Search for sub-millicharged particles at J-PARC
PPrepared for submission to JHEP
Search for sub-millicharged particles at J-PARC
Jeong Hwa Kim a In Sung Hwang a Jae Hyeok Yoo, a, a Korea University,145 Anam-ro, Seongbuk-gu, Seoul, 02841, Korea
E-mail: [email protected] , [email protected] , [email protected] Abstract:
We studied the feasibility of an experiment searching for sub-millichargedparticles ( χ s) using 30 GeV proton fixed-target collisions at J-PARC. The detector is com-posed of two layers of stacked scintillator bars and PMTs and is proposed to be installed280 m from the target. The main background is a random coincidence between two layersdue to dark counts in PMTs, which can be reduced to a negligible level using the timingof the proton beam. With N POT = 10 which corresponds to running the experiment forthree years, the experiment provides sensitivity to χ s with the charge down to × − in m χ < . GeV / c and × − in m χ < . GeV / c . This is the regime largely uncoveredby the previous experiments. We also explored a few detector designs to achieve an optimalsensitivity to χ s. The photoelectron yield is the main driver, but the sensitivity does nothave a strong dependence on the detector configuration in the sub-millicharge regime. Corresponding author. a r X i v : . [ h e p - e x ] F e b ontents Electric charge quantization is a long-standing question in particle physics. While GrandUnified Theories (GUTs) have typically been thought to preclude the possibility for par-ticles that do not have integer multiple electron charge (millicharged particles hereafter),well-motivated dark-sector models [1, 2] have been proposed to predict the existence ofmillicharged particles while preserving the possibility for unification. Such models can con-tain a rich internal structure, providing candidate particles for dark matter. Recent resultsfrom the EDGES experiment [3] suggest that the observed 21-cm absorption profile can beexplained if a fraction of dark matter is composed of millicharged particles [4].One well-motivated mechanism that leads to millicharged particles is to introduce a new U (1) in the dark sector with a massless dark-photon and a massive dark-fermion ( χ ) [5, 6].In this scenario, the dark-photon and the photon in the Standard Model kinematically mixand the charge of χ is determined by the size of the mixing. Therefore, depending on thestrength of mixing, χ can have an electric charge that is not integer multiple. Hereafter, χ is used to denote millicharged particles.A number of experiments have searched for millicharged particles, including in an elec-tron fixed-target experiment [7], proton-proton colliders [8–10], proton fixed-target experi-ment [11] and neutrino experiments [12, 13]. A comprehensive review is in Reference [14].In the parameter space of the charge ( Q ) and mass ( m χ ), the region of m χ > . GeV / c and Q < − e is largely unexplored.Proton fixed-target experiments provide a solid testing ground for χ s. The particleflux is much larger than the collider experiments and they can reach a higher energy regimethan electron fixed-target experiments. The sensitivity of such experiments to χ s can reachbeyond Q ∼ − e for a wide mass range from a few MeV / c to a few GeV / c . This letter– 1 –roposes a new experiment, SUBMET (SUB-Millicharge ExperimenT), which utilizes the GeV proton beam at Japan Proton Accelerator Research Complex (J-PARC) to searchfor χ s in this unexplored region. At proton fixed-target collisions at J-PARC, χ s with charge Q can be produced from thedecay of π , η and J/ψ neutral mesons. The Υ production is not relevant because thecenter-of-mass energy is . GeV for the collisions between the GeV proton beam andthe fixed target. The lighter mesons ( m = π , η ) decay through photons ( π , η → γχ ¯ χ ),while the J/ψ decays to a pair of χ s directly ( J/ψ → χ ¯ χ ). In both cases, m χ up to m m / iskinematically allowed. The number of produced χ s ( N χ ) can be calculated by the equationin [15], N χ ∝ c m (cid:15) N POT × f (cid:32) m χ m m (cid:33) (2.1)where c m is the number of mesons produced per proton-on-target (POT), N POT is the totalnumber of POT, (cid:15) = Q/e , and f is a phase space related integral. The c m of each mesonis extracted using PYTHIA8 [16] and the estimated values are c π = 1 . , c η = 0 . , and c J/ψ = 5 × − . Assuming N POT = 10 that corresponds to running the experiment for years [17], the expected number of χ s that reach the detector is in the order of at (cid:15) = 1 and at (cid:15) = 10 − . Beam dumpMuon monitor Decay volume Target χ SUBMET 30 GeV proton0 m120 m280 mNeutrino Monitor building
