The Higgs boson’s big brother

A heavy variant of the Higgs boson would decay primarily into W bosons or Z bosons. This is a decay mode newly added to the search.

Evidence is mounting that the particle discovered last year is the long-sought Higgs boson. When it was announced, no one seemed more cautious of claiming that than its discoverers. But now, as experimental uncertainties shrink, they can confidently say that the particle has no intrinsic spin, it is mirror-symmetric, and it couples to other particles in rough proportion to their masses. These are all properties that the boson predicted by the Higgs mechanism must satisfy.

One property that the theory does not predict well, however, is the mass of that boson. All predictions relied on assumptions about physics beyond the Standard Model, but generally they were in the few-hundred-GeV range. When the LHC experiments began their search, they cast as wide a net as possible and seem to have made a catch at the low end, 125 GeV.

That’s not the end of the story: Even if the 125-GeV boson gives mass to the fundamental particles, it may not be acting alone. Nothing in the theory forbids multiple Higgs bosons. In fact, many of the predictions for a low-mass Higgs were based on supersymmetric extensions of the Standard Model, and these extensions require at least five Higgs bosons. So while some physicists study the properties of the boson in hand, others scour the net for more.

CMS scientists recently published a search for additional Higgs bosons from 145 to 1,000 GeV. This mass range requires different search techniques, since a heavy Higgs would decay differently than a light Higgs. Larger data sets and new search modes both contribute to making this the most stringent limit yet—heavy Higgs bosons with Standard Model decay patterns are now ruled out up to 710 GeV.

It is remarkable to live in a time when such sweeping statements can be made with data. The few-hundred-GeV region has long been marked on physicists’ maps as “thar be dragons,” the energy scale at which something breaks the unity between electromagnetism and the weak force. We’ve already caught one dragon, why not two?

—Jim Pivarski

This paper represents a combination of a large number of independent analyses. Alexander Savin (Wisconsin) and Phil Dudero (Texas Tech) were editors of the paper, which represents work of the physicists pictured here along with many other researchers.