Charm-Produktion in Tiefinelastischer Streuung am ZEUS-Experiment in der Datennahmeperiode von HERA-II

Transcription

1 Charm-Produktion in Tiefinelastischer Streuung am ZEUS-Experiment in der Datennahmeperiode von HERA-II DISSERTATION zur Erlangung der Doktorwürde der Fakultät für Mathematik und Physik der Albert-Ludwigs-Universität in Freiburg i.br. vorgelegt von Falk Karstens geb. in Berlin September 26

2 Dekan: Prof. Dr. J. Honerkamp Gutachter: Prof. Dr. A. Bamberger, Prof. Dr. K. Jakobs Disputation: 8. November 26

3 To my parents and Ulrike

4

5 Kurzfassung Das ZEUS-Experiment ist eines von zwei Strahlkollisionsexperimenten mit vielseitiger Zielsetzung an der Hadron-Elektron-Ringanlage (HERA-Beschleuniger) am Deutschen Elektronensynchrotron (DESY), Hamburg. Ein neuer Mikro-Vertex-Detektor (MVD) und ein neuer Straw-Tube-Tracker (STT) werden am ZEUS-Experiment seit dem Luminositätsupgrade 21 betrieben. Beide Komponenten sind Teil eines neuen globalen Spurrekonstruktionssystems. Die verbesserte Spurrekonstruktion wird genutzt, um D ± - Mesonen in tiefinelastischer Streuung (DIS) im kinematischen Bereich 5 < Q 2 < 1 GeV 2 und.2 < y <.7 zu analysieren. D ± (21)-Mesonen sind angeregte Zustände von Mesonen mit Charmanteil, welche in einer Open-Charm-Produktion der Boson-Gluon- Fusion produziert werden. D ± -Mesonen werden im Goldenen Kanal untersucht. Bei diesem Zerfallsmodus zerfällt das D ± -Meson in starker und schwacher Wechselwirkung in ein Kaon und zwei Pionen: D ± D ( K + π ± ) + π ± S. Anhand der invarianten Masse können D ± -Mesonen identifiziert werden, welche unter Annahmen für die Ruhemassen von Kaon und Pionen und deren Impulse der Spuren berechnet wird. Der totale und differenzielle Wirkungsquerschnitte werden berechnet unter Verwendung von Monte-Carlo-Simulationen. Die Wirkungsquerschnitte werden mit Vorhersagen des theoretischen Modells Fixed-flavour-number-scheme (FNNS) in Next-to-leading-order (NLO) verglichen. Damit ist einer direkter Test der Gluon-Verteilungsfunktion im Proton möglich, welche mit diesem theoretischen Modell berechnet werden kann. Weiterhin ist die Bestimmung der Wirkungsquerschnitte Voraussetzung, um den Charmanteil in der Strukturfunktion des Protons zu bestimmen. v

6 Kurzfassung vi

7 Abstract The ZEUS experiment is one of two general purpose colliding-beam experiments at the Hadron-Elektron-Ringanlage (HERA collider) at Deutsches Elektronensynchrotron (DESY), Hamburg. The ZEUS experiment has a new Micro-Vertex Detector (MVD) and Straw-Tube Tracker (STT), which have been in operation since the luminosity upgrade in 21. Both components are part of a new global tracking system. This improved tracking has been used in a D ± analysis in deep inelastic scattering (DIS), in the kinematic range, 5 < Q 2 < 1 GeV 2 and.2 < y <.7. The D ± (21) mesons are excited states containing charm and are produced at HERA in open-charm production in boson gluon fusion. The D ± have been investigated in the so-called golden channel, where the D ± decays via strong and weak interaction into a kaon and two pions: D ± D ( K + π ± ) + π S ±. The D ± mesons are identified using its invariant mass, which is calculated with the rest mass assumption and the momentum of the tracks. The total and differential cross sections have been calculated using Monte Carlo simulations. They have been compared with theoretical fixed-flavor-number-scheme (FNNS) nextto-leading order (NLO) predictions. That is a direct test of the gluon density function calculated with that theoretical model. Furthermore the cross-section measurements are the presupposition to determine the charm contribution to the structure function of the proton. vii

