Electron-Positron Collisions at LEP

Members of the UCR High-Energy Physics Experimental Group study electron-positron collisions using the OPAL detector at the LEP storage ring at CERN, the European Laboratory for Particle Physics located near Geneva, Switzerland. LEP is the highest energy electron-positron collider in the world and - with a circumference of 27 kilometers - the world's largest scientific instrument. The OPAL experiment is a collaboration of about 30 universities and laboratories from Europe, North America, and Japan. The UCR group has built and maintains a fine grained readout system for the hadron calorimeter, essential for muon detection and hadronic energy measurements, and has created a "Monte Carlo farm" based on HP RISC workstations to provide OPAL with large samples of simulated events needed for data analysis.

Since the beginning of LEP physics program in 1989, the OPAL experiment has recorded over four million events in which the Z0 boson decays into leptons or hadrons. Based on this data sample, we are improving our knowledge of many of the important parameters of the so-called "Standard Model" of elementary particle physics, such as the mass and decay widths of the Z0, while looking for deviations from the Standard Model which could provide clues on the nature of physical laws at very high energies. The group is also active in the study of strong interactions, in particular in performing detailed and precise tests of Quantum Chromodynamics, the gauge theory of strong interactions. Starting in 1996, the emphasis of the LEP program is on the highest possible energies. The electron-positron collision energy will be increased to produce W+W- boson pairs. This "LEP2" phase of the program will allow precise measurements of the W boson mass and decay characteristics and will extend the search range for new particles such as the Higgs boson, possibly leading to their discovery.

(Faculty and research staff: D.A. Chrisman, Prof. J.W. Gary, P. Giacomelli, W. Gorn, Adj. Prof. J.G. Layter, L. Del Pozo, and Prof. B.C. Shen)

Recent Publications

D. Chrisman, J.W. Gary, P. Giacomelli, W. Gorn, J.G. Layter, B.C. Shen, et al., "A Comparison of b and uds Quark Jets to Gluon Jets," Z. Phys. C69, 543-560 (1996).

D. Chrisman, J.W. Gary, P. Giacomelli, W. Gorn, J.G. Layter, B.C. Shen, et al., "D++ Production in Hadronic Decays," Phys. Lett. B358, 162-172 (1995).

The CMS Detector at LHC

Members of the UCR team are taking part in the design and construction of the Compact Muon Solenoid (CMS) facility, one of the two large detectors that will exploit CERN's Large Hadron Collider (LHC), to be built in the LEP tunnel at the end of the LEP2 period,, after the year 2000. In the new energy region explored by LHC, one will hope to finally see signs of physics beyond the Standard Model, but among the prime programmatic goals of LHC will be the search for the Higgs boson, believed to be responsible for electroweak symmetry breaking in the Standard Model.

The ability of a detector to trigger on muons in very high energy hadron collisions is crucial to extract the physics in the LHC environment, which will be characterized by very high luminosity and very high data rates. American physicists from 16 university and national laboratory groups are responsible for muon detection in the endcap region of the CMS detector. The UCR group is involved in building and testing prototypes of a fast and precise type of chamber for this region. Members of the group permanently at CERN lead the muon chamber beam test operations there in a continuation of the RD5 detector development experiment in which UCR participated. And a cosmic ray test facility has been set up at a site on campus for continuous chamber testing. Analysis of data from these test sites is being carried out, and longer term projects are under way to develop detector-specific software. It is expected that LHC and its detectors will receive US Government project funding beginning in 1998, after which the group will enter an extended period of chamber construction and testing before installation at LHC begins around the year 2004.

(Faculty and research staff: D.A. Chrisman, Prof. J.W. Gary, P. Giacomelli, W. Gorn, Prof. J.G. Layter, and Prof. B.C. Shen)

Recent Publications

C. Albajar, et al., (RD5 Collaboration), "Measurement of Hadron Shower Punchthrough in Magnetic Field," Z. Phys. C69, 415-425 (1996).

C. Albajar, et al,. (RD5 Collaboration), "Measurement of Momentum and Angular Distribution Punchthrough Muons at the RD5 Experiment," CERN/PPE/95-195, Accepted for publication in Nucl. Inst. and Meth. in Phys. Res.

C. Albajar, et al., (RD5 Collaboration), "Electromagnetic secondaries in the detection of high energy muons," Nucl. Inst. and Meth. in Phys. Res., A364, 473-487 (1995).

The CMS Collaboration, "Compact Muon Solenoid Technical Proposal," CERN/LHCC 94-38.

Milagro

UCR is a member of a collaboration studying and particle astrophysics with the Milagro detector.

Observations of very high energy (VHE: 100 GeV to 100 TeV) and ultra high energy (UHE: >100 TeV) cosmic gamma rays gives us the opportunity to study fundamental issues in elementary particle physics and astrophysics at unprecedented high energy scales. As is well known, magnetic fields in the galaxy deflect the trajectories of charged particles so that information about their point of origin is lost. Cosmic gamma rays, on the other hand, come to earth directly from astrophysical sources so that experiments that detect gamma rays have the ability to identify and study these sources.

Milagro will be the first water-Cerenkov detector specifically built ot study extensive air showers from high energy gamma rays. Milagro will be totally sensitive to the electrons, photons, hadrons,and muons in the air shower. Milagro will have the ability to detect lower energy air showers than any previous ground-based array. In addition, it will have excellent angular resolution by measuring the arrival time of the shower particles. The multilayer design of Milagro allows it to separate showers which contain muons and hadrons induced by background cosmic rays from purely electromagnetic showers from gamma-ray sources.

The Milagro detector is being deployed in an existing 60m × 80m × 8m pond located at Fenton Hill near Los Alamos National Laboratory. The first stage of Milagro has been installed and operating since the beginning of 1997. The installation of the full Milagro detector is scheduled for completion in 1998.

(Faculty and research staff: Adj. Prof. C.M. Hoffman, Prof. B.C. Shen, A. Smith, I. Stancu, T. Tumer, and Prof. G.J.)

Recent Publications

J.A. Goodman, et al., "The Milagro Gamma Ray Observatory", Proceedings of the American Physical Society, Division of Particles and Fields Meeting, Albuquerque, August 2-6, 1994, 1555-1561 (1995).

G.E. Allen, et al., "Gamma Ray Bursters: Detection and Distance Estimates with Milagro", Proceedings of the 24th International Cosmic Ray Conference, ICRC-95, 28 August - 8 September, Rome, Vol. 3 516-519 (1995).

G. Gisler, et al., "Solar Physics with the Milagro Telescope", Proceedings of the 24th International Cosmic Ray Conference, ICRC-95, 28 August - 8 September, Rome, Vol. 4, 1308-1311 (1995).