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An optical discriminator for hyperon physics
&*H__
Nuclear Instruments and Methods in Physics Research A 387 (1997) 88-91
m-_
NUCLEAR INSTRUMENTS 8 METHODS IN PIIVSICS RESEARCH Sectm A
An optical discriminator for hyperon physics N. Lerosa3*, J.P. Perrouda, R. Chipauxb, A. Delbartb, J. Derreb, Y. Giomatarisb, C. Kochowskyb, S. Loucatosb “Lausanne Universi@, IPN, BSP, 1015 Dorigny, Switzerland “DAPNIA, SPP, C.E. Saclay, F-91191 Gifsur-Yvette, France
Abstract The use of an optical discriminator for fast hyperon selection has been investigated by the RD30 Collaboration. A realistic Monte-Carlo simulation of this device based on the detection of the Cherenkov light produced in a thin crystal shows quite promising performances. Such a discriminator has been built and the first results obtained in a pion beam will be presented.
1. Introduction Many fixed target experiments at high luminosity hadron accelerators require an efficient and fast selection of rare events within a typical 50 to 1OOMHz total rate. For events which contain long lived unstable particles (beauty, charm or strangeness physics) which are produced in a small target, the proposed [l] optical discriminator solves this problem. The optical discriminator consists of a spherical crystal shell centered on the target. For particles crossing the crystal sphere of radius R with an angle B with the normal (impact parameter b = OR), a fraction of the Cherenkov photons produced in the crystal are trapped and can be detected at its edge. Particles originating from the target do not lead to any photon detection. The development of such a device started in 1992 as R&D experiment CERN-RD30 [3]. A detailed description of the principle of the optical discriminator together with a first experimental result obtained with a LiF crystal can be found in Ref. [2]. The selection occurs at the downstream interface of the crystal with refractive index n, and the surrounding medium with refractive index nz. The minimum crossing angle of the particle with the crystal which can produce trapped Cherenkov light is e,,,,, = 42n,. E is a small positive number simply given by n: - n: = 1 - E. The sensitivity to low impact parameters was shown to be limited by the dispersion of the refractive index of LiF and by a thickness effect due to the rotation of the normal along the photon path inside a small radius crystal. To reach sensitivity to impact parameters of a few hundred km necessary for B-physics [4], two improvements have been investigated [5,6]. An achromatic pair has been found using a sapphire crystal and a siloxane cladding liquid such *Corresponding author. [email protected]. 0168~9002/97/$17.00 PII SO 168-9002(
Fax:
Copyright
96)00968-O
+41
21
692
0 1997 Elsevier
3605,
e-mail:
that E becomes almost independent of the wavelength and the use of a multilayer crystal of the same total thickness but with several thinner shells to diminish the thickness effect. In this paper we describe an optical discriminator suitable for the E871-FNAL fixed-target experiment at the Tevatron [7]. This experiment will perform a search for CP violation in hyperon decays. The aim is to fully reconstruct 4 X lo9 decays in order to achieve a sensitivity for the relevant asymmetry in decay angular distribution parameters of about 10m4.
2. Monte-Carlo simulation In experiment E871, unpolarized E hyperons are produced at zero degree by protons and momentum selected with a magnetic channel. The decay sequences 5 +ITA and R-+rp are detected with a simple wire chamber spectrometer with high-rate capability. The optical discriminator for this experiment consists of a spherical crystal shell of sapphire with a radius of curvature of 16 m, located at the end of a decay vacuum chamber, 13 m downstream of the hyperon channel exit. The sapphire crystal diameter of 250mm just covers the acceptance for the charged particles produced in the decay chain which have both vertices in the decay region. Background primary particles of the right selected momentum (mostly pions) seem to come from an almost point-like virtual source located around the middle of the hyperons channel. They cross the crystal which is centered on this virtual source at small angles and within a radius 60 mm from the shell center. The hatched area in Fig. 1 shows the distribution in angles for these primary background particles. The distributions in angles for the pions and proton, n(s), a(A) and p(A) from the E and A decay vertices are also shown. These distributions extend to much larger
Science B.V. All rights reserved
N. Leros
0
0
2
I
et al.
3
4
I Nucl.
and Meth.
Instr.
5
6
Track angle [mrad] Fig. I. Normalized distributions of the angle at the crossing point with the normal to the crystal. Hatched histogram is for primary particles, other histograms are for the proton and the pions from E decay chain. The open circles represent, for a 3 mm thick crystal, the average photoelectron yield per particle as a function of this angle.
in
Phys.
