Posts tagged positron

     You are hovering some planet in a galaxy far far away, uncertain whether it is made of matter or antimatter and hence whether or not it will be safe to land. The planet is inhabited by friendly aliens with whom you have made radio contact. They are very intelligent and understand you, and being advanced, know all about matter and antimatter.

     Naturally, they insist that they are made of matter; after all, it would be surprising if anyone chose to define their own stuff as ‘anti.’ How can we decide if their dictionary and ours coincide? What questions will unambiguously reveal whether they are made of the same stuff as us, or are anti-aliens?

     If matter and antimatter were always perfectly symmetrically counterpoised, there would be no way to settle the issue, other than gambling with a close approach of firing a tiny unmanned probe and seeing what  happens when it hits the atmosphere or anti-atmosphere. However, we know that there is an asymmetry, small but measurable, and that is what the electrically neutral variety of K mesons can reveal. They do so when they decay, producing a pion that is either positively or negatively charged accompanied by an electron or positron respectively. If matter and antimatter were perfect opposites, these two decays would also be precisely matched, the chance of each being the same. In reality, they are slightly different.

     The neutral K and anti-K are welded together in nature in such a way that they sometimes die quickly, but at other times live longer. The two possibilities are quite distinct and are known as the short- and long-lived versions. Each of these shows an asymmetry between matter and antimatter, but it is the long-lived one where the effect is biggest, they decay that leads to a positron being slightly more likely to happen than giving an electron: out of every two-thousand examples, on the average, 1,003 will give a positron and 997 give an electron. Now at last we have something to discuss with the alien.

     First, identify the K. It is no use giving its name, since the alien will certainly call it something else, but we can identify it by something we will agree about: its mass. It weighs in at slightly more than half the mass of a proton or antiproton and there are no other particles than can be confused with it. So tell the alien that we are interested in a particle whose mass is slightly more than half that of the massive particle that exists in the ‘nucleus’ at the center of the alien’s simplest atom, the proton in the hydrogen atom (or antiproton in an atom of antihydrogen.) That identifies the K.

     In addition to the neutral K, with no electric charge, there are also a K-plus and K-minus with positive or negative charge. So we much make sure that the alien and we are talking about the electrically neutral version. We must say that the property that holds the atom together is what we call ‘charge’ and that we are interested in the K that has no charge. The alien will be aware that this neutral K has two forms: one with a short life and one with a relatively long one. It is the latter that we will focus on.

     Now we come to the critical bit. In our world of matter, when the long-lived K decays into a pion and an electron or positron, it is the positron mode that is the most likely. So we ask the alien: ‘Is the lightweight particle that is produced most often in these decays the same as you find in your atoms, or is it the opposite?’ If the alien answers that it is the same, it is a positron, the alien is made of antimatter and we should look but not touch. If the alien replies that it is the opposite, an electron, then we are all made of matter and it is safe to land.

Antimatter, Frank Close

11:50 30 April 2009 by Amanda Gefter

At the moment physicists are having enough difficulty just taming antihydrogen, the simplest possible anti-atom. Can we ever expect them to make antihelium, and then organic antimolecules made from anticarbon and a whole anti-periodic table, too?

The problem here is that every anti-atom has to be built one subatomic antiparticle at a time. For example, if you want to make antideuterium - like antihydrogen, but with an added antineutron - you first have to make the antineutron. Antineutrons are neutral, making them impossible to steer in the conventional way with electromagnetic fields, so you just have to make great numbers of them and hope that for every million or so antineutrons you make, one ends up in the right place to make an antideuterium atom. “And for every further antineutron or antiproton you add, you lose another factor of a million,” says Michael Doser, spokesman for CERN’s AEGIS experiment studying the properties of antimatter.

While no one’s cracked that problem yet, one experiment at CERN is making use of a neat short cut to at least make something other than antihydrogen. ASACUSA has created atoms of “antiprotonic helium”, in which one of the electrons orbiting a helium nucleus is replaced by an antiproton. By studying the light spectra emitted by this composite matter-antimatter atom, the electrical and magnetic properties of the antiproton can be measured with great precision - and compared with those of a regular proton.

As for our chances of making anything more complex, Frank Close, a particle physicist at the University of Oxford, is pessimistic, saying it will take a billion years, give or take. “It depends on how long the human race lasts,” he says. It seems that our best bet for spying more exotic elements of the anti-periodic table is to look up at the sky - and hope that somewhere antistars are busy churning them out for us.

[Read More]

The reports began circulating a few weeks ago, and today’s publication in the journal Nature makes it official: Physicists have detected the heaviest bits of antimatter ever found on Earth. And that record is likely to stand for a long, long time.

Members of the STAR collaboration at the Relativistic Heavy Ion Collider, based at Brookhaven National Laboratory in New York, say they’ve seen the traces of 18 nuclei of antihelium-4 among about half a trillion particles produced by almost a billion gold-ion collisions at RHIC. These nuclei are like regular helium nuclei, except that instead of having two protons and two neutrons, they have two negatively charged antiprotons and two antiprotons.

The particles existed for only about 10 billionths of a second before they came in contact with ordinary matter particles and were annihilated, but that was long enough to register on STAR’s detectors. Physicists can routinely produce antihydrogen nuclei (basically, antiprotons), and last year a research team reported the first detection of antihydrogen atoms (a positron going around an antiproton). Scientists have even detected antihelium-3 nuclei (two antiprotons and an antineutron). But until now, antihelium-4 has eluded them.

[Read More]

(source: incomprehensibleuniverse)