Warwick-led study announces first dedicated measurement of Z boson mass
Analysis led by Warwick physicists, as part of the LHCb experiment, reports a new measurement of the mass of the Z boson, displaying the Large Hadron Collider’s growing role in precision physics.
The Large Hadron Collider beauty (LHCb) experiment is a particle physics detector collecting data at the Large Hadron Collider at CERN. The University of Warwick has been part of the LHCb collaboration since 2008, and has played a leading role in the collaboration’s work of characterising fundamental particles.
In a new paper submitted to Physical Review Letters, the LHCb collaboration announces the first dedicated measurement of the Z boson mass, using data from high-energy collisions between protons recorded at the Large Hadron Collider (LHC) in 2016. Warwick researchers performed the key analysis and have played a primary role in characterising the Z boson in this leap in precision physics.
The Z boson is an electrically neutral particle that mediates the weak nuclear force – one of nature’s fundamental forces. Discovered at CERN over 40 years ago, alongside the W boson, the Z boson played a central role in confirming the Standard Model of particle physics.
Unlike the photon, the mediator of the electromagnetic force, the Z boson has mass. This explains why the weak interaction is weak and short-ranged. Improving the precision of the Z boson mass measurement therefore improves understanding of the weak interaction.
On the importance of this work, Emir Muhammad, PhD Student in the Elementary Particle Physics Group at University of Warwick who performed the analysis said: “We wouldn't exist without the weak-nuclear force, and its weakness is directly related to the large mass of the Z boson and its partner, the W boson. Precisely measuring the mass of the Z boson remains essential for testing the Standard Model and searching for signs of new physics.”
The new LHCb measurement is based on a sample of 174,000 Z bosons decaying into pairs of muons, heavier relatives of the electron. The measurement resulted in a mass of 91,184.2 million electronvolts (MeV) with an uncertainty of just 9.3 MeV – or about a hundredth of a per cent.
The result is in line with measurements from the electron–positron LEP collider, the LHC’s predecessor, and the CDF experiment at the former proton–antiproton Tevatron collider in the US. What’s more, it matches the precision of the Standard Model prediction, which has an uncertainty of 8.8 MeV.
The LHCb measurement shows that this level of precision can be achieved at the LHC despite the complex environment of proton–proton collisions, in which many particles are produced simultaneously.
Associate Professor Mika Vesterinen, Department of Physics, University of Warwick said: “I'm really pleased by the impressive work done by my team in Warwick. The Z boson mass measurement from the LEP collider had seemed untouchable, but this result shows that we may have a path to challenging it with more data from the LHC, further advancing our understanding of the weak-nuclear force."
The achievement opens the door to more Z boson mass studies at the LHC and the future High-Luminosity LHC, including much-anticipated analyses from the ATLAS and CMS experiments. Importantly, the experimental uncertainties on Z boson mass measurements are largely independent across the LHC experiments, meaning that an average of the measurements will have a reduced uncertainty.
LHCb spokesperson Vincenzo Vagnoni said: “The High-Luminosity LHC has the potential to challenge the precision of the Z boson mass measurement from LEP – something that seemed inconceivable at the beginning of the LHC programme. This will pave the way for proposed future colliders, such as the FCC-ee, to achieve an even bigger leap in precision.”
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Notes to Editors
CERN press release
Image Credit (icon & top right): The LHCb detector in 2018. Credit: CERN
Image Credit (bottom left): Warwick group at LHCB, far left - Emir Muhammad. Credit: University of Warwick