Space Phenomenon

Matter in the Universe

1.11.3 Antimatter

Antimatter was first proposed in 1929 by the English Physicist Paul Dirac. In attempting to combine quantum mechanics with special relativity he found that the solutions of the equation showed unequivocally that if matter is created from energy then an equal amount of antimatter is also created. This showed that every proton in the universe there should be an antiproton, and so on for all other types of matter. The combined theory was called Quantum field theory.

On August 2, 1932 Anderson photographed a vapour trail in a magnetic cloud chamber left by a particle that had the same mass as an electron but whose path bent in accordance with positively charged particles. This was a positron the antimatter equivalent of an electron and had been created in the upper atmosphere by a high energy cosmic ray (proton or electron with an atmospheric nucleus). By the 1950’s a particle accelerator called the bevatron was built at Lawrence Berkeley Laboratory which possessed sufficient energy to create its own antimatter. They demonstrated the symmetrical conversion of energy into particles of matter and particles of antimatter.

Unfortunately the work opened up a problem with the current understandings of cosmology. If the explosions at the bevatron created equal amounts of matter and antimatter then the original big bang should have done likewise. However, there was nothing to suggest that antimatter existed in anything near the quantity of normal matter. In the 1960’s Alfvén proposed a theory for the matter-antimatter asymmetry. He argued that "leiden frost layers" could exist that separated matter and antimatter. Even though the two should annihilate on contact he proposed that at boundaries between the concentrations of the antithetical particles energy and thus heat could be generated that formed a pressure boundary between the matter and antimatter. He stated that sometimes these layers are breached and small numbers of antiparticles pass through and survive until they reach Earth whereas in other instances the leiden frost layers might be caused to collapse completely thus creating massive explosions. Alfvén, believed that the idea of a single big bang was apocryphal and stated that the expansion of the universe could be accounted for by a number of much smaller leiden frost layers collapsing and forcing matter apart.

This prompted considerable work into high atmosphere particle observation. By lifting equipment as high as possible it was accepted that antimatter detected was most likely to have originated from deep space as opposed to from interactions in the atmosphere. Unfortunately in all the ballooning experiments not one detected antimatter above the atmosphere. To this day the work of Smoot Buffington and Orth remains the most stringent limit published on complex antimatter nuclei in cosmic rays. It was stated that antimatter nuclei such as carbon and oxygen are rarer than one in ten thousand in our part of the universe and this contradicted Alfvén’s hypothesis.

Smoot and Davidson 1995

The most promising theory as to how the imbalance came about is described by the Sakharov theory. This states that in the first one millionth of a second the anti-particles (note atoms, protons etc did not exist until later) were slightly less abundant than the matter particles and there was an annihilation of matter and antimatter creating abundant energy and leaving only matter. This left a situation where energy photons out numbered the baryonic matter by billions to one. This theory not only explained why matter is dominant over antimatter but why energy in the form of photons is dominant over matter.

Unfortunately, CP symmetry states that matter can not be created in any excess unless the two types of baryons behave differently. The physicists at the Rutherford Appelton laboratories in Didcott, which I visited recently have been doing work in to the possible differences between matter and antimatter:

"Naively, one would expect antimatter to be exactly the opposite of matter in all respects, including in the early Universe," said Ken Peach.

However, to create the Universe we see today, he says, a preference for matter must have arisen a fraction of a second after the big bang. It only needed to be a tiny imbalance, with as little as one extra particle of matter surviving out of every billion created in the primordial inferno. Though previously confined to thought experiments the empirical data for the preference of matter might be determine from the behavior of peculiar particles called mesons, unstable mixtures of both matter and antimatter. Researchers first discovered that matter and antimatter could act differently by studying the K meson, or kaon, in 1964. Kaons, however, are relatively simple particles and reveal only a small part of the needed theory. It has been commonly held for some time that the true picture might only be revealed by the study of the larger B meson. Unfortunately, it was three decades before enough B mesons could be generated to allow proper study. Within months, however, not just one but a host of particle accelerators will be capable of creating B mesons in sufficient quantity to test the theory. It is possible that the B meson experiments will have effects beyond describing antimatter-matter behavior. The standard model, the theoretical edifice that describes nature's fundamental forces and particles could be challenged by the results of the experiment. The standard model has barely changed for more than a quarter of a century-but to explain matter's supremacy, physicists may need to invoke forces beyond it. One possibility that may have led to the Sakharov theory was that a minuscule difference in the rate of decay of matter and antimatter at high energies resulted in there being a fraction more matter than antimatter. Then, after all the antimatter was annihilated, some matter would have been left over to form today's Universe. The crucial initial difference, Sakharov speculated, could have been a phenomenon called charge-parity violation CP violation for short which had first been seen three years earlier.

