The Review summarizes much of particle physics and cosmology. Using data from previous editions, plus 2,717 new measurements from 869 papers, we list, evaluate, and average measured properties of gauge bosons and the recently discovered Higgs boson, leptons, quarks, mesons, and baryons. We summarize searches for hypothetical particles such as supersymmetric particles, heavy bosons, axions, dark photons, etc. Particle properties and search limits are listed in Summary Tables. We give numerous tables, figures, formulae, and reviews of topics such as Higgs Boson Physics, Supersymmetry, Grand Unified Theories, Neutrino Mixing, Dark Energy, Dark Matter, Cosmology, Particle Detectors, Colliders, Probability and Statistics. Most of the 120 reviews are updated, including many that are heavily revised. The Review is divided into two volumes. Volume 1 includes the Summary Tables and 97 review articles. Volume 2 consists of the Particle Listings and contains also 23 reviews that address specific aspects of the data presented in the Listings. The complete Review (both volumes) is published online on the website of the Particle Data Group (pdg.lbl.gov) and in a journal. Volume 1 is available in print as the PDG Book. A Particle Physics Booklet with the Summary Tables and essential tables, figures, and equations from selected review articles is available in print, as a web version optimized for use on phones, and as an Android app.

HELIX is a new NASA-sponsored instrument aimed at measuring the spectra and composition of light cosmic-ray isotopes from hydrogen to neon nuclei, in particular the clock isotopes 10Be (radioactive, with 1.4 Myr lifetime) and 9Be (stable). The latter are unique markers of the production and Galactic propagation of secondary cosmic-ray nuclei, and are needed to resolve such important mysteries as the proportion of secondary positrons in the excess of antimatter observed by the AMS-02 experiment. By using a combination of a 1 T superconducting magnet spectrometer (with drift-chamber tracker) with a high-resolution time-of-flight detector system and ring-imaging Cherenkov detector, mass-resolved isotope measurements of light cosmic-ray nuclei will be possible up to 3 GeV/n in a first stratospheric balloon flight from Kiruna, Sweden to northern Canada, anticipated to take place in early summer 2024. An eventual longer Antarctic balloon flight of HELIX will yield measurements up to 10 GeV/n, sampling production from a larger volume of the Galaxy extending into the halo. We review the instrument design, testing, status and scientific prospects.

We report on a long-lasting, elevated gamma-ray flux state from VER J0521+211 observed by VERITAS, MAGIC, and Fermi-LAT in 2013 and 2014. The peak integral flux above 200 GeV measured with the nightly-binned light curve is $(8.8 \pm 0.4) \times 10^{-7} \;\text{ph}\;\text{m}^{-2}\; \text{s}^{-1}$, or ~37% of the Crab Nebula flux. Multiwavelength observations from X-ray, UV, and optical instruments are also presented. A moderate correlation between the X-ray and TeV gamma-ray fluxes was observed, and the X-ray spectrum appeared harder when the flux was higher. Using the gamma-ray spectrum and four models of the extragalactic background light (EBL), a conservative 95% confidence upper limit on the redshift of the source was found to be z<=0.31. Unlike the gamma-ray and X-ray bands, the optical flux did not increase significantly during the studied period compared to the archival low-state flux. The spectral variability from optical to X-ray bands suggests that the synchrotron peak of the spectral energy distribution (SED) may become broader during flaring states, which can be adequately described with a one-zone synchrotron self-Compton model varying the high-energy end of the underlying particle spectrum. The synchrotron peak frequency of the SED, as well as the radio morphology of the jet from the MOJAVE program, are consistent with the source being an intermediate-frequency-peaked BL Lac object.

The Galactic Center region hosts a variety of powerful astronomical sources and rare astrophysical processes that emit a large flux of non-thermal radiation. We present the analysis of the very-high-energy gamma-ray emission above 2 TeV of the region around the Galactic Center known as the Central Molecular Zone using 125 hours of data taken with the VERITAS imaging-atmospheric Cherenkov telescope between 2010 and 2018. This analysis employs new shower reconstruction algorithms and instrument response functions optimized for data taken at large zenith angles such as the Galactic Center sources. We report positions and spectra for point sources VER J1745-290, G0.9+0.1, and HESS J1746-285, along with a light curve for VER J1745-290, the brightest source in the region consistent with the position of the supermassive black hole Sagittarius A*. We also measure the spectrum of the diffuse emission from the Galactic Center ridge region, which has been claimed as evidence of a Galactic PeVatron.

