S. Ahlen, A. Marin, B. Zhou
The Large Electron Positron (LEP) collider, which has been operating since August 1989,is located at the CERN laboratory near Geneva, Switzerland. LEP is one of the largest facilities in the world developed for experimental high energy physics. The L3 experiment at LEP is based on a large magnetic detector optimized for the precision measurement of photons, electrons, muons, and hadron jets. L3 has already collected a large amount of data from decays of the neutral gauge boson (Z0), enabling its study with high precision. The LEP energy will cross the W+W- threshold in early 1995 (LEP200 phase), which will allow the study of the charged gauge bosons in detail, and to search for the Standard Model Higgs boson up to a mass of 90 GeV. The Boston University-L3 group is carrying out the data analysis on the Higgs search and on tau-pair decays from Z0, and is studying the LEP200 physics capabilities with the L3 detector. The BU hardware task involves the new Silicon Micro-strip Detector, which is the key device for b-quark physics and for the LEP200 Higgs searches. The BU group is responsible for the Silicon radiation monitors and for developing the SMD data analysis software.
MACRO (Monopole Astrophysics and Cosmic Ray
Observatory)
S. Ahlen, J. Stone, L. Sulak, W. Worstell
The deep-underground MACRO detector is currently operating at the Gran Sasso Laboratory in Italy. MACRO has a geometrical acceptance of ~10000 m2/sr at an average depth of 3.8 km.w.eq. (kilometers of water equivalent), and comprises the largest and most sensitive cosmic ray tracking detectors ever constructed. Due to the large fiducial volume and high sensitivity to minute energy depositions which generate sequences of single photoelectrons in the phototubes, flux limits for Grand Unified Theory magnetic monopoles and for new particles predicted by superstring and supersymmetric theories are expected to greatly exceed any similar search to date. These same features also allow MACRO to function as an observatory of the low energy neutrinos radiated in stellar gravitational collapse. MACRO's high resolution tracking system (<0.2o intrinsic angular resolution and ~1cm spatial resolution) affords refined searches for celestial point sources of gamma rays and high energy neutrinos. By virtue of the detector's depth underground, fine-grain tracking capability, and large acceptance, MACRO is also ideally suited for a comprehensive study of multiple muon bundles that reflect the nature of the high energy cosmic ray primaries interacting at the top of the atmosphere.
High-Altitude Scientific Balloon Experiments
S. Ahlen, B. Zhou
Our group is involved in the analysis of several collaborative projects that have been built at Boston University and flown to the top of the atmosphere by balloons to study our astrophysical environment.
An experiment that measures the elemental and isotopic composition of cosmic rays from helium to oxygen through the use of a magnetic spectrometer using electronic particle detectors. This information enables one to determine the source composition, acceleration and propagation characteristics of the cosmic rays, which play an important role in the dynamical aspects of our Galaxy. SMILI flew in 1989, and data has been recently published. SMILI-II, optimized for the elements from lithium to oxygen, flew in Manitoba in 1991 and analysis of its data has just gotten underway.
An experiment searching for heavy antinuclei in the cosmic rays, which would demonstrate the existence of anti-galaxies in the universe. Data from a 1987 balloon flight in northern Saskatchewan are being analyzed and should be finished in 2 years.
A balloon experiment flown in the Antarctic in December 1991. MAGPIE uses CR-39 track etch detectors to measure the bending of particles and their velocity in a magnetic field. The high resolution of this detector allows adjacent isotopes of iron to be cleanly separated. The isotopic composition of iron and other heavy elements offers a window on cosmic ray nucleosynthesis. Analysis involves measurements of precision passive-track detectors with automated microscope systems.
