A windowless hydrogen gas target of nominal thickness 10^19 cm^-2 is an essential component of the DarkLight experiment, which is designed to utilize the megawatt electron beam at an Energy Recovery Linac (ERL). The design of such a target is challenging because the pressure drops by many orders of magnitude between the central, high-density section of the target and the surrounding beamline, resulting in laminar, transitional, and finally molecular flow regimes. The target system was assembled and operated at Jefferson Lab's Low Energy Recirculator Facility (LERF) in 2016, and subsequently underwent several revisions and calibration tests at MIT Bates in 2017. The system at dynamic equilibrium was simulated in COMSOL to provide a better understanding of its optimal operation at other working points. We have determined that a windowless gas target with sufficiently high density for DarkLight's experimental needs is feasible in an ERL environment.

The study of B meson decay appears to offer a unique opportunity to measure basic parameters of the Standard Model, probe for interactions mediated by higher mass particles, and investigate the origin of CP violation. These opportunities have been enhanced by the results of two measurements. The first is the measurement of a long B meson lifetime. In addition to allowing a simpler identification of B mesons and a measurement of the time of their decay, this observation implies that normal decays are suppressed, making rare decays more prevalent. The second measurement is that neutral B mesons are strongly mixed. This enhances the possibilities for studying CP violation in the B system. The CESR storage ring is likely to dominate the study of B physics in e{sup +}e{sup {minus}} annihilations for about the next five years. First, CESR has already reached a luminosity of 10{sup 32} cm{sup {minus}1} sec{sup {minus}1} and has plans for improvements which may increase the luminosity by a factor of about five. Second, a second-generation detector, CLEO II, will start running in 1989. Given this background, the main focus of this working group was to ask what is needed for the mid- to late-1990 s. Manymore » laboratories are thinking about new facilities involving a variety of techniques. To help clarify the choices, we focused on one example of CP violation and estimated the luminosity required to measure it using different techniques. We will briefly describe the requirements for detectors matched to these techniques. In particular, we will give a conceptual design of a possible detector for asymmetric collisions at the {Upsilon}(4S) resonance, one of the attractive techniques which will emerge from this study. A discussion of accelerator technology issues for using these techniques forms the second half of the B-factory Group report, and it follows in these proceedings. 34 refs., 2 figs., 2 tabs.« less

This white paper summarizes the workshop "U.S. Cosmic Visions: New Ideas in Dark Matter" held at University of Maryland on March 23-25, 2017.

We describe the current status of the DarkLight experiment at Jefferson Laboratory. DarkLight is motivated by the possibility that a dark photon in the mass range 10 to 100 MeV/c$^2$ could couple the dark sector to the Standard Model. DarkLight will precisely measure electron proton scattering using the 100 MeV electron beam of intensity 5 mA at the Jefferson Laboratory energy recovering linac incident on a windowless gas target of molecular hydrogen. The complete final state including scattered electron, recoil proton, and e+e- pair will be detected. A phase-I experiment has been funded and is expected to take data in the next eighteen months. The complete phase-II experiment is under final design and could run within two years after phase-I is completed. The DarkLight experiment drives development of new technology for beam, target, and detector and provides a new means to carry out electron scattering experiments at low momentum transfers.

It is generally accepted that certain astronomical and cosmological observations can be explained by invoking the concepts of Dark Matter and Dark Energy (DM/DE). Applying straightforward extensions of the Standard Model to DM/DE, results in scalar fields and predictions of particles generation via photo-magnetic coupling . Under the right conditions, these particles should be observable in earth-bound laboratory settings. Although many attempts have been made to observe these particles, none have succeeded. Heretofore, most searches have focused on detecting multi-GeV Dark Matter WIMPS. Recently, however, searches have been conducted in the lighter dark matter, sub-eV, WISP mass range. By comparison, little has been done to search for dark energy particles. The ALPs-Chameleon Experiments Stations (ACES) program, described herein, proposes a compact station that would search for both dark sector particles. Finally, it is noted that both "species" of particles - dark energy and dark matter - could be generated at the same time in the same magnetic field with the possibility of interaction between DM and DE particles. Thus, by using standard matter tools to produce particles from both dark sectors, ACES potentially could provide tri-sector discoveries with huge results for very little investment.

