9 Symmetry Violations and Quark Flavour Physics
583
9.6.2 Production and Detection of B-Mesons
The precise measurement of oscillation and CP violation in the neutral B meson
system requires a high number of produced B mesons. A good reconstruction of
the B meson decay vertex, necessary for an excellent decay time resolution, is
needed to resolve the time-dependent effects. To perform the CP measurements
the determination of the production flavor of the B meson, often referred as flavortagging, is necessary.
9.6.2.1 e + e − B-Factories
The first experiments which have systematically addressed these requirements are
the BABAR experiment [69] at the e + e − collider PEP-II at Stanford and the Belle
experiment [70] at the Japanese e + e − collider KEKB. Both e + e − machines were
operating at a centre-of-mass energy of
√
s = 10.58 GeV, corresponding to the
mass of the Υ (4S), an excited b ¯
b resonance which decays to B 0 B 0 (∼ 50%) and
B + B − (∼50%). The cross section for the Υ (4S) production is 1.1 nb which is about
a quarter of the total hadronic cross section.
BABAR was operated from 1999 to 2008 and collected a data set corresponding
to an integrated luminosity of 550 fb −1 . The Belle experiment has also been started
in 1999 and was taking data until the end of 2009, collecting about 1 ab −1 of data.
Due to the large number of B mesons the two experiments have recorded, they are
often referred as B-factories. 3 To continue this successful path, the KEKB collider
as well as the Belle experiment have both undergone an upgrade. The new Belle-II
experiment has started data-taking in 2018 and will be operated at a 40 times larger
instantaneous luminosity [72].
The mass of the ϒ(4S) lies only 11 MeV/c 2 above the sum of the two produced
B mesons which would therefore be produced essentially at rest and would not fly.
The vanishing decay length would prevent a measurement of the decay time. To
overcome this problem both machines have been operated with slightly asymmetric
beam energies resulting in a small boost of the ϒ(4S) and thus of the produced B
mesons. The boost factor βγ was 0.56 (0.425) for the BABAR (Belle) experiment,
resulting into typical decay lengths of about 250 μm for the two B mesons with
lifetimes of 1.5 ps.
If the ϒ(4S) with J P = 1 − decays into a pair of neutral B mesons, the two
mesons are produced in a coherent B 0 B 0 state with negative parity,
B
0 (θ 1 )B 0 (θ 2 ) − B
0 (θ 2 )B 0 (θ 1 ) ,
(9.199)
3 A comprehensive description of the two e + e − machines, the two detectors, the operation and the
physics results can be found in [71].
583
9.6.2 Production and Detection of B-Mesons
The precise measurement of oscillation and CP violation in the neutral B meson
system requires a high number of produced B mesons. A good reconstruction of
the B meson decay vertex, necessary for an excellent decay time resolution, is
needed to resolve the time-dependent effects. To perform the CP measurements
the determination of the production flavor of the B meson, often referred as flavortagging, is necessary.
9.6.2.1 e + e − B-Factories
The first experiments which have systematically addressed these requirements are
the BABAR experiment [69] at the e + e − collider PEP-II at Stanford and the Belle
experiment [70] at the Japanese e + e − collider KEKB. Both e + e − machines were
operating at a centre-of-mass energy of
√
s = 10.58 GeV, corresponding to the
mass of the Υ (4S), an excited b ¯
b resonance which decays to B 0 B 0 (∼ 50%) and
B + B − (∼50%). The cross section for the Υ (4S) production is 1.1 nb which is about
a quarter of the total hadronic cross section.
BABAR was operated from 1999 to 2008 and collected a data set corresponding
to an integrated luminosity of 550 fb −1 . The Belle experiment has also been started
in 1999 and was taking data until the end of 2009, collecting about 1 ab −1 of data.
Due to the large number of B mesons the two experiments have recorded, they are
often referred as B-factories. 3 To continue this successful path, the KEKB collider
as well as the Belle experiment have both undergone an upgrade. The new Belle-II
experiment has started data-taking in 2018 and will be operated at a 40 times larger
instantaneous luminosity [72].
The mass of the ϒ(4S) lies only 11 MeV/c 2 above the sum of the two produced
B mesons which would therefore be produced essentially at rest and would not fly.
The vanishing decay length would prevent a measurement of the decay time. To
overcome this problem both machines have been operated with slightly asymmetric
beam energies resulting in a small boost of the ϒ(4S) and thus of the produced B
mesons. The boost factor βγ was 0.56 (0.425) for the BABAR (Belle) experiment,
resulting into typical decay lengths of about 250 μm for the two B mesons with
lifetimes of 1.5 ps.
If the ϒ(4S) with J P = 1 − decays into a pair of neutral B mesons, the two
mesons are produced in a coherent B 0 B 0 state with negative parity,
B
0 (θ 1 )B 0 (θ 2 ) − B
0 (θ 2 )B 0 (θ 1 ) ,
(9.199)
3 A comprehensive description of the two e + e − machines, the two detectors, the operation and the
physics results can be found in [71].