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 .

A measurement is presented, using data taken with the H1 detector at HERA, of the contribution of diiractive interactions to deep{inelastic electron{proton (ep) scattering in the kinematic range 8:5 < Q 2 < 50 GeV 2 , 2:410 ?4 < Bjorken{x < 0:0133, and 3:7 10 ?4 < x IP < 0:043. The diiractive contribution to the proton structure function F 2 (x; Q 2) is evaluated as a function of the appropriate deep{inelastic scattering variables x IP , Q 2 , and (= x=x IP) using a class of deep{inelastic ep scattering events with no hadronic energy ow in an interval of pseudo{rapidity adjacent to the proton beam direction. The dependence of this contribution on x IP is measured to be x ?n IP with n = 1:19 0:06(stat:) 0:07(syst:) independent of and Q 2 , which is consistent with both a diiractive interpretation and a factorisable ep diiractive cross section. A rst measurement of the deep{inelastic structure of the pomeron in the form of the Q 2 and dependences of a factorised structure function is presented. For all measured , this structure function is observed to be consistent with scale invariance.

An analysis is presented of scaling violations of the proton structure function F 2 (x; Q 2) measured with the H1 detector at HERA in the range of Bjorken x values between x = 310 ?4 and 10 ?2 for four-momentum transfers Q 2 larger than 8.7 GeV 2. The structure function F 2 (x; Q 2) is observed to rise linearly with lnQ 2. Under the assumption that the observed scaling violations at small x 0:01 are described correctly by perturbative QCD, an estimate is obtained of the gluon distribution function G(x; Q 2 o) at Q 2 o = 20 GeV 2 .

Technical aspects of the three major components of the H1 detector at the electron-proton storage ring HERA are described. This paper covers the detector status up to the end of 1994 when a major upgrading of some of its elements was undertaken. A description of the other elements of the detector and some performance figures from luminosity runs at HERA during 1993 and 1994 are given in a paper previously published in this journal.

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.

We have built a fast, pipelined first-level trigger processor for the H1 experiment at HERA. The trigger finds tracks in the forward-muon spectrometer which point back to the interaction vertex. The inputs to the trigger come from drift chambers with a 6 cm drift space corresponding to a maximum drift time of 1.2 μs. These chambers consist of two layers of drift cells whose staggered configuration makes it possible to extract the time at which a particle traversed the chambers to better than one HERA bunch-crossing period, 96 ns. The trigger hardware exploits this characteristic in a real-time operation, and is able to find pointing tracks and associate them to production in a specific electron-proton crossing. The trigger processor must be deadtime free. It operates in a pipelined mode with 48 ns steps and has a latency of about 22 HERA bunch-crossing periods. The compact design is based on two semi-custom integrated circuits. Both are field-programmable 32 × 32 coincidence matrices, one having serial loading of its inputs and the other using parallel loading. The system was installed in the H1 experiment early in 1993 and has run successfully since then.

Events with no hadronic energy flow in a large interval of pseudo-rapidity in the proton direction are observed in photon-proton interactions at an average centre of mass energy 〈√sγp〉 of 200 GeV These events are interpreted as photon diffractive dissociation. Evidence for hard scattering in photon diffractive dissociation is demonstrated using inclusive single particle spectra, thrust as a function of transverse energy, and the observation of jet production. The data can be described by a Monte Carlo calculation including hard photon-pomeron scattering.

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.

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.

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.

Cross sections are presented for the inclusive production of charged particles measured in electron-proton collisions at low Q2 with the H1 detector at HERA. The transverse momentum distribution extends up to 8 GeV/c. Its shape is found to be harder than that observed in pp collisions at comparable centre-of-mass energies √Sγp ≈ √Spp ≈ 200GeV, and also harder than in γp collisions at lower energies √Sγp ≈ 18 GeV. Results from quantum chromodynamics (QCD) calculations agree with the measured transverse momentum and pseudorapidity cross sections.

