arXiv subjects
Publications and source records attributed to S. Nandi.
We present an SU(9) model of family unification with three light chiral families, and a natural hierarchy of charged fermion masses and mixings. The existence of singlet right handed neutrions with masses about two orders of magnitude smaller than the GUT scale, as needed to understand the light neutrinos masses via the see-saw mechanism, is compelling in our model.
We report the results of flux density measurements in the extended components of thirteen giant radio galaxies, made with the RATAN-600 in the centimeter range. Supplementing them with the WENSS, NVSS and GB6 survey data we constructed the spectra of the studied galaxy components. We computed the spectral indices in the studied frequency range and demonstrate the need for a detailed account of the integral contribution of such objects into the background radiation.
Results of RATAN-600 centimeter-wavelength flux-density measurements of the extended components in five giant radio galaxies are reported. The spectra of the components of these radio galaxies have been constructed using the data of the WENSS, NVSS, and GB6 surveys together with new RATAN-600 data. Spectral indices in the studied frequency range have been calculated.
The magnetic structures adopted by the Fe and Sm sublattices in SmFeAsO have been investigated using element specific x-ray resonant and non-resonant magnetic scattering techniques. Between 110 and 5 K, the Sm and Fe moments are aligned along the c and a directions, respectively according to the same magnetic representation $Γ_{5}$ and the same propagation vector (1, 0, 0.5). Below 5 K, magnetic order of both sublattices change to a different magnetic structure and the Sm moments reorder in a magnetic unit cell equal to the chemical unit cell. Modeling of the temperature dependence for the Sm sublattice as well as a change in the magnetic structure below 5 K provide a clear evidence of a surprisingly strong coupling between the two sublattices, and indicate the need to include anisotropic exchange interactions in models of SmFeAsO and related compounds.
We investigate the prospects for the discovery of massive color-octet vector bosons at the CERN Large Hadron Collider with $\sqrt{s} = 14$ TeV. A phenomenological Lagrangian is adopted to evaluate the cross section of a pair of colored vector bosons (colorons, $\tildeρ$) decaying into four colored scalar resonances (hyper-pions, $\tildeπ$), which then decay into eight gluons. We include the dominant physics background from the production of $8g,7g1q, 6g2q$, and $5g3q$, and determine the masses of $\tildeπ$ and $\tildeρ$ where discovery is possible. For example, we find that a 5$σ$ signal can be established for $M_{\tildeπ} \alt 495$ GeV ($M_{\tildeρ} \alt 1650$ GeV). More generally we give the reach of this process for a selection of possible cuts and integrated luminosities.
There is increasing evidence to suggest that AGN activity may be episodic, with a wide range of possible time scales. Radio galaxies exhibit the most striking examples of episodic activity, with two or three distinct pairs of lobes on opposite sides of the active nucleus. Radio emission from earlier cycles of activity are expected to have steep radio spectra due to radiative losses, and hence be detected more easily at low radio frequencies. Inverse-Compton scattered cosmic microwave background radiation could in prinicple probe even lower Lorentz-factor particles, revealing an older population. We illustrate the time scales of episodic activity by considering different radio galaxies, discuss the possiblity of episodic activity in cluster radio sources, and a possible trend for a high incidence of H{\sc i} absorption in sources with evidence of episodic activity.
We present radio continuum observations at frequencies ranging from $\sim$150 to 5000 MHz of the misaligned double-double radio galaxy, DDRG, 3C293 (J1352+3126) using the Giant Metrewave Radio Telescope (GMRT) and the Very Large Array (VLA). The spectra of the outer lobes and the central source are consistent with being straight, indicating spectral ages of $\lapp17-$23 Myr for the outer lobes, and $\lapp$0.1 Myr for the central source. The north-western lobe has a prominent hotspot suggesting that the interruption of jet activity is $\lapp$0.1 Myr, consistent with the age of the inner double. The time scale of interruption of jet activity appears significantly smaller than observed in most other DDRGs which are often associated with giant radio sources. These observations suggest that there is a wide range of time scales of interruption of jet activity in active galaxies.
