| Compact Muon Solenoid Experiment

Jet Physics

Standard Model Physics - Jets

Compact Muon Solenoid Experiment

Standard Model Physics - Jets

Our activities are within the Standard Model Physics (SMP) group.

We are presently addressing the following areas:

Inclusive Jet measurement at 13 TeV
Measurement of jets over the full range in |eta| < 4.7 and from pt > 100 GeV to the kinematic limit. This measurement is THE benchmark test for QCD. The cross section falls over many orders in magnitude as a function of pt. At present NLO QCD predictions describe the measurement reasonably well. The challenge lies in precision at very large pt, where electroweak effects play a role

The measurement is complemented with QCD analyses for better constraining the parton distributions at small and large momentum fractions x (together with using measurements from HERA) and a determination of the strong coupling alphas.
In addition to predictions from fixed NLO and NNLO QCD, comparisons with parton level predictions at NLO matched with parton showers are a challenge. Some studies indicate significant contributions from parton showers, more investigations are needed.

Measurement of inclusive B-jets, and ratio to inclusive jets.
This will give information on the heavy flavor treatment of jet cross sections as well as further constraints to the parton density, especially the gluon at small and large x.

Measurement of inclusive Top-jets, and ratio to inclusive jets.
This will give information on the heavy flavor treatment of jet cross sections as well as further constraints to the parton density. Intersting is a comparison at very large pt -- is QCD flavor blind ?

Measurements of Multi-jets
Multi-jet measurements over a wide range in |eta| and pt offer the challenge for detailed understanding of multi-parton radiation in QCD. The azimuthal correlations of two jets in dijet, tri-jet and 4-jet measurements are a direct probe of higher order QCD (and eventually also electro-weak) interactions at a rather inclusive level. At present, NLO QCD predictions described the measurements well, a challenges lies at high center-of-mass energies and higher jet multiplicities.
Future extensions:
The measurement will be first performed in a limited eta range, where the tracker allows to identify jets from pile-up. With an extension into the HF region, contributions from pile-up jets can play a role and a special treatment is needed. A proposal how to treat such a scenario is given in arXiv:1508.07811.

Measurement of Delta phi in high pt jets (pt > 3 TeV) in the back to back region of Delta phi ~ pi. This is the region, where fixed order calculations are not applicable and resummation of soft gluons to all orders has to be performed. The measurement will include a jet multiplicity of jets with pt > 30 GeV in addition to the high pt dijets: those jets are soft parton "mini-jets" (in comparison to the high pt jets)
Measurement of BBar + associated jets
BBar and associated jets will be measured for BBar jets within |eta| < 2.4 and the other jets within |eta| < 4.7. Of particular interest is the pt distribution of the BBar system in a mass region around the Z and Higgs mass. While BBar production is dominated by gluon fusion, Z production and its subsequent decay into BBar is coming from quark annihilation. The x-section of Z->BBar is more than one order of magnitude smaller than the QCD process, but differences in the pt spectrum might be visible.
With 2 jets in additon to the BBar jets, a high mass system can be defined and the evolution from the high mass dijets system towards the lower mass BBar system can be studied. In the extreme case, threshold resummation when m_BB -> M_jj can be studied.