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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 - PowerPoint PPT Presentation
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PVLAS Day - Giuseppe Ruoso www.ts.infn.it/experiments/pvlas 1
Gas measurements in the PVLAS experiment
Giuseppe RUOSO INFN - Laboratori Nazionali di Legnaro
PVLAS GroupM. 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 Ruoso 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)
PVLAS Day - Giuseppe Ruoso www.ts.infn.it/experiments/pvlas 3
Apparatus at LNL
PVLAS Day - Giuseppe Ruoso 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
PVLAS Day - Giuseppe Ruoso www.ts.infn.it/experiments/pvlas 5
Test with gases Gases are ideal test for the apparatus due to the Cotton-Mouton effect:Magnetic birefringence nu of a gas at pressure P in a dipole magnetic field BGas nu ( T ~ 293 K)
Nitrogen - (2.47± 0.04) x 10-13
Oxygen - (2.52± 0.04) x 10-12
Carbon Oxide - (1.83± 0.05) x 10-13
€
Ψ=πNL
λΔn sin 2θ( )
With N ~ 50000 a few mbar of nitrogen gives ellipticity ~ 10-4
Ellipticity Ψ due to birefringence
L = 1 m = laser wavelength (532 nm, 1064 nm)
€
n = n|| − n⊥ = Δnu
B T[ ]1T
⎛
⎝ ⎜
⎞
⎠ ⎟
2P
Patm
⎛
⎝ ⎜
⎞
⎠ ⎟
Eγ
Bext
E'γ
k
zona di campo
a
b
L
Ψ =ab=
πL
nsin2θ
θ€
Ψ=a
b
PVLAS Day - Giuseppe Ruoso www.ts.infn.it/experiments/pvlas 6
Heterodyne detection - ellipticity
ITR ≈I 0 σ2+ η+Ψ( )
2
[ ] =I0 σ 2 ηcosω0t+ΨcosωMt( )
2
[ ]
≈I0 L +ηΨ cosω0 −ωM( )t+cosω0 +ωM( )t[ ]+η2
2cos2ω0t+ L
⎧⎨⎩
⎫⎬⎭
ITR(ω)
α η2/2
α ηΨ
ω0 + ωMω0 - ωM
ωM 2ω0
ω
In the heterodyne detection, using a beat with a calibrated effect, we have
• Signal linear in the birefringence• Smaller 1/f noise
High sensitivity
€
ωM = 2ωROT
I0
polariser magnetic field ellipticity modulator (SOM)
analyser
Ψ ωM η ω0
ITr
ω0
PVLAS Day - Giuseppe Ruoso www.ts.infn.it/experiments/pvlas 7
Heterodyne detection - rotation
ITR ≈I 0 σ2+ η+Ψ( )
2
[ ] =I0 σ 2 ηcosω0t+ΨcosωMt( )
2
[ ]
≈I0 L +ηΨ cosω0 −ωM( )t+cosω0 +ωM( )t[ ]+η2
2cos2ω0t+ L
⎧⎨⎩
⎫⎬⎭
ITR(ω)
α η2/2
α ηΨ
ω0 + ωMω0 - ωM
ωM 2ω0
ω
In the heterodyne detection, using a beat with a calibrated effect, we have
• Signal linear in the birefringence• Smaller 1/f noise
High sensitivity
€
ωM = 2ωROT
I0
polariser magnetic field ellipticity modulator (SOM)
analyser
Ψ ωM η ω0
ITr
ω0
QWP
PVLAS Day - Giuseppe Ruoso 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/rotationThe phase is related to the triggers position and magnetic field direction. True physical signal must have a definite phase
10-8
10-7
10-6
10-5
10-4
0 1 2 3 4 5
RUN 965, neon 15 mbarB = 5.5 T, finesse = 61 000
10-19
10-18
10-17
10-16
Frequency (units of magnet rotation frequency)
PVLAS Day - Giuseppe Ruoso www.ts.infn.it/experiments/pvlas 9
Apparatus test with nitrogen
• Measure of Nitrogen CME
• Fabry-Perot: finesse F amplification factor control
€
N = 2F
π= 2
τc
d= 47800
nu (N2) = -(2.4±0.1)10-13
Run 573 FP, ~ 510 s, B = 5.0 T, P = 0.5 mbar Ψ = 3.77 10-4
Run 580 NO FP, B = 5.3 T, P = 85.7 mbar Ψ = 1.52 10
Phase = 195 degree
Expected amplification Measured amplification
€
N = 48150
= cavity decay time d = 6.4 m cavity length
PVLAS Day - Giuseppe Ruoso 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
PVLAS Day - Giuseppe Ruoso www.ts.infn.it/experiments/pvlas 11
Measurement of CME for Xe, Kr, HeDue to the extremely high sensitivity of the apparatus we were able to perform precise measurement of very small CME in noble gases
Gas nu ( T ~ 290 K, =1064 nm)
Xenon (2.44±0.22)x10-15
Kripton (8.61±0.35)x10-15
Helium (1.75±0.07)x10-16
1.4 10-16
1.6 10-16
1.8 10-16
2 10-16
2.2 10-16
HeG1 HeG2 HeG3 HeG4 HeG5 HeIR2HeIR3HeIR4
Data set
Infrared
Green
Stability of the apparatus: Helium CME for measurements performed over a time > 1 year
0
1 10-18
2 10-18
3 10-18
4 10-18
5 10-18
6 10-18
7 10-18
8 10-18
0 10 20 30 40 50
( )Pressure mbar
2 HeG set
Typical pressure plot: each point 100 s data record
PVLAS Day - Giuseppe Ruoso www.ts.infn.it/experiments/pvlas 12
