Vacuum Dichroism Research Articles

Di-lepton production from a single photon in strong magnetic fields: vacuum dichroism

We study di-lepton production from a single photon in the presence of a strong constant magnetic field. By the use of the Ritus-basis formalism, we analytically evaluate the photon-to-di-lepton conversion vertex with fully taking into account the non-perturbative interactions between the produced fermions and the strong magnetic field. We show that the di-lepton spectrum becomes anisotropic with respect to the magnetic-field direction and depends on the photon polarization as a manifestation of the vacuum dichroism in a strong magnetic field. According to the energy conservation in the presence of the Landau quantization, not only the transverse momentum of the produced fermions but also the longitudinal momentum is discretized, and the di-lepton spectrum exhibits spike structures as functions of the incident photon energy and the magnetic field strength. We also show that the di-lepton production is strictly prohibited for massless fermions in the lowest Landau levels as an analogue of the so-called helicity suppression.

AXION SEARCH WITH Q & A EXPERIMENT

Dark matter is a focused issue in galactic evolution and cosmology. Axion is a viable particle candidate for dark matter. Its interaction with photon is an effective way to detect it, e.g., pseudoscalar-photon interaction will generate vacuum dichroism in a magnetic field. Motivated to measure the QED vacuum birefringence and to detect pseudoscalar-photon interaction, we started to build up the Q & A experiment (QED [Quantum Electrodynamics] and Axion experiment) in 1994. In this talk, we first give a brief historical account of planet hunting and dark matter evidence. We then review our 3.5 m Fabry-Perot interferometer together with our results of measuring vacuum dichroism and gaseous Cotton-Mouton effects. Our first results give (-0.2 ± 2.8) × 10-13 rad/pass, at 2.3 T with 18,700 passes through a 0.6 m long magnet, for vacuum dichroism measurement. We are upgrading our interferometer to 7 m arm-length with a new 1.8 m 2.3 T permanent magnet capable of rotation up to 13 cycles per second. We aim at [Formula: see text] optical sensitivity with 532 nm cavity finesse around 100,000. When achieved, vacuum dichroism would be measured to 8.6 × 10-17 rad/pass in about 50 days, and QED birefringence would be measured to 28%.

Measurement of the Cotton–Mouton effect in nitrogen, oxygen, carbon dioxide, argon, and krypton with the Q & A apparatus

Experiments for vacuum birefringence and vacuum dichroism have been set up with high-finesse high magnetic experimental apparatuses, which seem to be ideal for small gaseous Cotton–Mouton effect (CME) measurements. PVLAS Collaboration has measured CMEs in krypton, xenon and neon at the wavelength of 1064 nm. In this Letter, we report on our measurement of CMEs in nitrogen, oxygen, carbon dioxide, argon, and krypton at the same wavelength in a magnetic field B = 2.3 T at pressure P = 0.5–300 Torr and temperature T = 295−298 K. Our results agree with the PVLAS results in the common cases.

New PVLAS results and limits on magnetically induced optical rotation and ellipticity in vacuum

In 2006 the PVLAS collaboration reported the observation of an optical rotation generated in vacuum by a magnetic field. To further check against possible instrumental artifacts, several upgrades to the PVLAS apparatus have been made during the past year. Two data taking runs, at the wavelength of 1064 nm, have been performed in the new configuration with magnetic field strengths of 2.3 and 5 T. The 2.3 T field value was chosen in order to avoid stray fields. The new observations do not show the presence of a rotation signal down to the levels of $1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\text{ }\text{ }\mathrm{rad}$ at 5 T and $1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\text{ }\text{ }\mathrm{rad}$ at 2.3 T (at 95% C.L.) with 45 000 passes in the magnetic field zone. In the same conditions no ellipticity signal was detected down to $1.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ at 2.3 T (at 95% C.L.), whereas at 5 T a signal is still present. The physical nature of this ellipticity as due to an effect depending on ${B}^{2}$ can be excluded by the measurement at 2.3 T. These new results completely exclude the previously published magnetically induced vacuum dichroism results, indicating that they were instrumental artifacts. These new results therefore also exclude the particle interpretation of the previous PVLAS results as due to a spin-zero boson. The background ellipticity at 2.3 T can be used to determine a new limit on the total photon-photon scattering cross section of ${\ensuremath{\sigma}}_{\ensuremath{\gamma}\ensuremath{\gamma}}l4.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}34}\text{ }\text{ }\mathrm{barn}$ at 95% C.L.

Q & A EXPERIMENT TO SEARCH FOR VACUUM DICHROISM, PSEUDOSCALAR–PHOTON INTERACTION AND MILLICHARGED FERMIONS

A number of experiments are underway to detect vacuum birefringence and dichroism — PVLAS, Q & A, and BMV. Recently, PVLAS experiment has observed optical rotation in vacuum by a magnetic field (vacuum dichroism). Theoretical interpretations of this result include a possible pseudoscalar–photon interaction and the existence of millicharged fermions. Here, we report the progress and first results of Q & A (QED [quantum electrodynamics] and Axion) experiment proposed and started in 1994. We use a 3.5-m high-finesse (around 30,000) Fabry–Perot prototype detector extendable to 7-m with the cavity mirrors suspended using X-pendulum-double pendulums. To polarize the vacuum, we use a 2.3-T dipole permanent magnet rotated at 5–10 rev/s, with 27-mm-diameter clear borehole and 0.6-m field length. Our ellipsometer/polarization-rotation-detection-system is formed by a pair of Glan–Taylor type polarizing prisms with extinction ratio lower than 10-8 together with a polarization modulating Faraday Cell with/without a quarter wave plate. Our first results give (-0.2 ± 2.8) × 10-13 rad/pass with 18,700 passes through a 2.3 T 0.6 m long magnet for vacuum dichroism measurement. We present our experimental limit on pseudo-scalar-photon interaction and millicharged fermions.