Repository logo

Parton Distributions in Beyond the Standard Model Theories



Change log



Parton distributions are a key ingredient of precise predictions for collider experiments. They are usually determined from fits to experimental data under the assumption that the Standard Model (SM) of particle physics is complete; however, this can bias studies of beyond the Standard Model (BSM) physics if these SM-like PDFs are used in these analyses. It is important to quantify the extent to which this occurs, in order to avoid making incorrect conclusions about BSM physics.

We begin in Chapter 1 with a review of perturbative quantum chromodynamics (QCD) and parton distribution functions (PDFs), providing a definition of the PDFs at next- to-leading order in QCD perturbation theory. At the end of the Chapter, in Sect. 1.4, we introduce the main problem that this thesis aims to address in a variety of special cases, namely the simultaneous extraction of PDFs together with other theory parameters (specifically BSM theories).

In Chapters 2, 3 and 4, we describe the interplay between PDFs and the parameters of various BSM models. In more detail, in Chapter 2, we perform an approximate simultaneous extraction of PDFs together with the parameters of a dark photon model; in particular, we use projected high-luminosity LHC (HL-LHC) data to investigate the sensitivity of the HL-LHC to our particular class of light, leptophobic dark photons. Subsequently, in Chapter 3, we introduce the Standard Model Effective Field Theory (SMEFT), and carry out a simultaneous determination of PDFs together with two parameters drawn from the SMEFT; we show that at the HL-LHC, there will be significant interplay between extraction of PDFs and SMEFT parameters. In Chapter 4, we perform a much more comprehensive analysis of the PDF-SMEFT interplay in the top sector, using a new efficient methodology, SIMUnet. Importantly in Sect. 4.7, we also comment on the efficacy of the Monte Carlo replica method for error propagation, which forms the heart of the uncertainty calculation in both the NNPDF and SIMUnet methodologies.

In the second half of this thesis, we focus on future issues in PDF fitting, related to the work presented in the previous chapters. In Chapter 5, we explore how New Physics in the data might be inadvertently ‘fitted away’ into the PDFs, if the data is treated as SM-like. We also recommend strategies for disentangling PDFs and BSM effects. Finally, in Chapter 6, we discuss the Monte Carlo replica method used in many of the previous chapters, and discuss the need for its replacement in future PDF and BSM fits.





Ubiali, Maria


BSM physics, high energy physics, particle phenomenology, parton distributions


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
My PhD was funded by the Sims fund studentship, together with a European Research Council grant (n.95024).