Showing papers by "Tata Institute of Fundamental Research published in 2000"

Review of Particle Physics

Abstract: This biennial Review summarizes much of particle physics. Using data from previous editions., plus 2778 new measurements from 645 papers, we list, evaluate, and average measured properties of gauge bosons, leptons, quarks, mesons, and baryons. We also summarize searches for hypothetical particles such as Higgs bosons, heavy neutrinos, and supersymmetric particles. All the particle properties and search limits are listed in Summary Tables. We also give numerous tables, figures, formulae, and reviews of topics such as the Standard Model, particle detectors., probability, and statistics. Among the 108 reviews are many that are new or heavily revised including those on CKM quark-mixing matrix, V-ud & V-us, V-cb & V-ub, top quark, muon anomalous magnetic moment, extra dimensions, particle detectors, cosmic background radiation, dark matter, cosmological parameters, and big bang cosmology.

Effect of the size-induced structural transformation on the band gap in CdS nanoparticles

Abstract: The interrelation between particle size, crystal structure and optical properties in semiconductor quantum dots has elicited widespread interest. We report the first attempt at relating the size-induced transformation from a hexagonal to a cubic structure in CdS nanoparticles to a change in the band gap. CdS nanoparticles with particle size in the 0.7-10 nm range were prepared by chemical precipitation using thiophenol as a capping agent. Whereas the band gap for bulk hexagonal CdS is about 2.5 eV, that for 1 nm cubic CdS nanoparticles was found to be almost 3.9 eV. We also suggest a simple mechanism (based on the periodic insertion of stacking faults) for the transformation from the cubic zinc blende structure to the hexagonal wurtzite structure.

Structural and conformational stability of horseradish peroxidase: effect of temperature and pH.

Abstract: Detailed circular dichroism and fluorescence studies at different pHs have been carried out to monitor thermal unfolding of horseradish peroxidase isoenzyme c (HRPc). The change in CD in the 222 nm region corresponds to changes in the overall secondary structure of the enzyme, while that in the 400 nm region (Soret region) corresponds to changes in the tertiary structure around the heme in the enzyme. The temperature dependence of the tertiary structure around the heme also affected the intrinsic tryptophan fluorescence emission spectrum of the enzyme. The results suggested that melting of the tertiary structure to a pre-molten globule form takes place at 45 degrees C, which is much lower than the temperature (T(m) = 74 degrees C) at which depletion of heme from the heme cavity takes place. The melting of the tertiary structure was found to be associated with a pK(a) of approximately 5, indicating that this phase possibly involves breaking of the hydrogen-bonding network of the heme pocket, keeping the heme moiety still inside it. The stability of the secondary structure of the enzyme was also found to decrease at pH below 4.5. A 'high temperature' unfolding phase was observed which was, however, independent of pH. The stability of the secondary structure was found to drastically decrease in the presence of DTT (dithiothreitol), indicating that the 'high temperature' form is possibly stabilized due to interhelical disulfide bonds. Depletion of Ca(2+) ions resulted in a marked decrease in the stability of the secondary structure of the enzyme.

Picosecond optical nonlinearity in monolayer-protected gold, silver, and gold-silver alloy nanoclusters

Abstract: Monolayer-protected Au, Ag, and Au:Ag alloy nanoclusters have been synthesized using octanethiol and octadecanethiol as capping agents. The particle-size distribution is narrow with an average core size of 3--4 nm. Optical nonlinearity induced by 35 ps pulses at 532 nm has been investigated in these samples using the Z-scan technique. It is found that in general, they behave either as saturable absorbers or reverse saturable absorbers depending on the intensity of excitation. Au and Ag clusters show nearly the same efficiency for optical limiting, but the alloy clusters are found to be less efficient in limiting and are less photostable. The observed effects are explained in terms of the electron dynamics of the excited-state species.

