REVIEW
Primordial Hypermagnetic Fields and Triangle Anomaly
High Energy Physics - Phenomenology · Submitted 3 Oct 1997 · 53 references
Pith's verdict: A clean, honest derivation of anomalous MHD with a self-consistent BBN bound, but the abstract oversells the fluctuation amplitude and the promised nonlinear check never arrives.
T0 · AI review
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Referee report
The paper
The paper claims
A tangled primordial hypercharge field could have written fermion number into the early plasma, leaving matter-antimatter domains that standard nucleosynthesis would forbid.
The reading
What carries the argument
The carrying object is the anomalous magnetohydrodynamics (AMHD) system: the hypermagnetic diffusivity equation $∂H_Y/∂t = -(4α'/πσ) ∇ × (µ_R H_Y) + (1/σ_c)∇^2 H_Y$, derived from a generalized Ohm's law whose extra term $(α'/π)µ_R H_Y$ is a current parallel to the field, fed by finite fermion density. Coupled to the kinetic equation $∂(µ_R/T)/∂t = -(g'^2/4π^2σ_cT^3)(783/88) H_Y · ∇ × H_Y - (Γ+Γ_H)(µ_R/T)$, this closes into a system in which the field and the fermion number exchange energy. The anomaly-induced current is the formal analogue of the dynamo $α$-term of ordinary MHD, and the authors use that analogy explicitly: both are mean ohmic currents along the field, one fed by vorticity, the other by fermion density. Solving in the adiabatic approximation $∂µ_R/∂t ~ 0$ gives the chemical potential as a local functional of the field configuration, which is what makes the whole construction predictive for a generic stochastic background.
Extended reading notes
Core claim
The central claim is Eq. (2.30) together with Eq. (2.32): the baryon number released at the electroweak phase transition is fixed by the time-integrated anomaly, $n_B(t_c) = -(n_f/2) Δn_{CS}(t_c)$ with $Δn_{CS} = -y_R^2 g'^2/(16π^2) ∫_0^{t_c} E_Y · H_Y dt$, and in the adiabatic regime this evaluates to a fluctuation $δ(n_B/s) = (α'/2πσ_c)(n_f/s) (H_Y · ∇ × H_Y)/(Γ+Γ_H) (Γ M_0/T_c^2)$, where $Γ_H = (783/22)(α'^2σ_c/π^2)|H_Y|^2T^2$. Two features carry the argument. First, the anomaly source is the helicity density $H_Y · ∇ × H_Y$ rather than the field strength alone, so a parity-invariant stochastic background with zero mean helicity still produces non-zero fluctuations with zero mean, giving inhomogeneities rather than a net asymmetry. Second, the time integration gives an amplification $Γ M_0/T_c^2$, and in the regime $Γ_H < Γ$ the chirality-flip rate cancels out entirely, leaving a fluctuation fixed by the spatial shape of the field rather than its magnitude. For a flat spectrum with field energy comparable to radiation energy at the smallest surviving scale $r_σ ~ 10^{-9}$ of the electroweak horizon, the authors estimate $Δ ~ 10^3$, far above the observed $n_B/s ~ 10^{-10}$, and they state plainly that this number is large enough that back-reaction must matter. Requiring $Δ(L_{diff},t_c) < n_B/s$ at the neutron diffusion scale instead yields the exclusion curve of Eq. (5.4) in the amplitude-slope plane, which the authors show lies below the corresponding bounds from microwave-background isotropy (Eq. 5.7), Faraday rotation (Eq. 5.8) and the direct nucleosynthesis limit on magnetic energy density (Eq. 5.9) for blue spectra.
Load-bearing premise
The derivation assumes the right-electron chemical potential adjusts adiabatically ($∂µ_R/∂t ~ 0$) and ignores back-reaction of the fermions just created on the field, an approximation the authors themselves flag: they call the $Δ ~ 10^3$ estimate of Eq. (5.1) 'unlikely to be correct' and say that numerical solutions of the nonlinear system are required once $Δ ~ 1$, which is exactly the regime the headline numbers occupy.
Editorial extensions
If this is right
- A stochastic hypermagnetic background with a sufficiently flat (blue) energy spectrum produces matter-antimatter domains at and above the neutron diffusion scale, which standard homogeneous nucleosynthesis does not cover.
- The requirement that homogeneous Big Bang nucleosynthesis be unchanged converts into the exclusion curve of Eq. (5.4) on the slope-amplitude plane, which is more restrictive than the microwave-background, Faraday-rotation and direct magnetic-energy bounds for spectra with $ε > 0.05$.
