###### Abstract

In this article, we calculate the electromagnetic form-factor of the meson with the light-cone QCD sum rules. The numerical value is in excellent agreement with the experimental data (extrapolated to the limit of zero momentum transfer, or the normalization condition ). For large momentum transfers, the values from the two sum rules are all comparable with the experimental data and theoretical estimations.

Electromagnetic form-factor of the meson with light-cone QCD sum rules

[2mm]
Zhi-Gang Wang ^{1}^{1}1 E-mail,wangzgyiti@yahoo.com.cn. , Zhi-Bin Wang

Department of Physics, North China Electric Power University, Baoding 071003, P. R. China

College of Electrical Engineering, Yanshan University, Qinhuangdao 066004, P. R. China

PACS numbers: 12.38.Lg; 13.40.Gp

Key Words: Electromagnetic form-factor, Light-cone QCD sum rules

## 1 Introduction

The meson, as both Nambu-Goldstone boson and quark-antiquark
bound state, plays an important role in testing the quark models and
exploring the low energy QCD. Its electromagnetic form-factor and
electromagnetic radius are important parameters, and have been
extensively studied both experimentally
[1, 2, 3, 4, 5, 6, 7] and theoretically, for
examples, the QCD sum rules
[8, 9, 10, 11], the
light-cone QCD sum rules [12, 13, 14, 15],
perturbative QCD
[16, 17, 18, 19, 20, 21],
Schwinger-Dyson equation [22, 23, 24], etc^{2}^{2}2In
Ref.[16], Radyushkin introduces the distribution
amplitude of the meson for the first time, expresses the
form-factor of the meson in terms of the distribution
amplitudes asymptotically, and formulates the perturbative QCD parton picture
for hard exclusive processes. .

In Refs.[12, 13, 14, 15], the axial-current is used to interpolate the meson, in Refs.[13, 14], the radiative corrections and higher-twist effects are taken into account. In this article, we choose the pseudoscalar current to interpolate the meson and calculate the electromagnetic form-factor of the meson with the light-cone QCD sum rules. In our previous works, we have studied the vector form-factors and scalar form-factors of the and mesons, the form-factors of the nucleons, and obtain satisfactory results [25, 26, 27, 28, 29]. The light-cone QCD sum rules carry out the operator product expansion near the light-cone instead of short distance , while the non-perturbative matrix elements are parameterized by the light-cone distribution amplitudes (which classified according to their twists) instead of the vacuum condensates [30, 31, 32, 33, 34, 35, 36]. The non-perturbative parameters in the light-cone distribution amplitudes are calculated with the conventional QCD sum rules and the values are universal [37, 38].

The article is arranged as: in Section 2, we derive the electromagnetic form-factor with the light-cone QCD sum rules; in Section 3, the numerical result and discussion; and in Section 4 is reserved for conclusion.

## 2 Electromagnetic form-factor of the meson with light-cone QCD sum rules

In the following, we write down the definition for the electromagnetic form-factor ,

(1) |

where the is the electromagnetic current and . We study the electromagnetic form-factor with the two-point correlation function ,

(2) |

where we choose the pseudoscalar current to interpolate the meson. The correlation function can be decomposed as

(3) |

due to Lorentz covariance. In this article, we derive the sum rules with the tensor structures and , respectively.

According to the basic assumption of the current-hadron duality in the QCD sum rules approach [37, 38], we can insert a complete series of intermediate states with the same quantum numbers as the current operator into the correlation function to obtain the hadronic representation. After isolating the ground state contribution from the pole term of the meson, the correlation function can be expressed in the following form,

(4) | |||||

where we introduce the indexes and to denote the electromagnetic form-factor from the tensor structures and respectively, and we use the standard definition for the decay constant ,

In the following, we briefly outline
the operator product expansion for the correlation function
in perturbative QCD theory. The calculations are
performed at the large space-like momentum regions and , which correspond to the small light-cone
distance required by validity of the operator
product expansion approach^{3}^{3}3In the frame where the
meson has a finite 3-vector , , the and can be approximated as
and
, where
, we obtain the relation and .
, we take the values and to avoid
strong oscillation, . For more
details, one can consult Ref.[36] . We write down the
propagator of a massive quark in the external gluon field in the
Fock-Schwinger gauge firstly [39],

