A new particle has popped into existence at CERN’s Large Hadron Collider, a heavier proton-like particle that contains two charm quarks.

Protons and neutrons are examples of a class of particles called baryons, which each contain three fundamental subatomic particles called quarks that come in a variety of so-called flavours. In the case of a proton, there are two “up” quarks and one “down” quark that make up the particle.

But heavier quarks, like those known as charm quarks, can also combine to make baryons. However, because these unusual quark combinations are heavier and so more unstable, they often have fleetingly short lifetimes and quickly decay into other particles.

In 2017, physicists working at CERN’s LHCb experiment glimpsed one of these exotic baryons, memorably named Xi_cc⁺⁺, that was made up of two charm quarks and an up quark. This particle lived for only a trillionth of a second. Now, physicists working on the LHCb experiment have spotted the charm-filled sister particle to Xi_cc⁺⁺, called the Xi_cc⁺particle, which contains a down quark instead of an up, making it a heavier analogue of the proton.

This particle had a predicted lifetime of six times shorter than that of the Xi_cc⁺⁺, making it much harder to detect. It was found only after the LHCb experiment was upgraded to carry out more sensitive particle searches. The finding has a statistical significance of over 7 sigma, a measure that physicists use to state how confident they are that the result isn’t a random fluke, which easily clears the 5-sigma bar required to claim a discovery.

“Not only is it interesting discovering the particle in its own right – the Xi_cc⁺ has been searched for for a long time – but it also really shows the power that these upgrades to the LHC are having,” says at the University of Manchester in the UK. “In one year’s data sample, we were able to see something that we couldn’t see with 10 years of data from the previous generation.”

Subscriber-only newsletter

Sign up to Lost in Space-Time

Sign up to newsletter

Spotting this particle could teach us about how the strong nuclear force, which describes how quarks bind together, glues together heavier quarks than those we see in protons and neutrons, says Parkes. But it also resolves a 20-year-old mystery.

In 2002, physicists working on the SELEX experiment at the Fermi National Accelerator Laboratory in Illinois thought that they had that looked very much like Xi_cc⁺, but with a much lower mass than predicted at only a 4.7 sigma level of confidence. “Now we’ve found it, but it’s at a mass which is similar to its partner [Xi_cc⁺⁺] that we found a few years ago, and not at the mass that was predicted by SELEX,” says Parkes. The strength of the new discovery closes the door on the question of this particle’s mass.

“It’s a very interesting measurement, but it’s unclear what we learn from it,” says at Vrije University Amsterdam in the Netherlands. “There is no rule in quantum chromodynamics which prevents this hadron from existing, but now we’ve measured it exists, we are left not particularly illuminated.”

Part of this, says Rojo, is because our current theories don’t predict well how heavier quarks inside baryons should interact or what their masses should be. “The data is now ahead of the theory for these kinds of particles, but it could be that in five years from now, this measurement is able to answer some very important theory questions,” says Rojo, such as what different combinations of quarks mean for particle masses.

CERN and Mont Blanc, dark and frozen matter: Switzerland and France

Prepare to have your mind blown by CERN, Europe's particle physics centre, where researchers operate the famous Large Hadron Collider, nestled near the charming Swiss lakeside city of Geneva.

Find out more

Peter Higgs lived a singular life. He developed a physics theory that stood a chance of radically advancing our understanding of the universe, and lived to see generations of experimentalists chase after and eventually triumphantly corroborate his work in the lab. He died in his home at age 94.

“Without Higgs’s work, we wouldn’t understand why there are atoms. Some pretty basic features of our world would not be understandable,” says at King’s College London.

Peter Higgs, physicist who theorised the Higgs boson, has died aged 94

Nobel prizewinning theoretical physicist Peter Higgs has died aged 94. He proposed the particle that gives other particles mass – now named the Higgs boson and discovered by the Large Hadron Collider at CERN in 2012

Higgs started that work at the University of Edinburgh in the UK in the 1960s. He was thinking about a branch of physics called quantum field theory, and in July of 1964, he took about a week to write a short paper on the topic. Physics Letters accepted the study but rejected Higgs’s more detailed follow-up work just a week later. Even though Physical Review Letters eventually published a revised version of the second paper, it received no fanfare and remained overlooked for years.