P. Hansson Adrian, S. Andreas, T. Averett, O. Baker, B. Batell, M. Battaglieri, J. Beacham, T. Beranek, J. D. Bjorken, F. Bossi, J. R. Boyce, G. D. Cates , A. Celentano, A. S. Chou, R. Cowan, F. Curciarello, H. Davoudiasl, P. deNiverville, R. De Vita, A. Denig, R. Dharmapalan, B. Dongwi, B. Döbrich, B. Echenard, D. Espriu, S. Fegan, P. Fisher, G. B. Franklin, A. Gasparian, Y. Gershtein, M. Graham, P. W. Graham, A. Haas, A. Hatzikoutelis, M. Holtrop, I. Irastorza, E. Izaguirre, J. Jaeckel, Y. Kahn, N. Kalantarians, M. Kohl, G. Krnjaic, V. Kubarovsky, H-S. Lee, A. Lindner, A. Lobanov, W. J. Marciano, D. J. E. Marsh, T. Maruyama, D. McKeen, H. Merkel, K. Moffeit, P. Monaghan, G. Mueller, T. K. Nelson, G.R. Neil, M. Oriunno, Z. Pavlovic, S. K. Phillips, M. J. Pivovaroff, R. Poltis, M. Pospelov, S. Rajendran, J. Redondo, A. Ringwald, A. Ritz, J. Ruz, K. Saenboonruang, P. Schuster, M. Shinn, T. R. Slatyer, J. H. Steffen, S. Stepanyan, D. B. Tanner, J. Thaler, M. E. Tobar, N. Toro, A. Upadye, R. Van de Water, B. Vlahovic, J. K. Vogel, D. Walker, A. Weltman, B. Wojtsekhowski, S. Zhang, K. Zioutas

Tests were performed to pass a 100MeV, 430kWatt c.w. electron beam from the energy-recovery linac at the Jefferson Laboratory's FEL facility through a set of small apertures in a 127mm long aluminum block. Beam transmission losses of 3p.p.m. through a 2mm diameter aperture were maintained during a 7h continuous run.

Dark sectors, consisting of new, light, weakly-coupled particles that do not interact with the known strong, weak, or electromagnetic forces, are a particularly compelling possibility for new physics. Nature may contain numerous dark sectors, each with their own beautiful structure, distinct particles, and forces. This review summarizes the physics motivation for dark sectors and the exciting opportunities for experimental exploration. It is the summary of the Intensity Frontier subgroup "New, Light, Weakly-coupled Particles" of the Community Summer Study 2013 (Snowmass). We discuss axions, which solve the strong CP problem and are an excellent dark matter candidate, and their generalization to axion-like particles. We also review dark photons and other dark-sector particles, including sub-GeV dark matter, which are theoretically natural, provide for dark matter candidates or new dark matter interactions, and could resolve outstanding puzzles in particle and astro-particle physics. In many cases, the exploration of dark sectors can proceed with existing facilities and comparatively modest experiments. A rich, diverse, and low-cost experimental program has been identified that has the potential for one or more game-changing discoveries. These physics opportunities should be vigorously pursued in the US and elsewhere.

We give a short overview of the DarkLight detector concept which is designed to search for a heavy photon A' with a mass in the range 10 MeV/c^2 < m(A') < 90 MeV/c^2 and which decays to lepton pairs. We describe the intended operating environment, the Jefferson Laboratory free electon laser, and a way to extend DarkLight's reach using A' –> invisible decays.

