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N. D. Hari Dass

Publications and source records attributed to N. D. Hari Dass.

At least 55 records · Page 3Linked to original sources
arXiv2005

High accuracy simulations of d=4 SU(3) qcd-string

We present here the results of our high accuracy simulations of $q\bar q$ potential in $d=4$ SU(3) Yang-Mills theory. We measure this quantity by measuring the {\it Polyakov Loop Correlators} using the {\it exponential variance reduction technique(multilevel)} of Luscher and Weisz. We further use numerical integration of the one-link integrals of SU(3) theory by adopting techniques proposed by de Forcrand and Roiesnell achieving significant speed-up in the process. Measurements were done at $β= 5.7$ on $24^3x32$ as well as $32^4$ lattices for separations between the Polyakov loops in the range $r = 2-9$ in lattice units. We analyse the results in terms of the force between $q\bar q$ pair as well as in terms of a {\em scaled second derivative} of the potential. We show that the data is accurate enough to distinguish between different string models and that it seems to favour the expression for ground state energy of a Nambu-Goto string. We conclude by discussing future plans.

hep-lat↗
arXiv2005

Three Lectures on Chiral Symmetry

In these lectures I cover various aspects of Chiral Symmetry in the hadronic world from a pre-QCD perspective. I also discuss the absence of spontaneous symmetry breaking in d=4 large N O(N) models.

hep-ph↗
arXiv2004

An alternate model for protective measurements of two-level systems

In this article we propose an alternate model for the so called {\it protective measurements}, more appropriately {\it adiabatic measurements} of a spin 1/2 system where the {\it apparatus} is also a quantum system with a {\em finite dimensional Hilbert space}. This circumvents several technical as well as conceptual issues that arise when dealing with an infinite dimensional Hilbert space as in the analysis of conventional Stern-Gerlach experiment. Here also it is demonstrated that the response of the detector is continuous and it {\it directly} measures {\em expectation values without altering the state of the system}(when the unknown original state is a {\it nondegenerate eigenstate of the system Hamiltonian}, in the limit of {\em ideal} adiabatic conditions. We have also computed the corrections arising out of the inevitable departures from ideal adiabaticity i.e the time of measurement being large but finite. To overcome the {\em conceptual} difficulties with a {\it quantum apparatus}, we have simulated a {\it classical apparatus} as a {\em large} assembly of spin-1/2 systems. We end this article with a conclusion and a discussion of some future issues.

quant-ph↗
arXiv2004

Purification of Single Qubits and reconstruction from Post-Measurement State

Purification of mixed states in Quantum Mechanics, by which we mean the transformation into pure states, has been viewed as an {\it Operation} in the sense of Kraus et al and explicit {\it Kraus Operators} \cite{kra1,kra2,kra3} have been constructed for two seperate purification protocols. The first one, initially due to Schrodinger \cite{sch} and subsequently elaborated by Sudarshan et al \cite{sudar}, is based on the {\it preservation of probabilities}. We have constructed a second protocol here based on {\it optimization of fidelities}. Both purification protocols have been implemented on a single qubit in an attempt to improve the fidelity of the purified post measurement state of the qubit with the initial pure state. We have considered both {\it complete} and {\it partial} measurements and have established bounds and inequalities for various fidelities. We show that our purification protocol leads to better state reconstruction, most explicitly so, when partial measurements are made.

quant-ph↗
arXiv2004

A New Relation between post and pre-optimal measurement states

When an optimal measurement is made on a qubit and what we call an Unbiased Mixture of the resulting ensembles is taken, then the post measurement density matrix is shown to be related to the pre-measurement density matrix through a simple and linear relation. It is shown that such a relation holds only when the measurements are made in Mutually Unbiased Bases- MUB. For Spin-1/2 it is also shown explicitly that non-orthogonal measurements fail to give such a linear relation no matter how the ensembles are mixed. The result has been proved to be true for arbitrary quantum mechanical systems of finite dimensional Hilbert spaces. The result is true irrespective of whether the initial state is pure or mixed.

