- Email: [email protected]
The HERMES recoil detector
ELSEVIER
Nuclear PhysicsB (Proc. Suppl.) 125 (2003) 189-192
The HERMES
SUPPLEMENTS
Recoil Detector
A. Borysenkoa* (On behalf of the HERMES
Collaboration)
“INFN - Laboratori Nazionali 00044 Frascati, Italy
di Frascati,
A Recoil Detector is proposed for the HERMES experiment for the investigation the detection of all Distributions of the this contribution. It background suppression
of exclusive reactions, where generated particles can provide an access to the recently introduced Generalised Parton nucleon. The design and the expected performances of Recoil Detector are presented in is shown that the Recoil Detector will ensure an effective detection of exclusive events with up to 99% in a wide kinematical range.
1. INTRODUCTION
ALU(@)
Recently, the study of hard exclusive reactions has been recognized as a powerful tool to investigate the structure of the nucleon. In particular exclusive processes can provide access to the socalled Generalised Parton Distributions (GPDs) of the nucleon and as a result can give information about the orbital angular momentum of quarks [l]. In this context HERMES performed the first measurement of a beam-spin asymmetry for the exclusive electroproduction of photons arising from the interference of Deeply Virtual Compton Scattering and the Bethe-Heitler process (DVCS-BH) (Figure 1) [2]. The measurement was performed by detecting the scattered lepton and the produced photon (ep+eyX). Due to the limited resolution of the HERMES spectrometer the exclusive events were selected with a missing mass cut of A4, < 1.7 GeV. To ensure a better identification of the exclusive process ep-+eyp a detector for the recoiling proton has been proposed. 2. The Recoil experiment
Detector
Figure 1. Projection of statistical accuracies (close circles) for measurements of the beam-spin asymmetry ALU for hard electroproduction of photons as a function of the azimuthal angle 4 between the production and the scattering plans; the first HERMES results [2] are shown as open circles; the curves represent various GPDs model predictions.
for the HERMES
erator
with high density gas targets in the region in front of the HERMES spectrometer [3]. The products of this reaction are detected in the forward angle HERMES spectrometer, where sequential tracking detectors of several types provide a good angular (A0 < 0.6 mrad) and momentum (AP/P = 0.7-
The HERMES experiment runs at DESY to investigate the spin structure of the nucleon. The 27.5 GeV positrons of the HERA accel*E-mail: 0920-5632/$
artem.borysenkoQlnf.infn.it - see front
interact
(H, D,4 He, N, Kr)
matter
0 2003 Elsevier
B.V
All rights
reserved.
190
A. Borysenko/Nuclear
Physics B (Proc. Suppl.) 125 (2003) 189-192
1.3%) resolutions. Particle identification is provided by a lead-glass calorimeter, a preshower detector, a transition radiation detector and a RICH (Ring Imaging Cherenkov Detector) [3]. The detection of hadrons in coincidence with the scattered positron and the identification of pions and kaons makes the forward HERMES spectrometer well suited for inclusive and semi-inclusive reactions. For the identification of all recoil particles generated in exclusive events a Recoil Detector was proposed to be located around the target region [4]. The 3D design of the Recoil Detector is presented in Figure 2. It consists of a Silicon Detector, a Scintillating Fiber Detector, a Photon Detector and a Superconducting Magnet [5].
momentum is presented versus the azimuthal angle. To achieve a full kinematic coverage it is necessary to use at least two detectors: silicon detector for the momentum region 0.1-0.5 GeV and scintillating fiber detector for 0.3-1.4 GeV.
P, GeV ..: DVCS-BH .,.;:
2 t
0, rad
of proton momentum Figure 3. Distribution P versus azimuthal angle 0 for DVCS-BH process (black points) and A reaction (gray points). Boxes represent the kinematic coverage of the Recoil Detector (l-silicon; 2-scintillating fiber detector).
2.1.
Figure 2. 3D design of the Recoil Detector: 1) Silicon Detector, 2) Scintillating Fiber Detector, 3) Photon Detector, 4) Superconducting Magnet of 1 T magnetic field.
