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the @xmath0-jet mass spectrum in @xmath67 scattering has been investigated with the zeus detector at hera . an excess of events relative to standard model expectations has previously been reported by the h1  @xcite and zeus  @xcite collaborations in neutral current deep inelastic scattering at high @xmath68 and high @xmath5 . these events contain high - mass @xmath0-jet final states . several models have been discussed  @xcite as possible sources of these events , including leptoquark production  @xcite and r - parity - violating squark production  @xcite . this paper presents an analysis of zeus data specifically aimed at searching for high mass states decaying to @xmath0-jet . candidate events with high transverse energy , an identified final - state positron , and at least one jet are selected . the measured energies ( @xmath69 ) and angles of the final - state positron and the jet with highest transverse momentum are used to calculate an invariant mass @xmath70 where @xmath71 is the angle between the positron and jet . the angle between the outgoing and incoming positron in the @xmath0-jet rest frame , @xmath72 , is also reconstructed using the measured energies and angles . no assumptions about the production process are made in the reconstruction of either @xmath73 or @xmath72 . the search was performed using @xmath2  pb@xmath3 of data collected in the 1994 - 1997 running periods . in the following , expectations from the standard model , leptoquark production and r - parity - violating squark production are summarized . after a discussion of the experimental conditions , the analysis is described and the @xmath73 and @xmath74 distributions presented . since these distributions do not show a clear signal for a narrow resonance , limits on the cross section times branching ratio are extracted for the production of such a state . limits are also presented in the mass versus coupling plane which can be applied to leptoquark and squark production . high - mass @xmath0-jet pairs , produced in the standard model ( sm ) via neutral current ( nc ) scattering , form the principal background to the search for heavy states . this process is reviewed first . leptoquark ( lq ) production and squark production in r - parity - violating ( @xmath75 ) supersymmetry are used as examples of physics beyond the sm that could generate the @xmath0-jet final state . the diagrams for nc and lq processes are shown in fig . [ fig : nclq ] . the squark production diagrams are similar to the lq diagrams , but different decay modes are possible , as discussed below . the kinematic variables used to describe the deep inelastic scattering ( dis ) reaction @xmath76 are @xmath77 where @xmath78 and @xmath79 are the four - momenta of the incoming and outgoing positron , respectively , and @xmath80 is the four - momentum of the incoming proton . the center - of - mass energy is given by @xmath81 . the nc interaction occurs between the positron and a parton ( quark ) inside the proton ( see fig .  [ fig : nclq ] ) . the production of the large @xmath0-jet masses of interest requires high @xmath68 partons , where the valence quarks dominate the proton structure .    in leading - order electroweak theory , the cross section for the nc dis reaction can be expressed as  @xcite @xmath82\ ] ] with @xmath83 and @xmath84 the fine structure constant . the contribution from the longitudinal structure function , @xmath85 , is expected to be negligible in the kinematic range considered here . the @xmath68 dependence of the nc cross section is very steep . in addition to the explicit @xmath86 factor , the structure functions @xmath87 and @xmath88 are dominated at large @xmath68 by valence - quark densities that fall quickly for @xmath89 . the @xmath90 dependence of the cross section is dominated by the @xmath91 term . the structure functions vary slowly with @xmath90 at fixed @xmath68 . the uncertainty in the nc cross section predicted by eq .  [ eq : dis ] is dominated by the uncertainty in the structure functions ( parton densities ) , and is small , about @xmath92% at the high-@xmath68 and moderate-@xmath90 ranges of this analysis  @xcite . the quantity of interest in this paper is the @xmath0-jet cross section , which is sensitive to qcd corrections . the uncertainty arising from these corrections has been estimated to be small for this analysis .    for dis or lq events produced via the diagrams shown in fig . [ fig : nclq ] ( i.e. assuming no qed or qcd radiation ) , the mass of the @xmath93 system is related to @xmath68 via    @xmath94    and @xmath95 is related to @xmath90 via    @xmath96    the steeply - falling @xmath68 and @xmath90 dependences of dis events will therefore produce distributions falling sharply with mass and peaking towards @xmath97 . leptoquark production is an example of new physics that could generate high - mass @xmath0-jet pairs . the set of leptoquarks with @xmath98-invariant couplings has been specified @xcite . only lqs with fermion number @xmath99 are considered here , where @xmath100 and @xmath101 denote the lepton and baryon number , respectively . these leptoquarks are listed in table  [ tab : lq ] together with some of their properties . the @xmath102 lqs have higher cross sections in @xmath103 scattering than @xmath104 scattering since in the @xmath67 case a valence quark can fuse with the positron . @xmath105    in principle , additional lq types can be defined  @xcite  which depend on the generations of the quarks and leptons to which they couple . only lqs which preserve lepton flavor and which couple to first - generation quarks are considered in this analysis .    as shown in fig . [ fig : nclq ] , leptoquark production can generate an s - channel resonance provided @xmath106 . contributions to the @xmath67 cross section would also result from u - channel exchange and interference of lq diagrams with photon and @xmath107 exchange . the cross section in the presence of a leptoquark can be written as    @xmath108    the first term on the right - hand side of eq . [ eq : terms ] represents the sm contribution discussed previously . the second ( third ) term arises from the interference between the sm and u - channel ( s - channel ) lq diagram , and the fourth ( fifth ) term represents the u - channel ( s - channel ) lq diagram alone . the additional contributions to the sm cross section depend on two parameters : @xmath109 , and @xmath110 or @xmath111 , the coupling to @xmath112 and quark . leptoquarks of well - defined helicity ( @xmath113 ) are assumed for simplicity in the limit - setting procedure , and one species of lq is assumed to dominate the cross section . the @xmath74 dependence varies strongly for the different terms : it is flat for scalar - lq production in the s - channel , and for vector - lq exchange in the u - channel , while it varies as @xmath114 for vector - lq production in the s - channel or scalar - lq exchange in the u - channel . the interference terms produce a @xmath74 dependence which is steeper due to the sharply - peaking @xmath74 distribution in nc dis . in general , the s - channel term dominates the additional contributions to the sm cross section if @xmath106 , the coupling @xmath115 is small , and the lq is produced from a quark rather than an antiquark . however , there are conditions for which the other terms can become significant , or even dominant  @xcite , leading to important consequences for the expected mass spectra and decay angular distributions . the u - channel and interference terms can not produce a resonance peak in the mass spectrum and the angular distributions from such terms can behave more like those of nc deep inelastic scattering . limits are presented in this paper for narrow - width lq and under conditions for which the s - channel term dominates . the width of a lq depends on its spin and decay modes , and is proportional to @xmath109 times the square of the coupling . in the narrow - width approximation , the lq production cross section is given by integrating the s - channel term  @xcite : @xmath116 where @xmath117 represents the spin of the lq , @xmath118 is the quark density evaluated at @xmath119 and with the scale @xmath120 . in the limit - setting procedure ( sect . [ sec : limits ] ) , this cross section was corrected for expected qed and qcd ( for scalar lq only ) radiative effects . the qcd corrections  @xcite enhance the cross section by 20 - 30% for the @xmath102 lq considered here . the effect of qed radiation on the lq production cross section was calculated and was found to decrease the cross section by 5 - 25% as @xmath109 increases from @xmath121  gev .      in the supersymmetry ( susy ) superpotential , @xmath122-parity - violating terms of the form @xmath123 @xcite are of particular interest for lepton - hadron collisions . here , @xmath124 , @xmath125 , and @xmath126 denote left - handed lepton and quark doublets and the right - handed down - type quark - singlet chiral superfields , respectively . the indices @xmath127 , @xmath128 , and @xmath78 label their respective generations .    for @xmath129 , which is the case for @xmath130 collisions , these operators can lead to @xmath131- and @xmath132-type squark production . there are 9 possible production couplings probed in @xmath67 collisions , corresponding to the reactions  @xcite @xmath133 for production and decay via the @xmath134 coupling , squarks behave like scalar leptoquarks and the final state is indistinguishable , event by event , from standard model neutral and charged current events . however , as for the scalar leptoquarks , the angular distributions of the final - state lepton and quark will be different and this fact can be exploited in performing searches . limits derived for scalar lq production can then be directly related to limits on squark production and decay via @xmath135 . in addition to the yukawa couplings , gauge couplings also exist whereby @xmath136 can decay by radiating a neutralino or chargino which can subsequently decay . the final - state signature depends on the properties of the neutralino or chargino . the search for such decay topologies from a squark is outside the scope of this analysis . in 1994 - 97 , hera operated with protons of energy @xmath137  gev and positrons of energy @xmath138  gev . the zeus detector is described in detail in  @xcite . the main components used in the present analysis were the central tracking detector ( ctd ) positioned in a 1.43  t solenoidal magnetic field and the uranium - scintillator sampling calorimeter ( cal ) . the ctd was used to establish an interaction vertex with a typical resolution along ( transverse to ) the beam direction of @xmath139  cm . it was also used in the positron - finding algorithm that associated a charged track with an energy deposit in the calorimeter . the cal was used to measure the positron and hadronic energies . the cal consists of a forward part ( fcal ) , a barrel part ( bcal ) and a rear part ( rcal ) , with depths of @xmath140 interaction lengths , respectively . the fcal and bcal are segmented longitudinally into an electromagnetic section ( emc ) , and two hadronic sections ( hac1,2 ) . the rcal has one emc and one hac section . the cell structure is formed by scintillator tiles ; cell sizes range from @xmath141  @xmath142 ( femc ) to @xmath143  @xmath142 at the front face of a bcal hac2 cell . the light generated in the scintillator is collected on both sides of the module by wavelength - shifter bars , allowing a coordinate measurement based on knowledge of the attenuation length in the scintillator . the light is converted into an electronic signal by photomultiplier tubes . the cells are arranged into towers consisting of @xmath144 emc cells , a hac1 cell and a hac2 cell ( in fcal and bcal ) . the transverse dimensions of the towers in fcal are @xmath145  @xmath142 . one tower is absent at the center of the fcal and rcal to allow space for passage of the beams . the outer boundary of the inner ring of fcal towers , used to define a fiducial cut for the jet reconstruction , defines a box of @xmath146  @xmath142 .    under test beam conditions , the cal has energy resolutions of @xmath147 for positrons hitting the center of a calorimeter cell and @xmath148 for single hadrons , where energies are in gev . in the zeus detector , the energy measurement is affected by the energy loss in the material between the interaction point and the calorimeter . for the events selected in this analysis , the positrons predominantly strike the bcal , while the jets hit the fcal . the in - situ positron - energy resolution in the bcal has been determined to average @xmath149 while the jet - energy resolution in the fcal averages @xmath150 . the jet - energy resolution was determined by comparing reconstructed jet energies in the calorimeter with the total energy of the particles in the hadronic final - state using monte carlo simulation , and therefore includes small contributions from the jet - finding algorithm .    in the reconstruction of the positron and jet energies , corrections were applied for inactive materials located in front of the calorimeter and for non - uniformities in the calorimeter response @xcite . for the high energies important in this analysis , the overall energy scale is known to @xmath151% for positrons in bcal and @xmath152% for hadrons in fcal and bcal . the electromagnetic energy scale was determined by a comparison with momentum measurements in the central tracking detector ( using lower - energy electrons and positrons ) . its linearity was checked with energies reconstructed from the double angle ( da ) method  @xcite . the hadronic - energy scales in the fcal and bcal were determined by using transverse - momentum balance in nc dis events . the angular reconstruction was performed using a combination of tracking and calorimeter information . from monte carlo studies , the polar - angle resolutions were found to be @xmath153  mrad for positrons and approximately ( @xmath154 )  mrad for jets with energies above @xmath155  gev . the luminosity was measured from the rate of the bremsstrahlung process @xmath156 @xcite , and has an uncertainty of @xmath157% . the zeus coordinate system is right - handed and centered on the nominal interaction point , with the @xmath158 axis pointing in the direction of the proton beam ( forward ) and the @xmath159 axis pointing horizontally toward the center of hera . the polar angle @xmath160 is defined with respect to the @xmath158 axis . the events of interest with large @xmath0-jet mass contain a final - state positron at a large angle and of