Of Particular Significance

Searching For SUEP at the LHC

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 03/15/2024

Recently, the first completed search for what is nowadays known as SUEP — a Soft-Unclustered-Energy Pattern, in which large numbers of low-energy particles explode outward from one of the proton-proton collisions at the Large Hadron Collider [LHC] — was made public by physicists working at the CMS experiment. As a theoretical idea, SUEP has its origin in 2006-2008, but it was this paper from 2016 that finally brought the possibility to widespread attention. (However, the name they gave it was unfortunate. To replace it, the acronym “SUEP” was invented.)

How can SUEP arise? If a proton-proton collision produces currently-unknown types of particles that

  • do not interact with ordinary matter directly (i.e. they are immune to the electromagnetic, strong nuclear and weak nuclear forces),
  • but do interact with each other, via their own, ultra-powerful force,

they can cause that collision to turn to SUEP.

While the familiar strong nuclear force mainly produces large numbers of particles in narrow sprays, known as jets, a new ultra-strong force could produce even larger numbers of particles, with relatively less energy-per-particle, arranged in near-spherical blasts. I gave a somewhat detailed description of SUEP in this post. (In fact, SUEP is a prediction of string theory — though, I hasten to add, one that has nothing to do with whether string theory describes quantum gravity in our universe.)

Below are shown two events, one with SUEP and one without, simulated back in 2007. Can you see the difference? (In these crude images, the darker lines represent higher-energy particles, and energy deposits are drawn in orange. You can see that the picture at right has lower-energy particles, more numerous and distributed more symmetrically, than the picture on the left.)

(Left) A busy non-SUEP event, with many jets of high-energy particles. (Right) A SUEP-like event, with particles that have lower energy, are more numerous, and are more broadly spread around. Simulation by the author in 2007, using a modified form of PYTHIA 6; originally presented here and here.

Now here’s some real data. Below is a typical (though still quite active) proton-proton collision at CMS. The yellow tracks show where the particles went. You can see that not all the tracks are straight (in contrast to my simulated events above). That’s because inside CMS is a magnetic field, which bends the paths of charged particles. The less energy a particle has, the more it curves. In this event, a substantial fraction of the tracks are straight and are clustered into narrow sprays (with orange cones drawn around them to guide the eye). These are the typical jets of mostly-high-energy particles created by the Standard Model’s strong nuclear force.

Now, here’s another real event observed at CMS, a truly amazing proton-proton collision that created an exceptional number of particles. Although there is a chance that it is SUEP, it’s probably just an extraordinary, rare process created by the strong nuclear force. Notice that almost all the tracks curve — these particles each have relatively low energy — and there are hardly any clusters of tracks similar to the ones above.

As was the case for the Higgs boson, a single suggestive picture is not enough. Discovery of SUEP would require many such SUEP-y proton-proton collisions be observed, in order that they could be distinguished, statistically, from known phenomena. (To be fair, there are some types of SUEP where just two or three events would suffice. But that’s a story for another day.)

No Discovery… But Still, Congratulations

Had this search actually found some evidence of SUEP, you would have seen it in news headlines. But it came up empty, as is the case for most scientific quests for new things. Nevertheless, despite a lack of a discovery, congratulations to CMS are due. This was a first-of-its-kind search, employing novel methods. Here’s CMS’s own description of their search.

Meanwhile, the story of SUEP is not over. CMS only looked for certain kinds of SUEP, and there are many more. A variety of hunting strategies will be needed in future, in order to cover all the possibilities.

The Current Status of the LHC Program

More generally, I want to highlight the significance and role of novel search strategies at the LHC experiments. This issue is often underestimated or misunderstood.

At the moment, and for the last few years, the central question facing LHC experimenters and their theoretical-physicist colleagues is this:

In 2012-2016, the discovery and initial examination of the Higgs boson completed the Standard Model. Since that time, nothing outside the Standard Model has been observed at the LHC. But it’s crucial to remember that although finding something proves that it exists, not finding it does not prove it does not exist.

It’s similar to trying to find a set of keys that might be in your house. If you find them right away, your search is over. But if you don’t find them right away, you can’t conclude they’re not in the house. You need to keep looking; maybe you haven’t looked in the right place yet. You have to search as carefully as you can, covering all locations and considering all possibilities, before you conclude that they simply must be elsewhere.

A single failure of the Standard Model would bring its reign to an end, and answer the central question in the negative. But to answer it with a reasonably confident “Yes” will require a thorough plan of searches for a wide variety of possible phenomena. If our search strategy leaves loopholes, we simply won’t be able to answer the central question with either a “No” or a “Yes”! And then we’ll be left in limbo.

Importantly, making the LHC search programs more thorough isn’t expensive. In fact, it’s more expensive not to make them thorough.

