121 research outputs found
Detection of a Hypercharge Axion in ATLAS
This Master of Science thesis treats the hypercharge axion, which is a hypothetical pseudo-scalar particle with electroweak interactions. First, the theoretical context and the motivations for this study are discussed. In short, the hypercharge axion is introduced to explain the dominance of matter over antimatter in the universe and the existence of large-scale magnetic fields. Second, the phenomenological properties are analyzed and the distinguishing marks are underlined. These are basically the products of photons and s with high transverse momenta and invariant mass equal to that of the axion. Third, the simulation is carried out with two photons producing the axion which decays into s and/or photons. The event simulation is run through the simulator ATLFAST of ATLAS (A Toroidal Large Hadron Collider ApparatuS) at CERN. Finally, the characteristics of the axion decay are analyzed and the criteria for detection are presented. A study of the background is also included. The result is that for certain values of the axion mass and the mass scale (both in the order of a TeV), the hypercharge axion could be detected in ATLAS
Dust Distribution during Reionization
The dust produced by the first generation of stars will be a foreground to
cosmic microwave background. In order to evaluate the effect of this early
dust, we calculate the power spectrum of the dust emission anisotropies and
compare it with the sensitivity limit of the Planck satellite. The spatial
distribution of the dust is estimated through the distribution of dark matter.
At small angular scales () the dust signal is found to be
noticeable with the Planck detector for certain values of dust lifetime and
production rates. The dust signal is also compared to sensitivities of other
instruments. The early dust emission anisotropies are finally compared to those
of local dust and they are found to be similar in magnitude at mm wavelengths.Comment: 6 pages, 6 figures; Typos fixed. Clarifications in the abstract,
sections 2 and 4.1 and fig
The Infrared Luminosity of Galaxy Clusters
The aim of this study is to quantify the infrared luminosity of clusters as a
function of redshift and compare this with the X-ray luminosity. This can
potentially constrain the origin of the infrared emission to be intracluster
dust and/or dust heated by star formation in the cluster galaxies. We perform a
statistical analysis of a large sample of galaxy clusters selected from
existing databases and catalogues.We coadd the infrared IRAS and X-ray RASS
images in the direction of the selected clusters within successive redshift
intervals up to z = 1. We find that the total infrared luminosity is very high
and on average 20 times higher than the X-ray luminosity. If all the infrared
luminosity is to be attributed to emission from diffuse intracluster dust, then
the IR to X-ray ratio implies a dust-to-gas mass abundance of 5e-4. However,
the infrared luminosity shows a strong enhancement for 0.1 < z < 1, which
cannot be attributed to cluster selection effects. We show that this
enhancement is compatible with a star formation rate in the member galaxies
that is typical of the central Mpc of the Coma cluster at z = 0 and evolves
with the redshift as (1+z)^5. It is likely that most of the infrared luminosity
that we measure is generated by the ongoing star formation in the member
galaxies. From theoretical predictions calibrated on extinction measurements
(dust mass abundance equal to 1e-5), we expect only a minor contribution, of a
few percent, from intracluster dust.Comment: 9 pages, 7 figures, accepted july 31st 2008 for publication in
Astronomy and Astrophysics, language improved for this versio
Dust during the Reionization
The possibility that population III stars have reionized the Universe at
redshifts greater than 6 has recently gained momentum with WMAP polarization
results. Here we analyse the role of early dust produced by these stars and
ejected into the intergalactic medium. We show that this dust, heated by the
radiation from the same population III stars, produces a submillimetre excess.
The electromagnetic spectrum of this excess could account for a significant
fraction of the FIRAS (Far Infrared Absolute Spectrophotometer) cosmic far
infrared background above 700 micron. This spectrum, a primary anisotropy
() spectrum times the dust emissivity law, peaking in the
submillimetre domain around 750 micron, is generic and does not depend on other
detailed dust properties. Arcminute--scale anisotropies, coming from
inhomogeneities in this early dust, could be detected by future submillimetre
experiments such as Planck HFI.Comment: 6 pages, 8 figures, accepted by A&A; clarifications made, typos
fixed, results more exactly calculate
Mass limits for heavy neutrinos
Neutrinos heavier than GeV are not excluded by particle
physics data. Stable neutrinos heavier than this might contribute to the cosmic
gamma ray background through annihilation in distant galaxies as well as to the
dark matter content of the universe. We calculate the evolution of the heavy
neutrino density in the universe as a function of its mass, , and then the
subsequent gamma ray spectrum from annihilation of distant (from
). The evolution of the heavy neutrino density in the universe is
calculated numerically. In order to obtain the enhancement due to structure
formation in the universe, we approximate the distribution of to be
proportional to that of dark matter in the GalICS model. The calculated gamma
ray spectrum is compared to the measured EGRET data. A conservative exclusion
region for the heavy neutrino mass is 100 to 200 GeV, both from EGRET data and
our re-evalutation of the Kamiokande data. The heavy neutrino contribution to
dark matter is found to be at most 15%.Comment: 8 pages, 10 figures (Major revision
Measurement of the charm structure function F_{2,c)^{γ} of the photon at LEP
The production of charm quarks is studied in deep-inelastic electron–photon scattering using data recorded by the OPAL detector at LEP at nominal e⁺e⁻ centre-of-mass energies from 183 to 209 GeV. The charm quarks have been identified by full reconstruction of charged D* mesons using their decays into D⁰π with the D⁰ observed in two decay modes with charged particle final states, Kπ and Kπππ. The cross-section σ^{D*} for production of charged D* in the reaction e⁺e⁻→e⁺e⁻D*Χ is measured in a restricted kinematical region using two bins in Bjorken x, 0.00140.1 the perturbative QCD calculation at next-to-leading order agrees perfectly with the measured cross-section. For x<0.1 the measured cross-section is 43.8±14.3±6.3±2.8 pb with a next-to-leading order prediction of 17.0⁺²·⁹_₂.₃ pb
European Consensus Statement on Expert Colposcopy
Peer reviewedPublisher PD
Search for the Standard Model Higgs Boson with the OPAL Detector at LEP
This paper summarises the search for the Standard Model Higgs boson in e+e-
collisions at centre-of-mass energies up to 209 GeV performed by the OPAL
Collaboration at LEP. The consistency of the data with the background
hypothesis and various Higgs boson mass hypotheses is examined. No indication
of a signal is found in the data and a lower bound of 112.7GeV/C^2 is obtained
on the mass of the Standard Model Higgs boson at the 95% CL.Comment: 51 pages, 21 figure
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