Showing papers by "Tata Institute of Fundamental Research published in 2003"

Geant4—a simulation toolkit

Abstract: G eant 4 is a toolkit for simulating the passage of particles through matter. It includes a complete range of functionality including tracking, geometry, physics models and hits. The physics processes offered cover a comprehensive range, including electromagnetic, hadronic and optical processes, a large set of long-lived particles, materials and elements, over a wide energy range starting, in some cases, from 250 eV and extending in others to the TeV energy range. It has been designed and constructed to expose the physics models utilised, to handle complex geometries, and to enable its easy adaptation for optimal use in different sets of applications. The toolkit is the result of a worldwide collaboration of physicists and software engineers. It has been created exploiting software engineering and object-oriented technology and implemented in the C++ programming language. It has been used in applications in particle physics, nuclear physics, accelerator design, space engineering and medical physics.

De Sitter vacua in string theory

Abstract: We outline the construction of metastable de Sitter vacua of type IIB string theory. Our starting point is highly warped IIB compactifications with nontrivial NS and RR three-form fluxes. By incorporating known corrections to the superpotential from Euclidean D-brane instantons or gaugino condensation, one can make models with all moduli fixed, yielding a supersymmetric AdS vacuum. Inclusion of a small number of $\overline{\mathrm{D}3}$-branes in the resulting warped geometry allows one to uplift the AdS minimum and make it a metastable de Sitter ground state. The lifetime of our metastable de Sitter vacua is much greater than the cosmological time scale of ${10}^{10}\mathrm{yr}.$ We also prove, under certain conditions, that the lifetime of dS space in string theory will always be shorter than the recurrence time.

Towards inflation in string theory

Abstract: We investigate the embedding of brane inflation into stable compactifications of string theory. At first sight a warped compactification geometry seems to produce a naturally flat inflaton potential, evading one well known difficulty of brane?antibrane scenarios. Careful consideration of the closed string moduli reveals a further obstacle: superpotential stabilization of the compactification volume typically modifies the inflaton potential and renders it too steep for inflation. We discuss the non-generic conditions under which this problem does not arise. We conclude that brane inflation models can only work if restrictive assumptions about the method of volume stabilization, the warping of the internal space, and the source of inflationary energy are satisfied. We argue that this may not be a real problem, given the large range of available fluxes and background geometries in string theory.

Observation of a Narrow Charmoniumlike State in Exclusive B±→K±π+π-J/ψ Decays

Abstract: We report the observation of a narrow charmoniumlike state produced in the exclusive decay process B+/--->K+/-pi(+)pi(-)J/psi. This state, which decays into pi(+)pi(-)J/psi, has a mass of 3872.0+/-0.6(stat)+/-0.5(syst) MeV, a value that is very near the M(D0)+M(D(*0)) mass threshold. The results are based on an analysis of 152M B-Bmacr; events collected at the Upsilon(4S) resonance in the Belle detector at the KEKB collider. The signal has a statistical significance that is in excess of 10sigma.

An increased estimate of the merger rate of double neutron stars from observations of a highly relativistic system

Abstract: The merger of close binary systems containing two neutron stars should produce a burst of gravitational waves, as predicted by the theory of general relativity. A reliable estimate of the double-neutron-star merger rate in the Galaxy is crucial in order to predict whether current gravity wave detectors will be successful in detecting such bursts. Present estimates of this rate are rather low, because we know of only a few double-neutron-star binaries with merger times less than the age of the Universe. Here we report the discovery of a 22-ms pulsar, PSR J0737-3039, which is a member of a highly relativistic double-neutron-star binary with an orbital period of 2.4 hours. This system will merge in about 85 Myr, a time much shorter than for any other known neutron-star binary. Together with the relatively low radio luminosity of PSR J0737-3039, this timescale implies an order-of-magnitude increase in the predicted merger rate for double-neutron-star systems in our Galaxy (and in the rest of the Universe).

Moduli Stabilization from Fluxes in a Simple IIB Orientifold

Abstract: We study novel type IIB compactifications on the T{sup 6}/Z{sub 2} orientifold. This geometry arises in the T-dual description of Type I theory on T{sup 6}, and one normally introduces 16 space-filling D3-branes to cancel the RR tadpoles. Here, we cancel the RR tadpoles either partially or fully by turning on three-form flux in the compact geometry. The resulting (super)potential for moduli is calculable. We demonstrate that one can find many examples of {Nu} = 1 supersymmetric vacua with greatly reduced numbers of moduli in this system. A few examples with {Nu} > 1 supersymmetry or complete supersymmetry breaking are also discussed.

