Branching fractions off the menu

The Higgs boson may decay into a Z boson and a photon through an intermediate pair of charged particles.

A particle’s branching fractions — that is, the probability that it will decay into one set of particles rather than another — are a good way to see if physicists really understand what’s happening at microscopic scales. Many things can affect a particle’s decision to decay into, say, electrons rather than photons. If the physicists’ predictions match the observed probability of decay, then the underlying mechanisms may be well understood, especially if it is a tight balance between opposing forces. If not, then there might be a new intermediate particle involved or some other new phenomena to be discovered.

Some branching fractions are determined by extremely complicated processes while others are relatively simple. The Z boson, for instance, decays into electron-positron, muon-antimuon and tau-antitau pairs with equal probability. It is as though the Z has a menu of everything less massive than it and blindly chooses from the menu.

The Higgs boson’s decays are also pretty simple: It can only decay directly into particle-antiparticle pairs, but with a probability that depends on the mass of the decay products. To fulfill its role as origin of mass, the Higgs must couple to matter particles in proportion to their masses and force particles by the squares of their masses, and hence it decays mostly into heavy particles. It is as though the Higgs’ menu is biased toward heavy final states.

However, the Higgs sometimes decides to decay in ways that aren’t even on this menu. It was discovered, in part, by its decay into two photons. Photons are massless — their coupling to the Higgs (and hence branching fraction from the Higgs) ought to be zero. Physicists believe that this decay is possible because the Higgs first decays to a pair of heavy charged particles that then re-collide to produce two photons. (Charged particles couple to photons.) It’s somewhat more complicated, but not unprecedented.

With this interpretation, other final states, such as a Z and a photon, are also possible. (Charged particles also couple to Z bosons.) This Z-photon pair isn’t even a particle-antiparticle pair, but it should be allowed in the same way that photon pairs are. A group of CMS physicists are exploring that possibility now, searching for Z-photon pairs with the mass of a Higgs. If the theory is well-understood, they should have enough data to see this “off-the-menu” decay mode in the near future.

Jim Pivarski

The U.S. physicists pictured above made major contributions to the search for Higgs decaying into a Z boson and a photon.
These U.S. collaborators are upgrading the core and detector software for the CMS detector in preparation for the return of operations in 2015. Because of the long lead time these upgrades require, the work began over a year ago and will continue through the data taking period.