8 Abstract viii

9 Contents Kurzfassung Abstract v vii 1. Introduction QCD physics DESY research center HERA storage ring ZEUS CTD tracking detector Micro-vertex detector Dead material MVD alignment Calorimeter system Trigger system and data storage Luminosity upgrade Data analysis Monte Carlo production Straw-tube tracker STT design Mechanical tension test with resonance method First running experience and STT review Pulser test Test set-up Measurement Channel characterization ix

10 Contents 4.5. Cooling system T calibration Occupancy Efficiency Test beam measurement Probability from data STT tracking Pattern recognition Track fit Track matching Tracking VCTRAK program package Kalman filter Impact parameter Beam spot Kinematics of deep inelastic scattering Structure functions Charm structure functions D ± physics Decay mechanism Next-to-leading order processes D ± approach for HERA II Measurement preparation Reconstruction of DIS variables Resolution Regular and ZTT tracking mode charm finders Charm finders with revertexing Energy loss The forward η bin Peak width in the forward bin Data quality monitoring Selection of D ± candidates Cut strategy x

11 Contents 9.7. Acceptance Forward bin Results D ± rates in e p and e + p interactions Visible cross section Differential cross sections Systematic uncertainties Cut variations Track reconstruction efficiency Fit method pt weighting Theoretical predictions Conclusions and outlook 95 A. Straw-tube tracker 97 B. Measurement preparation 11 B.1. Resolution of DIS variables B.2. Tracking studies in the forward η bin B.3. Regular and ZTT tracking mode comparison B.4. Decay length B.5. Data quality monitoring B.6. Box cut B.7. Acceptance corrections B.8. Results B.9. Systematic studies Glossary 125 Acknowledgments 145 xi

12 Contents xii

13 1. Introduction Matter is comprised of atoms that, in part, consist of elementary electrons, which are located in the atomic shell. At the center of the atom is a nucleus made from the nucleons protons and neutrons. The nucleons themselves are made of elementary quarks. Electrons and quarks are the base ingredients of the Standard model. The theory that best describes the interactions between quarks is Quantum Chromo Dynamics (QCD). It incorporates color charges and gluons as force carrier. The visible matter existing in real world is made out of three particles: electrons, protons and neutrons. Protons and neutrons can be described in terms of two quarks Figure 1.1.: The fundamental particles of the Standard model sorted into generations. All particles have corresponding anti-particles with exactly opposite quantum numbers. called up and down. Both are unified in the first quark generation. Quarks cannot be observed as free particles but instead appear in combined states. The observation of 1

14 1. Introduction light quarks u d s (up) (down) (strange) mass MeV MeV MeV charge [e] 2/3 1/3 1/3 heavy quarks c b t (charm) (bottom) (top) mass GeV GeV ± 5.1 GeV charge [e] 2/3 1/3 2/3 Table 1.1.: Mass and charge of the six different quark flavours are given [1]. radioactivity led to the discovery of a fourth particle the (electron-) neutrino, that closes up the first lepton generation. Systematic studies in accelerators with high interaction energies gave two more quark and lepton generations. These are short lived particles which are heavier but have similar properties than their smaller brothers (fig. 1.1). Some properties of light and heavy quarks are given in table 1.1. More particle generations are not expected, as can be seen for instance from ALEPH measurements at CERN (fig. 1.2). All particles have an anti-matter copy. As the universe developed, an imbalance in matter anti-matter reactions led to the fact that matter dominates the current universe. Anti-matter has the same properties than matter, except that it has opposite quantum numbers like charge and isospin. Anti-matter can be produced in the laboratory but annihilates with matter generating photons. Hadronic anti-matter can be found in antibaryons or mesons. Mesons are a bound state of matter and anti-matter made out of two valance quarks. Bound states of baryons and mesons are currently under investigation. So far no clear signal is seen, however many indications have been measured [2, 3]. Particle physics deals with three out of four fundamental forces. The boson of the strong interaction is the massless gluon. Gluons were observed for the first time in the Tasso experiment at DESY in 1979, (fig. 1.3). The electro-magnetic force is mediated by massless photons, γ. The weak interaction is represented by two massive bosons: W and Z. The mass of these bosons makes the interaction range very short. Electro-magnetic and weak interaction have been unified in electro-weak theory, part of the Standard Model. An experimental confirmation can be seen in the cross-section measurements for neutral current and charge current events at ZEUS (fig. 1.4). It is hypothesized that masses might be created because of spontaneous symmetry breaking, which involves 2

