586
K. Kleinknecht and U. Uwer
Cherenkov counter (ACC). Photon and electron energies were measured in the
electromagnetic calorimeter (ECL) consisting of thallium-doped CsI crystals. The
energy resolution varied from 4% at 100 MeV to about 1.6% at 8 GeV [71]. All
detector components were placed inside a super-conducting solenoid magnet of
cylindrical shape which was providing a magnetic field of 1.5 T. The iron return
yoke is instrumented with resistive plate chambers (KLM) and served to identify
muons and K L with a momentum above 600 MeV/c.
9.6.2.2 The LHCb Experiment at the Large Hadron Collider
The multi-purpose experiments CDF and D0 [73–76] operated at the p ¯
p collider
Tevatron at Fermilab established that—despite of the harsh and high-multiplicity
environment of the hadron collisions—precision measurements of B mesons competitive with those performed at e + e − B-factories are possible at hadron machines.
Both experiments have pioneered studies of the B 0
s system which was hardly studied
at the e + e − machines, as kinematically the heavier B 0
s system is only accessible
through the decays of the ϒ(5S). Only the Belle collaboration collected a small
data sample of ϒ(5S) decays.
At the Large Hadron Collider (LHC) at CERN, the LHCb experiment [77]
and to a lesser extent also the ATLAS and CMS experiments [78, 79] followed
the successful path of precision B meson studies explored at Tevatron. The cross
section for b ¯
b pair production in the pp collision of the LHC is huge, about 500 μb
for proton-proton center-of-mass energies of 13 TeV. As only small fractions of
the proton energies are needed to produce the b ¯
b pair, the momentum factions
x 1 and x 2 of the colliding partons are in general very different and the relatively
light b ¯
b system is boosted in the laboratory frame into either the forward or the
backward direction. The LHCb experiment has therefore been designed as a singlearm forward spectrometer. With a pseudo-rapidity coverage of 2 < η < 5 about
35% of all produced b ¯
b pairs lie within the detector acceptance. The average boost
of the b hadrons is large, βγ ≈ 25, resulting into average flight distances of about
1 cm. Although the b ¯
b cross section at the LHC is large, the total inelastic cross
section is about a factor 200 larger. The huge rate of non b events together with the
fact that the decay products of the b hadrons are comparably soft makes the trigger
to the primary challenge for any B meson experiment. Typical combined trigger
and reconstruction efficiencies vary between 0.1 and 10% depending on the decay
channel. Channels with muons in the final state are in general easier to trigger and
reconstruct.
At a hadron collider the two produced b quarks hadronise separately and no
quantum correlation between the two b hadrons exists. It is therefore harder to
conclude from the flavor of the second b hadron (tagging B), which for the case
that it is a neutral B meson oscillates independently, on the production flavor of the
signal B. Effective tagging efficiencies therefore stay significantly below 10%.
The LHCb forward spectrometer is shown in Fig. 9.26 and includes a highprecision tracking system consisting of a silicon-strip vertex detector (Velo) sur-
K. Kleinknecht and U. Uwer
Cherenkov counter (ACC). Photon and electron energies were measured in the
electromagnetic calorimeter (ECL) consisting of thallium-doped CsI crystals. The
energy resolution varied from 4% at 100 MeV to about 1.6% at 8 GeV [71]. All
detector components were placed inside a super-conducting solenoid magnet of
cylindrical shape which was providing a magnetic field of 1.5 T. The iron return
yoke is instrumented with resistive plate chambers (KLM) and served to identify
muons and K L with a momentum above 600 MeV/c.
9.6.2.2 The LHCb Experiment at the Large Hadron Collider
The multi-purpose experiments CDF and D0 [73–76] operated at the p ¯
p collider
Tevatron at Fermilab established that—despite of the harsh and high-multiplicity
environment of the hadron collisions—precision measurements of B mesons competitive with those performed at e + e − B-factories are possible at hadron machines.
Both experiments have pioneered studies of the B 0
s system which was hardly studied
at the e + e − machines, as kinematically the heavier B 0
s system is only accessible
through the decays of the ϒ(5S). Only the Belle collaboration collected a small
data sample of ϒ(5S) decays.
At the Large Hadron Collider (LHC) at CERN, the LHCb experiment [77]
and to a lesser extent also the ATLAS and CMS experiments [78, 79] followed
the successful path of precision B meson studies explored at Tevatron. The cross
section for b ¯
b pair production in the pp collision of the LHC is huge, about 500 μb
for proton-proton center-of-mass energies of 13 TeV. As only small fractions of
the proton energies are needed to produce the b ¯
b pair, the momentum factions
x 1 and x 2 of the colliding partons are in general very different and the relatively
light b ¯
b system is boosted in the laboratory frame into either the forward or the
backward direction. The LHCb experiment has therefore been designed as a singlearm forward spectrometer. With a pseudo-rapidity coverage of 2 < η < 5 about
35% of all produced b ¯
b pairs lie within the detector acceptance. The average boost
of the b hadrons is large, βγ ≈ 25, resulting into average flight distances of about
1 cm. Although the b ¯
b cross section at the LHC is large, the total inelastic cross
section is about a factor 200 larger. The huge rate of non b events together with the
fact that the decay products of the b hadrons are comparably soft makes the trigger
to the primary challenge for any B meson experiment. Typical combined trigger
and reconstruction efficiencies vary between 0.1 and 10% depending on the decay
channel. Channels with muons in the final state are in general easier to trigger and
reconstruct.
At a hadron collider the two produced b quarks hadronise separately and no
quantum correlation between the two b hadrons exists. It is therefore harder to
conclude from the flavor of the second b hadron (tagging B), which for the case
that it is a neutral B meson oscillates independently, on the production flavor of the
signal B. Effective tagging efficiencies therefore stay significantly below 10%.
The LHCb forward spectrometer is shown in Fig. 9.26 and includes a highprecision tracking system consisting of a silicon-strip vertex detector (Velo) sur-