Oddball antics in proton–proton collisions

The TOTEM collaboration at CERN has uncovered possible evidence for a subatomic three-gluon compound called an odderon, first predicted in 1973. The result derives from precise measurements of the probability of proton–proton collisions at high energies, and has implications for our understanding of data produced by the LHC and future colliders.

In addition to probing the proton structure, TOTEM is designed to measure the total cross section of proton–proton collisions. Physically it is by far the longest experiment at the LHC, comprising two detectors located 220 m on either side of the CMS experiment. While most proton–proton interactions at the LHC cause the protons to break into their constituent quarks and gluons, TOTEM detects the roughly 25% of elastic collisions that leave the protons intact. Such collisions merely cause the path of the protons to deviate, by around a millimetre over a distance of 200 m.

Elastic scattering at low-momentum transfer and high energies has long been successfully explained by the exchange of a pomeron – a colour-neutral state made up of an even number of gluons – between the incoming protons. But TOTEM’s latest results seem to be incompatible with this traditional picture.

The discrepancy came to light via measurements of a parameter called ρ, which represents the ratio of the real and imaginary parts of the nuclear elastic-scattering amplitude when there is minimal gluon exchange between the colliding protons and thus almost no deviation in their trajectories (corresponding to a vanishing squared four-momentum transfer, t). TOTEM measured the differential elastic proton–proton scattering cross section down to t = 8 × 10⁻⁴ GeV² at an energy of 13 TeV during a special LHC run involving “β^∗ = 2.5 km” optics and, exploiting Coulomb–nuclear interference, determined ρ with unprecedented precision: 0.09 ± 0.01.

While conventional models based on various pomeron exchanges and related “even-under-crossing” scattering amplitudes can describe ρ and the total proton–proton cross-section in the energy range 0.01–8 TeV, none can describe simultaneously TOTEM’s latest ρ measurement (which is lower than predicted by conventional models) and TOTEM’s total cross-section measurements ranging from 2.76 to 13 TeV (see figure). Combining the two measurements, TOTEM finds better agreement with models that indicate the exchange of three aggregated gluons.

The odderon started out in the early 1970s as a purely a mathematical concept. After the advent of QCD, however, theorists showed that QCD not just allowed but required the existence of such a three-gluon compound.

Although the new data favour the existence of the odderon, the TOTEM collaboration prefers to emphasise all the possible meanings and consequences its results might have – in particular concerning the behaviour of the total proton–proton cross section at high energies. If it turns out that the odderon is not entirely responsible for the observed decrease in ρ at 13 TeV, then it could be the first observation that the proton–proton cross-section growth slows down at energies beyond this. Either way, claims the TOTEM team, the results would constitute an important discovery.

“The TOTEM result is in a reasonable agreement with what is expected within the QCD picture, and the inclusion of the odderon certainly improves our description of the existing data on the high-energy elastic proton–proton scattering,” says theorist and QCD expert Valery Khoze of Durham University in the UK. “Conservatively, I would say that this is a strong indication in favour of the experimental observation of a long-awaited but so far experimentally elusive object predicted by QCD.”

Basarab Nicolescu of Babes-Bolyai University in Romania – who co-invented the odderon with the late Leszek Lukaszuk – and Evgenij Martynov of the Bogolyubov Institute for Theoretical Physics in Ukraine go further. In a paper published shortly after the TOTEM result, they write that the new data “can be considered as the first experimental discovery of the odderon”.

TOTEM researchers say they will continue to refine their measurements of ρ and explore how this ratio of scattering amplitudes evolves as a function of the squared four-momentum transfer. A similar “forward” experiment at the LHC called ALFA, which is part of the ATLAS experiment, is also taking part in such t-channel studies of the proton–proton cross section.

However, if a three-gluon compound is being produced in proton–proton collisions, it should also appear in other scattering experiments via direct s-channel production. Such a signature of the odderon could be detected, for example, by the LHCb experiment and also the COMPASS experiment at CERN.

“The discovery of the odderon would signal another bright manifestation of the predictive power of the QCD theory and confirm again that perturbative QCD allows for quite fair predictions in the experimentally available domain,” says Khoze.