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Papers by Stephen Poprocki

Phys. Rev. Accel. Beams, 2019

We report on extensive measurements at the Cornell Electron-Positron Storage Ring of electron-clo... more We report on extensive measurements at the Cornell Electron-Positron Storage Ring of electron-cloud-induced betatron tune shifts for trains of positron bunches at 2.1 and 5.3 GeV with bunch populations ranging between
0.64×10^10 and 9.6×10^10. Measurements using a witness bunch with variable distance from the end of the train and variable bunch population provide information on cloud decay and cloud pinching during the bunch passage. We employ Monte Carlo simulations of the reflection and absorption of synchrotron radiation photons to determine the pattern of absorption sites around the circumference of the storage ring. The Geant4 simulation toolkit is used to model the interactions of the photons with the beampipe wall and determine the production energy and location distributions of the photoelectrons which seed the electron cloud. An electron cloud buildup model based on fitted ring-averaged secondary-yield properties of the vacuum chamber predicts tune shifts in good agreement with the measurements.

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2019

Short range transverse wake fields are excited by charged particles passing through a vacuum cham... more Short range transverse wake fields are excited by charged particles passing through a vacuum chamber. The wake fields can act back on a bunch of particles and alter its trajectory. We have measured the distortion of the closed orbit due to transverse wake fields in the Cornell Electron–positron Storage Ring. A transverse monopole wake field is induced by an asymmetric impedance associated with the insertion of a single vertical scraper. A transverse dipole wake field is generated by displacing the bunch from the symmetry axis of the chamber. The orbit is offset vertically through the gap of a pair of symmetrically inserted scrapers and also in a narrow-gap undulator chamber. A fit to the distorted orbits yields the average kick of the transverse monopole wake of the asymmetric scrapers and the kick factor of the symmetric scrapers; the scraper results are in reasonable agreement with the values from numerical simulation of the wake fields. The kick factor extracted from the closed orbit distortion of the dipole wake induced by the narrow-gap undulator chamber is significantly greater than that predicted by the numerical simulation. Possible sources of the discrepancy are discussed.

We report on extensive measurements at the Cornell Electron-positron Storage Ring of electron-clo... more We report on extensive measurements at the Cornell Electron-positron Storage Ring of electron-cloud-induced betatron tune shifts for trains of positron bunches at 2.1 and 5.3 GeV with bunch populations ranging between 0.4x10^10 and 9.5x10^10. Measurements using a witness bunch with variable distance from the end of the train and variable bunch population inform the study of cloud decay and cloud pinching during the bunch passage. We employ Monte Carlo simulations of the reflection and absorption of synchrotron radiation photons to determine the pattern of absorption sites around the circumference of the storage ring. The Geant4 simulation toolkit is used to model the interactions of the photons with the beampipe wall and determine the production energy and location distributions of the photoelectrons which seed the electron cloud. An electron cloud buildup model based on these distributions successfully describes the tune shift measurements after determining ring-averaged secondary-yield properties of the vacuum chamber using an iterative parameter optimization procedure.

The synchrotron-radiation-induced buildup of low-energy electron densities in positron and proton... more The synchrotron-radiation-induced buildup of low-energy electron densities in positron and proton storage rings limits performance by causing betatron tune shifts and incoherent emittance growth. The Cornell Electron Storage Ring (CESR) Test Accelerator project includes extensive measurement and modeling programs to quantify such effects and apply the knowledge gained to the design of future accelerator projects. We report on improved measurements of betatron tune shifts along a train of positron bunches, now accurate in both horizontal and vertical planes. Improved electron cloud buildup modeling uses detailed information on photoelectron production properties obtained from recently developed simulations and successfully describes the measurements after determining ring-wide secondary-yield properties of the vacuum chamber by fitting the model to data with a multi-objective optimizer. Cloud splitting in dipole magnetic fields is seen to be the source of horizontal tune shifts decreasing at higher bunch populations.

