Abstract
The inclusion of the mixing effect is essential for a precise description of the pion electromagnetic form factor in the process, which quantifies the two-pion contribution to the anomalous magnetic moment of muon . In this study, we analyze the momentum dependence of mixing by considering loop contributions at the next-to-leading order in expansion within the framework of resonance chiral theory. We revisit a previous study [Y. H. Chen, D. L. Yao, and H. Q. Zheng, Commun. Theor. Phys. 69, 1 (2018)] and consider the contribution arising from the kaon mass splitting in the kaon loops and latest experimental data. We perform two types of fits (with momentum-independent or momentum-dependent mixing amplitude) to describe and data within the energy region of 600900 MeV and decay width of . Furthermore, we compare their results. Our findings indicate that the momentum-independent and momentum-dependent mixing schemes provide appropriate descriptions of the data. However, the momentum-dependent scheme exhibits greater self-consistency, considering the reasonable imaginary part of the mixing matrix element obtained. Regarding the contribution to the anomalous magnetic moment of the muon, , the results obtained from the fits considering the momentum-dependent mixing amplitude are in good agreement with those obtained without incorporating the momentum dependence of mixing, within the margin of errors. Furthermore, based on the fitted values of the relevant parameters, we observe that the decay width of is predominantly influenced by the mixing effect.
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I. INTRODUCTION
The anomalous magnetic moment of a muon, denoted as , plays a crucial role in the precision tests of the Standard Model (SM) [1, 2]. The long-standing discrepancy between the SM prediction of and its experimental measurement has recently been updated to 4.2 standard deviations [3, 4], and it has sparked numerous theoretical investigations. The SM uncertainty on is dominated by hadronic vacuum polarization (HVP), with the largest contribution originating from the intermediate states, accounting for over 70% of the HVP contribution. Theoretically, the two-pion low-energy contribution to is expressed as an integral over the modulus squared of the pion electromagnetic form factor, which can be extracted from -annihilation experiments. In principle, the two-pion contribution to can be evaluated accurately as long as the experimental data of are available everywhere at the required level of precision. Although it is known that a tension exists between the two most precise measurements by BaBar and KLOE Collaborations, the BaBar data lie systematically above the KLOE results in the dominant ρ region. Consequently, considerable efforts have been dedicated to finely describing the pion electromagnetic form factor [5−11]. In the dominant ρ region of the process, the isospin-breaking effect due to mixing, which becomes enhanced by the small mass difference between ρ and ω mesons, plays a significant role and should be considered appropriately.
Usually, the momentum dependence of mixing amplitude is neglected, and a constant mixing amplitude is used to describe data due to the narrowness of the ω resonance. The first study on the momentum dependence of mixing amplitude was conducted by Ref. [12]. Based on a quark loop mechanism of mixing, it was determined that the mixing amplitude significantly depends on momentum. Subsequently, the investigation of various loop mechanisms for mixing was initiated in different models, such as the global color model [13], extended Nambu-Jona-Lasinio (NJL) model [14, 15], chiral constituent quark model [16, 17], and hidden local symmetry model [18−20]. In our pervious study [21], we examined mixing using a model independent approach through Resonance Chiral Theory (RχT) [22]. Guided by chiral symmetry and large expansion, RχT provides us a reliable theoretical framework to study the dynamics with light flavor resonances and pseudo-Goldstone mesons in the intermediate energy region [23−28], and it has been successfully applied in the calculation of in the SM [9, 29−36]. In Ref. [21], we calculated the one-loop contributions to mixing, which are at the next-to-leading order (NLO) in the expansion [28, 37−40]. In this study, we update the previous study [21] by incorporating the contribution arising from the kaon mass splitting in the kaon loops.
Moreover, we focus on analysing the impact of the momentum dependence of mixing on describing the pion vector form factor data and its contribution to . Specifically, we perform two types of fits (with momentum-independent or momentum-dependent mixing amplitude) describing and and data in the energy region of 600–900 MeV, decay width of , and compare their results. The fit results demonstrate that the momentum-independent and momentum-dependent mixing schemes can effectively describe the data, while the momentum-dependent scheme exhibits higher self-consistency due to the reasonable imaginary part of the extracted mixing matrix element . Regarding the contribution to the anomalous magnetic moment of a muon, , which is evaluated between 0.6 GeV and 0.9 GeV, the results obtained from fits considering the momentum-dependent mixing amplitude are in good agreement with those from fits that do not include the momentum dependence of mixing, within the margin of errors.
