Searching for Sterile with the ICARUS Liquid Argon Time Projection Chamber 20120339ER

Searching for Sterile Neutrinos with the ICARUS Liquid Argon Time Projection Chamber PI: Mills, Geoffrey; P25; [email protected] Abstract A fascinating, natural extension to the standard model of particles and interactions would be the addition of three right-handed states to compliment the three known active, left-handed neutrinos (the so-called 3+3 model). Those “sterile” neutrino additions would be only observable through neutrino oscillations. Indeed, there are a number of hints that point toward the existence of light sterile neutrinos : terrestrial reactor and accelerator neutrino experiments; from fits to cosmological data 2); and the existence of dark matter in the cosmos. A new, neutrino-oscillation experiment, ICARUS-PS 1), is planned to begin taking data in 2013. The focus of the experiment will be to search for oscillation effects in the Δm2 ~ .1-10 eV2 range that would indicate the presence of new sterile neutrinos which have been indicated by recent experiments 2). The experiment will be revolutionary because of its capability to measure both appearance and disappearance for both antineutrinos and neutrinos of the electron and flavors. The new experiment will utilize the excess capacity (of the 19 GeV, CERN synchrotron (PS), and a refurbished PS neutrino beam line, in order to generate an intense flux of neutrinos suitable for a experiment. The neutrino beam and experimental facilities and infrastructure are the responsibility of CERN. The 600-ton, liquid-argon, ICARUS will be moved from its present location at the Grand Sasso underground laboratory in Italy (LNGS) to CERN, where it will be used as the far-detector located 850 meters from the PS neutrino proton target. A new ¼ scale version of ICARUS (150 tons of liquid-argon) will also be constructed over the next two years as a near detector located 127 meters from the proton target. The moving of the ICARUS facility from Grand Sasso (LNGS) to CERN and the construction of the new 150-ton near detector will be the responsibility of Italian INFN funding agency. We have been asked by Prof. Carlo Rubbia (Nobel prize recipient and former Director General of CERN) to join an international collaboration to accomplish that task. Our world-recognized expertise in short baseline neutrino oscillation physics and in the application photomultiplier systems to neutrino and dark matter experiments would be an invaluable contribution to that effort. We propose to design and construct the photomultiplier trigger system for the 150-ton near detector by using our accumulated expertise in the application of PMT systems to neutrino oscillation and dark matter experiments. We also propose use our expertise in pattern recognition a and reconstruction algorithms to extract neutrino event information from the liquid-argon neutrino data, information necessary for oscillation physics, a feat that has yet to be demonstrated. The analysis will include the existing ICARUS CNGS long baseline beam (CERN-to-Grand Sasso) data and the future ICARUS-PS data. The work related to this proposal would be performed over three years and lead to a publication of results from the CNGS data in FY2013, and from the PS data at the end of the first year of data taking, FY2014.

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Research Goals This proposal will take part in an international collaboration to search for active-sterile neutrino oscillations in the parameter range Δm2> ~ 0.1 eV2 at the 19 GeV, proton synchrotron (PS) at CERN. By measuring the neutrino interactions of each flavor of neutrino at two distances, 127 meters and 850 meters from the PS broad-band (0.5-3 GeV) neutrino source. Such a difference could only occur if neutrino oscillations were taking place. An observation of a difference in rate would indicate the presence of new sterile neutrino states, since the number of active neutrino states is known to be three 7), and since their known oscillation parameters do not come into play at this distance (Δm2 << ~ 0.2 eV2). A secondary goal will be to demonstrate that liquid-argon, time projection chamber (LAr-TPC) neutrino detectors are suitable for future long baseline experiments through the analysis of both ICARUS neutrino data samples (LNGS and CERN-PS). The proposed research would contribute significantly to the development of pattern recognition and reconstruction algorithms for LAr- TPC systems. At the present time, LAr-TPC technology appears very promising, and this would be the first significant physics result to be produced by such a neutrino detector system.

A natural, fascinating extension to the standard model of particles and interactions would be the addition of three right-handed neutrino states to compliment the three known active, left-handed neutrinos (the so-called 3+3 model). They would be indeed welcome, and in complete analogy with the right-handed components of the quark and charged fields, would not interact via the usual weak interactions, and hence be “sterile”. While the right-handed and left-handed com- ponents of the quark and charged lepton fields have rigorously identical masses, their uncharged neutrino counterparts need not have identical masses at all. Such sterile neutrino states would be exceedingly difficult to observe directly, as they might only interact via gravitational interac- tions. Neutrino oscillations, however, do offer a window of opportunity for probing for their ex- istence. Since there are an enormous range of possibilities for six (3+3) neutrino mass states and their unitary mixing with flavor states, the possibility of making an exciting discovery remains. The goal of this proposal is to join an international collaboration in order to employ LAr-TPC technology in a two-detector, near-far configuration, to allow the unambiguous measurement of neutrino oscillations in the Δm2 ~ .1-10 eV2 range. The experiment would be sensitive to the dis- appearance and appearance of both antineutrinos and neutrinos of the electron and muon flavors, and thus provide a powerful test of neutrino oscillations. In addition, it is vital to the US neutrino community that the LAr-TPC technology yields suc- cessful neutrino physics result. The ICARUS collaboration has demonstrated that LAr-TPC technology is capable of providing a massive neutrino detector, with extraordinarily detailed im- age information of the underlying neutrino events. It is a primary candidate for the giant neutrino detectors that will be employed in the US long-baseline neutrino oscillation program. This ex- periment will provide an ideal setting to prove and develop that technology with a large sample of neutrino interactions.

