Finding new answers to old questions

Last month, the NA63 collaboration presented its annual update at the 107th meeting of the SPS and PS experiments committee (SPSC). Among the many results presented were new insights into arenas of experimental quantum electrodynamics – including some that will have theorists heading back to the blackboard.


NA63 Collaboration members at work on the positron-production experiment. Image: NA63.

Based at CERN’s SPS North Area, the NA63 experiment uses high-energy beams for crystalline studies of various electromagnetic processes. “This year has been very productive for us, both in terms of data gathering and data analysis,” says Ulrik Uggerhoj, NA63 collaboration spokesperson. “We’ve released results based on previous runs, assembled and tested an entirely new experimental set-up, and have also gathered enough data to keep us occupied during the 2013 shutdown.”

In September, the collaboration announced a new result that considerably improves our understanding of beamstrahlung – the process experienced in colliders when there is an emission of synchrotron-like radiation due to interacting beams. By using crystalline targets in order to create virtual opposing bunches, NA63 was able to simulate the beamstrahlung effect using only a single beam. “We confirmed that there is a very important quantum effect that actually diminishes the beamstrahlung,” explains Ulrik. “If you calculate the beamstrahlung using classical electrodynamics, you get something like a factor of 2 more than quantum theory says. So, the correction is quite significant, particularly for accelerators such as CLIC.”

The complete positron-production experiment set-up: at the centre of the gold-coloured ring is the aligned diamond crystal that generates the positron beam. Image: NA63.

NA63 also examined the impact of the Landau–Pomeranchuk–Migdal (LPM) effect using low-Z targets. The LPM effect has been experimentally proved for high-Z materials for well over two decades, but its low-Z applicability had never been verified. Its effect at this level is especially important for analyses of extended air showers, as our nitrogen-heavy atmosphere is itself a low-Z material. Preliminary NA63 results indicate that the most widely used LPM theory is inaccurate for low-Z materials. While the extent of the theory correction that will have to be made is still unclear, this is a likely sign for theorists to reconsider the shelved problem.

The experiment’s main task for 2014 will be the exploration of a new positron production technique. By bombarding a diamond crystal with electrons, NA63 will be able to generate a low-emittance positron beam with high intensity specifically suited for the needs of CLIC. “For this purpose, it turns out that single crystals are a big advantage,” explains Ulrik. “It is likely that, in certain CLIC parameter regimes, the accelerator will be able to gain a factor of 2 in the combined emittance/intensity regime. We performed proof-of-principle measurements for this during the last run (see images), and we look forward to proving the technique.”

by Katarina Anthony