More information on SNO at PENN
Faculty: Gene Beier, Josh Klein
Postdoctoral researchers: Jeff Secrest, Gabriel Orebi Gann
Graduate Students: Tim Shokair, Richie Bonventre
Recent graduate students: Monica Dunford, Chris Kyba, Mark Neubauer, Vadim Rusu, Peter Wittich

The Sudbury Neutrino Observatory (SNO detector). Shown in the figure are Penn graduate student Doug McDonald, Professor Josh Klein and graduate student Peter Wittich (from left) and others with the acrylic vessel underground in Sudbury.

The Sudbury Neutrino Observatory project (SNO) has solved one of the great puzzles of twentieth-century physics and astrophysics---the anomalously low flux of neutrinos coming from the sun. Since the late 1960's when Ray Davis first announced that he detected about one-third the number of neutrinos predicted by models of stellar evolution, scientists were in a quandary regarding the source of the discrepancy. Was his experiment wrong? Was our understanding of stars wrong? Or was there something else, perhaps an inadequate understanding of the properties of neutrinos?

SNO has shown that Davis's experiment was correct, and that the model of the sun is also correct. The puzzle was solved when SNO showed that some of the Boron-8 electron-neutrinos that are produced in nuclear fusion reactions that power the sun transform to another type of neutrino which does not produce a signal in Davis's detector.

Unlike previous solar neutrino experiments, the SNO detector is sensitive to three different neutrino reactions. One of the reactions is, like Davis's experiment, only sensitive to the electron-neutrinos that the sun produces. The other two reactions are sensitive to electron-neutrinos, and, in different proportions, to mu-neutrinos and tau-neutrinos --- types that are not produced in the solar fusion reactions. The three reaction types can be separated using the position, angle, and energy information of the events observed.

By comparing measurements of the flux of solar Boron-8 electron neutrinos (nu_e) to the total flux of all neutrino types (nu_x) coming from the Sun, SNO has shown that Davis's original measurement was correct---the nu_e flux is suppressed---but that the flux of all types of neutrinos is in agreement with the predictions of the model. The conclusion is that some of the electron-neutrinos that were produced in the sun transform into the other types of neutrinos before they are detected on earth. The most likely mechanism for producing this transformation requires that neutrinos have small, but non-zero mass. This is an indication of exciting new physics beyond the Standard Model of elementary particle physics. Although the mass of the neutrinos is tiny, the total mass of all the neutrinos in the universe is comparable to that of all the visible stars.

The unique feature of SNO is the use of a kiloton of heavy water, D2O, as a neutrino target. The valuable D2O is securely contained in a spherical acrylic vessel which is twelve meters in diameter. The vessel is surrounded by light water, H2O, and is viewed by 9500 photomultiplier tubes. To limit backgrounds introduced by cosmic radiation at the earth's surface, the entire laboratory and detector are located two kilometers underground in a cavity in one of the world's most productive nickel mines. The SNO cavity, which is the size of a ten story apartment building, is maintained as a "clean room" to exclude trace contamination from mine dust.

The SNO experiment began taking calibration and neutrino data in May 1999. The program of calibrations determines the optical parameters, the spatial, angular, and energy responses of the detector, the response to signals from neutrinos and processes that produce background, and systematic effects which might bias interpretations. The calibrations are taken routinely to track the time dependence of the detector response.

Neutrino data will be acquired in at least three configurations of the detector. The initial configuration was the simplest, with only heavy water inside the acrylic vessel. In June, 2001, the detector configuration was altered to the first of two configurations that will enhance the detection capability for nu_x. Measurements in these two configurations will produce independent measures of the flux of nu_x and serve as checks on each other and on the result from the initial phase. The additional data will also permit accurate measurements of possible distortions in the electron energy spectrum and day-night spectral differences for nu_e induced events. These measurements will lead to precise evaluation of the physics parameters responsible for neutrino flavor transformation in the solar sector. Additional topics of study include atmospheric neutrinos and a search for anti-neutrino interactions. SNO's neutron detection capability is a unique asset for this work. A program of data acquisition and analysis lasting at least through 2005 is envisioned, in order to obtain the highest precision results possible.

The University of Pennsylvania group constructed and is responsible for maintaining all the front-end signal processing electronics for the detector. This includes PMT signal detection and digitization, triggering, and GPS timing electronics. The effort has required three custom designed integrated circuits and fourteen custom designed printed circuit boards. Graduate students contributed to or were solely responsible for nine of the circuit boards. This represents one of the many substantial and crucial contributions to the SNO experiment by students.

Penn researchers were deeply involved in commissioning the detector and are now active in operations and data analysis at all levels. The opportunities for learning a wide range of physics and experimental techniques---from hardware design to data acquisition software to data analysis---are great; Penn graduate students working on SNO get a broad exposure to both the hardware and analysis skills required to do effective research and an opportunity to work on one of the most exciting experiments in the particle physics.