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Research Collection Doctoral Thesis Resistive Multiplexed Micromegas Detectors to Search for Dark Sector Physics and Test the Weak Equivalence Principle for Anti- Matter at CERN Author(s): Banerjee, Dipanwita Publication Date: 2017-10-30 Permanent Link: https://doi.org/10.3929/ethz-b-000231250 Rights / License: In Copyright - Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library DISS. ETH NO. 24751 Resistive Multiplexed Micromegas Detectors to Search for Dark Sector Physics and Test the Weak Equivalence Principle for Anti-Matter at CERN A thesis submitted to attain the degree of DOCTOR OF SCIENCES of ETH ZURICH (Dr. sc. ETH Zurich) presented by Dipanwita Banerjee M.Sc in Physics, University of Sussex born on 21.11.1989 citizen of INDIA accepted on the recommendation of Supervisor: A. Rubbia Co-Supervisor: P. Crivelli, G. Dissertori 2017 I would like to dedicate this thesis to my late grandfather who would have been extremely happy to see me finish this work. Abstract In questa tesi vengono presentati gli esperimenti NA64 e GBAR che si pongono l'obiettivo di rispondere a due domande fondamentali ancora senza risposta nella fisica moderna: l'esistenza della materia oscura e l'asimmetria tra materia e antimateria. NA64 un esperimento svolto al CERN che ha come obiettivo la ricerca di un nuovo bosone gauge dato da una simmetria U 0(1), A0 (anche detto fotone oscuro) che potrebbe mediare l'interazione tra materia oscura e materia ordinaria tramite una nuova forza debole. L'esperimento sensibile a un intervallo ancora inesplorato di valori del mixing γ-A0, 10−5 < < 10−3 e della massa del fotone oscuro MA0 6 100 MeV. I risultati del primo test beam sono riportati in questa tesi e sono presentati i nuovi limiti sul parametro di mixing γ-A0 , questi risultati escludono l'esistenza di un A0 con massa 6 100 MeV nel caso di decadimento invisibile in grado di giustificare l'anomalia (g-2)µ. GBAR un esperimento situato presso l'accelleratore di antiprotoni al CERN che si pone l'obiettivo di misurare la caduta libera di un atomo di Anti-idrogeno con una precisione relativa dell' 1 % nella sua prima fase per testare in maniera diretta il principio di equivalenza per l'antimateria. L'esperimento user il fascio di particelle proveniente da ELENA per + produrre ioni H tramite linterazione con positronio, successivamente raffreddati a temperature ∼ 10 µK ed infine rilever gli atomi H dopo il photo-detaching di uno dei due positroni. Il segnale di detezione dato dallannichilazione del antiprotone rilevato tramite le tracce dei pioni prodotti nella reazione. L'esperimento attualmente in fase di preparazione al CERN con in programma un run di prova nel 2018. Per entrambi gli esperimenti un tracking preciso essenziale - NA64 richiede la ricostruzione precisa del momento delle particelle nel fascio per ridurre il background dovuto alle particelle a bassa energia presenti nello stesso. In GBAR un tracking preciso invece richiesto per individuare le tracce dei pioni prodotti durante l'annichilazione di H e ricostruire cos con precisione il vertice di interazione nonch ridurre il background dovuto ai cosmici. In entrambi gli esperimenti presentati Micromegas resistive con multiplexing in XY sono state scelte some soluzione al tracking. L'uso di moduli per tracking con multiplexing non era mai stato testato in ambienti ad alta intensit in passato per via delle possibili disambuigit causate dalle propriet del multiplexing. I primi risultati delle performance di questi moduli sono stati testati presso CERN SPS con un fascio di elettroni aventi una quantit di moto pari a 100 GeV/c e ad un'intensit di 3.3 × 105 e−/sec/cm2. A queste intensit il fattore 5 di multiplexing utilizzato per i moduli introduce un livello di disambuiguit pari al 50 %. I risultati qui ottenuti provano che utilizzando le informazione sulla dimensione e la carica totale integrata di ciascun cluster il livello di disambuigit pu essere ridotto sotto il 2%. Le performance attese dei moduli Micromegas in GBAR sono inoltre presentate utilizzando la simulazione di annichilazione di H utilizzando Geant4, tenendo conto dei parametri iniziali dell'atomo, l'accetanza della geometria e la risoluzione intrinseca dei moduli. Sono infine presentate la risoluzione della riconstruzione del vertice di annichilamento e la stima di rigezione del background. Abstract The experiments NA64 and GBAR aiming to explore the still unanswered questions in physics of the existence of dark matter and the matter- antimatter asymmetry, respectively, are presented in the scope of this thesis. NA64 is an experiment at CERN searching for a new U 0(1) gauge boson, A0 (dark photon) which may mediate the interaction of dark matter with ordinary matter via a very weak force. The experiment is sensitive to the still unexplored area 0 −5 −3 