Has the mystery of proton 'spin' been solved?
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In 1987 scientists at the European Muon Collaboration (EMC) at Cern were left baffled when their experiment to work out why and how protons 'spin' was unsuccessful.
The conundrum became known as the 'proton spin crisis' and has continued to prove puzzling for 27 years, but now researchers think they have the answer.
A study has shown that a subatomic particle known as a gluon may be responsible for giving the proton the majority of its spin, bringing to an end a decades-long puzzle.
Researchers using a collider in New York say they have solved the 'spin' mystery of protons. Since an experiment in 1987 the origins of proton spin have been unknown. It had once been thought to be cause exclusively by three quarks (illustrated), but now scientists think gluons play an important role
SUBATOMIC PHYSICS IN BRIEF
Atoms are usually made of protons, neutrons and electrons
These are made of even smaller elementary particles.
Elementary particles, also known as fundamental particles, are the smallest particles we know to exist.
They are subdivided into two groups, the first being fermions, which are said to be the particles that make up matter.
The second are bosons, the force particles that hold the others together.
Within the group of fermions are subatomic particles known as quarks.
When quarks combine in threes, they form compound particles known as baryons.
Protons are probably the best-known baryons.
Sometimes, quarks interact with corresponding anti-particles (such as anti-quarks), which have the same mass but opposite charges.
When this happens, they form mesons.
Mesons often turn up in the decay of heavy man-made particles, such as those in particle accelerators, nuclear reactors and cosmic rays.
Mesons, baryons, and other kinds of particles that take part in interactions like these are called hadrons.
The latest research was conducted by Dr Daniel de Florian from the University of Buenos Aires and colleagues using a collider at Brookhaven National Laboratory in Upton, New York.
Previously it had been thought that the proton's spin was caused exclusively by subatomic particles known as quarks.
But the experiment in 1987 had showed that quarks failed to account for the entirety of the proton's spin.
Quarks are the subatomic particles that make up larger particles such as protons, while gluons are the 'glue' that holds them together.
The term spin here is somewhat of a misnomer here, however.
It does not describe the process of actual spinning, like a ball rotating, but it refers to what is basically 'quantum spinning', also called 'nuclear spin'.
Spin at a quantum level - the smallest you can possibly get - is defined as a physical constant that explains how particles have a magnetic field, interact and so on.
How protons get this physical constant, however, was a mystery.
Subatomic particles are said to have different values of spin - for example quarks have a spin of ½ in either a positive or negative direction.
Protons have a spin of ½, which had led scientists to believe their spin could be accounted by two quarks of one orientation spin, and one the other.
This, however, was not the case, with only a quarter of the proton's spin coming from the quarks.
'That was the naïve idea 25 years ago,' Dr Daniel de Florian tells Scientific American.
'By the end of the '80s it was possible to measure the contribution of the spin of the quarks to the spin of the proton, and the first measurement showed it was 0 percent. That was a very big surprise.'
Later measurements would show this contribution to be up to 25 per cent of the proton's spin, leaving at least 75 per cent still accounted for.
WHAT IS A QUARK?
Quarks are elementary particles, the smallest particles we know to exist.
When they combine they form compound particles known as hadrons.
Quarks are said to have six 'flavours': Up, Down, Charm, Strange, Top and Bottom.
Combinations of quarks within these flavours gives rise to the 'larger' particles.
Groups of three quarks are known as baryons.
An example of a baryon is a proton, which is made of two 'Up' quarks and a 'Down' quark.
The latest research was conducted by Dr Daniel de Florian from the University of Buenos Aires and colleagues using a collider at Brookhaven National Laboratory in Upton, New York. The Relativistic Heavy Ion Collider (top, center) is 2.4 miles (3.9 kilometres) in circumference, and dominates Brookhaven's 5,265-acre campus
The new research shows that gluons, which have a spin of 1, contribute as much as half of the proton's spin.
This was based on proton-proton collisions at the Relativistic Heavy Ion Collider (RHIC).
When protons are smashed together their interaction is controlled by the strong force.
This is determined by gluons, meaning they are intricately involved in the collisions of protons.
The orientation of the protons' spins was then used to determine that gluons must indeed have an effect on spin.
More data is needed from collisions at lower momentum to confirm the result, but for now it looks like one of the great mysteries in subatomic physics might finally be solved.
And doing so will enable scientists to better understand how particles get their mass.
One of the other unsolved mysteries of subatomic physics is that of confinement - why quarks, gluons and so forth are only ever found within other subatomic particles like protons, and not by themselves.
Solving this would help explain how quarks and gluons in turn get their own spin.
This result could be an important factor in determining where proton mass comes from.
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