Author: vis

di Irene Celestino

“Perché siamo fatti di materia e non di antimateria?” Questa è sicuramente una delle domande più importanti e affascinanti della fisica moderna. Un quesito che riguarda intimamente l’esistenza dell’uomo e il ruolo che esso occupa nell’Universo. I ricercatori impegnati al CERN di Ginevra studiano le interazioni tra particelle sub-atomiche con sempre maggiore precisione e forse non bisognerà attendere ancora molto tempo prima di avere una risposta definitiva a questa domanda esistenziale. L’esperimento LHCb, istallato al Large Hadron Collider, è l’ultima frontiera nello studio di precisione delle proprietà della materia e dell’antimateria e sta mettendo a dura prova le certezze della fisica attuale, producendo un’innumerevole serie di risultati.

L’antimateria fu osservata sperimentalmente per la prima volta, quasi per caso, nel 1932 dal fisico americano Anderson [ref. 1] durante gli studi sui raggi cosmici, particelle provenienti dallo spazio profondo. I dati analizzati da Anderson rivelarono, in totale contrasto con ciò che lo scienziato americano aspettava, approssimativamente lo stesso numero di particelle con carica positiva e negativa. Anderson aveva osservato per la prima volta in assoluto il positrone [fig. 1], ovvero l’antiparticella dell’elettrone, dotata della stessa massa e delle stesse caratteristiche dell’elettrone, ma diversa per alcuni numeri quantici, come la carica elettrica, di segno opposto. Con il positrone entrava qualcosa di più che una nuova particella nel mondo della fisica: la relazione di simmetria con l’elettrone si annunciava come una nuova regola fondamentale del microcosmo, per cui ad ogni particella doveva venir associata un’antiparticella, persino alle particelle neutre, come il neutrone.  

Da allora i fisici sperimentali e teorici hanno fatto notevoli passi avanti nella comprensione delle particelle elementari e delle loro interazioni, tuttavia i dettagli più profondi del meccanismo di simmetria particella-antiparticella non sono del tutto noti, e non è ancora chiaro perché l’Universo, che osserviamo con i nostri potenti telescopi, sia dominato totalmente dalla materia; la presenza di antimateria è infatti rarissima.

Secondo le più moderne teorie accettate dalla comunità scientifica mondiale, nei primi istanti di vita, appena dopo il Big Bang, l’Universo doveva contenere lo stesso numero di particelle e antiparticelle. La notevole differenza nella quantità di materia e antimateria che si osserva oggi probabilmente ha avuto origine durante l’evoluzione dell’Universo dall’istante del Big Bang ai giorni nostri [fig. 2].

Nel 1967 il fisico russo Sakharov [ref. 2] dimostrò che le interazioni fondamentali tra le particelle, che hanno giocato un ruolo di primo piano durante il processo di evoluzione dell’Universo, avrebbero dovuto soddisfare alcune condizioni, tra cui violare la simmetria sotto l’operazione combinata di coniugazione di carica C (che trasforma una particella nella sua antiparticella, cambiando ovviamente anche il segno della sua carica elettrica) e la parità spaziale P (che inverte le coordinate spaziali e il moto delle particelle), la cosiddetta simmetria CP.

La violazione di questa simmetria fu osservata sperimentalmente per la prima volta nel 1964 dai fisici americani Cronin e Fitch [ref. 3], che aprirono la strada allo studio sperimentale di tale fenomeno. Fu un vero e proprio terremoto che distrusse dalle fondamenta le certezze dei fisici dell’epoca, dal momento che tutte le leggi della fisica note negli anni ‘60 erano indubitabilmente simmetriche sotto la trasformazione di simmetria di CP (gravità di Newton, elettromagnetismo di Maxwell, tutte le interazioni nucleari note all’epoca, ecc.). Da allora i fisici hanno lavorato instancabilmente alla ricerca della violazione della simmetria CP e questi studi sono tuttora di fondamentale importanza e sempre in corso nonostante gli enormi progressi ottenuti durante le ultime decadi. Attualmente si cerca spasmodicamente di scoprire sorgenti di violazione di CP di intensità molto maggiore di quella predetta nel Modello Standard, l’attuale teoria che spiega tutti i fenomeni che osserviamo oggi in Natura e che siamo in grado di creare in laboratorio nei potenti acceleratori di particelle. Infatti gli effetti dell’asimmetria presente nella teoria non sono abbastanza “forti” da spiegare perché siamo fatti di materia e non di antimateria. Trovare un processo che viola fortemente la simmetria di CP sarebbe una chiara indicazione della presenza di fisica “nuova”, non spiegabile con le conoscenze attuali.

