Precision measurement of the Z boson mass from proton-proton collision data recorded with the CMS experiment at the large hadron collider

• Applicant: Dr. David Walter, Ph.D.
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 558014196

The Z boson is an elementary particle and, alongside the massless photon, the massive electrically neutral exchange particle of the electroweak theory, which is part of the standard model of particle physics (SM). It was observed in 1983 at the Super Proton Synchrotron, a particle accelerator at the CERN research center in Geneva. Its mass is not predicted by the known theory, but its relation to other parameters of the SM, such as the mass of the electrically charged W boson. However, as yet unknown particles or effects could lead to a deviation of this relation. A precise measurement of the Z boson mass therefore allows the known theory to be validated and enables more accurate predictions of the SM. The Z boson mass was determined at the Large Electron-Positron Collider (LEP) with 2.1MeV accuracy; no measurement of the Z boson mass has yet been carried out at the currently most powerful particle accelerator, the Large Hadron Collider (LHC). Compared to the very clean detector signatures of collisions with electrons and positrons, which are themselves elementary particles, the LHC collisions of protons, which consist of quarks and gluons, produce far more by-products and the energy of the incident quark that contributes to Z boson production is unknown. Furthermore, several collisions take place simultaneously at the LHC in order to achieve the largest possible amount of data. This makes it difficult to measure the Z boson mass. Innovative methods of data analysis are required. In recent years, however, measurements of the W boson mass have shown that it is possible to precisely calibrate muons, the decay products of the Z boson which are particularly suitable for measurement. On the basis of the developed methods, a precision of 2 MeV can be aimed for, which would be a new record. The unprecedented number of LHC collisions allows a data-driven calibration based on hadrons such as the J/Psi particle, whose mass is precisely known. Recent advances in differential programming allow complex models with over 100,000 parameters describing the position of detector modules, their material and the magnetic field to be determined efficiently and accurately. The groundbreaking methods developed for this measurement will also become more relevant for other analyses in the coming years, when the amount of data at the High Luminosity Upgrade of the LHC (HL-LHC) increases by more than another order of magnitude, and most other relevant measurements are become limited by systematic uncertainties.

• DFG Programme: WBP Fellowship
• International Connection: USA
• Host: Professor Christoph Paus, Ph.D.