Roberto Frezzotti

Roberto Frezzotti is a theoretical particle physicist working on Quantum Field Theory, Monte Carlo simulations of lattice QCD, or QCD+QED, and their applications to problems of elementary Particle Physics, with special focus on Flavour and Beyond-Standard Model Physics. He carried his first steps in research within groups and/or collaborations led by prof. Roberto Petronzio and prof.  Martin Luscher. He subsequently collaborated with several European leading scientists, among whom prof. Giancarlo Rossi, Rainer Sommer, Karl Jansen, Chris Michael, Guido Martinelli, Olivier Pene, Vittorio Lubicz, Constantia Alexandrou. He held temporary research positions at DESY (Hamburg), Max-Planck-Institut fuer Physik (Munich), Univ Milano-Bicocca and CERN before joining as a staff member the Physics Department of the Univ. Roma Tor Vergata. He has served as deputy-coordinator at national level for the sector of theoretical physics within the Physics group evaluation experts (GEV) in Italy's procedure of research evaluation VQR 2015-2019.

He is a founding member of the European/Extended Twisted Mass Collaboration (ETMC), one of the world-level leading groups in the study of QCD (and QCD+QED) in its non-perturbative regime, hadron structure and flavour physics for precision tests of the Standard Model (SM). He served as a key scientist in 5 PRIN/IT research projects, ~10 PRACE/EU and EuroHPC projects on Tier0 computers and  as P.I. in the projects: "LIBETOV" (2014,UTOV), "PLepNuGam" (PRACE/EU, call 17) on flavour physics with isospin breaking effects; two EuroHPC Extreme Access projects on the muon g-2 Hadronic Vacuum Polarization contributions (2023-24, 2024-25). He was member of the LOC of the international conference Lattice2010. He is an expert referee of Physical Review D.

He is coauthor of 80 peer-reviewed papers + 83 publications with >8400 (two with 500+) citations and h-index=48 on HEP-INSPIRE; >4850 citations and h=41 on Scopus; >9100 citations and h=49 on Google Scholar; top-250 rank in the “Top Italian Scientists”. His main research results are summarized below and were presented in several seminars and about two dozens of invited talks, among which four plenary talks.

a) Fermionic regularizations for lattice QCD: he introduced and developed the formulation known as "twisted mass lattice QCD". With a 90 degree angle of "chiral twist" between the Wilson term and the soft quark mass terms in the Lagrangian this formulation allows to evaluate (by means of simulations whose numerical stability is well controlled by the quark mass) all physical quantities with sistematically reduced lattice artifacts. He contributed to the foundation of the "European Twisted Mass Collaboration", recently renamed as "Extended Twisted Mass Collaboration" (ETMC), one of the world-level leader groups in the study of QCD (and QCD+QED) in its non-perturbative regime, hadron structure and flavour physics for precision tests of the Standard Model (SM). 

b) Simulation algorithms for lattice QCD with two or four dynamical flavours: He developed and optimized the "Polynomial Hybrid Monte Carlo" simulation algorithm. He studied the phase structure of Wilson lattice QCD.

c) Development and application of non-perturbative renormalization methods that are necessary to obtain from lattice QCD simulations combined with experimental data first principle determinations of a few basic parameters of the SM, such as quark masses or CKM matrix elements, and of certain physical observables that constrain several beyond-SM models.

d) By means of numerical lattice simulations of isospin symmetric QCD he carried out:
- the computation of hadron masses, the QCD Lambda-parameter, the mass of u, d, s, c and b quarks, the leptonic decay constants of pions, kaons, D- and B-mesons; - the computation of some low energy constants of the effective chiral Lagrangian for QCD and the study of the pion form factor; - the computation of several matrix elements of the effective weak Hamiltonian, e.g. the ones that control the K0-antiK0 oscillations within the SM and its extensions; - the computation of the inclusive semileptonic decay rate of the Ds meson; - the computation of hadronic contributions to the photon vacuum polarization that are of utmost relevance for the SM prediction of the anomalous magnetic moment of the muon, as well as of related observables, such as the energy smeared R-ratio for e+e- into hadrons and the inclusive decay rate of the tau meson in the ud and us flavour hadronic channels (plus neutrino).

e) In the last few years within ETMC and the RM123 group, he has contributed to lattice studies of QCD+QED, i.e. to the inclusion of the leading isospin breaking effects due to electromagnetic corrections and up-down mass difference in the lattice computation of several observables These effects, although small (at the percent level), are crucial in certain hadronic quantities that are already very precisely computed in order to carry out indirect searches of new physics. Among these observables an outstanding phenomenological role is currently played by the hadronic vacuum polarization to anomalous magnetic moment of the muon, the energy smeared R-ratio and the inclusive hadronic decays of the tau meson - on which large scale computational projects within ETMC are ongoing and quite advanced.

f) Finally, together with collaborators in Italy and in Germany, he has also worked at the formulation and the numerical check via lattice simulations of an original mechanism for "natural" dynamical generation of elementary particle masses, based on a previously unnoticed "non-perturbative anomaly" in gauge theories with both fermion and scalar fields. How much fundamental or effective is the Higgs mechanism description of the masses of elementary fermions and weak gauge bosons and of their relation with the electroweak scale represents a key problem that remains open in particle physics, given success of the SM and the lack of evidence for new physics at the LHC accessible energy scales. Addressing such questions may be helpful to determine the scale of beyond-SM physics and have an impact on other open problems, such as the observed matter-antimatter asymmetry, the nature of Dark Matter or a possible unification of gauge couplings.

In the framework of the aforementioned activities he participated in several supercomputing projects at national (Italy,Germany, France) and EU (PRACE and EuroHPC calls) level. He recently led two Extreme Access Call EuroHPC projects  rooted in the ETMC research on the hadronic vacuum polarization contributions to the anomalous magnetic moment of the muon and carried out using the pre-Petaflop supercomputer Leonardo at Cineca, Bologna (in addition to several other machines accessible to ETMC worldwide).