Cosmic Ray Background Rejection with Wire-Cell LArTPC Event Reconstruction in MicroBooNE MICROBOONE-NOTE-1084-PUB The MicroBooNE Collaboration∗ (Dated: June 20, 2020) For a large Liquid Argon Time Projection Chamber (LArTPC) operating on or near the surface to detect neutrino interactions, the rejection of cosmic background is a critical and challenging task because of the large cosmic ray flux and the slow timing of the TPC. In this paper, we introduce a superior cosmic background rejection procedure based on the Wire-Cell 3D event reconstruction for LArTPCs. The foundational reconstruction techniques including the 3D imaging and clustering of TPC ionization charge data, processing of PMT light data, and matching of the TPC charge and PMT light information are reviewed. This is followed by a detailed description of track trajectory fitting and dQ=dx determination. These reconstruction tools further enable various methods to tag cosmic-ray muons in time with the neutrino beam spills. From a hardware trigger level 1:20k neutrino to cosmic-ray background ratio, high-performance generic neutrino selection, i.e. cosmic- ray background rejection, is achieved in the MicroBooNE experiment with a cosmic contamination of 14.9% (9.7%) for the visible energy region greater than 0 (200) MeV. High efficiencies of neutrino interactions are also retained, and they are 80.4% and 87.6% for νµ charged-current and νe charged- current interactions, respectively. CONTENTS B. Stopped muon examples 23 I. Introduction 1 References 24 II. MicroBooNE detector and readout 3 I. INTRODUCTION III. Review of foundational reconstruction techniques 4 The liquid argon time projection chamber [1{4] A. PMT light reconstruction 4 (LArTPC) is a three-dimensional tracking calorimeter B. TPC charge reconstruction 4 that is widely used in neutrino physics [5{12]. When 1. TPC digital signal processing 5 charged particles traverse the LAr detection medium, 2. Tomographic 3D image reconstruction 5 ionization electrons and scintillation photons are pro- 3. 3D clustering 6 duced. The detection of scintillation photons by a light C. Matching between charge and light 6 detector (e.g. a photomultiplier) provides the time of the activity. Under the influence of an external electric field, IV. Track trajectory and dQ=dx determination 6 the ionization electrons travel at a constant speed toward A. Track trajectory fit 7 the anode plane. The transverse position of ionization B. dQ=dx fit 8 electrons can be determined using position-sensitive de- C. Performance 9 tectors (e.g. multiple parallel wire planes with different wire orientations as shown in Fig. 1) at the anode. Given V. Rejecting in-beam cosmic-ray backgrounds 10 the drift velocity, the longitudinal position along the drift A. Effective boundary and fiducial volume 12 field can be calculated from the time delay, or drift time, B. Through-going muons (TGM) 12 between the time of the particle activity seen by the light C. Stopped muons (STM) 14 detectors and the arrival time of the ionization electrons D. Light-mismatched (LM) events 14 at the anode. Together, a 3D image of the particle ac- tivities with a millimeter-scale position resolution can be VI. Performance of the generic neutrino detection 16 achieved. In addition, the number of measured ionization electrons is proportional to the energy deposition of the VII. Summary and outlook 19 charged particle, which can provide particle identification (PID) information. Acknowledgments 19 Compared to the water Cherenkov or liquid-scintillator detector technology, the LArTPC is expected to have a A. Trajectory seed finding 19 higher efficiency in differentiating electrons from gamma rays in neutrino interactions through gap identification and dE=dx measurement [13]. Such a capability allows an excellent detection of νe charge-current interactions, ∗ MICROBOONE [email protected] (−) (−) which enables precision measurements of νµ ! νe os- 2 Sense Wires U V W V wire plane waveforms Liquid Argon TPC Charged Particles Cathode Plane Incoming Neutrino Edrift W wire plane waveforms t FIG. 1. Illustration of a LArTPC detector. Taken from Ref. [7]. cillations. Utilizing the LArTPC technology, the Micro- slow timing of the TPC (typical readout time of a few BooNE experiment [7] aims to understand the nature of ms), and the decoupling of the ionization charge and scin- the low-energy excess of νe-like events observed in the tillation light signals, since they are measured by separate MiniBooNE experiment [14] and to measure neutrino- detectors. In this paper, we present a high-performance argon interaction cross sections [15, 16]. The Short Base- neutrino detection (or cosmic background rejection) pro- line Neutrino (SBN) Program [17], consisting of three cedure based on the Wire-Cell LArTPC event reconstruc- large LArTPCs on the surface, is under construction