SPEAR SIGNED

Project "SPEAR" data sheet

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 Project objective

The Standard Model (SM) is widely accepted to be an approximation of a more complete theory of nature, but laboratory tests have failed to identify a conclusive deviation from its precise predictions. In the SM two of the four fundamental forces of nature, namely the electromagnetic and weak nuclear forces, are governed by three fundamental parameters. These parameters are precisely fixed by experimental determinations of the Z boson mass, the fine structure constant, and the Fermi constant. Other parameters are subsequently predictable and can therefore be confronted with experimental data, potentially exposing physics effects beyond the SM. Of notable importance are the W boson mass and the weak-mixing angle since these two parameters are not yet measured as precisely as they are predicted. A delicate feature of the SM, which could easily be perturbed by new physics, is the universality between the coupling strengths of the three known lepton generations to the gauge bosons. There is an intriguing set of beauty hadron measurements which suggest a violation of lepton universality, with a particularly large effect in the third generation. While inconclusive at this stage these results intensify the demand for a test of a potentially related anomaly in the partial decay width of the W boson to third generation leptons. Qualitatively new ideas are needed to tackle these scientific problems. LHCb is the first experiment of its kind, as a small-angle spectrometer detector at a hadron collider, and the SPEAR project is well timed to analyse existing data from LHCb and the full dataset from the first running period with the LHCb upgrade. The SPEAR project sets ambitious goals of (i) measuring the W mass for the first time with a small-angle spectrometer (ii), making the first weak mixing angle determination at a hadron collider that matches the precision of electron-positron colliders, and (iii) resolving the long-standing W boson lepton universality puzzle.