The magnet yoke and support tube of the L3 detector. Faculty Members -- S. Ahlen, J. Butler, F. Krienen, J. Miller, B.L. Roberts, J. Rohlf, J. Stone, L. Sulak, J.S. Whitaker, W. Worstell, B. Zhou
Research Faculty & Associates -- R. Carey, E. Kearns, J. Shank, J. Sullivan
Staff Scientist -- O. Johnson
Graduate Students -- D. Brown, J. Goldstein, D. Loomba, C. Okada, C. Orth, C. Wildgoose, J. Xu
Experimentalists in high-energy physics are probing at the smallest scales of length to learn about the fundamental nature of elementary particles and the interactions between them. In addition, they are performing precision tests of the standard model and searching for new physics beyond it. The goal of experimental astrophysics is to determine the nature of the universe through observations of radiation reaching the Earth from space. The major programs currently underway at Boston University are:
L3 Experiment at LEP -- S. Ahlen, 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.
The magnet yoke and support tube of the L3 detector.
Astrophysics Detectors Above the Earth's Atmosphere -- S. Ahlen, B. Zhou
Our group is currently involved in the analysis of two collaborative projects that have been built at Boston University and flown to the top of the atmosphere by balloons to study our astrophysical environment. We have recently proposed to build another balloon experiment which would be flown in 1999.
SMILI is 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 published. The results of this experiment proved that cosmic rays escape from our Galaxy. SMILI-II, optimized for the elements from lithium to boron, flew in Manitoba in 1991 and analysis of its data is nearly complete. The measurement of the abundance of the beryllium isotopes in this experiment will enable us to determine the amount of time it takes for high energy cosmic rays to escape the Galaxy.
MAGPIE was a balloon experiment flown in the Antarctic in December 1991. It used 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. Current data analysis involves measurements of precision passive-track detectors with automated microscope systems. This project will be completed within 12 months.
The CRO project is an advanced version of MAGPIE, in which particle velocity is determined by a novel ring imaging Cerenkov counter recently developed by our group. With this improvement, CRO will have better mass resolution than MAGPIE. Also, due to the reduced amount of nuclear fragmentation and to the wider energy range of the CRO technique, this experiment will collect about 10 times more events than MAGPIE. The primary goal of CRO is to measure the radioactive isotopes of aluminum and manganese at high energy, which complement the SMILI measurements of beryllium in determining cosmic ray lifetime. If approved, CRO will fly from the Arctic circle in 1999.
We are also participating in the AMS experiment to search for antimatter from outside of our Galaxy. This experiment consists of silicon tracking detectors placed in a large permanent magnet. The experiment will be attached to the International Space Station from 2001 to 2004, and will have a 10,000 fold improvement in sensitivity over previous experiments. Our group is responsible for data analysis and is also involved in a proposal to do significant gamma rat astronomy with AMS. There will be a test flight of AMS on the Space Shuttle in 1998.
The D0 Experiment -- J. Butler
The D0 Experiment studies proton-antiproton interactions at the world's highest energy accelerator, the Fermilab Tevatron. The D0 detector is a large, general purpose, hadron-collider detectors well suited to addressing the broad range of physics topics at square root of S = 1.8TeV. In the Standard Model of electroweak interactions, the basic constituents of matter are made up of three generations of quarks and leptons. In March 1995, the D0 and CDF collaborations simultaneously announced the observation of the top quark, the last constituent to be discovered in the Standard Model. The extraordinary large mass of the top quark, approximately the same as an entire gold atom, is very intriguing and could provide clues to the origins of mass. Some theorists have speculated that the top quark may be intimately connected to the electroweak symmetry breaking mechanism. The Boston University D0 group is involved in the top quark analysis using the data from the Tevatron Run I, which began in 1992 and is scheduled to end in early 1996. A rich program of top physics measurements becomes accessible with increased statistics. Higher luminosity in Run II, scheduled to begin in 1999, will result in thousands of reconstructed top events for study of this unique window on high-mass scale physics. To maximize the S0 top physics potential for Run II, an ambitious upgrade of the detector is underway. A major element of the upgrade is a substantial renovation of the muon system, including new forward tracking chambers, fast trigger counters, and electronics. The B.U. group has taken a leadership role in the upgrade of the D0 muon system, and will make significant contributions to the muon trigger electronics.
The g-2 Experiment -- R. Carey, 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.
A perspective view of the g-2 storage ring.
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.
The MACRO Experiment: (Monopole Astrophysics and Cosmic Ray Observatory) -- E. Kearns, J. Stone, L. Sulak
The deep-underground MACRO detector is currently operating at the Laboratori Nazionali del Gran Sasso in Abruzzo, Italy. MACRO has a geometrical acceptance of ~10000 m2/sr at an average depth of 3.8 kilometers of water equivalent under the mountainous overburden of the Gran Sasso d'Italia. We used this large area detector to research several topics.
The specialty of MACRO is the search for the magnetic monopoles: particles with bare north or south magnetic charge. These particles are a natural consequence of Grand Unified Theories, which also predict that the monopole will be very massive: perhaps 10^16 GeV. Such particles can only be produced by the intense energies available during the big bang. MACRO operates like a giant Time-Of-Flight counter to detect the unique signature of a slow moving but penetrating massive particle. It is equipped with tanks of liquid scintillator, planes of streamer tubes and plates of track etch material in the hopes of recording a convincing signature from a single candidate event.
