed. Future possible neutrino facilities using: i ) intense conventional ν ( − ) μ sources, ii ) superbeams and iii ) neutrino factories are discussed. Properties of CP-violating asymmetries are described. It is shown that our ability to measure the CP-violating phase δ is rather insensitive to the specific value of θ 13 for 0.01 ⩽ sin 2 2 θ 13 ≲ 0.20 , as well as the detector distance (for very long distances). The potential for making precision measurements of all neutrino oscillation parameters ( θ i j , Δ m i j 2 , δ ) using a wide band ν μ beam is explained using results of a Brookhaven-Homestake (2540km) study. An outlook on the future is given.

Finalstate phases inB→Dπ, D¯πdecaysandCPasymmetry

Final state phases \delta_f and \delta_f' in B-> D pi, barD pi decays are shown to be equal i.e. \delta =\delta_f-\delta_f' = 0. Thus CP-violating asymmetry A(t) is independent of final state phases. The estimate for the phases \delta_f and \delta_f' is also given.

Measurement of theB0→K2*(1430)0γandB+→K2*(1430)+γbranching fractions

We have investigated the exclusive, radiative B meson decays to K2*(1430) in 89* 10^6 BBbar events with the BaBar detector at the PEP-II storage ring. We measure the branching fractions BR(B0 -> K2*(1430)0 gamma) = (1.22 +-0.25 +-0.10)*10^(-5) and BR(B+ -> K2*(1430)+ gamma) = (1.45 +-0.40 +-0.15) * 10^(-5), where the first error is statistical and the second systematic. In addition, we measure the CP-violating asymmetry ACP(B0 -> K2*(1430)0 gamma) = -0.08 +- 0.15 +- 0.01.

Search forCPViolation in the DecayB0→D*±D∓

We report a search for CP-violating asymmetry in B0→D*±D∓ decays. The analysis employs two methods of B0 reconstruction: full and partial. In the full reconstruction method all daughter particles of the B0 are required to be detected; the partial reconstruction technique requires a fully reconstructed D− and only a slow pion from the D*+→D0πslow+ decay. From a fit to the distribution of the time interval corresponding to the distance between two B meson decay points we calculate the CP-violating parameters and find the significance of nonzero CP asymmetry to be 2.7 standard deviations.Received 13 August 2004DOI:https://doi.org/10.1103/PhysRevLett.93.201802©2004 American Physical Society

Measurement of Time-DependentCP-Violating Asymmetries inB0→K*0γK*0→KS0π0Decays

We present a measurement of the time-dependent CP-violating asymmetries in B0→K*0γ(K*0→KS0π0) decays based on 124×106 Υ(4S)→BB¯ decays collected with the BABAR detector at the PEP-II asymmetric-energy B Factory at the Stanford Linear Accelerator Center. In a sample containing 105±14 signal decays, we measure SK*γ=0.25±0.63±0.14 and CK*γ=−0.57±0.32±0.09, where the first error is statistical and the second, systematic.Received 31 May 2004DOI:https://doi.org/10.1103/PhysRevLett.93.201801©2004 American Physical Society

ProbingCPviolation inΩ→ΛK→pπKdecay

The sum of the CP-violating asymmetries A_Omega and A_Lambda in the decay sequence Omega->Lambda K, Lambda->p pi is presently being measured by the E871 experiment. We evaluate contributions to A_Omega from the standard model and from possible new physics, and find them to be smaller than the corresponding contributions to A_Lambda, although not negligibly so. We also show that the partial-rate asymmetry in Omega->Lambda K is nonvanishing due to final-state interactions. Taking into account constraints from kaon data, we discuss how the upcoming result of E871 and future measurements may probe the various contributions to the observables.

Transverse beam polarization and limits on leptoquark couplings in [formula omitted

It is shown that if electron and positron beams at a linear collider are transversely polarized, azimuthal asymmetries of the final-state top quark in e + e − → t t ¯ can be used to probe a combination of couplings of left and right chiralities in a scalar leptoquark model. The CP-conserving azimuthal asymmetry would be a sensitive test of the chirality violating couplings. A linear collider operating at s = 500 GeV and having transverse polarizations of 80% and 60%, respectively, for the e − and e + beams, can put a limit of the order of 0.025 on the product of the left and right chirality leptoquark couplings (in units of the electromagnetic coupling constant), with a leptoquark mass of 1 TeV and for an integrated luminosity of 500 fb −1. The CP-violating azimuthal asymmetry, which would provide a direct test of CP-violating phases in leptoquark couplings, can be constrained to the same level of accuracy. However, this limit is uninteresting in view of the much better indirect limit from the electric dipole moment of the electron.