- The Meissner pressure $\tfrac{1}{2}|\vec H_Y|^2\sin^2θ_W$ on the phase boundary can make the electroweak transition strongly first order even for Higgs masses up to about 160 GeV, removing a standard objection to electroweak baryogenesis in the minimal model.
- A Chern-Simons wave configuration with $\vec H_Y\cdot\vec∇\times\vec H_Y = k_0 H^2(t)$ yields a net asymmetry of the right order $10^{-10}$ for $k_0/T_c ~ 10^{-12}$ in the minimal model, or for $k_0/T_c ~ 10^{-16}$ and $H^2 ~ 10^{-4}T_c^4$ in extensions with a larger electron Yukawa coupling.
Reading between the lines
- The bound in Eq. (5.4) is a genuinely new handle on sub-horizon primordial fields: the microwave-background and Faraday constraints only reach fields coherent on enormous scales today, whereas this one probes the smallest scales that survive hyperconductivity, roughly $10^{-9}$ of the electroweak horizon.
- Because the fluctuation amplitude in the $Γ_H < Γ$ regime is independent of the field magnitude, an experimental or observational determination of small-scale magnetic helicity would translate directly into a prediction for baryon inhomogeneity, with no amplitude calibration needed.
- If matter-antimatter domains of this kind are allowed rather than excluded, the standard practice of quoting a single baryon-to-photon ratio from nucleosynthesis would need revision, since the local ratio would vary by orders of magnitude across the neutron diffusion length.
- The same anomaly coupling runs in reverse, so a pre-existing right-electron asymmetry should regenerate hypermagnetic field on the scales that survive diffusion, a channel worth checking against any claimed seed-field generation mechanism.
Original abstract
The high-temperature plasma above the electroweak scale $\sim 100$ GeV may have contained a primordial hypercharge magnetic field whose anomalous coupling to the fermions induces a transformation of the hypermagnetic energy density into fermionic number. In order to describe this process, we generalize the ordinary magnetohydrodynamical equations to the anomalous case. We show that a not completely homogeneous hypermagnetic background induces fermion number fluctuations, which can be expressed in terms of a generic hypermagnetic field configuration. We argue that, depending upon the various particle physics parameters involved in our estimate (electron Yukawa coupling, strength of the electroweak phase transition) and upon the hypermagnetic energy spectrum, sizeable matter-antimatter fluctuations can be generated in the plasma. These fluctuations may modify the predictions of the standard Big Bang nucleosynthesis (BBN). We derive constraints on the magnetic fields from the requirement that the homogeneous BBN is not changed. We analyse the influence of primordial magnetic fields on the electroweak phase transition and show that some specific configurations of the magnetic field may be converted into net baryon number at the electroweak scale.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
The high-temperature plasma above the electroweak scale $\sim 100$ GeV may have contained a primordial hypercharge magnetic field whose anomalous coupling to the fermions induces a transformation of the hypermagnetic energy density into fermionic number. In order to describe this process, we generalize the ordinary magnetohydrodynamical equations to the anomalous case. We show that a not completely homogeneous hypermagnetic background induces fermion number fluctuations, which can be expressed in terms of a generic hypermagnetic field configuration. We argue that, depending upon the various particle physics parameters involved in our estimate (electron Yukawa coupling, strength of the electroweak phase transition) and upon the hypermagnetic energy spectrum, sizeable matter-antimatter fluctuations can be generated in the plasma. These fluctuations may modify the predictions of the standard Big Bang nucleosynthesis (BBN). We derive constraints on the magnetic fields from the requirement that the homogeneous BBN is not changed. We analyse the influence of primordial magnetic fields on the electroweak phase transition and show that some specific configurations of the magnetic field may be converted into net baryon number at the electroweak scale.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | The high-temperature plasma above the electroweak scale $\sim 100$ GeV may have contained a primordial hypercharge magnetic field whose anomalous coupling to the fermions induces a transformation of the hypermagnetic energy density into fermionic number. | background 0.99 | 0.10 | 0.17 | hedged possibility 1.00 |
| s1 | In order to describe this process, we generalize the ordinary magnetohydrodynamical equations to the anomalous case. | method 0.93 | 0.91 | 0.03 | asserted 0.83 |
| s2 | We show that a not completely homogeneous hypermagnetic background induces fermion number fluctuations, which can be expressed in terms of a generic hypermagnetic field configuration. | result 0.88 | 0.94 | 0.12 | asserted 1.00 |
| s3 | We argue that, depending upon the various particle physics parameters involved in our estimate (electron Yukawa coupling, strength of the electroweak phase transition) and upon the hypermagnetic energy spectrum, sizeable matter-antimatter fluctuations can be generated in the plasma. | result 0.61 | 0.75 | 0.42 | hedged possibility 0.88 |
| s4 | These fluctuations may modify the predictions of the standard Big Bang nucleosynthesis (BBN). | interpretation 0.95 | 0.28 | 0.13 | hedged possibility 1.00 |
| s5 | We derive constraints on the magnetic fields from the requirement that the homogeneous BBN is not changed. | result 0.63 | 0.94 | 0.14 | asserted 0.99 |
| s6 | We analyse the influence of primordial magnetic fields on the electroweak phase transition and show that some specific configurations of the magnetic field may be converted into net baryon number at the electroweak scale. | result 0.66 | 0.93 | 0.34 | hedged possibility 0.61 |
What was examined, and what was not
- arXiv listing abstract v1
- full text
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Figures
Figures reproduced from arXiv:hep-ph/9710234 by the authors.
