In models with more than one Higgs doublet, the Yukawa couplings of the d-type (u-type) quarks are changed by a factor Ad (Au), which is often a ratio of vacuum expectation values. Abbott, Sikivie, and Wise showed that the KL-KS mass difference places stringent constraints on Au (Au10 for a charged-Higgs-boson mass of mW); however, this bound is very sensitive to the charged-Higgs-boson mass and to small Kobayashi-Maskawa mixing angles. We note that if the enhancement is greater than 4-5, then the top quark's Yukawa coupling will exceed its strong coupling, even if mt∼35 GeV. The effects of this enhancement on the e+e- cross section, on energy levels, leptonic widths, and decay rate of t-quarkonium, and on top-quark decays are discussed; the former two effects are sensitive to neutral-scalar masses only, the latter to the charged-Higgs-boson mass; all of the effects are insensitive to small mixing angles. We also note that the best limit on Ad comes from recent data on the semileptonic branching ratio of b quarks. © 1985 The American Physical Society.

Recently, the EMPRESS collaboration has included new data in the extraction of the primordial $^4$He abundance from Big Bang Nucleosynthesis (BBN), resulting in a determination that differs from the previous value and from theoretical expectations. There have been several studies attempting to explain this anomaly which involve variation of fundamental constants between the time of BBN and the present. Since the Higgs vacuum expectation value (vev) is the only dimensionful parameter in the Standard Model and it is already known to vary during the electroweak phase transition, we consider the possibility that the vev is slightly different during BBN compared to its present value. A modification of the vev changes not only particle masses but also affects, through mass thresholds, the QCD confinement scale. We use the recently developed PRyMordial program to study this variation and its impact on the $^4$He and deuterium abundances. We find that bounds on $|{\delta v}/{v}|$ are approximately $0.01$, and that the EMPRESS result can be explained within $2\sigma$ if $0.008 < {\delta v}/{v}< 0.02$, but at the cost of worsening the current $2\sigma$ discrepancy in the deuterium abundance to over $3\sigma$.

Recently, the EMPRESS Collaboration has included new data in the extraction of the primordial He4 abundance from big bang nucleosynthesis (BBN), resulting in a determination that differs from the previous value and from theoretical expectations. There have been several studies attempting to explain this anomaly which involve variation of fundamental constants between the time of BBN and the present. Since the Higgs vacuum expectation value (vev) is the only dimensionful parameter in the Standard Model and it is already known to vary during the electroweak phase transition, we consider the possibility that the vev is slightly different during BBN compared to its present value. A modification of the vev changes not only particle masses but also affects, through mass thresholds, the QCD confinement scale. We use the recently developed prymordial program to study this variation and its impact on the He4 and deuterium abundances. We find that bounds on |δv/v| are approximately 0.01, and that the EMPRESS result can be explained within 2σ if 0.008<δv/v<0.02, but at the cost of worsening the current 2σ discrepancy in the deuterium abundance to over 3σ.