Measurement of the Σ+ production and the p-Σ+ correlation function in proton-proton collisions with ALICE at the LHC

In the past decades, progress in the development of particle accelerators and detectors enabled scientists to study the production, interactions, and collective phenomena of many particle species in great detail. Ranging from light mesons to nuclei, even rare or short-lived probes could be measured over a broad range of collision energies. Nonetheless, data on one row of the baryon octet, the Σ baryons, is scarce. In previous measurements, mostly the excited states of Σ baryons were measured. Some results also exist on the Σ0. The charged ground-state Σ baryons are, however, mostly unexplored. The only measurements of the Σ+ production in high-energy collisions so far were performed by the DELPHI, OPAL, and L3 collaborations, which were all situated at the LEP collider at CERN and measured the Σ+ in hadronic decays of the Z0 boson. The reason for the scarce data situation is that the charged ground-state Σ baryons are extraordinarily challenging to measure, which will be discussed in detail in this thesis. The aforementioned measurements have been compared to different Monte Carlo event generators and showed sizeable deviations in the yield and shape of the obtained momentum spectra. It is well known that common event generators generally underpredict the production of strangeness, which also reflects in an underprediction of Σ baryons in these models. In this regard, the measurement of Σ+ cannot only provide a valuable cross check of the yields and spectral shape, but also serve as an input to improve those models. Moreover, a good knowledge about the production cross sections of Σ baryons is of importance for the measurements of protons and Λs, as the Σs feed down into the measured yields of these particles via their weak decays. The observed enhancement of the production of strangeness in hadronic collisions with increasing multiplicity was historically seen as a signature of the quark-gluon plasma. Currently, only one single strange ground-state baryon, the Λ, is measured multiplicity differentially and also in this regard, the measurement of Σ baryons can help to complete the picture. Apart from the measurement of the production cross section, the reconstruction of the Σ+ baryons opens up the possibility to study their interactions with other hadrons via the femtoscopy method. The interaction between nucleons and hyperons as well as the interaction between hyperons is generally not well known. This is related to the fact that neither targets nor beams can be created from hyperons, inhibiting scattering experiments. Knowledge about the aforementioned interactions is crucial for the equation of state of dense astrophysical objects like neutron stars, since the Pauli principle implies that the presence of hyperons might be energetically favorable at densities well above the nuclear saturation density. Among the interactions including hyperons, the N-Λ interaction is rather well known from femtoscopic measurements and the analysis of hypernuclei. These results have already created tension with astronomical observations, known as the “hyperon puzzle”. On the other hand, it has recently emerged that non-negligible three-body contributions might be responsible for the discrepancy and may even inhibit Λ baryons from appearing in neutron stars. This, however, does not rule out other hyperons like Σ or Ξ baryons from emerging, depending on their respective interactions with the surrounding neutron star matter. These interactions are, however, even less known and thus the extension of the femtoscopic studies to the Σ sector can provide valuable information. In this regard, the N-Σ (I=3/2) channel gives access to the partially Pauli-forbidden decuplet, which can neither be accessed by the N-N nor by the N-Λ interaction. Specifically, the p-Σ+ interaction studied in this thesis stands out, as it is closely related to the unmeasurable n-Σ- interaction, which is most important for neutron stars. Furthermore, given sufficient data, this channel allows to test the SU(3) flavor symmetry breaking in the 27-plet interaction. Moreover, knowledge about the p-Σ0 correlation function is an important input for the measurement of the p-Λ correlation function, as both systems are not fully separable experimentally. Previous femtoscopic studies focused on the p-Σ0 system. This seems natural, as the Σ0 is relatively easy to access experimentally. However, specific challenges arise which hinder the extraction of the relevant information in this system. This will be addressed in detail in this thesis. It will also be discussed why the p-Σ+ system is in fact more promising, both experimentally and theoretically, and which challenges there are. The difficulty of reconstructing the Σ+ baryon in high-energy collision experiments reflects itself in the lack of previous measurements and will take a prominent part in this thesis. Besides the results obtained from the available data, the methods which have been developed in the scope of this thesis will be useful for the measurement of other particle pairs and the data to come.