Das, Saurya

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    Planckian scattering of D-branes
    (Elsevier BV North Holland, 1988) Das, Saurya; Dasgupta, Arundhati; Ramadevi, P.; Sarkar, Tapobrata
    We consider the gravitational scattering of point particles in four dimensions, at Planckian centre of mass energy and low momentum transfer, or the eikonal approximation. The scattering amplitude can be exactly computed by modelling point particles by very generic metrics. A class of such metrics are black hole solutions obtained from dimensional reduction of p-brane solutions with one or more Ramond-Ramond charges in string theory. At weak string coupling, such black holes are replaced by a collection of wrapped D-branes. Thus, we investigate eikonal scattering at weak coupling by modelling the point particles by wrapped D-branes and show that the amplitudes exactly match the corresponding amplitude found at strong coupling. We extend the calculation for scattering of charged particles.
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    Electromagnetic charge-monopole versus gravitational scattering at Planckian energies
    (American Physical Society, 1994) Das, Saurya; Majumdar, Parthasarathi
    The amplitude for the scattering of a point magnetic monopole and a point charge, at center-of- mass energies much larger than the masses of the particles, and in the limit of low momentum transfer, is shown to be proportional to the (integer-valued) monopole strength, assuming the Dirac quantization condition for the monopole-charge system. It is demonstrated that, for small momentum transfer, charge-monopole electromagnetic effects remain comparable to those due to the gravitational interaction between the particles even at Planckian center-of-mass energies.
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    Electromagnetic and gravitational scattering at Planckian energies
    (American Physical Society, 1995) Das, Saurya; Majumdar, Parthasarathi
    The scattering of pointlike particles at a very large center-of-mass energies and fixed low momentum transfers, occurring due to both their electromagnetic and gravitational interactions, is reexamined in the particular case when one of the particles carries a magnetic charge. At Planckian center-of-mass energies, when gravitational dominance is usually expected, the presence of magnetic charge is show to produce dramatic modifications to the scattering cross section as well as to holomorphic structure of the scattering amplitude.
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    Shock wave mixing in Einstein and dilaton gravity
    (Elsevier BV North Holland, 1995) Das, Saurya; Majumdar, Parthasarathi
    We consider possible mixing of electromagnetic and gravitational shock waves, in the Planckian energy scattering of point particles in Minkowski space, By boosting a Reissner-Nordstriim black hole solution to the velocity of light, it is shown that no mixing of shock waves takes place for arbitrary finite charge carried by the black hole. However, a similar boosting procedure for a charged black hole solution in dilaton gravity yields some mixing: the wave function of even a neutral test particle, acquires a small additional phase factor depending on the dilalonic black hole charge. Possible implications for poles in the amplitudes for the dilaton gravity case are discussed
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    Eikonal particle scattering and dilaton gravity
    (American Physical Society, 1997) Das, Saurya; Majumdar, Parthasarathi
    Approximating light charged pointlike particles in terms of ~nonextremal! dilatonic black holes is shown to lead to certain pathologies in Planckian scattering in the eikonal approximation, which are traced to the presence of a ~naked! curvature singularity in the metric of these black holes. The existence of such pathologies is confirmed by analyzing the problem in an ‘‘external metric’’ formulation where an ultrarelativistic point particle scatters off a dilatonic black hole geometry at large impact parameters. The maladies disappear almost trivially upon imposing the extremal limit. Attempts to derive an effective three-dimensional ‘‘boundary’’ field theory in the eikonal limit are stymied by four-dimensional ~bulk! terms proportional to the light-cone derivatives of the dilaton field, leading to nontrivial mixing of electromagnetic and gravitational effects, in contrast with the case of general relativity. An eikonal scattering amplitude, showing decoupling of these effects, is shown to be derivable by resummation of graviton, dilaton, and photon exchange ladder diagrams in a linearized version of the theory for an asymptotic value of the dilaton field which makes the string coupling constant nonperturbative.
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    High energy effects on D-brane and black hole emission rates
    (American Physical Society, 1997) Das, Saurya; Dasgupta, Arundhati; Sarkar, Tapobrata
    We study the emission of scalar particles from a class of near-extremal five-dimensional black holes and the corresponding D-brane configuration at high energies. We show that the distribution functions and the black hole greybody factors are modified in the high energy tail of the Hawking spectrum in such a way that the emission rates exactly match. We extend the results to charged scalar emission and to four dimensions.
