Bentvelsen / Koch / Laenen Proceedings of the 31st International Conference on High Energy Physics ICHEP 2002

1. Auflage 2003
ISBN: 978-0-444-59916-2
Verlag: Elsevier Reference Monographs
Format: PDF
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, 1004 Seiten

The first precision measurements on CP violation in the B system are reported. Both the BELLE and the BABAR collaboration presented, among others, results for sin 2ß with much improved accuracy. Results from the Sudbury Neutrino Observatory, SNO, also deserve to be mentioned. The convincing evidence of solar neutrino oscillations had been presented by SNO prior to the conference; a full presentation was given at the conference. An incredibly precise measurement of the anomalous magnetic moment of the muon is reported, a fresh result from the Brookhaven National Laboratory. Apart from these distinct physics highlights, there are also the first results from the new Tevatron run and from the relativistic heavy ion collider RHIC. Theorists write of our ever better understanding of the Standard Model and of what might lie beyond. Risky as it is to highlight only a couple of exciting subjects, it is merely meant
to whet the appetite for further reading.

Autoren/Hrsg.

Bentvelsen, S.

Koch, J.

Laenen, E.

Leseproben

Neutrino Oscillation Results from the Sudbury Neutrino Observatory
Scott M. Osera for the SNO collaboration,     aDepartment of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104-6396, USA The Sudbury Neutrino Observatory (SNO) has determined the flavor content of the 8B solar neutrino flux by measuring the rates of charged current and neutral current neutrino interactions on deuterium. These results directly demonstrate neutrino flavor transformation at greater than 5s significance. The total flux of 8B neutrinos is found to be in good agreement with solar model predictions. Measurements of the day and night neutrino energy spectra probe models of neutrino oscillation. A global fit of SNO data and results from other solar neutrino experiments to neutrino oscillation models strongly favors the Large Mixing Angle (LMA) MSW solution. 1 Introduction
The Sudbury Neutrino Observatory (SNO) detects 8B solar neutrinos through the reactions: The charged current reaction (CC) is sensitive exclusively to electron-type neutrinos, while the neutral current reaction (NC) is equally sensitive to all active neutrino flavors (x = e, µ, t). The elastic scattering reaction (ES) is sensitive to all flavors as well, but with reduced sensitivity to vµ and vt. Sensitivity to these three reactions allows SNO to determine the electron and non-electron active neutrino components of the solar flux [1]. SNO [2] is a water Cherenkov detector located at a depth of 6010 m of water equivalent in the INCO, Ltd. Creighton mine near Sudbury, Ontario, Canada. The detector uses ultra-pure heavy water contained in a transparent acrylic spherical shell 12 m in diameter to detect solar neutrinos. Cherenkov photons generated in the heavy water are detected by 9456 photomultiplier tubes (PMTs) mounted on a stainless steel geodesic sphere 17.8 m in diameter. The geodesic sphere is immersed in ultra-pure light water to provide shielding from radioactivity in both the PMT array and the cavity rock. The data reported here represent a total of 306.4 live days, spanning the entire first phase of the experiment, in which only D2O was present in the sensitive volume. These analyses are described in more detail in [3], [4], and [5]. PMT times and hit patterns were used to reconstruct event vertices and directions and to assign to each event a most probable kinetic energy, Teff. The total flux of active 8B solar neutrinos with energies greater than 2.2 MeV (the NC reaction threshold) was measured with the NC signal (Cherenkov photons resulting from the 6.25 MeV ? ray from neutron capture on deuterium.) The analysis threshold was Teff = 5 MeV, providing sensitivity to neutrons from the NC reaction. Above this energy threshold, there were contributions from CC events in the D2O, ES events in the D2O and H2O, capture of neutrons (both from the NC reaction and backgrounds), and low energy Cherenkov background events. A fiducial volume was defined to only accept events which had reconstructed vertices within 550 cm from the detector center to reduce external backgrounds and systematic uncertainties associated with optics and event reconstruction near the acrylic vessel. The neutron response and systematic uncertainty was calibrated with a 252Cf source. The deduced efficiency for neutron captures on deuterium is 29.9±1.1% for a uniform source of neutrons in the D2O. The neutron detection efficiency within the fiducial volume and above the energy threshold is 14.4%. The energy scale uncertainty is 1.2%. 2 Backgrounds