of γ-A mixing strength 10 < < 10 and masses MA0 6 100 MeV. The results from the first beam run are reported and new limits were set on the γ-A0 mixing strength and the 0 results exclude the invisibly decaying A with a mass 6 100 MeV as an explanation for the (g-2)µ anomaly. GBAR is an experiment set up at the AD hall at CERN aiming to measure the free-fall of anti-hydrogen with a relative precision of 1 % in the first phase for a direct test of the equivalence principle for anti-matter. The experiment plans to use the ELENA + anti-proton beam to produce H ions from its interaction with positronium, cool the ions down to ∼ 10 µK temperature and eventually detect the free-fall of H after photo-detaching the excess positron from the ion. The signal of detection is given by its annihilation producing pion tracks. The experiment is being set up at CERN and is expected to start a commissioning run in 2018. For both experiments a tracker is essential - NA64 requires precise tracking of the incoming particles to reconstruct their momentum and suppress background from the low energy beam tail. In GBAR tracking is required to track the pion tracks and reconstruct the vertex of the H annihilation and reject cosmic ray background. Multiplexed XY Resistive Micromegas modules chosen for the tracking requirements of both the experiments are presented. The use of multiplexed modules in high intensity environments was not explored so far, due to the effect of ambiguities in the reconstruction of the hit point caused by the multiplexing feature. The first performance results of multiplexed modules tested at the CERN SPS 100 GeV/c electron beam at intensities up to 3.3 × 105 e−/sec/cm2 is reported. At these rates, a factor 5 multiplexing introduces more than 50 % level of ambiguity. The results prove that by using the additional information of cluster size and integrated charge of the induced XY signal clusters the ambiguities can be reduced to a level below 2%. The expected performances of the GBAR Micromegas tracker is also summarized from the simulation of H annihilation done with Geant4, taking into account the initial parameters of the atom, geometric acceptance and intrinsic resolution of tracker modules. The resolution of vertex reconstruction and estimation of the background rejection is also presented. 4 Contents 1 Motivation 7 2 NA64 - Search for invisible decays of sub-GeV dark photons in missing energy events 9 2.1 Introduction . 9 2.2 Principle of the Experiment . 10 2.3 Expected Results . 12 2.3.1 Extraction of MA0 and from missing energy spectrum in case of obser- vation of signal events . 12 2.3.2 Expected Sensitivity in case of non observation of signal events . 14 3 GBAR- Gravitational Behaviour of Antimatter at Rest 17 3.1 Introduction . 17 3.2 Principle of the experiment . 18 3.3 Statistical Precision on g ............................... 20 4 Micromegas Detectors 27 4.1 Basic Principle of Operation . 27 4.2 Improvements to Micromegas technology . 29 4.2.1 Resitive Layer . 30 4.2.2 Multiplexing . 31 5 NA64- Experimental Setup and Design Considerations 33 5.1 Experimental setup . 33 5.1.1 The SPS H4 Secondary Beam . 33 5.1.2 Trigger Scintillators . 34 5.1.3 Dipole magnet (MBPL) . 35 5.1.4 The BGO and fine-granularity Pb-Sc SRD detectors . 35 5.1.5 Micromegas Trackers . 39 5.1.6 ECAL . 48 5.1.7 HCAL . 49 5.1.8 VETO . 50 5.1.9 Readout Electronics . 52 5.2 Background . 53 5.2.1 Physical Background . 54 5.2.2 Beam Background . 55 5 6 Data Analysis and NA64 Results 58 6.1 Synchrotron Radiation Tagging . 59 6.2 Tracking with Micromegas modules . 61 6.2.1 Multiplexing ambiguity and correction . 62 6.2.2 MM Efficiency . 64 6.2.3 Hit Resolution and Tracking . 66 6.2.4 Tracking in NA64 and suppression of low energy electron tail . 67 6.3 ECAL Shower Development . 69 6.4 HCAL and Veto Response . 72 6.5 Results from July' 2016 Beam time . 73 7 GBAR setup 81 7.1 ELENA beam . 81 7.2 Antiproton decelerator . 82 7.3 Positron-Positronium Converter . 82 7.4 LINAC . 83 + 7.5 H trapping and cooling . 84 7.6 Micromegas Tracker . 85 7.7 Cosmic Test Bench . 86 7.8 Readout Electronics . 87 8 Simulation of the GBAR tracker and optimization of the free fall geometry 89 8.1 ANSYS study of free-fall cylinder . 89 8.2 Simulation of H annihilation . 90 8.3 Background Rejection with Micromegas tracking . 94 8.4 Shaper Simulation . 95 9 Summary and Outlook 103 9.1 NA64 . 103 9.2 GBAR . 105 6 Chapter 1 Motivation One of the main triumphs in the field of science and knowledge was a theory, developed in stages, through the works of various scientists along the latter half of the 20th century that explains nearly everything that rule our daily lives - from the fundamental structure of matter and energy to reactions that power the sun, from understanding the first moments of our universe's existence to the building blocks of our being.