The latest news from ATHENA 

October 9, 2006
ATHENA publishes evidence for the first formation of slow antiprotonic hydrogen (protonium) - the first antimatter chemistry.
The protonium has been synthesized following interaction between an antiproton and the positive molecular hydrogen ion H₂⁺. Careful analysis shows that the protnium is formed at sub-eV kinetic energies in states around n = 70 and with low angular momentum. The lifetime of this slow protonium was about 1.1 microseconds.
Physical Review Letters article in pdf-version: Evidence for the Production of Slow Antiprotonic Hydrogen in Vacuum

Protonium forms when antiprotons interacts with molecular hydrogen ions at the edge of the positron plasma.

Antihydrogen is formed all along the positron plasma and lives long enough to annihilate on the wall. Protonium is formed mainly at the center of the positron plasma where the molecular hydrogen ions are concentrated and does not live long enough to annihilate on the wall, but instead annihilates in flight towards the wall.

June 19, 2006
ATHENA publishes results on a new method for cooling and centering ions using a non-neutral buffer gas and demonstrates the method on antiprotons immersed in an electron buffer gas.
The method should allow us to get a similar control over the parameters for the antiprotons used in making antihydrogen as the rotating wall plasma method has given us for the positrons and electrons.
Physical Review A article in pdf-version: Sideband Cooling of Ions In a Non-Neutral Buffer Gas

July 7, 2005
ATHENA publishes experimental results of the most efficient method for generating large plasmas of cryogenic positrons so far. The method is one-and-a-half orders of magnitude more efficient than the previously reported most efficient method and almost 3 orders of magnitude more efficient than the method used by ATRAP.
Furthermore we have been able to generate the largest and the most dense positron plasma reported so far. The first result was obtained by stacking several shots of positrons from our positron accumulator while the density record was made using the rotating wall method.
Physical Review Letters article in pdf version: New Source of Dense, Cryogenic Positron Plasmas

 In the news
 
Webcast from ATHENA made by the Exploratorium in San Francisco - recorded and aired on September 20, 2002
(You need RealPlayer to view this)

Frequently Asked Questions about Antimatter 

Figure 1 Central part of the ATHENA apparatus and trapping potential. a, Schematic diagram, in axial section, of the ATHENA mixing trap and antihydrogen detector. The cylindrical electrodes and the position of the positron cloud (blue ellipse) are shown. A typical antihydrogen annihilation into three charged pions and two back-to-back 511-keV photons is also shown. The arrow indicates the direction of the magnetic field. The detector active volume is 16 cm long and has inner and outer diameters of 7.5 cm and 14 cm, respectively. b, The trapping potential is plotted against length along the trap. The dashed line is the potential immediately before antiproton transfer. The solid line is the potential during mixing.

Figure 2 Experimental data. a, The number of events passing the selection criteria is plotted against the cosine of the opening angle v gg (see text for definition). The histogram is for cold mixing data (grey background). A total of 7,125 events, out of a sample of  103,270 reconstructed vertices, have two clean (but not necessarily back-to-back), detected photons in the 511-keV energy window. The data represent 165 mixing cycles. Filled triangles represent hot mixing data and are scaled by 1.6 to depict the same number of mixing cycles. b, The opening angle distribution (grey histogram) for antiproton-only data (99,610 vertices reconstructed, 5,658 clean events plotted). The filled circles represent cold mixing data, analysed using an energy (E g ) window displaced upward so as not to include the 511-keV photo peak; no angular correlation of photons is seen.

The happy ATHENA crowd. (click on image for larger version)

Quicktime movie illustrating the capture of antiprotons from the CERN Antiproton Accelerator (red balls), the transfer of the positrons from the Positron Accumulator (green balls), the mixing of the two species and finally the formation of antihydrogen which escapes the trap, annihilates on the wall of the trap and subsequent detection of antihydrogen.
(For more details check the different subsections of this website)

 

