Project Description

LUX and High Energy Muon Induced Backgrounds in DUSEL

Project Description
I. Introduction
As part of the effort of pursuing large deep underground facilities by the particle physics community to solve some fundamental problems in physics, several experiments have been planned or proposed in the Deep Underground Science and Engineering Lab (DUSEL, [1]) in Homestake, Lead, South Dakota to address topics such as dark matter [2, 3], neutrino-less double beta decay [4,5], neutrino physics, and proton decay [6,7] (such as the Long-Baseline Neutrino Experiment, LBNE). All these underground experiments will benefit from the low background environment in deep underground caverns in DUSEL. Nevertheless, various backgrounds, such as gamma rays, neutrons from fissions, cosmic ray induced high energy muons and particles produced by them when they interact in the rocks and the detector materials still exist at a level that affects not only the experiment design but also the explanation of the experimental data. For most underground experiments that run at very low energy threshold, these particles form a complicated radiation environment for which we have to fit experimental data taken at the particular experiment sites in order to give precise predictions and estimate the backgrounds in the experiments.

Being the first dark matter experiment currently scheduled in DUSEL, the LUX experiment is primarily designed to look for signals from dark matter particles, such as the weakly interacting massive particles (WIMPs) predicted in supersymmetric and extra-dimensional extensions of the standard model [8]. The LUX detector consists of a 350 kg liquid xenon detector that is submerged in a 300-m³ water tank, see Figure 1. The water tank is designed to shield gamma rays and attenuate neutrons that are produced in the rocks and surrounding materials. In addition, the water inside the tank is monitored by 20 photomultiplier tubes (PMTs) so that muons and other charged particles with tracks of sufficient length in the water can be tagged. Due to its low threshold, capability of electron and nuclear recoil discrimination and very low internal radioactivity level in the LUX inner detector, systematic study of the data from the liquid Xenon (LXe) detector and the water tank will provide a benchmark in describing and characterizing various backgrounds in DUSEL, which will further benefit all other future experiments in DUSEL. The LUX collaboration is currently assembling its inner detector in the surface lab at Homestake Lead. Underground deployment to Davis Cavern at 4850 ft level (~ 4300 m.w.e. meter-water-equivalent overburden) is scheduled in the summer of 2011. A 1.5-ton scale two-phase Xenon dark matter experiment LZS using the same water shield technique is under planning, as is a 20-ton experiment, LZD.

Muons with energy above several TeV on the surface can reach labs at 4850 ft level in DUSEL. These multi-TeV muons originate from the interaction of high-energy cosmic ray particles in the Earth’s atmosphere. Our understanding of their production and propagation plays a critical role in the underground background estimation, and its association with other large scale environmental conditions, such as the seasonal changes in the Earth’s atmosphere, the position and moving direction of the Earth in space, etc. The Comic-ray Working Group in the IceCube Collaboration has a long history in cosmic ray physics, and it is currently studying high-energy muon signals using data collected with IceTop and the in-ice array in coincidence. Such data provides unique insight into not only cosmic rays from 300 TeV (1 TeV=10¹² eV) to 1 EeV (1 EeV=10¹⁸ eV), but also high-energy muon and muon bundle production in the atmosphere [9, 10, 11, 12, 13]. Rapid progress on ultra-high energy cosmic rays and muon production are being made in Pierre Auger Project [14] as well. Since the muons or muon bundles produced by comic rays in this energy range are the dominant muon component at the DUSEL depths, to apply the expertise and progress accumulated in IceCube to the study of underground muons at DUSEL is of great interest to both projects.

The proposed research work in this proposal will focus on the LUX experiment and the use of its data for a systematic study of underground muons and muon-induced background signals in Davis Cavern. Being the first dark matter experiment at DUSEL, LUX will provide a benchmark for improving and verifying the simulation of various backgrounds in DUSEL. The research also includes building a simulation scheme to describe the production of high-energy muons and the backgrounds they induce in DUSEL. Such a full Monte Carlo will not only produce more realistic fluctuations in muon-induced backgrounds, but also enable us to study possible correlations between muon-induced background effects and large-scale/long-term modulations in the Earth atmosphere or in cosmic ray flux. These two elements are essential in the explanation of experimental data; however, both are ignored to a large extent in the data analysis of many underground experiments.