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Background & Significance There are a growing number of hints indicating that sterile neutrino states may be missing from our standard model of particles and interactions. The analysis of cosmological data prefers the existence of extra, light degrees of freedom at the beginning of the universe 4), recent experimental results 2) have yielded anomalies in neutrino data from both accelerator based and reactor based neutrino experiments, and no convincing sign of a dark matter WIMP has been seen. The anomalies from reactor and accelerator are consistent with an interpretation as neutrino oscillations, and an experiment to unambiguously resolve those oscillation effects is warranted. Los Alamos is a world leader in short baseline neutrino oscillation experiments as exhibited by its major roles in the LSND experiment at LANSCE, in the MiniBooNE experiment at , and by its major role in the Long Baseline Neutrino Experiment (LBNE). Hence there is a major push to understand those anomalies by using the most advanced neutrino technology available, a LAr-TPC 1) in a two detector configuration. The 600-ton ICARUS detector will be moved to CERN in order to accomplish that goal. In addition, LAr-TPC technology is a high priority R&D effort in the US, because of its potential application in long baseline neutrino physics 3). Indications from a mounting number of directions point at the sterile neutrino hypothesis as a common solution: neutrino oscillation experiments 2) with reactor and accelerator based neutrino sources; fits to cosmological data4) from the number of light degrees of freedom derived from global; and as a natural candidate for the cosmological dark matter observed in the universe. All of those indications point toward the possible existence of extra sterile states, although no defini- tive experiment has yet been performed. The discovery of light sterile neutrinos would dramati- cally transform the landscape of present cosmology. The ICARUS experiment was constructed at the underground laboratory (LNGS) at Grand Sasso in Italy. It is a liquid argon, time projection chamber (LAr-TPC) containing ~ 600 tons of liquid argon as a neutrino target in two separate 300 ton modules. Each 4.2×3.9×19.9 m3 module, filled with ultra pure liquid argon, contains a cathode plane running down the center set at 75KV, and 1.5 meters to either side, three planes of anode sensing wires near ground potential. Ionizing radiation, such as particles created in neutrino interactions, create electrons that drift toward the anode sensing wires at 1.5 mm/µs, and create signals that are digitized every 400 ns by 10-bit waveform recorders. The signals are processed and reconstructed to produce an extraordinary 3D image of the ionization left by the neutrino interaction with a voxel size of ~ 3 mm3. The PMT system is required in order to provide an absolute time for the event, and hence provide an abso- lute scale in the electron drift direction. Figure 1 shows the ICARUS detector at the LNGS and an event display of a neutrino interaction in the detector. Additionally, long baseline neutrino oscillation physics is a top priority in the long-range planning 3) for particle physics in the United States and the worldwide particle physics community. LANL is committing significant resources to the Long Baseline Neutrino Experiment (LBNE), a DOE-HEP project to be carried out over the next two decades. Recent technological advances in liquid argon (LAr), time projection chamber (TPC), neutrino detector technology hold out the promise that we may be at the thresh- old of a new era in the resolution and reconstruction of neutrino interaction in ultra-massive neu- trino detectors, those required for long baseline oscillation physics. ICARUS is the only opera- tional neutrino detector employing the LAr-TPC technology.

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Figure 1 On the right, a photograch of the 600-ton ICARUS detector at the Grand Sasso underground lab (LNGS), Hall A. On the left, an event display of a low energy neutrino interaction in ICARUS showing the extraordinarily detailed im- age possible with the LAr-TPC.

