FERMILAB-PUB-18-668-A-PPD-T Proton Fixed-Target Scintillation Experiment to Search for Minicharged Particles Kevin J. Kelly1, ∗ and Yu-Dai Tsai2, 3, y 1Theoretical Physics Department, Fermilab, P.O. Box 500, Batavia, IL 60510, USA 2Fermilab, Fermi National Accelerator Laboratory, Batavia, IL 60510, USA 3University of Chicago, Kavli Institute for Cosmological Physics, Chicago, IL 60637 (Dated: December 17, 2018) We propose a low-cost and movable setup to probe minicharged particles (or milli-charged par- ticles) using high-intensity proton fixed-target facilities. This proposal, FerMINI, consists of a milliQan-type detector, requiring multi-coincident (nominally, triple-coincident) scintillation signa- tures within a small time window, located downstream of the proton target of a neutrino experiment. During the collisions of a large number of protons on the target, intense minicharged particle beams may be produced via meson photo-decays and Drell-Yan production. We take advantage of the high statistics, shielding, and potential neutrino-detector-related background reduction to search for minicharged particles in two potential sites: the MINOS near detector hall and the proposed DUNE near detector hall, both at Fermilab. We also explore several alternative designs, including the modifications of the nominal detector to increase signal yield, and combining this detector tech- nology with existing and planned neutrino detectors to better search for minicharged particles. The CERN SPS beam and associated experimental structure also provide a similar alternative. FerMINI can achieve unprecedented sensitivity for minicharged particles in the MeV to few GeV regime with −4 −1 fractional charge " = Qχ=e between 10 (potentially saturating the detector limitation) and 10 . I. INTRODUCTION Most recently, MCP as dark matter has been proposed to explain the anomalous 21 cm hydrogen absorption signal The quantization of electric charge, as currently ob- reported by the Experiment to Detect the Global Epoch served in nature, has been one of the longest-standing of Reionization Signature (EDGES) collaboration [34{ mysteries in particle physics. The Standard Model (SM) 36]. However, orthogonal constraints have been explored, U(1) hypercharge group in principle allows arbitrarily and it has been demonstrated that the favored MCP can- small charges, yet experiments so far suggest that elec- didates to explain the EDGES result do not comprise tric charge has a fundamental unit. This has inspired the entirety of the observed relic dark matter abundance the concept of Dirac quantization [1] and motivated sev- (see, e.g., Refs. [37{39]). The favored range of masses eral considerations of Grand Unification Theorie (GUT) of these MCP is below roughly a hundred MeV, which (see, e.g. Refs. [2, 3]), among other theoretical studies. is a region that could be explored in proton fixed-target The discovery of particles with electric charge magnitude experiments. less than the smallest quark charges, namely minicharged Probes of MCP and other weakly-interacting MeV- particles (MCP) would be a major paradigm shift in this GeV particles has been under intense study, due in large endeavor. MCP have been studied and searched for on part to the fact that many dark matter and dark sector various fronts (see, e.g., Refs. [4{16])1. hypotheses fall into these categories. Additionally, ex- Given that MCP interact feebly with the SM parti- perimental techniques to probe this region have matured cles through their small electric charge, they are also a significantly [18]. The most sensitive laboratory-based potential solution to another well-established mystery of probes of MCP are threefold: particle physics: dark matter. MCP may be low-energy Collider Probes arXiv:1812.03998v2 [hep-ph] 14 Dec 2018 consequences of fermions in the dark sector [18] that cou- • Electron Fixed-Target Experiments ple to the Standard Model via a massless dark photon • Proton Fixed-Target and Neutrino Experiments through kinetic mixing [19], and the dark sector parti- • cles may constitute the relic abundance of dark matter. Both the Tevatron and Large Hadron Collider (LHC) The theories and signatures of dark sector and dark pho- have provided constraints on MCP for the first cat- ton have been heavily explored (see, e.g., Refs. [20{33]). egory [6, 11]. Additionally, a dedicated experiment (milliQan) was specifically proposed to occupy the CMS P5 site to search for MCP [13, 14]. The Beijing Elec- tron{Positron Collider could also provide sensitivity to ∗ [email protected] the MCP particles [40]. Electron fixed-target experi- y [email protected]; ments have been historically the most sensitive searches 1 Although charge quantization could never truly be ruled out, for MCP below 100 MeV. The dedicated SLAC MCP ex- even with the detection of such particles, their discovery would periment [5, 7] still