============================================ March 7th 2018 - PDG chap. 42 selections - ============================================ Cheyenne chairs and Ben attends to guide discussion. ------------ Topics: - Collinear singularities - Resources for QCD - Color coherence - Generator differences - Dicussion about JC ----------- Details: - We start off with the 'Massive quarks' section - Collinear enhancement and singularities. Some discussion as to what that means. Useful link here: https://www.nikhef.nl/~h24/qcdcourse/section-7.pdf - Ben: if you have a particle radiating something you have a collinear enhancement, and soft enhancement, and obviously nothing divergesso how do you account for that in the theory? They are both cutoff. In QCD the cutoff for quarks is low mass. This also leads to the 'Dead cone' effect: because collinear singularity is cutoff by the mass of the top quark. - Patrick: Aside from Peskin & Schroeder, which deals with IR and collinear effects well, can Ben recommend a good QCD book? Ben: yes, the Pink book, i.e QCD and Collider Physics by Ellis, Stirling, and Webber. Link to the eBook (via cern login), here: https://www-cambridge-org.ezproxy.cern.ch/core/books/qcd-and-collider-physics/D0095E6D278BBBC74E9C3636AB4CB80C - Ben: this effect is even more important for the 100 TeV collider, imagine that you're now at the mass scale where you can radiate W and Z bosons? Here is a link to the paper Ben is talking about: https://arxiv.org/pdf/1407.2607.pdf - Next section: colour information. - Ben explains color coherence with his diagrams. See his thesis (page 201) for an example. - What is the leading color (LC) approximation? Over time I generate blue and anti-blue quarks in a simulation that don't interact. That's the approximation and it works well at lepton colliders but not so well at hadron colliders. - Pythia models this by setting Nc to ~O(1000) and just creating loads of colors that don't interact. - The top mass measurement is sensitive to this being mismodelled but not much else. Colour reconnection is done to account for this in pythia. - It's importnat to note that this color reconnection does happen in real events. e.g. gluons with an off-shell mass. - What's the real difference between pythia and sherpa? Ben: sherpa and herwig have made very similar, basically the same, physics choices. First though, what does time mean in parton showers? Time is equivalent to hardness - i.e. how long it takes to go from the hard-scale to the hadron-scale. Time is done with pT in Pythia and kT in Herwig. Formally, at the LO approximation, they're both making the same thing but the differences do manifest in the end. - Ben: Also, how do you turn your partons into hadrons? Sherpa uses a cluster model for this. Herwig is acutally the generator that did this first. - The evolution in a generator, is perturbative, but the hadronization is not (and this will be our topic for next week!). - Why does noone use Sherpa for showering? Sherpa is a black-box, you can't break it down into modular tasks. Sherpa has very few parameters to tune which isn't so useful. But, sherpa people did spend a lot of time working on their matrix elements, so they're calculated to a very high order. - Ben suggests the local theorist, Christian Bauer, is a good resource as he has made his own generator, called Geneva. - Rebecca: How do we like where this session went? I didn't realsie PDG was so sparse on this chapter, sorry. Next time we should have a more focused discussion. (There is general agreement and Ben & Greg suggest hadronization parameters in MC). Also, how do people feel about this divergence from our usual theme of reading analysis papers? People are generally happy with it, so we will proceed like this for the next 3 sessions and then go back to analysis. *********************************************** NEXT WEEK: Greg chairs, topic is hadronization Specifically, understanding the parameters of the clusters vs. strings model, and taking a look at the different generator documentation (where possible!!) to see how they parameterize. *********************************************** ---------- CliffNotes: - collinear singularities: singularities that can lead to divergent perturbative cross-sections arise from interactions that happen a long time after the creation of a quark-antiquark pair. - soft/collinear singularities arise from interactions that happen a long time after the creation of the quarkantiquark pair and that perturbation theory cannot handle this long-time physics. - We need measurements that are insensitive to longtime physics: infrared safe observables. - We have seen that soft/collinear singularities appear when 2->3 kinematics reduces to 2->2 kinematics at the boundaries of phase space. - Therefore a meaningful infrared safe observable must be insensitive to the indistinguishable 2->2, 2->3 origin of the long-distance interactions: https://www.nikhef.nl/~h24/qcdcourse/section-7.pdf - Quark masses act as a cut-off for collinear singularities. - if quark mass is low, its effect on a shower is small - for large quarks like charms, bottoms or top: the mass cuts off collinear radiation - so there is less collinear activity for heavy quarks than light ones - Her wig and Sherpa use a splitting Kernel by Altirelli-Parisi: https://npac2013.lal.in2p3.fr/2010-2011/Cours/Exam-IF/Altarelli-Parisi.pdf - Pythia uses a matrix-correction method, which includes mass effects - Previous parton shower algorithms simply limited the angle ==== Color info ===== - Angular-ordered showers: the angle of color-connected parsons determines the initial angle for shower development. - Dipole showers: dipoles are always color-connected partons. What are dipole showers?