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# Theoretical Physics

The theoretical physics group works on the study of fundamental interactions, beginning from first principles and arriving to the analysis of experimental data. In particular, we study the physics of elementary particles, theory of fields and strings, quantum mechanics, statistical physics, general relativity and quantum gravity with applications to cosmology.

Funding: UniBO, INFN

**Hawking radiation of black holes and condensed matter**

R. Balbinot (Assoc. Prof.)

Stephen Hawking in 1974 has shown that black holes emit thermal radiation the temperature of which is inversely proportional to their mass. While the Hawking effect is considered as a corner stone of modern theoretical physics, it was not observed experimentally because this temperature is very small, 8 orders smaller than the temperature of the cosmic microwave background radiation temperature (3 K).

An impressive analogy exists between gravity and some condensed matter systems. Indeed, the propagation of sound waves in inhomogeneous fluids is describable by means of the same equations which describe the propagation of light in a curved spacetime.

An acoustic analogue of the black hole can be constructed by means of connection between supersonic and subsonic regions. In this case, the boundary between them plays a role of the horizon. Flux of thermic phonons is analogue of Hawking radiation. The corresponding experiments have been already begun. The most promising systems are the Bose-Einstein condensates.

Collaborations: BEC Center – University of Trento, Laboratoire de Physique Théorique and Laboratoire de Physique Théorique et Modèles Statistiques, University of Paris-Sud, Orsay

http://www-th.bo.infn.it/activities/ts11/

**Particles, strings and branes**

F. Bastianelli (Assoc. Prof.), M. Cicoli (Assoc. Prof.), R. Zucchini (Assoc. Prof.), F.M. Soares Verissimo Gil Pedro (Researcher, junior), D. Ciupke (PostDoc.), V. Diaz (PhD Student), V. Guidetti (PhD Student)

One of the goals of theoretical physics is the formulation of a unified quantum theory of fundamental interactions. The string theory is one of the most advanced theories developed for this purpose. It provides a coherent description of quantum gravity, including supersymmetry, extra dimensions and extended objects (branes). It also has inspired new techniques for treatment of some standard problems of field theory. Our group works on the following particular directions:

- Application of the first quantization methods (worldline formalism) to the theory of particle interactions;
- Propagation of strings in curved spacetimes by use of nonlinear sigma models;
- Extension of gauge theories to study the dynamics of branes;
- Low-energy limits of string compactifications on the Calabi-You spaces; breaking of supersymmetry, realization of Standard Model by means of branes and cosmological applications of string theories.

Collaborations: International Center for Theoretical Physics, Trieste, University of Modena, University of Zurich, University of Cambridge, University of Oxford, University of Bonn

http://www-th.bo.infn.it/activities/pi14/

**Quantum and semiclassical gravity, black holes and cosmology**

R. Casadio (Assoc. Prof.), A. Kamenchtchik (Full Prof.), A. Giusti (PhD Student), M. Safari (Post Doc.), T. Vardanyan (PostDoc), G. P. Vacca (INFN), A. Tronconi (INFN), M. Lenzi (PhD Student)

In absence of the unified theory of fundamental interactions, physical systems at some reasonable scales of energy in astrophysics and cosmology can be studied by means of the semiclassical approach to gravity. Gravity is described by General Relativity while matter is described by means of quantum field theory, which plays the role of a source of gravitation.

One can use these methods to study both black hole physics and cosmology. The main directions of the research are the following:

1. Inflationary cosmology and anisotropy of the Cosmic Microwave Background Radiation (CMBR).

The inflation is a cornerstone of the modern cosmology which resolves many problems of the Big Bang theory of the Hot universe. The anisotropies of the CMBR represent a unique tool for studying of the physics of the very early universe, especially in light of such experiments as Planck.

2. Gravitational collapse, trans-Planckian physics and black holes.

New and important effects can arise when one takes into account the quantum nature of the process of the gravitational collapse. Trans-Planckian energies can appear in the process of evolution of black holes and can play an important role in the inflationary cosmology and in the study of the Hawking radiation.

3. Extra dimensions.

At the end of nineties some models with extra dimensions were suggested, where the fields of the Standard Model are confined on the four-dimensional brane, while gravity lives on the spacetime with more dimensions – bulk. One of their predictions is the possibility of existence of black holes with masses accessible for modern accelerators.

Collaborations: L.D. Landau Institute for Theoretical Physics, Moscow, Lebedev Physics Institute, Moscow, M.V. Lomonosov University, Moscow, Joint Institute for Nuclear Research, Dubna,

University of Los Angeles, University of Frankfurt, University of Cologne, University of Hamburg, University of Geneva.

http://www-th.bo.infn.it/activities/flag/

**Theory of multi-body quantum systems in low dimensions **

E. Ercolessi (Assoc. Prof.), L. Ferrari (Assoc. Prof.), C. Degli Esposti Boschi (CNR), G. Magnifico (PhD Student)

Recent experiments, realized due to the capacity of manipulation by ultracold atoms molecules in optical networks, have made possible the study in laboratory of new quantum effects for strongly correlated systems in low dimensionalities. These systems are examples of quantum simulators, being able to model such quantum systems as spin models, graphene, QCD, gauge theories etc.

