Of Particular Significance

At a Workshop on Hidden Particles at the LHC

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 07/07/2022

Cutting edge particle physics today:

I’ve been spending the week at an inspiring and thought-provoking scientific workshop. (Well, “at” means “via Zoom”, which has been fun since I’m in the US and the workshop is in Zurich; I’ve been up every morning this week before the birds.) The workshop brings together a terrific array of particle theorists and Large Hadron Collider [LHC] experimenters from the ATLAS and CMS experiments, and is aimed at “Semi-Visible Jets”, a phenomenon that could reveal so-far-undiscovered types of particles in a context where they could easily be hiding. [Earlier this week I described why its so easy for new particles to hide from us; the Higgs boson itself hid for almost 25 years.]

After a great set of kick-off talks, including a brand new result on the subject from ATLAS (here’s an earlier one from CMS) we moved into the presentation and discussion stage, and I’ve been learning a lot. The challenges of the subject are truly daunting, not only because the range of possible semi-visible jets is huge, but also because the scientific expertise that has to be gathered in order to design searches for semi-visible jets is exceptionally wide, and often lies at or beyond the cutting edge of research.

Semi-Visible Jets

In particle physics, an ordinary run-of-the-mill “jet” is a spray of ordinary “hadrons“, which are particles made from ordinary quarks, anti-quarks and gluons. These jets are themselves the result of a quark or anti-quark or gluon being produced with lots of motion-energy — typically moving at or near the speed of light — in a particle collision, such as those that are happening at the LHC (which just started producing collisions at a record energy-per-collision.) Below is an event from ATLAS showing an event with two back-to-back jets.

From ATLAS, a typical proton-proton collision with two ordinary jets. The proton beams are coming in and out of the screen; the collision point is at dead center. From the collision emerge two jets (plus a bit of other junk), the narrow groupings of nearly straight tracks shown in bright green and red; these leave lots of energy in the “calorimeters”, as shown by the dark green/yellow and dark red rectangles at the outer edges of the detector.

A “semi-visible” jet is the name given to something more complex. We don’t know if such jets occur in nature, but experimentalists at the LHC are now trying to look for them. These jets are sprays of currently-unknown, novel particles from a “hidden valley” or “dark sector” (almost equivalent names for more or less the same thing, and the words “hidden” and “dark” will be used almost interchangeably in this post.) These particles are typically themselves “dark hadrons”, made analogously to ordinary hadrons from “dark quarks”, “dark anti-quarks” and “dark gluons”. Why call these particles “dark?”

  • They are completely unaffected by the forces, other than gravity, that dominate daily life: the electromagnetic, weak-nuclear and strong-nuclear forces. That makes them undetectable; such particles would almost always pass through ordinary matter leaving no trace, even more “ghostly” than neutrinos.
  • Some of these types of undetectable dark hadrons might be the universe’s dark matter (or might decay to dark matter), though that’s a speculative issue that the LHC experiments won’t be able to determine by themselves.

If that were all there was to the story, these exotic jets would be completely invisible. What makes them visible, or “semi-visible”, is that while some of these dark hadrons may depart the scene leaving no trace, a certain fraction of these dark hadrons may decay, within a tiny fraction of a second, to ordinary particles that we know and love — quarks and anti-quarks, electrons and positrons, or others of the known elementary particles. If they decay to quarks and anti-quarks, each one of these makes its own ordinary jet. In that case, a semi-visible jet becomes, in a way, a jet of ordinary jets… along with some signs that something’s gone missing.

Left: Ordinary jets of hadrons will form from an ordinary, fast-moving quark; the total energy of the jet is approximately the total energy of the unobserved original quark. Center: A fast-moving hidden quark will make a jet of hidden hadrons; but these, in turn, may all decay to ordinary quark/anti-quark pairs, each of which leads to a jet of ordinary hadrons. The result is a jet of jets. Right: if only some of the hidden hadrons decay, while some do not, then the jet of jets is semi-visible; only some of the energy of the original dark quark will be transformed into observed particles, while some dark hadrons (grey dotted arrows) will escape the detector unobserved.

To give you a sense for this, here (from work I did in 2008) are a couple of simulated proton-proton collision events of these jets of hidden hadrons, both entirely visible (left) and semi-visible (right); on each side of each event is a jet of jets (and I promise you there are undetected particles, not shown, in the right-hand event.) Compare these with the jets in the real ATLAS event pictured at the top of the post.

