1,650 research outputs found
On causality, unitarity and perturbative expansions
We present a pedagogical case study how to combine micro-causality and
unitarity based on a perturbative approach. The method we advocate constructs
an analytic extrapolation of partial-wave scattering amplitudes that is
constrained by the unitarity condition. Suitably constructed conformal mappings
help to arrive at a systematic approximation of the scattering amplitude. The
technique is illustrated at hand of a Yukawa interaction. The typical case of a
superposition of strong short-range and weak long-range forces is investigated.Comment: 12 pages, 12 figure
A unitary and causal effective field theory
We report on a novel scheme based on the chiral Lagrangian. It is used to
analyze pion-nucleon scattering, pion photoproduction, and nucleon Compton
scattering. Subthreshold partial-wave amplitudes are calculated in chiral
perturbation theory and analytically extrapolated with constraints imposed by
electromagnetic-gauge invariance, causality and unitarity. Experimental
quantities are reproduced up to energies MeV in terms of
the parameters relevant at order .Comment: 4 pages, contribution to the proceedings of the MENU 2010 conference,
May 31-June 4, 2010, Williamsburg VA, US
Unitary and causal dynamics based on the chiral Lagrangian
Pion-nucleon scattering, pion photoproduction, and nucleon Compton scattering
are analyzed within a scheme based on the chiral Lagrangian. Partial-wave
amplitudes are obtained by an analytic extrapolation of subthreshold reaction
amplitudes computed in chiral perturbation theory, where the constraints set by
electromagnetic-gauge invariance, causality and unitarity are used to stabilize
the extrapolation. Experimental data are reproduced up to energies
MeV in terms of the parameters relevant at order . A
striking puzzle caused by an old photon asymmetry measurement close to the pion
production threshold is discussed.Comment: Invited plenary talk at Chiral 10 Workshop, Valencia (Spain), June
21-24, 201
Comparing 3C 120 jet emission at small and large scales
Context. Important information on the evolution of the jet can be obtained by
comparing the physical state of the plasma at its propagation through the
broad-line region (where the jet is most likely formed) into the intergalactic
medium, where it starts to significantly decelerate. Aims. We compare the
constraints on the physical parameters in the innermost ( pc) and outer
( kpc) regions of the 3C 120 jet by means of a detailed multiwavelength
analysis and theoretical modeling of their broadband spectra. Methods.The data
collected by Fermi LAT, Swift and Chandra are analyzed together and the
spectral energy distributions are modeled using a leptonic synchrotron and
inverse Compton model, taking into account the seed photons originating inside
and outside of the jet. The model parameters are estimated using the MCMC
method. Results. The -ray flux from the inner jet of 3C 120 was
characterized by rapid variation from MJD 56900 to MJD 57300. Two strong flares
were observed on April 24, 2015 when, within 19.0 minutes and 3.15 hours the
flux was as high as and
respectively. The broadband
emission in the quiet and flaring states can be described as SSC emission while
IC scattering of dusty torus photons cannot be excluded for the flaring states.
The X-ray emission from the knots can be well reproduced by IC scattering of
CMB photons only if the jet is highly relativistic (since even when
still ). These extreme requirements can be somewhat
softened assuming the X-rays are from the synchrotron emission of a second
population of very-high-energy electrons. Conclusions. We found that the jet
power estimated at two scales is consistent, suggesting that the jet does not
suffer severe dissipation, it simply becomes radiatively inefficient.Comment: Accepted for publication in Astronomy & Astrophysics. The abstract
has been shortened to comply with the size limit set by arXi
Three-nucleon force at large distances: Insights from chiral effective field theory and the large-N_c expansion
We confirm the claim of Ref. [D.R. Phillips, C. Schat, Phys. Rev. C88 (2013)
3, 034002] that 20 operators are sufficient to represent the most general local
isospin-invariant three-nucleon force and derive explicit relations between the
two sets of operators suggested in Refs. [D.R. Phillips, C. Schat, Phys. Rev.
C88 (2013) 3, 034002] and [H. Krebs, A.M. Gasparyan, E. Epelbaum, Phys.Rev. C87
(2013) 5, 054007]. We use the set of 20 operators to discuss the chiral
expansion of the long- and intermediate-range parts of the three-nucleon force
up to next-to-next-to-next-to-next-to-leading order in the standard formulation
without explicit Delta(1232) degrees of freedom. We also address implications
of the large-N_c expansion in QCD for the size of the various three-nucleon
force contributions.Comment: 15 pages, 6 figure
Lambda-N scattering length from the reaction gamma d -> K^+ Lambda n
The perspects of utilizing the strangeness-production reaction gamma d -> K^+
Lambda n for the determination of the Lambda n low-energy scattering parameters
are investigated. The spin observables that need to be measured in order to
isolate the Lambda n singlet (1S0) and triplet (3S1) states are identified.
Possible kinematical regions where the extraction of the Lambda n scattering
lengths might be feasible are discussed.Comment: 8 pages, 4 figure
- …