tion techniques [22] in the MicroBooNE experiment. to search for light sterile neutrinos [18]. Moreover, the The MicroBooNE detector [7] consists of a 2:56 m × Deep Underground Neutrino Experiment (DUNE) [19], 2:32 m × 10:36 m (∼85 metric tons of LAr) active TPC with ∼10,000 m3 detector modules, is planned to search for ionization charge detection and an array of 32 pho- for CP violation in leptons [20] and to determine the tomultiplier tubes (PMTs) [23] for scintillation light de- neutrino mass ordering [21]. To ensure the success of tection. It is located along the Booster Neutrino Beam these current and future physics programs, the current- (BNB) [24] of the Fermi National Accelerator Labora- generation large LArTPCs operating on the surface, such tory (FNAL) in Batavia, IL. Sitting on the beam axis, as MicroBooNE [7] and ProtoDUNE [12], are critical in 463 m from the beam target, MicroBooNE observes one developing and demonstrating the full capability of this neutrino interaction inside the TPC active volume per technology. ∼680 spills at the nominal beam intensity of 4.25×1012 For LArTPCs operating on the surface, the presence protons on target (POT) per pulse. Each proton pulse is of cosmic ray muons occurring at a rate of ∼0.2/m2/ms called a spill, and lasts 1.6 µs. When the BNB delivers is a major challenge to efficiently reconstructing neutrino a beam spill, a hardware trigger is initiated in Micro- interactions, the rate of which is generally several orders BooNE, which results in the recording of 4.8 ms of TPC of magnitude smaller. This challenge is the result of the data and 23.4 µs of PMT data covering the beam spill 3 window. This record is referred to as an event. In addi- drift distance. At the anode side, there are three parallel tion, self-discriminated PMT readouts are taken during wire readout planes (see Figure 1). In the drift direc- a period of 6.4 ms around the BNB trigger. Section II tion, these planes are labeled as the \U", \V", and \W" provides more details about the MicroBooNE detector planes, with each plane containing 2400, 2400, and 3456 and its readout. wires, respectively. The wire spacing within a plane is To reduce data size, a software trigger requiring signif- 3 mm, and the planes are spaced 3 mm apart. The wires icant PMT signals to be coincident with the beam spill is in the W plane run vertically and the wires in the U applied in data acquisition (DAQ) to decide whether to and V planes are oriented ±60◦ with respect to the ver- keep an event or not. After rejecting those events with tical direction. Different orientations of the wires allow low light output, therefore making them incompatible for determination of transverse positions of the ioniza- with particle activity from beam neutrino interactions, a tion electrons with respect to their drift direction. Bias data reduction by a factor of 22 is achieved. Still, after voltages for the U, V, and W planes are -110 V, 0 V, the software trigger, over 95% of the remaining events and 230 V, respectively, which satisfies the transparency have only cosmic rays within the trigger window. Fur- condition so that all drifting electrons pass through the thermore, at the rate of 5.5 kHz [25], there are on average U and V (induction) wire planes and are fully collected 26 cosmic-ray muons in the full 4.8 ms readout window. on the W (collection) plane. As the ionization electrons Such an overwhelming amount of cosmic rays creates sig- approach a wire plane, the induced current on each wire nificant challenges in selecting neutrino events [16, 26{ is amplified, shaped, and digitized through a custom de- 28]. In this work, an offline light reconstruction is first signed CMOS analog front-end ASIC [32] operating at applied to reject events triggered by cosmic rays arriv- 87 K in the liquid argon. The direct implementation of ing just before the beam spill, leading to a factor of 4 readout electronics in the cold liquid leads to a signif- reduction of triggered events. Then, a novel TPC-charge icantly reduced electronics noise. The equivalent noise to PMT-light matching algorithm, which requires digital charge (ENC) on each wire is generally below 400 elec- signal processing of the TPC data followed by the recon- trons [33]. struction of 3D images and activity clustering, is applied Figure 2b also shows the light collection system behind to remove TPC activity from cosmic rays outside of the the anode wire planes. Thirty-two 8-inch Hamamatsu beam spill. Section III briefly summarizes these tech- R5912-02MOD PMTs [23], providing approximately uni- niques. The rejection of stopped muons requires a new form coverage in the anode plane, are used to detect scin- set of tools to reconstruct the particle track trajectory tillation light from the LAr and provide the start time of and its dQ=dx, which is described in detail in Sec.