What do particle physicists see when collisions happen?

What physicists see when watching particle collisions is basically nothing like this reproduction.

Remember when the Large Hadron Collider – that massive particle-smasher located deep underground in the pastoral Swiss countryside – first started up in 2008? Remember how it destroyed our entire universe by creating a black hole that swallowed us whole and gulped us straight into the apocalypse?

Or perhaps you don't remember that.


Perhaps what you're thinking of is that time the LHC started up in the wake of nonstop hype about how it might destroy the planet. But then, it started up, and you ate a turkey sandwich for lunch and got a parking ticket that day. The world, it seemed, continued.

So let's just get one thing out of the way before we dive into the exciting world of particle collisions: Just like that first day of the first beam was for the typical non-physicist, they're not that exciting.

Now, before you armchair physicists and real physicists get in a huff, let's acknowledge that, of course, particle collisions are exciting on a fundamental, universal level. Particle collisions are the physicists' equivalent of grabbing the universe and whapping it on the head, asking if this thing is on. By studying particle collisions, we can gauge not just what might've occurred right after the birth of our universe, but we can judge how primary pieces of matter function and interact.

In other words: It's a big deal.

And yet. In spite of all the talk about accelerating and smashing, about protons traveling at almost the speed of light, about collisions so monumental that people used to think they'd tear us all to ribbons ... what scientists really see bears no resemblance to the last 30 fiery, destructive minutes of your typical summer blockbuster. Not even when you take into account that there are 600 million collisions per second happening when the thing is on [source: CERN].

It's not just the anticlimax of all that end-of-the-world prattle not panning out. It's that what physicists see when protons collide turns out to be ... data.

To be fair, it's lots and lots of data. While it would be awesome if physicists were watching a screen that showed protons bursting like fireworks – lit up with labels like "muon!" or "Higgs!" to easily identify themselves – it's really numbers and graphical representations collected by the detectors that "show" physicists what happens during collisions.

Physicists are looking for many different pieces of data when studying particle collisions. That means that there isn't just one signal to watch – or even just one type of detector to gauge from. Instead, they rely on several different kinds of detectors to give them clues about what they're observing.

First, they're looking at where the particles produced in the proton collision are going. A tracking device can immediately let them know a few things like the charge of the particle (positive will bend one way, negative the other) or the momentum of the particle (high momentum goes in a straight line, low spirals tight). Now remember, they're not looking at the actual track of a particle. Instead, they're looking at the electrical signals a computer has recorded, which can be graphed into a reproduction of the path [source: CERN].

A tracking device won't pick up neutral particles, so they're identified in a calorimeter instead. A calorimeter measures the energy as particles are stopped and absorbed. Thy can tell physicists pretty specific things, since a certain kind of calorimeter measures electrons and photons, while another is on the case for protons and pions [source: CERN]. Radiation detection also measures the velocity of particles. Physicists study all these small identifiers to determine what happens to particles during and shortly after a collision.

All of these tools and the evidence they collect are what scientists are watching to determine what happened during a collision. After that, it's time to investigate any strange or significant results they come across. A good example of this was the discovery of the Higgs boson, a tiny particle that permeates the universe, adding mass to particles. Physicists studied the data sets from the collisions to see if the Higgs field would shoot off a spare particle (a Higgs boson) when two protons were smashed together. The idea was kind of like watching two streams of water snake through a sandy beach: Each stream on its own might run smoothly through the sand, but if they crashed together suddenly, a grain of sand could kick up.

That grain of sand wasn't a flash on the screen. Instead, it was carefully plotted data collected from numerous collisions. These numbers were, to a certain extent, mathematical probabilities. Other experiments determined where we needed to look when finding the mass equivalent (and thus existence) of the Higgs [source: Preuss].

Scientists also knew that if the Higgs existed, it had to act a few specific ways (like how it decayed into other particles). So when they saw an excess of events beyond what was predicted on a data plot, they got excited – and they could start judging whether the signal they were seeing in the data was something new [source: CERN]. In the case of the Higgs, it was.

So, nope – particle physicists don't get to see black holes or even mini-Big Bangs when collisions occur. What they see instead is evidence that certain particles blasted off during the smash, and data that indicate that what they saw was part of a larger predictable model – or if they're even luckier, a whole new path of discovery.


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Author's Note: What do particle physicists see when collisions happen?

While it'd be great to just see an amazing "collision" on screen and then watch a neon green particle appear that's never been spotted before, don't discount how exciting it must be for particle physicists in reality. Getting a whole bunch of data that points to something spectacular must be thrilling, in its own right, even if it doesn't mean a particle waving at you on the big screen.

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  • "The Safety of the LHC." CERN. (July 15, 2014) http://press.web.cern.ch/backgrounders/safety-lhc
  • Boyle, Alan. "Doomsday fears spark lawsuit over collider." NBC News. March 28, 2008. (July 15, 2014) http://www.nbcnews.com/id/23844529/#.U8W2qY1dVEd
  • Butterworth, Jon. "Even the most boring collisions at the Large Hadron Collider tell us something." The Guardian. Jan. 26, 2014. (July 15, 2014) http://www.theguardian.com/science/life-and-physics/2014/jan/26/even-the-most-boring-collisions-at-the-large-hadron-collider-tell-us-something-this-time-about-cosmic-rays
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