Two simulated LHC events that produced “dark jets”, i.e. “jets of hidden hadrons”, entirely visible on the left, and semi-visible on the right. The wide groupings of narrow clusters of tracks represent “jets of jets”. [Technical details: the dark hadrons decay mainly to bottom quark-antiquark pairs. The simulated detector has no magnetic field. Grey tracks are charged hadrons; grey scale shows transverse momentum. Green tracks are leptons. Orange bars show the total energy deposited in the calorimeters.]

But this picture shows just one of many examples of what semi-visible jets can look like. There are tremendous variations from one semi-visible jet to another even within a single choice of hidden valley [you can actually see this in the figure above], and if you change to another hidden valley with, say, a different set of dark quarks or a different choice for their masses, you may find your semi-visible jets are qualitatively different. This is what makes looking for semi-visible jets so difficult; the space of possibilities is so large that we have to find strategies that are able to cover many possibilities at once, but we don’t yet have all the basic tools and techniques that we need in order to design those strategies.

That’s why we hold workshops: to bring the experts together to brainstorm and share ideas and knowledge. Hopefully we’ll solve some of these problems in the coming six months to a year, and perhaps in two to four years you’ll see the fruits of this labor — a range of new and more powerful searches for this phenomenon, and a chance for a discovery.

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13 Responses

  1. Matt, I have a question that may be a bit away from the main line of this article. Is it time to start thinking about the next generation collider beyond LHC? If so what form would it take? I’m thinking LHC is like the Hubble. Is it time for an equivalent to the Watt telescope? Are there any experiments we can get to with greater energies that might answer questions about string theory?

    Speaking of the new telescope, with its ability to reach back further toward the Big Bang what can we expect on its impact on particle physics and the Standard Model.

    1. I try to keep comments focused on the topic, but I will note (a) I wrote about related issues recently at https://profmattstrassler.com/2022/06/09/a-big-think-made-of-straw-bad-arguments-against-future-colliders/, and (b) it isn’t super likely that the Webb will impact particle physics and the Standard Model (any more than Hubble did); Webb is likely to change our understanding of the universe’s rather early history (first galaxies etc.), but that won’t feed back into particle physics unless some big surprises turn up.

  2. Thanks even more! ” In the rest frame of each hadron” (which is what I should have been thinking all along) and those (obviously classical, since they multiply to >c, numbers ) plus the numbers in the reference you listed will answer my question.

    1. Nah, I was just a little careless with numbers. Nothing violates relativity here, and in fact relativity and locality are the key to the argument. The precise numbers depend on the details of the relative speeds of the hadrons as they separate from one another.

  3. Thanks for the answer. At least I got that number in the e+e- collisions. Its in the “enough” regime and scales OK (because of course in pp collisions the indiviual partons are of lower energy).

    I should perhaps ask one more time, narrowing what I mean.
    The real number I need to know is the typical energy in the rest frame of one jet, well after the particles in that jet have reached a macroscopic distance away from each other, so they are interacting only electromagnetically. Or,
    perhaps just ask: are they all relativistic in that frame? I assume yes, but I need to ask.

    Alternatively, I suppose I could ask the overall question: at what energy regime (in the pairwise CM frame) are the individual hadron pairs in a jet when they reach the regime when their final strong force elastic scattering decouples? That’s really what I need to know for my project (which is the ultra-infamous “quantum measurement problem”). I should add that my view of that is that at least on a solar system or smaller scale, unitarity is **never ever** violated. I really don’t want to get into that morass here!

    Thanks for your patience!.

    1. In pp collisions the individual partons are often at much higher energy; we observe q q –> q q at several TeV.

      Again, your second question really is ill-posed. I can’t answer it; it’s ill defined.

      As for your last question, that’s clear; shortly after the hadrons form, at distances of order 10-14 meters, they decouple from each other. The process of formation *is* a process of decoupling; strong interactions tail off sharply after that point. But the “rest frame of the jet” is the wrong place to ask that question. In the rest frame of each hadron, all other hadrons move off at semi-relativistic speeds, and so within 10^-23 seconds (in the hadron’s frame) are so they are much further apart than their size — much too far apart to affect one another, except very slightly through long-range electromagnetic effects. There will be rare exceptions to this in particular jets where particles are accidentally traveling in almost exactly the same direction at almost exactly the same speed, but that’s very unusual; I’ve described the typical situation.