Gli studi più avanzati, finalizzati ad approfondire la conoscenza delle particelle già note e a scoprire tracce di nuove particelle o nuove interazioni, vengono attualmente effettuati attraverso il Large Hadron Collider (LHC), l’acceleratore di particelle più potente mai costruito dall’uomo. Questo strumento genera collisioni tra protoni ad altissima energia, durante le quali vengono riprodotte condizioni simili a quelle che l’Universo doveva avere negli istanti immediatamente successivi al Big Bang.

Uno degli esperimenti attualmente in funzione installati al LHC è il Large Hadron Collider beauty (LHCb) [fig. 4, fig. 5], dove alcuni dei ricercatori della Scuola Normale Superiore svolgono le loro ricerche più avanzate.  LHCb è specializzato nella misura precisa della violazione di CP attraverso lo studio dei decadimenti di particelle composte dal quark beauty (mesoni B dotati di “bellezza”), da cui il nome LHCb, e dal quark charm (mesoni D dotati di “fascino”).  Questi mesoni si creano nelle collisioni del LHC e sono instabili, per cui tendono a decadere in frazioni infinitesime di secondo, cioè dividersi in altre particelle più leggere e stabili, conservando la somma totale di energia e di quantità di moto. LHCb è sensibile ai prodotti finali dei decadimenti dei mesoni e ne misura con precisione la carica, la massa, la velocità, e l’energia. Dai prodotti di decadimento è possibile quindi risalire al mesone originario e misurarne accuratamente tutte le proprietà fisiche, tra cui il suo “sapore”, cioè se si tratta di una particella o di un’antiparticella. Dal conteggio di particelle e anti-particelle si riesce così a misurare la violazione della simmetria di CP con grande precisione.

Recentemente la collaborazione internazionale LHCb ha pubblicato sulla prestigiosa rivista Physical Review Letters [ref. 4, ref. 5] una delle misure più precise di sempre dell’asimmetria tra materia e antimateria nei decadimenti del mesone D⁰, che non è stata ancora mai sperimentalmente osservata, nonostante gli innumerevoli tentativi fatti negli ultimi decenni.  Alla guida dell’équipe di ricercatori che ha condotto tali studi ci sono il Dr Pietro Marino (studente di dottorato della Scuola Normale Superiore, attualmente post-doc al École Polytechnique Fédérale di Losanna), il Dr. Michael J. Morello (Scuola Normale Superiore) e il Prof. Giovanni Punzi (Università di Pisa), tutti e tre associati alla Sezione di Pisa dell’Istituto Nazionale di Fisica Nucleare. I ricercatori pisani, sfruttando il campione più abbondante mai raccolto di decadimenti del mesone D⁰ e della sua antiparticella, l’anti-mesone , hanno misurato con estrema precisione il tempo di vita di queste particelle (~0.5 pico-sec) dall’istante in cui sono state create sino all’istante in cui si sono disintegrate.  Hanno poi misurato l’asimmetria, data dal rapporto tra la differenza e la somma del numero misurato di e , ed in particolare la sua variazione in funzione del tempo di vita, anche nota come parametro , cercando segnali di violazione della simmetria CP in regioni mai esplorate prima dall’uomo. Un valore di  diverso da zero indicherebbe in modo inequivocabile violazione della simmetria di CP nel sistema dei mesoni .

Nonostante la rivoluzionaria precisione raggiunta dalla misura attuale, il valore ottenuto per il parametro  è ancora compatibile con zero all’interno dell’errore sperimentale, e quindi non spiega la situazione attuale, cioè la totale preponderanza della materia sull’antimateria.

LHCb però è solo all’inizio del suo cammino e nei prossimi anni raccoglierà molti più dati di quelli attualmente a disposizione, permettendo ai ricercatori di spingersi a precisioni ancora maggiori in un territorio mai esplorato prima.

