Project Description

C. Building a full Monte Carlo simulation of muon induced backgrounds in DUSEL, starting from cosmic ray primary particles and improving the simulation by applying up-to-date progress on high-energy muons or muon bundles made in IceCube and Auger.

Most current underground background simulations start from an average muon flux on the surface. The advantage of this approach is that one can alter parameters and make sure the surface muon profile gives accurate predictions for muon flux and spectrum. Nevertheless, this approach does not include several important features that are equally important for underground experiments searching for exotic events. One of them is the fluctuation (in both energy and multiplicity) related to the muon production in air shower development. This fluctuation is much larger than the fluctuations introduced in the muon propagation through the overburden, and can affect the dispersion of muon-induced backgrounds in the LXe detector. The other is the modulation in air shower development in the Earth atmosphere. Both of these have features visible in underground labs at DUSEL level.

The IceCube and Auger collaborations are making significant effort in the study of high-energy muon production by high-energy cosmic rays. In particular, the IceCube Cosmic-ray Working Group led by Bartol Research Institute and the University of Delaware is working on several topics using IceTop and in-ice coincident events that may eventually improve the calculation of muon production in the TeV region and above. One topic is the production of muons above 100 GeV. Despite the progress made through many years of both experimental and theoretical work, discrepancies still exist among different model predictions. See summary in Figure 6 [70, 71].



Figure 6. Left: Comparison of inclusive muon flux predictions to L3 data [70]. Shown are calculations using QGSJET, SIBYLL and TARGET as high-energy hadronic interaction model. Right: Vertical atmospheric muon (and neutrino) fluxes. Conventional muon and neutrino fluxes by solid and dashed lines marked “conven.” Other curves and shaded areas are the prompt muon flux predictions from different model calculations together with two experimental bounds from LVD and AKENO. See [71] and references therein for more details.

Another relevant topic is the high-energy muon bundle structure. For a long time, people have known high-energy air showers can produce multiple high energy muons in the form of muon bundle in which muons are highly collimated and close to each other in space [38]. An empirical description of the integral muon energy spectrum in air shower was given by the Elbert formula [72]:

in which A, E₀ and  are the mass, total energy, and zenith angle of the primary nucleus. p₁=0.757 and p₂=5.25 [38]. Nevertheless, most underground muon flux measurements have assumed single, uncorrelated muons. In the LUX water shield, the ratio of multiple-muon events to single muon events is about few percent. Study has shown that giving the energy carried by all muons in the bundle to a single muon makes a huge difference in IceCube [73].

Some efforts have been made to improve the IceTop-InIce coincidence data analysis [25, 74]. As IceCube completes its deployment in 2011, within several years, the 1-km² IceTop array will accumulate significant amount of air shower data from several hundred TeV up to ~ 1 EeV, among which there will be unprecedented coincident events between IceTop and the in-ice array. Progress is expected in the calculation of muon production and muon bundle characteristics in the energy region that dominates the muon flux in DUSEL. The PI of this proposal has been joining the cosmic-ray working group phone conferences and will follow the progress in IceCube and manage to apply it in the development of a full Monte Carlo simulation scheme to simulate muon-induced backgrounds in DUSEL.

With the advantage of working on LUX experiment, we will be able to cross check the simulation to great detail with the systematic measurements done with LUX data described in Section B, which is the only reliable way of developing a precise background models for a certain site.

4. 
   Broader Impacts
   

Benefits to science: The results of the proposed work will provide the underground physics community with the first systematic experimental muon background profile at the 4850 ft level in DUSEL and their signatures in a sensitive underground dark matter detector, which will provide a benchmark for all forth-coming underground experiments in DUSEL. The modulation study will help the physics community understand better this long debated phenomenon in dark matter experiment and its influence in the search for extremely rare events. By developing a full Monte Carlo background simulation scheme starting from cosmic ray flux, we expect to eventually build up a full Monte Carlo background library for DUSEL that also includes the fluctuations in air shower development and seasonal effect in the Earth atmosphere. After being improved over time, such a library can eventually serve all planned underground experiments in DUSEL. Before dark matter particles are detected, the search for them is basically to increase the detector sensitivity by increasing the detector size and understand/eliminate the backgrounds. It is worth to point out that significant uncertainties still exist in our understanding of various backgrounds in present dark matter search experiments [75]. Based on new results and ideas that may come out of our research, with the full Monte Carlo background simulation to be built, we will be able to re-evaluate more quantitatively the necessity of having an appropriate surface array for DUSEL [76].
Prepare for future DUSEL programs by integrating research and education at SDSMT: During the course of the proposed work, the postdoctoral fellow and graduate students will be expected to play major roles in all areas of the research, and will have the chance to be trained in many aspects of research including data analysis, simulation and some service tasks. Through participation in the large international collaborations LUX and IceCube, they will also have the opportunity to experience working with large teams of diverse scientists, helping them to develop important skills for their future careers. All group members will be encouraged to attend professional conferences to present their work, and to mentor undergraduate interns, thus improving their own communication skills.

