Searching for Dark Matter in Charge-Coupled Devices

1. 1 Searching for Dark Matter in Charge-Coupled Devices Kathleen Chinetti Illinois Mathematics and Science Academy Thomas Schwarz, Ph.D. Fermi National Accelerator Laboratory IMSAloquium April 25, 2012

2. 2 What is dark matter? • Around 80% of all matter in the universe is Dark Matter • Candidate model of DM is the Weakly-Interacting Massive Particle (WIMP) • WIMPs are exotic (‘non-baryonic’), subatomic particles • Interact with baryonic matter only weakly • Particles that have never been identified before by scientists Visible Matter >5% Images from http://www.dogbreedinfo.com/images17/NorthernInuitDogFreya14wks.JPG, http://noodlepie.typepad.com/photos/uncategorized/2008/03/07/img_1014filtered.jpg, http://www.popsci.com/files/imagecache/article_image_large/articles/tevatron.jpg, http://space.1337arts.com/wp-content/uploads/2009/09/thumb.jpg, http://static.ddmcdn.com/gif/space-collision-1.jpg, http://static.ddmcdn.com/gif/space-collision-1.jpg, http://www.fnal.gov/pub/inquiring/timeline/images/standardmodel.gif, http://www.lpi.usra.edu/resources/lunar_orbiter/images/moon.jpg, http://www.michigan.gov/images/j0395972_86320_7.jpg, http://www.visitingdc.com/images/white-house-picture.jpg, http://upload.wikimedia.org/wikipedia/commons/thumb/0/0d/Teddy_bear_27.jpg/250px-Teddy_bear_27.jpg IMSAloquium April 25, 2012 K. Chinetti Dark Matter ~23% Dark Energy ~72%

3. 3 Detecting WIMPs • WIMPs are, by definition, very difficult to detect • Rather than identifying the particle itself, detectors must register the energy release from a baryonic-nonbaryonic collision. • DAMIC is unique through its extremely high sensitivity to low mass DM • Most DM experiments search for ‘high-mass’ DM (50-100 GeV) • DAMIC detects ‘low mass’ DM (0-10 GeV) Figure 2. Direct detection of WIMPs. WIMPs require detectors who measure the scattering of energy off target nuclei. Their low mass and infrequent interactions require both sensitive and low-background detectors. Figure from http://www.unizar. es/lfnae/ipaginas/ip0300.html IMSAloquium April 25, 2012 K. Chinetti

4. 4 DAMIC’s Challenges • DAMIC’s high sensitivity creates problems with background particles • Some are physically blocked from the detector using lead, copper, and earth as shielding. • The remainder must be removed through analysis code. Figure 3. Shielding around the DAMIC detector. The actual silicon-based CCD panels are placed inside several levels of shielding, starting with a copper sheath and culminating in thick lead blocks. Pictured at the far right are DAMIC collaborators. (Physicist Ben Kilminster is on the far left and physicist Juan Estrada is second from right. ) Figure from Natalie Harrison (Fermilab), the DAMIC experiment profile, the FermilabToday archives IMSAloquium April 25, 2012 K. Chinetti

5. 5 Background reduction in DAMIC Figure 4. one hour exposures of CCD at sea level (top), at 350 feet underground (middle), and underground with lead shielding (bottom). The straight line signature of cosmic ray muons disappears underground. With shielding, the scattering from background radiation decreases, limiting the wiggly tracks and dots. A DM interaction looks like a white dot on these images. Figure and text from the FermilabToday archives IMSAloquium April 25, 2012 K. Chinetti

6. 6 My Task Because dark matter detectors utilizing charge-coupled devices (CCDs) are very sensitive to particle interaction, careful selection must separate the candidate dark matter events from background events. How can this selection be standardized and used to effectively search for a possible dark matter signal? IMSAloquium April 25, 2012 K. Chinetti

