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)

Start your day with The Independent, sign up for daily news emails
News
Bill Gates, founder of Microsoft and co-chair of the Bill and Melinda Gates Foundation
businessUber, Snapchat and Facebook founders among those on the 2015 Forbes Billionaire List
News
news... and what your reaction to the creatures above says about you
News
Homer’s equation, in an episode in 1998, comes close to the truth, as revealed 14 years later
science
News
news
ebooks
ebooksA special investigation by Andy McSmith
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

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

iJobs Job Widget
iJobs General

Ashdown Group: Finance Manager - Covent Garden, central London - £45k - £55k

£45000 - £55000 per annum + 30 days holiday: Ashdown Group: Finance Manager - ...

Ashdown Group: Systems Administrator - Lancashire - £30,000

£28000 - £30000 per annum: Ashdown Group: 3rd Line Support Engineer / Network ...

Recruitment Genius: Graduate Web Developer

£26000 - £33000 per annum: Recruitment Genius: A Web Developer is required to ...

Ashdown Group: PeopleSoft Developer - London - £45k

£45000 per annum: Ashdown Group: PeopleSoft Application Support & Development ...

Day In a Page

The difference between America and Israel? There isn’t one

The difference between America and Israel? There isn’t one

Netanyahu knows he can get away with anything in America, says Robert Fisk
Families clubbing together to build their own affordable accommodation

Do It Yourself approach to securing a new house

Community land trusts marking a new trend for taking the initiative away from developers
Head of WWF UK: We didn’t send Cameron to the Arctic to see green ideas freeze

David Nussbaum: We didn’t send Cameron to the Arctic to see green ideas freeze

The head of WWF UK remains sanguine despite the Government’s failure to live up to its pledges on the environment
Author Kazuo Ishiguro on being inspired by shoot-outs and samurai

Author Kazuo Ishiguro on being inspired by shoot-outs and samurai

Set in a mythologised 5th-century Britain, ‘The Buried Giant’ is a strange beast
With money, corruption and drugs, this monk fears Buddhism in Thailand is a ‘poisoned fruit’

Money, corruption and drugs

The monk who fears Buddhism in Thailand is a ‘poisoned fruit’
America's first slavery museum established at Django Unchained plantation - 150 years after slavery outlawed

150 years after it was outlawed...

... America's first slavery museum is established in Louisiana
Kelly Clarkson: How I snubbed Simon Cowell and become a Grammy-winning superstar

Kelly Clarkson: How I snubbed Simon Cowell and become a Grammy-winning superstar

The first 'American Idol' winner on how she manages to remain her own woman – Jane Austen fascination and all
Tony Oursler on exploring our uneasy relationship with technology with his new show

You won't believe your eyes

Tony Oursler's new show explores our uneasy relationship with technology. He's one of a growing number of artists with that preoccupation
Ian Herbert: Peter Moores must go. He should never have been brought back to fail again

Moores must go. He should never have been brought back to fail again

The England coach leaves players to find solutions - which makes you wonder where he adds value, says Ian Herbert
War with Isis: Fears that the looming battle for Mosul will unleash 'a million refugees'

The battle for Mosul will unleash 'a million refugees'

Aid agencies prepare for vast exodus following planned Iraqi offensive against the Isis-held city, reports Patrick Cockburn
Yvette Cooper: We can't lose the election. There's too much on the line

Yvette Cooper: We can't lose the election. There's too much on the line

The shadow Home Secretary on fighting radical Islam, protecting children, and why anyone in Labour who's thinking beyond May must 'sort themselves out'
A bad week for the Greens: Leader Natalie Bennett's 'car crash' radio interview is followed by Brighton council's failure to set a budget due to infighting

It's not easy being Green

After a bad week in which its leader had a public meltdown and its only city council couldn't agree on a budget vote, what next for the alternative party? It's over to Caroline Lucas to find out
Gorillas nearly missed: BBC producers didn't want to broadcast Sir David Attenborough's famed Rwandan encounter

Gorillas nearly missed

BBC producers didn't want to broadcast Sir David Attenborough's famed Rwandan encounter
Downton Abbey effect sees impoverished Italian nobles inspired to open their doors to paying guests for up to €650 a night

The Downton Abbey effect

Impoverished Italian nobles are opening their doors to paying guests, inspired by the TV drama
China's wild panda numbers have increased by 17% since 2003, new census reveals

China's wild panda numbers on the up

New census reveals 17% since 2003