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PEP-II accelerator has now delivered double the amount of data originally expected
:: 05 October, 2007
A steady push during the course of eight years, the PEP-II accelerator has now delivered double the amount of data originally expected when the BaBar experiment first
started up in May of 1999.
Between then and September 4, 2007, when Run 6 concluded, PEP-II has generated a remarkable half an inverse attobarn of integrated luminosity—a measure of the number of particle collisions in a given period of time. This corresponds to 500 million pairs of B and anti-B particles, whose decays have yielded major discoveries this decade. The sheer quantity of data is reshaping the kind of physics that can be investigated. PEP-II's achievement is a testament to hard work, as well as continual upgrades and adjustments to a machine built with the capacity for future improvements. The accelerator now regularly operates with four times the luminosity, or collision concentration, it was designed to deliver. Higher luminosity translates into more accurate results and the ability to find otherwise unobservable physics, such as matter–antimatter asymmetry in rare "penguin" type decays.
"Many groups at SLAC have worked very hard over a long time to make luminosity
high and reliability strong. We're very pleased," said John Seeman, head of the Accelerator Systems Division.
To reach half an inverse attobarn, accelerator physicists and operators needed to keep PEP-II humming stably, providing high luminosity day after day, month after month. Despite a series of challenging problems which resulted in reduced luminosity for the first half of the run, PEP-II supplied an average of just under 700 inverse picobarns per day in August, the best month of data collection ever and seven times the rate called for in the original goals.
A number of measures contributed to the success of PEP-II—the world's first storage ring to use
colliding beams
of different energies, which was the key to directly observing an asymmetry between B and anti-B decays. Over the years, the Accelerator Systems Division has doubled the number of radiofrequency (RF) stations that supply accelerating power to the beams; increased the amount of charge carried by the beams; improved the vacuum systems and beam focusing ability; and instigated continual beam injection, which alone accounts for a 40 percent boost in average luminosity.
Fortunately, the BaBar detector was also built with headroom—the ability to handle more data flow than PEP-II was originally expected to provide. In addition, upgrades to the triggering and other systems have enabled the collaboration to take advantage of the welcome flood of data.
"The science program at BaBar greatly expanded because of the increased luminosity and data. It has allowed a much richer exploration of matter–antimatter asymmetries than originally thought possible," said BaBar
Spokesperson Hassan Jawahery.
The final run for the PEP-II/BaBar duo begins Dec. 4 and wraps up on Sept. 30,
2008.
While the one half inverse attobarn so far accumulated is a tremendous milestone for the project, plans for the 2008 run remain even more ambitious. "SLAC is committed to maximizing luminosity in the next run. A final round of upgrades being implemented this fall will position PEP-II to assault all time records for storage ring performance," said
David MacFarlane, Deputy Director of PPA.
"With a good solid run, we could add 200 to 250 inverse femtobarns to the existing 500 (the same as half an inverse attobarn). We're aiming for 50 percent more data in one year," Seeman said.
News Inside News:
PEP-II: Asymmetric B Factory-
The PEP-II facility consists of two independent storage rings, one located atop the other in the PEP tunnel. The high-energy ring, which stores a 9-GeV electron beam, was an upgrade of the existing PEP collider; it reutilized all of the PEP magnets and incorporated a state-of-the-art copper vacuum chamber and a new radio-frequency system capable of supporting a stored beam of high current. The low-energy ring, which stores 3.1-GeV positrons, was newly constructed.
Injection is achieved by extracting electrons and positrons at collision energies from the SLC and transporting them each in a dedicated bypass line. The low-emittance SLC beams are used for the injection process. The collider was completed in July 1998.
The term "asymmetric" refers to the fact that the e- and e+ energies are not equal. This results in a collision center of mass which is moving in the laboratory. The motion of the center of mass is crucial for the study of the matter versus anti-matter discrepancy discussed below.
The construction of PEP-II was a collaboration of SLAC, LBNL, and LLNL.
A new detector, BaBar was constructed to take advantage of the high-intensity
collisions.
The B-Factory facility, PEP-II and BaBar, is pursuing a broad agenda of physics
involving the heavy quark and heavy lepton sector. At design luminosity, PEP-II
produces about 1E8 b and c quarks, and roughly the same number of tau's per
year.
In particular, the B-Factory is pursuing the question of why we live in a
matter-dominated Universe. At the beginning of the Big Bang, matter and
anti-matter were produced in equal amounts -- what tipped the balance in favor
of matter? Understanding the mechanism is crucial if we are to have a full
understanding of the evolution of our Universe. A small imbalance in the matter
to anti-matter ratio at primordial times, one part in about 1E9, would suffice.
In such a situation, all the matter and anti-matter, except the tiny unpaired
excess, would have annihilated to form photons (which we can detect now). The
unpaired excess, all matter, evolved to form the Universe.
To substantiate this paradigm, we need experimental evidence that conclusively
establishes the mechanism responsible for creating the tiny matter excess. There
are several competing theoretical models for this mechanism. The main role for
the B-Factory is to make a broad set of measurements capable of confronting the
crucial question of what happened to all the anti-matter. It is fascinating to
realize that with the B-Factory, we are able to confront crucial physics issues
which took place less than 1E-34 seconds after the start of the Big Bang.
Secondary Electron Emission Yields From PEP-II Accelerator Materials†-
The PEP-II B-Factory at SLAC operates with aluminum alloy and copper vacuum
chambers, having design positron and electron beam currents of 2 and 1 A,
respectively. Titanium nitride coating of the aluminum vacuum chamber in the
arcs of the positron ring is needed in order to reduce undesirable
electron-cloud effects. The total secondary electron emission yield of TiN-coated
aluminum alloy has been measured after samples of beam chamber material were
exposed to air and again after electron-beam bombardment, as a function of
incident electron beam angle and energy. The results may be used to simulate and better
understand electron-cloud effects under actual operating conditions. We also
present yield
measurements for other accelerator materials because new surface effects are
expected to arise as beam currents increase. Copper, in particular, is growing
in popularity
for its good thermal conductivity and self-radiationshielding properties. The
effect of electron bombardment, “conditioning”, on the yield of TiN and copper
is shown.
Details link for
Secondary Electron Emission Yields
In The Images-
1.The PEP-II accelerator has reached a milestone, delivering enough particle
collisions for the BaBar detector to observe 500 million pairs of B and anti-B
particles
2.Surface texture of alkali-cleaned LER chamber
material for PEP-II, uncoated. Etch pits are evident.
RMS roughness = 0.3 mm.
3. Low-energy ring, which stores 3.1-GeV positrons, was newly constructed.
4.Technicians work on reconnecting elements of the BaBar detector after a major upgrade. Bottom photo: Babar rests in the PEP-II storage ring tunnel, his namesake detector behind the wall in the background.
Release link: http://www.slac.stanford.edu/
Tags: PEP-II accelerator , BaBar experiment , anti-B particles , collision concentration , Accelerator Systems Division , inverse picobarns , anti-B decays , beam injection ,
