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This review paper describes the energy-upgraded CEBAF accelerator. This superconducting linac has achieved 12 GeV beam energy by adding 11 new high-performance cryomodules containing eighty-eight superconducting cavities that have operated CW at an average accelerating gradient of 20 MV/m. After reviewing the attributes and performance of the previous 6 GeV CEBAF accelerator, we discuss the upgraded CEBAF accelerator system in detail with particular attention paid to the new beam acceleration systems. In addition to doubling the acceleration in each linac, the upgrade included improving the beam recirculation magnets, adding more helium cooling capacity to allow the newly installed modules to run cold, adding a new experimental hall, and improving numerous other accelerator components. We review several of the techniques deployed to operate and analyze the accelerator performance, and document system operating experience and performance. In the final portion of the document, we present much of the current planning regarding projects to improve accelerator performance and enhance operating margins, and our plans for ensuring CEBAF operates reliably into the future. For the benefit of potential users of CEBAF, the performance and quality measures for beam delivered to each of the experimental halls is summarized in the appendix., Comment: 66 pages, 73 figures, 21 tables

The $f_1(1285)$ meson with mass $1281.0 \pm 0.8$ MeV/$c^2$ and width $18.4 \pm 1.4$ MeV (FWHM) was measured for the first time in photoproduction from a proton target using CLAS at Jefferson Lab. Differential cross sections were obtained via the $\eta\pi^{+}\pi^{-}$, $K^+\bar{K}^0\pi^-$, and $K^-K^0\pi^+$ decay channels from threshold up to a center-of-mass energy of 2.8 GeV. The mass, width, and an amplitude analysis of the $\eta\pi^{+}\pi^{-}$ final-state Dalitz distribution are consistent with the axial-vector $J^P=1^+$ $f_1(1285)$ identity, rather than the pseudoscalar $0^-$ $\eta(1295)$. The production mechanism is more consistent with $s$-channel decay of a high-mass $N^*$ state, and not with $t$-channel meson exchange. Decays to $\eta\pi\pi$ go dominantly via the intermediate $a_0^\pm(980)\pi^\mp$ states, with the branching ratio $\Gamma(a_0\pi \text{ (no} \bar{K} K\text{)}) / \Gamma(\eta\pi\pi \text{(all)}) = 0.74\pm0.09$. The branching ratios $\Gamma(K \bar{K} \pi)/\Gamma(\eta\pi\pi) = 0.216\pm0.033$ and $\Gamma(\gamma\rho^0)/\Gamma(\eta\pi\pi) = 0.047\pm0.018$ were also obtained. The first is in agreement with previous data for the $f_1(1285)$, while the latter is lower than the world average., Comment: 26 pages, 16 figures, Reference added, submitted to Physical Review C

The CLAS Collaboration provides a comment on the physics interpretation of the results presented in a paper published by M. Amaryan et al. regarding the possible observation of a narrow structure in the mass spectrum of a photoproduction experiment., Comment: to be published in Physical Review C

We have measured the nuclear transparency of the incoherent diffractive $A(e,e'\rho^0)$ process in $^{12}$C and $^{56}$Fe targets relative to $^2$H using a 5 GeV electron beam. The nuclear transparency, the ratio of the produced $\rho^0$'s on a nucleus relative to deuterium, which is sensitive to $\rho A$ interaction, was studied as function of the coherence length ($l_c$), a lifetime of the hadronic fluctuation of the virtual photon, and the four-momentum transfer squared ($Q^2$). While the transparency for both $^{12}$C and $^{56}$Fe showed no $l_c$ dependence, a significant $Q^2$ dependence was measured, which is consistent with calculations that included the color transparency effects., Comment: 6 pages and 4 figures

We present studies of single-spin asymmetries for neutral pion electroproduction in semi-inclusive deep-inelastic scattering of 5.776 GeV polarized electrons from an unpolarized hydrogen target, using the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. A substantial $\sin \phi_h$ amplitude has been measured in the distribution of the cross section asymmetry as a function of the azimuthal angle $\phi_h$ of the produced neutral pion. The dependence of this amplitude on Bjorken $x$ and on the pion transverse momentum is extracted with significantly higher precision than previous data and is compared to model calculations., Comment: to be submitted PLB