MACRO's high resolution tracking and timing are also used to perform high statistics measurements of cosmic ray muons; in particular the scintillator timing is used to distinguish upward going muons produced by neutrino interactions in the rock. This is an opportunity to investigate the possible flavor oscillation of massive neutrino's as suggested by the atmospheric neutrino puzzle, as well as possible astrophysical point sources of neutrinos. The large area and fine grain of the tracking affords unique measurements of large multiplicity muon showers. From these we hope to infer information about the nature of the primary particles in the high-energy range of the cosmic ray spectrum The large mass of liquid scintillator (~ 600 tons) is also instrumented to identify a burst of low energy pulses as might be caused by the flux of neutrinos from a supernova within our galaxy.
The deep underground MACRO detector at the Laboratori for Nazinalli del
Gran Sasso in Abruzzo, Italy.
The Super-Kamiokande Neutrino Detector -- E. Kearns, J. Stone, L. Sulak
Super-Kamiokande is a 50,000 ton water Cherenkov detector recently constructed in Toyama, Japan, that will begin taking experimental data in April 1996. It supersedes previous detectors (IMB and Kamiokande) both in size and resolution. Large volume water detectors were invented to discover proton decay, but so far have only set limits (well in excess of the first predictions of the SU(5) Grand Unified Theory). As Super-K is 6 to 10 times larger than the previous generation of detectors, it can reach a proton lifetime of 10^34 years, covering predictions by supersymmetric models ( which are themselves supported by the apparent convergence of running coupling constants as measured at LEP and elsewhere). Among the possible decay modes are very interesting signatures, such as p--> nu K+ which would provide evidence for mediation by the Higgs particle.
Since the Super-K detector is designed to investigate contained events of energy 1 GeV, it will naturally pursue the intriguing puzzle of atmospheric neutrinos first observed in the prior generation of water Cherenkov detectors. These neutrinos are produced in the shower of particles caused by cosmic ray interactions in our atmosphere. The puzzle is that the ratio of the flux of electron neutrinos have a small but finite mass and undergo flavor oscillation as they travel the long distance to the detector. The Super-K detector will have 2500 atmospheric neutrino interactions per year with which to investigate this possibility. The group is also planning a controlled beam of accelerator neutrinos from the KEK accelerator laboratory, 250 kilometers away.
Another hint for massive neutrinos is found in the apartment deficit of solar neutrinos, as recorded by many radio-chemical experiments as well as the predecessor Kamiokande experiment. The Super-K detector has been instrumented with these low energy interactions in mind, and will record 11,000 solar neutrino interactions per year. And perhaps neutrinos from another dying sun will be detected: the Super-K detector would record 8,000 neutrino interactions from a supernova in the center of our galaxy.
Looking up inside the Super-K detector during construction in Toyama, Japan.
The inside of the detector is lined with phototubes, and rises to a
height of over 40 meters.
Fiber Calorimetry for Hadron Colliders -- L. Sulak, J. Sullivan
Forward calorimeters in SSC/LHC detectors are necessary to cover the pseudorapidity range from ehta = 2.5 to at least ehta = 5. They allow computation of missing transverse energy and far forward jet tagging that is radiation-resistant to gigards, fast, and insensitive to radioactivity, especially to neutrons. This can be accomplished by embedding quartz optical fibers in a copper absorber. In this calorimeter, the shower particles produce light by the Cerenkov effect, generating a signal of 6ns duration.
Unique to this new technology, and unlike ionization detectors, the observed Cerenkov energy of hadronic showers has a transverse dimension nearly an order of magnitude smaller than that in conventional calorimeters that sample only ionization detector. All of these characteristics allow a quartz fiber calorimeter to be operated in the extreme working conditions of a forward detector for the LHC collider at CERN.
The primary physics goal of this project is to look for new physics(e.g., Higgs or Susy) at the higher energies available at LHC. The B.U. group plans to conduct an in-depth study of the performance of different kinds of silica fibers in the presence of radiation load, a study of customized ultra-violet photodetectors and pixelized hybrid photodiodes, and the construction and beam testing of a full containment calorimeter module. This hadronic module will test all the features required radiation resistance, speed, spatial resolution, transverse energy measurement , and implementation of a ET trigger. The end goal of this project will be a proof of feasibility and the collection of basic information necessary for the design of a forward calorimeter for the CMS detector at LHC.
The CMD-2 Experiment -- 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 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, the B.U. group is participating in the CMD-2 experiment at the VERR-2M electron-positron collider at Novosibirsk, Russia. CMD-2 is a general-purpose cryogenic magnetic detector to which the B.U. group will contribute data acquisition hardware, computer capabilities, and an independent analysis effort. CMD-2 data from the production of low-mass vector mesons (rho, omega, and psi) is the most precise yet available, and is also used in preparation for the future phi-meson factories.
Colliding Beams Group -- J.S. Whitaker
The Boston University Colliding Beams Group is participating in several experiments to study the electroweak and strong interactions by examining the collisions of high-energy particles with their antiparticles. Two efforts are underway: the SLD Experiment at the Stanford Linear Accelerator Center in California and the ATLAS Experiment at CERN, in Switzerland.
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 data analysis effort.
The Colliding Beams Project: Scale drawing of the SLD detector.
The ATLAS Experiment is a large detector system being developed by a collaboration of physicists from all around the world to study very-high-energy proton-proton interactions at the Large Hadron Collider at CERN. This experiment will probe the origins of electroweak symmetry breaking and the particles associated with the new physics that must appear at energies at the symmetry breaking scale. The BU participants are involved in the development of the muon detectors for ATLAS; including the design and testing of pressurized drift tube detectors and the development of custom CMOS integrated circuits for these devices.
Atlas detector system.