La vita dei ricercatori però non sarà affatto facile e libera da ostacoli. La misura attuale ha già richiesto di superare innumerevoli sfide sperimentali, come la necessità di raccogliere un enorme numero di decadimenti di interesse e di sviluppare tecniche sperimentali raffinate e innovative, in modo da eliminare tutte le asimmetrie spurie che possono pericolosamente falsificare la misura.

Nonostante queste difficoltà i ricercatori non vedono l’ora di analizzare i nuovi dati che LHCb fornirà nei prossimi anni ad alta luminosità e di navigare verso l’ignoto, affrontando e superando le difficoltà che si porranno davanti al cammino della conoscenza.

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[1] Carl D. Anderson, “The Positive Electron”,  Phys. Rev. 43, 491 – Published 15 March 1933. https://journals.aps.org/pr/abstract/10.1103/PhysRev.43.491

[2] A. D. Sakharov (1967). “Violation of CP invariance, C asymmetry, and baryon asymmetry of the universe”. Journal of Experimental and Theoretical Physics 5: 24–27., republished as A. D. Sakharov (1991). “Violation of CP invariance, C asymmetry, and baryon asymmetry of the universe”. Soviet Physics Uspekhi 34 (5): 392–393.

[3] J. H. Christenson, J. W. Cronin, V. L. Fitch, and R. Turlay, “Evidence for the 2π Decay of the K02 Meson”, Phys. Rev. Lett. 13, 138 – Published 27 July 1964. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.13.138

[4] LHCb Collaboration, Measurement of the CP violation parameter AΓ in D0→K+K− and D0→π+π− decays, Phys. Rev. Lett. 118, 261803, https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.261803

[5] P. Marino, Measurement of the CP violation parameter AΓ in D0→K+K− and D0→π+π− decays, CERN-THESIS-2017-007, http://cds.cern.ch/record/2248481?ln=en

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At the center of almost every galaxy around us, we found super-massive black holes with a mass of million or billion times that of the Sun. In our galaxy, the Milky Way, there is a black hole with about 4 million solar masses. Some of these black holes are active and devour huge amounts of gas, irradiating their own host galaxy. Others, like the Milky Way one, are more quiet.

Current theories successfully describe the formation process of small black holes, up to tens of solar masses. These objects are formed with the collapse of stars more massive than the Sun, at the end of their life. From small black holes, of “stellar mass”, it is possible to form super-massive black holes through a growth process, accreting gas from the surrounding environment or merging with other black holes. Nonetheless, this process requires a long time, billion years, to finally reach the size of super-massive black holes.

As usual, nature has some surprises for us: these super-massive black holes not only are commonly observed at the center of close-by galaxies, but they have been localized also in extremely far away galaxies, very old ones, existed when the Universe was 800 million years old. A small fraction when compared to the 13-14 billion years of its current age. Understanding how it was possible to form black holes of billion solar masses in such a short time is one of the biggest mystery in modern cosmology.

One of the theories proposed to solve this puzzle states that, in the peculiar conditions of the early Universe, black holes formed in the first billion years form the Big Bang were much more massive than the ones formed at the end of the life of a modern star. Not tens, but up to 100,000 times the mass of the Sun. These objects are named direct collapse black holes. As the name suggests, they should have been formed from the direct collapse, without fragmentation, of huge amounts of primordial gas. Instead of forming 100,000 stars similar to the Sun, they formed just a single, collapsed, object of 100,000 solar masses. The existence of these intermediate-mass objects would help in the understanding of the observations indicating the presence of super-massive black holes in the early Universe. They would then represent the missing link between small black holes, formed from stars, and super-giant ones, found at the center of galaxies. Direct collapse black holes are objects imagined by astrophysicists, but never observed before.

In a very recent study led by Fabio Pacucci and Andrea Ferrara, cosmologists at the Scuola Normale Superiore, the authors believe that they have found a simple and effective method to observe the first black holes formed in the Universe. This study suggests that it is possible to select these objects, among the billions populating the sky, looking at their colors. Observed through special photometric filters (that allow to select some wavelengths, similarly to photographic filters), these objects should appear much redder than other objects observable in the early Universe. Fabio Pacucci, first author of the study, says: “This reddening should be caused by the huge amount of gas present in the host galaxy, and collapsing toward the central object. The light emitted from the environment surrounding the central black hole would lose energy crossing it and would become redder, less energetic.” We predict that the reddening should decrease with time, because the gas present in the host galaxy decreases as well. Similarly to the daily cycle of the Sun, at the rise of their life these black holes are very red, progressively turning whiter with the approaching of noon.