Background

   Nature, as far as we know, consists of normal matter – atoms such as hydrogen and carbon, which in turn contain positively charged protons, uncharged neutrons and negatively charged electrons.  However, physicists have known since the 1930’s that for each of the particle types of normal matter, there exists an equivalent particle of antimatter.   Antimatter particles should have the same mass, but opposite charge, as their matter counterparts.  Matter and antimatter don’t like to co-exist; if a particle meets its antiparticle, they annihilate in a burst of energy – often creating new particle-antiparticle pairs.   The energy release during matter-antimatter annihilation is often the basis for futuristic propulsion schemes in popular science fiction literature, such as Star Trek.  Cosmologists believe that there were equal amounts of matter and antimatter at the beginning of the Universe – the so-called Big Bang.  Why the universe is now composed of matter is a topic of great interest in theoretical physics.  According to the laws of physics as we understand them today, it should be possible to build a universe containing only antimatter.
    The laws of physics that govern the interactions of fundamental particles are often collectively referred to as The Standard Model.  The Standard Model places some restrictive conditions on the relationship between matter and antimatter.  Thus, comparing the characteristics of matter and antimatter serves to test the underlying theory of the Standard Model.  Essential to the Standard Model is the so-called CPT theorem, which involves discrete symmetries.  The CPT theorem requires that the laws of physics be invariant under the following operation: all particles are replaced by their antiparticle counterparts (Charge conjugation), all spatial coordinates are reflected about the origin (Parity), and the flow of time is reversed (Time reversal).  The CPT theorem has important implications for antimatter, including the above-mentioned mass equivalence of particle and antiparticle.
    The CPT theorem also requires that atoms and their anti-atom equivalents behave in the same way.  For example, hydrogen and antihydrogen should have the same spectrum – the frequencies or colours of light that they emit and absorb.  It has long been a goal of physicists to be able to produce atoms of anti-hydrogen, in order to compare their spectra with that of hydrogen.  An antihydrogen atom consists of an antiproton (negatively charged) and a positron (the antimatter counterpart to the electron).  Antihydrogen atoms were first reported to be observed at CERN in 1996 and at Fermilab (near Chicago in the USA) in 1998, but these experiments produced very few antihydrogen atoms, and these at velocities close to the speed of light.  The antihydrogen lived for a very short time before colliding with normal matter and annihilating.  There was no possibility for making precision comparison measurements of hydrogen and antihydrogen in these experiments, which only demonstrated the existence of antihydrogen.

Cold Antihydrogen

    The ATHENA experiment takes a completely different approach to producing antihydrogen.  The idea is to produce antihydrogen atoms at low energy – essentially at rest – in order to be able to study their properties.  The AD machine at CERN was built to take antiprotons, which are produced in high-energy collisions, and decelerate them to more manageable energies.
    The ATHENA apparatus slows, cools, and traps antiprotons from the AD.  The antiprotons are trapped in high vacuum in an electromagnetic ‘bottle’ known as a Penning trap.  At the same time, positrons from a radioactive source are accumulated in another trap.  The two clouds of charged particles (about 10000 antiprotons and 70 million positrons) are mixed together to produce antihydrogen.  All of this takes place in a cryogenic environment at about 15 degrees above absolute zero.
    Antihydrogen atoms that are formed escape the electromagnetic trap because they have no net charge.  They then annihilate and are detected by a specially built detector, unique to the ATHENA experiment.  Antihydrogen produces a very characteristic annihilation signal in this detector, allowing researchers to confirm its production.
    To date, ATHENA has directly detected 131±22 atoms of antihydrogen.  This implies that about 50.000 anti-atoms were actually produced in the apparatus, since most of them escape detection.
    The next step for ATHENA is to try to make measurements of the spectrum of antihydrogen and try to compare these to hydrogen.  Any difference in these two spectra would require fundamental changes to our current model of matter and antimatter.  These experiments could begin as early as next year, when the AD physics program resumes in May.
    The ATHENA result is a significant milestone in antimatter science, and it opens the door to the anticipated  application of modern techniques of atom trapping, cooling, and manipulation to the realm of atomic antimatter.   Tests of the behaviour of antimatter under the influence of gravity are also an interesting future perspective.
 

Figure 3 Colour contour plots of the distribution (obtained by projecting into the plane perpendicular to the magnetic field) of the vertex positions of reconstructed events. a, Cold mixing. All reconstructed antiproton annihilation vertices from the mixing region are plotted—no crystal cuts are applied. The trap inner radius is 1.25 cm. The annihilations are centred on a slightly smaller radius, in agreement with our Monte Carlo simulations. (Some events appear to be outside of the trap radius owing to vertex reconstruction errors.) b, The same plot as above, but for hot mixing. These data are normalized to represent the same number of mixing cycles (165) as those in a.

Early July 2002   ATHENA starts studies of positron cooling of antiprotons.
Early July 2002   ATHENA simultaneously confines positrons and antiprotons.
 
 

LVJ - Last  modified October 4, 2008