Gas measurements in the PVLAS experiment

1. Gas measurements in the PVLAS experiment Giuseppe RUOSO INFN - Laboratori Nazionali di Legnaro • Summary • Apparatus and test with gases • Low pressure birefringence measurements • Mixing of the photon with low mass particles • PVLAS Group • M. Bregant, G. Cantatore, F. Della Valle, M. Karuza, E. Milotti, E. Zavattini, G. Raiteri (Trieste) • S. Carusotto, E. Polacco (Pisa), U. Gastaldi, P. Temnikov (INFN - LNL) • G. di Domenico, G. Zavattini (Ferrara), R. Cimino (INFN - LNF) • Technical support • S. Marigo (LNL), A. Zanetti, G. Venier (TS) www.ts.infn.it/experiments/pvlas

2. The PVLAS apparatus • Detect modifications of the polarisation state of a linearly polarised light beam traversing a dipole magnetic field in vacuum: • ellipticity due to birefringence • rotation of the polarisation plane The two measurements are independent: by inserting an optical element (Quarter Wave Plate) one can switch from one measure to the other OR using a Faraday Cell it is possible to perform measurement simultaneously (Only in recent data) A Fabry Perot cavity (FP) increases the effective optical path by a factor N ~ 5 104 Laser is green (532 nm) or infrared (1064 nm) www.ts.infn.it/experiments/pvlas

3. Apparatus at LNL www.ts.infn.it/experiments/pvlas

4. Detection method • A pair of crossed polarisers (P, A) is used to sense polarization changes • The optical path length is increased by means of a Fabry-Perot resonator (finesse ~105) (mirrors M1 and M2) • An intense magnetic field (~ 6 T) is generated by a superconducting dipole magnet • A removable quarter-wave plate (QWP) used to measure dichroisms • Heterodyne detection is employed to extract small signals • the interaction is time-modulated by rotating the magnet (this rotation also acts as a clock for all signals enabling phases to be measured) • a carrier ellipticity is introduced by means of a modulator (SOM) • Light intensity transmitted through the last polarizer is detected and Fourier-analysed: the resulting spectrum contains the physical information www.ts.infn.it/experiments/pvlas

5. Test with gases Gases are ideal test for the apparatus due to the Cotton-Mouton effect: Magnetic birefringenceDnu of a gas at pressure P in a dipole magnetic field B Ellipticity Y due to birefringence With N ~ 50000 a few mbar of nitrogen gives ellipticity ~ 10-4 L = 1 m l = laser wavelength (532 nm, 1064 nm) www.ts.infn.it/experiments/pvlas

6. ellipticity modulator (SOM) polariser analyser magnetic field ITr I0 h w0 Y wM Heterodyne detection - ellipticity • In the heterodyne detection, using a beat with a calibrated effect, we have • Signal linear in the birefringence • Smaller 1/f noise High sensitivity w0 www.ts.infn.it/experiments/pvlas

7. ellipticity modulator (SOM) polariser analyser magnetic field ITr I0 h w0 Y wM Heterodyne detection - rotation QWP • In the heterodyne detection, using a beat with a calibrated effect, we have • Signal linear in the birefringence • Smaller 1/f noise High sensitivity w0 www.ts.infn.it/experiments/pvlas

8. Measurements Heterodyne detection technique (Rotating Magnet) Measured effect given by Fourier amplitude and phase at signal frequency Vector in the polar plane The amplitude measure the ellipticity/rotation The phase is related to the triggers position and magnetic field direction. True physical signal must have a definite phase www.ts.infn.it/experiments/pvlas

9. Apparatus test with nitrogen • Measure of Nitrogen CME Dnu (N2) = -(2.4±0.1)10-13 Phase = 195 degree • Fabry-Perot: finesse F amplification factor control t = cavity decay time d = 6.4 m cavity length Run 573 FP, t ~ 510 ms, B = 5.0 T, P = 0.5 mbar Y = 3.77 10-4 Run 580 NO FP, B = 5.3 T, P = 85.7 mbar Y = 1.52 10-6 Expected amplification Measured amplification www.ts.infn.it/experiments/pvlas

10. B Square check with Neon During data taking the magnetic field diminishes and data must be normalized to a standard field value before making comparison. In order to do this we verified the B2 dependence of the effect The fit to a quadratic function optimizes the chi square www.ts.infn.it/experiments/pvlas

11. Measurement of CME for Xe, Kr, He Due to the extremely high sensitivity of the apparatus we were able to perform precise measurement of very small CME in noble gases Stability of the apparatus: Helium CME for measurements performed over a time > 1 year Typical pressure plot: each point 100 s data record www.ts.infn.it/experiments/pvlas