We describe machine operational issues encountered and solutions developed during the DarkLight Aperture Test conducted in July 2012. Machine Configuration – The DarkLight aperture test required a) high beam brightness, small momentum spread, and as much electron beam power as reliably available, b) minimum radiation background, c) implementation of a small beam size and betatron envelope (“minibeta”) at the test aperture so as to insure transmission of the core beam, and d) management of beam halo and control of collective effects such as resistive wall heating and the beam break-up instability (BBU). A hardware configuration that meets all requirements is discussed elsewhere [1]. The desire for high beam brightness and power was operationally constrained by the condition of the injector, which was performance-limited by two hardware problems. Firstly, a damaged bellows shield in the 350 keV room temperature beam line resulted in partial obstruction of the vacuum aperture just upstream of the superconducting booster cryounit. Secondly, high current operation of the (aging) superconducting booster engendered some risk; at elevated current (above 5 or 6 mA), there is a long-standing pattern of waveguide vacuum faults that precipitate RF and gun high voltage trips leading to catastrophic damage to the photocathode. To insure reliable beam delivery and reduce risk, we therefore operated the system using a UV FEL injector tuning [2] at a baseline bunch charge of 60 pC. With the bunch charge thus reduced from the IR value of 135 pC, the beam emittances are – given the known scaling with – smaller by ~ 60/135 ~ 1/√2. The resulting spatially smaller beam reduces the impact of the damaged bellows. While still providing a bright and high quality beam, this also limits the operating current to 4.5 mA at the maximum photocathode drive laser repetition rate of 75 MHz (60 pc X 75 MHz) – thereby avoiding the very high current regime (over 5 mA), in which there is dramatically increased risk of booster faults – while still providing opportunity to operate at high power. A small momentum spread was maintained by operating the linac “cross-phased” [3] – with the beam phased on the rising part of the RF waveform in the first and third (lower gradient) cryomodules, and on the falling portion of the waveform in the second (high gradient) module. The phase-energy correlations so induced by the three modules cancel one another, resulting in small relative energy spread (of order 2‰ full), while allowing beam operations to proceed using phasing procedures nearly identical to those employed during FEL operations. The resulting JLAB-TN-13-020 16 April 2013 Page 2 of 11 nearly mono-energetic full-energy bunch was, as a result, not significantly modulated in length as it traversed the beam transport system. It was therefore relatively long in the test aperture (~2 psec RMS or longer), significantly reducing any heating due to resistive wall effects. The vault radiation background is dominated by field emission from SRF cavities [4] and – at moderate to high CW current – loss of beam halo. The former was controlled by operating at 100 MeV total energy (which was also the energy specified by the user) rather than the 135 MeV nominal accelerator operating point; the reduction in SRF cavity gradients alleviated field emission and reduced backgrounds of some species/at some locations by nearly two orders of magnitude after optimization of the linac gradient profile [5]. Demonstration of halo management was of course an experimental requirement; this was supported in part by the choice of the 60 pC bunch charge, and in part by the design of the minibeta insertion used to transport the beam to and from the test aperture. Operation with small momentum spread also reduced the potential impact of dispersion errors and assisted in suppression of any momentum tails on the beam. Implementation of the “interaction region” and “mini-beta” tuning were critical components of the test. The design configuration embedded the test apertures in the center of a well-characterized alternating-gradient transport module in the IR recirculator. This section of transport line is well characterized and heavily instrumented with beam position monitors, beam viewers, and beam loss monitors. It exhibits reproducible halo behavior. Large amplitude beam components appear to reach maxima at the beginning and end of the region, and thus are at a minimum at the center – the location of choice for the test. The system has both reflective and translational symmetry (periodicity) and thus tends to suppress aberrations. This region also includes a phase-space exchange module, with five skew and two normal quads arranged symmetrically about the center of the line. This “five-quad rotator” was readily modified to accommodate the aperture test: the center skew quad was replaced by the test apparatus, and the skew quads immediately upand downstream rotated to a normal orientation. The resulting “interaction region” then provided multiple quadrupole triplets (including a pair immediately adjacent to the test assembly). This allowed strong focusing of the beam (to a small spot size) without generation of excessively large betatron mismatch, and had – in addition to the variables needed for the linear match – unconstrained focusing quad strengths that allowed optimization on target parameters such as phase advance and peak beam envelope values. Both of these were of use in halo management; the choice of phase advance could, in addition, be utilized as a means of controlling the beam breakup threshold [6]. Halo and instability control were thus integral components of both the full system and the interaction region design. Operation at 60 pC with a UV injector configuration improves emittance and notionally reduces halo effects in contrast to those occurring at higher charge. The choice of operation with small beam momentum spread meets user physics requirements, alleviates impact from beamline dispersion errors, suppresses momentum tails, mitigates chromatic effects (despite the aggravated chromaticity necessarily associated with strong focusing to small spot sizes), and – by virtue of the longitudinal matching process (and JLAB-TN-13-020 16 April 2013 Page 3 of 11 Liouville’s theorem...) forces a long bunch length at the test aperture, which in turn mitigates potential resistive wall heating and instability effects. The test assembly itself is located a point of “minimum halo amplitude” [7]. Multiplicity in available magnet channels allow quadrupole linear matching solutions that produce a small spot at the test aperture while constraining the maximum spot sizes upand downstream, thereby reducing halo effects. Free choice of phase advance – also provided by multiplicity of focusing channels – can be exercised to manage the halo pattern at the test location and beyond, and to modify the turn-to-turn phase advance as a means of controlling the BBU instability threshold. Given the constrained apertures, care in system alignment, beam steering, and betatron matching were vital to successful operation. The test assembly itself was mechanically characterized with high precision using a Faro arm, and additional steering dipoles and diagnostics were installed in the 3F region to provide the requisite orbit and envelope control. Two extra correctors were added to control the orbit into/out of the test assembly. Three higherresolution viewers were arranged symmetrically about the cube center, and replaced the single viewer originally nearby. A high dynamic range YAG screen was installed on the same actuator as the new central viewer to allow LDR beam properties measurements at the longitudinal position of the center of the aperture block. A schematic layout of ERL backleg showing the diagnostic and corrector configuration is shown in Figure 1; Figure 2 presents mechanical modeling of the region and a photograph of an installation in progress. Space constraints precluded use of a desirable corrector configuration at 3F07; instead of the design corrector pair on the BPM immediately upstream of the quadrupole, only the vertical corrector was installed. A horizontal corrector was installed downstream of the quad in the first available space. Figure 1a: Nominal 3F diagnostic and corrector configuration. Figure 1b: Modified 3F diagnostic and corrector configuration as used in aperture transmission test. Note polarity mismatch of correctors and quads at 3F04, 5, 8, and 9. JLAB-TN-13-020 16 April 2013 Page 4 of 11 Figure 2a: Plan view of “interaction region” with test cube, quadrupoles, and ancillary hardware. Figure 2b (left): perspective view from interior of ring looking down-beam; 2c (right): photo of installation, from exterior of ring. Care in alignment of the test apertures and the adjacent quadrupoles allowed us to center in MQX3F06 and MQX3F07 in order to define a baseline around which the beam could be steered to thread beam and optimize transmission through the apertures. We note that the need to limit hardware modifications (cost and time constraints) precluded installing a full suite of corrector pairs at all desired locations. As a result, there was a mismatch in the correction plane and quadrupole focusing plane at some locations (3F04, 5, 8, and 9) due to the reversal of quad polarity executed during the transition from the FODO (Figure 1a) to the minibeta (Figure 1b) configuration. This did not impose insurmountable difficulties in steering during the test, but did make the orbit correction and control procedure less transparent than if a more complete set of correctors (allowing in-focusing-plane steering at every quad for each tuning) were available. JLAB-TN-13-020 16 April 2013 Page 5 of 11 Machine Setup For Transmission Test – Machine operations used a nearly-standard sequence of procedures to restore, retune, and optimize a collection of previously established machine operating

