A pi-zero particle decay, as seen by the ALICE detector at the LHC. Image credit ALICE Collaboration
Textbooks were being rewritten during last week's Physics at LHC conference.
“I was sitting in the session, listening to the ALICE talk by Andrea Dainese from Padova on Wednesday morning, and suddenly I knew: I could replace all the textbook bubble-chamber pictures from the sixties in my lectures,” said DESY's Thomas Naumann, a member of the ATLAS collaboration.
Naumann's revelation was sparked by an image showing a neutral pion decaying into two photons that then convert into two electron-positron pairs in the the ALICE inner tracker. Generations of physicists have learned about the history and characteristics of neutral pions, also called pi zero particles, in their undergraduate classes and textbooks. Until now, the decay of a pi zero particles was always illustrated with a picture from a 1950s- or 1960s-era bubble chamber experiment.
Pi zeros aren’t rare; in fact their decays are responsible for most of the photons seen in the LHC detectors. They are used as standard candles to calibrate detectors, which is why every student of particle physics has to know them inside out. They're also one of the many known particles being rediscovered by particle physicists using the first LHC collision data.
Theorist Hitoshi Murayama's prediction for a page in the PDG in 2016.
The particle physicist’s bible, the booklet published by the Particle Data Group or PDG, played an important role at the Physics at LHC conference. It contains tables with all the possible data for all existing and hypothetical particles, such as their mass, charge, flavor, lifetime and decay modes. LHC physicists are rediscovering the known particle families from strange through charm, and bottom through (hopefully soon) top, and in a matter of weeks they have almost reached the statistical precision currently listed in the PDG for many measurements.
The known particles aren't the only ones being reassessed; theoretical physicists are also hard at work refining their predictions about particles and their behavior. In his talk, theorist Hitoshi Murayama of Japan's Institute for the Physics and Mathematics of the Universe showed his vision of what the PDG could look like in a few years. A slide from his talk—originally called ‘Theories of Beyond the Standard Model Physics’ but renamed “How stupid theorists are and Why LHC matters”—features a page from the PDG in 2016.
by Barbara Warmbein