Physicists had assumed that if one could magically change all particles in two fashions, by switching their charges (that is, swapping matter for antimatter) and reversing their "parities" (reflecting them in a mirror and flipping them upside down), the particles would still behave in exactly the same way. In other words, if the idea of CP conservation were right, there would be no way to distinguish between matter and antimatter flipped in this way. That looking-glass notion is indeed true for the dominant forces in the Universe: electromagnetism and the strong force. However, research on kaons shattered the notion that it was also true for the weak force, which governs radioactive decay. Kaons and other mesons exist as the fleeting union of an ordinary quark and an antiquark. In 1964, a sensational experiment at Brookhaven National Laboratory in New York by James Cronin and Val Fitch, who shared the Nobel prize in 1980, showed that one type of kaon occasionally turns into its antikaon. Crucially, this happens less often than the reverse process, by about 1 part in 500. That is, there is a small preference for matter over antimatter. However, as we have touched upon briefly above physicists now know that kaons cannot tell the whole story. The Cronin and Fitch experiment showed that this bias exists, but revealed little about its intricate details. So physicists at the Fermi National Accelerator Laboratory (Fermilab) in Illinois and the European laboratory for particle physics (CERN) in Switzerland are trying to fill in the blanks. They are engaged in what Cronin, now at the University of Chicago, calls "heroic efforts" to measure a second oddity in the decay rates of kaons and antikaons that pops up only about once in every 10 million events. But even if they do detect another CP violation in kaons, most theorists believe that the decays of kaons are too simple to flesh out the rest of the picture. Bulky B mesons, on the other hand, include "bottom" quarks, which are much heavier than the "strange" quarks in kaons. Also, when Bs decay they produce a far richer menagerie of particles than the lighter kaons. Physicists suspect that B meson decays involve many different types of CP violation, and will thus reveal aspects of the relationship between matter and antimatter that kaons cannot.

New Scientist February 1999

"The evidence is tantalizing but not conclusive;"

 

says Al Goshaw of Duke University and co-spokesperson of the Collider Detector at Fermilab (CDF) collaboration, whose paper on the work is soon to appear in Physical Review Letters. "If it's there (CP violation), we'll see it" with more data, he adds. A lack of data will soon be a non-issue with the Tevatron accelerator, when its Main Injector, which will increase the luminosity of the colliding beams of particles, becomes operational in 2000, producing hundreds of times more B mesons. The results should also be encouraging for physicists at the Stanford Linear Accelerator's so-called B factory, a particle accelerator specifically designed to study CP violation in B mesons, which could begin taking data by April next year.

Science December 1998

 

However as discussed below the Tevatron is looking to be replaced as the world's leading B factory as other accelerators go online in what will become a race to chart an important new area of physics.

 

"It's as if kaons represent a 3-note piano, whereas B mesons give you the whole keyboard," says Peach. "Each tune you can play gives you different information about the source of CP violation."

This is why physicists in California and Japan have built massive machines dedicated to producing B mesons and anti-B mesons. These "B factories" will have to produce many millions of the unusual particles. This is because most Bs and anti-Bs behave just as one would expect if CP conservation were valid. It appears only 1 in 10 000 decays will a particle decay in a way that might reveal differences in how matter and antimatter behave. This means that the imbalance is are in a particle that is already exceptionally difficult to observe. The various chains of particles produced by the decay of one particle are called channels.

"A B meson can decay in hundreds of ways, but for CP violation there are a few flagship channels,"

 

says Helen Quinn, a SLAC theorist. The first is the so-called "golden channel", in which a B meson yields two other mesons, a "J/psi" and a "K short", a type of kaon The rate at which that occurs from a B versus an anti-B may hint at a similar inequity that favoured matter over antimatter after the big bang.

 

"This will be the first measurement of CP violation outside of the kaon system, so everyone wants to do it first," says Quinn. "It's not too rare, not too hard to find and very cleanly predicted. We should see the rate difference in the decays relatively soon."

 

It is hoped the B-factories will produce significantly large enough portions to allow physicists go well beyond merely detecting CP violation. Measuring several rarer channels could vindicate the standard model's description of CP violation, or unveil types of matter-antimatter asymmetry that can be explained only by new theories. The standard model already falls short of explaining the Universe's preponderance of matter. The part of it that describes weak forces operating on light and heavy quarks does contain a smidgen of CP violation, based on the observed asymmetry between kaons and antikaons. But this isn't enough to explain the Universe: the model predicts about a million billion times fewer protons than we see today. The hope is that the B factories will bridge that gap by revealing what amounts to a stronger source of CP violation in the early Universe.
For instance, the B factories may show that a simple addition to the standard model would suffice to beef up the levels of CP violation it predicts in the early Universe. This could involve a force-carrying particle called the Higgs boson, which is believed to endow quarks, electrons and other particles with mass.

"The way CP violation comes about is intimately connected with the Higgs mechanism," Quinn says. But she adds: "When we try to look beyond the standard model today, we are speculating and nothing more. It's a frustratingly good theory."

Adding a new kind of Higgs boson, or even more than one, to the standard model might fix any shortcomings exposed by the B factories. Alternatively, CP violation might stem from a new fundamental force, the "superweak" force proposed in the 1960s by Lincoln Wolfenstein of Carnegie Mellon University. This force would operate only among certain quarks and would produce no other observable effects. "That's a haunting possibility," says Cronin. "If that plays a role, it would be extremely hard to learn anything more about CP violation."

