A new, extremely precise measurement of the mass of the W boson, the particle whose
discovery in 1983 earned Carlo Rubbia - a former student of the Scuola Normale - and Simon Van der Meer the Nobel Prize in Physics, was presented a few days ago at CERN.

PISA, 24/9/ 2024. The result is a new and important confirmation of the validity of the Standard Model, the currently accepted theory of elementary particle physics.

The long-awaited measurement was obtained using data collected by CMS, one of the large experimental apparatuses located along the Large Hadron Collider (LHC) tunnel. The CMS international collaboration involves thousands of physicists and engineers, a substantial number of whom are Italian.  The new value of 80360.2 +- 9.9 megaelectronvolts (the energy corresponding to the particle’s mass, according to the famous E= mc,2 equation was presented on Tuesday, September 17th, at a CERN seminar.

Since its discovery, the mass of the W boson has been measured with ever increasing precision. The result presented by the CMS collaboration last Tuesday is in very good agreement with theoretical predictions and with all the previously existing measurements, with one notable exception: in 2022 the CDF experiment, using data collected at the Tevatron collider at Fermilab (USA), published a very precise and surprisingly high value for the W mass, which to date has not been backed up by any other independent experiment. The CDF discrepancy was the reason for which the CMS result was eagerly awaited by the scientific community. The result confirms the Standard Model expectations, disagreeing with CDF.

The Scuola Normale's contribution to this amazing result is truly remarkable. It can be traced back to the determination of Prof. Luigi Rolandi, for many years professor of Experimental and High Energy Physics at the Scuola Normale, who over more than a decade succeeded in building a highly motivated working group, training many students, some of whom have continued to work on this measurement and have taken on key roles in the project. One of them is Prof. Lorenzo Bianchini, a former SNS student and now a UniPi professor, and Principal Investigator of the ERC ASYMOW project, devoted to the precision measurement of the W mass. Other notable examples are Dr. Elisabetta Manca and, among the more recent acquisitions,  PhD students Davide Bruschini and Cristina Alexe.

Elisabetta Manca, now a Postdoctoral Scholar at the University of California in Los Angeles, contributed significantly to the result, first as a PhD student and then as a research fellow at the Scuola Normale. With Prof. Rolandi as a mentor, she worked on the calibration of the CMS tracker with a precision of less than one part in ten thousand - the most precise ever achieved in the experiment in ten years of data taking - and on the design and implementation of a new measurement technique that enables us to infer the W production model directly from the observed data, and which has been used to validate the main result.

“I am very proud and excited to see this result, which I have worked on and contributed to for a decade, and which has marked the entirety of my academic career, from the beginnings as a young student, to the more scientifically mature phase. Aiming for such levels of precision in a measurement involves the need to re-examine everything that has been done so far by the scientific community, and often to attempt to solve problems that may not have a solution. Prof. Rolandi’s dedication and guidance is what gave me the stamina that allowed me to make it to the end.”

Cristina Alexe, PhD student at the Scuola Normale, is a member of the CMS collaboration and has been working on the W mass measurement within the ASYMOV project. “Having this data analysis as a subject for my PhD thesis is a unique learning experience. I have the opportunity to employ the most recent experimental tools to face challenges that are new in our field. The working environment in Pisa has been very stimulating and supportive: I know I can always depend on the experience of the people around me, and I feel my work as part of the group is being appreciated”.

Davide Bruschini, also a CMS member, is in the final stages of his PhD studies at the Scuola Normale, and his research work is within the ASYMOW project. “After so many years of work, this result from the CMS collaboration has reached a much higher precision than was expected in the early stages, especially if we consider that only a fraction of the available data has been used, which means there is room for substantial improvement. Taking part in the project in person has brought me a considerable body of knowledge which is certainly going to prove a precious asset for my future career. This result is just a starting point for even more precise measurements of the W mass, to which I hope I’ll be able to contribute”.

*Franco Ligabue is a Professor of Experimental Physics at the Scuola Normale Superiore. He is also a member of CMS