Study of the $K_1(1270)-K_1(1400)$ mixing in the decays $B\to J/\Psi K_1(1270), J/\Psi K_1(1400)$

We studied the B meson decays $B\to J/\Psi K_1(1270,1400)$ in the pQCD approach beyond the leading order. With the vertex corrections and the NLO Wilson coefficients included, the branching ratios of the considered decays are $Br(B^+\to J/\Psi K_1(1270)^+)=1.76^{+0.65}_{-0.69}\times10^{-3}, Br(B^+\to J/\Psi K_1(1400)^+)=7.03^{+2.70}_{-2.52}\times10^{-4}$, and $Br(B^0\to J/\Psi K_1(1270)^0)=(1.63^{+0.60}_{-0.64})\times10^{-3}$ with the mixing angle $\theta_{K_1}=33^\circ$, which can agree well with the data or the present experimental upper limit within errors. So we support the opinion that $\theta_{K_1}\sim33^\circ$ is much more favored than $58^{\circ}$. Furthermore, we also give the predictions for the polarization fractions, direct CP violations from the different polarization components, the relative phase angles for the considered decays with the mixing angle $\theta_{K_1}=33^\circ$ and $58^\circ$, respectively. The direct CP violations of the two charged decays $B^+\to J/\Psi K_1(1270,1400)^+$ are very small $(10^{-4}\sim10^{-5})$, because there is no weak phase until up to $\mathcal{O}(\lambda^4)$ with the Wolfenstein parameter $\lambda=0.22537$. These results can be tested at the running LHCb and forthcoming Super-B experiments.


I. INTRODUCTION
B meson exclusive decays into charmonia have been received a lot of attentions for many years. They are regarded as the golden channels in researching CP violation and exploring new physics. At the same time, they play the important roles in testing the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) triangle. Moreover, these decays are ideal modes to test the different factorization approaches. Compared with other factorization approaches, such as the naive factorization assumption (FA) [1], the QCD-improved factorization (QCDF) [2], the perturbtive QCD (pQCD) approach [3] has the unique advantage in solving the B meson charmed decays [4,5]. The Sudakov factor induced by the k T resummation [6] can eliminate the double logarithmic divergences. The jet function induced by the threshold resummation [7] can smear the end-point singularities. Without the divergences, one can evaluate all possible Feynman diagrams correctly, including the nonfactorizable emission diagrams and annihilation type diagrams. But it is difficult to calculate these two kinds of contributions by using other factorization approaches.
Some of the decays B → J/ΨK 1 (1270), J/ΨK 1 (1400) have been measured by Belle [8], where the first uncertainties are statistical and the second are systematic.
In view of the above situation, the motivations are in order: (a) Proving whether the pQCD approach can be used in our considered decays by comparing with the data. Several earlier works on B decays into charmonia [4,17,18] show that this approach can give the results in agreement with data, which encourage our attempt. (b)Exploring the inner structure of the axial vector mesons K 1 (1270, 1400), in other words, detecting which mixing angle shown in Eq.(4) is favored. (c) Studying of CP violation even new physics in these decays containing the charmonium state. Besides the full leading-order (LO) contributions, the next-to-leading-order (NLO) contributions are also included, which are mainly from the NLO Wilson coefficients and the vertex corrections to the hard kernel. Certainly, other NLO contributions, such as the quark loops and the magnetic penguin corrections, are also available in the literature [19,20], while they will not contribute to these considered decays.
