By the time it finished its first run, at the end of 2012, the Large Hadron Collider at CERN was providing about 500 million proton-proton collisions per second to each of the two largest particle detectors, ATLAS and CMS. When it resumes operation in 2015, it will produce collisions at even higher rates. Because of this, those of us wishing to record and analyse the data have to be very selective about which events we are going to save. We spend a lot of time and effort trying to filter out the common collisions, so we can focus on the rare events, when a Higgs Boson, or something else amazing, is produced. But even the supposedly dull stuff is helping to resolve some key questions about our universe.



The particle collisions at the LHC are at higher energies than we have ever achieved before in a laboratory. However, they are not the highest energy particle collisions we have ever seen. That distinction belongs to collisions in the upper atmosphere. Mysterious astrophysical accelerators spray super-high energy particles across the universe, and some of these bombard the Earth. (The fact that some of them hit us must be an accident, since anyone capable of producing such beams deliberately could surely have zapped us into plasma by now if they really wanted to.) These particles hit the constituent atoms of the upper atmosphere, shattering them and leading to enormous showers of secondary particles, which can be detected on the ground, or in balloons, by various technologies. Some of the particles are even measured in space, before they hit the atmosphere, by experiments such as the Alpha Magnetic Spectrometer on the International Space Station.



Such measurements allow us to estimate the energy of the incoming particle with varying degrees of accuracy. The rate at which they are arriving (in some rather bizarrely scaled units) is shown in this figure, as a function of the energy of the incoming particle.

The flux of high energy particles (so-called cosmic rays) arriving at the Earth as a function of the energy of the particle. The flux (on the vertical axis) has been scaled by a factor of the energy to the power 2.5. The equivalent centre-of-mass energies for various colliders are shown on the top axis. From: T. Pierog, EPJ Web Conf. 53 (2013) 01004. Photograph: T. Pierog/T. Pierog, EPJ Web Conf. 53 (2013) 01004

You can see that there are a few interesting features in that distribution. At very high energies, between 10¹⁹ and 10²⁰ electronvolts, the spectrum turns over and drops dramatically. This is thought to be due to one of two reasons. The universe is filled with a bath of very low-energy photons - the quanta of light - left over from the big bang. Protons at the high-energy end of the distribution start to have enough energy that, when they collide with these very low-energy photons, they can produce a new kind of particle (called a Delta - a particle like the proton but with a higher mass). Because this becomes possible, the probability of the collision goes up, and that means the cosmic rays are attentuated - hence the drop in the distribution. The other possibility is that at these energies the unknown cosmic accelerators simply run out of steam.



The second feature is between 10¹⁵ and 10¹⁶ electronvolts, where the gradient of the distribution changes. Astrophysicists call this "the knee". (There's also an "ankle" between 10¹⁸ and 10¹⁹ ). The reasons for this knee were thought to be either that the type of cosmic ray, or the way they are produced, changes at that energy, or that something weird happens in particle physics. The energy available in those collisions is higher than anything measured in a laboratory before the LHC, so we did not really know what particles at these energies were doing, or even what particles might be being produced. It was possible that our models were wrong, and something unexpected could change inside the shower of particles in the atmosphere. Since the properties of the shower of particles are used to infer the energy of the initial cosmic rays, this would lead to a change in the measured rate and energy, potentially explaining the knee.



On the upper horizontal axis of the plot you can see the centre-of-mass energies of the particle accelerators which are equivalent to the incoming cosmic ray energies on the lower horizontal axis. Note that the previous highest-energy collider, the Tevatron, was just below the knee. The LHC is just above it. The most common collisions (so-called "minimum bias") were measured by the main detectors at the LHC, and there are also relevant measurements from specialised detectors very close to the beam-line. The results have been compared to the expectations of our best models, and they agree - not perfectly, but pretty well. The models have been improved as a result, and that is important for the precision both of further LHC measurements and of cosmic rays studies. However, the main thing we learned was that nothing sudden or dramatic happens at these energies to cause the knee. So we know now that the knee has to be caused by a change in the type of particle (maybe from protons to alpha-particles or heavier nuclei) or something else to do with the cosmic ray source. So the LHC data, coupled with cosmic ray data, are telling us something about the violent astrophysics out there in the galaxy and beyond.

We also search for Dark Matter at the LHC, and the fact that we haven't found it yet also tells us something about astrophysics and the very large-scale features of the universe. Not bad for an experiment designed to study the tiniest known things.



There was a nice summary posted on the arXiv on Friday by David d'Enterria, which prompted me to write this. You can find it here if you want to know more, and this is also good. Other good things on the arXiv this week include ten chapters on the future of US particle physics, of which this is chapter one. Like some previous studies, these chapters break topics down into an "Energy Frontier", an "Intensity Frontier" and a "Cosmic Frontier". These are useful categories for a discussion, but the above results illustrate that fact that there are many interconnections between them, too.









