The top quark as a calibration tool at the LHC

Thanks to the large top quark pair production cross section and the relatively low background at the LHC, $t\bar{t}$ events can be used for calibration at ATLAS and CMS. Assuming the Standard Model prediction $BR(t\rightarrow bW)$=1 to be true, the heavy flavour content of $t\bar{t}$ events is well predicted, which allows to calibrate and measure the efficiency of $b$-tagging algorithms directly from the data with a precision of about 5\%. The light ($b$-) jet energy scale can also be extracted from $t\bar{t}$ events at the 1\% level using $W$ (and top) hadronic decays

Constraining SUSY Dark Matter with the ATLAS Detector at the LHC

In the event that R-Parity conserving supersymmetry (SUSY) is discovered at the LHC, a key issue which will need to be addressed will be the consistency of that signal with astrophysical and non-accelerator constraints on SUSY Dark Matter. This issue is studied for the SPS1a mSUGRA benchmark model by using measurements of end-points and thresholds in the invariant mass spectra of various combinations of leptons and jets in ATLAS to constrain the model parameters. These constraints are then used to assess the statistical accuracy with which quantities such as the Dark Matter relic density and direct detection cross-section can be measured. Systematic effects arising from the use of different mSUGRA RGE codes are also estimated. Results indicate that for SPS1a a statistical(systematic) precision on the relic abundance ~ 2.8% (3 %) can be obtained given 300 fb-1 of data.Comment: 11 pages, 10 encapsulated postscript figures. Minor modification to ref

Scenery from the Top: Study of the Third Generation Squarks at CERN LHC

In the minimal supersymmetric standard model (MSSM) properties of the third generation sfermions are important from the viewpoint of discriminating the SUSY breaking models and in the determination of the Higgs boson mass. If gluinos are copiously produced at CERN LHC, gluino decays into tb through stop and sbottom can be studied using hadronic decays of the top quark. The kinematical endpoint of the gluino decays can be evaluated using a W sideband method to estimate combinatorial backgrounds. This implies that fundamental parameters related to the third generation squarks can be reliably measured. The top-quark polarization dependence in the decay process may also be extracted by looking at the b jet distribution near the kinematical endpoint.Comment: 4 pages in PRL format, 4 Postscript figures, uses revtex

Measurement of the Superparticle Mass Spectrum in the Long-Lived Stau Scenario at the LHC

In supersymmetric scenarios with a long-lived stau, the LHC experiments provide us with a great environment for precise mass measurements of superparticles. We study a case in which the mass differences between the lightest stau and other sleptons are about 10 GeV or larger, so that the decay products of heavier sleptons are hard enough to be detected. We demonstrate that the masses of neutralinos, sleptons, and squarks can be measured with a good accuracy.Comment: 20 pages, 6 figure

Hidden SUSY at the LHC: the light higgsino-world scenario and the role of a lepton collider

While the SUSY flavor, CP and gravitino problems seem to favor a very heavy spectrum of matter scalars, fine-tuning in the electroweak sector prefers low values of superpotential mass \mu. In the limit of low \mu, the two lightest neutralinos and light chargino are higgsino-like. The light charginos and neutralinos may have large production cross sections at LHC, but since they are nearly mass degenerate, there is only small energy release in three-body sparticle decays. Possible dilepton and trilepton signatures are difficult to observe after mild cuts due to the very soft p_T spectrum of the final state isolated leptons. Thus, the higgsino-world scenario can easily elude standard SUSY searches at the LHC. It should motivate experimental searches to focus on dimuon and trimuon production at the very lowest p_T(\mu) values possible. If the neutralino relic abundance is enhanced via non-standard cosmological dark matter production, then there exist excellent prospects for direct or indirect detection of higgsino-like WIMPs. While the higgsino-world scenario may easily hide from LHC SUSY searches, a linear e^+e^- collider or a muon collider operating in the \sqrt{s}\sim 0.5-1 TeV range would be able to easily access the chargino and neutralino pair production reactions.Comment: 20 pages including 12 .eps figure

