RICH DETECTORS Giulia Meo University of Heidelberg 27 January 2017 - - PowerPoint PPT Presentation

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RICH DETECTORS Giulia Meo University of Heidelberg 27 January 2017 - - PowerPoint PPT Presentation

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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

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RICH DETECTORS

Giulia Meo

University of Heidelberg

27 January 2017

1/30

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2/30 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

RICH DETECTORS Giulia Meo

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3/30 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 θc = 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

dW dω = LZ2e2ω c2 (1 − 1 β2n2(ω)) = ⇒ LZ2e2ω c2 sin2 θc

RICH DETECTORS Giulia Meo

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SLIDE 4

4/30 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

RICH DETECTORS Giulia Meo

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SLIDE 5

5/30 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.

RICH DETECTORS Giulia Meo

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SLIDE 6

6/30 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 (βmin = 1

n and

βt =

1 √ n2−1) in which the velocity

  • f the particle is measured

1 n < β < 1 √ n2 − 1 βt is the velocity for the total reflection angle, in the case of a air (n ≃ 1) light guide.

RICH DETECTORS Giulia Meo

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SLIDE 7

7/30 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.

RICH DETECTORS Giulia Meo

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SLIDE 8

8/30 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

RICH DETECTORS Giulia Meo

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SLIDE 9

9/30 Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb:

  • Components
  • Reconstruction

RICH Detectors

RICH DETECTORS Giulia Meo

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SLIDE 10

10/30 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.

RICH DETECTORS Giulia Meo

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SLIDE 11

11/30 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.

RICH DETECTORS Giulia Meo

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12/30 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, C4F10 for the RICH1 and CF4 for the RICH2), spherical mirrors and flat mirrors, two photon detectors for each RICH.

RICH DETECTORS Giulia Meo

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13/30 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

  • f 25-300 mrad.
  • RICH2 covers the high

momentum region 15-100GeV/c over the angular range 15-120 mrad.

RICH DETECTORS Giulia Meo

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SLIDE 14

14/30 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.

RICH DETECTORS Giulia Meo

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15/30 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.

  • RICH DETECTORS

Giulia Meo

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16/30 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 mm2).

RICH DETECTORS Giulia Meo

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17/30 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%.

RICH DETECTORS Giulia Meo

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18/30 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

RICH DETECTORS Giulia Meo

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19/30 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 θC is computed reconstructing the trajectory
  • f the photon through the RICH optical system.

RICH DETECTORS Giulia Meo

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20/30 Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb:

  • Components
  • Reconstruction

Cherenkov Angle Resolution

  • Distribution of

∆θC = θrec − θexp 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 C4F10.

  • This value is in reasonable

agreement with the expectations from simulation

  • f 1.50 ± 0.02 mrad in

RICH 1.

RICH DETECTORS Giulia Meo

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21/30 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.

RICH DETECTORS Giulia Meo

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22/30 Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb:

  • Components
  • Reconstruction

Reconstruction

RICH DETECTORS Giulia Meo

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23/30 Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb:

  • Components
  • Reconstruction

Photoelectron yield

  • The ring Cherenkov angle θCR is the average of each angle
  • btained from each single hit in the ring. The resolution for

Npe photoelectrons is σθC =

σθ √ N , with σθ is the resolution for

a single photonelectron.

  • The number of detected photoelectrons is deduced by the

emitted spectrum considering the efficiencies of all the RICH components : Npe = N0Lsin2 θ, with N0 the figure of merit and L the radiating path lenght.

  • Npe is measured by fitting the ∆θC distributions of the

photoelectrons.

  • The ∆θC distribution of each individual track is fitted with a

Gaussian signal over a linear background PDF.

  • The individual track Npe is taken as the number of

photoelectron candidates under the Gaussian shape.

RICH DETECTORS Giulia Meo

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24/30 Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb:

  • Components
  • Reconstruction

Photoelectron yield

Figure: Individual track photon yield distributions for the C4F10 (left) and CF4 (right) radiators. The plot is produced from kaons and pions from D0 → K− π+ decays Radiator Npe from data Npe from simulation D0 →K− π+ pp→ppµ+µ− Aerogel 5.0 ± 3.0 4.3 ± 0.9 6.8 ± 0.3 C4F10 20.4 ± 0.1 24.5 ± 0.3 29.5 ± 0.5 CF4 15.8 ± 0.1 17.6 ± 0.2 23.3 ± 0.5

RICH DETECTORS Giulia Meo

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25/30 Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb:

  • Components
  • Reconstruction

Particle Identification

  • High occupancy environment: employ an overall log-likelihood

algorithm.

  • Compute the event likelihood from the distribution of photon

hits, the associated tracks (in both RICH), their errors and a set of mass hypotheses. Starting point: assume all particles to be pions.

  • For each track: recompute likelihood changing the mass

hypothesis to e, µ, π, K and proton.

  • Keep the mass hypothesis corresponding to the the largest

increase in the event likelihood.

  • Repeat procedure until all tracks have been set to their
  • ptimal hypotheses, and no further improvement in the event

likelihood is found.

RICH DETECTORS Giulia Meo

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26/30 Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb:

  • Components
  • Reconstruction

PID calibration samples

Figure: Invariant mass distributions of the (a) K0

S , (b) Λ and (c) D0 calibration

samples.

  • Need calibration samples

independent from the RICH PID information = ⇒ to avoid bias.

  • Reconstruct,through

purely kinematic selections independent of RICH information, exclusive decays of particles copiously produced and reconstructed at LHCb

RICH DETECTORS Giulia Meo

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27/30 Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb:

  • Components
  • Reconstruction

PID Performance

Utilizing the log-likelihood values one is able to study the discrimination achievable between any pair of track types by imposing requirements on their differences.

Figure: Kaon identification efficiency and pion misidentification rate measured on data (left) and using simulated events (right) as a function

  • f track momentum. Two different ∆logL(K − π) requirements have

been imposed on the samples

RICH DETECTORS Giulia Meo

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28/30 Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb:

  • Components
  • Reconstruction

PID Performance

  • ∆logL(K − π) > 0 and averaging over the momentum range

2 - 100 GeV/c = ⇒ the kaon efficiency and pion misidentification fraction are found to be ∼ 95% and ∼ 10%.

  • The alternative PID requirement of ∆logL(K − π) > 5 =

⇒ the misidentification rate can be significantly reduced to ∼ 3% for a kaon efficiency of ∼ 85%.

RICH DETECTORS Giulia Meo

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29/30 Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb:

  • Components
  • Reconstruction

Conclusions

RICH PID is broadly used in LHCb analyses

  • Invariant mass B→ h+ h− without (left) and with (right) RICH

PID.

  • Signal under study: B0 → π+ π− (turquoise dotted line).
  • Without RICH PID, there are many contributions and this channel

is completely dominated by B0 → Kπ (red dotted line), B0 →3-body (orange dashed line), Bs → KK (yellow line), Bs → Kπ (brown line), Λb → pK (purple line), Λb → pπ (green line), background (grey solid line).

RICH DETECTORS Giulia Meo

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30/30 Cherenkov Radiation Cherenkov Detectors RICH Detectors RICH system in LHCb:

  • Components
  • Reconstruction

Questions?

RICH DETECTORS Giulia Meo