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WARP an Argon Double Phase Technique for Dark Matter Search

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WARP an Argon Double Phase Technique for Dark Matter Search UNIVERSITY OF PAVIA Department of Nuclear and Theoretical Physics WARP An argon double phase technique for dark matter search External Refereee: Prof. E. Bellotti Doctoral Thesis of L. Grandi Dottorato di Ricerca in Fisica XVII Ciclo Abstract One of the most intriguing puzzles of modern physics involves the dark matter problem. Several experimental observations, obtained through very different experimental techniques, both at galactic and cosmic scale, suggest that the majority of matter, whose total amount is somehow fixed by the observed gravitational potentials, is not visible. This dark matter, to coherently satisfy the experimental results should be constituted by nonbaryonic, cold, neutral and relic elementary particles, known with the name of WIMPs (Weakly In- teracting Massive Particles). These particles could be in principle detectable through their possible interaction, although with a very low cross section, with ordinary matter, leading to the production within a target of nuclear recoils in the range up to 100 keV . The most promising WIMP candidate is represented by the Lightest Supersymmetric Particle (LSP) called, in the MSSM model, neutralino. The WARP programme (WIMP ARgon Programme) belongs to the so- called direct detection experiments class and it proposes to use liquid argon as target for WIMPs search. In order to be sensitive to the expected tiny energy depositions and to be able to discriminate nuclear recoils, eventually gener- ated by WIMPs scattering, from the dominant electromagnetic background, the standard Time Projection Chamber technique has been modified, foresee- ing a gaseous region above the sensitive volume where luminous multiplication can occur. An interaction in the medium produces scintillation light and a cer- tain amount of ionization: the application of opportune electric fields is used to drift a fraction of ionization electrons towards the liquid-gaseous interface, to extract and accelerate them in order to induce electroluminescence light. Photomultipliers are hence used to detect both prompt scintillation light and the delayed secondary light associated with ionization. The ratio between the two intensities can be used to discriminate the nature of the ionizing particle: a second and decoupled identification technique is provided by prompt scintil- lation signal shape analysis. These experimental techniques have been tested by the means of a 2.3 l double phase argon chamber prototype providing ex- tremely good background rejection power. On the basis of the results a 100 l sensitive volume chamber has been pro- jected. Material selection and the use of active and passive shielding strongly ii reduce the otherwise overwhelming background, mainly associated with neu- trons and electromagnetically interacting particles (photons and electrons) coming from outside and from inner materials. The most dangerous surviving background component is represented by the β-emitter 39Ar; this radioactive isotope is present in liquid argon with a typical activity, within the detec- tion window, of about 0.76 Bq/kg of Ar. This not negligible contamination, directly measured by the means of the prototype, implies very performing re- jection powers, in order to reduce as much as possible the number of β-events mislabeled as nuclear recoil (obviously the discrimination techniques will start to fails at some level). If the combination of the two proposed identification techniques is able to provide a rejection power of the order of 10−7-10−8 (as it seems possible), the surviving background events should be kept to a rate lower than 1 event in 10 100 days, leading to the possibility of exploring ÷ WIMP-nucleon cross sections approximately two orders of magnitude lower than actual limits. The proposed technique seems really promising mainly due to argon low cost and to the fact that its treatment is well supported even at industrial level: as a consequence huge mass detectors can be imagined, leading to tremendous increase of the sensitivity to WIMP search. To my beloved family and to my sweet and patient Giada iv Aknowledgements First of all I would like to thank the I.N.F.N. and in particular the sec- tion of Pavia (administrative office, mechanical and electronic laboratories) for having supported the experimental R.