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Universitá Degli Studi di Ferrara dottorato di ricerca in Fisica ciclo XXI Coordinatore prof. Filippo Frontera Internal polarized gas targets: systematic studies on intensity and correlated effects Settore

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Universitá Degli Studi di Ferrara dottorato di ricerca in Fisica ciclo XXI Coordinatore prof. Filippo Frontera Internal polarized gas targets: systematic studies on intensity and correlated effects Settore scientifico disciplinare FIS/01 Dottorando Luca Barion Relatore dott. Paolo Lenisa Relatore Aggiunto prof. Paola Ferretti Dalpiaz Anni Dedicata a... Contents 1 Spin-physics and polarized antiprotons The spin of the proton Polarized antiprotons The SpinLab facility in Ferrara Unpolarized Atomic Beam Source (ABS1) Dissociator Vacuum chambers Diagnostic system Motor speed stabilization circuit Time Of Flight (TOF) Polarized Atomic Beam Source (ABS2) Pumping system Dissociator Magnetic system Data Acquisition System (DAQ) ABS ABS Computer spinlab Fox Box Ntuples Elastic scattering of the ABS beam with residual gas. Evaluation of attenuation coefficients and cross-sections Motivation Historical background Rest Gas Attenuation Measurement of the attenuation coefficient A n Experimental procedure Cross section evaluation method New ideas part A: Finned injection tubes Intensity of polarized internal targets Finned injection tubes Measurements Injection tube conductance i 4.3.2 Atomic beam intensity Target thickness estimation Comparison to simulations Conclusions New ideas part B: Trumpet nozzle Introduction Monte-Carlo simulations Measurements Procedure for data-taking Data analysis and results Conclusions A Atomic Beam Sources magnetic system 83 B Beer law 87 C Files for trumpet nozzle simulation program DS2G 93 C.1 read 1000.f C.2 DS2GD-trumpet2b.TXT D Time Of Flight 107 D.1 Time Of Flight analysis procedure D.2 Program sources D.2.1 cleanup single.csh D.2.2 header.dat D.2.3 template.ctr D.2.4 conv.inc D.2.5 labparam.inc D.2.6 shared.inc D.2.7 convolute.f D.2.8 fitit.f D.2.9 runfit.csh Introduction The work presented in this thesis has been performed in the frame of the PAX collaboration. PAX is the acronym of Polarized Antiproton experiments. The collaboration has proposed an experimental program to create a stored intense and polarized antiproton beam at the future facility FAIR (Facility for Antiproton and Ion Research) at GSI - Darmstadt, Germany. See for reference the technical report [1]. The polarizing method proposed is based on the spin selective scattering of an initially unpolarized antiproton beam, stored in a ring, passing many times through a gas of polarized protons. This spin-selective filtering is obtained using an internal polarized gas target, installed on the antiproton storage ring. The filtering targets performance is crucial for the success of the PAX project. In particular, the experiment would highly benefit from an increase in the target thickness, on which, since more then 10 years, no substantial progresses have been accomplished. These gas targets consist of a light tube, called cell, coaxial with the circulating beam and filled at its center with nuclear polarized atomic gas from an Atomic Beam Source (ABS). Due to wall collisions, the thickness of these targets is a factor 100 larger then the atomic jet alone. This technique, suggested by W.Haeberli at the end of 1960s [2] has proven very successful in various experiments. Among others, it is worthwhile to mention HERMES, and its contributions to the understanding of the nucleon spin structure [3]. An exhaustive review of this technique is presented in ref. [4]. Gas targets have a high (80-90%) polarization without dilution and the polarization direction can be reversed in milliseconds. However, the experimental luminosity is limited by the low target thickness, typically atoms/cm 2. The thickness is limited both by the geometry of the cell and the maximum intensity available from the ABS that fills the cell. This thesis has been dedicated to systematic studies on the limiting mechanisms of the target thickness and to propose new ideas for overcoming these limits; the tests have been carried out at the Spinlab laboratory of Universitá di Ferrara. The beam attenuation in the focusing chambers is a limiting factor for existing and future ABS, not fully understood at present. This effect is due both to beam particle collision with the background gas particles (rest-gas) and to intra-beam scattering. The total scattering cross section for collisions of beam particles with rest-gas particles is the physical relevant quantity that allows to evaluate independently the rest-gas attenuation, disentangling this effect from intra-beam scattering. Up to now, no data or theoretical previsions exist on this cross section, in the temperature range of interest for ABS. In the frame of this thesis, this problem has been addressed. Using a combination of beam simulations and dedicated test bench measurements, the total