We review the LO order predictions for the decays B → J/ΨK 1 (1270), J/ΨK 1 (1400) including those for the main NLO contributions in Section II. We perform the numerical study in Section III, where the theoretical uncertainties are also considered. Section IV is the conclusion. at the leading order. The hard gluon connecting the four quark operator and the spectator quark is necessary to ensure the pQCD applicability. They are the same with those for B + → J/ΨK + 1B . If replacing the spectator u quark with d quark, we will obtain the Feynman Diagrams for the decays

II. THE LEADING-ORDER PREDICTIONS AND THE MAIN NEXT-TO-LEADING ORDER CORRECTIONS
The weak effective Hamiltonian H ef f for the decays B → J/ΨK 1 (1270, 1400) can be written as: where C i (µ) are Wilson coefficients at the renormalization scale µ, V represents for the Cabibbo-Kobayashi-Maskawa (CKM) matrix element, and the four fermion operators O i are given as: with i, j being the color indices. It is convenient to do the calculation in the rest frame of B meson because of the heavy b quark. Throughout this paper, we take the light-cone coordinate (P + , P − , P T ) to describe the meson's momenta with P ± = (p 0 ± p 3 )/ √ 2 and P T = (p 1 , p 2 ). Then the momenta of mesons B, J/Ψ and K 1 can be written as respectively. The mass ratios r 2 = m J/Ψ /m B , r 3 = m K 1 /m B . In the numerical calculation, the terms proportional to r 2 3 are neglected, as r 2 3 ∼ 0.06 is numerically small. Putting the (light) quark momenta in B, J/Ψ, K 1 mesons as k 1 , k 2 , k 3 , respectively, we have There are three kinds of polarizations of a vector or an axial-vector meson, namely longitudinal (L), normal (N) and transverse (T). So the amplitudes for the decay mode B(P 1 ) → V 2 (P 2 , ǫ * 2µ ) + A 3 (P 3 , ǫ * 3ν ) are characterized by those polarization states, which can be decomposed as follows: where M 2(3) is the mass of the vector (axial-vector) meson V 2 (A 3 ). The definitions of the amplitudes M j (j = L, N, T ) in terms of the Lorentz-invariant amplitudes a, b and c are given as: It is noticed that the subscript K 1 refers to the flavor eigenstate K 1A or K 1B . At the leading order, the relevant contributions are only from the factorizable and non-factorizable emission diagrams, as shown in Fig.1. We take the decay B + → J/ΨK + 1A(B) as an example. The emission particle is the vector meson J/Ψ, and the amplitude for the factorizable emission diagrams Fig.1(a) and Fig.1(b) from the longitudinal polarization can be written as: where the color factor C F = 4/3. φ K 1 and φ t,s K 1 are the twist-2 and twist-3 distribution amplitudes for the axial-vector meson K 1A or K 1B , which can be found in Appendix A. The evolution factors evolving the Sudakov factor, the hard function h e and the jet function S t (x) are given in Appendix B. Similarly, the normal and transverse polarization amplitudes are displayed as The longitudinal polarization amplitude for the non-factorizable spectator diagrams Fig.1(c) and Fig.1(d) is given as: where the twist-2 and twist-3 distribution amplitudes ψ L,t (x 2 ) for the J/Ψ meson (Type I) can be found in Appendix A. The other two polarization amplitudes are written as: By combining these amplitudes from the different Feynman diagrams and Eq.(4), one can get the total decay amplitude for the decay B + → J/ΨK 1 (1270) + : where M j and F j (j = L, N, T ) refer to the different helicity amplitudes. The combinations of the Wilson coefficients a 2 = C 1 + C 2 /3, a i = C i + C i+1 /3 with i = 3, 5, 7, 9. As for the decays B + → J/ΨK 1 (1400) + , the total amplitude can be obtained by replacing sin θ K 1 and cos θ K 1 with cos θ K 1 and − sin θ K 1 in Eq. (22), respectively.
Here only the vertex corrections need to be considered in the NLO calculations for the decays B + → J/ΨK + 1A,B . Since the vertex corrections can reduce the dependence of the Wilson coefficients on the renormalization scale µ, they usually play the important roles in the NLO analysis. It is well known that the nonfactorizable amplitude contributions are small [3], we concentrate only on the vertex corrections to the factorizable amplitudes, as shown in Fig.2. Furthermore, the infrared divergences from the soft and the collinear gluons in these Feynman diagrams can be canceled each other. That is to say, these corrections are free from the end-point singularity in the collinear factorization theorem, so we can quote the QCDF expressions for the vertex corrections: their effects can be combined into the Wilson coefficients, with the function f h I (h = 0, ±) defined as: As for the expressions of f I and g I are given in Appendix C. Certainly, the NLO Wilson coefficients will be used in the NLO calculations.