The Proton Decay and Neutrino Astrophysics
Experiment
S. Dye, J. Stone, L. Sulak
Exciting new experimental endeavors are available to students with interest in proton decay and/or neutrino astrophysics. Together with potential analyses of the complete data sample from the IMB proton decay detector, possibilities exist for involvement in the Super-Kamiokande experiment and the Deep Underwater Muon and Neutrino Detection (DUMAND) project. Super-Kamiokande , now under construction in Japan, offers unprecedented resolution and exposure for proton decay and neutrino space searches, and for studies of neutrinos from the atmosphere and the sun. In addition, important calibration experiments at KEK (the national accelerator laboratory of Japan) present an excellent opportunity for students to gain immediate hardware experience in both accelerator and non-accelerator physics. DUMAND is designed to discover sources of high-energy neutrinos. When installation in the ocean off Hawaii is completed, the DUMAND array will be the world's largest particle detector.
F. Krienen J. Miller, B.L. Roberts, L. Sulak, W. Worstell
Another way to explore the physics of very high energy (TeV) particles is to exploit the uncertainty principle of quantum mechanics. Conservation of energy in classical physics is expanded in quantum mechanics to allow for the production of evanescent "virtual" particles, which appear and disappear too quickly to allow for their direct measurement. Although their existence is fleeting, the effects produced by virtual particles are quite real and can be measured in high-precision experiments. The more precise the measurement, the more massive and short-ranged particles can be studied through their virtual interactions. This method is being used in a precision measurement of the muon anomalous magnetic moment (g-2) at Brookhaven National Laboratory. Its value is of great importance in determining the fundamental structure of the muon, and in elucidating the nature of the fundamental forces at the smallest distances thus far accessible. This project requires the construction of a muon storage ring made from a beam tube surrounded by a superconducting magnet. The 7-meter radius magnet requires a uniform field to within 1 ppm (part per million), and will store 3.09 GeV/c muons for 10 lifetimes, enabling their precession frequency to be studied to a precision of 0.35ppm. The contribution of the virtual intermediate vector bosons, W+_and Zo, is 1.7ppm, a prediction which is tied to the renormalizability of the standard model. After subtracting effects from the virtual production of known particles, it will be sensitive to the virtual production of new particles and interactions on a mass scale of several TeV.
Data collection from the g-2 project will begin in early 1996. The Boston University team has substantial responsibility for detector electronics, software and simulations, as well as beam dynamics issues.
W. Worstell
The muon g-2 experiment (described above) requires precise knowledge of standard model rates for virtual particle production. For leptons and photons, these rates can be calculated from the extremely successful QED theory. The virtual production rates for weak intermediate vector bosons are also precisely known through electroweak theory. The virtual production of hadrons, which are particles containing quarks, cannot, however, be precisely predicted from first principles. It must instead be extrapolated from experimentally measured quantities. In particular, measurements of cross-sections for hadron production in electron-positron collisions at center-of- mass energies in the range 400-1400 MeV are crucial to the interpretation of the results of the muon g-2 experiment. For this reason, we are participating in the CMD-2 experiment at the VEPP-2M electron-positron collider at Novosibirsk, Russia. CMD-2 is a general-purpose cryogenic magnetic detector to which we are contributing data acquisition hardware, computer capabilities, and an independent analysis effort. CMD-2 data from the production of low-mass vector mesons is the most precise yet available, and is also used in preparation for the future -meson factories.
The Boston University Colliding Beams Group is participating in experiments to study the electroweak and strong interactions by examining the collisions of high energy electrons and their antiparticles (positrons). Two efforts are underway: the SLD Experiment at the Stanford Linear Accelerator Center in California and the Beijing Spectrometer (BES) in Beijing, China.
The SLD Experiment is studying the production of the neutral vector boson Z0 and its decays into leptons and quarks. Polarized electron beams are being exploited to examine the detailed properties of the Z0 through study of asymmetries in Z0 decays. The Boston group has constructed sophisticated proportional detectors for the particle identification system and is active in the software effort.
The Beijing Spectrometer has been constructed to study the details of charm quark spectroscopy, possible gluonium states - bound states of gluons, the mediators of the strong interaction - and the properties of the tau lepton. The experiment studies electron-positron annihilation in the center-of-mass energy range from 3 to 5.6 GeV, providing millions of events for high-statistics studies. Recently, the mass of the tau lepton was measured with an error that is eight times smaller than obtained by previous measurements. An upgrade of the experiment is now under way in which Boston University researchers are participating in the construction of the new Main Drift Chamber.