Periodic Reporting for period 1 - KaonToALP (Search for axion-like particle produciton in kaon decays at CERN-NA62 experiment)
Berichtszeitraum: 2021-05-04 bis 2023-05-03
Particle physics studies sub-atomic constituents of matter and the laws of their mutual interactions. Our current knowledge is embodied in a mathematical description called the Standard Model (SM), which has been extremely successful and has withstood generations of experimental tests. With the Higgs boson discovery at the Large Hadron Collider (LHC), all the predicted constituents of the SM have been observed. Remarkably, the measured Higgs boson mass falls into a narrow region where consistency of the SM does not require new particles up to a very high energy scales. Moreover a large parameter space of new physics above the electroweak mass scale beyond the SM description has been explored at the LHC, with no new physics found so far.
Yet the SM is clearly incomplete, fundamentally contradicting the emerging Standard Model of cosmology. A clear piece of evidence for phenomena beyond the SM description is the conclusion from cosmological observations that the matter density of the universe is dominated by Dark Matter (DM) rather than the "normal" SM matter. The canonical "WIMP" DM scenario involving heavy particles interacting via the weak force is now almost ruled out experimentally by the LHC, as well as dedicated direct and indirect WIMP detection experiments. Therefore the hidden sector DM scenario is increasingly considered as a natural possibility, which has recently given rise to a new active field of research. Major advances in the exploration of this new paradigm can be achieved by exploiting data sets collected by the existing “intensity frontier” experiments.
The hidden sector is postulated to consist of new particles with masses in the broad vicinity of the electroweak scale that do not interact via the known electroweak and strong forces, and interact feebly with ordinary matter through light mediator particles charged under both SM and hidden sector fields, or mixing with their SM counterparts. In addition to providing DM candidates, hidden-sector models lead to simple and predictive cosmology, and offer solutions for astrophysical and low-energy anomalies.
It is proposed to perform the first search for production of a short-lived axion-like particle (ALP) in charged-kaon decays (K+ --> pi+a), which decaying promptly into a di-photon final state, using world's largest data set of charged kaon decays collected by the NA62 experiment at CERN in 2016-18. New powerful constraint on the ALP phase space will be established; a possible ALP discovery would be a breakthrough in particle physics. To achieve credible results on the ALP search, it is mandatory to understand the main Standard Model background process K+ --> pi+ gamma gamma. Therefore a comprehensive study of this process is also proposed, including a re-analysis of the inputs to the description of this decay within Chiral Perturbation Theory, and the most precise measurement of this process to date. This measurement by itself represents an important test of the theories describing low-energy dynamics. These novel measurements are based on an existing data sample collected recently, thereby minimizing the associated risks.
Yet the SM is clearly incomplete, fundamentally contradicting the emerging Standard Model of cosmology. A clear piece of evidence for phenomena beyond the SM description is the conclusion from cosmological observations that the matter density of the universe is dominated by Dark Matter (DM) rather than the "normal" SM matter. The canonical "WIMP" DM scenario involving heavy particles interacting via the weak force is now almost ruled out experimentally by the LHC, as well as dedicated direct and indirect WIMP detection experiments. Therefore the hidden sector DM scenario is increasingly considered as a natural possibility, which has recently given rise to a new active field of research. Major advances in the exploration of this new paradigm can be achieved by exploiting data sets collected by the existing “intensity frontier” experiments.
The hidden sector is postulated to consist of new particles with masses in the broad vicinity of the electroweak scale that do not interact via the known electroweak and strong forces, and interact feebly with ordinary matter through light mediator particles charged under both SM and hidden sector fields, or mixing with their SM counterparts. In addition to providing DM candidates, hidden-sector models lead to simple and predictive cosmology, and offer solutions for astrophysical and low-energy anomalies.
It is proposed to perform the first search for production of a short-lived axion-like particle (ALP) in charged-kaon decays (K+ --> pi+a), which decaying promptly into a di-photon final state, using world's largest data set of charged kaon decays collected by the NA62 experiment at CERN in 2016-18. New powerful constraint on the ALP phase space will be established; a possible ALP discovery would be a breakthrough in particle physics. To achieve credible results on the ALP search, it is mandatory to understand the main Standard Model background process K+ --> pi+ gamma gamma. Therefore a comprehensive study of this process is also proposed, including a re-analysis of the inputs to the description of this decay within Chiral Perturbation Theory, and the most precise measurement of this process to date. This measurement by itself represents an important test of the theories describing low-energy dynamics. These novel measurements are based on an existing data sample collected recently, thereby minimizing the associated risks.
