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Zachary Marshall | USLHC | USA

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Rediscovery! (kinda)

You’ve heard from us a few times that the first thing ATLAS (or CMS) will have to do is “rediscover” the physics that we’re pretty sure is there. ATLAS made a big step in that direction this week with the identification of the first W boson candidate events:

First candidate W boson decaying to a muon and neutrino

First candidate W boson decaying to a muon and neutrino

First W boson candidate decaying to an electron and neutrino

First W boson candidate decaying to an electron and neutrino

This is a pretty big deal for us. I think Flip will tell you more about the weak force in his next blog, but here’s the very quick version. A W boson can decay to an electron and a neutrino, or a muon and a neutrino, among other things (we have one of each!!). The electrons are those things that orbit a nucleus in an atom. A muon is, basically, a cosmic ray. They go through your body constantly (probably several thousand will have gone through you by the time you finish reading this sentence), and are one of the things that help evolution by kicking around your DNA. Neutrinos can’t be detected by ATLAS or CMS. They fly right out of the detector, completely unnoticed. Actually, they keep flying off into space. Neutrinos are produced by the billions in the sun, and several million will go through your body as you read this. They don’t do any harm – they basically do not interact with your body at all.

So what does a W boson look like?? We don’t see it directly – it decays too fast. We see the electron or muon in our detector, and we can measure that thing’s momentum. We see most of the other ‘stuff’ in the event, and add it all up. Once we’ve done that, the event doesn’t balance. The next trick comes from Newton: for every action there is an equal and opposite reaction. The protons come into ATLAS going East-West. If something comes out going North, then there must be something that comes out going South as well! That’s how we can “see” the neutrino. We look for what’s missing when we add up the rest of the event.

In both of these events, there is some missing piece (the red dotted line) and an electron (in yellow) or muon (in red). We know that’s what the W boson looks like – they’ve been seen many times at LEP and the Tevatron. So if we guess that the missing piece is a neutrino, and that the neutrino and electron/muon came from the same particle, we can check what the mass of the particle was. And if that mass comes out close to the mass of the W boson, then we can say that this was probably an event with a W boson in it. It could have been something else – we can’t be positive – which is why we call it a “candidate.”

Why is this a big deal? Well, we only expect one W boson for every million events!! So that we managed to pick this up so quickly is a great sign for the way our detector is behaving!! At the very least, it’s a great first step!

The next thing on the list is the Z boson (I’ll leave that to Flip), and then the top quark after that. And probably in the meantime we’ll find some good high energy “jets” (quarks and gluons). Once we have all of those down (or maybe even a little before), you may start to see limits – or even discovery – of new physics!!!

–Zach

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