much higher energy than that of the incident positron beam , as well as one or more energetic jets . the only important sm source of such events is nc scattering with large @xmath161 . other potential backgrounds , such as high transverse - energy ( @xmath162 ) photoproduction , were determined to be negligible . the following requirements selected events of the desired topology :    * a reconstructed event vertex was required in the range @xmath163  cm . * the total transverse energy , @xmath164 , was required to be at least @xmath165  gev . * an identified  @xcite positron was required with energy @xmath166  gev , located either in the fcal or bcal . the positron was required to be well - contained in the bcal or fcal and not to point to the bcal / fcal interface , at approximately @xmath167 . positrons within @xmath168  cm of the boundary between adjacent bcal modules , as determined by tracking information , were also discarded to remove showers developing in the wavelength - shifter bars . * a hadronic jet with transverse momentum @xmath169  gev , located in a region of good containment , was required . the jets were reconstructed using the longitudinally - invariant @xmath170-clustering algorithm  @xcite in the inclusive mode  @xcite . only jets with a reconstructed centroid outside the inner ring of fcal towers were considered . in events where multiple jets were reconstructed , the jet with highest transverse momentum was used . after all cuts , 12% of the events had more than one jet , both in the data and monte carlo simulation ( see below ) . the @xmath162 cut , the jet - containment cut and the positron - containment cut define the available kinematic region for further analysis , as shown in fig . [ fig : acceptance ] . the jet containment cut , in particular , limits the values of @xmath171 that can be measured at the highest @xmath0-jet masses . because most such events have @xmath171 near @xmath151 , the acceptance for nc dis events ( with @xmath172  gev ) falls below @xmath173% for masses beyond 220  gev . in the region allowed by the cuts shown in fig . [ fig : acceptance ] , the acceptance is typically @xmath174% . a total of @xmath175 events remained after applying all cuts , compared to @xmath176 events predicted by the nc monte carlo simulation based on the measured luminosity of @xmath2  pb@xmath3 ( the sources of uncertainty on the expected number of events are described in sect . [ sec : systematics ] ) . the @xmath162 distributions for data and nc simulation are compared in fig . [ fig : compare]a . the positron transverse - momentum ( @xmath177 ) spectrum , jet transverse - momentum ( @xmath178 ) spectrum and the ratio @xmath179 , where @xmath180 is the transverse momentum of the hadronic system , are shown in figs . [ fig : compare](b - d ) , respectively . the missing transverse momentum , , and the longitudinal momentum variable , @xmath181 , are compared in figs . [ fig : compare](e , f ) . the global properties of the events are well reproduced by the simulation . the sm deep inelastic scattering events were simulated using the heracles 4.5.2  @xcite program with the django 6 version 2.4  @xcite interface to the hadronization programs . in heracles , corrections for initial- and final - state electroweak radiation , vertex and propagator corrections , and two - boson exchange are included . the nc dis hadronic final state was simulated using the meps model of lepto 6.5  @xcite , which includes order @xmath182 matrix elements and models of higher - order qcd radiation . as a systematic check , the nc final state was simulated using the color - dipole model of ariadne 4.08  @xcite . the cteq4 parton - distribution set  @xcite was used to evaluate the expected number of events from nc dis scattering . the leptoquark events were generated using pythia 6.1  @xcite . this program takes into account the finite width of the lq , but only includes the s - channel diagrams . initial- and final - state qcd radiation from the quark and the effect of lq hadronization before decay are taken into account , as are initial- and final - state qed radiation from the positron . the generated events were input into a geant - based  @xcite program which simulated the response of the zeus detector . the trigger and offline processing requirements applied to the data were applied to the simulated events . the luminosity of the nc monte carlo samples ranges from @xmath183  pb@xmath3 at @xmath184  gev@xmath185 to @xmath186  pb@xmath3 at @xmath187  gev@xmath185 . the mass of each @xmath0-jet pair was reconstructed from the measured energies and angles of the positron and jet as described by eq . [ eq : mass ] . this formula makes no correction for the finite jet mass . possible mass shifts and the resolutions for resonant lepton - hadron states were estimated from pythia . narrow scalar lq events in the mass range @xmath189  gev were simulated . the mean mass for reconstructed events was found to be within 6% of the generated value , while the peak position as determined by a gaussian fit was typically lower than the generated value by only 1% . the average mass resolution , determined from a gaussian fit to the peak of the reconstructed mass spectrum , ranged from 5.5% to 3% for masses from 150 to 290  gev . the rms of the distribution was typically twice as large . the positron scattering angle in the @xmath0-jet rest frame , @xmath188 , was reconstructed as the angle between the incoming and outgoing positron directions in this frame . these directions were determined by performing a lorentz transformation using the measured positron and jet energies and angles in the laboratory frame . the resolution in @xmath190 near @xmath191 , as determined from a gaussian fit , was @xmath192 degrading to @xmath193 as @xmath194 decreases . the shift in @xmath190 was less than @xmath192 for both the nc mc and the leptoquark mc .    in order to determine limits on leptoquark and squark production , the mass of the electron - hadron system was reconstructed by the constrained - mass method . this method reconstructs the @xmath0-hadron mass as @xmath195 where @xmath196 is the sum of the energy and @xmath197 contributions from the positron and all jets satisfying @xmath198  gev and pseudorapidity @xmath199 ( with the highest @xmath200 jet required to be outside the fcal inner ring ) . the @xmath201 cut removes contributions from the proton remnant . the constraints @xmath202 and @xmath203 , which are satisfied by fully contained events , have been assumed in arriving at this equation . when using this mass - reconstruction method , events with measured @xmath204  gev were removed to avoid large initial - state qed radiation . the @xmath205 method gave , on average , improved resolution over the @xmath73 method for narrow lq mc events . the improved resolution occurred at smaller @xmath74 ( for @xmath206 the mass resolution determined from a gaussian fit to the reconstructed mass distribution for @xmath207  gev was about @xmath168% in the @xmath205 method and @xmath208% for the @xmath73 method ) ; at the larger @xmath74 values where nc dis events are concentrated , the resolutions of the two methods were similar ( about @xmath208% for @xmath207  gev ) . the @xmath205 method relies on constraints which do not necessarily apply to a resonant state whose