CP-violating Asymmetries Research Articles

Dalitz-plot analysis of B±→K±K+K− decays

We study B±→K±K+K− decays using the QCD factorization model with final state interactions between K+ and K− mesons taken into account. The parameters of the model are fitted to the data of the and LHCb collaborations. We describe the KK¯ effective mass distributions and examine the CP-violating asymmetry effects in the full range of the Dalitz plot. Published by the American Physical Society 2024

CP violation in light neutrino oscillations and heavy neutrino decays: A general and explicit seesaw-bridged correlation

With the help of a block parametrization of the canonical seesaw flavor textures in terms of the Euler-like rotation angles and CP-violating phases, we derive a general and explicit expression for the Jarlskog invariant of CP violation in neutrino oscillations and compare it with the CP-violating asymmetries of heavy Majorana neutrino decays within the minimal seesaw framework which contains two right-handed neutrino fields. Two simplified scenarios are discussed to illustrate how direct or indirect the correlation between these two types of CP violation can be.

Intrinsic and extrinsic CP violation in long-baseline neutrino experiments

The observation of CP violation in the long-baseline (LBL) neutrino experiments is complicated by the matter effects, wherein apart from the intrinsic/genuine CP violation the possibility of extrinsic/fake CP violation is also there. Using exact expressions of transition probabilities, we study the dependence of genuine and fake components of CP violation asymmetry on baseline length [Formula: see text] and neutrino energy [Formula: see text]. We examine the ongoing LBL experiments T2K, NO[Formula: see text]A and the next-generation experiment T2HK for their sensitivity towards genuine and fake components of CP violation and propose an experimental setup which not only has a very high CP discovery potential but also has well-separated sensitivities in matter and vacuum. In addition, the proposed setup facilitates the hierarchy and octant determination due to prominent matter effects.

{B}_c^{-}to J/psi left(to {mu}^{+}{mu}^{-}right){tau}^{-}left(to {pi}^{-}{nu}_{tau },{rho}^{-}{nu}_{tau },{ell}^{-}{overline{nu}}_{ell }{nu}_{tau}right){overline{nu}}_{tau } decays with visible final-state kinematics

The semitauonic {B}_c^{-}to J/psi {tau}^{-}{overline{nu}}_{tau } decay provides an ideal and clean mode to scrutinize possible new physics effects in bto c{tau}^{-}{overline{nu}}_{tau } transitions as indicated by the current data on R(D(*)) anomalies. In this work, we use the spin density matrix method to obtain the maximum information on the underlying physics of {B}_c^{-}to J/psi {tau}^{-}{overline{nu}}_{tau } decay with both polarized τ lepton and J/ψ meson. Their subsequent decays, with J/ψ → μ+μ− as well as τ− → π−ντ, τ− → ρ−ντ and {tau}^{-}to {ell}^{-}{overline{nu}}_{ell }{nu}_{tau } , are exploited to extract the energy and angular distributions of the charged final-state particles in the processes. Starting with the most general effective Hamiltonian relevant for the bto c{tau}^{-}{overline{nu}}_{tau } transitions, including all possible Lorentz structures of the dimension-six operators with both left- and right-handed neutrinos, we first derive the five-fold differential decay rate in terms of the visible final-state kinematics. From this distribution, we then construct in total 34 normalized observables, among which nine refer to the CP-violating triple product asymmetries that vanish within the Standard Model. We also construct five new observables based on the combinations of these normalized observables that can only be attributed to the right-handed neutrinos. On the other hand, considering the low statistics of the fully differential distribution, we introduce some integrated observables with only one kinematic variable left, which are more promising to be measured due to the largely increased statistics. The sensitivities of all these observables to the different new physics scenarios are investigated in detail. Finally, assuming an ideal circumstance, we give an estimate of the statistical uncertainties of the nine CP-conserving observables at LHCb and found that τ− → π−ντ has the highest analyzing power among the three τ decay channels.

