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The Sudbury Neutrino Observatory Confirms the Oscillation Picture

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The Sudbury Neutrino Observatory Confirms the Oscillation Picture NEWS SOLAR PARTICLES The Sudbury Neutrino Observatory confirms the oscillation picture The Sudbury Neutrino Observatory, from neutrino collisions produce which started taking data in 1999, flashes of light that are picked up has announced its first results on by 9500 photomultiplier tubes.The solar neutrinos, which confirm the detector is sensitive to those solar suspicion that something happens neutrinos produced via the beta to these particles on their 150 mil­ decay of boron-8. lion kilometre journey from the Sun The heavy water is the key - SNO to the Earth. is the first extraterrestrial neutrino Experiments have been monitor­ detector to use heavy water. In one ing solar neutrinos for some heavy water reaction (call it reac­ 40 years.To see neutrinos at all tion A), an electron-type neutrino demands a major effort, so meas­ can break up a target deuteron, urements are difficult and reliable Left: when a neutrino from the Sun hits a nucleus in the heavy producing two protons and an results take time to amass. As the water of the Sudbury Neutrino Observatory (SNO) detector, a faint emergent electron. Electrons can work continued, physicists began to cone of light spreads out and is picked up (green points) by the also appear from elastic scattering suspect that their experiments were surrounding light sensors. Right: schematic of the SNO detector. (reaction B), where an incoming not seeing as many solar neutrinos The detector cavity 34 m high by 22 m in diameter, is 2000 m neutrino bounces off an atomic as expected - there was a "solar underground in a nickel mine in Ontario, Canada. The 1000 tonne electron, which then recoils. neutrino problem". heavy water target is contained in a 12 m diameter acrylic vessel, However, reaction B can be pro­ Neutrinos are produced in the viewed by 9500 phototubes mounted on an 18 m diameter duced by any kind of neutrino. nuclear reactions in the Sun's core, concentric geodesic sphere. Over 241 days, SNO collected which provide the Sun's energy (the 1169 neutrino events, which were radiant light and heat which make life possible bizarre neutrino behaviour is the reason for the carefully analysed to classify them as being is only a by-product of the Sun's nuclear fur­ solar neutrino deficit - the particles are indeed due to reaction A or B. nace). If physicists think that they understand living up to their non conformist reputation. The apparent flux of solar neutrinos meas­ what happens inside the Sun, they should be Neutrinos come in three types - electron, ured via the observed rate for reaction A able to predict the number of neutrinos which muon and tau - according to their subnuclear ( 1.75 0.± 07 +0.12 - 0.11 ± 0.05 x 106 cm"2 s"\ arrive at the Earth. When measurements do parentage. When such distinct neutrino types where the three sets of errors are respectively not agree with the prediction, there is a were first discovered, it was initially believed statistical, systematic and theoretical) is dilemma - either we do not understand how that each type was immutable - a neutrino slightly lower than the precision measurement the Sun works, or neutrinos are perverse parti­ born with an electron (as in beta decay or the (2.32 ± 0.03 +0.08 - 0.07 x 106 cm"2 s"1) via cles that do not behave as expected. reactions deep inside the Sun) could continue reaction B, by the Superkamiokande detector In appraising these two alternatives, it is to show such electron character for ever. in Japan (CERN Courier September 2000 p8- important to remember that, 100 years ago, However, the non conformist reputation of SNO's measurement of the rate for reaction B physicists could not understand where the these particles led some far-sighted physicists has not yet attained this precision).The fluxes Sun got its energy from and why it hadn't yet to suspect that perhaps neutrinos were not as measured via the two reactions are differ­ burned out. Only the advent of nuclear physics immutable. Perhaps there was a small chance ent because some of the electron neutrinos in the 1930s showed how nuclear transforma­ that a neutrino could change its allegiance in produced in the Sun have "oscillated" into tions could supply such prodigious and flight. A neutrino that began its journey in other types of neutrino en route, and on arrival enduring outputs.The neutrino concept was electron class could 'oscillate' and upgrade to at SNO are no longer able to trigger reaction A. an initially hesitant postscript to this nuclear muon class. Such changed seating arrange­ Evidence for neutrino oscillations has been picture.To understand nuclear beta decay, ments en route could explain an observed seen in other situations (CERN