In the first part of the project implementation generators for the K+ --> pi+ gamma gamma and K+ --> pi+ a processes were developed and integrated into NA62 software. An event selection for the signal channel was developed. Information from multiple downstream detectors was combined to obtain the best possible resolution on the decay vertex position and the di-photon invariant mass, as the latter quantity determines the sensitivity to ALP production. To simulate very large MC samples of background events with the finite computing resources available, dedicated "biased" simulation methods were developed. These methods were allow full simulation of the potentially interesting background topologies only: K+ --> pi+ pi0 gamma and K+ --> pi+ pi0 pi0 decays, followed by prompt pi0 --> gamma gamma decays with merged photons. The simulation of the calorimeter response was improved as required for adequate description of photon detection inefficiency, and photon cluster merging. Estimates of the backgrounds from each possible source in the signal region were made using the simulated samples and validated by comparing to data in the control regions. Systematic uncertainties on the background estimates were evaluated. A sample of 3984 K+ --> pi+ gamma gamma decay candidates with an estimated background contamination of 291+/-14 events was obtained. The decay rate and spectrum within Chiral Perturbation Theory (ChPT) are determined by a single a priori unknown combination of fundamental low-energy constants. However, due to the dominance of the pion loop contribution, the model involves seven empirical external parameters extracted from K --> 3pi decay measurements. These parameters were last re-evaluated 17 years ago. A work was organised to evaluate new precise K --> 3pi decay amplitudes by a fit to all available measurements including high-precision inputs published after the previous update. This task was done in collaboration with the theoretical group of INFN Napoli.
Main results of this project are the following
1) a precision measurement of the K+ --> pi+ gamma gamma decay was performed within the updated ChPT framework. It was found that leading order of the ChPT is not sufficient to describe data and NLO contribution must be taken into account. The measured K+ --> pi+ gamma gamma decay rate and corresponding combination of fundamental low-energy constants are the most accurate in the world. This result was presented for the first time at KAON2022 international conference.
2) Using the measured K+ --> pi+ gamma gamma di-photon mass spectrum, search for the K+ --> pi+a, a --> gamma gamma decay chain on top of the K+ --> pi+ gamma gamma and other backgrounds was performed as a peak search in the reconstructed di-photon mass distribution in the ALP mass range 207-350 MeV/c^2. The hypothesis test procedure was developed, involving shape analysis and using the unbinned profile likelihood ratio test statistic. Final results were obtained in terms of confidence intervals for the K+ --> pi+a, a --> gamma gamma decay rate. The first search for the K+ --> pi+a in the BC11 framework (ALP with gluon coupling) was performed.
Main results of this project are the following
1) a precision measurement of the K+ --> pi+ gamma gamma decay was performed within the updated ChPT framework. It was found that leading order of the ChPT is not sufficient to describe data and NLO contribution must be taken into account. The measured K+ --> pi+ gamma gamma decay rate and corresponding combination of fundamental low-energy constants are the most accurate in the world. This result was presented for the first time at KAON2022 international conference.
2) Using the measured K+ --> pi+ gamma gamma di-photon mass spectrum, search for the K+ --> pi+a, a --> gamma gamma decay chain on top of the K+ --> pi+ gamma gamma and other backgrounds was performed as a peak search in the reconstructed di-photon mass distribution in the ALP mass range 207-350 MeV/c^2. The hypothesis test procedure was developed, involving shape analysis and using the unbinned profile likelihood ratio test statistic. Final results were obtained in terms of confidence intervals for the K+ --> pi+a, a --> gamma gamma decay rate. The first search for the K+ --> pi+a in the BC11 framework (ALP with gluon coupling) was performed.
New evaluation of ChPT empirical external parameters extracted from K --> 3pi decay measurements was done.
The new measurement of the di-photon mass spectra in the the K+ --> pi+ gamma gamma decay established that leading order of the Chiral Perturbation Theory is not sufficient to describe data and NLO contribution must be taken into account. It will motivate theoreticians to investigate further low-energy QCD processes (at NNLO). This measurement will have impact on the future dedicated experiments taking into account the used technique (kaon decay in flight) and established background sources.
The improvements and studies performed in this project also contributed to other publications by the NA62 collaboration. The scientific impact of the project: 4 journal publications plus 4 conference talks. New contacts were made with key theoretical physicists in the dedicated field.
The preparation of the final publication is in progress, and the results are expected to be published soon together with OpenData datasets. It will give an opportunity to use the project results for any researcher extending socio-economic impact of the project.
The new measurement of the di-photon mass spectra in the the K+ --> pi+ gamma gamma decay established that leading order of the Chiral Perturbation Theory is not sufficient to describe data and NLO contribution must be taken into account. It will motivate theoreticians to investigate further low-energy QCD processes (at NNLO). This measurement will have impact on the future dedicated experiments taking into account the used technique (kaon decay in flight) and established background sources.
The improvements and studies performed in this project also contributed to other publications by the NA62 collaboration. The scientific impact of the project: 4 journal publications plus 4 conference talks. New contacts were made with key theoretical physicists in the dedicated field.
The preparation of the final publication is in progress, and the results are expected to be published soon together with OpenData datasets. It will give an opportunity to use the project results for any researcher extending socio-economic impact of the project.