Figure 1 . Illustration of the experimental site. χ s are produced near the target and reach SUBMETafter penetrating the beam dump, the muon monitor and the sand. The detector is located mfrom the target and approximately m underground. In J-PARC a GeV proton beam is incident on a graphite target to produce hadronsthat subsequently decay to a pair of muon and muon neutrino in the decay volume. The– 2 –emaining hadrons are then dumped in the beam dump facility. Since they are MinimumIonizing Particles, muons can penetrate the beam dump and be identified by the muonmonitor located behind the beam dump facility. The on-axis near detector, InteractiveNeutrino GRID (INGRID) [18], is inside the Neutrino Monitor (NM) building located m from the target. The space between the muon monitor and INGRID is filled with sand.The experimental site is illustrated in Figure 1. The proton beam has a repetition rate of1.16 s and each spill contains 8 bunches which are separated by ns [19]. The beamtiming is available at the site and this allows for substantial suppression of backgrounds atthe level of O (10 − ) .If χ s are produced, they penetrate the space between the target and the detectorwithout a significant energy loss because of their feeble interaction with matter. Therefore,they can be detected at the NM building if a detector sensitive to identifying such particlesis installed. The area behind the V-INGRID on B2 ( ∼ m underground) is unoccupiedand can be a potential detector site. The distance from the axis of the neutrino beam is ∼ m. Figure 2 . Demonstration of the SUBMET detector. There are two layers of stacked scintillatorbars (blue). At one end of each bar, a PMT (black) is attached. χ s penetrate both layers in anarrow time window. The detector concept proposed for this experiment is based on a similar proposal madein [20], sharing the idea to use a segmented detector with large scintillator bars. To besensitive to charges below − e , a thick sensitive volume is needed. It is advantageous tosegment the large volume because it helps reducing backgrounds due to dark currents andshower particles from cosmogenic muons to a negligible level. It also allows for utilizing thedirectionality of the incident χ s to further suppress non-pointing particles. The detector,– 3 –s shown in Figure 2, is composed of 2 layers of stacked × × cm BC-408 plasticscintillator bars [24]. They are aligned such that the produced χ s pass through both layersin a narrow time window. In each layer there are × scintillator bars, so the area ofthe detector face is about . m . A prototype of a detector with a similar design has beeninstalled at the LHC, and shown robustness and sensitivity to χ s [10].At the end of each scintillator bar, a photodetector is attached to convert the photonsto an electronic signal. Photomultipliers (PMTs) are suitable for this experiment because oftheir large area coverage, low cost, and low dark current. The total volume of the detectoris approximately . × . × . m including the PMTs.The signal acceptance rate, the fraction of χ s that go into the detector area of . m at m from the target, is calculated as a function of distance from the beam axis to thedetector. It is in the order of O (10 − ) and does not depend on the position strongly, up toa few meters from the axis, since the detector is located far from the target. At 5 m therate is only ∼ % lower than the on-axis region. This provides some flexibility in selectingthe location of the detector. The effect of energy loss and multiple Coulomb scattering inthe sand is estimated to be negligible for the charge range of interest, particularly below − e , so they have a small impact on the sensitivity of the experiment. χ s that reach the detector will go through both layers within a ∼ ns time windowproducing a coincidence signal. In this section, the background sources that can mimicthis coincidence signal are discussed. They can be divided into three categories; randomcoincidence, beam-induced, and cosmic-induced backgrounds.In PMTs, spurious current pulses can be produced by thermal electrons liberated fromthe photocathode. Therefore, a random coincidence of such pulses in different layers canbe identified as a millicharge signal. The typical size of the pulses is very small and thismakes random coincidence the major background source in Q < − e regime. The rateof random coincidence can be large depending on the rate of the spurious pulses (darkcount rate, DCR) even if the time window for the coincidence signal is ns. The randomcoincidence rate is nN n τ n − where n is the number of layers, N is the DCR, and τ isthe coincidence time window. Using a typical PMT DCR of Hz at room temperature, n = 2 , and τ = 10 ns, the random coincidence rate of two bars is . per year. Therecan be ×
15 = 225 such coincidence signals, so the total coincidence rate is ∼ peryear. The liberation of electrons is a thermal activity, which can be reduced by cooling thecathodes. With N = 100 Hz, the random coincidence background is reduced to . eventsper year.Muons are produced from the pion decays in the decay volume together with neutrinos.The density of quartz, which typically takes up the largest fraction of sand, is . g / cm and dE/dx = 1 . MeVcm / g [21], so the energy loss of a MIP in > m of sand ismuch larger than GeV. Therefore, such beam-induced muons can’t reach the detector.Although the muons from the pion decays can’t reach the detector, neutrinos can and mayinteract with the scintillator material to produce small signals. The number of neutrino– 4 –nteraction events in INGRID is ∼ . × for N POT = 10 [22]. Since