We report on calculations of electron production by synchrotron radiation absorbed in the vacuum ... more We report on calculations of electron production by synchrotron radiation absorbed in the vacuum chamber walls of the Cornell Electron Storage Ring (CESR). These electrons are the source of electron clouds which limit the performance of storage rings by causing betatron tune shifts, instabilities and emittance growth. Until now, cloud buildup modeling codes have used ad hoc models of the production of the seed electrons. We have employed the photon scattering code Synrad3D to quantify the pattern of absorbed photons around the CESR ring, including the transverse distribution on the wall of the beam-pipe. These distributions in absorbed photon energy and incident angle are used as input to Geant4-based simulations of electron emission from the walls. The average quantum efficiency is found to vary dramatically with the location of the absorption site, owing to the distribution in impact energies and angles. The electron production energy spectrum plays an important role in the modeling of electron cloud buildup, where the interplay of production energy and acceleration by the beam bunches determines the time structure and multipacting characteristics of the cloud.

Transverse vertical wakefields can increase vertical emittance and distort the phase space of a b... more Transverse vertical wakefields can increase vertical emittance and distort the phase space of a bunch in a storage ring. Here we report recent measurements and simulations of these effects from wakefields from movable scrapers at the Cornell electron-positron storage ring test accelerator. Charge-dependent vertical beam size growth was observed with a single scraper inserted through the top of the storage ring vacuum chamber. No change in the beam size was observed with top and bottom scrapers inserted symmetrically. The apparent growth in the vertical beam size was due in large part to the yz coupling (vertical crabbing) induced by the transverse monopole wake of the asymmetric scraper configuration. We explored this yz coupling and thus the orientation at the observation point by varying the vertical betatron phase advance between the vertical beam size monitor and the scrapers. In addition, we found that existing residual, current-independent yz coupling, perhaps due to nonzero vertical dispersion in the rf cavities, could be compensated by the scraper wake. Predictions from tracking simulations are in good agreement with the measurements. Moreover, the vertical beam size as a function of vertical displacement at narrow-gap chambers (closed scrapers and undulator chamber) was measured. We found the transverse wakefield produced by the off-axis beam could also introduce a yz tilt as well as dilute the beam emittance.

We present the development and validation of a new multivariate b jet identification algorithm (“... more We present the development and validation of a new multivariate b jet identification algorithm (“b tagger”) used at the CDF experiment at the Fermilab Tevatron. At collider experiments, b taggers allow one to distinguish particle jets containing B hadrons from other jets. Employing feed-forward neural network architectures, this tagger is unique in its emphasis on using information from individual tracks. This tagger not only contains the usual advantages of a multivariate technique such as maximal use of information in a jet and tunable purity/efficiency operating points, but is also capable of evaluating jets with only a single track. To demonstrate the effectiveness of the tagger, we employ a novel method wherein we calculate the false tag rate and tag efficiency as a function of the placement of a lower threshold on a jet's neural network output value in Z+1 jet and candidate samples, rich in light-flavor and b jets, respectively.

Diboson production (WW+WZ+ZZ) has been observed at the Tevatron in hadronic decay modes dominated... more Diboson production (WW+WZ+ZZ) has been observed at the Tevatron in hadronic decay modes dominated by the WW process. This paper describes the measurement of the cross section of WZ and ZZ events in final states with large E̸T and using b-jet identification as a tool to suppress WW contributions. Because of the limited energy resolution, we cannot distinguish between partially hadronic decays of WZ and ZZ, and we measure the sum of these processes. The number of signal events is extracted using a simultaneous fit to the invariant mass distribution of the two jets for events with two b-jet candidates and events with fewer than two b-jet candidates. We measure a cross section σ(pp̅ →WZ,ZZ)=5.8-3.0+3.6 pb, in agreement with the standard model.

We present the development and validation of a new multivariate bbb jet identification algorithm ... more We present the development and validation of a new multivariate bbb jet identification algorithm ("$b$ tagger") used at the CDF experiment at the Fermilab Tevatron. At collider experiments, bbb taggers allow one to distinguish particle jets containing BBB hadrons from other jets. Employing feed-forward neural network architectures, this tagger is unique in its emphasis on using information from individual tracks. This tagger not only contains the usual advantages of a multivariate technique such as maximal use of information in a jet and tunable purity/efficiency operating points, but is also capable of evaluating jets with only a single track. To demonstrate the effectiveness of the tagger, we employ a novel method wherein we calculate the false tag rate and tag efficiency as a function of the placement of a lower threshold on a jet's neural network output value in Z+1Z+1Z+1 jet and tbartt\bar{t}tbart candidate samples, rich in light flavor and bbb jets, respectively.

