On the Erasure and Regeneration of the Primordial Baryon Asymmetry by Sphalerons

We show that a cosmological baryon asymmetry generated at the GUT scale, which would be destroyed at lower temperatures by sphalerons and possible new B- or L-violating effects, can naturally be preserved by an asymmetry in the number of right-handed electrons. This results in a significant softening of previously derived baryogenesis-based constraints on the strength of exotic B- or L-violating interactions.Comment: 10 pp. LaTex (2 figures, included) UMN-TH-1201/9

Reconciling Supersymmetric Grand Unification with αs(mZ)≈0.11\alpha_s(m_Z)\approx 0.11

We argue that supersymmetric grand unification of gauge couplings is not incompatible with small $\alpha_s$, even without large GUT-scale corrections, if one relaxes a usual universal gaugino mass assumption. A commonly assumed relation $M_2\simeq m_{\rm gluino}/3$ is in gross contradiction with $\alpha_s\approx 0.11$. Instead, small $\alpha_s$ favors $M_2\gg m_{\rm gluino}$. If this is indeed the case our observation casts doubt on another commonly used relation $M_1\simeq 0.5 M_2$ which originates from the same constraint of a common gaugino mass at the GUT scale. One firm prediction emerging within the small $\alpha_s$ scenario with the unconstrained gaugino masses is the existence of a relatively light gluino below $\sim$ 200\gev.Comment: 18 pages, LaTex format for text; epsf.sty needed for including 3 Postscript figures in the text. CHANGES: Comments on dark matter and non-minimal supergravity (see end of Sec. 2.3) and several references added; also some minor corrections made

Constraints on Baryon-Nonconserving Yukawa Couplings in a Supersymmetric Theory

The 1-loop evolution of couplings in the minimal supersymmetric standard model, extended to include baryon nonconserving $(B\!\!\!/)$ operators through explicit $R$-parity violation, is considered keeping only $B\!\!\!/$ superpotential terms involving the maximum possible number of third generation superfields. If all retained Yukawa couplings $Y_i$ are required to remain in the perturbative domain $(Y_i < 1)$ upto the scale of gauge group unification, upper bounds ensue on the magnitudes of the $B\!\!\!/$ coupling strengths at the supersymmetry breaking scale, independent of the model of unification. They turn out to be similar to the corresponding fixed point values reached from a wide range of $Y_i$ (including all $Y_i$ greater than unity) at the unification scale. The coupled evolution of the top and $B\!\!\!/$ Yukawa couplings results in a reduction of the fixed point value of the former.Comment: PRL-TH-94/8 and TIFR/TH/94-7, 15 pages, LaTe

Quark singlets: Implications and constraints

Quarks whose left- and right-handed chiral components are both singlets with respect to the SU(2) weak-isospin gauge group, offer interesting physics possibilities beyond the Standard Model (SM) already studied in many contexts. We here address some further aspects. We first collect and update the constraints from present data on their masses and mixings with conventional quarks. We discuss possible effects on $b\to s\gamma$ and $Z\to b\bar b$ decays and give fresh illustrations of CP asymmetries in $B^0$ decays differing dramatically from SM expectations. We analyse singlet effects in grand unification scenarios: $d$-type singlets are most economically introduced in ${\bf 5+5^*}$ multiplets of $SU(5)$, with up to three generations, preserving gauge coupling unification with perturbative values up to the GUT scale; $u$-type singlets can arise in ${\bf 10+10^*}$ multiplets of $SU(5)$ with at most one light generation. With extra matter multiplets the gauge couplings are bigger; we give the two-loop evolution equations including exotic multiplets and a possible extra $U(1)$ symmetry. Two-loop effects can become important, threatening unification (modulo threshold effects), perturbativity and asymptotic freedom of $\alpha_3$. In the Yukawa sector, top-quark fixed-point behaviour is preserved and singlet-quark couplings have infrared fixed points too, but unification of $b$ and $\tau$ couplings is not possible in a three-generation $E_6$ model.Comment: Revtex version 3.0, 49 pages. 10 postscript figures included, concatenated (not tarred) into one ps file, compressed and uuencoded. Compressed postscript version of entire paper available soon at http://phenom.physics.wisc.edu/pub/preprints/1995/madph-95-870.ps.Z or at ftp://phenom.physics.wisc.edu/pub/preprints/1995/madph-95-870.ps.