Measurements ofCP-Violating Asymmetries inB0→Ks0π0Decays

We present a measurement of the time-dependent CP-violating (CPV) asymmetries in B0→KS0π0 decays based on 124×106 Υ(4S)→BB¯ decays collected with the BABAR detector at the PEP-II asymmetric-energy B factory at SLAC. In a sample containing 122±16 signal decays, we obtain the magnitudes of the direct CPV asymmetry CKS0π0=0.40−0.28+0.27±0.09 and of the CPV asymmetry in the interference between mixing and decay SKS0π0=0.48−0.47+0.38±0.06 where the first error is statistical and the second systematic.Received 27 February 2004DOI:https://doi.org/10.1103/PhysRevLett.93.131805©2004 American Physical Society

CP violation in [formula omitted] production at a linear collider

We discuss the observability of CP-violating asymmetries in the process e + e − → χ ˜ 1 0 χ ˜ 2 0 → χ ˜ 1 0 χ ˜ 1 0 ℓ + ℓ − , with ℓ = e , μ . We consider two examples of supersymmetric scenarios: (i) with decays χ ˜ 2 0 → ℓ ˜ R ± ℓ ∓ → χ ˜ 1 0 ℓ + ℓ − ; (ii) with χ ˜ 2 0 three-body decays. The asymmetries can be of order 0.1 but they are partially washed out by the large backgrounds from W + W − and slepton pair production, being the observed asymmetries one order of magnitude smaller. However, with appropriate kinematical cuts they can be observed at an e + e − collider with a centre of mass energy of 500 GeV and high luminosity.

Direct CP violation in B∓→π∓ω, π∓ρ0, π0ρ∓, and in [formula omitted] with an enhanced branching ratio for π0ρ0

We present a novel dynamics for generating sizable CP-violating asymmetries in the decays of charged B∓→π∓ω,π∓ρ0,π0ρ∓, and in B0(B0)→π∓ρ±. The dynamics for the necessary final-state interactions involves the mixing of G-parity eigenstates of the system (D∗D,D∗D) with the G=±1 states of πω and πρ, respectively. The dynamical effect is enhanced by the empirically large branching ratio for decays to (D∗D,D∗D). A correlated result is a markedly enhanced branching ratio for B0(B0)→π0ρ0, which has now been observed in two experiments.

CP violation in selectron cascade decays [formula omitted

Selectron decays constitute a source of 100% polarised neutralinos, whose helicity is fixed by the charge and “chirality” of the decaying selectron. In SUSY scenarios where the second neutralino χ˜20 has three-body decays, the cascade decay e˜L→eχ˜20→eχ˜10μ+μ− provides a clean place to study CP violation in the neutralino sector, through the analysis of CP-violating asymmetries involving the χ˜20 spin s→ and the momenta of the two muons. We show that a CP-violating asymmetry in the triple product s→⋅(p→μ−×p→μ+) could be observable at a 800 GeV linear collider provided the gaugino mass M1 has a large phase at the electroweak scale.

Observation of large CP violation and evidence for direct CP violation in B0-->pi(+)pi(-) decays.

We report the first observation of CP violation in B0-->pi(+)pi(-) decays based on 152x10(6) gamma (4S)-->BB decays collected with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider. We reconstruct a B0-->pi(+)pi(-) CP eigenstate and identify the flavor of the accompanying B meson from its decay products. From the distribution of the time intervals between the two B meson decay points, we obtain A(pipi)=+0.58+/-0.15(stat)+/-0.07(syst) and S(pipi)=-1.00+/-0.21(stat)+/-0.07(syst). We rule out the CP-conserving case, A(pipi)=S(pipi)=0, at a level of 5.2 standard deviations. We also find evidence for direct CP violation with a significance at or greater than 3.2 standard deviations for any S(pipi) value.

