Searches for Physics Beyond the Standard Model at Colliders

All experimental measurements of particle physics today are beautifully described by the Standard Model. However, there are good reasons to believe that new physics may be just around the corner at the TeV energy scale. This energy range is currently probed by the Tevatron and HERA accelerators and selected results of searches for physics beyond the Standard Model are presented here. No signals for new physics have been found and limits are placed on the allowed parameter space for a variety of different particles.Comment: Proceedings for 2007 Europhysics Conference on High Energy Physics, Manchester, July 200

Prompt Decays of General Neutralino NLSPs at the Tevatron

Recent theoretical developments have shown that gauge mediation has a much larger parameter space of possible spectra and mixings than previously considered. Motivated by this, we explore the collider phenomenology of gauge mediation models where a general neutralino is the lightest MSSM superpartner (the NLSP), focusing on the potential reach from existing and future Tevatron searches. Promptly decaying general neutralino NLSPs can give rise to final states involving missing energy plus photons, Zs, Ws and/or Higgses. We survey the final states and determine those where the Tevatron should have the most sensitivity. We then estimate the reach of existing Tevatron searches in these final states and discuss new searches (or optimizations of existing ones) that should improve the reach. Finally we comment on the potential for discovery at the LHC.Comment: 41 pages, minor changes, added refs and discussion of previous literatur

Measurements of the Production, Decay and Properties of the Top Quark: A Review

With the full Tevatron Run II and early LHC data samples, the opportunity for furthering our understanding of the properties of the top quark has never been more promising. Although the current knowledge of the top quark comes largely from Tevatron measurements, the experiments at the LHC are poised to probe top-quark production and decay in unprecedented regimes. Although no current top quark measurements conclusively contradict predictions from the standard model, the precision of most measurements remains statistically limited. Additionally, some measurements, most notably the forward-backward asymmetry in top quark pair production, show tantalizing hints of beyond-the-Standard-Model dynamics. The top quark sample is growing rapidly at the LHC, with initial results now public. This review examines the current status of top quark measurements in the particular light of searching for evidence of new physics, either through direct searches for beyond the standard model phenomena or indirectly via precise measurements of standard model top quark properties

Model-independent extraction of ∣Vtq∣|V_{tq}| matrix elements from top-quark measurements at hadron colliders

Current methods to extract the quark-mixing matrix element $|V_{tb}|$ from single-top production measurements assume that $|V_{tb}|\gg |V_{td}|, |V_{ts}|$: top quarks decay into $b$ quarks with 100% branching fraction, s-channel single-top production is always accompanied by a $b$ quark and initial-state contributions from $d$ and $s$ quarks in the $t$-channel production of single top quarks are neglected. Triggered by a recent measurement of the ratio $R=\frac{|V_{tb}|^{2}}{|V_{td}|^{2}+|V_{ts}|^{2}+|V_{tb}|^{2}}=0.90 \pm 0.04$ performed by the D0 collaboration, we consider a $|V_{tb}|$ extraction method that takes into account non zero d- and s-quark contributions both in production and decay. We propose a strategy that allows to extract consistently and in a model-independent way the quark mixing matrix elements $|V_{td}|$, $|V_{ts}|$, and $|V_{tb}|$ from the measurement of $R$ and from single-top measured event yields. As an illustration, we apply our method to the Tevatron data using a CDF analysis of the measured single-top event yield with two jets in the final state one of which is identified as a $b$-quark jet. We constrain the $|V_{tq}|$ matrix elements within a four-generation scenario by combining the results with those obtained from direct measurements in flavor physics and determine the preferred range for the top-quark decay width within different scenarios.Comment: 36 pages, 17 figure

