Periodic Reporting for period 5 - ONEDEGGAM (The search for new physics through precision measurements of the CKM angle gamma)
Periodo di rendicontazione: 2024-06-01 al 2025-05-31
One of the most compelling mysteries surrounding the Standard Model is that our existence itself is a clear demonstration of the failings of the theory. We live in an asymmetric universe where matter has a profound dominance over anti-matter (everything is made of matter), although this is not observed with anti-matter or matter created in the laboratory where they have almost uniform properties. The Standard Model has been confirmed to incredible precision in a number of different tests. However these clear discrepancies, between the universe it would allow, and the one that does in fact exist, leads to the conviction that this theory is only a low energy approximation to a higher energy theory. Nearly all of these theories predict the existence of further families of fundamental particles that are heavy with respect to the known particles.
Central to nearly all particle physics research is the desire to find or characterize this new physics. The approach I will take within this project is to observe this new physics through the study of CP violation, that is, the difference in behaviour of matter and anti-matter. The lack of any direct observation of new physics particles so far at the higher collision energy of the LHC Run-2 suggests that perhaps new physics is at a mass scale beyond the reach of the LHC. In that case, the need for indirect searches for new physics through CP violation only intensifies. The aim of this project is a measurement of the Cabibbo-Maskawa-Kobayashi (CKM) angle γ with a precision of 1°. Increasing the precision on the least well known angle, γ, by almost an order of magnitude could for the first time, provide laboratory evidence of physics beyond the Standard Model.
Direct measurements of γ are made using decays such as B^∓→DK^∓ (other similar decays can be used too), where the charmed meson (D) is a superposition of the D^0 and (D^0 ) ̅. In the next few years a substantial number of these decays will be collected by the LHCb experiment. It is the interference of the decay paths B^∓→D^0 K^∓ and B^∓→(D^0 ) ̅K^∓ that gives sensitivity to the phase γ. Interference between two decay paths only occurs when the initial and final states are identical and hence it is required that both the D^0 and (D^0 ) ̅ decay to the final state. Hence to determine γ, information is required not only about the B decay, but the D decay too. The large samples of B decays will provide good information on the B decay but can only contribute weakly to the information on the D decay. The relevant D decay information is related to the difference in amplitudes and phases between the D^0 and (D^0 ) ̅ decay and is referred to as the charm strong-phase parameters.
It will not be possible to make full use of future datasets at either LHCb or Belle II if there is not a significant expansion and reoptimisation of the measurements of the charm strong-phase related parameters. The only experiment at which these charm strong-phases can be improved is BESIII. With a dedicated and coordinated effort, that brings the power and information contained within the BESIII and LHCb data sets together, I will make the most precise measurement of γ.
The project ushered in a new era of precision when in comes to charm strong-phase related parameters and the measurement of γ, with many decay channels fully exploited. At the end, 21 publications arise from this project with a further 3 in peer review. New, robust, techniques that will continue to be used as the research continues beyond this project have been pioneered through this project. Delays to operating schedules meant that there was insufficient data to reach the 1° target, but the project closed with a precision of 2.5°, with the expectation that the target precision will soon be reached.
The central value of γ turned out to be consistent with the Standard Model – therefore the mysteries of the universe only continue to deepen.
Central to nearly all particle physics research is the desire to find or characterize this new physics. The approach I will take within this project is to observe this new physics through the study of CP violation, that is, the difference in behaviour of matter and anti-matter. The lack of any direct observation of new physics particles so far at the higher collision energy of the LHC Run-2 suggests that perhaps new physics is at a mass scale beyond the reach of the LHC. In that case, the need for indirect searches for new physics through CP violation only intensifies. The aim of this project is a measurement of the Cabibbo-Maskawa-Kobayashi (CKM) angle γ with a precision of 1°. Increasing the precision on the least well known angle, γ, by almost an order of magnitude could for the first time, provide laboratory evidence of physics beyond the Standard Model.
Direct measurements of γ are made using decays such as B^∓→DK^∓ (other similar decays can be used too), where the charmed meson (D) is a superposition of the D^0 and (D^0 ) ̅. In the next few years a substantial number of these decays will be collected by the LHCb experiment. It is the interference of the decay paths B^∓→D^0 K^∓ and B^∓→(D^0 ) ̅K^∓ that gives sensitivity to the phase γ. Interference between two decay paths only occurs when the initial and final states are identical and hence it is required that both the D^0 and (D^0 ) ̅ decay to the final state. Hence to determine γ, information is required not only about the B decay, but the D decay too. The large samples of B decays will provide good information on the B decay but can only contribute weakly to the information on the D decay. The relevant D decay information is related to the difference in amplitudes and phases between the D^0 and (D^0 ) ̅ decay and is referred to as the charm strong-phase parameters.
