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A silicon microstrip detector for COMPASS and a first measurement of the transverse polarization of {_L63_1hn0-hyperons [Lambda-0-hyperons] from quasi-real photo-production [Elektronische Ressource] / Michael Wiesmann

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Published 01 January 2004
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PHYSIK-DEPARTMENT
A Silicon Microstrip Detector for COMPASS
and
A First Measurement of the
0Transverse Polarization of -Hyperons
from Quasi-Real Photo-Production
Dissertation
von
Michael Wiesmann
¨TECHNISCHE UNIVERSITAT
¨MUNCHENFakultat¤ fur¤ Physik der Technischen Universitat¤ Munchen¤
Physik Department E18
A Silicon Microstrip Detector for COMPASS
and
A First Measurement of the
0Transverse Polarization of -Hyperons
from Quasi-Real Photo-Production
Michael Wiesmann
Vollstandiger¤ Abdruck der von der Fakultat¤ fur¤ Physik der Technischen
Universitat¤ Munchen¤ zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften (Dr. rer. nat.)
genehmigten Dissertation.
Vorsitzender: Univ.-Prof. Dr. A. J. Buras
Prufer¤ der Dissertation:
1. Univ.-Prof. Dr. St. Paul
2. Hon.-Prof. Dr. S. Bethke
Die Dissertation wurde am 27.01.2004 bei der Technischen Universitat¤ Munchen¤
eingreicht und durch die Fakultat¤ fur¤ Physik am 13.02.2004 angenommen.Contents
Introduction 1
1 The COMPASS Experiment 5
1.1 Physics Program of COMPASS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1.1 Physics with a Muon Beam . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1.2 Hadronic Physics Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
01.2 Transverse Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.2.1 General Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.2.2 Experimental Phenomenology . . . . . . . . . . . . . . . . . . . . . . . . . . 18
1.2.3 Theoretical Ideas for Transverse Polarization . . . . . . . . . . . . . . . . . . 20
1.3 The COMPASS Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
1.3.1 General Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
1.3.2 Layout of the Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
1.3.3 The COMPASS Beams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
1.3.4 The Polarized Target . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
1.3.5 Setup along the beam line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
1.3.6 The Muon Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
1.3.7 The Data Acquisition System . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2 Silicon Microstrip Detectors 37
2.1 Basic Operation of Silicon Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.1.1 Production of Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
2.1.2 pn-Junctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.1.3 Silicon for Particle Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2.2 Radiation Damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
2.3 Lazarus Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
ICONTENTS
3 The COMPASS Silicon Detector 49
3.1 Silicon Wafer Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3.2 The Silicon Module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.2.1 The Frontend Chip APV25 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.2.2 The Frontend Boards L board . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.3 The Readout Chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.3.1 The Repeater Card . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.3.2 The ADC Cardsg adc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.3.3 The Data Concentrator GeSiCA . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.3.4 A Word on the Grounding Scheme . . . . . . . . . . . . . . . . . . . . . . . 61
3.4 The Silicon Station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3.5 Performance in the COMPASS beam . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
3.5.1 Multiplicity and Occupancy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.5.2 Pulse Height Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
3.5.3 Time Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
3.5.4 Ef ciency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
3.5.5 Correlation between p and n Side . . . . . . . . . . . . . . . . . . . . . . . 70
3.6 Summary and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
4 Event Reconstruction and Related Tools 75
4.1 The ROOT Analysis Framework . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.2 Reconstruction and Analysis with CORAL . . . . . . . . . . . . . . . . . . . . . . . 76
4.2.1 General Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
4.2.2 Selected Steps of the Reconstruction . . . . . . . . . . . . . . . . . . . . . . . 78
4.3 Physics Analysis with PHAST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.3.1 mDST Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.3.2 UserEvent Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
05 Reconstruction of in COMPASS 81
05.1 Event Topology V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
5.2 De nition of Coordinate System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
5.3 Invariant Mass Spectra and Side Band Correction . . . . . . . . . . . . . . . . . . . 86
5.4 Kaon Mass Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
05.5 Selection Criteria for V and Background Suppression . . . . . . . . . . . . . . . . . 89
5.5.1 Event Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
IICONTENTS
05.5.2 Enriching of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
0fl5.5.3 Sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
05.5.4 K Sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
5.6 Properties of the Virtual Photon Beam . . . . . . . . . . . . . . . . . . . . . . . . . . 101
5.7 Bias-Canceling Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
5.8 Extraction of Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
06 Transverse Polarization 111
0 0 0fl6.1 Average Transverse Polarization of , and K . . . . . . . . . . . . . . . . . . . 111
0V6.2 Dependence of P on x and p . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114F Tn
6.3 Interpretation of the Results and Comparison with other Experiments . . . . . . . 121
Summary and Outlook 125
List of Figures 127
List of Tables 131
Bibliography 133
Acknowledgments 139
Own Contributions 141
IIIIntroduction
Neutrons and protons are the basic building blocks of matter. They form the atomic nucleus
(hence also the name nucleons) and are responsible for the major part of the atomic mass. In our
current understanding they are composed of quarks bound by the strong color force.
Six different quarks, the avors, are known. Sorted according to their mass they are (from lightest
to heaviest): Up (u),Down (d),Strange (s),Charm (c),Bottom (b) andTop (t). In the naive constituent
quark model nucleons are described as a combination of three constituent quarks. Together they
de ne the properties of the nucleon like charge and mass. Different combinations of avors result
in different types of nucleons: protons consists of (uud), neutrons of (udd).
