RICH DETECTORS Giulia Meo University of Heidelberg 27 January 2017 1/30
Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction - Particle Identification 2/30 RICH DETECTORS Giulia Meo
Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction Cherenkov Radiation Cherenkov radiation is electromagnetic radiation emitted when a charged particle passes through a dielectric medium at a speed greater than the phase velocity of light in that medium. • Cherenkov light is emitted with cos θ c = 1 βn in which β is the velocity of the charged particle and n is the refractive index of the medium. • The light Cherenkov is produced for tracks with β > 1 /n . • Energy radiated dω = LZ 2 e 2 ω ⇒ LZ 2 e 2 ω dW 1 sin 2 θ c (1 − β 2 n 2 ( ω )) = c 2 c 2 3/30 RICH DETECTORS Giulia Meo
Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction Cherenkov Detectors • Cherenkov radiation is used in particle physics for particle identification (PID). • There are three types of Cherenkov detectors: 1. Threshold Counters 2. Differential Counters 3. RICH detectors 4/30 RICH DETECTORS Giulia Meo
Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction Threshold Counter Detectors used to select particles with a certain mass in a beam line with fixed momentum • From the choice of a medium with a suitable refractive index, we have a signal (Cherenkov light) only for particles with a certain mass. 5/30 RICH DETECTORS Giulia Meo
Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction Differential Counter Detectors used to measure the velocities of the particles in a beam line. • Selection interval ( β min = 1 n and 1 β t = n 2 − 1 ) in which the velocity √ of the particle is measured 1 1 n < β < √ n 2 − 1 β t is the velocity for the total reflection angle, in the case of a air (n ≃ 1) light guide. 6/30 RICH DETECTORS Giulia Meo
Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction RICH Detectors RICH → Ring Imaging Cherenkov • Detectors used to measure different values of β for several particles of different known momentum. • Accepted particles from various angles. • Imaging the Cherenkov cone into a ring, we can measure the ring radius. • The ring radius allows the Cherenkov angle to be determined. • From the Cherenkov angle we determine β . 7/30 RICH DETECTORS Giulia Meo
Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction RICH Detectors The main components of a RICH detector are: radiator, (mirror) and a photon detector. Figure: Focusing scheme and proximity focusing scheme 8/30 RICH DETECTORS Giulia Meo
Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction RICH Detectors 9/30 RICH DETECTORS Giulia Meo
Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction LHCb • LHCb is an experiment dedicated to the study of CP violation and the rare decay of heavy flavours. 10/30 RICH DETECTORS Giulia Meo
Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction RICH Detectors in LHCb RICH system in LHCb is used to identify charged hadrons ( π , K, p) from 2 to 100 GeV/c. Requirements from this experiment: • Reduction of combinatorial background. • Distinguish the final state of identical decay topologies B → h + h − where h is a charged hadron. 11/30 RICH DETECTORS Giulia Meo
Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction RICH Detectors in LHCb Main components of the two RICH detectors: 3 Radiators (Aerogel, a colloidal form of quartz solid, C 4 F 10 for the RICH1 and CF 4 for the RICH2), spherical mirrors and flat mirrors, two photon detectors for each RICH. 12/30 RICH DETECTORS Giulia Meo
Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction RICH Radiators • RICH1 covers the low and intermediate momentum region 1-60 GeV/c over the full spectrometer angular acceptance of 25-300 mrad. • RICH2 covers the high momentum region 15-100GeV/c over the angular range 15-120 mrad. 13/30 RICH DETECTORS Giulia Meo
Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction Mirrors • Particles produced in the collisions in LHCb travel through the mirrors. To reduce the amount of scattering, spherical mirrors have a carbon-fibre structure for RICH-1, and a special thin glass substrate for RICH-2. • The spherical mirrors of RICH1 (4 segments) are constructed in four quadrants, while those of RICH2 (56 segments), and all flat mirrors (16 and 40 segments in RICH1 and RICH2), are tiled from smaller mirror elements. 14/30 RICH DETECTORS Giulia Meo
Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction Photon Detectors • The RICH detectors utilize Hybrid Photon Detectors (HPDs) to measure the spatial positions of emitted Cherenkov photons. • The HPD is a vacuum photon detector in which a photoelectron, released from the conversion in a photocathode of an incident photon, is accelerated by an applied high voltage of typically 10 to 20 kV into a reverse-biased silicon detector. 15/30 RICH DETECTORS Giulia Meo •
Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction Photon Detectors • There are 196 HPDs in RICH1 and 288 in RICH2. • Each silicon detector contains a matrix of 32 rows and 32 columns of silicon pixel (1024 pixels per tube) 500 µm × 500 µm in size. • Silicon sensor surface < photocathode surface = ⇒ de-magnification by ∼ 5 (pixel size at the HPD entrance window of 2.5 × 2.5 mm 2 ). 16/30 RICH DETECTORS Giulia Meo
Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction Quantum Efficiency Figure: QE measurement for one of the best HPD and the average QE( % ) at 270 nm versus the HPD batch number. The QE curves show an average maximum of 31 % at 270 nm, above the specification minimum of 20 % . 17/30 RICH DETECTORS Giulia Meo
Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction Silicon Pixel - Threshold and Noise • Average signal charge at 20 kV: C = 5000 e − • Average threshold: T = 1065 e − • Average electronic noise: N = 145 e − • Signal over noise: S/N = (C-T)/N > 27 18/30 RICH DETECTORS Giulia Meo
Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction Cherenkov Photons Candidate Reconstruction • The exact emission point of each photon is unknown, the mid-point of the trajectory in the radiator is taken. • The candidate photons for each track are determined by combining the photon emission point with the measured hit positions of the photons. • Cherenkov angle θ C is computed reconstructing the trajectory of the photon through the RICH optical system. 19/30 RICH DETECTORS Giulia Meo
Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction Cherenkov Angle Resolution • Distribution of ∆θ C = θ rec − θ exp for each photon, fitted with a Gaussian plus a polynomial background. The Cherenkov angle resolution is determined to be 1.618 ± 0.002 mrad for C 4 F 10 . • This value is in reasonable agreement with the expectations from simulation of 1.50 ± 0.02 mrad in RICH 1. 20/30 RICH DETECTORS Giulia Meo
Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction Resolutions Resolution RICH1 RICH2 (in mrad) Chromatic 0.84 0.48 Pixel 0.60 0.19 Emission Point 0.61 0.27 Overall 1.45 0.65 Overall + Track 1.50 0.76 Table: Single photon resolution from all simulations. The main error sources contributing to the resolution are: • Chromatic Aberration = ⇒ due to the dependence n ( λ ). • Dimension of the silicon pixel. • Photon emission point as the mid-point in the trajectory. 21/30 RICH DETECTORS Giulia Meo
Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb: - Components - Reconstruction Reconstruction 22/30 RICH DETECTORS Giulia Meo
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