At high center-of-mass energy, √s, semi-hard parton interactions with transverse momenta pT much smaller than √s may start to play a substantial role. This high-energy region is supposed to be more appropriately described by the BFKL evolution equations. Events with at least two jets where the most forward and most backward jets have similar low-pT and are widely separated in rapidity (Mueller-Navelet (MN) dijet events) are sensitive to BFKL dynamics for which a stronger rise of the inclusive dijet cross section is expected with increasing rapidity separation between the jets. The azimuthal angle decorrelation of MN dijets is therefore expected to show effects beyond the DGLAP description. The reconstruction of jets in the rapidity region covered by the CASTOR calorimeter, −6.6 < η < −5.2, allows to increase the pseudorapidity separation between the jets by more than 1 unit and to measure jets with transverse momentum pT as low as 3 GeV. A measurement of jets reconstructed in the CASTOR calorimeter is presented to illustrate that the reach of such a kinematic region is a reality at CMS.

The CASTOR reconstruction software provides the following basic objects: the energy per channel and the total energy in each of the 16 φ-sectors, so-called CASTOR towers. Jets in the CASTOR acceptance are reconstructed with the anti-kT jet clustering algorithm with a distance parameter R = 0.7, applied to the collection of CASTOR towers. As the calorimeter has no segmentation in η, all towers are assigned to the same pseudorapidity value η = −5.9, and the jet reconstruction algorithm reduces to a 1-dimensional clustering in φ. As a consequence of the absence of segmentation in η, only the energy spectrum can be measured for jets reconstructed in CASTOR.

The normalized distributions of the jet multiplicity and jet energy spectrum are presented in the figure for jets reconstructed in the CASTOR acceptance, −6.6 < η < −5.2, with an energy E > 500 GeV. The data are compared to the predictions of the Monte Carlo (MC) event generator PYTHIA8 with tune 4C. The error band represents the 22% uncertainty on the absolute energy scale of the CASTOR calorimeter.