Res. A
89
387 (1997) 88-91
angles. The optical discriminator takes advantage of these differences to select within a very short time (several ns) events with detected trapped Cherenkov photons. A typical curve showing the photoelectrons yield per particles as a function of the crossing angle is also shown in Fig. 1. Below a threshold angle @,,, the photoelectron yield is very low. It then increases and reaches a saturation value. For the interface between sapphire and the siloxane Cargille liquid used as cladding, the dispersion curve of the threshold angle 0,,,,, at a temperature of 25°C is shown in Fig. 2. The threshold angle fI,,,,, can be adjusted for a given liquid by adjusting its temperature, the dashed line in Fig. 2 shows the curve at 38°C used in the simulation. The measured quantum efficiency of the phototubes looking at the crystal edge is also given. For the simulation we consider only photons with a wavelength larger than 320 nm. Within the range of sensitivity of the photodetector the value of O,,,m,, remains lower than about 3 mrad. The simulation predictions at the 40 MHz rate expected for the experiment E871 are very encouraging. If at least two fired phototubes with a threshold set at 0.3 photoelectrons are required and assuming an extra 25% of light losses, one expect a reduction of background by a factor of 25 while keeping 95% of the z events.
3. The experimental tests
20
“1 ‘:
l
’
.
The final 250 mm in diameter spherical sapphire crystal is still under construction and we present here experimental results obtained in a 10 GeV/c momentum CERN pion beam with a 64 mm in diameter, 3 mm thick flat sapphire crystal. Fig. 3 is a sketch of the crystal and its liquid cladding container. The crystal can precisely rotate around
Q.e.
4
.
0.175
yj 0.1s i .
/to
PM1
/
jO.12,
\ 10 -
\
T=25’C
0.1
Fig. 2. Threshold angle above which light can be trapped in the crystal as a function of the wavelength A. The solid line with the dots is obtained from measurements of the siloxane liquid refractive indices at 25” corrected to take into account the slight birefringence of the crystal. The dotted line is the curve adopted for the simulation. The black squares and the right scale is the measured quantum efficiency of the phototube used during our test IUIIS.
\ \ to
PMT
Fig. 3. Sketch of the crystal and liquid container.
II. VACUUM DEVICES
90
N. Loos et al. 1 Nucl. Instr. and Meth. in Phys. Res. A 387 (1997)
two orthogonal axis. The particle trajectory is reconstructed with two upstream drift chambers, each equipped with x-y planes and having a spatial resolution of 150 p.m. The exit edge of the crystal is seen by a set of 40 photomultipliers (PMT) RI398 from Hamamatsu, sitting on a cone at an angle of 76 degrees relative to the beam and at 184 mm from the crystal exit face. Hamamatsu phototubes RI398 have a peak to valley ratio of the single photoelectron response better than 2, a single photoelectron background rate at 20°C less than 1 kHz and a rise time better than 2 ns. The whole detector is enclosed in a light tight box stabilized at 20°C by air circulation. The cladding liquid flow is around 0.2 to 0.3 l/mn and its temperature is stabilized around the desired value. The pion beam has a divergence of 1.3 mrad and is almost normal to the crystal. Pions which cross the crystal within a radius of 4 mm around its center are selected. For each incident pion we record the number of phototubes having a signal of amplitude higher than 0.3 photoelectrons in coincidence within 7 ns with the incoming pion. Fig. 4 is a plot of the average number of hit phototubes per incident track as a function of the temperature of the cladding liquid. We have used a liquid with an index of refraction at 25°C of n, = 1.4562 at the sodium line. At low temperature the threshold angle e,,,,, is high and one observes essentially a background due to photons with an angle lower than the total reflexion angle which nevertheless reach the exit face of the crystal after multiple Fresnel reflexions. This background is quite high since only 3 reflexions on the downstream interface are necessary to reach the exit face. The threshold angle t?,,,,, diminishes at a rate of about 0.4 mrad per degree and the Fresnel background increases slowly until the value of @,,, = 0 is
88-91
Fig. 5. For two liquid cladding temperatures 24°C (open circles) and 30°C (black dots), the average number of hit phototubes as a function of the incident angle of the particle.
reached. At this point the rise becomes very fast and saturates when all the produced photons are totally reflected at the selecting interface. The data are in good agreement with the Monte-Carlo simulation. In Fig. 5 we present for two temperatures, 24°C and 30°C the average number of hit phototubes as a function of the angle of the incident particle. For these data the beam was kept fixed and the crystal support was rotated. These curves clearly show the expected behavior, a flat Fresnel background and a rise when the threshold angle 0,,,,, is reached. It also clearly shows the displacement of the threshold with temperature. With this small crystal the Fresnel background dominates and other small contributions cannot be estimated. With the 250 mm in diameter crystal this background will be reduced by at least a factor of 10 since 14 reflexions instead of 3 on the selecting interface will be necessary to reach the exit edge of the crystal.