The cosmic-ray electron and positron spectra measured at 1 au during solar minimum activity

Abstract: We report on a new measurement of the cosmic-ray electron and positron spectra. The data were collected by the balloon-borne experiment CAPRICE94, which was —own from Lynn Lake, Canada, on 1994 August 8¨9 at an altitude corresponding to 3.9 g cm~2 of average residual atmosphere. The experi- ment used the NMSU-WIZARD/CAPRICE94 balloon-borne magnet spectrometer equipped with a solid radiator Ring Imaging Cerenkov (RICH) detector, a time-of-—ight system, a tracking device consisting of drift chambers and multiwire proportional chambers, and a silicon-tungsten calorimeter. This was the —rst time a RICH detector was used together with an imaging calorimeter in a balloon-borne experi- ment. A total of 3211 electrons, with a rigidity at the spectrometer between 0.3 and 30 GV, and 734 positrons, between 0.3 and 10 GV, were identi—ed with small backgrounds from other particles. The absolute energy spectra were determined in the energy region at the top of the atmosphere between 0.46 and 43.6 GeV for electrons and between 0.46 and 14.6 GeV for positrons. We found that the observed positron spectrum and the positron fraction are consistent with a pure secondary origin. A comparison of the theoretically predicted interstellar spectrum of electrons shows that the injection spectrum of primary electrons is steeper than that of the nucleonic components of cosmic rays. Furthermore, the observed electron and positron spectra can be reproduced from the interstellar spectra by a spherically symmetric model for solar modulation; hence, the modulation is independent of the sign of the particle charge. Subject headings: balloonscosmic rayselementary particlesSun: activity

Dynamic instabilities and memory effects in vortex matter

Abstract: The magnetic flux line lattice in type II superconductors serves as a useful system in which to study condensed matter flow, as its dynamic properties are tunable. Recent studies have shown a number of puzzling phenomena associated with vortex motion, including: low-frequency noise and slow voltage oscillations; a history-dependent dynamic response, and memory of the direction, amplitude duration and frequency of the previously applied current; high vortex mobility for alternating current, but no apparent vortex motion for direct currents; and strong suppression of an a.c. response by small d.c. bias. Taken together, these phenomena are incompatible with current understanding of vortex dynamics. Here we report a generic mechanism that accounts for these observations. Our model, which is derived from investigations of the current distribution across single crystals of NbSe2, is based on a competition between the injection of a disordered vortex phase at the sample edges, and the dynamic annealing of this metastable disorder by the transport current. For an alternating current, only narrow regions near the edges are in the disordered phase, while for d.c. bias, most of the sample is in the disordered phase--preventing vortex motion because of more efficient pinning. The resulting spatial dependence of the disordered vortex system serves as an active memory of the previous history.

Magnetoresistance in ordered and disordered double perovskite oxide, Sr$_2$FeMoO$_6$

Abstract: We have prepared crystallographically ordered and disorder specimens of the double perovskite, Sr$_2$FeMoO$_6$ and investigated their magnetoresistance behaviour. The extent of ordering between the Fe and Mo sites in the two samples is determined by Rietveld analysis of powder x-ray diffraction patterns and reconfirmed by M\"{o}ssbauer studies. While the ordered sample exhibits the sharp low-field response, followed by moderate changes in the magnetoresistance at higher fields, the disordered sample is characterised by the absence of the spectacular low-field response. We argue that the low field response depends crucially on the half-metallic ferromagnetism, while the high-field response follows from the overall magnetic nature of the sample, even in absence of the half-metallic state.

The Structure of Dark Matter Halos in Hierarchical Clustering Theories

Abstract: During hierarchical clustering, smaller masses generally collapse earlier than larger masses and so are denser on the average. The core of a small-mass halo could be dense enough to resist disruption and survive undigested when it is incorporated into a larger object. We explore the possibility that a nested sequence of undigested cores in the center of the halo that have survived the hierarchical, inhomogeneous collapse to form larger and larger objects determines the halo structure in the inner regions. For a flat universe with P(k) ∝ kn, scaling arguments then suggest that the core density profile is ρ ∝ r-α, with α = (9 + 3n)/(5 + n). For any n < 1, the signature of undigested cores is a core density profile shallower than ρ ∝ 1/r2 and dependent on the power spectrum. For typical objects formed from a cold dark matter (CDM)-like power spectrum, the effective value of n is close to -2, and thus α could typically be near 1, the Navarro, Frenk, & White (NFW) value. Velocity dispersions should also decrease with decreasing radius within the core. However, whether such behavior holds depends on detailed dynamics. We first examine the dynamics using a fluid approach to the self-similar collapse solutions for the dark matter phase-space density, including the effect of velocity dispersions. We highlight the importance of tangential velocity dispersions to obtain density profiles shallower than 1/r2 in the core regions. If tangential velocity dispersions in the core are constrained to be less than the radial dispersion, a cuspy core density profile shallower than 1/r cannot hold in self-similar collapse. We then look at the profiles of the outer halos in low-density cosmological models in which the total halo mass is convergent. We find a limiting r-4 outer profile for the open case and a limiting outer profile for the Λ-dominated case, which approaches the form [1 - (r/λ)-3]1/2, where 3 is the logarithmic slope of the initial density profile. Finally, we analyze a suite of dark halo density and velocity dispersion profiles obtained in cosmological N-body simulations of models with n = 0, -1, and -2. The core-density profiles show considerable scatter in their properties, but nevertheless do appear to reflect a memory of the initial power spectrum, with steeper initial spectra producing flatter core profiles. These results apply as well for low-density cosmological models (Ωmatter = 0.2-0.3), since high-density cores were formed early, where Ωmatter ≈ 1.