HELIX (High Energy Light Isotope eXperiment) is a balloon-borne experiment designed to measure the chemical and isotopic abundances of light cosmic ray nuclei, especially the Be-10/Be-9 ratio over the energy range from 0.2 GeV/n to beyond 3 GeV/n. This is a key measurement for constraining cosmic-ray propagation models. The detector is a mass spectrometer, which is built around a 1 Tesla superconducting magnet and a high-resolution tracking system to determine particle rigidity. Time-of-flight counters and a ring-imaging Cherenkov detector (RICH) are used to measure velocities. The proximity-focused RICH consists of a radiator made of aerogel tiles (refractive index approximately 1.15) and a detector plane of silicon photomultipliers. For discrimination of the Be-9 and Be-10 isotopes at high energy, the refractive index of the aerogel must be known to a precision of 0.07%. Given the manufacturing tolerances in the production process, the index must be mapped over the lateral extent of aerogel tiles on a fine grid. In this contribution, we describe and show initial results from procedures developed for this task. These include laser-deflection and electron-beam measurements.

A hydrophobic, highly transparent silica aerogel with a refractive index of $\sim $1.15 was developed using sol–gel polymerization, pin drying, and supercritical carbon dioxide solvent extraction technologies. A total of 96 monolithic tiles with dimensions of 11 cm $\times $ 11 cm $\times $ 1 cm were mass produced with a high crack-free yield for use as Cherenkov radiators to be installed in the proximity-focusing ring-imaging Cherenkov (RICH) detector. The RICH detector, containing 36 aerogel tiles, will be installed in the High Energy Light Isotope eXperiment (HELIX) spectrometer and used to measure the velocity of cosmic-ray particles. HELIX is a balloon-borne experimental program designed to measure the chemical and isotopic abundances of light cosmic-ray nuclei. A water-jet cut test of the aerogel tiles and a gluing test of the trimmed tiles with dimensions of 10 cm $\times $ 10 cm $\times $ 1 cm in an aluminum frame were successful in the context of integration into the radiator module.

We report on the search for very-high-energy gamma-ray emission from the regions around three nearby supersonic pulsars (PSR B0355+54, PSR J0357+3205 and PSR J1740+1000) that exhibit long X-ray tails. To date there is no clear detection of TeV emission from any pulsar tail that is prominent in X-ray or radio. We provide upper limits on the TeV flux, and luminosity, and also compare these limits with other pulsar wind nebulae detected in X-rays and the tail emission model predictions. We find that at least one of the three tails is likely to be detected in observations that are a factor of 2-3 more sensitive. The analysis presented here also has implications for deriving the properties of pulsar tails, for those pulsars whose tails could be detected in TeV.

The High-Energy Light Isotope eXperiment (HELIX), is a new balloon-borne 1 Tesla magnet spectrometer to measure light isotopic abundances including Be-10 and Be-9 to energies of similar to 3 GeV/nuc. This requires precise measurements of the rigidity and velocity of the detected particles, necessitating a multi-system detector. The detectors include a magnet/particle tracker for measuring the rigidity, and both time-of-flight and ring imaging Cherenkov (RICH) detectors for measuring the particle velocity and charge. The energy range of interest, high altitude environment, and proximity to the magnet present a challenging environment for a RICH detector. HELIX uses a proximity-focused RICH with an aerogel radiator above a 1m(2) focal plane populated with 12,800 silicon photomultipliers (SiPMs). In this contribution we will describe the design of the RICH and discuss progress towards the fabrication and characterization of the focal plane components.

HELIX (High Energy Light Isotope eXperiment) is a balloon-borne experiment designed to measure the chemical and isotopic abundances of light cosmic ray nuclei. Detailed measurements by HELIX, especially of $^{10}$Be from 0.2 GeV/n to beyond 3 GeV/n, will provide an essential set of data for the study of propagation processes of the cosmic rays. HELIX consists of a 1 Tesla superconducting magnet with a high-resolution tracking system, time of flight detector, and a ring-imaging Cherenkov detector. The instrument is scheduled to have a long-duration balloon flight out of McMurdo Station during NASA's 2019/20 Antarctic balloon campaign. In this talk, we will discuss the scientific goals and the design of the experiment, and report on its current status.

