• N K Sharma

Articles written in Pramana – Journal of Physics

• Mesonic decays of τ lepton: Effects of neutrino mass and mass mixing

Experimentally established mesonic decays ofτ lepton have been reexamined with the inclusion of the effects of finite neutrino mass and the associated mass mixing in the form of Kobayashi-Maskawa mixing matrix. A comparison with the experimentally predicted decay probabilities provides limits for thevτ mass which are finite in all decays except for the lower limit in mass mixing case of the decayτK* (892)+vτ for which$$m_{v_\tau } = (420 \pm 610)$$ MeV. The large error in this value is because of (i) large errors in the experimental values of life time and branching ratio for this decay and (ii) thekm mixing used in the calculations. The ratio of parity-violating to parity-conserving terms in the differential decay probabilities of various decays differs slightly from their values corresponding to those with varishingvτ mass.

• Evaluation ofS,T, U parameters, triple gauge boson vertices and some oblique corrections in extraU(1) superstring inspired model

Explicit evaluation of the following parameters has been carried out in the extraU (1) superstring inspired model: (i) As Mz2 varies from 555 GeV to 620 GeV and (mt) CDF = 175.6 ± 5.7 GeV (Table 1): (a) SNew varies from -0.100 ± 0.089 to -0.130 ± 0.090, (b) TNew varies from -0.098 ± 0.097 to -0.129 ± 0.098, (c) UNew varies from -0.229 ± 0.177 to -0.253 ± 0.206, (d) Τz varies from 2.487 ± 0.027 to 2.486 ± 0.027, (e) ALR varies from 0.0125 ± 0.0003 to 0.0126 ± 0.0003, (f) AFBb remains constant at 0.0080 ± 0.0007. Almost identical values are obtained for (mt)D0 = 169 GeV (see table 2). (ii) Triple gauge boson vertices (TGV) contributions: AsMz2 varies from 555 GeV to 620 GeV and (mt) CDF = 175.6 ±5.7 GeV. (a)√s = 500 GeV, asymptotic case:$$\overline f _1^{Z_1 }$$ varies from -0.301 to -0.179;$$\overline f _{3|Z_1 }^{Z_1 }$$ varies from -0.622 to -0.379;$$f_5^{Z_1 }$$ varies from +0.0061 to 0.0056;$$\overline f _{3|Z_1 }^{\gamma _1 }$$ varies from -3.691 to -2.186.$$\overline f _z^{Z_2 }$$ varies from +0.270 to +0.118;$$\overline f _3^{Z_2 }$$ varies from +0.552 to 0.238;$$f_5^{Z_2 }$$ varies from +0.0004 to +0.0002;$$\overline f _{3|Z_2 }^{\gamma _1 }$$ remains constant at -0.110. (b)√s = 700 GeV, asymptotic case:$$\overline f _1^{Z_1 }$$ varies from -0.297 to -0.176;$$\overline f _3^{Z_1 }$$ varies from -0.609 to -0.370;$$\overline f _5^{Z_1 }$$ varies from -0.0082 to -0.0078;$$\overline f _{3|Z_1 }^{\gamma _1 }$$ varies from -3.680 to -2.171.√s = 700 GeV, nonasymptotic case:$$\overline f _1^{Z_2 }$$ varies from -0.173 to -0.299;$$\overline f _3^{Z_2 }$$ varies from-0.343 to -0.591;$$f_5^{Z_2 }$$ varies from -0.005 to -0.011;$$\overline f _{3|Z_2 }^{\gamma _1 }$$ remains constant at -0.110.

The pattern of form factors values for√s = 1000, 1200 GeV is almost identical to that of√s= 700 GeV. Further the values of the form factors for (mt)D0 (=169 GeV) follow identical pattern as that of (mt) CDF form factors values (see tables 5, 6, 9, 10).

We conclude that the values of all the form factors with the exception of these of$$f_5^{Z_1 }$$,$$f_5^{Z_2 }$$ are comparable or larger than theS, T values and therefore the TGV contributions are important while deciding the use of extraU (1) model for doing physics beyond standard model.

• Bounds on neutrino mixing with exotic singlet neutrinos

We examine the effects of mixing induced non-diagonal light-heavy neutrino weak neutral currents on the amplitude for the process $$v_a \overline v _a \to ZZ$$ (with a=e, μ or τ). By imposing constraint that the amplitude should not exceed the perturbative unitarity limit at high energy $$\left( {\sqrt s = \Lambda } \right)$$, we obtain bounds on light-heavy neutrino mixing parameter sin2$$\theta _L^{v_a }$$ where $$\theta _L^{v_a }$$ is the mixing angle. In the case of one heavy neutrino (mass mξ) or mass degenerate heavy neutrinos, for Λ=1 TeV, no bound is obtained for mξ&lt;0.50 TeV. However, sin2$$\theta _L^{v_a }$$≤3.8 × 10−6 for mξ=5 TeV and sin $$\theta _L^{v_a }$$≤6.0 × 10−8 for mξ=10 TeV. For Λ=∞, no constraint is obtained for mξ&lt;0.99 TeV and sin2$$\theta _L^{v_a }$$≤3.8 × 10−2 (for mξ=5 TeV) and sin2$$\theta _L^{v_a }$$≤9.6 × 10−3 (for mξ=10 TeV).

• Pramana – Journal of Physics

Volume 94, 2019
All articles
Continuous Article Publishing mode

• Editorial Note on Continuous Article Publication

Posted on July 25, 2019