Data recorded more than 30 years ago at CERN are now revealing a measurable trace of a fundamental quantum phenomenon.
Researchers reanalyzed collisions produced by DELPHI, one of the detectors at the former LEP collider. Between 1994 and 1995, electrons and their antiparticles collided there at an energy of 91.2 GeV. This energy favored the production of the Z boson, a particle that mediates one of the fundamental interactions of matter.
CERN’s DELPHI detector, used at the former LEP collider.
Source: Anna Pantelia / CERN, CC BY 4.0 via Wikimedia Commons
The Z boson disappears almost immediately after its creation and gives rise to other particles. In many cases, it produces quarks, which then form showers of charged particles. The researchers studied how these charges are distributed according to their direction around the collision point.
To understand the result, it is necessary to consider a particular property of quantum laws. Some symmetries that hold in a classical description are not exactly preserved once quantum effects are taken into account. Physicists then speak of an “anomaly.” Here, this term refers neither to an experimental error nor to a deviation from the Standard Model.
This anomaly leaves a very concrete consequence in Z boson decays. Positive and negative charges are not distributed in exactly the same way forward and backward. This imbalance had already been studied using other methods, but the new analysis directly measures the charge flow as a function of angle.
The total charge flow preserves information directly linked to the electroweak structure described by the Standard Model. Data corrected for detector effects show the expected angular modulation and agree with the simulations used by the researchers.
This mainly opens up a new way to exploit the old LEP data. More precise analyses are now being considered, along with similar measurements at future electron-positron colliders.