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Dark Matter, Millicharges, Axion and Scalar Particles, Gauge Bosons, and Other New Physics with LDMX

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Dark Matter, Millicharges, Axion and Scalar Particles, Gauge Bosons, and Other New Physics with LDMX PHYSICAL REVIEW D 99, 075001 (2019) Dark matter, millicharges, axion and scalar particles, gauge bosons, and other new physics with LDMX Asher Berlin,1 Nikita Blinov,1 Gordan Krnjaic,2 Philip Schuster,1 and Natalia Toro1 1SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA 2Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA (Received 1 October 2018; published 1 April 2019) The proposed LDMX experiment would provide roughly a meter-long region of instrumented tracking and calorimetry that acts as a beam stop for multi-GeV electrons in which each electron is tagged and its evolution measured. This would offer an unprecedented opportunity to access both collider-invisible and ultrashort-lifetime decays of new particles produced in electron (or muon) nuclear fixed-target collisions. In this paper, we show that the missing momentum channel and displaced decay signals in such an experiment could provide world-leading sensitivity to sub-GeV dark matter, millicharged particles, and visibly or invisibly decaying axions, scalars, dark photons, and a range of other new physics scenarios. DOI: 10.1103/PhysRevD.99.075001 I. INTRODUCTION energy search by NA64 at CERN [2] and provides roughly a meter-long region of instrumented tracking and calorim- Particle dark matter (DM) science is undergoing a etry that acts as a beam stop. The detector concept revolution, driven simultaneously by recent advances in allows each individual electron to be tagged and its theory and experiment. New theory insight has motivated evolution measured as it passes through a thin target, broadening the mass range for DM searches to include the tracking planes, and a high-granularity silicon-tungsten entire MeV–TeV range and beyond, extending the traditional calorimeter. Not only does this enable a model-independent weakly interacting massive particle (WIMP) paradigm where missing momentum and energy search, but it also offers an the DM relic abundance is set by the freeze-out of annihi- unprecedented opportunity to access remarkably short- lation reactions mediated by the weak interaction. The focus lifetime (cτ ∼ 10 μm) decays of new particles. Most on the MeV–TeV range retains a healthy emphasis on the existing studies of LDMX have focused on a specific known mass scales of the Standard Model (SM). It is also (and important) class of sub-GeV DM models [1,3–5].Our now widely recognized that it is important to search not only analysis finds that LDMX is sensitive to a much broader for DM itself, but also for particles that can mediate dark range of both thermal and nonthermal DM, and simulta- sector interactions with the SM, especially in the case of sub- neously to new particles like axions with either photon or GeV dark sectors. At the same time, ongoing advances on electron couplings, to very weakly coupled millicharges, key experimental fronts promise to unlock much of the well- to visibly and invisibly decaying dark photons and other motivated and unexplored sub-GeV DM territory in the gauge bosons, and to Higgs-like scalars, among other new coming years [1]. Therefore, now is an especially exciting physics possibilities. Our findings can be divided into three time to carefully scrutinize how to best leverage these broad categories of new physics studies in which we opportunities to achieve as much science with strong present sensitivity projections for LDMX (and a muon- discovery potential as possible. beam variant [6]) and comparisons to many other existing The Light Dark Matter eXperiment (LDMX) and planned experiments: Collaboration has recently proposed a new small-scale (1) Dark matter particles: In Sec. III, we provide new experiment to measure missing momentum in electron- calculations of thermal freeze-out scenarios for nuclear fixed-target collisions at high luminosity with a many simple (and viable) sub-GeV scalar and beam in the 4–16 GeV range. The LDMX setup builds on fermion DM models coupled through a dark photon, the demonstration of an electron fixed-target missing as well as the natural generalizations of these models to those with other vector or scalar mediators. We also provide calculations of the relic abundance for Published by the American Physical Society under the terms of viable freeze-in models with heavy dark photon the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to mediators and low reheat temperatures, comple- the author(s) and the published article’s title, journal citation, menting existing calculations performed with ultra- and DOI. Funded by SCOAP3. light dark photons. For these models, as well as 2470-0010=2019=99(7)=075001(38) 075001-1 Published by the American Physical Society BERLIN, BLINOV, KRNJAIC, SCHUSTER, and TORO PHYS. REV. D 99, 075001 (2019) representative strongly interacting massive particle While we have tried