Tuesday, August 9, 2016

New Particle at the Large Hadron Collider: It's Dead, Jim

For the past few months, the world of particle physics has been holding its breath.  The experimental groups at the Large Hadron Collider had reported possible evidence of a completely unexpected new particle with about 750 times the mass of the proton, and everyone was waiting for new data to either confirm or rule out this discovery. In the meantime, theorists churned out hundreds of papers explaining this particle. (I make no claim to moral superiority here -- I didn't write anything on this subject only because it's outside of my specialty). Well, the new data has come in, and the verdict is.... no such particle.

The Large Hadron Collider. Nothing to see here. Please move along.
Which leads to an interesting question: when was the last time that a completely unexpected new particle was discovered at an accelerator? The answer is shocking.

Here's a list of major discoveries, in reverse order:

2012:  The Higgs boson. This won a well-deserved Nobel Prize a year later. But it was certainly not a "surprise." The Higgs boson was the last missing piece of the standard model of particle physics.  It would have been more surprising if it hadn't been discovered.

1995:  The top quark.  Matter is composed of 6 particles, called quarks, and this was the last one to be discovered.  It was unexpectedly massive, but certainly no surprise -- everyone was confident that it was out there somewhere.

1983:  The W and Z bosons. These were certainly a major discovery, but they had been predicted earlier to explain the weak interaction, which is responsible for nuclear beta decay. So I would not call their discovery unexpected.

1977:  Discovery of the bottom quark.  There were good reasons to think the bottom quark existed, so no surprise here.

1975:  And here's our most recent surprise at a particle accelerator:  the tau particle. The particles we observe seem to be duplicated in "families" of particles, but until this discovery, it looked like there were only two families. The tau was the first evidence for a third family (which was then completed with the aforementioned discoveries of the bottom and top quarks).

That's right, the most recent unexpected particle to turn up at an accelerator was in 1975.  In retrospect, my high school years (1973-77) were something of a golden age for particle physics. I remember reading about these discoveries in the pages of Scientific American and thinking that this was just the normal progress of science. Little did I know that I wouldn't see the likes of it again for 40 years. I certainly haven't given up hope that the Large Hadron Collider will turn up something totally unexpected. I just hope they do it before I retire. Or die.


Kathy said...

I suppose the dearth of unexpected particles points to how robust the theory behind the Standard Model is.

But it leads to an inevitable question: are there no new particles left to find? And the corollary, can we know for sure there aren't?

Interesting times.

Robert Scherrer said...

The "nightmare scenario" (yes, it's really called that) is one in which the Large Hadron Collider simply confirms the Standard Model of particle physics by finding the Higgs boson and nothing else. Unfortunately, we seem to be drifting in that direction. Of course, we can never prove that there aren't more particles out there -- they might just be at energies too high for us to produce in a terrestrial accelerator. And astrophysics provides strong evidence for physics beyond the Standard Model, in the form of dark matter.

Kathy said...

I can see that. The Model explains a lot, but also leaves a lot unexplained (I assume). Ergo let's find what else is lurking in the subatomic wilderness. If nothing is found, then what the hell do we do next?

BTW, so much for all of Trek's mythical particles...

I use this analogy a lot, but it seems to fit. This reminds me of the hunt for the planet Vulcan in the late XIX Century. The answers aren't where we know how to find them. And remember that search dragged on for decades until Einstein figured out General Relativity. It may be we're in for another revolution in physics.

As I said, interesting times.