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At least 73 records · Page 4Linked to original sources

arXiv2007

Neutron Polarisabilities from Deuteron Compton Scattering in \chiEFT

Chiral Effective Field Theory is for photon energies up to 200 MeV the tool to accurately determine the polarisabilities of the neutron from deuteron Compton scattering. A multipole analysis reveals that dispersive effects from an explicit Delta(1232) prove in particular indispensable to understand the data at 95 MeV measured at SAL. Simple power-counting arguments derived from nuclear phenomenology lead to the correct Thomson limit and gauge invariance. At next-to-leading order, the static scalar dipole polarisabilities are extracted as identical for proton and neutron within the error-bar of available data: α^n=11.6\pm1.5_stat\pm0.6_Baldin, β^n=3.6\mp1.5_stat\pm0.6_Baldin for the neutron, in units of 10^-4 fm^3, compared to α^p=11.0\pm1.4_stat\pm0.4_Baldin, β}^p=2.8\mp1.4_stat\pm0.4_Baldin for the proton in the same framework. New experiments e.g. at MAXlab (Lund) will improve the statistical error-bar.

nucl-th↗

arXiv2007

Pion-nucleon P_33 and P_11 scatterings in the Lippmann-Schwinger approach

We study the pion-nucleon P_33 and P_11 scattering, where the Delta and the Roper resonances are seen, respectively. We use the Lippmann-Schwinger equation extended to couple to a one-body state, and investigate nature of these resonances by taking and omitting a one-baryon state into account. We see validity and puzzle of the standard quark model interpretation, for the Delta and the Roper resonances, respectively.

nucl-th↗

arXiv2007

$D$ and $\bar{D}$ mesons in hot and dense matter

The $D$ and $\bar {D}$ mesons are studied in hot dense matter within a self-consistent coupled-channel approach taking, as bare interaction, a broken SU(4) s-wave Tomozawa-Weinberg interaction supplemented by an attractive isoscalar-scalar term. The in-medium solution at finite temperature incorporates Pauli blocking effects, baryon mean-field bindings, and $π$ and open-charm meson self-energies. In the $DN$ sector, the $Λ_c$ and $Σ_c$ resonances remain close to their free-space position while acquiring a remarkable width. As a result, the $D$ meson spectral density shows a single pronounced peak close to the free mass that broadens with increasing density specially towards lower energies. The low-density theorem is not a good approximation for the repulsive $\bar D$ self-energy close to saturation density. We discuss the implications for the $J/Ψ$ suppression at CBM (FAIR).

nucl-th↗

arXiv2007

On a possible origin of a resonance-like structure in the two-photon invariant mass spectrum of the reaction pp->ppgg

We show that the resonance-like structure found by the \emph{CELSIUS-WASA Collaboration} in the two-photon invariant mass spectrum of the reaction $pp \to pp γγ$ is rather a signature of the $NN$-decoupled dibaryon resonance $d^\star_1$(1956) that is produced in the radiative process $pp \to γd^\star_1$ and then undergoes radiative decay into two protons $d^\star_1 \to pp γ$. It is found that a contribution of the dibaryon mechanism $pp \to γd^\star_1 \to pp γγ$ of the reaction $pp \to pp γγ$ to the invariant mass spectrum of its photon pairs can reasonably well reproduce the experimentally observed spectrum in the vicinity of the resonance-like structure.

hep-ph↗

arXiv2007

From Quark-Gluon Plasma to the Perfect Liquid

After reviewing some basic concepts of the theory of strongly interacting matter above nuclear energy density and reviewing some salient results of the experimental program at the Relativistic Heavy Ion Collider (RHIC), these lectures explain why the quark-gluon plasma observed in the RHIC experiments has been called a ``perfect liquid.'' They then give an introduction to some recent ideas concerning the possible origin of the nearly inviscid nature of the quark-gluon plasma and discuss the connection between low viscosity and strong parton energy loss of hot QCD matter.