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 99.9999% pure)An all metal gas insertion line ensures the sample purity
We also use a cryopanel to prevent contamination during gas filling
Chamber outgassing < 2 10-5 mbar/hourMain components: H2, CO, H2OTypical run lasts 3-4 hours
No contribution for measurements reported here
PVLAS Day - Giuseppe Ruoso 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
0
2 10-7
4 10-7
6 10-7
8 10-7
1 10-6
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
amp
Helium pressure (mbar)
Chamber filled with heliumCavity amplification = 33 000B = 5 T
-60
0
60
120
180
240
300
360
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Helium pressure (mbar)
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 Ruoso 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
10-8
10-7
10-6
10-5
10-8 10-6 10-4 10-2 100 102
Pressure (mbar)
Nitrogen Neon
Helium
November 2005 data
PVLAS Day - Giuseppe Ruoso 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)
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
0
70
140
210
280
350
10-5 10-4 10-3 10-2 10-1 100 101
Nitrogen phaseHelium phaseNeon phase
Gas pressure (mbar)Nov 2005
PVLAS Day - Giuseppe Ruoso www.ts.infn.it/experiments/pvlas 16
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 meV Inverse Coupling M ~ 4 105 GeV
Possible interpretation
PVLAS Day - Giuseppe Ruoso 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)
€
ε M,m, pgas( ) =FBext
2 L2
8πM 2
sin x
x
⎛
⎝ ⎜
⎞
⎠ ⎟2
€
x =L
2
pgas nstp −1( )ω
patm
+m2
2ω
⎡
⎣ ⎢ ⎢
⎤
⎦ ⎥ ⎥
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-3 10-2
Vacuum dichroismDichroism with p > 0 mbar (~ 10 mbar Ne)
mass (eV)
Curves for M = 4 10 5 GeV
PVLAS Day - Giuseppe Ruoso www.ts.infn.it/experiments/pvlas 18
Mixing of the photon with low mass particle IIIncreasing 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)
PVLAS Day - Giuseppe Ruoso 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
PVLAS Day - Giuseppe Ruoso www.ts.infn.it/experiments/pvlas 20
Measurements - gas dichroism I
0
2 10-13
4 10-13
6 10-13
8 10-13
1 10-12
1.2 10 -12
1.4 10 -12
0 2 4 6 8 10
Measured dataResidual gas effect
Neon pressure (mbar)
y = m0*m2+1/4*(195*5e6/m1/2)...
ErrorValue
5.8e+132.86e+14M (eV)
3.68e-151.69e-13m2
0.000110.00114mass (eV)
NA3.09Chisq
NA0.998R
- 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:
€
y M,m, pgas( ) = b ⋅ pgas + ε M,m, pgas( )
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 Ruoso www.ts.infn.it/experiments/pvlas 21
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?
0
5 10-13
1 10-12
1.5 10 -12
2 10-12
2.5 10 -12
3 10-12
3.5 10 -12
0 5 10 15 20
AF02NMs runs 821-829
Measured data
Residual gas effect
Neon pressure (mbar)
y = m4+m0*m2+1/4*(195*5e6/m1...
ErrorValue
7.88e+132.97e+14M (eV)
2.53e-151.78e-13m2
0.0001140.00117mass (eV)
3.66e-14-3.68e-13m4
NA6.96Chisq
NA1R
-5 10-14
0
5 10-14
1 10-13
1.5 10-13
2 10-13
2.5 10-13
3 10-13
Difference (Measured data - residual gas effect)
0
5 10-13
1 10-12
1.5 10-12
2 10-12
2.5 10-12
3 10-12
3.5 10-12
0 5 10 15 20 25
AF05NMs runs 952 - 959
Measured dataResidual gas effect
Neon pressure (mbar)
y = m4+m0*m2+1/4*(195*5e6/m1...
ErrorValue
4.27e+151.49e+15M (eV)
1.14e-141.61e-13m2
0.007160.00027mass (eV)
1.87e-13-1.19e-13m4
NA10.5Chisq
NA0.998R
Fit compatible with straight lineParticle parameters compatible with 0Errors values compatible with left side data
PVLAS Day - Giuseppe Ruoso www.ts.infn.it/experiments/pvlas 22
Gas dichroism III - helium
To reduce linear effect due to Cotton Mouton we performed measurements with helium
Gas (n -1) @ Patm CME: nu
Neon 67.1 10-6 (5.9 ± 0.1)x10-16
Helium 34.9 10-6 (1.75±0.07)x10-16
-5 10-13
0 100
5 10-13
1 10-12
1.5 10-12
0 5 10 15 20 25
Measured dataResidual gas effect
y = -4.32e-13 + 6.59e-14x R= 1
helium pressure (mbar)
y = m4+m0*m2+1/4*(195*5e6/m1...
ErrorValue
5.83e+131.9e+14M (eV)
9.24e-156.59e-14m2
0.0002390.00169mass (eV)
1.33e-13-4.32e-13m4
NA14.3Chisq
NA0.906R
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 Ruoso 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