scattering cross sections for atomic and molecular collisions of hydrogen and deuterium in the temperature range K 1 2 CONTENTS have been extracted. These cross sections are reported for the first time in terms of relative velocity of the colliding particles. They are therefore physical quantities, independent from the different systems conditions and can be used to design new ABS. Besides ABS applications, the extraction of these cross sections is interesting in many physics fields, as astrophysics (transport phenomena), Bose-Einstein condensation, hydrogen masers, or polarized fusion [5][6]. Concerning new ideas to increase the target thickness, a study on the effect of a modified injection tube has been undertaken, adding radial fins to a traditional cylindrical tube. In this way, ideally, the incoming ABS beam is not affected, but the atoms diffusing back out the injection tube itself are partially obstructed, increasing therefore the target thickness. No improvement is foreseen for the PAX target geometry. On the other hand, from an analysis that takes into account the results of the simulations, compared with the measured data, for the first time a non-zero azimuthal component of the velocity of atoms of the beam has been demonstrated. This result has been published in [7]. More promising are the effects on the ABS hydrogen jet intensity of a trumpet nozzle, as suggested by W. Kubischta, who deduced a particular shape for the nozzle from Monte Carlo simulations. The nozzle has been produced in the Ferrara mechanical workshop and tested on the Spinlab test bench. At present the measurements confirm the trends foreseen with the simulations. This thesis is organized as follows. In the first section of Chapter 1 an overview of the physic motivations at the basis of PAX proposal is presented. The second section contains a short report on the status of art of the antiprotons polarization, with particular emphasis on the polarization mechanism connected with the filtering method. Chapter 2 describes in detail the facilities available in Spinlab laboratory of Universitá di Ferrara. They consist in two different Atomic Beam Sources, a beam diagnostic system, a calibrated gas supply system and a data acquisition system. Chapter 3 discusses the rest-gas attenuation studies and the extraction of the cross sections for the molecular-molecular and atomic-molecular interactions for hydrogen and deuterium. In Chapters 4 and 5 the effects of low conductance injection tube and of the trumpet nozzle are treated. The acceptance of focusing magnetic system for ABS is addressed in Appendix A. Appendix B contains a detailed treatment of the Beer s law for the extraction of the attenuation coefficients. Appendix C contains an example of Monte-Carlo input file and the code necessary to use the MC simulation program DS2G, together with a ray tracking program. Finally Appendix D reports the Time Of Flight measurement procedure, and contains the listing of all the used codes. Chapter 1 Spin-physics and polarized antiprotons 1.1 The spin of the proton Mass, charge and spin are fundamental properties of particles and their origin is subject of intense studies. The experiments in this field of physics were started by Rutherford in 1911, with his studies on the internal constituents of atoms [8]. He explored the electromagnetic structure of the atom by analyzing α particles scattered off a gold foil. The angular distribution of scattered particles, in fact, can give information on the inner structure of target atoms. The observation of big deflection angles (even 180 ) induced the conclusion that inside atoms there was a hard core (nucleus) where the positive charge was concentrated. This led, in 1913, to the formulation of the Bohr Model, in which the atom is depicted as a small positively charged nucleus, surrounded by electrons that travel in circular orbits around it, with electrostatic forces providing attraction. Then experiments went further, focusing on the investigation of even smaller dimensions: the structure and composition of sub-atomic particles that constitutes the nucleus (protons p and neutrons n). By studying the deviations from Coulomb scattering of α particles, Rutherford had already found an approximation for the nuclear radius: about 10 5 times smaller than an atomic radius, but the exact size of nucleons was still unknown. The hint that also nucleons should have a finite size and an inner structure came in 1933 from the experiments by Frisch and Stern[9][10]: the measured magnetic moment of proton was larger (more than double) than the value predicted by Dirac s theory, thus incompatible with a point-like particle. In fact the gyro-magnetic ratio g ( µ = eg S/2m) predicted by Dirac model is 2 for p and 0 for n, while the measured value was 5.58 for proton and for neutron. To investigate in more detail the nucleon configuration, the key is to reach smaller scales, by increasing the beam energy; in fact the higher the energy of the probe is, the smaller the size it can access. In the 1950s Hofstadter, using