- Email: [email protected]
The CLEO III upgrade
Nuclear Instruments and Methods in Physics Research A 446 (2000) 97}105
The CLEO III upgrade Georg Viehhauser Physics Department, Syracuse University, 201 Physics Building, Syracuse, NY 13244-1130, USA
Abstract The electron storage ring CESR at Cornell University is presently undergoing a major upgrade together with its B-physics detector CLEO. Improved focusing and superconducting cavities will increase the luminosity of the collider to more than 2;10 cm s\. To accommodate the necessary accelerator elements the tracking region of CLEO has to be rebuilt. The new tracking system consists of a four-layer silicon vertex detector and a large volume drift chamber, which also provides dE/dx particle identi"cation information. The CLEO III upgrade also includes a new particle identi"cation system which allows full exploitation of the possibilities provided by the upgraded accelerator. This PID system is a proximity focusing RICH with lithium #uoride radiators and multi-wire chambers "lled with a methane/triethylamine mixture to detect UV Cherenkov photons. Installation of the new detector elements in the CLEO detector is taking place during summer 1999. 2000 Elsevier Science B.V. All rights reserved.
1. Introduction CP violation can be accommodated into the Standard Model through the Cabbibo} Kobayashi}Maskawa (CKM) mixing matrix [1,2], which describes the mixing of strong interaction quark eigenstates in weak processes. In the popular parametrization by Wolfenstein [3] < !)+
"
1!j/2
j
Aj(o!ig)
!j
1!j/2
Aj
Aj(1!o!ig)
!Aj
1
(1)
where A&1, the Cabibbo angle j"0.22. This parameterization is valid to O(j). CP violation is
E-mail address: [email protected] (G. Viehhauser).
introduced by the complex phase o#ig. Fig. 1 shows the current experimental limits on this parameter. Contributing measurements are studies of B mixing, measurements in the B system of < /< and limits from B mixing as well as measurements of e in K decays. The "gure also ) includes a triangle derived from the requirement that the CKM matrix is unitary [4]: < < H # < < H #< < H +< #j< #< H "0. The angles a, b, and c are all, in principle, measurable from decays of B mesons and provide an important proving ground for the Standard Model. The Cornell e>e\ Storage Ring CESR is a symmetric collider operating at the B(4S)PBBM resonance. Therefore, the Lorentz boosts for the emerging B mesons are small, making measurements of the unitarity triangle via time-dependent BBM mixing unlikely. However, the CLEO detector at CESR will be able to extend its valuable contribution to B physics research in its third phase of
0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 0 3 9 - 5
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G. Viehhauser / Nuclear Instruments and Methods in Physics Research A 446 (2000) 97}105
(like e.g. KHc and qc) calls for good particle identi"cation capability of the detector.
2. Cornell e>e\ storage ring upgrade
Fig. 1. The 1p experimental bounds on CP violating parameter o#ig.
operation by measuring some parameters of the CKM triangle using rare decay rates which are proportional to CKM matrix elements and by possible observation of &direct' CP violation in charged B decays. Some of the most promising measurements are E The error in the < can be reduced by exclusive measurements such as BPpll, BPoll and BPu ll over the whole q range. Previous measurements of these decays su!er from errors on form factors. E The dominant error in the e band is due to < . This parameter can be measured better through dC(B\PDHll)/dq, since as qP0 many hadronic form factor uncertainties are reduced. This measurement requires a high statistics sample, as the rate falls sharply at low q. E The ratio < /< can be measured using BPqc and BPKHc &penguin' decays. CLEO has already observed the latter mode at about 10\. If more than one amplitude does contribute to BPqc and if these amplitudes have di!erent weak (CKM) phases, then direct CP violation might be observable in decay rate asymmetries in B>Pq>c and B\Pq\c decays. These measurements require a signi"cant increase in the number of observed B decays from the 9 fb\ collected by CLEO so far. In addition, similarity in the event topology of some CP conserving modes to potentially CP violating modes
Of primary importance for CLEO's future physics potential is the luminosity increase of CESR. This goal will be pursued in Phase III by multibunch operation, superconducting RF cavities and a redesigned interaction region [5]. The "rst two parts of this plan have been introduced gradually during previous CESR operation. So far 9 trains of 4 bunches each have been successfully stored in the ring under CLEO II running conditions. A horizontal crossing angle at the interaction point of 2.3 mrad has been used. In parallel CESR's four 5-cell room-temperature copper RF cavities have gradually been replaced by superconducting Nb cavities. At the high beam currents projected in Phase III the coupling of the beam to the walls of the small aperture copper cavities introduces unwanted high-order modes in the stored beams. The new cavities have a larger bore and will deliver 325 kW/cavity as compared to 150 kW/Cavity for the previous design. With the present bunch operation and 2 out of 4 cavities swapped, CESR delivered an instantaneous luminosity of up to 8;10 cm s\ during CLEO II.V operation in early 1999. The third cavity was replaced after the CLEO II.V shutdown and successfully tested in a machine development run. Installation of the last superconducting RF cavity is foreseen for September 1999. The "nal design for the CLEO III interaction region includes a set of two superconducting Nb quadrupole magnets at 65 cm and one rare earth NdFeB quadrupole magnet at 35 cm from the interaction point on both sides of the detector. The new magnets provide higher "eld gradients, made necessary by the wider aperture required by the crossing angle, and should improve focusing at the IP, while reducing the e!ects of secondary crossings at $2.1 m from the IP. With this magnet layout bH is expected to reduce from 18 to 7 mm at the T collision point. The superconducting quadrupole magnets will not be available for the CLEO III startup expected in Fall 1999 and will be installed
G. Viehhauser / Nuclear Instruments and Methods in Physics Research A 446 (2000) 97}105
during the "rst half of 2000. Until then the present quadrupole focusing system will be used. With all the above improvements, CESR anticipates storing 500 mA/beam and luminosities in excess of 2;10 cm s\, which is just 2.5 times higher than achieved during operation in early 1999. During regular operation CLEO III hopes to accumulate 20}30 fb\ yr\.