properties can not be anticipated in detail . we therefore choose to use the @xmath73 method as our primary search method . the @xmath205 method is used in the limit - setting procedure . the reconstructed values of @xmath73 are plotted versus @xmath171 for the selected events in fig . [ fig : cost_mej ] . most of the events are concentrated at large @xmath74 and small mass , as expected from standard model nc scattering . the five events indicated as open circles are from data taken in 1994 - 96 , with total luminosity @xmath210  pb@xmath3 . they were the subject of a previous publication  @xcite . in this earlier analysis , the kinematic variables were reconstructed with the da method . the five events also stand out with the @xmath73 reconstruction technique . the average value of @xmath73 for these events is @xmath211  gev , or @xmath212  gev less than the corresponding mass calculated previously via @xmath213 , where @xmath214 is the estimator of bjorken-@xmath68 calculated with the da method . this mass shift is compatible with expectations based on resolution and initial state radiation effects . with the present luminosity of @xmath2  pb@xmath3 , 7 events are observed in the region of @xmath215  gev and @xmath216 , where @xmath217 events are expected .    the @xmath73 spectrum for events with @xmath218  gev is shown in fig .  [ fig : mej]a on a logarithmic scale . the high - mass part of the spectrum is shown on a linear scale in the inset . the predicted number of events ( @xmath219 ) from nc processes is shown as the histogram . the ratio of the measured mass spectrum to the expectation is shown in fig . [ fig : mej]b . the shaded band indicates the systematic uncertainty on the expectations . the uncertainty on @xmath219 varies with mass from @xmath212% at 100 gev up to 30% at 250 gev . the most important uncertainties are on the energy scale and the jet position . the nc dis cross section given in eq . [ eq : dis ] ( neglecting @xmath85 ) can be rewritten in terms of the @xmath220 invariant mass , @xmath221 , and the polar angle of the outgoing struck quark in the laboratory frame , @xmath222 : @xmath223\;\ ; .\ ] ] the mass dependence is very steep . in addition to the explicit @xmath224 dependence , there is also a strong suppression of high masses implicit in the structure functions . an incorrect energy scale will produce a shift in the mass spectrum and potentially a significant error in the number of expected events at a given mass . the dependence on the quark angle is also steep , approximately @xmath225 at small @xmath222 . the number of events passing the jet fiducial cut is therefore strongly dependent on the accuracy of the jet position reconstruction . the jet fiducial - volume cut requires the highest-@xmath200 jet to point outside the inner ring of fcal towers . many distributions from data and mc were compared to search for possible systematic biases . the dominant sources of uncertainty are itemized below in order of decreasing importance :    1 . knowledge of the calorimeter energy scales : + the scale uncertainties discussed in sect . [ sec : conditions ] are @xmath151% for bcal positrons and @xmath152% for hadrons , leading to an uncertainty of @xmath226% in @xmath219 at @xmath227  gev . 2 .   uncertainties in the simulation of the hadronic energy flow , including simulation of the proton remnant , the energy flow between the struck quark and proton remnant , and possible detector effects in the innermost calorimeter towers : + many distributions of data and mc were compared and no important systematic differences were found . figure  [ fig : eflow ] shows the fraction of the jet energy in the inner ring of fcal towers associated with the highest @xmath200 jet as a function of @xmath228 . this is shown for all events in fig . [ fig : eflow]a , as well as for those with @xmath229  gev in fig . [ fig : eflow]b . for the highest @xmath228 values considered , this ratio is about @xmath210% . the energy located in the innermost towers of the fcal and not associated with the highest @xmath200 jet is shown in fig . [ fig : eflow]c , d , and compared to the mc simulation . no large differences are seen between data and mc ( the lowest @xmath230 bin in fig  [ fig : eflow]d contains only five data events ) . the innermost towers of the fcal have a larger uncertainty in the energy scale than the rest of the fcal owing to their slightly different construction and proximity to the beam . the energy in these cells has been varied by @xmath231% . as a test of the simulation of the forward energy flow , the ariadne mc has been used instead of the lepto mc . these tests yielded variations in @xmath219 of 13% at @xmath232  gev . 3 .   uncertainty in the parton density functions : + the parton density functions were estimated as in  @xcite , and led to an uncertainty of 5% in @xmath219 at @xmath232  gev . 4 .   uncertainties in the acceptance : + the alignment of the fcal was determined to better than @xmath92  mm , and various jet position reconstruction algorithms were compared . these studies yielded an uncertainty of 2% in @xmath219 . 5 .   uncertainties in the energy resolution functions . + these were studied by comparing tracking information with calorimeter information for individual events , as well as by comparing different reconstruction methods . the mc energies were smeared by additional amounts to represent these uncertainties , leading to a variation of less than @xmath92% in @xmath219 . other uncertainties include positron finding efficiency , luminosity determination , vertex simulation , multijet production rates , and hadronization simulation . these were found to be small in comparison to the items listed above . the overall systematic uncertainty was obtained by summing the contributions from all these sources in quadrature .      the data in fig . [ fig : mej ] are in good agreement with the sm expectations up to @xmath233  gev . some excess is seen at higher masses . for @xmath229  gev , @xmath234 events were observed in the data , while @xmath235 events are expected . a careful study of individual events in this mass region uncovered no signs of reconstruction errors . rather , the events always contain clear examples of a high - energy positron ( typically @xmath236  gev ) near @xmath237 and a high - energy jet ( typically @xmath238  gev ) in the forward direction ( 2 events have a second jet , in accord with nc dis monte carlo expectations ) . the distributions shown in fig . [ fig : compare ] for all the data are restricted to the events with @xmath229 gev in fig . [ fig : compare_210 ] . whereas the shapes of the distributions are similar , the data lie systematically above the mc , which is normalized to the integrated luminosity . the events with large @xmath73 have characteristics similar on average to nc dis events . in particular , the @xmath239 projection of the events with @xmath229  gev is shown in fig . [ fig : cost ] and compared to the mc expectations for neutral current dis ( solid histogram ) . the expectations for narrow s - channel scalar and vector lq production are also shown for comparison . for @xmath102 lqs with @xmath240 , the u - channel and interference terms would not significantly affect these expectations . the shape of the data and nc mc @xmath74 distributions are qualitatively similar , peaking at high values of @xmath190 .    