588
K. Kleinknecht and U. Uwer
selection is performed by a trigger system consisting of a hardware stage, based
on information from the calorimeter and the muon systems, followed by a software
stage, which applies full event reconstruction.
The displacement of the B decay vertex from the primary interaction vertex
is an important identification requirement of B events. However, due to the large
boost in forward direction, the association of b hadrons to their production vertex is
challenging, in particular if the number of primary vertices is large. In order to keep
the average number of primary proton-proton interactions per bunch crossing at the
most optimal value (below 2) the instantaneous luminosity for the LHCb interaction
point was leveled by displacing the centres of the colliding beams slightly. Until
2018 LHCb has recorded data corresponding to an integrated luminosity of 3 fb
−1
at 7 and 8 TeV, and 6 fb
−1 at 13 TeV proton-proton centre-of-mass energy. This
data sample corresponds to more than 1 × 10 12 b ¯
b events produced inside the LHCb
acceptance.
9.6.3 Measurements of B Oscillations
Mixing in the B 0 −B 0 system was discovered in 1987 by the ARGUS collaboration [80] and the first time-integrated determinations of the mixing parameter x d
have been performed by the ARGUS and the CLEO collaborations [81, 82]. Timedependent measurements of the mixing frequency d became possible at the
electron-positron collider LEP. The silicon vertex detectors and the large boost of
the B mesons produced at the Z resonance allowed to observe the B 0 oscillations.
With their high statistics and low background B samples the BABAR and Belle
experiments improved the errors on the mixing frequency d significantly
[83, 84]. Exploiting the excellent time resolution as well as the large statistics
of recorded B mesons the LHCb experiment performed the so far most precise
measurement of d [88].
After the first B 0 mixing measurement it was clear that mixing was an important
effect also in the B 0
s −B 0
s system and theory predicted a much faster oscillation of the
B 0
s meson. While limits on x s existed from LEP and the two Tevatron experiments it
took until 2006 that the CDF collaboration resolved the fast mixing and performed
the first measurement of the mixing frequency [85]. The measurement was repeated
with much smaller uncertainties by the LHCb collaboration in 2012 [86].
The oscillation frequency of neutral B mesons is measured using flavor-specific
final states, i.e. final states like B 0 → D (∗)− + ν or B 0 → D − π + where the charge
of one of the final-state particles (e.g. the lepton or pion charge) indicate the flavor
of the decaying signal B 0 (e.g. ¯
b → ¯
cc + ν ). In case of incoherent production of the
two b hadrons, a flavor-specific decay of the second B (B tag ) is used to tag the flavor
at the time t = 0 of the production. For the coherent production at the B factories the
tagging B defines the flavor of the signal B at time t with respect to its decay—see
Fig. 9.24 and Eq. (9.201). B-factory experiments have also used explicitly so called
K. Kleinknecht and U. Uwer
selection is performed by a trigger system consisting of a hardware stage, based
on information from the calorimeter and the muon systems, followed by a software
stage, which applies full event reconstruction.
The displacement of the B decay vertex from the primary interaction vertex
is an important identification requirement of B events. However, due to the large
boost in forward direction, the association of b hadrons to their production vertex is
challenging, in particular if the number of primary vertices is large. In order to keep
the average number of primary proton-proton interactions per bunch crossing at the
most optimal value (below 2) the instantaneous luminosity for the LHCb interaction
point was leveled by displacing the centres of the colliding beams slightly. Until
2018 LHCb has recorded data corresponding to an integrated luminosity of 3 fb
−1
at 7 and 8 TeV, and 6 fb
−1 at 13 TeV proton-proton centre-of-mass energy. This
data sample corresponds to more than 1 × 10 12 b ¯
b events produced inside the LHCb
acceptance.
9.6.3 Measurements of B Oscillations
Mixing in the B 0 −B 0 system was discovered in 1987 by the ARGUS collaboration [80] and the first time-integrated determinations of the mixing parameter x d
have been performed by the ARGUS and the CLEO collaborations [81, 82]. Timedependent measurements of the mixing frequency d became possible at the
electron-positron collider LEP. The silicon vertex detectors and the large boost of
the B mesons produced at the Z resonance allowed to observe the B 0 oscillations.
With their high statistics and low background B samples the BABAR and Belle
experiments improved the errors on the mixing frequency d significantly
[83, 84]. Exploiting the excellent time resolution as well as the large statistics
of recorded B mesons the LHCb experiment performed the so far most precise
measurement of d [88].
After the first B 0 mixing measurement it was clear that mixing was an important
effect also in the B 0
s −B 0
s system and theory predicted a much faster oscillation of the
B 0
s meson. While limits on x s existed from LEP and the two Tevatron experiments it
took until 2006 that the CDF collaboration resolved the fast mixing and performed
the first measurement of the mixing frequency [85]. The measurement was repeated
with much smaller uncertainties by the LHCb collaboration in 2012 [86].
The oscillation frequency of neutral B mesons is measured using flavor-specific
final states, i.e. final states like B 0 → D (∗)− + ν or B 0 → D − π + where the charge
of one of the final-state particles (e.g. the lepton or pion charge) indicate the flavor
of the decaying signal B 0 (e.g. ¯
b → ¯
cc + ν ). In case of incoherent production of the
two b hadrons, a flavor-specific decay of the second B (B tag ) is used to tag the flavor
at the time t = 0 of the production. For the coherent production at the B factories the
tagging B defines the flavor of the signal B at time t with respect to its decay—see
Fig. 9.24 and Eq. (9.201). B-factory experiments have also used explicitly so called