ISBN 91-628-3661-7 LUNFD6/(NFFL-7173) 1999 Potential for B-physics measurements with a fixed-target proton-collision experiment Thesis submitted for the degree of Doctor of Philosophy in Physics by Jenny Ivarsson Department of Physics Lund University Professorsgatan 1 Box 118 SE-221 00 Lund Sweden Abstract HERA-B is a high-energy physics experiment at HERA. It is a fixed-target experi- ment with a forward spectrometer to benefit from the strong boost of beauty hadrons. The main goal of HERA-B is to detect and measure the degree of CP violation in 0 0 exclusive B-decays, with emphasis on the golden decay, B J/ψKS. A computer simulated study on this decay is presented and the accura→cy of the measurement is estimated. Necessary conditions for a detection of CP violation are investigated. Various other channels are reviewed, which can give a sign of CP violation within and beyond the Standard Model. Very high rates are required for measurements of the typically very much suppressed signals. The thesis includes a comparison of the situation with the B-factories and the hadron colliders. HERA-B has a very tight time schedule and physics measurements will be performed even before the detector is completed. In particular, the order of installation of detector parts is determined not only by technical factors but also by the feasibility to measure the b¯b cross- section, which is a crucial parameter in the precision of any B-physics measurements at HERA-B. A thorough study has been performed on the possibilities to measure the b¯b cross-section with a partially equipped detector. The emphasis is placed on the inclusive decay B J/ψX. Studies on double semileptonic decays and Υ decays are also presented. → Contents Preface 3 1 B Physics and CP Violation 7 1.1 Heavy-quarksymmetries.......................... 8 1.2 Openheavyflavourproductionanddecay ................ 11 1.3 Heavy-quarkonium production and decay ................ 17 1.4 CPviolationintheStandardModel ................... 18 1.5 Mixingofneutralmesons ......................... 22 1.6 ExperimentalobservationofCPviolation ................ 24 1.7 CPviolationbeyondtheStandardModel ................ 32 2 The HERA-B experiment 36 2.1 Requirements................................ 37 2.2 VertexDetector............................... 38 2.3 Trackingsystem .............................. 41 2.4 Particleidentificationdetectors ...................... 45 2.5 Trigger.................................... 48 2.6 OfflineSoftware............................... 52 2.7 ComparisonwithotherB-physicsexperiments.............. 63 3 Optimization for σb¯b measurements 68 3.1 Experimentalandtheoreticalpredictions. ................ 69 3.2 Theoptimalgeometry ........................... 70 3.3 Semileptonicdecays ............................ 91 3.4 Υreconstruction ..............................106 4 The golden decay 112 4.1 ExtractingaCPviolationsignature ...................112 4.2 Thesimulatedeventsample........................116 4.3 Triggersimulation .............................116 4.4 Analysisandefficiencies ..........................119 4.5 Background.................................125 4.6 B flavourdetermination ..........................127 4.7 The precision of sin 2β ...........................130 iv CONTENTS 5 Summary 135 Acknowledgements 138 A Kalman filter 141 Bibliography 145 Notice the World The sun shines on a flower stalk On the path where I do walk On the field where startled deer Leap into bush to disappear Each step the scenery’s new I could live for this and this view I could die for this and this vital fear. Spirited songs of greyish lark Cracklings in the aged bark The humming little flies and bees Whispering wind up in the trees Rustle from leaves on the ground I could live for this and this sound I could die for this and this unified peace. There’s a scent I can’t locate But it’s clearer near the lake Deep in the wood it’s more intense like moss that clings to a meadow fence And fir growing close by the well I could live for this and this smell I could die for this and this nuance of sense. 2 CONTENTS The gentle wind that heats and chills Fondles over grassy hills Caresses too my cheek and hair Dabbed by a wing so weak and fair I touch the nature ceiling I could live for this and this feeling I could die for this and this varying air. Sweet dynamics of death and birth Complexity engulfs the earth With all its souls that need refill Nature itself is rich in it still A superior beauty reigns too I could live for this and I do I could die for this and I know that I will Jenny Ivarsson Munich 1995 Preface Have you ever reflected on the endlessly rich complexity in nature? Have you ever walked in a bright friendly forest in early summer, when the wind rustles quietly in the leaves, little creatures run around in the grass and the sun shines on your skin and finds the way to your amygdala? How can a world which exhibits such complexity be built? If you want to describe it, paint