in summary , there is some excess of events with @xmath229  gev above the standard model predictions . the probability of observing such an excess depends strongly on possible systematic biases . the most important of these are biases in the energy scales . as a test , many mc experiments were generated where the jet energy scale was shifted by + 2% and the electron energy scale by + 1% . a window of width @xmath241 , where @xmath242 is the mass resolution at mass @xmath73 , was moved over the accessible mass range . for each simulated experiment , the number of observed events within the mass window was compared with the nominal expectations as a function of @xmath73 , seeking the excess which gave the largest statistical significance . the same procedure was applied to the data . as a result , it was found that 5% of the simulated experiments would observe , somewhere in the mass spectrum , an excess of statistical significance at least as large as the one found in the data . the excess is therefore not statistically compelling . furthermore , the events have the characteristics of neutral current scattering . limits are therefore set on the production of narrow scalar or vector states , as discussed below . limits are set on the production cross section times branching ratio into positron+jet , @xmath243 , for a narrow scalar or vector state . for definiteness , limits on coupling strength versus mass for @xmath102 leptoquarks are presented , as well as limits on @xmath244 versus mass for scalar states coupling to @xmath245 or @xmath246 quarks , such as @xmath75 squarks . the limits are extracted for @xmath247 , allowing the use of the narrow - width approximation assumed in eq . [ eq : nwa ] . the @xmath205 mass reconstruction method was used to set limits as described in sect . [ sec : reconstruction ] . the positron fiducial cuts were removed since this method is less sensitive to the positron - energy measurement , while the cut @xmath248  gev was applied to reduce radiative effects . the mass spectrum reconstructed with this technique is shown in fig . [ fig : mcj]a . in total , 8026 events passed all selection cuts while 7863 events are predicted by the mc . the leptoquark mc described in sect .  [ sec : mc ] was used to determine the event selection efficiency and the acceptance of the fiducial cuts , as well as to estimate the mass resolution . this mc and the nc  background simulation were used to calculate an optimal bin width , @xmath249 , for each @xmath205 , and optimal @xmath250 range , @xmath251 , to obtain on average the best limits on lq couplings . the bin widths were typically 20 gev . the values of @xmath252 for setting limits range from @xmath253 to @xmath254 for vector leptoquarks with masses between @xmath189  gev , and from @xmath255 to @xmath254 for scalar leptoquarks in the same mass range . the mass spectrum after applying the optimal @xmath74 cut for the scalar search is shown in fig . [ fig : mcj]b . no significant deviations from expectations are seen after applying this cut . the 95% confidence level ( cl ) limits on @xmath243 were obtained directly from the observed number of data events with @xmath256  in the particular mass window  @xcite . the procedure described in  @xcite was extended to include the systematic uncertainties in the numbers of predicted events . this was found to have negligible effect on the limits . the limits for a narrow scalar or vector state are shown in fig . [ fig : csl_mejet ] . these limits lie between @xmath151 and @xmath255  pb as the mass increases from @xmath8 to @xmath257  gev . the 95% cl exclusion limits for different species of lq are given in the coupling versus mass plane in fig . [ fig : lqlimits ] . the limits exclude leptoquarks with coupling strength @xmath258 for masses up to 280 gev for specific types of f=0 leptoquarks . the h1 collaboration has recently published similar limits  @xcite . in fig . [ fig : lqlimits ] , the zeus results are compared to recent limits from opal . at lep @xcite , sensitivity to a high - mass lq arises from effects of virtual lq exchange on the hadronic cross section . the hera and lep limits are complementary to tevatron limits  @xcite , which are independent of the coupling @xmath259 . the limits by d0 ( cdf ) extend up to @xmath260  gev for a scalar lq with 100% branching ratio to @xmath93 . the d0 limits are shown as vertical lines in fig . [ fig : lqlimits ] . the tevatron limits for vector lqs are model dependent  @xcite , but are expected to be considerably higher than for scalar lqs . the zeus limits presented in fig .  [ fig : lqlimits ] can also be applied to any narrow state which couples to a positron and a @xmath245 or @xmath246 quark and with unknown branching ratio to @xmath0-jet(s ) . these states correspond to the leptoquark types as labelled in the figure . for these states , the limits are on the quantity @xmath244 . examples of scalar states for which these limits apply are @xmath75-squarks ( e.g.,the limit on the @xmath261 lq can be read as a limit on the @xmath262 r - parity - violating coupling ) . data from @xmath2  pb@xmath3 of @xmath263 collisions at a center - of - mass energy of 300  gev have been used to search for a resonance decaying into @xmath0-jet . the invariant mass of the @xmath0-jet pair was calculated directly from the measured energies and angles of the positron and jet . this approach makes no assumptions about the production mechanism of such a state . the observed mass spectrum is in good agreement with standard model expectations up to @xmath0-jet masses of about @xmath4  gev . above this mass , some excess is seen . the angular distribution of these events is typical of high-@xmath5 neutral current events and does not give convincing evidence for the presence of a narrow scalar or vector state . by applying restrictions on the decay angle to optimize sensitivity to a narrow state in the presence of nc background , limits have been derived on the cross section times decay branching fraction for a scalar or vector state decaying into positron and jet(s ) . these limits can be interpreted , for example , as limits on leptoquark or r - parity - violating squark production . limits on the production of leptoquarks and squarks are presented in the coupling strength versus mass plane . at a coupling strength @xmath7 , new states are ruled out at 95% confidence level for masses between @xmath8 and @xmath9  gev . we thank the desy directorate for their strong support and encouragement , and the hera machine group for their diligent efforts . we are grateful for the support of the desy computing and network services . the design , construction and installation of the zeus detector have been made possible by the ingenuity and effort of many people from desy and home institutes who are not listed as authors . it is also a pleasure to thank w. buchmller , r. rckl and m. spira for useful discussions . j. l. hewett , ` single squark production at hera via r parity violating interactions ' , proc . of the 1990 dpf summer study on high - energy physics , snowmass , colorado , usa , eds . e. berger , world scientific ( 1992 )
the @xmath0-jet invariant mass spectrum produced in the reaction @xmath1 has been studied at a center - of - mass energy of 300  gev . 
