Do gluons carry half of the nucleon momentum?

1. Do gluons carry half of the nucleon momentum? X.S.Chen, X.F.Lu Dept. of Phys., Sichuan Univ. W.M.Sun, Fan Wang NJU and PMO Joint Center for Particle Nuclear Physics and Cosmology (J-CPNPC) T.Goldman T.D., LANL, USA

2. Outline • Introduction • Conflicts between Gauge invariance and Canonical Quantization • A new set of quark, gluon momentum, angular momentum, and spin operators III.0 A lemma:Decomposing the gauge field into pure gauge and physical parts III.1 Quantum mechanics III.2 QED III.3 QCD IV. Nucleon internal structure V. Summary

3. I.Introduction • A widely accepted nucleon internal momentum structure: quark and gluon carry half the nucleon momentum in the asymptotic limit. D.J.Gross and F. Wilczek, PRD9,980(1974); H.Georgi and H.D. Politzer, PRD9,416 (1974); H.D.Politzer, Phys.Rep. 14,129(1974). • This is a distorted picture, because it is obtained based on a momentum decomposition where both quark and gluon momentum operators are not proper.

4. A proper momentum operator must be gauge invariant and satisfy canonical momentum algebra. • The nucleon internal spin structure has the same problem. The quark gluon angular momentum operators used in the nucleon spin sum rule is not proper either. • The nucleon internal momentum and spin structure should be reexamined based on the proper quark and gluon momentum, orbital angular momentum and spin operators.

5. II. Conflicts between gauge invariance and canonical quantization

6. Quantum mechanics The classical canonical momentum of a charged particle moving in an electromagnetic field, an U(1) gauge field, is It is not gauge invariant! The gauge invariant one is

7. Gauge is an internal degree of freedom, no matter what gauge is used, the canonical momentum of a charged particle is quantized as The orbital angular momentum is The Hamiltonian is

8. Under a gauge transformation, the matrix elements transformed as They are not gauge invariant, even though the Schroedinger equation is.

9. We call The physical momentum. It is neither the canonical momentum nor the mechanical momentum

10. QED • The canonical momentum and orbital angular momentum of electron are gauge dependent and so their physical meaning is obscure. • The canonical photon spin and orbital angular momentum operators are also gauge dependent. Their physical meaning is obscure too. • Even it has been claimed in some textbooks that it is impossible to have photon spin and orbital angular momentum operators. V.B. Berestetskii, A.M. Lifshitz and L.P. Pitaevskii, Quantum electrodynamics, Pergamon, Oxford, 1982. C. Cohen-Tannoudji, J. Dupont-Roc and G. Grynberg, Photons and atoms, Wiley, New York,1987.

11. Multipole radiation Multipole radiation measurement and analysis are the basis of atomic, molecular, nuclear and hadron spectroscopy. If the spin and orbital angular momentum of photon is gauge dependent and not measurable or even meaningless , then all determinations of the parity of these microscopic systems would be meaningless!

12. Multipole field The multipole radiation theory is based on the decomposition of an em field into multipole radiation field with definite photon spin and orbital angular momentum quantum numbers coupled to a total angular momentum quantum number LM,

13. QCD • Because the canonical parton (quark and gluon) momentum is “gauge dependent”, so the present analysis of parton distribution of nucleon uses the covariant derivative operator instead of the canonical momentum operator ; uses the Poynting vector as the gluon momentum operator. They are not the proper momentum operators!

14. The quark spin contribution to nucleon spin has been measured, the further study is hindered by the lack of gauge invariant quark orbital angular momentum, gluon spin and orbital angular momentum operators. The present gluon spin measurement is even under the condition that “there is not a gluon spin can be measured”.

15. III. A New set of quark, gluon (electron, photon) momentum, orbital angular momentum and spin operators

16. III.0 Decomposing the gauge field into pure gauge and physical parts • There were gauge field decompositions discussed before, mainly mathematical. Y.S.Duan and M.L.Ge, Sinica Sci. 11(1979)1072; L.Fadeev and A.J.Niemi, Nucl. Phys. B464(1999)90; B776(2007)38. • We suggest a new decomposition based on the requirement: to separate the gauge field into pure gauge and physical parts. X.S. Chen, X.F.Lu, W.M.Sun, F.Wang and T.Goldman, Phys. Rev. Lett. 100(2008)232012.

