Could the Higgs Decay to New Z-like Particles?

Today I’m continuing with my series, begun last Tuesday (click here for more details on the project), on the possibility that the Higgs particle discovered 18 months ago might decay in unexpected ways.

I’ve finished an article describing how we can, with current and with future Large Hadron Collider [LHC] data, look for a Higgs particle decaying to two new spin one particles, somewhat similar to the Z particle, but with smaller mass and much weaker interactions with ordinary matter.  [For decays to spin zero particles, click here.] Just using existing published plots on LHC events with two lepton/anti-lepton pairs, my colleagues and I, in our recent paper, were able to put strong limits on this scenario: for certain masses, decays to the new particles can occur in at most one in a few thousand Higgs particles.  The ATLAS and CMS experiments could certainly do better, perhaps even to the point of making a discovery with existing data, if this process is occurring in nature.

The Higgs could decay to two new spin-one particles, here labelled ZD, which in turn could each produce a lepton/anti-lepton pair.  The resulting signature would be spectacular, but neither ATLAS nor CMS has done a optimizal search for this signal covering the full allowed ZD mass range.

The Higgs could decay to two new spin-one particles, here labelled ZD, which in turn could each produce a lepton/anti-lepton pair (e = electron, μ = muon). The resulting signature would be spectacular, but neither ATLAS nor CMS has yet published an optimal search for this signal across the full allowed ZD mass range.

You might wonder how particle physicists could have missed a particle with a mass lower than that of the Z particle; wouldn’t we already have observed it? A clue as to how this can occur: it took much longer to discover the muon neutrino than the muon, even though the neutrino has a much lower mass. Similarly, it took much longer to discover the Higgs particle than the top quark, even though the Higgs has a lower mass. Why did this happen?

It happened because muon neutrinos interact much more weakly with ordinary matter than do muons, and are therefore much harder to produce, measure and study than are muons. Something similar is true of the Higgs particle compared to the top quark; although the top quark is nearly 50% heavier than the Higgs, the Large Hadron Collider [LHC] produces 20 times as many top quarks and anti-quarks as Higgs particles, and the signature of a top quark is usually more distinctive. So new low-mass particles to which the Higgs particle can perhaps decay could easily have been missed, if they interact much more weakly with ordinary matter than do the Z particle, top quark, bottom quark, muon, etc.

The muon neutrino was discovered not because these neutrinos were directly produced in collisions of ordinary matter but rather because muons were first produced, and these then decayed to muon neutrinos (plus an electron and an electron anti-neutrino).  Similarly, new particles may be discovered not because we produce them directly in ordinary matter collisions, but because, as in the above figure, we first produce a Higgs particle in proton-proton collisions at the LHC, and the Higgs may then in turn decay to them.

I should emphasize that direct searches for these types of new particles are taking place, using both old and new data from a variety of particle physics machines (here’s one example.) But it is often the case that these direct searches are not powerful enough to find the new particles, at least not soon, and therefore they may first show up in unexpected exotic decays of the Higgs… especially since the LHC has already produced a million Higgs particles, most of them at the ATLAS and CMS experiments, with a smaller fraction at LHCb.

I hope that some ATLAS and CMS experimenters are looking for this signal… and that we’ll hear results at the upcoming Moriond conference.

10 responses to “Could the Higgs Decay to New Z-like Particles?

  1. So when you write “muon neutrinos interact much more weakly with ordinary matter than do muons,” is that just another way of saying that muons participate in the electromagnetic interaction as well as the weak interaction and gravity, while muon neutrinos only participate in the weak interaction and gravity? Or is the strength of a particle’s interactions an independent property, distinct from which interactions it feels?

    Thanks! and thanks for all the work you put into this site. This is the best source I’ve seen on current work in physics for lay readers.

  2. kashyap vasavada

    Matt: In an article “For decays to spin zero particles, click here” you mention changing sign of field (S-> –S) . What is this symmetry? Is it used often? It looks different from other symmetries like P,C,T, rotation or internal symmetries.

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  4. Douglas Gluntz

    Prof. Strassler: Q: What probability would you give– to the discovery by the LHC before it is finally retired/decommissioned–for actually finding SOME PARTICLE beyond the Std Model which carries the unique attribute of exhibiting/possessing negative gravity…thus increasing—perhaps by a few fractional percentage points by its happening for this [Low-TEV] particle— the probability now given (whatever it may be???) to the speculation that at the very 10E-35-seconds following the Big Bang “analogous negative-gravity Very Very High-TEV particles” were in existence …and which thereby mechanistically produced the conjectured “cosmological inflation episode” that seems necessary to resolve cosmology’s “horizon problem”?

    • Zero. Negative gravity is inconsistent.

      • Douglas Gluntz

        …So, if the graviton with spin +2 is the theorized boson by which an attractive force comes to be created and acting between all matter particles, am I to understand that there is no possibility for there to be some other kind of [within LHC’s Tev range] boson being discovered, perhaps with spin -2 (??), which brings-about a repulsive force (i.e., “negative gravity”—my term) between all matter particles and for which some “big brother” different boson but with analogous repulsive-agency attribute—plausibly being present very, very, very early following the Big Bang moment? If not, could you briefly relate your thoughts on the possible physics that would have produced the speculated “cosmological inflation phenonemon” starting about 10E-35 seconds which seemingly provides resolution to the cosmological horizon problem faced under the “standard Big Bang” (as contrasted from the “inflationary Big Bang “) modeling? Thank you!

        • Total spin is a positive quantity, just like the square of a real number is always positive. The spin along a particular axis can be negative, but when we say that the graviton has spin 2, we mean that its spin along a particular axis can be at most 2 or -2.

          In short, there is no such thing as a “negative spin” particle. Any attempt to define such a thing would give a theory that is physically inconsistent (it would give negative probabilities for many processes.)

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