Monopoles and Solitons in Fuzzy Physics

Abstract: Monopoles and solitons have important topological aspects like quantized fluxes, winding numbers and curved target spaces. Naive discretizations which substitute a lattice of points for the underlying manifolds are incapable of retaining these features in a precise way. We study these problems of discrete physics and matrix models and discuss mathematically coherent discretizations of monopoles and solitons using fuzzy physics and noncommutative geometry. A fuzzy σ-model action for the two-sphere fulfilling a fuzzy Belavin–Polyakov bound is also put forth.

The word and Riemannian metrics on lattices of semisimple groups

Abstract: Let G be a semisimple Lie group of rank ⩾2 and Γ an irreducible lattice. Γ has two natural metrics: a metric inherited from a Riemannian metric on the ambient Lie group and a word metric defined with respect to some finite set of generators. Confirming a conjecture of D. Kazhdan (cf. Gromov [Gr2]) we show that these metrics are Lipschitz equivalent. It is shown that a cyclic subgroup of Γ is virtually unipotent if and only if it has exponential growth with respect to the generators of Γ.

Vibration modes of giant gravitons

Abstract: We examine the spectrum of small vibrations of giant gravitons when the gravitons expand in anti\char21{}de Sitter space and when they expand on the sphere. For any given angular harmonic, the modes are found to have frequencies related to the curvature length scale of the background; these frequencies are independent of radius (and hence angular momentum) of the brane itself. This implies that the holographic dual theory must have, in a given R charge sector, low-lying non-BPS excitations with level spacings independent of the R charge.

Coronal structure and abundances of capella from simultaneous EUVE and ASCA spectroscopy

Abstract: We report analysis of the simultaneous 1996 March EUVE and ASCA observations of the spectroscopic binary Capella. The EUVE spectrum is dominated by lines of highly ionized Fe, requiring a continuous emission-measure distribution over a wide range of temperatures. The ASCA spectrum shows He-like line emission features of S, Si, and Mg, as well as unresolved L-shell emission lines of Fe and Ni and H-like and He-like Ne lines. The flux in these line features cannot be determined independently from the continuum flux. The ASCA spectrum is relatively soft, with few counts above 4 keV. The emission-measure distribution determined by Line-Based Analysis of the EUV Fe line intensities is well constrained from Te ~ 6 × 105 to 2 × 107 K, but it is not constrained above this range since Fe XXIV is the highest temperature line observed with EUVE. Since repeated observations of Capella by EUVE have shown that emission-line intensities of the hottest EUV-emitting material (Fe XXI to XXIV) vary by factors up to 4, the ASCA spectrum is important for extending the temperature coverage. Thus, the high-energy cut-off of the ASCA spectrum provides a constraint on the highest temperature emission measures. In principle, elemental abundances are determined from global fits to the ASCA spectrum; however, no well-fitting model has been found for the high signal-to-noise ASCA performance verification spectrum of Capella (1993 September 2). The newer ASCA spectrum of Capella (1996 March 3-4) shows a similar pattern of fitting difficulties. Using the EUVE measurements (1996 March 3-7) to constrain models, we have conducted sensitivity studies of the atomic data, source physics, and instrument calibration. The plasma spectral emission models (Raymond-Smith, MEKAL, SPEX) around 1.2 keV appear to have flux deficits relative to the observed ASCA count spectrum. New atomic models by Liedahl and Brickhouse, calculated with the HULLAC code, provide a set of lines—missing from the existing plasma codes—to fill in this flux deficit. Incorporating these additional lines dramatically improves the spectral model fits to the data, allowing reliable determination of elemental abundances. The successful application of the new atomic models to the Capella problem can have widespread implications, affecting spectral models of galaxies, cluster cooling flows, and supernova remnants, as well as other stellar coronae. Analysis with the new atomic models of the simultaneous ASCA and EUVE data confirms the previous EUVE results that the continuous emission-measure distribution of Capella has a strong enhancement at Te ~ 6 × 106 K. While a two-temperature model actually provides a better fit to the ASCA spectrum than the EUVE-derived continuous model, the EUVE data are not well fitted with only two temperatures. We find that the abundances of Mg, Si, S, and Fe are consistent with solar photospheric values, while Ne appears to be underabundant by a factor of ~3 to 4.