intense and energetic beams of electrons, investigated the 3 4 Spin-physics and polarized antiprotons electro-magnetic structure of the nucleons by studying elastic scattering reactions. e + p e + p (1.1) Figure 1.1: Elastic Scattering. In the hypothesis of one gamma exchange, the cross section of reaction eq. 1.1 (described by the Feynman diagram of fig. 1.1) is given by eq. 1.2: ( ) dσ = dω meas ( ) dσ F(q) 2 (1.2) dω Mott ) where ( ) dσ dω meas is the measured cross section, ( dσ dω is the Mott formula for the cross Mott section (point-like, spin-less target), q is the is the four-momentum transferred in the interaction and F(q) 2 are the electro-magnetic form factors of p. Form factors parameterize the difference between scattering from a point-like target and the measured data, and can be derived from the measured differential elastic cross sections. The experiments were performed at Stanford Linear Accelerator Center (SLAC), where an electron beam (accelerated by a linac) was scattered off a liquid hydrogen or deuterium target. For the results [11][12] published in , Hofstadter was awarded the Nobel Prize in Physics in Studies continued in that direction, increasing further the electron beam energy and so switching from elastic scattering to Deep Inelastic Scattering (DIS), in which the energy is so high that the reaction products are different from the colliding particles, which can break into their constituents; new particles can be created from the energy in excess. When passing from elastic scattering to DIS (fig. 1.2a), Structure Functions (SF) have to Figure 1.2: (a) Deep Inelastic Scattering, (b) Semi Inclusive Deep Inelastic Scattering. be introduced; the excitation energy of the proton adds a second degree of freedom, so SF are functions of two independent parameters. 1.1 The spin of the proton 5 The first DIS experiments (1969)[13][14] revealed different results from the previous elastic scattering experiments: the proton was not as supposed (a fuzzy ball of positive electric charge), but rather had an internal structure, made of hard scattering centers, that could explain the large deflection angles observed. These experiments showed that proton is made of much smaller particles, in a similar way as Rutherford found that atoms instead of being structure-less as initially supposed, are made of smaller particles. Feynman in 1969 introduced the parton model[15], in which protons are composed of an infinite number of point-like partons (later matched to quarks and gluons) and are considered in the infinite-momentum frame so, in DIS case, they have a very high energy. For this reason the following approximations can be introduced: the transverse momentum of the parton can be neglected (since it is small, it is possible to consider only the longitudinal momentum) and its mass can be approximated to zero. In this model, at high energy, interactions are described as occurring between the incident lepton and the point-like partons, considered as quasi-free constituents of the nucleon (fig. 1.2b). As a result the total cross section for the nucleon is obtained by the sum of the individual partons cross sections contributions. The point-like internal constituents of nucleons were identified as quarks, that had already been theoretically foreseen in 1964 by Gellmann[16]. These results opened the way to the development of a new theory of hadrons called Quantum Chromodynamics (QCD), that now is a part of the Standard Model. The resulting model for the structure of nucleons is the following: proton is composed of 3 quarks, two up quarks (u) with electric charge +2/3 and one down quark (d) with electric charge -1/3; in this way the net electric charge is +1 (in units of e). Neutron is composed of 3 quarks: one up quark (u) with electric charge +2/3 and two down quarks (d) with electric charge -1/3; in this way the net electric charge is 0. Quarks are bind together by massless and chargeless exchange bosons, called gluons. In 1980s, the EMC (European Muon Collaboration) experiments at CERN, revealed that only a small fraction of the nucleon total spin is carried by the quarks and thus opened the so called spin-crisis [17][18]. The result was subsequently confirmed by SLAC and the spin entered in the new scenario as the tool to understand the nucleons structure. The three already known quarks were called valence quarks, and the hypothesis of the existence of the sea quarks (couples of q q) and sea gluons was made. The single constituents spins, their corresponding angular motion and binding bosons (gluons) are now considered possible sources of the total nucleon spin, according the formula: S N z = 1 2 = q + G + Lq z + L g z (1.3) where Sz N is the z component of nucleon spin, q and G are the contributions from quarks and gluons spins, L q z and L g z are the contribution from quarks and gluons angular momenta. The importance of dedicated experiments to measure these quantities is evident. At present the theoretical models are based on the fact that, at leading-twist order, the quark