7. 7 Building a DM analysis code • All data were collected by the DAMIC detector and stored on a Dark Energy Survey (des05) machine. • Analyzed data using C++ language (inconjunctionwithROOT libraries). • Used existing colleagues’ code as a template Courtesy of Thomas Schwarz, FNAL T.Schwarz FNAL J.Estrada FNAL B.Kilminster FNAL J.Molina UNA N.Harrison UChicago IMSAloquium April 25, 2012 K. Chinetti

8. 8 Implementing Background Cuts Muon Neutron? X-ray? DM?? • My code removed background events using amount of nuclear recoil. • Cosmic ray muons and electrons (scattering from background radiation) identified by interaction length • React with each nearby silicon atom • Easily identified and removed using size-based cuts • Both neutrons and x-rays react with only one silicon atom, making them difficult to remove. • Neutrons can only be removed through shielding • X-rays can not be removed through shielding Electron Figure 5. Events as recorded by DAMIC’s CCDs. Figure from Thomas Schwarz, Fermilab IMSAloquium April 25, 2012 K. Chinetti

9. 9 Removing X-Ray Background • X-rays are created as secondary particles of a muon collision with the detector materials • Can be removed selecting diffusion limited hits (“Bikini cut”) Figure 5. Muon track through the DAMIC detector. Charge released increases during travel from collision to pixel. Consequently, depth of the hit is directly proportional to eventual rms value. X-rays only hit on the edges of the detector and will therefore only have either high or low rms values. Figure from Thomas Schwarz, Fermilab IMSAloquium April 25, 2012 K. Chinetti

10. 10 Searching for a DM Signal • The movement of Earth through the galaxy causes time-based fluctuations in DM events • In a plot of time and DM candidate events, should see sinusoidal, month-based dependence • Ultimately plotted data representing nearly a year of collection IMSAloquium April 25, 2012 K. Chinetti

11. 11 Searching for a DM Signal Figure 6. Candidate dark matter events plotted against time of year. The candidate dark matter events were normalized by dividing the entries after cuts by the total data collection time in kiloseconds. IMSAloquium April 25, 2012 K. Chinetti

12. 12 Analyzing the Results • No evidence of a DM signal • Data may be still heavily polluted by background events Br Au Figure 7. Background X-ray energy spikes. Figure from T.Schwarz, Fermilab Ge Mn Cu Sr IMSAloquium April 25, 2012 K. Chinetti

13. 13 Polyethylene Shielding Analysis • Polyethylene should thermalize neutrons • Reduces their energy by providing many targets • Located outside of lead shielding • DAMIC was run with and without polyethylene shielding • With polyethylene shielding, there should be fewer candidate events in the high energy ranges and more in the low energy ranges IMSAloquium April 25, 2012 K. Chinetti

14. 14 Polyethylene Shielding Analysis • Number of events after cuts/number of events before cuts – difference between polyethylene and no polyethylene data in the 2-5 KeV range (~0.06); in the 13-15 KeV range (~0.01) IMSAloquium April 25, 2012 K. Chinetti Figure 2. Energy plots of data collected with and without polyethylene shielding around the detector (left and right, respectively) in the 2-5 KeV range.

15. 15 Discussion of Future Plans • Further analysis of polyethylene shielding • Move the neutron gun back to MINOS tunnel • Implement ‘Anti-Bikini’ cuts to amplify the x-ray signal • Identify the sources of the three x-ray energy spikes • Move the detector deeper • SNOLAB has a 2 km deep facility • Increase the number of CCDs IMSAloquium April 25, 2012 K. Chinetti

16. 16 Acknowledgements I would like to heartily thank my advisor, Dr. Thomas Schwarz. I also would like to gratefully thank the remainder of the group: physicists Juan Estrada, Ben Kilminster, Jorge Molina, and Natalie Harrison. I would also like to thank both the Student Inquiry and Research staff and the Fermilab Education Office staff for their continued partnership and support of Fermi SIRs. IMSAloquium April 25, 2012 K. Chinetti

17. 17 Thank you Any questions? IMSAloquium April 25, 2012 K. Chinetti