This method was applied from the authors of the study. They found two very ancient objects that could be the first black holes of this class ever discovered. These objects were observed in the visible light and in the infrared from the NASA space telescopes Hubble and Spitzer, inside the southerner constellation of the Fornax. Furthermore, these objects are clearly detected in the X-rays from the NASA space telescope Chandra. The emission of high-energy radiation, like the X-rays, strongly suggests that these objects are really black holes. These two objects were formed when the Universe was younger than one billion years, less than one tenth of the current age.

Andrea Ferrara, Cosmology professor at the Scuola Normale Superiore, says: “If this discovery will be confirmed, out vision of the formation mechanisms of super-massive black holes would dramatically change”. Direct collapse black holes would represent the progenitors of local super-massive black holes, localized at the center of most galaxies. Consequently, also our understanding of the formation process of galaxies would be improved.

Author: Claudia Cristalli

On November the 4th and 5th the Scuola Normale Superiore in Pisa hosted the international meeting Science Outreach. A Chance for development. Perspectives in Italy and China, organized by VIS in collaboration with the Confucius Institute of Pisa, which sponsored the event. The cooperation and support of Confucio’s directors Wu Xueyan and Alberto di Minin and staff Giada Alì and Ester Armentano was precious for the good result of the event. The aim of the meeting was to explore what Outreach means and what can be done to render Outreach more effective. Chinese and Italian leading cultural institutions employ different strategies in the communication of scientific results to a larger, non-specialized public; the meeting was therefore a good occasion for comparison. The lively discussion that followed was moderated by Andrea Ferrara, VIS director. What emerged is a strong common perspective on how Outreach activity has to be conceived; its instances, however, are often quite different in practice.

1. The first question on the table was: What is Outreach? All participants in the meeting were unanimous in stressing the wide range of communication tools that this activity requires. “Outreach” is not meant to be just “diffusion” or “popularization” of knowledge, since the popularization-model implies a hierarchical structure, with information flowing from a privileged source (at the top) to a passive mass (lying at the bottom). In fact, what happens during effective outreach – as well as during effective teaching – is that the interest of the auditory is awaken, and at this point listening is anything but a passive activity.

Katherine Isaacs (UniPi), coordinator of the event Bright. The researcher’s night in Tuscany, an European outreach manifestation that finds place every fourth Friday of September, has highlighted three aspects that may define the aims of Outreach: (1) To spread the cultural influence of Universities beyond the boundaries of seminars, professor’s bureaus, and research labs, to the surrounding social tissue; (2) To stress how research and teaching activities are mutually interdependent and able to enrich each other; (3) To elaborate not only a program, but an attitude of responsibility and respect, which has to be a structural component of the developmental strategy of every scientific and cultural institution.

Michele Lanzinger, director of the MuSe of Trento, while sharing with Isaacs roughly the same theoretical background, gave his personal insight on Outreach as based on the MuSe experience. Therefore, Lanzinger spoke of (1) an effective positive repercussion on local economy of the Natural History Museum, of the Astronomic Observatory, and of the daily research pursued by geologist working within the walls of the MuSe or at their African dislocation, among the Udzungwa Mountains, a National Park in the Eastern Arc Mountains, Tanzania. Moreover, (2) the contemporary need of lifelong learning parallels (on the non-specialized side) the need to defend the interplay between teaching and research among professional scientists. A well-organized cultural institution can give some important contribution to both those needs. The awareness of having not only children but also professionalized adults among the public surely makes the case for communicating research outside the boundaries of the Academy even stronger. (3) As a consequence of this, it is of paramount importance to elaborate a new, non-hierarchical standard of communication. Also the terminology stressing the difference between specialized and non-specialized people should be abandoned, since it is of no use in improving understanding of any kind. The ideal science communicator will manage to address simultaneously a wide range of different people, keeping in mind the different backgrounds to which they belong.

