arXiv subjects

R. A. Johnson

Publications and source records attributed to R. A. Johnson.

At least 199 records · Page 11Linked to original sources

arXiv2000

Observation of Anomalous Dimuon Events in the NuTeV Decay Detector

A search for long-lived neutral particles (N^0) which decay into at least one muon has been performed using an instrumented decay channel at the E815 (NuTeV) experiment at Fermilab. The decay channel was composed of helium bags interspersed with drift chambers, and was used in conjunction with the NuTeV neutrino detector to search for N^0 decays. The data were examined for particles decaying into the muonic final states mu mu, mu e, and mu pi. Three mu mu events were observed over an expected background of 0.040 +/- 0.009 events; no events were observed in the other modes. Although the observed events share some characteristics with neutrino interactions, the observed rate is a factor of 75 greater than expected. No Standard Model process appears to be consistent with this observation.

hep-ex↗

arXiv2000

Multiwavelength study of the nuclei of a volume-limited sample of galaxies I: X-ray observations

We discuss ROSAT HRI X-ray observations of 33 very nearby galaxies, sensitive to X-ray sources down to a luminosity of approximately 10^38 erg/s. The galaxies are selected from a complete, volume limited sample of 46 galaxies with d<7 Mpc for which we have extensive multi-wavelength data. For an almost complete sub-sample with M_B<-14 (29/31 objects) we have HRI images. Contour maps and source lists are presented within the central region of each galaxy, together with nuclear upper limits where no nuclear source was detected. Nuclear X-ray sources are found to be very common, occurring in ~35% of the sample. Nuclear X-ray luminosity is statistically connected to host galaxy luminosity - there is not a tight correlation, but the probability of a nuclear source being detected increases strongly with galaxy luminosity and the distribution of nuclear luminosities seems to show an upper envelope that is roughly proportional to galaxy luminosity. While these sources do seem to be a genuinely nuclear phenomenon rather than nuclear examples of the general X-ray source population, it is far from obvious that they are miniature Seyfert nuclei. The more luminous nuclei are very often spatially extended, and HII region nuclei are detected just as often as LINERs. Finally, we also note the presence of fairly common super-luminous X-ray sources in the off-nuclear population - out of 29 galaxies we find 9 sources with a luminosity larger than 10^39 erg/s. These show no particular preference for more luminous galaxies. One is already known to be a multiple SNR system, but most have no obvious optical counterpart and their nature remains a mystery.

astro-ph↗

arXiv1999

Search for a 33.9 MeV/c^2 Neutral Particle in Pion Decay

The E815 (NuTeV) neutrino experiment has performed a search for a 33.9 MeV/c^2 weakly-interacting neutral particle produced in pion decay. Such a particle may be responsible for an anomaly in the timing distribution of neutrino interactions in the KARMEN experiment. E815 has searched for this particle's decays in an instrumented decay region; no evidence for this particle was found. The search is sensitive to pion branching ratios as low as 10^-13.

hep-ex↗

arXiv1999

The Lick Observatory Supernova Search

We report here the current status of the Lick Observatory Supernova Search (LOSS) with the Katman Automatic Imaging Telescope (KAIT). The progress on both the hardware and the software of the system is described, and we present a list of recent discoveries. LOSS is the world' most successful search engine for nearby supernovae.

astro-ph↗

arXiv1999

Nuclear Structure Functions in the Large x Large Q^2 Kinematic Region in Neutrino Deep Inelastic Scattering

Data from the CCFR E770 Neutrino Deep Inelastic Scattering (DIS) experiment at Fermilab contain events with large Bjorken x (x>0.7) and high momentum transfer (Q^2>50 (GeV/c)^2). A comparison of the data with a model based on no nuclear effects at large x, shows a significant excess of events in the data. Addition of Fermi gas motion of the nucleons in the nucleus to the model does not explain the excess. Adding a higher momentum tail due to the formation of ``quasi-deuterons'' makes some improvement. An exponentially falling F_2 \propto e^-s(x-x_0) at large x, predicted by ``multi-quark clusters'' and ``few-nucleon correlations'', can describe the data. A value of s=8.3 \pm 0.7(stat.)\pm 0.7(sys.) yields the best agreement with the data.

hep-ex↗

arXiv1999

Evidence for Diffractive Charm Production in nu_mu Fe and nubar_mu Fe Scattering at the Tevatron