nucl-th↗

arXiv2007

Two-pion-exchange effects in the $pp\to ppπ^0$ reaction

We study the pp \to ppπ^0 reaction near threshold based on heavy-baryon chiral perturbation theory. We show that the two-pion-exchange diagrams give much larger contribution than the one-pion-exchange diagram which is of lower chiral order in Weinberg's counting scheme. We also discuss the relation of our results to the momentum counting scheme.

nucl-th↗

arXiv2007

Electromagnetic reactions on light nuclei using chiral effective theory

I describe the use of chiral effective theory (ChiET) to compute electromagnetic reactions in two- and three-nucleon systems. I first explain how chiral perturbation theory can be extended to the few-nucleon sector. I then explain the predictions of the resulting ChiET for electron-deuteron scattering, and how they will be tested by forthcoming data from BLAST. I conclude by displaying predictions for elastic Compton scattering from deuterium and Helium-3 nuclei. These computations, in concert with future data from MAX-Lab and HIGS, should give significant new information on neutron polarizabilities, and hence yield insight into the structure of the nucleon.

nucl-th↗

arXiv2007

Multichannel chiral approach for kaonic hydrogen

We present an exact solution to the $K^{-}$-proton bound state problem formulated in the momentum space. The 1s level characteristics of the kaonic hydrogen are described together with the available low energy $\bar{K}N$ data.

nucl-th↗

arXiv2007

What we have learned so far from 3-fluid hydrodynamics

Available data on heavy-ion collisions at AGS and SPS energies are analyzed using a 3-fluid dynamical model within a purely hadronic scenario. We investigate the problems met in reproducing these data within this scheme. In particular, we try to indicate those data which could point towards the occurrence of a phase transition into the quark-gluon phase. We also discuss the success of the model in reproducing the transverse-mass spectra of various hadrons. We argue that the simultaneous reproduction of the inverse-slope parameters of all considered particles may imply that these particles belong to the same hydrodynamic flow at the instant of their freeze-out rather than that it signals the onset of a phase transition.

nucl-th↗

arXiv2007

Eta and Eta' physics

This talk describes the reasons why $η$ and $η^\prime$ decays are an interesting topic of study for both theory and experiment. The main part discusses the results of the recent calculation of $η\to3π$ at two-loop order in ChPT. Some puzzling aspects of the results compared to earlier dispersive calculations are highlighted. I also like to remind the reader of the use of $η$ and $η^\prime$ decays for studying the anomaly.

nucl-th↗

arXiv2007

$πN\to ηN$ process in a $χ$QM approach

A chiral quark model approach is used to investigate the $π^{-}p \to ηn$ process at low energies. The roles of the most relevant nucleon resonances in $n\leq 2$ shells are briefly discussed.

nucl-th↗

arXiv2007

Dynamics, Symmetries and Hadron Properties

We provide a snapshot of Dyson-Schwinger equation applications to the theory and phenomenology of hadrons. Exact results for pseudoscalar mesons are highlighted, with details relating to the U_A(1) problem. Calculated masses of the lightest J=0,1 states are discussed. We recapitulate upon studies of nucleon properties and give a perspective on the contribution of quark orbital angular momentum to the spin of a nucleon at rest.

nucl-th↗

arXiv2007

Hydrodynamics of transversally thermalized partons in ultra-relativistic heavy-ion collisions

The hydrodynamic description of transversally thermalized matter, possibly formed at the early stages of ultra-relativistic heavy-ion collisions, is developed. The formalism is based on the thermodynamically consistent approach with all thermodynamic variables referring to two-dimensional objects, the so-called transverse clusters, which are identified with the particles having the same rapidity. The resulting hydrodynamic equations for a single cluster have the form of the two-dimensional hydrodynamic equations of the perfect fluid. Since the clusters do not perform any work in the longitudinal direction, their energy is completely transformed and used to generate strong radial and elliptic flows that turn out to be compatible with the experimental data.

