Overview of the PhD work

PhD Summary
My thesis is about the measurement of the parameters A_e, A_mu and A_tau that describe the parity violation of the couplings of the Z⁰ boson to electrons, muons and tau leptons. The measurement was done with polarized asymmetries at SLD. I participated in the replacement of SLD's old vertex detector `VXD2' with `VXD3'. I have also worked on the BaBar experiment that will investigate CP violation in the neutral B meson system. I participated in the development of the DIRC, a new particle identification device of BaBar, and investigated the amount of radiation damage of the `drift chamber' using a prototype at Colorado State University.

SLD Experiment
The `SLAC Linear Collider' (SLC) is a linear accelerator accelerating electrons and positrons (the positively charged anti particles of the electrons) to the energy of 45.64 GeV. A GeV is 1,000,000,000 times the energy that the electron has after passing through a potential difference of 1 Volt. After being accelerated, both electron and positron beam pass through the so-called `arc' which bends them, so that they collide head-on in the `Collider Hall'. The effective energy is 91.28 GeV (twice the beam energy). If an electron collides with a positron in the `Collider Hall', this energy is used almost entirely to create a Z⁰ boson which is an elementary particle of a mass corresponding to the energy of 91.187 GeV. In the theory of interactions between elementary particles, the so-called standard model, this particle is the cause for the weak interaction, one of the four known forces of nature. The SLC and the `SLC Large Detector' (SLD) which is at the `Interaction Point' (where the collisions happen), were built to investigate the characteristics of this particle and the weak interaction. SLC is the only accelerator in this energy range that is capable of polarizing the electron beam, that is, it can control the direction of the electron spin (=quantum mechanical description of an apparent rotation of the electron about itself).

Since the weak interaction violates the mirror symmetry, there are asymmetries involved with the production and the decay of the Z⁰. From these asymmetries it is possible to determine fundamental parameters of the standard model. My thesis is about asymmetries in the decay of the Z⁰ in leptons. Leptons is the generic name for three similar particles that are distinguished only by their mass: electrons, muons and taus. The hypothesis of lepton universality claims that all characteristics of these three leptons are the same, so measuring the asymmetries for all three of them is an important test of this hypothesis. I helped to write and submit a paper to Physical Review Letters that describes the measurement. (SLAC-PUB-7418)

Next to this analysis, my work for SLD concerned mostly the improved vertex detector `VXD3'. A vertex detector allows precise measurements of the production and decay location of elementary particles and therefore facilitates the measurement of their lifetimes. Since different particles can have vastly different lifetimes, a vertex detector is also helpful to select certain particles out of the large quantity of collision events. VXD3 uses CCD chips, which are used mainly for electronic cameras. A CCD is divided into light sensitive pixels (=small quadratic areas which will react to incident light). For VXD3, a chip has 4000 x 800 pixels of size 0.02mm x 0.02mm. VXD3 is therefore the most precise vertex detector in a collision environment such as SLD.

BaBar Experiment
According to the standard model, the weak interaction does not only violate the mirror symmetry (named P symmetry), but also the so-called CP symmetry which unites the mirror symmetry with the sign-reversal of all charges (that is the conversion of matter into antimatter). So far, this CP violation has been observed only in the neutral K meson system. An investigation of this CP violation in the B meson system requires a precise comparison of the decay of the B⁰ with the decay of its anti particle; dependent on the decay time. This decay can be measured more easily if the B mesons are produced at high velocities. The storage ring `Positron Electron Project II (PEP II)' at SLAC uses high energy electrons (9.0 GeV) and collides them with low energy positrons (3.1 GeV).

The planned `BaBar' detector will analyze the collisions. To see CP violation, it is very important to distinguish with great accuracy the long-lived particles that leave their tracks in BaBar. In 1993 I started working for the BaBar collaboration as a member of the `DIRC group' to design a new device to identify the stable particles using the Cherenkov-Effect, that is the emission of light from high energy particles in a material with a sufficiently high index of refraction. The `Detector of Internally Reflected Cherenkov light' (DIRC) uses quartz bars as Cherenkov medium as well as to guide the light outside the detector like an optical fiber.

