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A New Force in Nature

An international team may have discovered a new particle which will challenge science's understanding of the nature of matter.

A multinational team of physicists, including a group of TAU researchers, may have discovered a deviation from the Standard Model in particle physics, which if confirmed might become the most important discovery of the past two decades in the field.

The standard model explains the interaction between elementary particles that compose natural matter. Although the model, in principle, gives a complete description of all known matter, it is regarded by most physicists as being incomplete. "Today, we believe that there must be a more comprehensive theory to explain physical phenomena that cannot yet be calculated according to known principles," says Prof. Aharon Levy of TAU's School of Physics and Astronomy, the Raymond and Beverly Sackler Faculty of Exact Sciences. "Our experiment is the first to yield results which have substantially deviated from the standard model predictions."

Major milestones in particle physics were discoveries of leptons and quarks - particles even smaller than protons and electrons. Physicists are constantly trying to identify even more elementary particles of matter, and the forces that operate between them. Until now, though, experiments designed to identify deviations from the standard model have produced no significant results.

An illustration showing the result of a collision between a positron and a proton. The collision takes place in the beam pipe around the detector. The positron enters from the left and collides with a proton coming from the right. It collides with one of the quarks from the proton, almost performing a U-turn, and leaves signs of its trajectory which register in the upper part of the detector. The quark produces a jet of particles which registers in the lower part of the detector.

The recent discovery was a result of two experiments involving a collision between atomic particles (positrons and protons), conducted at the particle accelerator HERA, at the DESY Research Center in Hamburg, Germany. Headed by Prof. Levy, the TAU research group includes Prof. Halina Abramovicz and Prof. Shmuel Dagan, also of the School of Physics and Astronomy.

The HERA accelerator, opened in 1990, allows researchers to observe particles of ever smaller dimensions and examine their substructure. Positrons (a type of lepton) and protons (complex particles consisting of quarks) were accelerated in a 6.3 km tunnel, each in a separate ring, and made to collide at two points where measurement detectors had been set up. Two different detectors, costing $100 million each, were especially constructed for the experiment.

"The standard model predicted that in some of the positron-proton collisions, the trajectory of the positron would change dramatically, as if the positron made a U-turn. To the researchers' surprise, the measurements registered by the detectors showed that the number of collisions in which the positron rebounded were about ten times greater than predicted," says Prof. Levy.

Researchers propose several possible explanations for the new phenomenon: one option is the discovery of a new substructure in elementary particles, while the other is the possible discovery of a new type of particle, composed of a lepton and quark - to be known as a leptoquark - and a new force in nature which connects the new particles.

To declare a new effect in physics, however, the risk of error must be minimized to well below one percent. This calls for additional experimentation and verification. Prof. Levy says that by the end of 1997 there should be enough data from the experiments to determine with greater certainty that a "new physics" - differing from the standard model - is indeed needed to explain the new force.

The Israeli researchers play a significant role in the huge project, which altogether comprises about 430 scientists from 50 research institutes in 12 countries. The recent breakthrough in particle physics was published in the German scientific journal, Zeitschrift fur Physik, and the experiments' scientific data have been posted on a special internet site (http://xxx.tau.ac.il).

The TAU group has enjoyed many years of support for its various research projects from the German-Israel Foundation for Scientific Research (GIF), Minerva Foundation, Israel Science Ministry and TAU Research Authority.