12. Gas system High purity gas samples has to be used in the measurements (Helium is 99.9999% pure) An all metal gas insertion line ensures the sample purity Lower vacuum chamber with optics Gas bottles and insertion line We also use a cryopanel to prevent contamination during gas filling Chamber outgassing < 2 10-5 mbar/hour Main components: H2, CO, H2O Typical run lasts 3-4 hours No contribution for measurements reported here www.ts.infn.it/experiments/pvlas

13. Gases at low pressure - ellipticity Studying the amplitude of the gas ellipticity for pressures close to zero it is possible to deduce the amplitude of the searched vacuum effect Helium Data indicates that vacuum is showing an effect which has sign opposite to helium and thus there exists an helium pressure at which the overall effect is zero! www.ts.infn.it/experiments/pvlas

14. Gases at low pressure - ellipticity - II We performed the same measurement with different gases Helium, Neon, Nitrogen Nitrogen has a CME with sign opposite to neon and helium and shows no zero crossing Data collected in two different periods give similar results but different vacuum amplitudes Log - Log scale www.ts.infn.it/experiments/pvlas

15. Gases at low pressure - ellipticity - summary • Zero pressure ellipticity effect of the order of 10-7 for 33000 passes in a 5 T field for 532 nm light • Similar results for infrared (lower statistics) • The sign of the ‘vacuum’ signal is opposite to noble gases birefringence (CME) and same as nitrogen Nov 2005 Gas data in any case do not suggest the nature of the vacuum signal. Explanation of this result is still unclear www.ts.infn.it/experiments/pvlas

16. Vacuum rotation Possible interpretation Rotation is actually a dichroism (selective absorption of a polarization component) due to the mixing of the photon with a low mass particle Particle mass m ~ 1 meV Inverse Coupling M ~ 4 105 GeV www.ts.infn.it/experiments/pvlas

17. Mixing of the photon with low mass particle If we suppose that the vacuum rotation signal is physical and due to a particle we can use a gas to change the effect due to a change of the effective mass of the photon (different index of refraction) www.ts.infn.it/experiments/pvlas

18. Mixing of the photon with low mass particle II Increasing the pressure from vacuum the expected signal will decrease following a [(sin x ) / x]2 function, with characteristic zeroes depending on the gas pressure P (index of refraction) Neon (n-1) = 67.1 10-6 (P / Patm) Helium (n-1) = 34.9 10-6 (P / Patm) www.ts.infn.it/experiments/pvlas

19. Fabry -Perot cavities and ellipsometers When an ellipticity is present in a Fabry-Perot cavity with birefringent mirrors, a spurious dichroism is also generated due to a leakage between resonant modes of the cavity that are almost degenerate Gas in cavity with magnetic field generates ellipticity linearly proportional to pressure through CME A dichroism is also generated linearly proportional to pressure that amounts ~ 5 - 10 % of the produced ellipticity www.ts.infn.it/experiments/pvlas

20. Measurements - gas dichroism I • gases do not generate rotation/dichroism • small dichroism proportional to pressure due to Cotton-Mouton effect via cavity birefringence (spurious effect) • to reduce spurious effect we choose gases with largest ratio (n-1)/CME First measurement: neon Fitting function: The y axis has the measured rotation/dichroism projected on the physical axis and divided by the number of passes in the cavity www.ts.infn.it/experiments/pvlas

21. Gas Dichroism II - still neon Several measurements performed, some data show effect, some other no: Fit compatible with straight line Particle parameters compatible with 0 Errors values compatible with left side data Difference (Measured data - residual gas effect) If the non linearity is correct, is due to particle mixing or is there another possible explanation? www.ts.infn.it/experiments/pvlas

22. Gas dichroism III - helium To reduce linear effect due to Cotton Mouton we performed measurements with helium First data showed the non linearity, but on following runs this was not clear Data analysis is still underway, also with the study of possible systematic effects that could mimic the non linear part www.ts.infn.it/experiments/pvlas

23. Conclusions • Gas measurements are very important in the PVLAS experiment: • Careful tests of the apparatus performances can be executed • Vacuum magnetic birefringence/ellipticity measurements receive a stronger validation from measurements with gas at low pressure • The particle hypothesis can be tested measuring rotation / dichroism in the presence of a gas. Regarding this point a clear result needs more statistics and a careful control of systematics www.ts.infn.it/experiments/pvlas