The collaboration of the CDF experiment (acronym for “Collider Detector at Fermilab”) has made known the measurement of the mass of the W boson equal to MW = 80 433.5 ± 9.4 MeV, the most accurate measurement ever made. To date, this is our strongest measure, and it manifests a discrepancy between the expected value in the Standard Model and the measured one. 

The result achieved rewards the ten-year effort of an experiment to which the Italians, with the constant support of INFN, have contributed decisively, since the beginnings dating back over 40 years ago.

CDF is a historic experiment in physics of elementary particles and fundamental interactions of nature carried out at the Tevatron proton and antiproton collider at the Fermilab high-energy laboratory in Chicago.

The CDF experiment has collected data for almost twenty years, achieving countless important physics results, including the discovery in 1995 of the top quark which is the last of the six quarks envisaged by the Standard Model of elementary particles and fundamental interactions.

Another fuamental particle of the Standard Model is the W boson revealed at CERN in Geneva in 1983. It plays an important role by acting as a mediator of weak interactions and the value of its mass M_W represents one of the fundamental parameters in particle physics. Measurements of its mass were carried out with increasing precision, first using experiments at the Tevatron, then at LEP at CERN, and finally at LHC at CERN.

The final step in the Standard Model validation was the observation of the Higgs boson, obtained 10 years ago from the ATLAS and CMS experiments conducted at the LHC accelerator.

The masses of the Higgs boson (M_H),  of the top quark (M_t) and M_W are intimately correlated with each other and their precision measurement allows the Standard Model itself to be tested.

The measurement of the mass of the W carried out by the CDF experiment, described in the article published in Science, is the result of an analysis work that lasted ten years and was based on a sample of about four million candidate W events collected between 2002 and 2011. For these events the distributions of some kinematic quantities obtained from the data were studied and compared with predictions obtained from simulations obtained with reference to different possible values for M_W. In this way, after having carried out careful calibrations, we finally obtained the measurement of the mass of the W boson equal to M_W = 80 433.5 ± 9.4 MeV. Relative accuracy, about 1 part in 10,000, surpasses all other previous combined measurements. This precision is also comparable with the theoretical one, making it possible to directly compare measurements and predictions.

To realize the relevance of this result, it is important to compare the M_W measured value together with the world average obtained for the mass of the top quark Mt (the red outline in the figure on the side) with the theoretical predictions based on the measured value for the mass of the boson of Higgs (the solid blue line). A significant discrepancy can be noted between the most recent M_W and M_t measurement and the predictions of the Standard Model. This discrepancy suggests the need for improvements in the predictions of the Standard Model, or the introduction of theoretical extensions with respect to the current Standard Model.

This does not come as a complete surprise because physicists believe that the description of nature provided by this model, however accurate it may be, may be incomplete. Proof of this are the recent discoveries of neutrino oscillations and indications of the possible presence of a type of matter not yet detected in our Universe, the so-called dark matter. In conclusion, this result could be the first indication built on solid experimental foundations and very high precision measures of the existence of new physics, or physics “beyond the Standard Model”, as it is used to say.                                                                                                 

Several professors and researchers of the Department of Physics and Astronomy of the University of Bologna, together with colleagues from other Italian universities (Padua, Pavia, Pisa, Rome Sapienza, Siena, Trieste and Udine) and researchers from the National Institute of Nuclear Physics have taken part in the construction of the CDF detector, in the data taking, in the writing of the reconstruction and simulation programs, and in the analysis of the events collected by CDF.                                                  

There have also been a number of students, doctoral students and post-docs who have trained within our group over the years.  Many of them have benefited of summer periods of stay at Fermilab within a CDF “summer student” program co-financed by INFN for advanced scientific training.

An immense gratitude and an affectionate memory go to Prof. Franco Rimondi o who led the group of researchers of CDF-Bologna until his death.

Publication on “Science”

News on INFN Nazional site

 

Sushanta Tripathy, a post-doc INFN fellow at INFN Bologna Unit, was appointed by the Phsyics Coordination of the ALICE Experiment convener of the Physics Working Group “Monte Carlo generators and minimum-bias physics”. The appointment has been endorsed officially from the ALICE Collaboration Board last 10th June.

Sushanta graduated at Pondicherry Central University, in India and obtained his Ph.D. at the prestigious Indian Institute of Technology at Indore in 2019. Before joining INFN Bologna in January 2020, he had positions as visiting researcher at Lund University in Sweden and at UNAM in Mexico City as post-doc.

Sushanta’s extensive research work covers the study of hadronic resonances from proton-proton to heavy-ion collisions, including the \phi meson and the debated f0(980) state, and the study of global observables as event shapes, multiplicity and spherocity. As INFN fellow, besides leading several ALICE publications, he authored on his own several papers with few collaborators, including a review in Scientific Reports on event topology and global observables in heavy-ion collisions and a study published on Physical Review D estimating the impact parameter and transverse spherocity using machine learning (Artificial Intelligence) techniques. Since 2021, Sushanta has also joined the ERC-CosmicAntiNuclei project at the University of Bologna, working on the development of an afterburner model for antinuclei formation using Monte Carlo event generators.

In this new role, Sushanta will be a member of the ALICE Physics Board and will coordinate analyses on luminosity measurements, multiplicity, underlying event and supervision of Monte Carlo generators used in ALICE analyses. He will take service effective since September 1st.

The INFN Italian National Institute for Nuclear Physics shares the strong and firm position of the European Union and of the Italian Government to condemn Russia’s aggression against Ukraine, as well as any form of oppression among states and people, in the name of respect, confrontation and cooperation. These are the only and essential tools to have democratic states, guarantee the freedom of people, and achieve the progress of society. This is what science shows us every day as researchers, this is what we want to represent every day as a public scientific institution of a free and democratic State, these are our values.

In this deeply dramatic moment, which leaves all of us bewildered by its senselessness and brutality, we express our full solidarity with the Ukrainian people. And we assure our willingness to support the initiatives for peace and in support of people in difficulty that will be promoted by the Italian Government and the Ministry of University and Research. We are working to offer support to our Ukrainian colleagues with welcoming initiatives, and we will help to spread initiatives to help the Ukrainian population promoted by our local structures, our staff and the scientific community. INFN will also be aligned with the decisions of the Government and the Ministry of University and Research regarding the management of international scientific collaborations involving Russia. We would also like to express our deep sadness for the condition of Russian colleagues and their fellow citizens who suffer the tragic choices of their government. Science has been, is and will always continue to be, by its very nature, a place of freedom and a ground for dialogue and collaboration among people: we believe in this, as people and as scientists, and we work for this as INFN.

 

Professor Federico Boscherini, Deputy Director of the Department of Physics and Astronomy “Augusto Righi” at the University of Bologna  has been appointed Chair of the  XFEL, ( European X-ray Free Electron Laser Facility),with effect as of 1 July 2022 for a first term of two years.

The Council, is the supreme organ of the European XFEL, which decides on all important issues of the company (like the annual financial statement and the annual operation budget, and important personnel matters as well as the further development of the facility).

The European XFEL is located in the metropolitan area of Hamburg, Germany and is organized as a non-profit company under German private law (GmbH) that is publicly funded (total construction budget: 1.54 B€; operation
budget for 2022: 141 M€) through its international shareholders from 12 Europeancountries, including CNR and INFN from Italy.