Rate of photon production from hot hadronic matter

Thermal photon emission rates from hot hadronic matter are studied to order $e^{2}g^{4}$, where $g$ 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 e+e−e^+e^- and γγ\gamma\gamma Colliders

We consider single production of leptoquarks (LQ's) at $e^+e^-$ and $\gamma\gamma$ colliders, for two values of the centre-of-mass energy, $\sqrt{s}=500$ 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 $e^+e^-$ and $\gamma\gamma$ colliders, for the LQ coupling strength equal to $\alpha_{em}$. The cross sections for single production of $2^{nd}$- and $3^{rd}$-generation LQ's at $e^+e^-$ colliders are too small to be observable. In $\gamma\gamma$ collisions, on the other hand, $2^{nd}$-generation LQ's with masses much larger than $\sqrt{s}/2$ can be detected. However, $3^{rd}$-generation LQ's can be seen at $\gamma\gamma$ colliders only for masses at most $\sim\sqrt{s}/2$, 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 ($0 < T < \infty$)? 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 $10^{-3}$ at $T=1/E_{gr}$ and then decreases to zero when $T \to \infty$. 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 $T$ 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 ($\sqrt{s}=20$ 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 $150 \leq T_c \leq 200$ 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