Limits on the AGN activities in X-ray underluminous galaxy groups

We have observed four X-ray underluminous groups of galaxies using the Giant Meterwave RadioTelescope. The groups NGC 524, 720, 3607, and 4697 are underluminous in relation to the extrapolation of the Lx - T relation from rich clusters and do not show any evidence of current AGN activities that can account for such a departure. The GMRT observations carried out at low frequencies (235 and 610 MHz) were aimed at detecting low surface brightness, steep-spectrum sources indicative of past AGN activities in these groups. No such radio emissions were detected in any of these four groups. The corresponding upper limits on the total energy in relativistic particles is about 3 X 10$^{57}$ erg. This value is more than a factor of 100 less than that required to account for the decreased X-ray luminosities (or, enhanced entropies) of these four groups in the AGN-heating scenario. Alternatively, the AGN activity must have ceased about 4 Gyr ago, allowing the relativistic particles to diffuse out to such a large extent (about 250 kpc) that their radio emission could have been undetected by the current observations. If the latter scenario is correct, the ICM was pre-heated before the assembly of galaxy clusters.Comment: 10 pages, 3 figures, accepted for publication in ApJ Letter

Suppression of Higgsino mediated proton decay by cancellations in GUTs and strings

A mechanism for the enhancement for proton lifetime in supersymmetric/supergravity (SUSY/SUGRA) grand unified theories (GUTs) and in string theory models is discussed where Higgsino mediated proton decay arising from color triplets (anti-triplets) with charges $Q=-1/3(1/3)$ and $Q=-4/3(4/3)$ is suppressed by an internal cancellation due to contributions from different sources. We exhibit the mechanism for an SU(5) model with $45_H+\bar{45}_H$ Higgs multiplets in addition to the usual Higgs structure of the minimal model. This model contains both $Q=-1/3(1/3)$ and $Q=-4/3(4/3)$ Higgs color triplets (anti-triplets) and simple constraints allow for a complete suppression of Higgsino mediated proton decay. Suppression of proton decay in an SU(5) model with Planck scale contributions is also considered. The suppression mechanism is then exhibited for an SO(10) model with a unified Higgs structure involving $144_H+\bar{144}_H$ representations.The SU(5) decomposition of $144_H+\bar{144}_H$ contains $5_H+\bar 5_H$ and $45_H+\bar{45}_H$ and the cancellation mechanism arises among these contributions which mirrror the SU(5) case. The cancellation mechanism appears to be more generally valid for a larger class of unification models. Specifically the cancellation mechanism may play a role in string model constructions to suppress proton decay from dimension five operators. The mechanism allows for the suppression of proton decay consistent with current data allowing for the possibility that proton decay may be visible in the next round of nucleon stability experiment.Comment: 26 pages, no figures. Revtex 4. To appear in Physical Review

The Role of Δ(1232)\Delta(1232) in Two-pion Exchange Three-nucleon Potential

In this paper we have studied the two-pion exchange three-nucleon potential $(2\pi E-3NP)$ using an approximate $SU(2) \times SU(2)$ chiral symmetry of the strong interaction. The off-shell pion-nucleon scattering amplitudes obtained from the Weinberg Lagangian are supplemented with contributions from the well-known $\sigma$-term and the $\Delta(1232)$ exchange. It is the role of the $\Delta$-resonance in $2\pi E-3NP$, which we have investigated in detail in the framework of the Lagrangian field theory. The $\Delta$-contribution is quite appreciable and, more significantly, it is dependent on a parameter Z which is arbitrary but has the empirical bounds $|Z| \leq 1/2$. We find that the $\Delta$-contribution to the important parameters of the $2\pi E-3NP$ depends on the choice of a value for Z, although the correction to the binding energy of triton is not expected to be very sensitive to the variation of Z within its bounds.Comment: 14 pages, LaTe

Predictions in SU(5) Supergravity Grand Unification with Proton Stability and Relic Density Constraints

It is shown that in the physically interesting domain of the parameter space of SU(5) supergravity GUT, the Higgs and the Z poles dominate the LSP annihilation. Here the naive analyses on thermal averaging breaks down and formulae are derived which give a rigorous treatment over the poles. These results are then used to show that there exist significant domains in the parameter space where the constraints of proton stability and cosmology are simultaneously satisfied. New upper limits on light particle masses are obtained.Comment: (An error in the reheating factor is corrected, strengthening the conclusions, i.e. the region in parameter space where the relic density constraints are satisfied is enlarged.

CP Violation and Dark Matter

A brief review is given of the effects of CP violation on the direct detection of neutralinos in dark matter detectors. We first summarize the current developments using the cancellation mechanism which allows for the existence of large CP violating phases consistent with experimental limits on the electron and on the neutron electric dipole moments in a broad class of SUSY, string and D brane models. We then discuss their effects on the scattering of neutralinos from quarks and on the event rates. It is found that while CP effects on the event rates can be enormous such effects are reduced significantly with the imposition of the EDM constraints. However, even with the inclusion of the EDM constraints the effects are still very significant and should be included in a precision prediction of event rates in any SUSY, string or D brane model.Comment: Based on an invited talk at the conference "Sources and Detection of Dark Matter in the Universe", at Marina del Rey, CA, Feb. 23-25, 2000; 12 pages, Latex including 2 figure

Magnetism and superconductivity in Eu0.2Sr0.8(Fe0.86Co0.14)2As2 probed by 75As NMR

We report bulk superconductivity (SC) in Eu$_{0.2}$Sr$_{0.8}$(Fe$_{0.86}$Co$_{0.14}$)$_{2}$As$_{2}$ single crystals by means of electrical resistivity, magnetic susceptibility, and specific heat measurements with $T$$_{\mathrm{c}}$ $\simeq$ 20 K with an antiferromagnetic (AFM) ordering of Eu$^{2+}$ moments at $T$$_{\mathrm{N}}$ $\simeq$ 2.0 K in zero field. $^{75}$As NMR experiments have been performed in the two external field directions (H$\|ab$) and (H$\|c$). $^{75}$As-NMR spectra are analyzed in terms of first order quadrupolar interaction. Spin-lattice relaxation rates (1/$T_{1}$) follow a $T^{3}$ law in the temperature range 4.2-15 K. There is no signature of Hebel-Slichter coherence peak just below the SC transition indicating a non s-wave or s$_{\pm}$ type of superconductivity. The increase of 1/$T_{1}T$ with lowering the temperature in the range 160-18 K following $\frac{C}{T+\theta}$ law reflecting 2D AFM spin fluctuations