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    Aspects of Planckian scattering beyond the eikonal
    (Indian Academy of Sciences, 1998) Das, Saurya; Majumdar, Parthasarathi
    We discuss an approach to compute two-particle scattering amplitudes for spinless light particles colliding at Planckian centre-of-mass energies, with increasing momentum transfer away from the eikonal limit. The leading corrections to the eikonal amplitude, in our 'external metric' approach, are shown to be vanishingly small in the limit of the source particle mass going to zero. For massless charged particles, the electromagnetic and gravitational interactions decouple in the eikonal limit, hut mix non-trivially for the leading order corrections.
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    Black hole emission rates and the AdS/CFT correspondence
    (SISSA, 1999) Das, Saurya; Dasgupta, Arundhati
    We study the emission rates of scalar, spinor and vector particles from a 5 dimensional black hole for arbitrary partial waves. The solution is lifted to 6 dimensions, and the near horizon BTZ S3 geometry of the black hole solution is probed to determine the greybody factors. We show that the exact decay rates can be reproduced from a (1 + 1)-dimensional conformal eld theory which lies on the boundary of the near horizon geometry. The AdS/CFT correspondence is used to determine the dimension of the CFT operators corresponding to the bulk elds. These operators couple to plane waves incident on the CFT from in nity to produce emission in the bulk.
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    Statistical entropy of Schwarzschild black strings and black holes
    (American Physical Society, 2000) Das, Saurya; Ghosh, Amit; Mitra, P.
    The statistical entropy of a Schwarzschild black string in five dimensions is obtained by counting the black string states which form a representation of the near-horizon conformal symmetry with a central charge. The statistical entropy of the string agrees with its Bekenstein-Hawking entropy as well as that of the Schwarzschild black hole in four dimensions. The string length which gives the Virasoro algebra also reproduces the precise value of the Bekenstein-Hawking entropy and lies inside the stability bound of the string.
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    Conserved quantities in Kerr-anti-de Sitter spacetimes in various dimensions
    (SISSA, 2000) Das, Saurya; Mann, Robert B.
    We compute the conserved charges for Kerr anti-de Sitter spacetimes in various dimensions using the conformal and the counterterm prescriptions. We show that the conserved charge corresponding to the global timelike killing vector computed by the two methods di er by a constant dependent on the rotation parameter and cosmological constant in odd spacetime dimensions, whereas the charge corresponding to the rotational killing vector is the same in either approach. We comment on possible implications of our results to the AdS/CFT correspondence.
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    New holographic entropy bound from quantum geometry
    (American Institute of Physics, 2001) Das, Saurya; Kaul, Romesh K.; Majumdar, Parthasarathi
    A new entropy bound, tighter than the standard holographic bound due to Bekenstein, is derived for spacetimes with nonrotating isolated horizons from the quantum geometry approach, in which the horizon is described by the boundary degrees of freedom of a three dimensional Chern-Simons theory.
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    Quantum mechanics of charged black holes
    (Elsevier, 2001) Barvinsky, Andrei; Das, Saurya; Kunstatter, Gabor
    We quantize the spherically symmetric sector of generic charged black holes. Thermal properties are incorporated by imposing periodicity in Euclidean time, with period equal to the inverse Hawking temperature of the black hole. This leads to an exact quantization of the area (A) and charge (Q) operators. For the Reissner–Nordström black hole, A = 4πG¯h(2n + p + 1) and Q= me, for integers n,p,m. Consistency requires the fine structure constant to be quantized: e2/¯h = p/m2. Remarkably, vacuum fluctuations exclude extremal black holes from the spectrum, while near extremal black holes are highly quantum objects. We also prove that horizon area is an adiabatic invariant.
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    Scalar field spacetimes and the anti-de Sitter space/conformal-field theory conjecture
    (American Physical Society, 2001) Das, Saurya; Gegenberg, J.; Husain, V.
