PVLAS Day - Giuseppe Ruoso Gas measurements in the PVLAS experiment Giuseppe RUOSO INFN - Laboratori...

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


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)


Apparatus at LNL


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


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


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


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


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)


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


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


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


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


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


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


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


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


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


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)


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


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


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


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



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


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


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


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

PVLAS Day - Giuseppe Ruoso Gas measurements in the PVLAS experiment Giuseppe RUOSO INFN - Laboratori...

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