Searching For SUEP at the LHC

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.)

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:

Does the Standard Model (the math that describes all the known elementary particles) correctly predict all observed data at the LHC?

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.