The main goals of the MicroBooNE experiment are: (1) to demonstrate the capabilities of a liquid argon TPC in the reconstruction of neutrino events; and (2) to determine whether the excess of events observed by the MiniBooNE experiment  are due to events with an energetic electron or photon.
MiniBooNE has observed a 2.8σ excess of antielectron-neutrino candidate events in antineutrino mode that is quite consistent with neutrino oscillations at Δm2 ~ 1 eV2 and with the LSND oscillation signal . In neutrino mode, MiniBooNE observes a 3.4σ excess of electron-neutrino candidate events that is consistent with the LSND oscillation signal if CP violation is included. MicroBooNE can test the combined evidence for oscillations from LSND and MiniBooNE by determining whether there is an excess of energetic electron events, as expected for oscillations, or an excess of energetic photon events, which would indicate some other process. MiniBooNE is unable to differentiate electrons from photons; however, with the MicroBooNE liquid argon TPC, shown in Figure 1, events with an energetic electron will look much different than energetic photon events. The LANL team has spent 20 years working on the LSND and MiniBooNE experiments, and MicroBooNE is a great opportunity to understand further the excess events observed in the two experiments.
LANL staff (Mills) wrote the original GEANT3 (later translated to GEANT4) based software to predict neutrino fluxes in the Booster Neutrino Beamline (BNB). LANL staff (Mills), through a Laboratory Directed Research and Development (LDRD) funded project, led the HARP-MiniBooNE collaboration, which produced well-measured pion yields for the MicroBooNE beamline. LANL is now leading the MicroBooNE beam group efforts. As part of the work, we have provided the initial calculations of the neutrino flux at MicroBooNE, and we continue to maintain them. These flux calculations were used to develop preliminary oscillation and cross section analyses. The calculations include both BNB and NuMI neutrino flux estimates. The primary source of neutrinos for MicroBooNE is BNB; however, the NuMI beam will provide higher electron neutrino and antineutrino event rates and a unique opportunity to study these events. Additionally, MicroBooNE can provide a measurement of the kaons produced in the NuMI beamline, which will be useful for other experiments using the NuMI beamline.
The MicroBooNE DAQ system is responsible for the collection of signals from the TPC and PMT readout systems, and then it builds and records the completed events based on a trigger decision. Additionally, the DAQ system is responsible for run control and monitoring each component of the detector. LANL is involved with many aspects of the DAQ system, but is most heavily contributing to the low-level DAQ applications responsible for receiving and properly handling data from the TPC and PMT readout electronics crates.
Both the TPC and PMT readout electronics systems organize into readout crates mounted onto racks in the detector hall. In total, there are nine TPC readout crates that collect data from the 8256 wires in the TPC, and one crate dedicated to the readout of the 30 PMTs and trigger decision. In a ``triggered'' mode suitable for performing neutrino physics analyses, a positive trigger decision prompts each readout crate to send its data out via an optical link to a custom-built PCIe card housed in a PC dedicated to handling data from the crate. These PCs, called ``sub-event buffer'' or SEB PCs, continuously check for the arrival of new data coming into the PCIe card and, upon arrival, move it to a large buffer on the PC. A separate process forms event fragments from the data and, once all of the data in an event from one crate is formed into a package, it is sent along via an ethernet connection to an event-builder (EVB) PC. The EVB PC collects data from each of the SEBs, puts it in the proper data format, attaches headers to specify identifying marks for the event (run and event number, trigger decision, the time from a GPS module, etc.), and writes the event to disk. Each full event will be $\sim20$~MB in size, and so in order to handle a 10 Hz rate from the BNB, we utilize lossless compression via Huffman coding to reduce the size by about a factor of 10.
In addition to this triggered mode, data from the readout crates are read out in a continuous matter. The data may be analyzed in the future to look for the presence of neutrinos resulting from a nearby core-collapse supernova. The data is compressed to a manageable size in each readout crate, and then sent to a separate PCIe card in the SEB via an optical link. The SEB extracts the data in a similar way to the triggered mode, but rather than send it to the EVB, it writes it to local disks to be held for up to two days. If a supernova is detected, the data corresponding to the time of the core-collapse will be collected from all the SEBs and analyzed offline.
