Strange particle production at the ZEUS detector

Thesis



The production of the neutral strange particles Λ, Λ[bar] and K⁰_s at HERA has been studied with the ZEUS detector, using an integrated luminosity of 121pb^-1 . Differential cross-sections are measured in a High Q² DIS sample (Q² ≥ 25GeV²), a Low Q² DIS sample (5 ≤ Q² ≤ 25GeV² ) and a photoproduction sample. The photoproduction sample has Q² <= 1GeV² and at least two jets that satisfy E_T^jet > 5GeV and |η^jet| < 2.4. Cross-sections are measured as a function of the particle P_T and η, and additionally as a function of the event Q² and x in the DIS samples, and as a function of x_γ in the photoproduction sample. The DIS cross-sections are compared to the ARIADNE Monte Carlo, which broadly describes the data. The photoproduction cross-sections are compared to PYTHIA, where the agreement is less good, particularly at low x_γ. The baryon-antibaryon asymmetry, defined as (Λ-Λ[bar])/(Λ+Λ[bar]), is measured and always found to be consistent with zero, indicating little transfer of the Baryon Number from the beam proton to the Λ system in the detector region. The polarization of Λ and Λ[bar] is measured and also found to be always consistent with zero, indicating that there is no preferred frame in Λ production. The baryon-meson ratio (Λ + Λ[bar])/K⁰_s is studied, and is found to generally agree with ARIADNE in DIS, but give an excess over PYTHIA in photoproduction. The excess is particularly prominent at low x_γ in events with no clear jet structure. This is interpreted as hadron production in this region depending on a more-than-one-dimensional overlap of quark wave functions. An excess of strangeness in this region at low x_γ with no high E_T jets would be significant. It is investigated by taking the ratio of strange particles to charged tracks but no excess is seen.

Authors: Cottrell, A

• Publication date: 2005
• DOI: 10.5287/ora-6rno8b77d
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Strange particles; Hadrons; Particles (Nuclear physics)