 the data were collected using the zeus detector operating at the hera collider , and correspond to an integrated luminosity of @xmath2  pb@xmath3 . 
 the observed mass spectrum is in good agreement with standard model expectations up to an @xmath0-jet mass of @xmath4  gev . above this mass , 
 some excess is seen . 
 the angular distribution of these events is typical of high-@xmath5 neutral current events and does not give convincing evidence for the presence of a narrow scalar or vector state . 
 limits are presented on the product of cross section and branching ratio for such a state and are interpreted as limits on leptoquark or r - parity - violating squark production . 
 specific leptoquark types are ruled out at @xmath6% confidence level for coupling strength @xmath7 for masses between @xmath8 and @xmath9  gev . 
 -1.5 cm -0.3 cm -0.3 cm 16.cm 0.cm 0.3 cm plus0.05 cm minus0.05 cm    desy 
 00 - 023 + february 2000 +    -1.cm -0.3 cm -0.3 
 cm 16.cm 0.cm 0.3 cm plus0.05 cm minus0.05 cm     the zeus collaboration    j.  breitweg , s.  chekanov , m.  derrick , d.  krakauer , s.  magill , b.  musgrave , a.  pellegrino , j.  repond , r.  stanek , r.  yoshida + _ argonne national laboratory , argonne , il , usa _  @xmath10    m.c.k . 
 mattingly + _ andrews university , berrien springs , mi , usa _ 
 g.  abbiendi , f.  anselmo , p.  antonioli , g.  bari , m.  basile , l.  bellagamba , d.  boscherini@xmath11 , a.  bruni , g.  bruni , g.  cara  romeo , g.  castellini@xmath12 , l.  cifarelli@xmath13 , f.  cindolo , a.  contin , n.  coppola , m.  corradi , s.  de  pasquale , p.  giusti , g.  iacobucci , g.  laurenti , g.  levi , a.  margotti , t.  massam , r.  nania , f.  palmonari , a.  pesci , a.  polini , g.  sartorelli , y.  zamora  garcia@xmath14 , a.  zichichi + _ university and infn bologna , bologna , italy _ 
 @xmath15    c.  amelung , a.  bornheim , i.  brock , k.  cobken , j.  crittenden , r.  deffner , h.  hartmann , k.  heinloth , e.  hilger , p.  irrgang , h .- 
 p . 
 jakob , a.  kappes , u.f . 
 katz , r.  kerger , e.  paul , h.  schnurbusch , + a.  stifutkin , j.  tandler , k.ch . 
 voss , a.  weber , h.  wieber + _ physikalisches 
 institut der universitt bonn , bonn , germany _ 
 @xmath16    d.s . 
 bailey , o.  barret , n.h . 
 brook@xmath17 , b.  foster@xmath18 , g.p . 
 heath , h.f . 
 heath , j.d . 
 mcfall , d.  piccioni , e.  rodrigues , j.  scott , r.j . 
 tapper + _ h.h . 
 wills physics laboratory , university of bristol , bristol , u.k . 
 _  @xmath19    m.  capua , a. mastroberardino , m.  schioppa , g.  susinno + _ calabria university , physics dept.and infn , cosenza , italy _  @xmath15    h.y . 
 jeoung , j.y . 
 kim , j.h . 
 lee , i.t . 
 lim , k.j . 
 ma , m.y . 
 pac@xmath20 + _ chonnam national university , kwangju , korea _ 
 @xmath21    a.  caldwell , w.  liu , x.  liu , b.  mellado , s.  paganis , r.  sacchi , s.  sampson , f.  sciulli + _ columbia university , nevis labs . 
 , irvington on hudson , n.y . , usa _ 
 @xmath22    j.  chwastowski , a.  eskreys , j.  figiel , k.  klimek , k.  olkiewicz , k.  piotrzkowski@xmath23 , m.b . 
 przybycie , p.  stopa , l.  zawiejski + _ inst . of nuclear physics , 
 cracow , poland _ 
 @xmath24    l.  adamczyk , b.  bednarek , k.  jele , d.  kisielewska , a.m.  kowal , t.  kowalski , m.  przybycie , + e.  rulikowska - zarbska , l.  suszycki , d.  szuba + _ faculty of physics and nuclear techniques , academy of mining and metallurgy , cracow , poland _ 
 @xmath24    a.  kotaski + _ jagellonian univ . , 
 dept . of physics , 
 cracow , poland _ 
 @xmath25    l.a.t . 
 bauerdick , u.  behrens , j.k . 
