Science: Sticky questions inside the proton: Brian Foster looks at emerging knowledge about subatomic particles that is exciting physicists

Hydrogen is the most abundant substance in the universe. It is also the simplest of all the atoms, its nucleus consisting of a single proton around which orbits a solitary electron. The proton is therefore important, if only because it is the major constituent of the most abundant element.

But protons are the constituents of all atomic nuclei. The structure and internal dynamics of the proton, although it may seem rather recherche a topic, is thus one of the most fundamental issues in the study of the physical world.

And things are sticky inside the proton. Experiments at the Hera accelerator in Hamburg are throwing new light on the nature of subatomic particles. When the results were reported to the 26th International Conference on High Energy Physics in Glasgow last month they caused great excitement among physicists.

The Hera accelerator is the biggest and most advanced machine at Germany's laboratory for fundamental particle physics. It collides electrons and protons circulating in opposite directions inside an vacuum pipe of 6.3km (4 miles) circumference. In a sense, Hera is the world's biggest electron microscope.

The electron appears to have no measurable size or internal structure, and so it behaves in a very simple way when it collides with other particles. The proton is more complex. By measuring how electrons scatter after colliding with protons, physicists can assess the proton's structure in a manner analogous to the way our eyes perceive objects by recording light scattered from them. Hera is so much larger than conventional electron microscopes because its electrons need to be about a billion times more energetic than in the conventional electron microscope in order to penetrate sufficiently deeply inside the proton to resolve its many tiny constituents.

What does the scattering of electrons from the proton reveal? In our present understanding, the proton consists of three point-like constituents known as quarks. These so-called 'valence' quarks are for ever confined inside the proton by the 'strong' nuclear force. The carrier of this force is itself a particle, which is known as the gluon because it carries an attractive force which sticks or glues the quarks together inside the proton.

Occasionally, the gluon spontaneously converts itself momentarily into a pair of entities. One is a quark and the other its antimatter equivalent, an 'antiquark' with the same mass but all other properties opposite. This violates the fundamental principle that energy and momentum are always conserved, but is allowed by the aspect of quantum mechanics known as Heisenberg's uncertainty principle. Processes that violate energy conservation are allowed, provided they take place so quickly that the energy violation multiplied by the time taken is less than a fundamental quantity h, known as Planck's constant.

The effect of this is that, at a particular instant, the proton can appear to consist of a large number of transitory quarks and antiquarks spun out of gluons, but obeying the Heisenberg uncertainty principle. The violent collision of an electron with a proton in Hera is sensitive to these brief fluctuations. The two experiments operating at Hera, known as H1 and Zeus, can infer the density and time-structure of quarks and antiquarks inside the proton and hence discover information about the gluon and the strong nuclear force.

Over the past year, the experiments have revealed surprising details. The number of quarks and antiquarks, and hence the number of gluons which give rise to them, is much larger than expected.

The implications of this behaviour are unclear. The theory of the strong force, known as quantum chromodynamics, is extremely complex, and can only be solved by approximations and simplifications. The rapid increase in the number of gluons gives the first indication that the approximation may fail and that other theoretical approaches may be necessary.

Another important question raised by the experimental results is that the number of gluons cannot continue to rise at the presently observed rate. If many more are produced, the proton will not be able to contain them and they will interact so strongly that some will be absorbed.

In the Sixties, lower energy experiments scattering electrons from protons at Stanford, California gave the first evidence that they contained point-like constituents, later identified as quarks. This was crucial to our understanding of matter. It seems possible that the Hera experiments may be able to provide another big step forward in our understanding of the strong interaction by observing the number of gluons inside the proton. The interior of the proton seems to be a sticky place indeed]

Brian Foster is a Reader in Physics in Bristol University.

(Photograph omitted)

Suggested Topics
Start your day with The Independent, sign up for daily news emails
ebooks
ebooksAn introduction to the ground rules of British democracy
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
SPONSORED FEATURES
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Recruitment Genius: Office / Sales Manager

£22000 - £32000 per annum: Recruitment Genius: Established and expanding South...

