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# Measurements of transverse momentum dependent azimuthal asymmetries in SIDIS at COMPASS

Sbrizzai, Giulio

2011-03-29

Abstract

The main topic of this thesis is the measurement of the transverse
momentum dependent (TMD) azimuthal asymmetries in the
inclusive hadron production in the deep inelastic scattering (DIS).
This work has been carried on in the framework of COMPASS, a
fixed target experiment at CERN's SPS, which started data taking in 2002.
One of the main topic of COMPASS is the study of the nucleon spin
structure using high energy (100-200 GeV) muon beam and either
transversely and longitudinally polarized proton and deuteron target.
Recent developments both from the experimental and theoretical side
pointed out the relevance of the transverse spin effects and
of new effects, related to the intrinsic transverse momentum of the
quark and its correlation with the spin.
The nucleon structure, in the non perturbative QCD formalism, can be
described at first order, in the collinear approximation,
by three parton distribution functions (PDF):
the unpolarized PDF, the helicity PDF and the transversity PDF.
When including the intrinsic transverse momentum of the quark
other 5 PDFs are needed at first order.
They are related to the correlation between the transverse momentum
of the quark and the spin and are practically unmeasured.
The semi inclusive DIS (SIDIS) allows to study all these functions.
Indeed the the SIDIS differential cross section, taking into account
the beam and target polarization, is characterized by 14
azimuthal modulations in independent linear combination
of the nucleon spin angle and of the hadrons angle,
as measured with respect to the virtual photon direction.
Their amplitudes are related to the structure functions
given by the convolution between the new PDFs and the
hadron fragmentation functions.
In this thesis the measurement of the azimuthal asymmetries
which appears in the SIDIS cross section on the unpolarized and
longitudinally polarized target. Of particular interest is the
information that this measurement give on the Boer-Mulders TMD PDF,
which is a very interesting topic.
The analysed data have been taken using the 160 GeV positive muon beam
on the deuteron target. The target configuration and the spectrometer
are optimized for the spin asymmetry measurements and are described
in the thesis as well as the data taking. Also, the data processing and
the software used for the analysis and the SIDIS
data selection are described.
The measurement of the azimuthal asymmetries on the longitudinally
polarized target have been done using the same data as the one
used for the $\Delta$G (the contribution to the nucleon spin of
the longitudinal spin of the gluons) measurement. To avoid acceptance
effects in the asymmetries extraction, the data collected from the
two oppositely polarized target cells (which form the target) have been
opportunely combined. Other possible systematic effects have been
studied in details and are described in the thesis. \\
The asymmetries have been extracted as functions of the
relevant kinematical variables and separately for positive and negative
hadrons. The final results, accepted for publication in European
Journal C, are compared with the results
produced by other experiments and with the calculations
of theoretical models of the TMD functions.
The measurement of the azimuthal asymmetries which appears in the
spin independent part of the SIDIS cross section is more difficult
since it cannot benefit from the periodically reversal of the target
polarization. The measured hadron azimuthal distributions must be
corrected for the possible effects introduced by the apparatus acceptance
(geometrical acceptance, detectors and trigger efficiencies). The Monte
Carlo simulation of the apparatus has been used to estimate those effects.
Extensive studies of the description of the azimuthal acceptance of
the apparatus have been done in the whole kinematical domain of
interest using different SIDIS events generations.
A reliable asymmetries extraction method has been developed and used.
A considerable effort has been put into the study and the control
of the systematics. Also in this case the asymmetries have
been extracted as functions of the relevant kinematical
variables and separately for positive and negative hadrons. \\
The strongest measured signals are related to the Cahn effect,
which is a kinematical effect proportional to the intrinsic
transverse momentum of the quarks, and to the Boer-Mulders TMD PDF,
describing the correlation between the intrinsic transverse
momentum of the quark and its transverse spin. \\
The results, in particular the kinematical and the charge
dependencies of the asymmetries, are interpreted in terms
of TMD quantities and higher effects in the non perturbative QCD.
The comparisons with the results
produced by the other experiments, and with the theoretical predictions
are also discussed.

Insegnamento

Publisher

Università degli studi di Trieste

Languages

en

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