Taming Unphysical Scales for Physical Predictions

Emmanuel College, Cambridge

Emmanuel College, Cambridge

University of Cambridge, Cambridge, UK

Workshop on renormalisation and factorisation scale choices for precision predictions of cross sections at the LHC


  • Perturbative predictions of any LHC observable depend on factorization and renormalization scales
  • Scale choices are important in any kinematics but will be critical for describing still-unexplored regimes like:
    • boosted regime/particle productions at high pT 
    • production and decay of high mass particles
    • jet physics, especially for jets at large rapidity
  • Guessing the right scale for a process is not an easy task
  • Is there such a thing as correct scale choice?
    • Or are there only more-or-less smart choices?
    • There are definitely very wrong choices, which lead to an unstable perturbative behavior


The convening of this meeting has been compelled by a convergence of new LHC-related developments:

  • The calculation of almost all major two-to-two processes at NNLO
  • NLO calculations now allow detailed access to even the most subtle effects
  • Mature LO and NLO merging techniques that have been very successful in bridging various multiplicities
  • Resummation at NNLL is turning into a bread-and-butter phenomenology

These new developments would make it possible to quantify the impact of various scale-setting procedures; this was not possible in the past.

Workshop goals:

  • To draw attention to the last missing piece in the precision physics puzzle: the choice of unphysical factorisation and renormalisation scales
  • Bringing together leading researchers in order to debate ways of setting scales
  • Be the first step towards harmonised scale-setting accord across LHC processes and for the benefit of precision LHC phenomenology

Related Workshop:

The Workshop will immediately follow the annual Workshop on Parton Showers and Resummation PSR2017. Both Workshops will be held in the same venue. Participants may wish to take advantage of this opportunity and attend both meetings. More information about PSR2017 is available here: https://indico.cern.ch/e/PSR2017.


  • Michal Czakon (RWTH Aachen University)
  • Joey Huston (Michigan State University)
  • Alexander Mitov (Cambridge University)
  • Bryan Webber (Cambridge University)
  • Malgorzata Worek (RWTH Aachen University)
  • Thursday, March 30
    • Morning Session: Jets + Top Quark
      • 1
        Speaker: Alexander Mitov (University of Cambridge (GB))
      • 2
        Scale choices, uncertainties and correlations for jet observables
        Speaker: Bogdan Malaescu (Centre National de la Recherche Scientifique (FR))
      • 10:30 AM
        Discussion 20'
      • 3
        Scales for single jet inclusive production at the LHC through NNLO
        Speaker: James Currie
      • 11:15 AM
        Discussion 20'
      • 4
        Scales for differential top-quark pair production through NNLO
        Speaker: Alexander Dimitrov Mitov (CERN)
      • 12:00 PM
        Discussion 20'
    • Lunch
    • Afternoon Session I: H/W/Z + jets
      • 5
        Scales for top quark production with a jet including decays and off-shell effects through NLO
        Speaker: Malgorzata Maria Worek (Rheinisch-Westfaelische Tech. Hoch. (DE))
      • 2:05 PM
        Discussion 20'
      • 6
        Lessons from Measurements of W/Z/H+jets production
        Speaker: Prof. Joey Huston (Michigan State University (US))
      • 2:55 PM
        Discussion 20'
      • 7
        Scales for Z/H in association with a jet through NNLO
        Speaker: Nigel Glover (Durham University)
      • 3:40 PM
        Discussion 20'
    • Coffee Break
    • Afternoon Session II: Alternative methods to scale setting
    • Reception
    • Conference dinner
    • Morning Session I: PDF’s and ratios
      • 11
        Scale variation and PDF uncertainties
        Speaker: Stefano Forte (Università degli Studi e INFN Milano (IT))
      • 9:20 AM
        Discussion 20'
      • 12
        Scale variation and scale error determination in ratios
        Speaker: Fernando Febres Cordero (University of Freiburg)
      • 10:00 AM
        Discussion 20'
    • Coffee break
    • Morning Session II: Ratios, NLO Merging and Resummation
      • 11:20 AM
        Discussion 20'
      • 14
        Scale variation for the ratio of W/Z pT's
        Speaker: Jonas Lindert
      • 12:00 PM
        Discussion 20'
    • Lunch
    • Afternoon Session: Resummation and Parton Showers
      • 15
        Scales for merging of jet multiplicities in top, H and W production
        Speakers: Stefan Hoche (SLAC National Accelerator Laboratory (US)), Stefan Hoeche (SLAC)
      • 1:55 PM
        Discussion 20'
      • 16
        Profile scales in the context of resummed distributions
        Speaker: Frank Tackmann (Deutsches Elektronen-Synchrotron (DE))
      • 2:35 PM
        Discussion 20'
      • 2:55 PM
        Coffee break 10'
      • 17
        Scales in SCET
        Speaker: Matthew Schwartz
      • 3:30 PM
        Discussion 20'
    • Closing discussions
      • 3:50 PM
        Workshop closing discussion 30'