Dark sectors, consisting of new, light, weakly-coupled particles that do not interact with the known strong, weak, or electromagnetic forces, are a particularly compelling possibility for new physics. Nature may contain numerous dark sectors, each with their own beautiful structure, distinct particles, and forces. This review summarizes the physics motivation for dark sectors and the exciting opportunities for experimental exploration. It is the summary of the Intensity Frontier subgroup New, Light, Weakly-coupled Particles of the Community Summer Study 2013 (Snowmass). We discuss axions, which solve the strong CP problem and are an excellent dark matter candidate, and their generalization to axion-like particles. We also review dark photons and other dark-sector particles, including sub-GeV dark matter, which are theoretically natural, provide for dark matter candidates or new dark matter interactions, and could resolve outstanding puzzles in particle and astro-particle physics. In many cases, the exploration of dark sectors can proceed with existing facilities and comparatively modest experiments. A rich, diverse, and low-cost experimental program has been identified that has the potential for one or more game-changing discoveries. These physics opportunities should be vigorously pursued in the US and elsewhere.

We report measurements of photon and neutron radiation levels observed while transmitting a 0.43MW electron beam through millimeter-sized apertures and during beam-off, but accelerating gradient RF-on, operation. These measurements were conducted at the Free-Electron Laser (FEL) facility of the Jefferson National Accelerator Laboratory (JLab) using a 100mev electron beam from an energy-recovery linear accelerator. The beam was directed successively through 6mm, 4mm, and 2mm diameter apertures of length 127mm in aluminum at a maximum current of 4.3mA (430kW beam power). This study was conducted to characterize radiation levels for experiments that need to operate in this environment, such as the proposed DarkLight Experiment. We find that sustained transmission of a 430kW continuous-wave (CW) beam through a 2mm aperture is feasible with manageable beam-related backgrounds. We also find that during beam-off, RF-on operation, multipactoring inside the niobium cavities of the accelerator cryomodules is the primary source of ambient radiation when the machine is tuned for 130mev operation.