BaBar uses a drift chamber as the main tracking system. With the help of a prototype built in 1996 by and run at Colorado State University, I investigated the amount of radiation damage to the drift chamber and its consequences that result from the operation of BaBar. For this purpose the efficiency and resolution of this prototype was measured with cosmic rays, after it was aged artificially with a radioactive Fe⁵⁵ source. I wrote programs that reconstruct these cosmic rays and measure the time-to-distance relationship for the drift chamber cells. This relationship is important, because the drift chamber can measure only times, but the interesting information from a tracking system is the location of a track left behind when a particle passes through the detector. This location can be determined from the distance of a track piece to a known position in the drift chamber, which is measured by converting the measured times to distances using the time-to-distance relationship.

Visiting Scholar at SLAC
During my stay as Visiting Scholar at the Stanford Linear Accelerator Center in Stanford, California, USA I mainly worked for the SLD experiment (especially during the `run', that is its operation during the first 6 months in 1996). I also took advantage of the presence of many other particle physicists as well as colloquium talks by invited speakers from all over the world.

Presentations and Publications
Since experiments in High Energy Physics are fairly expensive, for each of them a large number of physicists from different countries form a collaboration. The BaBar collaboration has about 400 members, the SLD collaboration about 100 members. All presentations and publications have to be approved by the entire collaboration; also, the entire collaboration is considered the author of all presentations and publications. Besides numerous collaboration meetings of BaBar and SLD, I participated in two conferences of the Division of Particles and Fields (DPF) of the American Physical Society (APS) in August 1994 and August 1996. At DPF 1996 I presented as SLD representative `Electroweak Asymmetries at SLD'. At the APS conference in Indianapolis in May 1996, I presented the preliminary results of my thesis work, the asymmetries involved with the decay of the Z⁰ in leptons. The title of this talk was `Measurement of Z⁰ Lepton Coupling Asymmetries'. Finally, I participated in the international tau lepton conference in Estes Park, Colorado. I also helped with the organization of that meeting.

Computer experience
Much of my work for the BaBar collaboration consisted of Monte-Carlo simulations. These programs simulate with a built-in random number generator first the productions and decays of the particles (done by the so-called event generator) and then the measurement of these particles by the detector taking into account measurement errors. I ported the Fortran program `AsLund' to the HP UNIX work station of the High Energy Physics group at Colorado State University. AsLund is a fast simulation for BaBar, all uncertainties are not truly simulated but described by parameters. I used AsLund to compare the DIRC with other suggested particle identification devices. More accurate simulations can be done with the slower program `BBSim' which is written in the computer languages of Fortran and C++. Again, I ported this program to the HP UNIX work station of our research group and adjusted parts of the package to the design of the DIRC prototype 0. I then compared the predictions of BBSim with the measured data from that prototype. Further, I participated in the development of a Fortran program to analyze the data of DIRC prototype I.

I also wrote programs for a prototype of the drift chamber used to predict the amount of radiation damage and to measure the consequences of this damage. For this purpose, one program part (reconstruction) calculates the tracks of cosmic rays, which can be measured with this prototype. It also does a graphical display of those tracks. Other parts calculate the efficiency and resolution of the prototype.

Programs for SLD are written in the computer languages Prepmort and IDA, both of them being Fortran variations that allow better structured programs than Fortran does. Also, they link Fortran with Jazelle, a fast relational data bank that controls the access to the SLD data. In addition to programs used for my thesis analysis, I participated in the development of the reconstruction code for the vertex detector VXD3. This code calculates the tracks of the long-lived particles through the detector using the raw data. I was furthermore responsible for the correct geometry and material input of VXD3 in the Monte Carlo simulation of SLD.