W-W+ ->mu-/nu & mu+/nu?

or am i dreaming?

Posted February 24, 2015 3:33 AM

and as if to feed my confirmation bias, this was my next event:
http://talk.higgshunters.org/#/subjects/AHH0000s4b

😛

Posted February 24, 2015 3:49 AM

One part of the event selection requires a Z boson decaying to a positive and negative muon. So this collision does produce a boson but its a Z boson and not two W bosons.

Posted February 24, 2015 11:29 PM

by markbakovic

Even though the slice view shows each muon track to have z components of different sign? I was guessing that decay products of the same particle would have to exit the collision area in the same "beam direction". Is it that the parent particle can have eta >0 and if the detected particles track like this it just indicates that in this case it was quite low? (and the way to tell the difference between the two scenarios is with the full momentum data?)

Posted February 27, 2015 4:19 AM

by peterwatkins scientist

In the decays of very massive particles such as Z bosons the decay particles often have z components of momentum with opposite sign. If the Z boson is produced with very high momentum in + z direction then the two decaying particles will sometimes be boosted enough to both be in the +z direction but this is less common.

Posted February 28, 2015 10:49 PM

by markbakovic

interesting... I'm guessing that's due to how much energy got converted into the mass in the boson, leaving not much left over from the momentum of the collision? (and that the boson's momentum has contributions from both beams, so the instinctive "pool ball deflection" analogy just doesn't cut it when visualising what happens?)

Posted March 3, 2015 8:00 AM

by peterwatkins scientist in response to markbakovic's comment.

Yes thats right. In terms of the deflection analogy the two colliding protons have equal and opposite momentum. However the collision that produces the Z boson is between gluons (or quarks/antiquarks) inside the protons and these usually will have different momenta. So usually the Z boson is produced with some momentum in the +/- z direction.

Posted March 3, 2015 11:46 AM

by markbakovic in response to peterwatkins's comment.

Awesome, thanks very much for elaborating and humouring my curiosity, I appreciate it 😃

Posted March 4, 2015 8:43 AM

by andy.haas scientist

I took a closer look at the event. There is just 44 GeV of missing transverse momentum (MET). So while it could be W+W- with each decaying to muons (and neutrinos giving the MET), it is far far likelier to be Z->mumu+jet(s), where the jets failed to be found / reconstructed. A ~40 GeV jet is not so difficult to miss. Also, the two muons are required to have a total mass close to the Z mass, which enhances the likelihood of it being a Z even further. But you never know! There should be ~10 W+W- in this data somewhere...

Posted March 4, 2015 7:08 PM

by markbakovic

Ooh, cool, thanks for that Andy. Yeah if there's a criterion that the muons have to add up to ~91GeV then even just the transverse momenta wouldn't add up if you say the MET is 44 GeV, as that would require about ~60 GeV per muon of transverse momentum (given the geometry of their radial separation, or whatever the correct term for that is) alone by my arithmetic, so too much even if they had no z momentum at all... If the criterion were >= 91 GeV, that would be a different story I suppose and would gladly revel in having picked one of the 10 cases were it true, but until then I'm happy to stand thoroughly corrected and slightly the wiser 😃

Posted March 7, 2015 6:55 AM

by andy.haas scientist

Actually, it's not that the momenta of the muons must add up to 91 GeV... it's the "invariant mass" of the two muons that has to add up to something near the Z mass of 91 GeV. (The range selected is 70-110 GeV... due to quantum mechanics, the energy of each Z is uncertain, by an amount inversely proportional to its lifetime, which is pretty short!).

The formula for invariant mass is a little more complicated than just adding, and depends on the angle between things. So two muons can have large momentum (like in this case) but still a smaller mass, as long as the angle between them is small. See more here:
http://en.wikipedia.org/wiki/Invariant_mass

But my point was that selecting events where the muons form an invariant mass close to that of the Z serves to enhance the fraction of Z events (relative to other processes like WW, or top-antitop decays, etc.). That doesn't mean it can't be WW... those muons can happen to fall in the range with a mass near the Z by chance...

Posted March 7, 2015 6:16 PM

by markbakovic

Ah of course, I forgot about that! My oversimplified calculations were entirely overly simplified...

Posted March 8, 2015 9:08 AM