Periodic Reporting for period 4 - ONEDEGGAM (The search for new physics through precision measurements of the CKM angle gamma)
Okres sprawozdawczy: 2022-12-01 do 2024-05-31
One of the most compelling mysteries surrounding our modern theories of particles and their interactions (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. This is a likely place in which to observe new physics since it is an area where the Standard Model description fails to describe the world around us. 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. If there is evidence for new physics, the way in which new physics alters flavor physics observables from their Standard Model values will provide great insight on the couplings of these new particles and allow for better understanding of their nature and properties.
The aim of this project is a measurement of the Cabibbo-Maskawa-Kobayashi (CKM) angle γ with a precision of 1°. The quark couplings are described by the CKM matrix, which provides the weak force coupling strength between the quarks. Within the Standard Model, this matrix is 3x3 and unitary, which means that the quark couplings can be represented as a triangle, with angles α, β, and γ. If measurements of the sides and angles of this triangle can show that it is, in fact, not a triangle then this is a clear demonstration of the existence of new physics. Experimentally this can be achieved through over-constraining the triangle. Increasing the precision on the least well known angle, γ, by almost an order of magnitude to 1° will allow for much tighter constraints and could be sufficient to provide evidence for discrepancies. This would, 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 year 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 need to observe new physics as soon as possible cannot be overstated. Any evidence or understanding of new physics that can be achieved within the next few years will shape and possibly transform the next generation of particle physics experiments, which are currently in the conceptual design phase. The observation of new physics would lead to a significant change in the goals of the field, both experimental and theoretical, as the focus will shift from finding evidence for new physics to understanding its features and couplings to the Standard Model 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. This is a likely place in which to observe new physics since it is an area where the Standard Model description fails to describe the world around us. 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. If there is evidence for new physics, the way in which new physics alters flavor physics observables from their Standard Model values will provide great insight on the couplings of these new particles and allow for better understanding of their nature and properties.
The aim of this project is a measurement of the Cabibbo-Maskawa-Kobayashi (CKM) angle γ with a precision of 1°. The quark couplings are described by the CKM matrix, which provides the weak force coupling strength between the quarks. Within the Standard Model, this matrix is 3x3 and unitary, which means that the quark couplings can be represented as a triangle, with angles α, β, and γ. If measurements of the sides and angles of this triangle can show that it is, in fact, not a triangle then this is a clear demonstration of the existence of new physics. Experimentally this can be achieved through over-constraining the triangle. Increasing the precision on the least well known angle, γ, by almost an order of magnitude to 1° will allow for much tighter constraints and could be sufficient to provide evidence for discrepancies. This would, 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 year 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 need to observe new physics as soon as possible cannot be overstated. Any evidence or understanding of new physics that can be achieved within the next few years will shape and possibly transform the next generation of particle physics experiments, which are currently in the conceptual design phase. The observation of new physics would lead to a significant change in the goals of the field, both experimental and theoretical, as the focus will shift from finding evidence for new physics to understanding its features and couplings to the Standard Model particles.
The work performed so far falls in 4 categories:
1. Phenomenological work on the higher order corrections from physics effects due to neutral K mesons on measurements of gamma.
This work is completed and published in JHEP. The results were very interesting. They show that the previous work on this area had overlooked an important correction which leads to these effects not being dominant for B-->Dpi decays. This new knowledge is effectively revolutionary as it allows for the B-->Dpi decay channel to be exploited simultaneously with the B-->DK. This means that all parameters except for the strong phases can be determined from the data itself. This is a major reduction in uncertainty
2. Work has proceeded as planned on the measurement of strong-phase parameters using BESIII data. The main decay mode is D->Kspipi. These measurements have already been completed and published. New techniques were used to make use of more of the data. The results lead to uncertainties on gamma a factor 4 smaller than those of previous measurement from CLEO. This is a great step forward in improvement. The results were felt to be of sufficient importance that two articles were prepared. A Physical Review Letters which provides a short broad spectrum overview of interest to the general physicist and a more detailed technical publication in Physical Review D that showcases the new techniques developed for this measurement. Work also proceeds on other strong phase measurements. For D->K3pi, which is the next most sensitive channel the paper draft is close to submission to the journal and in the final stages of internal BESIII review. The decay mode D->4pi is also well underway.
