After-dinner talk at Physical Society Club 4 Nov 1998

ARE ATOMS NEUTRAL ?

The obvious answer would appear to be "yes". Normal matter seems to be uncharged. It consists of atoms in which electrons orbit around nuclei, which in turn consist of protons and neutrons. Neutrons carry no net charge. There are equal numbers of protons and electrons in an atom, and they have equal and opposite electric charge. Hence atoms are neutral.

But just how equal are the proton and electron charges, and how neutral is the neutron? However satisfying aesthetically exact equality might be, ultimately this must be determined by experiment.

In 1959 R A Lyttelton and H Bondi [1] suggested that there might be a general excess of charge in the universe. If the proton and electron charges were not exactly equal, (and the neutron charge not precisely zero), the small difference would give bulk matter an excess charge. If this charge inequality were about 1 part in 1018 then, assuming equal numbers of protons and electrons, electrostatic repulsion could outweigh gravitational attraction and could cause the expansion of the universe! Remember that this was at a time when the steady-state theory was a serious, and perhaps the most popular, candidate model of cosmology, and the big bang was not established.

In that paper the authors carefully worked out some of the consequences. Within the Universe "units" which might be identified with galaxies or clusters of galaxies were ionised, hence conducting, so that excess charge would appear at the surface. Everything fitted well within the framework of the continuous creation model. An inequality of 2 x 10-18 gave the known expansion rate, and this value could not be excluded on the basis of existing experiments. A rather poorer limit of around 10-17 was inferred from Millikan oil drop experiments.

After the paper was submitted but before it was published, the Manchester Guardian referred to this work [2]. Very soon afterwards, and before the appearance of [1], an experiment was published by A M Hillas and T E Cranshaw [3] which showed that the proton-electron charge inequality was less than the above by a factor of about 50.

The experiment was beautiful in its simplicity. A cylinder of compressed gas was placed inside an aluminium box, which itself was placed inside another aluminium box but well insulated from it. An electrometer was used to observe any changes in potential between these boxes. The gas was then released from the cylinder to an external gasometer by passing through a small gap across which a potential was applied to remove ions from the gas. Experiments were carried out with argon (81 litres at SPT) and with nitrogen (58 litres). No significant amount of charge was found to be carried with the out-flowing gas leading to the conclusion that the neutron charge was

(-1 ± 3) x 10-20, and the hydrogen atom charge (1 ± 3) x 10-20 where the electron charge is -1.

In the 40 years since this experiment only small improvements in precision have been made, but several different methods have been used.

Levitometer experiments, in searches for isolated fractional charges, have given limits on the proton-electron charge inequality. M Marinelli and G Morpurgo [4] using steel balls obtained (0.8 ± 0.8) x 10-21 .

An automated Millikan liquid-drop method is in progress by M L Perl and E R Lee [5]. By 1997 they had studied nearly 107 drops, and hope to study 108 to 1010 in future, at which stage they may obtain a better lower limit for the charge difference.

An electro-acoustic method [6] has claimed comparable accuracy to the levitometer and molecular beam methods [7] have given somewhat less stringent limits.

The gas efflux method has been used [8] since the experiment of Hillas and Cranshaw and is also at the 10-21 level, although one measurement claims to be rather better than this.

The experiments all depend on some assumptions and of course have different problems, but I think we can safely assume that the proton-electron charge difference is less than about 10-21, a value that is given by the compilation of the Particle Data Group [9].

So the proton-electron charge difference is very small, consistent with being zero. This may seem a trivial conclusion. However we know that the proton charge comes from the charges of its valence quarks (+2/3, +2/3, -1/3) and the electron is a lepton. In the standard model of particle physics, the quarks and leptons are quite distinct: the quarks feel the strong interaction, the leptons do not. But the exact or near equality of the proton and electron charges is unlikely to be a coincidence. There must be some deep connection between the quarks and leptons.

There are of course some similarities: there are three generations of quarks and of leptons, with two members in each generation. It is possible that at some high energy the quarks and leptons couple with each other. For example at high energies as existed in the early universe there might have been some precursor objects, with lepton and quark properties. At lower energies this symmetry is broken in an analogous way to electroweak symmetry breaking. So leptoquarks would have decayed into the separate lepton and quark species.

Whether or not this scheme is correct, it seemed that to collide quarks with leptons at the highest possible energies might give us clues about connections between these fundamental fermions. This was my main personal motive for working on experiments at the world's only proton-electron collider, the HERA machine at the DESY Laboratory in Hamburg.

Since the machine started in 1992, our collaboration H1, and that of our friendly rivals ZEUS, have produced a lot of interesting physics. There have been well over 100 publications not connected with leptoquarks.

In 1997 both H1and ZEUS [10, 11, 12] published a handful of interesting events which were at least suggestive of the existence of leptoquarks. There was an excess of events at very high momentum-transfer between the colliding objects, and some suggestion of clustering around a high mass value. No strong claims were possible with such poor statistics, but following publication, a number of newspaper headlines appeared. I particularly liked one of them [13]: it was way over the top without actually being wrong !

With more running of the experiments at higher intensity, the peculiar events continued to appear, but their interpretation as leptoquarks seems less likely. I have now reached retirement age and have withdrawn from H1. I will follow with interest the work of my colleagues and others, and hope that at some stage we will have a better understanding of the connection between leptons and quarks, and hence of the charge neutrality of matter.

References

[1]R A Lyttelton and H Bondi Proc. Royal Soc. 252, 313-333, (1959)

Footnote: Paper received 3 March 1959,

Bondi elected FRS 19 March 1959

[2]Manchester Guardian13 May 1959

[3]A M Hillas and T E Cranshaw Nature 184, 892-893, (1959) 19 Sept

[4]M Marinelli and G MorpurgoPhys. Lett. 137B, 439-442, (1984)

[5]M L Perl and E R LeeAm. J. Phys 65, 698-706 (1997)

[6]H F Dylla and J G KingPhys. Rev. A 7, 1224-1229, (1973)

[7]L J Fraser, E R Carlson and V W Hughes

Bull. Amer. Phys. Soc. 13, 636, (1968)

[8]J G KingSee ref.5

[9]Particle Data GroupEurop. Phys J. 3, 50, (1998)

[10]H1 CollaborationZ. Phys. C 74, 191-205, (1997)

[11]ZEUS CollaborationZ. Phys. C 74, 207, (1997)

[12]B Straub18th International Symposium on

Lepton-Photon Interactions

World Scientific. p.3-25 (1997)

[13]Daily Telegraph20 Feb 1997

"Atom smasher hurls particle theory into chaos"

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