• Volume 40, Issue 6

      June 1993,   pages  1-501

    • Flavour democracy calls for the fourth generation

      Amitava Datta

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      It is argued with the help of an illustrative model, that the inter species hierarchy among the fermion masses and the quark mixing angles can be accommodated naturally in the standard model with (approximate) flavor democracy provided there are three exactly massless neutrinos and four families of sequential quark-leptons with all members of the fourth family having roughly equal masses. The special problem of light neutrino masses (if any) and possible solutions are also discussed.

    • Parton distributions, small-x physics and the spin structure of the nucleon

      E Reya

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      Several theoretical aspects in leading (1-loop) and higher (2-loop) order as well as various approaches of extracting leading twist-2 parton distributions from structure function measurements are discussed and summarized. Their implications for the small-x region (x⩽10−2) are analyzed and compared with alternative approaches where higher twist contributions (‘fans’) are added to the twist-2 LO terms in the evolution equations. The second part of these lectures deals with longitudinally polarized parton distributions related to the structure functiong1, in particular with various scenarios to explain the total spin structure of nucleons, including the gluon anomaly as well. Specific (realistic) tests for discriminating between these alternatives are discussed as well asx-dependent expectations, in particular for neutron targets in connection with the Bjorken sum rule. Furthermore, various theoretical expectations and sum rules for the transverse (chiral-even) structure functiong2 are presented and very recent developments of transverse chiral-odd (‘transversity’) distributions are briefly discussed.

    • WKB approximation in complex time

      S Biswas J Guha

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      The WKB approximation to the one particle Schrödinger equation in time is used to obtain the wavefunction at a given point as a sum of semiclassical terms, each corresponding to a different classical trajectory (real or complex) but ending up at the same point. A method to find out reflection coefficient for processes involving one and two turning points is developed and it is shown that the semiclassical complex analysis reproduces exactly the reflection coefficient that is obtained through the exact solution of the problem. The connection between pair production and reflection amplitude is also shown. The pair production amplitude in a time dependent gravitational background is calculated and it is shown that the vacuum considered in complex trajectory WKB analysis refers to adiabatic vacuum.

    • Stability of quark gluon plasma to nielsen-olesen mode

      Vishnu M Bannur Predhiman K Kaw

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      Nielsen and Olesen showed that perturbative vacuum with uniform chromomagnetic field in one space and one color direction is unstable. This instability is called Nielsen-Olesen instability (NOI), and leads to formation of a ‘spaghetti of flux tubes’ as a model for non-perturbative vacuum and confinement. We re-examine this instability in presence of color sources, quarks and gluons, at a finite temperature and find that at sufficiently high temperature NOI is stabilized due to an ‘effective mass’ of gluons arising through plasma effects. This explains how a QGP with no confinement effects may exist at high temperature. As the temperature is lowered, NOI reappears at a valueT=Tc, which is very close to confinement-deconfinement transition from hadrons to QGP..

    • Effective collision strengths among the fifteen lowest states of FeXVII

      Man Mohan

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      Collision strengths have been calculated for electron impact excitation of neon-like Fe XVII for all transitions within its 15 lowest states. Configuration interaction wavefunctions have been used to represent the target states. The standardR-matrix code has been used to calculate the contribution from the lower scattering partial waves (L⩽9), while the no exchange version of the same code has been used to compute efficiently the contribution of higher partial waves (L⩾10). Effective collision strengths for all the 105 transitions are tabulated for elected temperatures in the range logTe=5.40 to logTe=7.00 withTe expressed in °K.

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