4. Conclusions
Temperature
I-C]
Fig. 4. Average number of hit phototubes per track as a function of the liquid cladding temperature. The open circles are the data and the black dots the simulation prediction.
A realistic simulation has shown that a rather simple device consisting of a thin spherical sapphire crystal with a suitable liquid cladding interface can be used as a first level trigger in a hyperon experiment. For the FNAL hyperon experiment E871, a background rejection by a factor of 25 and an efficiency of 95% for 8 events can be achieved. Preliminary tests with a smaller crystal have given very encouraging results and confirmed simulation predictions. A final test with the 250 mm in diameter, 3 mm thick and with 16 m radius of curvature will begin soon.
N. Leros et ~1. I Nucl. Instr. and Meth. in Phys. Reu. A 387 (1997) 88-91
References [I] G. Charpak, Y. Giomataris and L. Lederman, Nucl. Instr. and Meth. A 306 (1991) 439. 121 G. Charpak et al.. Nucl. Instr. and Meth. A 332 (1993) 91. [3] CERN/DRDC/RD30 (20 August 91);
[4] [5] [6] [7]
91
CERN/DRDC/92-53 Addendum (5 November 1992); CERN/DRDC/93-56 Status Report (20 December 1993). P.D. Dauncey et al.. Nucl. Instr. and Meth. A 351 (1994) 147. C. Kochowski, Nucl. Instr. and Meth. A 351 (1994) 193. M. Atac et al., Nucl. Instr. and Meth. A 367 (1995) 372. I. Antos et al., Fermilab Proposal P-871 (March 26 1994).
II. VACUUM DEVICES
Nuclear Instruments and Methods in Physics Research A 387 (1997) 88-91
m-_
NUCLEAR INSTRUMENTS 8 METHODS IN PIIVSICS RESEARCH Sectm A
An optical discriminator for hyperon physics N. Lerosa3*, J.P. Perrouda, R. Chipauxb, A. Delbartb, J. Derreb, Y. Giomatarisb, C. Kochowskyb, S. Loucatosb “Lausanne Universi@, IPN, BSP, 1015 Dorigny, Switzerland “DAPNIA, SPP, C.E. Saclay, F-91191 Gifsur-Yvette, France
Abstract The use of an optical discriminator for fast hyperon selection has been investigated by the RD30 Collaboration. A realistic Monte-Carlo simulation of this device based on the detection of the Cherenkov light produced in a thin crystal shows quite promising performances. Such a discriminator has been built and the first results obtained in a pion beam will be presented.
1. Introduction Many fixed target experiments at high luminosity hadron accelerators require an efficient and fast selection of rare events within a typical 50 to 1OOMHz total rate. For events which contain long lived unstable particles (beauty, charm or strangeness physics) which are produced in a small target, the proposed [l] optical discriminator solves this problem. The optical discriminator consists of a spherical crystal shell centered on the target. For particles crossing the crystal sphere of radius R with an angle B with the normal (impact parameter b = OR), a fraction of the Cherenkov photons produced in the crystal are trapped and can be detected at its edge. Particles originating from the target do not lead to any photon detection. The development of such a device started in 1992 as R&D experiment CERN-RD30 [3]. A detailed description of the principle of the optical discriminator together with a first experimental result obtained with a LiF crystal can be found in Ref. [2]. The selection occurs at the downstream interface of the crystal with refractive index n, and the surrounding medium with refractive index nz. The minimum crossing angle of the particle with the crystal which can produce trapped Cherenkov light is e,,,,, = 42n,. E is a small positive number simply given by n: - n: = 1 - E. The sensitivity to low impact parameters was shown to be limited by the dispersion of the refractive index of LiF and by a thickness effect due to the rotation of the normal along the photon path inside a small radius crystal. To reach sensitivity to impact parameters of a few hundred km necessary for B-physics [4], two improvements have been investigated [5,6]. An achromatic pair has been found using a sapphire crystal and a siloxane cladding liquid such *Corresponding author. [email protected]. 0168~9002/97/$17.00 PII SO 168-9002(
Fax:
Copyright
96)00968-O
+41
21
692
0 1997 Elsevier
3605,
e-mail:
that E becomes almost independent of the wavelength and the use of a multilayer crystal of the same total thickness but with several thinner shells to diminish the thickness effect. In this paper we describe an optical discriminator suitable for the E871-FNAL fixed-target experiment at the Tevatron [7]. This experiment will perform a search for CP violation in hyperon decays. The aim is to fully reconstruct 4 X lo9 decays in order to achieve a sensitivity for the relevant asymmetry in decay angular distribution parameters of about 10m4.
