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UMWA Report Summary

Project ID: 280126
Funded under: FP7-IDEAS-ERC
Country: France

Final Report Summary - UMWA (Ultimate measurement of the W boson mass with ATLAS, at the LHC)

The W electroweak gauge boson was discovered in 1983 at the CERN SPS collider and its properties have been studied since more than 30 years. In the Standard Model (SM) of particle physics, the W boson mass is in particular sensitive to the top-quark and Higgs-boson masses, and measurements of this parameter test the consistency of the theory : a deviation of the measured value from the SM prediction would be a sign of new physics. The W boson mass was measured previously in e+e- and proton-anti-proton collisions, providing a combined world average of 80385 ± 15 MeV, driven by the TeVatron results, and consistent with the SM prediction 80358 ± 8 MeV.
The measurement of the W mass is particularly challenging at the LHC, due to the large number of interactions per beam crossing and to the significant contributions of second-generation quarks to W production. The W mass was measured by ATLAS using W-boson decays into an electron or muon and a neutrino, by reconstructing the kinematic distributions of the decay leptons in the final state.
The measurement requires an accurate calibration of the detector response. Using the precisely measured value of the Z boson mass at LEP, the large sample of Z-boson events at the LHC allowed measuring the electron-energy and muon-momentum scales with a relative accuracy of 10-4. The reconstruction of the transverse momentum of the neutrino relies on the precise measurement of the hadronic activity recoiling against the W boson. The recoil response was calibrated with Z-boson events by exploiting the expected momentum balance between the recoil and the Z-decay products.
An accurate description of W-boson production and decay is crucial for the measurement. The enhanced amount of heavy-quark-initiated production, and the ratio of valence and sea quarks in the proton affect the W-boson transverse momentum distribution and its polarisation. The measurement is thus particularly sensitive to the parton distribution functions (PDFs) of the proton. To address these issues, ATLAS developed innovative techniques, which combine the most advanced theoretical predictions with experimental constraints from precise measurements of Z- and W-boson differential cross sections and of Z-transverse momentum and polarisation.
The ATLAS collaboration reports the first measurement of the W boson mass using LHC pp collisions data at a centre-of-mass energy of 7 TeV, and corresponding to an integrated luminosity of 4.6 fb-1. The measured value is 80370 ± 19 MeV, consistent with the SM prediction and with the world average.
This result constitutes the most precise individual measurement, with a precision comparable to that of the CDF experiment.
Modelling uncertainties, which dominate the overall uncertainty on the present result, need to be reduced in order to fully exploit the larger data samples available at centre-of-mass energies of 8 and 13 TeV. A better knowledge of the PDFs, as achievable using recent precise measurements of W- and Z-boson rapidity cross sections, and improved QCD and electroweak predictions for Drell-Yan production, are therefore crucial for future measurements of the W -boson mass at the LHC.


Nathalie JUDAS, (Financial assistant to the head of IRFU)
Tel.: +33 1 69 08 77 88
Fax: +33 1 69 08 74 01
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