This paper is organized as follows. In Sec. II, we introduce the description of mixing and elaborate on the calculation of mixing amplitude up to the next-to-leading order in the expansion. In Sec. III, the fit results are shown and related phenomenologies are discussed. A summary is provided in Sec. IV.
II. CALCULATIONS IN RESONANCE CHIRAL THEORY
In the isospin basis , we define the pure isospin states and . The mixing between the isospin states of and can be implemented by considering the self-energy matrix
with and . The none-zero off-diagonal matrix element contains information on mixing. The mixing between the physical states of and ω, is obtainable by introducing the following relation
where and denote the mixing parameters. The matrix of dressed propagators corresponding to physical states is diagonal [41],
where abbreviations and are defined by the following:
The information of mixing is encoded in the off-diagonal element of the self-energy matrix, decomposed as follows:
where denotes the mass difference between u and d quarks, and α denotes the fine-structure constant. and denote the structure functions of the strong and electromagnetic interactions, respectively. In this study, the diagrams in Fig. 1 are calculated in RχT up to NLO in expansion.
In RχT, the vector resonances can be described in terms of antisymmetric tensor fields with normalization:
where denotes the polarization vector. Here, the vector mesons are collected in a matrix as follows:
The effective Lagrangian for the leading order strong isospin-breaking effect, corresponding to the tree-level contribution diagram (a) in Fig. 1, is as follows [42, 43]:
with and . The pseudo-Goldstone bosons originating from the spontaneous breaking of chiral symmetry can be filled nonlinearly into:
with the Goldstone fields
where F denotes the pion decay constant. Considering the mass relations of the vector mesons at in terms of the quark counting rule, the value of the coupling constant is determined as follows: [42, 43]. Thus, the tree-level strong contribution can be expressed as follows:
The Lagrangian describing the interactions between and electromagnetic fields or Goldstone bosons are as follows:
with the relevant building blocks defined as follows:
Here, denote field strength tensors composed of the left and right external sources and , and and denote real resonance couplings constants. The tree-level electromagnetic contribution from diagram (b) in Fig. 1 can be calculated using the Lagrangian in Eq. (12):
The physical decay constants and have been employed in the amplitude, and are differentiated by means of isospin breaking.
The loop contributions of diagrams (d)–(i) in Fig. 1 have been extensively discussed in our previous study [21]. However, a noteworthy distinction in our current study is the inclusion of the contribution from diagram (c), which arises from the kaon mass splitting within the kaon loops. To ensure comprehensiveness, we present the expressions for the loop contributions in the Appendix A. Furthermore, it should be noted that the ultimate expression for the renormalized mixing amplitude is presented in Eq. (A23).
III. PHENOMENOLOGICAL DISCUSSION
The mass and width of the ρ meson are conventionally determined by fitting to the experimental data of and [44], where various mechanisms are used to describe mixing effect. To prevent interference due to their mixing mechanisms, we treat mass and relevant couplings and as free parameters in our fit. Regarding its width, the energy-dependent form is constructed in a similar manner as introduced in [45]:
where and is the step function.
With respect to the ω mass, it has been indicated in Refs. [5, 7] that the result determined from is inconsistent with that from particle data group (PDG) [44], primarily determined by experiments involving and . Therefore, we treated the ω mass and width as free parameters and estimated them by fitting in our programme. The physical coupling can be determined from the decay width of . Using the Lagrangian formula in Eq. (12), the decay width can be derived as follows:
Hence, the expression for can be obtained. Based on the decay widths provided above, and in Eq. (4) can be rewritten as
The pion form-factor in decay, irrelevant to mixing effect, were thoroughly examined in Refs. [36, 46−48]:
Furthemorme, function
To incorporate isospin-breaking effects, one approach involves multiplying by factor , where corresponds to the short distance correction [49]. Additionally, accounts for the long-distance radiative correction, as described in [50]. Specifically, in our fit of decay data, we perform the following substitution.
The pion form-factor in annihilation is as follows:
As defined in Appendix A.2, parameter a is associated with the combined coupling constant of the direct interaction. In the first bracket of Eq. (21), the second term corresponds to the contribution from coupling, third term represents the contribution of mixing, and fourth term corresponds to contribution from the direct isospin-breaking coupling of ω to the pion pair.
The leading order contribution of intermediate state to the anomalous magnetic moment of the muon is as follows [51]:
where
and the kernel function is defined as follows:
with
It should be noted that in the formula for in Eq. (22), the integration is performed from 4 to . In this study, we focus on the momentum dependence of mixing. Therefore, we only describe the pion vector form factor up to 900 MeV. To extend the study by considering higher energies, we must consider the effects of excited resonances, including and . However, these effects are beyond the scope of this study. It is interesting to note that the enhancement factor in Eq. (22) provides higher weight to the lowest lying resonance that couples strongly to .