&D. activity we have started about four years ago in the Pavia laboratory that has led to the proposal of the WARP experiment. At the same time I would like to thank also the direction of the National Laboratory of Gran Sasso for the helpfulness shown to host the WARP programme. A number of people have played a crucial role in the experimental activity that has provided the results shown in this Ph.D. thesis. I would like to thank my supervisor Prof. C. Rubbia for the opportunities he gave me. A great thank to Doc. C. Montanari for the job done together, for his helpfulness and for his tenacity. For his approach to physics and for the interesting discus- sions I would like to thank also Doc. N. Ferrari. A thank also to the whole WARP collaboration and in particular to the Pavia group: thank to Prof. E. Calligarich and to Prof. M. Cambiaghi. Together with my collaborators, a lot of other people should be thanked: they are my family, my girlfriend and all my friends. Each of them knows exactly the special role he plays in my life. Thank to everyone. II Contents Preface VII 1 The dark matter 1 1.1 Introduction............................. 1 1.2 The Friedmann-Robertson-Walker Model . 2 1.2.1 The Robertson-Walker metric . 2 1.2.2 TheFriedmannModel . 5 1.3 Thedarkmatterpuzzle . .. .. .. .. .. .. 11 1.3.1 Nonbaryonic candidates . 17 1.3.2 Dark matter and galactic structures formation . 26 1.3.3 The galactic dark halo and its local density . 30 1.4 Summary .............................. 32 2 WIMP Direct detection 35 2.1 Generalconsiderations . 35 2.2 Expected nuclear recoils energy spectrum and rate . 36 2.3 Direct detection experimental techniques . 49 2.3.1 Experimentsreview . 53 2.4 Argon as proposed WIMP target: the WARP programme . 57 2.5 Summary .............................. 60 3 Liquid Argon Technology 63 3.1 Introduction............................. 63 3.2 Particles interaction in Argon . 64 3.2.1 Electron-ion pairs production in argon . 66 3.2.2 Ionization recombination process in liquid argon . 76 3.2.3 Scintillation light emission in liquid argon . 83 3.2.3.1 Scintillation light quenching processes . 87 IV CONTENTS 3.2.3.2 Ionization and luminescence time dependence . 97 3.2.4 Xenon doped Argon: general behaviour . 98 3.2.5 Electronegative impurities electrons attachment . 102 3.3 Double-phase implementation of LAr technology . 104 3.3.1 Electrons extraction through liquid-gas interface . 104 3.3.2 Proportional light emission in noble gases . 108 3.4 Summary .............................. 110 4 WARP R.&D. experimental results 115 4.1 WARP Prototype 2.3 l:phaseI.................. 116 4.1.1 Experimental Set-up . 117 4.1.1.1 Cryogenic and purification system . 119 4.1.1.2 Photomultiplier and light collection . 123 4.1.1.3 Fields configuration and grid transparency . 128 4.1.1.4 Signal description and data acquisition system 132 4.1.2 Experimental results . 141 4.1.2.1 Evaluation of light collection efficiency . 143 4.1.2.2 Analysis of extraction and multiplication data 147 4.1.2.3 Particle discrimination based on S2/S1 . 151 Detector response to X-rays............ 152 Detector response to nuclear recoils . 154 Detector response to γ and α-particles . 166 Comparison between α and nuclear recoils . 174 4.2 WARP Prototype 2.3 l:phaseII ................. 176 4.2.1 The modified experimental set-up . 176 4.2.1.1 New data acquisition and readout system . 179 4.2.2 Experimental results at LNGS . 182 4.2.2.1 39Ar activity measurement . 182 4.2.2.2 S2/S1 and pulse shape discrimination . 190 4.2.2.3 S2/S1 rejection power . 194 4.3 Summary .............................. 200 5 WARP proposed 100 l detector 203 5.1 Theexperimentallayout . 204 5.1.1 Innerdetector. 206 5.1.2 Active anti-coincidence shield . 211 5.1.3 External passive shields . 215 CONTENTS V 5.1.4 Cryogenics and purification systems . 217 5.1.5 Readout electronics and trigger system . 220 5.2 Backgroundestimate . 222 5.2.1 Estimate of γ-like background . 225 5.2.2 Estimate of neutrons-induced background . 233 5.2.3 Estimate of neutrinos-induced background . 241 5.3 Projected experimental sensitivity . 243 Bibliography 245 VI CONTENTS Preface In this PhD thesis I have tried to summarize the main results of almost four years spent to characterize the behaviour of a double phase argon chamber with the ultimate aim to develop a high performance particle discrimination technique, useful even for tiny energy depositions, to be eventually exported to a larger sized detector for dark matter search. Like in each small R&D activity, I have had the fortune to face almost every aspect of this research from the hardware to the data analysis. As far as the layout given to this PhD work, I have decided to be as much exhaustive as possible, trying to give the reader the elements to fully under- stand the liquid argon technology and to minutely explain the motivations that have suggested the various experimental choices. These guidelines could sometimes result as pedantic but, according to my personal taste, a detailed description of the work is more useful for those people trying to really get into the experimental problem. The reverse of the medal is that, at the same time, this work could result too detailed for readers just looking for the main exper- imental results. Trying to solve this problem I have inserted at the end of each chapter (apart from Chap. 5) a brief summary to focus the main information presented in the chapter itself.
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