High-power, relativistic electron beams from energy-recovering linacs have great potential to realize new experimental paradigms for pioneering innovation in fundamental and applied research. A major design consideration for this new generation of experimental capabilities is the understanding of the halo associated with these bright, intense beams. In this Letter, we report on measurements performed using the 100 MeV, 430 kW cw electron beam from the energy-recovering linac at the Jefferson Laboratory's Free Electron Laser facility as it traversed a set of small apertures in a 127 mm long aluminum block. Thermal measurements of the block together with neutron measurements near the beam-target interaction point yielded a consistent understanding of the beam losses. These were determined to be 3 ppm through a 2 mm diameter aperture and were maintained during a 7 h continuous run.

A. Afanasev, J.R. Armendariz, O. Baker, B. Batell, J. Beacham, F. Bossi, J. Boyce, M. Buckley, G. Carosi, R. Cowan, A. Denig, B. Echenard, A. Freyberger, A. Gasparian, M. Graham, P.W. Graham, A. Haas, J. Hartnett, I. Irastorza, J. Jaeckel, I. Jaegle, M. Lamm, A. Lindner, W.C. Louis, D. McKeen, H. Merkel, G. Mills, L.A. Moustakas, G. Mueller, M. Pivovaroff, R. Povey, S. Rajendran, J. Redondo, A. Ringwald, P. Schuster, M. Schwarz, K. Sigurdson, P. Sikivie, J.H. Steffen, S. Stepanyan, M. Strassler, D.B. Tanner, M. Tobar, N. Toro, A. Upadhye, S. Vahsen, R. Van de Water, J. Vogel, D. Walker, N. Weiner, A. Weltman, W. Wester, G. Wiedemann, B. Wojtsekhowski, K. Zioutas

New light particles that couple only weakly to ordinary matter are ubiquitous innew physics extensions of the Standard Model. Their existence is motivated byseveral theoretical and observational puzzles, many of which are central in ourquest to obtain a comprehensive understanding of the constituents of ouruniverse and their interactions. These include the nature of dark matter anddark energy, the strong CP problem, and a variety of astrophysical puzzles anddark matter-related anomalies. Our working group examines axions, axion-likeparticles, hidden-sector photons, milli-charged particles, chameleons, andrelated particles (see [1] for a recent review). Their masses can range anywherefrom sub-femto-eV to the weak scale (∼ 100 GeV), and they are characterized bytheir small coupling or mixing with the photon. This allows them to be producedwith intense beams of photons, electrons, or protons and detected with sensitiveequipment. This makes them, by definition, targets for the intensity frontier.We will sometimes refer to these weakly interacting sub-eV (or slim )particles as WISPs . Axions are pseudo-scalar particles that solve the strongCP problem. They have extremely small masses, because they arise as pseudo-Nambu-Goldstone bosons of an almost exact the Peccei-Quinn symmetry, which is spontaneously broken at a very high energy scale. Thespontaneous breaking of other, non-Peccei-Quinn global symmetries is common innew physics models (including string theory) and can give rise to light axion-like scalar or pseudo-scalar particles, called ALPs. Axions and ALPs canconstitute the dark matter of our universe and can explain a variety ofastrophysical observations. The axion couples to gluons, photons, and StandardModel fermions, and it is the latter two that are the most easily detected. ALPsalso naturally couple to photons, although this coupling is not guaranteed.Hidden-sector photons, called A bosons, are massive vector bosons that can mixwith the ordinary photon via kinetic-mixing. This mixing can permit photon-Aoscillations (observable for sub-eV A bosons) and produces a small coupling ofthe A to electrically charged matter. A sub-eV mass A could be the dark matterparticle or contribute to the observed number of relativistic degrees of freedomin the early universe. A MeV-GeV mass A could explain the discrepancy betweenthe measured and calculated muon anomalous magnetic moment in the StandardModel.