119203729
Measuring unification
If supersymmetry is observed at the LHC its model parameters can be measured at the electroweak scale. We discuss the expected precision on the parameter determination, including a proper treatment of experimental and theoretical errors. Particular attention is paid to degenerate solutions. Using the SFitter framework we perform a bottom–up reconstruction of the unified parameters at the high scale, including a full error propagation.
119257516
Di-boson and Tri-boson production at the LHC
The status of di-boson and tri-boson production is shortly review. Using the VBFNLO and the LOOPSIM package, approximated results at NNLO QCD are given for WZ production. Results for waa + jet at NLO QCD are also shown.
119114605
Physics beyond the standard model: supersymmetry
This collection of studies on new physics at the LHC constitutes the report of the supersymmetry working group at the Workshop 'Physics at TeV Colliders', Les Houches, France, 2007. They cover the wide spectrum of phenomenology in the LHC era, from alternative models and signatures to the extraction of relevant observables, the study of the MSSM parameter space and finally to the interplay of LHC observations with additional data expected on a similar time scale. The special feature of this collection is that while not each of the studies is explicitly performed together by theoretical and experimental LHC physicists, all of them were inspired by and discussed in this particular environment.
246159
Complete one-loop calculation of the T-violating D-parameter in neutron decay in the MSSM
Abstract.We investigate the violation of time reversal invariance in the decay of the free neutron in the framework of the minimal supersymmetric standard model (MSSM). The coefficient of the triple product of the neutron spin and the momenta of electron and neutrino, the so-called D parameter, is computed at one-loop order including all diagrams. We find that D is mainly sensitive to the trilinear A coupling in the squark sector and to the phase of the coefficient $\mu$ which mixes the two Higgs superfields. The maximal MSSM contribution using parameters still allowed by experiment is however at $D \approx 10^{-7}$, while QED final state interactions give a value of $D_{\mathrm {fsi}} = - 2.3 \cdot 10^{-5}$. Explicit expressions for all relevant diagrams are given in an appendix.
116260923
NLO QCD corrections to $W^\pm Z\gamma$ production with leptonic decays
We present a computation of the O(alpha_s) QCD corrections to W^\pm Z \gamma production at the Large Hadron Collider. The photon is considered as real, and we include full leptonic decays for the W and Z bosons. Based on the structure of the VBFNLO program package, we obtain numerical results through a fully flexible Monte Carlo program, which allows to implement general cuts and distributions of the final-state particles. The NLO QCD corrections are sizable and strongly exceed the theory error obtained by a scale variation of the leading-order result. Also, the shapes of relevant observables are significantly altered.
123906099
Helmholtz Alliance Linear Collider Forum
Supersymmetric scenarios with a very weakly interacting lightest superpartner (LSP) - like the gravitino or axino - naturally give rise to a long-lived next-to-LSP (NLSP). If the NLSP is a charged slepton it leaves a very distinct signature in a collider experiment. At the ILC/CLIC it will be possible to capture a significant fraction of the produced charged sleptons and observe their decays. These decays potentially reveal the nature of the LSP and thus provide a unique possibility to measure the properties of a very weakly interacting LSP which otherwise is most likely hidden from any other observation, like direct or indirect dark matter searches. We review the proposals that have been made to measure the LSP properties at the ILC/CLIC and compare its potential to the capability of the LHC.
13956524
Constraining reionization using the thermal history of the baryons
The thermal evolution of the intergalactic medium (IGM) depends on the reionization history of the universe. Numerical simulations indicate that the low density IGM, which is responsible for the low column density Ly-alpha forest, follows a well defined temperature-density relation. This results in a cut-off in the distribution of line widths as a function of column density. We use hydrodynamic simulations to calibrate the relation between the cut-off and the temperature-density relation and apply this relation to Keck spectra spanning a redshift range z=2-4.5. We find that the temperature peaks at z~3 and interpret this as evidence for reheating due to the reionization of helium.
118217379
Small Scale Structure in the Universe and the Distribution of Baryons at High Redshift
We briefly discuss the relevance of Lyman alpha forest observations for measuring cosmological parameters, comparing the properties of high z QSO spectra with those of artifical spectra from hydrodynamic simulations, based on hierarchical cosmologies. In particular, we describe a measurement of the baryon content of the universe obtained by matching the average opacity of the Lyman alpha forest from simulations to the observed one from a new dataset observed with the Keck telescope. We then report on a search for possible small scale structure in the intergalactic medium which could be unresolved by current cosmological simulations. Observing column density gradients in the spectra of lensed QSOs we find the typical intergalactic gas cloud to be structureless down to at least the 4 % level, on scales of 0.1 kpc. Extending the study of lensed absorption to higher column densities we begin to sample the velocity field internal to high redshift galaxies. Remarkable differences in column density (of order 50 %) and projected velocity (tens of km/s) between lines of sight separated by only a few hundred parsecs are found, and we may be observing structure in the early interstellar medium.
