How to create antiprotons

I was a post-doc at FERMILAB between 1988-1992. I worked on the Tevatron which was a proton-antiproton collider. In those days I took the presence of antiprotons for granted. I knew, of course, how difficult it was to produce, collect and cool antiprotons into bunches but I thought that the people of the antiproton source department were consummate professionals…of course they knew how to produce antiprotons! In my mature age, now, I see the production of antiprotons as miraculous.

It is miraculous because a proton consists of quarks and gluons. How is it possible to reverse the (fractional) electric charge of those quarks at the same time? If you think this way you cannot comprehend what is going on. But, if you remember that all fermions (spin=1/2 particles) (quarks are fermions) emerge as particle-antiparticle pairs then some comprehension is possible.

Particle-antiparticle pair creation may occur in the natural decays of unstable particles or  when energy is concentrated in a very small volume. This happens when particles collide. When the energy density is high in that small volume (interaction point) the quantum fluctuations result in the emergence of fermion-antifermion pairs (quark-antiquark, electron-antielectron) as well as photons and gluons which then combine to form protons, neutrons and sometimes antiprotons albeit with a very small probability.

Fermions are always created as fermion-antifermion pairs

This means that spin=1/2 property is deeply related to the phenomenon of pair creation. We currently do not understand this connection. Add this to your list of mysteries!

Other mysteries

The spin of a proton is 1/2. Same as the quark spin. How is it possible to have the resultant spin of exactly 1/2 from the quark/gluon mess inside the proton? For that matter, how is it possible to have exactly +1 electric charge from the mess of quarks and gluons inside the proton? These are mysteries.

Fermion-antifermion pair creation rule is not broken by neutrinos

Neutrinos are fermions too, but, they are very special. Neutrinos rarely interact with matter. Neutrinos do not carry any electric charge therefore they do not interact electromagnetically. Very rarely they collide with other particles. When that happens their presence can be detected. If the collision is energetic enough, neutrinos can interact via the “weak nuclear force” which may transform them into charged leptons: (electron type) neutrino into electron, (muon type) neutrino into muon, (tau type) neutrino into tau.

The 3 types of neutrinos can also transform into each other. This phenomenon is known as neutrino oscillations. In contrast, electrons, muons and tau particles to not transform into each other. Neutrino-less charged lepton flavor violation has never been observed.

Neutrinos being so special, the question arises as to whether they are created as fermion-antifermion pairs as well. The answer is yes. The diagram below shows the neutron decay (neutron decays into proton + electron + antineutrino).  There is a fermion-antifermion pair in this process. The fermion is the electron, the antifermion is the antineutrino.

The fermion-antifermion pair does not have to involve 2 charged fermions. One of them can be a neutrino (or antineutrino).  No problem…as long as the electric charge is conserved in the overall interaction.

Another example would be the creation of neutrinos in the Sun

proton + proton → deuteron + positron + neutrino,

Again, we see the fermion-antifermion pair. In this case, the fermion is the neutrino, the antifermion is the positron (antielectron).

How to collect protons (FERMILAB):

“One can obtain a proton by stripping an electron from a hydrogen atom because hydrogen consists of one proton and one electron. This is known as ionization. At Fermilab, we take hydrogen and add an extra electron. This results in negative hydrogen ions. A positive voltage is then used to accelerate these ions because a positive voltage will attract the negative ions. The electrons are accelerated to an energy of 75,000 electron volts. This is about the energy of the electron beam found in a television picture tube. This done at Fermilab in a machine known as a Cockcroft-Walton. The negative ions then enter the Linac or linear accelerator and are accelerated to an energy of 400 million electron volts. The ions then pass through a carbon foil that strips off the extra electrons and permits the protons to pass through. Now the protons continue to be accelerated by passing through the booster, main injector and the Tevatron.”

How to create antiprotons (FERMILAB):

“The antiprotons are a lot harder to make. They do not exist freely in nature like normal matter. To make the antiprotons Fermilab uses a 120 giga electron volt beam of protons taken from the main injector. This beam is incident through the side of a drum shaped target made of nickel. The drum is rotated quickly to prevent the beam from hitting the same spot over and over. This prevents the distruction of the target.

This collision with the target produces many particles a very small portion of which are antiprotons. The particles fly off a many angles and have to be focused to fit into the relatively small beam pipe. This focusing cannot be done with conventional quadrapole magnets. Fermilab uses what is known as a lithium lens. The lower atomic mass of lithium prevents scattering of the antiprotons. After passing though the lens the positive particles are separated from the negative particle containing the antiprotons with a pulsed magnet. The negative particles are sent down the beam pipe. The negatively charged particles consist of antiprotons, pions and other particles. After sometime the pions and other particles decay away and the antiprotons are left.”

What about CERN antiproton production?

There is an excellent educational material at the CERN website.

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