The Schwarzschild-Couder Telescope (SCT) is a telescope concept proposed for the Cherenkov Telescope Array. It employs a dual-mirror optical design to remove comatic aberrations over an 8^∘ field of view, and a high-density silicon photomultiplier camera (with a pixel resolution of 4 arcmin) to record Cherenkov emission from cosmic ray and gamma-ray initiated particle cascades in the atmosphere. The prototype SCT (pSCT), comprising a 9.7 m diameter primary mirror and a partially instrumented camera with 1536 pixels, has been constructed at the Fred Lawrence Whipple Observatory. The telescope was inaugurated in January 2019, with commissioning continuing throughout 2019. We describe the first campaign of observations with the pSCT, conducted in January and February of 2020, and demonstrate the detection of gamma-ray emission from the Crab Nebula with a statistical significance of 8.6σ.

The Cherenkov Telescope Array (CTA) is an international project for a next-generation ground-based gamma ray observatory, aiming to improve on the sensitivity of current-generation experiments by an order of magnitude and provide energy coverage from 30 GeV to more than 300 TeV. The 9.7m Schwarzschild-Couder (SC) candidate medium-size telescope for CTA exploits a novel aplanatic two-mirror optical design that provides a large field of view of 8 degrees and substantially improves the off-axis performance giving better angular resolution across all of the field of view with respect to single-mirror telescopes. The realization of the SC optical design implies the challenging production of large aspherical mirrors accompanied by a submillimeter-precision custom alignment system. In this contribution we report on the status of the implementation of the optical system on a prototype 9.7 m SC telescope located at the Fred Lawrence Whipple Observatory in southern Arizona.

High angular resolution observations at optical wavelengths provide valuable insights into stellar astrophysics1,2, and enable direct measurements of fundamental stellar parameters3,4 and the probing of stellar atmospheres, circumstellar disks5, the elongation of rapidly rotating stars6 and the pulsations of Cepheid variable stars7. The angular size of most stars is of the order of one milliarcsecond or less, and to spatially resolve stellar disks and features at this scale requires an optical interferometer using an array of telescopes with baselines on the order of hundreds of metres. We report on the implementation of a stellar intensity interferometry system developed for the four VERITAS imaging atmospheric Cherenkov telescopes. The system was used to measure the angular diameter of the two sub-milliarcsecond stars β Canis Majoris and ϵ Orionis with a precision of greater than 5%. The system uses an offline approach in which starlight intensity fluctuations that are recorded at each telescope are correlated post observation. The technique can be readily scaled onto tens to hundreds of telescopes, providing a capability that has proven technically challenging to the current generation of optical amplitude interferometry observatories. This work demonstrates the feasibility of performing astrophysical measurements using imaging atmospheric Cherenkov telescope arrays as intensity interferometers and shows the promise for integrating an intensity interferometry system within future observatories such as the Cherenkov Telescope Array. Stellar intensity interferometry (SII) is undergoing a revival. Here, data from the four 12 m optical reflectors of the VERITAS array are correlated post facto to determine the angular diameter of two stars to a high precision, laying the groundwork for SII at future large Cherenkov arrays.

The novel 9.7m Schwarzschild-Couder Telescope (SCT), utilizing aspheric dual-mirror optical system, has been constructed as a prototype medium size x-ray telescope for the Cherenkov Telescope Array (CTA) observatory. The prototype SCT (pSCT) is designed to achieve simultaneously the wide (≥ 8°) field of view and the superior imaging resolution (0.067 per pixel) to significantly improve scientific capabilities of the observatory in conducting the sky surveys, the follow-up observations of multi-messenger transients with poorly known initial localization and the morphology studies of x-ray sources with angular extent. In this submission, we describe the hardware and software implementations of the telescope optical system as well as the methods specifically developed to align its complex optical system, in which both primary and secondary mirrors are segmented. The pSCT has detected Crab Nebula in June 2020 during ongoing commissioning, which was delayed due to worldwide pandemic and is not yet completed. Verification of pSCT performance is continuing and further improvement of optical alignment is anticipated.

