Highlights of SIGHAD03

Can data on hadronic cross-sections from e⁺e^– annihilations and τ decays be reconciled? This was one of the main topics discussed at the Workshop on Hadronic Cross Section at Low Energy – SIGHAD03 – which was held in Pisa, Italy, on 8-10 October 2003, and attended by about 60 participants from a variety of countries.

Despite its well known successes, the Standard Model still has a number of weaknesses, one of them being the prediction of the anomalous magnetic moment of the muon, a_µ. Participants at SIGHAD03 heard the whole story from the main protagonists: from the pioneering experimental work carried out more than 40 years ago, which was nicely summarized by Francis Farley of Yale, to the impressive accuracy reached in recent years on both the experimental and theoretical sides.

The main issue, however, still concerns the evaluation of hadronic contributions to a_µ below 2 GeV. These cannot be calculated by perturbative quantum chromodynamics, and so rely almost entirely on data. In the 1990s data from hadronic decays of the τ (from the ALEPH experiment) were used to add information to that obtained directly from electron-positron annihilations. This method – which was pioneered by Michel Davier of Orsay and presented by him at the workshop – allowed a substantial improvement in the theoretical evaluation of a_µ. In the meantime, the CMD-2 experiment at the VEPP-2M collider complex in Novosibirsk was able to improve the measurement of the two-pion annihilation cross-section (leading to a 0.6% systematic error), and both the OPAL and CLEO experiments came up with independent measurements of the τ spectral function.

These improvements led to a comparison between the π⁺π^– spectral functions from e⁺e^– and τ data. However, after including isospin-violating effects, there is still a discrepancy of the order of 10 to 15% above the ρ resonance (see figure 1). The origin of this discrepancy, and more generally the results of the evaluation of a_µ using different approaches, was the central theme of one of the sessions at the workshop. Whether it is due to a missing correction in theory (the difference in mass and width between the charged and neutral ρ mesons, as discussed by Davier and Fred Jegerlehner of DESY) or whether it lies in the data, is still controversial.

SIGHAD03 also heard about the status of and the prospects for existing and planned colliders. Within a few years, upgrades of electron-positron colliders in Beijing (BEPCII/BESIII), Novosibirsk (VEPP-2000), Frascati (DAFNE-2) and Cornell (CLEO-C) should become operational and therefore provide new data. Results on τ spectra from B-factories are also expected; indeed, the first results from the Belle collaboration at KEK-B in Japan were presented at the workshop.

An improvement in the current situation will soon come from existing meson factories, with the KLOE, BaBar and Belle experiments. Here the use of the initial state radiation process (ISR), as recently proposed by the group of Johann Kühn in Karlsruhe, allows the whole available energy range to be scanned while working at a fixed energy. In particular, the KLOE collaboration at the DAFNE φ-factory in Frascati presented results on the hadronic cross-section below the φ peak, which agree with the Novosibirsk e⁺e^– data and thus confirm the 2σ discrepancy with the τ approach. Preliminary data were also presented by the BaBar collaboration (at SLAC), showing the feasibility of the ISR method at B-factories, where there is the advantage that a much wider energy range can be covered. Recent theoretical developments on ISR were also reviewed, and during the workshop a round table was organized in order to discuss the status of radiative corrections for luminosity measurements in an attempt to provide a unified picture of the current situation.

Precise measurements of R, the ratio of hadronic to muon-pair cross-sections in e⁺e^– , at low energy have a strong influence not only on the anomalous magnetic moment of the muon but also on the running electromagnetic coupling constant, whose uncertainty limits the prediction of the Higgs mass. The measurements also provide a test of the perturbative behaviour of the strong interaction and of low-energy effective field theories. The running of the electromagnetic and strong coupling constants, and the determination of charm and bottom masses, are two examples that were reviewed at the workshop, and whose progress has benefited from the latest experimental results from the electron-positron colliders in Beijing (BES) and Novosibirsk.

In summary, this was a short but very intensive workshop. However, there were also two moments of relaxation, with a visit to the Piazza dei Miracoli, where the leaning tower is located, and a delicious dinner in the lovely ancient Villa Toscana. During the dinner, Simon Eidelman proposed organizing the next workshop in Novosibirsk two years from now. By then, new theoretical and experimental results, expected in particular from the g-2 experiment at Brookhaven, as anticipated by Lee Roberts will clarify whether the discrepancy observed in a_µ will vanish, or whether it will remain, so requiring new physics.