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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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5 Responses to “Rediscovery! (kinda)”

  1. Abhishek says:

    Finding a candidate named W boson is exciting.According to article ,it is decided by momentom of electrons and neutrinos.Are there telescope like instruments like “fermi telescope” to see these events directly and in a better way.

  2. Zachary Marshall says:

    Hi Abhishek! It is very hard to detect neutrinos. There are experiments to do so, like IceCube, CHORUS, Super-Kamiokande, etc. They use the neutrino as an *incoming* particle, and let it produce (via a W-boson) an electron or a muon.

    Their rate, though, is only a few dozen events per year. Neutrinos really interact very rarely! So they do not give as precise a measurement of the W as we do. Even though ours is not as well-constrained a system, we will record a million times more W-boson events than they will in the next year.

  3. nouman shams says:

    Hi,
    Nice article,
    I have a daft question for anyone to answer; I heard that particles are travelling with 99.99 % of speed of light in LHC so according to Einstein’s theory if you travel that fast you live longer (slowness of time) so are you guys really see this kind of behaviour in particles?

    Sorry for inconvenience in advance

    Ta

    Nouman Shams

  4. Zachary Marshall says:

    Hi Nouman,

    Yep, we definitely do see that behavior!! Muons, in fact, have a very short lifetime. We see many of them at the surface of the earth, even though they are produced in the upper atmosphere. The only good explanation for the number observed is special relativity!

    We also can see some particles in our detector that have very short lifetimes but travel some measurable distance because of relativity (and their very high speed).

    Zach

  5. jonathan says:

    Hi Zach, Just wanted to let you know how much I appreciate you taking time to make such nice & readable articles. It really helps me understand and appreciate this fascinating experiment.

    Jonathan

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