# Presentation: 717011

## Recent Results From the PVLAS Experiment and Future Perspectives

- Guido Zavattini on behalf of the PVLAS collaboration
*Università di Ferrara and INFN sezione di Ferrara, Italy*

**University of Ferrara**

## PVLAS Collaboration

**M. Bregant**
**G. Cantatore**
**F. Della Valle**
**M. Karuza**
**E. Milotti***** (Udine)***
**E. Zavattini**
_**Lab. Naz. di Legnaro**_
**U. Gastaldi**
**G. Ruoso**

_**INFN - Pisa**_
**S. Carusotto**
**E. Polacco**
_**INFN -  Ferrara**_
**G. Di Domenico**
**G. Zavattini**
_**CERN**_
**G. Petrucci**** **
_**Lab. Naz. di Frascati**_
**R. Cimino**

**PVLAS Collaboration**
**(Polarizzazione del Vuoto con LASer)**** **

## Slide 3

## Classical Electromagnetism in vacuum

**Classical vacuum has no structure. The superposition principle is valid**

**Notes:**
c = velocita della luce
equazioni sono lineari

## Heisenberg’s Uncertainty Principle

- Vacuum is a minimum energy state and can
- fluctuate into anything compatible with vacuum

- Vacuum has a **structure** which can be observed by perturbing it and probing it.

- Evidence of microscopic structure of vacuum is known (Lamb Shift ....)
- Macroscopically observable (small) effects have been predicted since 1936 but have never been directly observed yet.

## Aim of PVLAS

- PVLAS was designed to obtain experimental information on VACUUM using optical techniques.
- The full experimental program is to detect and measure
_**LINEAR BIREFRINGENCE**_
_**LINEAR DICHROISM**_
- acquired by VACUUM induced by an external magnetic field **B**

## Reference

- The polariser P and analyser A define two perpendicular directions which we use as base.

## Linear Birefringence

- In coming beam can be expressed as

- After a phase delay of the component parallel to **B** by 

_**A signal is induced along the direction of the analyser A:**_** **
**Max. component along A: **

## Linear Dichroism

**Linear Dichroism**

## Linear Dichroism

- In coming beam can be expressed as

- After reduction of the component parallel to **B** by a factor q (q≤1)

_**A signal is induced along the direction of the analyser A:**_** **
**          ****Max. polarization rotation: **

## Linear dichroism and birefringence

**Dichroism**
**Ellipticity**

_**ellipticity **_****

_**apparent**_
_**rotation**_** ******

**Linear dichroism and birefringence**

## Today’s presentation - Dichroism

**Today’s presentation - Dichroism**

**We have observed consistently a dichroism signal generated by a 1.1 m long, 5.5 T magnet. The beam traverses the region N=52000 times.**

- There is a reduction of the component parallel to **B.**
**What has happened to the missing part?**
- Can we exclude a systematic error?
- Do we have a physical ‘handle’?
- What is the comparison of our result with other experiments?
- Future plans.

## PVLAS principle

- The optical ellipsometer consists of two crossed polarisers with an ellipticity modulator providing a carrier frequency for heterodyne detection
- The Fabry-Perot increases the optical path in the field region, where the **rotating** magnetic field causes a time-varying ellipticity which then beats with the carrier

## Data Acquisition

**Rotating table has 32 ticks equally spaced which trigger ADC + start tick.**
**Photodiode signal (I****Tr****) is demodulated at ********SOM****, 2********SOM**** with lockin amplifiers.**
**Also acquire**
**light intensity (I****0****) (AC and DC coupled)**
**Position Sensitive photoDiode of output beam**
**stray magnetic field in 3 positions**
**laser feedback signal**

**Notes:**
correlazione fra diodo e tavolo

## PVLAS: schematic drawing

**The turntable, holding the magnet, rests on a beam fixed to the floor (green in the drawing)**

**lower**
**optical **
**bench**

**magnet in **
**cryostat**

**rotating**
**turntable**

**upper**
**optical bench**

**laser beam**

**vacuum**
**chamber**

**floor level**

**PVLAS: schematic drawing**

**Notes:**
sabbia +pali
verticale

## Photo gallery - 1

- Lower optical bench

- Upper optical bench

- Top photodiode

## Photo gallery - 2

- Mode TEM00

- Mode TEM11

- Mirror mount

- Short test cavity

- Mirrors

## Photo gallery - 3

- Cryostat for magnet

- Magnet position

- Control room

## Running

**Running**

- Rotating
- magnet
- (notice the red streak)

- locked laser during rotation.