The Euler Class Group of a Noetherian Ring

Abstract: For a commutative Noetherian ring A of finite Krull dimension containing the field of rational numbers, an Abelian group called the Euler class group is defined. An element of this group is attached to a projective A-module of rank = dim A and it is shown that the vanishing of this element is necessary and sufficient for P to split off a free summand of rank 1. As one of the applications of this result, it is shown that for any n-dimensional real affine domain, a projective module of rank n (with trivial determinant), all of whose generic sections have n generated vanishing ideals, necessarily splits off a free direct summand of rank 1.

Quasiparticles in the superconducting state of Bi(2)Sr(2)CaCu(2)O(8+delta)

Abstract: Recent improvements in momentum resolution lead to qualitatively new angle-resolved photoemission spectroscopy results on the spectra of Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi2212) along the (pi, pi) direction, where there is a node in the superconducting gap. We now see the intrinsic line shape, which indicates the presence of true quasiparticles at all Fermi momenta in the superconducting state, and lack thereof in the normal state. The region of momentum space probed here is relevant for charge transport, motivating a comparison of our results to conductivity measurements by infrared reflectivity.

A tracked approach for automated NMR assignments in proteins (TATAPRO).

Abstract: A novel automated approach for the sequence specific NMR assignments of 1HN, 13Cα, 13Cβ, 13C′/1Hα and 15N spins in proteins, using triple resonance experimental data, is presented. The algorithm, TATAPRO (Tracked AuTomated Assignments in Proteins) utilizes the protein primary sequence and peak lists from a set of triple resonance spectra which correlate 1HN and 15N chemical shifts with those of 13Cα, 13Cβ and 13C′/1Hα. The information derived from such correlations is used to create a `master_list' consisting of all possible sets of 1HN i, 15Ni, 13Cα i, 13Cβ i, 13C′i/1Hα i, 13Cα i−1, 13Cβ i−1 and 13C′i−1/ 1Hα i−1 chemical shifts. On the basis of an extensive statistical analysis of 13Cα and 13Cβ chemical shift data of proteins derived from the BioMagResBank (BMRB), it is shown that the 20 amino acid residues can be grouped into eight distinct categories, each of which is assigned a unique two-digit code. Such a code is used to tag individual sets of chemical shifts in the master_list and also to translate the protein primary sequence into an array called pps_array. The program then uses the master_list to search for neighbouring partners of a given amino acid residue along the polypeptide chain and sequentially assigns a maximum possible stretch of residues on either side. While doing so, each assigned residue is tracked in an array called assig_array, with the two-digit code assigned earlier. The assig_array is then mapped onto the pps_array for sequence specific resonance assignment. The program has been tested using experimental data on a calcium binding protein from Entamoeba histolytica (Eh-CaBP, 15 kDa) having substantial internal sequence homology and using published data on four other proteins in the molecular weight range of 18–42 kDa. In all the cases, nearly complete sequence specific resonance assignments (> 95%) are obtained. Furthermore, the reliability of the program has been tested by deleting sets of chemical shifts randomly from the master_list created for the test proteins.

