REVIEW
Protecting the Primordial Baryon Asymmetry From Erasure by Sphalerons
High Energy Physics - Phenomenology · Submitted 5 Jan 1994 · 22 references
Pith's verdict: A careful, honest re-derivation of the e_R-decoupling effect on sphaleron erasure, with a real corrected number (20 keV neutrino bound) and a full Boltzmann network; the headline idea is from the authors' own PRL, and they say so.
T0 · AI review
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Referee report
The paper
The paper claims
The right-handed electron's tiny Yukawa coupling keeps it out of thermal equilibrium down to roughly a TeV, and that delay weakens bounds on exotic baryon- and lepton-number violation by as much as nine orders of magnitude.
The reading
What carries the argument
The machinery is a chemical-potential bookkeeping scheme layered on a Boltzmann network. Fast reactions (electroweak gauge interactions and sphalerons) are turned into equilibrium conditions among chemical potentials; charge neutrality and the sphaleron constraint close the system; and the slow reactions — $e_R ↔ e_L$ chirality flips and the exotic $B$- or $L$-violating operator — are kept as explicit rate equations. The load-bearing object is the $e_R$–$e_L$ equilibration rate $Γ_{RL}$, built from the decay integral $I_D$ plus the gauge and top-quark scattering integrals $I_G$ and $I_{top}$. The authors show that at small thermal Higgs mass the decays are phase-space suppressed and the scatterings dominate, which is what closes the loophole that a more accurate decay rate alone would have opened. Charge neutrality is what keeps a nonzero baryon number alive while the $e_R$ asymmetry persists, since the plasma must cancel the charge carried by the out-of-equilibrium electrons.
Extended reading notes
Core claim
The paper's central claim is that the decoupling of the right-handed electron sets the real clock on baryon erasure, and that this clock runs much slower than previously assumed. The authors build the full set of Boltzmann equations for the $e_R$ and $e_L$ asymmetries, compute every tree-level chirality-changing rate (Higgs decays, gauge and top-quark scatterings, with the on-shell s-channel resonance subtracted), and find that $e_R$ number comes into equilibrium only at $T_* = 1.3 f(x_H)$ TeV, where $x_H$ is the asymptotic thermal Higgs mass in units of temperature. Two consequences follow. First, in a universe with $B-L = 0$ the asymmetry is indeed erased, confirming earlier work, but for a different reason: $e_R$ relaxes not to zero but to a fixed value $-C_{e,p}/3$, and the baryon number dies exponentially as $B(t) = B_{eq} e^{-∫Γ dt}$. Second, and this is the point of the paper, when $B-L$ is violated by an exotic operator the erasure can only proceed between $T_*$ and the electroweak phase transition, so bounds on such operators relax dramatically: the dimension-nine operator limit falls from about $4 × 10^{12}$ GeV to about $30$ TeV, and the neutrino mass bound becomes $⟨m_{ei}⟩ ≲ 20$ keV.
Load-bearing premise
The claim rests on the right-handed electron remaining a pure spectator — the exotic $B$- and $L$-violating operators must not touch it — and on taking the largest imaginable primordial asymmetry, $2 × 10^{-3}$, as the starting value, since the quoted bound scales with the logarithm of the erasure factor.
Editorial extensions
If this is right
- Lower bounds on nonrenormalizable $B$- and $L$-violating operators weaken from roughly $4 × 10^{12}$ GeV to roughly $30$ TeV, so new physics at the TeV scale is no longer excluded by the requirement that a GUT-scale baryon asymmetry survive.
- The dimension-five operator that generates Majorana neutrino masses is bounded by $⟨m_{ei}⟩ ≲ 20$ keV, about an order of magnitude looser than earlier estimates, with the exact value depending on the assumed primordial asymmetry.
- A primordial lepton or $e_R$ asymmetry can generate a baryon asymmetry even when $B = 0$ initially, which turns the argument from a preservation statement into a production mechanism.
- In the $B - L = 0$ case the older negative conclusion stands: storing the asymmetry in $e_R$ cannot save it, so that route to protection is closed within the minimal standard model.
- For a dimension-nine operator such as $M^{-5}(\bar u_R d_R^c)(\bar d_R d_R^c)(\bar u_R d_R^c)$, the correct temperature to use in the bound is $T_* \sim 1$ TeV rather than the sphaleron turn-on temperature of $10^{12}$ GeV.
Reading between the lines
- The protection is conditional on the right-handed electron being a spectator under the new sector: any exotic interaction that couples directly to $e_R$ — a gauged $B-L$ boson light enough to be in equilibrium, for instance — would restore something close to the original stringent bounds, so the weakening is not a generic feature of all $B-L$ violation.
- Because $T_*$ scales with the thermal Higgs mass parameter $x_H$, the cosmological bound is a function of the top and Higgs masses; a heavier top quark or a lighter Higgs shortens the protective window, so collider mass measurements feed directly into a cosmological constraint in a way that was not available when the paper was written.
