Mass-independent Renormalization Scheme Research Articles

Standard model at 200 GeV

The Standard Model can be defined quantitatively by running parameters in a mass-independent renormalization scheme at a fixed reference scale. We provide a set of simple interpolation formulas that give the fundamental Lagrangian parameters in the $\overline{\mathrm{MS}}$ scheme at a renormalization scale of 200 GeV, safely above the top-quark mass and suitable for matching to candidate new physics models at very high mass scales using renormalization group equations. These interpolation formulas take as inputs the on-shell experimental quantities and use the best available calculations in the pure $\overline{\mathrm{MS}}$ scheme. They also serve as an accounting of the parametric uncertainties for the short-distance Standard Model Lagrangian. We also include an interpolating formula for the $W$-boson mass. This paper is based on results obtained with the publicly available computer program smdr.

Renormalization scheme and mass scale independence

We demonstrate that in the mass-independent renormalization scheme, the renormalization group equations associated with the unphysical parameters that characterize the renormalization scheme and the mass scale leads to summation that results in a cancellation between the implicit and explicit dependence on these parameters. The resulting perturbative expansion is consequently independent of these arbitrary parameters. We illustrate this by considering R, the cross section for e+e– → hadrons.

Renormalization mass scale and scheme dependence in the perturbative contribution to inclusive semi-leptonic b decays

We examine the perturbative calculation of the inclusive semi-leptonic decay rate Γ for the b quark, using mass-independent renormalization. To finite order of perturbation theory the series for Γ will depend on the unphysical renormalization scale parameter μ and on the particular choice of mass-independent renormalization scheme; these dependencies will only be removed after summing the series to all orders. In this paper we show that all explicit μ-dependence of Γ, through powers of ln(μ), can be summed by using the renormalization group equation. We then find that this explicit μ-dependence can be combined together with the implicit μ-dependence of Γ (through powers of both the running coupling a(μ) and the running b quark mass m(μ)) to yield a μ-independent perturbative expansion for γ in terms of a(μ) and m(μ) both evaluated at a renormalization scheme independent mass scale [Formula: see text] which is fixed in terms of either the “[Formula: see text] mass” [Formula: see text] of the b quark or its pole mass mpole. At finite order the resulting perturbative expansion retains a degree of arbitrariness associated with the particular choice of mass-independent renormalization scheme. We use the coefficients ci and gi of the perturbative expansions of the renormalization group functions β(a) and γ(a), associated with a(μ) and m(μ) respectively, to characterize the remaining renormalization scheme arbitrariness of Γ. We further show that all terms in the expansion of Γ can be written in terms of the ci and gi coefficients and a set of renormalization scheme independent parameters τi. A second set of renormalization scheme independent parameters σi is shown to play a very similar role in the perturbative expansion of mpole in terms of m(μ) and a(μ). We illustrate our approach by a perturbative computation of Γ using the [Formula: see text] renormalization scheme. Two other particular mass independent renormalization schemes are briefly considered.

Implications of extra dimensions on the effective charge and the beta function in quantum electrodynamics

A comprehensive analysis on the photon self-energy, the fermion self-energy, and the fermion vertex function is presented at one loop in the context of quantum electrodynamics (QED) with 1 extra dimension. In 5-dimensional theories, characterized by an infinite number of Kaluza-Klein fields, one-loop amplitudes involve discrete as well as continuous sums, $\sum^\infty_{n=1}\int d^4k$, that could diverge. Using dimensional regularization, we express such sums as products of gamma and Epstein functions, both defined on the complex plane, with divergences arising from poles of these functions in the limit as $ D \to 4$. Using the analytical properties of the Epstein function, we show that the ultraviolet divergences generated by the Kaluza-Klein sums can be consistently renormalized, which means that the corresponding renormalized quantities reduce to the usual ones of QED at the limit of a very large compactification scale $R^{-1}$. The main features of QED at the one-loop level were studied. We use the mass-dependent $\mu$-scheme to calculate, in QED with an arbitrary number $n$ of extra dimensions, a beta function fulfilling all desirable physical requirements. We argue that in this type of theories, with a large mass spectrum covering a wide energy range, beta functions should not be calculated by using mass-independent renormalization schemes. We show that the beta function is finite for any energy $\mu$. In particular, it reduces to the usual QED result $e^3/12\pi^2$ for $m\ll \mu \ll R^{-1}$ and vanishes for $m\gg \mu$, with $m$ the usual fermion mass. Throughout the work, the decoupling nature of all our results obtained from the analytical properties of the Epstein function is stressed.

