NA62 RICH Research Articles

Light detection system and time resolution of the NA62 RICH

A large RICH detector is used in NA62 to suppress the muon contamination in the charged pion sample by a factor of 100 in the momentum range between 15 and 35 GeV/c. Cherenkov light is collected by 1952 photomultipliers placed at the upstream end. In this paper the characterization of the photomultipliers and the dedicated Frontend and Data Acquisition electronics are described, the time resolution and the light detection efficiency measurement are presented.

Final performances of the NA62 RICH detector

A RICH detector was built in the NA62 experiment to separate pions from muons for particle momenta between 15 and 35 GeV/c. The NA62 experiment was commissioned in 2014 and 2015 and took data in the years 2016, 2017 and 2018. The NA62 RICH consists of a steel vacuum-tight cylindrical vessel, about 17 m long and 4 m wide, enclosing a central beam pipe, filled with neon gas. The Cherenkov light is reflected by a mosaic of 20 spherical mirrors with 17 m focal length, placed at the downstream end of the vessel, and it is collected by 1952 Hamamatsu R7400-U03 photomultipliers (PM) placed at the upstream end. A detailed description of the detector and of its performance during the data taking period are presented.

Performance of the NA62 ring imaging Cherenkov detector

The Ring Imaging Cherenkov detector of the NA62 experiment at the CERN SPS is a key element of particle identification in the NA62 experimental strategy. According to the NA62 requirements, the RICH identifies π+ and μ+ in the momentum range between 15 and 35 GeV/c with a muon rejection factor of O(102), measures the arrival time of charged particles with a precision better than 100 ps and is one of the main components of the NA62 trigger system. After the commissioning in 2014–2015, the RICH detector has been successfully operated during the 2016, 2017 and 2018 data taking periods of NA62. The main design aspects and operational characteristics of the NA62 RICH are described and results on its performance, directly measured with data, are presented.

The role of the NA62 RICH in the BR(K+→π+νν̄) measurement

Abstract The NA62 experiment aims to measure the B R ( K + → π + ν ν ) with a 10% precision. One of the main backgrounds comes from the decay K + → μ + ν , therefore a highly powerful pion/muon separation is needed. The NA62 RICH, together with the calorimeter system, provides an accurate particle identification which has been essential to obtain the first results of the NA62 K + → π + ν ν analysis, based on 2016 data. Two different algorithms have been exploited which, used in combination, allowed to get a pion reconstruction and identification efficiency of 75%, with a muon suppression factor of about 500, in the momentum range 15–35 GeV/c.

NA62 RICH performance: measurement and optimization

Abstract The Ring Imaging Cherenkov detector of the NA62 experiment at CERN SPS was commissioned in 2014, but the optimal performance was achieved in 2016 after the precise mirror alignment with reconstructed tracks. The measurement and monitoring of basic performance parameters is discussed: ring radius resolution, ring centre resolution, single hit resolution and mean number of hits per ring. The performance is measured with 2016 data on the positron sample. Different contributions to the resolutions are calculated.

Commissioning and performance of the NA62 RICH detector

NA62 is the last generation kaon experiment at CERN SPS aiming to study the decay K+→π+νν¯. The goal of the experiment is to measure the decay branching ratio, O(10−10), with 10% precision collecting about 100 K+→π+νν¯ events in three years of data taking and assuming a 10% signal acceptance. The weak signal and the huge background make the experiment very challenging. The NA62 detector must be able to reject background events from decay channels with branching ratios up to 10 orders of magnitude higher than the signal one and with similar experimental signature. In order to achieve the designed goal, good Particle Identification (PID), kinematic rejection and precise timing are required. The key element of the PID in NA62 is the Ring Imaging Cherenkov detector (RICH) to identify pions and muons and to measure the particle arrival time. After a pilot run in 2014, installation and commissioning of the detector were completed in 2015, when also the first physics run took place. Results on the detector performance will be presented together with preliminary results from the 2015 and 2016 physics runs.

