Category Archives: Dark Matter

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.

X-Rays From Dark Matter? A Little Hint For You To Enjoy

Well it’s not much to write home about, and I’m not going to write about it in detail right now, but the Resonaances blog has done so (and he’s asking for your traffic, so please click):

A team of six astronomers reports that when they examine the light (more specifically, the X-rays) coming from clusters of galaxies around the sky, and account for all the X-ray emission lines [light emitted in extremely narrow bands by atoms or their nuclei] they know about, there’s an excess of photons [particles of light] with energy E=(3.55-3.57)+/-0.03 keV, a “weak unidentified emission line”, that can’t easily be explained.  What could it be?

[A keV is 1000 eV; an eV is an electron-volt, an amount of energy typical of chemical reactions.  Note that physicists and astronomers commonly use the word “light” to refer not just to “visible light” — the light you can see — but to all electromagnetic waves, no matter what their frequency. ]

Well first: is this emission line really there?  The astronomers claim to detect it in several ways, but “the detection is at the limit of the current instrument capabilities and subject to significant modeling uncertainties” — in other words, it requires some squinting — so they are cautious in their statements.

Second: if it’s really there, what’s it due to?  Well, the most exciting and least likely possibility is that it’s from dark matter particles decaying to a photon with the above-mentioned energy plus a second, unobserved, particle — perhaps a neutrino, perhaps something else.   I’ll let Resonaances explain the sterile neutrino hypothesis, in which the dark matter particles are kind of like neutrinos — they’re fermions, like neutrinos, and they are connected to neutrinos in some way, though they aren’t as directly affected by the weak nuclear force.

But before you get excited, note that the authors state: “However, based on the cluster masses and distances, the line in Perseus is much brighter than expected in this model, significantly deviating from other subsamples.”  In other words: don’t get excited, because something very funny is going on in the Perseus cluster, and until that’s understood, the data can’t be said to be particularly consistent with a dark matter hypothesis.

One more anomaly — one more hint of dark matter — to put on the pile of weak and largely unrelated hints that we’ve already got!  I don’t suggest losing sleep over it… at least not until it’s confirmed by other groups and the Perseus cluster’s odd emissions are explained.