Comprehensive study of the background for the Pixel Vertex Detector at Belle II
Beschreibung
vor 9 Jahren
The highly successful Belle experiment was located at the KEKB
accelerator in Tsukuba, Japan. KEKB was an electron-positron ring
accelerator running at the asymmetric energies of 8 GeV (e-) and
3.5 GeV (e+). The Belle experiment took data from 1999 to 2010, but
was shut down in June 2010 in order to begin a major upgrade of the
accelerator and the detector. Belle played a crucial role in the
award of the 2008 Nobel Prize for Physics to M. Kobayashi and T.
Maskawa. The main physics goal of Belle was the measurement of
CP-violation in the B-meson system. This mission, as well as the
search for physics beyond the Standard Model, has been passed to
the Belle II experiment located at the SuperKEKB accelerator, the
direct successors of the Belle experiment and KEKB respectively.
The precise measurement of CP-violation and the search for rare or
"forbidden" decays of the B-meson and the tau-lepton as signals for
New Physics relies heavily on a large number of recorded events and
the precision with which B-meson and lepton decay vertices can be
reconstructed. Thus, the accelerator upgrade aims for an increase
of the luminosity by a factor of 40, resulting in a peak luminosity
of 8x10^35 cm^{-2} s^{-1}. This upgrade is scheduled to be finished
by 2017 and will result in asymmetric beam energies of 7 GeV (e-)
and 4 GeV (e+), provided by beams with a vertical size of only 48
nm ("nano-beam optics"), a size that has never been reached at any
particle collider before. The accelerator upgrade will result in
the desired increase of the collision rate of particles, while it
will also inevitably lead to an increase in the background for all
sub-detectors. The Belle detector would not have been able to
handle the new background conditions expected at SuperKEKB, hence
an upgrade of the Belle detector to the Belle II detector was
necessary. Additionally the upgrade aims to increase the physics
performance of the detector, making it more sensitive to the
effects of New Physics. The detector upgrade will see improvements
and redesigns of almost all subsystems as well as the inclusion of
a whole new sub-detector, the PiXel vertex Detector (PXD). The
introduction of the PXD will ensure that decay vertices are
reconstructed with an extremely high precision in the harsh
background conditions expected at Belle II. The PXD is a
semi-conductor based particle tracking detector and will be the
innermost sub-detector of Belle II. It offers excellent track and
vertex reconstruction capabilities, while having a thickness of
only 75 μm in order to minimise multiple scattering effects. Due to
the innovative concept of a high-luminosity nano-beam accelerator,
the scale of background being produced at the future SuperKEKB
cannot be derived from a traditional electron-positron collider and
has, therefore, to be simulated using first-principle Monte Carlo
techniques. This thesis focuses on a detailed study of the expected
background for the pixel vertex detector at the upcoming Belle II
experiment. It starts with a comprehensive summary of the key
components of the SuperKEKB accelerator and the Belle II detector
before delving into the details of the Belle II simulation and
reconstruction framework basf2. It was decided to develop the basf2
framework from scratch, rather than adapting the software framework
used at Belle. The changes made in the upgrade from the Belle to
the Belle II detector, would have required major modifications of
nearly all existing libraries. This thesis continues by explaining,
in detail, the measurement and analysis of an experiment conducted
at Belle in 2010, shortly before the KEKB accelerator and the Belle
detector were shut down. The experiment aimed at establishing the
validity of a major background for the PXD, namely the two-photon
process into an electron-positron pair, described by the
Monte-Carlo generators KoralW and BDK, which have never been tested
in the kinematical region relevant for the PXD. From a comparison
based on Monte Carlo data it is found that the difference between
KoralW and BDK in the high cross-section, low pt region (smaller
than 20 MeV) for the produced electron and positron is very small,
and that both Monte-Carlo generators agree with the experiment in
this important low momentum regime. However, the question arises as
to whether the delivered cross-section of the Monte Carlo
generators is correct over a wider phase space, but still below the
centre-of-mass energies where these generators have been verified
experimentally (e.g. at the e+e- colliders PETRA and LEP). In order
to answer this question, a comparison between recorded detector
data and Monte Carlo data is performed, an analysis that has never
been done for centre-of-mass energies of the order of those of the
Belle and Belle II experiments. From the results the conclusion is
drawn that both Monte Carlo generators, KoralW and BDK, agree very
nicely for low values of pt but differ significantly for
intermediate values where the total cross-sections are already very
small. The recorded data proved that for intermediate pt ranges the
behaviour of BDK is correct, while KoralW overshoots the data.
