PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas1 Gas measurements in the PVLAS experiment Giuseppe RUOSO INFN - Laboratori Nazionali di Legnaro 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) Summary Apparatus and test with gases Low pressure birefringence measurements Mixing of the photon with low mass particles

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas2 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 ~ Laser is green (532 nm) or infrared (1064 nm)

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas3 Apparatus at LNL

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas4 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 ~10 5 ) (mirrors M 1 and M 2 ) 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

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas5 Test with gases Gases are ideal test for the apparatus due to the Cotton-Mouton effect: Magnetic birefringence  n u of a gas at pressure P in a dipole magnetic field B Gas  n u ( T ~ 293 K) Nitrogen- (2.47± 0.04) x Oxygen- (2.52± 0.04) x Carbon Oxide- (1.83± 0.05) x With N ~ a few mbar of nitrogen gives ellipticity ~ Ellipticity  due to birefringence L = 1 m = laser wavelength (532 nm, 1064 nm)

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas6 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 I0I0 polariser magnetic field ellipticity modulator (SOM) analyser    M    0 I Tr 00

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas7 Heterodyne detection - rotation In the heterodyne detection, using a beat with a calibrated effect, we have Signal linear in the birefringence Smaller 1/f noise High sensitivity I0I0 polariser magnetic field ellipticity modulator (SOM) analyser    M    0 I Tr 00 QWP

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas8 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

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas9 Apparatus test with nitrogen Measure of Nitrogen CME Fabry-Perot: finesse F amplification factor control  n u (N 2 ) = -(2.4±0.1) Run 573 FP,  ~ 510  s, B = 5.0 T, P = 0.5 mbar  = Run 580 NO FP, B = 5.3 T, P = 85.7 mbar  =  Phase = 195 degree Expected amplification Measured amplification  = cavity decay time d = 6.4 m cavity length

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas10 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 B 2 dependence of the effect The fit to a quadratic function optimizes the chi square

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas11 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 Gas  n u ( T ~ 290 K, =1064 nm) Xenon(2.44±0.22)x Kripton(8.61±0.35)x Helium(1.75±0.07)x Stability of the apparatus: Helium CME for measurements performed over a time > 1 year Typical pressure plot: each point 100 s data record

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas12 Gas system Lower vacuum chamber with optics Gas bottles and insertion line High purity gas samples has to be used in the measurements (Helium is % pure) An all metal gas insertion line ensures the sample purity We also use a cryopanel to prevent contamination during gas filling Chamber outgassing < mbar/hour Main components: H 2, CO, H 2 O Typical run lasts 3-4 hours No contribution for measurements reported here

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas13 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 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! Helium

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas14 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

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas15 Gases at low pressure - ellipticity - summary Zero pressure ellipticity effect of the order of for passes in a 5 T field for 532 nm light Similar results for infrared (lower statistics) Gas data in any case do not suggest the nature of the vacuum signal. Explanation of this result is still unclear The sign of the ‘vacuum’ signal is opposite to noble gases birefringence (CME) and same as nitrogen Nov 2005

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas16 Vacuum rotation 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 meVInverse Coupling M ~ GeV Possible interpretation

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas17 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)

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas18 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) = (P / P atm ) Helium (n-1) = (P / P atm )

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas19 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 ~ % of the produced ellipticity

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas20 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 Fitting function: First measurement: neon The y axis has the measured rotation/dichroism projected on the physical axis and divided by the number of passes in the cavity

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas21 Gas Dichroism II - still neon Several measurements performed, some data show effect, some other no: If the non linearity is correct, is due to particle mixing or is there another possible explanation? Difference (Measured data - residual gas effect) Fit compatible with straight line Particle parameters compatible with 0 Errors values compatible with left side data

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas22 Gas dichroism III - helium To reduce linear effect due to Cotton Mouton we performed measurements with helium Gas(n P atm CME:  n u Neon (5.9 ± 0.1)x Helium (1.75±0.07)x 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

PVLAS Day - Giuseppe Ruosowww.ts.infn.it/experiments/pvlas23 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