CERN experiment reports first evidence of an ultra rare process that could lead to new physics

Scientists at CERN have reported on their first significant evidence of a process predicted by theory, paving the way for the search for evidence of new physics in particle processes that could explain dark matter and other mysteries of the universe.

Today, the CERN NA62 collaboration, which is partially funded by the UK Council for Science and Technology Facilities (STFC) and involves several UK scientists, presented at the ICHEP 2020 conference in Prague the first significant experimental evidence for the ultra rare decomposition of the charged kaon into a charged pion and two neutrinos, (i.e., K⁺ → π⁺νν).

The decomposition process is important in cutting-edge research in physics because it is very sensitive to deviations from theoretical predictions. This means that it is one of the most interesting things for physicists looking for evidence to support an alternative theoretical model in particle physics.

Professor Mark Thomson, particle physicist and CEO of STFC, said 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 at the 1970s:

“The Standard Model describes the fundamental forces and basic components of the universe. It is a very 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 of the standard model. Ultra-rare process measurements provide an exciting avenue to explore these possibilities, in hopes 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 built, with a significant contribution from the UK, specifically for the measurement of these ultra rare kaon decays, from kaons produced by a unique high intensity proton beam provided by the CERN accelerator complex. Kaons are created by colliding high-energy protons from CERN’s Super Proton Super Synchrotron (SPS) on a stationary beryllium target. This creates a secondary particle beam that contains and propagates nearly one billion particles per second, of which approximately 6% are kaons. The primary objective of NA62 is to accurately measure how the charged kaon particle breaks down into a pion and a neutrino-antineutrino pair. The UK has a strong leadership role in the K⁺ → π⁺Decomposition analysis.

“This Kaon decay process is called the ‘gold channel’ due to the combination of being extremely rare and excellently predicted in the Standard Model. It is very difficult to capture and is very promising for scientists seeking new physics.” explains Professor Cristina Lazzeroni, particle physics at the University of Birmingham, and NA62 spokesperson.

“This is the first time that we have been able to obtain meaningful experimental evidence for this decomposition process. It is an exciting time because it is a critical step in capturing the precise measurement of decomposition and identifying potential deviations from the Standard Model.”

“In turn, this will allow us to find new ways to understand our universe. The instruments and techniques developed in the NA62 experiment will lead to the next generation of rare kaon decay experiments.”

The new result, measured with an accuracy of 30%, provides the most accurate measurement to date of this process. The result is consistent with the expectations of the Standard Model, but still leaves room for the existence of new particles.

More data is needed to reach a definitive conclusion about the presence or not of new physics.

STFC member Ernest Rutherford Fellow, Dr. Giuseppe Ruggiero, from the University of Lancaster, has been the lead analyst for this measurement since 2016 and helped create the experiment. He said:

“Analyzing the data from the experiment posed a real challenge. We had to suppress a lot of unwanted data, roughly a billion times. And we had to do this without losing the little signal we wanted to detect. This is much more challenging than find a needle in a million haystacks! We use a method called blind analysis technique. So called, because the analysis is done without looking at the region, or “blind box” where the signal is supposed to be. “

STFC also funded two Ernest Rutherford Scholarships, one at the University of Liverpool and then Lancaster, and one at the University of Birmingham. In addition, three PhD students from the University of Birmingham were supported by STFC and one is now working as a postdoctoral researcher on the project. All five “early career” physicists have worked on the project.

The data used in the research was taken between 2016–2018 at CERN’s Prevessin site in France, and the research involves more than 200 scientists from 31 institutions. A new data collection period will begin in 2021 and will allow the NA62 collaboration to provide a more definitive answer to the question of new physics.

The results

The new result comes from a detailed analysis of the complete NA62 dataset collected so far, corresponding to the 6 × 10 exposure¹² Kaon breaks down. Because the process being measured is very rare, the team had to be especially careful not to do anything that could skew the result. For that reason, the experiment was conducted as a “blind analysis,” where physicists initially only look at the background to verify that their understanding of the various sources is correct.

Only once they are satisfied with that, they look at the data region where the signal is expected to be; This is called “blind analysis”. After blind analysis, seventeen K⁺ → π⁺Candidates are observed in the main dataset collected in 2018, revealing a significant excess over the expected fund of only 5.3 events.

This excess leads to the first evidence of this process (with a statistical significance above the level of “three sigma”). The decomposition rate, measured with an accuracy of 30%, provides the most accurate measurement to date of this process. The result is consistent with the expectations of the Standard Model, but still leaves room for new physical effects. More data is needed to reach a definitive conclusion about the presence or not of new physics.

The probability of this process occurring, called the “branching ratio”, for ultra rare K⁺ → π⁺The decay is very small and is predicted within the Standard Model of particle physics with high precision: (8.4 ± 1.0) × 10^-eleven. This leads to exceptional sensitivity to possible phenomena beyond the description of the Standard Model, making this decay a “golden mode”, that is, one of the most interesting observables on the precision frontier of particle physics. However, the experimental study is extremely challenging due to the small rate, a neutrino pair in the end state, and the huge potential background processes. Due to its characteristics, the NA62 experiment has excellent sensitivity to a variety of rare kaon decompositions and exotic processes.

Collaboration NA62 is preparing to collect an even larger data set in 2021–24, when CERN SPS will restart operation, taking data at a higher beam intensity with an improved beam line and detector configuration. The next objective is a “five sigma” observation of the K⁺ → π⁺νν decay, followed by a decay rate measurement with an accuracy of 10%, providing a powerful independent test of the standard model of particle physics. The horizon of a new physics program with a sensitivity to decomposition rates well below 10^-eleven level is now in sight.

For the long-term future, a high-intensity Kaon Beam program is beginning to take shape, with prospects of measuring K⁺ → π⁺νν decay at a precision percentage, to address the analog decay of the neutral kaon, KL → π^{0 0}νν, and to achieve extreme sensitivities to a wide variety of rare kaon disintegrations that are complementary to research in the beauty quark sector.

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