A bright future for HERA physics

23 February 2015

What can the data offer experiments now and in the future?

Even though HERA – the only electron–proton collider built so far – stopped running in mid-2007, analyses of the vast amounts of data from the Hermes, H1 and ZEUS experiments continue to produce important and high-impact measurements relevant to spin physics, the structure of the proton and other areas of QCD. Special efforts have been made to ensure that these unique data are safely preserved for future analyses for at least the next 10 years, within the framework of the Data Preservation in High-Energy Physics collaboration ( CERN Courier May 2009 p21).

In November 2014, DESY hosted a workshop on “Future Physics with the HERA Data for Current and Planned Experiments”, to pull together experts and ask questions about what the HERA data still have to say and how they are relevant to other facilities. The aim was, in effect, to create a list of subjects that are still to be investigated or exploited fully. Across two days, almost 30 presentations and lively discussions occupied around 70 participants, both experimentalists and theorists, from across the globe.

The most recent results from the collaborations and a perspective from theory were presented first in a special HERA symposium, starting with a presentation on recent results from Hermes by Charlotte Van Hulse of the University of the Basque Country. She highlighted the semi-inclusive deep-inelastic scattering (DIS) data collected on a transversely polarized hydrogen target that provides access to various transverse-momentum-dependent parton distribution functions (PDFs), which are sensitive to correlations between quark spin, proton spin, the transverse momentum of quarks and/or of final-state hadrons.

Two talks followed that showed results from H1 and ZEUS, the first on proton structure by Aharon Levy of Tel Aviv University and the second on diffraction and hadronic final states by Alice Valkárová of Charles University, Prague. All of the measurements of inclusive DIS from H1 and ZEUS have been combined recently and QCD fits to these data have been performed, providing a new set of PDFs of the proton (see figure). The 15 years of data taking at HERA have culminated in a combination of 3000 data points, and their impact on knowledge of the structure of the proton will last for years to come. Also, recent jet measurements at HERA have enabled the strong coupling constant to be extracted with an experimental precision of <1%. This has been achieved through the simultaneous measurement of inclusive-jet, dijet and trijet cross-sections.

Providing a theoretical perspective, Robert Thorne of University College London discussed the contribution that data from HERA have made to the understanding of electroweak physics, physics beyond the Standard Model and, in particular, QCD and the structure of the proton. With a crowded auditorium, the symposium went significantly over time because the results shown provoked much discussion that continued into the evening.

The workshop started with general talks from Elke Aschenauer of Brookhaven and Hannes Jung of DESY, both of whom highlighted the need to measure particle production – either inclusively or, even better, by tagging specific particle species – in electron–proton (ep) scattering, differential in four kinematic quantities. Such detailed measurements can be useful in model building and in tuning Monte Carlo simulations, but they can also pin down the transverse-momentum distributions of partons, which are more commonly considered in spin physics. Jung also stressed the contribution that HERA data can make to understanding the nature of multi-parton interactions, by virtue of the unique ability of being able to contrast events in which the colliding photon is either point-like or hadronic-like, thereby turning multi-parton interactions “off” and “on”, respectively, within the same experimental set-up.

Updates are needed to Monte Carlo simulations to include the more advanced models for underlying events in ep scattering, as has been done for pp interactions. Simon Plätzer of the Institute for Particle Physics Phenomenology, Durham, discussed recent advances made for the HERWIG++ event generator to include ep processes. He emphasized that the program is ready for comparison with DIS processes, even including the next-to-leading-order matrix elements. As well as a personal perspective, Achim Geiser of DESY provided an extensive list of topics yet to be covered, which anyone interested could look at to see what most excites them.

Given some tension seen between theory and the HERA inclusive data at low photon virtualities, Q2, and low Bjorken-x, as presented by Levy, several talks, including those by Joachim Bartels of Hamburg University and Amanda Cooper-Sarkar of Oxford University, discussed this region as an avenue for future work. Clearly a joint H1 and ZEUS extraction of the longitudinal structure function, FL, is needed. Also, the more precise combined data sets now available demand a phenomenological analysis in which the proton structure function, F2, is parameterized in terms of x–λ, where the dependence on –λ could reveal information on the Pomeron and the applicability of the parton-evolution schemes to describe the structure of the proton.

A highlight of the workshop was the status of next-to-next-to-leading-order (NNLO) QCD predictions of jet production at HERA, presented by Thomas Gehrmann of Zurich University. The use of such predictions will allow more precise comparisons with data and, for example, reduced uncertainties on the extractions of PDFs and the strong coupling constant. The first full predictions and comparison to data for the production of dijets in DIS will be the first NNLO final-state prediction at HERA, and is expected during 2015.

In a wide-ranging talk on diffractive processes, Marta Ruspa of the University of Piemonte Orientale highlighted the crucial questions still to be answered, which relate to the consistency and combination of the H1 and ZEUS data for measurements of inclusive diffraction in DIS. These data allow the extraction of diffractive PDFs (DPDFs) in analogy to the conventional PDFs for inclusive DIS. Using DPDFs, and because factorization should hold, predictions can be made for other processes. The experimental results on the holding of factorization are not conclusive, however, and further investigation of the HERA data would help to clarify this issue and give a better understanding of the mechanism at the LHC, as Bartels indicated.

Ronan McNulty of University College Dublin discussed the overlaps in physics from HERA and the LHCb experiment at CERN, in particular the complementary information on extraction of proton PDFs and the measurement of vector-meson production, particularly J/ψ production, and its sensitivity to the gluon distribution in the proton. Similarly, Sasha Glazov of DESY proposed ideas for common HERA–LHC analyses in the area of PDF extractions and jet physics, where HERA has particularly precise measurements.

Alessandro Bacchetta of the University of Pavia and Emanuele Nocera of the University of Genova highlighted the many pioneering measurements made by the Hermes collaboration in mapping out the helicity and 3D structure of the proton. Open issues include the strange-quark spin content of the proton, electroweak structure functions, etc. These speakers also discussed how the final Hermes analyses, together with results from experiments such as COMPASS at CERN and others at Jefferson Lab and at a future electron–ion collider, could lead to a greater understanding of the complete picture of the structure of the proton.

In a presentation that was relatively technical but very important for this workshop, Dirk Kruecker of DESY outlined the status of the long-term and safe preservation of the HERA data. To ensure that the most is made of this data legacy, the collaborations are open to people outside of the traditional institutes who are interested in analysing the data. To gain access to the data and work on a publication along with a collaboration, interested people should contact the respective spokesperson. A summary document of the workshop will be published in early 2015, and should act as a useful reference for anyone interested in future analyses with the HERA data.

A summary talk by John Dainton of the University of Liverpool provided a thought-provoking and entertaining résumé of some of the highlights of HERA physics, and how they relate to other facilities and fit into the broad context of particle physics. After two intense days, the talks and discussions gave the workshop delegates renewed vigour with which to exploit the HERA data fully during the years to come, and push back the understanding of a rich and wide variety of QCD processes, such as the nature of diffraction and the structure of the proton.


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