The PI on this proposal, Xinhua Bai, became a faculty member in the physics department at South Dakota School of Mines and Technology (SDSMT) in August of 2009. Since then, SDSMT has become an institutional member in the LUX Collaboration and an associate institutional member in the IceCube Collaboration. As a new member in LUX, SDSMT missed all project funding opportunities for the LUX S4 experiment R&D, hardware design, and fabrication. Funding requested in this proposal will support our work with LUX water shield calibration and simulation, LUX deployment, operation, data analysis and other service work including education outreach activities in South Dakota. The PI anticipates hiring one postdoctoral research fellow and supporting two master graduate students and one undergraduate summer student in the next three years. Funds for adding a computer cluster at SDSMT will enable the group to carry out the proposed analysis and simulation work more effectively. Since the LUX Collaboration does not have a centralized computing facility for data storage and analysis, the cluster will also be shared with the other LUX groups and visitors from IceCube. A new digital oscilloscope, a fast laser diode pulser (two wavelengths), an optical attenuator, two beam splitters and seven multimode fibers are needed in the PI’s lab and in Davis Cavern for forthcoming testing, calibration, and education outreach (EO) activities. More details about the personnel plan and the usage of these equipments/devices are given in the “Budget Justification”. Current resources in PI’s lab are summarized in “Facilities, Equipment, and Other Resources”. In addition to research work, the PI’s lab will become the only base on the SDSMT campus for physics major student’s training and education outreach activities related to DUSEL physics programs.

II. Results From Prior NSF Supported Research
While working as a post doctoral research fellow and then as a research scientist in Fermilab, University of Wisconsin-Madison, Bartol Research Institute and in the Department of Physics and Astronomy at the University of Delaware, the PI, Dr. Xinhua Bai was supported by several NSF grants listed in Table 1; however, being a non-faculty member, Dr. Bai was not a PI or Co-PI on these prior grants. This section only summarizes the major results of these grants in which the PI of current proposal played a significant role.
Table 1. Projects the PI of this proposal joined since 1998. Other projects funded by the Chinese National Science Foundation the PI participated before 1998 are not listed.

Title & Grant No. of the Projects	Support Period	Total Grant	PI/Award No.	Co-PIs
South Pole Air Shower Experiment -2 No. 9615101	May 01, 1997 - January 31, 2001	$665,000	Thomas Gaisser	Todor Stanev, Paul Evenson
Continued Operation of the South Pole Air Shower Experiment -2 No. 9980801	July 01, 2001 - June 30, 2004	$719,855	Thomas Gaisser	Todor Stanev
IceCube Startup and Construction Project No. 0236449	August 1, 2002 - March 31, 2011	$201,914,198	Francis Halzen
Air Showers in IceCube No. 0602679	June 01, 2006 - May 31, 2010	$750,000	Thomas Gaisser	Todor Stanev, David Seckel

(1) SPASE2 experiment (Grants No. 9615101, No. 9980801): The SPASE2 experiment [15] was an air shower experiment located at the South Pole. Dr. Bai first worked as a winter-over scientist at the South Pole for this experiment and AMANDA (Antarctica Muon and Neutrino Detector Array) during 1998-1999. In later years, he worked in almost all aspects in this experiment, including detector calibration, experiment operation, improving air shower reconstruction techniques, experiment simulation and physics analysis. He did the muon survey for AMANDA optical modules (OMs) with SPASE2-AMANDA coincident events [16]. In the cosmic ray composition study [17] and point source search work [18] with SPASE2 data, he made significant contributions by reconstructing air shower events and building up a Monte Carlo library at Bartol Research Institute during those two funding period.

(2) IceCube experiment (Grants No. 0236449, No. 0602679): Dr. Bai joined IceCube Project in its conceptual design phase. By carrying out tests at the South Pole and in the lab he set up on the campus at the University of Delaware, Dr. Bai made important contributions in the IceTop ice Cherenkov detector design, early R&D and simulation work [19] and IceCube DOM calibration work [20]. After the first IceCube string was successfully deployed, Dr. Bai carried out and led on various calibrations and performance verifications using IceTop and in-ice coincident data. Some of the results were reported in journal publications or international professional conference proceedings [10, 21, 22, 23]. Particularly and more relevant to the research work in this proposal, Dr. Bai was the first who carefully measured the muon flux on the surface at the South Pole [24]. He did the measurement using scintillate detectors together with a large ice Cherenkov detector. The Cherenkov detector was used as an absorber and as a detector in coincidence. The experiment measured muons with zenith angle from vertical to nearly horizontal. To eliminate the severe background for the horizontal events, various techniques were used, such as coincidence and anti-coincidence, time-of-flight (TOF), and waveform discrimination. In this work, the measured flux was also compared with muon flux from Monte Carlo simulations. Recently, in order to understand the energy loss of high-energy muons or muon bundles in deep under ice, Dr. Bai and his colleagues carried out a Monte Carlo study [25], in which several questions regarding muon bundle energy losses through different interaction channels and the reconstruction of muon bundle energy loss were addressed.