Wes Ketchum, a LANL Director's-funded postdoc, has taken on much of the effort to write the software directing the data flow in the SEB and EVB PCs. We make use of a DAQ test stand located at Fermilab, where we operate a readout electronics crate, SEB, and EVB PC system, which read signals from a mock TPC. We have developed much of the software to perform data-taking operations, made preliminary measurements of important quantities like data-throughput rates, total latency, and error rates. We have also validated many important aspects of the DAQ system, including the ability to maintain reliable connections between the EVB and SEBs, to monitor in real time the condition of the PCs and important properties of the data flow, and the ability to run simultaneously with the triggered and continuous readout streams.
LANL is also contributing heavily to the serial checkout tests MicroBooNE will conduct as the front-end electronics are mounted onto the TPC. These tests are not only a final check that the electronics are in good working condition when they are installed, but the testing procedure is very similar to the calibration of the electronics that will be done during commissioning and running of the experiment. So, by carrying out these quality tests, we will also exercise the DAQ to take electronics calibration data, quickly perform basic processing of the data, and store the results in a calibrations database. We also plan to perform a number of additional tests to validate the mapping of all the electronics components in the readout system.
In addition to its primary goal of investigating the low energy excess observed with MiniBooNE, MicroBooNE will have sensitivity to sterile neutrino oscillations comparable to MiniBooNE. The preliminary oscillation sensitivities have been calculated using the software framework developed at LANL for the final MiniBooNE appearance results. Zarko Pavlovic has provided the fitting code and worked on adapting it to the MicroBooNE software environment. The fitting technique includes the simultaneous fit to both muon and electron neutrino charged current quasi-elastic (CCQE) event samples, allowing for reduced systematic errors by constraining the fit to the high statistics muon neutrino CCQE sample.
Zarko Pavlovic has done preliminary studies to estimate the sensitivity to sterile neutrino oscillations by using the combined MiniBooNE and MicroBooNE data sets. The excess of events observed in MiniBooNE neutrino mode has 6? statistical significance; however, when systematic errors are included, it is reduced to a significance of 3.4σ. The systematic error is dominated by uncertainties in the gamma-induced backgrounds. MicroBooNE will provide an important measurement of these backgrounds. Unlike Cherenkov detectors, the LArTPC offers powerful resolution between electron and gamma-induced events. By combining the MiniBooNE and MicroBooNE data sets, it is possible to achieve an improved sensitivity to oscillations in the Δm2 ~ 1 eV2 region.
For a complete sterile neutrino picture, it is necessary to measure both electron neutrino appearance and muon neutrino disappearance. MicroBooNE and MINOS+ will be the next two experiments to run in the near future that will provide more information on these two crucial measurements. The Los Alamos neutrino group is involved with both measurements. Zarko Pavlovic is playing a major role in developing the oscillation analysis on MicroBooNE, and on MINOS+ he is leading the reconstruction group. MINOS+ is a continuation of the MINOS experiment in the NOvA era. One important aspect of the experiment is the much higher neutrino event rate in the near detector compared to previous running. Over the last six months after taking the lead of the reconstruction group, Zarko has made changes in reconstruction, which have led to a significant improvement in reconstruction speed.
Given that the origin of the MiniBooNE 3.8σ excess in neutrino and antineutrino mode is unknown (e.g. oscillations, backgrounds, exotic physics, etc.), it is prudent to consider all possible explanations. A new opportunity to exploit the specific features of the BNB configuration arises from the physics of certain dark matter particle candidates. Recent theoretical work suggests that sub-GeV weakly interacting massive particles (WIMPs) are viable candidates for the dark matter in the Universe, but fall into a mass range that is difficult to probe using direct searches. Various dark sector models predict that low mass WIMP production and subsequent scattering in MiniBooNE or MicroBooNE is a detectable signal . Richard Van de Water and Zarko Pavlovic have been working with theorists from the Universities of Victoria and Chicago who are investigating this possibility. Detailed signal sensitivities for WIMP-Nucleon scattering in MicroBooNE have been calculated and are shown in Figure 3. The neutrino background swamps these neutral current like signals; however, by steering the beam off target, the neutrino backgrounds can be reduced by two orders of magnitude, while leaving the WIMP production mechanism unaffected.
We are currently working on improvements to the sensitivity plots by developing detailed event generator for the beam and detector simulations. MicroBooNE benefits from higher efficiency and lower backgrounds, but has 7-8 times less fiducial mass than MiniBooNE. We are investigating ways to exploit the exquisite precision of the MicroBooNE TPC to possibly enhance sensitivity to a WIMP signal.