With such a recoil detector it will be possible to identify and reconstruct several kinds of exclusive reactions such as: Deeply Virtual Compton Scattering, Bethe-Heitler process and the analogous reaction with delta resonance in the final state. The kinematic coverage of the Recoil Detector is presented in Figure 3, where the recoil proton
Recoil
proton
detection
The silicon detector will be located inside the vacuum beam pipe to provide a high efficiency for the detection of protons with low momenta. It consists of 16 double sided (99 mm x 99 mm x 300 pm) semiconductor TIGRE sensors, with 128 strips per side. The readout is performed with 64 HELIX 3.0 chips [5]. A readout scheme with a coupling capacitor, which divides the signal into a high and low gain readout channel, was used and a dynamic rage of 40-60 mip of readout was achieved. A signal/noise ratio of about 6.3 for minimal ionizing particle (mip) was obtained in an 1 GeV electron beam test. This detector will allow the detection of protons with a minimal momentum of Pmin = 135 Mev/c, a large
A. Bolysenkzo/Nuclear
Physics B (Proc. Suppl.) 125 (2003) 189-192
angular acceptance (0 = 0.4 - 1.35 rad, 4 N 27r) with good angular resolution (0.07 rad). The second detector, which will be used to provide the full kinematic coverage, is a scintillating fiber detector [5]. This detector is located between the beam pipe and the super conducting magnet to ensure an effective detection of protons with an energy between 0.3 GeV and 1.6 GeV. It consists of two cylindrical 4 mm thick layers of scintillating fibers SCSF-78M (8 1 mm) of total number 10648 in the first and 4508 in the second layer. The fibers are positioned with a lo0 angle between the inner and outer layers. The readout is performed with 64 channels Hamamatsu R5900-OO-M64 photomultipliers and a GASSIPLEX chip. A 10 % momentum resolution of the detector in a 1 T longitudinal magnetic field of a superconducting magnet was shown in a Monte Carlo simulation (Figure 4). The combination of the scintillating fiber detector with the silicon provides a lo0 resolution for the whole recoil proton spectrum of 0.1-1.6 GeV.
00
0.5
1
1.5
P, GeVlc
Figure 4. Recoil proton momentum resolution with the silicon (1) and the scintillating fiber (2) detectors.
2.2.
Background
suppression
The photon detector is designed for the identification of events with the intermediate delta resonance (ep + eyA+ -+ emno --+ em2r). The
0
100
200
300
400 ADC
500
191
600
700
800
900
channels
Figure 5. The results of (1) PMT calibration pedestal and 1 photoelectron peak (9 ADC channel/phe). The light output of scintillation detectors: (4) 95-100 phe/mip - scintillator directly connected to PMT; (2) 18-20 phe/mip - scintillator with light collection by two fibers glued in the groove; (3) 35-40 phe/mip - after painting the scintillator with reflecting paint.
photon detector is located between the scintillating fiber detector and the magnet. It consists of three layers of tungsten converters and scintillator bars. Each layer of scintillators consists of sixty 10x20~280 mm BC-408 strips arranged in a barrel. The readout is performed with 2 m long BCF-92A (01.5 mm) WLS fibers connected to 64 channels Hamamatsu R5900-OO-M64 photomultipliers, that will allow to install PMTs in a region, where field of superconducting magnet is less then 0.02 T. The results of test of light output of the scintillation detectors with cosmic muons are presented in Figure 5. A light output of 35-40 photoelectrons per minimal ionization particle was obtained for the scintillator with light collection by two WLS fibers. A efficiency of 70 % for photon detection and an overall efficiency of 12 % for x0 identification was shown in a MonteCarlo simulation. The difference in energy deposition for pions
192
A. Borysenko/Nucleur
AE,MeV
0
Physics B (Proc. Suppl.) 125 (2003) 189-192 AE,MeV
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6’1.8
2
0
0.2
0.40.6-i.S
1
1.2
P, G&/c
Figure 6. Energy deposition
in the scintillating
1.6
1.8
2
P, GeVlc
fiber detector (left) and the photon detector (right).
and protons with the same momentum will be used for the identification of events with the intermediate delta resonance and the following genThe eration of r+ and n (ep + eyA+ --+ enyn+). energy deposition in the scintillating fiber detector and the photon detector is presented in Figure 6, as a function of the particle momentum. As shown the combination of these two detectors can provide an effective particle identification in the full momentum range of 0.1-1.6 GeV. The combined information from the Recoil Detector with co-planarity cuts, i.e., ensuring that the recoiling proton is in the same plane as the produced meson or photon, will provide a background suppression up to 99 % and will allow effective selection of exclusive reactions.
3. SUMMARY
1.4
and OUTLOOK
The design of Recoil Detector for the HERMES experiment is presented in this contribution. It is shown that the Recoil Detector will ensure an effective detection of exclusive events with background suppression up to 99 %. This will provide the unique opportunity to study exclusive events in order to gain access to Generalised Parton Distributions. The installation of the Recoil Detector is planed in 2004 and its running in 2005-2006.
REFERENCES 1. X.Ji, Phys. Rev. D 55 (1997) 7114. 2. HERMES collaboration, A. Airapetian et al., Phys. Rev. Lett. 87 (2001) 182001. 3. HERMES collaboration, K. Ackerstaff et al., Nucl. Inst. and Meth. A417 (1998) 230. 4. HERMES collaboration, A Large Acceptance Recoil Detector for HERMES Addendum to the Proposal, DESY-PRC Ol- 01, 2001. 5. HERMES collaboration, Recoil detector Technical design report, DESY-PRC 02-01, 2002.