provides leading sensitivity for MCP weaken the concept. It would also test the predictions of theories in this range. Several proposed electron-fixed target ex- related to charge quantization (e.g. [2, 3, 17]). periments (e.g. LDMX [41] and NA64 [42]) can further 2 improve the sensitivity of MCP, but the mass reach would that couple to the SM through a massive vector field, as be limited by the beam energy. Finally, using neutrino demonstrated in Refs. [46, 47]. Outside of probing MCPs experiments and protons-on-fixed-targets to study MCP and light dark matter scenarios, one can also utilize this has been long proposed [8{10], but a dedicated analysis proposal to probe the electric dipole moment of a heavy considering all the current and proposed near-future ex- neutrino, as was proposed utilizing the milliQan facility periments has only been done recently [16], followed by [48]. We leave the detailed analysis on these fronts to a the study based on reactor neutrino experiment for the future study [45]. lower mass MCP [43]. Here, we propose a Fermilab-based experiment to probe minicharged particles, FerMINI, that combines the II. PRODUCTION techniques of dedicated searches at SLAC [5, 7] and the LHC [13] with the advantages of neutrino facility We consider minicharged particles χ with electric sites [16]. We utilize the intense proton beams, for exam- charge Qχ and define " Qχ=e. For mχ < 10 GeV, ≡ −1 ple, the existing Neutrinos at the Main Injector (NuMI) existing constraints bound " . 10 , and even stronger beamline and the future Long-Baseline Neutrino Facility constraints exist for mχ . 100 MeV. In proton fixed- (LBNF) beamlines, and place more than one (the nomi- target experiments, minicharged particles are produced nal design is 3) groups of scintillator arrays downstream via neutral meson decays and Drell-Yan processes, dis- of the intense beam, shielded from strong electromagnetic cussed below: radiation. The signature for MCP is the detection of one Meson Decays: We consider the following meson de- or a few photoelectrons (PE) produced when the par- cays to the millicharged particle χ: ticle traverses the scintillator, causing small ionizations 0 π γχχ¯ (m 0 = 135 MeV) and producing photons collected by the photomultiplier π • η !γχχ¯ (m = 548 MeV) tubes (PMT). We require such a detection in contiguous η • J=! χχ¯ (m = 3:1 GeV) scintillator-PMT sets in each of the three detector groups • ! J= Υ χχ¯ (mΥ = 9:4 GeV) in order to greatly reduce background. The two sites we • ! explore are the existing Main Injector Neutrino Oscilla- When produced in proton-proton collisions, each of these tion Search (MINOS) near detector hall and the proposed mesons m may decay into millicharged particles with Deep Underground Neutrino Experiment (DUNE) near masses up to mm=2. detector hall. We show the potential reach of such setups For m = π0; η, the decay proceeds similar to that of in Figs. 2a and 2b, respectively, and see that these can m γe+e−. We may write the total number of χ pro- provide leading sensitivity to MCP searches for masses duced! via these decays as in the range of 10 MeV to 5 GeV. M 2 The nominal FerMINI setup substantially benefits 2 (3) χ Nχ 2cmBr(m γγ)" αEM NPOT I 2 : (1) from the large fluxes of MCP from the intense proton ' ! × mm ! collisions. We will also discuss new ideas to combine the MCP detector with the neutrino detectors in Section V. Here, cm is the number of meson m produced per proton- FerMINI serves as an example to demonstrate that the on-target (POT, total number NPOT) in the target hall, 2 (3) proton-fixed target facilities could be natural habitats " αEM is proportional to the γχχ¯ coupling, and I (x) characterizes the three-body decay2 m γχχ¯, for the dedicated low-cost detectors, including milliQan ! (mostly proposed for LHC recently), searching for weakly 1 (3) 2 4x 1 z interacting and long-lived particles. I (x) = dz 1 −2 3π 4x − z z × Note that, in the analysis, we limit our attention to Z r 12x3 + 6x2(3z 2) + x(5z 2)(z 1) + z(z 1)2 : the minimal theoretical assumption that the MCP we − − − − (2) are searching for are simply fermions with small U(1)Y hypercharges with masses between MeV-GeV (if the We find, using PYTHIA8 [49], cπ0 4:5 and cη 0:5 for minicharged particle is a scalar instead, the sensitivity 120 GeV protons on target. ' ' is largely similar). The model and constraints do not The J= and Υ mesons may decay directly via m rely on the existence of dark photons nor assumptions χχ¯, and ! of MCP abundance and velocity distributions in the lo- cal galaxy, but the bounds we derive certainly serve as 2 2 + − 2 (2) Mχ me Nχ 2cmBr(m e e )" NPOT I ; ; conservative constraints to the MCP-related dark matter ' ! × m2 m2 and dark sector scenarios. Interestingly, finding MCP m m ! (3) without an accompanying massless dark photon would have implications on not only GUT theories (again see, e.g., Refs.