The tale of two detectors

Editor’s note: This is a modified version of an article previously posted by the CERN Courier.

The TOTEM collaboration at the LHC, in collaboration with the DØ collaboration at the Tevatron collider at Fermilab, have announced the discovery of the odderon – an elusive state of three fundamental particles called gluons that was predicted almost 50 years ago. The result was presented on Friday 5 March during a meeting at CERN, and follows the joint publication in December 2020 of a CERN/Fermilab preprint by TOTEM and DØ reporting the observation.

“This result probes the deepest features of the theory of quantum chromodynamics, notably that gluons interact between themselves and that an odd number of gluons are able to be “colourless”, thus shielding the strong interaction,” says TOTEM spokesperson Simone Giani of CERN. “A notable feature of this work is that the results are produced by combining the LHC and Tevatron data at different energies.”

States comprising two, three or more gluons are usually called “glueballs”, and are peculiar objects made only of the carriers of the strong force. The advent of quantum chromodynamics (QCD) led theorists to predict the existence of the odderon in 1973. Proving its existence has been a major experimental challenge, however, requiring detailed measurements of protons as they glance off one another in high-energy collisions.

While most high-energy collisions cause protons to break into their constituent quarks and gluons, roughly 25% are elastic collisions where the protons remain intact but emerge on slightly different paths (deviating by around a millimetre over a distance of 200 m at the LHC). TOTEM measures these small deviations in proton–proton scattering using two detectors located on either side of the CMS experiment 220 m from the interaction point , while DØ employed a similar setup at the Tevatron proton–antiproton collider.

At lower energies, differences in proton–proton vs proton–antiproton scattering are due to the exchange of different virtual mesons – particles made up of a quark and an antiquark. At multi-TeV energies, on the other hand, proton interactions are expected to be mediated purely by gluons. In particular, elastic scattering at low-momentum transfer and high energies has long been explained by the exchange of a pomeron – a “colour-neutral” virtual glueball made up of an even number of gluons.

However, in 2018, TOTEM reported measurements at high energies that could not easily be explained by this traditional idea. Instead, a further QCD object seemed to be at play, supporting models in which a three-gluon compound, or one containing higher odd numbers of gluons, was being exchanged. The results were sufficient to claim evidence for the odderon, although not yet its definitive observation.

The new work is based on a model-independent analysis of data at medium-range momentum transfer. The TOTEM and DØ teams compared LHC proton–proton data (recorded at collision energies of 2.76, 7, 8 and 13 TeV and extrapolated to 1.96 TeV), with Tevatron proton–antiproton data measured at 1.96 TeV, and found evidence again for the odderon. When the teams combined the result with measurements at different scattering angles at 13 TeV by the TOTEM collaboration, the significance of the result was boosted to the discovery level.

“When combined with the measurements at 13 TeV, the significance of the result is in the range of 5.2–5.7 standard deviations and thus constitutes the first experimental observation of the odderon,” said Christophe Royon of the University of Kansas, who presented the results on behalf of DØ and TOTEM last week. “This is a major discovery by CERN and Fermilab.”

In addition to the new TOTEM-DØ model-independent study, several theoretical papers based on data from the Intersecting Storage Rings, the Super Proton Synchrotron, the Tevatron and the LHC, and on model-dependent inputs, provide additional evidence supporting the conclusion that the odderon exists.

 

 

Beneath its surface, a proton teems with activity.

You’ve probably heard that particles such as protons have mass and charge. They also have a property called spin. Technologies such as magnetic resonance imaging harness the proton’s spin to see inside the human body.

While scientists know how to harness spin, they’re still piecing together the story of where a proton’s spin comes from. The SpinQuest experiment at the Department of Energy’s Fermilab may help us understand what the components of spin actually are.

A brief history of how scientists see a proton and its spin

Spin is represented using unitless numbers such as ½ or 1, and, like mass or charge, is an intrinsic property of particles. Protons have a spin of ½ and two orientations: spin up and spin down.

Protons are composed of even smaller particles called quarks and gluons, which have their own spin.

Early proton spin models represented rice porridge. A glutinous rice, signifying the gluons, formed a base for tastier ingredients, namely three quarks bubbling to the surface. In this model, the surface quarks that contributed to the proton’s spin are united by gluons.