Alternative technique for Standard Model estimation of the rare kaon decay branchings $$\left. {BR(K \to \pi \nu \bar \nu )} \right|_{SM} $$

We estimate $$BR(K \to \pi \nu \bar \nu )$$ in the context of the Standard Model by fitting for λ t≡V tdV ts * of the “kaon unitarity triangle” relation. To find the vertex of this triangle, we fit data from |ɛ K|, the CP-violating parameter describing K mixing, and a ψ,K , the CP-violating asymmetry in B d 0 → J/ψK 0 decays, and obtain the values $$\left. {BR(K \to \pi \nu \bar \nu )} \right|_{SM} = (7.07 \pm 1.03) \times 10^{ - 11} $$ and $$\left. {BR(K_L^0 \to \pi ^0 \nu \bar \nu )} \right|_{SM} = (2.60 \pm 0.52) \times 10^{ - 11} $$ . Our estimate is independent of the CKM matrix element V cb and of the ratio of B-mixing frequencies $${{\Delta m_{B_s } } \mathord{\left/ {\vphantom {{\Delta m_{B_s } } {\Delta m_{B_d } }}} \right. \kern-\nulldelimiterspace} {\Delta m_{B_d } }}$$ . We also use the constraint estimation of λ t with additional data from $$\Delta m_{B_d } $$ and |V ub|. This combined analysis slightly increases the precision of the rate estimation of $$K^ + \to \pi ^ + \nu \bar \nu $$ and $$K_L^0 \to \pi ^0 \nu \bar \nu $$ (by ⋍10 and ⋍20%, respectively). The measured value of $$BR(K^ + \to \pi ^ + \nu \bar \nu )$$ can be compared both to this estimate and to predictions made from $${{\Delta m_{B_s } } \mathord{\left/ {\vphantom {{\Delta m_{B_s } } {\Delta m_{B_d } }}} \right. \kern-\nulldelimiterspace} {\Delta m_{B_d } }}$$ .

Measurement of time-dependent CP asymmetries in B0-->D(*)+/-pi-/+ decays and constraints on sin(2beta+gamma).

We present a measurement of CP-violating asymmetries in fully reconstructed B0-->D(*)+/-pi-/+ decays in approximately 88 x 10(6) upsilon(4S)-->BBmacr; decays collected with the BABAR detector at the PEP-II asymmetric-energy B factory at SLAC. From a time-dependent maximum-likelihood fit we obtain the following for the CP-violating parameters: a=-0.022+/-0.038 (stat)+/-0.020 (syst), a*=-0.068+/-0.038 (stat)+/-0.020 (syst), c(lep)=+0.025+/-0.068 (stat)+/-0.033 (syst), and c*(lep)=+0.031+/-0.070 (stat)+/-0.033 (syst). Using other measurements and theoretical assumptions we interpret the results in terms of the angles of the Cabibbo-Kobayashi-Maskawa unitarity triangle, and find |sin((2beta+gamma)|>0.69 at 68% confidence level. We exclude the hypothesis of no CP violation [sin(2beta+gamma)=0] at 83% confidence level.

Measurement of time-dependent CP asymmetries and constraints on sin(2beta+gamma) with partial reconstruction of B0-->D*-/+pi+/- decays.

We present a measurement of time-dependent CP-violating asymmetries in decays of neutral B mesons to the final states D(*-/+)pi(+/-), using approximately 82x10(6) BBmacr; events recorded by the BABAR experiment at the PEP-II e(+)e(-) storage ring. Events containing these decays are selected with a partial reconstruction technique, in which only the high-momentum pi(+/-) from the B decay and the low-momentum pi(-/+) from the D(*-/+) decay are used. We measure the amplitude of the asymmetry to be -0.063+/-0.024(stat)+/-0.014(syst) and compute bounds on |sin((2beta+gamma)|.