III. NUMERICAL RESULTS AND DISCUSSIONS
We use the following input parameters for the numerical calculations [21,22]: For the CKM matrix elements, we adopt the Wolfenstein parametrization and the updated values A = 0.814, λ = 0.22537,ρ = 0.117 ± 0.021 andη = 0.353 ± 0.013 [23]. With the total amplitudes, one can write the decay width as: where P c is the three momentum of either of the two final state mesons, and the three helicity amplitudes are defined as: for the longitudinal, parallel, and perpendicular polarizations, respectively, and the ratio Then the polarization fractions f σ (σ = L, , ⊥) are written as: With the above transversity amplitudes, one can defined the relative phases φ and φ ⊥ as: For the charged B meson decays, the direct CP violation A dir CP is written as: where A f is the total decay amplitude. If where the first error comes from ω b = 0.4 ± 0.1 GeV for B meson, the second error is from the decay constants f K 1A = 0.250 ± 0.013 GeV and f K 1B = 0.190 ± 0.01 GeV, the third error comes from the Gegenbauer momentums given in Appendix A, and the last one comes from the c quark mass 1.275 ± 0.025 GeV. When the mixing angle is taken as θ K 1 = 33 • , the pQCD prediction for the decay B + → J/ΨK 1 (1270) + can agree well with the experimental measurement (1.80 ± 0.52) × 10 −3 , at the same time, the result for the decay B + → J/ΨK 1 (1400) + is near the experimental upper limit 5.4×10 −4 . So we suggest our experimental colleagues to measure carefully the branching ratio of the decay B + → J/ΨK 1 (1400) + at LHCb. It is helpful to determine the mixing angle θ K 1 between K 1A and K 1B accurately. Considering that the difference of the branching ratios for the neutral and charged decay modes is mainly from the B meson lifes τ B + and τ B 0 , one can obtain easily the branching ratios Br(B 0 → J/ΨK 1 (1270) 0 ) = (1.63 +0.60 −0.64 ) × 10 −3 and Br(B 0 → J/ΨK 1 (1400) 0 ) = (6.52 +2.50 −2.34 ) × 10 −4 for the mixing angle θ K 1 = 33 • . The former is consistent with the experimental value (1.30 ± 0.46) × 10 −3 within errors, and the latter can be tested at the present LHCb experiment. So comparing our predictions and the present data, one can find that the mixing angle θ K 1 = 33 • is much more favored than 58 • . In Fig.3(a), we give the dependences of the branching ratios Br(B + → J/ΨK 1 (1270) + ) and Br(B + → J/ΨK 1 (1400) + ) on the mixing angle θ K 1 . The predictions for the branching ratios of the decays B + → J/ΨK 1 (1270) + and B + → J/ΨK 1 (1400) + near the mixing angle 33 • can explain the data at the same time.
When comparing the LO and NLO results, one can find that the NLO corrections are necessary. The LO branching ratio for the decay B + → J/ΨK 1 (1270) + is about 3.42 × 10 −3 , which is almost two times of the experimental value. After including the NLO contributions, one can find that all of the real parts of the amplitudes decrease consistently (shown in Table I). Furthermore, this downward trend is dominant by comparing with the changes of each imaginary part. So the NLO branching ratio for the decay B + → J/ΨK 1 (1270) + will decrease significantly and converge with the experimental value. While the branching ratio of the decay B + → J/ΨK 1 (1400) + has a tiny increase compared with the LO result 6.38 × 10 −4 with the mixing angle θ K 1 = 33 • .