Scientists realized that the rice porridge model oversimplified reality when their experiments yielded an unexpected result: The spins of a proton’s surface quarks did not add up to a proton’s spin-½.

The closer a proton’s surface quarks are to each other, the more mischief they drum up. As it turns out, when the surface quarks get close enough, they exchange gluons, and the gluons themselves generate even more quarks in the form of quark-antiquark pairs. The quark-antiquark pairs, called “sea quarks,” are like carbonation bubbles, disappearing as quickly as they emerge.

Today, spin models are more akin to a potato salad within which many ingredients, including surface quarks, gluons, sea quarks and the interactions among them, meld and contribute to the spin of a proton.

SpinQuest will be one of the first experiments to directly test whether the sea quarks, through their own orbiting motion, are components of proton spin. SpinQuest is supported by the DOE Office of Science.

Studying the sea quarks

Scientists theorize that the sea quarks might balance the books on proton spin. To investigate, they will look oppositely charged muons produced in the SpinQuest experiment.

Wait, muons?

Yes, muons.

SpinQuest starts at Fermilab’s Main Injector accelerator, which will fire our familiar protons at a polarized target. A quark from a proton in the proton beam and an antiquark from a proton in the target will interact, eventually producing a pair of oppositely charged muons, heavier cousins of the electron.

Scientists will then measure two different angles: the angle between a flat surface formed from the muon tracks and the orientation of the target proton’s spin when it points up, and another angle when the target proton’s spin points down.

“The angle between the muon plane and the proton spin polarization is believed to be directly connected to how the sea quarks contribute to proton spin,” said Kun Liu, co-spokesperson for SpinQuest and scientist at Los Alamos National Laboratory.

If SpinQuest scientists see that the angle changes with target proton spin orientation, they can conclude that sea quarks contribute to a proton’s spin in some way, pointing researchers to the sea quarks for the missing components of a proton’s spin. No difference, and scientists need to reexamine the theoretical underpinnings of proton spin.

Dark sector searches

Another project for scientists will be to analyze data from the SpinQuest spectrometer in a search for dark sector particles. This separate experiment could revolutionize our understanding of fundamental structures and interactions in the universe.

Specifically, scientists will look for two dark sector particles – dark photons and dark Higgs bosons, which have ordinary-matter counterparts in photons (light) and Higgs bosons (which confer mass to particles). If discovered, dark photons and dark Higgs bosons could begin to answer questions about the 25% of matter in our universe we know is there but cannot see — dark matter.

“We are able to conduct this dark sector search without diverting any of the proton beam or requiring additional technology,” Liu said. “Because this area of study has such great scientific and discovery potential, we thought it imperative that we also pursue this other fascinating line of inquiry, called a parasitic mode, while SpinQuest is running.”

Setting records in a reuse and recycle effort

Although the experiment will use existing equipment with minor modifications, it was still challenging to build.

“SpinQuest leverages the power of Fermilab’s Main Injector accelerator, but this has created some challenges for the custom-built, polarized target,” said SpinQuest co-spokesperson Dustin Keller and professor at the University of Virginia. “In overcoming these, we might very well set some records.”

The target, which will operate at 1 kelvin, will be subject to the highest-intensity proton beam ever to hit such a target. Once installed and operational, the target and the superconducting magnet that polarizes it will be under constant threat. Bombarding protons will generate heat in both the target and superconducting magnet, excess energy that may limit the potential of the experiment.

To combat the heat load, scientists and engineers assembled a large pumping system that works in tandem with a cryogenic refrigerator developed at the University of Virginia to produce one of the highest-cooling-power evaporation systems to date. The system will keep the target cold and polarized even while under the eye of the intense proton beam.

Such challenges only bring the collaboration together.

“This experiment really gets back to Fermilab’s roots,” said Rick Tesarek of Fermilab, one of the lead scientists on SpinQuest. “It’s a small experiment making a hard measurement with a dedicated group of people, mostly students and postdocs, working to produce a result they only have read about in textbooks.”

After first beam, which is expected in spring 2021, scientists will commission their detector, turn on the target and make sure both operate well under the high-intensity proton beam.

Then, they will start collecting data.

Scientists around the world await results that may change our theoretical understanding of spin, spark new experiments, discover a dark realm of particles or all of the above.

The SpinQuest collaboration consists of 50 individuals from 13 institutions from around the world.

The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.