arXiv subjects

A. Das

Publications and source records attributed to A. Das.

At least 127 records · Page 7Linked to original sources

arXiv2010

Measurements of Branching Fractions for B0 --> Ds+pi- and B0-bar --> Ds+K-

We present improved measurements of the branching fractions for the decays B0 --> Ds+pi- and B0-bar --> Ds+K- using a data sample of 657x10^6 BB-bar events collected at the Y(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider. The results are BF(B0 --> Ds+pi-) = (1.99 +/- 0.26 +/- 0.18)x10^-5 and BF(B0-bar --> Ds+K-) = (1.91 +/- 0.24 +/- 0.17)x10^-5, where the uncertainties are statistical and systematic, respectively. Based on these results, we determine the ratio between amplitudes of the doubly Cabibbo suppressed decay B0 --> D+pi- and the Cabibbo favored decay B0 --> D-pi+, R_Dpi = [1.71 +/- 0.11(stat) +/- 0.09(syst) +/- 0.02(theo)]%, where the last term denotes the theory error.

hep-ex↗

arXiv2010

Search for pair production of the scalar top quark in the electron-muon final state

We report the result of a search for the pair production of the lightest supersymmetric partner of the top quark ($\tilde{t}_1$) in $p\bar{p}$ collisions at a center-of-mass energy of 1.96 TeV at the Fermilab Tevatron collider corresponding to an integrated luminosity of 5.4 fb$^{-1}$. The scalar top quarks are assumed to decay into a $b$ quark, a charged lepton, and a scalar neutrino ($\tildeν$), and the search is performed in the electron plus muon final state. No significant excess of events above the standard model prediction is detected, and improved exclusion limits at the 95% C.L. are set in the the ($M_{\tilde{t}_1}$,$M_{\tildeν}$) mass plane.

hep-ex↗

arXiv2010

The Dielectric Response of La0.5Ca0.5-xSrxMnO3 (0.1 <= x <= 0.4) Manganites with Different Magnetic Ground States

The dielectric behavior of half doped manganites La0.5Ca0.5-xSrxMnO3 (0.1 \leq \times \leq 0.4) with varying magnetic ground states has been studied. The real part of relative permittivity as a function of temperature ε^'(T), exhibits a maximum around the ferromagnetic (TC) and charge ordering transition (TCO) temperatures accompanied with high dielectric losses. The activation energies obtained for x = 0.1 and 0.3 samples below TCO are the same ~ 0.12eV, whereas the relaxation time constant varies in the range 2.8 \times 10-9 s - 6.03 \times 10-11 s. In contrast to samples having x \leq 0.3, for x = 0.4 doping the dielectric permittivity exhibits a strong temperature dependence in the vicinity of magnetic phase transitions. This behavior may be correlated with the presence of competing magnetic interactions (magnetic polarons) close to the magnetic transitions.

cond-mat.mtrl-sci↗

arXiv2010

Determination of the width of the top quark

We extract the total width of the top quark, Gamma_t, from the partial decay width Gamma(t -> W b) measured using the t-channel cross section for single top quark production and from the branching fraction B(t -> W b) measured in ttbar events using up to 2.3 fb^-1 of integrated luminosity collected by the D0 Collaboration at the Tevatron ppbar Collider. The result is Gamma_t = 1.99 +0.69 -0.55 GeV, which translates to a top-quark lifetime of tau_t = (3.3 +1.3 -0.9) x 10^-25 s. Assuming a high mass fourth generation b' quark and unitarity of the four-generation quark-mixing matrix, we set the first upper limit on |Vtb'| < 0.63 at 95% C.L.

hep-ex↗

arXiv2010

Measurement of CP violating asymmetries in B^0 -> K^+K^- K^0_S decays with a time-dependent Dalitz approach