Diboson production ($WW+WZ+ZZ$) has been observed at the Tevatron in hadronic decay modes dominat... more Diboson production ($WW+WZ+ZZ$) has been observed at the Tevatron in hadronic decay modes dominated by the WWWWWW process. This paper describes the measurement of the cross section of WZWZWZ and ZZZZZZ events in final states with large mett\mettmett and using bbb-jet identification as a tool to suppress WWWWWW contributions. Due to the limited energy resolution, we cannot distinguish between partially hadronic decays of WZWZWZ and ZZZZZZ, and we measure the sum of these processes. The number of signal events is extracted using a simultaneous fit to the invariant mass distribution of the two jets for events with two bbb-jet candidates and events without two bbb-jet candidates. We measure a cross section sigma(pbarptoWZ,ZZ)=5.8+3.6−3.0\sigma(p\bar{p}\to WZ,ZZ) = 5.8^{+3.6}_{-3.0}sigma(pbarptoWZ,ZZ)=5.8+3.6−3.0 pb, in agreement with the standard model.

Autonomous gravitational-wave searches -- fully automated analyses of data that run without human... more Autonomous gravitational-wave searches -- fully automated analyses of data that run without human intervention or assistance -- are desirable for a number of reasons. They are necessary for the rapid identification of gravitational-wave burst candidates, which in turn will allow for follow-up observations by other observatories and the maximum exploitation of their scientific potential. A fully automated analysis would also circumvent the traditional "by hand" setup and tuning of burst searches that is both labourious and time consuming. We demonstrate a fully automated search with X-Pipeline, a software package for the coherent analysis of data from networks of interferometers for detecting bursts associated with GRBs and other astrophysical triggers. We discuss the methods X-Pipeline uses for automated running, including background estimation, efficiency studies, unbiased optimal tuning of search thresholds, and prediction of upper limits. These are all done automatically via Monte Carlo with multiple independent data samples, and without requiring human intervention. As a demonstration of the power of this approach, we apply X-Pipeline to LIGO data to search for gravitational-wave emission associated with GRB 031108. We find that X-Pipeline is sensitive to signals approximately a factor of 2 weaker in amplitude than those detectable by the cross-correlation technique used in LIGO searches to date. We conclude with the prospects for running X-Pipeline as a fully autonomous, near real-time triggered burst search in the next LSC-Virgo Science Run.

Phys. Rev. Accel. Beams, 2019

We report on extensive measurements at the Cornell Electron-Positron Storage Ring of electron-clo... more We report on extensive measurements at the Cornell Electron-Positron Storage Ring of electron-cloud-induced betatron tune shifts for trains of positron bunches at 2.1 and 5.3 GeV with bunch populations ranging between
0.64×10^10 and 9.6×10^10. Measurements using a witness bunch with variable distance from the end of the train and variable bunch population provide information on cloud decay and cloud pinching during the bunch passage. We employ Monte Carlo simulations of the reflection and absorption of synchrotron radiation photons to determine the pattern of absorption sites around the circumference of the storage ring. The Geant4 simulation toolkit is used to model the interactions of the photons with the beampipe wall and determine the production energy and location distributions of the photoelectrons which seed the electron cloud. An electron cloud buildup model based on fitted ring-averaged secondary-yield properties of the vacuum chamber predicts tune shifts in good agreement with the measurements.

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2019

Short range transverse wake fields are excited by charged particles passing through a vacuum cham... more Short range transverse wake fields are excited by charged particles passing through a vacuum chamber. The wake fields can act back on a bunch of particles and alter its trajectory. We have measured the distortion of the closed orbit due to transverse wake fields in the Cornell Electron–positron Storage Ring. A transverse monopole wake field is induced by an asymmetric impedance associated with the insertion of a single vertical scraper. A transverse dipole wake field is generated by displacing the bunch from the symmetry axis of the chamber. The orbit is offset vertically through the gap of a pair of symmetrically inserted scrapers and also in a narrow-gap undulator chamber. A fit to the distorted orbits yields the average kick of the transverse monopole wake of the asymmetric scrapers and the kick factor of the symmetric scrapers; the scraper results are in reasonable agreement with the values from numerical simulation of the wake fields. The kick factor extracted from the closed orbit distortion of the dipole wake induced by the narrow-gap undulator chamber is significantly greater than that predicted by the numerical simulation. Possible sources of the discrepancy are discussed.