Temporal variations of the rotation rate in the solar interior

Abstract: The temporal variations of the rotation rate in the solar interior are studied using frequency splittings from Global Oscillations Network Group (GONG) data obtained during the period 1995-1999. We find alternating latitudinal bands of faster and slower rotation that appear to move toward the equator with time—similar to the torsional oscillations seen at the solar surface. This flow pattern appears to persist to a depth of about 0.1 R☉, and in this region its magnitude is well correlated with solar activity indices. We do not find any periodic or systematic changes in the rotation rate near the base of the convection zone.

Nielsen-Olesen vortices in noncommutative abelian Higgs model

Abstract: We construct Nielsen-Olesen vortex solution in the noncommutative abelian Higgs model. We derive the quantized topological flux of the vortex solution. We find that the flux is integral by explicit computation in the large-θ limit as well as in the small-θ limit. In the context of a tachyon vortex on the brane-antibrane system we demonstrate that it is this topological charge that gives rise to the RR charge of the resulting BPS D-brane. We also consider the left-right-symmetric gauge theory which does not have a commutative limit and construct an exact vortex solution in it.

Are bitvectors optimal

Abstract: We study the it static membership problem: Given a set S of at most n keys drawn from a universe U of size m, store it so that queries of the form "Is u in S?" can be answered by making few accesses to the memory. We study schemes for this problem that use space close to the information theoretic lower bound of $\Omega(n\log(\frac{m}{n}))$ bits and yet answer queries by reading a small number of bits of the memory. We show that, for $\epsilon > 0$, there is a scheme that stores $O(\frac{n}{\epsilon^2}\log m)$ bits and answers membership queries using a randomized algorithm that reads just one bit of memory and errs with probability at most $\epsilon$. We consider schemes that make no error for queries in S but are allowed to err with probability at most $\epsilon$ for queries not in S. We show that there exist such schemes that store $O((\frac{n}{\epsilon})^2 \log m)$ bits and answer queries using just one bitprobe. If multiple probes are allowed, then the number of bits stored can be reduced to $O(n^{1+\delta}\log m)$ for any $\delta > 0$. The schemes mentioned above are based on probabilistic constructions of set systems with small intersections. We show lower bounds that come close to our upper bounds (for a large range of n and $\epsilon$): Schemes that answer queries with just one bitprobe and error probability $\epsilon$ must use $\Omega(\frac{n}{\epsilon\log(1/\epsilon)} \log m)$ bits of storage; if the error is restricted to queries not in S, then the scheme must use $\Omega(\frac{n^2}{\epsilon^2 \log (n/\epsilon)}\log m)$ bits of storage. We also consider deterministic schemes for the static membership problem and show tradeoffs between space and the number of probes.

Gravitational Collapse: The Story So Far

Abstract: We discuss here some recent developments in the theory of gravitational collapse, examining the issue of the final fate of continual collapse of a matter cloud. It is pointed out that it is basically the nature of the regular initial data that decides whether the collapse ends in a black hole or a naked singularity. We outline here some problems which remain as yet unresolved regarding the naked singularities and cosmic censorship.

Implications of 21-cm observations for damped Ly α systems

Abstract: We present Giant Metrewave Radio Telescope H i 21-cm absorption observations, of candidate and confirmed damped Lyα systems. The derived spin temperatures Tspin are high, in all cases, ∼1000 K or higher. We have also collated from the published literature a list of damped absorbers for which 21-cm observations exist, and discuss the implications of the observations for the nature of these systems. A cross-comparison of the H i 21-cm profiles (which trace the cold gas) with the low ionization metal profiles (which trace all the neutral H i, both cold and warm) shows that in all cases for which both spectra are available, the 21-cm absorption coincides in velocity with the deepest metal line feature. This is consistent with models in which the deep metal line features arise from discrete clouds but not with models where the deepest features are the result of velocity crowding. We also find that the typical derived spin temperatures of damped Lyα systems are considerably higher than those in the Milky Way or nearby spiral galaxies. The only exceptions are systems that are known to be associated with the discs of spirals; these do, in fact, show low spin temperature. In a multi-phase medium the derived spin temperature is a weighted average of the temperatures of the individual phases. High apparent Tspin values are hence to be expected from small, low metallicity objects since these objects should (as per existing theoretical models of the formation of a multi-phase interstellar medium (ISM) in the Milky Way and high-redshift proto-galaxies) have a lower fraction of the cold phase in their ISM as compared with large galaxies. The high Tspin is hence consistent with the observed low metallicities of damped Lyα absorbers as well as with recent findings that damped absorption is associated with a variety of galaxy types (as opposed to being confined to the discs of large spirals). Finally, although the number of systems for which observations are available is small, we suggest that the following trend may be identified: at low redshift, damped Lyα absorption arises from a range of systems, including spiral galaxy discs, while, at high redshift, absorption occurs predominantly in smaller systems.