Exotic searches in both MiniBoonE and MicroBooNE are important. The main physics goal of MicroBooNE is to determine if the MiniBooNE oscillation excess is due to electrons or photons. If the excess is electron-like, then this is evidence for oscillations. If the excess is photon-like, then this is evidence for Standard Model neutral current like backgrounds with final state photons. However, dark sector models also predict photons in the final state via WIMP-nucleon scattering through a ? resonance. This can be tested in detail with a beam off target run. We are developing a proposal for such a run after the initial three-year neutrino run.
LANL staff (Garvey and Mills) were responsible for the construction of the Booster Neutrino Beamline (BNB) Beryllium target and, working closely with FNAL staff and LANL staff Richard Van de Water, was heavily involved in the monitoring and debugging of the various elements of the BNB during its ten years of running for MiniBooNE. The LANL team has successfully ensured that the beamline has run at its full capacity. In 2004 we helped diagnose the horn failure and played an important role in the subsequent study to understand and fix the problem. In 2006 we spearheaded the effort to resolve the fallen 25 m absorber plate problem caused by hydrogen embrittlement of high grade steel chains, and helped develop a new hanging structure to permanently secure the blocks in place. More recently, in 2010 when the horn water lines became clogged, Gerry Garvey was instrumental in the understanding of the problem and the realization that the horn shorts were due to the collection of water in the sump pan, thus not requiring a change out of the second horn. The solution was to pump water from the pan, and we were able to continue running with two water lines out of six valved off. The Be target that LANL designed has worked flawlessly over ten years of running. Geoff Mills continues to monitor its performance carefully, including temperature versus proton rates, to ensure there are no signs of degradation.
A few deficiencies in the beamline operation have been identified through careful understanding and analysis of the MiniBooNE neutrino data MiniBooNE. The following upgrades/improvements have been designed in conjunction with FNAL accelerator division staff. One such improvement is the development of dual low mass beam position multiwires one meter upstream of the target. These multiwires can stay in the beamline during running and will have a position resolution on the order of 0.5 mm. This will allow better determination of the proton position and direction, which will result in better positioning of the beam on target during operation. This also allows better determination on an event-by-event basis of the direction/angle of the proton beam relative to events reconstructed in the detector. This will improve sensitivity for exotic physics that are related to the proton direction, such as bremsstrahlung type processes. The multiwires are built and were installed during the 2013 accelerator shutdown. The locations of the multiwires are shown in Figure 2. We are currently commissioning and analyzing data to determine how well the multiwires are working.
A second improvement is the development of a new beam timing system to improve the absolute beam time determination and RF structure. This will involve using fiber optic transmission of the Resistive Wall Monitor (RWM) beam crossing time to transmit the signal to the detector. In the past, for MiniBooNE, this was done with an RG59 copper wire, which caused major stability issues related to lightning storms. (yes, 0.5km of buried copper wire makes a good lightning strike monitor!) The new hardware has been purchased, tested, and installed. A new system has been developed to capture the RWM waveform that will allow recording of the 53MHz timing structure of the proton beam. This involves a fast digital oscilloscope and readout of the data into ACNET for storage in the new intensity frontier beam database. This new timing system will be more robust and have better timing characteristics to allow searches for exotic particle production in the beam.
The MicroBooNE data analysis requires information about the beam in addition to the detector data. A new beam database has been developed at Fermilab to collect all of the beamline instrumentation data and provide it to experiments. Zarko Pavlovic has developed a software package that polls the data from the database and stores it with the detector data. The stored beam data includes all of the beam information necessary for analysis, including the proton beam intensity and beam quality data on a spill-by-spill basis. The preliminary beam DAQ data format has been defined in accordance to the MicroBooNE DAQ, and the first version of the code required to collect, store and merge, beam data with detector data has been developed. The merging process requires that each detector beam trigger data set be matched with beam instrumentation data from the same spill. Data from both the Booster Neutrino Beamline (BNB) and the Neutrinos at the Main Injector (NuMI) beamline will be acquired. These two neutrino beamlines are the sources of neutrinos for the MicroBooNE experiment.
There is a strong team of LANL staff members working on MicroBooNE from the Subatomic Physics Group (P-25).
Staff Scientists and Postdoctoral Scholars:
- Wesley Ketchum
- William Louis
- Geoff Mills
- Zarko Pavlovic
- Richard Van de Water
- A. A. Aguilar-Arevalo et al., Phys. Rev. Lett. 110, 161801 (2013).
- A. Aguilar et al., Phys. Rev. D 64, 112007 (2001).
- R. Dharmapalan et al. [MiniBooNE Collaboration], arXiv:1211.2258 [hep-ex] (2012).