MEET SPACE’S NEW PARTICLE

First proposed in the 1970s, the LHC may have finally found evidence of the elusive odderon

There are very rare cases in the history of physics that a scientific and testable idea is neither proved nor disproved 33 years after its invention,” wrote Basarab Nicolescu in a 2007 paper, describing over a quarter of a century of debate over the odderon. A virtual – or quasi – particle, Nicolescu believed it played a vital role in forming the matter of the universe by keeping atomic nuclei locked together.

However, having never been observed, did it really exist? Another decade on and Nicolescu may finally have closure on his proposed particle, following a recent experiment carried out at the Large Hadron Collider (LHC). These results suggest it is finally time to welcome a new virtual member to our particular family of fundamental components of cosmic stuff that have been observed inside the world's largest and most powerful particle accelerator. But what do we mean by a virtual particle?

The story starts in quantum field theory, a set of equations that give physical form to subatomic particles by describing them as excited states of a more fundamental underlying field. In this framework the interactions between ordinary, familiar particles – what we tend to think of as forces – are described as the exchange of virtual particles, which are similar to ordinary particles, except their excited states last only momentarily. Electromagnetic repulsion or attraction, for example, can be thought of as the exchange of virtual photons, where the virtual photon is the force carrier.

The odderon was first proposed by Leszek Lukaszuk and Basarab Nicolescu in 1973 as a potential force carrier for the strong interaction – or nuclear – force. This is the attractive force between protons and neutrons which tends to overcome the electromagnetic repulsion of similarly charged protons in the atomic nucleus, holding it all together. Despite this vital role, some have tended to demote the odderon and its like to that of ‘quasiparticles’.

Paul M. Sutter, an astrophysicist at Ohio State University, writing for livescience.com this year, described quasiparticles as “brief, effervescent patterns or ripples of energy that appear in the

“The odderon is a compound of gluons as a proton is a compound of quarks and gluons” Simone Giani

“Physicists take some shortcuts and pretend that these patterns are their own particles, so quasiparticles are treated like particles even though they aren't” Paul M. Sutter

midst of a high-energy particle collision. Since it takes a lot of legwork to fully describe that situation mathematically, physicists take some shortcuts and pretend that these patterns are their own particles. So, quasiparticles are treated like particles even though they definitely aren't.”

Not everyone likes this term, nor the implications for the status of the particles given it. “I do not agree with the concept of the so-called quasiparticles,” says CERN’s Simone Giani. “The odderon is a compound of gluons as a proton is a compound of quarks and gluons; as an atom is a compound of protons, neutrons and electrons; as a molecule is a compound of atoms.”

Whatever name you prefer, the work of

Lukaszuk and Nicolescu suggested the odderon could appear fleetingly in specific, high-energy but non-destructive, low-angle collisions of protons and antiprotons. Such interactions produce complex sequences of events involving the exchange of energy between colliding components, and its conversion into matter in the form of new particles.

Timothy Raben, a post doctoral theoretical physicist at Michigan State

University and colleague of Giani, described these mash-ups “like two big semi trucks that are transporting cars, the kind you see on the motorway. If those trucks crashed together, after the crash you'd still have the trucks, but the cars would now be outside, no longer aboard the trucks – and also new cars are produced as energy and transformed into matter.”

However, before exploring ways to test their theory, Lukaszuk and Nicolescu first had to face the backlash. Back in the 1970s their odderon theory went against a lot of conventional wisdom of the time. “Once I started a conference on hadronic elastic scattering saying that proton-proton versus proton-antiproton strong interactions have always been assumed to be identical, and never observed experimentally to be identical,” recalls Giani. “The old community is very much against it.”

“Revolutionary, if not heretical” was the phrase Nicolescu used in a recent paper published in Physics Letters B with Evgenij Martynov to described how his propositions sat among the scientific consensus. However, it wasn't long before supporting evidence came to light. Seven years after its introduction to the world of particle physics, the odderon was rediscovered in a second set of equations. This time it was as an essential prediction of quantum chromodynamics (QCD), the theory that describes the strong interaction between quarks and gluons, the fundamental components that make up hadrons such as protons and neutrons.

QCD theory described the odderon as a compound of three, or a larger odd number, of gluons which are exchanged between protons interacting via the strong force. However, odderons are not the only one to facilitate this interaction. Compounds of an even number of gluons can do the same job, with such virtual particles instead named pomerons.

However, the QCD evidence was supportive rather than definitive, and the theory still very much had its critics. Back in 1990 at the annual Rencontres de Moriond conference of new ideas in physics, Nicolescu explained his continued belief in

his theory by quoting 19th-century French scientist Claude Bernard: “When a fact arises which is in contradiction with a dominant theory, one has to accept the fact and to give up the theory, even if this theory is supported by famous names and it is generally accepted.” In the same talk Nicolescu had some choice words for the criticism his theory had faced, which he said had sometimes taken the form of a religious language.

