At the HERA electron-proton collider an event has been observed in the H1 detector which shows an isolated muon recoiling against a hadronic system, both of high transverse momentum. The event was registered in a total integrated luminosity of 4 pb ?1 .

Evidence is presented using data taken with the H1 detector at HERA for a class of deep inelastic electron{proton scattering (DIS) events (5 < Q 2 < 120 GeV 2) at low Bjorken-x (10 ?4 < x < 10 ?2) which have almost no hadronic energy ow in a large interval of pseudo-rapidity around the proton remnant direction and which cannot be attributed to our present understanding of DIS and uctuations in-nal state hadronic fragmentation. From an integrated luminosity of 273 nb ?1 , 734 events, that is about 5% of the total DIS sample, have no energy deposition greater than 400MeV forward of laboratory pseudo-rapidity max = 1:8 up to the largest measurable pseudo-rapidity of about 3:65. Evidence that about 10% of observed rapidity gap events are exclusive vector meson electroproduction is presented. Good descriptions of the data are obtained using models based either on a vector meson dominance like picture, which includes a large fraction of inelastic virtual photon dissociation, or on deep inelastic electron{pomeron scattering in which the partonic sub{structure of the latter is resolved.

ISSN 0418-9833 August 1994 A Search for Heavy Leptons at HERA H1 Collaboration Abstract: A search for direct production of new leptons in the mass range from 10 GeV up to 225 GeV is presented by the H1 experiment at HERA. The data were obtained during 1993 and correspond to an integrated luminosity of 528 nb 1. The search includes heavy lepton decays to nal states e ( ) and e ( )W , e ( )Z with the subsequent decay of the W and Z bosons into jets or lepton pairs. No evidence was found for the production of new massive electrons or neutrinos in any of the decay channels. Rejection limits for excited electrons and neutrinos are derived.

The impact of the ARGUS experiment to elementary particle physics is reviewed. More than ten years of data taking has allowed ARGUS to contribute significantly to our understanding of beauty and charmed hadrons, τ Leptons, ϒ mesons, ϒϒ interactions and fragmentation processes. In particular the ARGUS measurements of CKM matrix elements opened up a new window on the Standard Model.

The electron beam lifetime in the HERA storage ring is not only uniformly lower than the positron beam lifetime, but the electron beam lifetime curves display complicated structures not present in the smoothly behaved positron beam lifetime curves. Summaries of characterising quantities are presented for large numbers of runs in 1993 and 1994, enabling the identification of trends in the beam lifetime behaviour. The lifetime reduction seems to be due to many discrete lifetime reduction events. The lifetime achieved at given current depends in a complicated (and stochastic) manner on the filled current, energy and integrated ion getter pump voltage. The properties of the lifetime problem are overwhelmingly consistent with the capture of material by the electron beam (most likely dust particles of size 0.1-1 /spl mu/m) which-being then highly positively charged-scatters beam electrons. The source of this material, or the mechanism of its liberation from the chamber or pump walls, is not entirely understood. Moves towards curing the electron beam lifetime problem are discussed.

Properties of the hadronic final state in photoproduction events with large transverse energy are studied at the electron-proton collider HERA. Distributions of the transverse energy, jets and underlying event energy are compared to\(\bar pp\) data and QCD calculations. The comparisons show that theγp events can be consistently described by QCD models including — in addition to the primary hard scattering process — interactions between the two beam remnants. The differential jet cross sectionsdσ/dE T jet anddσ/dη jet are measured.

Deep inelastic scattering (DIS) events, selected from 1993 data taken by the H1 experiment at HERA, are studied in the Breit frame of reference. The fragmentation function of the quark is compared with those of e+e− data. It is shown that certain aspects of the quarks emerging from within the proton in e−p interactions are essentially the same as those of quarks pair-created from the vacuum in e+e− annihilation. The measured area, peak position and width of the fragmentation function show that the kinematic evolution variable, equivalent to the e+e− squared centre of mass energy, is in the Breit frame the invariant square of the four-momentum transfer. We comment on the extent to which we have evidence for coherence effects in pArton showers.

Photoproduction of 2-jet events is studied with the H1 detector at HERA. Parton cross sections are extracted from the data by an unfolding method using leading order parton-jet correlations of a QCD generator. The gluon distribution in the photon is derived in the fractional momentum range 0.04 ⩽ xγ ⩽ 1 at the average factorization scale 75 GeV2.

Jet rates in deep inelastic electron proton scattering are studied with the H1 detector at HERA for momentum transfers squared between 10 and 4000 GeV2. It is shown that they can be quantitatively described by perturbative QCD in next to leading order making use of the parton densities of the proton and with the strong coupling constant αs as a free parameter. The measured value, αs(MZ2) = 0.123 ± 0.018, is in agreement both with determinations from e+e− annihilation at LEP using the same observable and with the world average.

Photon-proton (γp) interactions with Q2 < 10−2 GeV2 and deep-inelastic scattering (γ∗p) interactions with photon virtualities Q2 > 5 GeV2 are studied at the high energy electron-proton collider HERA. The transverse energy flow and relative rates of large rapidity gap events are compared in the two event samples. The observed similarity between γp and γ∗p interactions can be understood in a picture where the photon develops as a hadronic object. The transverse energy density measured in the central region of the collision, at η∗ = 0 in the γ∗p centre of mass frame, is compared with data from hadron-hadron interactions as function of the CMS energy of the collision.

