ALGORITMI DI TRIGGER PARALLELI PER LA RICERCA DI DECADIMENTI RARI DEL MESONE K

NA62 presso i laboratori del CERN di Ginevra. L’obiettivo finale di questa tesi ha riguardato la creazione di un algoritmo di trigger per le camere a straw dell’esperimento NA62 e la possibilità di una sua implementazione su una GPU (scheda grafica). La tesi presenta all’inizio (primo capitolo) un introduzione riguardante l’esperimento NA62 in cui viene brevemente descritto il suo obiettivo, la misura del branching ratio del decadimento ultra raro K+ -> pi+ nu nubarra con un rapporto segnale su fondo pari a 10 per ottenere un test stringente sul Modello Standard delle particelle elementari: sono esposti cenni della fisica del decadimento con le motivazioni teoriche che hanno portato alla sua scelta e le strategie sperimentali che verranno utilizzate nella reiezione dei fondi e nella costruzione dei rivelatori. Nel secondo capitolo viene poi descritto l’apparato sperimentale, suddiviso in una parte a monte della regione di decadimento, in cui sono utilizzati i rivelatori per identificare i K+ all’interno del fascio e misurarne il tempo, la direzione e l’impulso, ed un parte a valle della regione di decadimento. In tale regione è presente un sistema di rivelatori di veto per i fotoni che permette una copertura angolare totale per angoli inferiori a 50 mrad, un rivelatore Cherenkov, un sistema di veto per i muoni, un odoscopio carico ed uno spettrometro magnetico composto da camere a deriva di tipo STRAW, che sarà descritto con particolare attenzione in quanto direttamente coinvolto nel lavoro di tesi. Successivamente, nel terzo capitolo, vengono descritte le GPU, le loro modalità di programmazione, il linguaggio usato (OpenCL) e i vantaggi del loro utilizzo al posto di trigger hardware (basati solitamente su FPGA) come i loro minori costi, la maggiore flessibilità e la loro comodità ai fini delle manutenzioni e riprogrammazioni. Nel quarto capitolo viene descritto il lavoro da me svolto per l’ottimizzazione di un algoritmo di trigger di livello 0 per la ricostruzione di singoli cerchi nel RICH per una GPU in modo da ridurre al minimo il suo tempo di esecuzione. Infine nell’ultima parte della tesi, il quindo capitolo, viene presentato lo studio, la simulazione e l’implementazione di un possibile algoritmo di trigger per le camere a STRAW con lo scopo di rigettare gli eventi di fondo K+->pi+ pi+ pi- perdendo una minima quantità di segnale. Lo studio dell’algoritmo di trigger si è sviluppato in due parti: nella prima si è considerato ogni evento di segnale K+ -> pi+ nu nubarra e di fondo K+->pi+ pi+ pi- singolarmente,mentre nella seconda parte si è considerato l’effetto del pileup, ovvero della sovrapposizione di eventi avvenuti a tempi diversi all’interno del tempo risolutivo del rivelatore. Dopo aver esposto i risultati dell’algoritmo simulato vengono descritti i cambiamenti apportati in seguito alla sua parallelizzazione e implementazione sulla GPU. I risultati dell’algoritmo e il suo tempo di esecuzione sulla GPU mostrano l’utilità dell’utilizzo di questo algoritmo e la possibilità dell’utilizzo delle GPU nel trigger dell’esperimento

Online trigger processing for the NA62 rare kaon decay experiment

The work presented in this thesis covers almost all the aspects of the common Trigger and Data Acquisition of the NA62 experiment that has as main goal the measurement the Branching Ratio of the ultra-rare K+ -> pi+ nu nubar decay, very useful to obtain a stringent test of the Standard Model. This PhD work began with the development and the testing of the firmware of common boards of the NA62 TDAQ system: TDCB and TEL62. The TDCB is a daughter-board of the TEL62 and measures the detector hit times. The TEL62 processes and stores these detector data in a buffer memory; at the arrival of a L0 trigger request, it extracts the data within a programmable time window around the trigger time to send them to the PC farm. The TEL62s of some detectors also take care of producing the L0 trigger primitives that are merged to generate L0 trigger requests. In this thesis is described the significant contribution given to the developing, the testing and the commissioning of the TDCB and TEL62 firmware. Since the 2012 Technical Run to the 2015 Run the system was tested, and evolved to be compatible with the detector input rate and the beam at growing intensity up to nominal. After three main versions the system composed by TDCB and TEL62 manages to cope with the design rate. Once the work on the Data Acquisition system was concluded, I focuses on the Trigger system and the analysis of the L0 and L1 triggers. The goal of this work was to study the detector response and the trigger conditions required to obtain the needed rejection factor with the minimum amount of signal loss. Starting from 13 MHz of event rate, the L0 trigger must provide a factor 13 of rejection to reach the design L0 output rate of 1 MHz; the L1 trigger should provide a rejection of a factor 10 to achieve the goal of 100 KHz of L1 event output rate. The starting L0 and L1 scheme analysed failed to reach the output rate request by a factor 5. This gap could be filled only by using the STRAW spectrometer at the L1. For this reason in the last part of this work the development of a L1 STRAW algorithm is described, starting from a Monte Carlo simulation and validating with the use of real data samples used for the L0 and L1 analysis. The results reached by the STRAW algorithm plus other improvements allows to achieve the required L1 rejection factor