(5) |

where the is the gluonic field strength. Substituting the above and quark propagators and the corresponding meson light-cone distribution amplitudes into the correlation function , and completing the integrals over the variables and , finally we obtain the representation at the level of quark-gluon degrees of freedom,

(6) |

the explicit expressions of the and are given in the appendix. In calculation, we have used the two-particle and three-particle light-cone distribution amplitudes of the meson [30, 31, 32, 33, 34, 39, 40, 41, 42, 43], the explicit expressions of the light-cone distribution amplitudes are also presented in the appendix. The parameters in the light-cone distribution amplitudes are scale dependent and estimated with the QCD sum rules [30, 31, 32, 33, 34, 39, 40, 41, 42, 43]. In this article, the energy scale is chosen to be .

We take the Borel transformation with respect to the variable for the correlation functions and . After matching with the hadronic representation below the threshold, we obtain the following two sum rules for the electromagnetic form-factors and respectively,

(8) | |||||

where

(9) |

and the is threshold parameter.

## 3 Numerical result and discussion

The input parameters of the light-cone distribution amplitudes are taken as , , , , , , , [30, 31, 32, 33, 34, 39, 40, 41, 42, 43]; and , , . The threshold parameter is chosen to be , which can reproduce the value of the decay constant in the QCD sum rules.

In this article, we take the values of the coefficients of the twist-2 light-cone distribution amplitude from the conventional QCD sum rules [40, 43]. The has been analyzed with the light-cone QCD sum rules and (non-local condensates) QCD sum rules confronting with the high precision CLEO data on the transition form-factor [44, 45, 46, 47, 48, 49]. We also study the electromagnetic form-factors and with the values and at GeV, which are obtained via one-loop renormalization group equation for the central values and at from the (non-local condensates) QCD sum rules with improved model [49].

The Borel parameters in the two sum rules are taken as , in this region, the values of the electromagnetic form-factors and are rather stable. In this article, we take the special value in numerical calculations, although such a definite Borel parameter cannot take into account some uncertainties, the predictive power cannot be impaired qualitatively.

In the two sum rules in Eqs.(7-8), the dominant contributions come from the two-particle twist-3 light-cone distribution amplitudes and due to the pseudoscalar current . The different values of the coefficients of the obtained in Ref.[43] and Ref.[49] respectively can lead to results of minor difference. If we choose the axial-vector current to interpolate the meson, the main contributions come from the twist-2 light-cone distribution amplitude [17, 18, 19, 20, 21]. The uncertainties concerning the denominator are canceled out with each other, see Eqs.(7-8), which result in small net uncertainties.

Taking into account all the uncertainties, finally we obtain the numerical values of the electromagnetic form-factors and , which are shown in the Figs.1-2, at zero momentum transfer,

(10) |

the parameters of the twist-2 light-cone distribution amplitude obtained in Ref.[49] can reduce the values of the form-factors and slightly, about .

Comparing the experimental data (extrapolated to the limit , or the normalization condition ) [1, 2, 3, 4, 5, 6, 7] and theoretical estimation with the vector meson dominance theory [50], our numerical value is excellent. The value is too large to make any reliable prediction, however, it is not un-expected. From the two sum rules, we can see that the terms of the are companied with an extra factor , for example,

where we have taken the asymptotic distribution amplitude . The value of the is greatly enhanced in the region of small- due to the extra , in the limit , , the dominant contributions come from the end-point of the light-cone distribution amplitudes. We should introduce extra phenomenological form-factors (for example, the Sudakov factor [18, 19]) to suppress the contribution from the end-point. The value of the is more reliable at small momentum transfers.