A systematic expansion of running couplings and masses

As an alternative to directly integrating their defining equations to find the running coupling [Formula: see text] and the running mass [Formula: see text], we expand these quantities in powers of [Formula: see text] and their boundary values [Formula: see text] and [Formula: see text]. Renormalization group summation is used to partially sum these logarithms. We consider this approach using both the [Formula: see text] and ’t Hooft renormalization schemes. We also show how the couplings and masses in any two mass independent renormalization schemes are related.

Renormalization scheme dependence and the renormalization group beta function

The renormalization that relates a coupling "a" associated with a distinct renormalization group beta function in a given theory is considered. Dimensional regularization and mass independent renormalization schemes are used in this discussion. It is shown how the renormalization $a^*=a+x_2a^2$ is related to a change in the mass scale $\mu$ that is induced by renormalization. It is argued that the infrared fixed point is to be a determined in a renormalization scheme in which the series expansion for a physical quantity $R$ terminates.

Probing QCD perturbation theory at high energies with continuum extrapolated lattice data

Precision tests of QCD perturbation theory are not readily available from experimental data. The main reasons are systematic uncertainties due to the confinement of quarks and gluons, as well as kinematical constraints which limit the accessible energy scales. We here show how continuum extrapolated lattice data may overcome such problems and provide excellent probes of renormalized perturbation theory. This work corresponds to an essential step in the ALPHA collaboration's project to determine the $\Lambda$-parameter in 3-flavour QCD. I explain the basic techniques used in the high energy regime, namely the use of mass-independent renormalization schemes for the QCD coupling constant in a finite Euclidean space time volume. When combined with finite size techniques this allows one to iteratively step up the energy scale by factors of 2, thereby quickly covering two orders of magnitude in scale. We may then compare perturbation theory (with $\beta$-functions available up to 3-loop order) to our non-perturbative data for a 1-parameter family of running couplings. We conclude that a target precision of 3 percent for the $\Lambda$-parameter requires non-perturbative data up to scales where $\alpha_s\approx 0.1$, whereas the apparent precision obtained from applying perturbation theory around $\alpha_s \approx 0.2$ can be misleading. This should be taken as a general warning to practitioners of QCD perturbation theory.

Three-loop corrections to the lightest Higgs scalar boson mass in supersymmetry

I evaluate the largest three-loop corrections to the mass of the lightest Higgs scalar boson in the minimal supersymmetric standard model in a mass-independent renormalization scheme, using effective field theory and renormalization group methods. The contributions found here are those that depend only on strong and Yukawa interactions and on the leading and next-to-leading logarithms of the ratio of a typical superpartner mass scale to the top-quark mass. The approximation assumes that all superpartners and the other Higgs bosons can be treated as much heavier than the top quark, but does not assume their degeneracy. I also discuss the consistent addition of the three-loop corrections to a complete two-loop calculation.

Refined gluino and squark pole masses beyond leading order

The physical pole and running masses of squarks and gluinos have recently been related at two-loop order in a mass-independent renormalization scheme. I propose a general method for improvement of such formulas, and argue that better accuracy results. The improved version gives an imaginary part of the pole mass that agrees exactly with the direct calculation of the physical width at next-to-leading order. I also find the leading three-loop contributions to the gluino pole mass in the case that squarks are heavier, using effective field theory and renormalization group methods. The efficacy of these improvements for the gluino and squarks is illustrated with numerical examples. Some necessary three-loop results for gauge coupling and fermion mass beta functions and pole masses in theories with more than one type of fermion representation, which are not directly accessible from the published literature, are presented in an Appendix.