Certainly, the mixing angle θ K 1 has also been checked in other B meson decays. For example, the charged decays B + → φK 1 (1270) + and B + → φK 1 (1400) + have been measured by BaBar Collaboration [24] with the branching ratio (6.1±1.9)×10 −6 and an upper limit 3.2 × 10 −6 , respectively. In order to explaining these data, many works support the smaller mixing angle (∼ 33 • ) although suffering severe interference from the annihilation type contributions. The authors of Refs. [25,26] found that the theoretical predictions for the decay B + → φK 1 (1270) + could explain the data by taking θ K 1 ∼ 33 • , while the values of Br(B + → φK 1 (1400) + ) arrived at 10 −5 order and would overshoot the upper limit greatly. In Ref. [27] the authors studied these two charged decays within the generalized factorization approach (GFA). With the annihilation type contributions turned off, their predictions about these two channels could agree with the data with N ef f c = 5 being the effective color number containing the nonfactorizable effects. The similar situation also happened in the decays B + → K 1 (1270) + γ and B + → K 1 (1400) + γ. In Ref. [28] the authors explained well the data Br(B + → K 1 (1270) + γ) = (4.3 ± 1.3) × 10 −5 and Br(B + → K 1 (1400) + γ) < 1.5 × 10 −5 with θ K 1 = (34 ± 13) • . Among of these decays B → K 1 (1270, 1400)V (V refers to a vector meson or a photon), the branching ratios of decays B → K 1 (1270)V are always larger than those of decays B → K 1 (1400)V , because of the constructive (destructive) interference between the modes B → K 1A V and B → K 1B V through Eq.(4) for the former (latter).
We also calculate the polarization fractions f σ (σ = L, , ⊥), the direct CP violations A dir,L CP , A dir, CP , A dir,⊥ CP from the different polarization components, and the relative phases φ ,⊥ defined in Eqs. (32)(33)(34), respectively. The results for the decay B + → J/ΨK 1 (1270) + are listed in Table II and for the decay B + → J/ΨK 1 (1400) + in Table III. Comparing with the longitudinal polarization fractions for the decays B + → J/ΨK 1 (1270) + and B + → J/ΨK 1 (1400) + , we find that the former decreases monotonically with the increase of the mixing angle θ K 1 from 0 • to 90 • , while the latter decreases firstly then increases within The direct CP violation from the longitudinal component is much smaller than those from the two transverse components for the decay B + → J/ΨK 1 (1270) + . As for the dependences of the total direct CP violations for these two charged decays on the mixing angle θ K 1 are shown in Fig.3(b). The total direct CP violation values corresponding to the mixing angle θ K 1 = 33 • and 58 • are listed as following: The NLO predictions for the polarization fractions (f L , f , f ⊥ ), the direct CP violations from the different polarization components, and the relevant phase angles (φ , φ ⊥ ) for the decays B + → J/ΨK 1 (1270) + with the mixing angle θ K 1 = 33 • and 58 • . The first uncertainty comes from the ω b = 0.4 ± 0.1 for B meson, the second and the third uncertainties are from the decay constants f K 1A and f K 1B and the Gaigenbuar momentums in the wave functions of K 1A and K 1B . The last one comes from c quark mass 1.275 ± 0.025 GeV.   III: Same as Table II but

Decay Mode
where the errors are the same with those in Eqs.(35) and (36). We adopt the Wolfenstein parametrization up to O(λ 4 ) in our calculations. The weak phase will appear in the CKM    matrix element V cs = −Aλ 2 + 1 2 Aλ 4 (1 − 2(ρ + iη)), where these Wolfenstein parameters are given at the start of this section. So such small CP asymmetries are in accordance with our expectation.
