A Search for Selectrons and Squarks at HERA

Data from electron-proton collisions at a center-of-mass energy of 300 GeV are used for a search for selectrons and squarks within the framework of the minimal supersymmetric model. The decays of selectrons and squarks into the lightest supersymmetric particle lead to final states with an electron and hadrons accompanied by large missing energy and transverse momentum. No signal is found and new bounds on the existence of these particles are derived. At 95% confidence level the excluded region extends to 65 GeV for selectron and squark masses, and to 40 GeV for the mass of the lightest supersymmetric particle.Comment: 13 pages, latex, 6 Figure

Search for excited fermions with the H1 detector

We present a search for excited electrons, neutrinos and quarks using the H1 detector at the $ep$ collider HERA, based on data taken in 1994 with an integrated luminosity of 2.75 pb$^{-1}$. Radiative decays of excited quarks and neutrinos have been investigated as well as decays of excited electrons into all possible electroweak gauge bosons. No evidence for new particle production is found and exclusion limits are derived.Comment: 20 pages, latex, 6 Figure

Determination of the longitudinal proton structure function F(L) (x, Q**2) at low x

A measurement of the inclusive cross section for the deep-inelastic scattering of positrons off protons at HERA is presented at momentum transfers $8.5 \leq Q^2 \leq 35 GeV^2$ and large inelasticity $y = 0.7$, i.e. for the Bjorken-x range $0.00013 \leq x \leq 0.00055$. Using a next-to-leading order QCD fit to the structure function F_2 at lower y values, the contribution of F_2 to the measured cross section at high y is calculated and, by subtraction, the longitudinal structure function F_{L} is determined for the first time with an average value of $F_L=0.52 \pm 0.03 (stat)$^ {+0.25}_{-0.22}$(syst) at$Q^2=15.4 GeV^2$and$x=0.000243$.Comment: 17 pages, latex, 4 Figure

Measurement of charged particle transverse momentum spectra in deep inelastic scattering

Transverse momentum spectra of charged particles produced in deep inelastic scattering are measured as a function of the kinematic variables x_B and Q2 using the H1 detector at the ep collider HERA. The data are compared to different parton emission models, either with or without ordering of the emissions in transverse momentum. The data provide evidence for a relatively large amount of parton radiation between the current and the remnant systems.Comment: 21 pages, latex, 7 Figure

A Search for leptoquarks at HERA

A search for leptoquarks at HERA was performed in H1 using 1994 $e^+ p$ data corresponding to an integrated luminosity of about 3 \pb^{-1}. Single leptoquarks were searched for in direct positron-quark fusion processes taking into account possible decays into lepton-quark pairs of either the first, the second, or the third generation. No significant deviation from the Standard Model predictions is found in the various final states studied and mass dependent exclusion limits are derived on the Yukawa couplings of the leptoquarks. Compared with earlier results from an analysis of $e^-p$ data, exclusion limits are considerably improved for leptoquarks which could be produced via $e^+$-{\it valence~quark} fusion. For leptoquarks with lepton flavour conserving couplings, masses up to 275 \GeV (depending on the leptoquark type) are excluded for coupling values larger than $\sqrt{ 4 \pi \alpha_{em}}$. For leptoquarks with lepton flavour violating couplings, masses up to 225 \GeV are excluded for the first time in a direct search for couplings with leptons of the second or third generation larger than $\sqrt{ 4 \pi \alpha_{em}}$. Fourteen possible combinations of couplings are studied and stringent exclusion limits comparable or better than any existing direct or indirect limits are obtained for each leptoquark type.Comment: 16 pages, latex, 4 figures appended as uuencoded fil

Strangeness production in deep inelastic positron - proton scattering at HERA

Measurements are presented of $K^0$ meson and $\Lambda$ baryon production in deep-inelastic positron-proton scattering (DIS) in the kinematic range $10 < Q^2 < 70$GeV$^2$ and $10^{-4} < x < 10^{-2}$. The measurements, obtained using the H1 detector at the HERA collider, are discussed in the light of possible mechanisms for increased strangeness production at low Bjorken-$x$. Comparisons of the $x_F$ spectra, where $x_F$ is the fractional longitudinal momentum in the hadronic centre-of-mass frame, with results from electron-positron annihilation are made. The $x_F$ spectra and the $K^0$ ``seagull'' plot are compared with previous DIS results. The mean $K^0$ and $\Lambda$ multiplicities are studied as a function of the centre-of-mass energy $W$ and are observed to be consistent with a logarithmic increase with $W$ when compared with previous measurements. A comparison of the levels of strangeness production in diffractive and non-diffractive DIS is made. An upper limit of $0.9$nb, at the 95% confidence level, is placed on the cross-section for QCD instanton induced events.Comment: 36 pages, latex, 15 Figure

Elastic photoproduction of rho0 mesons at HERA

The cross section for the elastic photoproduction of \r0 mesons ($\gamma p \to \rho^0 p$) has been measured with the H1 detector at HERA for two average photon-proton centre-of-mass energies of 55 and 187 GeV. TheFcenterline lower energy point was measured by observing directly the $\rho^{0}$ decay giving a cross section of 9.1\pm 0.9 (\stat)\pm 2.5 (\syst) \mub. The logarithmic slope parameter of the differential cross section, ${\rm d}\sigma/{\rm d}t$, is found to be 10.9 \pm 2.4 (\stat) \pm 1.1 (\syst) GeV$^{-2}$. The \r0 decay polar angular distribution is found to be consistent with s-channel helicity conservation. The higher energy cross section was determined from analysis of the lower part of the hadronic invariant mass spectrum of diffractive photoproduction and found to be 13.6\pm 0.8 (\stat)\pm 2.4 (\syst) \mub.Comment: 25 pages, latex, 9 Figures appended as uuencoded fil