PVLAS

Collaboration

- INFN: Ferrara, LNL, Padova, Pisa, Trieste

Laboratory: Legnaro National Laboratory of INFN

National Responsible: G. Cantatore (Ts)

1. Goal of the experiment

Several effects, such as photon-photon scattering and particle production from a two-photon interaction, contribute to the complex structure of the quantum vacuum. Pursuing a detection strategy based on optical techniques, these interactions are studied in PVLAS by sending a linearly polarized laser beam through a transverse magnetic field, and by measuring changes in the polarization state of the light.

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2.

Activities during 2002

General

Four measurement runs, for a total of about 30 hours of data taking, have been conducted in 2002. Data have been taken in several different experimental conditions, including field off/field on at various intensities, cavity under vacuum or with low pressure nitrogen gas for calibration, quarter-wave plate inserted or removed, cavity mirrors inserted or removed. The aim has been to clarify the nature of the signal peak that is always observed under vacuum when the magnetic field is turned on. Preliminary analysis of the data indicates that the peak observed without the quarter wave plate (QWP) is due to a physical ellipticity originating in the Fabry-Perot cavity, while the peak observed when the QWP is present is due, at least in part, to spurious effects.

Cryogenics

A new 3500 l liquid–He dewar has been tested with satisfactory result and will be online for the upcoming runs. LNL is installing a new He compressor for the gas recovery line, which will double the present capacity.

Apparatus structure

A structure in aluminum and polycarbonate, allowing safe access to the liquid–He cryostat for routine operations, has been designed and installed.

Optics and vacuum

A single-piece vacuum chamber for the lower optical bench, intended to replace the present three-piece chamber, and a new flange with a moveable holder for the QWP have been designed and contracted to outside companies for construction.

The following achievements have been obtained:

Magnetic shielding tests completed. Validation tests on the vibration reduction feedback system (100 %)

Frequency-doubled laser installed at LNL and relative tests completed (50 % -(comment 2))

30 hours of data acquisition with frequency-doubled laser (postponed - (comment 3))

Comments:

1. global soft iron shields have been tested and found to have a negligible effect. Feedback system tested along a single axis, however a correlation between beam movements and ellipticity signal has not been established;

2. 532 nm laser has been successfully tested and locked to a trial Fabry-Perot cavity in Trieste; however, installation at LNL will be delayed until checks on the observed signal peak are completed at the present wavelength of 1064 nm;

3. since a signal peak, with a possible physical origin, has already been observed, the installation of the frequency-doubled laser has been postponed until the origin and nature of the signal have been further clarified. 30 hours of data acquisition have been completed at the normal wavelength of 1064 nm.

Milestones 2003

Assembly of the new access structure completed | 31-12-2003
Installation of new lower vacuum chamber and of upgraded optics movement stages | 30-04-2003
Completion of 20 hours data acquisition time in the new configuration | 30-10-2003

3. INFN contribution to the experiment in terms of manpower and financial support

- Manpower: 11 researchers (6.5 FTE), 1 technician (0.4 FTE), technical support from Legnaro Laboratories, technical support form the Trieste electronics service.
- Budget for the Year 2003: 1 % of the CSN2 total budget for 2003

4. Publications in refereed journals (in 2002): 1

5. Number of talks at conferences (in 2002): 3

6. Number of undergraduate and doctoral theses on the experiment: 0

7. Leadership roles in the experiment

- E. Zavattini (CERN, INFN and University of Trieste) is the PVLAS senior scientific coordinator
- G. Cantatore (INFN and University of Trieste) is the collaboration spokesman

8. Innovative instruments

- Electro-optic frequency locking system for high-Q, high-Finesse optical resonators.
- Liquid-He rotating cryostat housing a superconducting, high-field, dipole magnet.
- High-sensitivity ellipticity modulator with external control capability based on mechanical stress optical birefringence.

9. Competing experiments

PVLAS is at the moment the only operating experiment in the world having direct access to low energy photon-photon scattering processes.

Main PVLAS technological leads:

- very high Q optical resonators
- liquid He rotating cryostat

10. International committee which has reviewed the experiment

PVLAS is annually reviewed by INFN