arXiv subjects

Biswajoy Brahmachari

Publications and source records attributed to Biswajoy Brahmachari.

47 records · Page 3Linked to original sources

arXiv1996

$α_s(m_Z)$, intermediate scales and Fermion masses in supersymmeric theories; two-loop results

One way to generate an intermediate scale being consistent with gauge coupling unification is to add new Higgs scalars above the intermediate scale. We classify such scenarios according to their degree of departure from minimal supersymmetric standard model. Thereafter, we summarize the results of a two-loop renormalization group analysis of the gauge and the Yukawa sectors of such scenarios, and their sensitivity to the {\it input} $α_3(m_Z)$. The presence of an adjoint color octet above the intermediate scale can raise the gauge coupling unification scale to the $string$ scale, acommodate a suitable intermediate scale, provide a left-handed $τ$-neutrino$ mass of the order of few electron volts and also have the right prediction of the ratio of $m_b /m_τ$.

hep-ph↗

arXiv1996

Signatures of Baryon non-conserving Yukawa couplings in a supersymmetric theory

Renormalization effects of large baryon-nonconserving Yukawa couplings $λ^{\prime\prime}_{ijk} \overline{U_i} \overline{D_j} \overline{D_k}$ lower the right handed squark masses keeping the left-handed squark masses virtually untouched at the lowest order. At low energy they enhance the mass-splitting between left and right handed squarks of the same generation as well as intergenerational mass splitting among squarks, potentially detectable in future colliders or in rare decays. The predicted mass of the lightest stop squark becomes lower than the experimental bound for larger ranges of parameter space than that of the Baryon-conserving case, hence, further constraining the parameter space of a supersymmetric theory when baryon violation is included.

hep-ph↗

arXiv1996

A low α_s: Hint of new physics at the GUT scale?

In a SUSY GUT having an extra reverse doublet-triplet splitting near the GUT scale, where the mass of an extra doublet is greater than the mass of an extra triplet by two orders of magnitude, a low prediction of $α_s$ can be achieved through threshold corrections via the heavy scalars in the fundamental representations, making the prediction consistent with the values being suggested by low energy measurements. We implement this mechanism in a SUSY SU(5) GUT minimally extended to suppress Higgsino mediated proton decay. We also point out that as a consequence of this extension a natural scenario arises with a large hierarchy in Yukawa couplings ($ λ_t / λ_b \sim 40$). An experimental low value of $α_s(m_Z)$ along with the non-observation of dimension-five proton decay modes at SuperKamiokande detector will favor such an extension of the SUSY SU(5) GUT over the minimal case.

hep-ph↗

arXiv1996

Two-loop renormalization group analysis of supersymmetric SO(10) models with an intermediate scale

Two-loop evolutions of the gauge couplings in a class of intermediate scale supersymmetric SO(10) models including the effect of third generation Yukawa couplings are studied. The unification scale, the intermediate scale and the value of the unification gauge coupling in these models are calculated and the gauge boson mediated proton decay rates are estimated. In some cases the predicted proton lifetime turns out to be in the border-line of experimental limit. The predictions of the top quark mass, the mass ratio $m_b(m_b)/ m_τ(m_τ)$ from the two-loop evolution of Yukawa couplings and the mass of the left handed neutrino via see-saw mechanism are summarized. The lower bounds on the ratio of the VEVs of the two low energy doublets ($\tanβ$) from the requirement of the perturbative unitarity of the top quark Yukawa coupling up to the grand unification scale are also presented. All the predictions have been compared with those of the one-step unified theory.

hep-ph↗

arXiv1995

A low $α_s$ and its consequences for unified model building

We review various ways of obtaining consistency between the idea of supersymmetric grand unification and an apparent low value of $α_s \sim 0.112$ indicated by several low energy experiments. We argue that to reconcile the low value of $α_s$ with the predictions of supersymmetric GUTs, we need to go beyond the standard minimal supersymmetric GUT scenario and invoke new physics either at $10^{11}-10^{12}$ GeV, or at the GUT scale.

