Eur. Phys. J. C (2024) 84: 720
https://doi.org/10.1140/epjc/s10052-024-13003-1

Letter

Observation and differential cross section measurement of neutral current DIS events with an empty hemisphere in the Breit frame

¹ I. Physikalisches Institut der RWTH, Aachen, Germany
² University of Michigan, 48109, Ann Arbor, MI, USA
³ LAPP, Université de Savoie, CNRS/IN2P3, Annecy-le-Vieux, France
⁴ Inter-University Institute for High Energies ULB-VUB, Brussels and Universiteit Antwerpen, Antwerp, Belgium
⁵ Lawrence Berkeley National Laboratory, 94720, Berkeley, CA, USA
⁶ Department of Physics, University of the Basque Country UPV/EHU, 48080, Bilbao, Spain
⁷ School of Physics and Astronomy, University of Birmingham, Birmingham, UK
⁸ Horia Hulubei National Institute for R &D in Physics and Nuclear Engineering (IFIN-HH), Bucharest, Romania
⁹ University of Illinois, 60607, Chicago, IL, USA
¹⁰ STFC, Rutherford Appleton Laboratory, Didcot, Oxfordshire, UK
¹¹ Institut für Physik, TU Dortmund, Dortmund, Germany
¹² Institute for Particle Physics Phenomenology, Durham University, Durham, UK
¹³ CERN, Geneva, Switzerland
¹⁴ IRFU, CEA, Université Paris-Saclay, Gif-sur-Yvette, France
¹⁵ II. Physikalisches Institut, Universität Göttingen, Göttingen, Germany
¹⁶ Institut für Theoretische Physik, Universität Göttingen, Göttingen, Germany
¹⁷ Deutsches Elektronen-Synchrotron DESY, Hamburg and Zeuthen, Germany
¹⁸ Physikalisches Institut, Universität Heidelberg, Heidelberg, Germany
¹⁹ Rice University, 77005-1827, Houston, TX, USA
²⁰ Institute of Nuclear Physics, Polish Academy of Sciences, Krakow, Poland
²¹ Department of Physics, University of Lancaster, Lancaster, UK
²² Argonne National Laboratory, 60439, Lemont, IL, USA
²³ Department of Physics, University of Liverpool, Liverpool, UK
²⁴ School of Physics and Astronomy, Queen Mary University of London, London, UK
²⁵ Aix Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France
²⁶ Max-Planck-Institut für Physik, Munich, Germany
²⁷ National Institute of Science Education and Research, Jatni, Odisha, India
²⁸ Joint Laboratory of Optics, Palacký University, Olomouc, Czech Republic
²⁹ IJCLab, Université Paris-Saclay, CNRS/IN2P3, Orsay, France
³⁰ LLR, Ecole Polytechnique, CNRS/IN2P3, Palaiseau, France
³¹ Faculty of Science, University of Montenegro, Podgorica, Montenegro
³² Institute of Physics, Academy of Sciences of the Czech Republic, Praha, Czech Republic
³³ Faculty of Mathematics and Physics, Charles University, Praha, Czech Republic
³⁴ University of California, 92521, Riverside, CA, USA
³⁵ Dipartimento di Fisica, Université di Roma Tre, INFN Roma 3, Rome, Italy
³⁶ Shandong University, Shandong, People’s Republic of China
³⁷ Fakultät IV, Department für Physik, Universität Siegen, Siegen, Germany
³⁸ Stony Brook University, 11794, Stony Brook, NY, USA
³⁹ Physics Department, University of Tennessee, 37996, Knoxville, TN, USA
⁴⁰ Institute of Physics and Technology of the Mongolian Academy of Sciences, Ulaanbaatar, Mongolia
⁴¹ Ulaanbaatar University, Ulaanbaatar, Mongolia
⁴² Brookhaven National Laboratory, 11973, Upton, NY, USA
⁴³ Paul Scherrer Institut, Villigen, Switzerland
⁴⁴ Department of Physics and Astronomy, Purdue University, 47907, West Lafayette, IN, USA
⁴⁵ Fachbereich C, Universität Wuppertal, Wuppertal, Germany
⁴⁶ Yerevan Physics Institute, Yerevan, Armenia
⁴⁷ Departamento de Fisica Aplicada, CINVESTAV, Mérida, Yucatán, Mexico
⁴⁸ Institut für Teilchenphysik, ETH, Zürich, Switzerland
⁴⁹ Physik-Institut der Universität Zürich, Zürich, Switzerland
⁵⁰ Affiliated with an Institute Covered by a Current or Former Collaboration Agreement with DESY, Hamburg and Zeuthen, Germany

^dz stefan.schmitt@desy.de

Received: 15 March 2024
Accepted: 7 June 2024
Published online: 22 July 2024

Abstract

The Breit frame provides a natural frame to analyze lepton–proton scattering events. In this reference frame, the parton model hard interactions between a quark and an exchanged boson defines the coordinate system such that the struck quark is back-scattered along the virtual photon momentum direction. In Quantum Chromodynamics (QCD), higher order perturbative or non-perturbative effects can change this picture drastically. As Bjorken-x decreases below one half, a rather peculiar event signature is predicted with increasing probability, where no radiation is present in one of the two Breit-frame hemispheres and all emissions are to be found in the other hemisphere. At higher orders in ${\alpha }_s$ or in the presence of soft QCD effects, predictions of the rate of these events are far from trivial, and that motivates measurements with real data. We report on the first observation of the empty current hemisphere events in electron–proton collisions at the HERA collider using data recorded with the H1 detector at a center-of-mass energy of 319 GeV. The fraction of inclusive neutral-current DIS events with an empty hemisphere is found to be $0.0112\pm 3.9{\%}_{\text{stat}}\pm 4.5{\%}_{\text{syst}}\pm 1.6{\%}_{\text{mod}}$ in the selected kinematic region of $150<Q^2<1500{\text{GeV}}^2$ and inelasticity $0.14<y<0.7$. The data sample corresponds to an integrated luminosity of 351.1 pb$_{}^{-1}$, sufficient to enable differential cross section measurements of these events. The results show an enhanced discriminating power at lower Bjorken-x among different Monte Carlo event generator predictions.