References
How later papers used it
81 later papers cite it
1 later paper by other authors is listed below: 1 compared its own results with it.
Compares with it 1 of 3 papers with this label
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Said that its values of the magnetic field and correlation length for the collision dominated case are similar to those given in this paper, and believed they are also consistent with BBN constraints.
Origin and dynamics of the Primordial Magnetic field in a parity violating plasma (2017) · 0.84 · paraphrased: the licence does not allow quoting
How the literature uses it
65 arXiv papers cite this in 134 sentences
Papers citing it · 98 sentences without a confident label
Builds on it 1 paper
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This work was soon extended by Giovannini and Shaposhnikov [ref] to consider the generation of baryon-number isocurvature fluctuations from a preexisting stochastic hypermagnetic field.
Evolution of the Baryon Asymmetry through the Electroweak Crossover in the Presence of a Helical Magnetic Field · Introduction · 0.83
Compares with it 2 papers
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In summary, we obtain the two-point correlation function of the helicity in terms of the power spectrum as [equation] We can easily check that Eq. [ref] is consistent with the one given in Ref. [ref] .
Baryon isocurvature constraints on the primordial hypermagnetic fields · Correlation functions in the position space · 1.00
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Thus the values of the magnetic field and the correlation length scale estimated by us can be consistent with the current bounds obtained from CMB observation and necessary for current observed baryon asymmetry Since the values of $ℬ_p$ and $λ_B$ for the collision dominated case are similar to those given in the Ref. [ref] , we believe that they are also consistent with BBN constraints.
Origin and dynamics of the Primordial Magnetic field in a parity violating plasma · Discussion · 0.84
Cites it as support 2 papers
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We find that since the baryon isocurvature perturbation is generated even from non-helical hypermagnetic fields [ref] , the constraint becomes severer for less helical hypermagnetic fields, in which case hypermagnetic fields with longer coherence length are needed for the BAU.
Baryon isocurvature constraints on the primordial hypermagnetic fields · Introduction · 0.96
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Once we assume that helical hypermagnetic fields are generated before the electroweak symmetry breaking at $T≳100 GeV$, it is found that the baryon asymmetry is generated automatically through the hypermagnetic helicity decay without introducing any new physics [ref] (see also Ref. [ref] ).
Chiral effects in astrophysics and cosmology · Baryogenesis · 0.97
Cites it as background 23 papers
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Recently, increasing efforts and interests have been devoted to the baryogenesis mechanisms via the decay of magnetic helicity of the $U(1)_Y$ gauge field in the standard model [ref] .
Leptogenesis from magnetic helicity of gauged $\text{U}(1)_{B-L}$ · Introduction · 0.94
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Here, $g'$ is the U$_Y$(1) gauge coupling [ref] .
The effects of the U$_\textrm{Y}$(1) Chern-Simons term and its baryonic contribution on matter asymmetries and hypermagnetic fields · Introduction · 0.83
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In the presence of these anomalous effects, the ordinary magnetohydrodynamic (MHD) equations are generalized to the equations of anomalous magnetohydrodynamics (AMHD) [ref] .
Contribution of the thermal mass to the chiral vortical effect and magnetobaryogenesis · Introduction · 0.81
Discussion
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