Antimatter - Palahalli Vishwanath (Deccn Herald 1/12/2015)

ANTIMATTER

Palahalli R Vishwanath

( Two major expriments in the last few months have made interesting observations on antimatter ...)

The discovery of a particle with curious properties in August 1932 was probably the first time

a piece of antimatter was found in nature A particle detector called clo ud chamber where an incoming praticle would leave a trail of drops was being used at that time to record particles. In the presence of a magnetic field , particles would curve in oppostie directions depending on their charge . Carl Andersen , a physicist from Caltech in USA and working with Cosmic Rays found a track in the cloud chamber which was opposite in direction to the ones caused by the electron. Anderson called it a positron which seemed to be very similar to the electron except for the charge.

This particle had been actually anticipated by the scientific community. In 1897 J J Thomson discovered the electron , a negative charged particle , a constituent of all matter. In 1928, the great theorist Paul Dirac purblished the famous Dirac equation which allowed electrons to have both postive charge and negative energy. While for sometime he thought that proton could be that particle, three years later he predicted the particle would be an anti electron with all qualities same as electron but with a positive charge. Dirac and Anderson got the Nobel prize for teh predictiona ndthe discovery in 1933 and 1936 respectively. Thus it was accepted that all particles should have antiparticles.

In the postulation and discovery of the positron , physicists were enucniating a fundamental concept that equal amount of matter and antimater shjould be present in the universe. Dirac had also postulated that when matter and antimatter combine they would annhilate each other resulting in the creation of energy which would present itself in the form of high energy photons like gamma rays.. The energy of one of the first big particle acclerators BEVATRON was tuned specifically to produce antiprotons. It started functioning in fall of 1954 and after one year of the experiment and sifting through nearly 2 million particle events, the group had detectred 38 particles with same mass as proton but with negative charge. This reserach also fetched the Nobel Prize for the discovery of Antiproton. Neutral particles also have anti particles ; for eg neutron and anti nutron have differing signs for their magnetic moment. Also just like matter, there has to be antimatter also in existence. Just as hydrogen atom has a proton in the center and an electron in the outer ring, anti hydrogen atom would have anti proton in the center and a positron in the outer ring, The two big accelator labs - CERN and FRMILAB- produced anti hydrogen atoms for the first time inthe 1990s. However it is expected to take 100 billion years to manufacture one gram of anti hydrogen !

The preponderance of matter over antimatter has been a mystery of nature. If these had been in the same propotin the univese as we know would not have come into existence. Somehere in the intial stages of the BigBang due to certain processes we have basically an assymetric universe . In 1967 Adres Sakharaov proposed three processes which could result in an excess of matter. The most importnat is the so called CP symmetry violation in decay proceses which inplies that physical laws must have acted differently for matter and antimatter. This symmetry violation hs been studied extenisvely in particle physics. Its first demosntarion in 1964 fetched the Nobel Prize for its discoverers. However there is the possibility that there are some regions of the unvierse where antimatter dominates. Thus how much antimatter exists is one of the fundamental questions of the origin and nature of the Universe. Another imprtant question is about the possible difference between matter and antimatter. There have been two ineresting experiments addressing these questions in the last few months.

First is the AMS (Alpha Magnetic Spectrometer) experiment located on the International Space Station which looks for primary anti protons in cosmic rays. Its aim, according to its spokesman Nobel prize winner Samuel Ting is " to search for phenomena that so far we have not had the imagination or the technology to disocver"! The standard picture is that antiprotons are produced in collisions of cosmic ray protons with nuclei in interstellar maater. New results from the experiment presented in mid April disagree with current models of anit proton production. The ratio of antiprotons to protons has been obtained across a wide energy range and the experiment finds that this proportion does not decrease at higher energies as predicted, but stays almost constant . Earlier the samegroup had found an anomalous result for the proportion of positrons to electrons, a higher fraction than expected but it could be undestood invoking conventional physis. The authors beleive that dark matter could be producing these antiprotons but only more data can give a better udnestanding of the results.

The second experiment seeks to find possible differences abetween matter and animatter. While they can differ, for example, in the way they decay , other fundamental properties, such as the absolute value of their electric charges and masses, are predicted to be exactly equal.It is with this aim that an experimetn called BASE1 ( the Baryon Antibaryon Symmetry Experiment ) conducted in CERN laboratiory in Geneva. started some time ago . The experiment looks for precise comparison of the charge-to-mass ratio ofhe proton to that of the antiproton. They published their results two weeks ago in the prestigious journal Nature. The new result , the result of an intese 35 day experiment with 13000 measurments, shows no difference between the proton and the antiproton, They state " We found that the charge-to-mass ratio is identical to within 69 parts
per thousand billion, supporting a fundamental symmetry between matter and
antimatter" . This is a test of ' New Physics' that goes beyond ' The Standard Model of particle physics ' which gives satisfactory explanations for most of the obsrvations in particle phsyics Any difference –however small — between the charge-to-mass ratio of protons and
antiprotons would break a fundamental symmetry law, a difference that would constitute a dramatic challenge to the basic concepts of particle physics.

Figures

1. The incominga gamma ray ( a netural particle with no trail) gives out a pair of particles - an aelectron and a positron . This is a picture from a particle detector called buble chamber placed in amgnetic field. Ihe particles lose energy and thus spiral inwards

2. Hydrogen nd antihydrogen ; proton and antiprotonhave diferent quark constituents