3. The use of the results of work items 1 and 2 above have been incorporated into a new measurement of gamma using current LHCb data. These inputs alongside other detailed work on the B decay lead to a measurement of gamma of 5 degrees precision. This is equivalent to the precision of all other gamma measurements made to date. It is the most sensitive measurement. At the end of the reporting period it had been submitted to JHEP for publication. It has since been accepted, though not yet published. The results were first presented at the leading conference of the field ICHEP (Prague though online).
4. LHCb is gearing up for a brand new experiment where nearly all the detector is new and a revolutionary new trigger strategy will be implemented to significantly increase the rate at which interesting signal decays can be collected. Significant work has been done to prepare for the commencement of data taking. This includes work on implementing this new trigger and coordinating and contributing to the alignment and calibration of the new detector. The work is going well, but there is far more to do than anticipated.
1. Phenomenological work on the higher order corrections from physics effects due to neutral K mesons on measurements of gamma.
This work is completed and published in JHEP. The results were very interesting. They show that the previous work on this area had overlooked an important correction which leads to these effects not being dominant for B-->Dpi decays. This new knowledge is effectively revolutionary as it allows for the B-->Dpi decay channel to be exploited simultaneously with the B-->DK. This means that all parameters except for the strong phases can be determined from the data itself. This is a major reduction in uncertainty
2. Work has proceeded as planned on the measurement of strong-phase parameters using BESIII data. The main decay mode is D->Kspipi. These measurements have already been completed and published. New techniques were used to make use of more of the data. The results lead to uncertainties on gamma a factor 4 smaller than those of previous measurement from CLEO. This is a great step forward in improvement. The results were felt to be of sufficient importance that two articles were prepared. A Physical Review Letters which provides a short broad spectrum overview of interest to the general physicist and a more detailed technical publication in Physical Review D that showcases the new techniques developed for this measurement. Work also proceeds on other strong phase measurements. For D->K3pi, which is the next most sensitive channel the paper draft is close to submission to the journal and in the final stages of internal BESIII review. The decay mode D->4pi is also well underway.
3. The use of the results of work items 1 and 2 above have been incorporated into a new measurement of gamma using current LHCb data. These inputs alongside other detailed work on the B decay lead to a measurement of gamma of 5 degrees precision. This is equivalent to the precision of all other gamma measurements made to date. It is the most sensitive measurement. At the end of the reporting period it had been submitted to JHEP for publication. It has since been accepted, though not yet published. The results were first presented at the leading conference of the field ICHEP (Prague though online).
4. LHCb is gearing up for a brand new experiment where nearly all the detector is new and a revolutionary new trigger strategy will be implemented to significantly increase the rate at which interesting signal decays can be collected. Significant work has been done to prepare for the commencement of data taking. This includes work on implementing this new trigger and coordinating and contributing to the alignment and calibration of the new detector. The work is going well, but there is far more to do than anticipated.
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.
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.
All together this has led to the world's most precise measurement of gamma. The precision is equal that to all other measurements before it combined. Hence this is a marked step forward in the knowledge of this parameter. The precision here is 5 degrees, and when combined with other independent measurements that came before it the current world knowledge stand at just under 4 degrees.
The aim of the project was to bring down the uncertainty to 1 degree. While the initial work done by the project has certainly pushed forward to 1 degree faster than expected the impact of the Coronavirus pandemic can not be understated. Delays to operating schedules now mean that insufficient data will be available to meet the 1 degree target, however with further innovation and use of data currently available it is hoped that a precision of ~2.5 degrees could still be obtained with the 1 degree precision following swiftly after the end of the project.
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.
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.
All together this has led to the world's most precise measurement of gamma. The precision is equal that to all other measurements before it combined. Hence this is a marked step forward in the knowledge of this parameter. The precision here is 5 degrees, and when combined with other independent measurements that came before it the current world knowledge stand at just under 4 degrees.
The aim of the project was to bring down the uncertainty to 1 degree. While the initial work done by the project has certainly pushed forward to 1 degree faster than expected the impact of the Coronavirus pandemic can not be understated. Delays to operating schedules now mean that insufficient data will be available to meet the 1 degree target, however with further innovation and use of data currently available it is hoped that a precision of ~2.5 degrees could still be obtained with the 1 degree precision following swiftly after the end of the project.