A Classic Thesis Style - Johannes Gutenberg-Universität Mainz

C O N C L U S I O N S
The effort to understand the universe is one
of the very few things that lifts human life a
little above the level of farce, and gives it
some of the grace of tragedy.
— Steven Weinberg
For the first physical measurement of the Kaos spectrometer, the
elementary Λ and Σ 0 electroproduction cross section was chosen. The
coincident detection of kaons and electrons allowed the reconstruction
of the undetected hyperon mass, and consequently, the identification
and separation of the two reaction channels, included in the combined
acceptance of the spectrometers tandem. One of the existing spectrometers,
B, was used for electron detection at the minimum attainable
angle with respect to the electron beam, in order to maximize the
virtual photon flux. Two data taking periods were approved for the
years 2008 and 2009. Very low photon four momentum values were
chosen at Q 2 = 0.03-0.055 (GeV/c) 2 with central values Q 2 = 0.036 and
0.005 (GeV/c) 2 , and corresponding degrees of polarization ɛ = 0.4
and ɛ = 0.54. The hadronic system was excited to mean invariant
energies of W = 1.75 GeV and W = 1.67 GeV.
Being Q 2 a measure of photon virtuality, this experiments used
almost real photons, although with a relatively large degree of transverse
linear polarization. Longitudinal and longitudinal-transverse
interference terms are strongly suppressed in the total cross section
by the small value of ɛL = Q2
ω 2 ɛ ≈ 0.01. The remaining transversetransverse
interference term does indeed take into account, that this
virtual photons are partially linearly polarized, as explained in appendix
A. Strictly speaking, then, electroproduction cross section at
low Q 2 is not equivalent to unpolarized photoproduction. Experiments
with real photons can measure σT T by combining linearly polarized
and unpolarized measurements, since it can be proven that the
asymmetry observable defined as Σ = σ⊥−σ �
σ⊥+σ � (where σ⊥(σ �) is the differential
cross section for the photon polarization normal (parallel) to
the production plane) is related with σT and σT T by Σ = −σT T (Q 2 =0)
σT (Q 2 =0) .
Isobaric model predictions for σT T (Q 2 = 0) are very close to zero for
our kinematics so we consider informative a direct comparison with
unpolarized photoproduction results.
As we shall see, this timid departure from photon reality, although
far from the “hot” small kaon angle region discussed in Chapter 1,
still might hinder important features of the reaction dynamics.
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