Scalar-mediated ttˉt\bar t forward-backward asymmetry

A large forward-backward asymmetry in $t\bar t$ production, for large invariant mass of the $t\bar t$ system, has been recently observed by the CDF collaboration. Among the scalar mediated mechanisms that can explain such a large asymmetry, only the t-channel exchange of a color-singlet weak-doublet scalar is consistent with both differential and integrated $t\bar t$ cross section measurements. Constraints from flavor changing processes dictate a very specific structure for the Yukawa couplings of such a new scalar. No sizable deviation in the differential or integrated $t\bar t$ production cross section is expected at the LHC.Comment: 22 pages, 1 figure and 2 tables. v2: Corrected Eqs.(50,51,74), adapted Fig.1, Tab.1 and relevant discussions. Extended discussion of top decay and single to

Precision measurements of the top quark mass from the Tevatron in the pre-LHC era

The top quark is the heaviest of the six quarks of the Standard Model. Precise knowledge of its mass is important for imposing constraints on a number of physics processes, including interactions of the as yet unobserved Higgs boson. The Higgs boson is the only missing particle of the Standard Model, central to the electroweak symmetry breaking mechanism and generation of particle masses. In this Review, experimental measurements of the top quark mass accomplished at the Tevatron, a proton-antiproton collider located at the Fermi National Accelerator Laboratory, are described. Topologies of top quark events and methods used to separate signal events from background sources are discussed. Data analysis techniques used to extract information about the top mass value are reviewed. The combination of several most precise measurements performed with the two Tevatron particle detectors, CDF and \D0, yields a value of \Mt = 173.2 \pm 0.9 GeV/$c^2$.Comment: This version contains the most up-to-date top quark mass averag

Testing the gaugino AMSB model at the Tevatron via slepton pair production

Gaugino AMSB models-- wherein scalar and trilinear soft SUSY breaking terms are suppressed at the GUT scale while gaugino masses adopt the AMSB form-- yield a characteristic SUSY particle mass spectrum with light sleptons along with a nearly degenerate wino-like lightest neutralino and quasi-stable chargino. The left- sleptons and sneutrinos can be pair produced at sufficiently high rates to yield observable signals at the Fermilab Tevatron. We calculate the rate for isolated single and dilepton plus missing energy signals, along with the presence of one or two highly ionizing chargino tracks. We find that Tevatron experiments should be able to probe gravitino masses into the ~55 TeV range for inoAMSB models, which corresponds to a reach in gluino mass of over 1100 GeV.Comment: 14 pages including 6 .eps figure

Axigluon as Possible Explanation for ppˉ→ttˉp\bar{p} \to t\bar{t} Forward-Backward Asymmetry

A flavor-nonuniversal chiral color model is introduced. It is used for comparison to the recent data on $\bar{p} p \to \bar{t} t$. We concluded that the data are consistent with interpretation as an axigluon exchange within 1$\sigma$ and a unique rise and fall behavior is predicated with regard to the asymmetry $A^t_{FB}$ as a function of $t \bar{t}$ invariant mass, which can distinguish our model from others before one discovers the axigluon resonance. Further aspects of the model are discussed.Comment: reference update

Multi-lepton signals from the top-prime quark at the LHC

We analyze the collider signatures of models with a vector-like top-prime quark and a massive color-octet boson. The top-prime quark mixes with the top quark in the Standard Model, leading to richer final states than ones that are investigated by experimental collaborations. We discuss the multi-lepton final states, and show that they can provide increased sensitivity to models with a top-prime quark and gluon-prime. Searches for new physics in high multiplicity events are an important component of the LHC program and complementary to analyses that have been performed.Comment: 7 pages, 4 figures, 2 table

Constraints on Randall-Sundrum model from top-antitop production at the LHC

We study the top pair production cross section at the LHC in the context of Randall-Sundrum model including the Kaluza-Klein (KK) excited gravitons. It is shown that the recent measurement of the cross section of this process at the LHC restricts the parameter space in Randall-Sundrum (RS) model considerably. We show that the coupling parameter ($\frac{k}{\bar{M}_{pl}}$) is excluded by this measurement from 0.03 to 0.22 depending on the mass of first KK excited graviton ($m_1$). We also study the effect of KK excitations on the spin correlation of the top pairs. It is shown that the spin asymmetry in $t\bar{t}$ events is sensitive to the RS model parameters with a reasonable choice of model parameters.Comment: 17 pages, 6 figure