Smokeless tobacco and health in India and South Asia

Abstract: South Asia is a major producer and net exporter of tobacco. Over one-third of tobacco consumed regionally is smokeless. Traditional forms like betel quid, tobacco with lime and tobacco tooth powder are commonly used and the use of new products is increasing, not only among men but also among children, teenagers, women of reproductive age, medical and dental students and in the South Asian diaspora. Smokeless tobacco users studied prospectively in India had age-adjusted relative risks for premature mortality of 1.2-1.96 (men) and 1.3 (women). Current male chewers of betel quid with tobacco in case-control studies in India had relative risks of oral cancer varying between 1.8-5.8 and relative risks for oesophageal cancer of 2.1-3.2. Oral submucous fibrosis is increasing due to the use of processed areca nut products, many containing tobacco. Pregnant women in India who used smokeless tobacco have a threefold increased risk of stillbirth and a two- to threefold increased risk of having a low birthweight infant. In recent years, several states in India have banned the sale, manufacture and storage of gutka, a smokeless tobacco product containing areca nut. In May 2003 in India, the Tobacco Products Bill 2001 was enacted to regulate the promotion and sale of all tobacco products. In two large-scale educational interventions in India, sizable proportions of tobacco users quit during 5-10 years of follow-up and incidence rates of oral leukoplakia measured in one study fell in the intervention cohort. Tobacco education must be imparted through schools, existing government health programmes and hospital outreach programmes.

New supersymmetric string compactifications

Abstract: We describe a new class of supersymmetric string compactifications to 4d Minkowski space. These solutions involve type II strings propagating on (orientifolds of) non Calabi-Yau spaces in the presence of background NS and RR fluxes. The simplest examples have descriptions as cosets, generalizing the three-dimensional nilmanifold. They can also be thought of as twisted tori. We derive a formula for the (super) potential governing the light fields, which is generated by the fluxes and certain "twists" in the geometry. Detailed consideration of an example also gives strong evidence that in some cases, these exotic geometries are related by smooth transitions to standard Calabi-Yau or G(2) compactifications of M-theory.

Observation of the DsJ(2317) and DsJ(2457) in B Decays

Abstract: We report the first observation of the B-->(D) over barD(sJ)(2317) and B-->(D) over barD(sJ)(2457) decays based on 123.8x10(6) B (B) over bar events collected with the Belle detector at KEKB. We observe the D-sJ(2317) decay to D(s)pi(0) and the D-sJ(2457) decay to the D(s)(*)pi(0) and D(s)gamma final states. We also set 90% C.L. upper limits for the decays D-sJ(2317)-->D-s*gamma, D-sJ(2457)-->D-s*gamma, D-sJ(2457)-->D(s)pi(0), and D-sJ(2457)-->D(s)pi(+)pi(-).

Duality twists, orbifolds, and fluxes

Abstract: We investigate compactifications with duality twists and their relation to orbifolds and compactifications with fluxes. Inequivalent compactifications are classified by conjugacy classes of the U-duality group and result in gauged supergravities in lower dimensions with nontrivial Scherk-Schwarz potentials on the moduli space. For certain twists, this mechanism is equivalent to introducing internal fluxes but is more general and can be used to stabilize some of the moduli. We show that the potential has stable minima with zero energy precisely at the fixed points of the twist group. In string theory, when the twist belongs to the T-duality group, the theory at the minimum has an exact CFT description as an orbifold. We also discuss more general twists by nonperturbative U-duality transformations.

Changes in solar dynamics from 1995 to 2002

Abstract: Data obtained by the GONG and MDI instruments over the last 7 years are used to study how solar dynamics—both rotation and other large scale flows—has changed with time. In addition to the well-known phenomenon of bands of faster and slower rotation moving toward the equator and pole, we find that the zonal flow pattern rises upward with time. Like the zonal flows, the meridional flows also show distinct solar activity–related changes. In particular, the antisymmetric component of the meridional flow shows a decrease in speed with activity. We do not see any significant temporal variations in the dynamics of the tachocline region where the solar dynamo is believed to be operating. Subject headings: Sun: helioseismology — Sun: interior — Sun: oscillations — Sun: rotation