The bare cross section, including final-state radiation, takes the following form [5, 52−54]:
where
The experimental data considered in this study are the pion form factor of the process measured by the OLYA [55], CMD [56], BaBar [57], BESIII [58], KLOE [59], CLEO [60], and SND [61] Collaborations, the form factor of decay measured by the ALEPH [62] and CLEO [63] Collaborations, and the decay width of [44]. It should be noted that in the experimentally published form factor data , the vacuum polarization effects have been excluded through the subtraction of the hadronic running of . Thus, in our fitting of the form factor data , the one-photon-reducible Fig. 1(b) should not be considered. Given that we focus on the analysis of the mixing effect, we only take into account the form factors and data in the energy region of 600–900 MeV. It should be noted that for the pion form factor , a tension is observed between the two most precise measurements from BaBar and KLOE in the ρ peak region. However, other measurements align with theirs within the stated uncertainties. This highlights the impact of momentum dependence of mixing, and to avoid the tension between BaBar and KLOE data, we conduct four separate fits. Specifically, in Fits Ia and Ib, we fit all data sets excluding BaBar with momentum-independent and momentum-dependent , respectively. In Fits IIa and IIb, we fit all data sets excluding KLOE with momentum-independent and momentum-dependent , respectively. Fits Ia and IIa involve eight free parameters: , , , , , a, and the real and imaginary part of constant . There are nine free parameters in Fits Ib and IIb: , , , , , a, , , and . As defined in the Appendix A, , , and are the corresponding parameters for the counterterms.
In Fig. 2, the fitted results of the fits using momentum-independent (Fits Ia and IIa) and momentum-dependent (Fits Ib and IIb) are shown as red dotted lines and black solid lines, respectively. The fitted parameters as well as are listed in Table 1. It is intriguing to compare the results obtained from fits utilising momentum-independent and momentum-dependent for the same datasets. Specifically, we compare Fit Ia and Ib and Fit IIa and IIb. When examining pion form factors and , we observe that the differences between the theoretical predictions of the fits using momentum-independent and the corresponding ones using momentum-dependent are low. Furthermore, it should be noted that for the pion form factor in Fits Ia and Ib, the theoretical predictions are much higher than the KLOE data at ρ peak, and these deviations contribute significantly to their value of . Thus, we conclude that the momentum-independent and momentum-dependent can describe the data well, and the discordances among different collaborations contribute significantly to values in the fits.
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Standard imageTable 1. Fit results of the parameters. Fits Ia and Ib fit all data sets excluding BaBar, and Fits IIa and IIb fit all data sets excluding KLOE. Fits Ia and IIa use momentum-independent , while Fits Ib and IIb use momentum-dependent .
Fit Ia | Fit Ib | Fit IIa | Fit IIb | |
---|---|---|---|---|
/MeV | ||||
/MeV | ||||
/MeV | ||||
/MeV | ||||
/MeV | ||||
− | − | |||
− | − | |||
− | − | |||
− | − | |||
− | − | |||
In the last line of Table 1, we provide the results of , evaluated between 0.6 GeV and 0.9 GeV. The differences between the results using the momentum-independent and the results using the momentum-dependent for the same datasets, namely the differences between Fits Ia and Ib and Fits IIa and IIb, respectively, are negligible.
In Fig. 3, we plot the real and imaginary parts of the mixing amplitudes in Fits Ib and IIb. It is determined that the real part is dominant within mixing region. The real part in Fit IIb demonstrates a significant momentum dependence, whereas the real part in Fit Ib displays a smooth momentum dependence. Additionally, it should be noted that the real parts of the two fits nearly reach the same point at . In comparison to the real part, the imaginary part is rather small. At , in Fit Ib the mixing amplitude MeV, and in Fit IIb MeV. The minimal magnitude of the imaginary part aligns with the findings presented in Refs. [64, 65]. However, therein the effect of direct was not considered. It is worth mentioning that larger imaginary part is obtained in [13, 17] by using global color model and a chiral constituent quark model, respectively. By utilising our fitted parameter results, we proceed to calculate the ratio of the two-pion couplings associated with the isospin-pure ω and ρ.