We present evidence for the diffractive processes nu_mu Fe -> mu^- D_s^+ (D_s^*+) Fe and nubar_mu Fe -> mu^+ D_s^- (D_s^*-) Fe using the Fermilab SSQT neutrino beam and the Lab E neutrino detector. We observe the neutrino trident reactions nu_mu Fe -> nu_mu mu^- mu^+ Fe and nubar_mu Fe -> nubar_mu mu^+ mu^- Fe at rates consistent with Standard Model expectations. We see no evidence for neutral-current production of J/psi via either diffractive or deep inelastic scattering mechanisms.

hep-ex↗

arXiv1999

Low Q^2 low x structure function analysis of CCFR data for F2

Analyses of structure functions (SFs) from neutrino and muon deep inelastic scattering data have shown discrepancies in F2 for x < 0.1. A new SF analysis of the CCFR collaboration data examining regions in x down to x=.0015 and 0.4 < Q^2 < 1.0 is presented. Comparison to corrected charged lepton scattering results for F2 from the NMC and E665 experiments are made. Differences between muon and neutrino scattering allow that the behavior of F2 from muon scattering could be different from F2 from neutrino scattering as Q^2 approaches zero. Comparisons between F2 muon and F2 neutrino are made in this limit.

hep-ex↗

arXiv1999

Investigating the process of star formation in young LMC star clusters

The rich star clusters in the Large Magellanic Cloud (LMC) are ideal for studying the process of star formation. Here we focus on the determination of age spreads amongst the massive stars in two young clusters, NGC1818 and NGC1805. We present colour magnitude diagrams (derived from HST data) for these clusters, and discuss the difficulties in age spread determination.

astro-ph↗

arXiv1999

Charm Production at NuTeV

Neutrino deep-inelastic scattering provides a means to study both the strange and charm content of the nucleon. The NuTeV experiment (Fermilab E-8Neutrino deep-inelastic scattering provides a means to study both the strange and charm content of the nucleon. The NuTeV experiment (Fermilab E-815) takes full advantage of separated neutrino and anti-neutrino beams to probe the nucleon. The strange sea is studied with charged-current charm production resulting in an opposite-signed two muon final state. The charm content of the nucleon is probed via neutral-current charm production creating an event with a single wrong-signed muon. Preliminary results are presented for both analyses.

hep-ex↗

arXiv1999

Strange Content of the Nucleon (NuTeV)

The NuTeV experiment uses neutrino deep-inelastic scattering from separate neutrino and anti-neutrino beams to study the structure of the nucleon. Charged-current production of charm is sensitive to the strange content of the nucleon while neutral-current charm production probes the charm content. Preliminary analyses of both topics are presented along with discussion of possible momentum asymmetry in the strange sea.

hep-ex↗

arXiv1999

A Measurement of the Weak Mixing Angle in Neutrino-Nucleon Scattering at NuTeV

The NuTeV experiment at Fermilab presents a determination of the electroweak mixing angle. High purity, large statistics samples of muon-neutrino and muon-antineutrino events allow the use of the Paschos-Wolfenstein relation. This considerably reduces systematic errors associated with charm production and other sources. With Standard Model assumptions, this measurement of sin2thw indirectly determines the W boson mass to a precision comparable to direct measurements from high energy e+e- and p-pbar colliders. NuTeV measures sin^2theta_W (on-shell) = 0.2253 +/- 0.0019(stat) +/- 0.0010(syst) which implies M_W = 80.26 +/- 0.11 GeV.

hep-ex↗

arXiv1999

Measurement of sin^2 theta_W in nu N Scattering from NuTeV

We report the measurement of sin^2 theta_W in nu N deep inelastic scattering from the NuTeV experiment. By using separate neutrino and anti-neutrino beams, NuTeV is able to determine sin^2 theta_W with smaller systematic and similar or smaller statistical errors when compared to previous neutrino experiments. NuTeV measures sin^2 theta_W (on-shell) = 0.2253 +/- 0.0019(stat.) +/- 0.0010(sys.), which implies M_W = 80.26 +/- 0.11 GeV/c^2.