We combine analytical techniques such as exact solutions and effective field theories with numerical simulations, based on the algorithm of Density Matrix Renormalization Group to study exotic phases of matter and phase transitions in low-dimensional strongly correlated systems. We study also the properties of entanglement and dynamical behavior of such systems.

Collaborations: University of Bari, University of Naples, MIT - Cambridge

http://www-th.bo.infn.it/activities/quantum/Homepage-ITALIANO.html

**Theoretical nuclear physics. **

P. Finelli (Assist. Prof.)

This research activity is concentrated on the applications of the methods of theoretical physics to the physics of strong interactions responsible for the internal structure of protons and neutrons and, more generally, for the nuclear structure. The topics under investigation are

- Structure of nuclei studied by means of the mean field approaches such as density functional theory, random phase approximation and optimized potential method.
- Elastic and quasi-elastic scattering of electrons as a method for measuring of the distribution of protons and neutrons.
- Physics of hypernuclei, i.e. nuclei with nonzero quantum number of strangeness.
- Nuclear superconductivity.

Collaborations: University of Pavia, University of Lecce, Technical University of Munchen,

University of Seattle.

http:/www-th.bo.infn.it/activities/Nuclear_Physics/

**Quantum field theories**

R. Soldati (Assoc. Prof.)

This activity is devoted to the study of internal consistency and analysis of possible manifestations of a modification of quantum electrodynamics which breaks the Lorentz and CPT symmetries in the presence of a Chern-Simons term or of the big galactic or extragalactic magnetic fields.

Such an area of research is relevant because the existence of big magnetic fields is confirmed and the recent observation of optical rotations, generated in the open space by magnetic fields render necessary to study the possible birefringence of vacuum, which can bring to discovery of new physics.

Graphene is a crystal lattice, essentially bidimensional, of the carbon atoms with the lattice structure of regular hexagons. From the theoretical point of view graphene can be described by a pseudo-relativistic theory of quantized field, whose elementary excitations or quasiparticles behaves as massless Dirac fermions or as charged neutrinos in 2+1 spacetime dimensions, which move in a plane with a velocity which is two orders less than the speed of light in vacuum. Recently the group looked for a theoretical explanation of the experimental results, connected with quantum Hall effect for the monolayer, the bilayer and the multilayer and for explanation of the high mobility of quasiparticles in the presence of a uniform electrostatic field.

Collaborations: University of Saint Petersburg, Rutgers University, Piscataway

http://www-th.bo.infn.it/activities/pi14/

**Integrability**

F. Ravanini (Assoc. Prof.), E. Ercolessi (Assoc. Prof.), N. Vernazza (PhD Student), D. Fioravanti (INFN)

Integrability is a fundamental property of different classical and quantum systems, which permits to find some quantities exactly and to approach non-perturbative problems, crucial for the understanding of the physical world by means of field-theoretical or statistical-mechanical methods.

The strategical goal of the project is to understand the algebraic, geometrical and analytical structures, connected to integrability and apply them to such areas as condensed matter, spin chains and their quantum entanglement, string theories, gauge theories, confinement in quantum chromodynamics, non-linear field models.

Collaborations: City University, London

#### Contacts

**Roberto Balbinot**

Associate Professor

Dipartimento di Fisica e Astronomia

Bologna (BO)

tel: +39 051 20 9 1158

**Fiorenzo Bastianelli**

Associate Professor

Dipartimento di Fisica e Astronomia

Bologna (BO)

tel: +39 051 20 9 1186

**Roberto Casadio**

Associate Professor

Dipartimento di Fisica e Astronomia

Bologna (BO)

tel: +39 051 20 9 1112

**Michele Cicoli**

Associate Professor

Dipartimento di Fisica e Astronomia

Bologna (BO)

tel: +39 051 20 9 1011

**David Simon Maria Ciupke**

Research fellow

Dipartimento di Fisica e Astronomia - DIFA

Bologna (BO)

**Elisa Ercolessi**

Associate Professor

Dipartimento di Fisica e Astronomia

Bologna (BO)

tel: +39 051 20 9 1088

**Loris Ferrari**

Associate Professor

Dipartimento di Fisica e Astronomia

Bologna (BO)

tel: +39 051 20 9 5109

**Paolo Finelli**

Assistant professor

Dipartimento di Fisica e Astronomia

Bologna (BO)

tel: +39 051 20 9 1188

**Alexandr Kamenchtchik**

Professor

Dipartimento di Fisica e Astronomia

Bologna (BO)

tel: +39 051 20 9 1114

**Francesco Ravanini**

Associate Professor

Dipartimento di Fisica e Astronomia

Bologna (BO)

tel: +39 051 20 9 1045

**Francisco Manuel Soares Verissimo Gil Pedro**

Junior assistant professor (fixed-term)

Dipartimento di Fisica e Astronomia - DIFA

Bologna (BO)

**Roberto Soldati**

Associate Professor

Dipartimento di Fisica e Astronomia

Bologna (BO)

tel: +39 051 20 9 1153

**Tereza Vardanyan**

Research fellow

Dipartimento di Fisica e Astronomia - DIFA

Bologna (BO)

**Roberto Zucchini**

Associate Professor

Dipartimento di Fisica e Astronomia

Bologna (BO)

tel: +39 051 20 9 1118