First Measurement of Three Jet Cross Sections in DIS at HERA H 1 Collaboration

The production of three jets with high transverse momentum has been studied in deep inelastic positron-proton scattering with the H1 detector at HERA. Jets are defined by the inclusive Ì algorithm in the Breit frame. Cross sections as a function of the squared transferred-momentum É ¾ , the Bjorken scaling variable Ü , the mean transverse energy Ì ÖÖÖØ and the invariant mass Å ¿¿¿Ø of the three jets have been measured over a wide É ¾ range from to ¼¼¼ Î ¾. Moreover, the ratio of the three jet cross section with respect to the two jet cross section Ê ¿¾ has been measured as a function of É ¾ and the mass of the hard process Å ÒÒÒØ for É ¾ ½½¼ Î ¾ and Å ÒÒÒØ ¾¾ GeV. The shape of the Dalitz variables ¿ ¾ ¿ Å ¿¿¿Ø , ¾ Å ¿¿¿Ø , where ¿ () is the energy of the (second) highest energetic jet in the three jet rest frame, and two angular distributions characterising the angular orientation of the jets are studied. The data are compared to Ç´« ¾ × µ perturbative QCD (pQCD) calculations and to QCD Monte Carlo models. Ç´« ¾ × µ pQCD calculations describe the data well for É ¾ ¼ Î ¾ and for transverse jet energies Ì ÖÖÖØ ½¼ GeV. For lower É ¾ and lower Ì they fall below the data. Monte Carlo models have difficulties in describing the cross sections, but are able to reproduce the shape normalised angular distributions in the three jet centre of mass system. The jet with the lowest energy in the three jet centre of mass frame is most likely to be emitted in the direction of the proton beam. This contrasts with a uniform distribution of the available phase space, but it is consistent with the observation that in the QCD models this jet mainly stems from initial state radiation processes. Abstract: 157 Parallel sessions: 1 Plenary sessions: 1