CP violation and the CKM matrix: assessing the impact of the asymmetric B factories

We present an up-to-date profile of the Cabibbo-Kobayashi-Maskawa matrix with emphasis on the interpretation of recent CP-violation results from the B factories. For this purpose, we review all relevant experimental and theoretical inputs from the contributing domains of electroweak interaction. We give the “standard” determination of the apex of the Unitarity Triangle, namely the Wolfenstein parameters $\overline\rho$ and $\overline\eta$ , by means of a global CKM fit. The fit is dominated by the precision measurement of $\sin 2 \beta$ by the B factories. A detailed numerical and graphical study of the impact of the results is presented. We propose to include $\sin 2\alpha$ from the recent measurement of the time-dependent CP-violating asymmetries in $B^0\to\rho^ + \rho^-$ , using isospin relations to discriminate the penguin contribution. The constraint from $\varepsilon^\prime/\varepsilon$ is discussed. We study the impact from the branching fraction measurement of the rare kaon decay $K^ + \to\pi^ + \nu\overline\nu$ , and give an outlook into the reach of a future measurement of $K^0_L\to\pi^0\nu\overline\nu$ . The B system is investigated in detail. We display the constraint on $2\beta + \gamma$ and $\gamma$ from $B^0\to D^{(*)\pm}\pi^\mp$ and $B^ + \to D^{(*)0}K^ + $ decays, respectively. A significant part of this paper is dedicated to the understanding of the dynamics of B decays into $\pi\pi$ , $K\pi$ , $\rho\pi$ , $\rho\rho$ and modes related to these by flavor symmetry. Various phenomenological approaches and theoretical frameworks are discussed. We find a remarkable agreement of the $\pi\pi$ and $K\pi$ data with the other constraints in the unitarity plane when the hadronic matrix elements are calculated within QCD Factorization, where we apply a conservative treatment of the theoretical uncertainties. A global fit of QCD Factorization to all $\pi\pi$ and $K\pi$ data leads to precise predictions of the related observables. However sizable phenomenological power corrections are preferred. Using an isospin-based phenomenological parameterization, we analyze separately the $B\to K\pi$ decays, and the impact of electroweak penguins in response to recent discussions. We find that the present data are not sufficiently precise to constrain either electroweak parameters or hadronic amplitude ratios. We do not observe any unambiguous sign of New Physics, whereas there is some evidence for potentially large non-perturbative rescattering effects. Finally we use a model-independent description of a large class of New Physics effects in both $B^0\overline B^0$ mixing and B decays, namely in the $b\to d$ and $b\to s$ gluonic penguin amplitudes, to perform a new numerical analysis. Significant non-standard corrections cannot be excluded yet, however Standard Model solutions are favored in most cases. In the appendix to this paper we propose a frequentist method to extract a confidence level on $\Delta m_s$ from the experimental information on $B^0_s \overline B_s^0$ oscillation. In addition we describe a novel approach to combine potentially inconsistent measurements. All results reported in this paper have been obtained with the numerical analysis package CKMfitter, featuring the frequentist statistical approach Rfit.

Stokes vector of photon in the decays B0→ρ0γ and [formula omitted

We consider a model for the decay B0→ρ0γ in which the short-distance amplitude determined by the Hamiltonian describing b→dγ is combined with a typical long-distance contribution B0→D+D−→ρ0γ. The latter possesses a significant dynamical phase which induces a CP-violating asymmetry ACP, as well as an important modification of the Stokes vector of the photon. The components S1 and S3 of the Stokes vector S→=(S1,S2,S3) can be measured in the decay B0→ρ0γ∗→π+π−e+e− where they produce a characteristic effect in the angular distribution dΓ/dφ, φ being the angle between the π+π− and e+e− planes. A similar analysis is carried out for the decays B0→K∗γ and B0→K∗γ∗→π+K−e+e−.

Measurements of branching fractions and CP-violating asymmetries in B meson decays to charmless two-body states containing a K(0).

We present measurements of branching fractions and CP-violating asymmetries in decays of B mesons to two-body final states containing a K0. The results are based on a data sample of approximately 88 x 10(6) Upsilon(4S)-->BB decays collected with the BABAR detector at the PEP-II asymmetric-energy B Factory at SLAC. We measure B(B+-->K(0)pi(+))=(22.3+/-1.7+/-1.1)x10(-6), B(B0-->K(0)pi(0)=(11.4+/-1.7+/-0.8)x10(-6), B(B+-->K(0)K+)<2.5 x 10(-6), and B(B0-->K(0)K(0)<1.8 x 10(-6), where the first uncertainty is statistical and the second is systematic, and the upper limits are at the 90% confidence level. In addition, the following CP-violating asymmetries have been measured: A(CP)(B+-->K(0)pi(+))=-0.05+/-0.08+/-0.01 and A(CP)(B0-->K(0)pi(0)=0.03+/-0.36+/-0.11.