Two-photon physics

Abstract: Gupta has introduced an alternative method of quantization for the Maxwell field which differs from the usual one in that the scalar part of the field is quantized by means of the indefinite metric of Dirac. It is shown that this method can be extended into a general and consistent theory, including the case of interaction with electrons. Some of the advantages of the new method are the following: The well known difficulty of normalizing a state vector satisfying the Lorentz condition no longer occurs. For processes taking place within long time intervals the photon vacuum can consistently be stated in the form Aμ (x)ψ0 = 0, a condition which could not be fulfilled in the ordinary theory. Gauge invariance is exhibited in a peculiar direct way. It is shown, by a canonical transformation, that the theory is equivalent with the reduced theory where the longitudinal field is eliminated, and replaced by the static Coulomb interaction. All physical results are therefore identical with those of the ordinary theory. Lorentz invariance is exhibited in a simple way. †Original Ref.: K. Bleuler, “Eine neue Methode zur Behandlung der longitudinalen und skalaren Photonen,” Helv. Phys. Acta 23, 567–586 (1950). This version of the article was typeset in 2009 using LTEX2e and REVTEXv4.0. Translated by Dale Alan Woodside in 1995 using German Assistant for Windows Vers. 1.0B. Translation is Copyright 1995, 2009 by Dr. Dale Alan Woodside, all rights reserved. German Assistant is a trademark of MicroTac Software, Inc. Windows is a registered trademark of Microsoft Corporation.

Abstract: Two-photon exchange contributions to elastic electron-scattering are reviewed. The apparent discrepancy in the extraction of elastic nucleon form factors between unpolarized Rosenbluth and polarization transfer experiments is discussed, as well as the understanding of this puzzle in terms of two-photon exchange corrections. Calculations of such corrections both within partonic and hadronic frameworks are reviewed. In view of recent spin-dependent electron scattering data, the relation of the two-photon exchange process to the hyperfine splitting in hydrogen is critically examined. The imaginary part of the two-photon exchange amplitude as can be accessed from the beam normal spin asymmetry in elastic electron-nucleon scattering is reviewed. Further extensions and open issues in this field are outlined.

Abstract: We show that photons with energies above the GZK cutoff can reach us from very distant sources if they mix with light axions in extragalactic magnetic fields. The effect which enables this is the conversion of photons into axions, which are sufficiently weakly coupled to travel large distances unimpeded. These axions then convert back into high energy photons close to the Earth. We show that photon?axion mixing facilitates the survival of super-GZK photons most efficiently with a photon?axion coupling scale M??1011 GeV, which is in the same range as the scale for the photon?axion mixing explanation for the dimming of supernovae without cosmic acceleration. We discuss possible observational consequences of this effect.

Abstract: The book is based on a laboratory course in nuclear physics given to advanced students. It treats the experimental techniques and instrumentationmost often used in nuclear and particle physics experiments as well as in various other experimental sciences. It provides most useful results and formulae, technical know-how and informative details on -interactionof radiation in matter; - radion protection and radioactive sources; - statistics for the interpretation and analysis of data; - principles and operation of the main types of detectors (ionization, scintillation and semiconductor detectors); - nuclear electronics instrumentation (NIM, CAMAC); - various systems and techniques for experiments. Thanks to the author's long teaching experience the material is presented in a verypractical, hands-on way making the book a useful text and lab companion for students and experienced scientists.