Astroparticle Physics

18 1 Historical IntroductionHiggs particleThe oscillation scenario for solar neutrinos was confirmedin 2001 by the SNO 19 experiment by showing thatthe total flavour-independent neutrino flux from the Sun arrivingat Earth (ν e ,ν µ ,ν τ ) was consistent with solar-modelexpectations, demonstrating that some of the solar electronneutrinos had oscillated into a different neutrino flavour.The generation of the masses for elementary particles isstill an open question. In the standard model of electroweakand strong interactions, the mass generation is believed tocome about by a spontaneous symmetry breaking, thesocalledHiggs mechanism. This process favours the existenceof at least one additional massive neutral boson. Whether theLEP experiments at CERN have seen this enigmatic particleat the kinematic limit of LEP, with a mass of about 115 GeV,needs to be confirmed by future hadron colliders.A very recent, equally exciting discovery, is the measurementof the acceleration parameter of the universe.Based on the ideas of the classical Big Bang, one would assumethat the initial thrust of the explosion would be sloweddown by gravitation. Observations on distant supernova ex-plosions (1998) however, appeared to indicate that in earlycosmological epochs, the rate of expansion was smaller thantoday. The finding of an accelerating universe –whichisnow generally accepted – has important implications forcosmology. It suggests that the largest part of the missingdark matter is stored as dark energy in a dynamical vacuum(‘quintessence’).Finally, it should be highlighted that the discovery of ex-trasolar planets (Mayor and Queloz 1995) has led to the resumptionof discussions on the existence of extraterrestrialintelligence. So far, the nearest extrasolar planet (‘Millennium’)has been observed in the Tau Boötis solar system,by the Herschel telescope on the Canary Islands. The planetis twice as large as Jupiter and eight times as heavy, and issituated at a distance of 55 light-years. Until now about 100extrasolar planets have been discovered. Possibly we are notthe only intelligent beings in the universe pursuing astroparticlephysics.accelerating universe?extrasolar planets19 SNO – Sudbury Neutrino Observatory