18420729
The Distribution of column densities and b values in the Lyman-alpha forest
We describe the properties of the Ly$\alpha$ forest in the column density range $\nhi \geq 2\times10^{12}$ cm$^{-2}$ based on 1056 lines in the wavelength range 4300--5100\AA measured in extremely high S/N, $R=36,000$ spectra of four quasars. The column density distribution is well described by a -1.5 power law to $2\times10^{12}$ cm$^{-2}$, below which limit confusion becomes too severe to measure a spectrum of individual clouds. The distribution of $b$ values shows a well-defined lower envelope with a cutoff at $b=20\kms$ corresponding to a cloud temperature of 24,000 K. There is only a very small fraction (less than 1%) of narrow line clouds which cannot be identified with metal-lines. From modeling the Ly$\alpha$ absorption lines as complexes of clouds each with thermal broadening corresponding to $b_c$ we find the $b$ distribution can be understood if there is a mean of 3.25 clouds per absorption line with a spread in velocity centroids characterized by a dispersion of $10.75\kms$.
119386569
The Observed Probability Distribution Function, Power Spectrum, and Correlation Function of the Transmitted Flux in the Lyα Forest*
A sample of eight quasars observed at high resolution and signal-to-noise ratio is used to determine the transmitted flux probability distribution function (TFPDF), and the power spectrum and correlation function of the transmitted flux in the Lyα forest, in three redshift bins centered at z = 2.41, 3.00, and 3.89. All the results are presented in tabular form, with full error covariance matrices, to allow for comparisons with any numerical simulations and with other data sets. The observations are compared with a numerical simulation of the Lyα forest of a ΛCDM model with Ω = 0.4, known to agree with other large-scale structure observational constraints. There is excellent agreement for the TFPDF if the mean transmitted flux is adjusted to match the observations. A small difference between the observed and predicted TFPDF is found at high fluxes and low redshift, which may be due to the uncertain effects of fitting the spectral continuum. Using the numerical simulation, we show how the flux power spectrum can be used to recover the initial power spectrum of density fluctuations. From our sample of eight quasars, we measure the amplitude of the mass power spectrum to correspond to a linear variance per unit ln k of Δ(k) = 0.72 ± 0.09 at k = 0.04(km s-1)-1 and z = 3, and the slope of the power spectrum near the same k to be np = -2.55 ± 0.10 (statistical error bars). The results are statistically consistent with those of Croft et al., although our value for the rms fluctuation is lower by a factor of 0.75. For the ΛCDM model we use, the implied primordial slope is n = 0.93 ± 0.10, and the normalization is σ8 = 0.68 + 1.16(0.95 - n) ± 0.04.
14687651
Metallicity of the Intergalactic Medium Using Pixel Statistics. II. The Distribution of Metals as Traced by C IV
We measure the distribution of carbon in the intergalactic medium as a function of redshift z and overdensity δ. Using a hydrodynamical simulation to link the H I absorption to the density and temperature of the absorbing gas, and a model for the UV background radiation, we convert ratios of C IV to H I pixel optical depths into carbon abundances. For the median metallicity this technique was described and tested in Paper I of this series. Here we generalize it to reconstruct the full probability distribution of the carbon abundance and apply it to 19 high-quality quasar absorption spectra. We find that the carbon abundance is spatially highly inhomogeneous and is well described by a lognormal distribution for fixed δ and z. Using data in the range log δ = -0.5-1.8 and z = 1.8-4.1, and a renormalized version of the 2001 Haardt & Madau model for the UV background radiation from galaxies and quasars, we measure a median metallicity of [C/H] = -3.47 + 0.08(z - 3) + 0.65(log δ - 0.5) and a lognormal scatter of σ([C/H]) = 0.76 + 0.02(z - 3) - 0.23(log δ - 0.5). Thus, we find significant trends with overdensity but no evidence for evolution. These measurements imply that gas in this density range accounts for a cosmic carbon abundance of [C/H] = -2.80 ± 0.13 (ΩC ≈ 2 × 10-7), with no evidence for evolution. The dominant source of systematic error is the spectral shape of the UV background, with harder spectra yielding higher carbon abundances. While the systematic errors due to uncertainties in the spectral hardness may exceed the quoted statistical errors for δ < 10, we stress that UV backgrounds that differ significantly from our fiducial model give unphysical results. The measured lognormal scatter is strictly independent of the spectral shape, provided the background radiation is uniform. We also present measurements of the C III/C IV ratio (which rule out temperatures high enough for collisional ionization to be important for the observed C IV) and of the evolution of the effective Lyα optical depth.