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CESR-c Achieves New Performance Record

The Cornell Electron Storage Ring (CESR-c), operating at the Wilson Laboratory at Cornell University, has achieved a new luminosity record at energies designed to optimize studies of the charm quark. The new daily record was achieved on Monday, April 15. It is 4.07 pb-1 delivered to the experiment.

New Performance Record
Luminosity is the measure of the probability of collision of the electrons and anti-electrons(positrons) produced in the CESR accelerator complex. The quantity, pb-1, or inverse picobarns, is the unit in which luminosity is measured.

In order to understand why this record is important, we must discuss the nature of the scientific investigation carried on at Wilson Laboratory. CESR-c is a particle accelerator used to study the basic building blocks of matter and the forces that bind them. The theory that describes both the matter and forces is known as The Standard Model, an extremely successful theory. So far, all of its predictions have been verified by the experimental data. However, an important aspect of science is that we must always subject our theories to more and more stringent tests to insure ourselves that our theories are indeed correct. The purpose of the experiment now being performed at Wilson Laboratory is to make precision tests of the Standard Model. The experiment is being carried out by a group of 126 physicists representing 23 colleges and universities in the United States and Canada, known as the experimental group CLEO-c. CLEO is not an acronym, but CLEO -patra, of course, has been associated throughout history with Julius Caesar (CESR). The "c" denotes that we are studying the c(charm) quark, one of the basic building blocks of matter.

In a storage ring such as CESR, electrons and their anti-matter partners, positrons, are brought into collision. When matter and anti-matter collide, they annihilate into pure energy. This pure energy can then reform into new states of matter. This is an example of Einstein's famous equation E= mc2 that established the correspondence between matter and energy. It is by this process that we produce the charm quarks we want to study. In order to make precision measurements we must produce many charm quarks. The higher the probability that the electrons and positrons will collide, the more charm quarks can be produced. The measure of the probability of collision is the luminosity. The number of quarks produced is directly proportional to the luminosity. The more charm quarks produced, the more precise our results will be, resulting in more precise tests of the Standard Model. This is why our luminosity record is so important. In fact, due to a new piece of apparatus (the anti-solenoid magnet) that enabled us to achieve this new record, we hope to obtain even more luminosity records in the near future.

On March 24, a two month run was completed operating at an energy that was optimized for the production of the Ds meson which is comprised of a charm and strange quark(cs). So far, a total of 400,000 Ds mesons have been accumulated. Previously, CLEO-c operated at a lower energy which is optimum for producing the D meson, the lightest particle that contains the charm quark. D mesons can be electrically charged being made up of a charm and d quark(cd), or electrically neutral, being made up of a charm and u quark(cu).At present about 4,000,000 D mesons have been accumulated. On March, 2008, when the CLEO-c experiment will end, it is hoped that the number of produced D and Ds mesons will be doubled or tripled. This will allow very accurate, precise tests of the Standard Model.

The Inverse Picobarn
In order to understand this unit of luminosity, we should first discuss its inverse, the picobarn(pico is the prefix for 10-12, or one millionth of a millionth), or better, the barn. The barn is a cross-sectional area of size 10-24cm2 so a picobarn is 10-12barns (see below for the genesis of the name barn). The cross section is directly related to the probability of producing a particular reaction when particles collide. Note, while the luminosity measures the probability of collision, the cross section measures the probability of production. Thus, in a storage ring, such as CESR, the number of particles produced in a particular reaction is directly proportional to the product of these two probabilities: Luminosity x cross-section. If, for instance, the cross-section is 1 picobarn and the accelerator has accumulated a luminosity of 1 inverse picobarn(pb-1), the number of produced events is 1.0. For the Ds production, the cross section is 1 nbarn (n=nano= 10-9 =1000(103)pico). So with the luminosity record of 4.07 pb-1 on April 15, approximately 4000 Ds mesons were produced on that day.

To give a bit of an historical insight into the barn unit of cross-section, it was Enrico Fermi who named it. The area corresponding to a barn is the approximate area of a typical atomic nucleus which has a size of 10-12 cm. For most sub-nuclear processes, this is a very large cross-section, so Fermi suggested "it was as big as a barn".

The Anti-Solenoid Magnet
One of the main components of the CLEO detector is the superconducting CLEO solenoid magnet. It produces the magnetic field inside the detector which allows measurement of both the path and sign of the charge of all electrically charged particle produced in the electron-positron collisions. This information is vital to the analysis of the data. Unfortunately, the particles in the electron and positron beams also pass through this magnet field, distorting the beams. This distortion was limiting the luminosity that could be achieved. The CESR accelerator group designed the anti-solenoid magnet which applies a reverse magnetic field to cancel the effect of the CLEO magnet. When installed, there was an almost immediate increase of 20% in the luminosity resulting in the new perfomance record.

LEPP Staff at anti-solenoid magnet

The anti-solenoid magnet that was responsible for the new luminosity record (see the text). There are identical magnets on opposite sides of the CLEO detector. Shown in the picture is the outer jacket(blue) of one of the magnets with three LEPP staff who were instrumental in the installation of the magnet. The arrow over the letter B indicates the direction of the anti-solenoid magnetic field. The vertical aluminum tube in the center of the magnet, surrounds the transfer lines for the liquid helium which is used to make the magnet superconducting.

Cut-away rendering of anti-solenoid magnet

A cut-away rendering of the anti-solenoid magnet. The electron and positron beams pass through the center of the horizontal cylindrical section. Also indicated is how the liquid helium is brought into the magnet from above.