Il Nobel per la Fisica 2021 è stato assegnato all’italiano Giorgio Parisi, fisico teorico dell’Università Sapienza di Roma e dell’Istituto Nazionale di Fisica Nucleare (Infn) e vicepresidente dell’Accademia dei Lincei. Parisi è stato premiato per le sue ricerche sui sistemi complessi.

Giorgio Parisi divide il premio Nobel per la Fisica a metà con Syukuro Manabe e Klaus Hasselmann. I due ricercatori hanno avuto il riconoscimento per le loro ricerche su modelli climatici e il riscaldamento globale.

Il Presidente della Repubblica Sergio Mattarella ha espresso grande soddisfazione per il conferimento del Premio Nobel per la fisica al professor Giorgio Parisi e gli ha rivolto le piu’ grandi congratulazioni per questo altissimo riconoscimento che rende onore all’Italia e alla sua comunita’ scientifica. Lo rende noto il Quirinale.

ECCO IL CURRICULUM INTEGRALE DI GIORGIO PARISI

Giorgio Parisi was born in Rome August 4, 1948, he completed his studies at the university of Rome and he graduated in physics in 1970 under the direction of Nicola Cabibbo. He carried out his research at the National Laboratories of Frascati, first as fellow of the CNR (1971-1973) and later as a researcher of the INFN (1973-1981). During this period he made long stays abroad: Columbia University, New York (1973-1974), Institut des Hautes Etudes Scientifiques, Bures-sur-Yvettes (1976-1977), Ecole Normale Superieure, Paris (1977-1978). He was nominated full professor at the University of Rome in February 1981; from 1981 to 1992 he was a professor of Theoretical Physics at the University of Rome Tor Vergata. Currently (since 1992) is Professor of Theoretical Physics at University of Rome La Sapienza. He wrote more than six hundred articles and contributions to scientific conferences and he has authored four books. In his scientific career, he worked mainly in theoretical physics, addressing topics as diverse as particle physics, statistical mechanics, fluid dynamics, condensed matter, the constructions of scientific computers. He also wrote some papers on neural networks, immune system and the movement of groups of animals. His works are extremely well known. In the Google Scholar database (http://scholar.google.com/citations?hl=en&user=TeuEgRkAAAAJ&pagesize=100&view_op=lis t_works) we can count about 700 works with more than 70.000 citations and a H-index 110. The text of the last 385 works can be found in the archives (http://arxiv.org/find/all/1/all:+parisi_g/0/1/0/all/0/1). In 1992 he was awarded the Boltzmann Medal (awarded every three years by I.U.P.A.P. on Thermodynamics and Statistical Mechanics) for his contributions to the theory of disordered systems and the Max Planck Medal in 2011, from the German physical society. He also received the Feltrinelli Prize for Physics in 1987, the Italgas Prize in 1993, the Dirac Medal for theoretical physics in 1999, the Italian Prime Ministe Prize in 2002, the Enrico Fermi Award in 2003, the Dannie Heineman Prize in 2005, the Nonino Prize in 2005, the Galileo Prize in 2006, the Microsoft Prize in 2007, the Lagrange Prize in 2009, the Vittorio De Sica Prize in 2011, the Prix des Trois Physiciens in 2012, the Nature Award for Mentoring in Science in 2013 and the 2015 High Energy and Particle Physics Prize by the EPS HEPP Board, the Lars Onsager Prize in 2016. He received in 2010 a first senior ERC grant and in 2016 a second senior ERC grant. He is a member of the Accademia dei Lincei, the Accademia dei Quaranta, the Académie des Sciences, the U.S. National Academy of Sciences, the European Academy, the Academia Europea and the American Philosophical Society.

**Research highlights**

In his research career, Giorgio Parisi has given many seminal and widely recognized contributions in different areas of physics that have been widely recognized. Some of his most interesting contributions to physics are presented below. They are divided into areas in spite of the fact that different subjects are strongly related and many ideas came from experiences doner om the experience in other fields, as sometimes can be seen from the names (e.g. quenched gauge theories, where the name quenched comes from spin glasses).

**Particle Physics**

Giorgio Parisi started the phenomenological study of scaling violations in deep inelastic electron scattering in a field theory framework: this study culminated in the equations (with Altarelli) for the evolution of the parton densities. The Altarelli-Parisi equations are at the basis of perturbative QCD computations in proton-proton collisions that have been recently verified with very high accuracy at LHC, in the same experiments where the Higgs have been discovered. Giorgio Parisi has introduced the flux tube model of quark confinement based on the analogy of magnetic confinement of monopoles in superconductors, that is the best explanation of quark confinement. With Brézin, Itzykson, Zuber Giorgio Parisi started the detailed study of the planar diagram approximation in field theory. This approach has been the starting point of nonperturbative studies of quantum gravity in one dimensions. In this context, he has also introduced and studied the one-dimensional supersymmetric string. With Fucito, Hamber, Marinari and Rebbi, Giorgio Parisi started the study of lattice gauge theories with Fermions, both in the quenched approximation and in the unquenched case, introducing the first numerical method of Bosonization (i.e. pseudofermions). Giorgio Parisi was one of the proponents and the scientific coordinator of the APE Project, that was one of the first dedicated supercomputers for lattice gauge theories.