 bienlein , c.  burgard@xmath26 , k.  desler , g.  drews , , u.  fricke , f.  goebel , p.  gttlicher , r.  graciani , t.  haas , w.  hain , g.f . 
 hartner , d.  hasell@xmath27 , k.  hebbel , k.f . 
 johnson@xmath28 , m.  kasemann@xmath29 , w.  koch , u.  ktz , h.  kowalski , l.  lindemann@xmath30 , b.  lhr , m.  milite , t.  monteiro@xmath23 , m.  moritz , d.  notz , f.  pelucchi , m.c . 
 petrucci , m.  rohde , p.r.b . 
 saull , a.a . 
 savin , , f.  selonke , m.  sievers , s.  stonjek , e.  tassi , g.  wolf , u.  wollmer , + c.  youngman , + _ deutsches elektronen - synchrotron desy , hamburg , germany _    c.  coldewey , h.j . 
 grabosch , , a.  meyer , , p.b .  straub + _ desy zeuthen , zeuthen , germany _    g.  barbagli , e.  gallo , p.  pelfer + _ university and infn , florence , italy _  @xmath15    g.  maccarrone , l.  votano + _ infn , laboratori nazionali di frascati , frascati , italy _ 
 @xmath15    a.  bamberger , a.  benen , s.  eisenhardt@xmath31 , p.  markun , h.  raach , s.  wlfle + _ fakultt fr physik der universitt freiburg i.br . 
 , freiburg i.br . 
 , germany _ 
 @xmath16    p.j . 
 bussey , a.t . 
 doyle , s.w . 
 lee , n.  macdonald , g.j . 
 mccance , d.h . 
 saxon , l.e . 
 sinclair , + i.o . 
 skillicorn , r.  waugh + _ dept . of physics and astronomy , university of glasgow , glasgow , 
 u.k . 
 _  @xmath19    i.  bohnet , n.  gendner , u.  holm , a.  meyer - larsen , h.  salehi , k.  wick + _ hamburg university , i. institute of exp . 
 physics , hamburg , germany _ 
 @xmath16    d.  dannheim , a.  garfagnini , i.  gialas@xmath32 , l.k . 
 gladilin@xmath33 , d.  kira@xmath34 , r.  klanner , e.  lohrmann , g.  poelz , f.  zetsche + _ hamburg university , ii . 
 institute of exp . 
 physics , hamburg , germany _ 
 @xmath16    r.  goncalo , k.r . 
 long , d.b . 
 miller , a.d . 
 tapper , r.  walker + _ imperial college london , high energy nuclear physics group , london , u.k . 
 _  @xmath19    u.  mallik + _ university of iowa , physics and astronomy dept . , 
 iowa city , usa _ 
 @xmath10    p.  cloth , d.  filges 
 + _ forschungszentrum jlich , institut fr kernphysik , jlich , germany _    t.  ishii , m.  kuze , k.  nagano , k.  tokushuku@xmath35 , s.  yamada , 
 y.  yamazaki + _ institute of particle and nuclear studies , kek , tsukuba , japan _ 
 @xmath36    s.h . 
 ahn , s.b . 
 lee , s.k . 
 park + _ korea university , seoul , korea _ 
 @xmath21    h.  lim , i.h .  park , d.  son + _ kyungpook national university , taegu , korea _ 
 @xmath21    f.  barreiro , g.  garca , c.  glasman@xmath37 , o.  gonzalez , l.  labarga , j.  del  peso , i.  redondo@xmath38 , j.  terrn + _ univer . 
 autnoma madrid , depto de fsica terica , madrid , spain _ 
 @xmath39    m.  barbi , f.  corriveau , d.s . 
 hanna , a.  ochs , s.  padhi , m.  riveline , d.g . 
 stairs , m.  wing + _ mcgill university , dept . of physics , montral , qubec , 
 canada _ 
 @xmath40  @xmath41    t.  tsurugai + _ meiji gakuin university , faculty of general education , yokohama , japan _ 
 v.  bashkirov@xmath42 , b.a . 
 dolgoshein + _ moscow engineering physics institute , moscow , russia _ 
 @xmath43    r.k . 
 dementiev , p.f . 
 ermolov , yu.a . 
 golubkov , i.i . 
 katkov , l.a . 
 khein , n.a . 
 korotkova , + i.a . 
 korzhavina , v.a . 
 kuzmin , o.yu . 
 lukina , a.s . 
 proskuryakov , l.m . 
 shcheglova , a.n . 
 solomin , + n.n . 
 vlasov , s.a . 
 zotkin + _ moscow state university , institute of nuclear physics , moscow , russia _  @xmath44    c.  bokel , m.  botje , n.  brmmer , j.  engelen , s.  grijpink , e.  koffeman , p.  kooijman , s.  schagen , a.  van  sighem , h.  tiecke , n.  tuning , j.j . 
 velthuis , j.  vossebeld , l.  wiggers , e.  de  wolf + _ nikhef and university of amsterdam , amsterdam , netherlands _ 
 @xmath45    d.  acosta@xmath46 , b.  bylsma , l.s . 
 durkin , j.  gilmore , c.m . 
 ginsburg , c.l . 
 kim , t.y . 
 ling + _ ohio state university , physics department , columbus , ohio , usa _ 
 @xmath10    s.  boogert , a.m.  cooper - sarkar , r.c.e . 
 devenish , j.  groe - knetter@xmath47 , t.  matsushita , o.  ruske , + m.r . 
 sutton , r.  walczak + _ department of physics , university of oxford , oxford u.k . _  @xmath19    a.  bertolin , r.  brugnera , r.  carlin , f.  dal  corso , u.  dosselli , s.  dusini , s.  limentani , m.  morandin , m.  posocco , l.  stanco , r.  stroili , c.  voci + _ dipartimento di fisica dell universit and infn , padova , italy _  @xmath15    l.  iannotti@xmath48 , b.y . 
 oh , j.r . 
 okrasiski , w.s . 
 toothacker , j.j . 
 whitmore + _ pennsylvania state university , dept . of physics , 
 university park , pa , usa _ 
 @xmath22    y.  iga + _ polytechnic university , sagamihara , japan _ 
 @xmath36    g.  dagostini , g.  marini , a.  nigro + _ dipartimento di fisica , univ . 
 la sapienza and infn , rome , italy _  @xmath49    c.  cormack , j.c . 
 hart , n.a . 
 mccubbin , t.p . 
 shah + _ rutherford appleton laboratory , chilton , didcot , oxon , u.k . 