15 1.1. QCD physics Figure 1.2.: Hadronic cross-section measurements from the Aleph experiment at CERN, as function of c.m. energy around the Z boson resonance. The cross-section is described best with a three lepton generation model. another particle, the Higgs boson [4, 5]. In a minimal super-symmetric standard model extension (MSSM) all particles might be copied another time. So far no evidence has been found, but theory strongly predicts these particles as they overcome the parameter fine tuning problem in particle physics [6]. Since these particles have not been experimentally observed it is assumed that they are found at energies above current limits but possibly, they will be found by one of the upcoming experiments at the large hadron collider (LHC). Experiments at LHC might also discover extra dimensions predicted by string theory [7] QCD physics The theory of QCD successfully explains the strong interaction. The fundamental parameter is the coupling constant α s, which describes its strength. It turned out during the last decades of α s measurements, that α s depends on a scale µ and is in fact not constant (fig. 1.5). The following parametrization introduces a constant Λ of the standard 3

16 1. Introduction Figure 1.3.: A three jet event of an e + e collision seen in the Tasso detector at DESY in Topographic correlations give evidence that one jet must originate from a gluon emitted from a quark an experimental proof of the existence of the gluon. model, which has to be defined by the experiment: α s (Q 2 ) = 4π β ln (Q 2 /Λ 2 ) (1.1) The variable µ separates the hard and the soft scale. Interactions that occur at the hard scale can be calculated with perturbative QCD (pqcd). In pqcd, the quarks are asymptotically free particles. At soft scales there are scatterings on the whole proton, where no quark structure is visible. That regime is also theoretically understood. In the transition region between hard and soft scale, a rigorous mathematical treatment is possible with increased order of the perturbation expansion. QCD is a non-abelian theory. It implies that the gluons are self-interacting. That leads to the fact that the quarks are confined and are not freely visible. 4

17 1.1. QCD physics ZEUS ep DIS cross section dσ/dq 2 (pb/gev 2 ) Preliminary e + neutral current e charged current Q 2 (GeV 2 ) Figure 1.4.: Electroweak unification seen in jet cross-section measurements for neutral (NC) and charge current (CC) events measured by ZEUS above Q 2 = 1, GeV 2 [9]. The cross-sections become equal at that limit. The results are compared to Standard Model expectations separately for e p and e + p scattering..3 α s (µ) µ GeV Figure 1.5.: The strong coupling constant α s measurements shown for different kinematic regions. The one point below the curve was added from lattice calculations [1]. 5

18 1. Introduction 6

19 2. DESY research center DESY s history stretches back to the 195s. It was founded in 1959 with backing from the German government and the city of Hamburg, represented by S. Balke and M. Brauer (fig. 2.1). The DESY laboratory (fig. 2.2) is member of the Helmholtz organization of German research centers. The goal of DESY is the development and operation particle accelerator systems, with the main research topics being High Energy Physics (HEP) and the research with synchrotron radiation. DESY (DESY Zeuthen) employees some Figure 2.1.: The signing of the foundation contracts of DESY in 1959 with by S. Balke and M. Brauer in the city chambers of Hamburg. 156 people (17), of which 19 (15) work in administration and workshops. DESY it home of about 29 guest researchers who come from more than 33 nations. There are around 9 diploma students, 45 PhDs and 24 young researchers at DESY and it has an annual budget of about e145 millions (e15 millions). DESY Zeuthen in Berlin 7

20 2. DESY research center Figure 2.2.: DESY area in Hamburg seen from the air. The storage rings PETRA and HERA lie underground. The PETRA ring surrounds the DESY areal. is specialized in neutrino physics, participating in, for example, the Amanda experiment at the south pole [11]. Furthermore it is a center for parallel computing. One of the current developments is the photo injector PITZ [12] HERA storage ring The principles of particle acceleration have been found already at the beginning of the last century by R. Wideröe [13]. The build of large scale accelerators begun after the Second World War. At Deutsches Elektronensynchrotron DESY an accelerator system was built up in the last 4 years to collide electrons and protons at large energies in the Hadron-Elektron Ringanlage HERA. The ZEUS experiment is beside H1 a colliding experiment to probe the proton at the super microscope HERA. It is worldwide the largest ep collider. Starting with separate particle creation electrons 1 and protons undergo different steps of preacceleration in linear and circular accelerators. The maximum achievable energies of the accelerated particles are given with the balance of inserted and emitted energy. The radiated energy depends on the velocity v = βc and the energy E = γmc 2 in the storage ring. The velocity is rather constant but the motion on a circular trajectory of radius R implies radial acceleration and Synchrotron radiation. The classical energy loss 1 Hereafter, both electrons and positrons are referred to as electrons, unless explicitly stated otherwise. 8