**General Statistical Physics**

At the beginning of the 1970’s the field-theoretical renormalization group approach to second order phase transitions was formulated only in 4-epsilon dimensions. Giorgio Parisi presented a formulation of the renormalization group at fixed dimension, both clarifying many fundamental aspects and paving the way to very accurate determinations of the critical exponent in 3 dimensions. With Sourlas Giorgio Parisi introduced the dimensional reduction that allows to connect properties of some systems in dimensions D with those of other systems in dimensions D-2. Giorgio Parisi started the study of multifractals, (multifractals are a generalization of fractals). With Benzi, Paladin and Vulpiani, he introduced the mechanism of Stochastic Resonance. Both contributions had a very large influence. New algorithms have been introduced to simulate the equilibrium behavior of complex systems: the simulated tempering, that Giorgio Parisi invented with Marinari, simulated tempering (with Marinari), that evolved into the parallel tempering algorithm (which is now the state-of-the-art in the field).

**Collective Animal Behavior**

Giorgio Parisi and his collaborators have been recently the first ones to obtain data on the three-dimensional behavior of large animal groups. They have measured the three-dimensional positions of flocks of starlings. The number of birds simultaneously observed was of the order of few thousands and this result extends previous measurements of two orders of magnitude. The techniques developed open the possibility of developing a quantitative study of the collective three-dimensional behaviour of large animal groups.

**Disordered Systems**

In the framework of Anderson localization Giorgio Parisi introduced (simultaneously and independently from Wegner) the symmetry group O(n|n), that has been the starting point of most of later investigations. With Kardar and Zhang, Giorgio Parisi introduced a model for growth of surfaces in a random media (or in presence of a random deposition). The KPZ model became a standard in this field. Very carefully planned recent experiments agree very well with the theoretical predictions. Many of his most original contributions are related to spin glasses, starting from the analytic solution of the Sherrington-Kirkpatrick model, a model that became the prototype of a physical complex system. In this case Giorgio Parisi introduced the technique of breaking the replica symmetry was successfully introduced for the first time. In the following years, this technique widely spread to many different research areas (e.g. neural networks, optimization theory, glass physics). Giorgio Parisi also found the physical interpretation of the solution, where two unexpected phenomena were discovered: the fluctuations of intensive quantities and the ultrametricity. These results have been rigorously proved after 30 years of effort by Talagrand (2003) and Panchenko (2013). Giorgio Parisi also gave important contributions to the study of finite dimensional spin glass systems. He obtained key results both for the equilibrium states and for the out of equilibrium behavior. He gave a general argument for the validity of the Generalized Fluctuation-Dissipation relation, conjectured by Cugliandolo and Kurchan: this relation has been verified first in numerical simulations and later by Hérrison and Ocio in experimental spin glass samples.

**Optimization**

Giorgio Parisi obtained analytic results for matching, bipartite matching and traveling salesman problems in the random case. Some of these results were rigorously provend 15 years later by Aldous. He has also presented a conjecture on the average minimum cost of bipartite matching over N elements. This conjecture was finally provend, leading to very interesting mathematical developments in combinatorial analysis. With Mézard, Giorgio Parisi started the study of the cavity approach on random Bethe lattices that opened the possibility of solving a large number of models of wide interest in combinatorial optimization. In particular, using these methods, it was possible to compute exactly the threshold for satisfiability in the celebrated random K-Sat model. The rigorous proof of these results has been obtained by Ding, Sly and Sun in 2015. The previous analytic studies have prompted the development of new very fast solving algorithms: the decimation method based on survey propagation (eventually with backtracking) is much more efficient than other solving algorithms on random problems. Very recently these methods have been successfully applied to the study of compressing sensing, that is a very important theoretical problem with many practical applications.

**Structural Glasses**

The replica theory developed by Giorgio Parisi it at the bases of the Random First Order theory of structural glasses, that is one of the most promising theories of the glass transition. Giorgio Parisi has strongly contributed to the development of this theory. With Mézard, he has done a first-principles computation of the thermodynamical behaviour in the glassy phase of soft spheres, thatspheres that has been later extended to binary mixtures and hard spheres by him and Zamponi. In 2012 he has developed a renormalization group approach to the study of the glass transition that allows us to estimate the range of validity of the mean field computations. As a consequence of these works, our understanding of the glass transition is reaching the same quality of that of standard phases order transition. In 2015 Giorgio Parisi and his collaborators have been able to define and to solve the mean field theory for jamming of hard spheres by considering a hard spheres fluid in the infinite dimensional limit. This hard spheres model can be solved when the space dimension d goes to infinity using the replica symmetry breaking theory that was developed by him in the eighties.

The results were fully unexpected: • There is an unexpected phase transition at high pressure: the Gardner (Gross-KanterSompolinski) transition characterized by a divergence of the correlation time and by a higher temperature phase where there are many ergodic components (replica symmetry breaking). Stimulated by this work both numerical and experimental evidence (Seguin Dauchot, 2016) has been found for this transition. • In the infinite pressure limit we reach jamming. At the end of the day one finds that the exponents that characterize jamming con be computed analytically, in very good agreement with the results of the numerical simulations and of the experiments. For example, the exponent γ (that control the distribution of the gaps between the spheres) has analytically the value γ = 0.41269, to be compared to the expected value γ = 0.40 ± .02. It is the first time that the exponents of a mean field theory are not simple rational number. These exponents are in very good agreement with experiments (in dimensions 2 and 3) and with accurate numerical simulations in dimensions from 2 to 8. These results have arisen a wide scientific interest, as proved by a recent large grant from the Simons foundation to a collaboration (Cracking the Glass Transition) that aims to elucidate aspects of this approach. Furthermore the paper The simplest model of jamming (written with Silvio Franz) has been chosen as one of three winners of the Journal of Physics A Best Paper Prize in 2017.