Two-loop scalar self-energies in a general renormalizable theory at leading order in gauge couplings

I present results for the two-loop self-energy functions for scalars in a general renormalizable field theory, using mass-independent renormalization schemes based on dimensional regularization and dimensional reduction. The results are given in terms of a minimal set of loop-integral basis functions, which are readily evaluated numerically by computers. This paper contains the contributions corresponding to the Feynman diagrams with zero or one vector propagator lines. These are the ones needed to obtain the pole masses of the neutral and charged Higgs scalar bosons in supersymmetry, neglecting only the purely electroweak parts at two-loop order. A subsequent paper will present the results for the remaining diagrams, which involve two or more vector lines. I also include an illustrative example, featuring several nontrivial consistency checks.

Matching functions for heavy particles

We introduce matching functions as a means of summing heavy-quark logarithms to any order. Our analysis is based on Witten's approach, where heavy quarks are decoupled one at a time in a mass-independent renormalization scheme. The outcome is a generalization of the matching conditions of Bernreuther and Wetzel: we show how to derive closed formulas for summed logarithms to any order, and present explicit expressions for leading order and next-to-leading order contributions. The decoupling of heavy quarks in theories lacking asymptotic freedom is also considered.

Complete two-loop effective potential approximation to the lightest Higgs scalar boson mass in supersymmetry

I present a method for accurately calculating the pole mass of the lightest Higgs scalar boson in supersymmetric extensions of the Standard Model, using a mass-independent renormalization scheme. The Higgs scalar self-energies are approximated by supplementing the exact one-loop results with the second derivatives of the complete two-loop effective potential in Landau gauge. I discuss the dependence of this approximation on the choice of renormalization scale, and note the existence of particularly poor choices which fortunately can be easily identified and avoided. For typical input parameters, the variation in the calculated Higgs mass over a wide range of renormalization scales is found to be of order a few hundred MeV or less, and is significantly improved over previous approximations.

An efficient renormalization group improved implementation of the MSSM effective potential

In the context of MSSM, a novel improving procedure based on the renormalization group equation is applied to the effective potential in the Higgs sector. We focus on the one-loop radiative corrections computed in Landau gauge by using the mass independent renormalization scheme $\overline{\mathrm{DR}}$ . Thanks to the decoupling theorem, the well-known multimass scale problem is circumvented by switching to a new effective field theory every time a new particle threshold is encountered. We find that, for any field configuration, there is a convenient renormalization scale $\tilde{Q}^*$ at which the loop expansion respects the perturbation series hierarchy and the theory retains the vital property of stability.

Nonperturbative renormalization of a fermion mass term in the mass-independent renormalization scheme

We develop a new nonperturbative renormalization scheme for the fermion mass term in the Dyson-Schwinger equation (DSE) approach for the study of the fermion propagator. The renormalized DSE for the fermion propagator is derived in the mass-independent renormalization (MIR) scheme. Our method is suitable for studying the spontaneous breaking of chiral symmetry in gauge field theories, since the MIR scheme connects the system with a finite renormalized mass and the chiral limit of the theory in a smooth way. We demonstrate that our method correctly reproduces the known asymptotic behavior of the renormalized quark propagator when applied to asymptotic-free quark models for QCD. We apply the MIR scheme to the dual Ginzburg-Landau theory and demonstrate its applicability.

Effective field theory of vacuum tilting

Simple models of topcolor and topcolor-assisted technicolor rely on a relatively strong U(1) gauge interaction to ``tilt'' the vacuum. This tilting is necessary to produce a top-condensate, thereby naturally obtaining a heavy top-quark, and to avoid producing a bottom-condensate. We identify some peculiarities of the Nambu-Jona-Lasinio approximation often used to analyze the topcolor dynamics. We resolve these puzzles by constructing the low-energy effective field theory appropriate to a mass-independent renormalization scheme. We construct the power-counting rules for such an effective theory. By requiring that the Landau pole associated with the U(1) gauge theory be sufficiently above the topcolor gauge boson scale, we derive an upper bound on the strength of the U(1) gauge-coupling evaluated at the topcolor scale. The upper bound on the U(1) coupling implies that these interactions can shift the composite Higgs boson mass-squared by only a few per cent and, therefore, that the topcolor coupling must be adjusted to equal the critical value for chiral symmetry breaking to within a few per cent.

