LHC Data: Where do we look after the Higgs?

In 2012, the two largest experiments at CERN, ATLAS and CMS, announced that they had discovered the Higgs boson. It was an extraordinarily exciting moment for the particle physics community—we had finally found all the particles in the Standard Model! And then it was a disturbing moment for the community: where do we look next?

How have we searched for new particles so far?

The Standard Model in particle physics is a catalog of all the known particles in our universe. It contains the fundamental particles that make the atom (electrons, and quarks which comprise the protons and neutrons) as well as their heavier cousins that decay too quickly to form stable things like atoms. It also contains what we call ‘bosons,’ or particles that mediate interactions. As an example: macroscopically, we see two positively charged objects repel each other. In the particle physics picture, the repulsion happens as bosons (photons in this case) are being thrown from one electron to another, pushing them away.

When the Large Hadron Collider (LHC) was built at CERN, we expected to find the Higgs as the Standard Model needed the Higgs to explain how particles could have mass. Theoretical predictions told us at what energy we needed to build the collider. Lo and behold, after enough data was collected, we found the Higgs.

higgs_PB
Image from CMS Collaboration. The deviation from data (black dots) to the null hypothesis (green band) at 125 GeV is due to the Higgs Boson.

That being said, nothing about discovering a new particle is straightforward. Unfortunately, there is no microscope that can make the Higgs boson visible to the human eye. Instead, experimental particle physicists must be much cleverer. The Higgs decays far too quickly to be seen even if we had a means of observing it directly. Instead, we look for the decay products of the Higgs.

Imagine you plant a tomato seed in the early spring and spend the next few months traveling. When you return to check on your plant, you don’t look for a seed. You look for a long stem with several tomatoes hanging off it. Looking for particles at high energy colliders is the same idea, only instead of months it takes fractions of a second for a particle to decay and produce many new particles. If you expect to see the original particle, just like expecting to see a single seed, you simply won’t find it.

The Higgs decays into almost all massive particles. Einstein has told us that energy is equal to mass, and we know we can’t pull energy out of nowhere, so we just need to find the particles with energy that equal the mass of the Higgs. Simple conservation of energy, right?

Again, we need to be much cleverer. The Higgs can decay into particles that also decay, meaning we now have four particles to look for. And maybe some of those particles decay a lot and produce tens or hundreds of particles clustered together (what we call a jet). These objects are certainly more complicated than a simple two particle search, but if the detector is empty except for the decaying Higgs, it shouldn’t matter.

Again, we are reminded of how difficult the job of an experimentalist is! The detector, as you may have guessed, is far from empty. Nearly 40 proton-proton collisions are occurring at the same time in the detector, spaced only millimeters away. There are thousands of particles to sort through if you want to consider any one collision. Finding the Higgs seems impossible at this point. But experimentalists did it. Using the theoretical predictions regarding the rates of how often we get a Higgs boson in these collisions and how often they decay into certain types of fundamental particles, experimentalists were able to pick out enough Higgs bosons to claim a discovery.

At this point it should be clear that finding a new particle is hard. It seems the only reason we could find the Higgs was that we knew exactly what we were looking for. But now that the Higgs has been discovered, particle physicists are left with the terrifying notion that we have no idea what we are looking for.

Why are we still looking?

Data and theory agree extremely well, which is why we continue to use the Standard Model as the guiding principle in particle physics. But they also disagree enough that we know it can’t be the full story. Particle physicists often try to develop new theoretical models that can be tested at the LHC, but without a beacon as bright as something like the Higgs, it is doubtful that any specific model will fix all the discrepancies we observe.

This has brought several physicists to consider a different paradigm when searching for new physics: instead of letting theory drive how we search, why not search for absolutely anything that we can, and fill out the theory details later?

First Data, then Theory

An uncomfortable truth about experiments like the LHC that are high energy and large data output is that how we look for new particles limits what kind of new particles we can find.

This problem starts at the trigger. Almost all the collisions that happen at the LHC are ‘uninteresting.’ They are uninteresting mostly because they occur very often and we already understand the physics. It’s processes that are uncommon that we still need to study. Because electronics physically cannot transfer the amount of data being collected for every collision fast enough, we have to be picky. In fact, we throw away over 99.999% of data produced at the LHC. The choice of what events we save and what we throw away is fixed by the trigger.

atlas_PB
Image from ATLAS Collaboration. A visualization of a proton-proton collision at the LHC. There are many particles in this event, including a muon (red) and an electron (blue) that might have been used for triggering.

The trigger has specifications like “keep all the data from collisions that produce an electron with energy about 20 GeV” or “keep all events with two muons and two electrons.” When the trigger condition is met, whatever it may be, the data in the detector is stored. While this helps us wade through the swamp of data, it could also be throwing away events that signal the existence of new particles. There is no way around this problem besides finding triggers that are robust enough to be unlikely to throw away interesting events. Until electronics get faster and data storage capabilities increase, we will have to use triggers.

The next problem is deciding on the signature, or what particles in the detector actually signal the existence of a new particle. The signal of a particle is the list of particles that are observable after the particle is produced. For example, if a particle called a Z boson is produced, one of the signatures for the Z boson is an electron and an anti-electron. So to find the Z, we look for electron-anti-electron pairs whose energy adds up to the mass of the Z.

If we have no idea what kind of particle we are looking for, or how massive it is, or what other particles it interacts with, we have no theoretical motivation for what particle to look for. So far, the most popular way to search for these new, mysterious particles is by first building models that predicts properties of the particle. Therefore, we assume that maybe this particle decays into something we can look for, like three jets. This kind of search is easy in one way: we know exactly what to look for! Anyone who has been to a hardware store knows that knowing the name or shape of what you want makes it infinitely easier to find. However, just because we know what we are looking for doesn’t mean it was the right thing to look for. Thus, a new trend in particle searches is rising. Instead of building models that predict very specific signals, why don’t we just look at the most general things we can possibly look at?

This concept of data-first-then-theory is not a function of physicists getting lazy. Instead, one should view it as scientific environmentalism. There are several enormous, expensive experiments that have run or are still running and collecting data. Clearly the analysis done on the data they’ve already collected hasn’t led to any recent discoveries. But does that mean it’s bad data or a worthless experiment? Absolutely not! It could just mean that we blinded ourselves to new physics by looking in the wrong direction.

Imagine you’re locked out of a room and given a huge ring of keys of all shapes and sizes. First, you’d try the keys that seem most likely to fit into the lock—maybe they have the same number on the door written on the handle, maybe the metal matches the lock. But when these don’t work, you have to start considering the keys that might look a bit funny or seem too bizarre to fit in the lock. After all, the only tools you have are the keys in your hand. This is what particle physicists are trying to do at colliders like the LHC. The amount of data we have access to is overwhelmingly big, and we had to start looking somewhere. But just because we’ve looked at some data doesn’t mean we’ve completely exhausted the possibility of finding a new particle in this data. It just means maybe we haven’t tried the right key.

In my next post, I’ll describe how a few particle theorists, including myself, performed an agnostic search for new physics in LHC data nearly a decade old. In our search we restricted ourselves to looking at one of the simplest triggers, and were able to look for new physics no one else had yet searched for.