We report on extensive measurements at the Cornell Electron-positron Storage Ring of electron-clo... more We report on extensive measurements at the Cornell Electron-positron Storage Ring of electron-cloud-induced betatron tune shifts for trains of positron bunches at 2.1 and 5.3 GeV with bunch populations ranging between 0.4x10^10 and 9.5x10^10. Measurements using a witness bunch with variable distance from the end of the train and variable bunch population inform the study of cloud decay and cloud pinching during the bunch passage. We employ Monte Carlo simulations of the reflection and absorption of synchrotron radiation photons to determine the pattern of absorption sites around the circumference of the storage ring. The Geant4 simulation toolkit is used to model the interactions of the photons with the beampipe wall and determine the production energy and location distributions of the photoelectrons which seed the electron cloud. An electron cloud buildup model based on these distributions successfully describes the tune shift measurements after determining ring-averaged secondary-yield properties of the vacuum chamber using an iterative parameter optimization procedure.

The synchrotron-radiation-induced buildup of low-energy electron densities in positron and proton... more The synchrotron-radiation-induced buildup of low-energy electron densities in positron and proton storage rings limits performance by causing betatron tune shifts and incoherent emittance growth. The Cornell Electron Storage Ring (CESR) Test Accelerator project includes extensive measurement and modeling programs to quantify such effects and apply the knowledge gained to the design of future accelerator projects. We report on improved measurements of betatron tune shifts along a train of positron bunches, now accurate in both horizontal and vertical planes. Improved electron cloud buildup modeling uses detailed information on photoelectron production properties obtained from recently developed simulations and successfully describes the measurements after determining ring-wide secondary-yield properties of the vacuum chamber by fitting the model to data with a multi-objective optimizer. Cloud splitting in dipole magnetic fields is seen to be the source of horizontal tune shifts decreasing at higher bunch populations.

We report on calculations of electron production by synchrotron radiation absorbed in the vacuum ... more We report on calculations of electron production by synchrotron radiation absorbed in the vacuum chamber walls of the Cornell Electron Storage Ring (CESR). These electrons are the source of electron clouds which limit the performance of storage rings by causing betatron tune shifts, instabilities and emittance growth. Until now, cloud buildup modeling codes have used ad hoc models of the production of the seed electrons. We have employed the photon scattering code Synrad3D to quantify the pattern of absorbed photons around the CESR ring, including the transverse distribution on the wall of the beam-pipe. These distributions in absorbed photon energy and incident angle are used as input to Geant4-based simulations of electron emission from the walls. The average quantum efficiency is found to vary dramatically with the location of the absorption site, owing to the distribution in impact energies and angles. The electron production energy spectrum plays an important role in the modeling of electron cloud buildup, where the interplay of production energy and acceleration by the beam bunches determines the time structure and multipacting characteristics of the cloud.

Transverse vertical wakefields can increase vertical emittance and distort the phase space of a b... more Transverse vertical wakefields can increase vertical emittance and distort the phase space of a bunch in a storage ring. Here we report recent measurements and simulations of these effects from wakefields from movable scrapers at the Cornell electron-positron storage ring test accelerator. Charge-dependent vertical beam size growth was observed with a single scraper inserted through the top of the storage ring vacuum chamber. No change in the beam size was observed with top and bottom scrapers inserted symmetrically. The apparent growth in the vertical beam size was due in large part to the yz coupling (vertical crabbing) induced by the transverse monopole wake of the asymmetric scraper configuration. We explored this yz coupling and thus the orientation at the observation point by varying the vertical betatron phase advance between the vertical beam size monitor and the scrapers. In addition, we found that existing residual, current-independent yz coupling, perhaps due to nonzero vertical dispersion in the rf cavities, could be compensated by the scraper wake. Predictions from tracking simulations are in good agreement with the measurements. Moreover, the vertical beam size as a function of vertical displacement at narrow-gap chambers (closed scrapers and undulator chamber) was measured. We found the transverse wakefield produced by the off-axis beam could also introduce a yz tilt as well as dilute the beam emittance.

We present the development and validation of a new multivariate b jet identification algorithm (“... more We present the development and validation of a new multivariate b jet identification algorithm (“b tagger”) used at the CDF experiment at the Fermilab Tevatron. At collider experiments, b taggers allow one to distinguish particle jets containing B hadrons from other jets. Employing feed-forward neural network architectures, this tagger is unique in its emphasis on using information from individual tracks. This tagger not only contains the usual advantages of a multivariate technique such as maximal use of information in a jet and tunable purity/efficiency operating points, but is also capable of evaluating jets with only a single track. To demonstrate the effectiveness of the tagger, we employ a novel method wherein we calculate the false tag rate and tag efficiency as a function of the placement of a lower threshold on a jet's neural network output value in Z+1 jet and candidate samples, rich in light-flavor and b jets, respectively.

