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.
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 10 7 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.
2
In 1997 both H1 and 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 Guardian
[3]
A M Hillas and T E Cranshaw Nature 184, 892-893, (1959) 19 Sept
[4]
M Marinelli and G Morpurgo
Phys. Lett. 137B, 439-442, (1984)
[5]
M L Perl and E R Lee
Am. J. Phys 65, 698-706 (1997)
[6]
H F Dylla and J G King
Phys. 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 King
See ref.5
[9]
Particle Data Group
Europ. Phys J. 3, 50, (1998)
[10]
H1 Collaboration
Z. Phys. C 74, 191-205, (1997)
[11]
ZEUS Collaboration
Z. Phys. C 74, 207, (1997)
[12]
B Straub
18th International Symposium on
Lepton-Photon Interactions
World Scientific. p.3-25 (1997)
[13]
Daily Telegraph
20 Feb 1997
"Atom smasher hurls particle theory into
chaos"
13 May 1959
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