quant-ph↗
arXiv2002

A Hundred Years of Larmor Formula

Sir Joseph LARMOR showed in 1897 that an oscillating electric charge emits radiation energy proportional to (acceleration)$^2$. At first sight,the result appears to be valid for arbitrary accelerations. But, perpetual uniform acceleration has been a case of nagging doubts, as radiation reaction vanishes and the equivalence principle, as also conformal symmetry of Maxwell equations each require nil energy loss. Special hypotheses are devised by some to justify the assumption of radiation loss for both perpetual and non-perpetual (uniform) accelerations which, as in the case of (uniform) velocities, are really different. The problem is here simply resolved by an explicit computation to show absence of radiation for the perpetual case and by illustrating that Larmor formula makes sense {\it only if} there is {\it change} in acceleration, just as kinetic energy has nontrivial quantitative sense, only when there is change in velocity.

physics.ed-ph↗
arXiv2002

A No-go theorem for de Sitter compactifications?

A general framework for studying compactifications in supergravity and string theories was introduced by Candelas, Horowitz, Strominger and Witten. This was further generalised to take into account the warp factor by de Wit, Smit and Hari Dass. Though the prime focus of the latter was to find solutions with nontrivial warp factors (shown not to exist under a variety of circumstances), it was shown there that de Sitter compactifications are generically disfavoured. In this note we place these results in the context of a revived interest in de Sitter spacetimes .

hep-th↗
arXiv2002

An Elementary (Pseudo)Scalar at all scales

We consider an extended linear $σ$ model in which the fermions are quarks and are coupled to gluons. Equivalently, this is QCD extended by coupling the quarks to a colour singlet chiral multiplet of ($ σ, \vec π$) fields. This theory has a phase governed by a UV fixed point where all couplings are AF (asymptotically free). This implies that the scalars are elementary at high energies (UV) and, as they are colour singlets, they are not confined at low energy (IR). Thus, the scalar particles are elementary at all scales.

hep-th↗
arXiv2002

On the Emergence of the Microcanonical Description from a Pure State

We study, in general terms, the process by which a pure state can ``self-thermalize'' and {\em appear} to be described by a microcanonical density matrix. This requires a quantum mechanical version of the Gibbsian coarse graining that conceptually underlies classical statistical mechanics. We introduce some extra degrees of freedom that are necessary for this. Interaction between these degrees and the system can be understood as a process of resonant absorption and emission of ``soft quanta''. This intuitive picture allows one to state a criterion for when self thermalization occurs. This paradigm also provides a method for calculating the thermalization rate using the usual formalism of atomic physics for calculating decay rates. We contrast our prescription for coarse graining, which is somewhat dynamical, with the earlier approaches that are intrinsically kinematical. An important motivation for this study is the black hole information paradox.

cond-mat.stat-mech↗
arXiv2002

Information cloning of harmonic oscillator coherent states and its fidelity

We show that in the case of unknown {\em harmonic oscillator coherent states} it is possible to achieve what we call {\it perfect information cloning}. By this we mean that it is still possible to make arbitrary number of copies of a state which has {\it exactly} the same information content as the original unknown coherent state. By making use of this {\it perfect information cloning} it would be possible to estimate the original state through measurements and make arbitrary number of copies of the estimator. We define the notion of a {\em Measurement Fidelity}. We show that this information cloning gives rise, in the case of $1\to N$, to a {\em distribution} of {\em measurement fidelities} whose average value is ${1\over 2}$ irrespective of the number of copies originally made. Generalisations of this to the $M\to MN$ case as well as the measurement fidelities for Gaussian cloners are also given.

quant-ph↗
arXiv2001

Vacuum polarisation induced coupling between Maxwell and Kalb-Ramond Fields

We present here a manifestly gauge invariant calculation of vacuum polarization to fermions in the presence of a constant Maxwell and a constant Kalb-Ramond field in four dimensions. The formalism is a generalisation of the one used by Schwinger in his famous paper on gauge invariance and vacuum polarization. We get an explicit expression for the vacuum polarization induced effective Lagrangian for a constant Maxwell field interacting with a constant Kalb-Ramond field. In the weak field limit we get the coupling between the Maxwell field and the Kalb-Ramond field to be $(\tilde{H}.\tilde{F})^2$, where ${\tilde H}_μ= {1\over {3!}}ε_{μαβλ}H^{αβλ}$ and $\tilde F$ is the dual of $F_{μν}$.