R&D Approach A neutrino oscillation experiment employing both a near and a far LAr-TPC detector con- structed at the refurbished CERN-PS neutrino beam will begin operations in 2013. The near far (near) detector will be installed 850 (127) meters from the neutrino source, and combined with the ~ 1 GeV average energy of the neutrino beam, will have a sensitivity to neutrino oscillations with Δm2 in the range ~ 0.1 eV2 to 10 eV2. The extraordinary, “bubble chamber like” imaging capability of the LAr-TPC detectors should allow the unambiguous determination the flavor of the neutrino reactions on an event-by-event basis, and thus enable the measurement of both the appearance and the disappearance of antineutrinos and neutrinos, for both electron and muon fla- vors. The work proposed here would be to design and construct the critical PMT trigger system for the near detector. That system is essential for the operation of the LAr-TPC as it provides a ref- erence time, or T0, for the TPC, especially in the high rate, ~ 30 kHz, of cosmic rays present at the CERN facility. We will also develop efficient pattern recognition algorithms and reconstruc- tion techniques and test them on the existing ICARUS data from the CERN-to-Grand-Sasso (CNGS) data and eventually use them for the CERN-PS analysis. . Preliminary studies The near detector PMT system design will require initial investigations of both scintillation and Cherenkov light production in the and the expected yield for events typical of the CERN-PS. Liquid argon emits a substantial amount of scintillation light, ~ 2.5×104 photons/MeV, however,

Mills, Geoffrey B. CUI Page Markings (if applicable) 4 Searching for Sterile Neutrinos with the ICARUS Liquid Argon Time Projection Chamber 20120339ER is typically in the UV at 128nm wavelength. Standard phototube face materials will generally not transmit light to the photocathode at that wavelength, so that an appropriate window material and/or wavelength-shifting material will need to be investigated. Initial investigations will de- termine the required surface area, window material, gain required of the PMTs, and also the most appropriate signal processing for the system. Our initial plan is to deploy a system of around 30 8-inch Hamamatsu R5912 PMTs in the near detector. Analysis of the ICARUS data and development simulation tools for the ICARUS configuration will be aided by existing software (LArSoft) already under construction for the MicroBooNE project at Fermilab. This will form a basis for a close connection between the ICARUS devel- opment and the US lead LBNE effort. The geometry of the ICARUS near detector will initially be implemented in LArSoft and allow studies of light yields for realistic events and aid in the PMT system design. Methods, Technical Challenges & Alternatives For the PMT system there are a number of decisions that need to be made prior to purchasing the hardware. The model we have in mind is based tests made here at Los Alamos and else- where. The Hamamatsu Photonics Division has developed special photomultipliers, enhanced to operate at the 89 degrees Kelvin of the liquid argon. The main difficulty has been the conductiv- ity of the photocathode material at low temperatures, and a special, platinum, photocathode un- dercoating has been developed to mitigate that problem. There are several approaches to observing the UV scintillation light, but the most common is to coat the photocathode face with a wave-shifter, tetra phenyl butadiene (TPB), which has been shown to work in liquid argon cryogenic applications with good quantum efficiency. The tech- nique has been developed for the MiniClean/DEAP LDRD effort here at Los Alamos, and this proposal would benefit directly from that effort both in knowledge and equipment facilities for coating the PMTs. The PMT mechanical support and cabling design will performed in conjunction with engineers at CERN and other groups in the ICARUS collaboration. The initial ideas for readout of the PMTs will be to use 1 GHz waveform digitizers with circular buffering, built-in peak detection, zero suppression, that also have the capability to generate an external trigger signal. Those prop- erties will enable the PMTs to act as an independent trigger, however, the primary trigger for the experiment will be generated by the accelerator. This trigger will come every 2.1 seconds, the repetition rate of the PS accelerator, which provides beam with a single-turn extraction pulse of 2.1 µs. As its primary task, the PMT system will detect the event time inside this 2.1 µs window. The PMT trigger will be used in logical coincidence with the beam pulse trigger in order to re- duce that amount of data recorded. A cosmic ray rate of ~ 30 kHz is expected to generate an ac- cidental trigger inside the 2.1 µs beam window ~ 6% of the time, while true neutrino interactions will happen much less frequently. Thus 94% of the beam triggers may be safely discarded which is especially useful given the 100MB size of a single, compressed ICARUS event. The ~ 30 PMTs must be purchased towards the end of FY 2012. They must be coated with TPB, have the bases and cables installed, tested in liquid argon, and finally packaged and sent to CERN. Installation of the PMTs will take place in FY2013 and be supported by CERN techni- cians. The development of the pattern recognition and reconstruction algorithms necessary to extract event properties such as muon, electron, photon, , and proton energy and direction, particle