Fast distributed algorithms for (weakly) connected dominating sets and linear-size skeletons

Abstract: Motivated by routing issues in ad hoc networks, we present polylogarithmic-time distributed algorithms for two problems. Given a network, we first show how to compute connected and weakly connected dominating sets whose size is at most O(logΔ) times optimal, Δ being the maximum degree of the input network. This is best-possible if NP ⊈ DTIME[nO(log log n)] and if the processors are limited to polynomial-time computation. We then show how to construct dominating sets which satisfy the above properties, as well as the "low stretch" property that any two adjacent nodes in the network have their dominators at a distance of at most O(log n) in the network. (Given a dominating set S, a dominator of a vertex u is any v ∊ S such that the distance between u and v is at most one.) We also show our time bounds to be essentially optimal.

Pressure and nonlinear susceptibilities in QCD at finite chemical potentials

Abstract: When the free energy density of QCD is expanded in a Taylor series in the chemical potential $\ensuremath{\mu},$ the coefficients are the nonlinear quark number susceptibilities. We show that these depend on the prescription for putting the chemical potential on the lattice, making all extrapolations in the chemical potential prescription dependent at finite lattice spacing. To put bounds on the prescription dependence, we investigate the magnitude of the nonlinear susceptibilities over a range of temperature, T, in QCD with two degenerate flavors of light dynamical quarks at lattice spacing $1/4T.$ The prescription dependence is removed in quenched QCD through a continuum extrapolation, and the dependence of the pressure P on $\ensuremath{\mu}$ is obtained.

The Amyloid β Peptide (Aβ1-40) Is Thermodynamically Soluble at Physiological Concentrations†

Abstract: Precipitation of the 39−43-residue amyloid β peptide (Aβ) is a crucial factor in Alzheimer's disease (AD). In normal as well as in AD-afflicted brain, the Aβ concentration is estimated to be a few nanomolar. Here we show that Aβ1-40 precipitates in vitro only if the dissolved concentration is >14 μM. Using fluorescence correlation spectroscopy, we further show that the precipitation is complete in 1 day, after which the size distribution of Aβ monomer/oligomers in the solution phase becomes stationary in time and independent of the starting Aβ concentration. Mass spectra confirm that both the solution phase and the coexisting precipitate contain chemically identical Aβ molecules. Incubation at 68 °C for 1 h reduces the solubility by <12%. Together, these results show that the thermodynamic saturation concentration (Csat) of Aβ1-40 in phosphate-buffered saline (PBS) at pH 7.4 has a well-defined lower limit of 15.5 ± 1 μM. Divalent metal ions (believed to play a role in AD) at near-saturation concentrations...

Compactification with Flux on K3 and Tori

Abstract: We study compactifications of type-IIB string theory on a K3 × T2/Z2 orientifold in the presence of RR and NS flux We find the most general supersymmetry preserving, Poincare invariant, vacua in this model All the complex structure moduli and some of the Kahler moduli are stabilised in these vacua We obtain in an explicit fashion the restrictions imposed by supersymmetry on the flux, and the values of the fixed moduli Some T-duals and Heterotic duals are also discussed, these are non-Calabi-Yau spaces A superpotential is constructed describing these duals Finally, we consider some susy breaking vacua where the flux gives rise to large extra dimensions with vanishing tree-level cosmological constant

Observation of minority spin character of the new electron doped manganite La0.7Ce0.3MnO3 from tunneling magnetoresistance

Abstract: We report the magnetotransport characteristics of a trilayer ferromagnetic tunnel junction built of an electron doped manganite (La0.7Ce0.3MnO3) and a hole doped manganite (La0.7Ca0.3MnO3). At low temperatures the junction exhibits a large positive tunneling magnetoresistance (TMR), irrespective of the bias voltage. At intermediate temperatures below T(C) the sign of the TMR is dependent on the bias voltage across the junction. The magnetoresistive characteristics of the junction strongly suggest that La0.7Ce0.3MnO3 is a minority spin carrier ferromagnet with a high degree of spin polarization, i.e., a transport half-metal.

