This year will be a fallow one for the Large Hadron Collider (LHC). The accelerator and its experiments are still being upgraded and the 27-km-circumference collider is not due to restart until 2015. However, all is not quiet at the CERN particle-physics lab near Geneva: the accelerators that feed protons into the LHC – the Proton Synchrotron (PS) and the Super Proton Synchrotron (SPS) – will both be fired up in the second half of 2014, which means that lots of experiments at CERN should be taking data this year, including some that are entirely new.
One new experiment firing up is the rather prosaically named NA62, and physicists working on it are now in the final stages of installing their 270-m-long experiment on the SPS. The SPS itself has a circumference of 7 km and its experiments are in CERN’s “North Area”, from which NA62 takes part of its name (62 being simply an incremental experiment number). The NA62 collaboration is small by CERN standards but it still comprises about 150 physicists at 20 institutes worldwide. Their primary aim is to make an extremely precise measurement of the probability that a positively charged kaon will decay to a positively charged pion plus a neutrino/antineutrino pair.
The decay probability might seem an arcane value to measure, but as collaboration member John Fry of the University of Liverpool in the UK explains, the decay itself is “one of the few ways open to us to actually challenge the Standard Model of particle physics”. Unfortunately, measuring the decay is far from easy. “This is a very rare process and the probability that it happens is about one in 10 billion,” explains CERN’s Giuseppe Ruggiero, who is physics co-ordinator of NA62.
Tiny quantum fluctuations
Challenging the Standard Model is also what the experiments on the LHC are trying to do, but NA62 is taking a different approach. Instead of smashing protons together at high energies and looking for hints of new physics in the vast numbers of particles that fly off in all directions, NA62 is looking for evidence of tiny quantum fluctuations in a specific decay process. A kaon comprises an up quark and an anti-strange quark. The up quark is a “spectator” that does not take part in the decay, while the anti-strange quark is transformed into an anti-down quark. According to the Standard Model, this occurs via a quantum loop and the probability of the transition has been calculated to a high degree of precision.
However, hitherto unknown particles not predicted by the Standard Model could also contribute to the quantum loop. These particles could, for example, be “sparticles” that are predicted to exist by supersymmetric models of particle physics. Rather than revealing themselves in the final products of the decay, these particles would appear as quantum fluctuations and then contribute to the quantum loop before vanishing. These fluctuations could cause a significant deviation from the Standard Model decay rate – and measuring that discrepancy is the primary goal of NA62.
The NA62 experiment begins by smashing an intense beam of 400 GeV protons from the SPS into a beryllium target that is 40 cm long. This collision creates a mixed beam of about 800 million charged particles per second. Most of these particles are pions and protons, with just 6% being the kaons of interest.
Stamping kaons
The beam is then sent through a Cherenkov detector called KTAG, which identifies individual kaons by the Cherenkov radiation they create. Although all the particles in the beam have the same momentum, protons, pions and kaons have different masses and so are travelling at different velocities. They therefore create Cherenkov radiation at different angles, and by measuring these trajectories, KTAG can identify individual kaons and give each one a “time stamp” that allows a kaon to be followed through the rest of the experiment.
The kaons next encounter the Gigatracker, which is a silicon pixel detector that measures the momentum of each kaon to a high precision. This precision is needed to match the “mother” kaon to the “daughter” pion that it decays to. The kaons then drift through a 65 m section of the experiment where about 10% of them will decay. At this point the attention shifts to measuring the momentum of the daughter pions, which is done using a device dubbed the Straw Tracker. It has more than 7000 narrow drift tubes that are arranged into modules containing rows at right angles to each other.
Finally, a second Cherenkov detector called RICH measures the speed of each pion. This value allows the physicists to confirm that they were actually tracking a daughter pion rather than a daughter muon, which is produced in much greater numbers. There are in addition several other detectors that measure the decay products to ensure that only events associated with the desired decay channel are captured.
Beyond the Standard Model
The NA62 collaboration expects to see about two decay events in the eight weeks that it plans to run in late 2014. The experiment will then run continually for a further three years, which should yield a total of about 100 events. If the measured decay rate from that predicted by the Standard Model differs by a factor of two, NA62 will be able to measure this with a statistical uncertainty of 5σ, which is the gold standard of a discovery in particle physics. In other words, by the end of 2017, NA62 could discover physics beyond the Standard Model, something that its much bigger sister experiments on the LHC – ATLAS and CMS – have so far not managed to do.
In addition to its primary goal, NA62 will also be seeking evidence of “flavour violation” by looking for kaons that decay to final states that cannot occur in the Standard Model. The collaboration will also be making a very precise comparison of two similar kaon decay processes – one into an electron and neutrino and the other into a muon and neutrino. The ratio of the decay rates of these two processes is sensitive to physics beyond the Standard Model.
Soon we will be ready to ask our questions to nature
Augusto Ceccucci, CERN
“After years of construction everything is coming together, the software is coming together and the students are getting the physics programme ready and soon we will be ready to ask our questions to nature,” says Augusto Ceccucci of CERN who is spokesperson for NA62.
In the above video, Ceccucci and three colleagues talk about the physics of NA62 and how the experiment was planned and built.
A year of new start-ups
In addition to NA62, many other projects will also be starting up at CERN this year. They include new experiments studying the properties of antimatter, both of which will exploit CERN’s existing Antiproton Decelerator (AD). This facility fires protons from the Proton Synchrotron into a block of metal to create high-energy antiprotons, which are then slowed down before being used. CERN’s antiprotons should be available once again from 1 August and the AD’s four existing experiments – ACE, ALPHA, ASACUSA and ATRAP – are all expected to make use of them.
One of the new experiments on the AD is AEGIS, which is the first designed specifically to measure Earth’s gravitational pull on antimatter. This will be done by measuring the vertical distance a beam of antihydrogen atoms falls as it travels a set horizontal distance. Discovering even the tiniest of differences between the gravitational behaviour of matter and antimatter could shed light on mysteries such as why there is so little antimatter in the universe. But creating antihydrogen, which consists of an antiproton and a positron, is no mean feat and the AEGIS team will spend most of its time in 2014 fine-tuning its antihydrogen generator and its beam-creation set-up.
Running for the first time at the AD is an antiproton experiment called BASE, which aims to make the most precise measurement ever of the magnetic moment of the antiproton. By trapping a single antiproton using magnetic and electric fields, BASE physicists aim to improve the current experimental value of the magnetic moment by several orders of magnitude. By making the same measurements on protons, BASE could reveal a tiny difference in their values of their magnetic moments. Such a difference would imply that CPT symmetry – a fundamental symmetry of nature as far as we know – is violated and this would point to physics beyond the Standard Model.
Meanwhile, over at CERN’s Super Proton Synchrotron, existing experiments such as the Common Muon and Proton Apparatus for Structure and Spectroscopy (COMPASS) will be up and running in 2014.
