REHOVOT, ISRAEL—April 14, 2011—AnInternational team of scientists in the XENON collaboration, includingseveral from the Weizmann Institute, announced on Thursday the resultsof their search for the elusive component of our universe known as darkmatter. This search was conducted with greater sensitivity than everbefore. After one hundred days of data collection in the XENON100experiment, carried out deep underground at the Gran Sasso NationalLaboratory of the INFN, in Italy, they found no evidence for theexistence of Weakly Interacting Massive Particles – or WIMPs – theleading candidates for the mysterious dark matter. The three candidateevents they observed were consistent with two they expected to see frombackground radiation. These new results reveal the highest sensitivityreported as yet by any dark matter experiment, while placing thestrongest constraints on new physics models for particles of darkmatter. Weizmann Institute professors Eilam Gross, Ehud Duchovni andAmos Breskin, and research student Ofer Vitells, made significantcontributions to the findings by introducing a new statistical methodthat both increases the search sensitivity and enables new discovery.

Anydirect observation of WIMP activity would link the largest observedstructures in the universe with the world of subatomic particle physics.While such detection cannot be claimed as yet, the level of sensitivityachieved by the XENON100 experiment could be high enough to allow anactual detection in the near future. What sets XENON100 apart fromcompeting experiments is its significantly lower background radiation.The XENON100 detector, which uses 62 kg of liquid xenon as its WIMPtarget, and which measures tiny charges and light signals produced bypredicted rare collisions between WIMPs and xenon atoms, continues itssearch for WIMPs. New data from the 2011 run, as well as the plan tobuild a much larger experiment in the coming years, promise an excitingdecade in the search for the solution to one of nature's mostfundamental mysteries.

Cosmologicalobservations consistently point to a picture of our universe in whichthe ordinary matter we know makes up only 17% of all matter; the rest –83% – is in an as yet unobserved form – so-called dark matter. Thiscomplies with predictions of the smallest scales; necessary extensionsof the Standard Model of particle physics suggest that exotic newparticles exist, and these are perfect dark matter candidates.  WeaklyInteracting Massive Particles (WIMPs) are thus implied in both cosmologyand particle physics. An additional hint for their existence lies inthe fact that the calculated abundance of such particles arising fromthe Big Bang matches the required amount of dark matter. The search forWIMPs is thus well-founded; a direct detection of such particles wouldprovide the central missing piece needed to confirm this new picture ofour universe.

Theproperties of dark matter have been addressed through a variety ofapproaches and methods; these have provided the scientists with indirecthints of what to search for. WIMPs are expected to have a masscomparable to that of atomic nuclei, with a very low probability ofinteracting with normal matter. Such particles are thought to bedistributed in an enormous cloud surrounding the visible disk of theMilky Way. Earth is moving through this cloud, along with the Sun, onits journey around the Galaxy center. This movement results in a ‘WIMPwind,' which may occasionally scatter off atomic nuclei in anEarth-bound detector, releasing a tiny amount of energy, which can thenbe detected with ultra-sensitive devices.

Inthe XENON100 experiment, 62 kg of liquid xenon acts as a WIMP target.The liquid, at a temperature of about -90° C, is contained in astainless steel cryostat equipped with a cryo-cooler to maintain highlystable operating conditions. The experiment is located in the Gran SassoUnderground Laboratory (LNGS) in Italy where it is shielded from cosmicradiation by 1400 meters of rock. Further shielding from radioactivityin the detector itself and its surroundings is provided by layers ofactive and passive absorbers surrounding the target. These include 100kg of active liquid xenon scintillator, 2 tons of ultra-pure copper, 1.6tons of polyethylene and 34 tons of lead and water. The radio-purematerials used to produce the detector components assure an ultra-lowbackground radiation environment.