Towards Inflation in String Theory

Abstract: We investigate the embedding of brane inflation into stable compactifications of string theory. At first sight a warped compactification geometry seems to produce a naturally flat inflaton potential, evading one well-known difficulty of brane-antibrane scenarios. Careful consideration of the closed string moduli reveals a further obstacle: superpotential stabilization of the compactification volume typically modifies the inflaton potential and renders it too steep for inflation. We discuss the non-generic conditions under which this problem does not arise. We conclude that brane inflation models can only work if restrictive assumptions about the method of volume stabilization, the warping of the internal space, and the source of inflationary energy are satisfied. We argue that this may not be a real problem, given the large range of available fluxes and background geometries in string theory.

Metal nanoplasmas as bright sources of hard X-ray pulses.

Abstract: We report significant enhancements in light coupling to intense-laser-created solid plasmas via surface plasmon and "lightning rod" effects. We demonstrate this in metal nanoparticle-coated solid targets irradiated with 100 fs, 806 nm laser pulses, focused to intensities approximately 10(14)-10(15) W cm(-2). Our experiments show a 13-fold enhancement in hard x-ray yield (10-200 keV) emitted by copper nanoparticle plasmas formed at the focal volume. A simple model explains the observed enhancement quantitatively and provides pointers to the design of structured surfaces for maximizing such emissions.

Dynamic spatiotemporal expression of LIM genes and cofactors in the embryonic and postnatal cerebral cortex.

Abstract: LIM-Homeodomain (LIM-HD) genes encode a family of transcription factors known to be involved in development and patterning in several systems. Previously, we have shown that LIM-HD gene Lhx2 is required for the formation of a crucial boundary in the dorsal telencephalon (Bulchand et al. [2001] Mech Dev 100:165-175). To further explore the role of LIM-HD genes as well as the broader LIM gene family in dorsal telencephalic development, we examined the expression pattern of the members of this gene family and their cofactors in the developing mouse cerebral cortex. Transcription factor activity of the LIM-HD proteins requires the formation of a tetrameric complex consisting of two LIM-HD molecules linked by a dimer of cofactor (Clim) molecules. LIM-only (Lmo) proteins can interfere with this process by competing for the cofactors. LIM-HD protein function, thus, can be modulated by the presence of the appropriate Clim or Lmo molecules. At least 13 LIM-HD, 4 Lmo, and 2 Clim genes have been identified in the mouse. Several of these genes exhibit complex spatiotemporal patterns spanning different stages of cortical development, from embryonic to postnatal ages. Noteworthy features of the expression patterns include delineation of boundaries within the developing cortex, up- or down-regulation during formation of selected cortical layers, and a striking complementarity of expression of several members consistent with specific functions in cortical development. Significantly, in some cases, Lmo or Clim gene expression is robust where no LIM-HD gene expression is detectable. These results suggest multiple and distinct roles for LIM-HD, Lmo, and Clim genes in cortical development, and also support a LIM-HD-independent role for some Lmo and Clim members.

A direct sum theorem in communication complexity via message compression

Abstract: We prove lower bounds for the direct sum problem for two-party bounded error randomised multiple-round communication protocols. Our proofs use the notion of information cost of a protocol, as defined by Chakrabarti et al. [CSWY01] and refined further by Bar-Yossef et al. [BJKS02]. Our main technical result is a 'compression' theorem saying that, for any probability distribution µ over the inputs, a k-round private coin bounded error protocol for a function f with information cost c can be converted into a k-round deterministic protocol for f with bounded distributional error and communication cost O(kc). We prove this result using a Substate Theorem about relative entropy and a rejection sampling argument. Our direct sum result follows from this 'compression' result via elementary information theoretic arguments. We also consider the direct sum problem in quantum communication. Using a probabilistic argument, we show that messages cannot be compressed in this manner even if they carry small information.

Self-Assembly of Template-Directed J-Aggregates of Porphyrin

Abstract: This communication describes the first observation of J-aggregate formation of a porphyrin by a polymer template. The chirality of the polymer is superimposed on the J-aggregate. Regular micrometer...

Higgsino dark matter in a supergravity model with nonuniversal gaugino masses

Abstract: We study a specific SUGRA model with nonuniversal gaugino masses as an alternative to the minimal SUGRA model in the context of supersymmetric dark matter. The lightest supersymmetric particle in this model turns out to be a Higgsino-dominated instead of a B-ino-dominated lightest neutralino. The thermal relic density of this Higgsino dark matter is somewhat lower than the cosmologically favored range, which means that it may be only a subdominant component of cold dark matter. Nonetheless, it predicts favorable rates of indirect detection, which can be seen in square-km-size neutrino telescopes.