Electron-Positron Mass Asymmetries

• Electron-Positron Mass Asymmetries are differences between electrons' and positrons' effective masses, probing violations of the Weak Equivalence Principle and CPT symmetry.
• High-energy collider experiments and cosmological tests, including BBN, impose stringent bounds on these asymmetries via forbidden processes and modified dispersion relations.
• Field-theoretic models with temperature-dependent CPT-violating effects show that mass asymmetries vanish at low temperatures while allowing keV-level gaps in the early universe.

Electron-Positron Mass Asymmetries are experimentally constrained differences between the effective gravitational or inertial masses of the electron ($e^-$) and positron ($e^+$), as well as between their fundamental dispersion relations at finite temperature or in the presence of external backgrounds. These asymmetries directly probe foundational principles such as the Weak Equivalence Principle (WEP), CPT symmetry, and extensions of General Relativity and Quantum Field Theory. Modern limits derive from both high-energy collider searches for forbidden processes and cosmological tests based on Big Bang Nucleosynthesis (BBN), spanning laboratory and early-universe regimes. Field-theoretic models that permit electron-positron mass splitting have been developed, including temperature-dependent CPT-violating backgrounds with distinctive $T^2$ scaling. The anomalous mass difference, denoted $\Delta m \equiv m_{e^-} - m_{e^+}$ (or, in the gravitational sector, $\Delta m_g$), is subject to stringent experimental and observational bounds.

1. Theoretical Framework

The WEP postulates equivalence between inertial mass $m$ and gravitational mass $m_g$ for all particle species; General Relativity assumes $m = m_g$ universally, with extensive experimental confirmation at low energies for normal matter. For antimatter, and in particular for electrons/positrons at relativistic energies, direct experimental confirmation is lacking. Violations of WEP permit $m_g \neq m$ for $e^-$ and $e^+$, resulting in modified dispersion relations in external gravitational potentials.

Deviations are parametrized via $\Delta m \equiv m_g - m$ and expressed as a small expansion parameter $\kappa \equiv 2|\Phi| (\Delta m/m)$, where $\Phi$ is the gravitational potential. The background metric in weak-field approximation takes the form: $$ds^2 = H^2 dt^2 - H^{-2} (dx^2 + dy^2 + dz^2), \quad H^2 \equiv 1 + 2\Phi$$ For a test particle, the effective potential is $\Phi_m = \Phi \cdot (m_g / m)$, leading to the first-order dispersion relation

$$p(\mathcal{E}) = \mathcal{E}\left[1 + 2|\Phi| \frac{\Delta m}{m}\right]\sqrt{1 - \frac{m^2}{\mathcal{E}^2}}$$

This formalism underpins indirect experimental bounds via high-energy processes (Kalaydzhyan, 2015).

Temperature-dependent CPT-violating backgrounds introduce further asymmetries. A universal CPT-violating parameter $b_0(T) = \alpha T^2$ (with $\alpha$ of mass dimension $-1$) shifts the electron and positron masses: $$m_e(T) = m_0 + b_0(T), \quad m_{e^+}(T) = m_0 - b_0(T), \quad \Delta m(T) = 2\alpha T^2$$ This allows for sizable early-universe mass gaps ($\sim$ keV at $T \sim$ MeV) while automatically suppressing asymmetry at $T \to 0$ (Barenboim et al., 9 Jan 2026).

2. Forbidden Processes and Experimental Constraints

High-energy collider experiments enable stringent indirect tests of mass asymmetry through kinematic investigations of forbidden processes:

• Vacuum Cherenkov Radiation ($e^\pm \to e^\pm \gamma$): If $\kappa < 0$, the electron (or positron) group velocity exceeds $c$, enabling spontaneous emission of real photons in vacuum above energy threshold $\mathcal{E}_\text{th} = m / \sqrt{-2\kappa}$. Absence of this process for $e^\pm$ at $104.5$ GeV at LEP sets $\mathcal{E}_\text{th} > 100$ GeV and thus $\kappa_\text{min} \simeq -1.31 \times 10^{-11}$.
• Photon Decay ($\gamma \to e^+ e^-$): For $\kappa > 0$, photon decay to $e^+ e^-$ becomes kinematically allowed above threshold $\omega_\text{th} = \sqrt{2 m^2 / \kappa}$. Non-observation for photons with $\omega > 300$ GeV at Tevatron implies $\kappa_\text{max} \simeq +5.80 \times 10^{-12}$.

Combining these with astrophysical potentials yields the key bounds (Kalaydzhyan, 2015):

Potential Choice	Lower Bound	Upper Bound	$m_g/m$ Bound
Sun's Potential ($\Phi_\odot$)	$\kappa_\text{min} = -1.31 \times 10^{-11}$	$\kappa_\text{max} = +5.80 \times 10^{-12}$	$0.96 < m_g/m < 1.04$
Local Supercluster ($\Phi_\text{LS}$)	$\kappa_\text{min} = -1.31 \times 10^{-11}$	$\kappa_\text{max} = +5.80 \times 10^{-12}$	$1 - 4\times10^{-7} < m_g/m < 1 + 2\times10^{-7}$

A plausible implication is that antigravity-type scenarios (where antimatter is repelled by Earth) are excluded within these empirical limits.