Recruitment Genius: Administrative Assistant / Order Fulfilment

£14000 per annum: Recruitment Genius: An exciting opportunity to join a thrivi...

SThree: Trainee Recruitment Consultant

£18000 - £23000 per annum + Uncapped OTE: SThree: Trainee Recruitment Consulta...

SThree: Trainee Recruitment Consultant

£18000 - £23000 per annum + Uncapped OTE: SThree: Trainee Recruitment Consulta...

Day In a Page

Refugee crisis: David Cameron lowered the flag for the dead king of Saudi Arabia - will he do the same honour for little Aylan Kurdi?

Cameron lowered the flag for the dead king of Saudi Arabia...

But will he do the same honour for little Aylan Kurdi, asks Robert Fisk
Our leaders lack courage in this refugee crisis. We are shamed by our European neighbours

Our leaders lack courage in this refugee crisis. We are shamed by our European neighbours

Humanity must be at the heart of politics, says Jeremy Corbyn
Joe Biden's 'tease tour': Could the US Vice-President be testing the water for a presidential run?

Joe Biden's 'tease tour'

Could the US Vice-President be testing the water for a presidential run?
Britain's 24-hour culture: With the 'leisured society' a distant dream we're working longer and less regular hours than ever

Britain's 24-hour culture

With the 'leisured society' a distant dream we're working longer and less regular hours than ever
Diplomacy board game: Treachery is the way to win - which makes it just like the real thing

The addictive nature of Diplomacy

Bullying, betrayal, aggression – it may be just a board game, but the family that plays Diplomacy may never look at each other in the same way again
Lady Chatterley's Lover: Racy underwear for fans of DH Lawrence's equally racy tome

Fashion: Ooh, Lady Chatterley!

Take inspiration from DH Lawrence's racy tome with equally racy underwear
8 best children's clocks

Tick-tock: 8 best children's clocks

Whether you’re teaching them to tell the time or putting the finishing touches to a nursery, there’s a ticker for that
Charlie Austin: Queens Park Rangers striker says ‘If the move is not right, I’m not going’

Charlie Austin: ‘If the move is not right, I’m not going’

After hitting 18 goals in the Premier League last season, the QPR striker was the great non-deal of transfer deadline day. But he says he'd preferred another shot at promotion
Isis profits from destruction of antiquities by selling relics to dealers - and then blowing up the buildings they come from to conceal the evidence of looting

How Isis profits from destruction of antiquities

Robert Fisk on the terrorist group's manipulation of the market to increase the price of artefacts
Labour leadership: Andy Burnham urges Jeremy Corbyn voters to think again in last-minute plea

'If we lose touch we’ll end up with two decades of the Tories'

In an exclusive interview, Andy Burnham urges Jeremy Corbyn voters to think again in last-minute plea
Tunisia fears its Arab Spring could be reversed as the new regime becomes as intolerant of dissent as its predecessor

The Arab Spring reversed

Tunisian protesters fear that a new law will whitewash corrupt businessmen and officials, but they are finding that the new regime is becoming as intolerant of dissent as its predecessor
King Arthur: Legendary figure was real and lived most of his life in Strathclyde, academic claims

Academic claims King Arthur was real - and reveals where he lived

Dr Andrew Breeze says the legendary figure did exist – but was a general, not a king
Who is Oliver Bonas and how has he captured middle-class hearts?

Who is Oliver Bonas?

It's the first high-street store to pay its staff the living wage, and it saw out the recession in style
Earth has 'lost more than half its trees' since humans first started cutting them down

Axe-wielding Man fells half the world’s trees – leaving us just 422 each

However, the number of trees may be eight times higher than previously thought
60 years of Scalextric: Model cars are now stuffed with as much tech as real ones

60 years of Scalextric

Model cars are now stuffed with as much tech as real ones