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Could the Higgs have an anti-particle?

Is it conceivable that there is a Higgs boson anti-particle?

The recent discovery of a new particle has been described by many scientists as the most important development in particle physics in the last century.

"Now physicists need to understand exactly what it is that they've got," says Professor Geoff Taylor from the University of Melbourne and part of the physics team working with the Atlas Detector on the Large Hadron Collider at CERN, the European Centre for Nuclear Research.

Scientists are almost certain it's the elusive Higgs boson, a particle that gives all other particles their mass through the Higgs field.

If it is the Higgs it will not have an anti-particle, says Taylor.

"At the elemental particle level bosons do not have anti-particles."

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Bosons and fermions

There are two types of particles in the Standard Model: fermions, the matter particles, and bosons, the force carriers.

Only fermions, which are subdivided into quarks and leptons, have anti-particles, says Taylor.

"Really fermions are the things where we have this idea of a particle and anti-particle pair," says Taylor, "anti-particles at the fundamental level are fermions with the opposite charge."

"That's why electrons, which have a negative charge, can have anti-matter counterparts called positrons which carry a positive charge."

In addition to the Higgs, bosons include the photon, the gluon, the W and Z bosons, and the yet-to-be-discovered, graviton. These particles carry the fundamental forces of electromagnetic energy, strong and weak nuclear forces, and gravity.

Some people say bosons that have no electrical charge, such as the photon, Higgs and Z boson, are their own anti-particle. But it is a mistake to refer to them this way, says Taylor.

Nor is it correct to say that the W+ and W- bosons are anti-particles of each other, he says.

"The W+ and W- bosons only differ by charge so it's an easy mistake to talk about it that way, but it's just a pair of different charges."

"While they behave in some sense like particle and anti-particle, we don't think of one as the anti-particle counterpart of the other because they're force carriers," says Taylor

So what sets the different types of particles apart?

"Fermions have conservation laws associated with them, so for example they are created in particle-anti-particle pairs, the sum of their quantum numbers cancelling to maintain the conservation laws," explains Taylor.

"Bosons operate under different laws and can be created singly. This is a crucial distinction and is in nature of being either matter particles or force carriers."

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Is it the Higgs?

Bosons have an integer spin of zero, one or two, while fermions have a half integer spin.

"To have spin means it has angular momentum at the quantum level, which is the spinning momentum of a rotating object," says Taylor.

Along with having no electrical charge the Higgs particle has zero integer spin.

"This makes it different from other bosons. It's called a scalar particle because the Higgs field it mediates (and which gives particles their mass), causes it to have zero spin."

So how does the new particle stack up with the Higgs?

"The new particle has no electric charge and does have integer spin, probably zero, but further experimentation is required to confirm that," says Taylor.

Professor Geoff Taylor from the University of Melbourne and part of the physics team working with the Atlas Detector on the Large Hadron Collider at CERN. He was interviewed by Stuart Gary.