The HERA particle accelerator in Germany is set to call it quits in June, but a lone physicist is now campaigning for HERA to have one last hurrah. He claims it could discover a particle believed by many to account for the unseen dark matter that constitutes the bulk of the universe鈥檚 mass.
The particle in question is the axion. Proposed in 1977 to solve a problem with the strong nuclear force, these hypothetical entities have been considered as strong candidates for dark matter, because they have very little mass and barely interact with matter.
One of the most tantalising hints of their existence came from the PVLAS experiment at the National Laboratories of Legnaro in Italy. Last July, the PVLAS team announced a slight shift in the polarisation of a laser beam fired through a strong magnetic field. The shift was 10,000 times larger than expected by standard physics, but could be explained if a tiny fraction of photons from the laser had turned into axions (快猫短视频, 15 July 2006, p 35).
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However, the CERN Axions Solar Telescope (CAST) near Geneva, which searches for axions produced in the sun, has failed to corroborate the PVLAS result. 鈥淭he axion community is still excited by the PVLAS result, but it needs independent experimental verification,鈥� says CAST spokesman Konstantin Zioutas at the University of Patras in Greece. 鈥淚f someone found direct evidence for the presumed axion-like particle interpretation of the PVLAS data, both groups would deserve the Nobel prize,鈥� he says.
Now Krzysztof Piotrzkowski of the Catholic University of Louvain (UCL) in Belgium is proposing to do just that. What鈥檚 more, it would be cheap, easy to set up, and could deliver results in a week, he says.
Piotrzkowski鈥檚 idea is to exploit apparatus already in place at the Hadron-Electron Ring Accelerator in Hamburg, Germany. At HERA, an intense beam of photons is generated as a by-product of the accelerator. This beam passes through HERA鈥檚 strong magnetic field, which runs parallel to the photons鈥� electric field. According to theory, if the energy of the photon beam is much larger than the equivalent mass predicted of axions, some of the photons will convert to axions, says Piotrzkowski.
To isolate the axions, he suggests a 50-centimetre lead shield that would block the photons but let axions through. Once the axions exit the lead, some will revert back to photons, which can then be detected ().
Unfortunately, HERA is due to shut down at the end of June, so it鈥檚 now or never for Piotrzkowski. 鈥淭he experiment could in theory be carried out at other accelerators, but not without massive modifications to the apparatus, which really isn鈥檛 feasible,鈥� he says.
Zioutas says Piotrzkowski鈥檚 method could provide a direct test for axions, but he isn鈥檛 convinced that it can produce definitive results within a week. 鈥淎s an experimentalist, I can assure you, we always seem to underestimate the length of time needed by a factor of 10, preparatory work not included,鈥� Zioutas says.
Meanwhile, HERA鈥檚 managers are examining Piotrzkowski鈥檚 proposal. 鈥淲e are certainly interested in the axion search and this is something that we could do,鈥� says Manfred Fleischer of DESY, the German research centre that runs HERA. 鈥淚t鈥檚 unfortunate that we鈥檙e only hearing this proposal so close to the end of our run. It will be very, very tough to fit it into our schedule.鈥�
Piotrzkowski thinks it鈥檚 to HERA鈥檚 advantage to squeeze in the test. 鈥淗ERA helped confirm many of the predictions of the standard model of physics, but as yet it hasn鈥檛 had any spectacular results,鈥� he says. 鈥淚f they find the axion, then they could go out in a blaze of glory.鈥�
