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progress page 1 Particle Physics - Progress, 2 of 3 progress page 3

Detection of tetra-neutrons - April 2002
The first evidence of nuclear cluster containing only four neutrons (tetra-neutron) with no proton may have found at the GANIL (Grand Accelerator National D'ions Lourds) accelerator in France (Physical Review Letters, C65 044006 (2002)). This confirms the prediction made by scientists back in 1960s, that a lump of four or more neutrons might stable enough to exist. The experiment was carried out using lithium-11, beryllium-14 and boron-15 beams and smash these neutron-rich nuclei on a carbon target. The resulting fragments were then allowed to collide with protons. Neuton clusters were indirectly detected by measuring the time for fragments to reach the protons and the recoil energy of the protons after the collisions in a liquid scintillator.


Production of anti-atoms - September 2002
Physicists from the ATHENA collaboration at CERN have manged to produced 50 000 cold antihydrogen atoms. A normal hydrogen consists of a positively charged proton and a negatively charged electron 'orbiting' the proton. The antimatter counterpart consists of a negatively charged antiproton surrounds by a positively charged positron. The antimatter can be used to study the conservation of CPT symmetry (charge, parity and time reversal), by comparing the frequency of the elctronic transition between the ground state and the first excited state in hydrogen and the corresponding transition in antihydrogen. The frequency would differ slightly if there is a violation of CPT symmetry.

To make antihydrogen atoms, the antiprotons are shot into a cloud of positron, produced from the radioactive decay of sodium-22. The particles are confined in a trap that uses a magnetic field and various electric fields. Antihydrogen atoms that manage to escape the trap will annihilate to prodce pions (by antiproton) and gamma rays (by positron) as they hit the surrounding electrodes.


Symmetry confirmation - December 2002
Scientists at the Lawrence Berkeley National Loboratory and the University of California have confirmed most precised ever the charge conjugation symmetry measurement between matter and antimatter. The results is reported in Phys. Rev. A, 66, 052505. The study was based on the observation of rare decays of positronium. Positronium has a similar atomic structure to that of hydrogen atom: a proton and an electron. However, in case of positronium, the proton is replaced by a positron. According to the charge-conjugation symmetry invariance, the rate of decay into photons for matter and antimatter should be the same. Of a few events out of a billion positronium decays found no violation to the charge-conjugate symmetry. These findings is in good aggreement with the predictions of quantum electrodynamics (QED).


Absence of extra dimensions ? - February 2003
According to the string theory prediction, there exists six extra spatial dimensions beyond the usual three already observe in the real world. It is believed that these extra dimensions curled up (compaction) into small length scales and it is thought that they may generate forces with strength comparable to gravity over the distance of about 100 mm. To test this prediction, scientists at the University of Colorado (Nature, 421, 922 (2003)) have set up an experiment by monitoring the mutual attraction of two thin sheets of tungsten placed 108 mm apart. However, no new force can be detected over this distance, apart from gravity.


Discovery of a new meson - April 2003
New particle called Ds (2317) has been unexpectedly discovered using BaBar detector while analysing result to study the difference between matter and antimatter using bottom quarks. The new mesonic particle has a mass of 2.32 GeV/c2 and consists of a charm quark and an anti-strange quark. The mass is lower than theoretical prediction and the decay is found to violate isospin conservation. This could either mean that the quark potential model need some modification or, even more dramatic, the state is of an entirely different kind (four quark state?)


Discovery of five-quark particles - July 2003
Scientists in Japan and United State has confirmed the discovery of a particle containing two up quarks, two down quarks and an anti-strange quark. It has a mass of about 1.5 times the mass of a proton (Physical Review Letters, 91, 012002). In fact the existence of such a particle has already been predicted back in 1997 by Russian scientists at the Petersburg Nuclear Physics Institute.

The pentaquark, created from a high-energy gamma-ray (2.4 GeV) interact with a neutron, has only a life-time of the order of 10-20 seconds before decaying into a meson (positive kaon) and a neutron. It may hold clues to the early Universe, just after big bang and the nature of strong force. More studies will be needed to determine the physical, structural nature of the particle.

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