Commissioning and first results of the NA62 RICH

The NA62 experiment at CERN has been constructed to measure the ultra rare charged kaon decay into a charged pion and two neutrinos with a 10% uncertainty. The main background is the charged kaon decay into a muon and a neutrino which is suppressed by kinematic tools using a magnetic spectrometer and by the different stopping power of muons and pions in the calorimeters. A RICH detector is needed to further suppress the μ+ contamination in the π+ sample by a factor of at least 100 between 15 and 35 GeV/c momentum, to measure the pion crossing time with a resolution of about 100 ps and to produce the trigger for a charged track. The detector consists of a 17 m long tank (vessel), filled with neon gas at atmospheric pressure. Cherenkov light is reflected by a mosaic of 20 spherical mirrors with 17 m focal length, placed at the downstream end, and collected by 1952 photomultipliers (PMTs) placed at the upstream end. The RICH detector installation was completed in the summer of 2014 and the detector was used for the first time during the pilot run at the end of 2014. The RICH was then operated during the NA62 Commissioning Run in 2015 and in the 2016 Physics Run. It must be noted that in 2014 and 2015 the RICH mirror alignment was not optimal and the need of a better performance in the pion-muon separation was the main reason for the detector maintenance carried out in the 2015-2016 winter shutdown. In this contribution the construction of the detector will be described and the performance reached during the 2014-2015 data-taking will be discussed; preliminary results from the 2016 data-taking will also be shown.

NaNet-10: a 10GbE network interface card for the GPU-based low-level trigger of the NA62 RICH detector.

A GPU-based low level (L0) trigger is currently integrated in the experimental setup of the RICH detector of the NA62 experiment to assess the feasibility of building more refined physics-related trigger primitives and thus improve the trigger discriminating power. To ensure the real-time operation of the system, a dedicated data transport mechanism has been implemented: an FPGA-based Network Interface Card (NaNet-10) receives data from detectors and forwards them with low, predictable latency to the memory of the GPU performing the trigger algorithms. Results of the ring-shaped hit patterns reconstruction will be reported and discussed.

Almagest, a new trackless ring finding algorithm

A fast ring finding algorithm is a crucial point to allow the use of RICH in on-line trigger selection. The present algorithms are either too slow (with respect to the incoming data rate) or need the information coming from a tracking system. Digital image techniques, assuming limited computing power (as for example Hough transform), are not perfectly robust for what concerns the noise immunity. We present a novel technique based on Ptolemy׳s theorem for multi-ring pattern recognition. Starting from purely geometrical considerations, this algorithm (also known as “Almagest”) allows fast and trackless rings reconstruction, with spatial resolution comparable with other offline techniques. Almagest is particularly suitable for parallel implementation on multi-cores machines. Preliminary tests on GPUs (multi-cores video card processors) show that, thanks to an execution time smaller than 10μs per event, this algorithm could be employed for on-line selection in trigger systems. The user case of the NA62 RICH trigger, based on GPU, will be discussed.

Studies of the Effects of CO$_2$ Contamination of the Neon Gas Radiator on the Performance of the NA62 RICH Detector

The NA62 RICH detector is used for the separation of pions and muons in the momentum range 15-35 GeV/c and is expected to provide a muon suppression factor better than 10-2. A prototype of the final detector equipped with about 400 PMs (RICH-400 prototype) was built and tested in a dedicated run in 2009. The π-μ separation was tested, as well as the effect of the contamination of the neon radiator with different amounts of oxygen and CO2. The μ misidentification probability is about 0.7% and the time resolution better than 100 ps in the whole momentum range. We did not observe any absorption of the light due to the contamination of the radiator, however an effect on the ring radius is clearly observed due to the change of the refractive index of the medium. The conclusion of the studies is that the amount of CO2 in the final detector should be known at the 10-3 level of precision or the quality of the pion identification could be seriously compromised.

The NA62 RICH detector

The NA62 RICH must separate charged pions from muons in a 15–35 GeV/ c momentum range. The detector will have a 17 m long, 4 m wide cylindrical vessel, filled with neon at an atmospheric pressure. A mosaic of 20 mirrors, with 17 m focal length will be placed at the downstream end of the vessel while 2000 single anode photomultipliers will be placed at the upstream part. Two test beams were held at CERN in 2007 and 2009 with a RICH prototype. A muon suppression factor greater than 100 was validated.