3. Cosmological Tests: BBN Constraints

Early-universe electron-positron mass asymmetries are tightly constrained by their effects on Big Bang Nucleosynthesis (BBN). The approach introduces: $$b_0(T) = \alpha T^2, \quad \Delta m(T) = 2\alpha T^2$$ Modified dispersion relations impact several key steps:

• Friedmann Equation: The Hubble rate $H^2 = (8\pi G / 3) \rho_\text{tot}$ depends on separate electron and positron contributions with shifted masses.
• Chemical Potential Evolution: Charge neutrality is enforced via

$n_{e^-}(m_e, \mu, T) = n_{e^+}(m_{e^+}, -\mu, T)$

• Weak Interactions: Neutron-proton conversion rates incorporate $m_e$ or $m_{e^+}$, modifying freeze-out conditions.
• Nuclear Reaction Network: Observables $Y_P$ (Helium-4 fraction), $D/H$ (deuterium-to-hydrogen ratio), and $N_\text{eff}$ (effective relativistic degrees of freedom) are sensitive to $\Delta m(T)$.

Numerical implementation is realized in the modified \texttt{PRyMordial} BBN code, with full finite-mass and chemical potential effects (Barenboim et al., 9 Jan 2026). Linearized sensitivities are: $$\Delta Y_P \simeq S_Y \alpha, \quad S_Y \approx -0.002$$

$$\Delta(D/H) \simeq S_D \alpha, \quad S_D \approx +1.2 \times 10^{-5}$$

$$\Delta N_\text{eff} \simeq S_N \alpha, \quad S_N \approx +0.02$$

Observational constraints (2$\sigma$) lead to $|\alpha| \lesssim 1.1 \times 10^{-6}$ GeV$^{-1}$, $|\Delta m (1\,\text{MeV})| \lesssim 2.2$ keV.

4. Field-Theoretic Models for Mass Asymmetry

Explicit field-theoretic mechanisms for temperature-dependent electron-positron mass asymmetry with $T^2$ scaling have been constructed:

(a) Cubic Proca-Potential Model

A massive vector field $B_\mu$ is introduced with the Lagrangian

$$\mathcal{L} = -\frac{1}{4} F^{\mu\nu} F_{\mu\nu} - V(B_\mu, T) + \overline{\psi}(i \gamma^\mu \partial_\mu - m_\psi - g B_\mu \gamma^\mu \gamma^5)\psi$$

Analyzing the effective finite-$T$ potential: $$V_\text{eff}(B_0, T) = -\frac{\lambda}{6} B_0^3 + \frac{1}{2} c T^2 B_0^2$$ Minimization yields $\langle B_0 \rangle \sim T^2$ and thus mass asymmetry $b_0(T) = g \langle B_0 \rangle \propto T^2$.

(b) Scalar-Vector EFT with Phase Transition

A real scalar $\phi$ couples to the vector via

$$V_\phi = \frac{1}{2} \mu_\phi^2 \phi^2 + \frac{1}{4} \lambda_\phi \phi^4 - \frac{1}{2} c_\phi T^2 \phi^2$$

$$\mathcal{L} \supset -\frac{\kappa}{M} \phi^2 u^\mu B_\mu - \frac{1}{4} \lambda_B (B_\mu B^\mu)^2$$

For $T > T_c \equiv \mu_\phi / \sqrt{c_\phi}$, the induced vacuum expectation value gives $\langle B_0 \rangle \propto T^2$.

(c) PT-Symmetric Quantum-Mechanical Model

A non-Hermitian coordinate $B_0$ with PT-symmetric Hamiltonian: $$H = \frac{1}{2} p^2 - i \lambda B_0^3 + \frac{1}{2} c T^2 B_0^2$$ High-temperature thermal effective potential yields saddle-point $\langle B_0 \rangle \sim T^2$.

In all models, $b_0(T)$ naturally vanishes as $T \to 0$, ensuring restoration of CPT symmetry today.

5. Methodological Implementation and Computational Tools

Simulation of BBN with electron-positron mass asymmetries utilizes the public \texttt{PRyMordial} code, with key modifications:

• Separate computation of electron/positron energy density and pressure for $m_e \neq m_{e^+}$.
• Dynamical solution of charge neutrality condition for $\mu(T)$.
• Accurate weak $n \leftrightarrow p$ rates including radiative and finite-mass effects, using $m_e(T), m_{e^+}(T)$.
• Incorporation of neutrino-electron collision terms with finite-mass corrections from the NUDEC_BSM_v2 tables, weighted by actual particle masses.
• Rescaling of QED plasma corrections using $m_\text{eff} = (m_e + m_{e^+})/2$.
• Enforcement of standard atomic mass-excess benchmarks to maintain nuclear binding energy normalization.

This approach permits extraction of constraints on $\alpha$ and $\Delta m(T)$ from observed $Y_P$, $D/H$, and $N_\text{eff}$ abundances (Barenboim et al., 9 Jan 2026).

6. Implications, Limits, and Outlook

Current laboratory and cosmological data exclude large electron-positron mass asymmetries and antigravity scenarios at the levels probed. Collider bounds yield $0.96 < m_g/m < 1.04$ (solar potential) and $1-4\times10^{-7} < m_g/m < 1+2\times10^{-7}$ (Local Supercluster potential) (Kalaydzhyan, 2015). In the early universe, BBN measurements constrain potential CPT-violating mass asymmetries to $\Delta m(T\sim 1\,\mathrm{MeV}) \lesssim$ 1.6--2.6 keV, depending on observable (Barenboim et al., 9 Jan 2026).

A plausible implication is that any field-theoretic model aiming to explain observable effects via electron-positron mass splitting must satisfy stringent $\alpha$ bounds and vanish identically at laboratory temperatures. Complementary accelerator experiments at future facilities (ILC, CLIC) and precision cosmological surveys could further tighten constraints or uncover small violations. These tests probe both particle physics extensions and foundational principles in gravity and cosmology.