Effective scalar field theory and reduction of couplings

A general discussion of the renormalization of the quantum theory of a scalar field as an effective field theory is presented. The renormalization group equations in a mass-independent renormalization scheme allow us to identify the possibility to go beyond the renormalizable ${\ensuremath{\varphi}}^{4}$ theory without losing its predictive power. It is shown that there is a minimal extension with just one additional free parameter (the mass scale of the effective theory expansion) and some of its properties are discussed.

Further results on O( a) improved lattice QCD to one-loop order of perturbation theory

We present results at one-loop order of perturbation theory for various improvement coefficients in on-shell O( a) improved lattice QCD. In particular we determine the additive counterterm required for on-shell improvement of the isovector vector current. Employing a general mass-independent renormalization scheme we also obtain the coefficients of the O( a) counterterms which are proportional to the quark mass in the improved isovector pseudo-scalar, axial vector and vector operators. In the latter case a comparison with a recent non-perturbative study is made.

A renormalization group analysis of the NCG constraints mtop = 2 mw, mHiggs = 3.14 mw

We study the evolution under the renormalization group of the restrictions on the parameters of the standard model coming from Non-Commutative Geometry, namely m top = 2 m w and m Higgs = 3.14 m w . We adopt the point of view that these relations are to be interpreted as tree level constraints, and, as such, can be implemented in a mass independent renormalization scheme only at a given energy scale μ 0. We show that the physical predictions on the top and Higgs masses depend weakly on μ 0.

More on the sigma to fermion mass ratio in the linear sigma model and Nambu-Jona-Lasinio model.

We calculate the two-loop contributions to the $\ensuremath{\beta}$ functions for the Yukawa coupling constant $G$ and the four-scalar coupling constant $\ensuremath{\lambda}$ in the linear $\ensuremath{\sigma}$ model using a mass-independent renormalization scheme. The results are used to obtain a correction to the infrared fixed point value for $\frac{\ensuremath{\lambda}}{{G}^{2}}$. We argue that this is the leading correction to the fixed point value for this ratio in the regime of fine-tuning, and use our results to complete a calculation of the ratio of the $\ensuremath{\sigma}$ mass to the fermion mass to order ${G}^{2}$. Our calculation also applies to the Nambu---Jona-Lasinio model.

Renormalization-scheme-invariant QCD and QED: The method of effective charges

We review, extend, and give some further applications of a method recently suggested to solve the renormalization-scheme-dependence problem in perturbative field theories. The use of a coupling constant as a universal expansion parameter is abandoned. Instead, to each physical quantity depending on a single scale variable is associated an effective charge, whose corresponding St\uckelberg---Peterman---Gell-Mann---Low function is identified as the proper object on which perturbation theory applies. Integration of the corresponding renormalization-group equations yields renormalization-scheme-invariant results free of any ambiguity related to the definition of the kinematical variable, or that of the scale parameter $\ensuremath{\Lambda}$, even though the theory is not solved to all orders. As a by-product, a renormalization-group improvement of the usual series is achieved. Extension of these methods to operators leads to the introduction of renormalization-group-invariant Green's function and Wilson coefficients, directly related to effective charges. The case of nonzero fermion masses is discussed, both for fixed masses and running masses in mass-independent renormalization schemes. The importance of the scale-invariant mass $\stackrel{^}{m}$ is emphasized. Applications are given to deep-inelastic phenomena, where the use of renormalization-group-invariant coefficient functions allows to perform the factorization without having to introduce a factorization scale. The Sudakov form factor of the electron in QED is discussed as an example of an extension of the method to problems involving several momentum scales.