Marcos Valdes, VIS coordinator, maintains that the Outreach program of the Scuola Normale Superiore of Pisa is going in the same direction: “Here in Pisa the people responded very well to our events, with a strong participation both at the public conferences [I’ll tell you the discovery that changed my life] and at the CAVE3D-visits. It means that VIS (and Outreach in general) meets a real need, that is, to get closer to the world of research, either scientific or humanistic (I am thinking now at our digital reconstruction of Segesta’s archaeological site, Sicily)”. Valdes draws on Isaacs’ points while bearing Lanzinger’s results in mind: first of all, (1) the integration between research and communication must also act as a firewall against the diffusion of viral scientific disinformation, which diverts and manipulates the public opinion. Therefore, Outreach has (1.1) to develop a policy of information on the web and (1.2) to foster a critical attitude towards any content prior and above the transmission of any particular content. Secondly, (2) there is an ethical dimension in Outreach which Valdes reports in very down-to-earth terms: since public research is a tax-funded activity, it is the duty of the research institutions to give a feedback to the citizens who are financially sustaining them. Thirdly, (3) a public opinion positively concerned about research and sensible to “good science” may influence political choices and the allocation of resources. Outreach therefore will promote a virtuous cycle between the spreading of scientific information and the assignment of research grants to virtuous institutions, which on their turn will contribute to the general level of scientific awareness in society. And so on.

Briefly put, Outreach can be defined as that attitude of responsibility and respect which pushes a cultural institution to open its doors both locally (via conferences and meetings) and globally (via the World Wide Web). The Outreach activities focus on researchers and scientists as individuals, and actively promote the cultural and economical development of the territory. Moreover, the Outreach program has a political and ethical relevance, since it is committed to the diffusion and defense of a scientific mentality and – more broadly but most importantly – a critical attitude in society.

2. To open Universities and research spaces to the general public: this is what characterizes mostly the Outreach activities in Chongqing. Chongqing is a very populous metropolitan area (29.914.000 inhabitants in 2014) situated in the South-West of China. Together with Beijing, Shanghai and Tianjin, Chongqing is a direct-controlled municipality, which does not mean independent governance but a direct control by the central government; it is the highest possible administrative status for cities and their surrounding territory. In particular, Chongqing is by far the largest (82,403 km2) and the most populous municipality. It has a very good Science and Technology Museum, with free entrance for children and high school students, and many other Museums, such as the Natural History Museum. Liu Hui, Physics professor at Chongqing University, described the structure and the timing of Outreach activities: the National Science Popularization Day on the third week-end of September, and the Science and Technology National Week on the third week of May. In both occasions Universities open their doors to the general public, and visitors – mostly children accompanied by parents and high-school students – explore the various panels and scientific installations arranged by both undergraduate and graduate students. Conferences and talks are held by professors, but the greater part of scientific communication happens among quasi-peers, all still school-aged. Also the conferences or open lessons held by professors are mostly visited by school-aged boys and girls: “Adults and retired people are not really interested in those manifestations, and they also come to visit Science Museums only when drawn in by their children or grandchildren. Adults may also actively bring their children or grandchildren to the museum, but only in so far they realize that it is good for their formation. However, since they do not see any immediate utility of such a knowledge in their daily work, they tend not to be interested in it”, Hui said. This attitude towards culture, which, according to Han Zhong, is diffused in the whole Country, has already begun to change: starting from Hong-Kong. As Ming-Chung Chu, cosmologist, reports, “Twenty years ago it was the same by us as well. Our public was almost entirely composed by students. When I gave an open lesson on a scientific topic, my audience was mostly composed by students who were thinking about enrolling in the Department of Physics. Now the situation has changed, and roughly 50% of our audience is composed by adults, workers or retired people, who come without children. They come on their own because they have a personal interest in what we have to say”.

This can be considered the very first step of any Outreach activity: to engage the people’s interest and to awake their curiosity in science and in the persons who do science. How to obtain this first fundamental result is however still problematic.

3. When the question is how to awake the interest of people in science, there are several different approaches that can be considered. Each has both advantages and disadvantages.