Given that it will take quite some time for the Large Hadron Collider (LHC) to reach its desired luminosity, it is important to investigate the most favorable scenario in which supersymmetry (SUSY) may be discovered at the early runs at the LHC. Our aim in this work is to find such a scenario within the gravity mediated SUSY breaking (SUGRA) framework and select a class of final states that warrant a discovery at the very early runs of the LHC. It turns out that such a situation can be associated with a scenario where gluinos are sufficiently light and so are the third generation scalars while the first two family scalars are heavy. We find that this can be achieved from a high-scale set-up with scalar mass non-universality in the third family and gaugino mass non-universality with $M_3 < M_1, M_2$. We show that the final state channels which are most favorable in such a region of parameter space are $4b+{E_{T}}\!\!\!\!/$, $4b+\ell+{E_{T}}\!\!\!\!/$ and $2b+2\ell+{E_{T}}\!\!\!\!/$. We also justify our claim by comparing the results with a minimal supergravity (mSUGRA) scenario with similar gluino mass.
We investigate the prospects for the discovery of massive hyper-gluons using data from the early runs of the CERN Large Hadron Collider with $\sqrt{s} = 7$ TeV and assuming an integrated luminosity of 1 fb$^{-1}$. A phenomenological Lagrangian is adopted to evaluate the cross section of a pair of colored vector bosons (coloron, $\tildeρ$) decaying into four colored scalar resonances (hyper-pion, $\tildeπ$), which then decay into eight gluons. We include the dominant physics background from the production of $8g$, $7g1q$, $6g2q$, and $5g3q$. We find an abundance of signal events and that realistic cuts reduce the background enough to establish a $5σ$ signal for $m_{\tildeπ} \alt 220$ GeV or $m_{\tildeρ} \alt 733$ GeV.
In this talk, I present a new mechanism for the generation of neutrino masses via dimension 7 operators: llHH(H*H)/M^3. This leads to new formula for the light neutrino masses, m_ν~v^4/M^3. This is distinct from the usual see-saw formulae: m_ν~v^2/M. The scale of new physics can naturally be at the TeV scale. Microscopic theory that generated d=7 operator has an isospin 3/2 Higgs multiplet Φ, which contains a triply charged Higgs boson with mass around ~TeV or less. These particles can be produced at the LHC (and possibly at the Tevatron) with distinctive multi-W and multi-lepton final states. For some choice of the parameter space, these particles can also be long-lived with the possibility of displaced vertices, or even escaping the detector. Their leptonic decay modes carry information about the nature of the neutrino mass hierarchy.
In this paper we explore models in which we add a singlet scalar Higgs to the Standard Model. All of these models explain the origin of the mass hierarchy amongst the fermion masses and mixing angles. We discuss 24 different variations on this model, and explore the different phenomenological possibilities. We find that the phenomenological implications of all the models are very similar except in the Higgs sector. Higgs decays and signals can be altered very significantly in all the models, but break up into two distinct classes. We also describe a systematic method for generating these models from higher order interactions involving vector-like quarks and flavon scalars.
Measurements of the flux densities of the extended components of seven giant radio galaxies obtained using the RATAN-600 radio telescope at wavelengths of 6.25 and 13 cm ar e presented. The spectra of components of these radio galaxies are constructed using these new RA TAN-600 data together with data from the WENSS, NVSS, and GB6 surveys. The spectral indices in the stu died frequency range are calculated, and the need for detailed estimates of the integrated contributi on of such objects to the background emission is demonstrated.
We explore a scenario in the Standard Model in which dimension four Yukawa couplings are either forbidden by a symmetry, or happen to be very tiny, and the Yukawa interactions are dominated by effective dimension six interactions. In this case, the Higgs interactions to the fermions are enhanced in a large way, whereas its interaction with the gauge bosons remains the same as in the Standard Model. In hadron colliders, Higgs boson production via gluon gluon fusion increases by a factor of nine. Higgs decay widths to fermion anti-fermion pairs also increase by the same factor, whereas the decay widths to photon photon and gamma Z are reduced. Current Tevatron exclusion range for the Higgs mass increases to ~ 142-200 GeV in our scenario, and new physics must appear at a scale below a TeV.