2. Monte-Carlo simulation In experiment E871, unpolarized E hyperons are produced at zero degree by protons and momentum selected with a magnetic channel. The decay sequences 5 +ITA and R-+rp are detected with a simple wire chamber spectrometer with high-rate capability. The optical discriminator for this experiment consists of a spherical crystal shell of sapphire with a radius of curvature of 16 m, located at the end of a decay vacuum chamber, 13 m downstream of the hyperon channel exit. The sapphire crystal diameter of 250mm just covers the acceptance for the charged particles produced in the decay chain which have both vertices in the decay region. Background primary particles of the right selected momentum (mostly pions) seem to come from an almost point-like virtual source located around the middle of the hyperons channel. They cross the crystal which is centered on this virtual source at small angles and within a radius 60 mm from the shell center. The hatched area in Fig. 1 shows the distribution in angles for these primary background particles. The distributions in angles for the pions and proton, n(s), a(A) and p(A) from the E and A decay vertices are also shown. These distributions extend to much larger
Science B.V. All rights reserved
N. Leros
0
0
2
I
et al.
3
4
I Nucl.
and Meth.
Instr.
5
6
Track angle [mrad] Fig. I. Normalized distributions of the angle at the crossing point with the normal to the crystal. Hatched histogram is for primary particles, other histograms are for the proton and the pions from E decay chain. The open circles represent, for a 3 mm thick crystal, the average photoelectron yield per particle as a function of this angle.
in
Phys.
Res. A
89
387 (1997) 88-91
angles. The optical discriminator takes advantage of these differences to select within a very short time (several ns) events with detected trapped Cherenkov photons. A typical curve showing the photoelectrons yield per particles as a function of the crossing angle is also shown in Fig. 1. Below a threshold angle @,,, the photoelectron yield is very low. It then increases and reaches a saturation value. For the interface between sapphire and the siloxane Cargille liquid used as cladding, the dispersion curve of the threshold angle 0,,,,, at a temperature of 25°C is shown in Fig. 2. The threshold angle fI,,,,, can be adjusted for a given liquid by adjusting its temperature, the dashed line in Fig. 2 shows the curve at 38°C used in the simulation. The measured quantum efficiency of the phototubes looking at the crystal edge is also given. For the simulation we consider only photons with a wavelength larger than 320 nm. Within the range of sensitivity of the photodetector the value of O,,,m,, remains lower than about 3 mrad. The simulation predictions at the 40 MHz rate expected for the experiment E871 are very encouraging. If at least two fired phototubes with a threshold set at 0.3 photoelectrons are required and assuming an extra 25% of light losses, one expect a reduction of background by a factor of 25 while keeping 95% of the z events.
3. The experimental tests
20
“1 ‘:
l
’
.
The final 250 mm in diameter spherical sapphire crystal is still under construction and we present here experimental results obtained in a 10 GeV/c momentum CERN pion beam with a 64 mm in diameter, 3 mm thick flat sapphire crystal. Fig. 3 is a sketch of the crystal and its liquid cladding container. The crystal can precisely rotate around
Q.e.
4
.
0.175
yj 0.1s i .
/to
PM1
/
jO.12,
\ 10 -
\
T=25’C
0.1
Fig. 2. Threshold angle above which light can be trapped in the crystal as a function of the wavelength A. The solid line with the dots is obtained from measurements of the siloxane liquid refractive indices at 25” corrected to take into account the slight birefringence of the crystal. The dotted line is the curve adopted for the simulation. The black squares and the right scale is the measured quantum efficiency of the phototube used during our test IUIIS.