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arXiv1995

Low $α_{strong}(M_Z)$, Intermediate Scale SUSY SO(10) GUT and Its Implications

We show that one of the ways of obtaining consistency between the idea of supersymmetric grand unification and an apparent low value of $α_{strong}(M_Z)\simeq .11$ indicated by several low energy experiments is to have an intermediate scale corresponding to a local $B-L$ symmetry breaking around the mass scale of $10^{10}$ to $10^{12}$ GeV. We discuss the realization of this idea within the framework of supersymmetric $SO(10)$ grand unified theories with spectra of particles derivable from simple superstring-inspired versions of this model. We then study the $b-τ$ mass unification within this class of models and show that due to the influence of new gauge and Yukawa interactions beyond the intermediate scale, the prediction of the $b$-quark mass comes out well within the presently accepted values. We also discuss an un-orthodox class of SUSY models inspired by some theoretical considerations having two pairs of Higgs doublets at low energy and show that they also can lead to unification with intermediate scales and low $α_{s}(M_Z)$ as desired.

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arXiv1995

Predictions of the Top mass in Mininal Supersymmetric Left-Right Model

The one-loop evolution of Yukawa couplings in the minimal supersymmetric left-right model (MSUSYLR) model with a wide variation of the right handed breaking scale $M_R$ from 1 TeV to $10^{18}$ GeV is studied assuming that all third generation Yukawa couplings are equal and in the fixed point domain of the top quark Yuakwa coupling ($h_t$) at the Plank scale. We show that: (1) The top quark Yukawa coupling $h_t$ displays a fixed point behaviour that is similar to that of the minimal supersymmetric standard model (MSSM). (2) The MSUSYLR model predicts a value of the top mass in the interval 177 to 184 GeV for $α_s$ in the interval 0.11 to 0.12. (3) A large value of $tanβ$ is required to reproduce the correct mass of the bottom quark and tau lepton. (4) With the experimental value of the ratio ${m_b(m_b) \over m_τ(m_τ)}$ as an input the range of the right handed symmetry breaking scale $M_R$ can be predicted. (5) The numerical value of the Majorana Yukawa coupling $h_M$ can be calculated which is otherwise a completely free parameter.

hep-ph↗

arXiv1995

A Model for the Three Lepton Decay Mode of the Proton

An extension of the left--right symmetric model has been constructed which gives in a natural way the three lepton decay modes of the proton which have been suggested as an explanation for the atmospheric neutrino anomaly. We write down the potential which after minimization gives the proper choice of the Higgs spectrum. With this Higgs spectrum we then study the evolution of the gauge coupling constants and point out that for consistency one has to include effects of gravity.

hep-ph↗

arXiv1994

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.

hep-ph↗

arXiv1994

Potential Minimization in Left-Right Symmetric Models

We study the Higgs potentials in the Left-Right symmetric model for various choices of Higgs fields. We give emphasis to the cases when the Higgs field $ξ=(2,2,15)$ is included to give the correct relations of quark masses and a singlet field $η$ which breaks the left-right parity. As special cases we also include $ξ^\prime=(2,2,15)$ and $χ=(2,2,6)$ (which are interesting in the context of the three lepton decay mode of the proton) and field $δ=(3,3,0)$ none of which acquire {\it vev}. We show that the linear couplings of these fields upon minimization put fine tuning conditions on the parameters of the model. We carry out the minimization of these potentials explicitly. In all the cases the relationship between the {\it vev$\;$}s of the left and right handed triplets $v_L$ and $v_R$ are given. The phenomenological consequences of this minimization regarding the neutrino masses are also studied.

hep-ph↗

arXiv1993

Mass Limits of Invisibly Decaying Higgs Particles from the LEP Data

In the Majoron models the $SU(2)$ doublet Higgs can decay invisibly into a Majoron pair via its mixing with a singlet. An analysis of the LEP data shows the invisible decay mode to be more visible than the SM decay. For these models, the dominantly doublet Higgs $H$ is shown to have a mass limit within $\pm 6$ GeV of the SM limit irrespective of the model parameters. But the dominantly singlet one $S$ can be arbitrarily light for sufficiently small mixing angle.

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