We report a measurement of $CP$ violating asymmetries in $B^0(\overline{B}^0) \to K^+ K^- K^0_S$ decays with a time-dependent Dalitz approach. This analysis is based on a data sample of $657\times 10^6$ $B\overline{B}$ pairs accumulated at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. As the result of an unbinned maximum likelihood fit to the selected candidates, the mixing-induced and direct $CP$ violation parameters, $ϕ^{\rm eff}_1$ and ${\cal A}_{CP}$ are obtained for $B^0 \to ϕ(1020) K^0_S$, $B^0 \to f_0(980) K^0_S$ and other $B^0 \to K^+ K^- K^0_S$ decays. We find four solutions that describe the data. There are \{eqnarray*} ϕ_1^{\rm eff}(B^0\to ϕ(1020) K^0_S) & = & (32.2 \pm 9.0 \pm 2.6 \pm 1.4)^{\circ}; ϕ_1^{\rm eff}(B^0\to ϕ(1020) K^0_S) & = & (26.2 \pm 8.8 \pm 2.7 \pm 1.2)^{\circ};\\ ϕ_1^{\rm eff}(B^0\to ϕ(1020) K^0_S) & = & (27.3 \pm 8.6 \pm 2.8 \pm 1.3)^{\circ}\; {\rm and}\\ ϕ_1^{\rm eff}(B^0\to ϕ(1020) K^0_S) & = & (24.3 \pm 8.0 \pm 2.9 \pm 5.2)^{\circ}.{eqnarray*}\ The values for the $CP$ violating phase in $B^0\to ϕ(1020) K^0_S$ are similar but other properties of the Dalitz plot are quite different for the four solutions. These four solutions have consistent $ϕ^{\rm eff}_1$ values for all three $B$ meson decay channels and none of them deviates significantly from the values measured in $B \to (c\bar{c}) K^0$ decays with the currently available statistics. In addition, we find no significant direct $CP$ violation.

hep-ex↗

arXiv2010

Disorder Induced Negative Magnetization in LaSrCoRuO6

This paper reports effect of thermally induced disorder on the magnetic properties of LaSrCoRuO6 double perovskite. While the ordered sample is antiferromagnetic, the disordered sample exhibits negative values of magnetization measured in low applied fields. Isothermal magnetization on this sample shows hysteresis due to presence of ferromagnetic interactions. Based on neutron diffraction and X-ray Absorption Fine Structure (XAFS) studies, these results have been interpreted to be due disorder in site occupancy of Co and Ru leading to octahedral distortions and formation of Ru-O-Ru ferromagnetic linkages. Below 150K these ferromagnetic Ru spins polarize the Co spins in a direction opposite to that of the applied field resulting in observed negative magnetization.

cond-mat.str-el↗

arXiv2010

Diffuse Neutron Scattering Study of Magnetic Correlations in half-doped La0.5Ca0.5-xSrxMnO3 (x = 0.1, 0.3 and 0.4) Manganites

The short range ordered magnetic correlations have been studied in half doped La0.5Ca0.5-xSrxMnO3 (x = 0.1, 0.3 and 0.4) compounds by polarized neutron scattering technique. On doping Sr2+ for Ca2+ ion, these compounds with x = 0.1, 0.3, and 0.4 exhibit CE-type, mixture of CE-type and A-type, and A-type antiferromagnetic ordering, respectively. Magnetic diffuse scattering is observed in all the compounds above and below their respective magnetic ordering temperatures and is attributed to magnetic polarons. The correlations are primarily ferromagnetic in nature above T\_N, although a small antiferromagnetic contribution is also evident. Additionally, in samples x = 0.1 and 0.3 with CE-type antiferromagnetic ordering, superlattice diffuse reflections are observed indicating correlations between magnetic polarons. On lowering temperature below T\_N the diffuse scattering corresponding to ferromagnetic correlations is suppressed and the long range ordered antiferromagnetic state is established. However, the short range ordered correlations indicated by enhanced spin flip scattering at low Q coexist with long range ordered state down to 3K. In x = 0.4 sample with A-type antiferromagnetic ordering, superlattice diffuse reflections are absent. Additionally, in comparison to x = 0.1 and 0.3 sample, the enhanced spin flip scattering at low Q is reduced at 310K, and as temperature is reduced below 200K, it becomes negligibly low. The variation of radial correlation function, g(r) with temperature indicates rapid suppression of ferromagnetic correlations at the first nearest neighbor on approaching TN. Sample x = 0.4 exhibits growth of ferromagnetic phase at intermediate temperatures (~ 200K). This has been further explored using SANS and neutron depolarization techniques.

cond-mat.mtrl-sci↗

arXiv2010

Pressure effects on the magnetic structure in La0.5Ca0.5-xSrxMnO3 (0.1 -< x -< 0.4) manganites

