Last Update: October 2000

PVLAS (Ferrara, LNL, Padova, Pisa, Trieste)

Laboratory: Legnaro National Laboratory of INFN

1. Goal of the experiment

The experimental exploration of the complex structure of the quantum vacuum is the research goal of the PVLAS collaboration (Universities of Trieste, Pisa, Ferrara, Padova, and I.N.F.N. Legnaro Laboratories). Several effects predicted by Quantum Electrodynamics (QED), such as photon-photon scattering and particle production from a two-photon interaction, contribute to this structure. Since any neutral light particle couples to two photons, with a strength depending on its particular nature, a possible detection strategy can be pursued by optical techniques: among such particles there could be those considered to be possible dark matter candidates, and the axion is an example of this class of objects.

These interactions are explored by the PVLAS experiment by sending a linearly polarised laser beam through a transverse magnetic field, and by measuring changes in the polarisation state of the light. The PVLAS collaboration is at present running at the Laboratori Nazionali di Legnaro of I.N.F.N. a very sensitive optical ellipsometer capable of measuring the small rotations or ellipticities which can be acquired by the beam as a consequence of the interaction with the external field. The apparatus consists of a very high finesse (F~140000), 6.4 m long, vertical Fabry-Perot optical resonant cavity. The magnetic field is provided by a superconducting, 1 m long, ~6.5 T, dipole magnet housed in a warm bore, liquid He, cryostat. The magnet-cryostat assembly can be rotated to provide the time-modulation of the effect necessary for heterodyne detection, and it is located within a granite tower-like structure holding the optics setup. A linearly polarised laser beam is frequency locked to the cavity and its polarisation state is studied, using a heterodyne technique, for rotation and/or ellipticity acquired within the magnetic field. Light transmitted through crossed polarisers is detected by a photodiode and the polarisation parameters are extracted from a Fourier analysis of the resulting current signal. These measured parameters, besides giving direct access to such fundamental quantities as the fine structure constant and the Compton wavelength of the electron, contain mass and coupling constant to two photons of possible produced particles

2. Physics achievements during 2000

The construction of the apparatus has been completed and the first frequency locking between the 1064 nm laser and the Fabry-Perot optical resonator (length 6.4 m, finesse ~100000) has been achieved. This performance is sufficient for the data taking and the corresponding quality factor Q ~ 1012 is the highest published up to this time. A detailed study of the mechanical noise sources within the apparatus has led to the installation of a hydraulic motor able to actuate the rotating table holding the PVLAS magnet-cryostat assembly. With this modified set-up cavity resonance can be maintained for an indefinite length of time while the magnet is rotating: long integration times are then possible.

A commissioning and a data run with the apparatus functioning as a single integrated unit have been conducted during the year 2000. The commissioning run has demonstrated the principle of the experiment and evidenced the noise sources, which will be studied in subsequent runs. The first data run, conducted with a 4.5 T magnetic field, modulated at ~ 0.3 Hz, and a Fabry-Perot finesse of ~100000 (total optical path within the field zone ~100 km), has yielded ellipticity data with a sensitivity of 2.5x10-7 1/Hz1/2, and reached, in a few hours integration time, the limits on scalar/pseudoscalar particle couplings to two photons obtained in the BFRT precursor experiment (see R. Cameron et al, Phys. Rev. D Vol. 47, n. 9, p. 3707, 1993).

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

- Manpower: 11 researchers (7.80 FTE), technical support from Legnaro Laboratories, technical support form the Trieste electronics service
- Financial support: 390 MLit.

4. Number of publications in refereed journals: 7

5. Number of talks at conferences: 13

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

- n. 3 Ph.D. theses
- n. 8 Laurea Diploma theses

7. Leadership roles in the experiment

- Senior spokesman of the PVLAS collaboration is E. Zavattini (Cern and University of Trieste) assisted by G. Cantatore (University of Trieste).
- A natural leadership role within the collaboration is played by the Trieste group.
- The LNL and Padova groups are leading the cryogenic studies.

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 the INFN National Committee on non-accelerator particle physics