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April 4th 2018 - Journal club meeting minutes
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New paper: "Measurement of the inclusive 3-jet production differential cross section in proton–proton collisions at 7 TeV and determination of the strong coupling constant in the TeV range"
Rebecca chairs.
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Topics:
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The methodology in this paper, even the concepts, were very basic so this paper was finished quite fast.
Included below are the discussion points.
* Establish what we’re measuring: the production cross-section of a QCD process.
We then compare that measured cross-section to the calculated one, changing the value of alpha-s until it minimizes the chi-squared.
* Why 3 jets? Awe’ve seen measurements like this in the past that use the ratios of cross-sections to eliminate statistical uncertainties
and a popular one is the R32 ratio that divides the 3 and 2-jet production cross-sections.
However, this is only proportional to alpha at LO and so is sensitive to experimental uncertainties.
Presumably it gets prohibitively hard to calculate the multi-jet multiplicity spectrum for higher-order jets.
* Particle flow: CMS can do this and we can’t, so presumably they have better JES factors than us?
* Selection: we have these single-jet triggers with varying bands of pT - the important thing is that they are mutually exclusive,
so if you get a hit in two of them, it’s from 2? That doesn’t make sense.. discuss?
* JET ID: They make sure each jet has at least 2 particles, one of which is charged, so it can be correctly matched to the PV with tracks.
* JetEnergyScale (JES): seems to be a calibration factor. Why is it so high? I talked to Ben about this.
JES is calculated from a. Bunch of different techniques and the dominating factor is ‘in-situ’ uncertainty.
When you actually look at the nuisance parameters that make up this in-situ uncertainty it all comes back down to modeling again - which cannot easily be reduced.
* Not much to say about modeling: other than that they use the Lund string model we talked about a few weeks ago.
* World average: computed from 6 steps:
* 1. Comparing to lattice QCD calculations
* 2. Ratio of hadronic to leptonic tau decaus
* 3. Analyses of Proton Structure functions
* 4. LEP event ‘shapes’ and rates
* 5. Jet cross-sections, top cross-sectios
* Anything notable in the plots? Cesar notes that Figure 3, the theory uncertainties are smaller in the positive than the negative: why? It’s not clear.
Other topics:
* Why is the Z-mass chosen as the scale to measure the strong coupling constant at?
The general consensus is that it’s not too small at this mass and it’s also a mass scale that was attainable by colliders in the past.
* Is the inclusive 3-jets exactly 3 or 3 or more?
We determined it was 3 or more, otherwise their specification about leading pT jets and picking the top 3 wouldn’t make sense.
* We talk about the energy and position resolution of CMS and ATLAS and Neha tells us they are very different designs but effectively similar.
Emily notes that CMS is better at measuring the charge-flip in same-sign WW.
* Neha: Table 2 - the mass of the top is sometimes listed as 10^10! And the references they point to do not explain this number.
Marjorie says the reason for this is they’re not including the top mass in the loop calculation by having such a large mass.
When you run over the loop, top quark gives you a small contribution because of its large mass, so this table essentially says its just not being used in the calculation.