Periodic Reporting for period 2 - ASYMOW (Power to the LHC data: an ASYmptotically MOdel-independent measurement of the W boson mass)
Reporting period: 2023-05-01 to 2024-10-31
The ASYMOW project aims at providing a precise measurement of the mass of the W boson, one of the fundamental mediators of the electroweak force.
A precision measurement of this fundamental parameter of Nature is important because it would allow for a stringent test of the Standard Model, the theory which explains the interactions between elementary particles.
More importantly, a possible deviation between the direct measurement of the W boson mass and the value predicted by the SM would indicate the existence of new physics beyond the Standard Model, which in turn could explain some of the puzzles that riddle the current theory.
The novelty of the project lies in the experimental approach proposed to extract the value of the W boson mass.
While all measurements to date made use of selected, but necessarily "smaller", samples of collider data, this project wants to use the full data sample collected by the CMS experiment during the second run of the LHC.
To do so, a new approach, which we called theory-agnostic, is pursued. The idea behind the theory-agnostic approach is to leverage the need for a precise prediction of the W boson production mechanism, which have been so far the limiting factor in the measurement of the mass, by making use of a generic parametrization based solely on quantum-mechanical and geometrical symmetries. This generic model will be then constrained directly from the data, simultaneously with the mass of the W particle.
If all the other experimental systematics will be kept under control, this new approach can circumvent the limiting source of systematic uncertainties, paving the way towards a measurement with less than 10 MeV uncertainty, which would be a primer at the LHC and crucial to corroborate, or discard, the existing tension between theory and experiments.
A precision measurement of this fundamental parameter of Nature is important because it would allow for a stringent test of the Standard Model, the theory which explains the interactions between elementary particles.
More importantly, a possible deviation between the direct measurement of the W boson mass and the value predicted by the SM would indicate the existence of new physics beyond the Standard Model, which in turn could explain some of the puzzles that riddle the current theory.
The novelty of the project lies in the experimental approach proposed to extract the value of the W boson mass.
While all measurements to date made use of selected, but necessarily "smaller", samples of collider data, this project wants to use the full data sample collected by the CMS experiment during the second run of the LHC.
To do so, a new approach, which we called theory-agnostic, is pursued. The idea behind the theory-agnostic approach is to leverage the need for a precise prediction of the W boson production mechanism, which have been so far the limiting factor in the measurement of the mass, by making use of a generic parametrization based solely on quantum-mechanical and geometrical symmetries. This generic model will be then constrained directly from the data, simultaneously with the mass of the W particle.
If all the other experimental systematics will be kept under control, this new approach can circumvent the limiting source of systematic uncertainties, paving the way towards a measurement with less than 10 MeV uncertainty, which would be a primer at the LHC and crucial to corroborate, or discard, the existing tension between theory and experiments.
During the first reporting period, the main scientific targets foreseen by the project have been successfully attained. The recruitment of personnel has progressed as expected, leading to the creation of a well-balanced and motivated team of researchers.
Postdoctoral researchers and PhD students have been assigned scientific roles within the team and have already delivered important results. The project is now well acknowledged by the CMS Collaboration as the main driver for future W boson mass measurements. At the same time, the project team has taken a major stake in the ongoing CMS measurement of mW, albeit on a partial sample of collision data. This first measurement is expected to be published in the coming months. The research group involved in the ASYMOW project is fully integrated within this early effort, contributing with original and high-profile work and taking responsibilities of several aspects of the analysis. The main achievements obtained so-far include: i) the identification of an efficient analysis framework which we deemed suitable to sustain the full-Run2 analysis; ii) the finalization of the framework needed to measure the muon reconstruction and selection efficiencies and a thorough study of the residual systematic uncertainties thereof; iii) an important contributing role to the effort of delivering a muon momentum scale calibration in CMS to the desired level of accuracy for the 2016 data, allowing us to secure its applicability to the full Run2 data; iv) the production of MC samples needed to extend the measurement to the full-Run2 data and a detailed study of the residual theoretical uncertainties from state-of-the-art codes; v) the validation of reconstruction performances in data and simulatation for the 2017 and 2018 data-taking periods, including the derivation of the most relevant data-to-simulation correction factors; vi) a detailed statistical study of the theory-agnostic method leading to the definition of a new analytic model which superseeds the originally proposed one; vii) a direct involvement of the group in the data-taking, data-quality validation, and performance studies in the early Run3 of the LHC in sight of a possible inclusion of Run3 data.
Postdoctoral researchers and PhD students have been assigned scientific roles within the team and have already delivered important results. The project is now well acknowledged by the CMS Collaboration as the main driver for future W boson mass measurements. At the same time, the project team has taken a major stake in the ongoing CMS measurement of mW, albeit on a partial sample of collision data. This first measurement is expected to be published in the coming months. The research group involved in the ASYMOW project is fully integrated within this early effort, contributing with original and high-profile work and taking responsibilities of several aspects of the analysis. The main achievements obtained so-far include: i) the identification of an efficient analysis framework which we deemed suitable to sustain the full-Run2 analysis; ii) the finalization of the framework needed to measure the muon reconstruction and selection efficiencies and a thorough study of the residual systematic uncertainties thereof; iii) an important contributing role to the effort of delivering a muon momentum scale calibration in CMS to the desired level of accuracy for the 2016 data, allowing us to secure its applicability to the full Run2 data; iv) the production of MC samples needed to extend the measurement to the full-Run2 data and a detailed study of the residual theoretical uncertainties from state-of-the-art codes; v) the validation of reconstruction performances in data and simulatation for the 2017 and 2018 data-taking periods, including the derivation of the most relevant data-to-simulation correction factors; vi) a detailed statistical study of the theory-agnostic method leading to the definition of a new analytic model which superseeds the originally proposed one; vii) a direct involvement of the group in the data-taking, data-quality validation, and performance studies in the early Run3 of the LHC in sight of a possible inclusion of Run3 data.
The calibration of the muon momentum scale on 2016 data has reached an outstanding level of precision, definitely beyond the state of the art. This gives confidence that similar performances can be achieved on the full-Run2 data sample.
Also, the production of sufficiently large samples of MC-simulated events, which is mandatory for the success of the full Run2 analysis, has been recently completed, mostly thanks to the committment of the team.
Also, the production of sufficiently large samples of MC-simulated events, which is mandatory for the success of the full Run2 analysis, has been recently completed, mostly thanks to the committment of the team.