Lepton flavour violation in future linear colliders in the long-lived stau NLSP scenario

We analyze the prospects of observing lepton flavour violation in future e-e- and e+e- linear colliders in scenarios where the gravitino is the lightest supersymmetric particle, and the stau is the next-to-lightest supersymmetric particle. The signals consist of multilepton final states with two heavily ionizing charged tracks produced by the long-lived staus. The Standard Model backgrounds are very small and the supersymmetric backgrounds can be kept well under control by the use of suitable kinematical cuts. We discuss in particular the potential of the projected International Linear Collider to discover lepton flavour violation in this class of scenarios, and we compare the estimated sensitivity with the constraints stemming from the non-observation of rare decays.Comment: 30 pages, 12 figures. Discussion extended to include the efficiency of identifying long-lived staus, references added. To appear in JHE

Squark anti-squark pair production at the LHC: the electroweak contribution

We present the complete NLO electroweak contribution of $\mathcal{O}(\alpha^2_s \alpha)$ to the production of diagonal squark--anti-squark pairs in proton--proton collisions. Compared to the lowest-order $\mathcal{O}(\alpha_s\alpha + \alpha^2)$ electroweak terms, the NLO contributions are also significant. We discuss the LO and NLO electroweak effects in cross sections and distributions at the LHC for the production of squarks different from top squarks, in various supersymmetric benchmark scenarios.Comment: 38 pages, 21 figures. Replaced with the version published in JHE

Top quark and Electroweak measurements at the Tevatron

We present recent preliminary measurements at the Tevatron of t-tbar and single top production cross section, top quark mass and width, top pair spin correlations and forward-backward asymmetry. In the electroweak sector, we present the Tevatron average of the W boson width, and preliminary measurements of the W and Z forward-backward asymmetries and WZ, ZZ diboson production cross sections. All measurements are based on larger amount of collision data than previously used and are in agreement with the standard model.Comment: 6 pages, 6 figures; In proceedings of the 16th International Symposium on Particles, Strings and Cosmology, Valencia (Spain) 201

Long-term Running Experience with the Silicon Micro-strip Tracker at the D{\O} detector

The SiliconMicro-strip Tracker (SMT) at the D{\O} experiment in the Fermilab Tevatron collider has been operating since 2001. In 2006, an additional layer, referred to as 'Layer 0', was installed to improve impact parameter resolution and compensate for detector degradation due to radiation damage to the original innermost SMT layer. The SMT detector provides valuable tracking and vertexing information for the experiment. This contribution will highlight aspects of the long term operation of the SMT, including the impact of the silicon readout test-stand. Due to the full integration of the test-stand into the D{\O} trigger framework, this test-stand provides an advantageous tool for training of new experts and studying subtle effects in the SMT while minimizing impact on the global data acquisition.Comment: Proceedings of TIPP 2011 (Technology and Instrumentation for Particle Physics 2011), June 9-14 2011, Chicago, US

Non-universal gauge boson Z′Z' and the spin correlation of top quark pair production at e−e+e^{-}e^{+} colliders

In the off-diagonal basis, we discuss the contributions of the non-universal gauge boson $Z'$ predicted by the topcolor-assisted technicolor ($TC2$) model to the spin configurations and the spin correlation observable of the top quark pair production via the process $e^{-}e^{+}\to t\bar{t}$. Our numerical results show that the production cross sections for the like-spin states, which vanish in the standard model, can be significantly large as $M_{Z'}\approx \sqrt{S}$. With reasonable values of the $Z'$ mass $M_{Z'}$ and the coupling parameter $k_{1}$, $Z'$ exchange can generate large corrections to the spin correlation observable.Comment: 16 pages, 5 figure

Radiative corrections to the semileptonic and hadronic Higgs-boson decays H -> W W/Z Z -> 4 fermions

The radiative corrections of the strong and electroweak interactions are calculated for the Higgs-boson decays H -> WW/ZZ -> 4f with semileptonic or hadronic four-fermion final states in next-to-leading order. This calculation is improved by higher-order corrections originating from heavy-Higgs-boson effects and photonic final-state radiation off charged leptons. The W- and Z-boson resonances are treated within the complex-mass scheme, i.e. without any resonance expansion or on-shell approximation. The calculation essentially follows our previous study of purely leptonic final states. The electroweak corrections are similar for all four-fermion final states; for integrated quantities they amount to some per cent and increase with growing Higgs-boson mass M_H, reaching 7-8% at M_H \sim 500 GeV. For distributions, the corrections are somewhat larger and, in general, distort the shapes. Among the QCD corrections, which include corrections to interference contributions of the Born diagrams, only the corrections to the squared Born diagrams turn out to be relevant. These contributions can be attributed to the gauge-boson decays, i.e. they approximately amount to \alpha_s/\pi for semileptonic final states and 2\alpha_s/\pi for hadronic final states. The discussed corrections have been implemented in the Monte Carlo event generator PROPHECY4F.Comment: 29 pages, LaTeX, 30 postscript figure