Courier there had to be a particle that would be very deficit of electron-type solar neutrinos. September 2000 p8).The SNO result is the difficult to detect - if it could be detected at The Sudbury Neutrino Observatory (SNO) is first direct evidence for solar neutrinos oscil­ all. From the start, neutrinos acquired a repu­ a vessel containing 1000 tonnes of heavy lating on their journey to Earth. When an tation for being non conformist. water, 2000 m underground in an active nickel experiment makes its debut with such impor­ The new Sudbury results confirm that mine in Ontario, Canada. Particles resulting tant results, its future looks assured. CERN Courier September 2001 5 NEWS JAPAN Japan's KEKB offers unprecedented Unprecedented luminosity (a measure of the machine's collision rate) at the KEKB Japanese B-factory electron-positron collider: the top two charts show the increase in peak and daily performance, with the milestones achieved by machine improvements. Integrated luminosity, like age, can only go in one direction, but it has effectively doubled this year at KEKB. The KEKB Japanese B-factory collider is deliv­ due to the photoelectron cloud up to about luminosity per month are a little higherthan ering unprecedented luminosity (a measure of 900 mA.The second improvement came from those of PEP-ll/BaBar at SLAC, Stanford, the machine's electron-positron collision rate) the installation of new movable masks on the KEKB needs further improvements to continue to the international collaboration running the moving chamber in the high-energy ring to be competitive with its rival in the long run Belle experiment. Since the commissioning of (HER).This replacement, already verified at (even assuming the present luminosity of the machine in November 1998, the KEK LER in the previous year, has raised the HER PEP-II). KEKB has significant obstacles in machine team has solved many difficulties stored current limit HER 580 to 770 mA. running more than nine months a year due to and has recently made major progress - it has Third, a state-of-the-art setting was the periodic inspection of the refrigeration achieved the highest luminosity in collider achieved in the betatron tunes - very close to system required by law, expensive summer history: 4.49 x 1033 cm"2 s'1. the half-integer resonance.The vertical tunes electricity, a weak cooling system incapable of Integrated luminosities (a measure of the were raised beyond half-integer resonance handling summer heat and so on. collision "dose" administered) are 232 pb"1 lines in both rings to gain stability of the orbits Several improvements are planned during per day, 1.50 fb"1 per week and 4.83 fb"1 per as well as a wider high-luminosity area in the this summer's shutdown. More solenoid wind­ month.These are all numbers recorded by the tune spaces.The horizontal tunes, especially ings are planned in the LER. Currently the Belle detector.Total data so far collected by in LER, were set even closer to the half-integer electron-cloud effect still looks to be the Belle had reached 33.1 fb"1 by mid-July. resonance to gain the dynamic focusing effect dominant restriction on the number of KEKB luminosity has been nearly doubled of the beam-beam interaction without sacri­ bunches circulating in the LER. this year, as seen in the figure above.This was ficing the machine aperture. Other notable Currently, the collision is carried out with brought about by several machine improve­ improvements are in the orbit control, beta­ four-bucket spacing. A shorter spacing is ments. First, 1300 out of 1800 m of field-free tron tune monitor and control, beam-size essential to achieve higher luminosity, region of the arcs in the low-energy ring (LER) control, the beam-abort system, the logging because the bunch current is limited in both has been covered with solenoid windings.This system and the injectors. rings for various reasons. suppressed the vertical blow-up of the beam Though the peak luminosity and integrated The replacement of the vacuum chamber is 6 CERN Courier September 2001 NEWS CERN luminosity New CMS visitor centre CP-violation parameter proves a star attraction In time for the summer conference season, both of the big experiments measuring the charge-parity (CP) asymmetry in the decays of B-mesons reported impressive results. The BaBar experiment at the PEP-II electron-positron collider at SLAC, Stanford, based on a sample of 32 million B pairs, reported a value for the sin26 CP-violation parameter of 0.59 ± 0.14. The BELLE experiment at the KEKB electron-positron collider at the Japanese KEK laboratory, with 31.3 million B pairs, measures the parameter as 0.99 ± 0.14 ± 0.06. These are the most precise measurements of this parameter so far. For statistical sticklers, the results are now clearly non-zero - physicists can say with confidence that CP violation happens in B decays.
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