“One can ask ourselves if, in the light of recent QCD results, the heretical is becoming now sacred. In any case, I cannot agree with such a religious language,” he declared. It was clear the only way to end the speculation was a direct observation of the odderon. However, in all previous experiments only ‘pomeronic’, even-numbered gluon compounds were detected being exchanged during collisions.

One option was to look for products of subsequent odderon bonding. The three-gluon virtual particle was thought to be able to fuse with other particles emitted from the smashed-up proton – such as a light-carrying photon – creating mesons, another family of subatomic particles composed of one quark and one antiquark. This could be detected with the particle accelerators of the time. However, such processes are dominated by pomerons and pomeron-photon exchanges, making the observation of a three-gluon state difficult.

Another approach proposed comparing the cross sections of particle-antiparticle collisions with that of proton-proton collisions to look for predicted differences in the profiles. All that was required were machines capable of repeatedly smashing protons and protons and protons and antiprotons together at the requisite energies.

However, by the mid-2000s there was a lack of facilities available to perform such experiments. The Intersecting Storage Rings (ISR), a particle accelerator at CERN and the world's first hadron collider, had offered the first strong hint for the existence of the odderon, but it closed shortly after in the mid-1980s. “This is the main reason of the non-observation till now of the odderon,” wrote Nicolescu in 2007, shortly after the death of Lukaszuk. However, his lament at the limitations of experimental set-ups, was accompanied with hope for the future: “Another crucial and spectacular test of the possibility at high energies would be offered by the LHC collider. It would be a pity not to continue the comparison of p-ap and p-p scattering which is, of course, of great importance to an understanding of strong interaction dynamics.”

Though the first experiments were run at CERN’s Large Hadron Collider (LHC) in 2008, it would take until 2015 for it to be operating at sufficiently high energy to look for the odderon. In the meantime the DZero experiment at the Fermi National Accelerator Laboratory in the US was providing comparative data with precise studies of the interactions of protons and antiprotons at the highest available energies offered by its Tevatron collider. If the LHC could collect proton-proton

collision data performed at similar energies then direct comparison would allow the team to identify any differential elastic scattering cross sections caused dominantly by an odderon contribution.

The LHC instrument for the job was the TOTal Elastic and diffraction dissociation Measurement experiment (TOTEM). TOTEM has detectors spread across almost half a kilometre from one of the four points in the supercollider where proton beams are directed into each other, causing billions of collisions every second. Rather than observing the obliteration of full-on collisions, the TOTEM experiment was designed to detect protons that are only slightly deviated by collisions. “They represent about 25 per cent probability of what may happen in an LHC collision between protons. In the remaining 75 per cent they break,” says Giani.

Analysis of these interactions at TOTEM was compared with the highest energy results from the DZero in a paper published in December 2018. Almost half a century after the odderon was first predicted, and despite an 800 GeV gap in the energies of collisions, Giani and his colleagues were able to announce with an “extremely high degree of confidence and significance” that they had their virtual particle. “The difference is characterised by the oscillation into a dip and then a local max, which are present in the p-p differential cross section [recorded at TOTEM] and not in the pp-bar differential cross section [recorded at DZero],” says Giani.

However, that 800 GeV difference still represents a small leap of faith, so Giani and his colleagues will soon publish a joint paper with the DZero team

“This doesn’t break the Standard Model, but there are very opaque regions of the Standard Model, and this shines a light on one of those opaque regions”

Timothy Raben

to confirm the results after extrapolating them to a common energy.

“The DZero data by itself cannot establish evidence for the odderon. But when one takes both DZero data at 1.96 TeV for antiproton-proton and TOTEM data for proton-proton at the close value of 2.76 TeV, one finds a strong evidence for the odderon,” says Nicolescu.

Further modelling since published claims to confirm that for any model to be consistent with TOTEM’s data, a three-gluon-state exchange was essential. Nicolescu himself, writing with Martynov in Physics Letters B in March 2018, confirmed their own analysis of the results showed that the new TOTEM datum can be considered as the first experimental discovery of the odderon. “The odderon must be present in order to explain the new TOTEM datum,” they wrote.

So what does the odderon confirmation mean for particle physics and our wider understanding of the cosmos? “The main implications are, both mathematically and empirically, that when a particle exists in its virtual state to mediate interactions, it should also exist as a standalone particle,” says Giani. “This doesn’t break the Standard Model, but there are very opaque regions of the Standard Model, and this work shines a light on one of those opaque regions,” Raben said in the press release that accompanied their 2018 paper.

Jerome Luine, principal scientist at Northrop Grumman Next Basic Research, told Now magazine – the company's research title – that it is unlikely we will know all the consequences of this observation right away. “When electrons were discovered, no one had a clue as to what they would eventually be important for. Now, of course, it’s the basis for all our electronics. We’re in a similar situation right now with QCD; we don’t know what we’ll do with this knowledge in the future, but it could be critical information for building in future technologies.”

However, perhaps simply ending nearly half a century of speculation is reason to celebrate in its own right as we welcome a new member to our (virtual) particle family.