\ \ to
PMT
Fig. 3. Sketch of the crystal and liquid container.
II. VACUUM DEVICES
90
N. Loos et al. 1 Nucl. Instr. and Meth. in Phys. Res. A 387 (1997)
two orthogonal axis. The particle trajectory is reconstructed with two upstream drift chambers, each equipped with x-y planes and having a spatial resolution of 150 p.m. The exit edge of the crystal is seen by a set of 40 photomultipliers (PMT) RI398 from Hamamatsu, sitting on a cone at an angle of 76 degrees relative to the beam and at 184 mm from the crystal exit face. Hamamatsu phototubes RI398 have a peak to valley ratio of the single photoelectron response better than 2, a single photoelectron background rate at 20°C less than 1 kHz and a rise time better than 2 ns. The whole detector is enclosed in a light tight box stabilized at 20°C by air circulation. The cladding liquid flow is around 0.2 to 0.3 l/mn and its temperature is stabilized around the desired value. The pion beam has a divergence of 1.3 mrad and is almost normal to the crystal. Pions which cross the crystal within a radius of 4 mm around its center are selected. For each incident pion we record the number of phototubes having a signal of amplitude higher than 0.3 photoelectrons in coincidence within 7 ns with the incoming pion. Fig. 4 is a plot of the average number of hit phototubes per incident track as a function of the temperature of the cladding liquid. We have used a liquid with an index of refraction at 25°C of n, = 1.4562 at the sodium line. At low temperature the threshold angle e,,,,, is high and one observes essentially a background due to photons with an angle lower than the total reflexion angle which nevertheless reach the exit face of the crystal after multiple Fresnel reflexions. This background is quite high since only 3 reflexions on the downstream interface are necessary to reach the exit face. The threshold angle t?,,,,, diminishes at a rate of about 0.4 mrad per degree and the Fresnel background increases slowly until the value of @,,, = 0 is
88-91
Fig. 5. For two liquid cladding temperatures 24°C (open circles) and 30°C (black dots), the average number of hit phototubes as a function of the incident angle of the particle.
reached. At this point the rise becomes very fast and saturates when all the produced photons are totally reflected at the selecting interface. The data are in good agreement with the Monte-Carlo simulation. In Fig. 5 we present for two temperatures, 24°C and 30°C the average number of hit phototubes as a function of the angle of the incident particle. For these data the beam was kept fixed and the crystal support was rotated. These curves clearly show the expected behavior, a flat Fresnel background and a rise when the threshold angle 0,,,,, is reached. It also clearly shows the displacement of the threshold with temperature. With this small crystal the Fresnel background dominates and other small contributions cannot be estimated. With the 250 mm in diameter crystal this background will be reduced by at least a factor of 10 since 14 reflexions instead of 3 on the selecting interface will be necessary to reach the exit edge of the crystal.
4. Conclusions
Temperature
I-C]
Fig. 4. Average number of hit phototubes per track as a function of the liquid cladding temperature. The open circles are the data and the black dots the simulation prediction.
A realistic simulation has shown that a rather simple device consisting of a thin spherical sapphire crystal with a suitable liquid cladding interface can be used as a first level trigger in a hyperon experiment. For the FNAL hyperon experiment E871, a background rejection by a factor of 25 and an efficiency of 95% for 8 events can be achieved. Preliminary tests with a smaller crystal have given very encouraging results and confirmed simulation predictions. A final test with the 250 mm in diameter, 3 mm thick and with 16 m radius of curvature will begin soon.
N. Leros et ~1. I Nucl. Instr. and Meth. in Phys. Reu. A 387 (1997) 88-91
References [I] G. Charpak, Y. Giomataris and L. Lederman, Nucl. Instr. and Meth. A 306 (1991) 439. 121 G. Charpak et al.. Nucl. Instr. and Meth. A 332 (1993) 91. [3] CERN/DRDC/RD30 (20 August 91);
[4] [5] [6] [7]
91
CERN/DRDC/92-53 Addendum (5 November 1992); CERN/DRDC/93-56 Status Report (20 December 1993). P.D. Dauncey et al.. Nucl. Instr. and Meth. A 351 (1994) 147. C. Kochowski, Nucl. Instr. and Meth. A 351 (1994) 193. M. Atac et al., Nucl. Instr. and Meth. A 367 (1995) 372. I. Antos et al., Fermilab Proposal P-871 (March 26 1994).
II. VACUUM DEVICES