Of Particular Significance

A Hidden Gem At An Old Experiment?

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 10/21/2016

This summer there was a blog post from   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  It has appeared in a new paper (a preprint, so not yet peer-reviewed) by an experimentalist named Arno Heister, who is evaluating 20-year old data from the experiment known as ALEPH.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.

ALEPH data as analyzed in Heister’s paper, showing the number of Z particle decays with two bottom quark jets and a muon/anti-muon pair, as a function of the invariant mass of the muon/anti-muon pair.  The bump at around 30 GeV is unexpected; might it be a new particle?  Not likely, but not impossible.

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.

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25 Responses

  1. Matt:
    I am a late-comer to your blog and really appreciate your explanations and updates on the science. I found your blog by looking up an explanation for virtual particles [very good] back in 2012 and have dipped in on various topics coming up to your latest here. It made me think of something you wrote in the blog about virtual particles about the fields. I have pasted it below. Could you explain more about the ‘metric’ of space-time and how some of the fields *could* form part of the metric for dimensions beyond our three dimensions and time. Or probably much simpler, direct me to articles or books that would explain these theories in more detail?

    Thank you.

    “Most fields are best thought of as pervading three-dimensional-space and time, except for the graviton field, also known as the “metric” of space-time (the object that is needed to decide how far part two points are) which is really intrinsic to space-time.

    In most theories with extra dimensions, some of the fields that we observe would actually form a part of the metric of the higher-dimensional space time. In other words, one explanation as to why there are so many fields in nature might be that we live in a world that has some of its dimensions wrapped up (think of how a hose has a large dimension along the hose and small dimension around the hose) and that the metric of the full space-time looks to us, in three-dimensional space, like a metric for three-dimensional space and time along with many other fields whose explanation seems non-obvious.”

  2. Highly skeptical of any signal of a relatively low energy particle that shows up as a bump in one old study and has not been revealed in any subsequent experiments in any notable way. A look elsewhere effect limited the LEP doesn’t take into account the fact that there have been other experiments conducted since then, and that a failure to find it them greatly lowers the significance of the result if it doesn’t rule it out entirely. For example, the constraint on possible BSM Z boson decays is very tight.

    1. There are plenty of reasons to be skeptical of this observation, but the objection you raised is not one of them. The constraints on BSM Z bosons ***all*** assume that their coupling constants are close to those of the Standard Model, and are thus greater than 0.1. A BSM Z boson with coupling constant much below 0.001 is simply not constrained at all at most masses above 10 MeV or so, and above 10 GeV any boson with a coupling below 0.01 is allowed; above the Z mass it is even worse. Hidden valleys often have fundamental or composite bosons with couplings to Standard Model fields that are much less than 0.1 .

  3. I think the paper does not treat the LEE correctly. Either the author scanned thousands of plots and stumbled on this, and the LEE is much larger, or there was a “Look-right-there” effect going on.

    1. It is no more promising than the diphoton bump. It is a bit less, in fact. However, when the Higgs first appeared in December 2011, it was also, in my opinion, not very promising. Most small bumps go away. All big bumps that stick around start as small bumps. Deal with it.

  4. I think the situation is bad, Matt. Particle physics is not in a good place. And I don’t think a 20-year-old bump on a graph is going to help at all. In fact, I think things are going to get worse. I’ve been thinking a lot about this recently because I’ve been doing some writing. Which reminds me: what’s happening with that book you said you were going to write?

  5. I laughed my way down the street imagining Physicists have trouble with ALEPHS as they did with Cantor’s infinities – aleph0, aleph1 etc. Found myself going down the wrong street as a result of that.

  6. Thanks for update, Professor.

    I wish we searched for hidden gems among politicians as well,
    so we could prevent ( hopefully !! ) ww-3.


  7. I remember once, when eating a salad, I thought I had consumed all of my carrots. Upon further inspection, and digging my fork more deeply into my Hidden Valley ranch dressing, I found, much to my surprise, one more carrot! Had I remained skeptical and not looked more closely for myself, I would have denied myself the joy of dicovering This lovely vegetable. So the moral of this anecdote? Dig deeper into the salad, as Professor Strassler advocates. V may indeed be there, even if masked under the likeness of Guy Fawkes.

    1. You know, it is a sad comment on the United States that when people learn about hidden valleys they think of salad dressing. It’s like thinking of The Wiz when asked about Oz, or thinking of a certain stereotype-driven sitcom when asked about the history of the universe. However, if you prefer to swim in a creamy vinaigrette instead of a cool alpine lake, be my guest.

  8. I concur with you on how often politicians introduce interference (the pun is intended, as we are talking about quantum physics!) with science in general, and in this case, with HEP.

    But let’s stay positive about this.

    The Standard Model, even though is the theory that we (actually, you and your fellow researchers) have come up with so far, needs to be fixed, or to be perfected, or even better, to be replaced, eventually, by an all-encompassing theory that includes gravity and what we so far call dark energy and dark matter.

    We could be just one experiment away from getting the first clue that may lead us to the doorway to this revised or new theory.

    Kind regards, GEN

  9. I will consider the fact that the LHC will discover nothing more as a great result. It means that the foundation of reality is severely restricted and that we get a chance to comprehend a large part of the lower levels of physical reality.
    In my conviction the foundation of reality IS VERY SIMPLE and its extension is significantly restricted. Discovering the how and why of this fact is already a great achievement.
    See: docs,com/hans-van-leunen

  10. Matt, even though it makes sense to move with caution, it is also exciting that this kind of news shows up every so often.

    As you say, there is still lots of experiments to be done at the LHC, in particular, testing these “hidden valleys” that you mention, and other kinds of “sideline” theories.

    With the LHC (and ATLAS, CMS, Alice and LHCb), and an immense set of big data tools available, theorists and experimentalists have an enormous opportunity to go after many roads of enquiry, so, it is not a time to shy away from going after “what we don’t know yet that we don’t know!”.

    Kind regards, GEN

    1. 🙂 My only mild objection to your comment, GEN, is that hidden valleys are only “sidelined” for political reasons. There is nothing odd or weird about the possibility of new particles and forces; for heaven’s sake, that’s what the neutrino and the weak interactions were, back in the 1920s!

  11. Scientists are often adventurous when they do research with government interference…the real magic starts when scientists don’t feel institutional pressures. The cost of innovation that sometimes causes a reset of what we know is peanuts once realised compared to the economic and knowledge growth that emanates from scientific freedoms.

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