Since, however, the cross-section peaks strongly for low values of
pt both generators can be used for further background studies.
Furthermore, this thesis includes a detailed basf2 simulation study
of the major beam and QED backgrounds that are expected at Belle II
and their impact on the PXD. Various figures of merit are
estimated, such as particle flux, radiation dose and occupancy. On
average the inner layer experiences a particle flux of 6.1 MHz
cm^{-2} and the outer layer of 2.5 MHz cm^{-2}. The distribution of
the particle flux along the global z-axis is fairly flat meaning
that the radiation damage is evenly distributed along the PXD
ladders. The simulation shows that the inner layer of the PXD is
exposed to a radiation dose of 19.9 kGy/smy and the outer layer to
a dose of 4.9 kGy/smy. Irradiation tests of DEPFET sensors with 10
MeV electrons showed that the sensors work reliably for a dose of
at least 100 kGy. It is believed that they can even cope with up to
200 kGy. Using the radiation dose values obtained from the
simulation, the numbers translate to a lifetime of roughly 10 years
for the PXD sensors, the typical operation time of a high energy
physics detector. The study shows that the expected PXD occupancy,
summing over all background sources, is given by inner layer: 1.28
+- 0.03 % outer layer: 0.45 +- 0.01 % The upper limit for the PXD,
imposed by the data acquisition and the track reconstruction, is
3%. The estimated values are well below this limit and, thus, the
PXD will withstand the harsh background conditions that are
expected at Belle II.
accelerator in Tsukuba, Japan. KEKB was an electron-positron ring
accelerator running at the asymmetric energies of 8 GeV (e-) and
3.5 GeV (e+). The Belle experiment took data from 1999 to 2010, but
was shut down in June 2010 in order to begin a major upgrade of the
accelerator and the detector. Belle played a crucial role in the
award of the 2008 Nobel Prize for Physics to M. Kobayashi and T.
Maskawa. The main physics goal of Belle was the measurement of
CP-violation in the B-meson system. This mission, as well as the
search for physics beyond the Standard Model, has been passed to
the Belle II experiment located at the SuperKEKB accelerator, the
direct successors of the Belle experiment and KEKB respectively.
The precise measurement of CP-violation and the search for rare or
"forbidden" decays of the B-meson and the tau-lepton as signals for
New Physics relies heavily on a large number of recorded events and
the precision with which B-meson and lepton decay vertices can be
reconstructed. Thus, the accelerator upgrade aims for an increase
of the luminosity by a factor of 40, resulting in a peak luminosity
of 8x10^35 cm^{-2} s^{-1}. This upgrade is scheduled to be finished
by 2017 and will result in asymmetric beam energies of 7 GeV (e-)
and 4 GeV (e+), provided by beams with a vertical size of only 48
nm ("nano-beam optics"), a size that has never been reached at any
particle collider before. The accelerator upgrade will result in
the desired increase of the collision rate of particles, while it
will also inevitably lead to an increase in the background for all
sub-detectors. The Belle detector would not have been able to
handle the new background conditions expected at SuperKEKB, hence
an upgrade of the Belle detector to the Belle II detector was
necessary. Additionally the upgrade aims to increase the physics
performance of the detector, making it more sensitive to the
effects of New Physics. The detector upgrade will see improvements
and redesigns of almost all subsystems as well as the inclusion of
a whole new sub-detector, the PiXel vertex Detector (PXD). The
introduction of the PXD will ensure that decay vertices are
reconstructed with an extremely high precision in the harsh
background conditions expected at Belle II. The PXD is a
semi-conductor based particle tracking detector and will be the
innermost sub-detector of Belle II. It offers excellent track and
vertex reconstruction capabilities, while having a thickness of
only 75 μm in order to minimise multiple scattering effects. Due to
the innovative concept of a high-luminosity nano-beam accelerator,