Global properties of the hadronic final state in deep inelastic scattering events at HERA are investigated. The data are corrected for detector effects and are compared directly with QCD phenomenology. Energy flows in both the laboratory frame and the hadronic centre of mass system and energy-energy correlations in the laboratory frame are presented. Comparing various QCD models, the colour dipole model provides the only satisfactory description of the data. In the hadronic centre of mass system the momentum components of charged particles longitudinal and transverse to the virtual boson direction are measured and compared with lower energy lepton-nucleon scattering data as well as withe+e− dat from LEP.

We present a study of Jψ meson production in collisions of 26.7 GeV electrons with 820 GeV protons, performed with the H1-detector at the HERA collider at DESY. The Jψ mesons are detected via their leptonic decays both to electrons and muons. Requiring exactly two particles in the detector, a cross section of σ(ep → JψX) = (8.8±2.0±2.2) nb is determined for 30 GeV ≤ Wγp ≤ 180 GeV and Q2 ≲ 4 GeV2. Using the flux of quasi-real photons with Q2 ≲ 4 GeV2, a total production cross section of σ(γp → J/ψX) = (56±13±14) nb is derived at an average Wγp=90 GeV. The distribution of the squared momentum transfer t from the proton to the Jψ can be fitted using an exponential exp(−b∥t∥) below a ∥t∥ of 0.75 GeV2 yielding a slope parameter of b = (4.7±1.9) GeV−2.

A search in the H1 experiment at HERA for scalar and vector leptoquarks, leptogluons and squarks coupling to first generation fermions is presented in a data sample corresponding to an integrated luminosity of 425 nb−1. For masses ranging up to ∼275 GeV, no significant evidence for the direct production of such particles is found in various possible decay channels. At high masses and beyond the centre of mass energy of 296 GeV a contact interaction analysis is used to further constrain the couplings and masses of new vector leptoquarks and to set lower limits on compositeness scales.

Multi-jet production is observed in deep-inelastic electron proton scattering with the H1 detector at HERA. Jet rates for momentum transfers squared up to 500 GeV 2 are determined using the JADE jet clustering algorithm. They are found to be in agreement with predictions from QCD based models.

An analysis is presented of scaling violations of the proton structure function F2(x,Q2) measured with the H1 detector at HERA in the range of Bjorken x values between x = 3 × 10−4 and 10−2 for four-momentum transfers Q>2 larger than 8.7 GeV2. The structure function F2(x,Q2) is observed to rise linearly with ln Q2. Under the assumption that the observed scaling violations at small x ⩽ 0.01 are described correctly by perturbative QCD, an estimate is obtained of the gluon distribution function G(x,Q02) at Q22 = 20 GeV2.

Evidence is presented using data taken with the H1 detector at HERA for a class of deep inelastic electron-proton scattering (DIS) events (5 < Q2 < 120 GeV2) at low Bjorken-x (10−4 < x < 10−2) which have almost no hadronic energy flow in a large interval of pseudo-rapidity around the proton remnant direction and which cannot be attributed to our present understanding of DIS and fluctuations in final 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 400 MeV forward of laboratory pseudo-rapidity ηmax = 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.

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 final states e (ν) γ and e (ν) W, e(ν) 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.

We present a study of J== meson production in collisions of 26:7 GeV electrons with 820 GeV protons, performed with the H1-detector at the HERA collider at DESY. The J== mesons are detected via their leptonic decays both to electrons and muons. Requiring exactly two particles in the detector, a cross section of (ep ! J==X) = (8:8 2:0 2:2) nb is determined for 30 GeV W p 180 GeV and Q 2 < 4 GeV 2. Using the ux of quasi-real photons with Q 2 < 4 GeV 2 , a total photoproduction cross section of (p ! J==X) = (56 13 14) nb is derived at an average W p = 90 GeV. The distribution of the squared momentum transfer t from the proton to the J== can be tted using an exponential exp (?bjtj) below a jtj of 0:75 GeV 2 yielding a slope parameter of b = (4:7 1:9) GeV ?2 .