The effect of high pressure (0 - 8 GPa) on the magnetic structure of polycrystalline samples of La0.5Ca0.5-xSrxMnO3 (0.1 -< x -< 0.4) manganites at 5 K is investigated using neutron diffraction technique. Application of pressure is found to modify the previously reported magnetic structure, observed under ambient conditions, in these compounds [I. Dhiman et al., Phys. Rev. B 77, 094440 (2008)]. In x = 0.1 composition, at 4.6(2) GPa and beyond, A-type antiferromagnetic structure is found to coexist with CE-type antiferromagnetic phase, observed at ambient pressure, with TN ~ 150 K. For x = 0.3 sample, as a function of pressure the CE-type phase is fully suppressed at 2.3(1) GPa and A-type antiferromagnetic phase is favored. Further Sr doping at x = 0.4, the A-type antiferromagnetic phase is observed at ambient pressure and for T < TN (~ 250K). This phase is retained in the studied pressure range. However, the magnetic moment progressively reduces with increasing pressure, indicating the suppression of A-type antiferromagnetic phase. The present study brings out the fragile nature of the CE-type antiferromagnetic state in half doped manganites as a function of pressure and disorder σ2. We observe that pressure required for destabilizing the CE-type antiferromagnetic state is reduced with increasing disorder σ2. External pressure and changing A-site ionic radii have analogous effect on the magnetic structure.

cond-mat.mtrl-sci↗

arXiv2009

Stable two--brane models with bulk tachyon matter

We explore the possibility of constructing stable, warped two--brane models which solve the hierarchy problem, with a bulk non--canonical scalar field (tachyon matter) as the source term in the action. Among our examples are two models--one with a warp factor (denoted as $e^{-2f(σ)}$) which differs from that of the standard Randall--Sundrum by the addition of a quadratic piece in the $f(σ)$ and another, where the warping is super-exponential. We investigate the issue of resolution of hierarchy and perform a stability analysis by obtaining the effective inter-brane potentials, in each case. Our analysis reveals that there does exist stable values of the modulus consistent with hierarchy resolution in both the models. Thus, these models, in which the bulk scalar field generates the geometry and also ensures stability, provide viable alternatives to the standard Randall--Sundrum two-brane scenario.

hep-th↗

arXiv2008

Discrete phase space - I: Variational formalism for classical relativistic wave fields

The classical relativistic wave equations are presented as partial difference equations in the arena of covariant discrete phase space. These equations are also expressed as difference-differential equations in discrete phase space and continuous time. The relativistic invariance and covariance of the equations in both versions are established. The partial difference and difference-differential equations are derived as the Euler-Lagrange equations from the variational principle. The difference and difference-differential conservation equations are derived. Finally, the total momentum, energy, and charge of the relativistic classical fields satisfying difference-differential equations are computed.

math-ph↗

arXiv2008

Discrete phase space - II: The second quantization of free relativistic wave fields

The Klein-Gordon equation, the Maxwell equation, and the Dirac equation are presented as partial difference equations in the eight-dimensional covariant discrete phase space. These equations are also furnished as difference-differential equations in the arena of discrete phase space and continuous time. The scalar field and electromagnetic fields are quantized with commutation relations. The spin-1/2 field is quantized with anti-commutation relations. Moreover, the total momentum, energy and charge of these free relativisitic quantized fields in the discrete phase space and continuous time are computed exactly. The results agree completely with those computed from the relativisitic fields defned on the space-time continuum.

math-ph↗

arXiv2008

Discrete phase space - III: The Divergence-free S-matrix elements

In the arena of the discrete phase space and continuous time, the theory of S-marix is formulated. In the special case of Quantum-Electrodynamics (QED), the Feynman rules are precisely developed. These rules in the fourmomentum turn out to be identical to the usual QED, except for the vertex function. The new vertex function is given by an infinite series which can only be treated in an asymptotic approximation at the present time. Preliminary approximations prove that the second order self-energies of a fermion and a photon in the discrete model have convergent improper integrals. In the final section, a sharper asymptotic analysis is employed. It is proved that in case the number of external photon or fermion lines is at least one, then the S-matrix elements converge in all orders. Moreover, there are no infra-red divergences in this formulation.

math-ph↗

arXiv2008

Phonon renormalisation in doped bilayer graphene

We report phonon renormalisation in bilayer graphene as a function of doping. The Raman G peak stiffens and sharpens for both electron and hole doping, as a result of the non-adiabatic Kohn anomaly at the $Γ$ point. The bilayer has two conduction and valence subbands, with splitting dependent on the interlayer coupling. This results in a change of slope in the variation of G peak position with doping, which allows a direct measurement of the interlayer coupling strength.

cond-mat.mes-hall↗

arXiv2008

The Influence of A-site Ionic Radii on the Magnetic Structure of Charge Ordered La0.5Ca0.5-xSrxMnO3 Manganites

