Category Archives: Dark Matter

Dark Matter Debates

Last week I attended the Eighth Harvard-Smithsonian Conference on Theoretical Astrophysics, entitled “Debates on the Nature of Dark Matter”, which brought together leading figures in astronomy, astrophysics, cosmology and particle physics. Although there wasn’t much that was particularly new, it was a very useful conference for taking stock of where we are. I thought I’d bring you a few selected highlights that particularly caught my eye. Continue reading

Which Parts of the Big Bang Theory are Reliable, and Why?

Familiar throughout our international culture, the “Big Bang” is well-known as the theory that scientists use to describe and explain the history of the universe. But the theory is not a single conceptual unit, and there are parts that are more reliable than others.

It’s important to understand that the theory — a set of equations describing how the universe (more precisely, the observable patch of our universe, which may be a tiny fraction of the universe) changes over time, and leading to sometimes precise predictions for what should, if the theory is right, be observed by humans in the sky — actually consists of different periods, some of which are far more speculative than others.  In the more speculative early periods, we must use equations in which we have limited confidence at best; moreover, data relevant to these periods, from observations of the cosmos and from particle physics experiments, is slim to none. In more recent periods, our confidence is very, very strong.

In my “History of the Universe” article [see also my related articles on cosmic inflation, on the Hot Big Bang, and on the pre-inflation period; also a comment that the Big Bang is an expansion, not an explosion!], the following figure appears, though without the colored zones, which I’ve added for this post. The colored zones emphasize what we know, what we suspect, and what we don’t know at all.

History of the Universe, taken from my article with the same title, with added color-coded measures of how confident we can be in its accuracy.  In each colored zone, the degree of confidence and the observational/experimental source of that confidence is indicated. Three different possible starting points for the "Big Bang" are noted at the bottom; different scientists may mean different things by the term.

History of the Universe, taken from my article with the same title, with added color-coded measures of how confident we can be in our understanding. In each colored zone, the degree of confidence and the observational/experimental source of that confidence is indicated. Three different possible starting points for the “Big Bang” are noted at the bottom; note that individual scientists may mean different things by the term.

Notice that in the figure, I don’t measure time from the start of the universe.  That’s because I don’t know how or when the universe started (and in particular, the notion that it started from a singularity, or worse, an exploding “cosmic egg”, is simply an over-extrapolation to the past and a misunderstanding of what the theory actually says.) Instead I measure time from the start of the Hot Big Bang in the observable patch of the universe.  I also don’t even know precisely when the Hot Big Bang started, but the uncertainty on that initial time (relative to other events) is less than one second — so all the times I’ll mention, which are much longer than that, aren’t affected by this uncertainty.

I’ll now take you through the different confidence zones of the Big Bang, from the latest to the earliest, as indicated in the figure above.

Continue reading

Dark Matter: Unseen, But Yet Again in the Limelight

The past two weeks have been busy!  I was on the road, consulting with and learning from particle experimenters and theorists at Caltech and the University of California at Irvine. And I’ve been giving talks: at the University of California Santa Barbara (for the Joe Polchinski Fest conference), at the University of California at Irvine, and yesterday in Boston at M.I.T. The Santa Barbara talk was only semi-technical, and is on-line.  The latter two, much more technical, focused on the two big projects that I completed this fall (one on whether searches for supersymmetry have been comprehensive, one on looking for unusual things the Higgs particle might do.)

While this has all been going on, there have been two big stories developing in dark matter searches, and those of you who already have heard about them will have noticed I have not written much about them yet.  (In fact I only wrote about one of them, and very partially.)  These stories are important, and also have some subtleties, which I want to make sure I understand fully before I try to explain them.  After consultations with some of the experts (including Kev Abazajian of U.C. Irvine and Tracey Slatyer of M.I.T) I’m a lot closer to that point, so an explanation will come soon, after I’ve done a bit more reading and learning.

For the moment let me just note that there are two completely different excesses —

  • one in X-ray photons (specifically photons with energies of about 3500 eV) noticed by two groups of scientists in a number of different galaxies, and
  • one in gamma ray photons (specifically photons with energies of 1 – 10 GeV [GeV = 1,000,000,000 eV]), extracted with care by one group of scientists from a complex set of astrophysical gamma ray sources, coming from a spherical region around, and extending well beyond, the center of our own galaxy.

These seem to the experts I’ve spoken with to be real excesses, signs of real phenomena — that is, they do not appear to be artifacts of measurement problems or to be pure statistical flukes. This is in contrast to yet another bright hint of dark matter — an excess of photons with energy of about 130 GeV measured by the Fermi satellite — which currently is suspected by some experts, though not all, to be due to a measurement problem.

But even if the experts are right about that, it still leaves the big question: are these excesses signals of previously unknown astrophysical phenomena, or are they signals of decaying or annihilating dark matter particles?  New astrophysics would be interesting too, but probably not Nobel-worthy, as dark matter would be.  There are arguments against astrophysical explanations in both cases, but they don’t seem by any means airtight yet.

Since the two excesses are completely different, it is highly likely that at least one of them is due to astrophysics.   [You can invent types of dark matter that would give you both signals — but it would take a small miracle for two signals of the same dark matter particles to show up in the same year.]  In fact, it is quite likely, in my mind, that they’re both due to astrophysics, not particle physics. But dark matter might show up in this way, so these excesses have to be explored fully.  It could be that this is the moment when dark matter is finally revealed.  If so — would the real dark matter excess please stand up?

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.