it, make a map of it, you can spend your whole lifetime without getting all the details, because once you notice the little needle, you will soon have to accept that it contains a whole world by itself. I pondered this. In physics books, they talked about atoms, protons and even mysterious things like quarks. In biology, the science of the living things, they talked about cells, that cells build up all this. But that dead and living things must be fundamentally the same, I was certain. The burning question I had during my first years as a thinking teenager was, whether cells were made up of atoms or atoms made up of cells. But I was lucky; I asked this question in a time when the wisdom of other people who had already thought about everything was documented in book after book. There was Democritus from ancient Greece with the idea of something undividable, Rutherford who found that the atom has a nucleus, and Gell-Mann postulating the quarks as the constituents of protons and neutrons. And there were Scheiden and Schwann who in 1838 proposed that the cell was the smallest form of life, and Mendel who got the idea of genes. You just need luck to find a book where both atom and cell are mentioned in the same chapter. The cell has a kernel in which there are chromosomes made up of genes, which in turn are made up of bases, and each base consists of five atoms. This sentence gave me my second scientific revelation (the first was when I understood how the earth can be round when it looks flat). No wonder that nature is so complex when it is constructed of such small building blocks. The number of possible combinations is just incredible. Perhaps you also sometimes wonder about the vast empty sky. When you lift your eyes upwards at night, do you then behold infinity? How can the universe be infinite? And if it is not, what lies beyond? And has the universe existed for an eternity? If not, what was there before? These are a couple of very uncomfortable paradoxes. And then along comes Einstein and tells us that time is relative. This concept is very hard to accept, but if you do, you can get a relieving solution to the space–time paradox. If space is bent in an additional dimension, just like the surface 4 CONTENTS of the earth and time is our perception of the extra dimension, then the universe has no boundary but is still not infinite. In my interpretation, space and time were created from a singularity and questions like ‘before’ or ‘outside’ are irrelevant. The history of the universe can tell us a lot, if not all about the world we are living in. In the beginning, when the available space was very small, everything was very concentrated and the energy available for physics processes enormous. In particle physics we try to reconstruct those processes by building particle accelerators, to achieve high energies concentrated on small spots. Under those conditions a lot of particles are created which do not exist in our normal low temperatures. Early in history, in the Big Bang theory, there existed electrons and positrons, quarks and antiquarks in equal proportions. Conservation laws required balance be- tween them. As the universe expanded it cooled down and some processes became impossible. At a certain threshold pair production was no longer possible and an- nihilation of matter and antimatter became an irreversible process. Electrons and positrons annihilated and released energy in the form of photons. Light quarks and antiquarks annihilated after 10−4 seconds, the heavier b-quarks and anti-b quarks after 10−9 seconds. If the universe had not expanded and cooled down, the annihi- lation processes would have continued until there was only energy left, which would have led to a terribly boring world without life or complexity. In the end there was more quarks than antiquarks. How could this happen? All those conservation laws required that there should be equal concentrations of both. I read in a book that some very heavy gauge boson (a force mediator), translated more often to a quark pair than the corresponding antiboson translated to an antiquark pair, although their total decay rates were the same. When the temperature decreased so that the reverse reaction was no longer possible, all of those gauge bosons eventually decayed and we were left with more quarks than antiquarks. I thought this was ingenious, but I was still uneasy. What the author of the popular book had not bothered to tell me was that in fact, some of the symmetry laws must be broken to lead to this decay. In this way, one question brings the other and at this stage you are addicted to knowledge. Then you go to university and learn the mathematical formulas for everything and how all the fantastic world of elementary particles and forces and their interactions can be described with mathematics; and all the time new questions arise, like how the particles get their masses and what happens at even smaller distances, at even higher energies.