17,395 research outputs found
String Effects on Fermi--Dirac Correlation Measurements
We investigate some recent measurements of Fermi--Dirac correlations by the
LEP collaborations indicating surprisingly small source radii for the
production of baryons in -annihilation at the peak. In the
hadronization models there are besides the Fermi--Dirac correlation effect also
a strong dynamical (anti-)correlation. We demonstrate that the extraction of
the pure FD effect is highly dependent on a realistic Monte Carlo event
generator, both for separation of those dynamical correlations which are not
related to Fermi--Dirac statistics, and for corrections of the data and
background subtractions. Although the model can be tuned to well reproduce
single particle distributions, there are large model-uncertainties when it
comes to correlations between identical baryons. We therefore, unfortunately,
have to conclude that it is at present not possible to make any firm conclusion
about the source radii relevant for baryon production at LEP
Color separate singlets in annihilation
We use the method of color effective Hamiltonian to study the properties of
states in which a gluonic subsystem forms a color singlet, and we will study
the possibility that such a subsystem hadronizes as a separate unit. A parton
system can normally be subdivided into singlet subsystems in many different
ways, and one problem arises from the fact that the corresponding states are
not orthogonal. We show that if only contributions of order are
included, the problem is greatly simplified. Only a very limited number of
states are possible, and we present an orthogonalization procedure for these
states. The result is simple and intuitive and could give an estimate of the
possibility to produce color separated gluonic subsystems, if no dynamical
effects are important. We also study with a simple MC the possibility that
configurations which correspond to "short strings" are dynamically favored. The
advantage of our approach over more elaborate models is its simplicity, which
makes it easier to estimate color reconnection effects in reactions which are
more complicated than the relatively simple annihilation.Comment: Revtex, 24 pages, 7 figures; Compared to the previous version, 1 new
figure is added and Monte-Carlo results are re-analyzed, as suggested by the
referee; To appear in Phys. Rev.
Study of color connections in annihilation
We replace in the event generator JETSET the color singlet chain connection
with the color separate state one as the interface between the hard and soft
sectors of hadronic processes. The modified generator is applied to produce the
hadronic events in annihilation. It describes the experimental data
at the same level as the original JETSET with default parameters. This should
be understood as a demonstration that color singlet chain is not the unique
color connection. We also search for the difference in special sets of
three-jet events arising from different color connections, which could subject
to further experimental test.Comment: 23 pages, 8 figures, 4 tables, Revtex
Dynamics of correlations due to a phase noisy laser
We analyze the dynamics of various kinds of correlations present between two
initially entangled independent qubits, each one subject to a local phase noisy
laser. We give explicit expressions of the relevant quantifiers of correlations
for the general case of single-qubit unital evolution, which includes the case
of a phase noisy laser. Although the light field is treated as classical, we
find that this model can describe revivals of quantum correlations. Two
different dynamical regimes of decay of correlations occur, a Markovian one
(exponential decay) and a non-Markovian one (oscillatory decay with revivals)
depending on the values of system parameters. In particular, in the
non-Markovian regime, quantum correlations quantified by quantum discord show
an oscillatory decay faster than that of classical correlations. Moreover,
there are time regions where nonzero discord is present while entanglement is
zero.Comment: 7 pages, 3 figures, accepted for publication in Phys. Scripta,
special issue for CEWQO 2011 proceeding
Forward-Backward Correlations and Event Shapes as probes of Minimum-Bias Event Properties
Measurements of inclusive observables, such as particle multiplicities and
momentum spectra, have already delivered important information on
soft-inclusive ("minimum-bias") physics at the Large Hadron Collider. In order
to gain a more complete understanding, however, it is necessary to include also
observables that probe the structure of the studied events. We argue that
forward-backward (FB) correlations and event-shape observables may be
particulary useful first steps in this respect. We study the sensitivity of
several different types of FB correlations and two event shape variables -
transverse thrust and transverse thrust minor - to various sources of
theoretical uncertainty: multiple parton interactions, parton showers, colour
(re)connections, and hadronization. The power of each observable to furnish
constraints on Monte Carlo models is illustrated by including comparisons
between several recent, and qualitatively different, PYTHIA 6 tunes, for pp
collisions at sqrt(s) = 900 GeV.Comment: 13 page
The time to extinction for an SIS-household-epidemic model
We analyse a stochastic SIS epidemic amongst a finite population partitioned
into households. Since the population is finite, the epidemic will eventually
go extinct, i.e., have no more infectives in the population. We study the
effects of population size and within household transmission upon the time to
extinction. This is done through two approximations. The first approximation is
suitable for all levels of within household transmission and is based upon an
Ornstein-Uhlenbeck process approximation for the diseases fluctuations about an
endemic level relying on a large population. The second approximation is
suitable for high levels of within household transmission and approximates the
number of infectious households by a simple homogeneously mixing SIS model with
the households replaced by individuals. The analysis, supported by a simulation
study, shows that the mean time to extinction is minimized by moderate levels
of within household transmission
Energy Scaling of Minimum-Bias Tunes
We propose that the flexibility offered by modern event-generator tuning
tools allows for more than just obtaining "best fits" to a collection of data.
In particular, we argue that the universality of the underlying physics model
can be tested by performing several, mutually independent, optimizations of the
generator parameters in different physical regions. For regions in which these
optimizations return similar and self-consistent parameter values, the model
can be considered universal. Deviations from this behavior can be associated
with a breakdown of the modeling, with the nature of the deviations giving
clues as to the nature of the breakdown. We apply this procedure to study the
energy scaling of a class of minimum-bias models based on multiple parton
interactions (MPI) and pT-ordered showers, implemented in the Pythia 6.4
generator. We find that a parameter controlling the strength of color
reconnections in the final state is the most important source of
non-universality in this model.Comment: 17 pages, 3 figures, 4 table
General K=-1 Friedman-Lema\^itre models and the averaging problem in cosmology
We introduce the notion of general K=-1 Friedman-Lema\^itre (compact)
cosmologies and the notion of averaged evolution by means of an averaging map.
We then analyze the Friedman-Lema\^itre equations and the role of gravitational
energy on the universe evolution. We distinguish two asymptotic behaviors:
radiative and mass gap. We discuss the averaging problem in cosmology for them
through precise definitions. We then describe in quantitative detail the
radiative case, stressing on precise estimations on the evolution of the
gravitational energy and its effect in the universe's deceleration. Also in the
radiative case we present a smoothing property which tells that the long time
H^{3} x H^{2} stability of the flat K=-1 FL models implies H^{i+1} x H^{i}
stability independently of how big the initial state was in H^{i+1} x H^{i},
i.e. there is long time smoothing of the space-time. Finally we discuss the
existence of initial "big-bang" states of large gravitational energy, showing
that there is no mathematical restriction to assume it to be low at the
beginning of time.Comment: Revised version. 32 pages, 1 figur
Anti-Hyperon Enhancement through Baryon Junction Loops
The baryon junction exchange mechanism recently proposed to explain valence
baryon number transport in nuclear collisions is extended to study midrapidity
anti-hyperon production. Baryon junction-anti-junction (J anti-J) loops are
shown to enhance anti-Lambda, anti-Xi, anti-Omega yields as well as lead to
long range rapidity correlations. Results are compared to recent WA97 Pb + Pb
-> Y + anti-Y + X data.Comment: 10 pages, 4 figure
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