The Standard Model of particle physics is alive and well. But it is not complete, so physicists continue to search for new particles and forces that could help complete the model and also explain some tensions with the model - or "anomalies" - in the behaviour of known particles. In a paper accepted for publication in Physical Review Letters, the NA64 collaboration describes how a search for new unknown particles - lightweight "X bosons" that could carry a new force - has allowed it to set bounds on how much these particles could contribute to a fundamental property of the electron, in which an apparent anomaly has recently emerged.
The property in question is the anomalous magnetic moment. The magnetic moment of a particle is a measure of how the particle interacts with a magnetic field. The anomalous magnetic moment is the part of the magnetic moment caused by the interaction of the particle with "virtual" particles that continually pop into and out of existence. These virtual particles comprise all the known particles, predicted by the Standard Model, but they could also include particles never before observed. Therefore, a difference between the Standard Model prediction of the anomalous magnetic moment of a particle and a high-precision measurement of this property could be a sign of new physics in the form of new particles or forces.
The most striking example of such an anomaly is the muon's anomalous magnetic moment, for which Fermilab in the US recently announced a difference with theory at a significance level of 4.2 standard deviations - just a little below the 5 standard deviations required to claim a discovery of new physics. But there is another example, although at a lower significance level: the Standard Model's prediction of the electron's anomalous magnetic moment, based on the measurement of the fundamental constant of nature that sets the strength of the electromagnetic force, differs from the direct experimental measurement at a level of 1.6 or 2.4 standard deviations, depending on which of the two measurements of the fundamental constant is used.
Like other anomalies, this anomaly may fade away as more measurements are made or as theoretical predictions improve, but it could also be an early indication of new physics, so it is worth investigating. In its new study, the NA64 collaboration set out to investigate whether new lightweight X bosons could contribute to the electron's anomalous magnetic moment and explain this apparent anomaly.
NA64 is a fixed-target experiment that directs an electron beam of 100-150 GeV energy, generated using a secondary beamline from the Super Proton Synchrotron, onto a target to look for new particles produced by collisions between the beam's electrons and the target's atomic nuclei. In the new study, the NA64 team searched for lightweight X bosons by looking for the "missing" collision energy they would carry away. This energy can be identified by analysing the energy budget of the collisions.
Analysing data collected in 2016, 2017 and 2018, which in total corresponded to about three hundred billion electrons hitting the target, the NA64 researchers were able to set bounds on the strength of the interaction of X bosons with an electron and, as a result, on the contributions of these particles to the electron's anomalous magnetic moment. They found that X bosons with a mass below 1 GeV could contribute at most between one part in a quadrillion and one part in ten trillions, depending on the X boson's mass.
"These contributions are too small to explain the current anomaly in the electron's anomalous magnetic moment," says NA64 spokesperson Sergei Gninenko. "But the fact that NA64 reached an experimental sensitivity that is better than the current accuracy of the direct measurements of the electron's anomalous magnetic moment, and of recent high-precision measurements of the fine-structure constant, is amazing. It shows that NA64 is well placed to search for new physics, and not only in the electron's anomalous magnetic moment."
