Studies of exotic hadron states at the LHCb experiment

Abstract

In this thesis, two analyses of data collected by the LHCb collaboration are presented. The first is a search for promptly produced pentaquarks in a range of combinations of Σ꜀ baryons (or the Λ꜀⁺ baryon) with D mesons, carried out using 5.7 fb⁻¹ of data collected at the LHCb experiment during Run 2. No signal is found, and upper limits are set on the yield relative to the Λ꜀⁺ →pK⁻ π⁺ decay mode in a mass range from the threshold of the state to 600 MeV/c² above threshold for each mode. The results of this work will be useful for theorists to make predictions concerning pentaquarks and some of the results may give indications of signal channels to be studied in the future with a larger dataset.

The second analysis is on the observation of the χ꜀₁ (3872) → J/ψ μ⁺μ⁻ decay, and measurement of its relative branching fraction to the known χ꜀₁ (3872) → J/ψ π⁺ π⁻ decay. The ratio is measured as

R= B(χ꜀₁ (3872) → J/ψ μ⁺μ⁻ ) / B(χ꜀₁ (3872) → J/ψ π⁺ π⁻) = (1.95 ± 0.41 ± 0.09) × 10⁻³

where the first error is statistical and the second is systematic. The result of this work may add to and elucidate on the discussion of the nature of the χ꜀₁ (3872)state, and provide further points of study in the future. The evidence of an enhancement consistent with the X(3915) state may also be useful in the current discourse around this state and other X states that exist around it.

Finally, LS2 work carried out for the upgrade of the LHCb RICH system is discussed, with a particular focus on the new photodetector system.