A special proton-proton run with larger beam sizes at the interaction point is intended to probe the p-p elastic scattering regime at small angles

Usually, the motto of the LHC is “maximum luminosity”. But for a few days per year, the LHC ignores its motto to run at very low luminosity for the forward experiments. This week, the LHC will provide the TOTEM and ATLAS/ALFA experiments with data for a broad physics programme.

The TOTEM experiment at Point 5 and the ATLAS/ALFA experiment at Point 1 study the elastic scattering of protons, which are not observable in normal operation runs. In the elastic scattering process, the two protons survive their encounter intact and only change directions by exchanging momentum.

To allow this special run, the operators play with the so-called beta-star parameter. The higher the beta star, the more de-squeezed the beams are, and the more parallel the beams are when they arrive at the interaction point. For this special run, the beta-star had to be raised to 2.5 km (whereas in normal runs it is as small as 0.4 m).

Running with such a high beta-star parameter is an achievement: during Run 1, at 8 TeV, a value of 1 km was reached. But with a higher energy, the two incoming protons deviate by smaller angles, for equal transferred momentum. Since the TOTEM and ATLAS/ALFA Roman Pot detectors cannot be moved closer to the beams, the beta-star parameter must be raised to even higher values to provide acceptance for the smaller angles. “The effort that is required for the machine to deliver beams with such a high value of the beta-star parameter is extremely challenging,” says Simone Giani, spokesperson of the TOTEM Collaboration. “We are very thankful to the LHC team for having pushed the machine to such extreme settings,” adds Karlheinz Hiller, ALFA project leader.

The TOTEM physics programme foreseen for this special high beta-star run features many interesting measurements. In addition to the precise determination of the total proton-proton interaction probability (closely related to the “cross-section”) at 13 TeV, TOTEM will focus on a detailed study of the region of low transferred momentum of the elastic scattering, that is, when the two protons barely interact and the scattering angles are very small.

An in-depth study of this region is important for many different reasons. First of all, the interaction probability seems to diverge for very small transferred momenta, but as this should not be physically possible, a detailed study of that region will shed light on what is happening when the two protons almost don’t interact.

Secondly, in the same region, the contribution of the electromagnetic interaction (“Coulomb” scattering) interferes with the nuclear part of the elastic interaction. Studying this interference zone can shed light on the internal structure of the protons, and on which part of the protons (either the peripheral or the inner part) is actually responsible for the elastic scattering process.

Part of the ATLAS/ALFA experiment apparatus at Point 1 in the LHC tunnel. (Image: Ronaldus Suykerbuyk/CERN )

Moreover, it is also possible to get information on the probability that two protons pass through each other without interfering, transparently. “This might appear awkward if you think of a proton as a billiard ball,” notes Simone Giani. “But the protons should be thought as multi-body quantum systems.”

In other words, to use a metaphor, one can imagine the two scattering protons as two large “galaxies” (made internally of tiny moving particles) launched at high speed against each other: there is a finite probability that the two “galaxies” will pass through each other without the inner particles interacting significantly.

Finally, the TOTEM collaboration plans to conduct physics studies looking for evidence of special states formed by three gluons, which are theoretically predicted but for which the experimental evidence is still weak.

The physics goal of the ATLAS/ALFA experiment is also to perform a precision measurement of the proton-proton total cross section, but then to use this to determine the absolute LHC luminosity at Point 1 for the 2.5 km run.

For ATLAS/ALFA, the interesting part of the spectrum is at low values of transferred momentum, where Coulomb scattering is dominant: since the Coulomb scattering cross-section is theoretically known, its measurement gives an independent estimate of the absolute luminosity of the LHC. The luminosity measurements are otherwise normally done via Van der Meer scans, during the standard high-luminosity runs.

“With good statistics – such as 10 million good elastic events – we hope to be able to measure the absolute luminosity with a 3% precision,” says Patrick Fassnacht, deputy project leader of the ATLAS/ALFA project.

The latest results published by the TOTEM Collaboration include a first observation of deviations from a pure exponential form of the elastic cross-section at 8 TeV. More information on the TOTEM website.

The latest result published by the ATLAS/ALFA Collaboration is the measurement of the total cross section from proton-proton elastic scattering at 7 and 8 TeV with the beta-star parameter at 90 m, whereas the data with beta-star at 1 km are still under analysis. More information on the ATLAS/ALFA website.