̽��ֱ�� of Cambridge - US Department of Energy

A peek inside the box that could help solve a quantum mystery

sc604 — Tue, 19 Nov 2024 15:22:24 +0000

Appearing as ‘bumps’ in the data from high-energy experiments, these signals came to be known as short-lived ‘XYZ states.’ They defy the standard picture of particle behaviour and are a problem in contemporary physics, sparking several attempts to understand their mysterious nature.

But theorists at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility in Virginia, with colleagues from the ̽��ֱ�� of Cambridge, suggest the experimental data could be explained with fewer XYZ states, also called resonances, than currently claimed.

̽��ֱ��team used a branch of quantum physics to compute the energy levels, or mass, 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.

̽��ֱ��researchers found that multiple particle states sharing the same degree of spin – or angular momentum – are coupled, meaning only a single resonance exists at each spin channel. This new interpretation is contrary to several other theoretical and experimental studies.

̽��ֱ��researchers have presented their results in a pair of companion papers published for the international Hadron Spectrum Collaboration (HadSpec) in Physical Review Letters and Physical Review D. ̽��ֱ��work could also provide clues about an enigmatic particle: X(3872).

̽��ֱ��charm quark, one of six quark ‘flavours’, was first observed experimentally 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.

“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 from Cambridge’s Department of Applied Mathematics and Theoretical Physics (DAMTP).

Thanks to the power of modern particle accelerators, scientists have detected a hodgepodge of exotic charmonium candidate states over the past two decades.

“High-energy experiments started seeing bumps, interpreted as new particles, almost everywhere they looked,” said co-author Professor Jozef Dudek from William & Mary. “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 researchers 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.

Despite the tiny volumes they were working with, the team required enormous computing power to simulate all the possible behaviours and masses of quarks.

̽��ֱ��researchers used supercomputers at Cambridge and the 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.”

̽��ֱ��results can be understood in terms of just a single short-lived particle 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.”

Now that the team has proved this type of calculation is feasible, they are ready to apply it to the mysterious particle X(3872).

“ ̽��ֱ��origin of X(3872) is an open question,” said Wilson. “It appears very close to a threshold, which could be accidental or a key part of the story. This is one thing we will look at very soon."

Professor Christopher Thomas, also from DAMTP, is a member of the Hadron Spectrum Collaboration, and is a co-author on the current studies. Wilson’s contribution was made possible in part by an eight-year fellowship with the Royal Society. ̽��ֱ��research was also supported in part by the Science and Technology Facilities Council (STFC), part of UK Research and Innovation (UKRI). Many of the calculations for this study were carried out with the support of the Cambridge Centre for Data Driven Discovery (CSD3) and DiRAC high-performance computing facilities in Cambridge, managed by Cambridge’s Research Computing Services division.

Reference:
David J. Wilson et al. ‘Scalar and Tensor Charmonium Resonances in Coupled-Channel Scattering from Lattice QCD.’ Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.132.241901

David J. Wilson et al. ‘Charmonium xc0 and xc2 resonances in coupled-channel scattering from lattice QCD.’ Physical Review D (2024). DOI: 10.1103/PhysRevD.109.114503

Adapted from a Jefferson Lab story.

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 tied to the millionths of a second after the Big Bang.

gremlin via Getty Images

Abstract colourful lines

̽��ֱ��text in this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

Yes

New design points a path to the ‘ultimate’ battery

sc604 — Thu, 29 Oct 2015 13:46:09 +0000

Scientists have developed a working laboratory demonstrator of a lithium-oxygen battery which has very high energy density, is more than 90% efficient, and, to date, can be recharged more than 2000 times, showing how several of the problems holding back the development of these devices could be solved.

Lithium-oxygen, or lithium-air, batteries have been touted as the ‘ultimate’ battery due to their theoretical energy density, which is ten times that of a lithium-ion battery. Such a high energy density would be comparable to that of gasoline – and would enable an electric car with a battery that is a fifth the cost and a fifth the weight of those currently on the market to drive from London to Edinburgh on a single charge.

However, as is the case with other next-generation batteries, there are several practical challenges that need to be addressed before lithium-air batteries become a viable alternative to gasoline.

Now, researchers from the ̽��ֱ�� of Cambridge have demonstrated how some of these obstacles may be overcome, and developed a lab-based demonstrator of a lithium-oxygen battery which has higher capacity, increased energy efficiency and improved stability over previous attempts.

