52 research outputs found
Heavy flavours in AA collisions: production, transport and final spectra
A multi-step setup for heavy-flavour studies in high-energy nucleus-nucleus
(AA) collisions --- addressing within a comprehensive framework the initial
Q-Qbar production, the propagation in the hot medium until decoupling and the
final hadronization and decays --- is presented. The initial hard production of
Q-Qbar pairs is simulated using the POWHEG pQCD event generator, interfaced
with the PYTHIA parton shower. Outcomes of the calculations are compared to
experimental data in pp collisions and are used as a validated benchmark for
the study of medium effects. In the AA case, the propagation of the heavy
quarks in the medium is described in a framework provided by the relativistic
Langevin equation. For the latter, different choices of transport coefficients
are explored (either provided by a perturbative calculation or extracted from
lattice-QCD simulations) and the corresponding numerical results are compared
to experimental data from RHIC and the LHC. In particular, outcomes for the
nuclear modification factor R_AA and for the elliptic flow v_2 of D/B mesons,
heavy-flavour electrons and non-prompt J/\psi's are displayed
Extending a previous analysis on a possible modulation effect in WIMP direct search
In this note we present an extension of our previous analysis of some
preliminary experimental results of the DAMA/NaI Collaboration which might be
indicative of a yearly modulation effect. Here we present a direct way for
obtaining from the experimental data the relevant cosmological implications for
relic neutralinos. We find that some of the configurations singled out by the
DAMA/NaI results would have cosmological properties compatible with a
neutralino as a dominant component of cold dark matter (on the average in the
Universe and in our galactic halo)
Supersymmetric candidates for dark matter
Direct and indirect detection rates of relic neutralinos are reviewed in the
framework of the Minimal Supersymmetric Standard Model. The theoretical
estimates are compared with the most recent experimental limits from
low-background detectors and neutrino telescopes. The properties of neutralino
under the hypothesis that preliminary experimental results of the DAMA/NaI
Collaboration may be indicative of a yearly modulation effect are examined
Some Applications of the ADM Formalism
The ADM Formalism is discussed in the context of 2+1--dimensional gravity,
uniting two areas of relativity theory in which Stanley Deser has been
particularly active. For spacetimes with topology R x T^2 the partially reduced
and fully reduced ADM formalism are related and quantized, and the role of
"large diffeomorphisms" (the modular group) in the quantum theory is
illustrated
Doorway States in the Random-Phase Approximation
By coupling a doorway state to a see of random background states, we develop
the theory of doorway states in the framework of the random-phase approximation
(RPA). Because of the symmetry of the RPA equations, that theory is radically
different from the standard description of doorway states in the shell model.
We derive the Pastur equation in the limit of large matrix dimension and show
that the results agree with those of matrix diagonalization in large spaces.
The complexity of the Pastur equation does not allow for an analytical approach
that would approximately describe the doorway state. Our numerical results
display unexpected features: The coupling of the doorway state with states of
opposite energy leads to strong mutual attraction
Analyzing high-energy factorization beyond next-to-leading logarithmic accuracy
We provide a complete and detailed study of the high-energy limit of four-parton scattering amplitudes in QCD, giving explicit results at two loops and higher orders, and going beyond next-to-leading logarithmic (NLL) accuracy. Building upon recent results, we use the techniques of infrared factorization to investigate the failure of the simplest form of Regge factorization, starting at next-to-next-to-leading logarithmic accuracy (NNLL) in ln( s/ | t |). We provide detailed accounts and explicit expressions for the terms responsible for this breaking in the case of two-loop and three-loop quark and gluon amplitudes in QCD; in particular, we recover and explain a known non-logarithmic double-pole contribution at two-loops, and we compute all non-factorizing single-logarithmic singular contributions at three loops. Conversely, we use high-energy factorization to show that the hard functions of infrared factorization vanish in d = 4 to all orders in the coupling, up to NLL accuracy in ln( s/ | t |). This provides clear evidence for the infrared origin of high-energy logarithms. Finally, we extend earlier studies to t -channel exchanges of color representations beyond the octet, which enables us to give predictions based on the dipole formula for single-pole NLL contributions at three and four loops
A simple solution for marginal deformations in open string field theory
We derive a new open string field theory solution for boundary marginal deformations generated by chiral currents with singular self-OPE. The solution is algebraically identical to the Kiermaier-Okawa-Soler solution and it is gauge equivalent to the TakahashiTanimoto identity-based solution. It is wedge-based and we can analytically evaluate the Ellwood invariant and the action, reproducing the expected results from BCFT. By studying the isomorphism between the states of the initial and final background a dual derivation of the Ellwood invariant is also obtained
A different kind of string
In U(1) lattice gauge theory in three spacetime dimensions, the problem of confinement can be studied analytically in a semi-classical approach, in terms of a gas of monopoles with Coulomb-like interactions. In addition, this theory can be mapped to a spin model via an exact duality transformation, which allows one to perform high-precision numerical studies of the confining potential. Taking advantage of these properties, we carried out an accurate investigation of the effective string describing the low-energy properties of flux tubes in this confining gauge theory. We found striking deviations from the expected Nambu-Goto-like behavior, and, for the first time, evidence for contributions that can be described by a term proportional to the extrinsic curvature of the effective string worldsheet. Such term is allowed by Lorentz invariance, and its presence in the infrared regime of the U(1) model was indeed predicted by Polyakov several years ago. Our results show that this term scales as expected according to Polyakov’s solution, and becomes the dominant contribution to the effective string action in the continuum limit. We also demonstrate analytically that the corrections to the confining potential induced by the extrinsic curvature term can be related to the partition function of the massive perturbation of a c = 1 bosonic conformal field theory. The implications of our results for SU( N ) Yang-Mills theories in three and in four spacetime dimensions are discussed
Anomalies in Diffractive Electroproduction of 2S Radially Excited Light Vector Mesons at HERA
We present the color dipole phenomenology of diffractive photo- and
electroproduction of vector mesons () and their radial excitations (). The main emphasis is related to
light vector mesons. We discuss how the energy dependence of the color dipole
cross section in conjuction with the node of the radial wave function of the
states can lead to an anomalous and energy dependence of diffractive
production of vector mesons. The color dipole model predictions for
light vector meson production are compared with the experimental data
from the EMC, NMC, ZEUS and H1 collaborations
On extremal limits and duality orbits of stationary black holes
With reference to the effective three-dimensional description of stationary, single center solutions to (ungauged) symmetric supergravities, we complete a previous analysis on the definition of a general geometrical mechanism for connecting global symmetry orbits (duality orbits) of non-extremal solutions to those of extremal black holes. We focus our attention on a generic representative of these orbits, providing its explicit description in terms of D = 4 fields. As a byproduct, using a new characterization of the angular momentum in terms of quantities intrinsic to the geometry of the D = 3 effective model, we are able to prove on general grounds its invariance, as a function of the boundary data, under the D = 4 global symmetry. In the extremal under-rotating limit it becomes moduli-independent. We also discuss the issue of the fifth parameter characterizing the four-dimensional seed solution, showing that it can be generated by a transformation in the global symmetry group which is manifest in the D = 3 effective description
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