Ironically, these papers contained a key ingredient that was sorely lacking from the theory of all particles in the universe: the reason why they have mass.

Almost all known particles need some mass in order to bind to each other and form the structures, like atoms, that comprise our physical world. But physicists understand all particles as excitations of invisible fields that permeate everything – electrons, for example, are excitations of the electromagnetic field – and even the best theories at the time could not explain where these masses come from.

Subscriber-only newsletter

Sign up to Lost in Space-Time

Sign up to newsletter

Higgs theorised that particles would acquire mass by interacting with a new type of field. That field had a very special excitation of its own, another particle called the Higgs boson. The Higgs field solved a huge question in theoretical particle physics, and the Higgs boson was a tantalising target that experimentalists could hunt for in order to tie theory to reality.

“If you remove everything from the vacuum, all matter or quantum fluctuations, all electromagnetic stuff, all gravity, you will be left with the Higgs field,” says at the University of Oxford. “And we need that just like a goldfish needs water. It stabilises empty space.”

Working independently from Higgs, physicists François Englert and Robert Brout reached the same conclusion, also in 1964.

However, according to Close, who wrote a biography of Higgs in 2022, Higgs did not necessarily set out to write a groundbreaking paper. He simply followed a line of rigorous and often solitary scholarship, which led him to worry deeply about what seemed to be a technical issue that plagued quantum field theory. Other physicists had previously resolved a similar issue in systems with less cosmic implications, such as perfect conductors of electricity. Higgs figured out how to generalise their mathematics to all of particle physics.

But quantum field theory was unfashionable at the time, and when he lectured about his work at prestigious institutions like Harvard University in 1965, Higgs was largely met with scepticism, says Ellis. In 1976, Ellis and two of his colleagues at the CERN particle physics laboratory in Geneva, Switzerland, drawing attention to how the Higgs boson could show up in some experiments at the facility.

“No one really seemed to care, but to us, [the Higgs boson] was extremely important
 And I was absolutely sure that the Higgs boson will be found,” says at Texas A&M University, who co-authored the paper. He was a very young researcher at the time, but that study was prescient about the future of particle physics. By 1984, views among physicists had shifted and they were eager to hunt for the Higgs boson. Leadership at CERN discussed building a new particle collider, in large part to help with the search.

Peter Higgs on knowledge, immortality and the future of physics

In a rare interview, the Nobel laureate who predicted the Higgs boson talks about the search for new physics and why we need knowledge for knowledge’s sake

That detector – the Large Hadron Collider (LHC) – found the Higgs boson in 2012. Within the LHC, researchers engineered a careful head-on collision of two incredibly fast protons, a crash capable of producing a Higgs boson. But the boson only lasts for less than a billionth of a billionth of a second before becoming a shower of other particles. Analysis of the collision’s wreckage showed those particles had come from a Higgs boson with such high certainty that the odds of it being a fluke were just 5 in 10 million.

Physicists around the world were rapturous, and Higgs and Englert shared a Nobel prize in physics the next year.

Close and Ellis both say that even before the LHC started to operate, other colliders had obtained less direct evidence vindicating Higgs’s theory, such as very precise measurements of masses of other exotic particles. Higgs was aware of these findings, as he explained to żìĂš¶ÌÊÓÆ” in 2012: “I had faith in the theory behind the mechanism as other features of it were being verified in great detail at successive colliders. It would have been very surprising if the remaining piece of the jigsaw wasn’t there.”

Still, the direct search for the Higgs boson at the LHC had a strong influence on particle physics. It bolstered efforts to build new infrastructure like accelerators, and cemented the large collaborations that manage this equipment as a standard approach for conducting scientific research.

Since 2012, the LHC has been upgraded to produce even more energetic collisions, and researchers have set out to answer lingering questions about not only particles, including the Higgs boson itself, but also dark energy and dark matter, the unexplained phenomena that make up most of the universe.