We have measured the 3He(e,e'pp)n reaction at an incident energy of 4.7 GeV over a wide kinematic range. We identified spectator correlated pp and pn nucleon pairs using kinematic cuts and measured their relative and total momentum distributions. This is the first measurement of the ratio of pp to pn pairs as a function of pair total momentum, $p_{tot}$. For pair relative momenta between 0.3 and 0.5 GeV/c, the ratio is very small at low $p_{tot}$ and rises to approximately 0.5 at large $p_{tot}$. This shows the dominance of tensor over central correlations at this relative momentum., Comment: 4 pages, 4 figures, submitted to PRL

We report the first measurement of the transverse momentum dependence of double spin asymmetries in semi-inclusive production of pions in deep inelastic scattering off the longitudinally polarized proton. Data have been obtained using a polarized electron beam of 5.7 GeV with the CLAS detector at the Thomas Jefferson National Accelerator Facility (JLab). A significant non-zero $\sin2\phi$ single spin asymmetry was also observed for the first time indicating strong spin-orbit correlations for transversely polarized quarks in the longitudinally polarized proton. The azimuthal modulations of single spin asymmetries have been measured over a wide kinematic range.

A 10-arcmin field around the HDF(N) contains 92 radio sources >40 uJy, resolved by MERLIN+VLA at 0".2-2".0 resolution. 55 have Chandra X-ray counterparts including 18 with a hard X-ray photon index and high luminosity characteristic of a type-II (obscured) AGN. >70% of the radio sources have been classified as starbursts or AGN using radio morphologies, spectral indices and comparisons with optical appearance and MIR emission. Starbursts outnumber radio AGN 3:1. This study extends the VO methods previously used to identify X-ray-selected obscured type-II AGN to investigate whether very luminous radio and X-ray emission originates from different phenomena in the same galaxy. The high-redshift starbursts have typical sizes of 5--10 kpc and star formation rates of ~1000 Msun/yr. There is no correlation between radio and X-ray luminosities nor spectral indices at z>~1.3. ~70% of both the radio-selected AGN and the starburst samples were detected by Chandra. The X-ray luminosity indicates the presence of an AGN in at least half of the 45 cross-matched radio starbursts, of which 11 are type-II AGN including 7 at z>1.5. This distribution overlaps closely with the X-ray detected radio sources which were also detected by SCUBA. Stacked 1.4-GHz emission at the positions of radio-faint X-ray sources is correlated with X-ray hardness. Most extended radio starbursts at z>1.3 host X-ray selected obscured AGN. Radio emission from most of these ultra-luminous objects is dominated by star formation but it contributes less than 1/3 of their X-ray luminosity. Our results support the inferences from SCUBA and IR data, that at z>1.5, star formation is an order of magnitude more extended and more copious, it is closely linked to AGN activity and it is triggered differently, compared with star formation at lower redshifts., Comment: 24 pages, 12 figures, uses graphicx, rotating, natbib, supertabular packages and aa.cls. Accepted for publication in A&A

We present spectroscopic and polarimetric observations of a complete, optically unbiased sample of 2Jy radio galaxies at intermediate redshifts (0.15 < z < 0.7). These data -- which cover the nuclear regions of the target galaxies -- allow us to quantify for the first time the various components that contribute to the UV excess in the population of powerful, intermediate redshift radio galaxies. We find that, contrary to the results of previous surveys -- which have tended to be biased towards the most luminous and spectacular objects in any redshift range -- the contribution of scattered quasar light to the UV excess is relatively minor in most of the objects in our sample. Only 7 objects (32% of the complete sample) show significant polarization in the rest-frame UV, and none of the objects in our sample is polarized in the near-UV at the P > 10% level. Careful measurement and modelling of our spectra have allowed us to quantify the contributions of other components to the UV excess. We show that nebular continuum (present in all objects at the 3 -- 40% level), direct AGN light (significant in 40% of objects), and young stellar populations (significant in 15 -- 50% of objects) all make important contributions to the UV continuum in the population of powerful radio galaxies. These results serve to emphasise the multi-component nature of the UV continuum in radio galaxies. The results also point to an interesting link betweeen the optical/UV and far-IR properties of our sample objects, in the sense that the objects with the clearest evidence for optical/UV starburst activity are also the most luminous at far-IR wavelengths. This supports the idea that the cooler dust components in radio galaxies are heated by starbursts rather than by AGN., Comment: Accepted for publication in MNRAS. 21 pages, 6 figures

