Exclusive production: shedding light with grazing protons

When two protons approaching each other pass close enough together, they can “feel” each other, similar to the way that two magnets can be drawn closely together without necessarily sticking together. According to the Standard Model, at this grazing distance, the protons can produce a pair of W bosons.

When two protons approaching each other pass close enough together, they can “feel” each other, similar to the way that two magnets can be drawn closely together without necessarily sticking together. According to the Standard Model, at this grazing distance, the protons can produce a pair of W bosons.

As its name implies, the primary mission of the Large Hadron Collider is to generate collisions of protons for study by physicists at experiments such as CMS. It may surprise you to find out that the vast majority of protons accelerated by the LHC never collide with one another. Some of these fly-by protons, however, still interact with each other in such a way as to help physicists shed light on the nature of the universe.

The LHC accelerates bunches of protons, with more than 10 billion protons in each bunch, in opposite directions around the ring. As those protons arrive at a detector, such as CMS, magnets focus the beams to increase the density of protons and thus increase the chance of a coveted collision. Despite what seems like overwhelming odds, only a few of these protons actually collide with each other: tens to hundreds per each beam “crossing.” An even smaller fraction of the remaining protons pass close enough to other protons to “feel” each other, even if they do not directly collide.

Finn Rebassoo of Lawrence Livermore National Laboratory was one of the primary analysts for this result.

Finn Rebassoo of Lawrence Livermore National Laboratory was one of the primary analysts for this result.

Think of two toy magnets on a tabletop: A north end and a south end moved close enough to each other will rather firmly stick to each other. However, you can also move one magnet just close enough to the other that you can make it wiggle without drawing it all the way over. This exchange of energy is mediated by the exchange of photons, the carrier particle of the electromagnetic force. Similarly, two protons in the LHC that get just the right distance from each other will exchange photons without colliding.

Now for the part that gets really interesting to particle physicists. The photons generated by these near-miss proton interactions can be billions of times more energetic than those of visible light, and as a result they carry enough energy to create particles in their own right. The Standard Model predicts the production of massive particles, such as pairs of W bosons, from these interacting photons without any of the additional activity that is seen in the messier proton-proton collision events. In a detector such as CMS, this pair of W bosons is said to be produced “exclusively.” However, “exclusive production” is an apt name in another way – creating a pair of W bosons from interacting photons is a rare occurrence in an even rarer sample of photons generated from near-miss proton interactions.

CMS scientists performed such a search for such W boson pairs emanating from interacting photons. In a data set consisting of 7- and 8-TeV collisions, 15 candidate events for this process were observed. While it may not seem like much, the expected background was considerably smaller, allowing the CMS team to claim that they have evidence of the process. (In the particle physics world, evidence is a three-standard-deviation departure from background, as explained here). Furthermore, these results helped place stringent results on a number of models which predict a greater rate of this process.

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