Nuclear PhysicsB (Proc. Suppl.) 125 (2003) 189-192
The HERMES
SUPPLEMENTS
Recoil Detector
A. Borysenkoa* (On behalf of the HERMES
Collaboration)
“INFN - Laboratori Nazionali 00044 Frascati, Italy
di Frascati,
A Recoil Detector is proposed for the HERMES experiment for the investigation the detection of all Distributions of the this contribution. It background suppression
of exclusive reactions, where generated particles can provide an access to the recently introduced Generalised Parton nucleon. The design and the expected performances of Recoil Detector are presented in is shown that the Recoil Detector will ensure an effective detection of exclusive events with up to 99% in a wide kinematical range.
1. INTRODUCTION
ALU(@)
Recently, the study of hard exclusive reactions has been recognized as a powerful tool to investigate the structure of the nucleon. In particular exclusive processes can provide access to the socalled Generalised Parton Distributions (GPDs) of the nucleon and as a result can give information about the orbital angular momentum of quarks [l]. In this context HERMES performed the first measurement of a beam-spin asymmetry for the exclusive electroproduction of photons arising from the interference of Deeply Virtual Compton Scattering and the Bethe-Heitler process (DVCS-BH) (Figure 1) [2]. The measurement was performed by detecting the scattered lepton and the produced photon (ep+eyX). Due to the limited resolution of the HERMES spectrometer the exclusive events were selected with a missing mass cut of A4, < 1.7 GeV. To ensure a better identification of the exclusive process ep-+eyp a detector for the recoiling proton has been proposed. 2. The Recoil experiment
Detector
Figure 1. Projection of statistical accuracies (close circles) for measurements of the beam-spin asymmetry ALU for hard electroproduction of photons as a function of the azimuthal angle 4 between the production and the scattering plans; the first HERMES results [2] are shown as open circles; the curves represent various GPDs model predictions.
for the HERMES
erator
with high density gas targets in the region in front of the HERMES spectrometer [3]. The products of this reaction are detected in the forward angle HERMES spectrometer, where sequential tracking detectors of several types provide a good angular (A0 < 0.6 mrad) and momentum (AP/P = 0.7-
The HERMES experiment runs at DESY to investigate the spin structure of the nucleon. The 27.5 GeV positrons of the HERA accel*E-mail: 0920-5632/$
artem.borysenkoQlnf.infn.it - see front
interact
(H, D,4 He, N, Kr)
matter
0 2003 Elsevier
B.V
All rights
reserved.
190
A. Borysenko/Nuclear
Physics B (Proc. Suppl.) 125 (2003) 189-192
1.3%) resolutions. Particle identification is provided by a lead-glass calorimeter, a preshower detector, a transition radiation detector and a RICH (Ring Imaging Cherenkov Detector) [3]. The detection of hadrons in coincidence with the scattered positron and the identification of pions and kaons makes the forward HERMES spectrometer well suited for inclusive and semi-inclusive reactions. For the identification of all recoil particles generated in exclusive events a Recoil Detector was proposed to be located around the target region [4]. The 3D design of the Recoil Detector is presented in Figure 2. It consists of a Silicon Detector, a Scintillating Fiber Detector, a Photon Detector and a Superconducting Magnet [5].
momentum is presented versus the azimuthal angle. To achieve a full kinematic coverage it is necessary to use at least two detectors: silicon detector for the momentum region 0.1-0.5 GeV and scintillating fiber detector for 0.3-1.4 GeV.
P, GeV ..: DVCS-BH .,.;:
2 t
0, rad
of proton momentum Figure 3. Distribution P versus azimuthal angle 0 for DVCS-BH process (black points) and A reaction (gray points). Boxes represent the kinematic coverage of the Recoil Detector (l-silicon; 2-scintillating fiber detector).
2.1.
Figure 2. 3D design of the Recoil Detector: 1) Silicon Detector, 2) Scintillating Fiber Detector, 3) Photon Detector, 4) Superconducting Magnet of 1 T magnetic field.
With such a recoil detector it will be possible to identify and reconstruct several kinds of exclusive reactions such as: Deeply Virtual Compton Scattering, Bethe-Heitler process and the analogous reaction with delta resonance in the final state. The kinematic coverage of the Recoil Detector is presented in Figure 3, where the recoil proton
Recoil
proton
detection
The silicon detector will be located inside the vacuum beam pipe to provide a high efficiency for the detection of protons with low momenta. It consists of 16 double sided (99 mm x 99 mm x 300 pm) semiconductor TIGRE sensors, with 128 strips per side. The readout is performed with 64 HELIX 3.0 chips [5]. A readout scheme with a coupling capacitor, which divides the signal into a high and low gain readout channel, was used and a dynamic rage of 40-60 mip of readout was achieved. A signal/noise ratio of about 6.3 for minimal ionizing particle (mip) was obtained in an 1 GeV electron beam test. This detector will allow the detection of protons with a minimal momentum of Pmin = 135 Mev/c, a large
A. Bolysenkzo/Nuclear
Physics B (Proc. Suppl.) 125 (2003) 189-192
angular acceptance (0 = 0.4 - 1.35 rad, 4 N 27r) with good angular resolution (0.07 rad). The second detector, which will be used to provide the full kinematic coverage, is a scintillating fiber detector [5]. This detector is located between the beam pipe and the super conducting magnet to ensure an effective detection of protons with an energy between 0.3 GeV and 1.6 GeV. It consists of two cylindrical 4 mm thick layers of scintillating fibers SCSF-78M (8 1 mm) of total number 10648 in the first and 4508 in the second layer. The fibers are positioned with a lo0 angle between the inner and outer layers. The readout is performed with 64 channels Hamamatsu R5900-OO-M64 photomultipliers and a GASSIPLEX chip. A 10 % momentum resolution of the detector in a 1 T longitudinal magnetic field of a superconducting magnet was shown in a Monte Carlo simulation (Figure 4). The combination of the scintillating fiber detector with the silicon provides a lo0 resolution for the whole recoil proton spectrum of 0.1-1.6 GeV.
00
0.5
1
1.5
P, GeVlc
Figure 4. Recoil proton momentum resolution with the silicon (1) and the scintillating fiber (2) detectors.
2.2.
Background
suppression
The photon detector is designed for the identification of events with the intermediate delta resonance (ep + eyA+ -+ emno --+ em2r). The
0
100
200
300
400 ADC
500
191
600
700
800
900
channels
Figure 5. The results of (1) PMT calibration pedestal and 1 photoelectron peak (9 ADC channel/phe). The light output of scintillation detectors: (4) 95-100 phe/mip - scintillator directly connected to PMT; (2) 18-20 phe/mip - scintillator with light collection by two fibers glued in the groove; (3) 35-40 phe/mip - after painting the scintillator with reflecting paint.
photon detector is located between the scintillating fiber detector and the magnet. It consists of three layers of tungsten converters and scintillator bars. Each layer of scintillators consists of sixty 10x20~280 mm BC-408 strips arranged in a barrel. The readout is performed with 2 m long BCF-92A (01.5 mm) WLS fibers connected to 64 channels Hamamatsu R5900-OO-M64 photomultipliers, that will allow to install PMTs in a region, where field of superconducting magnet is less then 0.02 T. The results of test of light output of the scintillation detectors with cosmic muons are presented in Figure 5. A light output of 35-40 photoelectrons per minimal ionization particle was obtained for the scintillator with light collection by two WLS fibers. A efficiency of 70 % for photon detection and an overall efficiency of 12 % for x0 identification was shown in a MonteCarlo simulation. The difference in energy deposition for pions
192
A. Borysenko/Nucleur
AE,MeV
0
Physics B (Proc. Suppl.) 125 (2003) 189-192 AE,MeV
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6’1.8
2
0
0.2
0.40.6-i.S
1
1.2
P, G&/c
Figure 6. Energy deposition
in the scintillating
1.6
1.8
2
P, GeVlc
fiber detector (left) and the photon detector (right).
and protons with the same momentum will be used for the identification of events with the intermediate delta resonance and the following genThe eration of r+ and n (ep + eyA+ --+ enyn+). energy deposition in the scintillating fiber detector and the photon detector is presented in Figure 6, as a function of the particle momentum. As shown the combination of these two detectors can provide an effective particle identification in the full momentum range of 0.1-1.6 GeV. The combined information from the Recoil Detector with co-planarity cuts, i.e., ensuring that the recoiling proton is in the same plane as the produced meson or photon, will provide a background suppression up to 99 % and will allow effective selection of exclusive reactions.
3. SUMMARY
1.4
and OUTLOOK
The design of Recoil Detector for the HERMES experiment is presented in this contribution. It is shown that the Recoil Detector will ensure an effective detection of exclusive events with background suppression up to 99 %. This will provide the unique opportunity to study exclusive events in order to gain access to Generalised Parton Distributions. The installation of the Recoil Detector is planed in 2004 and its running in 2005-2006.
REFERENCES 1. X.Ji, Phys. Rev. D 55 (1997) 7114. 2. HERMES collaboration, A. Airapetian et al., Phys. Rev. Lett. 87 (2001) 182001. 3. HERMES collaboration, K. Ackerstaff et al., Nucl. Inst. and Meth. A417 (1998) 230. 4. HERMES collaboration, A Large Acceptance Recoil Detector for HERMES Addendum to the Proposal, DESY-PRC Ol- 01, 2001. 5. HERMES collaboration, Recoil detector Technical design report, DESY-PRC 02-01, 2002.