7,625 research outputs found

### Transverse Mass Distribution Characteristics of $\pi^0$ Production in $^{208}$Pb-induced Reactions and the Combinational Approach

The nature of invariant cross-sections and multiplicities in some
$^{208}Pb$-induced reactions and some important ratio-behaviours of the
invariant multiplicities for various centralities of the collision will here be
dealt with in the light of a combinational approach which has been built up in
the recent past by the present authors. Next, the results would be compared
with the outcome of some of the simulation-based standard models for multiple
production in nuclear collisions at high energies. Finally, the implications of
all this would be discussed.Comment: 14 pages, 14 figures, a few changes have been made in the tex

### A note on the time evolution of generalized coherent states

I consider the time evolution of generalized coherent states based on
non-standard fiducial vectors, and show that only for a restricted class of
fiducial vectors does the associated classical motion determine the quantum
evolution of the states. I discuss some consequences of this for path integral
representations.Comment: 9 pages. RevTe

### Constrained Dynamics for Quantum Mechanics I. Restricting a Particle to a Surface

We analyze constrained quantum systems where the dynamics do not preserve the
constraints. This is done in particular for the restriction of a quantum
particle in Euclidean n-space to a curved submanifold, and we propose a method
of constraining and dynamics adjustment which produces the right Hamiltonian on
the submanifold when tested on known examples. This method we hope will become
the germ of a full Dirac algorithm for quantum constraints. We take a first
step in generalising it to the situation where the constraint is a general
selfadjoint operator with some additional structures.Comment: 49 pages, TEX, input files amssym.def, amssym.te

### On the Groenewold-Van Hove problem for R^{2n}

We discuss the Groenewold-Van Hove problem for R^{2n}, and completely solve
it when n = 1. We rigorously show that there exists an obstruction to
quantizing the Poisson algebra of polynomials on R^{2n}, thereby filling a gap
in Groenewold's original proof without introducing extra hypotheses. Moreover,
when n = 1 we determine the largest Lie subalgebras of polynomials which can be
unambiguously quantized, and explicitly construct all their possible
quantizations.Comment: 15 pages, Latex. Error in the proof of Prop. 3 corrected; minor
rewritin

### Description of isolated macroscopic systems inside quantum mechanics

For an isolated macrosystem classical state parameters $\zeta(t)$ are
introduced inside a quantum mechanical treatment. By a suitable mathematical
representation of the actual preparation procedure in the time interval
$[T,t_0]$ a statistical operator is constructed as a solution of the Liouville
von Neumann equation, exhibiting at time $t$ the state parameters $\zeta(t')$,
$t_0\leq t' \leq t$, and {\it preparation parameters} related to times $T \leq
t'\leq t_0$. Relation with Zubarev's non-equilibrium statistical operator is
discussed. A mechanism for memory loss is investigated and time evolution by a
semigroup is obtained for a restricted set of relevant observables, slowly
varying on a suitable time scale.Comment: 13 pages, latex, romp31 style, no figures, to appear in the
Proceedings of the XXXI Symposium on Mathematical Physics (Torun, Poland), to
be published in Rep. Math. Phy

### Relaxation properties of the quantum kinetics of carrier-LO-phonon interaction in quantum wells and quantum dots

The time evolution of optically excited carriers in semiconductor quantum
wells and quantum dots is analyzed for their interaction with LO-phonons. Both
the full two-time Green's function formalism and the one-time approximation
provided by the generalized Kadanoff-Baym ansatz are considered, in order to
compare their description of relaxation processes. It is shown that the
two-time quantum kinetics leads to thermalization in all the examined cases,
which is not the case for the one-time approach in the intermediate-coupling
regime, even though it provides convergence to a steady state. The
thermalization criterion used is the Kubo-Martin-Schwinger condition.Comment: 7 pages, 8 figures, accepted for publication in Phys. Rev.

### Self-induced decoherence approach: Strong limitations on its validity in a simple spin bath model and on its general physical relevance

The "self-induced decoherence" (SID) approach suggests that (1) the
expectation value of any observable becomes diagonal in the eigenstates of the
total Hamiltonian for systems endowed with a continuous energy spectrum, and
(2), that this process can be interpreted as decoherence. We evaluate the first
claim in the context of a simple spin bath model. We find that even for large
environments, corresponding to an approximately continuous energy spectrum,
diagonalization of the expectation value of random observables does in general
not occur. We explain this result and conjecture that SID is likely to fail
also in other systems composed of discrete subsystems. Regarding the second
claim, we emphasize that SID does not describe a physically meaningful
decoherence process for individual measurements, but only involves destructive
interference that occurs collectively within an ensemble of presupposed
"values" of measurements. This leads us to question the relevance of SID for
treating observed decoherence effects.Comment: 11 pages, 4 figures. Final published versio

### Decoherence time in self-induced decoherence

A general method for obtaining the decoherence time in self-induced
decoherence is presented. In particular, it is shown that such a time can be
computed from the poles of the resolvent or of the initial conditions in the
complex extension of the Hamiltonian's spectrum. Several decoherence times are
estimated: $10^{-13}-$ $10^{-15}s$ for microscopic systems, and
$10^{-37}-10^{-39}s$ for macroscopic bodies. For the particular case of a
thermal bath, our results agree with those obtained by the einselection
(environment-induced decoherence) approach.Comment: 11 page

### Hadronization corrections to helicity components of the fragmentation function

In the hadronic decays of Z, gluon emission leads to the appearance of the
longitudinal component of the fragmentation function, F_L. Measurement of F_L
and the transverse component, F_T, could thus provide an insight into the gluon
fragmentation function. However, hadronization corrections at low x can be
significant. Here we present a method of accounting for such corrections, using
the JETSET event generator as illustration.Comment: 11 pages, 5 figure

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