hep-th↗
arXiv2001

Simplified Method for Trace Anomaly Calculations in d=6 and d<6

We discuss a simplified method for computing trace anomalies in d=6 and d<6 dimensions. It is known that in the quantum mechanical approach trace anomalies in d dimensions are given by a (1+d/2)-loop computation in an auxiliary 1d sigma model with arbitrary geometry. We show how one can obtain the same information using a simpler d/2-loop calculation on an arbitrary geometry supplemented by a (1+d/2)-loop calculation on the simplified geometry of a maximally symmetric space.

hep-th↗
arXiv2000

Quasi-local Update Algorithms for Numerical Simulations of d=3 SU(2) LGT in the Dual Formulation

In the dual formulation of d=3 SU(2) LGT, the link variables are group representations and valid configurations are those satisfying a number of triangle inequalities. In \cite{lat99} algorithms for local updates that automatically respect these constraints were described. It was also pointed out there that these local updates were not ergodic. In this presentation, we describe two different quasi-local updating algorithms which, in conjunction with the local updates, appear to be ergodic.

hep-lat↗
arXiv2000

Current Status of the Numerical Simulations of d=3 SU(2) Lattice Gauge Theory in the Dual Formulation

We have continued our systematic investigations of the numerical simulations of lattice gauge theories in the dual formulation. These include: i) a more practical implementation of the quasi-local updating technique, ii) a thorough investigation of the sign problem, iii) issues related to the ergodicity of the various update algorithms, iv) a novel way of measuring conventional observables like plaquette in the dual formalism and v) investigations of thermalisation. While the dual formulation holds out a lot of promises in principle, there are still some ways to go before it can be made into an attractive alternative lattice formulation.

hep-lat↗
arXiv2000

Axial anomaly and Ginsparg-Wilson fermions in the Lattice Dirac Sea picture

The axial anomaly equation in 1+1 dimensional QED is obtained on the lattice for fermions obeying the Ginsparg-Wilson relation. We make use of the properties of the Lattice Dirac sea to investigate the connection between the anomaly and the Ginsparg-Wilson operator in the Hamiltonian picture. The correct anomaly is reproduced for gauge fields whose characteristic time is much larger than the lattice spacing, which is the regime where the adiabatic approximation applies. A non-zero Wilson $r$ parameter is necessary to get the correct anomaly. The anomaly is shown to be independent of $r$ for $r>0.5$. The generalization to 3+1 dimensions is also discussed.

hep-lat↗
arXiv1999

Asymptotically Free Non-Abelian Gauge Theories With Fermions and Scalars As Alternatives to QCD

In this paper we construct non-Abelian gauge theories with fermions and scalars that nevertheless possess asymptotic freedom.The scalars are taken to be in a chiral multiplet transforming as $(2,2)$ under $SU(2)_L\otimes SU(2)_R$ and transforming as singlets under the colour SU(3) group. We consider two distinct scenarios, one in which the additional scalars are light and another in which they are heavier than half the Z-boson mass. It is shown that asymptotic freedom is obtained without requiring that all additional couplings keep fixed ratios with each other. It is also shown that both scenarios can not be ruled out by what are considered standard tests of QCD like R- parameter, g-2 for muons or deep inelastic phenomena. The light mass scenario is however ruled out by high precision Z-width data (and only by that one data).The heavy mass scenario is still viable and is shown to naturally pass the test of flavour changing neutral currents. It also is not ruled out by precision electroweak oblique parameters. Many distinctive experimental signatures of these models are also discussed.

hep-th↗
arXiv1999

Numerical Simulations of d=3 SU(2) LGT in the Dual Formulation

We have developed the techniques necessary for a numerical simulation of d=3 SU(2) Lattice Gauge theories in the dual formulation as originally developed by Anishetty et al . These include updating techniques that preserve the constrained configuration space, efficient evaluation of 6-j symbols and a certain problem associated with the positive indefiniteness of the weight factors.

hep-lat↗