21 2.1. HERA storage ring Accelerated particle e + p Energy [GeV] Center-of-mass energy[gev] 318 Injection energy [GeV] 12 4 Maximum current [ma] Number of bunches Time between bunch crossings [ns] 96 Horizontal beam size [mm] Vertical beam size [mm].3.3 Longitudinal beam size [mm] 7 14 Specific luminosity [cm 2 s 1 ma 2 ] Peak luminosity [cm 2 s 1 ] Integrated luminosity per year [pb 1 ] 44.9 Table 2.1.: HERA parameters are given for run II (23/4) [14]. per revolution δe is: δe = 4π 3 e 2 R β3 γ 4 (2.1) The ratio of velocity and energy depends on the rest mass, which is higher for protons than for electrons and hence the Lorentz boost γ is much higher for electrons for comparable energies, therefore the radiative loss is smaller for protons. Super conducting magnets which provide higher magnetic fields than conventional magnets used for electrons apply a strong Lorentz force the keep the protons on a circular orbit. The protons and electrons fly in opposite directions in separated beam tubes in a tunnel approximately 2 m under the earth surface with a circumference of approximately 6 km. Some HERA parameters are given in table 2.1. During a long shutdown in 2-1, the specific luminosity delivered by HERA was increased and hence so too was the integrated luminosity. The luminosity is now about five times higher in the post-upgrade period. Figure 2.3 shows integrated luminosities taken with the ZEUS experiment for different run periods. 9

22 2. DESY research center ZEUS Luminosity Integrated Luminosity (pb -1 ) Days of running Figure 2.3.: A long shutdown 2-1 was used at HERA to increase the specific luminosity and hence the integrated luminosity. Integrated luminosities at ZEUS for different data periods are given in the diagram. 1

23 3. ZEUS The ZEUS experiment is beside H1 a colliding experiment to probe the proton at the super microscope HERA. The ZEUS detector is designed according to the onion principle around the interaction point (fig. 3.1). The moving center of mass system in the direction of flight of the proton due to the kinematic imbalance between proton and electron that gives the experiment a priority direction. The asymmetric concept of the detector covers almost the full solid angle 4π with 98 %. The ZEUS detector holds a big variety of different components with different tasks. There are two main classes of detectors. Close to the interaction point tracking detectors are located. The momentum from the tracks of charged particles can be calculated from the curvature in a magnetic field. It is the ionizing of gas of charged particles which makes the detection possible. Beyond the track detection calorimetric systems measure the energy first of electromagnetic interacting particles and then hadronically interacting particles (sec. 3.3). The quantum mechanical nature of the observed physics processes is visible in probabilities. Measurements are done with representative samples. The error of the measurement consists of statistical and systematical contributions. The first one can be improved with high repetitions of the measurement of the same physics process. Processes with low probability are always covered by processes of higher probability. A sophisticated trigger system at the ZEUS detector offers enrichment of interesting processes (sec. 3.4) CTD tracking detector The Central tracking drift chamber (CTD) is the most important tracking chamber in the ZEUS detector [15]. The CTD is placed within a homogeneous field of a solenoid with B = 1.43 T. It measures position and momentum of charged particles. Additionally, energy loss de/dx information provides particle identification (sec. 9.3). The CTD sense wire specification effectively defines the ZEUS coordinate system. The active length of 11