Besides an electrical charge, the quarks also carry a strong charge, the color: quarks can be
red (r), blue (b) or green (g), anti-quarks carry the respective anti-colors anti-red (r),
anti-blue (b) or anti-green (g). Since its introduction to particle physics, color has never been
found with a free particle: quarks always appear as white clusters called hadrons. Two ways to
get white hadrons have so far been observed: mesons are built of one quark and one anti-quark in
such a way that their colors neutralize each other, baryons are made up of three quarks whose color
combination (rgb), in the same way as in the optical analogon, also makes white. In this scheme
the nucleons are only two hadrons out of many: they are the three-quark-systems consisting ofu-
andd-quarks only and are therefore the lightest baryons.
In the framework of Quantum Chromodynamics (QCD) the interaction of the quarks and their color
elds is described in analogy to the very successful Quantum Electrodynamics (QED) by the ex-
change of eld quanta, so-called gluons. The strength of the interaction between quarks and
gluons is described in the coupling constant . But while in QED the eld quanta do not carrys
charge and therefore cannot interact among each other, in QCD gluons are colored and do show
self-interaction, which leads to a much more complex interaction scheme than in QED.
It turned out that the interaction of quarks is rather weak when they come very close together,
i.e. when their kinetic energies are high. It can then be nicely described assuming a one-gluon-
exchange similar to the one-photon-exchange in the QED case. Due to a small coupling constant
, more-gluon-exchanges are said to be unlikely: at around 100 GeV, was measured to be onlys s
1of the order of =10. It is therefore suf cient for many applications to calculate just the one-gluon
exchange and add more gluons only as small disturbance. This is called perturbative QCD.
Surprisingly, in the case of large distances and, accordingly, small quark energies, the interaction
gets stronger. To allow for more gluons in such interactions, the coupling constant is said to bes
not constant but assumed to get larger for larger distances. Hence it is called a running coupling
constant . As a consequence this makes it impossible to separate two quarks, as the force eld
acquires so much energy that it nally bursts into a qqfl-pair under conservation of the white
color. The fact that quarks are not separable is called con nement.
1INTRODUCTION
At the nucleon’s energy scale of 1 GeV, is already more than 0.3. Consequently the behaviors
of quarks and gluons and therefore the structure of the nucleon cannot be described in the same
perturbative way of a simple one-gluon-exchange anymore, but many-gluon-exchanges have to
be taken into account as well. The method breaks down anyway at 1 at the latest. In thes
last few years much effort has been spent on nding new ways of describing low-energy QCD.
The most promising ones are Lattice QCD and Chiral Perturbation Theory. In Lattice QCD the eld
equations are solved exactly on a grid with a nite spacing, using a huge amount of computer
power. Chiral Perturbation Theory uses the chiral symmetry of QCD at low quark momenta, which
is approximately valid since the quark masses can be neglected because they are still much smaller
than the quark momenta.
In the nucleon and in all baryons in general quarks move at distances for which the low-energy-
models mentioned above are just starting to be applicable. In this region many peculiar features of
QCD show up. Thus inside the nucleons a large number of so-called sea quarks were found, which
have their origin in gluons uctuating for short times into quark anti-quark pairs. Furthermore,
while in atoms the energy stored in the binding of the electrons to the protons with opposite elec-
1trical charge is very small , the binding energy is surprisingly large in the nucleon. Despite
the large number of sea quarks, the contribution of quarks to the total momentum of the nu-
cleon was measured to be only around 50%, the rest is contributed by the binding , the gluons.
These results helped considerably to advance the theoretical understanding of the nucleon: from
a vacuum lled with three point-like quarks one moved to some kind of plum-pudding model
where quarks are embedded into a background of gluons like raisins in the pudding. This back-
ground, however, transforms constantly back and forth into quarks and anti-quarks as well. The
small excess of three valence quarks over the large number of quark anti-quark-pairs in the sea
de nes the type of the nucleon. The constituent quark mentioned at the beginning nally became
only an effective particle consisting of a valence quark with a large cloud of gluons and sea
quarks around.
A very sensitive probe for the forces reigning the nucleon is another static property of quarks and
nucleons, the spin. It seems that it plays a more important role in high-energy particle produc-
tion than expected. Traditionally the spin of particles was considered little interesting as it was
thought not to in uence the particle production at all. At any rate in high-energy multi-particle
production the particle’s mass is small compared to its energy, and thus, it was assumed, its spin
behavior should be as simple as that of massless particles. Surprisingly, experimental data did
not con rm this assumption: more than 25 years ago in the collision of high energetic protons
0of 300 GeV with beryllium nuclei, scientists found Lambdas particles (uds) for which the spin
direction was not distributed homogeneously, but mainly perpendicular (i.e. transverse) to the
0production plane spanned by proton and , even though neither proton nor beryllium were
polarized themselves [Bun76].
Since then such spontaneous polarization has been found with other interactions as well, exhibiting
a regular pattern, which should help to uncover the underlying production mechanism. Yet the
observed polarization still cannot be comprehensively described and is likely to originate from
several sources, being related to either the structure of the baryon itself or to the production
process. Measuring this quantity therefore gives an important insight into the world of the quarks
and how they put themselves together to make up baryons.
While much data on transverse polarization phenomena have already been collected in proton-
proton, kaon-proton or pion-proton collisions, no data is available from photo-production.
COMPASS, a state of the art experiment currently running at the high-energy particle physics
1Compare, for example, the binding energy of the electron of 13.6 eV with the hydrogen mass of 938 890 076.4 eV.
2