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The CERN experiment provides the first evidence of an extremely rare process that could lead to new physics



The CERN experiment provides the first evidence of an extremely rare process that could lead to new physics

The NA62 experiment is 270 meters long and includes the 120-meter-long vacuum tank shown here, which contains several particle detectors. (Note: the axis of the experiment is a straight line, the curvature of the reservoir is the optical effect of the photo.) Credit: CERN

CERN scientists reported their first significant evidence of a predictable process in theory, paving the way for the search for new physics evidence in particle processes that could explain dark matter and other mysteries of the Universe.

Today, the CERN NA62 collaboration, co-funded by the UK Council for Science and Technology Institutions (STFC) and involving a number of UK scientists, presented the first significant experimental evidence for an extremely rare charged caon in a charged pion and two neutrinos (ie K+ → π+νν).

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

Particle physicist and STFC executive chairman Professor Mark Thomson said it was an exciting development because the result shows how accurate measurements of this process can lead to new physics beyond the standard model of particle physics developed in the 1970s:

“The standard model describes the basic forces and basic elements of the universe. This is a very successful theory, but there are some mysteries of the universe that the standard model does not explain, such as the nature of dark matter and its origin. matter is the imbalance of antimatter in the universe.

“Physicists have been looking for theoretical extensions of the standard model. Measurements of extremely rare processes provide an exciting way to explore these possibilities, with the hope of discovering a new physics than the standard model.”

In the UK, participants in this study are from the Universities of Birmingham, Bristol, Glasgow and Lancaster. It was funded by the STFC, which is part of UK research and innovation, as well as the Royal Society and the European Research Council (ERC).

The CERN experiment provides the first evidence of an extremely rare process that could lead to new physics

Historical evolution of theoretical predictions (red areas) and experimental boundaries (black triangles). There are 3 measurements in blue: the first is from an experiment in the US. These two data are from NA62 with previous data and this measurement. Increased current measurement accuracy is clearly visible. Credit: NA62 collaboration

The NA62 experiment was designed and constructed with significant UK input, in particular by measuring these extremely rare kaon decays from canons derived from the unique high-intensity proton beam provided by the CERN accelerator complex. Kaons are created by colliding high-energy protons from the CERN Super Proton Synchrotron (SPS) to a stationary beryllium target. This creates a fiber of secondary particles that contains and propagates nearly a billion particles per second, of which about 6% are kaons. The main purpose of NA62 is to accurately measure how a charged kaon particle decomposes into a pion and a neutrino-antineutrine pair. The UK plays an important role in K+ → π+νν decomposition analysis.

“This process of kaon decay is called the ‘golden channel’ because alignment is quite rare and perfectly predictable in the standard model. It is very difficult to capture and maintain a real promise for scientists looking for new physics,” the professor explains. Cristina Lazzeroni, Particle Physicist at the University of Birmingham and representative of NA62.

“This is the first time we have been able to obtain significant experimental evidence for this degradation process. This is an exciting moment as it is an essential step in accurately measuring the degradation measurements and identifying possible deviations from the standard model.

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

The new result, measured with 30% accuracy, allows the most accurate measurement of the process to date. The result meets the expectations of the standard model, but still leaves room for the existence of new particles.

More data are needed to draw a definitive conclusion about the existence of new physics or not.

STFC Ernest Rutherford collaborator dr. Giuseppe Ruggiero from the University of Lancaster since 2016. Is a key analyst for this measurement and helped create the experiment. He said:

The CERN experiment provides the first evidence of an extremely rare process that could lead to new physics

17 events from 2018 The data set is visible inside the red boxes (during the analysis, the red boxes were kept blind and their contents were revealed only as a last step). Other black dots that are not in the red boxes are mainly due to the background, i.e. other processes. Credit: NA62 collaboration

“Analyzing the experimental data was a real challenge. We had to stifle a huge amount of unwanted data, about a thousand billions of times. And we had to do it without losing the tiny signal we wanted to detect. It’s a much more complicated fact. , called the blind analysis method, so-called because the analysis is performed regardless of the region or “blind box” where the signal should be. “

STFC also funded two Ernest Rutherford Fellowships: one at the Universities of Liverpool, later Lancaster, and the other at the University of Birmingham. In addition, three doctoral students from the University of Birmingham have received STFC support, and one is now working on a project as a doctoral student. All five “early career” physicists worked on the project.

The data used in the study were taken from 2016–2018. CERN Prevessin on site in France. More than 200 researchers from 31 institutions participated in the study. The new data collection period will start in 2021. And will allow NA62 collaboration to provide a more accurate answer to the question of new physics.

Findings

The new result is obtained by examining in detail the entire NA62 data set collected so far, corresponding to a 6 × 10 exposure.12 kaonas decomposes. Because a measurable process is so rare, the team had to be extra careful not to do anything that could skew the result. For this reason, the experiment was performed as a “blind analysis,” where physicists initially only looked into the background and checked that their understanding of the various sources was correct.

Just satisfied with that, they review the area of ​​data where the signal is expected; this is called “blind analysis”. After blind analysis, seventeen K+ → π+νν candidates are monitored in a core dataset collected in 2018, revealing a significant excess of only 5.3 expected events.

This surplus provides the first evidence of this process (whose statistical significance exceeds the “three-digit” level). The degree of rot, measured with an accuracy of 30%, allows the most accurate measurement of the process to date. The result meets the expectations of the standard model, but still leaves room for new physics effects. More data are needed to draw a definitive conclusion about the existence of new physics or not.

The probability of the occurrence of this process, called the “bifurcation ratio,” is particularly rare in K+ → π+νν decay is very small and predicted very accurately according to the standard model of particle physics: (8.4 ± 1.0) × 10–11. This results in an exceptional sensitivity to possible phenomena that are not included in the description of the standard model, so this decay becomes a “golden mode”, ie one of the most interesting precision in the physics of observed particles. However, the experimental study is very complex due to the low velocity, the neutrino pair in the final state, and the huge potential background processes. Due to its properties, the NA62 experiment has excellent sensitivity to a variety of rare kaon decay and exotic processes.

The NA62 is preparing to collect an even larger data set in 2021-24, when CERN SPS will be operational again, taking data with higher light intensities, improved fiber line and detector setup. Another target is the observation of the “five sigmas” K+ → π+νν decomposition, after which the decay rate is measured with an accuracy of 10%, thus obtaining a powerful independent standard model of particle physics. The horizon of a new physics program with a sensitivity to decay rate less than 10–11 level is in sight.

For a longer period, a high-intensity kaon beam program begins to form, providing the ability to measure K+ → π+νν decomposition to several percent accuracy to solve an analogous neutral decay decomposition, KL → π0νν, and to achieve a special sensitivity to many rare kaona decays, which are complementary to research in the beauty quartz sector.


Extremely rare decay of kaon may lead to evidence of new physics


Citation: The CERN experiment provides the first evidence of an ultra-rare process that could lead to new physics (July 28, 2020), obtained in 2020. July 28 From https://phys.org/news/2020-07-cern-evidence-ultra-rare- physics.html

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