Lorentz-Violating Parameter in QFT

• Lorentz-violating parameters are coupling constants that quantify explicit symmetry breaking in quantum field theories, defined within the Standard-Model Extension.
• They are tightly bounded by high-precision lab experiments, astrophysical observations, and cosmological measurements across photon, fermion, and higher-dimensional sectors.
• A metric-based approach using the Zamolodchikov method rigorously quantifies the 'distance' to Lorentz invariance, aiding model comparisons and experimental design.

A Lorentz-violating parameter is a coupling constant or tensor coefficient that quantifies explicit violations of Lorentz invariance in quantum field theories. Such parameters arise when extending the Standard Model (SM) via effective field theory—most systematically codified by the Standard-Model Extension (SME)—and appear in both renormalizable (dimension ≤4) and non-renormalizable operators. Empirically, Lorentz-violating parameters are bounded to be extremely small, with state-of-the-art constraints arising from high-precision laboratory experiments, astrophysical observations, and cosmological measurements.

1. Formal Construction: Parameter Space and Symmetry Distance

Consider a quantum field theory specified by a Lagrangian ℒ(λ) depending on a set of couplings λ^I. In the presence of explicit symmetry breaking—such as Lorentz violation—one organizes λ^I into a symmetric subspace S (e.g., Lorentz-invariant couplings) and a set of Lorentz-violating (LV) directions. The "distance" of a theory from the Lorentz-invariant surface S is defined via a Riemannian metric in coupling space, constructed à la Zamolodchikov using the two-point function of the Lagrangian perturbation operators: $$d\ell^2 = 2\pi^4 \hat{x}^8 \langle \delta\mathcal{L}(\hat{x}_v)\,\delta\mathcal{L}^\theta(0) \rangle$$ where δℒ(λ) = δλ^I O_I, the O_I are local operators, and θ is the Osterwalder–Schrader reflection. The explicit metric is

$$g_{IJ}(\hat{x},\lambda) = 2\pi^4 \hat{x}^8 \langle O_I(\hat{x}_v)\,O_J^\theta(0) \rangle$$

The finite distance to the Lorentz-invariant submanifold S is obtained by minimizing the length of all possible paths in parameter space connecting the theory to S. To ensure this measure is physically meaningful (not an artifact of coordinate choices, total derivatives, or field redefinitions), one performs additional minimizations over these equivalence classes, with the physical distance given by: $$d(\tau_1, \tau_2) = \min_{\mathcal{L}_1',\mathcal{L}_2'} d_{\mathcal{L}}(\tau_{\mathcal{L}_1'}, \tau_{\mathcal{L}_2'})$$ This construction robustly quantifies the severity of Lorentz violation induced by the set of parameters.

2. Lorentz-Violating Operators and Parameter Taxonomy

In the SME and its generalizations, Lorentz-violating parameters appear in various sectors. In the fermion sector, representative terms include

• CPT-odd, dimension-3: $a_\mu \bar{\psi} \gamma^\mu \psi$, $b_\mu \bar{\psi} \gamma_5 \gamma^\mu \psi$
• CPT-even, dimension-4: $c_{\mu\nu} \bar{\psi} \gamma^\mu i D^\nu \psi$, $$d_{\mu\nu} \bar{\psi} \gamma_5 \gamma^\mu i D^\nu \psi$$, $H_{\mu\nu} \bar{\psi} \sigma^{\mu\nu} \psi$

In the photon sector:

• CPT-even: $$-\frac{1}{4} (k_F)_{\kappa\lambda\mu\nu} F^{\kappa\lambda} F^{\mu\nu}$$
• CPT-odd: $$(k_{AF})^\kappa \epsilon_{\kappa\lambda\mu\nu} A^\lambda F^{\mu\nu}$$

In higher-dimensional or nonminimal sectors, dimension-5 and dimension-6 operators also arise, often suppressed by a high scale (Planck or beyond)—the coefficients remain the key LV parameters.

Several LV parameters serve as benchmarks due to their clear physical roles or their appearance in phenomenology:

• The photon-sector isotropic "deformation parameter" $\widetilde{\kappa}_{\rm tr}$ modifies the phase velocity of electromagnetic waves (0809.3217).
• The neutrino-sector velocity parameter δ modifies the "maximum speed" and thus the dispersion relation for neutrinos, tightly constrained by IceCube and atmospheric experiments (Somogyi et al., 2019).
• In supersymmetric LV theories, the parameter c enters the supersymmetry algebra and acts as an anisotropy between spatial and temporal directions; it is strictly constrained by experiment and shown not to renormalize (Redigolo, 2011).

3. Elimination of Ambiguities and the Reduced Metric

A central subtlety is the nonuniqueness of operator bases and the possible redundancy of parameters under coordinate/frame redefinitions or total derivative terms. For a Lagrangian

$$\mathcal{L} = \mathcal{L}_{LI}(\lambda^a) + \zeta^i O_i^{LV} + \xi^a O_a^{LI}$$

one constructs the reduced metric by projecting out tangent (Lorentz-invariant) directions: $$\gamma_{ij} = g_{ij} - g_{ia} h^{ab} g_{bj}, \quad h^{ab} = (g_{ab})^{-1}$$ so that the squared distance to S becomes $d_L^2 = \zeta^i \gamma_{ij}(\zeta) \zeta^j$. In concrete models, this projection eliminates spurious dependence on unphysical parameterizations or total derivatives. Subsectors such as the photon or fermion sector may employ standard conventions (e.g., symmetric, traceless c^{\mu\nu}, dual-trace-free k_F) to encapsulate only physical Lorentz-violating parameters.