The primary backgrounds to the NC signal are due to low levels of uranium and thorium decay chain daughters (214Bi and 208Tl) in the detector materials. These activities generate free neutrons in the D2O, from deuteron photodisintegration (pd), and low energy Cherenkov events. Exsitu assays and in-situ analysis of the low energy (4 – 4.5 MeV) Cherenkov signal region provide independent uranium and thorium photodisintegration background measurements. Two ex situ assay techniques were employed to determine average levels of uranium and thorium in water. Radium ions were directly extracted from the water onto either MnOx or hydrous Tioxide (HTiO) ion exchange media. Radon daughters in the U and Th chains were subsequently released, identified by a spectroscopy, or the radium was concentrated and the number of decay daughter ß-a coincidences determined. These techniques provide isotopic identification of the decay daughters and contamination levels in the assayed water volumes, presented in Fig. 1(a). Secular equilibrium in the U decay chain was broken by the ingress of long-lived (3.8 day half-life) 222Rn in the experiment. Measurements of this background were made by periodically extracting and cryogenically concentrating 222Rn from water degassers. Radon from several tonne assays was subsequently counted in ZnS(Ag) scintillation cells [6]. The Radon results are presented (as mass fractions in g(U)/g(D2O)) in Fig. 1(b). Figure 1 Thorium (a) and uranium (b) backgrounds (equivalent equilibrium concentrations) in the D2O deduced by in situ and ex situ techniques. Independent measurements of U and Th decay chains were made by analyzing Cherenkov light produced by the radioactive decays. The ß and ß-? decays from the U and Th chains dominate the low energy monitoring window. Events in this window monitor ? rays that produce photodisintegration in these chains (E? > 2.2 MeV). Cherenkov events fitted within 450 cm from the detector center and extracted from the neutrino data set provide a time-integrated measure of these backgrounds over the same time period and within the fiducial volume of the neutrino analysis. Statistical separation of in situ Tl and Bi events was obtained by analyzing the Cherenkov signal isotropy. Tl decays always result in a ß and a 2.614 MeV ?, while in this energy window Bi decays are dominated by decays with only a ß, and produce, on average, more anisotropic hit patterns. Results from the ex situ and in situ methods are consistent with each other as shown on the right hand side of Figs. 1(a) and 1(b). For the 232Th chain, the weighted mean (including additional sampling systematic uncertainty) of the two determinations was used for the analysis. The 238U chain activity is dominated by Rn ingress which is highly time dependent. Therefore the in-situ determination was used for this activity as it provides the appropriate time weighting. The average rate of background neutron production from activities in the D2O region is 1.0 ± 0.2 neutrons per day, leading to detected background events. The production rate from external activities is neutrons per day, which leads to 27 ± 8 background events since the neutron capture efficiency is reduced for neutrons born near the heavy water boundary. The total photodisintegration background corresponds to approximately 12% of the number of NC neutrons predicted by the standard solar model from 8B neutrinos. Low energy backgrounds from Cherenkov events in the signal region were evaluated by using acrylic encapsulated sources of U and Th deployed throughout the detector volume and by Monte Carlo calculations. Probability density functions (pdfs) in reconstructed vertex radius derived from U and Th calibration data were used to determine the number of background Cherenkov events from external regions which either entered or mis-reconstructed into the fiducial volume. Cherenkov event backgrounds from activities in the D2O were evaluated with Monte Carlo calculations. Together the low energy background from Cherenkov events totaled events. Other sources of free neutrons in the D2O region are cosmic ray events and atmospheric neutrinos. To reduce these backgrounds, a neutron background cut imposed a 250-ms deadtime (in software) following every event in which the total number of PMTs which registered a hit was greater than 60. In addition, a “muon follower” cut removed every event following within 20 s of a throughgoing muon. The number of remaining NC atmospheric neutrino events and background events generated by sub-Cherenkov threshold muons, reactor anti-neutrinos, and a-induced disassociation of deuterons is estimated to be . 3 Integral Flux Analysis
The data recorded during the pure D2O detector phase are shown in Figure 2. There are 2928 events in the energy region selected for analysis, 5 to 20 MeV. Fig. 2(a) shows the distribution of selected events in the cosine of the angle between the Cherenkov event direction and the direction from the sun (cos??) for the analysis threshold of Teff= 5 MeV and fiducial volume selection of R = 550 cm, where R is the reconstructed event radius. Fig. 2(b) shows the distribution of events in the volume-weighted radial variable (R/RAV)3, where RAV = 600 cm is the radius of the acrylic vessel. Figure 2(c) shows the kinetic energy spectrum of the selected...