Quest for anomaly in fundamental physics has drawn another blank, says S.Ananthanarayanan.
Like any theory, the imposing edifice of physics stands on assumptions, howsoever evident. And then, physics has run into an impasse and is waiting for a way forward. Trials to test whether the basic symmetries which physics assumes to exist are true are then doubly important, both to validate all that is going on, and also to show us if a new path needs to be taken.
We are aware that there are two kinds of charge – positive and negative. Atoms have the positive charge on the nucleus and the negative charge evenly distributed outside, around the nucleus, so that the two kinds of charge exactly cancel out. And based on this model have all of physics and the technological marvels of our times been constructed. But, underlying the model is the assumption that the units that make up the positive and negative charge, the charge on the electrons, the tiny particles at the exterior of atoms, and the charge on the protons, the more massive particles that lie at the centre, are equal, although opposite. A team working at CERN in Geneva, has reported in the journal, Nature Communications, that the most accurate verification, to date, of this assumption has proved that it is still valid.
This assumption itself corresponds to basic symmetries that are assumed in all physical laws – the assumption that the laws should be the same wherever and however events are viewed. There are three such basic symmetries – one that physics should work the same way even if the charges of the components of an experiment are interchanged, second if the experiment were viewed through a mirror, which is to say that left were changed to right, and third, if the experiment were viewed backwards in time – which is to say, an apple bounding off the ground should rise just till the height of the branch and attach itself to stalk from which it fell! These assumptions do seem to be wholly reasonable and generally valid. The three symmetries are called ‘C’, for charge, ‘P’, for parity and ‘T’, for time, or the CPT symmetry.
But nature contains some instances where the symmetries are not conserved. In the case of beta decay, a neutron (a neutral particle in the nucleus) decays into a proton and an electron (to conserve charge). It is seen however, that direction of emission of the electron is related to the sense of spin of the neutron which is undergoing decay. Now, if this event were viewed as a reflection in a mirror, the sense of spin of the neutron would be reversed, but the direction of emission of the electron would stay the same. This would be a case of behavior changing on reflection and a violation of ‘P’.
But this violation gets resolved if we change the nature of the particles, along with reflection. In the subatomic world, particles also have ‘antiparticles’, which are the same as the particles except for charge and some internal parameters. Thus, for electrons and protons, there are the positrons and antiprotons and there can be antiatoms and an anti-universe. Particles and antiparticles can arise spontaneously from a photon that has sufficient energy and if a pair of antiparticles should meet, they annihilate, emitting photons. If beta decay, on reflection, were viewed in terms of antiparticles, as ‘positron decay’, where the positively charged antiparticle of the electron was emitted, then the fact that the direction is unchanged with the reversal of the sense of spin is no longer a problem, as charge has also changed. The combined symmetry of charge and parity, or CP, is then said to be conserved, although ‘P’ alone is not, in this kind of interaction. It was also seen that a neutral particle called the neutrino, which is always emitted in beta decay, had a given sense of ‘spin’. The neutrino was then assigned an ‘intrinsic parity’, and when the ‘antineutrino’ was considered, there was a case of ‘C’ violation, but again with CP invariance.
This state of CP invariance also did not last long, as a case of kaon decay was discovered, where even CP was violated. It was found that there were two forms of kaon decay, with emission of neutrinos and antineutrinos, and these were equivalent to each other under the ‘CP’ reversal. This should suggest that both forms be equally likely. But experiment showed that one form of decay was slightly more frequent than the other, which suggests that there may a basic difference between particles and antiparticles. And also suggests why the real universe is made of particles.
But the one bastion that has not been breached is ‘CPT’ invariance. This symmetry also lies at the base of the equality of opposite charges and the observation that atoms, which are built of positive and negative charges, are truly neutral. That atoms, which are available in plenty, are neutral has been verified to a very great degree, but the real test would be if atoms made of antiparticles were also isolated and shown to be neutral.
The hydrogen atom, which is one proton and one electron, is the simplest and the best understood atom. In principle, the forces between an antiproton and a positron should be the same and they should also form an atom, which should have the same energy levels and should hence emit the same spectral lines as normal hydrogen. Such antihydrogen atoms have in fact been created in the lab, but the problem is to get a good enough number of them for long enough. Antihydrogen is created by bringing together equal numbers of antiprotons, created in a particle accelerator and then slowed down, and positrons, from a radioactive source. The constituent particles are guided to the place of merger with carefully designed electric and magnetic fields and good numbers of antihydrogen atoms do get produced. But the problem is that once the atoms form, they become neutral and do not respond to the confining electric and magnetic fields. The new-born atoms just spread out and annihilate as soon as they contact the container walls and that is the end of their short existence!
The Geneva team used the equipment of the Antihydrogen Laser Physics Apparatus (ALPHA) international collaboration based at CERN. The arrangement consists of an octuple, or eight poled magnet and a pair of coils carrying current, which create a pocket of low magnetic field, where the charged particles meet to form antihydrogen. Depending on the movement energy of the antiatoms, their production can be confined to a small volume for a short while, by magnetic fields, as even electrically neutral atoms still have magnetic properties. After the short confinement and switching off the fields, the atoms rush out to annihilate against the walls of the container. Annihilation creates radiation, which is detected by silicon based imaging equipment, resulting in data of each antiatom annihilation event.
As the equipment was first designed to create numbers of antiatoms, it is provided with a pair of opposite electric fields, which would sweep away any leftover antiprotons, while leaving the antiatoms undisturbed. As the present experiment was to test the charge neutrality of the antiatoms, this pair of electric fields became a useful means to test the linearity of the antiatom paths. Out of come 1,300 events considered, 386 were proved anithydrogen annihilation events and their position was distributed, 241 on one side and 145 on the other. Any charge on the antiatoms would result in a level of deviation. After providing the necessary allowances for statistical errors, the distribution was found to be closely centred and the extent of deviation allowed calculation of the how much the charge on the antiatom could have been.
The result was that the charge, if there were any, was less than a hundred millionth of the charge on the electron. As the antihydrogen atom consists of an antiproton and a positron, this means the charge o the two particles does not differ by a fraction more than one in hundred million. This is the best assessment made so far in respect of the rare and short-lived antihydrogen atom.
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