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The results are in Fit Ib, and in Fit IIb. It should be noted that the value of G is expected to be of the order in Ref. [65]. The central values of our results of G are in good agreement with the expectation in Ref. [65], while they are lower than other two estimations, namely in [66] and in [67]. As listed in Table 1, the differences in between the momentum-dependent fits and momentum-independent fits for the same data sets are minimal. The of Fit IIa is slightly lower than the of Fit IIb. However, Fit IIa contains one less fitting parameter than Fit IIb. It can be observed that the magnitude of the imaginary part of in Fit IIa is significantly greater than those in other three fits. In our framework, the imaginary part of arises from and real intermediate states. By considering the decay widths of and , the imaginary part of , contributed from intermediate state, can be estimated to be approximately −150 MeV2 [7, 65]. If the estimated ratio of the two-pion couplings of the isospin-pure ω and ρ are used: [65], then the intermediate state contribution to the imaginary part of can be obtained in the order of several hundred MeV2. In our momentum-dependent scheme, the imaginary part of due to and intermediate states are explicitly computed, and the numerical results of Im in Fits Ib and IIb are of the order of one hundred MeV2, as expected. However, in the momentum-independent Fits Ia and IIa, the imaginary part of is a free fitting parameter. As listed in Table 1, the fitted results of Im and parameter "a" in Fit IIa are unreasonably high. The fitted results of Im and parameter "a" in Fit Ia exhibit large error bars. Consequently, we conclude that both momentum-independent and momentum-dependent mixing schemes can describe data well. However, the momentum-dependent mixing scheme is more self-consistent, especially given the reasonable imaginary part of , which is extracted.
We wish to emphasize that the direct coupling is generally an unknown quantity, and it impacts in two ways, both through the third term in the first bracket of Eq. (21), appearing as real intermediate state in the contributions to and through the fourth term in that bracket. Conventionally, the direct is assumed to be neutralized in due to the fact that ω and ρ are quasidegenerate and that 2π channel dominates the ρ decay [65]. Theoretical models that do not neglect direct coupling may be more comprehensive, especially given the availability of high-precision data available currently. It should be noted that in Refs. [5, 7], the pion form factor has been examined in a model-independent way using dispersion theory. Specifically, mixing is subsumed in one parameter , which should contain a small imaginary part originating from the radiative intermediate states (with an estimated phase of approximately 4 degrees). Furthemrore, given that direct coupling is not considered in Refs. [5, 7], term is actually a combination of mixing and direct . Therefore, it cannot be directly compared to discussed in this context. (At , our in Fits Ib and IIb contains negative phase.) Nevertheless, the ratio between the on-ω-mass-shell transition amplitude and transition amplitude (without final state interaction) should be model independent. With , the ratio between the second term and first term in Eq. (2.5) of [7] yields , using Re and obtained therein. It can be observed that the difference between the phase of and is minimal. In this study, the ratio between the sum of the third term and fourth terms and the sum of the first and second terms in the first bracket of Eq. (21) predicts and in Fit Ib and IIb, respectively. It can be observed that our results of approximately agree with that in [7].
Using the central values of the fitted parameters of our best fit (Fit IIb) in Table 1, we calculate the decay width of
Based on Eq. (29), we can determine that the first term due to direct is smaller than the second term due to mixing by an order of magnitude. Within 1σ uncertainties, our theoretical value of the branching fraction is , which is consistent with the values provided in PDG [44] and with those reported in the recent dispersive analysis [68].
Regarding the mass of the ω meson, previous studies [5, 7, 57] indicated that the result extracted from is substantially lower than the current PDG average [44], which primarily relies on and experiments. The discrepancy amounts to approximately 1 MeV, corresponding to around 5 σ considering the current precision. It has been observed that the fitted value for and phase of are strongly correlated [5, 7, 57]. It should be noted that direct coupling has not been considered in [5, 7, 57]. As indicated in Table 1 above, our fitted results for the mass of ω are in good agreement with the value in PDG: MeV, and this agreement remains unaffected by the inclusion or exclusion of the momentum dependence of . Furthermore, we can observe that a strong correlation (80%) exists between parameter "a," which quantifies the direct coupling, and the mass of ω. As mentioned earlier, the direct coupling influences the imaginary part and real part of the amplitude, and thereby, affects the phase of . It should be noted that the phase of approximately agrees with the phase of . Thus, our observations align with with those in Refs. [5, 7, 57]. Hence, a strong correlation exists between the mass of the omega meson and phase of . Our findings suggest that the inclusion of direct coupling is likely crucial in the analysis aimed at extracting the ω mass from the process.
