I am performing undergraduate research in particle physics.
In our research, we are using two event generators (PYTHIA8 and MC@NLO with HERWIG++) to study open bottom production.
I am unsure of the difference between leading-order event generators and next-to-leading-order event generators. A graduate student I am working with told me that the difference is that NLO event generators include the next-to-highest-order term in its calculations.
Is this limited to only the parton distribution functions (PDF's) or is there more to it?
Best Answer
It is not limited to the Parton distribution functions (pdfs) only. The difference between LO and NLO event generators is that at the hard scattering level the formers use tree level matrix elements while the latter use one loop matrix elements*. The utility is that when you generate events with NLO matrix elements you improve the precision by lowering the dependencies on the renormalisation and factorization scales. You can also estimate higher orders effects from the NLO cross section uncertainties...etc *just to explain we say LO is the lowest order for a process to occur. So LO might be one loop level. Think for example about Higgs production in gluon gluon fusion which cannot occur at tree level. When you talk about NLO (in strong coupling for example) you should include real emission diagrams and define a subtraction method to remove soft and collinear singularities.