Present Collider Experiments Research Articles

Impact of nuclear gluon distributions on leptoquark production by neutrinos

We investigate non-standard interactions of neutrinos with atomic nuclei through excitations of leptoquarks. A leptoquark term in the Lagrangian admits the possibility that neutrinos interact with gluons. The current lower limits on the leptoquark masses are of the order of 1 TeV depending on the leptoquark quantum numbers and couplings. Such heavy states can be produced in ultra-high energy cosmic neutrino scattering processes. The four-momentum transfer squared and the Bjorken variable simultaneously probed in these processes may reach values kinematically inaccessible at present collider experiments. We study the impact of the gluon density in a nucleus on the cross section for the neutrino-induced leptoquark production. We show that taking into account the nuclear parton distributions shifts the production threshold to significantly lower neutrino energies. As a particular case we consider the interaction with oxygen, which is abundant in water targets used in neutrino detection experiments.

The Higgs Sector in the Next-to-MSSM

We study the Higgs sector in the Next-to-Minimal Supersymmetric Standard Model with and without explicit CP violation, focusing on the case of weak scale expectation value of the singlet field. We scan a wide range of the parameter space to find out allowed regions by requiring that the electroweak vacuum be the global minimum of the effective potential and that the neutral Higgs bosons with moderate gauge coupling be heavier than the lower bound on the Higgs boson in the standard model. Among the allowed parameters, some sets admit the situation in which the light Higgs bosons couple with the $Z$ boson too weak to be found in present collider experiments. For such parameter sets, we find an upper bound on the charged Higgs mass which is reachable in LHC.

Higgs as a slepton

In theories with TeV scale quantum gravity there is a logical possibility where the electroweak Higgs can be a fourth generation slepton. Despite maximal supersymmetry breaking such a scenario turns out to be tightly constrained, and might be testable already at present collider experiments. Also, a light Higgs is no longer a prediction of supersymmetry.

Probing supergravity models with the b-->s gamma microscope.

We present a calculation of the branching ratio $B(b\ensuremath{\rightarrow}s\ensuremath{\gamma})$ in two well-motivated supersymmetric models: the minimal SU(5) and the no-scale flipped SU(5) supergravity models. We find that the improved CLEO upper bound [$B(b\ensuremath{\rightarrow}s\ensuremath{\gamma})<5.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ at 95% C.L.] does not yet constrain the minimal SU(5) supergravity model, where $B{(b\ensuremath{\rightarrow}s\ensuremath{\gamma})}_{\mathrm{minimal}}=(2.3\ensuremath{-}3.6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. In the flipped SU(5) model the CLEO bound is constraining, although still not very significantly, even for light charged Higgs boson masses. An improvement in sensitivity by a factor of 2 will probe all (more than half) of the parameter space of the minimal (flipped) SU(5) supergravity model. This "resolution" of the $b\ensuremath{\rightarrow}s\ensuremath{\gamma}$ microscope far surpasses that of present collider experiments. In the flipped model there exists a significant region of parameter space where $B(b\ensuremath{\rightarrow}s\ensuremath{\gamma})$ is highly suppressed due to a new phenomenon involving a complicated cancellation against the QCD correction parameter.

Observability of charge asymmetries for lepton pairs produced in present collider experiments

Now that collider experiments at CERN (√ s=0.63 TeV) and Fermilab (√ s=1.8 TeV) each are accumulating data samples of several events per picobarn, it becomes possible to observe Drell-Yan production of lepton pairs in the continuum below and above the Z, where the standard electroweak theory predicts forward-backward asymmetries to be large. The conditions needed to observe an effect are estimated, and the unique character of the measurement is stressed.

Squark decays into gauginos at the p [formula omitted] collider

Conventional analyses of missing p T events due to squark production at the CERN p p collider assume q̃ → q γ decays dominate. In principle, the monojet and dijet cross sections could be reduced by competition from q̃ → qW̃ ± and q̃ → qZ̃ 0 decays. We compute this reduction factor for two mass scenarios: m q ̃ > m W ̃ > m o ̃ l and m ℓ ≈ m q ̃ > m W ̃ . The monojet and dijet cross sections for squarks light enough to be observed in present collider experiments are reduced by no more than 55%, while there may exist an observable cross section for jet(s) + charged lepton(s) + missing p T events. Thus the lower bounds on m q ̃ usually derived from q̃ → q γ decays remain valid.