Scientists at CERN, including experts from the University of Birmingham, have reported on their first significant evidence for a process predicted by theory. The findings pave the way for searches for evidence of new physics in particle processes that could explain dark matter and other mysteries of the universe.
The CERN NA62 collaboration , which is part-funded by the UK’s Science and Technology Facilities Council (STFC) and involves a number of UK scientists, presented at the ICHEP 2020 conference in Prague the first significant experimental evidence for the ultra-rare decay of the charged kaon into a charged pion and two neutrinos, (i.e. K+ +++++).
The decay process is important in cutting-edge physics research because it is so sensitive to deviations from theoretical predictions. This means that it is one of the most interesting things to observe for physicists looking for evidence to supports alternative theoretical model in particle physics.
Professor Mark Thomson, particle physicist and Executive Chair of STFC, said that this was exciting progress because the result shows how precise measurements of this process could lead to new physics, beyond the Standard Model of particle physics developed in the 1970s:
“The Standard Model describes the fundamental forces and building blocks of the universe. It is a highly successful theory, but there are several mysteries of the universe that the Standard Model does not explain, such as the nature of dark matter and the origins of the matter-antimatter imbalance in the universe.
“Physicists have been searching for theoretical extensions to the Standard Model. Measurements of ultra-rare processes provide an exciting avenue for exploring these possibilities, with the hope of discovering new physics beyond the Standard Model.”
The UK participants in this research are from the Universities of Birmingham, Bristol , Glasgow and Lancaster , and have been funded by STFC which is part of UK Research and Innovation , as well as by the Royal Society and the European Research Council (ERC).
The NA62 experiment has been designed and constructed, with a significant UK contribution, specifically for measurement of these ultra-rare kaon decays, from kaons produced by a unique high-intensity proton beam provided by the CERN accelerator complex. The kaons are created by colliding high-energy protons from CERN’s Super Proton Synchrotron (SPS) into a stationary beryllium target. This creates a beam of secondary particles which contains and propagates almost one billion particles per second, about 6% of which are kaons. The main aim of NA62 is to measure precisely how the charged kaon particle decays into a pion and a neutrino-antineutrino pair. The UK has a strong leading role in the K++++++ decay analysis.
“This kaon decay process is called the ‘golden channel’ because of the combination of being both ultra-rare and excellently predicted in the Standard Model. It is very difficult to capture and holds real promise for scientists searching for new physics,” explains Professor Cristina Lazzeroni , Particle Physicist at the University of Birmingham, and spokesperson for NA62.
“This is the first time we have been able to obtain significant experimental evidence for this decay process. It is an exciting moment because it is a fundamental step towards capturing the precise measurement of the decay and identifying possible deviations from the Standard Model.
“In turn, this will enable us to find new ways of understanding our universe. The instruments and techniques developed in the NA62 experiment will lead to the next generation of rare kaon decays experiments."
The new result measured to a 30% precision, gives the most precise measurement to date of this process. The result is consistent with the Standard Model expectation, but still leaves room for the existence of new particles.
This excess leads to the first evidence for this process (with a statistical significance above the “three sigma” level). The decay rate, measured to a 30% precision, gives the most precise measurement to date of this process. The result is consistent with the Standard Model expectation, but still leaves room for new physics effects. More data are needed to reach a definitive conclusion on the presence or not of new physics.
The probability for this process to happen, called “branching ratio”, for the ultra-rare K++++++ decay is very small and predicted within Standard Model of particle physics to a high precision: (8.4 1.0)×10×11. This leads to exceptional sensitivity to the possible phenomena beyond the Standard Model description, making this decay a “golden mode”, i.e. one of the most interesting observables at the precision frontier of particle physics. The experimental study is however extremely challenging due to the tiny rate, a neutrino pair in the final state, and huge potential background processes. Due to its characteristics, the NA62 experiment has excellent sensitivity to a variety of rare kaon decays and exotic processes.