HERA and LHC workshops help prepare for the future

After the major luminosity upgrade of DESY’s electron-proton collider HERA in 2001, experiments at the accelerator are now producing data for Run II, which will last until the end of HERA operation in 2007. The results obtained by the two collider experiments H1 and ZEUS will have a profound impact on the physics to be explored at CERN’s Large Hadron Collider (LHC). Since March 2004, members of the communities working at HERA and preparing for the LHC have been meeting regularly at CERN and DESY in a series of workshops intended to promote co-operation between the two communities. The aim of the six “HERA and the LHC” workshops, the final meeting of which was held at DESY during the week before Easter 2005, was to investigate the exact implication of HERA results on the physics at the LHC.

The goals of the series of workshops, which had more than 200 registered participants, were as follows:
• to identify and prioritize those measurements to be made at HERA that have an impact on the physics reach of the LHC;
• to encourage and stimulate the transfer of knowledge between both communities and establish an ongoing interaction;
• to encourage and stimulate theoretical and phenomenological efforts;
• to examine and improve theoretical and experimental tools;
• to increase the quantitative understanding of the implication of HERA measurements on LHC physics.

At the final meeting of the series, the speakers summarized the results and presented the conclusions from studies and discussions carried out during the past year by working groups on parton density functions, multijet final states and energy flows, heavy quarks, diffraction and Monte Carlo tools. In general it was made very clear that there is a strong interest from the LHC physics community in detailed studies at HERA. Several general talks on physics at the LHC and HERA outlined the importance of the results obtained at HERA, with special emphasis on the measurements that have still to be done and that will have a significant impact on the physics reach of the LHC. “Clearly, to calculate properly the production rates of Higgs and supersymmetry we absolutely need to understand quantum chromodynamics [QCD] as well as possible,” said John Ellis from CERN. It also became evident that much more theoretical, phenomenological and experimental investigations would be desirable, and to this end several projects were launched during the workshop.

Speakers repeatedly stressed the importance for LHC physics of precise measurements of the parton densities, i.e. the densities of the various types of quarks and the gluons within the proton. In particular, the whole issue of parton density functions (PDFs), from the standard integrated ones to unintegrated and generalized PDFs and eventually to diffractive PDFs, is a rich field for theoretical and experimental studies. These include not only a precise experimental determination of the PDFs, but also address the more fundamental question of the universality of the PDFs and in particular whether those obtained at HERA can be applied to the LHC without further modification beyond evolution effects in QCD.

In the multijet working group, one of the main topics was the issue of multiple scatterings and underlying events. The understanding of these effects has an impact on, for example, the Higgs cross-section measurements in the boson-fusion channel at the LHC. A major step towards a deeper understanding of multiple scatterings is their relation to diffractive scattering: they are simply different facets of the large density of partons at high energies. The dynamics of these high-density systems require extensions of the concept of parton densities from transverse-momentum dependent (unintegrated) to generalized and diffractive parton densities, which can be measured precisely at HERA. These parton densities will be essential for analysing diffractive Higgs production at the LHC, a very clean and promising channel. However, to study this process and also problems of parton dynamics at low x that are still unsolved, the forward region of the LHC detectors needs further instrumentation. This is a task for which the experiments at HERA have accumulated both technical and physics experience over recent years.

Heavy quark production at the LHC is also interesting in terms of QCD. The densities of heavy quarks will play an important role at the LHC, for example in Higgs production channels, and they will be accurately measured at HERA in the high-luminosity programme. In the forward production of heavy quarks, as will be the case in the LHCb experiment, effects coming from high parton densities and the saturation of the cross-section might be observed directly.

All of these studies require adequate tools and simulation programs. The working groups made measurements from HERA, the Tevatron at Fermilab and the SPS at CERN available in the form of easy-to-use computer codes. These will be useful for any tuning of Monte Carlo generators. New concepts were also investigated and user-friendly interfaces to simulation programs were developed.

A unique machine

During the year of the workshops, co-operation between experimenters at HERA and the LHC and the interest from the theoretical and phenomenological side have continuously increased. It has become clear that not only can the LHC profit from HERA (i.e. with exact measurements of parton densities), but also that HERA will profit from investigations carried out for the LHC, such as the application of next-to-leading (NLO) calculations in Monte Carlo event generators (MC@NLO).

HERA is a unique machine; it is the only high-energy electron-proton collider in the world. During the workshop meetings, it became obvious that for many topics, it is the only place today where many of the necessary measurements and studies can be performed. HERA is a machine for precision QCD measurements, just as the Large Electron-Positron collider was for the electroweak sector, with the difference that QCD is richer but also more difficult. Many questions are still unanswered, for example those concerned with the understanding of diffraction and issues in parton evolution with all its consequences for the LHC.

The workshops have critically assessed the physics programme of HERA and made suggestions for further measurements and investigations, in particular those that will be important for the physics reach of the LHC and that cannot be performed anywhere other than at HERA. One example is the precise measurement of the gluon density using the longitudinal structure function F_L, which is important for clarifying uncertainties in the present knowledge of the gluon density and the formulation of QCD at high parton densities.

In view of the prospects for further progress emerging from the high-statistics HERA Run II data, a continuation of the workshop series is now planned on an annual basis. The next meeting will be held at CERN in March 2006.