In order to check whether the results are sensitive to the wave functions (WFs) of J/Ψ meson, we also calculate them by using the harmonic-oscillator type wave functions for the J/Ψ meson, which are listed in Appendix A. The results for the decays B + → J/ΨK 1 (1270) + and B + → J/ΨK 1 (1400) + are given in Table IV and Table V, respectively. Through comparing these two sets of results corresponding the two type WFs of J/Ψ meson, we can see that • The branching ratios will decease about 30% by using the harmonic-oscillator type wave functions of J/Ψ meson except for that of the decay B + → J/ΨK 1 (1400) + with mixing angle θ = 58 • , but anyway they keep in the same order by changing the wave functions for J/Ψ meson.
• For the decay B + → J/ΨK 1 (1400) + , the polarization fractions are sensitive to the wave functions of J/Ψ meson. If taking the mixing angle θ = 33 • , the longitudinal component is less than the transverse components by using Type I WFs, but it is contrary in the case of using the harmonic-oscillator type WFs. If taking the mixing angle θ = 58 • , the longitudinal polarization fraction is close to the sum of other two transverse polarization fractions in Type I WFs, while the longitudinal polarization component is more dominant than the transverse ones in the harmonic-oscillator type WFs.
• In most cases, the values of these two relative strong phases are similar to each other in each decay mode. But for the case of the decay B + → J/ΨK 1 (1400) + with the mixing angle θ = 58 • , the relative strong phases φ and φ ⊥ are with opposite signs. It is valuable for us to determine the mixing angle by measuring these relative phases from the future experiments.
• In most cases, the values of the direct CP asymmetries are in the order of 10 −4 by using both of these two type WFs of J/Ψ meson. But still for the case of the decay B + → J/ΨK 1 (1400) + with the mixing angle θ = 58 • , there is a smaller direct CP violation value.

IV. SUMMARY
We study the B meson decays B → J/ΨK 1 (1270, 1400) in the pQCD approach beyond the leading order. With the vertex corrections and the NLO Wilson coefficients included, the branching ratios of the considered decays are Br(B + → J/ΨK 1 (1270) + ) = 1.76 +0.65 −0.69 × 10 −3 , Br(B + → J/ΨK 1 (1400) + ) = 6.47 +2.50 −2.34 × 10 −4 , and Br(B 0 → J/ΨK 1 (1270) 0 ) = (1.63 +0.60 −0.64 ) × 10 −3 with the mixing angle θ K 1 = 33 • . These results can agree well with the data or the present experimental upper limit within errors. So we support the opinion that θ K 1 ∼ 33 • is much more favored than 58 • . We suggest our experimental colleagues to measure carefully the branching ratio of the decay B + → J/ΨK 1 (1400) + at LHCb. It is important to determine the mixing angle θ K 1 between K 1A and K 1B accurately. On the experimental side, we find that the branching ratios of the decays B → K 1 (1270)V (V refers to a vector or a photon) are usually much larger than those of B → K 1 (1400)V . It is because of the constructive (destructive) interference between B → K 1A V and B → K 1B V for the former (latter). In order to check the dependence of our predictions on the wave functions of J/Ψ meson, we also give the results by using the harmonic-oscillator type wave functions for the J/Ψ meson, and find that these two type WFs can give the consistent results in most cases, while some values are sensitive to the type of wave functions of the J/Ψ meson.
where K 1 refers to the flavor state K 1A or K 1B , and the corresponding distribution functions can be calculated by using light-cone QCD sum rule and listed as following: The upper formulas are for the longitudinal polarization wave functions, and the transverse polarization ones are given as: where the Gegenbauer moments are given as [22,32]: The evolution factors are given by: where the hard scales (t) are chosen as: The Sudakov exponents are defined as: where the quark anomalous dimension is γ q = −α s /π, and the expression of the s(Q, b) in one-loop running coupling coupling constant is listed as: here the variables are defined byq = ln[Q/( √ 2Λ)],b = ln[1/(bΛ)] and the coefficients A (1,2) and β 1 are given as: where n f is the number of the quark flavors and γ E the Euler constant.