24 3. ZEUS Figure 3.1.: A schematic view of the ZEUS detector is shown. The ZEUS detector is made in onion principle around the interaction point. The asymmetric built up of the detector covers almost the full solid angle 4π with 98 %. the wires is 22.4 cm between the end-plates. It covers a radial range from r = 19. cm (innermost sense wire) to r = 78.5 cm (outermost sense wire). The detector has a polar angular coverage of 15 < ϑ < 164 and a complete azimuthal angular coverage. The CTD has 9 superlayers (SL) and each layer has 8 layers inside. 5 axial superlayers parallel to the beam pipe are interleaved by 4 stereo layers which are rotated by ±5 o to get z information. Additional z information is available from the timing at both ends of the wire (ZbyT) for superlayer 1, 3 and 5 (resolution σ z 4 cm). It is used for trigger purposes mainly. 8 sense wires are slanted to get ambiguity breaking (fig. 3.2). The two track resolution is about 2 mm [16]. To recognize a secondary vertex using the CTD, it has to be separated from the primary vertex by at least.5 1 cm. The CTD is filled with a mixture of argon, CO 2 and ethane. Water was added for the HERA II period to resolve aging of the chamber caused by ethane [17]. Traversing charged particles ionize the gas mixture along their trajectory. The minimal ionizing particles ionize 1-2 atoms per centimeter. The electrons drift to the positive sense 12

25 3.1. CTD tracking detector wires with a typical velocity of 5 µm/ns, whereas the positive ions are attracted by the negative field wires. In the field of the sense wire avalanche-like multiplication occurs with an implication factor of about 1 4. The momentum resolution in HERA II for tracks (with transverse momentum p T > 15 MeV ) coming from the vertex and passing at least three superlayers was parameterized for two different tracking modes, regular tracking mode and ZTT tracking mode, which are described in chapter 5, to [18] σ reg (p T )/p T =.38 p T /p T (3.1) σ ZT T (p T )/p T =.34 p T.15.34/p T (3.2) For HERA II the CTD was starting point for combined pattern recognition for MVD (sec. 3.2) and STT (sec. 4). (b) field wire shaper wire guard wire ground wire sense wire (a) X-Y SECTION THROUGH THE CTD (b) A TYPICAL CELL IN THE CTD showing ionisation drift paths Figure 3.2.: A cut through the CTD detector is given in a). The CTD has 9 superlayers and each layer has 8 layers inside. In b) a cell of the CTD is enlarged. 8 sense wires are slanted to get ambiguity breaking. 13

26 3. ZEUS 3.2. Micro-vertex detector The micro-vertex detector (MVD) is part of the HERA II upgrade programme and was installed in 2-1 [19,2]. The MVD is made of double sided silicon strip modules with low noise occupancy. Each module consists of two half modules with p-sided strips and 3 µm thickness. The silicon strips are implanted at 2 µm distance from each other. Aluminium read-out strips are placed above each sixth silicon strip. A full module is formed by glueing a mirror imaged half module on top of another half module. Information from the inner and outer half module delivers a 3-dimensional space point. There are a total of 361, strips on 2.9 m 2 silicon in 26 modules that cover the geometric acceptance of 1 < ϑ < 17 [21]. The MVD consists of a barrel and a wheel part. The angle between strips of a half module in the barrel part is 9. The barrel part has two or three layers to fit around the oval beam pipe (fig. 3.3). The barrel modules are put onto ladders. The forward section consists of four wedge shaped wheels, each of them made of two layers of 14 silicon sensors. In each wheel the two layers are parallel but strips are tilted by 18 /14 in opposite directions. In general the signal of a charged particle is seen as a cluster of charge on more than one strip. Clusters are reconstructed with the η-algorithm [22]. Neighbouring strips above a strip threshold are grouped into clusters. The MVD provides at least three space points per track and has hit efficiency of about 97 %. Impact parameter resolution of about 1 µm is required for efficient charm tagging (sec. 5.3). The alignment needs a accuracy of 2 µm to achieve this goal (sec ). The intrinsic hit resolution is 2 µm [23]. Efforts have been made to simulate the dead material introduced by the MVD in the detector simulation. Together with the MVD a new track reconstruction model was introduced (ZTT tracking mode). This track reconstruction model is based on the Kalman filter [24] and should deliver a better accuracy for tracks since the MVD introduces an improved resolution around the vertex. The two track resolution is about 2 µm Dead material All material known from the construction is put into the detector simulation in Monte Carlo. Physics processes can be used to verify the description, e.g. photon conversions or secondary vertices. They have an increased probability in the material. A scatter plot with real data and MC events show deviations from a perfect description (fig. 3.4). Small deviations come from small differences in positions. In general all material is included. 14