Detecting heavy charged Higgs bosons at the CERN LHC with four $b$ quark tags

Abstract: We investigate the signature of a heavy charged Higgs boson of the minimal super-symmetric standard model in the lepton plus multijet channel at the CERN Large Hadron Collider with four b tags. The signal is the gluon-gluon fusion process $g\stackrel{\ensuremath{\rightarrow}}{g}t\overline{b}{H}^{\ensuremath{-}},$ followed by the ${H}^{\ensuremath{-}}\ensuremath{\rightarrow}\overline{t}b$ decay, while the main background is from $g\stackrel{\ensuremath{\rightarrow}}{g}t\overline{t}b\overline{b}.$ We find that the two can be separated effectively by kinematic cuts and mass reconstruction, but the signal size is not very large in the end. Nonetheless, with a good b-tagging efficiency ${\ensuremath{\epsilon}}_{b}\ensuremath{\sim}50%,$ this channel can provide a viable signature over a limited but interesting range of the parameter space.

Magnetic Moments of Branes and Giant Gravitons

Abstract: We study the magnetic analogue of Myers' Dielectric Effect and, in some cases, relate it to the blowing up of particles into branes, first investigated by Greevy, Susskind and Toumbas. We show that D0-branes or gravitons in M-theory, moving in a magnetic four-form field strength background expand into a non-commutative two sphere. Both examples of constant magnetic field and non-constant fields in curved backgrounds generated by branes are considered. We find, in all cases, another solution, consisting of a two-brane wrapping a classical two-sphere, which has all the quantum numbers of the D0-branes. Motivated by this, we investigate the blowing up of gravitons into branes in backgrounds different from AdSm × Sn. We find the phenomenon is quite general. In many cases with less or even no supersymmetry we find a brane configuration which has the same quantum numbers and the same energy as a massless particle in supergravity.

Extraction of the width of the W boson from measurements of σ(pp̄→W+X)×B(W→eν) and σ(pp̄→Z+X)×B(Z→ee) and their ratio

Abstract: We report on measurements on inclusive cross sections times branching fractions into electrons for W and Z bosons produced in p (p) over bar collisions at root s = 1.8 TeV. From an integrated luminosity of 84.5 pb(-1) recorded in 1994-1995 using the D empty set detector at the Fermilab Tevatron, we determine sigma(p (p) over bar--> W + X) x B(W-->e nu) = 2310 +/- 10(stat) +/- 50(syst) +/- 100(lum)pb and sigma(p (p) over bar-->Z + X) x B(Z-->ee)=221+/-3(stat)+/-4(syst)+/-10(lum)pb. From these, we derive sigma(p (p) over bar-->W + X) x B(N-->e nu)/sigma(p (p) over bar-->Z + X) x B(Z-->ee) = 10.43+/-0.15(stat) +/- 0.20(syst) +/- 0.10(NLO), B(W-->e nu) = 0.1044+/-0.0015(stat)+/-0.0020(syst)+/-0.0017(theory)+/-0.0010(NLO). and Gamma(w) = 2169 +/- 0.03 l(stat) +/- 0.042(syst) +/- 0.041(theory)+/- 0.022(NLO)GeV. We use the latter to set a 95% confidence level upper limit on the partial decay width of the W boson into nonstandard model final states, Gamma(W)(inv), of 0.213 GeV. Combining these results with those from the 1992-1993 data gives sigma(p (p) over bar-->W+X) x B(W-->e nu)/sigma(p (p) over bar-->Z+X) x B(Z-->ee) = 10.51+/-0.25, Gamma(W) = 2.152 +/- 0.066 GeV, and a 95% C.L. upper limit an Gamma(W)(inv) of 0.191 GeV. Using a sample with a luminosity of 505 nb(-1) taken at root s = 630 GeV, we measure sigma(p (p) over bar--> W+ X) x B(W --> e nu) = 658 +/- 67 pb.