Dr. Bai is also well experienced in using computer clusters. He has been using clusters for his data analysis and simulation work for many years in IceCube. He helped manage the cluster from the science side at Bartol Research Institute for nearly two years. He also worked closely with the computer system administer at Bartol Research Institute in the extension and upgrade of the Bartol IceCube cluster in 2008.

In addition to his research activities, Dr. Bai has participated and led several educational outreach activities for SPASE2, AMANDA and IceCube Project; for example, the presentation for IceCube at the “Antarctic Treaty Meeting Displays” at the Maryland Science Center in April 2009. At “2010 Engineer's Week” on February 19^th in Rapid City, SD, the PI led the physics department effort introducing dark matter detection in DUSEL in five sessions to an audience of about 80 students from regional middle schools including Spearfish Middle School, Newell Middle School and Dakota Middle School. Dr. Bai has also served as a member on the IceCube publication committee between 2007 and early 2010.

Located in Rapid City, 50 miles from the DUSEL, SDSMT is expanding its research interest into experimental astro-particle physics, dark matter search, and neutrino physics. Using the startup fund (total $100K) the PI received from SDSMT, the group has transformed three storage rooms into a particle physics lab that consists of an analysis room, an optical lab and a general electronics/assembly room. We are playing an active role in LUX, such as LXe PMT internal review, water shield PMT unit assembly, testing and calibration. In addition to a master degree student, Mark Hanhardt, currently dedicated to the LUX experiment, the PI is also tutoring an undergraduate student, Douglas Tiedt, in GEANT4 detector simulation using IceTop ice Cherenkov detector simulation package as an example. This student is planning to participate in LUX water shield simulations and to become a graduate student here. The PI and the current graduate student also support and participate in the LUX detector integration in the surface lab. The requested funding in this proposal will allow this local group to continue all the work already started with our university funds and to make more contributions to the LUX experiment deployment, operation and physics analysis.

III. Proposed Work
The proposed work for this project includes: A) LUX water shield calibration, monitoring, simulation and operation; B) analysis of the data from LUX experiment to systematically characterize underground muons and muon-induced background signals in the Davis Cavern; C) building up a full Monte Carlo scheme to simulate muon induced backgrounds in DUSEL starting from cosmic ray primary fluxes, and improving the simulation by applying progress in high energy muon production anticipated in IceCube. As part of this proposal, we will also participate in educational outreach programs through the administration of the Education Outreach (EO) Office at Sanford Lab.
A. Work toward the first dark matter experiment at DUSEL: LUX water shield calibration, simulation and operation.

The water shield used in the LUX experiment is a 300-ton water Cherenkov detector that is designed to shield gamma rays and attenuated neutrons in the Davis Cavern. With the water volume monitored by 20 PMTs (10” Hamamatsu R7081), the water shield can also provide a veto to the LUX inner detector to be free from particles (and their secondary particles) that trigger the water shield. In order to estimate the precise veto efficiency and extract more physics results using the signal in the water shield, one has to carry out very careful calibration and simulation of the PMTs and the integrated water Cherenkov detector. Given the similarity between the LUX water shield and the Ice Cherenkov detector of IceTop and the water Cherenkov detector in Pierre Auger surface array, the experience and knowledge the PI has with these two cosmic ray experiments are unique among LUX member institutions in developing a research and service plan for the LUX experiment.

Currently, we are doing the PMT testing and calibration in the PI’s lab at SDSMT. This work includes taking the single photon electron (SPE) spectrum and waveform data for all 20 PMTs to be used in the water shield, the calibration of their gains, linearity, timing and dark noise rates in the PI’s Particle Physics Lab at SDSMT. The PI’s lab now has the equipment needed to perform most of the calibration work: a data acquisition system, a dark box that can test three PMTs at a time, high voltage supplies, necessary NIM electronics. The water shield signal/ HV splitter box and pre-amplifier box were designed and fabricated by another LUX collaborator UC Davis. Supported by the PI’s start-up fund, a programmable LED pulser recently fabricated at SDSMT (modified from the design used in IceCube PMT calibration work [26]) is under testing. It can be used to study the PMT response to extremely bright light pulses. A fast (~ tens of pico- second) laser diode (such as Hamamatsu PLP-10 or PicoQuant PDL-800 plus diode heads) is requested in the proposal for the timing calibration.