Some physicists caution that they are unlikely to answer all the questions. According to Bruce Winstein of the University of Chicago, who studies kaons at Fermilab, painting the full picture of CP violation will probably require machines that produce Bs even more copiously than the B factories. A planned B meson experiment at CERN's Large Hadron Collider (LHC), which will eventually become the world's most powerful physics machine in 2005, some have rather colloquially commented that it will "blow everyone else out of the water".

 

"It will produce a trillion Bs per year," says Neville Harnew of the University of Oxford. "The disadvantages of the huge number of background events, as compared with those at an electron-positron collider such as SLAC and KEK, are completely outweighed by the huge advantage in statistics."

 

LHC will also spit out heavier B mesons composed of bottom and strange quarks and anti-quarks, possibly yielding exotic new types of matter-antimatter asymmetry. Fermilab's potent Tevatron machine will churn out 10 billion B mesons per year after an upgrade in 2000, although they will be buried in an avalanche of other particles. Researchers are learning how to unravel those horribly complex collisions and could get results within two to three years.

 

A novel experiment at the Hadron Electron Ring Accelerator (HERA) in Hamburg, Germany, also will produce a flood of Bs. "If we are delayed and have to cope with competition from those groups, we'll really be in trouble," says Stephen Olsen of the University of Hawaii, who is part of the KEK team.

 

As the two B factories prepare to turn on their machines, physicists on both sides of the Pacific are expressing delight with the friendly nature of the race so far. "It's a very open competition," says Dorfan of SLAC. "We share everything with each other. Right now it's an even race, and there's no reason to believe they won't be as successful as we are." However, the pressure to be first is still there. "Physicists in the high-energy community here think that if we don't do a really good job on the B factory, our next project will suffer tremendously in terms of funding," says Kazuo Abe of KEK. Theorist Anthony Sanda of Nagoya University agrees: "We can't keep coming in second and asking for more money from the government."

New Scientist February 1999

Some of the most recent work by Harvard physicists Gerald Gabrielse and Anton Khabbaz and their Bonn collaborators have shown that to within one part in 10 billion protons and antiprotons have the same mass. The team have been able to make this comparison by loading a single antiproton and a single proton coupled with two electrons, in order to make the proton into a negatively charged object, and allowed them to orbit (simultaneously) around an ion trap under the influence of a strong magnetic field. Though as expressed above one would initially have no reason to believe the proton and antiproton masses would be different, but this stringent new measurement constitutes the best test yet of the CPT theorem, which says that physics should not discriminate between particles, on the one hand, and antiparticles moving backwards in time on the other.

Physics News Update

The American Institute of Physics Bulletin of Physics News

At present the research is waiting for the technology to either catch up or to create the data but eraly reports on CP violation are promising:

"We're not claiming a detection," says Barry Wicklund, a CDF collaborator at Argonne National Laboratory--just a hint of indecorous behavior to come.

Science December 1998

Antimatter As Energy

At present the work in antimatter at Fermilab uses far more energy than it could ever supply. Though large amounts of energy are liberated when two particles collide the amount of energy needed to produce the antiparticle is far more than can be gained from the annihilation. The accelerator at Fermilab creates antiprotons but in order to create an antiproton it is required for the proton to accelerate to a very high energy and collide with a metal target. Fermilab's Main Ring (synchrotron ring) accelerates protons to 120 GeV of energy. An eV (electron volt) = 1.6 x 10^-19 Joules of energy, a very small amount. A (Giga-)ev or GeV is 10⁹ or a billion eV. A (Mega-)ev or MeV is 10⁶ eV. In order to accelerate the proton, hundreds of magnets for bending and focusing the beam as well as radio-frequency cavities have to powered. 10's of MegaWatts are needed for this in addition to similar beamline elements in the proton production stage. Once at the right energy, the protons are diverted onto a metal target. The results of the nuclear reaction are such that for each 200,000 protons hitting, one antiproton is produced. This factor includes the fact that the antiprotons are produced within a large angle cone and only those whose direction is close to the original proton direction can be accepted. After this the particles enter the Antiproton Debuncher and Accumulator rings which make the beam profiles and other distributions smaller and more manageable. They actually stack antiprotons until up to about 10¹² of them are kept in this storage ring (the Accumulator). Consider that one proton-antiproton annihilation both at rest will yield 2 photons (X-rays) with a combined energy of only 1.88 GeV. On the other hand, the maximum antiproton energy after reaccelerating it in the Tevatron is 1.0 TeV, a factor of 1 million greater. So 2.0 TeV = 2x10² eV = 3.2 x 10⁷ Joule can be produced from one collision. Unfortunately, the Tevatron has a maximum energy of 1.0 TeV per particle and it is the highest energy proton accelerator in the world. To make one larger you either have to make a ring larger than 4 miles around or make magnets with larger magnetic field. The state of the art for magnet design still does not allow for generation of consistent fields larger by more than a factor of ten.

It takes about 2 days to get 10¹² antiprotons and consider a watt = Joule/sec so with a beam of 10¹² protons and the same of antiprotons colliding given they all collide when you want them to which never happens, you get:

10¹² x 3.2 x 10^-7 / 2 x 24 x 3600 ~ 2 Watt

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