Similarly, advanced undergraduate students will gain valuable research experience by working closely with the postdoctoral fellow and graduate students on subtasks such as maintaining and upgrading the software and the study of simpler problems in major tasks. They will also accompany the group to the DUSEL site to participate in experiment deployment, calibration, maintenance and operation. They will be encouraged to present research results at undergraduate research symposia at professional conferences.

Moreover, since water Cherenkov detector is one of the favored options in the Long-Baseline Neutrino Experiment design, to grow a local group with experience on the LUX 300-ton water shield will benefit DUSEL science programs in the future.
Formal education: The Education Department at Sanford Laboratory is in the planning stages for a major science education center, the Sanford Center for Science Education (SCSE), to be built as part of the DUSEL Laboratory. The mission for the SCSE is, in part, to draw upon the science and engineering to be pursued at DUSEL to inspire and prepare the next generation of scientists, engineers and educators. They are in the process of developing prototype programs that would transition to and build capacity for the programs to be run by the SCSE, both onsite, offsite and virtual. The programs will be grounded in education research best practices and employ rigorous evaluation.

The planning team for the SCSE has need of content experts in designing these programs. From scientific point of view, the detection of dark matter would open a new window to the hidden portion in the Universe that is nearly ten times heavier than what we know today. It would be a major scientific discovery with wide-ranging implications for all of particle physics, cosmology and astrophysics. The search for dark matter has gained broad public attention. To help impose more positive repercussions for the role of science in society, the PI will work in partnership with the planning team for the SCSE to help devise and prototype models for hands-on or virtual delivery of astroparticle physics content. The PI is beginning that partnership in the summer of 2010 by being a lecturer for the Davis-Bahcall and Fermilab-BNL-Homestake summer scholars programs, which will bring twenty of the brightest science students (rising freshmen and sophomore undergraduates) in South Dakota to Sanford Lab for one week each. While there, the students will learn modern physics, participate in tours and hand-on activities, and perform an experiment of their choosing underground. These students will be tracked and mentored through the rest of their undergraduate careers and will be prime candidates for internships at SDSMT, Sanford Lab and elsewhere (students from 2009 have secured DUSEL-related internships for 2010 at Princeton University, Brookhaven National Laboratory, Colorado School of Mines and Sanford Lab)

Involvement of underrepresented groups: Located in the Great Plains, residents in South Dakota have a long tradition of mining and farming, the population of 800,000 being 67% rural. South Dakota is also the home to nine Native American tribes, comprising 9% of the population. From family structures to spirituality, people in South Dakota have a rich and colorful culture. In general the perception among all groups is that if one is interested in a career in Science, Technology, Engineering or Math (STEM) fields, one must move out of the state.

The reservations located in South Dakota are among the poorest counties in the country. American Indian students have high drop-out rates and only small numbers typically attend college; even fewer finish college, and only a very few pursue degrees in STEM fields. The presence of DUSEL in western South Dakota provides an opportunity to involve more American Indians in the design, construction, science and engineering of the facility.

SDSMT only has about 2.5% Native Americans on campus and many of them are the first generation to attend college in their family history. In order to improve this, SDSMT hosts several programs, which include NSF Tiospaye in Engineering Program, and the SD GEAR UP Honors Program, etc. Some of those programs (such as the SD GEAR UP Honors Program [77]) are designed to prepare Native American students to be successful in the science and college setting. Being the first astroparticle physicist in the history of the SDSMT, the PI will participate in local educational outreach programs for all groups with different cultural backgrounds. The PI and other group members will develop mini-lectures related to the research work with the IceCube and LUX experiments for interested SD GEAR UP students during their six week residential program on campus every summer. Tours will be provided of the PI’s lab on campus. This work will be coordinated with the work that Sanford Lab Education Department is also doing with GEAR-UP students, in order to provide an integrated approach to activities and lectures based on DUSEL and the science and engineering to take place there. This integrated approach will touch the GEAR-UP students at every grade level, and prepare them to graduate into other enrichment activities such as the Davis-Bahcall Scholars program, which would give the students an opportunity to visit Gran Sasso Laboratory in Italy and to study physics at Princeton for three weeks during the summer after their senior year of high school.