RE: Citizen Science Entry 4
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Thanks for this contribution and sorry for this late comment. My life is a bit hectic at the moment, as the deadline for a big grant proposal is approaching (and the application takes most my time). Typical academic life, somehow… ;)
Anyway, please find below a few comments. In short, this report is excellent and I will highlight it in the next episode (I will try to release it this week, but no promise). Congratulations for all the work done to understand what was going on, and your personal research allowing you to understand the dynamics of the process we studied.
I also found myself retyping commands in MadAnalysis5 quite a bit. As it turns out, scripts are available in MadAnalysis5! These scripts are plain-text files that contain a list of commands to execute in the interpreter.
That’s true. We can always start the programme with a script as you mentioned in the blog. I didn’t write it in the post but I probably should have. This will make it easier for the rest. I will introduce this in the next episode dealing with MadAnalysis5 (for the reasons mentioned above, some waiting time is in order).
What do the '% underflow' and '% overflow' mean? My guess was that these percentages indicate the amount of data that is not graphed.
That’s correct! Nice way to find out what they were, by the way.
Let’s now move on with the physics:
But does the b-quark inherit the momentum from the collision (and therefore from the top-quark)? And is there more momentum transfer along the vector of the collision beam?
As mentioned in the references you quoted, the (very large amount of) energy associated with each produced top quark is shared between its decay products. Some part goes to the W boson and some part to the b quark. Calculations tell us how the energy-momentum is shared in detail, and I won’t do them here (because it is not needed to qualitatively understand what is happening in our collisions). In most cases, the b quark gets a momentum larger than 25 GeV because the produced top quarks are so massive that we have a big reservoir of energy to be shared between the decay products (as you mentioned).
But it turns out plotting 'PT(b)' returns values on a per-jet basis (not all the energies of the b-jets summed together).
You can also type
plot PT(b[1]) …to plot the properties of the leading b jet, i.e. the one with the largest amount of transverse momentum.
The harder energy value to understand was MET itself (77 GeV). I clearly can't add the lepton, b-jet, and MET energy to get the energy of a single top-quark. The arithmetic doesn't work: 52 + 86 + 77 = 215 GeV. Whereas the top quark is 172 GeV.
You should not do this addition. As we plot distributions, we don’t know if a configuration corresponding to the peak of the lepton PT is that corresponding to the peak of the MET, and so on. We observe that the order of magnitude of the three peaks is similar, which is what is expected as we can see our top quarks as particles decaying into three other roughly massless particles. Consequently, the energy can be seen as split in three, in a first-order approximation.
While staring at the plot of top-quark decay it occurred to me that MET doesn't represent the energy of one neutrino, but two! The other top-quark decay also emits a neutrino! And that is also included in MET as total missing energy. So, dividing MET by 2 yields: 77/2 = 38. Then 52 + 86 + 38 = 176 GeV! That's very close to the known energy of a top quark (172 GeV). Maybe I just got lucky in the calculation, but that's my justification for MET.
Can you explain to me where the second neutrino would come from? I didn’t get this. I guess you just had luck here. As soon as we selected events containing a single lepton, then we have a single neutrino. A second neutrino must always come together with a second lepton. Note that it is possible that we have two leptons, and that one of the leptons is lost. This however does not represent the majority of our events.
Don't hesitate to come back to me if further clarifications are needed.
Cheers!
And thank you! I really do appreciate how much time and effort you put into reviewing and commenting on these posts. The effort is clear. Especially given you don't have a lot of time with your grant proposal deadline looming.
I hope the grant proposal goes well. :)
Yes! The energy associated with each top quark is shared with it's decay products. Great to know that the decay process itself is the primary avenue in which the byproducts are granted their energy. I was initially stuck in my thinking regarding the momentum of the protons in the beam.
Ahhh! You pointed out the magnitude of the peaks in your post. This explanation clears up my thinking on why the graph of MET is justified.
Yeah, I kept thinking about this after-the-fact and posted a comment on the second neutrino:
And you already answered the question I had regarding the neutrino only showing up alongside a second lepton :) In all, it came down to my unexamined thinking that the decay products of the top quark would be symmetrical. I realized my error upon digging deeper into the top quark decay products. It's clear, even in the top-quark decay products diagram I linked to, that a neutrino may not necessarily appear as a byproduct of top quark decay.