At the time of proposal writing, one Ph.D. student, Nick Walsh, from UC Davis works part time with one master degree student, Mark Handardt, of SDSMT on the PMT calibration in the PI’s lab. A calibration database will be created for each of the PMTs. This database is crucial in signal simulation, veto trigger and data acquisition (DAQ) system design, and the estimation of the energy loss by particles in the water. To optimize the operation of the water shield and use the data from it for physics analysis described in Section B below, a lot more calibration and simulation work are required in the lab and at the experiment site in Davis Cavern. Having a local team will provide great convenience during the experiment. Since the funding for the physics Ph.D. program at SDSMT was delayed, both UC Davis and SDSMT agreed to cooperate on this project closely (letter of agreement from LUX group at UC Davis is attached). In the following years, Mark Hanhardt will continue working together with Nick Walsh from UC Davis on the LUX water shield calibration, monitoring and operation, with Douglas Tiedt to join in the LUX water shield simulation and analysis in year 2011.

A-1: Water shield calibration, simulation, and monitoring using various sources:

In the design, the water shield should have 100% trigger efficiency for particles that have their Cherenkov radiation path length greater than one meter in water [27]. This trigger can be used as a veto to the background produced by particles such as muons that hit the water shield. Signals from the water shield can also be used to veto muons that stop and decay in the water volume (see analysis in Section B-1). To determine the background in the final dark matter search results, one has to quantitize the actual veto efficiency and its dependence on the energy and the type of the incoming particles. This cannot be done without precise Monte Carlo simulations. Along with the PMT testing, the SDSMT group will participate in building the water shield simulation package.

At present, many other parameters in the simulation, such as the reflectivity of the Tyvek liner and water property are taken from published results [28]. In reality, even with the properties of individual detector components measured and incorporated in the simulation, the integrated detector’s performance may still be different from what the simulation predicts. One main reason for the discrepancy is the chance in the interface between the detector components after they are integrated into a system under operation condition. A practical way to overcome this difficulty is to calibrate the detector during its operation. Unlike water Cherenkov detectors in air shower experiment on the surface where background muons suffice for effective calibration and monitoring purposes [19, 29], the water shield for LUX has to be calibrated with artificial sources due to the very low muon flux at 4850 ft level. We plan to calibrate and monitor the water shield with optical light. The optical light will be produced with a LED pulser (large pulses for linearity, saturation and after-pulse study) or laser diode (fast pulses for timing calibration) and guided into the tank through several optical fibers (to be added under this proposal). We can make several measurements at different wavelengths by choosing different LEDs or laser diodes. Calibration run using the optical light can happen periodically to monitor the long-term stability of the water shield without interrupting LUX dark matter data taking.

An additional R&D project with the large water shield is to study how to use it for the study of muon-generated small showers in Davis Cavern. The physics of those sub-MeV showers is poorly known due to the lack of our knowledge about muon photonuclear cross sections in the range of materials constituting the overburden in underground laboratories. Gamma ray with energy higher than several MeV (like those in muon induced local showers) can also be detected after Compton scattering or pair production in the water. In the present design, since the water shield is one continuous volume, except the pulse size and time, little other information can be used to identify different particles. On the other hand, since the water volume is a lot larger than the radiation length (6 m – 8 m versus ~ 40 cm) and Moliere radius (6 m - 8 m versus ~10 cm), small electromagnetic showers can be well contained in a small portion of the water volume. One improvement in the study of muon-induced underground showers may be achieved by adding several more layers or sections in the water volume [30] in future experiments. The R&D work during this funding period will only focus on optimizing the design with full Monte Carlo simulation.

A-2: Work toward the first dark matter search experiment in DUSEL and prepared to make more contributions in the future:

Since the LUX integration campaign started in Lead at the end of 2009, SDSMT group has contributed shifts and provided support to the LUX surface integration with resources based on SDSMT campus, such as sharing electronics and equipment in the PI’s lab, and cleaning LUX internal pieces using facilities in our chemistry department. The SDSMT group will participate in the LUX detector deployment to the underground lab, the water shield maintenance, operation, monitoring, and LUX physics data analysis.

After reliable calibration and simulation tools are developed for the water shield, we will carry out measurements using this 300-ton water shield alone and in coincidence with the inner LXe detector (see proposed research work in Section B below). As described in the “LZ Governance Structure” [31], SDSMT is expected to take the responsibility for the water shield in LUX future development such as LZS (1.5 or 3.0-ton liquid Xenon) and LZD (20-ton liquid Xenon). It also became clear at the last “Fall Workshop on DUSEL Science and Development of the MREFC” that the water shield at present size or bigger will be used in all dark matter experiments being planned for DUSEL [32]. Systematic analysis and simulations of the water shield and LUX experiment fits in DUSEL dark matter search plan. To grow a strong local group through the actual research activities with LUX will benefit the forthcoming projects in DUSEL.