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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
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)
Experimental perspectives in (low-energy) photon-photon scattering
No full textThe possibility of photon-photon scattering is a striking difference between classical and quantum electrodynamics. This genuinely quantum feature is made possible by the fluctuations of charged fields, and it makes quantum vacuum a nonlinear optical medium. Photon-photon scattering is thus a delicate probe into the structure of quantum electrodynamics and any departure from the expected behavior would be a powerful signal of "new physics". To date this process has never been observed – except as a radiative correction to other processes – and several experiments are trying to detect it at very low energy, in the scattering of real photons in powerful light beams off the virtual photons of intense magnetic fields. Here we briefly review the experimental state-of-the-art, with special emphasis on the PVLAS experiment, and we describe a new proposal to observe photon-photon scattering in the range 1 – 2 MeV
A polarisation modulation scheme for measuring vacuum magnetic birefringence with static fields
Get PDFA 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
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The PVLAS experiment: measuring vacuum magnetic birefringence and dichroism with a birefringent Fabry–Perot cavity
Get PDFVacuum magnetic birefringence was predicted long time ago and is still lacking a direct experimental con- firmation. Several experimental efforts are striving to reach this goal, and the sequence of results promises a success in the next few years. This measurement generally is accompa- nied by the search for hypothetical light particles that couple to two photons. The PVLAS experiment employs a sensitive polarimeter based on a high finesse Fabry–Perot cavity. In this paper we report on the latest experimental results of this experiment. The data are analysed taking into account the intrinsic birefringence of the dielectric mirrors of the cavity. Besides a new limit on the vacuum magnetic birefringence, the measurements also allow the model-independent exclu- sion of new regions in the parameter space of axion-like and milli-charged particles. In particular, these last limits hold also for all types of neutrinos, resulting in a laboratory limit on their charge
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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