17. U(1) Abelian gauge field

18. Other solution

19. Under a gauge transformation, The physical and pure gauge parts will be transformed as

20. SU(3) non-Abelian gauge field

21. The above equations can be rewritten as a perturbative solution in power of g through iteration can be obtained

22. Under a gauge transformation,

23. III.1 Quantum mechanics The classical canonical momentum of a charged particle moving in an electromagnetic field, an U(1) gauge field, is It is not gauge invariant! The gauge invariant one is

24. Gauge is an internal degree of freedom, no matter what gauge is used, the canonical momentum of a charged particle is quantized as The orbital angular momentum is The Hamiltonian is

25. New momentum operator • The canonical momentum is, It satisfies the canonical momentum commutation relation, but its matrix element is not gauge invariant. The new momentum operator is, It satisfies the canonical momentum commutation relation and its matrix element is gauge invariant.

26. We call The physical momentum. It is neither the canonical momentum nor the mechanical momentum

27. Hamiltonian of hydrogen atom Coulomb gauge Gauge transformed one

28. Follow the same recipe, we introduce a new Hamiltonian, which is gauge invariant, i.e., This means the hydrogen energy calculated in Coulomb gauge is physical.

29. A rigorous derivation Start from a QED Lagrangian including electron, proton and em field, under the heavy proton approximation, one can derive a Dirac equation and a Hamiltonian for electron and proved that the time evolution operator is different from the Hamiltonian exactly as we obtained phenomenologically. The nonrelativistic one is the Schroedinger or Pauli equation.

30. III.2 QED Different approach will obtain different energy-momentum tensor and four momentum, they are not unique: Noether theorem They are not gauge invariant. Gravitational theory (Weinberg) or Belinfante tensor It appears to be perfect , but individual part does not satisfy the momentum algebra.

31. New momentum for QED system We are experienced in quantum mechanics, so we introduce They are both gauge invariant and momentum algebra satisfied. They return to the canonical expressions in Coulomb gauge.

32. We proved the renowned Poynting vector is not the proper momentum of em field It includes photon spin and orbital angular momentum

33. Electric dipole radiation field

35. Each term in this decomposition satisfies the canonical angular momentum algebra, so they are qualified to be called electron spin, orbital angular momentum, photon spin and orbital angular momentum operators. • However they are not gauge invariant except the electron spin. Therefore the physical meaning is obscure.

37. However each term no longer satisfies the canonical angular momentum algebra except the electron spin, in this sense the second and third term is not the electron orbital and photon angular momentum operator. The physical meaning of these operators is obscure too. • One can not have gauge invariant photon spin and orbital angular momentum operator separately, the only gauge invariant one is the total angular momentum of photon. The photon spin and orbital angular momentum had been measured!

38. Dangerous suggestion It will ruin the multipole radiation analysis used from atom to hadron spectroscopy, where the canonical spin and orbital angular momentum of photon have been used. It is unphysical!

39. New spin decomposition for QED system

40. Multipole radiation • Photon spin and orbital angular momentum are well defined now and they will take the canonical form in Coulomb gauge. • Multipole radiation analysis is based on the decomposition of em vector potential in Coulomb gauge. The results are physical and these multipole field operators are in fact gauge invariant.

41. III.3 QCDthree decompositions of momentum three

42. Three decompositions of angular momentum 1. From QCD Lagrangian, one can get the total angular momentum by Noether theorem:

43. 2. One can have the gauge invariant decomposition,

44. 3.New decomposition

45. IV. Nucleon internal structure it should be reexamined! • The present parton distribution is not the real quark and gluon momentum distribution. In the asymptotic limit, the gluon only contributes ~1/5 nucleon momentum, not 1/2 ! arXiv:0904.0321[hep-ph],Phys.Rev.Lett. in press. • The nucleon spin structure should be reexamined based on the new decomposition and new operators.

46. Consistent separation of nucleon momentum and spin

47. Quantitative example:Old quark/gluon momentum in the nucleon

48. Proper quark/gluon momentum in nucleon

49. One has to be careful when one compares experimental measured quark gluon momentum and angular momentum to the theoretical ones. • The proton spin crisis is mainly due to misidentification of the measured quark axial charge to the nonrelativistic Pauli spin matrix elements. D. Qing, X.S. Chen and F. Wang,Phys. Rev. D58,114032 (1998)

50. To clarify the confusion, first let me emphasize that the DIS measured one is the matrix element of the quark axial vector current operator in a nucleon state, Here a0= Δu+Δd+Δs which is not the quark spin contributions calculated in CQM. The CQM calculated one is the matrix element of the Pauli spin part only.