The radio source 3C 264, hosted by the giant elliptical galaxy NGC 3862, was observed with the Very Energetic Radiation Imaging Telescope Array System (VERITAS) between 2017 February and 2019 May. These deep observations resulted in the discovery of very high energy (VHE;E> 100 GeV)gamma-ray emission from this active galaxy. An analysis of similar to 57 hr of quality-selected live time yields a detection at the position of the source, corresponding to a statistical significance of 7.8 standard deviations above background. The observed VHE flux is variable on monthly timescales, with an elevated flux seen in 2018 observations. The VHE emission during this elevated state is well characterized by a power-law spectrum with a photon index Gamma = 2.20 0.27 and fluxF(>315 GeV) = (7.6 +/- 1.2(stat) +2.3(syst)) x 10(13) cm(-2) s(-1), or approximately 0.7% of the Crab Nebula flux above the same threshold. 3C 264 (z = 0.0217) is the most distant radio galaxy detected at VHE, and the elevated state is thought to be similar to that of the famously outbursting jet in M87. Consequently, extensive contemporaneous multiwavelength data were acquired in 2018 at the time of the VHE high state. An analysis of these data, including Very Long Baseline Array, Very Large Array, Hubble Space Telescope, Chandra, and Swift observations in addition to the VERITAS data, is presented, along with a discussion of the resulting spectral energy distribution.

This paper reports the successful fabrication of silica aerogel Cherenkov radiators produced in the first batches from a 96-tile mass production performed using pin-drying technique in our laboratory. The aerogels are to be used in a ring-imaging Cherenkov detector in the spectrometer of a planned balloon-borne cosmic-ray observation program, HELIX (High Energy Light Isotope eXperiment). A total of 36 transparent, hydrophobic aerogel tiles with a high refractive index of 1.16 and dimensions of 10 cm x 10 cm x 1 cm will be chosen as the flight radiators. Thus far, 40 out of the 48 tiles fabricated were confirmed as having no tile cracking. In the first screening, 8 out of the first 16 tiles were accepted as flight-qualified candidates, based on basic optical measurement results. To fit the aerogel tiles into a radiator support structure, the trimming of previously manufactured prototype tiles using a water-jet cutting device was successful.

We report on variability and correlation studies using multiwavelength observations of the blazar Mrk 421 during the month of February, 2010 when an extraordinary flare reaching a level of $\sim$27~Crab Units above 1~TeV was measured in very-high-energy (VHE) $\gamma$-rays with the VERITAS observatory. This is the highest flux state for Mrk 421 ever observed in VHE $\gamma$-rays. Data are analyzed from a coordinated campaign across multiple instruments including VHE $\gamma$-ray (VERITAS, MAGIC), high-energy (HE) $\gamma$-ray (Fermi-LAT), X-ray (Swift}, RXTE, MAXI), optical (including the GASP-WEBT collaboration and polarization data) and radio (Mets\"ahovi, OVRO, UMRAO). Light curves are produced spanning multiple days before and after the peak of the VHE flare, including over several flare `decline' epochs. The main flare statistics allow 2-minute time bins to be constructed in both the VHE and optical bands enabling a cross-correlation analysis that shows evidence for an optical lag of $\sim$25-55 minutes, the first time-lagged correlation between these bands reported on such short timescales. Limits on the Doppler factor ($\delta \gtrsim 33$) and the size of the emission region ($ \delta^{-1}R_B \lesssim 3.8\times 10^{13}\,\,\mbox{cm}$) are obtained from the fast variability observed by VERITAS during the main flare. Analysis of 10-minute-binned VHE and X-ray data over the decline epochs shows an extraordinary range of behavior in the flux-flux relationship: from linear to quadratic to lack of correlation to anti-correlation. Taken together, these detailed observations of an unprecedented flare seen in Mrk 421 are difficult to explain by the classic single-zone synchrotron self-Compton model.

Compilation of papers presented by the VERITAS Collaboration at the 36th International Cosmic Ray Conference (ICRC), held July 24 through August 1, 2019 in Madison, Wisconsin.

