arXiv subjects

J. Wagner

Publications and source records attributed to J. Wagner.

At least 73 records · Page 4Linked to original sources

arXiv2014

On Deeply Virtual Compton Scattering at next-to-leading order

Deeply Virtual Compton Scattering in the near forward kinematic region is the golden access to Generalized Parton Distributions. We studied the O(alphaS) corrections to the scattering amplitude for both spacelike and timelike kinematics relevant respectively to the leptoproduction of a real photon and to the photoproduction of a lepton pair. It turns out that these corrections are phenomenologically important and that the gluonic contributions are by no means negligible, even in the moderate energy range of JLab12 and of the COMPASS-II experiment at CERN.

hep-ph↗

arXiv2014

NLO QCD corrections for DVCS and TCS

The inclusion of QCD corrections to the Born amplitude of deeply virtual Compton scattering in both spacelike (DVCS) and timelike (TCS) regimes modifies the extraction process of generalized parton distributions (GPDs) from observables. In particular, gluon contributions are by no means negligible even in the medium energy range accessible at JLab12. We emphasize the complementarity of spacelike and timelike measurements and raise the question of factorization scale dependence of the O(alpha_S) results.

hep-ph↗

arXiv2013

22 GHz Water Maser Search in 37 Nearby Galaxies - Four New Water Megamasers in Seyfert 2 and OH Maser/Absorber Galaxies

We report four new 22 GHz water masers found in a Green Bank Telescope search toward 37 nearby objects. Our goal was to find new maser galaxies, AGN disk-masers, and objects where both hydroxyl and water maser species coexist. We observed 37 sources within 250 Mpc that were selected by high X-ray luminosity (L_X > 10^40 W) and high absorbing column density (N_H ~ 10^22 cm^-2). Sources also included dual or triple AGN and interacting systems. We further included objects detected in hydroxyl (OH). The selection consisted of 16 new sources, 13 previous non-detections to follow up with a factor ten higher sensitivity, 10 OH masers and one deep OH absorber, of which 37 were observed. Water megamasers were detected towards the Sy 2 galaxy 2MFGC 13581 (a disk-maser), towards the 6 GHz OH absorber NGC 4261 (3C 270 with a twin-jet and dusty torus; broad water maser emission), and towards the two 1.6 GHz OH maser sources IRAS 17526+3253 and IRAS 20550+1656 (possible star formation water masers). We set upper limits on 33 non-detections. The search increased the number of known "dual-species" objects containing both OH and H2O masers to eight. The detection rate was 25 % in OH galaxies and 11 % overall. Combined with other searches, a total of 95 objects have now been searched for both OH and H2O masers. We found the overall dual-species detection rate (8 in 95) to be of the order of the joint probability of both species independently occurring in the same object (1 % lower bound). However, this needs to be verified by a more detailed analysis of selection criteria. Lastly, we see a lack of H2O kilomasers in OH megamaser objects already noted by Tarchi et al. (2011). This may be due to sensitivity bias rather than astrophysical reasons.

astro-ph.GA↗

arXiv2013

Next to leading order analysis of DVCS and TCS

The study of $O(α_s)$ QCD contributions to the timelike and spacelike virtual Compton scattering amplitudes in the generalized Bjorken scaling regime demonstrates that gluonic contributions are by no means negligible even in the medium energy range which will be studied intensely at JLab12 and in the COMPASS-II experiment at CERN.

hep-ph↗

arXiv2013

Timelike vs spacelike DVCS from JLab, Compass to ultraperipheral collisions and AFTER@LHC

Timelike and spacelike virtual Compton scattering in the generalized Bjorken scaling regime are complementary tools to access generalized parton distributions. We stress that the gluonic contributions are by no means negligible, even in the medium energy range which will be studied intensely at JLab12 and in the COMPASS-II experiment at CERN. Ultraperipheral collisions with proton or ion beams may also be used at RHIC and at collider or fixed target experiments at LHC.

hep-ph↗

arXiv2013

On the importance of gluon contributions to timelike and spacelike DVCS

We emphasize how large O(α_s) corrections to timelike and spacelike virtual Compton scattering amplitudes are in the generalized Bjorken scaling regime, and in particular the gluonic contributions, even in the medium energy range which will be studied intensely at JLab12 and in the COMPASS-II experiment at CERN. We stress that the timelike and spacelike cases are complementary and that their difference deserves much special attention.