Higgs himself was interested in some of those questions and kept working on them even after he retired in 1996. “The machine at Geneva – which was not designed just to discover the Higgs boson, though sometimes you get that impression – is expected to go on and improve our understanding of the links between particle physics and what happened in the early universe,” he told żìĂš¶ÌÊÓÆ” in 2013.

Finding the Higgs boson was the end of one chapter, but not the whole book, says Nanopoulos.

After his retirement, Higgs kept working on his own research. He was particularly interested in supersymmetry, which is a theory that posits the existence of heavy counterparts for every particle that we have detected already. Physicists who share this interest and want to find its experimental signatures hope that the LHC will discover dozens of new particles.

In addition to the Nobel prize, Higgs received several other accolades, including the Paul Dirac Medal and Prize, the Wolf Prize in Physics and the American Physical Society J. J. Sakurai Prize. In 1999, he turned down a knighthood, an act that fit his general rejection of fame. He did not want titles and was embarrassed by the media attention his work garnered over the years, particularly disliking the Higgs boson’s sensational nickname, the “God particle”.

The story of how Higgs even tried to evade the call from the Royal Swedish Academy of Sciences informing him of his Nobel win – by leaving his home without a cell phone – is well-known lore among physicists. Ellis also recalls that Higgs initially turned down the invitation to come to CERN for the official announcement of the discovery of his eponymous boson. But colleagues eventually convinced him to attend the festivities.

Close titled his biography of Higgs “Elusive”, which he says described both the man and the boson. Physicists widely agree that he was one of a kind and respected him for it.

Higgs died in his home in Edinburgh on 8 April after a short illness. He leaves behind two sons, a reinvigorated field of particle-seeking physicists and a clearer understanding of the forces that hold the universe together.

Researchers have measured the strong force, which binds together the particles that make up protons and neutrons, to the highest degree of precision ever. Despite being the most powerful of all the fundamental forces of nature, its strength is more uncertain than any of the others. Measuring it exactly is key to understanding the nature of the world around us.

The other fundamental forces – gravity, the electromagnetic force and the weak force – all get weaker as the particles they’re acting on get further apart. But the strong force gets even more powerful. This causes exotic effects that neutralise it, making it tough to measure directly.

“The only way we can observe the strong force is indirectly,” says at the CERN particle physics laboratory near Geneva, Switzerland. “This measurement is particularly difficult, and the improvement that we’ve had since the mid-80s has been quite slow.”

Camarda and his colleagues used the ATLAS experiment at the Large Hadron Collider (LHC) to make a leap in precision, bringing the relative uncertainty in the force’s strength down to 0.8 per cent. “This measurement represents an improvement of a factor of 2 to 3 with respect to the previous best experimental measurements,” says at the University of Maryland.

The researchers measured the strong force by slamming pairs of protons together, which produced a particle called a Z boson. If there were no force mediating the interactions between the protons, the final Z boson would be at a standstill. But the strong force imparted a small “kick” to this particle. Its resulting momentum depended on the strong force’s magnitude.

This is important to study because the value of the strong force is one of the largest sources of uncertainty remaining in the standard model of particle physics. “Anything we measure at the LHC, any prediction that we compute, depends on the value of the strong [force],” says Camarda. Unless we decrease the uncertainty in the strong force, it will be difficult to tell whether the LHC spots evidence of physics beyond the standard model, he says.

Subscriber-only newsletter

Sign up to Lost in Space-Time

Sign up to newsletter

The strong force is also crucial to our understanding of the fate of the universe. There is a small possibility that eventually the universe will end through a phenomenon called vacuum decay, in which a quantum fluctuation leads to a small bubble of unusual space-time called pure vacuum, which would then quickly grow and devour the entire cosmos. “The probability that the universe will disappear in a quantum bubble is very low,” says Camarda. “But we have uncertainty in this statement, and that uncertainty is driven by the value of this force.”

Even with this new measurement, our knowledge of the strong force still falls short of our precise calculations of the other fundamental forces. And the measurements are so difficult that it’s unlikely we will reach that same exactness anytime soon, even with better data. But there are proposals for a new collider at CERN that will be purpose-built to study the Z boson. If it is constructed, then perhaps we will reach that level of precision after all.