    We describe a class of asymptotically AdS scalar field spacetimes, and calculate the associated conserved charges for three, four and five spacetime dimensions using the conformal and counterterm prescriptions. The energy associated with the solutions in each case is proportional to AM22k2, where M is a constant and k is a scalar charge. In five spacetime dimensions, the counterterm prescription gives an additional vacuum ~Casimir! energy, which agrees with that found in the context of AdS conformal-field theory ~CFT! correspondence. We find a surprising degeneracy: the energy of the ‘‘extremal’’ scalar field solution M5k equals the energy of global AdS. This result is discussed in light of the AdS/CFT conjecture.
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    Adiabatic quantum computation and Deutsch's algorithm
    (American Physical Society, 2002) Das, Saurya; Kobes, Randy; Kunstatter, Gabor
    We show that by a suitable choice of a time-dependent Hamiltonian, Deutsch’s algorithm can be implemented by an adiabatic quantum computer. We extend our analysis to the Deutsch-Jozsa problem and estimate the required running time for both global and local adiabatic evolutions.
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    Quantum mechanical spectra of charged black holes
    (Elsevier, 2003) Das, Saurya; Ramadevi, P.; Yajnik, U. A.; Sule, A.
    It has been argued by several authors, using different formalisms, that the quantum mechanical spectrum of black hole horizon area is discrete and uniformly spaced. Recently it was shown that two such approaches, namely the one involving quantization on a reduced phase space, and the algebraic approach of Bekenstein and Gour are equivalent for spherically symmetric, neutral black holes (hep-th/0202076). That is, the observables of one can be mapped to those of the other. Here we extend that analysis to include charged black holes. Once again, we find that the ground state of the black hole is a Planck size remnant.
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    Relativistic particle in a three-dimensional box
    (2015-12-15) Alberto, Pedro; Das, Saurya; Vagenas, Elias C.
    We generalize the work of Alberto, Fiolhais and Gil and solve the problem of a Dirac particle confined in a 3-dimensional box. The non-relativistic and ultra-relativistic limits are considered and it is shown that the size of the box determines how relativistic the low-lying states are. The consequences for the density of states of a relativistic fermion gas are briefly discussed.
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    Das and Vagenas reply
    (2015-12-15) Das, Saurya; Vagenas, Elias C.
    A reply to the Comment by M. M. Ettefaghi and S. M. Fazeli
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    Where are the degrees of freedom responsible for black hole entropy?
    (2015-12-15) Das, Saurya; Shankaranarayanan, S.; Sur, Sourav
    Considering the entanglement between quantum field degrees of freedom inside and outside the horizon as a plausible source of black hole entropy, we address the question: where are the degrees of freedom that give rise to this entropy located? When the field is in ground state, the black hole area law is obeyed and the degrees of freedom near the horizon contribute most to the entropy. However, for excited state, or a superposition of ground state and excited state, power-law corrections to the area law are obtained, and more significant contributions from the degrees of freedom far from the horizon are shown.
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    Can MOND type hypotheses be tested in a free fall laboratory environment?
    (2015-12-15) Das, Saurya; Patitsas, Steve
    The extremely small accelerations of objects required for the the onset of modified Newtonian dynamics, or MOND, makes testing the hypothesis in conventional terrestrial laboratories virtually impossible. This is due to the large background acceleration of Earth, which is transmitted to the acceleration of test objects within an apparatus. We show however, that it may be possible to test MOND-type hypotheses with experiments using a conventional apparatus capable of tracking very small accelerations of its components, but performed in locally inertial frames such as artifi- cial satellites and other freely falling laboratories. For example, experiments involving an optical interferometer or a torsion balance in these laboratories would show nonlinear dynamics, and dis- placement amplitudes larger than expected. These experiments may also be able to test potential violations of the strong equivalence principle by MOND and to distinguish between its two possible interpretations (modified inertia and modified gravity).
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    Stability and hierarchy problems in string inspired braneworld scenarios
    (2015-12-15) Das, Saurya; Dey, Anindya; SenGupta, Soumitra
    We generalise the RS braneworld model by taking into account a general stringy bulk containing the scalar dilaton field and the two-form Kalb-Ramond field, apart from gravity. Assuming small fluctuations around a RS background, the back-reacted warp factor is obtained. It is shown that the fine tuning problem in connection with the Higgs mass reappears in a new guise and the effective modular potential fails to stabilise the braneworld.