hep-ex↗

arXiv1999

Optical and X-ray Variability in The Least Luminous AGN, NGC4395

We report the detection of optical and X-ray variability in the least luminous known Seyfert galaxy, NGC4395. The featureless continuum changed by a factor of 2 in 6 months, which is typical of more luminous AGN. The largest variation was seen at shorter wavelengths, so that the spectrum becomes `harder' during higher activity states. In a one week optical broad band monitoring program, a 20% change was seen between successive nights. In a 1 month period the spectral shape changed from a power law with spectral index alpha ~0 (characteristic of quasars) to a spectral index alpha ~2 (as observed in other dwarf AGN). ROSAT HRI and PSPC archive data show a variable X-ray source coincident with the galactic nucleus. A change in X-ray flux by a factor \~2 in 15 days has been observed. When compared with more luminous AGN, NGC4395 appears to be very X-ray quiet. The hardness ratio obtained from the PSPC data suggests that the spectrum could be absorbed. We also report the discovery of weak CaIIK absorption, suggesting the presence of a young stellar cluster providing of the order of 10% of the blue light. Using HST UV archive data, together with the optical and X-ray observations, we examine the spectral energy distribution for NGC4395 and discuss the physical conditions implied by the nuclear activity under the standard AGN model. The observations can be explained by either an accreting massive black hole emitting at about 10^(-3) L_(Edd) or by a single old compact SNR with an age of 50 to 500 yr generated by a small nuclear starburst.

astro-ph↗

arXiv1998

The Formation and Evolution of LMC Globular Clusters: the database

We present details of the database from a large Cycle 7 HST project to study the formation and evolution of rich star clusters in the Large Magellanic Cloud (see Elson et al., this volume). Our data set, which includes NICMOS, WFPC2 and STIS images of the cores and outer regions of 8 clusters, will enable us to derive deep luminosity functions for the clusters and to investigate the universality of the stellar initial mass function. We will look for age spreads in the youngest clusters, quantify the population of binary stars in the cores of the clusters and at the half-mass radii, and follow the development of mass segregation.

astro-ph↗

arXiv1998

Deep STIS Luminosity Functions for LMC Clusters

We present deep luminosity functions derived from HST STIS data for three rich LMC clusters (NGC 1805, NGC 1868, and NGC 2209), and for one Galactic globular cluster (NGC 6553). All of the LMC cluster luminosity functions are roughly consistent with a Salpeter IMF or with the solar neighbourhood IMF from Kroupa, Tout & Gilmore (1993). They continue to rise at least to 0.7 Msun. NGC 1868 shows evidence for mass segregation which may be primordial. A comparison of deep luminsoisty functions for seven Galactic globulars shows that the luminosity functions are eroded at low masses by amounts that are strongly correlated with distance from the Galactic plane.

astro-ph↗

arXiv1998

Measurements of the longitudinal structure function and |V_cs| in the CCFR experiment

Measurements of charged current neutrino and anti-neutrino nucleon interactions in the CCFR detector are used to extract the structure functions, F_2, xF_3(nu), xF_3(nubar) and R(longitudinal) in the kinematic region 0.01<x<0.6 and 1<Q^2<300 GeV^2. The new measurements of R in the x<0.1 region provide a constraint on the level of the gluon distribution. The x and Q^2 dependence of R is compared with a QCD based fit to previous data. The CKM matrix element |V_cs| is extracted from a combined analysis of xF_3 and dimuon data.

hep-ex↗

arXiv1998

Measurement of sin2thetaW from Neutrino-Nucleon Scattering at NuTeV

We report the measurement of sin2thetaW in neutrino-nucleon deep inelastic scattering from the NuTeV experiment. Using separate neutrino and anti-neutrino beams, NuTeV is able to determine sin2thetaW with low systematic errors by measuring the Paschos-Wolfenstein variable R-minus, a ratio of differences of neutrino and anti-neutrino neutral-current and charged-current cross-sections. NuTeV measures sin2thetaW(on-shell)= 0.2253+/-0.0019(stat)+/-0.0010(syst), which implies a W mass of 80.26+/-0.11 GeV.

hep-ex↗

arXiv1997

A letter of intent for an experiment to measure nu_mu to nu_e oscillations and nu_mu disappearance at the Fermilab Booster (BooNE)

This Letter of Intent to the Fermilab Program Advisory Committee describes a search for neutrino oscillations at the Fermilab booster. It is motivated by the LSND observation of nu_mu -> nu_e appearance. The booster neutrino experiment (BooNE) will be capable of observing both nu_mu -> nu_e appearance and nu_mu disappearance. In addition, the experiment will be able to measure Delta m^2 and sin^2(2theta) and search for CP violation in the lepton sector.

nucl-ex↗