SUSY Parameter Analysis at TeV and Planck Scales

Coherent analyses at future LHC and LC experiments can be used to explore the breaking mechanism of supersymmetry and to reconstruct the fundamental theory at high energies, in particular at the grand unification scale. This will be exemplified for minimal supergravity.Comment: 7 pages, 3 figures, uses espcrc2.sty (included), Proceedings, Loops and Legs 2004, Zinnowitz on Usedo

Slepton mass-splittings as a signal of LFV at the LHC

Precise measurements of slepton mass-splittings might represent a powerful tool to probe supersymmetric (SUSY) lepton flavour violation (LFV) at the LHC. We point out that mass-splittings of the first two generations of sleptons are especially sensitive to LFV effects involving $\tau-\mu$ transitions. If these mass-splittings are LFV induced, high-energy LFV processes like the neutralino decay {\nt}_2\to\nt_1\tau^{\pm}\mu^{\mp} as well as low-energy LFV processes like $\tau\to\mu\gamma$ are unavoidable. We show that precise slepton mass-splitting measurements and LFV processes both at the high- and low-energy scales are highly complementary in the attempt to (partially) reconstruct the flavour sector of the SUSY model at work. The present study represents another proof of the synergy and interplay existing between the LHC, i.e. the {\em high-energy frontier}, and high-precision low-energy experiments, i.e. the {\em high-intensity frontier}.Comment: 11 pages, 5 figures. v2: added discussion on backgrounds, added references, version to be published on JHE

Measurement of SUSY masses via cascade decays for SPS 1a

If R-parity conserving supersymmetry exists below the TeV-scale, new particles will be produced and decay in cascades at the LHC. The lightest supersymmetric particle will escape the detectors, thereby complicating the full reconstruction of the decay chains. In this paper we expand on existing methods for determining the masses of the particles in the cascade from endpoints of kinematical distributions. We perform scans in the mSUGRA parameter space to delimit the region where this method is applicable. From the examination of theoretical distributions for a wide selection of mass scenarios it is found that caution must be exerted when equating the theoretical endpoints with the experimentally obtainable ones. We provide analytic formulae for the masses in terms of the endpoints most readily available. Complications due to the composite nature of the endpoint expressions are discussed in relation to the detailed analysis of two points on the SPS 1a line. Finally we demonstrate how a Linear Collider measurement can improve dramatically on the precision of the masses obtained

Slepton Oscillation at Large Hadron Collider

Measurement of Lepton-Flavor Violation (LFV) in the minimal SUSY Standard Model (MSSM) at Large Hadron Collider (LHC) is studied based on a realistic simulation. We consider the LFV decay of the second-lightest neutralino, $\tilde{\chi}^0_2 \to \tilde{l} l' \to l l' \tilde{\chi}^0_1$, in the case where the flavor mixing exists in the right-handed sleptons. We scan the parameter space of the minimal supergravity model (MSUGRA) and a more generic model in which we take the Higgsino mass $\mu$ as a free parameter. We find that the possibility of observing LFV at LHC is higher if $\mu$ is smaller than the MSUGRA prediction; the LFV search at LHC can cover the parameter range where the $\mu \to e \gamma$ decay can be suppressed by the cancellation among the diagrams for this case.Comment: 29 pages, 10 figure

Scrutinizing LSP Dark Matter at the LHC

We show that LHC experiments might well be able to determine all the parameters required for a prediction of the present density of thermal LSP relics from the Big Bang era. If the LSP is an almost pure bino we usually only need to determine its mass and the mass of the SU(2) singlet sleptons. This information can be obtained by reconstructing the cascade $\tilde{q}_L \to \tilde{\chi}_2^0 q \to \tilde{\ell}_R \ell q \to \tilde{\chi}_1^0 \ell^+ \ell^- q$. The only requirement is that $m_{\tilde{\ell}_R} < m_{\tilde{\chi}_2^0}$, which is true for most of the cosmologically interesting parameter space. If the LSP has a significant higgsino component, its predicted thermal relic density is smaller than for an equal--mass bino. We show that in this case squark decays also produce significant numbers of $\tilde{\chi}_4^0$ and $\tilde{\chi}_2^\pm$. Reconstructing the corresponding decay cascades then allows to determine the higgsino component of the LSP