Landau pole

In physics, the Landau pole is the momentum (or energy) scale at which the coupling constant (interaction strength) of a quantum field theory becomes infinite. Such a possibility was pointed out by the physicist Lev Landau and his co-workers[1] The fact that coupling constants depend on the momentum (or length) scale is one of the basic ideas behind the renormalization group.

Landau poles appear in theories that are not asymptotically free, such as quantum electrodynamics (QED) or  \phi^4 theory, i.e. a scalar field with a quartic interaction, such as may describe the Higgs boson. In these theories, the renormalized coupling constant grows with energy. A Landau pole appears when the coupling constant becomes infinite at a finite energy scale. In a theory intended to be complete, this could be considered a mathematical inconsistency. A possible solution is that the renormalized charge could go to zero as the cut-off is removed, meaning that the charge is completely screened by quantum fluctuations (vacuum polarization). This is a case of quantum triviality, which means that quantum corrections completely suppress the interactions in the absence of a cut-off.

Since the Landau pole is normally calculated using perturbative one-loop or two-loop calculations, it is possible that the pole is merely a sign that the perturbative approximation breaks down at strong coupling. Lattice field theory provides a means to address questions in quantum field theory beyond the realm of perturbation theory, and thus has been used to attempt to resolve this question. Numerical computations performed in this framework seems to confirm Landau's conclusion.[2][3][4] Recent developments give a new insight into this problem.

Contents

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Brief history

According to Landau, Abrikosov, Khalatnikov,[5] the relation of the observable charge  g_{obs} with the “bare” charge  g_0 for renormalizable field theories is given by expression

 g_{obs}=\frac{g_0}{1%2B\beta_2 g_0 \ln \Lambda/m} \,,   \qquad \qquad\qquad      (1)

where m is the mass of the particle, and \Lambda is the momentum cut-off. For finite  g_0  and \Lambda \to\infty the observed charge g_{obs} tends to zero and the theory looks trivial. In fact, the proper interpretation of Eq.1 consists in its inversion, so that  g_0 (related to the length scale  \Lambda^{-1}) is chosen to give a correct value of  g_{obs} :

  g_0=\frac{g_{obs}}{1-\beta_2 g_{obs} \ln \Lambda/m}\,. \qquad\qquad\qquad (2)

With growth of  \Lambda the bare charge  g_0 increases and turn to infinity at the point

  \Lambda_{Landau} = m \exp\left\{ \frac{1}{\beta_2 g_{obs}} \right\} \,.
 \qquad\qquad\qquad (3)

The latter singularity is the Landau pole. In fact, the growth of  g_0 invalidates Eqs.1,2 in the region  g_0 \approx  1  (since they were obtained for  g_0 \ll 1 ) and reality of the Landau pole becomes doubtful.

The actual behavior of the charge  g(\mu) as a function of the momentum scale  \mu  is determined by the Gell-Mann–Low equation[6]

 \frac{dg}{d \ln \mu} =\beta(g)=\beta_2 g^2%2B\beta_3 g^3%2B\ldots   \qquad\qquad\qquad (4)

which gives Eqs.1,2 if it is integrated under conditions  g(\mu)=g_{obs} for  \mu=m and  g(\mu)=g_0 for  \mu=\Lambda , when only the term with  \beta_2 is retained in the right hand side. The general behavior of  g(\mu) depends on the appearance of the function  \beta(g) . According to classification by Bogoliubov and Shirkov,[7] there are three qualitatively different situations:

(a) if  \beta(g) has a zero at the finite value  g* , then growth of  g is saturated, i.e.  g(\mu)\to g* for  \mu\to\infty  ;

(b) if  \beta(g) is non-alternating and behaves as  \beta(g) \propto g^\alpha with  \alpha\le 1 for large  g , then the growth of  g(\mu) continues to infinity;

(c) if  \beta(g) \propto g^\alpha with  \alpha>1 for large  g, then  g(\mu)  is divergent at finite value  \mu_0 and the real Landau pole arises: the theory is internally inconsistent due to indeterminacy of  g(\mu) for  \mu>\mu_0 .

