arXiv subjects

C. Clarke

Publications and source records attributed to C. Clarke.

At least 109 records · Page 6Linked to original sources

arXiv2012

Measurements of Angular Distributions of Muons From Upsilon Meson Decays in p p-bar Collisions at sqrt(s)=1.96 TeV

The angular distributions of muons from Upsilon(1S,2S,3S) to mu+mu- decays are measured using data from proton anti-proton collisions at sqrt(s)=1.96 TeV corresponding to an integrated luminosity of 6.7 /fb and collected with the CDF II detector at the Fermilab Tevatron. This analysis is the first to report the full angular distributions as functions of transverse momentum pT for the Upsilon mesons in both the Collins-Soper and s-channel helicity frames. This is also the first measurement of spin alignment of Upsilon(3S) mesons. Within the kinematic range of Upsilon rapidity |y|<0.6 and pT up to 40 GeV/c, the angular distributions are found to be nearly isotropic.

hep-ex↗

arXiv2012

Search for a low mass Standard Model Higgs boson in the tau-tau decay channel in $p\bar{p}$ collisions at $\sqrt{s}$ = 1.96 TeV

We report on a search for the standard model Higgs boson decaying into pairs of tau leptons in $p\bar{p}$ collisions produced by the Tevatron at $\sqrt{s}$ = 1.96 TeV. The analyzed data sample was recorded by the CDFII detector and corresponds to an integrated luminosity of 6.0 fb$^{-1}$. The search is performed in the final state with one tau decaying leptonically and the second one identified through its semi-hadronic decay.Since no significant excess is observed, a 95% credibility level upper limit on the production cross section times branching ratio to the tau-tau final state is set for hypothetical Higgs boson masses between 100 and 150 GeV/$c^2$. For a Higgs boson of 120 GeV/$c^2$ the observed (expected) limit is 14.6 (15.3) the predicted value.

hep-ex↗

arXiv2011

Search for a Higgs Boson in the Diphoton Final State in p-pbar Collisions at sqrt(s) = 1.96 TeV

A search for a narrow Higgs boson resonance in the diphoton mass spectrum is presented based on data corresponding to 7.0 fb^-1 of integrated luminosity from p-pbar collisions at sqrt(s) = 1.96 TeV collected by the CDF experiment. No evidence of such a resonance is observed, and upper limits are set on the cross section times branching ratio of the resonant state as a function of Higgs boson mass. The limits are interpreted in the context of the standard model and one fermiophobic benchmark model where the data exclude fermiophobic Higgs bosons with masses below 114 GeV/c^2 at a 95% Bayesian credibility level.

hep-ex↗

arXiv2011

Measurement of ZZ production in leptonic final states at {\surd}s of 1.96 TeV at CDF

In this paper we present a precise measurement of the total ZZ production cross section in pp collisions at {\surd}s= 1.96 TeV, using data collected with the CDF II detector corresponding to an integrated luminosity of approximately 6 fb-1. The result is obtained by combining separate measurements in the four-charged (lll'l'), and two-charged-lepton and two-neutral-lepton (llvv) decay modes of the Z. The combined measured cross section for pp {\to} ZZ is 1.64^(+0.44)_(-0.38) pb. This is the most precise measurement of the ZZ production cross section in 1.96 TeV pp collisions to date.

hep-ex↗

arXiv2011

Search for new phenomena in events with two $Z$ bosons and missing transverse momentum in $p\bar{p}$ collisions at $\sqrt{s}=1.96$ TeV

We present a search for new phenomena in events with two reconstructed $Z$ bosons and large missing transverse momentum, sensitive to processes $p\bar{p}\rightarrow X_2X_2 \rightarrow Z Z X_1 X_1$, where $X_2$ is an unstable particle decaying as $X_2\rightarrow ZX_1$ and $X_1$ is undetected. The particles $X_1$ and $X_2$ may be, among other possibilities, fourth generation neutrinos or supersymmetric particles. We study the final state in which one $Z$ boson decays to two charged leptons and the second decays hadronically. In data corresponding to an integrated luminosity of 4.2 fb$^{-1}$ from proton-antiproton collisions recorded by the CDF II detector at the Tevatron, with center-of-mass energy of 1.96 TeV, we find agreement between data and standard-model backgrounds. We calculate 95% confidence level upper limits on the cross section of the process $p\bar{p}\rightarrow X_2X_2 \rightarrow Z Z X_1 X_1$ ranging from 50 fb to 1 pb, depending on the masses of $X_1$ and $X_2$.

