Computational Cluster Programs

Reserch Conducted using the UCLA Physics Saxon Cluster

New World's Best 95% C.L. Upper Limit of the t -> Zq Branching Ratio -- Feb. 2008

On February 14, 2008 a group led by Charles Plager of UCLA, which has been searching for the Flavor Changing Neutral Current (FCNC) decay of the top quark t -> Zq, obtained the new world's best 95% C.L. upper limit of the t -> Zq branching ratio ( BR(t -> Zq) < 3.7% ). The previous world's best limit (11%) was also obtained by the same group. The new number is more than 3.5 times better than the best-published limit on this vertex. The group will be submitting a paper to Physical Review Letters.

CERN, the European Organization for Nuclear Research, headquartered in Geneva Switzerland, has been in the process of building and bringing on line the Large Hadron Collider. The Large Hadron Collider will be the world's highest energy particle accelerator ("atom smasher") when it is switched on in the summer of 2008. The Large Hadron Collider is being built in a circular underground tunnel 27 km in circumference straddling the Swiss and French borders on the outskirts of Geneva. The Compact Muon Solenoid (CMS) experiment, one of a number of experiments being built as part of the Large Hadron Collider, is a large general-purpose particle physics detector. CMS is being run as a worldwide collaboration of over 2500 scientists and engineers from around the world. The UCLA researchers are participating in the CMS experiment.

Professor David Saltzberg of the UCLA Department of Physics and Astronomy is the owner of the Saxon Custer, a Tier 3 grid computing resource for the CMS project. Grid computing resources are ranked in size and function from the large Tier 0 resources at CERN itself through Tier 3 resources, intended primarily to be used by local groups. The Saxon Cluster, named after David Saxon, UCLA physics researcher, teacher and president of the University of California, has twelve dual-core dual-CPU 64-bit AMD Opteron nodes. The Saxon Cluster, including operating system, and grid and CMS-related software was installed and is maintained by ATS staff as part of the IDRE Cluster Hosting Program.

The UCLA researchers who are participating in CMS hope to find a new particle Z' (Z Prime). String Theory and other models predict these particles. Z' would be a cousin of the Z-boson which won CERN a Nobel Prize when it was discovered about 25 years ago. UCLA researchers will probably spend years analyzing data from the CMS experiment: finding critical particles, measuring their momenta, and simulating what signal and background would look like. This cannot be done without massive computing power.

Until the Large Hadron Collider starts up, the Tevatron collider at the Department of Energy's Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois remains the highest energy particle collider in the world. Tevatron which accelerates protons and antiprotons in a 6.3 km ring, was completed in 1983 and a substantial addition, the Main Injector was added, during the five year period starting in 1994. Fermilab's mission is to advance the understanding of the fundamental nature of matter and energy through basic research.

On March 2, 1995, physicists, working on the CDF and D0 experiments, at Fermilab announced the discovery of a subatomic particle called the top quark, the last to be discovered particle of the six-member quark family. Scientists had been searching for the top quark since the discovery of the bottom quark at Fermilab in 1977. The top quark is the third-generation up-type quark with a charge of +(2/3)e. As of 2007, its mass was measured at 170.9 +/- 1.8 GeV/c2, nearly as heavy as a gold nucleus. The top quark almost exclusively decays to a W boson and a bottom quark. Its lifetime is roughly 1x10**-25 seconds. The second and third generations of charged particles do not occur in normal matter and are seen only in extremely high-energy environments. Currently Fermilab produces the only samples of top quarks in the world.

Although physicists can't see the top quark itself, they can recognize the signatures of its decay products. The top quark has many decay channels, and thus more than one possible signature. The UCLA team that made the Feb 14, 2008 announcement, used the Saxon Cluster as part of an effort to search for a rare decay of top quarks, that would signal an as-yet unknown internal structure to the top quark. The experimental apparatus and signal efficiency was modeled by running 12 million pseudo-experiments on the Saxon Cluster. The cluster worked extremely well under a heavy load.