- The same bookkeeping should apply to $R$-parity-violating supersymmetric operators, which the paper mentions only in passing; it would be worth checking whether their limits weaken by a comparable factor or whether the extra colored fields change the equilibrium conditions.
- An $e_R$ that stays out of equilibrium down to about a TeV is a small, in-principle computable shift in the relativistic degrees of freedom during big bang nucleosynthesis, which offers an independent, if delicate, cross-check on the decoupling temperature.
Original abstract
If the baryon asymmetry of the universe was created at the GUT scale, sphalerons together with exotic sources of $(B-L)$-violation could have erased it, unless the latter satisfy stringent bounds. We elaborate on how the small Yukawa coupling of the electron drastically weakens previous estimates of these bounds.
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
If the baryon asymmetry of the universe was created at the GUT scale, sphalerons together with exotic sources of $(B-L)$-violation could have erased it, unless the latter satisfy stringent bounds. We elaborate on how the small Yukawa coupling of the electron drastically weakens previous estimates of these bounds.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | If the baryon asymmetry of the universe was created at the GUT scale, sphalerons together with exotic sources of $(B-L)$-violation could have erased it, unless the latter satisfy stringent bounds. | background 1.00 | 0.06 | 0.20 | hedged possibility 0.99 |
| s1 | We elaborate on how the small Yukawa coupling of the electron drastically weakens previous estimates of these bounds. | contribution 0.54 | 0.90 | 0.57 | asserted 1.00 |
What was examined, and what was not
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Figures
Showing 4 of the paper's 7 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:hep-ph/9401208 by the authors.
References
How the literature uses it
35 arXiv papers cite this in 53 sentences
Papers citing it · 38 sentences without a confident label
Contests it 2 papers
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However, this argument fails in the presence of interference terms and the prescription [ref] has been found to give superficial negative cross sections [ref] , which never happens in the subtraction scheme [ref].
Momentum distributions of cosmic relics: Improved analysis · RIS SUBTRACTION · contest 0.86
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Therefore we recompute $c _ t$ in [ref] using Maxwell-Boltzmann statistics for all particles, leading to $c _ t ^ MB = 2.14$, which is a relative error of $24%$ compared to the correct quantum statistics, as anticipated in [ref] .
Equilibration of right-handed electrons · Results and discussion · contest 0.71
Uses its method or tool 7 papers
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The CP asymmetry matrix $ε_αβ^(j)$ denoting CP asymmetry in the decay of $j^th$ neutrino in terms of Yukawa coupling and right-handed neutrino masses are given by [ref] , [equation] with [equation] Moreover, $P^i_αβ$ is the projection matrix describing how a particular combination of flavor asymmetry gets washed out via $i^th$ right-handed neutrino, and its tree-level value is given by [ref] , [equation] The last two terms in <ref> describe the effect of charged lepton interaction [ref] [equation] The $y_τ$ dependent interaction comes into thermal equilibrium when the temperature drops below $10^12 GeV$, leading to the decoherence of $τ$ lepton states.
Fermion mass, Axion dark matter, and Leptogenesis in SO(10) GUT · Leptogenesis · 0.80
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We follow the procedure from Ref. [ref] (see also Refs. [ref] ) and subtract $σ_s$ by [equation] For the Boltzmann equations we require the thermally averaged cross sections, summed over initial and final flavors.
Testing Dirac leptogenesis with the cosmic microwave background and proton decay · Involved cross sections · 1.00
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Here, we follow Refs. [ref] with a real-intermediate-state subtraction by splitting the Breit-Wigner form of the scalar propagator as [equation] where $Γ_ϕ$ is the decay width of the thermal scalar.
Scattering versus Forbidden Decay in Dark Matter Freeze-in · Introduction · 1.00
Uses its data or values 1 paper
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[ref] to be $γ_E≈5×10^-3T^4/6$ where thermal corrections and scatterings involving gauge fields and quark fields are taken into account.
Cosmic Evolution of Lepton Flavor Charges · Lepton flavor effect · 1.00
Compares with it 1 paper
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With the values for the gauge couplings of [ref] , our result is about $50%$ larger [Cf. equations (25) through (27) in [ref] .] which could be due to the use of classical statistics and of zero-momentum thermal fermion masses in [ref] .
Equilibration of right-handed electrons · Results and discussion · 0.98
Cites it as background 2 papers
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A matter-antimatter asymmetry created at some very high temperature like, e.g. in GUT baryogenesis, can be protected from washout if the right-handed electrons [ref] are not yet in equilibrium.
Equilibration of right-handed electrons · Introduction · 0.86
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[See also Refs. [ref] for related earlier work as well as Refs. [ref] for some related discussions in the more recent literature.]
Wash-in leptogenesis after axion inflation · Introduction · 0.93
Discussion
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