Of vacuum and gas

A new LHCb programme is delving into uncharted waters for the LHC: exploring how protons interact with noble gases inside the machine pipe. While, at first glance, it may sound risky for the overall quality of the vacuum in the machine, the procedure is safe and potentially very rich in rewards. The results could uncover the high-energy helium-proton cross-section (with all the implications thereof), explore new boundaries of the quark-gluon plasma and much more.


As the beam passes through LHCb, interactions with neon gas allow the experiment to measure the full beam profile. In this diagram, beam 1 (blue) and beam 2 (red) are measured by the surrounding VELO detector.

It all begins with luminosity. In 2011, LHCb set out to further improve its notoriously precise measurements of the beam profile, using the so-called Beam-Gas Imaging (BGI) method. BGI does exactly what it says on the tin: a small amount of gas is inserted into the vacuum, increasing the rate of collisions around the interaction point, thus allowing LHCb to measure the beam profile without displacing the beams themselves. “To accomplish this, we obtained support from our vacuum group colleagues to use different noble gases, first neon, then helium, and finally argon,” says Massimiliano Ferro-Luzzi, a physicist in the LHCb collaboration. “During the first few weeks of Run 2, using neon gas, we were able to measure the luminosity to a precision of 3.9% in one short LHC fill.” This adds to LHCb’s growing catalogue of high-precision measurements using neon, including those taken during Run 1.

While these results are admirable achievements on their own, they have also opened the door to a whole new domain of physics explorations. Upon learning of this gas-injection system, cosmic-ray and heavy-ion physicists approached the LHCb team – all eager to develop new types of beam-gas analysis. Now, only five months into Run 2, LHCb has already had special proton-helium, proton-neon and proton-argon runs.

“In fact, we are exploring the full range of noble gases, as they can be safely injected into the LHC vacuum,” explains Colin Barschel, an LHCb physicist. “Noble gases are not absorbed by the NEG coating and can be easily pumped out of the vacuum. Any remaining gas continues down the vacuum to the cold magnets, where it is ‘captured’ by the walls of the magnets. We’ve been able to do up to 24 hours of injection without any detrimental effect on the LHC performance.”

Helium: understanding antimatter in cosmic rays

One of the most exciting parts of LHCb’s new beam-gas programme is undoubtedly the proton-helium analysis. Its story, however, begins far above the LHC… in detectors outside of Earth's atmosphere, hunting for antimatter in cosmic rays.

“Recent cosmic-ray measurements, notably by the AMS-02 detector, have shown an excess of antiprotons compared to protons in the cosmos,” says Patrick Robbe, LHCb run coordinator and a physicist at LAL Orsay. “While these antiprotons may come from new physics processes, they may also be due to proton collisions with interstellar medium (primarily made up of helium and hydrogen). And while our understanding of proton-hydrogen interactions is quite good, the proton-helium cross-section is not well known.”

The gas injection system installed near the VELO detector at LHCb.

That’s where LHCb comes into play: “Our cosmic-ray colleagues who are also involved with the PAMELA experiment quickly recognised the potential of the gas injection system: it could allow us to simulate the cosmic environment and measure the proton-helium cross-section in the relevant energy range,” says Giacomo Graziani, who is responsible for the LHCb team at INFN Firenze. “These results should decrease the uncertainties on the computation of the antiproton flux, allowing cosmic-ray experiments to improve the interpretation of their measurements.” Giacomo’s team is now examining the first proton-helium data, which was gathered in early October. Read more about the analysis in the box below.

Argon: transforming LHCb into a fixed-target experiment

Moving down the periodic table, we find argon – an ideal candidate for heavy-ion physics. “With its higher number of nucleons, injecting argon into the vacuum increases the energy density of collisions,” says Patrick. “During this coming lead-ion run, we will collide lead beams against this heavy argon ‘fixed target’. The aim is to have a very high energy density, comparable to that of the fixed-target experiments performed at the SPS in the 80s and 90s.” These collisions will have lower multiplicities than lead-lead collisions, and so should be easier to analyse. 

After the ion run, the LHC will enter the Christmas shutdown and its cold magnets, if required, will be brought up in temperature. At that time, the accumulated gases will be released and pumped out of the machine – wiping the slate clean for LHCb, which will continue to explore proton-gas collisions in 2016.

Did you know?

Developed solely for luminosity measurements, LHCb’s gas-injection system does not measure the gas density at the interaction point precisely. Without this key factor to disentangle their data, the LHCb team had to develop an unconventional approach to “measuring”. 

The team is reverse engineering the gas-density, calculating the number of nuclei by studying known processes – namely, single-electron scattering. “We are looking for single electrons scattered from helium, neon and argon atoms when hit by proton beams,” says Giacomo. “The number of electrons allows us to work out the density of the gas at the collision point.” This is accomplished with the help of a very “open” trigger that records almost 100% of the collision event data – including these single electrons.


by Katarina Anthony