a large fractionof INGRID material is iron, the rate of neutrino interaction in INGRID can be used as anupper bound for SUBMET. One layer of SUBMET is approximately times smaller, sothe rate is ∼ × for N POT = 10 in one layer of SUBMET. Requiring coincidence intwo layers, the expected number of this background becomes negligible. The interaction ofthe neutrinos and the material of the wall of the NM building in front of the detector canproduce muons that go through the detector. These muons can be identified and rejectedby installing scintillator plates between the wall and the detector or by using the very largescintillation yield of a muon that can be separated from the millicharge signal.Cosmic muons that penetrate the cavern or the materials above the detector can pro-duce a shower of particles that is large enough to hit both layers simultaneously. In suchevents, the hits in multiple layers can be within the coincidence time window and will looklike a signal event. The particles in the shower generate more photons than χ s, so thesignals from cosmic muon showers can be rejected by vetoing large pulses. As done to tagthe muons produced in the wall of the NM building in front of the detector, scintillatorplates can be installed covering the whole detector to tag any ordinary-charged particlesor photons incident from top and sides of the detector. These auxiliary components wereproven to be effective in rejecting events with such particles [10]. In addition, the cosmicshower penetrates the detector sideways, leaving hits in multiple bars in the same layer,while χ s will cause a smaller number of hits. A cosmic shower and signals from radioactivedecays overlapping with dark current can be another source of background. Since the rateof this background depends on the environment strongly, a precise measurement can beperformed in situ only.To estimate the sensitivity of the experiment, we assume that the total background( N bkg ) over three years of running is events. The probability of detecting a χ in an n -layer detector is given by Poisson distribution P = (1 − e − N PE ) n where N PE is the number of photoelectrons. N PE is proportional to thequantum efficiency (QE) of PMT, (cid:15) , and the number of photons that reach the end ofthe scintillator ( N γ ). The (cid:15) term comes from the fact that the energy loss of a chargedparticle in matter is proportional to Q . In order to calculate N γ a GEANT4 [23] simulationis performed. Using a × × cm BC-408 scintillator with a surface reflectivity of98%, N γ is . × . Taking QE into account, N PE is . × (cid:15) . Once we have N PE and P , The total number of signal events measured by the detector can be calculated as s = N PE P .Figure 3 shows the % CL exclusion curve for N POT = 10 . SUBMET provides theexclusion down to (cid:15) = 5 × − in m χ < . GeV / c and (cid:15) = 8 × − in m χ < . GeV / c .Systematic uncertainty on b is not considered because it does not have a significant impacton the exclusion limit (Table 1). The sudden degradation of sensitivity at m χ = 0 . GeV / c is because of the small production rate of J/ψ with the GeV proton beam.– 5 – m [GeV/c ]10 = Q / e SUBMETFerMINICollidersBEBCCharm IISLACArgoNeuTMilliQan demonstrator
Figure 3 . Exclusion at % CL for N POT = 10 . The constraints from previous experiments areshown as shaded areas. The expected sensitivity of FerMINI [15] is drawn in the gray dotted line.There are other proposed experiments [20, 25], but only FerMINI with the NuMI beam is includedbecause it is in a similar time scale of SUBMET (within next 5 years). The number of signal events recorded by the detector drops rapidly in (cid:15) < − due tosmall N PE . Therefore, increasing N PE or N χ does not have a large impact on the sensitivityin this phase space. This will be discussed in a quantitative way in the next section. The sensitivity of the experiment depends on the configuration of the detector. This sectiondescribes the impact of a few key parameters for the detector design, focusing on the sub-millicharge regime. If further optimization of the detector is needed, this quantitative studycan serve as a guide.Searches in the sub-millicharge regime rely on the Poissonian fluctuation of small N PE .Approximating P (cid:39) N PE to the first order for small (cid:15) , we arrive at the following relation s = N (cid:15) =1 ,χ (cid:15) ( N (cid:15) =1 , PE (cid:15) ) n ≥ s (6.1)– 6 – χ (relative) N PE (relative) b (relative) Exclusion limit on (cid:15) for m χ = 10 MeV / c . × − . × − /