In the light-cone QCD sum rules, we carry out the operator product
expansion near the light-cone , which corresponds to
and , the two sum rules for the form-factors
and are valid at large momentum transfers.
We take the analytical expressions of the and in Eqs.(7-8)
as some functions which model the electromagnetic
form-factor at large momentum transfers, then
extrapolate the and to zero
momentum transfer (or beyond zero momentum transfer) with
analytical continuation in hope of obtaining some interesting
results ^{4}^{4}4We can borrow some ideas from the electromagnetic
-photon form-factor . The value of
is fixed by partial conservation of
the axial current and the effective anomaly lagrangian,
. In the limit of
large-, perturbative QCD predicts that .
The Brodsky-Lepage interpolation formula [51]
can reproduce both the value at
and the behavior at large-. The energy scale () is numerically close to
the squared mass of the meson, . The Brodsky-Lepage interpolation formula is similar to
the result of the vector meson dominance approach, . In
the latter case, the calculation is performed at the timelike energy
scale and the electromagnetic current is
saturated by the vector meson , where the mass
serves as a parameter determining the pion charge radius. With a
slight modification of the mass parameter,
, the experimental data can be
well described by the single-pole formula at the interval
[52]. In Ref.[27], the
four form-factors of have satisfactory behaviors at
large , which are expected by naive power counting rules, and
they have finite values at . The analytical expressions of
the four form-factors , , and
are taken as Brodsky-Lepage type of interpolation
formulae, although they are calculated at rather large , the
extrapolation to lower energy transfer has no solid theoretical
foundation. The numerical values of , , and
are
compatible with the experimental data and theoretical
calculations (in magnitude).
In Ref.[28], the vector form-factors
and are
also taken as Brodsky-Lepage type of interpolation formulae, the behaviors of low momentum
transfer are rather good in some channels.. It is obvious that the model
functions and may
have good or bad low- behaviors, although
they have solid theoretical foundation at large momentum
transfers. We extrapolate the model functions tentatively to zero
momentum transfer, systematic errors maybe very large and the
results maybe unreliable. The predictions merely indicate the
possible values of the light-cone QCD sum rules approach, they
should be confronted with the experimental data or other theoretical
approaches. The numerical results indicate that the small-
behavior of the is better than that of the
, so we take the value of the at .

The electromagnetic form-factors and are complex functions of the input parameters, in principle, they can be expanded in terms of Taylor series of for large-. At large momentum transfer, for example, , the central values of the two form-factors and can be fitted numerically as

(11) |

which are comparable with the experimental data [1, 2, 3, 4, 5, 6, 7] and theoretical estimations, for examples, the light-cone QCD sum rules [12, 13, 14, 15], perturbative QCD [17, 18, 19, 20, 21]. In Fig.2, we plot the electromagnetic form-factor comparing with the experimental data in Refs. [3, 6, 7] and prediction of the light-cone QCD sum rules with the axial-vector current in Ref.[14]. For more literatures, one can consult Ref.[53].

The large- behavior is expected from the naive power counting rules [54, 55, 56]. At large-, the -th term in the form-factors and respectively can be expanded as , the terms proportional to with are canceled out approximately with each other, i.e. , , . Finally we obtain and for the and respectively. Due to partial conservation of the axial-vector current, the axial-vector current has no-vanishing coupling with the meson, we can choose either the axial-vector current or the pseudoscalar current to interpolate the meson. They can lead to different sum rules, in the case of the axial-vector current, the soft contributions proportional to manifest themselves at large- [13, 14], see Fig.2, more experimental data are needed to select the pertinent sum rules.

In the limit of large-, , which is consistent with the prediction of perturbative QCD theory, i.e. hard-gluon exchange between the and quarks dominates over Feynman mechanism.

## 4 Conclusion

In this article, we calculate the electromagnetic form-factor of the meson with the light-cone QCD sum rules. Our numerical value is in excellent agreement with the experimental data (extrapolated to the limit or the normalization condition ). For large momentum transfers, the values from the two sum rules are all comparable with the experimental data and theoretical estimations.

## Appendix

The explicit expressions of the correlation functions,

(12) | |||||

(13) | |||||