Landau and Pomeranchuk [8] tried to justify the possibility (c) in the case of QED and   \phi^4 theory. They have noted that the growth of  g_0 in Eq.1 drives the observable charge  g_{obs} to the constant limit, which does not depend on  g_0 . The same behavior can be obtained from the functional integrals, omitting the quadratic terms in the action. If neglecting the quadratic terms is valid already for  g_0\ll 1 , it is all the more valid for  g_0 of the order or greater than unity : it gives a reason to consider Eq.1 to be valid for arbitrary  g_0 . Validity of these considerations on the quantitative level is excluded by non-quadratic form of the  \beta -function. Nevertheless, they can be correct qualitatively. Indeed, the result  g_{obs} =const(g_0) can be obtained from the functional integrals only for  g_0\gg 1 , while its validity for  g_0\ll 1 , based on Eq.1, may be related with other reasons; for  g_0\approx 1 this result is probably violated but coincidence of two constant values in the order of magnitude can be expected from the matching condition. The Monte Carlo results [9] seems to confirm the qualitative validity of the Landau–Pomeranchuk arguments, though a different interpretation is also possible.

The case (c) in the Bogoliubov and Shirkov classification corresponds to the quantum triviality in full theory (beyond its perturbation context), as can be seen by a reductio ad absurdum. Indeed, if  g_{obs} is finite, the theory is internally inconsistent. The only way to avoid it, is to tend  \mu_0 to infinity, which is possible only for  g_{obs}\to 0 . It is a widespread opinion, that QED and  \phi^4 theory are trivial in the continuum limit. In fact, available information confirms only “Wilson triviality”, which is equivalent to positiveness of  \beta(g) for  g\ne 0 and can be considered as firmly established. Indications of “true” quantum triviality are not numerous and allow different interpretation.

Phenomenological aspects

In a theory intended to represent a physical interaction where the coupling constant is known to be non-zero, Landau poles or triviality may be viewed as a sign of incompleteness in the theory. For example, QED is usually not believed to be a complete theory on its own, and the Landau pole could be a sign of new physics entering via its embedding into a Grand Unified Theory. The grand unified scale would provide a natural cutoff well below the Landau scale, preventing the pole from having observable physical consequences.

The problem of the Landau pole in QED is of pure academic interest. The role of  g_{obs} in Eqs.1,2 is played by the fine structure constant α ≈ 1/137 and the Landau scale for QED is estimated as 10283keV/c2, which is far beyond any energy scale relevant to observable physics. For comparison, the maximum energies accessible at the Large Hadron Collider are of order 1013 eV, while the Planck scale, at which quantum gravity becomes important and the relevance of quantum field theory itself may be questioned, is only 1028 eV.

The Higgs boson in the Standard Model of particle physics is described by  \phi^4 theory. If the latter has a Landau pole, then this fact is used in setting a "triviality bound" on the Higgs mass.[10] The bound depends on the scale at which new physics is assumed to enter and the maximum value of the quartic coupling permitted (its physical value is unknown). For large couplings, non-perturbative methods are required. Lattice calculations have also been useful in this context.[11]

Recent developments

Solution of the Landau pole problem needs calculation of the Gell-Mann–Low function  \beta(g) at arbitrary  g and, in particular, its asymptotic behavior for  g\to\infty . This problem is very difficult and was considered as hopeless for many years: by diagrammatic calculations one can obtain only few expansion coefficients  \beta_2, \beta_3,\ldots , which do not allow to investigate the  \beta function in the whole. The progress became possible after development of the Lipatov method for calculation of large orders of perturbation theory[12] : now one can try to interpolate the known coefficients  \beta_2, \beta_3,\ldots with their large order behavior and to sum the perturbation series. The first attempts of reconstruction of the  \beta function witnessed on triviality of  \phi^4 theory. Application of more advanced summation methods gave the exponent  \alpha in the asymptotic behavior  \beta(g) \propto g^\alpha a value close to unity. The hypothesis for the asymptotics  \beta(g) \propto g was recently confirmed analytically for  \phi^4 theory and QED [13] .[14] Together with positiveness of \beta(g)  , obtained by summation of series, it gives the case (b) of the Bogoliubov and Shirkov classification, and hence the Landau pole is absent in these theories. Possibility of omitting the quadratic terms in the action suggested by Landau and Pomeranchuk is not confirmed.