The FPGA based trigger and data acquisition system for the CERN NA62 experiment

The main goal of the NA62 experiment at CERN is to measure the branching ratio of the ultra-rare K+ → π+vv decay, collecting about 100 events to test the Standard Model of Particle Physics. Readout uniformity of sub-detectors, scalability, efficient online selection and lossless high rate readout are key issues. The TDCB and TEL62 boards are the common blocks of the NA62 TDAQ system. TDCBs measure hit times from sub-detectors, TEL62s process and store them in a buffer, extracting only those requested by the trigger system following the matching of trigger primitives produced inside TEL62s themselves. During the NA62 Technical Run at the end of 2012 the TALK board has been used as prototype version of the L0 Trigger Processor

A high-resolution TDC-based board for a fully digital trigger and data acquisition system in the NA62 experiment at CERN

A Time to Digital Converter (TDC) based system, to be used for most sub-detectors in the high-flux rare-decay experiment NA62 at CERN SPS, was built as part of the NA62 fully digital Trigger and Data AcQuisition system (TDAQ), in which the TDC Board (TDCB) and a general-purpose motherboard (TEL62) will play a fundamental role. While TDCBs, housing four High Performance Time to Digital Converters (HPTDC), measure hit times from sub-detectors, the motherboard processes and stores them in a buffer, produces trigger primitives from different detectors and extracts only data related to the lowest trigger level decision, once this is taken on the basis of the trigger primitives themselves. The features of the TDCB board developed by the Pisa NA62 group are extensively discussed and performance data is presented in order to show its compliance with the experiment requirements.Comment: 6 pages, 7 figures, presented to IEEE RT 2014 Conference and I want to publish in TN

Search for heavy neutral lepton production in K+ decays

A search for heavy neutral lepton production in K + decays using a data sample collected with a minimum bias trigger by the NA62 experiment at CERN in 2015 is reported. Upper limits at the 10−7 to 10−6 level are established on the elements of the extended neutrino mixing matrix |Ue4| 2 and |Uμ4| 2 for heavy neutral lepton mass in the ranges 170–448 MeV/c2 and 250–373 MeV/c2, respectively. This improves on the previous limits from HNL production searches over the whole mass range considered for |Ue4|2 and above 300 MeV/c2 for |Uμ4|2

Measurement of the very rare K+→π+ννˉK^+ \to \pi^+ \nu \bar\nu decay

The decay K+→π+νν¯ , with a very precisely predicted branching ratio of less than 10−10 , is among the best processes to reveal indirect effects of new physics. The NA62 experiment at CERN SPS is designed to study the K+→π+νν¯ decay and to measure its branching ratio using a decay-in-flight technique. NA62 took data in 2016, 2017 and 2018, reaching the sensitivity of the Standard Model for the K+→π+νν¯ decay by the analysis of the 2016 and 2017 data, and providing the most precise measurement of the branching ratio to date by the analysis of the 2018 data. This measurement is also used to set limits on BR(K+→π+X ), where X is a scalar or pseudo-scalar particle. The final result of the BR(K+→π+νν¯ ) measurement and its interpretation in terms of the K+→π+X decay from the analysis of the full 2016-2018 data set is presented, and future plans and prospects are reviewed

Search for K+→π+ννˉK^+ \to \pi^+ \nu \bar{\nu} at NA62

The decay $K^{+} \rightarrow \pi^{+}\nu\overline{\nu}$ has a very precisely predicted branching ratio ($\sim 8\times 10^{-11}$) and it is one of the best candidates to reveal indirect effects of new physics. The NA62 experiment at CERN SPS is designed to measure the branching ratio of the $K^{+} \rightarrow \pi^{+}\nu\overline{\nu}$ with a decay-in-flight technique, novel for this channel. NA62 has taken data firstly in 2016 with the aim to reach the SM sensitivity. It has then collected 10 times more statistics in 2017 and a similar amount of data is expected from the 2018 run. The preliminary result on the full 2016 data set and the prospects for future developments are presented here

Results and prospects of the NA62 experiment at CERN

$K^{+}\rightarrow \pi ^+v\bar{v}$ is one of the theoretically cleanest meson decay whereto look for indirect effects of new physics complementary to LHC searches. TheNA62 experiment at CERN SPS is designed to measure the branching ratio of thisdecay with 10% precision. NA62 started to take date with a good beam intensity in 2016. The analysis of the 5% of 2016 data acquired, in view of the final measurement, will be presented

NA62 RICH

photo of NA62 RIC