187 research outputs found
Polarisation dynamics of a birefringent Fabry-Perot cavity
No full textOptical Fabry–Perot cavities always show a non-degeneracy of two orthogonal polarisation states. This is due to the unavoidable birefringence of dielectric mirrors whose effects are extremely important in Fabry–Perot-based high-accuracy polarimeters: in birefringent cavities, ellipticities and rotations mix. We have developed and present here a theory of the polarisation state dynamics in a birefringent Fabry–Perot resonator, and we validate it through measurements performed with the polarimeter of the PVLAS experiment. The measurements are performed while a laser is frequency-locked to the cavity, and provide values for the phase difference between the two orthogonal polarisation components introduced by the combination of the two cavity mirrors (equivalent wave-plate) and for the finesse of the cavity. The theoretical formulas and the experimental data agree well showing that the consequences of the mirror birefringence must be taken into account in this and in any other similar experiment
A polarisation modulation scheme for measuring vacuum magnetic birefringence with static fields
No full textA novel polarisation modulation scheme for polarimeters based on Fabry–Perot cavities is presented. The application to the measurement of the magnetic birefringence of vacuum with the HERA superconducting magnets in the ALPS-II configuration is discussed
PROGRESS TOWARDS A FIRST MEASUREMENT OF THE MAGNETIC BIREFRINGENCE OF VACUUM WITH A POLARIMETER BASED ON A FABRY-PEROT CAVITY
No full textThis dissertation work was carried out in the context of the PVLAS experiment, financed by INFN and MIUR, which has the ultimate goal of measuring the magnetic birefringence of vacuum. Photon-photon interaction and therefore magnetic birefringence of vacuum anticipated Quantum Electrodynamics (QED): these effects, already studied since 1936 by Euler, Heisenberg and Weisskopf, are associated with the fluctuations of electron-positron pairs in vacuum. The effective Lagrangian density derived by these scientists was later confirmed by Schwinger in 1951 within the QED formalism. For a 2.5 T magnetic field it is found that the induced vacuum magnetic birefringence is:
∆nEHW = 2.47 × 10^−23 @ 2.5 T.
This birefringence is extremely small and is still waiting for a direct experimental confirmation.
This thesis mainly concerns the new high sensitive polarimeter of the PVLAS experiment. The polarimeter consists of a pair of crossed polarisers, a Fabry-Perot cavity with a high finesse (F = 7 × 10^5) and uses the heterodyne technique to minimise the noise and the systematic effects. The anisotropy of vacuum with respect to electromagnetic radiation is produced using two rotating dipole permanent magnets characterised globally by the parameter integral B^2 dl = 10.25 T2m.
In this thesis work, the experimental apparatus is characterised and the sources of spurious signals and excess wide band noise are studied. As the principal source of spurious signals, we have identified diffused light modulated by a mechanical coupling between the tube and the magnet. A detailed description of the methods used to minimise this magneto-mechanical coupling, as well as other noises and systematic effects, is given. The improvements thus obtained have allowed acquiring data for rather extended periods of time - of a few weeks and this has allowed the improvement of the existing current limits on magnetic vacuum birefringence, reaching a noise floor (1σ c.l.)
∆n(PVLAS) = (12 ± 17)×10−23 @ 2.5 T,
a factor 7 above the predicted QED value.
To reach the predicted QED value a ten fold improvement in sensitivity is necessary. The cause of the excess wide band noise, which is more than a factor 50 above the expected budget, is still unknown. Experimentally it is observed that the noise has a trend ≈ 1/ f α with α between 0.5 and 1.
The problem of the excess wide band noise is common to all the experimental efforts, past and present, intended to measure the vacuum magnetic birefringence. The sensitivity of these experiments and the slope of the spectral noise seem to suggest that the limit may be due to the thermal noise of the mirrors. These remarks, to which this thesis work has also contributed, perhaps will lead to a new understanding of the residual noise of such devices and to the development of useful new experimental methods.Questo lavoro di tesi è stato realizzato nell'ambito dell'esperimento PVLAS, che ha l'obiettivo finale di misurare la birifrangenza magnetica del vuoto. L’esperimento è finanziato dall’INFN e dal MIUR.
La birifrangenza magnetica del vuoto, così come lo scattering fotone-fotone, sono previsti dall'Elettrodinamica Quantistica (QED): questi effetti, studiati già a partire dal 1936 da Euler, Heisenberg e Weisskopf, sono associati alle fluttuazioni delle coppie elettrone-positrone nel vuoto. Per un campo magnetico di 2.5 T, la birifrangenza magnetica indotta è:
∆n(EHW) = 2.47 × 10^−23 @ 2.5 T.
Si tratta quindi di un effetto estremamente piccolo che è ancora in attesa di una conferma sperimentale diretta.