**Notes:**
non si sgancia mai!!!!!
motore elettrico non andava

## PVLAS Experiment

**High sensitivity ellipsometer based on;**
**Fabry-Pérot cavity for path length;**
**superconducting magnet for high field;**
**rotating magnet to reduce 1/f noise;**
**heterodyne technique.**

## Ellipticity measurement principle

**Ellipticity measurement principle**

- Ideally,

- Static measurement is excluded:
- Modulate the effect and add a carrier (t) to signal at SOM
- Rotating the field at Mag produces an ellipticity at 2Mag

- Main frequency components at SOM±2 Mag and 2SOM

**I****0**

**polariser**

**magnetic field**

**modulator**

**analyser**

******at********Mag**

******at********SOM**

**I****Tr**

## In practice, nearly static birefringences s(t) generate a 1/f noise around SOM.

**I**

**TR**

**(**

****

**)**

- 

****

**2**

**/2**

- 

****

****

**SOM **

**+ **

****

**Mag**

****

**SOM **

**- **

****

**Mag**

****

**SOM**

****

**SOM**

****

- A small, time-varying signal can be extracted from a large noise background with the heterodyne tecnique

**In practice, nearly static birefringences ********s****(t) generate a 1/f noise around ********SOM****.**

**Normalization**

**Desired signal**

**Birefringence noise**

## Calibration with gases - Cotton Mouton

**Calibration with gases - Cotton Mouton**

- Measurement time

- 192 s

- Sensitivity

- Spectrum of the signal demodulated at the carrier frequency.
- The signal is expected at twice the magnet rotation frequency.

**Corresponds to ********n = 2.6·10****-16**

## Polar plot of the phase and amplitude of the fourier signal at 2Mag for N2 and Kr

_**N**_**2**_** and Kr have opposite signs**_
** **
**Gases define the phase for a ‘real’ magnetically induce effect.**

## Dichroism measurements

**QWP can be inserted to transform a rotation into an ellipticity with the same amplitude. It can be oriented in two positions: 0 ̊ and 90 ̊.**

- Main frequency components at SOM±2 Mag and 2SOM

## Sensitivity and Result

## Slide 27

## Results for the measured dichroism in vacuum

**Observed signal in vacuum with B≠ 0 and cavity present**
**The data distribution changes sign when rotating the QWP**
**The average vectors lie along the physical axis**

## Results for the measured dichroism in vacuum

**QWP 90 ̊: Result of weighted average with the quarter**
**wave plate at 90 ̊**
**QWP 0 ̊: Result of weighted average with the quarter**
**wave plate at 0 ̊**
** ****N****2****: Physical axis defined by measuring the **
**Cotton-Mouton effect in Nitrogen**** **

- HalfDifference :                           = Dichroism
-     HalfSum :                            = Spurious signal

**Dichroism signal = (2.2±0.3)·10****-7**** rad**

## Error or physical signal?

| Candidate | Test | Comment |
| --- | --- | --- |
| residual gas | pressure measurement | excluded |
| mirror coating birefringence | direct measurement | excluded |
| electrical pick-up | measurement without the cavity | excluded |
| beam pointing instability | correlation with measured position signal | possibility. |
| polarizer movement | measurement without the cavity | excluded |
| diffusion from magnetised surfaces | pinhole insertion | excluded |
| physical signal | must satisfy signal conditions | NOT excluded |

## Final Vacuum Result

**Final Vacuum Result**

_**Questions**_**: What is the origin of this dichroism?**
- Possible systematic error.
- If it is physical, is it an absorption or a mixing?
- QED-QCD interference leading to photon splitting?
- .............?

## Vacuum as a Medium - QED

- The propagation of light will be affected by the polarized vacuum fluctuations.
- It seems that the leading term is due to *e**+**e*- pairs.
- This effect can be calculated by the leading term in the Euler-Heisenberg effective lagrangian.

- Scheme:
**perturb** the vacuum state with an external field
**probe** the **perturbed** vacuum state with a polarized laser beam
**deduce **information on the structure of the** **vacuum state

## Euler-Heisenberg Effective Lagrangian

**= ****4/3·10****-32**** cm****3****/erg**

**For fields much smaller than the critical field**
**(B << 4.4·10****13**** gauss**
**E << 4.4·10****13**** statvolt/cm) one can write**

- Higher order terms are neglected
- virtual pairs other than *e**+**e**-* are neglected

## Induced Magnetic Birefringence of Vacuum

- Light propagation is still described by Maxwell’s equations in media. They no longer are linear due to E-H correction.
- By applying the constitutive relations to L*EH* one finds

