• CMS
• gluon splitting
• inclusive b production
• jet clustering
• LHC

Several high-mass particles decay into b-quarks. Top quarks always do so, while the Higgs boson is expected to decay into b-quarks more than into any other particles. Distinguishing b-quarks from the QCD background helps to identify the decays of these particles.

At the CERN Large Hadron Collider (LHC) b-quarks are abundantly produced by strong interactions in pp collisions. The hadroproduction of bb̄ pairs is measured to have a large cross section (of the order of 1 mb). In heavy particles studies, the bb̄ pairs produced by QCD cause large backgrounds in wide regions of the phase space. The predictions of those backgrounds have theoretical limits because QCD is a non-perturbative theory and has very large errors in regions where b-quarks are produced with a small angle between them.

The aim of this thesis is the measurement of the momenta and directions of b-quarks in events in which there is one bb̄ pair. There are several variables of the bb̄ pair system that can provide information about non-perturbative bb̄ production. It has been chosen to provide a measurement of the event distribution in the angular distance between the B hadrons and in the transverse momentum (pT) of the bb̄ pair system.

When the b-quarks are produced with small angular separation they are reconstructed as a single jet because the typical angular dimension of a jet is ∆R = 0.4. Identifying the presence of b-quarks using the B hadron decay secondary vertices gives the possibility to detect events where b-quarks are produced at small angular separation. However, it is not possible to measure the transverse momentum of the b-quark from the charged particles contributing to the B hadron vertex since they carry only a fraction of the energy of the quarks.

The energy is measured by merging the information of secondary vertices and jets. The vertices are used as seeds for the construction of the jets and with a new clustering procedure it is possible to assign the jet energy to the two vertices also when b-quarks are produced with a small angular separation.
Several jet triggers have been used in this analysis in order to measure the cross section as a function of the pT of the bb̄ pair in a large pT range. ZeroBias data are also used to cover the very small pT region.

An unfolding procedure with D’Agostini method has been used to remove detector effects from the measured distributions.

The study has been carried out on 2.7/fb of proton-proton collisions recorded by CMS at a centre of mass energy of 13 TeV during 2015. The results of this analysis show that measurements and prediction are compatible in the perturbative region while less agreement is present in the non-perturbative region.