Their demonstrator relies on a highly porous, ‘fluffy’ carbon electrode made from graphene (comprising one-atom-thick sheets of carbon atoms), and additives that alter the chemical reactions at work in the battery, making it more stable and more efficient. While the results, reported in the journal Science, are promising, the researchers caution that a practical lithium-air battery still remains at least a decade away.

“What we’ve achieved is a significant advance for this technology and suggests whole new areas for research – we haven’t solved all the problems inherent to this chemistry, but our results do show routes forward towards a practical device,” said Professor Clare Grey of Cambridge’s Department of Chemistry, the paper’s senior author.

Many of the technologies we use every day have been getting smaller, faster and cheaper each year – with the notable exception of batteries. Apart from the possibility of a smartphone which lasts for days without needing to be charged, the challenges associated with making a better battery are holding back the widespread adoption of two major clean technologies: electric cars and grid-scale storage for solar power.

“In their simplest form, batteries are made of three components: a positive electrode, a negative electrode and an electrolyte,’’ said Dr Tao Liu, also from the Department of Chemistry, and the paper’s first author.

In the lithium-ion (Li-ion) batteries we use in our laptops and smartphones, the negative electrode is made of graphite (a form of carbon), the positive electrode is made of a metal oxide, such as lithium cobalt oxide, and the electrolyte is a lithium salt dissolved in an organic solvent. ̽��ֱ��action of the battery depends on the movement of lithium ions between the electrodes. Li-ion batteries are light, but their capacity deteriorates with age, and their relatively low energy densities mean that they need to be recharged frequently.

Over the past decade, researchers have been developing various alternatives to Li-ion batteries, and lithium-air batteries are considered the ultimate in next-generation energy storage, because of their extremely high energy density. However, previous attempts at working demonstrators have had low efficiency, poor rate performance, unwanted chemical reactions, and can only be cycled in pure oxygen.

What Liu, Grey and their colleagues have developed uses a very different chemistry than earlier attempts at a non-aqueous lithium-air battery, relying on lithium hydroxide (LiOH) instead of lithium peroxide (Li₂O₂). With the addition of water and the use of lithium iodide as a ‘mediator’, their battery showed far less of the chemical reactions which can cause cells to die, making it far more stable after multiple charge and discharge cycles.

By precisely engineering the structure of the electrode, changing it to a highly porous form of graphene, adding lithium iodide, and changing the chemical makeup of the electrolyte, the researchers were able to reduce the ‘voltage gap’ between charge and discharge to 0.2 volts. A small voltage gap equals a more efficient battery – previous versions of a lithium-air battery have only managed to get the gap down to 0.5 – 1.0 volts, whereas 0.2 volts is closer to that of a Li-ion battery, and equates to an energy efficiency of 93%.

̽��ֱ��highly porous graphene electrode also greatly increases the capacity of the demonstrator, although only at certain rates of charge and discharge. Other issues that still have to be addressed include finding a way to protect the metal electrode so that it doesn’t form spindly lithium metal fibres known as dendrites, which can cause batteries to explode if they grow too much and short-circuit the battery.

Additionally, the demonstrator can only be cycled in pure oxygen, while the air around us also contains carbon dioxide, nitrogen and moisture, all of which are generally harmful to the metal electrode.

“There’s still a lot of work to do,” said Liu. “But what we’ve seen here suggests that there are ways to solve these problems – maybe we’ve just got to look at things a little differently.”

“While there are still plenty of fundamental studies that remain to be done, to iron out some of the mechanistic details, the current results are extremely exciting – we are still very much at the development stage, but we’ve shown that there are solutions to some of the tough problems associated with this technology,” said Grey.

̽��ֱ��authors acknowledge support from the US Department of Energy, the Engineering and Physical Sciences Research Council (EPSRC), Johnson Matthey and the European Union via Marie Curie Actions and the Graphene Flagship. ̽��ֱ��technology has been patented and is being commercialised through Cambridge Enterprise, the ̽��ֱ��’s commercialisation arm.

Reference:
Liu, T et. al. ‘Cycling Li-O2 Batteries via LiOH Formation and Decomposition.’ Science (2015). DOI: 10.1126/science.aac7730

Researchers have successfully demonstrated how several of the problems impeding the practical development of the so-called ‘ultimate’ battery could be overcome.

What we’ve achieved is a significant advance for this technology and suggests whole new areas for research

Clare Grey

Reprinted with permission from T Liu et al., Science 350: 530 (2015)

False-colour microscopic view of a reduced graphene oxide electrode (black, centre), which hosts the large (on the order of 20 micrometers) lithium hydroxide particles (pink) that form when a lithium-oxygen battery discharges.

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes