- Email: [email protected]
Particle identification in LHCb
Nuclear Instruments and Methods in Physics Research A 494 (2002) 409–415
Particle identification in LHCb C. Matteuzzia,b,* a
Universita" di Milano-Bicocca and INFN, Piazza della Scienza 3, 20126 Milano, Italy b CERN, Geneva 23, 1211 Geneva, Switzerland For the LHCb Collaboration
Abstract A brief description of the particle identification system (RICH) of the LHCb experiment is given. Preliminary results are given on a test of the solid radiator, the aerogel. The radiation hardness has been tested with g’s and protons. The photoelectron yield, the Cherenkov angle resolution, and p=p separation were determined with a p and a mixed pþ / proton beam of momentum between 3 and 10 GeV=c: Four large hybrid photodiodes tubes were used as photodetectors. r 2002 Elsevier Science B.V. All rights reserved. PACS: 10; 40; 80 Keywords: Aerogel; HPD; Cherenkov; LHCb; Particle identification
1. The RICH’s of LHCb The LHCb detector is a single arm spectrometer dedicated to the study of the mechanism of CP violation in the B system. It will operate in pit 8 on the LHC machine (Fig. 1). The luminosity will be locally controlled to a value of 2 1032 in order to keep low the probability of multiple interactions in a beam crossing. The angular acceptance of 300ð250Þ mrad in the bending (non-bending) plane has been defined to % exploit the bb-production which is very forward at 14 TeV: *Corresponding author. CERN, Geneva 23, 1211 Geneva, Switzerland. E-mail address: [email protected] (C. Matteuzzi).
The main characteristics of LHCb is a powerful system for particle identification, which will allow the separation of several signal channel and their tagging. An example of decays which will require a highly performant particle identification is given in Fig. 2. The decay Bs -Kþ K must be separated from the more abundant Bd;s -Kp and from other topologies which mimic a two prong decay, like Bd -pþ p or L-pp: Another typical case is to separate Bs -Ds K; which gives access to the angle g; from the 15 times more abundant decay Bs -Ds p (see Fig. 3). The full range to be covered, if one wants to identify low-multiplicity decay channels (high momenta range) and kaons capable of tagging the flavor (low momenta range, c.f. Fig. 4) is from about 2 to 150 GeV=c:
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 1 5 1 1 - 5
410
C. Matteuzzi / Nuclear Instruments and Methods in Physics Research A 494 (2002) 409–415
RICH-2
RICH-1
Fig. 1. The LHCb spectrometer in pit 8.
Fig. 2. The B invariant mass spectrum without (left) and with (right) RICH.
Exploiting the correlation between the emission angle and the momentum of a particle (Fig. 4), the RICH system of LHCb is divided into two subdetectors, RICH1 and RICH2, the first cover-
ing low momenta range (2–50 GeV; 25–300 mrad of acceptance), while the second, with an horizontal (vertical) acceptance of 10–120ð100Þ mrad covers the range of momenta 20–150 GeV [1].
C. Matteuzzi / Nuclear Instruments and Methods in Physics Research A 494 (2002) 409–415
411
Fig. 3. The B invariant mass spectrum without (left) and with (right) RICH.
Fig. 4. Range of momenta (left). Emission angle vs momentum of a particle (right).
The Cherenkov radiators chosen are 1 solid (aerogel) with refraction index of 1.03 to identify particles between 2 and 10 GeV; and two gases, C4 F10 and CF4 ; with n ¼ 1:0014 and 1.0005, respectively. The geometrical configuration of the two subdetectors is shown in Fig. 5. RICH1 has a 5 cm thick aerogel radiator and a 85 cm long C4 F10 radiator. The expected number of detected photoelectrons are 7 and 35, respectively. RICH2 has about 180 cm long CF4 radiator, yielding about 20 p.e. The Cherenkov photons are focused by mirrors to detector planes located outside the spectrometer acceptance.
Hybrid photodiodes (HPDs) with pixel have been chosen as photodetectors, for they allow: (1) a coverage of 75% of the total area of 2:6 m2 ; (2) a good single photolectron sensitivity, (3) a quantum efficiency (QE) larger than 20% over the range 200–600 nm; (4) a spatial granularity of 2:5 2:5 mm2 ; and (5) a readout speed of 40 MHz: All this at an affordable cost for the 340 000 electronic channels. The HPDs, manufactured by DEP, will have a 75 mm diameter photocathode, with a total external diameter of 83 mm: The Si anode is divided in 1024 pixel of 50 500 mm2 : The demagnification factor of the optics is 5, corresponding to the required granularity at the photocathode surface.
412
C. Matteuzzi / Nuclear Instruments and Methods in Physics Research A 494 (2002) 409–415
rad
300 m
Photo detectors
d
300
C4F10
mra
CF4 gas
120 mrad Beam pipe
mirror
Aerogel
Spherical mirror
Beam pipe
Flat mirror
θC
Track Photodetector housing 1
2
(m)
10
11
12 m
Fig. 5. RICH1 and RICH2.
2. The aerogel radiator In this talk, I will concentrate on the results of recent tests of aerogel, the first Cherenkov radiator of RICH1 (c.f. Fig. 5). Aerogel is a very light (its density is around r ¼ 0:15 g=cm3 ), solid quartz-like material. It can be produced with a refractive index n ¼ 1 þ 0:20r: The transmitted light is typically in the visible region, and the optical quality of aerogel is expressed in term of the clarity parameter C (appearing in the transmittance T ¼ A expðCt=l4 )). Two kinds of aerogel were tested with a particle beam and with respect to their hardness to radiation. One is produced by Matsushita (Japan) [2], is hydrophobic, is produced in tiles of 1 or 2 cm thickness, and has a clarity around C ¼ 0:01 mm4 cm1 : The second is produced at the Boreskov Institute of Catalysis in Novosibirsk [3], is hydrophilic, has up to 4 cm of thickness, and has a higher quality C around 0:006 mm4 cm1 : The radiation hardness of aerogel was tested using two different kinds of particles: g’s from a 60 Co source (Eg ¼ 1:3 and 1:7 MeV) delivering
about 420 rad=min; and protons of 24 GeV and a fluence of 9 109 p=cm2 =s: The results are given in Fig. 6 which shows the change in transmittance at 500 nm as a function of the absorbed radiation. While no change in T is observed when the aerogel is irradiated with up to about 23 Mrad of g’s, a degradation in T is visible under irradiation with protons. The change corresponds to a loss of 15% in T at l ¼ 500 nm after a dose of 12 1012 p=cm2 ; which represents about 4 years of running of LHCb at the nominal luminosity and efficiency. No significant difference has been observed for the two kinds of aerogel with respect to this behavior. The aerogel produced in Novosibirsk has been tested in terms of humidity absorption, being hygroscopic. A tile of 10 10 2 cm3 was put in a fitotron, where humid air can be flown at a fixed humidity rate and temperature. The absorbed water was measured through the difference in weight of the tile. Transmittance was then measured, in the range 200–800 nm; at interval of time of exposure to humid (70%) air.
C. Matteuzzi / Nuclear Instruments and Methods in Physics Research A 494 (2002) 409–415 140
λ=500 nm
P1 P2
100
2.511 / 7 84.97 0.1518
/ 3 53.07 -0.6839
413
4.210 P1 P2
2.170 0.2197
120
1.289 0.1881
λ=400 nm
80 100 60
T
T
80
60
40
40 20 20
Novosibirsk tile 10x10x2 cm
3
Novosibirsk tile 5x5x2.3 cm
0
3
0 0
2.5
5
7.5
10
12.5
15
17.5
20
22.5
dose(MRad)
25
0
2
4
6
8 2
10
12
14
12
dose(p/cm x 10 )
Fig. 6. T vs g dose absorbed (left) and T vs protons dose (right).
Fig. 7. Sketch of the vessel (left) and quantum efficiencies of the 4 HPDs used (right).
A loss was observed, dependent on the wavelength. At 500 nm it amounts to 8% loss, at 400 nm to 15%. In the final detector, however, nitrogen will be flown to keep a dry environment. Aerogel was also tested in a particle beam in order to measure the photoelectron yield, the Cherenkov angle resolution and the p=p separation power. The CERN-PS beam of p’s and protons in the momentum range 1–10 GeV was used. The experimental setup is sketched in Fig. 7. The Cherenkov light from aerogel was reflected by a spherical mirror back to the focal plane, where it was read out by four large (12 in: diameter) padHPDs [4], with bialkali photocathodes, with a QE shown in Fig. 7 (right).
The silicon anode was divided into 2048 pads 1 1 mm2 : Two electrodes created a fountain shape electric field determining a demagnification factor of 2.3. The tubes were designed and manufactured at CERN. The test setup was simulated with GEANT4, taking into account the geometrical description of the vessel, the characteristics of the photodetectors, as well as the physics of the aerogel. A typical signal observed in a group of 16 pads is shown in Fig. 8. The number of photoelectrons detected from a 4 cm thick tile produced in Novosibirsk has been measured to be ð8:370:4Þ; which is in good agreement with the Monte Carlo expectation of ð8:970:5Þ:
414
C. Matteuzzi / Nuclear Instruments and Methods in Physics Research A 494 (2002) 409–415
Fig. 8. (Left) ADC counts in pads on the ring. The pedestals are subtracted. The peak is the single photoelectron signal. The shadowed area is the background as measured outside the beam spill. (Right) Distribution of the fitted Cherenkov radius for 4 cm Novosibirsk aerogel. The dashed line corresponds to the MC expectation.
Fig. 9. Projection of the Cherenkov ring radius for positive p=proton beams of increasing energies 6, 8 and 10 GeV:
The Cherenkov ring has been reconstructed on an event by event basis [5]. The solid line of Fig. 8 (right) shows the distribution of R for a data run of about 30k events with the Novosibirsk aerogel. A glass filter was put at the aerogel exit window, in
order to minimize the chromaticity spread contribution, while the superimposed dashed line corresponds to the MC simulation. The width of the Gaussian-like distribution is taken as the uncertainty in the determination of the radius R:
C. Matteuzzi / Nuclear Instruments and Methods in Physics Research A 494 (2002) 409–415
The Cherenkov angle was reconstructed event by event using a retracking procedure. The 4 HPDs were assumed to be in the focal plane of the spherical mirror. The Cherenkov angle obtained with a 9 GeV=c p beam with 4 cm of Novosibirsk aerogel was (249:674:6Þ mrad; showing a poorer resolution than expected by Monte Carlo: (246:673:5Þ mrad: The Cherenkov angle measured with 4 cm of Matsushita aerogel was (252:276:6Þ mrad: The p–p separation is shown in Fig. 9. More work is necessary to conclude the analysis of the data, these results are therefore only preliminary.
415
Acknowledgements We acknowledge the support of INTAS through the Contract INTAS-679.
References [1] LHCb RICH TDR CERN/LHCC/2000-0037, September 2000. [2] H. Yokogawa, M. Yokoyama, J. Non-Cryst. Solids 186 (1995) 23. [3] A.F. Danilyuk, et al., Nucl. Instr. and Meth. A 433 (1999) 406. [4] A. Braem, et al., Nucl. Instr. and Meth. A 442 (2000) 128. [5] J.F. Crawford, Nucl. Instr. and Meth. 211 (1983) 223.
Particle identification in LHCb C. Matteuzzia,b,* a
Universita" di Milano-Bicocca and INFN, Piazza della Scienza 3, 20126 Milano, Italy b CERN, Geneva 23, 1211 Geneva, Switzerland For the LHCb Collaboration
Abstract A brief description of the particle identification system (RICH) of the LHCb experiment is given. Preliminary results are given on a test of the solid radiator, the aerogel. The radiation hardness has been tested with g’s and protons. The photoelectron yield, the Cherenkov angle resolution, and p=p separation were determined with a p and a mixed pþ / proton beam of momentum between 3 and 10 GeV=c: Four large hybrid photodiodes tubes were used as photodetectors. r 2002 Elsevier Science B.V. All rights reserved. PACS: 10; 40; 80 Keywords: Aerogel; HPD; Cherenkov; LHCb; Particle identification
1. The RICH’s of LHCb The LHCb detector is a single arm spectrometer dedicated to the study of the mechanism of CP violation in the B system. It will operate in pit 8 on the LHC machine (Fig. 1). The luminosity will be locally controlled to a value of 2 1032 in order to keep low the probability of multiple interactions in a beam crossing. The angular acceptance of 300ð250Þ mrad in the bending (non-bending) plane has been defined to % exploit the bb-production which is very forward at 14 TeV: *Corresponding author. CERN, Geneva 23, 1211 Geneva, Switzerland. E-mail address: [email protected] (C. Matteuzzi).
The main characteristics of LHCb is a powerful system for particle identification, which will allow the separation of several signal channel and their tagging. An example of decays which will require a highly performant particle identification is given in Fig. 2. The decay Bs -Kþ K must be separated from the more abundant Bd;s -Kp and from other topologies which mimic a two prong decay, like Bd -pþ p or L-pp: Another typical case is to separate Bs -Ds K; which gives access to the angle g; from the 15 times more abundant decay Bs -Ds p (see Fig. 3). The full range to be covered, if one wants to identify low-multiplicity decay channels (high momenta range) and kaons capable of tagging the flavor (low momenta range, c.f. Fig. 4) is from about 2 to 150 GeV=c:
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 1 5 1 1 - 5
410
C. Matteuzzi / Nuclear Instruments and Methods in Physics Research A 494 (2002) 409–415
RICH-2
RICH-1
Fig. 1. The LHCb spectrometer in pit 8.
Fig. 2. The B invariant mass spectrum without (left) and with (right) RICH.
Exploiting the correlation between the emission angle and the momentum of a particle (Fig. 4), the RICH system of LHCb is divided into two subdetectors, RICH1 and RICH2, the first cover-
ing low momenta range (2–50 GeV; 25–300 mrad of acceptance), while the second, with an horizontal (vertical) acceptance of 10–120ð100Þ mrad covers the range of momenta 20–150 GeV [1].
C. Matteuzzi / Nuclear Instruments and Methods in Physics Research A 494 (2002) 409–415
411
Fig. 3. The B invariant mass spectrum without (left) and with (right) RICH.
Fig. 4. Range of momenta (left). Emission angle vs momentum of a particle (right).
The Cherenkov radiators chosen are 1 solid (aerogel) with refraction index of 1.03 to identify particles between 2 and 10 GeV; and two gases, C4 F10 and CF4 ; with n ¼ 1:0014 and 1.0005, respectively. The geometrical configuration of the two subdetectors is shown in Fig. 5. RICH1 has a 5 cm thick aerogel radiator and a 85 cm long C4 F10 radiator. The expected number of detected photoelectrons are 7 and 35, respectively. RICH2 has about 180 cm long CF4 radiator, yielding about 20 p.e. The Cherenkov photons are focused by mirrors to detector planes located outside the spectrometer acceptance.
Hybrid photodiodes (HPDs) with pixel have been chosen as photodetectors, for they allow: (1) a coverage of 75% of the total area of 2:6 m2 ; (2) a good single photolectron sensitivity, (3) a quantum efficiency (QE) larger than 20% over the range 200–600 nm; (4) a spatial granularity of 2:5 2:5 mm2 ; and (5) a readout speed of 40 MHz: All this at an affordable cost for the 340 000 electronic channels. The HPDs, manufactured by DEP, will have a 75 mm diameter photocathode, with a total external diameter of 83 mm: The Si anode is divided in 1024 pixel of 50 500 mm2 : The demagnification factor of the optics is 5, corresponding to the required granularity at the photocathode surface.
412
C. Matteuzzi / Nuclear Instruments and Methods in Physics Research A 494 (2002) 409–415
rad
300 m
Photo detectors
d
300
C4F10
mra
CF4 gas
120 mrad Beam pipe
mirror
Aerogel
Spherical mirror
Beam pipe
Flat mirror
θC
Track Photodetector housing 1
2
(m)
10
11
12 m
Fig. 5. RICH1 and RICH2.
2. The aerogel radiator In this talk, I will concentrate on the results of recent tests of aerogel, the first Cherenkov radiator of RICH1 (c.f. Fig. 5). Aerogel is a very light (its density is around r ¼ 0:15 g=cm3 ), solid quartz-like material. It can be produced with a refractive index n ¼ 1 þ 0:20r: The transmitted light is typically in the visible region, and the optical quality of aerogel is expressed in term of the clarity parameter C (appearing in the transmittance T ¼ A expðCt=l4 )). Two kinds of aerogel were tested with a particle beam and with respect to their hardness to radiation. One is produced by Matsushita (Japan) [2], is hydrophobic, is produced in tiles of 1 or 2 cm thickness, and has a clarity around C ¼ 0:01 mm4 cm1 : The second is produced at the Boreskov Institute of Catalysis in Novosibirsk [3], is hydrophilic, has up to 4 cm of thickness, and has a higher quality C around 0:006 mm4 cm1 : The radiation hardness of aerogel was tested using two different kinds of particles: g’s from a 60 Co source (Eg ¼ 1:3 and 1:7 MeV) delivering
about 420 rad=min; and protons of 24 GeV and a fluence of 9 109 p=cm2 =s: The results are given in Fig. 6 which shows the change in transmittance at 500 nm as a function of the absorbed radiation. While no change in T is observed when the aerogel is irradiated with up to about 23 Mrad of g’s, a degradation in T is visible under irradiation with protons. The change corresponds to a loss of 15% in T at l ¼ 500 nm after a dose of 12 1012 p=cm2 ; which represents about 4 years of running of LHCb at the nominal luminosity and efficiency. No significant difference has been observed for the two kinds of aerogel with respect to this behavior. The aerogel produced in Novosibirsk has been tested in terms of humidity absorption, being hygroscopic. A tile of 10 10 2 cm3 was put in a fitotron, where humid air can be flown at a fixed humidity rate and temperature. The absorbed water was measured through the difference in weight of the tile. Transmittance was then measured, in the range 200–800 nm; at interval of time of exposure to humid (70%) air.
C. Matteuzzi / Nuclear Instruments and Methods in Physics Research A 494 (2002) 409–415 140
λ=500 nm
P1 P2
100
2.511 / 7 84.97 0.1518
/ 3 53.07 -0.6839
413
4.210 P1 P2
2.170 0.2197
120
1.289 0.1881
λ=400 nm
80 100 60
T
T
80
60
40
40 20 20
Novosibirsk tile 10x10x2 cm
3
Novosibirsk tile 5x5x2.3 cm
0
3
0 0
2.5
5
7.5
10
12.5
15
17.5
20
22.5
dose(MRad)
25
0
2
4
6
8 2
10
12
14
12
dose(p/cm x 10 )
Fig. 6. T vs g dose absorbed (left) and T vs protons dose (right).
Fig. 7. Sketch of the vessel (left) and quantum efficiencies of the 4 HPDs used (right).
A loss was observed, dependent on the wavelength. At 500 nm it amounts to 8% loss, at 400 nm to 15%. In the final detector, however, nitrogen will be flown to keep a dry environment. Aerogel was also tested in a particle beam in order to measure the photoelectron yield, the Cherenkov angle resolution and the p=p separation power. The CERN-PS beam of p’s and protons in the momentum range 1–10 GeV was used. The experimental setup is sketched in Fig. 7. The Cherenkov light from aerogel was reflected by a spherical mirror back to the focal plane, where it was read out by four large (12 in: diameter) padHPDs [4], with bialkali photocathodes, with a QE shown in Fig. 7 (right).
The silicon anode was divided into 2048 pads 1 1 mm2 : Two electrodes created a fountain shape electric field determining a demagnification factor of 2.3. The tubes were designed and manufactured at CERN. The test setup was simulated with GEANT4, taking into account the geometrical description of the vessel, the characteristics of the photodetectors, as well as the physics of the aerogel. A typical signal observed in a group of 16 pads is shown in Fig. 8. The number of photoelectrons detected from a 4 cm thick tile produced in Novosibirsk has been measured to be ð8:370:4Þ; which is in good agreement with the Monte Carlo expectation of ð8:970:5Þ:
414
C. Matteuzzi / Nuclear Instruments and Methods in Physics Research A 494 (2002) 409–415
Fig. 8. (Left) ADC counts in pads on the ring. The pedestals are subtracted. The peak is the single photoelectron signal. The shadowed area is the background as measured outside the beam spill. (Right) Distribution of the fitted Cherenkov radius for 4 cm Novosibirsk aerogel. The dashed line corresponds to the MC expectation.
Fig. 9. Projection of the Cherenkov ring radius for positive p=proton beams of increasing energies 6, 8 and 10 GeV:
The Cherenkov ring has been reconstructed on an event by event basis [5]. The solid line of Fig. 8 (right) shows the distribution of R for a data run of about 30k events with the Novosibirsk aerogel. A glass filter was put at the aerogel exit window, in
order to minimize the chromaticity spread contribution, while the superimposed dashed line corresponds to the MC simulation. The width of the Gaussian-like distribution is taken as the uncertainty in the determination of the radius R:
C. Matteuzzi / Nuclear Instruments and Methods in Physics Research A 494 (2002) 409–415
The Cherenkov angle was reconstructed event by event using a retracking procedure. The 4 HPDs were assumed to be in the focal plane of the spherical mirror. The Cherenkov angle obtained with a 9 GeV=c p beam with 4 cm of Novosibirsk aerogel was (249:674:6Þ mrad; showing a poorer resolution than expected by Monte Carlo: (246:673:5Þ mrad: The Cherenkov angle measured with 4 cm of Matsushita aerogel was (252:276:6Þ mrad: The p–p separation is shown in Fig. 9. More work is necessary to conclude the analysis of the data, these results are therefore only preliminary.
415
Acknowledgements We acknowledge the support of INTAS through the Contract INTAS-679.
References [1] LHCb RICH TDR CERN/LHCC/2000-0037, September 2000. [2] H. Yokogawa, M. Yokoyama, J. Non-Cryst. Solids 186 (1995) 23. [3] A.F. Danilyuk, et al., Nucl. Instr. and Meth. A 433 (1999) 406. [4] A. Braem, et al., Nucl. Instr. and Meth. A 442 (2000) 128. [5] J.F. Crawford, Nucl. Instr. and Meth. 211 (1983) 223.