Questo lavoro di tesi riguarda principalmente il nuovo ellissometro ad altissima sensibilità dell’esperimento PVLAS. L'ellissometro è formato da una coppia di polarizzatori incrociati, una cavità Fabry-Perot ad altissima finesse (F = 7 × 10^5), ed utilizza la tecnica eterodina per minimizzare gli effetti di rumore e sistematici. Per rendere il vuoto birifrangente si fa uso di due magneti dipolari rotanti caratterizzati complessivamente dal parametro integrale del B^2 dl = 10.25 T2m.
Nel corso del mio lavoro di tesi ho caratterizzato l’apparato sperimentale, ho studiato le sorgenti di segnali spuri, e quelle di rumore in eccesso presente nel polarimetro. Come sorgente principale di segnali spuri è stata individuata la luce diffusa modulata dall’accoppiamento meccanico tra tubo e magnete. Nella tesi descrivo i metodi utilizzati per minimizzare questo accoppiamento magnetomeccanico, così come altri rumori ed effetti sistematici. I miglioramenti così ottenuti hanno permesso di acquisire dati per periodi piuttosto lunghi – dell'ordine di qualche settimana – e ciò ha permesso di migliorare i limiti attualmente esistenti sulla birifrangenza magnetica del vuoto, raggiungendo un livello di rumore (1σ c.l.)
∆n(PVLAS) = (12 ± 17)×10^−23 @ 2.5 T,
che è un fattore 7 al di sopra dell'obiettivo costituito dalla previsione della QED.
Per arrivare al livello della previsione della QED si dovrà migliorare la sensibilità almeno di un fattore 10. La causa del rumore, che è in eccesso di un fattore 50 rispetto al budget teorico previsto, è tuttora ignota. Sperimentalmente si osserva che il rumore ha un andamento ≈ 1/ f^α con un valore di α compreso tra 0.5 a 1.
Il problema del rumore a larga banda è comune a tutti gli apparati, passati e presenti, che vogliono misurare la birifrangenza magnetica del vuoto. L’esame delle sensibilità di questi esperimenti e l'andamento spettrale del rumore sembrano suggerire che il limite possa essere dovuto ad un rumore termico dello specchio. Queste osservazioni, a cui ho contribuito con il mio lavoro di tesi, forse consentiranno di arrivare ad una nuova comprensione del rumore residuo e allo sviluppo di nuovi metodi sperimentali
Upper limits on the amplitude of ultra-high-frequency gravitational waves from graviton to photon conversion
No full textIn this work, we present the first experimental upper limits on the presence of stochastic gravitational waves in a frequency band with frequencies above 1 THz. We exclude gravitational waves in the frequency bands from (2.7 - 14) × 10 14 Hz and (5 - 12) × 10 18 Hz down to a characteristic amplitude of hcmin≈6×10-26 and hcmin≈5×10-28 at 95% confidence level, respectively. To obtain these results, we used data from existing facilities that have been constructed and operated with the aim of detecting weakly interacting slim particles, pointing out that these facilities are also sensitive to gravitational waves by graviton to photon conversion in the presence of a magnetic field. The principle applies to all experiments of this kind, with prospects of constraining (or detecting), for example, gravitational waves from light primordial black-hole evaporation in the early universe
Gravitational waves from the early Universe and their detection
No full textIn this thesis we study several aspects regarding the nature of gravitational waves and their detection. Gravitational waves in the present days are object of intensive theoretical studies and are interesting from the detection point of view. At present there are several detectors aiming their detection from many compact objects such as neutron stars and black holes.
In connection with that, in this thesis we study graviton emission by primordial black holes in the early universe and predict their spectrum. After the Bing Bang the density of the primordial plasma which composed the universe was too high and the probability of gravitational collapse of matter into compact object, namely black holes could have been high. Once these objects are being formed they start interacting effectively between each other emitting gravitons by several mechanisms.
We study graviton emission by light primordial black holes as a result of quantum and classical scattering between them, graviton emission by binary systems of primordial black holes and graviton evaporation through Hawking mechanism. The spectrum of emitted gravitons is quite wide starting from the lower part of few Hz and ending at very high frequencies above the GHz. The predicted spectrum could be reveled by planned space interferometers such as DECIGO in the lower part of it.
The high frequency part of the spectrum is difficult to observe because there are no such a high sensitive detectors in order to detect it. Because of this difficulty we constructed a new way in order to observe it through the mechanism of graviton to photon oscillation. This mechanism is possible whenever a gravitational wave couples to a background magnetic field and as a result is generated a non trivial energy-momentum tensor which curves the space-time.
