Two Dutch physicists, whose theoretical breakthroughs led to the discovery of new sub-atomic particles, have won the 1999 Nobel Prize for Physics. It marks the third time in ten years that contributions to particle physics have won the award.

Professor Gerardus 't Hooft, at the University of Utrecht, and Professor Emeritus Martinus Veltman, formerly of Utrecht and Michigan Universities, share the prize of nearly �600,000.

Particle master: Professor t'Hooft has spent nearly his whole career at Utrecht

For thirty years they have been developing a series of mathematical procedures to explain the various families of sub-atomic particles. Their breakthrough paper was published in 1971.

The Royal Swedish Academy of Sciences' citation said the prize was "for elucidating the quantum structure of electroweak interactions in physics."

Well-deserved

Paul Guinnessy, Editor of Physicsweb, told BBC News Online: "It is not a surprise that t'Hooft and Veltman have won the prize, they have done some outstanding work in particle physics.

The BBC's Roland Pease reports

"It has been suggested for some time that they should win and a lot of people will be very happy that they did."

The physicists' work has allowed a deeper understanding of the various families of sub-atomic particles that comprise the atom.

Atoms are made of electrons swarming around a nucleus of protons and neutrons. The electrons are members of a family of fundamental particles that are not made of anything else.

The protons and neutrons however, are made of smaller particles called quarks. In addition these, and other related particles, exchange yet more particles when they interact.

The result is a bewildering number of particles out of which scientists attempt to extract some order.

Notorious science

It is an area of physics that has been notorious for the difficulty in making calculations and predictions. Over the past few decades many physicists have won Nobel prizes for their attempts to explain mathematically the properties of the sub-atomic world and make predictions.

"Their work allowed them to know under what conditions they would find the particles," said Mr Guinnessy. "There are so many collisions happening in a particle accelerator, it is very difficult to spot the ones you are looking for."

Commenting on t'Hooft and Veltman's work Lars Brink, a professor at Chalmers University of Technology Institute and a member of the Swedish Academy of Sciences said, "This is the entire framework we (particle physicists) use when calculating. We'll get finite answers. Earlier calculations only resulted in nonsense."

But Professor Veltman has a diffferent perspective on his work: "It is a difficult and abstract subject and something that I have never been able to explain to my wife and children."

Spot the particle

Their predictions led to the discovery of the W and Z particles in the 1980's and the top quark in 1995.

Their work also predicts the properties of the much desired Higgs particle. Physicists believe that the existence of this particle is responsible for endowing other particles with mass.

There is a chance that the Higgs particle could be detected using today_s particle accelerators but most scientists believe their best hope will be a new machine being built at CERN in France. Called the Large Hadron Collider it will come online in 2005.