We propose a simple extension of the Standard Model (SM) by adding an extra U(1) symmetry which is hidden from the SM sector. Such a hidden U(1) has not been considered before, and its existence at the TeV scale can be explored at the LHC. This hidden U(1) does not couple directly to the SM particles, and couples only to new SU(2)_L singlet exotic quarks and singlet Higgs bosons, and is broken at the TeV scale. The dominant signals at the high energy hadron colliders are multi lepton and multi b-jet final states with or without missing energy. We calculate the signal rates as well as the corresponding Standard Model background for these final states. A very distinctive signal is 6 high p_T b-jets in the final state with no missing energy. For a wide range of the exotic quarks masses the signals are observable above the background at the LHC.
We present a new approach for generating tiny neutrino masses. The Dirac neutrino mass matrix gets contributions from two new Higgs doublets with their vevs at the electroweak (EW) scale. Neutrino masses are tiny not because of tiny Yukawa couplings, or very heavy ($\sim 10^{14}\textrm{GeV}$) right handed neutrinos. They are tiny because of a cancelation in the Dirac neutrino mass matrix (fine tuning). After fine tuning to make the Dirac neutrino mass matrix at the $10^{-4}$ GeV scale, light neutrino masses are obtained in the correct scale via the see-saw mechanism with the right handed neutrino at the EW scale. The proposal links neutrino physics to collider physics. The Higgs search strategy is completely altered. For a wide range of Higgs masses, the Standard Model Higgs decays dominantly to $ν_L N_R$ mode giving rise to the final state $\barν ν\bar{b} b$, or $\barν ντ^+τ^-$. This can be tested at the LHC, and possibly at the Tevatron.
We use magnetic long range order as a tool to probe the Cooper pair wave function in the iron arsenide superconductors. We show theoretically that antiferromagnetism and superconductivity can coexist in these materials only if Cooper pairs form an unconventional, sign-changing state. The observation of coexistence in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ then demonstrates unconventional pairing in this material. The detailed agreement between theory and neutron diffraction experiments, in particular for the unusual behavior of the magnetic order below $T_{c}$, demonstrates the robustness of our conclusions. Our findings strongly suggest that superconductivity is unconventional in all members of the iron arsenide family.
We have studied a EuFe2As2 single crystal by neutron diffraction under magnetic fields up to 3.5 T and temperatures down to 2 K. A field induced spin reorientation is observed in the presence of a magnetic field along both the a and c axes, respectively. Above critical field, the ground state antiferromagnetic configuration of Eu$^{2+}$ moments transforms into a ferromagnetic structure with moments along the applied field direction. The magnetic phase diagram for Eu magnetic sublattice in EuFe2As2 is presented. A considerable strain ($\sim$0.9%) is induced by the magnetic field, caused by the realignment of the twinning structure. Furthermore, the realignment of the twinning structure is found to be reversible with the rebound of magnetic field, which suggested the existence of magnetic shape-memory effect. The Eu moment ordering exhibits close relationship with the twinning structure. We argue that the Zeeman energy in combined with magnetic anisotropy energy is responsible for the observed spin-lattice coupling.
Neutron diffraction and high-resolution x-ray diffraction studies find that, similar to the closely related underdoped Ba(Fe[1-x]Cox)2As2 superconducting compounds, Ba(Fe0.961Rh0.039)2As2 shows strong evidence of competition and coexistence between superconductivity and antiferromagnetic order below the superconducting transition, Tc = 14 K. The transition temperatures for both the magnetic order and orthorhombic distortion are in excellent agreement with those inferred from resistivity measurements, and both order parameters manifest a distinct decrease in magnitude below Tc. These data suggest that the strong interaction between magnetism and superconductivity is a general feature of electron-doped Ba(Fe[1-x]TMx)2As2 superconductors (TM = Transition Metal).