The possible contribution of [formula omitted] meson decay into X(3872) and ϕ(1020)

In this paper, we present the branching ratio and polarization fraction of two body decay of Bs0 to the vector mesons ϕ(1020) and X(3872) in which the X(3872) meson can be considered as an intermediate resonance in the final decay Bs0→J/ψπ+π−ϕ(1020). For this aim, the naive factorization approach is used to calculate the decay amplitude with the different helicities due to the combination of polarization states of two mesons X(3872) and ϕ(1020). Within the framework of the factorization approach, we find the branching fraction for the two body channel of Bs0→X(3872)ϕ(1020)=(6.31±0.01)×10−5 and also Br(X(3872)→J/ψπ+π−) and Br(X(3872)→J/ψπ+π−)Br(Bs0→X(3872)ϕ(1020)) are calculated which are in well agreement with CMS collaboration data. Furthermore, we estimate the decay width of resonance X(3872) into final states J/ψπ+π− which is within the range of experimental data. By these results and available experimental data, we render the branching ratio of decays B+ and Bs0 into X(3872)K+ and X(3872)K0, respectively. Also, in the nonleptonic two body decay of Bs0→X(3872)ϕ(1020), the direct and indirect CP-violating asymmetries are estimated to indicate the potential role of the such decays to measure CP-violation.

Light meson decays at BESIII

Due to the high production rate of light mesons in J/ψ radiative and hadronic decays, the largest sample of J/ψ events accumulated at the BESIII detector offers a unique laboratory for investigating light meson decays. In recent years the BESIII experiment has made significant progresses in the study of η′ decays, including the search for the η′→γγη and η′→π0π0π0π0 decays, the measurement of the branching fraction of and the search for a CP-violating asymmetry in the η′→π+π−e+e− decay, and the observation of the η′→π+π−μ+μ− decay. This paper gives brief introductions to these works.

Study of the four-body decays BS0→ππππ in the perturbative QCD approach

In this work, we analyse the CP-averaged branching ratios and direct CP-violating asymmetries of the four-body decays B_{S} \rightarrow \pi\pi\pi\pi decay from the S-wave resonances, f_{0}(980) and f_{0}(500) and P-wave resonances, \rho(770) by introducing the S-wave and P-wave \pi\pi distribution amplitudes within the framework of the perturbative QCD approach. We also calculate branching ratios of the two-body decays B^{0}_{S}\rightarrow\rho^{0}\rho^{0}, B^{0}_{S}\rightarrow\rho^{+}\rho^{-} from the corresponding quasi-two-body decays models and compare our results with those obtained in previous perturbative QCD approach, QCD factorization approach and FAT approach, it is found that the predictions are consistent with present data within errors. The branching ratios of our calculations for the four-body decays B_{S}\rightarrow \pi\pi\pi\pi are at the order of the 10^{-7}. For the CP-violating asymmetries, we found that CP-violating asymmetry can be enhanced largely by the \rho-\omega mixing resonances when \pi\pi pairs masses are in the vicinity of \omega resonance.

Patterns of C- and CP-violation in hadronic η and η′ three-body decays

We construct hadronic amplitudes for the three-body decays η(′) → π+π−π0 and η′ → ηπ+π− in a non-perturbative fashion, allowing for C- and CP-violating asymmetries in the π+π− distributions. These amplitudes are consistent with the constraints of analyticity and unitarity. We find that the currently most accurate Dalitz-plot distributions taken by the KLOE-2 and BESIII collaborations confine the patterns of these asymmetries to a relative per mille and per cent level, respectively. Our dispersive representation allows us to extract the individual coupling strengths of the C- and CP-violating contributions arising from effective isoscalar and isotensor operators in η(′) → π+π−π0 and an effective isovector operator in η′ → ηπ+π−, while the strongly different sensitivities to these operators can be understood from chiral power counting arguments.

CP violation in the rare Higgs decays via exchange of on-shell almost degenerate Majorana neutrinos, Hto {v}_k{N}_jto {v}_k{mathrm{ell}}^{-}Uoverline{D} and Hto {v}_k{N}_jto {v}_k{mathrm{ell}}^{+}overline{U}D

We investigate rare decays of Higgs via exchange of two almost degenerate heavy on-shell Majorana neutrinos Nj (j = 1, 2): Γ± = Γ(h → νkNj→ νkℓ±π∓), and into the open quark channels Γ± = Γ(h → νkNj→ νkℓ±UD), where UD are two jets of open quarks ( overline{U}D , or Uoverline{D} , where U = u, c and D = d, s). The related CP violation asymmetry ACP = (Γ−− Γ+)/(Γ− + Γ+) is studied in detail. We take into account the N1-N2 overlap and oscillation effects. We can see that for certain, presently acceptable, range of input parameters, such decays with open quark channels, and their asymmetries, could be detected in the International Linear Collider (ILC).