Particlesinteracting within the active liquid xenon space excite and ionizeatoms. This results in light emission in the deep ultraviolet. Aselectrons drift across the liquid xenon, they create a delayed,luminescent signal on the top of the detector, due to the experiment'sstrong electric field. Both primary and secondary scintillation lightsignals are detected via two arrays of photosensors – one located in theliquid xenon at the bottom, and one in the gas above the liquid (Figure1). The simultaneous measurement of these two light signals enables theresearchers to infer both the energy and the spatial coordinates of theparticles' interaction, while providing information on their nature.This analysis of ratio of the two light signals and their preciselocalization in space is an extremely accurate method of distinguishingWIMP signals from background events.

Manyof the technologies and methods used in the XENON100 experiment havebeen built on the research and development efforts of the XENON DarkMatter Search program, which produced, in 2006, the XENON10 prototype.For XENON100, a ten-fold increase in fiducial target mass, combined with100-fold reduction in background, translates into a substantialimprovement in sensitivity to WIMP-nucleon elastic scattering. Anextensive calibration using various sources of gammas and neutrons wasperformed to demonstrate that XENON100 reached its goals for sensitivityand for low background radiation.

Resultsfrom a preliminary analysis from 11.2 days worth of data, taken duringthe experiment's commissioning phase in October and November 2009, havealready set new upper limits on the interaction rate of WIMPs – theworld's best for WIMP masses below about 80 times the mass of a proton (Physical Review Letters 105 (2010) 131302).

Anew dark matter search was performed between January and June, 2010,and 100 days worth of data from this run have been analyzed. Threecandidate events were found within the pre-defined parameters in whichthe WIMP signal is expected to appear. However, these events, whilecoming from true particle interactions in the detector, are consistentwith predictions of two such events resulting from radioactivebackgrounds. Thus evidence for dark matter cannot be claimed, but a newupper limit for the strength of its interaction with normal matter couldbe calculated. These results represent the best limits to date. Theynarrow the possibilities open to supersymmetric particle physicstheories that predict the nature of dark matter.

XENON100 has achieved the lowest background among all dark matter experiments worldwide (Physical Review D (2011),arXiv:1101.3866). Since the data presented here were collected, theintrinsic background from radioactive krypton in the xenon fillingXENON100 has been reduced to an unprecedented low level and thedetectors' performance has been improved as well. Even as new data arebeing collected in these improved conditions, the scientific team ispreparing a next-generation dark matter search experiment featuring adetector that will contain more than 1000 kg of liquid xenon as afiducial WIMP target. With further reduction in overall backgroundradiation, XENON1T promises to be a hundred times more sensitive thanXENON100.

TheXENON collaboration consists of 60 scientists from 14 institutions inthe USA (Columbia University New York, University of California LosAngeles, Rice University Houston), China (Shanghai Jiao TongUniversity), France (Subatech Nantes), Germany (Max-Planck-InstitutHeidelberg, Johannes Gutenberg University Mainz, Willhelms UniversitätMünster), Israel (Weizmann Institute of Science), Italy (LaboratoriNazionali del Gran Sasso, INFN e Università di Bologna), Netherlands(Nikhef Amsterdam), Portugal (Universidade de Coimbra) and Switzerland(Universität Zürich).

XENON100is supported by the collaborating institutions and by the NationalScience Foundation and the Department of Energy in the USA, by the SwissNational Foundation in Switzerland, by l'Institut national de physiquedes particules et de physique nucléaire and La Région des Pays de laLoire in France, by the Max-Planck-Society and by DeutscheForschungsgemeinschaft in Germany, by the Weizmann Institute of Science,by FOM in the Netherlands, by the Fundação para a Ciência e Tecnologiain Portugal, by the Instituto Nazionale di FIsica Nucleare in Italy andby STCSM in China.

Prof. Amos Breskin's research is supported by the Nella and Leon Benoziyo Center for High Energy Physics; and the estate of Richard Kronstein. Prof. Breskin is the incumbent of the Walter P. Reuther Chair of Research in Peaceful Uses of Atomic Energy. Prof. Ehud Duchovni's research is supported by the Nella and Leon Benoziyo Center for High Energy Physics; and the Yeda-Sela Center for Basic Research. Prof. Duchovni is the incumbent of the Professor Wolfgang Gentner Chair of Nuclear Physics. Prof. Eilam Gross' research is supported by the estate of Richard Kronstein.