Global Analysis of New Physics Interactions


Thomas Cepollina


Wouter Dekens (, Vincenzo Cirigliano (

What Students Will Do:

Students will program an algorithm that can be used to perform fits of the effective interactions to measurements as described above. The resulting code will be made public and should allow users to extend it by adding interactions and measurements. Students will learn the basics of the (beyond the) Standard Model physics that this analysis will investigate.


Basic knowledge of quantum mechanics and basic coding skills, which will be refined during the project.

Expected Length:

One year

Figure for INTURN 24-8

Image Description and Credit: Experimental determinations of the CKM elements V_ud and V_us, adapted from Ref. [1]. The red and pink bands show the constraints from neutron and nuclear β decays, while the blue (green) band is derived from (semi)leptonic meson decays. These determinations are in tension with the black line that denotes the SM prediction of unitarity.

Project Description:

The Standard Model of particle physics (SM) is a very successful theory that has so far passed all experimental tests. However, the theory has a number of shortcomings, such as its inability to explain the matter-antimatter asymmetry and the lack of a Dark Matter candidate. Moreover, several hints of discrepancies between SM predictions and experiments have recently emerged in low-energy precision tests. In particular, precision measurements of beta decays of the neutron, nuclei, and mesons have revealed a tension in testing the unitarity of the Cabibbo-Kobayashi- Maskawa matrix, which relates the quark mass eigenstates to their weak interaction eigenstates. This situation is depicted in the image above and has led to a large number of papers that try to explain this discrepancy in terms of new particles and interactions. The current project aims to assess the viability of these new-physics explanations in a general way that capture large classes of new physics models.

The effects of beyond-the-SM (BSM) physics can be captured by effective interactions between SM particles, as long as the BSM particles are heavy compared to the SM fields. This allows for a model-independent analysis of the low-energy tests described above in terms of effective BSM interactions. However, the same BSM interactions also induce new effects in processes at higher energies, such as e+e− and pp collisions measured at high energy colliders such as LEP and the Large Hadron Collider. In order to make sure these measurements do not already exclude the proposed BSM explanations, such probes need to be taken into account. This complicates the analysis as a significant number of effective interactions and observables need to be included.

The goal of the project is to develop a program that can be used by the wider community to perform this type of model-independent analysis, following [1], and eventually extend the included experiments (for example, by including measurements of parity-violating processes).


[1] V. Cirigliano, A. Crivellin, M. Hoferichter and M. Moulson, Scrutinizing CKM unitarity with a new measurement of the Kµ3/Kµ2 branching fraction, Phys. Lett. B 838 (2023) 137748, [arXiv:2208.11707].

[2] V. Cirigliano, W. Dekens, J. de Vries, E. Mereghetti and T. Tong, Anomalies in global SMEFT analyses: a case study of first-row CKM unitarity, arXiv:2311.00021.