A contact interaction analysis is presented to search for new phenomena beyond the Standard Model in deep inelastic $e^\pm p \rightarrow e^\pm \, hadrons$ scattering. The data are collected with the H1 detector at HERA and correspond to integrated luminosities of $0.909 \ {\rm pb}^{-1}$ and $2.947 \ {\rm pb}^{-1}$ for electron and positron beams, respectively. The differential cross sections $d\sigma / dQ^2$ are measured in the $Q^2$ range bet\-ween $160 \ \GeV^2$ and $20,000 \ \GeV^2$. The absence of any significant deviation from the Standard Model prediction is used to constrain the couplings and masses of new leptoquarks and to set limits on electron--quark compositeness scales and on the radius of light quarks.

Using the ARGUS detector at the DORIS II electron-positron storage ring at DESY, we have measured BR(D*+→D0π+)=(68.8±2.4±1.3)%, BR(D*+→D+π0)=(31.2±1.1±0.8)%, BR(D*+→D+γ)<5.2% (90%CL), BR(D*0→D0π0)=(59.6±3.5±2.8)%, and BR(D*0→D0γ)=(40.4±3.5±2.8)%, and the mass difference Δm0=m(D*0)−m(D0)=(142.2±0.3±0.2)MeV/c2.

A measurement is presented, using data taken with the H1 detector at HERA, of the contribution of diffractive interactions to deep-inelastic electron-proton (ep) scattering in the kinematic range 8.5 < Q2 < 50GeV2, 2.4 × 10−4 < Bjorken-x < 0.0133, and 3.7 × 10−4 < χp < 0.043. The diffractive contribution to the proton structure function F2(x,Q2) is evaluated as a function of the appropriate deep-inelastic scattering variables χp, Q2, β (= χχp) using a class of deep-inelastic ep scattering events with no hadronic energy flow in an interval of pseudo-rapidity adjacent to the proton beam direction. the dependence of this contribution on χp is measured to be χp−n with n = 1.19 ± 0.06 (stat.) ± 0.07 (syst.) independent of β and Q2, which is consistent with both a diffractive interpretation and a factorisable ep diffractive cross section. A first measurement of the deep-inelastic structure of the pomeron in the form of the Q2 and β dependences of a factorised structure function is presented. For all measured β, this structure function is observed to be consistent with scale invariance.

The cross sections for the charged current processes {ie566-1} and, for the first time, {ie566-2} are measured at HERA for transverse momenta larger than 25 GeV.

We present a QCD analysis of the proton structure function F2 measured by the H1 experiment at HERA, combined with data from previous fixed target experiments. The gluon density is extracted from the scaling violations of F2 in the range 2 · 10−4 < x < 3 · 10−2 and compared with an approximate solution of the QCD evolution equations. The gluon density is found to rise steeply with decreasing x.

Properties of the hadronic final state in photoproduction events with large transverse energy are studied at the electron-proton collider HERA. Distributions of the transverse energy, jets and underlying event energy are compared to pp data and QCD calculations. The comparisons show that the γp events can be consistently described by QCD models including – in addition to the primary hard scattering process – interactions between the two beam remnants. The differential jet cross sections dσ/dE T and dσ/dη jet are measured.

We present an experimental study of theep→eγ+p andep→eγ+X processes using data recorded by the H1 detector in 1993 at the electron-proton collider HERA. These processes are employed to measure the luminosity with an accuracy of 4.5 %. A subsample of theep→eγ+X events in which the hard photon is detected at angles θ γ ′ ≤ 0.45 mrad with respect to the incident electron direction is used to verify experimentally the size of radiative corrections to theep→eX inclusive cross section and to investigate the structure of the proton in theQ2 domain down to 2 GeV2, lower than previously attained at HERA.

The production of transverse energy in deep inelastic scattering is measured as a function of the kinematic variables x and Q^2 using the H1 detector at the ep collider HERA. The results are compared to the different predictions based upon two alternative QCD evolution equations, namely the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) and the Balitsky-Fadin-Kuraev-Lipatov (BFKL) equations. In a pseudorapidity interval which is central in the hadronic centre of mass system between the current and the proton remnant fragmentation region the produced transverse energy increases with decreasing x for constant Q^2. Such a behaviour can be explained with a QCD calculation based upon the BFKL ansatz. The rate of forward jets, proposed as a signature for BFKL dynamics, has been measured.

A measurement of the proton structure function F2(x, Q2) is reported for momentum transfers squared Q2 between 4.5 GeV2 and 1600 GeV2 and for Bjorken x between 1.8 × 10−14 and 0.13 using data collected by the HERA experiment H1 in 1993. It is observed that F2 increases significantly with decreasing x, confirming our previous measurement made with one tenth of the data available in this analysis. The Q2 dependence is approximately logarithmic over the full kinematic range covered. The subsample of deep inelastic events with a large pseudo-rapidity gap in the hadronic energy flow close to the proton remnant is used to measure the “diffractive” contribution to F2.

We present a new measurement of the total photoproduction cross section performed with the H1 detector at HERA. For an average centre of mass energy of 200 GeV a value of σ tot λp =165±2±11µb has been obtained. A detailed analysis of the data in adequate kinematic regions enabled a decomposition of the total cross section in its elastic, single diffractive dissociation and remaining non-diffractive parts, based on safe assumptions on the double diffractive dissociation contribution.