the scale of background being produced at the future SuperKEKB
cannot be derived from a traditional electron-positron collider and
has, therefore, to be simulated using first-principle Monte Carlo
techniques. This thesis focuses on a detailed study of the expected
background for the pixel vertex detector at the upcoming Belle II
experiment. It starts with a comprehensive summary of the key
components of the SuperKEKB accelerator and the Belle II detector
before delving into the details of the Belle II simulation and
reconstruction framework basf2. It was decided to develop the basf2
framework from scratch, rather than adapting the software framework
used at Belle. The changes made in the upgrade from the Belle to
the Belle II detector, would have required major modifications of
nearly all existing libraries. This thesis continues by explaining,
in detail, the measurement and analysis of an experiment conducted
at Belle in 2010, shortly before the KEKB accelerator and the Belle
detector were shut down. The experiment aimed at establishing the
validity of a major background for the PXD, namely the two-photon
process into an electron-positron pair, described by the
Monte-Carlo generators KoralW and BDK, which have never been tested
in the kinematical region relevant for the PXD. From a comparison
based on Monte Carlo data it is found that the difference between
KoralW and BDK in the high cross-section, low pt region (smaller
than 20 MeV) for the produced electron and positron is very small,
and that both Monte-Carlo generators agree with the experiment in
this important low momentum regime. However, the question arises as
to whether the delivered cross-section of the Monte Carlo
generators is correct over a wider phase space, but still below the
centre-of-mass energies where these generators have been verified
experimentally (e.g. at the e+e- colliders PETRA and LEP). In order
to answer this question, a comparison between recorded detector
data and Monte Carlo data is performed, an analysis that has never
been done for centre-of-mass energies of the order of those of the
Belle and Belle II experiments. From the results the conclusion is
drawn that both Monte Carlo generators, KoralW and BDK, agree very
nicely for low values of pt but differ significantly for
intermediate values where the total cross-sections are already very
small. The recorded data proved that for intermediate pt ranges the
behaviour of BDK is correct, while KoralW overshoots the data.
Since, however, the cross-section peaks strongly for low values of
pt both generators can be used for further background studies.
Furthermore, this thesis includes a detailed basf2 simulation study
of the major beam and QED backgrounds that are expected at Belle II
and their impact on the PXD. Various figures of merit are
estimated, such as particle flux, radiation dose and occupancy. On
average the inner layer experiences a particle flux of 6.1 MHz
cm^{-2} and the outer layer of 2.5 MHz cm^{-2}. The distribution of
the particle flux along the global z-axis is fairly flat meaning
that the radiation damage is evenly distributed along the PXD
ladders. The simulation shows that the inner layer of the PXD is
exposed to a radiation dose of 19.9 kGy/smy and the outer layer to
a dose of 4.9 kGy/smy. Irradiation tests of DEPFET sensors with 10
MeV electrons showed that the sensors work reliably for a dose of
at least 100 kGy. It is believed that they can even cope with up to
200 kGy. Using the radiation dose values obtained from the
simulation, the numbers translate to a lifetime of roughly 10 years
for the PXD sensors, the typical operation time of a high energy
physics detector. The study shows that the expected PXD occupancy,
summing over all background sources, is given by inner layer: 1.28
+- 0.03 % outer layer: 0.45 +- 0.01 % The upper limit for the PXD,
imposed by the data acquisition and the track reconstruction, is
3%. The estimated values are well below this limit and, thus, the
PXD will withstand the harsh background conditions that are
expected at Belle II.
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