Recent results on branching fraction and CP asymmetry in B → ϕϕK decays

We present precise measurements of the branching fraction and CP-violation asymmetry in the B meson system, specifically in the charmless decays B → ϕϕK, ϕ → K+K−. This study is based on the full ϒ(4S) data set of 772×106BB‾ pairs collected by the Belle detector at KEKB asymmetric energy e+e− collider. These decays are mediated by the b → s flavor-changing neutral current (FCNC) transition, which could exhibit large CP violations due to interference of potential new-physics amplitudes appearing in the quantum loop with the b → c tree-level transition of B → ηcK, ηc → ϕϕ. Therefore, an observation of direct CP violation in B → ϕϕK decays would be a clear sign of physics beyond the standard model. In addition to being an NP probe, the decay is sensitive to possible production of glue-ball candidate that later decays as ξ → ϕϕ. Our results obtained with nine times larger statistics and refined analysis techniques significantly improve upon, and supersede, the Belle's earlier result with due consideration of the nonresonant K+K− contribution underneath the ϕ resonance.

Measurement of the branching fraction of and search for a CP -violating asymmetry in η′→π+π−e+e− at BESIII

The rare decay $\eta' \to \pi^+ \pi^- e^+ e^-$ is studied using a sample of $1.3 \times 10^9$ $J/\psi$ events collected with the BESIII detector at BEPCII in 2009 and 2012. The branching fraction is measured with improved precision to be $\left( 2.42\pm0.05_{stat.}\pm0.08_{syst.} \right) \times 10^{-3}$. Due to the inclusion of new data, this result supersedes the last BESIII result on this branching fraction. In addition, the CP-violating asymmetry in the angle between the decay planes of the $\pi^+\pi^-$-pair and the $e^+e^-$-pair is investigated. A measurable value would indicate physics beyond the standard model; the result is $\mathcal{A}_{CP} = \left(2.9\pm3.7_{stat.}\pm1.1_{syst.} \right)\%$, which is consistent with the standard model expectation of no CP-violation. The precision is comparable to the asymmetry measurement in the $K^0_L\to\pi^+\pi^-e^+e^-$ decay where the observed $(14\pm2)\%$ effect is driven by a standard model mechanism.

Pure annihilation decays of B s 0 → a 0 + a 0 − and B d 0 → in the PQCD approach

We study the CP-averaged branching fractions and the CP-violating asymmetries in the pure annihilation decays of and , where denotes the scalar a 0(980) and a 0(1450) [ and ], with the perturbative QCD factorization approach under the assumption of two-quark structure for the a 0 and states. The numerical results show that the branching ratios of the decays are in the order of 10−6, while the decay rates of the modes are in the order of 10−5. In light of the measured modes with the same quark components in the pseudoscalar sector, namely, and , the predictions for the considered decay modes in this work are expected to be measured at the Large Hadron Collider beauty and/or Belle-II experiments in the (near) future. Meanwhile, it is of great interest to find that the twist-3 distribution amplitudes ϕ S and ϕ T with inclusion of the Gegenbauer polynomials for the scalar a 0(1450) and states in scenario 2 contribute slightly to the branching ratios while significantly to the CP violations in the and decays, which indicates that, compared to the asymptotic ϕ S and ϕ T , these Gegenbauer polynomials could change the strong phases evidently in these pure annihilation decay channels. These predictions await for the future confirmation experimentally, which could further provide useful information to help explore the inner structure of the scalars and shed light on the annihilation decay mechanism.