to consider a broad survey of models (SIMP) and asymmetric DM scenarios, we present discussed in the literature, covering many of the basic sensitivity estimates for LDMX (using the missing scenarios listed in the U.S. Cosmic Visions report [1],we momentum channel) and other applicable experi- have not been exhaustive. For example, in the minimal dark ments. We find that important sensitivity targets sectors investigated below, we have focused largely on the can be reached for viable DM models spanning the predictive parameter space with a mediator heavier than entire keV–GeV mass range. We also illustrate the twice the DM mass. However, most of the signals we sensitivity of complementary experiments such as consider have a near-threshold counterpart (with an off- low-threshold direct detection, beam dump, and shell mediator) that would extend beyond this regime, and B-factory experiments. In the case that DM this would be interesting to study in more detail in a future freeze-out proceeds through nonresonant direct an- work. Additionally, sub-GeV supersymmetric hidden sec- nihilations to SM particles (among the most pre- tors [9,10], as well as variations of the strongly interacting dictive scenarios), the expected LDMX sensitivity is models [11–13] can also be probed with the missing close to decisive when combined with Belle II. momentum and displaced visible decay approaches. (2) Millicharged particles and invisible decays of new This paper is organized as follows: After briefly describ- particles: In Sec. IV, we provide calculations of ing the LDMX setup in Sec. IA, we discuss the theoretical millicharge production, rates for invisibly decaying motivation for DM and light mediators in Sec. II.In gauge bosons, simplified models of sub-GeV scalars, Sec. III, we analyze the thermal relic targets for several and muonic forces motivated by the ðg − 2Þμ anomaly minimal dark sectors and highlight the sensitivity of [7]. We show sensitivity estimates for LDMX (using LDMX and other experiments. We show that the science the missing momentum channel) and other applicable case for LDMX extends beyond DM in Secs. IV and V, experiments. We find that LDMX could provide where we study the discovery potential of LDMX for a leading sensitivity to millicharged particles below variety of beyond-the-SM (BSM) scenarios, including the 500 MeV mass range. We also find that sensitivity millicharges, gauge bosons, and scalars, in the missing to the invisible decays of dark photons and minimal momentum and visible channels, respectively. We conclude gauged B − L (and similarly Li − Lj, B − 3Li) sym- in Sec. VI. metries will be enhanced by many orders of magni- tude compared to existing searches in the entire A. Experimental comments sub-GeV mass range, and cover unexplored param- As the above summary emphasizes, an important con- eter space that can address the ðg − 2Þμ anomaly. clusion from this work is that LDMX-like experiments (3) Axion particles, dark photons, and visible displaced designed to measure missing momentum also provide decays of new particles: In Sec. V, we provide new sensitivity to long-lived particles with lifetimes typically calculations for the production and visible displaced shorter than what have been probed in beam dump experi- decays of axion and dark photon particles. We ments such as E141 [14], Orsay [15], E137 [16], NuCal consider both electron and photon coupled axions. [17], CHARM [18], and others. This is reflected in the In these cases, the signal is an electromag- range of reach projections shown in this paper, with new netic shower in the back of the LDMX electromag- territory covered at larger couplings than beam dumps, yet netic calorimeter (ECAL), where the residual well below the magnitude of couplings probed by collider potential backgrounds are dominated by hard- experiments like BABAR [19], Belle [20], or those at the neutron-initiated hadronic showers. While the final LHC [21]. To see why this should be the case, it is worth background levels cannot be precisely determined reviewing a few experimental aspects of LDMX, as this without detailed experimental study, we find that for will help the reader understand later sections of the paper. a well-motivated range of performance, LDMX will LDMX is designed primarily to measure missing cover significant new territory for the minimal dark momentum in electron-nuclear fixed-target collisions with photon with a mass less than 100 MeV and lifetime a4–16 GeV electron beam, though the use of a muon beam larger than 10 μm. In addition, axionlike particles has also been suggested [6]. To facilitate this measurement, with similar mass and lifetime will be explored, the beam options under consideration are all high repetition including part of the parameter space for the QCD rate (more than 40 MHz) and have a large beam spot (at axion discussed in Ref. [8]. Estimates of sensitivity least a few cm2). In this way, an appreciable number of to SIMP DM with displaced decay signatures are individual electrons can be separated and measured.
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