This summer there was a blog post from Sabine Hossenfelder claiming that “The LHC `nightmare scenario’ has come true” — implying that the Large Hadron Collider [LHC] has found nothing but a Standard Model Higgs particle (the simplest possible type), and will find nothing more of great importance. With all due respect for the considerable intelligence and technical ability of the author of that post, I could not disagree more; not only are we not in a nightmare, it isn’t even night-time yet, and hardly time for sleep or even daydreaming. There’s a tremendous amount of work to do, and there may be many hidden discoveries yet to be made, lurking in existing LHC data. Or elsewhere.
I can defend this claim (and have done so as recently as this month; here are my slides). But there’s evidence from another quarter that it is far too early for such pessimism. It has appeared in a new paper (a preprint, so not yet peer-reviewed) by an experimentalist named Arno Heister, who is evaluating 20-year old data from the experiment known as ALEPH.
In the early 1990s the Large Electron-Positron (LEP) collider at CERN, in the same tunnel that now houses the LHC, produced nearly 4 million Z particles at the center of ALEPH; the Z’s decayed immediately into other particles, and ALEPH was used to observe those decays. Of course the data was studied in great detail, and you might think there couldn’t possibly be anything still left to find in that data, after over 20 years. But a hidden gem wouldn’t surprise those of us who have worked in this subject for a long time — especially those of us who have worked on hidden valleys. (Hidden Valleys are theories with a set of new forces and low-mass particles, which, because they aren’t affected by the known forces excepting gravity, interact very weakly with the known particles. They are also often called “dark sectors” if they have something to do with dark matter.)
For some reason most experimenters in particle physics don’t tend to look for things just because they can; they stick to signals that theorists have already predicted. Since hidden valleys only hit the market in a 2006 paper I wrote with then-student Kathryn Zurek, long after the experimenters at ALEPH had moved on to other experiments, nobody went back to look in ALEPH or other LEP data for hidden valley phenomena (with one exception.) I didn’t expect anyone to ever do so; it’s a lot of work to dig up and recommission old computer files.
This wouldn’t have been a problem if the big LHC experiments (ATLAS, CMS and LHCb) had looked extensively for the sorts of particles expected in hidden valleys. ATLAS and CMS especially have many advantages; for instance, the LHC has made over a hundred times more Z particles than LEP ever did. But despite specific proposals for what to look for (and a decade of pleading), only a few limited searches have been carried out, mostly for very long-lived particles, for particles with mass of a few GeV/c² or less, and for particles produced in unexpected Higgs decays. And that means that, yes, hidden physics could certainly still be found in old ALEPH data, and in other old experiments. Kudos to Dr. Heister for taking a look.
Now, has he actually found something hidden at ALEPH? It’s far too early to say. Dr. Heister is careful not to make a strong claim: his paper refers to an observed excess, not to the discovery of or even evidence for anything. But his analysis can be interpreted as showing a hint of a new particle (let’s call it the V particle, just to have a name for it) decaying sometimes to a muon and an anti-muon, and probably also sometimes to an electron and an anti-electron, with a rest mass about 1/3 of that of the Z particle — about 30 GeV/c². Here’s one of the plots from his paper, showing the invariant mass of the muon and anti-muon in Z decays that also have evidence of a bottom quark and a bottom anti-quark (each one giving a jet of hadrons that has been “b-tagged”). There’s an excess at about 30 GeV.
The simplest physical effect that would produce such a bump is a new particle; indeed this is how the Z particle itself was identified, over three decades ago.
However, the statistical significance of the bump is still only (after look-elsewhere effect) at most 3 standard deviations, according to the paper. So this bump could just be a fluke; we’ve seen similar ones disappear with more data, for example this one. There are also a couple of serious issues that will give experts pause (the width of the bump is surprisingly large; the angular correlations seem consistent with background rather than a new signal; etc.) So the data itself is not enough to convince anyone, including Dr. Heister, though it is certainly interesting.
Conversely it is intriguing that the bump in the plot above is observed in events with bottom quarks. It is common for hidden valleys (including everything from a simple abelian Higgs models to more complex confining models) to contain
- at least one spin-one particle V (which can decay to muon/anti-muon or electron/positron) and
- at least one spin-zero particle S (which can decay to bottom/anti-bottom preferentially, with occasional decays to tau/anti-tau.)
For example, in such models, a rare decay such as Z ⇒ V + S, producing a muon/anti-muon pair plus two bottom quark/anti-quark jets, would often be a possibility.*
*[In this case the bottom and anti-bottom jets would themselves show a peak in their invariant mass, but unfortunately their distribution in the presence of a candidate V was not shown. One other obvious prediction of such a model is a handful of striking Z ⇒ V + S ⇒ muon/anti-muon + tau/anti-tau events; but the expected number is very small and somewhat model-dependent.]
Another possibility (also common in hidden valleys) is that the bottom-tagged jets aren’t actually from real bottom quarks, and are instead fake bottom jets generated by one or two new long-lived hidden valley particles.
But clearly, before anyone gets excited, far more evidence is required. We’ll need to see similar studies done at one or more of the three other experiments that ran concurrently with ALEPH — L3, OPAL, and DELPHI. And of course ATLAS, CMS, and LHCb will surely take a look in their own data; for instance, ATLAS and CMS could search for a dilepton resonance in events with at least two bottom-tagged jets, where the whole system of bottom-tagged jets and dileptons has a invariant mass not greater than about 100 GeV/c². [[IMPORTANT NOTE ADDED: It has been pointed out to me (thanks Matt Reece) that there was a relevant CMS search from 2015 that had somehow entirely escaped my attention, in which one b-tag was required and a di-muon bump was sought between 25 and 60 GeV. Although not aimed at hidden valleys, it provides one of the few constraints upon them in this mass range. And at first glance, it seems to disfavor any signal large enough to explain the ALEPH excess. But there might be subtleties, so let me not draw firm conclusions yet.]] They should also look for the V particle in other ways — perhaps following the methods I’ve suggested repeatedly (see for example pages 40-45 of this 2008 talk) — since the V might not only appear in Z particle decays. [That is: look for boosted V’s; look for V’s in high-energy events or high missing-energy events; look for V’s in events with many jets, possibly with bottom-tags; etc.] In any case, if anything like the V particle really exists, several (and perhaps all) of the experiments should see some evidence for it, and in more than just a single context.
Though we should be skeptical that today’s paper on ALEPH data is the first step toward a major discovery, at minimum it is important for what it indirectly confirms: that searches at the LHC are far from complete, and that discoveries might lie hidden, for example in rare Z decays (and in rare decays of other particles, such as the top quark.) Neither ATLAS, CMS nor LHCb have ever done a search for rare but spectacular Z particle decays, but they certainly could, as they recently did for the Higgs particle; and if Heister’s excess turns out to be a real signal, they will be seen to have missed a huge opportunity. So I hope that Heister’s paper, at a minimum, will encourage the LHC experiments to undertake a broader and more comprehensive program of searches for low-mass particles with very weak interactions. Otherwise, my own nightmare, in which the diamonds hidden in the rough might remain undetected — perhaps for decades — might come true.