Final Report Summary - PRECISIONJETS4LHC (Precise Predictions for Higgs and New Physics Signals with Jets at the Large Hadron Collider)
Many of the Higgs and new physics searches at the Large Hadron Collider involve signals with jets of energetic hadrons. This project has improved the theoretical predictions for such processes, by focusing on three different aspects:
1) Exclusive N-jet cross sections at NNLL:
As backgrounds depend on the number of jets, LHC analyses are often separated into bins with a particular number of jets to improve the signal sensitivity. The goal was to calculate the effect of jet binning at next-to-next-to-leading logarithmic order. The study of the 0-jet bin has already been studied extensively. Moving beyond that requires treating correlations between the transverse momentum of the jets and the veto on additional jets. Such a framework was developed [JHEP 1502 (2015) 117] and a study of its implications for Higgs + 1 jet is currently ongoing. The expected significant reduction in the theoretical uncertainty will increase the precision with which the Higgs couplings can be extracted. Separately, the resummation of jet binning in supersymmetry searches has been studied, where the large uncertainty due to jet binning has not yet been taken into account.
2) Calculations of jet substructure:
Given the excellent performance of the ATLAS and CMS detectors at the LHC, there has been an expanding interest in exploiting the substructure of jets. Many jet substructure studies rely solely on Monte Carlo simulations. The goal was to improve their description by using factorization and resummation to achieve higher precision. This has been carried out for a range of jet observables:
• Jet charge: The potential of jet charge at the LHC was studied and a method to calculate it was developed [Phys. Rev. Lett. 110 (2013) 212001], prompting studies by the ATLAS and CMS collaborations.
• Jet mass: A next-to-next-to-leading order calculation and preliminary comparison to ATLAS measurements was carried out [Phys. Rev. D 88, 054031 (2013)]. A study of the effect of hadronization and multiple parton interactions on jet mass, employing factorization rather than some specific model, providing a new way to experimentally disentangle these nonperturbative effects [Phys. Rev. Lett. 114, 092001 (2015)].
• Jet angularities: A next-to-leading logarithmic calculation of the simultaneous measurement of two generalized jet angularities was carried out, in order to determine their combined quark/gluon discrimination power [JHEP 1411 (2014) 129]. On comparing with the Pythia8 and Herwig++ Monte Carlo programs significant differences were found between all three, requiring further study.
3) Collinear radiation in jets at NNLO and jet algorithms:
The goal was to study the effect of jet algorithms on the collinear radiation in jets through a next-to-next-to-leading order calculation. In an important first step, a relationship with the splitting functions of Catani and Grazzini was established, and exploited to reproduce the known inclusive jet function (i.e. without a jet algorithm) and for the first time calculate the fragmenting jet function (describing fragmentation in a jet) at next-to-next-to-leading order [Phys. Rev. D 90, 054029 (2014)]. The determination of jet algorithm effects is still ongoing, but is challenging due to the complicated nature of the phase-space restrictions and various divergences.
1) Exclusive N-jet cross sections at NNLL:
As backgrounds depend on the number of jets, LHC analyses are often separated into bins with a particular number of jets to improve the signal sensitivity. The goal was to calculate the effect of jet binning at next-to-next-to-leading logarithmic order. The study of the 0-jet bin has already been studied extensively. Moving beyond that requires treating correlations between the transverse momentum of the jets and the veto on additional jets. Such a framework was developed [JHEP 1502 (2015) 117] and a study of its implications for Higgs + 1 jet is currently ongoing. The expected significant reduction in the theoretical uncertainty will increase the precision with which the Higgs couplings can be extracted. Separately, the resummation of jet binning in supersymmetry searches has been studied, where the large uncertainty due to jet binning has not yet been taken into account.
2) Calculations of jet substructure:
Given the excellent performance of the ATLAS and CMS detectors at the LHC, there has been an expanding interest in exploiting the substructure of jets. Many jet substructure studies rely solely on Monte Carlo simulations. The goal was to improve their description by using factorization and resummation to achieve higher precision. This has been carried out for a range of jet observables:
• Jet charge: The potential of jet charge at the LHC was studied and a method to calculate it was developed [Phys. Rev. Lett. 110 (2013) 212001], prompting studies by the ATLAS and CMS collaborations.
• Jet mass: A next-to-next-to-leading order calculation and preliminary comparison to ATLAS measurements was carried out [Phys. Rev. D 88, 054031 (2013)]. A study of the effect of hadronization and multiple parton interactions on jet mass, employing factorization rather than some specific model, providing a new way to experimentally disentangle these nonperturbative effects [Phys. Rev. Lett. 114, 092001 (2015)].
• Jet angularities: A next-to-leading logarithmic calculation of the simultaneous measurement of two generalized jet angularities was carried out, in order to determine their combined quark/gluon discrimination power [JHEP 1411 (2014) 129]. On comparing with the Pythia8 and Herwig++ Monte Carlo programs significant differences were found between all three, requiring further study.
3) Collinear radiation in jets at NNLO and jet algorithms:
The goal was to study the effect of jet algorithms on the collinear radiation in jets through a next-to-next-to-leading order calculation. In an important first step, a relationship with the splitting functions of Catani and Grazzini was established, and exploited to reproduce the known inclusive jet function (i.e. without a jet algorithm) and for the first time calculate the fragmenting jet function (describing fragmentation in a jet) at next-to-next-to-leading order [Phys. Rev. D 90, 054029 (2014)]. The determination of jet algorithm effects is still ongoing, but is challenging due to the complicated nature of the phase-space restrictions and various divergences.