Improved Limits on Millicharged Particles Using the ArgoNeuT Experiment at Fermilab

Abstract

A search for millicharged particles, a simple extension of the standard model, has been performed with the ArgoNeuT detector exposed to the Neutrinos at the Main Injector beam at Fermilab. The ArgoNeuT Liquid Argon Time Projection Chamber detector enables a search for millicharged particles through the detection of visible electron recoils. We search for an event signature with two soft hits (MeV-scale energy depositions) aligned with the upstream target. For an exposure of the detector of 1.0 × 10 20 protons on target, one candidate event has been observed, compatible with the expected background. This search is sensitive to millicharged particles with charges between 10 −3 e and 10 −1 e and with masses in the range from 0.1 GeV to 3 GeV. This measurement provides leading constraints on millicharged particles in this large unexplored parameter space region.Millicharged particles (mCPs), i.e. particles (χ) with an electric charge Q χ = · e much smaller than the elementary charge ( 1), are a particularly simple, wellmotivated, extension of the standard model (SM). In their simplest form they are just new particles that violate the quantization of charge seen in the SM. They can also arise in the low-energy limit of models in which charge is quantized but there exists a kinetically mixed dark photon [1]. In addition, these particles could make up part of the dark matter in the universe [2][3][4][5][6][7][8][9][10].Millicharged particles can be produced at any intense fixed-target-produced beam via the decays of neutral mesons or direct Drell-Yan pair production arising from proton interactions in the target [11, 12] 1 . Produced mCPs are relativistic in the lab frame. For example, for a 120 GeV proton beam striking a target (as in the case of ArgoNeuT), the boost factors of the produced mCPs are in the range of 10-100. The opening angle of the mCP beam is large, of order 0.1 radians. Neutrino detectors located downstream of an intense proton beam striking 1 The bremsstrahlung contribution to mCP production is not included in this study, which may further enhance the sensitivity. a target, nominally used to produce the neutrino beam, may be exposed to a large flux of mCPs that were produced there. When traveling through matter, mCPs will lose energy by atomic excitation and ionization like any charged particle but with ionization and excitation rates reduced by 2 . Therefore, the mCP ionization track is undetectable except when knock-on electrons energetic enough to themselves produce a visible signal are emitted. The distribution of electron recoil energies scales with the inverse squared of the electron recoil energy,where we have taken the relativistic mCP limit. Lowenergy thresholds are therefore key to detect these "δrays" produced by mCPs. The expected deflection of mCPs after each interaction is small. Therefore, mCPs will travel to the detector in an approximately straight path and will point back to the target [11]. Searches for mCPs have been conducted, with low-mass regions covered by low-energy experimen...

Understanding the energy resolution of liquid argon neutrino detectors

“…This would record all charges in the event, including low-energy hadrons and the spray component. To this end, we recall again that the hits from the spray are already seen in ArgoNeuT [19,[31][32][33], a small surface detector with a level of noise that is higher than what is planned for DUNE.…”

“…• We lower the thresholds to the values recently demonstrated by the ArgoNeuT collaboration [19]. These low thresholds have already been considered in Fig.…”

“…Our simulation framework in this paper is built on two community-based packages: we use GENIE (version 2.12.8) [16] for primary neutrino interactions and FLUKA (version 2011.2x.2) [17,18] for particle propagation in liquid argon. GENIE is the default generator code used in DUNE studies (as well as all other Fermilab-based neutrino experiments), while FLUKA is a well-tested package for calculations of particle transport and interactions with matter, which has been proven to provide good description of MeV hadronic physics [19].…”

“…In fact, an important proof-of-principle experimental observation of this phenomenon already exists, thanks to the work by the Ar-goNeuT collaboration [33]. Moreover, very recently, an ArgoNeuT paper [19] demonstrated that even hits from 0.5 MeV recoil electrons can be detected with 50% efficiency. Moreover, at 0.8 MeV the energy resolution is as good as 14%.…”

White Paper on New Opportunities at the Next-Generation Neutrino Experiments (Part 1: BSM Neutrino Physics and Dark Matter)

“…[92], searches for MCP in numerous past (LSND, MiniBooNE, and MiniBooNE DM run), current (MicroBooNE), and future (SBND, DUNE and SHiP) proton fixed target and neutrino experiment was performed. A more involved background analysis based on double-hit signatures is discussed in [439,440] based on the ArgoNeuT experiment. Recently, a dedicated experiment, Fer-MINI [441], is proposed utilizing the neutrino facilities and a dedicated scintillator-based detector to search for MCP.…”