A small workshop held recently in Switzerland considered the challenges to be overcome before accurate cross-section measurements can be made at the LHC.
For three days in October the population of Binn, a beautiful village in the Oberwallis in Switzerland, increased by almost 20% when 23 experimental and theoretical particle physicists attended a workshop on cross-section measurements at the Large Hadron Collider (LHC).
The main purpose of this small workshop, organized by Günther Dissertori and Michael Dittmar of the CMS group at ETH Zürich, was to investigate how well the different types of physics reactions and reaction ratios expected at the LHC can be measured and calculated. About half the time at the workshop was devoted to thought-provoking review talks, while the rest remained free for questions, discussion and critical comments.
The workshop began with an introduction to general aspects and problems of cross-section measurements at the LHC. The participants were reminded that in addition to the experimental uncertainties from efficiency, backgrounds and the machine luminosity, there are potentially important theoretical uncertainties in the calculations, such as those arising from uncertainties in the parton distribution functions (PDFs) and from unknown higher-order corrections. It was also pointed out that normalizing various interesting high-Q2 reactions to the well-understood and abundant production of W and Z bosons at the LHC could dramatically reduce systematic errors. This might particularly help with errors arising from the absolute-luminosity measurement and from PDF uncertainties. For some reactions the estimated theoretical and experimental uncertainties are of similar size, and an especially large effort will be needed to understand and, if possible, reduce the uncertainties.
A look ahead
The next two sessions were devoted to the theoretical and experimental aspects of cross-section calculations and their potential limitations at the LHC. While today it is obviously not possible to know exactly how the ATLAS and CMS general-purpose detectors will perform once the LHC starts, it is already clear that many important measurements and calculations will be much more difficult than corresponding ones at previous high-energy colliders, such as the Large Electron_Positron Collider (LEP). Several talks therefore addressed the question of systematic limitations at previous high-energy collider experiments, particularly for reactions that are relevant to the LHC. For example, there was general agreement that absolute measurements of αs with a precision comparable to that achieved at LEP will be impossible at the LHC.
Since all the high-Q2 reactions of interest at the LHC result from collisions of the quark and gluon constituents of the proton beams, the accuracy of a cross-section calculation relies on a precise knowledge of the parton distribution functions – that is, the quantities that describe how the momentum of a fast-moving proton is shared among its constituents. There were presentations on the latest PDF results from the H1 and ZEUS experiments at HERA, which clearly demonstrated the impressive precision that has already been achieved. The precise information on the quark and gluon PDFs that can be extracted from the HERA structure function data leads in turn to very accurate (±1-2%) predictions for the W and Z production cross-sections, and their ratios, at the LHC.
Even so, it is possible that measurements at the LHC could constrain the PDFs even further. For example, the ratio of W+/W– production at the LHC is very sensitive to the u/d quark PDF ratio in the proton. Using the most up-to-date PDFs yields an uncertainty of about 1.5% on the prediction for the W+/W– ratio. A measurement error smaller than this at the LHC will further constrain the u/d ratio. In principle a precision measurement of the kinematic distribution of charged leptons from Z decay could yield information on the weak mixing angle sin2θW, but it is likely that the residual PDF uncertainties will in practice preclude any improvement in the accuracy of sin2θW achieved so far at LEP and the SLAC Linear Collider.
Another session focused on the status of higher-order perturbative corrections in quantum chromodynamics (QCD). Although there has been steady progress in the calculation of these corrections in recent years, with some cross-sections now known to next-to-next-to leading order (NNLO) accuracy, there are still some important background processes that are known only to leading order (for example, tbart and bbarb production in QCD). Without at least the full next-to leading order (NLO) corrections to such processes, it is very difficult to estimate the uncertainty in the prediction, and this can have important implications for the unambiguous identification of new physics signals at the LHC.
An important development in the past year has been the implementation of exact NLO contributions in parton-shower Monte Carlo event generators. In particular, the MC@NLO program, which is based on the HERWIG Monte Carlo event generator, already includes many of the most important processes. However, the experimenters were quick to come up with a long wish-list of additional processes. It was not difficult to agree on a common list of “most wanted” QCD corrections, but the theorists were adamant that without a breakthrough in calculational techniques, the existing theoretical technology will not be able to cope with providing many of the required NLO or NNLO corrections before the LHC starts.
LHC processes in detail
During the second day of the workshop, four sessions were devoted to more detailed presentations and discussions of particular LHC processes. These were the production of Drell-Yan lepton pairs, the production of events with massive boson pairs and subsequent leptonic decays, and the measurement of jet-production processes that arise from quark-quark, quark-gluon and gluon-gluon scattering. During the second half of the afternoon the participants split up into more specialized working groups to focus on PDFs, high-mass Drell-Yan lepton pairs, the physics of jets and the systematics of potential backgrounds for exotic processes. The final Saturday evening session addressed the question of systematic uncertainties in background processes, which could limit the LHC’s potential to make discoveries.
Using a few examples from ATLAS and CMS search simulations for (SUSY) Higgs particles and other exotics, some particularly problematic signatures could be identified and analysed. Unfortunately it became obvious to most participants that the hypothetical large statistical significance of some of these exotic particles is not the full story, as in reality these signatures will be completely hidden in the uncertainties of the backgrounds.
While it is obviously difficult to quantify fully the limits of each individual signature, it is possible to conclude that to achieve signal-to-background ratios below, for example, 0.5 (0.25) – excluding the few exotic signatures that provide narrow mass peaks – the backgrounds need to be controlled to better than 10% (5%). Only in rare cases can such precision be obtained from Monte Carlo simulations, therefore data-based background estimates will almost certainly be required. The Saturday evening session concluded with short reports from the working groups about outstanding problems that should be addressed in detail during the coming years.
At the final summary session on Sunday morning, there was unanimous agreement that the meeting had been a great success, particularly because it enabled participants to step outside the routine of daily work and to think in depth about some of the most important issues facing experimenters and theorists in preparing for the start of the LHC. The success of the meeting could also be judged by a comment from one participant during an additional “special session” on Sunday afternoon: “This was the first conference where I not only participated in all the sessions but even listened to all the talks.” Falling asleep would in any case have been difficult during this last “session”: a four-hour hike to the Mässersee and back!