hep-ex↗

arXiv2011

Search for high-mass resonances decaying into ZZ in p$\bar{p}$ collisions at $\sqrt{s}=1.96$\,TeV

We search for high-mass resonances decaying into Z boson pairs using data corresponding to 6 fb^-1 collected by the CDF experiment in p\bar{p} collisions at sqrt{s}=1.96 TeV. The search is performed in three distinct final states: ZZ --> l^+l^-l^+l^-, ZZ --> l^+l^-νν, and ZZ --> l^+l^-jj. For a Randall-Sundrum graviton G*, the 95% CL upper limits on the production cross section times branching ratio to ZZ, sigma(p\bar{p} --> G^* --> ZZ), vary between 0.26 pb and 0.045 pb in the mass range 300 < M_{G*} < 1000 GeV/c^2.

hep-ex↗

arXiv2011

Measurement of branching ratio and Bs0 lifetime in the decay Bs0 -> J/psi f0(980) at CDF

We present a study of Bs0 decays to the CP-odd final state J/psi f0(980) with J/psi -> mu+ mu- and f0(980) -> pi+ pi-. Using ppbar collision data with an integrated luminosity of 3.8/fb collected by the CDF II detector at the Tevatron we measure a Bs0 lifetime of tau(Bs0 -> J/psi f0(980)) = 1.70 -0.11+0.12(stat) +-0.03(syst) ps. This is the first measurement of the Bs0 lifetime in a decay to a CP eigenstate and corresponds in the standard model to the lifetime of the heavy Bs0 eigenstate. We also measure the product of branching fractions of Bs0 -> J/psi f0(980) and f0(980) -> pi+ pi- relative to the product of branching fractions of Bs0 -> J/psi phi and phi -> K+ K- to be R_f0/phi = 0.257 +_0.020(stat) +-0.014(syst), which is the most precise determination of this quantity to date.

hep-ex↗

arXiv2011

Spiral structure in nearby galaxies I. Sample, data analysis, and overview of results

This paper, the first of two, introduces an observational study of spiral structure in galaxies chosen from the SINGS survey. Near infrared (NIR) and optical data are used to produce mass surface density maps, and from these the morphology of the disc is examined. The aim of this work is to characterise the prevalence of spiral structure in this sample and, in the cases where a clear spiral pattern is found, include the findings in a comparative study (reported in paper II). A two-armed (`grand design') spiral pattern is found in approximately half the galaxies studied, including all those that are designated as grand design in the optical, but also including some, but not all, optically flocculent galaxies. It is found that the level of non-axisymmetric structure in the galaxies' mass distributions is only modestly higher in those galaxies that are classified as `grand design' compared with those that are not, implying that non-grand design galaxies possess significant power in higher order modes. There is no evidence that bars preferentially trigger the spirals, but they do appear to stir up non-axisymmetric structure in the disc. In contrast, there is evidence that strong/close tidal interactions with companion galaxies are associated with strong two-armed spiral structure in the infrared, though there are a number of galaxies with relatively weak infrared spiral structure that do not possess such companions.

astro-ph.GA↗

arXiv2010

Cluster mass dependent truncation of the upper IMF: evidence from observations and simulations

We attempt to evaluate whether the integrated galactic IMF (IGIMF) is expected to be steeper than the IMF within individual clusters through direct evaluation of whether there is a systematic dependence of maximum stellar mass on cluster mass. We show that the result is sensitive to observational selection biases and requires an accurate knowledge of cluster ages, particularly in more populous clusters. At face value there is no compelling evidence for non-random selection of stellar masses in low mass clusters but there is arguably some evidence that the maximum stellar mass is anomalously low (compared with the expectations of random mass selection) in clusters containing more than several thousand stars. Whether or not this effect is then imprinted on the IGIMF then depends on the slope of the cluster mass function. We argue that a more economical approach to the problem would instead involve direct analysis of the upper IMF in clusters using statistical tests for truncation of the mass function. When such an approach is applied to data from hydrodynamic simulations we find evidence for truncated mass functions even in the case of simulations without feedback.

astro-ph.SR↗

arXiv2009

Limits on the location of planetesimal formation in self-gravitating protostellar discs

