What Flavour is the Higgs?

A particle matching the description of the Higgs boson has been found at the LHC experiment in Geneva. Many theorists are rather surprised because the Higgs has no spin and theorists have good arguments for why such particles don't exist as fundamental particles. A well known resolution is for the spinless particle not to be fundamental but made of particles of other spin bound together. Thus the Higgs may be made of a particle and an anti-particle with spin ½ - these particles are called fermions and common examples are the electron or quarks.  If this is true though the Higgs would have to be made of new fermions as yet undiscovered.  To bind the fermions together to make a Higgs we would need a new force of nature as well.

Nature already uses this mechanism - the strong nuclear force (QCD) between quarks binds quarks and anti-quarks together to make spinless particles such as the sigma meson and pions. If we assume the Higgs is emerging from a new exact copy of QCD but acting at higher energies then the Higgs mass would be at least ten times greater than the mass observed at LHC though. To make this idea work we need a variant of QCD that gives a lighter Higgs mass.

A very simple way to change QCD is just to change the number of flavours of quarks. QCD is a remarkable theory because it is asymptotically free - the quarks interact less the closer together they get, more the further apart they are. This property is what keeps quarks confined in protons and neutrons.  If the number of quarks becomes greater than 16 (it's 6 in nature) though that property is lost and quarks would break free - such a world would be very different from our own. Theorists have suggested that a third regime exists when the number of quarks lies between about 12 and 16. In this state the quarks are asymptotically free but do not form bound states. Instead they exist in a strange so called "unparticle" state. Such a  theory would be no good for the Higgs either since there would be no Higgs particle... however on the interface around the number of quarks being 12 strange things might happen with a light Higgs emerging.

Such speculation has been hard to follow up on - the quarks in these theories are very strongly interacting and traditional techniques for their study breakdown.  Recently new ideas have been developed to study such theories in which a remarkable duality to string theory is used to compute. The physics of quarks can be recast as a gravitational problem where it's much easier to calculate. A recent paper by Nick Evans (Southampton) and Timo Alho & Kimmo Tuominen (Helsinki) has brought these tools to bear on the Higgs. "We've shown that by tuning the number of flavours towards the transition to unparticles the Higgs does become very much lighter - more easily than we expected," says Evans. Is the Higgs made of new fermions then? "It’s not yet clear, we'll need some data to back up the idea, but it would be a nice resolution of the existence of a spinless particle".

Read more at arXiv:1307.4896.

Date: 
Thursday, July 18, 2013