Based on present day limits on the strength of large scale magnetic field we predict an isotropic electromagnetic background of extragalactic origin which could fill the universe. Such a background arises due to the fact that gravitons evaporated by primordial black holes could oscillate into photons in magnetic field. Moreover we show that this background can be the main component of the Cosmic X-ray Background for massive black holes or even explain it without requiring obscured AGNs.
Keywords: Gravitational waves, Black Holes, Graviton to photon oscillation, Primordial magnetic fields
Relic gravitational waves from light primordial black holes
No full textThe energy density of relic gravitational waves (GWs) emitted by primordial black holes (PBHs) is calculated. We estimate the intensity of GWs produced at quantum and classical scattering of PBHs, the classical graviton emission from the PBH binaries in the early Universe, and the graviton emission due to PBH evaporation. If nonrelativistic PBHs dominated the cosmological energy density prior to their evaporation, the probability of formation of dense clusters of PBHs and their binaries in such clusters would be significant and the energy density of the generated gravitational waves in the present-day universe could exceed that produced by other known mechanisms. The intensity of these gravitational waves would be maximal in the GHz frequency band of the spectrum or higher and makes their observation very difficult by present detectors but also gives a rather good possibility to investigate it by present and future high-frequency gravitational waves electromagnetic detectors. However, the low-frequency part of the spectrum in the range f∼0.1-10Hz may be detectable by the planned space interferometers DECIGO/BBO. For sufficiently long duration of the PBH matter-dominated stage, the cosmological energy fraction of GWs from inflation would be noticeably diluted. © 2011 American Physical Society
Erratum to: A polarisation modulation scheme for measuring vacuum magnetic birefringence with static fields (The European Physical Journal C, (2016), 76, 5, (294), 10.1140/epjc/s10052-016-4139-0)
No full textIn the original article unfortunately we missed two typos in the equation for I(δ) at the top of the fourth page of the article. © 2017, The Author(s)
Progress toward a direct experimental detection of γγ interactions
No full textA fundamental quantum electrodynamics prediction which has so far not yet been confirmed experimentally by a direct observation in a laboratory experiment is γγ interactions. Such a direct observation requires a scenario where both the beam and the target are made of bosons, while so far experiments have exploited matter particles as beam and/or target. A consequence of the existence of γγ interactions is that vacuum features magnetic birefringence as a macroscopic property. Magnetic Birefringence of Vacuum (MBV) is due to interactions of beam photons with virtual photons of a magnetic field. These interactions are mediated by loops of electron-positron and (with extremely weaker effects) by loops of muons and loops of hadrons, and could possibly be mediated by hypothetical very light particles with a coupling to two photons. Experimentation to detect MBV not only has started much later than experiments that have provided magnificent validations of QED, like g-2 and Lamb shift, but has not yet matched the performances necessary to observe MBV. A summary of the main properties and performances of experiments aiming at MBV detection is given with focus on recent results of the PVLAS experiment. The time evolution of the missing factor which monitors the capability of an experiment to observe MBV is reported. This evolution points to MBV detection in a near future. MBV experimentation could evolve from detection to precision measurements modulo a change in scale of the experiments, if it will be possible to exploit together the peak performances achieved separately in components of different MBV experiments. Data collected with the aim of detecting MBV provide at present the best model independent limits on the coupling to two photons of (so far hypothetical) very light scalar and pseudoscalar particles
Probing dark matter with polarimetry techniques
No full textIn this work, we propose polarimetry experiments to search for low-mass
(sub-eV) bosonic field dark matter, including axions and axion-like particles.
We show that a polarimetry configuration consisting of a thick birefringent
solid inside a Fabry-P\'erot cavity is exceptionally sensitive to scalar field
dark matter, which may cause oscillatory variations in the solid's thickness
and refractive index. In addition, we show that a reconfiguration of this
polarimetry experiment, in which two quarter-wave plates are placed inside the
Fabry-P\'erot cavity instead of a thick birefringent solid, is very sensitive
to axion-like particles. We investigate the possibility of using
cross-correlation of twin polarimeters to increase the sensitivity of the
experiment, which in turn could allow us to explore unexplored parts of the
parameter space and potentially detect a signal in either dark matter scenario
The PVLAS experiment: A 25 year effort to measure vacuum magnetic birefringence
No full textThis paper describes the 25 year effort to measure vacuum magnetic birefringence and dichroism with the PVLAS experiment. The experiment went through two main phases: the first using a rotating superconducting magnet and the second using two rotating permanent magnets. The experiment was not able to reach the predicted value from QED. Nonetheless the experiment has set the current best limits on vacuum magnetic birefringence and dichroism for a field of Bext=2.5 T, namely, Δn(PVLAS)=(12±17)×10−23 and |Δκ|(PVLAS)=(10±28)×10−23. The uncertainty on Δn(PVLAS) is about a factor 7 above the predicted value of Δn(QED)=2.5×10−23 @ 2.5 T
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