4. Applications: QED Sector and Current Constraints

In the SME's minimal QED subsector, after minimizing over redundant structures, one obtains physical distances governed by genuine LV coefficients. For instance, in the photon sector (before coordinate minimization): $d\ell_\gamma^2 = \sum (k_F^{\mu\nu\rho\sigma})^2$ After imposing dual-trace-free and tracelessness conditions, the independent parameters reduce to the well-known sets $\tilde{k}_{e\pm}$, $\tilde{k}_{o\pm}$, and $\tilde{k}_{tr}$, with sensitivity to O($10^{-32}$) in some birefringent components. In the fermion sector, minimizing over spinor redefinitions leads to distances determined by symmetric, traceless c^{\mu\nu} and d^{\mu\nu}, with current electron-sector bounds yielding $d\ell_e \lesssim 2 \cdot 10^{-14}$ for probe energies below $m_e$.

The total CPT-even QED-sector LV distance, after rescaling-minimization, is approximately $d\ell_{QED} \sim 2 \cdot 10^{-13}$ (in conventional normalization where a deep-IR scalar mass jump is order unity) (Anselmi et al., 2011). This suggests that current laboratory and astrophysical experiments have confirmed Lorentz invariance in QED to at least 13 decimal places, and in the photon sector to $\sim10^{-32}$ in certain components.

5. Higher-Dimensional and Weighted-Power-Counting Sectors

Nonminimal SME and weighted power counting permit Lorentz-violating operators of higher dimension, such as: $$- \frac{1}{4}\delta_2 (F^{ij})^2 - \frac{\tau_1}{4\Lambda_L^2} F^{ij} \partial_k^2 F^{ij} - \frac{\tau_0}{4\Lambda_L^4} (\partial_k^2 F^{ij})^2$$ Current bounds (e.g., $$\tau_0/\Lambda_L^4 \lesssim 10^{-24}\,\mathrm{GeV}^{-4}$$) imply the corresponding distance

$$d\ell^2 \lesssim 6 \times 10^{-28} [1 + O(10^{-6}q^2 + 10^{-15}q^8)]$$

for $q \lesssim 1$ GeV, and the distance grows with $q$ as unbounded higher-derivative LV operators become increasingly weakly constrained at high energies. This reflects an increasing lack of experimental sensitivity to certain LV operators as energy increases (Anselmi et al., 2011).

6. Illustrative Model Cases and Physical Interpretation

Simple free field theories further illuminate the structure of the LV parameter distance. For a free massive scalar, the perturbation $\delta\mathcal{L} = m\,dm\,\phi^2$ yields a finite distance $d(m_2,m_1)$ vanishing in both UV and IR as long as both masses are nonzero. For a massive vector $W_\mu$, the UV behavior scales logarithmically with mass.

The finite distance is an RG-invariant quantity: under RG flow, both the metric $G_{IJ}(t, \hat{\lambda})$ and the running of the couplings conspire to ensure $d\ell$ remains scale invariant, reflecting only the physical departure from Lorentz symmetry, independent of the renormalization scale or parameterization.

Summary Table: Representative Lorentz-Violating Parameters and Distance Constraints

Parameter/Class	Example in SME terms	Phenomenological Sensitivity
Photon sector	$(k_F)_{\mu\nu\rho\sigma}$ (nonbirefringent)	$<10^{-32}$ (birefringent), $10^{-16}$ (isotropic)
Fermion sector	$c^{\mu\nu}$, $d^{\mu\nu}$	$10^{-14}$ (electron)
QED sector distance	$d\ell_{QED}$	$2 \cdot 10^{-13}$
Higher-dim. (e.g., $\tau_{1,0}$)	Dimension-6,8 photon operators	$<10^{-24}$ GeV$^{-4}$

A plausible implication is that, given the RG invariance and operational definition via the Zamolodchikov metric, the global "distance to Lorentz invariance" provides a universal, normalization-independent, and experimentally meaningful measure of symmetry breaking in QFT.

7. Implications and Outlook

The use of distance in coupling space to quantify Lorentz violation provides a rigorous, positive-definite metric for assessing both the absolute and relative severity of different LV effects. Current experimental data imply that the physically meaningful LV distance in the QED sector is $d\ell_{QED} \sim 2 \cdot 10^{-13}$. Inclusion of higher-dimensional operators increases this distance at higher energies, reflecting the relatively weak bounds on such operators.

The methodology is applicable not only for parameter interpretation within SME but also for comparing fundamentally different models of Lorentz violation, benchmarking prospective improvements in experimental design, and informing theory building in quantum gravity and physics beyond the Standard Model (Anselmi et al., 2011).