With a total of 304 talks, Deep Inelastic Scattering 2017 (DIS17) demonstrated how deep inelastic scattering (DIS) and related topics permeate most aspects of high-energy physics and how we still have a huge amount to learn about strong interactions. Held at the University of Birmingham in the UK from 3–7 April, more than 300 participants from 41 countries enjoyed a week of lively scientific discussion and largely unanticipated sunshine.
The first of this series of annual international workshops on DIS and related topics took place in Durham, UK, in the Spring of 1993, when the first results from the world’s only lepton-hadron collider, HERA at DESY, were discussed by around 80 participants. A quarter of a century later, the workshop series has toured the globe, digested data from the full lifetime of HERA and numerous fixed-target DIS experiments, as well as playing a major role in the development and understanding of hadron-collider physics.
The dominant theme of DIS17 this year was the relevance of strong interactions, parton densities (PDFs) and DIS to the LHC. But a wide and eclectic range of other topics was included, notably new results from experiments at the Relativistic Heavy Ion Collider (RHIC), JLab and HERA, as well as theoretical advances and future plans for the field.
Following plenary review talks covering the latest news from the field, there followed two and a half days during which seven working groups operated in up to six simultaneous parallel sessions, covering: PDFs; low proton momentum fraction (Bjorken-x) physics; Higgs and beyond-the-Standard Model (BSM) studies in hadron collisions; hadronic, electroweak and heavy-flavour observables; spin and 3D hadron structure; and future facilities. The Birmingham event included a topical lecture on probing ultra-low-x QCD with cosmic neutrinos at IceCube and Auger, and a special session was devoted to the status and scientific opportunities offered by future proposed DIS facilities at CERN such as the Large Hadron electron Collider, LHeC) and at BNL or JLab in the US (the Electron Ion Collider, EIC).
All aspects of proton–proton collisions at the LHC featured during this year’s DIS event, from the role of parton densities and perturbative QCD dynamics in beyond-the Standard Model searches and Higgs boson studies, through the measurement and interpretation of processes that are sensitive to parton densities (such as electroweak gauge boson production), to topics that challenge our understanding of strong-interaction dynamics in the semi- and non-perturbative regimes. Ten years after HERA completed data-taking, the collider still featured strongly. The final round of combined inclusive DIS data published in 2016 by the H1 and ZEUS experiments have been integrated into global PDF fits, and also for a handful of new measurements and combinations. Heavy-ion collision results from RHIC and the LHC were also well represented, as were insights into 3D proton structure and hadron spin from semi-inclusive DIS and polarised proton–proton collisions at COMPASS, JLab and RHIC, and current and future DIS measurements with neutrinos.
Data from HERA and the LHC have brought a new level of precision to the parton densities of the proton, with associated theoretical advances including the push towards higher order (next-to-next-to-next-to leading order) descriptions. Taming the “pathological” rise of the proton gluon density at low-x in the perturbative domain remains a major topic, which is now being addressed experimentally in ultra-peripheral collisions and forward measurements at the LHC, as well as through theoretical modelling of low-x, low-Q2 HERA data with nonlinear parton dynamics and resummation techniques. The related topic of diffractive electron–proton scattering and the heavily gluon-dominated diffractive PDFs is benefiting from the full HERA statistics. New insights into elastic and total cross-sections, such as TOTEM’s observation of a non-exponential term in the four-momentum transfer dependence of the elastic cross-section, are emerging from the LHC data. Uncertainties in PDFs remain large at high x, and intense work is ongoing to understand LHC observables such as top-quark pair production, which are sensitive in this region. New data and theoretical work are revealing the transverse structure of the proton for the first time in terms of transverse-momentum-dependent parton densities. The LHC’s proton–lead collision data are also constraining nuclear PDFs in an unprecedented low-x kinematic region.
Concerning the future of DIS, potential revolutions in our understanding could be made with polarised proton and heavy-ion targets and with step changes in energy and luminosity becoming abundantly clear. The EIC offers 3D hadron tomography and an unprecedented window on the spin and flavour structure of protons and ions. Its eA scattering programme would probe low-x parton dynamics in a region where collective effects ultimately leading to gluon saturation are expected to become important. The LHeC offers a standalone Higgs production programme complementary to that of the LHC, as well as a new level in precision in PDFs that could be applied to extend the sensitivity to new physics at the LHC. The ep and eA scattering programme also would probe low-x parton dynamics in the region where gluon saturation is expected to be firmly established. Together, the proposed facilities open up an exciting set of new windows on hadronic matter with relevance to major questions such as quark confinement and hadronic mass generation.
The next instalment of DIS in April 2018, to be held in Kobe, Japan, is eagerly awaited.