Diboson production (WW+WZ+ZZ) has been observed at the Tevatron in hadronic decay modes dominated... more Diboson production (WW+WZ+ZZ) has been observed at the Tevatron in hadronic decay modes dominated by the WW process. This paper describes the measurement of the cross section of WZ and ZZ events in final states with large E̸T and using b-jet identification as a tool to suppress WW contributions. Because of the limited energy resolution, we cannot distinguish between partially hadronic decays of WZ and ZZ, and we measure the sum of these processes. The number of signal events is extracted using a simultaneous fit to the invariant mass distribution of the two jets for events with two b-jet candidates and events with fewer than two b-jet candidates. We measure a cross section σ(pp̅ →WZ,ZZ)=5.8-3.0+3.6 pb, in agreement with the standard model.

We present the development and validation of a new multivariate bbb jet identification algorithm ... more We present the development and validation of a new multivariate bbb jet identification algorithm ("$b$ tagger") used at the CDF experiment at the Fermilab Tevatron. At collider experiments, bbb taggers allow one to distinguish particle jets containing BBB hadrons from other jets. Employing feed-forward neural network architectures, this tagger is unique in its emphasis on using information from individual tracks. This tagger not only contains the usual advantages of a multivariate technique such as maximal use of information in a jet and tunable purity/efficiency operating points, but is also capable of evaluating jets with only a single track. To demonstrate the effectiveness of the tagger, we employ a novel method wherein we calculate the false tag rate and tag efficiency as a function of the placement of a lower threshold on a jet's neural network output value in Z+1Z+1Z+1 jet and tbartt\bar{t}tbart candidate samples, rich in light flavor and bbb jets, respectively.

Diboson production ($WW+WZ+ZZ$) has been observed at the Tevatron in hadronic decay modes dominat... more Diboson production ($WW+WZ+ZZ$) has been observed at the Tevatron in hadronic decay modes dominated by the WWWWWW process. This paper describes the measurement of the cross section of WZWZWZ and ZZZZZZ events in final states with large mett\mettmett and using bbb-jet identification as a tool to suppress WWWWWW contributions. Due to the limited energy resolution, we cannot distinguish between partially hadronic decays of WZWZWZ and ZZZZZZ, and we measure the sum of these processes. The number of signal events is extracted using a simultaneous fit to the invariant mass distribution of the two jets for events with two bbb-jet candidates and events without two bbb-jet candidates. We measure a cross section sigma(pbarptoWZ,ZZ)=5.8+3.6−3.0\sigma(p\bar{p}\to WZ,ZZ) = 5.8^{+3.6}_{-3.0}sigma(pbarptoWZ,ZZ)=5.8+3.6−3.0 pb, in agreement with the standard model.

Autonomous gravitational-wave searches -- fully automated analyses of data that run without human... more Autonomous gravitational-wave searches -- fully automated analyses of data that run without human intervention or assistance -- are desirable for a number of reasons. They are necessary for the rapid identification of gravitational-wave burst candidates, which in turn will allow for follow-up observations by other observatories and the maximum exploitation of their scientific potential. A fully automated analysis would also circumvent the traditional "by hand" setup and tuning of burst searches that is both labourious and time consuming. We demonstrate a fully automated search with X-Pipeline, a software package for the coherent analysis of data from networks of interferometers for detecting bursts associated with GRBs and other astrophysical triggers. We discuss the methods X-Pipeline uses for automated running, including background estimation, efficiency studies, unbiased optimal tuning of search thresholds, and prediction of upper limits. These are all done automatically via Monte Carlo with multiple independent data samples, and without requiring human intervention. As a demonstration of the power of this approach, we apply X-Pipeline to LIGO data to search for gravitational-wave emission associated with GRB 031108. We find that X-Pipeline is sensitive to signals approximately a factor of 2 weaker in amplitude than those detectable by the cross-correlation technique used in LIGO searches to date. We conclude with the prospects for running X-Pipeline as a fully autonomous, near real-time triggered burst search in the next LSC-Virgo Science Run.