Mills, Geoffrey B. CUI Page Markings (if applicable) 5 Searching for Sterile Neutrinos with the ICARUS Liquid Argon Time Projection Chamber 20120339ER interaction vertices, neutrino energy will commence in FY2012 with the ICARUS CNGS data, and continue with the CERN-PS data in FY2013. Initial work will be to investigate the use of self-annealing clustering techniques in conjunction with MIXT fit techniques that promise effi- cient processing of the large amount of information contained in the image data. Simultaneous development of the simulation and analysis software will be coordinated with the LArSoft effort at Fermilab and with the ICARUS collaboration. Expected results We expect to have completed the installation and commissioning of the detectors in late 2013 and then begin physics running, most likely in neutrino mode, where the around the proton target is set to focus positive and . We expect to have developed pattern recognition and reconstruction algorithms that are suffi- cient for extraction relevant physics information from LAr-TPC neutrino detectors. The successful completion of this proposal will produce detailed search for neutrino oscillation in the Δm2 ~ 0.1-10 eV range in both appearance and disappearance modes. The first year of the run, spanning 2013 and part of 2014, will most likely be in neutrino mode (positive horn focus), and thus directly address the anomalies in reactor neutrino data and the MiniBooNE, neutrino- mode, low energy excess of events. We will also have validated the LAr-TPC technology in a neutrino physics environment. This will have implications for the US long baseline program and future neutrino oscillation experi- ments. It is hoped that this proposal will lead to enhancements in the HEP base grant we have at the present time and dovetail with the end of the LDRD-ER support. Initial discussions last year with DOE/HEP representatives were not adverse to this idea. Schedule and Milestones The schedule for the PMT system will span roughly two years and include the design and con- struction of the PMT trigger. The design and purchase of conponents will occur in FY12 and early FY13. The installation of the system into the near detector is expected to take place in the middle FY13, while the check-out and operation will happen in late FY13 and FY14. At the same time, the software development schedule will commence in FY12 with the setup of simula- tion tools, proceed to pattern recognition and reconstruction development using the ICARUS- CNGS data as a testing ground, finally continue through FY14 with the analysis of the neutrino data and publication of results at the end of FY14. Mission Relevance & Program Development Plan This proposal directly builds core mission capabilities at the laboratory. Its primary goal of search for sterile neutrino oscillations is a component of the “Beyond the Standard Model” core missions of research at the laboratory. At a national level it provides direct experience with one of the primary technologies proposed in the Long Baseline Neutrino Experiment (LBNE). The proposal also utilizes present LDRD developments in photomultiplier coatings and operations at cryogenic temperatures, already developed at the laboratory for dark matter research. There is also significant overlap with threat reduction R&D where liquid argon TPC systems are being developed for nuclear materials detection.

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One of the grand challenges put forward for the laboratory is to seek new phenomena beyond the standard model of particle and their interactions. This proposal will make a sensitive, unam- biguous search for active-sterile oscillation effects outside the standard, three flavor neutrino os- cillation framework. Sterile neutrinos may hold the secret to a number of unexplained results, currently beyond the standard model of cosmology. The proposal will also provide a concrete demonstration of LAr-TPC technology as a neutrino detection system and explore its capabilities. This will provide crucial information on the effi- cacy of LAr-TPCs for decision makers in the LBNE program that the laboratory is currently en- gaged in. Additionally, there is now active R&D inside the lab in area of threat reduction (TR), for SNM detection technologies that plan to utilize LAr-TPC technology. The work in this proposal this will overlap significantly with their efforts. Qualifications of Research Team & Budget Justification The key contributors are Victor Gehman (P23) and Geoffrey Mills (P25). Dr. Gehman has been working on the MiniClean/DEAP dark matter search project for a number of years and is an ex- pert in the utilization of photomultiplier tubes in liquid argon, he is developing equipment to coat photomultiplier tubes with TPB, and has published articles on PMT operation in liquid argon. Dr. Mills has been working the field of neutrino oscillations for 16 years and has a long publica- tion history in the area. He was specifically approached by the ICARUS collaboration to join their effort. The budget calls for two FTE of postdoctoral researchers to perform much of the research activi- ties under the supervision of two staff members with a total of 0.3 FTE. From our experience this will be adequate for the proposed tasks of PMT system development and construction and analy- sis software development. The effort will be part of a larger, worldwide collaborative effort to develop and use LAr-TPC detectors in neutrino physics. We are requesting $250k in supplemental M&S for this project. The extra funds will be used to primarily to purchase the hardware components for the PMT trigger system. The PMTs are ex- pected to cost ~ $100k, their readout digitizers and associated equipment another $100k, plus shipping and install costs of another $50k. All of the M&S costs are central to our proposal which would not be viable without them.

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______References 1) ICARUS-PS proposal 2) LSND, MiniBooNE, Reactor anomalies 3) LRP-P5 4) Global cosmological fits 5) Dark matter review 6) PDG neutrino oscillation discussion 7) Giunti, Karagiorgi,… 8)

CVs for PIs and Key Investigators

Each CV should not exceed two pages. PI, Co-PI, and Co-Investigator CVs are used (together with the section titled “Qualifications of the Research Team”) to judge the ability of the research team to successfully care out the proposed work. CV’s do not count in the proposal length. ______

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