Non-leptonic two-body decays of [formula omitted] in light-front quark model

We study the non-leptonic two-body weak decays of Λb0→pM with M=(π−,K−) and (ρ−,K⁎−) in the light-front quark model under the generalized factorization ansatz. By considering the Fermi statistic between quarks and determining spin-flavor structures in baryons, we calculate the branching ratios (Bs) and CP-violating rate asymmetries (ACPs) in the decays. Explicitly, we find that B(Λb0→pπ−,pK−)=(4.18±0.15±0.30,5.76±0.88±0.23)×10−6 and ACP(Λb0→pπ−,pK−)=(−3.60±0.14±0.14,6.36±0.21±0.18)% in comparison with the data of B(Λb0→pπ−,pK−)=(4.5±0.8,5.4±1.0)×10−6 and ACP(Λb0→pπ−,pK−)=(−2.5±2.9,−2.5±2.2)% given by the Particle Data Group, respectively. We also predict that B(Λb0→pρ−,pK⁎−)=(12.13±3.27±0.91,2.58±0.87±0.13)×10−6 and ACP(Λb0→pρ−,pK⁎−)=(−3.32±0.00±0.14,19.25±0.00±0.80)%, which could be observed by the experiments at LHCb.

Inflationary Universe with a Coleman-Weinberg potential meets non-thermal leptogenesis

Baryon asymmetry via non-thermal leptogenesis is explained in the framework of the standard Coleman-Weinberg inflationary model. The CP-violation asymmetry factor arises from the out-of-equilibrium decays of heavy right-handed neutrinos in supersymmetric SO(10) GUT models. All quantities of interest are computed in terms of the two parameters of the inflationary potential, namely the vacuum expectation value of the inflaton, M, and the amplitude A. For successful baryogenesis we obtain analytical expressions relating A to M, and also the reheating temperature after inflation as well as the mass of the inflaton to M. Assuming typical values of CP-violation asymmetry obtained in concrete supersymmetric SO(10) unified models, we show graphically all three quantities of interest versus M. Interestingly enough, we obtain for (A,M) numerical values comparable to the ones previously obtained in the slow-roll approximation of inflationary dynamics upon comparison with CMB data, such as amplitude of scalar perturbations and spectral index. Proton decay is also briefly discussed. Our results may be of interest to model builders.

Nonleptonic two-body weak decays of Λb in a modified MIT bag model

We study the nonleptonic two-body weak decays of $\Lambda_b$ by modifying the MIT bag model without introducing new parameters to construct the momentum eigenstates of the baryons. We find that the branching ratios of $\Lambda_b^0 \to \Lambda_c^+ \pi^-$, $\Lambda_c^+ K^-$, $p \pi^-$ and $pK^-$ are $(4.5\pm0.2)\times 10^{-3}$, $(3.4\pm 0.1)\times 10^{-4}$, $(5.0\pm 0.5)\times 10^{-6}$ and $(6.0\pm 0.7) \times 10^{-6}$, which are all well consistent with the current experimental data, respectively. We also explore P and CP asymmetries for the decays of $\Lambda_b^0 \to p (\pi^-,K^-)$. In particular, we obtain that the direct CP-violating rate asymmetries in $\Lambda_b^0 \to p \pi^-$ and $\Lambda_b^0 \to p K^-$ are around $-4.4\%$ and $6.7\%$, in comparison with $(-2.5\pm2.9)\%$ and $(-2.5\pm2.2)\%$ from the Particle Data Group in 2020, respectively.

Exploring Supersymmetric CP Violation after LHC Run\xa02 with Electric Dipole Moments and B Observables

We consider the prospects for measuring distinctive signatures of the CP-violating phases in the minimal supersymmetric extension of the Standard Model (MSSM) in light of the limits on sparticle masses from searches at the LHC. We use the CPsuperH code to evaluate model predictions and scan the parameter space using a geometric approach that maximizes CP-violating observables subject to the current upper limits on electric dipole moments (EDMs). We focus on the possible CP-violating asymmetry A_{mathrm{CP}} in b rightarrow s gamma decay and on a possible CP-violating contribution to the B_s - {overline{B}}_s mass difference Delta M^{NP}_{B_s}, as well as future measurements of the EDMs of the proton, neutron and electron. We find that the current LHC and EDM limits are consistent with values of A_{mathrm{CP}}, Delta M^{NP}_{B_s} and the proton EDM that are measurable with the Belle-II detector, LHCb and a proposed measurement of the proton EDM using a storage ring, respectively. Measurement of a non-zero proton EDM would constrain A_{mathrm{CP}} significantly, but it and a CP-violating contribution to Delta M^{NP}_{B_s} could still be measurable, along with neutron and electron EDMs. A more accurate measurement of A_{mathrm{CP}} with the current central value would favour stop and chargino masses within reach of future LHC runs as well as a potentially measurable value of Delta M^{NP}_{B_s}.