27 3.2. Micro-vertex detector Figure 3.3.: A cut through the barrel part of the MVD detector is given. The barrel part has two or three layers to suit the oval beam pipe MVD alignment A priori the exact position of the tracking detectors are not known and has to be measured. The MVD has to be aligned against the CTD globally for efficient combining of tracks. Local alignment is needed to align the modules of the MVD itself against each other. The strip positions have six degrees of freedom (three rotations and three translations) to correct for. The MVD is aligned in three steps: laser alignment, cosmic alignment and ep collision alignment. An infra-red laser is used to align MVD alignments with a precision of 1 µm. Straight muon tracks created in the higher atmosphere of the earth provide improved alignment of horizontal layers of the barrel part of the MVD. The cosmic alignment gives shifts and rotations of the modules iteratively. About 1, tracks taken in two weeks with about 12 hits per track provide enough statistics to get track residuals σ 5 µm. In a later approach tracks from ep interactions are used to achieve also good alignment constants for vertical angles. In general these tracks have six hits. The Millepede program [26] provides an efficient way to align about 3, parameters in one step. The 15

28 3. ZEUS Figure 3.4.: The probability of photon interactions in the MVD are plotted for real data (black) and Monte Carlo (red) on top of each other. In general the description is good and all material is included. Small deviations come from differences in the positions [25]. alignment corrects for two shifts and three rotations. The hit residuals improve from about 3 µm with cosmic alignment to about 2 µm with ep track alignment. The quality of the alignment is seen with the impact parameters of ep collision tracks. The results of both approaches are seen in fig. 3.5 in comparison to the Monte Carlo prediction. The alignment with cosmic muons of the forward part of the MVD has difficulties as the vertical barrel part of the MVD. It can also be aligned with ep interaction tracks. For the reaction D ± D ( K + π ± ) + π S ±, the mass difference M = M(D ) M(D ) signal width improves with the ep alignment from.93 MeV to.76 MeV because of improved track fits [27] Calorimeter system The calorimeter system (CAL) measures the energies of all charged particles [29] except of neutrinos (and muons). All other particles stop in the calorimeter and deposit their energy completely. The particles shower in a material with 3 mm thick high density depleted uranium (98.1 % U 238, 1.7 % Nb,.2 % U 235 ) layers. The uranium is interleaved with 2.6 mm thick scintillation layers (SCN38). The light produced by the shower and read-out via wave length shifters with photo multipliers tubes (PMT) is equivalent to the absorbed energy. Not only the signal of each cell is measured, also the arrival time of the pulse is recorded with the precision of a few ns. It is a powerful tool 16

29 3.3. Calorimeter system Figure 3.5.: The impact parameter resolution with cosmic muon alignment shows an excess for vertical azimuthal angles (left). The impact parameter resolution improves with ep collision tracks alignment (right) [28]. The red curve gives the prediction of the Monte Carlo simulation. for background rejection at trigger level. The calorimeter is built around the tracking system. It consists of the forward (FCAL), barrel (BCAL) and rear part (RCAL) (fig. 3.6). It is made of modules, which have an electro-magnetic (EMC) and hadronic part (HAC). One EMC section has a depth of 25 X (radiation lengths). This is similar to one hadronic interaction length (1λ). One HAC section has a depth of 3 λ. The smallest subdivision of the calorimeter is called a cell. A tower contains four electro-magnetic cells of 5 2 cm 2 in the FCAL and BCAL and two electro-magnetic cells of 1 2 cm 2 in the RCAL. The tower contains one hadronic cell of 2 2 cm 2 in the RCAL and two in the BCAL and FCAL. The thickness of the scintillator and depleted uranium layers were chosen such that the CAL is compensating (e/h = 1. ±.2). This means that on average the response of the calorimeter (that is the measured light output) for hadrons and electrons is equal when these particles have the same energy. The energy resolution is superior to non compensating calorimeters, since the energy sharing between electromagnetic part and hadronic part of the cascade fluctuates. The CAL energy resolutions, as measured under test beam conditions, are σ(e)/e =.18/ E for electrons and σ(e)/e =.35/ E for hadrons (E in GeV ) [3]. The calorimeter is calibrated with the natural radioactivity of U 238 daily. 17