3. CLEO III overview Fig. 2 shows a quadrant of the longitudinal cross-section of CLEO III. One of the key considerations in the CLEO III design is to keep the proven components of the successful CLEO II detector [6] wherever possible. This is possible for the whole outer section of CLEO consisting of the superconducting solenoid magnet, the CsI electromagnetic calorimeter and the muon chamber system. In addition, CLEO III has to provide space for the focusing accelerator elements protruding towards the IP. This requires a new drift chamber
99
with a new silicon vertex detector. The redesign of the tracking system also allows at the same time for changes in the material distribution resulting in less material inside the tracking volume and before the endcap calorimeters. The CLEO III tracking system covers 93% of the solid angle. Furthermore, the particle identi"cation capability of CLEO III will be improved by a RICH detector replacing the TOF system just outside of the drift chamber. Its design is driven by the limited space available inside the existing CsI calorimeter. It therefore uses a proximity focusing design with triethylamine (TEA) gas as the photosensitive material. In addition the trigger and DAQ of the experiment have to be replaced to prepare CLEO III for the higher luminosity.
4. Silicon vertex detector Tracking in CLEO III should achieve comparable quality to CLEO II.V (p /p"0.6% at 2 GeV).
Fig. 2. CLEO III quarter section. Only components inside the barrel electromagnetic calorimeter are shown.
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To maintain this performance even with a reduced outer radius of the tracking volume the new silicon vertex detector for CLEO III will be of increased importance for precision tracking. In addition it will provide e$cient stand-alone reconstruction for low momentum particles such as slow pions in DHPDp decays. Reconstruction of secondary vertices will be of less importance as the Lorentz boosts at a symmetric e>e\ collider operating at the B(4S) do not provide su$cient separation from the primary vertex (j&30 lm for B mesons). An additional new task for the CLEO III silicon vertex detector will be to provide information on the zcoordinate needed for the reconstruction in the RICH detector. Table 1 and Fig. 3 give the mechanical layout of the CLEO III silicon vertex detector. Its design Table 1 CLEO III silicon vertex detector parameters Layer number
1
2
3
4
Radius (cm) Detectors in azimuth Detectors along z Channel noise (e\) (no irradiation) Collected signal (10e\) S/N ratio (no irradiation)
2.50 7 3 360 1.8 48
3.75 10 4 470 1.7 36
7.00 18 7 600 1.6 27
10.00 26 10 740 1.5 24
facilitates construction and operation by the use of only one kind of detector. Each detector is double sided with an active area of 5.12;2.56 cm and a thickness of 0.3 mm. Each has 511 channels in both directions, r and z, resulting in strip widths of 50 lm in r and 100 lm in z. These silicon vertex detectors are chained in z both mechanically and electronically. Up to 5 detectors form a mechanical entity called ladder held together by v-shaped beams of CVD diamond. This beam structure provides a thin (0.1% X ) yet sti! support with a sag of less than 25 lm and a resonance frequency larger than 100 Hz, su$cient protection from any excitation related to line frequency. In total there are 447 identical detectors combined into 61 ladders. Ladders will be mounted at the ends on support cones made of copper to e$ciently remove the 0.5 kW of heat generated by the readout electronics. Much e!ort has gone into reducing the noise in all layers of the silicon vertex detector. Each detector ladder is coupled by a #ex circuit to a hybrid card which holds RC, preampli"er/shaper and digitizer/sparsi"er chips. The #ex circuit decouples the readout chips from the detectors mechanically as well as thermally, allowing the detector to run at lower temperature thus reducing noise. Detector capacitances have been kept below 10 pF/strip for r and z strips by choosing the detector p-side for
Fig. 3. CLEO III silicon vertex detector layout.