50 1 1 8 . × − . × − . × − . . × − Table 1 . Various detector configurations and their sensitivity. N χ (relative) is the yields of sig-nal events within the acceptance relative to the baseline, N PE (relative) is the number of photonelectrons relative to N PE = 2 . × , and b (relative) is the number of background events relative N bkg = 5 . where s is the number of signal events, the subscript (cid:15) = 1 refers to the values at (cid:15) = 1 and s is the number of signal events that provides 95% exclusion limit. Reordering in termsof (cid:15) , the exclusion limit at 95% CL is (cid:15) = (cid:18) N n(cid:15) =1 , PE N (cid:15) =1 ,χ s (cid:19) − n +2 . (6.2)Due to the (cid:15) n +1) term in 6.1, there is a sharp cutoff in s around (cid:15) ∼ O (10 − ) . This limitsthe sensitivity to that regime regardless of the detector configuration.Table 1 shows different detector configurations and the corresponding exclusion lim-its for m χ = 10 MeV / c . The default configuration is in the first row; N χ (relative)=1, N PE (relative)=1, and b (relative) = 1 where “relative” means relative to the values of thebaseline configuration discussed in Section 3.The improvements achieved by extending the duration of data collection or makingthe detector area larger, e.g. , adding more bars to each layer, are modest. Extending theduration or the detector area by a factor of 2 increases N χ by the same amount. Sensitivityis improved by less than % ( nd row in Table 1). If the area of the detector is reducedto × cm (a factor of ) or the duration of the data-taking period is shortened by / (roughly 3 weeks), the exclusion limit moves to . × − ( rd row in Table 1). Theimpact of b is limited as well. If b is increased by a factor of which corresponds to theDCR of Hz, the limit is degraded by only % ( th row in Table 1).The th row in Table 1 shows that the most effective component to enhance sensitivityis N PE . Increasing N PE by a factor of improves the sensitivity by %. This can beachieved by using a scintillator material with a higher light yield or extending the length ofthe scintillator. However, improvement using longer scintillators is limited in length > cm due to the effect of light attenuation. The th row corresponds to the case of scintillatorlength of cm. Degradation of sensitivity is only 10 %, which allows for a smaller-scaledetector without a significant loss of performance.– 7 –hough N PE plays the main role in enhancing sensitivity to sub-millicharged particles,the exclusion limit with the . × . m detector is still in the range of (cid:15) = (3 . − . × − with the variations considered. This indicates that the sensitivity does not depend on theconfiguration of the detector strongly.In case of an unexpectedly large number of background events, installing additionallayers can be considered to control them. Using 3 layers and assuming 0 background events,the exclusion limit reaches (cid:15) = 1 . × − . The experiment still outperforms previoussearches in this configuration. We propose a new experiment, SUBMET, sensitive to millicharged particles produced atthe GeV proton fixed-target collisions at J-PARC. The detector, inspired by the milliQanexperiment, is based on long scintillators and is located in the Neutrino Monitor building280 m from the target. With the number of protons on target of , the experimentis sensitive to particles with electric charge × − e for mass less than . GeV / c and × − e for mass less than . GeV / c .SUMBET places the best limit in low mass region m χ < . GeV / c among the existingand the proposed experiments. In this regime, the N P E is very small so the probabilityto observe a photon produced by millicharged particles per layer ( P layer = (1 − e − N PE )) isextremely small. Since the total probability is a power of P layer by the number of layers,using two layers significantly enhances the probability compared to the detector designswith 3 or 4 layers.Note that this experiment is complementary to the existing proposals [15, 20, 26] sincethe main interest is in the low mass region. The center of mass energy of the proton-targetcollisions is . GeV and this limits the mass reach of the experiment to below m J/ψ / whileother proposals can cover higher mass regions. Compared to the FerMINI experiment, theproduction rate of J/ψ is much smaller due to lower beam energy. So, the sensitivity tothe χ s from J/ψ decay is slightly worse though it is still competitive.A few detector designs to achieve an optimal sensitivity were considered in Section 6 andwe found that the configuration of the detector generally does not affect the sensitivity. Inaddition, the operation of the upgraded proton beam at J-PARC will start in early 2022 [19].These indicate that it is very important to install the detector as early as possible to fullyexploit the upgraded power of the beam.
Acknowledgments
Authors thank Tsutomu Mibe, Yoshiaki Fujii, Takeshi Nakadaira, and Toshifumi Tsukamotofor the useful discussions on the detector site. In particular, we thank Toshifumi Tsukamotofor taking photographs of the Neutrino Monitor building so that we understand the spa-tial constraints inside the building. We thank the members of the milliQan collaboration,particularly, Andy Hass, Christopher S. Hill, and David Stuart for the discussions at var-ious stages of this study. We thank Matthew Citron, Albert De Roeck, Seung Joon Lee,– 8 –nd Eunil Won for providing comments on the draft. We thank Masashi Yokoyama fordiscussions regarding the schedule of the neutrino beamline. We also thank Hong Joo Kimfor the information on the property of various scintillation materials. This work has beensupported by a Korea University Grant.
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