ONEDEGGAM

The standard model describes the properties of known particles, yet there are plenty of reasons to believe that it does not tell the full story of how fundamental particles interact. Researchers in the ONEDEGGAM project are exploring the asymmetry between matter and anti-matter, which could open up a window to new physics, as Dr Sneha Malde explains.

The standard model of particle physics describes the particles that are known to exist within the universe and accurately predicts their properties, yet it leaves questions unanswered which suggests that it doesn’t tell the full story of how fundamental particles interact. One interesting example is a set of particles called quarks, which make up protons and neutrons. “Protons and neutrons are composite particles made up of quarks, but they’re composed of the first generation of quarks only. There are three generations of quarks,” explains Dr Sneha Malde, a researcher in particle physics at Oxford University in the UK. The interactions of quarks are described using the Cabibbo-Maskawa-Kobayashi (CKM) matrix. “There are also heavier versions of the up and down quarks inside protons and neutrons. In total there are six quarks, and the CKM matrix describes how they interact with each other,” outlines Dr Malde.

These quarks all have corresponding antiquarks, which might be expected to decay in the same way. However, certain asymmetries have been observed in this respect. “You would expect a particle and an anti-particle to decay in the same way, with the same rate or distribution, just with the opposite charge configuration. However, there are places where that seems to break,” says Dr Malde. This topic is central to the ONEDEGGAM project, in which Dr Malde is studying differences between how certain types of hadron – a composite particle comprised of quarks – decay, bringing together data from the LHC and the BES III experiment in China. “Most particle physics measurements are done using data from one place. However, in order to measure the angle γ, which describes some of this asymmetry between matter and anti-matter, I need two sets of inputs,” she explains. “We record a very high rate of beauty hadron decays from the LHCb experiment.”

Researchers from Dr Malde’s team have been able to measure the decays of these beauty hadrons very precisely. A beauty hadron can decay to a charm hadron alongside a strange hadron. “Data from LHCb shows us that the distributions from positive and negative beauty hadrons are different. But in order to interpret that within the standard model, we need more information about exactly what’s going on in the subsequent decay of the charm particle,” continues Dr Malde. This information is quite obscure and can be accessed via the process of quantum correlation, through which Dr Malde aims to gain deeper insights into the phase of the particles. “In physics, every process has an amplitude and a phase, and in general we don’t have access to the phase,” she outlines. “There are systems however where you can

introduce interference, and by doing that you can then gain access to the phase.”

One way of producing interference is via quantum correlation which is exactly how the data at the BESIII experiment is generated. Part of the ONEDEGGAM project’s work involves analysing this data. While relatively small numbers of particles have been generated at BESIII, they nevertheless hold great interest to Dr Malde. “They’re created in a quantum-entangled way. It is amazing to see this microscopic phenomenon materialise in a macroscopic way that you can even see,” she says. The predecessor of BESIII was an experiment called CLEO, and both have produced measurements for analysis at the LHC. “The numbers come with a level of uncertainty, and that uncertainty propagates through,” continues Dr Malde. “We’re trying to measure this angle γ at LHCb. We want to reach a precision of 1º, and if we can get there, then maybe we can start to see something break down in the standard model. However, if you want to have a precision of 1º, then you can’t have large sources of uncertainty coming from CLEO. The CLEO data gives us large uncertainties, because the data set is very small.”

ERC funding

This was a major motivating factor behind Dr Malde’s decision to apply for ERC funding, as it has essentially enabled her to join the BESIII experiment and gain direct access to the data it generates. The wider aim here is to make a more researchers uncover evidence of new physics beyond the standard model. “One of the key ideas about the CKM matrix is that if you’ve only got these three generations of quarks, and they all interact with each other, then the overall probability of something happening has to be one. So they can essentially only turn from one quark to another quark,” explains Dr Malde. This matrix can be represented as a triangle, where the three angles – ά, ß, and γ – should add up to 180º. “One of the key goals of the LHCb experiment is to try and measure these angles very accurately, and to measure the lengths of the sides very accurately,” continues Dr Malde.

Evidence that the angles don’t add up to 180º, or that the sides of the triangle are too long or short for example, would point towards the existence of physics beyond the standard model. While this is an exciting prospect, at this stage Dr Malde’s focus is primarily on measuring γ more precisely. “In general, measurements become more precise with time, and as scientists we of course want to make progress as fast as possible. I’m trying to accelerate that by bringing the BESIII and LHCb experiments together,” she outlines. The sub-detectors at the LHCb experiment are currently being upgraded, and Dr Malde hopes to make further progress when it returns to operation. “We will start taking data next year, and my project will analyse the data that comes out in the first two years. I would

precise measurement of γ, which could help hope that with that data we would be able to

achieve greater precision than we have now, and move from 5º down to 1º. That would be outstanding,” she says.

A precise measurement of γ could then open up new avenues of research. If a standard model parameter is precisely measured, and found to be different to what was expected, then that suggests the existence of new physics, while confirmation that predictions are actually correct would also be interesting. “If we were to find that this angle γ is exactly where we expect it to be, then it tells us something about this new physics. It means that it has to respect the standard model in this area,” explains Dr Malde.

To explore the phenomenon of CP violation, the difference between the properties of matter and anti-matter, through measurements of the CKM angle γ, with the hope of achieving the precision of one degree. The measurements are performed using both the large exquisite dataset from the LHCb experiment and the unique quantum-entangled dataset of the BESIII experiment.

European Research Council Starting grant. Funded under: H2020-EU.1.1. • Overall budget: € 1 499 955

Project Coordinator, Sneha Malde University of Oxford Department of Physics Denys Wilkinson Building Keble Road Oxford OX1 3RH T: + 01865 (2)73357 E: sneha.malde@physics.ox.ac.uk W: https://www2.physics.ox.ac.uk/contacts/ people/malde

https://link.springer.com/article/10.1007%2 FJHEP07%282019%29106 https://arxiv.org/abs/1904.01129J. High Energy Phys. (2019) 2019: 106 https://arxiv.org/abs/2003.00091

Sneha Malde

Sneha Malde is a researcher in particle physics at the University of Oxford. She holds a Dorothy Hodgkin Fellowship with the Royal Society, which enables her to pursue her research interests. Her main interests are in high-energy frontier physics, using data from the LHCb experiment.

The notable differences between these two diagrams of simulated data show how quantum-entanglement, or Einstein’s “spooky action at a distance”, would manifest in BESIII data. The data can be used to determine critical parameters related to the decay of the charm meson.