 _  @xmath19    d.  epperson , c.  heusch , h.f .- w . 
 sadrozinski , a.  seiden , r.  wichmann , d.c . 
 williams + _ university of california , santa cruz , ca , usa _ 
 @xmath10    n.  pavel + _ fachbereich physik der universitt - gesamthochschule siegen , germany _ 
 @xmath16    h.  abramowicz@xmath50 , s.  dagan@xmath51 , s.  kananov@xmath51 , a.  kreisel , a.  levy@xmath51 + _ raymond and beverly sackler faculty of exact sciences , school of physics , tel - aviv university , + 
 tel - aviv , israel _ 
 @xmath52    t.  abe , t.  fusayasu , k.  umemori , t.  yamashita + _ department of physics , university of tokyo , tokyo , japan _  @xmath36    r.  hamatsu , t.  hirose , m.  inuzuka , s.  kitamura@xmath53 , t.  nishimura + _ tokyo metropolitan university , dept . of physics , tokyo , japan _ 
 @xmath36    m.  arneodo@xmath54 , n.  cartiglia , r.  cirio , m.  costa , m.i . 
 ferrero , s.  maselli , v.  monaco , c.  peroni , m.  ruspa , a.  solano , a.  staiano + _ universit di torino , dipartimento di fisica sperimentale and infn , torino , italy _  @xmath15    m.  dardo + _ ii faculty of sciences , torino university and infn - alessandria , italy 
 _  @xmath15    d.c . 
 bailey , c .- p . 
 fagerstroem , r.  galea , t.  koop , g.m . 
 levman , j.f . 
 martin , r.s . 
 orr , s.  polenz , a.  sabetfakhri , d.  simmons + _ university of toronto , dept . of physics , toronto , ont . 
 , canada _  @xmath55    j.m . 
 butterworth , c.d . 
 catterall , m.e . 
 hayes , e.a . 
 heaphy , t.w . 
 jones , j.b . 
 lane , b.j . 
 west + _ university college london , physics and astronomy dept . , 
 london , u.k . 
 _  @xmath19    j.  ciborowski , r.  ciesielski , g.  grzelak , r.j . 
 nowak , j.m . 
 pawlak , r.  pawlak , b.  smalska , + t.  tymieniecka , a.k . 
 wrblewski , j.a . 
 zakrzewski , a.f . 
 arnecki + _ warsaw university , institute of experimental physics , warsaw , poland _ 
 @xmath24    m.  adamus , t.  gadaj + _ institute for nuclear studies , warsaw , poland _ 
 @xmath24    o.  deppe , y.  eisenberg@xmath51 , d.  hochman , u.  karshon@xmath51 + _ weizmann institute , department of particle physics , rehovot , israel _  @xmath56    w.f . 
 badgett , d.  chapin , r.  cross , c.  foudas , s.  mattingly , d.d . 
 reeder , w.h . 
 smith , a.  vaiciulis@xmath57 , t.  wildschek , m.  wodarczyk + _ university of wisconsin , dept . of physics , 
 madison , wi , usa _ 
 @xmath10    a.  deshpande , s.  dhawan , v.w . 
 hughes + _ yale university , department of physics , new haven , ct , usa _ 
 @xmath10    s.  bhadra , c.  catterall , j.e . 
 cole , w.r . 
 frisken , r.  hall - wilton , m.  khakzad , s.  menary , w.b . 
 schmidke + _ york university , dept . of physics , toronto , ont . , 
 canada _ 
 @xmath55    @xmath58 now visiting scientist at desy + @xmath59 also at iroe florence , italy + @xmath60 now at univ . of salerno and infn napoli , italy + @xmath61 supported by worldlab , lausanne , switzerland + @xmath62 pparc advanced fellow + @xmath63 also at university of hamburg , alexander von humboldt research award + @xmath64 now at dongshin university , naju , korea + @xmath65 now at cern + @xmath66 now at barclays capital plc , london + @xmath27 now at massachusetts institute of technology , cambridge , ma , usa + @xmath28 visitor from florida state university + @xmath29 now at fermilab , batavia , il , usa + @xmath30 now at sap a.g . , walldorf , germany + @xmath31 now at university of edinburgh , edinburgh , u.k 
 . + @xmath32 visitor of univ . of crete , 
 greece , partially supported by daad , bonn - kz . 
 + @xmath33 on leave from msu , supported by the gif , contract i-0444 - 176.07/95 + @xmath34 supported by daad , bonn - kz . 
 + @xmath35 also at university of tokyo + @xmath37 supported by an ec fellowship number erbfmbict 972523 + @xmath38 supported by the comunidad autonoma de madrid + @xmath42 now at loma linda university , loma linda , ca , usa + @xmath46 now at university of florida , gainesville , fl , usa + @xmath47 supported by the feodor lynen program of the alexander von humboldt foundation + @xmath48 partly supported by tel aviv university + @xmath50 an alexander von humboldt fellow at university of hamburg + @xmath51 supported by a minerva fellowship + @xmath53 present address : tokyo metropolitan university of health sciences , tokyo 116 - 8551 , japan + @xmath54 now also at universit del piemonte orientale , i-28100 novara , italy + @xmath57 now at university of rochester , rochester , ny , usa +    [ cols= " > , < " , ]     -1.5 cm -0.3 cm -0.3 cm 16.cm 0.cm 0.3 cm plus0.05 cm minus0.05 cm