IV. CONCLUSIONS
We utilized the resonance chiral theory to examine mixing. Specifically, we analyzed the impact of the momentum dependence of mixing on describing the pion vector form factor in the process and its contribution to the anomalous magnetic moment of muon . The incorporation of momentum dependence arises from the calculation of loop contributions, which corresponds to the next-to-leading orders in expansion. Based on fitting to the data of and processes within the energy range of 600–900 MeV and decay width of , we determine that the mixing amplitude is dominated by its real part, and its imaginary part is relatively small. Although momentum-independent and momentum-dependent mixing schemes yield satisfactory data descriptions, the latter proves to be more self-consistent due to the reasonable imaginary part of the mixing matrix element . Regarding the contribution to anomalous magnetic moment of muon , the results obtained from fits considering the momentum-dependent mixing amplitude align well with those from corresponding fits that exclude the momentum dependence of mixing, within the margin of error. Additionally, we provide the ratio of the isospin-pure ω and ρ two-pion couplings, denoted as , and observe that mixing plays a crucial role in the decay width of . Furthermore, we ascertain that including direct coupling is essential in analyzing the extraction of the mass of the ω meson from the process.
ACKNOWLEDGMENTS
We are grateful to Pablo Roig for helpful discussions and valuable suggestions.
APPENDIX A: LOOP CONTRIBUTIONS
1. Diagram (c): kaon loops
Using and vertexes obtained via the Lagrangian in Eq. (12): , we can calculate the charged and neutral kaon loops contribution to amplitude
and
where and with and is the Euler constant.
The persistence of a non-zero structure function arises from the mass difference between the charged and neutral kaons as described:
2. Diagram (d): loop
For the isospin-violating vertex of , we construct the Lagrangian:
For convenience, we define the combination The -loop contribution to the structure function can be calculated as follows:
3. Diagram (e): π-tadpole loop
According to the Lorentz, P and C invariances, the Lagrangian corresponding to the interaction of can be constructed as follows:
For simplicity, we define the combinations,
The mass difference between the charged and neutral pions in the internal lines of loops can be disregarded due to its higher-order magnitude beyond our scope of consideration. Consequently, the expanded expression of Lagrangian (A6) can be simplified as follows:
With the aforementioned Lagrangian, the π-tadpole contribution to mixing can be derived as follows:
4. Diagrams (f)-(i): loops
In the loop diagrams (f)-(i), the resonance chiral effective Lagrangian describing vector-photon-pseudoscalar (VJP) and vector-vector-pseudoscalar (VVP) vertices are provided in Ref. [69]:
and
The couplings involved, or their combinations, can be estimated by matching the leading operator product expansion of Green function to the same quantity evaluated within RχT. This procedure leads to high energy constraints on the resonance couplings [69]:
Using the the effective vertices stated in Eqs. (A10) and (A11), theloop contribution, i.e., the summation of the loops diagrams (f)-(i), can be expressed as
The subsequent calculation is straightforward. However, the result of the extracted electromagnetic structure function is too extensive to present here. It should be noted that in our numerical computation we employ the high energy constraints in Eq. (A12) along with the fitted parameters provided in Ref. [25]. Therefore, all the parameters involved in are known.
5. Counterterms and renormalized amplitude
Given that the ω meson predominantly decays into the three-pion state, its two-loop self energy diagram contributes beyond NLO in and is not relevant for our current consideration. The self-energy diagrams for ρ meson are depicted in Fig. A1. The Lagrangian required to renormalize ρ meson one-loop self-energy has been provided in Ref. [38],
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Specifically, only the combination of couplings is relevant for this purpose. Using Lagrangians in Eqs. (12) and (A14), ρ self-energy takes the form:
The renormalized ρ mass fulfills:
Given that physical is finite, the following holds:
The wave-function renormalization constant of ρ meson is obtained from:
In our calculation of mixing, the tree amplitudes can only absorb the ultraviolet divergence that is proportional to . To neutralize , , and ultraviolet divergence originating from loop contribution , , , and , we construct the counterterms as follows:
We adopt subtraction scheme and absorb the divergent pieces proportional to by the bare couplings in the counterterms. Consequently, the remanent finite pieces of counterterms can be expressed as
with
In summary, at the NLO in , the UV-renormalized mixing amplitude is as follows:
where a bar denotes that the divergences are subtracted.
As discussed in Ref. [41], the mixing amplitude should vanish as . Thus, the final expression of the renormalized mixing amplitude is obtained as follows:
where an additional finite shift is imposed to guarantee that constraint is satisfied. It should be noted that due to the finite shift performed in Eq. (A23), our numerical calculation is actually independent of the coupling . In our numerical computation, scale μ will be set to , and we use MeV provided by PDG [44].
Footnotes
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Supported in part by the Fundamental Research Funds for the Central Universities (FRF-BR-19-001A), and the National Natural Science Foundation of China (11975028, 11974043)