The influence of the A-site ionic radii ( ) on the magnetic structure has been investigated in La0.5Ca0.5-xSrxMnO3 compounds (0 .GE. x .LE. 0.5) using neutron diffraction, magnetization, and resistivity techniques. All compounds in the composition range x .LE. 0.3 crystallize in the orthorhombic structure (Space Group Pnma). No further structural transition is observed as temperature is lowered below 300K. The compound x = 0.4, is a mixture of two orthorhombic phases crystallizing in Pnma and Fmmm space group. The x = 0.5 compound has a tetragonal structure in the space group, I4/mcm. The charge ordered (CO) state with CE-type antiferromagnetic order remains stable for x .LE. 0.3. Above x = 0.3, the CE-type antiferromagnetic state is suppressed. In x = 0.4 compound, A-type antiferromagnetic ordering is found at temperatures below 200K. Orbital ordering accompanying the spin ordering is found in all the samples with x .LE. 0.4. The system becomes ferromagnetic at x = 0.5 and no signature of orbital ordering is observed. As a function of , the charge ordered state is stable up to ~ 1.24A, and is suppressed thereafter. The magnetic structure undergoes a transformation from CE-type antiferromagnetic state to a ferromagnetic state with an intermediate A-type antiferromagnetic state.

cond-mat.str-el↗

arXiv2007

Electrochemically Top Gated Graphene: Monitoring Dopants by Raman Scattering

We demonstrate electrochemical top gating of graphene by using a solid polymer electrolyte. This allows to reach much higher electron and hole doping than standard back gating. In-situ Raman measurements monitor the doping. The G peak stiffens and sharpens for both electron and hole doping, while the 2D peak shows a different response to holes and electrons. Its position increases for hole doping, while it softens for high electron doping. The variation of G peak position is a signature of the non-adiabatic Kohn anomaly at $Γ$. On the other hand, for visible excitation, the variation of the 2D peak position is ruled by charge transfer. The intensity ratio of G and 2D peaks shows a strong dependence on doping, making it a sensitive parameter to monitor charges.

cond-mat.mtrl-sci↗

arXiv2007

The D0 Run II Impact Parameter Trigger

Many physics topics to be studied by the D0 experiment during Run II of the Fermilab Tevatron ppbar collider give rise to final states containing b--flavored particles. Examples include Higgs searches, top quark production and decay studies, and full reconstruction of B decays. The sensitivity to such modes has been significantly enhanced by the installation of a silicon based vertex detector as part of the DO detector upgrade for Run II. Interesting events must be identified initially in 100-200 microseconds to be available for later study. This paper describes custom electronics used in the DO trigger system to provide the real--time identification of events having tracks consistent with the decay of b--flavored particles.

physics.ins-det↗

arXiv2006

Path integrals and wavepacket evolution for damped mechanical systems

Damped mechanical systems with various forms of damping are quantized using the path integral formalism. In particular, we obtain the path integral kernel for the linearly damped harmonic oscillator and a particle in a uniform gravitational field with linearly or quadratically damped motion. In each case, we study the evolution of Gaussian wavepackets and discuss the characteristic features that help us distinguish between different types of damping. For quadratic damping, we show that the action and equation of motion of such a system has a connection with the zero dimensional version of a currently popular scalar field theory. Furthermore we demonstrate that the equation of motion (for quadratic damping) can be identified as a geodesic equation in a fictitious two-dimensional space.

quant-ph↗

arXiv2006

General Solutions of Cosmological Equations with Inequalities for Perfect Fluid and Scalar Field

Spherically symmetric cosmological equations in the usual FLRW coordinates are explored, with different sources. The first couples a perfect fluid with a quintessence scalar field and the second couples a perfect fluid to a tachyonic scalar field. In both cases, in the inflationary regime, the scale factor a(t) and its first two time derivatives are positive definite. Both sources in the matter phase yield a scale factor and its first derivative as positive definite. In both cases and in each phase, the general solutions of the differential equations together with the algebraic and differential inequalities are obtained. As special cases, exponential, hyperbolic, and power law inflation, as well as power law expansion for the matter phase are all derived from the general solutions. With recent data on baryonic matter, Hubble parameter and deceleration parameter, the relative percentages of baryonic matter, daek matter and dark energy are calculated. The quintessence model yields: 4% baryonic matter, 18% dark matter, and 78% dark energy. The tachyonic model gives: 4% baryonic matter, 36% dark matter, and 60% dark energy.

gr-qc↗