In this Letter we show that if planetesimals form in spiral features in self-gravitating discs, as previously suggested by the idealised simulations of Rice et al, then in realistic protostellar discs, this process will be restricted to the outer regions of the disc (i.e. at radii in excess of several tens of A.U.). This restriction relates to the requirement that dust has to be concentrated in spiral features on a timescale that is less than the (roughly dynamical) lifetime of such features, and that such rapid accumulation requires spiral features whose fractional amplitude is not much less than unity. This in turn requires that the cooling timescale of the gas is relatively short, which restricts the process to the outer disc. We point out that the efficient conversion of a large fraction of the primordial dust in the disc into planetesimals could rescue this material from the well known problem of rapid inward migration at a $\sim$ metre size scale and that in principle the collisional evolution of these objects could help to re-supply small dust to the protostellar disc. We also point out the possible implications of this scenario for the location of planetesimal belts inferred in debris discs around main sequence stars, but stress that further dynamical studies are required in order to establish whether the disc retains a memory of the initial site of planetesimal creation.

astro-ph.SR↗

arXiv2009

Pseudo-viscous modeling of self-gravitating discs and the formation of low mass ratio binaries

We present analytic models for the local structure of self-regulated self-gravit ating accretion discs that are subject to realistic cooling. Such an approach can be used to predict the secular evolution of self-gravitating discs (which can usefully be compared with future radiation hydrodynamical simulations) and to define various physical regimes as a function of radius and equivalent steady state accretion rate. We show that fragmentation is inevitable, given realistic rates of infall into the disc, once the disc extends to radii $> 70$ A.U. (in the case of a solar mass central object). Owing to the outward redistribution of disc material by gravitational torques, we also predict fragmentation at $> 70$ A.U. even in the case of low angular momentum cores which initially collapse to a much smaller radius. We point out that 70 A.U. is close to the median binary separation and propose that such delayed fragmentation, at the point that the disc expands to $> 70$ A.U., ensures the creation of low mass ratio companions that can avoid substantial further growth and consequent evolution towards unit mass ratio. We thus propose this as a promising mechanism for producing low mass ratio binaries, which, while abundant observationally, are severely underproduced in hydrodynamical models.

astro-ph.SR↗

arXiv2009

Latest Beam Test Results of the FONT4 ILC Intra-train Feedback System Prototype

We present the design and preliminary results of a prototype beam-based digital feedback system for the Interaction Point of the International Linear Collider. A custom analogue front-end processor, FPGA-based digital signal processing board, and kicker drive amplifier have been designed, built, and tested on the extraction line of the KEK Accelerator Test Facility (ATF). The system was measured to have a latency of approximately 140 ns.

physics.acc-ph↗

arXiv2008

Tracing spiral density waves in M81

We use SPITZER IRAC 3.6 and 4.5micron near infrared data from the Spitzer Infrared Nearby Galaxies Survey (SINGS), optical B, V and I and 2MASS Ks band data to produce mass surface density maps of M81. The IRAC 3.6 and 4.5micron data, whilst dominated by emission from old stellar populations, is corrected for small-scale contamination by young stars and PAH emission. The I band data are used to produce a mass surface density map by a B-V colour-correction, following the method of Bell and de Jong. We fit a bulge and exponential disc to each mass map, and subtract these components to reveal the non-axisymmetric mass surface density. From the residual mass maps we are able to extract the amplitude and phase of the density wave, using azimuthal profiles. The response of the gas is observed via dust emission in the 8micron IRAC band, allowing a comparison between the phase of the stellar density wave and gas shock. The relationship between this angular offset and radius suggests that the spiral structure is reasonably long lived and allows the position of corotation to be determined.

astro-ph↗

arXiv2007

The response of self-graviting protostellar discs to slow reduction in cooling timescale: the fragmentation boundary revisited

A number of previous studies of the fragmentation of self-gravitating protostellar discs have modeled radiative cooling with a cooling timescale (t_{cool}) parameterised as a simple multiple (beta_{cool}) of the local dynamical timescale. Such studies have delineated the `fragmentation boundary' in terms of a critical value of beta_{cool} (beta_{crit}), where the disc fragments if beta_{cool} < beta_{crit}. Such an approach however begs the question of how in reality a disc could ever be assembled with beta_{cool} < beta_{crit}. Here we adopt the more realistic approach of gradually reducing beta_{cool}, as might correspond to changes in thermal regime due to secular changes in the disc density profile. We find that when beta_{cool} is gradually reduced (on a timescale longer than t_{cool}), the disc is stabilised against fragmentation, compared with models in which beta_{cool} is reduced rapidly. We therefore conclude that a disc's ability to remain in a self-regulated, self-gravitating state (without fragmentation) is partly dependent on its thermal history, as well as its current cooling rate. Nevertheless, a slow reduction in t_{cool} appears only to lower the fragmentation boundary by about a factor two in t_{cool} and thus only permits maximum alpha values (parameterising the efficiency of angular momentum transfer in the disc) that are about a factor two higher than determined hitherto. Our results therefore do not undermine the notion of a fundamental upper limit to the heating rate that can be delivered by gravitational instabilities before the disc is subject to fragmentation. An important implication of this work, therefore, is that self-gravitating discs can enter into the regime of fragmentation via secular evolution and it is not necessary to invoke rapid (impulsive) events to trigger fragmentation.