This paper presents the requirements and design of a Timing Synchronization System (TSS) for the Continuous Electron Beam Accelerator Facility (CEBAF) control system at Thomas Jefferson National Accelerator Facility. A clock module has been designed which resides in a VME crate. The clock module can be a communications master or a slave depending on its configuration, which is software and jumper selectable. As a master, the clock module sends out messages in response to an external synchronization signal over a serial fiber optic line. As a slave, it receives the messages and interrupts an associated computer in its VME crate. The application that motivated the development of the TSS, the Accelerator 30 Hz Measurement System, will be described. Operational experience with the TSS will also be discussed., Comment: PSN# TUAP070, ICALEPCS 2001, San Jose, CA

We present new spectroscopic observations of the southern radio galaxy PKS1549-79 (z =0.152).Despite the flat spectrum character of the radio emission from this source, our optical spectra show no sign of the broad permitted lines and non-stellar continuum characteristic of quasar nuclei and broad line radio galaxies. However, the high ionization forbidden lines, including [OIII]5007,4959, are unusually broad for a narrow line radio galaxy (FWHM 1350 km/s), and are blueshifted by 600 km/s relative to the low ionization lines such as[OII]3726,3729. The [OII] lines are also considerably narrower (FWHM 650 km/s) than the [OIII] lines, and have a redshift consistent with that of the recently-detected HI 21cm absorption line system. Whereas the kinematics of the [OIII] emission lines are consistent with outflow in an inner narrow line region, the properties of the [OII] emission lines suggest that they are emitted by a more extended and quiescent gaseous component. We argue that, given the radio properties of the source, our line of sight is likely to be lying close to the direction of bulk outflow of the radio jets. In this case it is probable that the quasar nucleus is entirely obscured at optical wavelengths by the material responsible for the HI absorption line system.The unusually broad [OIII] emission lines suggest that the radio source is intrinsically compact. Overall, our data are consistent the idea that PKS1549-79 is a radio source in an early stage of evolution., Comment: Accepted for publication in MNRAS

As compared to electron storage rings, one advantage of recirculating linear accelerators is that the beam properties at target are no longer dominated by the equilibrium between quantum radiative diffusion and radiation damping because new beam is continually injected into the accelerator. This allows the energy spread from a CEBAF-type machine to be relatively small; the measured energy spread from CEBAF at 4 GeV is less than 100 parts per million accumulated over times of order several days. In this paper, the various subsystems contributing to the energy spread of a CEBAF-type accelerator are reviewed, as well as the machine diagnostics and controls that are used in CEBAF to ensure that a small energy spread is provided during routine running. Examples of relevant developments are (1) stable short bunches emerging from the injector, (2) precision timing and phasing of the linacs with respect to the centroid of the beam bunches on all passes, (3) implementing 2 kHz sampling rate feedback systems for final energy stabilization, and (4) continuous beam energy spread monitoring with optical transition radiation devices. We present measurement results showing that small energy spreads are achieved over extended periods., Comment: 5 pages, 5 figures, Invited Paper TH205 at 2000 International Linac Conference

The nature of the optical--radio correlations for powerful radio galaxies is investigated using spectroscopic observations of a complete sample of southern 2Jy radio sources. In line with previous work, we find that significant correlations exist between the luminosities of the [OIII]5007, [OII]3727 and Hbeta emission lines and the radio luminosity. However, our observations are not easily reconciled with the idea that these correlations are caused by the increase in the power of the photoionizing quasar as the jet power increases, with average ISM properties not changing appreciably with redshift or radio power: not only do we find that the scatter in the L_[OIII] vs. L_radio correlation is significantly larger than in L_[OII]} vs. L_radio and L_Hbeta vs. L_radio correlations, but the ionization state deduced from the emission lines does not increase with radio power as predicted by the simple, constant ISM, photionization model. We conclude that: (a) there exists a considerable range in the quasar ionizing luminosity at a given redshift; and (b) that the mean density of the emission line clouds is larger in the high redshift/high power radio sources. The latter density enhancement may either be a consequence of the increased importance of jet-cloud interactions or, alternatively, due to a higher pressure in the confining hot ISM, in the high redshift objects. Deep spectra show that many of the sources in our sample are broad line radio galaxies (BLRG). The fact that the BLRG are observed out the redshift limit of the survey, overlapping in redshift with the quasars, argues against the idea that BLRG are simply the low radio power counterparts of high power, high redshift quasars., Comment: 37 pages, LateX, including 11 figures and 3 tables. To appear in MNRAS