Each experiment’s data is collected as a giant pile, and each search for new phenomena involves examining one of the already-assembled giant piles through a particular lens. If we don’t hunt for everything reasonable in those data sets, then we’re partly wasting the time, effort and money that we spent to obtain them!

And there’s no reason for undue pessimism that none of these searches will find anything. Even a dramatic new phenomenon like SUEP can lie hidden in a vast data set, undetected until the moment that someone searches the data in just the right way.

That’s why this first SUEP search is important: it’s a novel way of exploring the LHC’s data. It pushes the boundary of what we know in a previously unexplored direction, and sets a new frontier for future investigation.

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8 Responses

  1. Are these SUEP searches compatible with triggers? AFAIK the triggers ignore similar events. Are these searches based on the events stored periodically even without triggers?

    1. No, the trigger used requires what is known as an “initial radiation jet”, meaning a quark or gluon which is radiated off before the main mini-collision that creates the SUEP.

  2. Well, it seems we are getting closer rather than being stuck, don’t you say?

    ” – but do interact with each other, via their own, ultra-powerful force”

    … “Ultra-powerful force” … I’m willing risk my house, this is quantum gravity.

    ” The less energy a particle has, the more it curves.”

    … Is this because the higher energy particles are moving faster and hence the magnetic field doesn’t interact as “fast”, i.e. velocity dependent?

    This is just a guess, are the majority of particles in the second collision, the likey SUEP event, bosons and/or very, very light virtual particles?

    Thank you for giving us, non-physists, a window into this exciting world of theoretical physics.

    1. @ADM : I’m not a particle physicist, so caveat emptor.

      First, regarding “… “Ultra-powerful force” … I’m willing risk my house, this is quantum gravity.”

      This may be useful here: “What about gravity? Well, for the particles we know about, gravity is amazingly weak.” “That’s about 100,000,000,000,000,000,000,000,000,000,000 times smaller than the electric force between two top quarks.” https://profmattstrassler.com/articles-and-posts/particle-physics-basics/the-known-forces-of-nature/the-strength-of-the-known-forces/

      Second, on slower moving particles in general I think it is helpful to consider that they will be subjected to a force on their trajectory for a longer time. Hence they will get higher change of momentum (change of momentum I = integral F(t)*dt). It is true that the electromagnetic Lorenz force is velocity dependent from magnetism being a low velocity relativistic effect F = q(E + vxB) (q charge, E and B electric and magnetic field vectors, v velocity vector). But that would tend to counteract the general effect – higher particle velocities would tend to feel higher effective magnetic fields (so to speak).

      The misnomer virtual particles would not make tracks, they form a constant noise background everywhere in the detector volume (I would think). It is likely true that they would be more densely appearing around the collision, but not in the form of particle tracks.

      “The best way to approach this concept, I believe, is to forget you ever saw the word “particle” in the term. A virtual particle is not a particle at all. It refers precisely to a disturbance in a field that is not a particle. A particle is a nice, regular ripple in a field, one that can travel smoothly and effortlessly through space, like a clear tone of a bell moving through the air. A “virtual particle”, generally, is a disturbance in a field that will never be found on its own, but instead is something that is caused by the presence of other particles, often of other fields.”
      “This unpleasant jiggling motion — this disturbance of the swing — is different from the swing’s natural and preferred back-and-forth regular motion just as a “virtual particle” disturbance is different from a real particle. If something makes a real particle, that particle can go off on its own across space. If something makes a disturbance, that disturbance will die away, or break apart, once its cause is gone. So it’s not like a particle at all, and I wish we didn’t call it that.” https://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/

      1. Sorry, Torbjörn, but your premise is wrong; this is not quantum gravity. It is just quantum field theory. Your guesses are fun to read, but they are quite far off the mark. Did you read this post yet? https://profmattstrassler.com/2022/03/20/a-prediction-from-string-theory/

        [When you sent this, did you not see something like “comment held subject to approval”? Maybe there’s something missing from the current website set-up. I hold comments with lots of links to avoid spam.]

        1. Matt, I think I was in a hurry, I don’t read popups then. I’m sorry for the extra work, I am now better informed.

          Maybe I misunderstand your response here, but it was ADM that suggested it was quantum gravity, not I. I tried to answer his general questions in the quantum field theory setting. (But I’m happy that echoing your own posts make it fun to read for you.)

    2. A SUEP-like event produces known particles in great abundance, though exactly which ones depends on the type of SUEP. What’s new in SUEP is the process behind the creation of those particles.

      1. Re: Diagram: “Newer Thinking About Confining Forces”

        Is confinement ~ symmetry?

        So, as some very same distances (radius) in the proton, the the ultra strong forces become polarized hence creating “spin” and hence particles we see in LHC and as described in the Standard Model?

        This symmetry breaking could just be a freak of nature at the Big Bang. Did we got lucky?

        Ref: https://profmattstrassler.com/2022/03/20/a-prediction-from-string-theory/

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