  4. A question, which I’ve not been able to find an answer to: what is the typical energy of the various particles inside a single jet, in the reference frame of the jet’s center of mass. Presumably this would be proportional to the jet’s energy in the lab frame (for a standard pp or ppbar collider like LHC) energy but inversely poportional to the number of particles in the jet. I have no idea of how many total particles are in a jet. Am I off base?

    1. Jets are far more subtle than that; you can’t treat a jet like a decaying particle. You can’t even specify what a jet is without specifying the algorithm you are using to define it. Even the mass of a jet is subtle, and your presumptions about it aren’t really right. Almost never have I seen a calculation in the rest frame of a jet, because it’s the wrong frame for understanding how jets form. Crudely speaking, a quark’s “fragmentation” or “hadronization” into a jet of hadrons depends on where the corresponding anti-quark is going. That means that even if we go to the jet’s rest frame, the physics we observe depends upon the anti-quark’s energy and direction as seen in the jet’s frame. Indeed, the quark-antiquark invariant mass is far more important to the hadronization than what eventually will turn out to be the jet mass. The correct rest frame for understanding the jets, consequently, is the quark-antiquark rest frame. [That, too, is a severe understatement of the complexity, because there are gluons around too and one needs to follow them as well… but I’ll never finish this answer if I don’t sweep that under the rug.]

      In particular, your notion that the “typical energy of the various particles inside a single jet, in the reference frame of the jet’s center of mass” is “proportional to the jet’s energy in the lab frame” is wrong. If you consider a whole set of events, in each of which the quark (and corresponding jet) have energy of, say, 100 GeV, and even if the corresponding anti-quark is going exactly in the opposite direction with the same energy, each jet in these events will have a different mass, and ***the energies of the particles as seen in the jet’s rest frame will be proportional to that mass***, not to the energy of the jet as seen in the lab frame. (It’s true that the *average* jet mass increases with the energy of the jet, but still it’s the jet’s mass, event by event, that regulates the particle energies in the jet rest frame.)

      Meanwhile, even if I fix the quark’s jet, so I take a set of events where its energy and direction is always the same, when I vary the antiquark’s direction and energy in the lab frame I will change the quark-antiquark invariant mass, and the number of particles in the quark jet will be related (roughly logarithmically) to the invariant mass of the quark-antiquark pair, even though the energy of the quark in the lab frame isnt changing. So that will change how much energy per particle there is, as seen in the jet’s rest frame, even if I compare two jets with the same jet mass. In other words, your question does not have an answer if you don’t tell me what the antiquark is doing.

      You also wanted to know how many particles there are per jet. That fluctuates wildly and the distribution depends on the quark-antiquark invariant mass as well as on the jet algorithm. But if you look at page 2 of https://pdg.lbl.gov/2019/reviews/rpp2018-rev-cross-section-plots.pdf you will find the total number of particles in electron-positron collisions, which produce quark/antiquark pairs of fixed center-of-mass energy, for four different energy ranges. Roughly, if you divide by 2 that would give you the number of particles per jet. Note that this number is not something that anyone can calculate, not even with a computer program. It is something we learn only from data. [And that is a big problem when you want to do the physics of hidden jets, because there is no reason for them to be identical to the the ordinary ones we see in data; the hidden forces involved may be somewhat different and the hidden particle masses will surely be different.] *However*, these numbers do not necessarily apply in a proton-proton machine because the directions and relative energy of the quarks and anti-quarks in the lab frame of a proton-proton machine are highly variable, whereas in an electron positron machine the energy is fixed and quark and anti-quark are most often back-to-back.

      Jets, in short, are a highly advanced topic. There’s a reason I don’t get into too much detail on this blog.

  5. Love your posts, the fundamental source of all things is just amazing. With that said, in quantum field theory, fundamental particles have their own fields which permeate the universe. Electron field, quark field …..etc. I believe, there are something like 36 fundamental fields? Does that sound correct? When a particle decays, I have always thought of that decaying particle shoving its energy into another field….hence the emergence of other particles. So, does the discovery of these new particles mean that there are more fundamental fields? Or are they composite particles?

    1. Yes, the new particles require new fields. The hidden quarks may be elementary particles and their fields may thus be elementary (though of course we can’t verify that easily anymore than we can for ordinary quarks.) The hidden hadrons are definitely composites, held together by hidden gluon fields, in analogy to ordinary hadrons.

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