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A search for Theta^+(1540) and Xi^{--}_{3/2}(1862) pentaquark candidates has been performed in proton-induced reactions on C, Ti and W targets at sqrt(s) = 41.6 GeV studying the pK_s resp. Xi^-pi^- and Xi^-pi^+ (and charge conjugated) decay channels at mid-rapidity. With sensitivities of (Br)x(dsigma/dx_F) < 5 to 25 microbarn/nucleon, we find no evidence for narrow pentaquark peaks in any of the studied final states. Preliminary values for the upper limit of relative yield ratios at mid-rapidity are (Theta^+(1540)) / (Lambda(1520)) < 0.02, (Br)x(Xi^{--}_{3/2}(1862)) / (Xi(1530)^0) < 0.077, and (Br)x(Xi_bar^{++}_{3/2}(1862)) / (Xi_bar(1530)^0) < 0.058 at 95% CL.
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We have searched for Theta^+(1540) and Xi^{--}(1862) pentaquark candidates in proton-induced reactions on C, Ti and W targets at mid-rapidity and sqrt{s} = 41.6 GeV. In 2x10^8 inelastic events we find no evidence for narrow (sigma~5 MeV) signals in the Theta^+ -> pK_s and Xi^{--} -> Xi^-pi^- channels; our 95% CL upper limits (UL) for the inclusive production cross section times branching fraction Bx(dsigma/dy)|_{y~0} are 3.7 and 2.5 microb/N. The UL of the yield ratio of [Theta^+ / Lambda(1520)] < 2.7% is significantly lower than model predictions. Our UL of [BxXi^{--} / Xi(1530)^0] < 4% is at variance with the results that have provided first evidence for the Xi^{--} signal.
Investigations of the existence of pentaquark states containing a single $b$ (anti)quark decaying weakly into four specific final states J/$psi K^+pi^- p$, J/$psi K^- pi^- p$, J/$psi K^- pi^+ p$, and $J/psi phi (1020) p$ are reported. The data sample corresponds to an integrated luminosity of 3.0/fb in 7 and 8 TeV pp collisions acquired with the LHCb detector. Signals are not observed and upper limits are set on the product of the production cross section times branching fraction with respect to that of the $Lambda_b$.
Signals of QCD instanton-induced processes are searched for in neutral current deep-inelastic scattering at the electron-proton collider HERA in the kinematic region defined by the Bjorken-scaling variable $x > 10^{-3}$, the inelasticity $0.2< y < 0.7$ and the photon virtuality $150 < Q^2 < 15000$ GeV$^2$. The search is performed using H1 data corresponding to an integrated luminosity of ~$351$ pb$^{-1}$. No evidence for the production of QCD instanton-induced events is observed. Upper limits on the cross section for instanton-induced processes between $1.5$~pb and $6$~pb, at $95%$~ confidence level, are obtained depending on the kinematic domain in which instantons could be produced. Compared to earlier publications, the limits are improved by an order of magnitude and for the first time are challenging theory predictions.
Recent results on searches for new particles at the electron-proton collider HERA are reported. Based on roughly 40pb-1 of e^+p data taken in the years 1994-1997, the H1 and ZEUS collaborations have derived new exclusion limits for the direct production of excited fermion states and of leptoquarks in different decay channels, including lepton-flavor violating decays. The results of searches for contact interactions further constrain the parameter space for such particles and their couplings in the high-mass regime, where direct production is kinematically prohibited. Also preliminary analyses of the e^-p data taken in 1998 and 1999 do not find signals of new physics.
Recent results on the searches for new physics in ep collisions at HERA using the ZEUS and H1 detectors are presented. No evidence for excited fermions or supersymmetric particles within the context of the Minimal Supersymmetric Standard Model nor R-parity violation has been found using an integrated luminosity of up to 20 inv pb. New limits have therefore been established.
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