structure of hadrons is completely described only by three structure functions. These functions are the quark momentum distribution F 1 (x, Q 2 ), the helicity distribution g 1 (x, Q 2 ) and transversity distribution h q 1 (x, Q2 ). Quark momentum distribution F 1 (x, Q 2 ) describes the number of quarks (or antiquarks) 6 Spin-physics and polarized antiprotons of designated flavor that carry a momentum fraction between x and x+dx of the proton s momentum, in the infinite-momentum frame. Helicity distribution g 1 (x, Q 2 ) describes the difference between the probability to find a quark (with momentum fraction x) with spin aligned and spin anti-aligned to the spin of the longitudinally polarized nucleon ( Prob( ) Prob( ) ). Transversity h 1 describes the difference between the probability to find a parton (with momentum fraction x) with spin aligned and spin anti-aligned to the spin of the transversely polarized nucleon ( Prob( ) Prob( ) ). Using Deep Inelastic Scattering, experimental and theoretical studies led to improved knowledge of F 1 (x, Q 2 ) and g 1 (x, Q 2 ); in order to complete the description of the partonic structure of the nucleon, today one piece is still missing: transversity. Up to now, little is known about transversity because it is the most difficult SF to measure, since it is a chiral-odd function. For this reason it is not possible to measure transversity in DIS experiments. Electroweak and strong interactions conserve chirality, so in DIS events, h 1 cannot occur alone, but has to be combined to a second chiral-odd quantity. Thus it is necessary to use one of the following reactions: Semi-Inclusive Deep Inelastic Scattering (SIDIS) (h 1 coupled to a chiral-odd fragmentation function called Collins function); transverse Single Spin Asymmetries (SSA) in inclusive processes like p p πx (h 1 coupled to Collins function); polarized Drell-Yan processes (product of two transversity distributions). Recently Anselmino and collaborators have extracted h 1 with a simultaneous analysis of DIS experiments (HERMES and COMPASS) and the annihilation experiment BELLE [19]. The value obtained for h 1 was found to be rather small respect the calculations of the quark models. This opened very interesting speculations and made more urgent the direct measurement of the transversity with the Drell-Yan events produced in the annihilation of protons and antiprotons, both polarized: p + p e + + e + X (1.4) Measuring the double transverse spin asymmetry A TT, a direct measurement of transversity can be obtained. It should be noted that it is necessary to use p beam, because a proton beam (available for example at RHIC) would give a much lower useful event-rate; in fact the proton beam would probe the sea quarks, instead of the valence quarks, therefore leading to a much lower cross section. 1.2 Polarized antiprotons Polarized antiprotons The most challenging part of the PAX program is to find a way to produce an intense circulating stored beam of polarized antiprotons. In fact, such a tool is still not existing, despite the efforts lasted many years (see [20]). For instance it is interesting to remind that, in 1985, O. Chamberlain and A. Krish organized a workshop at Bodega Bay (CA, USA)[21], with the aim to provide a deep discussion on all the ideas proposed to polarize antiprotons. Prof A. Krish, in his review talk, listed twelve ideas. 1. Polarized antiprotons from in flight decay of antihyperons. 2. Spin filtering of antiprotons by a polarized hydrogen target. 3. Stochastic technique (see stochastic cooling). 4. DNP in flight using polarized electrons and microwave radiation. 5. Spin-Flip from synchrotron radiation induced by X-ray laser. 6. Spontaneous Spin-Flip from synchrotron radiation. 7. Polarization by scattering. 8. Repeated Stern-Gerlach deflections. 9. Polarized antiprotons via the formation of anti-hydrogen, and the application of ABS method. 10. Polarization during storage in a penning trap. 11. Polarization by channeling. 12. Polarization through interaction with polarized X-rays from a diamond crystal. Several of this ideas survived the workshop for a significant period of time. In particular idea (1) has been successfully applied at FNAL, by the E704 Collaboration [22], leading to secondary 500 GeV/c beams of protons and antiprotons. However, no accumulation and storage have ever been tried. Idea (2), the spin filtering of stored beams, was rated as practical and promising. In fact in 2007 Daresbury Workshop[23] emerged that the only method survived, among the ones proposed in the Bodega workshop, was the spin filtering. It is interesting to observe that this idea is certainly not new. There were a number of examples of production of polarized nucleons via interactions with polarized targets. In 1966 Shapiro[24] showed that, due to strong spin dependence in the (n,p) cross section, polarized neutrons could be produced by means of spin-selective attenuation in a solid polarized proton target. In 1970 a test[25] successfully used
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