hep-ph↗

arXiv2013

Zooming towards the Event Horizon - mm-VLBI today and tomorrow

Global VLBI imaging at millimeter and sub-millimeter wavelength overcomes the opacity barrier of synchrotron self-absorption in AGN and opens the direct view into sub-pc scale regions not accessible before. Since AGN variability is more pronounced at short millimeter wavelength, mm-VLBI can reveal structural changes in very early stages after outbursts. When combined with observations at longer wavelength, global 3mm and 1mm VLBI adds very detailed information. This helps to determine fundamental physical properties at the jet base, and in the vicinity of super-massive black holes at the center of AGN. Here we present new results from multi-frequency mm-VLBI imaging of OJ287 during a major outburst. We also report on a successful 1.3mm VLBI experiment with the APEX telescope in Chile. This observation sets a new record in angular resolution. It also opens the path towards future mm-VLBI with ALMA, which aims at the mapping of the black hole event horizon in nearby galaxies, and the study of the roots of jets in AGN.

astro-ph.HE↗

arXiv2013

Demonstration of Angle Dependent Casimir Force Between Corrugations

The normal Casimir force between a sinusoidally corrugated gold coated plate and a sphere was measured at various angles between the corrugations using an atomic force microscope. A strong dependence on the orientation angle of the corrugation is found. The measured forces were found to deviate from the proximity force approximation and are in agreement with the theory based on the gradient expansion including correlation effects of geometry and material properties. We analyze the role of temperature. The obtained results open new opportunities for control of the Casimir effect in micromechanical systems.

quant-ph↗

arXiv2013

On timelike and spacelike deeply virtual Compton scattering at next to leading order

We study timelike and spacelike virtual Compton scattering in the generalized Bjorken scaling regime at next to leading order in the strong coupling constant, in the medium energy range which will be studied intensely at JLab12 and in the COMPASS-II experiment at CERN. We show that the Born amplitudes get sizeable O(α_s) corrections and, even at moderate energies, the gluonic contributions are by no means negligible. We stress that the timelike and spacelike cases are complementary and that their difference deserves much special attention.

hep-ph↗

arXiv2012

Three Body Fluctuation-Induced Interaction at Fluid Interfaces: A Strong Deviation from the Pairwise Summation

We present a new method based on the scattering technique to investigate fluctuation-induced forces at a fluid interface. The scattering approach, well suited to the study of many body systems of arbitrary geometries, is augmented to include boundary fluctuations. Using this method, we study the deviation of the total fluctuation-induced interaction from the sum of pairwise energies for three colloidal particles. We consider both frozen and fluctuating colloids and obtain a very good agreement between analytical and numerical results. We find a marked difference in the three body fluctuation-induced free energy between the frozen and fluctuating colloids, both in sign and relative size.

cond-mat.soft↗

arXiv2012

On timelike and spacelike hard exclusive reactions

We show to next-to-leading order accuracy in the strong coupling alpha_s how the collinear factorization properties of QCD in the generalized Bjorken regime relate exclusive amplitudes for spacelike and timelike hadronic processes. This yields simple space--to--timelike relations linking the amplitudes for electroproduction of a photon or meson to those for photo- or meso-production of a lepton pair. These relations constitute a new test of the relevance of leading twist analyzes of experimental data.

hep-ph↗

arXiv2012

Timelike Compton Scattering from JLAB to RHIC and LHC energies

Timelike Compton scattering (TCS) i.e. the exclusive photoproduction of a lepton pair with large invariant mass nicely complements the already successful experimental study of deeply virtual Compton scattering (DVCS). The same Generalized Parton Distributions enter both amplitudes, which offer a promissing way to access the quark and gluon nucleon structure. We review recent progress in this domain, emphasizing the fact that analyticity and factorization properties dictate the relation of the NLO corrections to TCS to those of DVCS. We also stress that data on TCS at high energy should be available soon thanks to the proposed experimental program at JLab at 12 GeV, and that, before the future high energy electron ion colliders become reality, the study of ultraperipheral collisions at the RHIC and LHC may open a window on quark and gluon GPDs at very small skewness. .

hep-ph↗

arXiv2012

PT-Symmetric Quantum Electrodynamics and Unitarity

More than 15 years ago, a new approach to quantum mechanics was suggested, in which Hermiticity of the Hamiltonian was to be replaced by invariance under a discrete symmetry, the product of parity and time-reversal symmetry, $\mathcal{PT}$. It was shown that if $\mathcal{PT}$ is unbroken, energies were, in fact, positive, and unitarity was satisifed. Since quantum mechanics is quantum field theory in 1 dimension, time, it was natural to extend this idea to higher-dimensional field theory, and in fact an apparently viable version of $\mathcal{PT}$-invariant quantum electrodynamics was proposed. However, it has proved difficult to establish that the unitarity of the scattering matrix, for example, the Källén spectral representation for the photon propagator, can be maintained in this theory. This has led to questions of whether, in fact, even quantum mechanical systems are consistent with probability conservation when Green's functions are examined, since the latter have to possess physical requirements of analyticity. The status of $\mathcal{PT}$QED will be reviewed in this report, as well as the general issue of unitarity.