Leptogenesis in models with multi-Higgs bosons

We study the leptogenesis scenario in models with multi-Higgs doublets. It is pointed out that the washing-out process through the effective dimension five interactions, which has not been taken into account seriously in the conventional scenario, can be effective, and the resultant baryon asymmetry can be exponentially suppressed. This fact implies new possible scenario where the observed baryon asymmetry is the remnant of the washed out lepton asymmetry which was originally much larger than the one in the conventional scenario. Our new scenario is applicable to some neutrino mass matrix models which predict too large CP-violating parameter and makes them viable through the washing-out process.Comment: Latex 2e, 11 pages, 2 figures. Many parts in the original manuscript have been revised, but conclusions are unchange

Charmless hadronic Bc→VA,AAB_c \to VA, AA decays in the perturbative QCD approach

In this work, we calculate the branching ratios (BRs) and the polarization fractions of sixty two charmless two-body $B_c$ meson decays into final states involving one vector and one axial-vector meson ($VA$) or two axial-vector mesons($AA$) within the framework of perturbative QCD (pQCD) approach systematically, where $A$ is either a $^3P_1$ or $^1P_1$ axial-vector meson. All considered decay channels can only occur through the annihilation topologies in the standard model. Based on the perturbative calculations and phenomenological analysis, we find the following results: (i) the CP-averaged BRs of the considered sixty two $B_c$ decays are in the range of $10^{-5}$ to $10^{-9}$; (ii) since the behavior for $^1P_1$ meson is much different from that of $^3P_1$ meson, the BRs of $B_c \to A(^1P_1) (V, A(^1P_1))$ decays are generally larger than that of $B_c \to A(^3P_1) (V, A(^3P_1))$ decays in the pQCD approach; (iii) many considered decays modes, such as $B_c\to a_1(1260)^+ \omega$, $b_1(1235) \rho$, etc, have sizable BRs within the reach of the LHCb experiments; (iv) the longitudinal polarization fractions of most considered decays are large and play the dominant role; (v) the pQCD predictions for several decays involving mixtures of $^3P_1$ and/or $^1P_1$ mesons are highly sensitive to the values of the mixing angles, which will be tested by the ongoing LHC and forthcoming Super-B experiments; (vi) the CP-violating asymmetries of these considered $B_c$ decays are absent in the standard model because only one type tree operator is involved.Comment: 31 pages, 1 figur

A simple quantum cosmology

A simple and surprisingly realistic model of the origin of the universe can be developed using the Friedmann equation from general relativity, elementary quantum mechanics, and the experimental values of h, c, G and the proton mass. The model assumes there are N space dimensions (with N > 6) and the potential constraining the radius r of the invisible N -3 compact dimensions varies as r^4. In this model, the universe has zero total energy and is created from nothing. There is no initial singularity. If space-time is eleven dimensional, as required by M theory, the scalar field corresponding to the size of the compact dimensions inflates the universe by about 26 orders of magnitude (60 e-folds). If the Hubble constant is 65 km/sec Mpc, the energy density of the scalar field after inflation results in Omega-sub-Lambda = 0.68, in agreement with recent astrophysical observations.Comment: To be published in General Relativity and Gravitation, August 200

Flavor Changing Neutral Currents involving Heavy Quarks with Four Generations

We study various FCNC involving heavy quarks in the Standard Model (SM) with a sequential fourth generation. After imposing $B\to X_s\gamma$, $B\to X_sl^+l^-$ and $Z\to b\bar{b}$ constraints, we find ${\cal B}(Z\to s\bar{b}+\bar{s}b)$ can be enhanced by an order of magnitude to $10^{-7}$, while $t\to cZ, cH$ decays can reach $10^{-6}$, which are orders of magnitude higher than in SM. However,these rates are still not observable for the near future.With the era of LHC approaching, we focus on FCNC decays involving fourth generation $b^\prime$ and $t^\prime$ quarks. We calculate the rates for loop induced FCNC decays $b^\prime\to bZ, bH, bg, b\gamma$, as well as t^\prime\to tZ,\tH, tg, t\gamma. If $|V_{cb'}|$ is of order $|V_{cb}| \simeq 0.04$, tree level $b^\prime\to cW$ decay would dominate, posing a challenge since $b$-tagging is less effective. For $|V_{cb'}| \ll |V_{cb}|$, $b'\to tW$ would tend to dominate, while $b'\to t^\prime W^*$ could also open for heavier $b'$, leading to thepossibility of quadruple-$W$ signals via $b'\bar b'\to b\bar b W^+W^-W^+W^-$. The FCNC $b'\to bZ, bH$ decays could still dominate if $m_{b'}$ is just above 200 GeV. For the case of $t'$, ingeneral $t^\prime\to bW$ would be dominant, hence it behaves like a heavy top. For both $b'$ and $t'$, except for the intriguing light $b'$ case, FCNC decays are in the $10^{-4} -10^{-2}$ range, and are quite detectable at the LHC.For a possible future ILC, we find the associated production of FCNC $e^+e^-\to b\bar s$, $t\bar c$ are below sensitivity, while $e^+e^-\to b^\prime\bar b$ and$t^\prime\bar t$ can be better probed.Tevatron Run-II can still probe the lighter $b'$ or $t'$ scenario. LHC would either discover the fourth generation and measure the FCNC rates, or rule out the fourth generation conclusively.Comment: 31 pages, 15 eps figures, version to appear in JHE