An elusive particle that first formed in the hot, dense early universe has puzzled physicists for decades. Following its discovery in 2003, scientists began observing a slew of other strange objects in high-energy particle collisions.
Appearing as 'bumps' in the data from high-energy experiments, these signals defy the standard picture of particle behaviour and are a leading problem in contemporary physics, sparking several attempts to understand their mysterious nature. To date, no theoretical picture has explained the complete pattern of observed states, short-lived particles called resonances, which have been dubbed XYZ states.
But new work by researchers in the High Energy Physics group in the Department of Applied Mathematics and Theoretical Physics (DAMTP), and colleagues at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility in Virginia, suggests the experimental data could be explained with fewer XYZ states than currently claimed.
The team used a branch of quantum physics - lattice quantum chromodynamics (QCD) - to compute the masses of particles containing a specific 'flavour' of the subatomic building blocks known as quarks. Quarks, along with gluons, a force-carrying particle, make up the Strong Force, one of the four fundamental forces of nature. QCD is the theory of interacting quarks and gluons, and lies within the standard model of particle physics.
The researchers include Dr David Wilson, Royal Society University Research Fellow and Professor Christopher Thomas, both from the High Energy Physics group in DAMTP, who are part of the international Hadron Spectrum Collaboration. The team found that multi-particle states sharing the same total angular momentum are coupled, meaning only a single resonance exists at each spin channel. In contrast to previous theoretical and experimental studies, this new insight suggests that instead of several supposed XYZ particles the experimental data might actually show just one particle - but seen in different ways.
The work could also provide clues about an enigmatic particle which has defied explanation since its discovery two decades ago: X(3872).
Charm, anticharm and a jumble of data
The charm quark, one of six quark 'flavours', was first observed experimentally 50 years ago in 1974. It was discovered alongside its antimatter counterpart, the anticharm, and particles paired this way are part of an energy region called charmonium.
In 2003, Japanese researchers discovered a new charmonium candidate dubbed X(3872): a short-lived particle state that appears to defy the present quark model.
The mysterious X(3872) appears similar to a meson (a quark-antiquark state), but not where any mesons were expected. Some scientists claim X(3872) could be a tetraquark, which is a composite particle (hadron) made up of two quarks and two antiquarks. For comparison, protons and neutrons are hadrons with three quarks. Other possible explanations for X(3872) include a molecule-like bound system of two mesons, each containing two quarks, or some sort of quark-gluon hybrid.
"X(3872) is now more than 20 years old, and we still haven't obtained a clear, simple explanation that everyone can get behind," said lead author Dr David Wilson, Royal Society University Research Fellow in the High Energy Physics group in DAMTP.
Following an explosion in the amount of data captured by modern particle accelerators including the LHCb experiment at CERN, scientists have detected a hodgepodge of other exotic charmonium candidate states over the past two decades, with names such as Y(4260) and Zc(3900) - hence the label 'XYZ'. In fact, so many were being discovered that in 2017 the Particle Data Group revamped its naming scheme.
"High-energy experiments began measuring processes that are hundreds of times fainter," said co-author Professor Jozef Dudek from the Jefferson Lab and William & Mary University in Virginia. "They started seeing bumps, interpreted as new particles, almost everywhere they looked. And very few of these states agreed with the model that came before."
But now, by creating a tiny virtual 'box' to simulate quark behaviour, the research team has discovered that several supposed XYZ particles might actually be just one particle seen in different ways. This could help simplify the confusing jumble of data scientists have collected over the years.
Supercomputers and new ways of seeing
The study was a complex undertaking. Despite the tiny volumes they were working with, the team required enormous computing power to simulate all the possible behaviours and masses of quarks. Many of the calculations for this study were carried out with the support of the Cambridge Service for Data Driven Discovery (CSD3) and STFC's DiRAC high-performance computing facilities in Cambridge.
Quantum chromodynamics (QCD) describes quarks' interactions with gluons, the photon-like carriers of the strong force. Supercomputers, with their ability to crunch huge numbers of numbers, can be applied to QCD by placing the theory on a lattice.
Think of the lattice as a small, tightly packed grid of points representing space and time. Theorists can use the possible configurations of quarks and gluons inside this 'box' to predict properties of hadrons such as their masses and lifetimes.
The lattice box is tiny – nearly a million times smaller than a single atom. But even such a tiny volume requires an enormously powerful computer to sample the possible behaviours of quarks and gluons.
The researchers used the supercomputers at Cambridge and Jefferson Lab to infer all the possible ways in which mesons – made of a quark and its antimatter counterpart – could decay. To do this, they had to relate the results from their tiny virtual box to what would happen in a nearly infinite volume – that is, the size of the universe.
"In our calculations, unlike experiment, you can't just fire in two particles and measure two particles coming out," said Wilson. "You have to simultaneously calculate all possible final states, because quantum mechanics will find those for you."
The results can be understood in terms of just a single short-lived resonance whose appearance could differ depending upon which possible decay state it is observed in.
"We're trying to simplify the picture as much as possible, using fundamental theory with the best methods available," said Wilson. "Our goal is to disentangle what has been seen in experiments."
The researchers have presented their results in a pair of companion papers published in Physical Review Letters and Physical Review D. Now that the team has proved this type of calculation is feasible, they are ready to apply it to the mysterious particle X(3872).
"This work shows how a confusing set of experimental observations could be understood in a simple underlying picture using the fundamental theory," says DAMTP's Professor Christopher Thomas. "It will be very interesting to apply these methods to other systems that have been studied by experiments at the LHC and elsewhere to see if a similar picture emerges, and improve our understanding of one of the fundamental forces of nature."
This article is adapted from news releases from the University of Cambridge and Jefferson Lab.