Experimental Particle Physics and Astrophysics (HEE)

Members of this group:

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.

Precision measurements at low energies provide an alternate path to the frontier in particle physics, and provide complementary information to that obtained at the highest-energy colliders.
Both kinds of information will be necessary if we are to understand the data obtained at the Large Hadron Collider. See the medium-energy page for more details.

Research Descriptions

superk

As a sequel to the combined effort of the two earlier deep underground ring-imaging detectors, IMB and Kamiokande, Super-Kamiokande started recording data deep inside a lead and zinc mine in the Japanese Alps in 1996. The detector is a 40-meter high by 40-meter diameter stainless steel tank, filled with 50,000 tons of ultra-pure water. The walls of the tank are lined with 11,200 photomultiplier tubes, each an enormous 50 centimeters in diameter. One half of the surface of the tank is covered by photosensitive material. These tubes record the Cherenkov light from charged particles as they pass through the water.

Physicists in the US and in Japan designed Super-K to search for, in part, the radioactive decay of the proton, a rare event never before observed. Detecting proton decay would confirm the Grand Unified Theory of particle physics.

Though Super-K has yet to detected a single candidate, it observed something equally intriguing: convincing evidence that neutrinos have mass. Because neutrinos carry no charge, they rarely interact with other particles. Billions go through each of us every second without effect. But since Super-K is so large, it detects a handful of these neutrinos each week. Using the data collected by Super-K, researchers including Boston University physicists Jim Stone (US co-spokesman), Larry Sulak and Ed Kearns confirmed results from the two precursor experiments that about half the neutrinos expected were unseen.

Neutrinos come in three species – the tau neutrino, the muon neutrino, and the electron neutrino. Super-K discovered that these neutrino species transform into each other. If oscillations occur, muon neutrinos could transform into tau neutrinos, and the missing neutrinos observed by Super-K would be explained. This was documented in the 1999 Ph.D. thesis of Boston University graduate student Mark Messier. Quantum mechanically, for these oscillations to occur, the neutrino must have mass. Several experiments since this revelation in 1998 have reinforced this interpretation.

The discovery of massive neutrinos has forced theorists to rethink the Standard Model of particle physics; neutrino mass is not anticipated in this most accurate and predictive of theories. Further, with mass, the neutrinos in the universe account for nearly as much mass as all the stars. Hence, they would influence the formation of galaxies in the early universe. This discovery is the first indication of new physics beyond the Standard Model.

Neutron EDM

L. Roberts, J. Miller,

The ATLAS Experiment at CERN

S. Ahlen, J. Butler, U. Heintz, S. Whitaker, A. Marin, J. Shank, S. Youssef, N. Nation,

atlas_det

cms

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 (LHC) at CERN, a laboratory for high energy physics near Geneva, Switzerland. It will be completed in 2007. This experiment will probe the origins of electroweak symmetry breaking and the particles associated with the new physics (such as the hypothetical Higgs Boson) that must appear at energies at the symmetry breaking scale. A figure illustrating the various parts of ATLAS is shown below.

Boston University personnel are involved in the construction and installation of the muon detectors for ATLAS. The muon detector will occupy a region the size of a five-story building and will measure the trajectories of muons in a magnetic field with a precision of better than 1/10 of a millimeter. This permits the determination of the muon momentum, which will be an important ingredient in searches for new phenomena at the LHC’s energy scale, which will be an order of magnitude greater than currently available. The detectors are now built, and are currently being assembled into “sectors” that will be moved to the ATLAS experimental hall in early 2006. A photo of the first sector is shown below.

Boston University is also playing a leading role in the development of computing and analysis tools that will be crucial when data begins to flow from the experiment in 2007. It is expected that many important discoveries in particle physics will be made at the LHC in the coming decade. These discoveries will improve our understanding of the fundamental particles and their interactions, and also of the nature of the early universe. One important goal of the LHC is to search for particles that may be responsible for the so-called “dark matter” of the universe. One possible type of particle that could account for this mysterious phenomenon are the particles associated with so-called Supersymmetry Theories.

The Compact Muon Solenoid

J. Rohlf, E. Hazen, S. Wu,

solenoid

The Compact Muon Solenoid (CMS) is a 15 kiloton detector designed to search for new physics at an unprecedented distance scale of 10-19 m at the CERN Large Hadron Collider (LHC). The CMS experiment will probe such tiny distance scales with proton-proton collisions at a center-of-mass energy of 14 TeV with first collisions expected in 2007. The detector consists of 220 square meters of silicon pixels and strips (80 million channels) for precision charged particle tracking, 75k lead-tungsten crystals for precision electron and photon measurement, a highly segmentated1000-ton brass hadron-calorimeter plus a quartz-fiber forward calorimeter to measure jets from quark and gluon scattering and energy balance, all surrounded by a precision muon chambers embedded in the return yoke of the magnet.

The GHz collision rate at the LHC presents enormous technical challenges on the design of readout electronics due to the intense radiation environment and the high speed at which millions of channels of data must be processed. The Boston University group, which includes not only our excellent staff in the Electronics Design Facility but also several experts resident at CERN, has a leadership role in calorimeter electronics and software for physics analysis of jets and missing energy. The group has designed and built the data concentrator, a sophisticated piece of digital electronics based on modern field programmable gate arrays (FPGAs) to read out the hadron calorimeter. Arjan Heering has led the custom design of the 18-channel hybrid photo-diode used to convert scintillation light from the calorimeter into electrical signals. We have also designed the electronics to feed calorimeter signals into the muon trigger to greatly reduce the backgrounds in the online event selection.

The Boston University group is also taking a leadership role in physics analysis of jets and missing energy. This work is closely tied to a number of dedicated runs in the test beam at CERN (coordinated by D. Lazic). While the exact physics to appear at the LHC is unknown, we have a strong belief that it will involve measurement of muons and quark jets. The timing of the CMS project presents an opportunity for new graduate students not seen since the discovery of the W and Z at CERN nearly 25 years ago.

The DØ Experiment

J. Butler, U. Heintz,

The DØ experiment studies proton-antiproton collisions at the world’s highest energy accelerator, the Fermilab Tevatron. These collisions release energy equivalent to 2000 times the proton mass. The DØ detector is a large, highly sophisticated instrument that measures the fragments of these collisions and allows scientists to study the structure of matter at these high energies.

According to our current understanding, the basic constituents of matter are quarks and leptons. All the matter surrounding us is made of the lightest quarks, called up and down, and the lightest leptons, the electron and its neutrino. However there exist two additional families of quarks and leptons with identical properties, except much larger masses. The heaviest of the quarks, the “top” quark, was discovered in 1995 by the DØ and CDF collaborations at Fermilab. The top quark turned out to have an extraordinarily large mass, approximately the same as an entire gold atom. Particle physicists believe that its further study will provide clues to the origins of mass.

The members of the Boston University DØ group were actively involved in the discovery of the top quark and the study of the carrier of the weak force, the “W boson”. The group is now participating in the second data-taking run which began in 2001 and will continue until 2009. During this run, thousands of top quarks will be created, allowing a detailed study of the properties of this intriguing particle. The data from the Fermilab Tevatron will also provide the best opportunity until the LHC begins operation to find the Higgs boson and new physics beyond the standard model.

The Boston University group has taken leadership roles in the construction of the muon detector system and the silicon microstrip tracker, the development of algorithms to identify bottom quarks and muons, and the “Top and Higgs Physics” analysis group. The group has designed and built a significant fraction of the electronics for the muon system trigger, the silicon track trigger, and the central fiber tracker trigger. The group’s physics interests center on the top quark and the search for new particles and forces beyond the standard model.

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