References

  1. ^ Lev Landau, in Wolfgang Pauli, ed (1955). Niels Bohr and the Development of Physics. London: Pergamon Press. 
  2. ^ Göckeler, M.; R. Horsley, V. Linke, P. Rakow, G. Schierholz, and H. StĂŒben (1998). "Is There a Landau Pole Problem in QED?". Physical Review Letters 80 (19): 4119–4122. arXiv:hep-th/9712244. Bibcode 1998PhRvL..80.4119G. doi:10.1103/PhysRevLett.80.4119. http://link.aps.org/doi/10.1103/PhysRevLett.80.4119. 
  3. ^ Kim, S.; John B. Kogut and Lombardo Maria Paola (2002-01-31). "Gauged Nambu–Jona-Lasinio studies of the triviality of quantum electrodynamics". Physical Review D 65 (5): 054015. arXiv:hep-lat/0112009. Bibcode 2002PhRvD..65e4015K. doi:10.1103/PhysRevD.65.054015. http://link.aps.org/doi/10.1103/PhysRevD.65.054015. 
  4. ^ Gies, Holger; Jaeckel, Joerg (2004-09-09). "Renormalization Flow of QED". Physical Review Letters 93 (11): 110405. arXiv:hep-ph/0405183. Bibcode 2004PhRvL..93k0405G. doi:10.1103/PhysRevLett.93.110405. http://link.aps.org/doi/10.1103/PhysRevLett.93.110405. 
  5. ^ L. D. Landau, A. A. Abrikosov, and I. M. Khalatnikov, Dokl. Akad. Nauk SSSR 95, 497, 773, 1177 (1954).
  6. ^ Gell-Mann, M.; Low, F. E. (1954). "Quantum Electrodynamics at Small Distances". Physical Review 95 (5): 1300–1320. Bibcode 1954PhRv...95.1300G. doi:10.1103/PhysRev.95.1300. http://link.aps.org/doi/10.1103/PhysRev.95.1300. 
  7. ^ N. N. Bogoliubov and D. V. Shirkov, Introduction to the Theory of Quantized Fields, 3rd ed. (Nauka, Moscow, 1976; Wiley, New York, 1980).
  8. ^ L.D.Landau, I.Ya.Pomeranchuk, Dokl. Akad. Nauk SSSR 102, 489 (1955); I.Ya.Pomeranchuk, Dokl. Akad. Nauk SSSR 103, 1005 (1955).
  9. ^ B.Freedman, P.Smolensky, D.Weingarten, Phys. Lett. B 113, 481 (1982).
  10. ^ Gunion, J.; H. E. Haber, G. L. Kane, and S. Dawson (1990). The Higgs Hunters Guide. Addison-Wesley. 
  11. ^ For example, Heller, Urs; Markus Klomfass, Herbert Neuberger, and Pavols Vranas (1993-09-20). "Numerical analysis of the Higgs mass triviality bound". Nuclear Physics B 405 (2–3): 555–573. arXiv:hep-ph/9303215. Bibcode 1993NuPhB.405..555H. doi:10.1016/0550-3213(93)90559-8. , which suggests MH < 710 GeV.
  12. ^ L.N.Lipatov, Zh.Eksp.Teor.Fiz. 72, 411 (1977) [Sov.Phys. JETP 45, 216 (1977)].
  13. ^ I. M. Suslov, JETP 107, 413 (2008); JETP 111, 450 (2010); http://arxiv.org/abs/1010.4081, http://arxiv.org/abs/1010.4317.
  14. ^ I. M. Suslov, JETP 108, 980 (2009), http://arxiv.org/abs/0804.2650.