Hardly any antimatter

Particle physicists are wondering why there is no t much antimatter in the universe, when it is actually crucial for our very existence in

Just fifty years ago (in 1964) an important discovery was made in particle physics which implied that the decay of the so-called long-lived neutral kaon particles violates a symmetry dubbed CP (to be explained below).

This was soon seen as an important step in understanding the so-called matter-antimatter asymmetry in the universe i.e. why the universe is practically devoid of antimatter.

If the universe began with equal amounts of matter and antimatter, in the high density conditions in the beginning, matter and antimatter would have annihilated completely to radiation, so that no stars or planets can form with any possibility of life.

We simply wouldn’t be around! That is the significance of the discovery of CP violation, in that it enables a possible solution to avoid such a disastrous equality between matter and antimatter, which would turn everything to radiation.

The antimatter story began with Dirac, whose seminal equation (unifying special relativity and quantum mechanics) predicted that for every particle there is a corresponding antiparticle having same mass but opposite in all other respects such as electric charge and the like.

Around this time, the basic elementary particles constituting matter such as the electron, proton and neutron were already known.

Just four years after Dirac’s prediction, Anderson discovered the anti-electron or positron (having a positive electric charge, identical in value to the electron negative charge) in cosmic rays.

Anti-proton and anti-neutron were experimentally discovered in 1955-56 using a then powerful particle accelerator, called the ‘Bevatron’.

So, if positrons and anti-protons are in close proximity they can form anti-hydrogen atoms and indeed experiments, in CERN laboratory, like the ATHENA, have produced millions and millions of such anti-hydrogen atoms whose properties are being studied.

However, when matter and antimatter,  interact, they annihilate, producing pure radiant energy.

For instance, electrons and positrons annihilate to two gamma ray photons. It’s the same with protons and anti-protons, which go to neutral Pi particles which decay to gamma rays.

Incidentally, a few tons of antimatter can produce enough energy to last mankind for several years.

Although in high energy experiments, particles and antiparticles are produced in equal numbers, the proportion of antimatter to matter, in the universe, is miniscule.

For instance, the cosmic ray protons, that bombard the earth every instant, outnumber anti-protons hundred thousand to one!

From the intensity of the intergalactic gamma ray background radiation one, can infer that the ratio is less than one in ten million!

It is fortunate that the universe (under the extreme conditions in the big bang) did not begin with equal amounts of matter and antimatter.

Otherwise, we would be left with only radiation. It turns out that an initial asymmetry of one part in a billion made all the difference to our existence!

In other words, for every one billion anti-protons produced in the beginning, there were one billion and one protons.

Thus, the extra proton survived the annihilation. Particle physicists dub all heavier
particles like protons and neutrons as baryons and antiparticles as antibaryons.

The conservation of baryon number is crucial in high energy interactions. Proton has a baryon number of plus one while antiproton has a baryon number of minus one.

That is why they are produced in equal numbers in laboratory high energy experiments.

So in the earliest phases of the evolution of the universe, the conservation of baryon number must have been violated, albeit by a miniscule amount.

Indeed, experiments have looked for proton decay which would violate baryon number conservation, but so far, there has been no positive result.

Coming to CP violation, C and P stand for charge and parity, respectively. Parity or P violation implies that physical processes are asymmetric with respect to a mirror image, i.e. left-right symmetry is violated.

Indeed in 1957, it was discovered that the decay of particles by weak interaction processes, violates parity, i.e. the electrons produced in the decay are predominantly left-handed (their spin being in the same direction as their motion).

This implied that all neutrinos are ‘left handed’ particles. This discovery of maximal non-conservation of P in weak decays leading to a disturbing conclusion that laws
of physics depend on the frame of reference was soon evaded, because experiments soon showed that symmetry under charge-conjugation (C) was also
maximally violated.

Thus, as long as the combined operation CP was conserved, the possibility of an
absolute distinction between left-handed and right-handed co-ordinate systems would be prevented, being compensated exactly by asymmetry between particles and antiparticles.

For instance, positrons in weak decays would be right-handed and antineutrinos would have right-handed helicity.

So CP invariance restores overall symmetry conservation. In 1964, Fitch and Cronin presented results of an experiment studying particles called neutral K mesons or kaons,
showing that the long-lived kaon can decay to two Pi mesons (or Pions) implying
violation of CP symmetry.

Charged kaons, being particle and antiparticle, have same mean lifetime but for neutral kaons two different lifetimes are observed; the state called K(S) has short life (one-tenth nanosecond) and state called K(L) has long life of about fifty nanoseconds.

If CP is to be preserved, K(L) should decay to three Pions, while K(S) should decay to two Pions. Their experiment commenced with a beam of neutral Kaons.

After coasting for several seconds, the experimenters were left with a pure K(L) beam. It was observed that a small portion of the K(L) decays to two pions, i.e. about one in five hundred decays, thus violating the CP symmetry.

This violation was also later demonstrated in decays of the K(L) to two channels, one involving a positron and a neutrino and the other an electron and antineutrino.

Here again the channel involving positrons was more in one, out of 300 decays, again suggesting violation of the CP symmetry.

The significance of this bias in favour of positrons can now be explained. Here on earth, we define the positron (antimatter) as having positive charge and electron as negative.

But these are just names and what we define as positive and negative charge is quite arbitrary!

All physical results would have been the same if we had defined the electron as positive and positron as negative. So, we need an unambiguous way of defining what we call matter and antimatter.

The positron is now defined as the charged particle which is 0.3 per cent more prolific in the long-lived neutral kaon (K(L)) decay.

It is very interesting that this now enables us to explain to an intelligent being, in say some remote corner of the universe, not only that we are made up of matter, but even to communicate that our heart is on the left side (or liver on the right side). Left and right are again arbitrary definitions.

But in the K(L) decays, the positron and the associated neutrino are right-handed.

So right is the direction associated with the more (1 in 500) prolific positron decay mode of the K(L), while left is the helicity of the electron in the slightly less abundant
electron decay mode of the  K(L).

It was first pointed out by the Russian physicist Andrei Sakharov that three
fundamental conditions are essential to create matter-antimatter asymmetry in the early evolution of the universe.

First there should be a baryon number violation, so that assuming a zero baryon number initially, this violation would obviously develop a baryon asymmetry.

In addition one would definitely need a CP violation, so that decay of massive exotic particles in the high energy early phase of the universe, would be biased in favour of baryons (matter) rather than antibaryons (antimatter).

We need a violation of only one part in a billion to create the matter excess. Again we should also have a non-equilibrium situation (this is ensured by the expansion of the universe).

Although there are a plethora of models and theories to realise the above,
conditions there has been no quantitative estimate agreeing with observations and moreover B violation interactions (like in proton decay) are yet to be observed.

All in all, the discovery of the violation of a basic symmetry in physics, in the
decays of neutral kaons, just fifty years ago, could provide us with an explanation of why there is no antimatter in the universe, which as we have seen is indeed crucial for our very existence in the first place!