We conduct a search for periodic emission in the very high-energy (VHE) gamma-ray band ( E > 100 GeV) from a total of 13 pulsars in an archival VERITAS data set with a total exposure of over 450 hr. The set of pulsars includes many of the brightest young gamma-ray pulsars visible in the Northern Hemisphere. The data analysis resulted in nondetections of pulsed VHE gamma-rays from each pulsar. Upper limits on a potential VHE gamma-ray flux are derived at the 95% confidence level above three energy thresholds using two methods. These are the first such searches for pulsed VHE emission from each of the pulsars, and the obtained limits constrain a possible flux component manifesting at VHEs as is seen for the Crab pulsar.

The angular size of a star is a critical factor in determining its basic properties 1 . Direct measurement of stellar angular diameters is difficult: at interstellar distances stars are generally too small to resolve by any individual imaging telescope. This fundamental limitation can be overcome by studying the diffraction pattern in the shadow cast when an asteroid occults a star 2 , but only when the photometric uncertainty is smaller than the noise added by atmospheric scintillation 3 . Atmospheric Cherenkov telescopes used for particle astrophysics observations have not generally been exploited for optical astronomy due to the modest optical quality of the mirror surface. However, their large mirror area makes them well suited for such high-time-resolution precision photometry measurements 4 . Here we report two occultations of stars observed by the Very Energetic Radiation Imaging Telescope Array System (VERITAS) 5 Cherenkov telescopes with millisecond sampling, from which we are able to provide a direct measurement of the occulted stars’ angular diameter at the ≤0.1 mas scale. This is a resolution never achieved before with optical measurements and represents an order of magnitude improvement over the equivalent lunar occultation method 6 . We compare the resulting stellar radius with empirically derived estimates from temperature and brightness measurements, confirming the latter can be biased for stars with ambiguous stellar classifications.

We propose a mission concept for a space-based gamma-ray and cosmic-ray explorer known as the Advanced Particle-astrophysics Telescope (APT). We consider two realizations of the instrument; a 3m×3m detector that would fall just under the $500M threshold for a small instrument, and a larger 3m×6m instrument that would be a candidate for a probe-class mission. The instrument design was driven by two scientific objectives: (1) to confirm or rule out the thermal WIMP dark matter paradigm and (2) to promptly localize the electromagnetic counterparts of gravitywave/neutron-star mergers. These goals motivate a gamma-ray instrument with ten times the sensitivity of Fermi at GeV energies, simultaneously providing sub-degree MeV transient localization over the largest possible field of view. A Sun–Earth Lagrange orbit for the instrument would remove Earth obscuration, allowing a view of the entire sky. The very large area needed to achieve this sensitivity coupled with a high-Earth orbit dictate the use of an electromagnetic calorimeter with limited depth (<6 radiation lengths) to reduce mass. Such an instrument would also be a powerful cosmic-ray detector capable of measuring the elemental abundances of very rare, ultra-heavy r-process cosmic-ray nuclei for material originating outside our solar system, connecting to the n-star merger science. With the addition of foam radiators, the CsI detectors could detect the transition radiation X-rays from very-high-energy light cosmic rays, specifically Boron and Carbon, needed to differentiate models of cosmic ray propagation of importance to indirect dark matter detection. The APT detector design would incorporate 20 layers of 5mm thick CsI:Na with crossed wavelength shifting fiber (WLS fiber) readout, interspersed with 20 x− y scintillating optical fiber tracker (SOFT) layers using interleaved 1.5mm round scintillating fibers. The 3m×3m×2.5m detector volume consisting of passive plastic scintillating fibers and CsI crystals would be read out on the sides with SiPM photodetectors and analog-pipeline waveform digitizers. Detailed simulations and laboratory measurements show that the APT instrument could achieve ~10 times the sensitivity of the Fermi LAT for pair-events from 60 MeV–TeV gamma-rays and provide more than an order of magnitude improvement in sensitivity over any other proposed gamma-ray experiment in the MeV energy range with prompt GRB source localization to better than 1◦ uncertainty. Likewise, the cosmic-ray detector would improve statistics on rare heavy elements and rare very-high-energy lighter nuclei by orders of magnitude compared to any extant experiment. 1Washington University (WU), 2Stockholm U., 3U. Minnesota, 4Louisianna State University, 5Erlangen Center for Astroparticle Physics, 6Fermi National Accelerator Laboratory,7Goddard Space Flight Center, 8University of California, Santa Cruz, 9Jet Propulsion Laboratory, 10U. Hawaii, 11U. Chicago