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Study of the distribution of gluons and quarks inside the nucleon through ultra-peripheral collisions

Periodic Reporting for period 1 - pANUCSTR (Study of the distribution of gluons and quarks inside the nucleon through ultra-peripheral collisions)

Reporting period: 2018-08-01 to 2020-07-31

It is astonishing but true that we do not fully understand 95% of our mass and that of the visible universe. Visible matter is made of atoms, the atom's nucleus is made of protons and neutrons (collectively referred to as nucleons) and these nucleons consist of a highly-energetic soup of very-fast moving and constantly interacting quarks and gluons. Gluons have no mass and quarks a negligibly small one, yet together they generate most of the mass we can see. Understanding how that occurs is the objective of the present project.

The project provides crucial information on the quarks and gluons inside the nucleon: where they are located, how they move and how they orbit each other, and the pressure distribution they generate. This work is of fundamental importance in our investigations into the origin of mass and will influence our understanding of the nuclear elements and nuclear astrophysics.

I use data from the LHCb experiment at the Large Hadron Collider at CERN, obtained when protons and nuclei are smashed together at unprecedented energies. In glancing collisions, the projectiles do not break up, but turn some of their energy into mass, through Einstein’s famous E=mc2. By studying the newly created system, the structure of the original protons and nuclei can be inferred.

In particular, I have been studying the exclusive process in which the newly created system consists of exactly one J/psi meson and the process in which the newly system consists of a pair of oppositely charged muons.
The former process provides information about gluons, while the latter process is also sensitive to quarks.
The production of J/psi mesons has already been studied by the LHCb experiment in proton-proton collisions. I provide the first measurement, using LHCb data, in the collision of protons and lead ions.
The process with pairs of muons has never previously been studied and my work breaks new ground in the measurement of this process and what we know about the structure of quarks in the nucleon.
The first step In performing the measurements described above, was to study the production of the states using simulated data. This work was done partly in collaboration with theorists, and in the course of this, I improved the theoretical description of the process.
Using this simulation, I then developed a methodology to select the data of interest, and created a fitting algorithms to the extract the signal.

Several terabytes of data had been collected by the LHCb experiment from 2015 to 2018, and this is stored on computers around the world in a distributed data warehouse.
I interrogated this data and filtered it to extract my signals of interest.
Critical to the signal extraction was the use of a sub-detector of the LHCb experiment called HERSCHEL. This little-used sub-detector was particularly important for my analysis but had not been calibrated. Consequently, I spent several months understanding the detector's behaviour and provided calibrations that were not alone necessary for my analysis, but also allowed this detector to be used by several other physics analyses in the LHCb collaboration.

Having produced a cleaned and filtered subset of the data containing my physics signal, I calculated the various efficiencies with which this data was collected. These efficiencies include the original data-taking efficiency with which data was written to disk during proton-proton and proton-lead collisions, as well as the reconstruction efficiency and my selection. From these efficiencies, it was possible to work back and measure the probability of my signal events being produced in the original beam collisions. This quantity, known technically as the cross-section for the process, is what can be compared directly to theory, and which allows us to extract the distribution of quarks and gluons in the nucleon.

The LHCb collaboration consists of about 800 physicists who self-organise into smaller teams, depending on their physics interest. I was appointed convenor of the LHCb QCD/CEP working group, which studies the strong interaction between quarks and gluons and reactions where only one or a few more particles are created.

In order to understand the physics better, to discuss with theory and experimental colleagues, and to disseminate my ideas and results, I participated in 14 workshops and conferences and was involved in the organisation and convenorship of four workshops related to the study of the strong force.
The LHC has the potential to already explore various aspects now and for the first time. There is a clear interest from the hadron-physics community to investigate nucleon structure and hadron formation using different techniques. The usage of proton-ion collisions as a photon beam to probe the nucleon is a relatively novel aspect but with a recognised high potential.

My measurement of exclusive J/psi production occurs in a so far unmeasured kinematic region, at very high energy.
My measurement of exclusive pairs of muons is the first in hadron collisions.

Both measurements provide new and complementary information on the internal nucleon structure with, in particular, the exclusive production of muons offering a new probe for the study of the nucleon structure.
The measurements will trigger more theoretical and experimental activity, at currently existing experiments (RHIC, LHC), but also at future experiments, and in particular, the EIC (electron-ion collider).
Candidate event for the exclusive production and decay of a J/psi meson into two muons.