Gran Sasso becomes a workshop WONDERland

The INFN’s Gran Sasso National Laboratory provides the world’s largest underground infrastructure for astroparticle physics. It currently hosts four operational dark-matter experiments – CRESST, DAMA-LIBRA, WArP and XENON – and was therefore a fitting venue for WONDER, the Workshop On Next Dark-matter Experimental Research. Designed to generate fruitful discussions about the future of the exciting field of dark-matter physics, the workshop was held on 22–23 March and attracted around 100 participants.

As is well known, “dark matter” is the name given to 23% of the “inventory” of the universe, the existence of which is indicated by several experimental facts, the first and most famous being the anomalous behaviour of the radial velocity of galaxies. Although some alternative models still survive to explain these unexpected effects, the most fascinating explanation – at least for particle physicists – is the existence of stable, massive particles that interact only weakly with ordinary matter and permeate all galaxies, including ours. Supersymmetry provides a nice theoretical framework for such an explanation, and the lightest supersymmetric particle, the neutralino, could be a viable candidate for dark matter. First, however, someone has to observe some experimental evidence to pin down the characteristics of the “dark” particles, which are often referred to as “WIMPs” – weakly interacting massive particles. The question is: how to identify the particles?

One way is to look for the production of WIMPs in collisions at the LHC at CERN. Other “indirect” techniques look for likely signatures of annihilations of WIMPs occurring in the Sun, Earth or galactic halo; these could appear, for example, as anomalous neutrino or gamma-ray fluxes. A third method is to observe the direct interactions of WIMPs with ordinary matter. Underground laboratories are the ideal place to carry out this quest. Anywhere else on the surface of the Earth, the overwhelming cosmic radiation would drown out the tiny signal (if it exists), making the search as hopeless as trying to spot a distant star in daylight.

Even amid the “cosmic silence” at the heart of a mountain (as at Gran Sasso), dark-matter experiments struggle to attain the best sensitivity with elaborate techniques and, above all, by trying to reduce the residual gamma and neutron backgrounds to unprecedentedly low levels.

DAMA-LIBRA, one of the first experiments at Gran Sasso, does in fact observe a significant modulation signal in its scintillators of high-purity sodium iodine, which is identical to the one that the motion of the Earth through the dark-matter halo is supposed to cause. The DAMA collaboration presents this signal as evidence for the discovery of dark matter and the scientific community waits for a confirmation, possibly with new, different techniques. The problem is that, until to now, the other experiments seem to rule out DAMA-LIBRA’s result, although the comparison between different techniques is far from straightforward. Theoretical models still survive that reconcile all current experimental results with a positive discovery by DAMA-LIBRA.

Among today’s technologies, detectors employing cryogenic noble liquids occupy a pre-eminent position. These seem to allow for excellent signal-to-background discrimination, coupled with the possibility to build massive detectors. The Gran Sasso National Laboratory provided a natural location to discuss the future of these searches because it hosts three experiments, other than DAMA-LIBRA, that are competing for the discovery of dark matter, namely CRESST, WArP and XENON. The race is particularly interesting between the latter two of these because they use the same “double-phase” technique, but with different targets. XENON employs 160 kg of its homonymous noble element in liquid form, while WArP has a similar amount of liquid argon, a medium with which research groups at INFN have considerable expertise.

Carlo Rubbia, the spokesperson of the WIMP Argon Programme (WArP), opened the workshop with an excellent and comprehensive overview of the experimental landscape. This was followed by theoretical talks that helped to set up the general framework of the field. With regard to experimental activities, preliminary results from the XENON100 detector provided a highlight of the workshop. About 11 days of data have been analysed and were presented by the XENON spokesperson Elena Aprile, from Columbia University. The data show an extremely low background – the lowest ever reached – and raise even stronger expectations for future results.

Claudio Montanari of INFN presented the status of WArP, which has just started data-taking, while Wolfgang Seidel of the Max Planck Institute talked about interesting results from CRESST, a detector made from scintillating calcium-tungstate crystals. Activities beyond Gran Sasso were also discussed. Masaki Yamashita of Kamioka/Tokyo presented Xmass, a particularly promising detector based on liquid xenon, which is close to its commissioning phase in the Kamioka mine in Japan. Newer techniques also seem to be interesting and promising. These include the directional detectors that Neil Spooner of Sheffield University described and, in particular, the bubble chambers COUPP and PICASSO, which Nigel Smith of SNOLAB discussed in his extensive overview of dark-matter activities around the world.

Two stimulating talks were dedicated to the problem of backgrounds, especially from neutrons. Frank Calaprice of Princeton University and Vitaly Kudryavtsev of Sheffield University described these issues. The final session covered, in depth and in a critical manner, the issues of backgrounds, sensitivity and stability for each group of techniques.

Overall, the workshop revealed an extremely lively field, with existing detectors producing new results, others about to enter their commissioning phase, advanced projects being proposed for new underground facilities and intense theoretical activity. We all “wonder” if a discovery is just round the corner.