It will not be possible to make full use of future datasets at either LHCb or Belle II if there is not a significant expansion and reoptimisation of the measurements of the charm strong-phase related parameters. The only experiment at which these charm strong-phases can be improved is BESIII. With a dedicated and coordinated effort, that brings the power and information contained within the BESIII and LHCb data sets together, I will make the most precise measurement of γ.
The project ushered in a new era of precision when in comes to charm strong-phase related parameters and the measurement of γ, with many decay channels fully exploited. At the end, 21 publications arise from this project with a further 3 in peer review. New, robust, techniques that will continue to be used as the research continues beyond this project have been pioneered through this project. Delays to operating schedules meant that there was insufficient data to reach the 1° target, but the project closed with a precision of 2.5°, with the expectation that the target precision will soon be reached.
The central value of γ turned out to be consistent with the Standard Model – therefore the mysteries of the universe only continue to deepen.
Through out the project there has been data analysis on two fronts. Data from LHCb in a variety of B decay channels has been analysed to look for CP violation. In order to interpret this it is necessary to determine strong-phase parameters from the BESIII data. Throughout the project new state-of-the-art techniques have been developed and then applied to subsequent measurements. Two particularly ground-breaking developments have been the new measurement strategy that combines the control channel with higher-order corrections, and the investigation of e+e- collision data above the charm threshold. A large amount of work was also devoted to the preparation and delivery of the new large LHCb dataset, with particular attention on the alignment and calibration required for high quality data.
In combination, the world average now stands at 65.7 \pm 2.5 degrees which is a remarkable achievement given the lack of the large datasets that had initially been envisaged by the project proposal. Work has commenced on the larger datasets but was not completed by the end of the project. It will continue, albeit more slowly and the target precision will be reached.
The results have been disseminated primarily through publication in physics journals with high impact factors. There are 21 publication peer-reviewed and published to date and 3 more are in peer review. There will be further publications as the work unfinished during the project period comes to conclusion. The results have also been presented at numerous conferences in the field. 15 of these were attended by either the PI or team members over the course of the project. The work from the project, and the progress it has made in the area of understanding the Standard Model was highlighted in the keynote talk at the leading conference in the field in the summer of 2024.
In combination, the world average now stands at 65.7 \pm 2.5 degrees which is a remarkable achievement given the lack of the large datasets that had initially been envisaged by the project proposal. Work has commenced on the larger datasets but was not completed by the end of the project. It will continue, albeit more slowly and the target precision will be reached.
The results have been disseminated primarily through publication in physics journals with high impact factors. There are 21 publication peer-reviewed and published to date and 3 more are in peer review. There will be further publications as the work unfinished during the project period comes to conclusion. The results have also been presented at numerous conferences in the field. 15 of these were attended by either the PI or team members over the course of the project. The work from the project, and the progress it has made in the area of understanding the Standard Model was highlighted in the keynote talk at the leading conference in the field in the summer of 2024.
The results from the phenomenological work bring new understanding to the biases and impact the higher order corrections can have on gamma measurements. A by-product of this is that it is now possible to use the B-->Dpi decay mode as signal and control simultaneously. This strategy has then been deployed in the latest gamma measurement and removed the dominant systematic experimental uncertainty. This strategy was further developed for use in other similar decay channels to large success. Over the course of the project this has now become embedded as the best way to perform gamma measurements and the only way to ensure small systematic uncertainties which is crucial in precision measurements.
The strong-phase measurements that are completed are world leading and have significant impact on future gamma measurements. They have new partial reconstruction techniques and new tag decay modes that have not been utilised previously. They also exploit the C-even quantum-correlated data for the first time. This opens an entirely new field of study.
All together this has led to the world's most precise measurement of gamma. A precision of ~2.5 degrees has been obtained with the 1 degree target firmly in sight once the analysis from larger datasets from LHCb and BESIII is complete. The figure shows the improvement from 2016-->2024 which are largely the result of this project. The final results from the project are not included as the combination is only run periodically, and requires permission of the LHCb collaboration to be externally shown. The central value of gamma is around 65 degrees which places it firmly in line with the Standard Model.
The strong-phase measurements that are completed are world leading and have significant impact on future gamma measurements. They have new partial reconstruction techniques and new tag decay modes that have not been utilised previously. They also exploit the C-even quantum-correlated data for the first time. This opens an entirely new field of study.
All together this has led to the world's most precise measurement of gamma. A precision of ~2.5 degrees has been obtained with the 1 degree target firmly in sight once the analysis from larger datasets from LHCb and BESIII is complete. The figure shows the improvement from 2016-->2024 which are largely the result of this project. The final results from the project are not included as the combination is only run periodically, and requires permission of the LHCb collaboration to be externally shown. The central value of gamma is around 65 degrees which places it firmly in line with the Standard Model.