Magnetic Field Dependent Mass & Coupling Beta Functions

• Magnetic Field Dependent Mass and Coupling Beta Functions describe how effective masses and couplings evolve with an external magnetic field via modified renormalization group flows.
• The framework uses an environmentally friendly RG approach where the magnetic scale replaces traditional UV scales, leading to phenomena like magnetic catalysis and asymptotic freedom-type running.
• Applications in scalar models, QCD, and NJL theories reveal non-monotonic behavior in dynamical masses and effective couplings, with implications for meson properties and quark matter stability.

Magnetic field dependent mass and coupling beta functions describe the behavior of dynamically generated masses and effective interaction couplings as functions of an external magnetic field within quantum field theory. When quantum fields—be they scalars, fermions, or gauge bosons—are exposed to a magnetic background, the standard renormalization group (RG) running with respect to the ultraviolet (UV) scale is supplanted or supplemented by a flow with respect to the magnetic field strength. This environmental dependence modifies the structure of the relevant beta functions, the ordinary differential equations that dictate how couplings and masses evolve as the system's external conditions change. Such magnetic-field-induced running plays a central role in phenomena including magnetic (and inverse magnetic) catalysis, chiral phase transitions, and the stability of exotic matter in astrophysical and heavy ion collision environments.

1. Scalar Models: Environmentally Friendly Renormalization Group Analysis

The prototypical setting for magnetic-field-dependent beta functions is a model with three real scalar fields: a neutral field $\pi_0$ and two charged fields $\pi^+, \pi^-$ with Lagrangian

$$\mathcal{L} = (D_\mu \pi^+)(D^\mu \pi^-) + \frac{1}{2} \partial_\mu \pi_0 \partial^\mu \pi_0 - \frac{1}{2} m^2 (2 \pi^+ \pi^- + \pi_0^2) - \frac{\lambda}{4} (2 \pi^+ \pi^- + \pi_0^2)^2$$

The covariant derivative $D_\mu = \partial_\mu \mp iqA_\mu$ (for $\pi^\pm$), and the external uniform magnetic field $B$ is taken along $z$ with $A_y = Bx$.

The "Environmentally Friendly Renormalization Group" (EFRG) approach employs the magnetic scale $b^2 \equiv |qB|$ as the flow parameter instead of the traditional subtraction scale $\mu$. At one-loop order, Schwinger's proper-time representation is used for charged scalar propagators in the magnetic background. This framework enables computation of the $B$-dependent mass and quartic coupling corrections for the neutral scalar via charged-loop vacuum polarization (Ayala et al., 22 Dec 2025).

The one-loop beta functions satisfied by the neutral scalar mass $m^2(B)$ and coupling $\lambda(B)$ are:

$$\begin{align*} \beta_{m^2}(B) &\equiv b \frac{dm^2}{db} = \frac{\lambda(b)}{2} b\, \frac{\partial A(m(b),b)}{\partial b} \ \beta_{\lambda}(B) &\equiv b \frac{d\lambda}{db} = -\frac{3}{2} \lambda^2(b)\, b\, \frac{\partial C(m(b),b)}{\partial b} \end{align*}$$

where $A$ and $C$ are calculated from proper-time integrals involving gamma and digamma functions; see Table 1 for explicit expressions.

Beta Function	Explicit Form
$\beta_{m^2}(B)$	$(\lambda/2) b \,\partial_b A(m,b)$
$\beta_{\lambda}$	$-(3/2)\lambda^2 b\,\partial_b C(m,b)$

Numerical integration (e.g., fourth-order Runge-Kutta) shows that $m^2(B)$ increases monotonically with $B$—reflecting a bosonic analogue of magnetic catalysis—while $\lambda(B)$ decreases, suggestive of asymptotic freedom-type running for the quartic self-interaction. Analytical solutions in the $m^2 \ll b^2$ regime yield

$$\lambda(B) = \frac{\lambda_0}{1 + \frac{3\lambda_0}{4\pi^2}\ln(b/b_0)}\,,$$

and

$$m^2(B) \simeq m^2_0 + \frac{\ln 2}{8\pi^2}\lambda_0 (b^2-b_0^2)$$

for large $B$ (Ayala et al., 22 Dec 2025).