hep-th↗

arXiv2012

Investigating the Spectral Geometry of a Soft Wall

The idealized theory of quantum vacuum energy density is a beautiful application of the spectral theory of differential operators with boundary conditions, but its conclusions are physically unacceptable. A more plausible model of a reflecting boundary that stays within linear spectral theory confines the waves by a steeply rising potential function, which can be taken as a power of one coordinate, z^α. We report investigations of this model with considerable student involvement. An exact analytical solution with some numerics for α=1 and an asymptotic (semiclassical) analysis of a related problem for α=2 are presented.

quant-ph↗

arXiv2012

Wedges, Cones, Cosmic Strings, and the Reality of Vacuum Energy

One of J. Stuart Dowker's most significant achievements has been to observe that the theory of diffraction by wedges developed a century ago by Sommerfeld and others provided the key to solving two problems of great interest in general-relativistic quantum field theory during the last quarter of the twentieth century: the vacuum energy associated with an infinitely thin, straight cosmic string, and (after an interchange of time with a space coordinate) the apparent vacuum energy of empty space as viewed by an accelerating observer. In a sense the string problem is more elementary than the wedge, since Sommerfeld's technique was to relate the wedge problem to that of a conical manifold by the method of images. Indeed, Minkowski space, as well as all cone and wedge problems, are related by images to an infinitely sheeted master manifold, which we call Dowker space. We review the research in this area and exhibit in detail the vacuum expectation values of the energy density and pressure of a scalar field in Dowker space and the cone and wedge spaces that result from it. We point out that the (vanishing) vacuum energy of Minkowski space results, from the point of view of Dowker space, from the quantization of angular modes, in precisely the way that the Casimir energy of a toroidal closed universe results from the quantization of Fourier modes; we hope that this understanding dispels any lingering doubts about the reality of cosmological vacuum energy.

math-ph↗

arXiv2011

NLO corrections to timelike and spacelike DVCS

Generalized Parton Distributions (GPDs) offer a new way to access the quark and gluon nucleon structure. We advocate the need to supplement the experimental study of deeply virtual Compton scattering by its crossed version, timelike Compton scattering. We review recent progress in this domain, emphasizing the need to include NLO corrections to any phenomenological program to extract GPDs from experimental data.

hep-ph↗

arXiv2011

New results in exclusive hard reactions

Generalized Parton Distributions offer a new way to access the quark and gluon nucleon structure. We review recent progress in this domain, emphasizing the need to supplement the experimental study of DVCS by its crossed version, timelike Compton scattering (TCS), where data at high energy should appear thanks to the study of ultraperipheral collisions at the LHC. This will open the access to very low skewness quark and gluon GPDs. Our leading order estimates show that the factorization scale dependence of the amplitudes is quite high. This fact demands the understanding of higher order contributions with the hope that they will stabilize this scale dependence. The magnitudes of the NLO coefficient functions are not small and neither is the difference of the coefficient functions appearing respectively in the DVCS and TCS amplitudes. The conclusion is that extracting the universal GPDs from both TCS and DVCS reactions requires much care. We also describe the extension of the GPD concept to three quark operators and the relevance of their nucleon to meson matrix elements, namely the transition distribution amplitudes (TDAs) which factorize in hard exclusive pion electroproduction off a nucleon in the backward region and baryon-antibaryon annihilation into a pion and a lepton pair. We discuss the main properties of the TDAs.

hep-ph↗

arXiv2011

Exploring the nucleon structure through GPDs and TDAs in hard exclusive processes

Generalized Parton Distributions (GPDs) offer a new way to access the quark and gluon nucleon structure. We review recent progress in this domain, emphasizing the need to supplement the experimental study of deeply virtual Compton scattering by its crossed version, timelike Compton scattering. We also describe the extension of the GPD concept to three quark operators and the relevance of their nucleon to meson matrix elements, namely the transition distribution amplitudes (TDAs) which factorize in backward meson electroproduction and related processes. We discuss the main properties of the TDAs. \

hep-ph↗