DOUGLAS R BERGMAN

Curriculum Vitae

DOUGLAS R BERGMAN portrait
  • Associate Professor, Physics And Astronomy

Research

Research Summary

I study the spectrum and composition of cosmic rays at the highest energies. How the cosmic rays can have such high energies remains uncertain, but it is thought to involve the most extreme astrophysical environments. We observe the cosmic rays via the extensive air showers produced when they strike the Earth.

Research Statement

The field of Ultra-High Energy Cosmic Ray (UHECR) physics is poised to change from exploratory observations and vague speculations to precision measurements and well-constrained theoretical explanations, such as happened in the field of Cosmology a decade ago. The observation of the Griessen-Katsepin-Kuzmin (GZK) Cutoff in the UHECR energy spectrum by the High Resolution Fly’s Eye (HiRes) experiment, of which I was a part, was the beginning of this shift, along with the measurement of changing composition in the Knee region of the cosmic ray energy spectrum by the Kascade Experiment. To continue this move towards precision measurements strongly constraining theoretical models, we must not just measure the features of the UHECR more accurately, but measure the relationships between the features more accurately. The systematic uncertainties involved in comparing different experiments covering different energy ranges currently limits our ability to make these sort of comparisons. A single experiment covering as many of the features of the UHECR spectrum as possible will reduce these uncertainties. The Telescope Array (TA) is just such a project. My current and near future research involves TA and its Low Energy Extension (TALE). TA Monocular Analysis: I am currently analyzing fluorescence detector data from the first three years of TA operation, along with my graduate student Sean Stratton and postdoc Tom Stroman, with the aim of producing an energy spectrum, in the monocular mode, in the very near future. This analysis specifically uses data from the two new fluorescence detector (FD) sites which employ a Flash ADC (FADC) data acquisition system. The third FD site uses reconditioned mirrors from HiRes and a different data acquisition system, so the analysis of that data is a completely separate endeavor. There are a number of spectrum analyses ongoing in TA: monocular, stereo, hybrid, surface detector. The monocular analysis is challenging in that the geometry of air showers is harder to reconstruct using only the timing of signals observed at one site, but it provides the widest energy range of any of the analyses above. Again, it’s only by studying the widest energy range possible that we will improve our understanding of the sources of cosmic rays. The heart of this analysis is to understand the detector itself in all details which are important to observing and reconstructing air showers. We achieve this understanding through simulation of the detector, producing “Monte Carlo” (MC) data in the same format as the actual data coming from the instrument. Our understanding of the detector is only as good as the agreement between distributions of variables in the data and those in the MC. If there is a disagreement between distributions in data and MC, e.g. in the distribution of distances between the shower and the detector, it is a sign of something which should be corrected in the simulation. The extent to which they agree allows us to estimate the systematic uncertainty in our aperture calculation. The great advantage of requiring this kind of understanding of the detector is that we can measure the biases introduced by cuts made to analyze the data. This includes not just cuts made to analyze the data after it has been collected, but “cuts” made by the detector itself when recoding the data, i.e., the cuts imposed by the trigger. TALE Development & Construction: TA itself, using monocular analyses, will only cover energies from 0.3 EeV to just above 100 EeV. This covers the features of the GZK Cutoff and the Ankle. To cover energies an order of magnitude lower, which is necessary to observe the 2nd Knee and the change of composition which should accompany the Galactic-Extragalactic transition of UHECR sources, we have to augment TA with more FDs and an infill array of SDs. I have been primarily involved with the design of the new fluorescence mirrors, which must look higher in the sky to observe the full extent of close-by showers, and I will be extensively involved in the immanent deployment of these detectors. I expect to perform an analysis of the data from the TALE FDs similar to my analysis of TA and HiRes fluorescence data. This analysis will be eased by the fact that the TALE FDs will actually be the reconditioned HiRes-II mirrors and FADC electronics. Non-Imaging Cherenkov Array: To extend the range of TA/TALE even further, I have been designing a non-imaging Cherenkov detector array to work with FDs of the TALE detector. Traditionally non-imaging Cherenkov detectors have been used at lower energies, at around 3 PeV in the Knee region. By extending the usable energy range of Cherenkov arrays to overlap with fluorescence detectors, one gains the ability to cross-calibrate the Cherenkov composition measurement with the unambiguous Xmax measurement available in fluorescence telescopes. The cone of Cherenkov light from an EAS falls primarily within a ring of 120 m about the shower core. The transverse momentum of shower particles broadens this cone, so there is a power law drop-off in intensity outside of the central region. Even with this broadening, the individual counters cannot be placed much more than 150 m apart, so that it is difficult to cover the large areas necessary to see the low fluxes at higher energies. Cherenkov array composition measurements have traditionally used the distribution of light intensity within the ring to determine the depth of shower maximum. This requires very close spacing of detectors. However, the width of the Cherenkov front is also dependent on the depth of shower maximum, and this effect gets bigger with radius, allowing us to have a robust composition measurement even with widely spaced detectors. My preliminary estimates show that we can measure Xmax to about 20 g/cm2 at 300 PeV, comparable to what one can do in fluorescence. The combination of fluorescence with Cherenkov for shower detection has never been done before. If we can achieve it, it promises to give us access to the early development of the air shower, earlier than is visible to the FD alone. Measurement of the early development of the shower will allow us to measure properties of particle interactions at and above the CM energies at the LHC.

Research Keywords

  • Ultra high energy cosmic rays, Interest Level: 5

Research Groups

  • Greg Furlich, Graduate Student. Physics & Astronomy. 08/2015 - present.
  • Bingran Wang, Undergraduate Student. 12/2012 - 04/2015.
  • Jian Lan, Graduate Student. 09/2012 - 04/2016.
  • Thomas Stroman, Postdoc. Physics & Astronomy. 07/2010 - 08/2015.
  • Rhett Zollinger, Graduate Student. Physics & Astronomy. 05/2010 - 07/2011.

Grants, Contracts & Research Gifts

  • Experimental Research in Ultrahigh Energy Cosmic Ray Physics. PI: Gordon Thomson. Co-PI(s): Douglas Bergman. National Science Foundation, 05/01/2007 - 04/30/2010. Total project budget to date: $1,173,500.00

Research Equipment and Testing Expertise

  • Photomulitplier tubes, FADC data acquisition.

Software Titles

  • TRUMP. Simulation software for UHECR fluorescence detectors. Release Date: 01/2011. Inventors: Douglas Bergman, Sean Stratton, Tom Stroman, Elliott Barcikowski.
  • TRUMP: TA Rationalized & Upgradable MC Program. A Monte Carlo simulation program for the Telescope Array (TA) fluorescence detectors. Release Date: 2009. Inventors: Douglas Bergman, Lauren Scott, Sean Stratton. Distribution List: Within the TA Collaboration.