2. QCD Coupling and Dynamical Mass in Magnetic Backgrounds

In quantum chromodynamics (QCD), the application of a strong magnetic field reorganizes quark and gluon propagators into Landau levels, effectively replacing the standard RG scale $\mu^2$ by $eB$ (the product of the elementary charge and magnetic field). The QCD coupling and its beta function become

$$\alpha_s(eB) = \frac{12\pi}{(11N_c-2N_f)\ln(eB/\Lambda_{\text{QCD}}^2)}$$

and

$$\beta(\alpha_s, B) = -b_0 \alpha_s^2, \quad b_0 = \frac{11N_c-2N_f}{6\pi}$$

where $N_c$ is the number of colors, $N_f$ is the number of active flavors, and $\Lambda_{\text{QCD}}$ is the QCD scale. This leading-order expression coincides with the vacuum result—except that now the RG flow is with respect to $\ln(eB)$ rather than $\ln \mu^2$ (Li et al., 2016).

Practical model treatments, such as the Nambu–Jona-Lasinio (NJL) framework, implement a $B$-dependent running of the four-fermion coupling $G(eB)$ to match lattice and Dyson–Schwinger results:

$$G'(eB) = \frac{G}{1 + \alpha \ln(1+\beta |eB|/\Lambda_{\text{QCD}}^2)}$$

for SU(2), or

$$G'(eB) = \frac{G}{\ln(e + |eB|/\Lambda_{\text{QCD}}^2)}$$

for SU(3). The dynamical mass $M_f(eB)$ then follows from a modified gap equation (Li et al., 2016):

$eB$ (GeV$^2$)	$M_u(G)$ (MeV)	$M_u(G')$ (MeV)	$M_s(G')$ (MeV)
0.00	330	330	469
0.05	355	310	460
0.10	370	290	450
0.20	395	260	430

For moderate to strong $B$, the fall-off in $G'(eB)$ dominates over the Landau-level-driven enhancement, driving $M_f(eB)$ non-monotonically. This produces a crossover from "magnetic catalysis" (mass increases with $B$ at fixed $G$) to "inverse magnetic catalysis" (mass decreases for running $G(eB)$ at high fields) (Li et al., 2016).

3. Magnetic Field Beta Functions for Effective Couplings and Masses

One-loop background field calculations in the NJL model rigorously establish the $B$-dependent corrections to the effective four-fermion coupling $G(B)$, as well as $B$-dependent meson masses:

$$\beta_{G(B)} = \frac{dG(B)}{dB}, \quad \beta_{M_\pi}(B) = \frac{d M_\pi(B)}{dB}, \quad \beta_{M_K}(B) = \frac{d M_K(B)}{dB}$$

For each flavor $f$, the scalar-channel coupling is

$G_{ff}(B) = G_0 + \bar G_{ff}(B)$

with $\bar G_{ff}(B)$ given by

$$\frac{G_0^2 N_c (M_f^*(B))^2}{2\pi^2} \left[ 1 + \frac{|q_fB|}{(M_f^*)^2}\ln\left(\frac{(M_f^*)^2}{4\pi|q_fB|}\right) + \frac{2|q_fB|}{(M_f^*)^2}\ln\Gamma\left(\frac{(M_f^*)^2}{2|q_fB|}\right) + 2\psi\left(\frac{(M_f^*)^2}{2|q_fB|}\right) - 3\ln\left(\frac{(M_f^*)^2}{2|q_fB|}\right) + 2\frac{|q_fB|}{(M_f^*)^2} \right]$$

where $M_f^*(B)$ is determined self-consistently via the modified gap equation (Moreira et al., 2022).

The beta function $\beta_{G_{ff}(B)}$ then involves both the explicit $B$-derivative of the prefactor and the implicit derivative through $M_f^*(B)$, evaluated using the chain rule. For moderate $|B|$, $\beta_{G_{ff}(B)} < 0$, confirming a monotonic decrease of the effective coupling with increasing field strength.

For neutral pion and kaon masses, the $B$-dependent mass is obtained from the Bethe–Salpeter equation, and its flow is given by the implicit function theorem. The beta functions for the meson masses are negative for low–to–moderate $B$: the mass decreases with increasing field, in agreement with lattice QCD for $eB\lesssim 0.2$–$0.4\,\text{GeV}^2$ (Moreira et al., 2022).

