Electron - Positron Physics


At record breaking centre-of-mass energies in excess of 207 GeV, the data analysis of the Helsinki group probes still unexplored domains of masses for new physics, among them the theoretically favoured region for the Higgs boson. The group's final analysis of the Z0 data and its contribution to the DELPHI charged Higgs analysis directly contribute to the present limits of the validity of the Standard Model.

Furthermore the group has been an active participant in physics feasibility studies, detector design and R&D's aimed at future linear e+e- colliders (TESLA, CLIC).

Since the year 2000 marked the end of operation of the CERN LEP collider, the emphasis during 2001 has been to ensurethat the LEP2 data taken between years 1996 and 2000 with the DELPHI experiment are reconstructed with the most optimal reconstruction algorithms and detector calibrations. This to obtain the most from the collected data and especially to answer the very entangling question whether a light Higgs boson, which manifests the electro-weak symmetry breaking, exists or not? Very preliminary results at the end of year 2000 showed an excess of signal events as expected from a Standard Model Higgs boson with a mass of about 115 GeV. New preliminary results in Summer 2001 confirmed this excess however with a slightly lower significance. Final LEP results on the Higgs search are expected in 2002. The sensitivity of the combined LEP data is not enough to claim a discovery so the final answer has to wait for sufficient data from either the LHC or the Tevatron.

In the field of heavy flavour physics, the group is pursuing an inclusive measurement of the lepton momentum spectra in semileptonic B decays based on the large set of Z0 decays collected by DELPHI during 1991-95 using recently achieved significant improvements of the B hadron reconstruction algorithms at LEP. Such an inclusive measurement is very difficult to do at the B-factories despite the larger statistics of B hadrons decays available and hence the DELPHI measurement provides complementary information on for example the extraction of the expectation value of the b quark kinetic energy operator inside the B hadron.

The group has actively studied QCD coherence phenomena and developed a novel method of reconstructing colour dipoles and partons in hard scattering final states. A publication, demonstrating for the first time the existence of the dead cone effect for gluon radiation in heavy quark final states, is in preparation.

The group has also looked for pair-produced charged Higgs bosons, predicted by several extensions of the Standard Model, in the high energy LEP2 data. The group has had a coordinating role in the overall DELPHI activity on charged Higgs bosons. The most recent results exclude mass values almost up to the W mass. The search effectively applies the algorithms for event colour structure reconstruction as well as jet flavour tagging that both have been developed inside the group. The analysis of the 1999 data has been completed and published in 2001 and a publication on the 2000 data is in preparation including the final LEP charged Higgs results.

With the LEP analysis being finalized, the focus of high energy e+e- physics is moving towards a high luminosity linear collider, which will cover the energy range from the Z0 pole up to about 800 GeV. Based on the experience gained from silicon vertex detector and Higgs analysis contributions to DELPHI, the group has concentrated on defining the precision and discovery potential of the Higgs sector at such a collider. Precise investigation of the Higgs sector, beyond the discovery potential of the LHC or the Tevatron, are required for unambiguous identification of the mechanism responsible for the electro-weak symmetry breaking, which could be achieved at such a collider with the anticipated detector response.

The group has also completed a conceptual design of a vertex tracker based on a novel type of silicon pixel detectors with capacitive charge sharing. The working principle of these novel detectors has been proved within an R&D effort made in collaboration with the Milano, Krakow and Warsaw Universities. The group has also studied in detail the full reconstruction of the decays of a pair of charged Higgs bosons at such a collider. All these studies were performed in the framework of a series of European Committee for Future Accelerators (ECFA) and DESY sponsored workshops and the results have been included in the Technical Design Report for the TESLA project, published in 2001. Furthermore, the group has been involved in feasibility studies for a few TeV linear collider at CERN (CLIC).