Giordano Mancini works for the SNS’s Center DreamsLab which collaborates with VIS on outreach activities, and reported his experience as follows: “We wanted to captivate the interest of our public; therefore, we worked to create a very immersive experience, that is, to present some most interesting results of contemporary research within an extraordinary perceptual context. Therefore, I worked at programming the software needed to exploit the potentiality of our CAVE3D and of the Oculus Rift. We can now visualize the structure of a molecule or of a neural net, walk into a reconstruction of Segesta’s Agorà or travel through time in a simulation of much earlier stages of the Universe’s development. All reconstructions are firmly grounded on scientific data (or biological samples, in the case of the neural net): it is not a big game, even if it may look like a game”. Actually, this is the greatest risk of this communication style: to make science and scientific research nothing but a pleasant entertainment, or an emotional experience. A second risk is that of shifting the attention of the public from the scientific content to the technology used to display it.

Davide Dalpiaz works as cultural mediator and is in charge of the digitalization at MuSe. He has therefore an experience of the use of technology to communicate science on a much larger scale than VIS. Even if his experience is very positive, both Dalpiaz and Lanzinger are looking towards new challenges: “We wanted to give each visitor a digital instrument that would have allowed her/him to organize the visit in a free and autonomous way. Thanks to this instrument [iPad mini] it is possible to watch at a story on many of our exhibits, just while stepping by them. However, we now have many visitors alone looking at a screen and not interacting with each other. How can we make the MuSe experience to become not only a private experience but also a shared experience?”. Technology seems to need the corrective action of games, especially role-playing games. However, this strategy also presents several limits, among which the most obvious one is the fact that it is not clear how much of the scientific attitude towards a problem can be satisfactorily communicated by a game.

4. A different but complementary proposal comes form Elisabetta Baldanzi (INO, National Optic Institute – CNR, National Research Center). Baldanzi is a researcher in photometry (the measurement of the intensity of light or relative illuminating power) with interests in vision’s psychophysics and spectrophotometry, and an important figure in the Italian Outreach’s landscape: recently she has been awarded – ex-aequo with Marcos Valdes – the SIF [Italian Society of Physics] prize for Scientific Communication. Her approach to Outreach is based on the will to integrate different culture domains. Enhancing science does not mean to promote scientism, and Baldanzi’s Outreach events are projected together with the exhibition of some works of art or with a concert. The main advantage of this strategy is that, thanks to the composite nature of the content it offers, it is able to stimulate the curiosity of people with very different interests. One person may come because of an interest in lasers, the other because of an interest in the physiology of perception, a third because of Rembrandt’s The Night Watch. Moreover, this strategy also gives visibility to the historical dimension of science, thereby contributing to bridging the cultural gap between scientific sciences and human sciences.

5. The meeting thus closes with a number of good intentions from all parts. In particular, everybody hopes for a close cooperation between the CNR, INO, & VIS in Italy, and Chongqing & Hong-Kong Universities in China. In this context, technology could actually have an important role in making the physical distance between the two Countries practically negligible. Marcos Valdes proposed therefore to focus this initial stage of cooperation on the development of more software solutions for visualizing scientific results, especially for Oculus Rift. That is all for now, and the details of the program remain to be written.

Dietro le quinte della scoperta del Bosone di Higgs.

Il 12 marzo 2014 il Prof. Luigi Rolandi della SNS, uno dei ricercatori principali al CERN di Ginevra, ha inaugurato il ciclo di Conferenze Pubbliche di VIS. Oltre 150 persone hanno assistito alla conferenza tenutasi in Sala Azzurra alle ore 18:00 e almeno 70 hanno poi partecipato alla mostra guidata a tema nella storica sede della Biblioteca nel Palazzo del Capitano. Di seguito il video integrale della conferenza.

La Conferenza di presentazione di VIS si terrà mercoledì 5 marzo alle ore 11:00 in Sala Azzurra (SNS, Palazzo della Carovana) alla presenza di stampa, autorità e di vari invitati tra cui i presidi degli Istituti Superiori di Pisa e della regione. Lo scopo è di illustrare le attività del progetto VIS e il suo ruolo nell’ambito della divulgazione scientifica e della formazione degli insegnanti e degli studenti. L’incontro sarà aperto a tutto il pubblico interessato.