4. Physical Interpretation and Phenomenological Consequences

The principal outcome of charged-boson or quark-loop vacuum polarization in a magnetic field is a modification of the effective scalar mass and interaction strength. In scalar models, $m^2(B)$ grows monotonically with $B$ (“magnetic catalysis”), while $\lambda(B)$ is suppressed, behaving as $[\ln(B)]^{-1}$ for asymptotically large $B$ (Ayala et al., 22 Dec 2025). In QCD and NJL settings, increasing $B$ typically suppresses the four-fermion coupling, and, depending on the interplay with Landau level enhancement, can reverse magnetic catalysis (“inverse magnetic catalysis”). This impacts phase structure, catalysis of symmetry breaking, and stability properties of dense matter.

For oriented applications, such as the stability of strange quark matter and the free energy per baryon in the presence of $B$:

• A running $G'(eB)$ can reduce free energy per baryon by $\sim 50$ MeV at $eB\sim 0.1\,\mathrm{GeV}^2$, potentially stabilizing strange quark matter ($F/A < 930$ MeV) relevant for compact stars (Li et al., 2016).
• $B$-dependent corrections to meson masses closely track lattice findings at low field, with discrepancies at higher fields presumably arising from omitted higher-loop or non-perturbative physics (Moreira et al., 2022).

5. Extension to Gauge Theories and Lattice Validation

The EFRG and background field methods can be extended to theories with gauge and/or fermionic degrees of freedom:

• In QED, both electron and photon loops contribute; the RG scale $b$ set by $|eB|$ enters Schwinger propagators for electrons and photons, with corresponding $\beta_e(B)$ and $\beta_m(B)$.
• In QCD, external $B$ drives the separation of color and flavor sectors, and Landau level sums become central for both gluon and quark polarization. The running coupling $g_s(B)$ satisfies a flow equation $dg_s/db = \beta_g(B)$. Nonperturbative phenomena such as inverse magnetic catalysis at finite temperature demand going beyond leading order.
• Beta functions obtained from effective field theory or model analysis can be directly compared with lattice QCD computations; for instance, extracted effective $G(B)$ and $M_\pi(B)$ agree quantitatively with lattice up to $eB \simeq 0.4\,\text{GeV}^2$ (Moreira et al., 2022).

Quantity	Trend with $B$	Field/Model
$m^2(B)$	Increases	Scalar EFRG (Ayala et al., 22 Dec 2025)
$\lambda(B)$	Decreases	Scalar EFRG (Ayala et al., 22 Dec 2025)
$G_{ff}(B)$	Decreases	NJL background field (Moreira et al., 2022)
$M_f^*(B)$	Non-monotonic	NJL gap eq. (Li et al., 2016)
$M_{\pi}(B)$	Decreases (modest)	NJL Bethe‐Salpeter (Moreira et al., 2022)
$F/A$	Decreases	Quark matter (Li et al., 2016)

6. Limitations and Outlook

The primary limitation of current scalar and NJL-type analyses is the restriction to one-loop order and the neglect of gauge field fluctuations, higher-order corrections, and full gauge invariance. In strongly-coupled QCD, additional effects—such as gluon loop-induced inverse magnetic catalysis and flavor-dependent vacuum polarization—become relevant at large fields. The observed trends in $B$-dependent beta functions, particularly the suppression of effective couplings and non-monotonic mass evolution, motivate further study in the context of gauge theories and at finite temperature or density. Extensions to non-Abelian gauge theories require worldline or proper-time techniques that preserve gauge invariance and systematically incorporate Landau-level dynamics (Ayala et al., 22 Dec 2025, Moreira et al., 2022, Li et al., 2016).

A plausible implication is that environmental RG approaches, using $B$ as the flow variable, offer a unifying framework for analyzing quantum field responses to strong external conditions, with direct applicability to lattice QCD, heavy ion collisions, and compact star physics. Quantitative discrepancies at high field strengths and in the pseudoscalar sector highlight the need for higher-order and nonperturbative treatments. The calculable shifts in meson mixing angles, such as the $\eta$–$\eta^\prime$ angle, provide potential signatures of magnetic field effects in precision hadronic experiments (Moreira et al., 2022).