A measurable angular distribution for overline{B}to {D}^{ast }{tau}^{-}{overline{v}}_{tau } decays

At present, the measurements of {R}_{D^{left(ast right)}} and RJ/ψ hint at new physics (NP) in bto c{tau}^{-}overline{v} decays. The angular distribution of overline{B}to {D}^{ast}left(to Dpi right){tau}^{-}{overline{v}}_{tau } would be useful for getting information about the NP, but it cannot be measured. The reason is that the three-momentum {overrightarrow{p}}_{tau } cannot be determined precisely since the decay products of the τ− include an undetected ντ. In this paper, we construct a measurable angular distribution by considering the additional decay τ−→ π−ντ. The full process is overline{B}to {D}^{ast}left(to D{pi}^{prime}right){tau}^{-}left(to {pi}^{-}{v}_{tau}right){overline{v}}_{tau } , which includes three final-state particles whose three-momenta can be measured: D, π′, π−. The magnitudes and relative phases of all the NP parameters can be extracted from a fit to this angular distribution. One can measure CP-violating angular asymmetries. If one integrates over some of the five kinematic parameters parametrizing the angular distribution, one obtains (i) familiar observables such as the q2 distribution and the D∗ polarization, and (ii) new observables associated with the π− emitted in the τ decay: the forward-backward asymmetry of the π− and the CP-violating triple-product asymmetry.

Inspection of new physics in Bs→K+K− decay mode

We scrutinize the penguin dominated $ B_s \to K^+ K^-$ decay mode involving $b \to s$ quark level transition in family non-universal $Z^ \prime$ and vector-like down quark model. There is discrepancy in the standard model branching ratio value of this mode with the experimental results reported by Belle, CDF and LHCb Collaborations. Additionally, the measured values of CP-violating asymmetries $ C_{K^+K^-}$ and $ S_{K^+K^-}$ deviate from the SM results. We constrain the new parameter space by using the existing experimental limits on leptonic $ B_s\to \ell \ell$ ($\ell = e, \mu, \tau$) processes. We then check the effects of new physics on the branching ratio and CP violating parameters of the $ B_s \to K^+ K^-$ process.

CP -violating dark photon interaction

We introduce a scenario for CP-violating (CPV) dark photon interactions in the context of non-abelian kinetic mixing. Assuming an effective field theory that extends the Standard Model (SM) field content with an additional $U(1)$ gauge boson ($X$) and a $SU(2)_L$ triplet scalar, we show that there exist both CP-conserving and CPV dimension five operators involving these new degrees of freedom and the SM $SU(2)_L$ gauge bosons. The former yields kinetic mixing between the $X$ and the neutral $SU(2)_L$ gauge boson (yielding the dark photon), while the latter induces CPV interactions of the dark photon with the SM particles. We discuss experimental probes of these interactions using searches for permanent electric dipole moments (EDMs) and di-jet correlations in high-energy $pp$ collisions. It is found that the experimental limit on the electron EDM currently gives the strongest restriction on the CPV interaction. In principle, high energy $pp$ collisions provide a complementary probe through azimuthal angular correlations of the two forward tagging jets in vector boson fusion. In practice, observation of the associated CPV asymmetry is likely to be challenging.

First observation and study of the K ± → π ± π 0 e + e − decay with the NA48/2 experiment at CERN

The first observation of the rare decay K ± → π ± π 0 e + e − is reported by the NA48/2 experiment at CERN, based on 4919 candidates with 4.9% background contamination. From the analysis of 1.7 × 1011 kaon decays collected in 2003–2004, the branching ratio in the full kinematic region is measured to be BR(K ± → π ± π 0 e + e −)= (4.24 ± 0.14) × 10−6. A detailed study of the kinematic space shows evidence for a structure dependent contribution. Both measured BR and structure dependent contribution are in perfect agreement with theoretical predictions based on Chiral Perturbation Theory. Several P- and CP-violating asymmetries are also evaluated.