1,548 research outputs found
Rate of photon production from hot hadronic matter
Thermal photon emission rates from hot hadronic matter are studied to order
, where indicates a strong-interaction coupling constant.
Radiative decay of mesons, Compton and annihilation processes for hadrons, and
bremsstrahlung reactions are all considered. Compared to the standard rates
from the literature, one finds two orders of magnitude increase for low photon
energies stemming mainly from bremsstrahlung and then a modest increase (factor
of 2) for intermediate and high energy photons owing to radiative decays for a
variety of mesons and from other reactions involving strangeness. These results
could have important consequences for electromagnetic radiation studies at
RHIC.Comment: 5 pages LaTeX, 4 Postscript figure
Single Leptoquark Production at and Colliders
We consider single production of leptoquarks (LQ's) at and
colliders, for two values of the centre-of-mass energy,
GeV and 1 TeV. We find that LQ's which couple within the first
generation are observable for LQ masses almost up to the kinematic limit, both
at and colliders, for the LQ coupling strength equal to
. The cross sections for single production of - and
-generation LQ's at colliders are too small to be observable.
In collisions, on the other hand, -generation LQ's with
masses much larger than can be detected. However,
-generation LQ's can be seen at colliders only for
masses at most , making their observation more probable via the
pair production mechanism.Comment: plain TeX, 14 pages, 6 figures (not included but available on
request), some minor changes to the text, one reference added, figures and
conclusions unchanged, UdeM-LPN-TH-93-152, McGill-93/2
Test of Quantum Action for Inverse Square Potential
We present a numerical study of the quantum action previously introduced as a
parametrisation of Q.M. transition amplitudes. We address the questions: Is the
quantum action possibly an exact parametrisation in the whole range of
transition times ()? Is the presence of potential terms beyond
those occuring in the classical potential required? What is the error of the
parametrisation estimated from the numerical fit? How about convergence and
stability of the fitting method (dependence on grid points, resolution, initial
conditions, internal precision etc.)? Further we compare two methods of
numerical determination of the quantum action: (i) global fit of the Q.M.
transition amplitudes and (ii) flow equation. As model we consider the inverse
square potential, for which the Q.M. transition amplitudes are analytically
known. We find that the relative error of the parametrisation starts from zero
at T=0 increases to about at and then decreases to zero
when . Second, we observe stability of the quantum action under
variation of the control parameters. Finally, the flow equation method works
well in the regime of large giving stable results under variation of
initial data and consistent with the global fit method.Comment: Text (LaTeX), Figures(ps
Word Embeddings for Entity-annotated Texts
Learned vector representations of words are useful tools for many information
retrieval and natural language processing tasks due to their ability to capture
lexical semantics. However, while many such tasks involve or even rely on named
entities as central components, popular word embedding models have so far
failed to include entities as first-class citizens. While it seems intuitive
that annotating named entities in the training corpus should result in more
intelligent word features for downstream tasks, performance issues arise when
popular embedding approaches are naively applied to entity annotated corpora.
Not only are the resulting entity embeddings less useful than expected, but one
also finds that the performance of the non-entity word embeddings degrades in
comparison to those trained on the raw, unannotated corpus. In this paper, we
investigate approaches to jointly train word and entity embeddings on a large
corpus with automatically annotated and linked entities. We discuss two
distinct approaches to the generation of such embeddings, namely the training
of state-of-the-art embeddings on raw-text and annotated versions of the
corpus, as well as node embeddings of a co-occurrence graph representation of
the annotated corpus. We compare the performance of annotated embeddings and
classical word embeddings on a variety of word similarity, analogy, and
clustering evaluation tasks, and investigate their performance in
entity-specific tasks. Our findings show that it takes more than training
popular word embedding models on an annotated corpus to create entity
embeddings with acceptable performance on common test cases. Based on these
results, we discuss how and when node embeddings of the co-occurrence graph
representation of the text can restore the performance.Comment: This paper is accepted in 41st European Conference on Information
Retrieva
The Exact Evolution Equation of the Curvature Perturbation for Closed Universe
As is well known, the exact evolution equation of the curvature perturbation
plays a very important role in investigation of the inflation power spectrum of
the flat universe. However, its corresponding exact extension for the non-flat
universes has not yet been given out clearly. The interest in the non-flat,
specially closed, universes is being aroused recently. The need of this
extension is pressing. We start with most elementary physical consideration and
obtain finally this exact evolution equation of the curvature perturbation for
the non-flat universes, as well as the evolutionary controlling parameter and
the exact expression of the variable mass in this equation. We approximately do
a primitive and immature analysis on the power spectrum of non-flat universes.
This analysis shows that this exact evolution equation of the curvature
perturbation for the non-flat universes is very complicated, and we need to do
a lot of numerical and analytic work for this new equation in future in order
to judge whether the universe is flat or closed by comparison between theories
and observations.Comment: 10 pages, no figures, Late
Thermal photon production in high-energy nuclear collisions
We use a boost-invariant one-dimensional (cylindrically symmetric) fluid
dynamics code to calculate thermal photon production in the central rapidity
region of S+Au and Pb+Pb collisions at SPS energy ( GeV/nucleon).
We assume that the hot matter is in thermal equilibrium throughout the
expansion, but consider deviations from chemical equilibrium in the high
temperature (deconfined) phase. We use equations of state with a first-order
phase transition between a massless pion gas and quark gluon plasma, with
transition temperatures in the range MeV.Comment: revised, now includes a_1 contribution. revtex, 10 pages plus 4
figures (uuencoded postscript
Soft Electromagnetic Radiations from Relativistic Heavy Ion Collisions
The production of low mass dileptons and soft photons from thermalized Quark
Gluon Plasma (QGP) and hadronic matter in relativistic heavy ion collisions is
evaluated. A boost invariant longitudinal and cylindrically symmetric
transverse expansion of the systems created in central collision of lead nuclei
at CERN SPS, BNL RHIC, and CERN LHC, and undergoing a first order phase
transition to hadronic matter is considered. A large production of low mass (M<
0.3 GeV) dileptons, and soft photons (p_T< 0.4 GeV) is seen to emanate from the
bremsstrahlung of quarks and pions. We find an increase by a factor of 2--4 in
the low mass dilepton and soft photon yield as we move from SPS to RHIC
energies, and an increase by an order of magnitude as we move from SPS to LHC
energies. Most of the soft radiations are found to originate from pion driven
processes at SPS and RHIC energies, while at the LHC energies the quark and the
pion driven processes contribute by a similar amount. The study of the
transverse mass distribution is seen to provide interesting details of the
evolution. We also find a unique universal behaviour for the ratio of M^2
weighted transverse mass distribution for M= 0.1 GeV to that for M= 0.2 and 0.3
GeV, as a function of M_T, for SPS, RHIC, and LHC energies, in the absence of
transverse expansion of the system. A deviation from this universal behaviour
is seen as a clear indication of the flow.Comment: Revtex fil
- âŠ