nucl-th↗

arXiv2007

Strange atoms, strange nuclei and kaon condensation

Analyses of comprehensive sets of strong-interaction data consisting of level shifts, widths and yields in strange atoms of K mesons and Sigma hyperons are reviewed. The introduction of density dependent optical potentials, in both cases, improves significantly the fit to the data. A strongly attractive antikaon-nuclear potential of order 150-200 MeV in nuclear matter is suggested by fits to the kaonic-atom data, with interesting possible repercussions on kaon condensation and on the evolution of strangeness in high-density stars. The case for relatively narrow, deeply bound antikaon atomic states is made, essentially independent of the nuclear potential depth. Dynamical models for calculating binding energies and widths of antikaon-nuclear states are discussed and a lower bound of approximately 50 MeV on the width is established. For Sigma atoms, the fitted potential becomes repulsive inside the nucleus, in agreement with recently reported (pi-,K+) spectra from KEK, implying that Sigma hyperons generally do not bind in nuclei. This repulsion significantly affects calculated compositions and masses of neutron stars.

nucl-th↗

arXiv2007

A "Domain Wall" Scenario for the AdS/QCD

We propose a scenario for bottom-up gravity dual picture of QCD-like theories, which consists of two near-AdS$_5$ domains separated by the ``domain wall'' at which the effective coupling relatively rapidly switches from small perturbative value at its UV side to strong at its IR side. Its consequence are corresponding jumps in 5-d masses of the bulk fields, related to different anomalous dimensions of the corresponding operators. Inclusion of both weakly and strongly coupled parts of bulk wave functions allows for consistent inclusion of QCD hard processes. We further discuss how transitions from strong to weak coupling in hard observables should look like, exemplified by pion diffractive dissociation, which seems to show this transition experimentally. Then we propose a dynamical mechanism for this jump related to instantons, which are point-like bulk objects located at/near the domain wall. We further argue that in the limit of large number of colors $N_c\to\infty$ the "domain wall" is becoming a true singularity. Instanton-induced contributions to correlators and partition function of the instanton ensemble is reformulated in the $AdS_5$ language. Among other applications are lowest mesons as collective vibrations of the ``domain wall''.

hep-ph↗

arXiv2008

A QCD chiral critical point at small chemical potential: is it there or not?

For a QCD chiral critical point to exist, the parameter region of small quark masses for which the finite temperature transition is first-order must expand when the chemical potential is turned on. This can be tested by a Taylor expansion of the critical surface (m_{u,d},m_s)_c(mu). We present a new method to perform this Taylor expansion numerically, which we first test on an effective model of QCD with static, dense quarks. We then present the results for QCD with 3 degenerate flavors. For a lattice with N_t=4 time-slices, the first-order region shrinks as the chemical potential is turned on. This implies that, for physical quark masses, the analytic crossover which occurs at mu=0 between the hadronic and the plasma regimes remains crossover in the mu-region where a Taylor expansion is reliable, i.e. mu less than or similar to T. We present preliminary results from finer lattices indicating that this situation persists, as does the discrepancy between the curvature of T_c(m_c(mu=0),mu) and the experimentally observed freeze-out curve.

hep-lat↗

arXiv2008

Plasma Electromagnetic Fluctuations as an Initial Value Problem

Fluctuations of electric and magnetic fields in the collisionless plasma are found as a solution of the initial value linearized problem. The plasma initial state is on average stationary and homogeneous. When the state is stable, the initial fluctuations decay exponentially and in the long time limit a stationary spectrum of fluctuations is established. For the equilibrium plasma it reproduces the spectrum obtained from the fluctuation-dissipation relation. Fluctuations in the unstable two-stream system are also discussed.

physics.plasm-ph↗

arXiv2007

Excited Baryons in Large Nc QCD: Matching the 1/Nc expansion to quark models using the permutation group S_N

We show how to match quark models to the 1/Nc expansion of QCD. As an example we discuss in detail the mass operator of orbitally excited baryons and match it to the one-gluon exchange and the one-boson exchange variants of the quark model. The matching procedure is very general and makes use of the transformation properties of states and operators under S_N^{orb} \times S_N^{sp-fl}, the permutation group acting on the orbital and spin-flavor degrees of freedom of N quarks.

hep-ph↗