astro-ph↗

arXiv2007

eta Chameleontis: abnormal initial mass function or dynamical evolution?

Eta Chamaeleontis is a unique young (~9 Myr) association with 18 systems concentrated in a radius of ~35 arcmin, i.e. 1pc at the cluster distance of 97pc. No other members have been found up to 1.5 degrees from the cluster centre. The cluster mass function is consistent with the IMF of other rich young open clusters in the higher mass range but shows a clear deficit of low mass stars and brown dwarfs with no objects below 0.1Msun. The aim of this paper is to test whether this peculiar mass function could result from dynamical evolution despite the young age of the cluster. We performed N-body numerical calculations starting with a log-normal IMF and different initial conditions in terms of number of systems and cluster radius using the code NBODY3. We simulated the cluster dynamical evolution over 10 Myr and compared the results to the observations. We found that it is possible to reproduce eta Cha when starting with a very compact configuration (with Ninit=40 and R0=0.005pc) which suggests that the IMF of the association might not be abnormal. The high initial density might also explain the deficit of wide binaries that is observed in the cluster.

astro-ph↗

arXiv2006

The role of the energy equation in the fragmentation of protostellar discs during stellar encounters

In this paper, we use high-resolution smoothed particle hydrodynamics (SPH) simulations to investigate the response of a marginally stable self-gravitating protostellar disc to a close parabolic encounter with a companion discless star. Our main aim is to test whether close brown dwarfs or massive planets can form out of the fragmentation of such discs. We follow the thermal evolution of the disc by including the effects of heating due to compression and shocks and a simple prescription for cooling and find results that contrast with previous isothermal simulations. In the present case we find that fragmentation is inhibited by the interaction, due to the strong effect of tidal heating, which results in a strong stabilization of the disc. A similar behaviour was also previously observed in other simulations involving discs in binary systems. As in the case of isolated discs, it appears that the condition for fragmentation ultimately depends on the cooling rate.

astro-ph↗

arXiv2006

Star formation bursts in isolated spiral galaxies

We study the response of the gaseous component of a galactic disc to the time dependent potential generated by N-body simulations of a spiral galaxy. The results show significant variation of the spiral structure of the gas which might be expected to result in significant fluctuations in the Star Formation Rate (SFR). Pronounced local variations of the SFR are anticipated in all cases. Bursty histories for the global SFR, however, require that the mean surface density is much less (around an order of magnitude less) than the putative threshold for star formation. We thus suggest that bursty star formation histories, normally attributed to mergers and/or tidal interactions, may be a normal pattern for gas poor isolated spiral galaxies.

astro-ph↗

arXiv2005

The photometric evolution of FU Orionis objects: disc instability and wind-envelope interaction

We present the results of a photometric monitoring campaign of three well studied FU Orionis systems (FU Orionis, V1057 Cygni and V1515 Cygni) undertaken at Maidanak Observatory between 1981 and 2003. When combined with photometric data in the literature, this database provides a valuable resource for searching for short timescale variability - both periodic and aperiodic - as well as for studying the secular evolution of these systems. In the case of V1057 Cyg (which is the system exhibiting the largest changes in brightness since it went into outburst) we compare the photometric data with time dependent models. We show that prior to the end of the `plateau' stage in 1996, the evolution of V1057 Cyg in the $V-(B-V)$ colour-magnitude diagram is well represented by disc instability models in which the outburst is triggered by some agent - such as an orbiting planet - in the inner disc. Following the end of the plateau phase in 1996, the dimming and irregular variations are consistent with occultation of the source by a variable dust screen, which has previously been interpreted in terms of dust condensation events in the observed disc wind. Here we instead suggest that this effect results from the interaction between the wind and an infalling dusty envelope, the existence of this envelope having been previously invoked in order to explain the mid infrared emissio of FU Orionis systems. We discuss how this model may explain some of the photometric and spectroscopic characteristics of FU Orionis systems in general.

astro-ph↗