## n = 3AeB02 ≈ 4·10-32B02   (B0 in gauss)

**anisotropy**

******n = 3*****A******e*****B****0****2**** ≈ 4·10****-32****B****0****2****   (B****0**** in gauss)**

**Linearly polarized light passing through a transverse external magnetic field.**

***A******e***** can be determined by measuring the magnetic birefringence of vacuum.**

## Numbers

**High field**
**B = 6T **

******n = *****3A******e******B******0******2****** = *****1.2·10****-22**

**Long optical path length in field**

**High finesse optical cavity **

**With these numbers ******** ≈ 3·10****-11**

***F***** ≈ 10****5****; *****L***** = 1.1 m**

## What else?

- _QED_
- Photon splitting? Much smaller than birefringence.
- Higher order corrections are ~ 1%
- _OTHER_
- quark-gluon contribution: QED-QCD interference?
*low mass, neutral particle search: axion-like*

## Axion-like contribution

- One can add extra terms [*] to the E-H lagrangian to include contributions from hypothetical _neutral light particles interacting weakly with two photons_

**scalar case**

**pseudoscalar case**

**[L.Maiani, R. Petronzio, E. Zavattini,  Phys. Lett B, Vol. 173, no.3 1986]**
**[E. Massò and R. Toldrà,  Phys. Rev. D, Vol. 52, no. 4, 1995]**

*M, M**s* are inverse coupling constants

## Effect of Axion-like particle

**Effect of Axion-like particle**

- Dichroism
- Ellipticity

## Both M and km can be disentangled

-  Both  and ** are proportional to B2
-   depends only on *M* for small *x*
-  _**the ratio **__******__****__** depends only on k**_**m****2**

**Dichroism**
**Ellipticity**

**Both *****M***** and *****k******m***** can be disentangled**

**Induced linear birefringence and dichroism**

## M Vs. m for Dichroism

- With very small x, M must be greater than 7·105GeV

## Field dependence

**Preliminary**

## Using gases as physical ‘handle’ in axion search

- If we assume that the effect is due to a light neutral particle we can use a gas to change its effective mass causing oscillations of the signal as a function of pressure.

## Dichroism Vs. Neon gas pressure

- Gasses do not generate dichroism
- Small dichroism proportional to pressure due to Cotton Mouton effect, cavity transfer function and mirror birefringence - understood

## Dichroism Vs. Neon gas pressure

**Dichroism Vs. Neon gas pressure**

**First Neon gas data**

## More Ne gas measurements

## Ellipticity?

- Everthing is birefringent. Furthermore birefringences are not uniform
- Small movements will cause variable signal
- The physical ‘handle’ available for dichroism does not work here
- Cotton Mouton effect
- Dependence of boson induced ellipticity is small and does not have a strong dependence on index of refraction
- What I can say is that here too we always have a signal at twice the magnet rotation frequency. Variability is greater than dichroism signal. Analysis is still on going.
**Per pass we have 1.4·10****-12**** < ******** < 9·10****-12**** (Preliminary)**

## Speculation

- If we believe in the signals in vacuum seen by PVLAS (B = 5.5T; N = 52000)
- 3·10-8 < ellipticity < 6·10-7;      dichroism = (2.2±0.3)·10-7 rad
- and we interpret the signals as due to a pseudoscalar particle of mass *m* and inverse coupling constant *M* to two photons

- For vacuum dichroism and gas measurements a 3  interval is given.

## Other experiments

**Other experiments**

**Two types of limits**
_**model dependent**_
**"microwave cavity expt."(BNL-Rochester-Fermilab) based on the existence of galactic halo axions. Very low masses and very narrow bands**
**"solar axion expt." (BNL-Rochester-Trieste-Fermilab) based on the conversion of solar axions in a magnetic field**
**CAST solar axion experiment: M > 8.6·10****9 ****GeV for m < 20 meV**
_**model independent**_
**"laser expt." (BNL-Rochester-Trieste-Fermilab) PVLAS precursor**
**PVLAS**

**E. Massò proposes solution by introducing boson form factor which suppresses solar (high energy) production.**
**hep-ph/0504202 v2 31 May 2005**

## 	BFRT - PVLAS

## Near future and on going activities

**Continue with different wavelength: ******** = 532 nm**
**First measurements have already been performed. Apparatus is functioning. Next run in October.**
**Change gas: Measured with He and 532 nm. Will repeat in October**
**Input beam has been stabilized. Cavity will also be stabilized**
**Search for unexpected photon splitting**
_**Regeneration experiment**_

## Regeneration

**Regeneration**

- Laser

**Production**

**Detection**

**Shining wall**

## Slide 53

## Current axion limits

**Current axion limits**

## Localization of the effect

**Localization of the effect**

- Moreover by substituting one of the mirrors with a short cavity the signal disappears.
- By introducing the QWP before the modulator the signal diminishes

- Other property

- By removing the F.P. the **signal** disapears.

**The signal is generated within the cavity**

## Outline

- Experimental aim of PVLAS
- Method and experimental setup
- Present results
- Possible interpretation and future plans