Unexpected Decays of the Higgs Particle: What We Found

A few weeks ago, I reported on the completion of a large project, with which I’ve been personally involved, to investigate how particle physicists at the Large Hadron Collider [LHC] could be searching, not only in the future but even right now, for possible “Exotic Decays” of the newly-discovered Higgs particle .

By the term “exotic decays” (also called “non-Standard-Model [non-SM] Decays”), we mean “decays of this particle that are not expected to occur unless there’s something missing from the Standard Model (the set of equations we use to describe the known elementary particles and forces and the simplest possible type of Higgs field and its particle).”  I’ve written extensively on this website about this possibility (see here, here,  here, here, here, here, here and here), though mostly in general terms. In our recent paper on Exotic Decays, we have gone into nitty-gritty detail… the sort of detail only an expert could love.  This week I’m splitting the difference, providing a detailed and semi-technical overview of the results of our work.  This includes organized lists of some of the decays we’re most likely to run across, and suggestions regarding the ones most promising to look for (which aren’t always the most common ones.)

Before I begin, let me again mention the twelve young physicists who were co-authors on this work, all of whom are pre-tenure and several of whom are still not professors yet.  [ When New Scientist reported on our work, they unfortunately didn't even mention, much less list, my co-authors.] They are (in alphabetical order): David Curtin, Rouven Essig, Stefania Gori, Prerit Jaiswal, Andrey Katz, Tao Liu, Zhen Liu, David McKeen, Jessie Shelton, Ze’ev Surujon, Brock Tweedie, and Yi-Ming Zhong.

The Project

With an immense variety of possible exotic decays of the observed Higgs particle, a thorough search poses a severe challenge.  Experimenters at ATLAS and CMS, the two general-purpose experiments at the LHC, have their work cut out for them. But the payoff would also be huge; if any such decay turned up, it would mean a revolutionary discovery of new forces and (almost always) new particles, ones that lie outside the Standard Model, which has stood as the predictive framework for particle physics for nearly 40 years.

Writing a paper that completely covers all the possibilities would have been impossible. We had to choose a more limited goal, and so the decision was to restrict ourselves to

• decays which do not produce any unknown long-lived particles (except ones that are always completely invisible to the detector);
• decays in which the number of observable electrons, muons, taus, photons, quarks, or gluons (or their anti-particles) is at most 4.

It’s important to realize, then, that we left out a lot. But at least, with these restrictions, we had a finite list of cases (about 20 sub-classes) to consider, and it took us under 200 pages to do so.

Perhaps our most important observations are that:

• Discoveries are still possible with the existing data set. CMS and ATLAS could add to our knowledge by looking more carefully at the data they already have, rather than waiting a year or two for more.
• Some completed ATLAS and CMS searches for other phenomena, which weren’t intended for this purpose, nevertheless, according to our estimates, already put interesting and even sometimes stringent limits on non-SM Higgs decays. Of course, dedicated searches would certainly do even better.
• There are many opportunities coming when the LHC turns on again in 2015. One of the big challenges is to assure that the trigger system, which faces even greater strain as the LHC collision energy and collision rate increase, can avoid discarding certain types of non-SM Higgs decays. This important issue must be studied this year!

Today, I’ve created a page summarizing what we know about possible Higgs decays to two new spin-zero particles, which in turn decay to quark pairs or lepton pairs according to our general expectation that heavier particles are preferred in spin-zero-particle decays. A number of theories (including models with more Higgs particles, certain non-minimal supersymmetric models, some Little Higgs models, and various dark matter models) predict this possibility.

Over the coming week or so, I’ll be adding more pages with other classes of decays, and pointing you to them as they’re ready.