G. Viehhauser / Nuclear Instruments and Methods in Physics Research A 446 (2000) 97}105
the measurement of the z coordinate. The p-stops have an &atoll' shape surrounding the strips with an insulating barrier between p-stops for di!erent strips [7]. Reverse biasing the p-stops by about 10 V reduces the detector capacitance, allows for a lower detector bias voltage and widens the charge distribution, which improves the position measurement using the charge center of gravity. The custom preampli"er/shaper chip (FEMME) [8] based on the Viking design uses shaping with a shaping time of 2 ls to achieve the low noise of ENC"149 e\#5.5 e\/pF. Even though the shaping time corresponds to 36 beam crossings, we expect only about 10\ occupancy per strip. The last chip in the readout chain is the SVX}CLEO digitizer/sparsi"er based on the FNAL/LBL SVX}II(b) which digitizes the signal with an 8-bit ADC. Taking into account measured parameters wherever available and assuming reasonable noise contributions from other sources the signal-to-noise ratio is expected to be better than 20 even after irradiation with an integrated dose of 300 krad, more than the expected dose for 10 yr of CLEO III operation. Silicon vertex detectors of the CLEO III &atoll' design have been tested in the RD42 beam at CERN with a readout consisting of VA2 hybrids and a VME-based ADC system. Track residuals of 8 lm have been observed after reconstruction of the charge center-of-gravity (Fig. 4). At present layers 1 and 2 of the CLEO III silicon vertex detector are completed and mounted to the beampipe. All support elements are at hand and ladders for layers 3 and 4 are being assembled.
5. Drift chamber The goal for the CLEO III drift chamber is to provide the same or better momentum resolution and dE/dx performance as the CLEO II drift chamber while giving up more than 10 cm in outer radius to the RICH. During Phase II the momentum resolution was dominated by multiple scattering. Consequently, to recover the loss in lever arm, the design for the CLEO III drift chamber reduces the amount of scattering material in the tracking volume.
101
Fig. 4. Testbeam track residuals of the CLEO III silicon vertex detector. The Gaussian "t yields 8.4 lm position resolution.
Fig. 5. CLEO III drift chamber.
As shown in Fig. 5, the mechanical design for the CLEO III drift chamber features conical endplates with a tapered inner section (&wedding cake') to accommodate the focusing elements of the accelerator. The conical shape of the endplates transfers the wire tension load to the outer surface of the drift chamber, allowing the inner support tube closing the detector volume towards the silicon vertex detector to be made of lightweight Rohacell. This shape also allows for a reduced thickness, lowering the amount of material in front of the endcap electromagnetic calorimeter. The inner tapered section of the drift chamber contains 16 axial super-layers, while the conical
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outer section houses 8 stereo u and v super-layers. Each of the 14;14 mm square cells contains 3 "eld wires per sense wire. In addition to reducing the amount of support material in the tracking volume the drift chamber gas was also switched from Ar/C H 50/50 to He/C H 60/40 as the argon-based gas accounted for half of the scattering material in the chamber during CLEO II operation. While having a much longer radiation length, helium-based gas mixtures also have a smaller Lorentz angle, which increases the detection e$ciency at the cell edges inside the magnetic "eld (Fig. 6). He/C H 60/40 was chosen for its primary ionization yield which matches the one for the CLEO II gas, maintaining the dE/dx performance of the chamber. The new helium-based gas mixture was introduced already during the last period of CLEO II operation after its properties were discovered in test chamber studies [9].
Fig. 6. He/C H and Ar/C H in the CLEO II drift chamber. The upper plot shows the improved resolution of the DH!D mass di!erence. The lower plot shows the improved e$ciency at cell edges.
At present the drift chamber has passed "nal acceptance tests and is ready for installation into the CLEO III detector.
6. RICH detector The goal of the CLEO III particle identi"cation system is to achieve 4pp/K separation at 2.65 GeV/c, the average maximum momentum for B daughter particles at a symmetric collider operating at the B(4S) resonance. At such momenta the dE/dx measurement by the drift chamber will provide better than 2p separation, so that an additional 3p will have to be provided by the CLEO III RICH. For momenta between 0.9 and 2 GeV where dE/dx will provide almost no particle separation, the RICH should achieve in excess of 4p by itself. The design of the CLEO III RICH is driven by the limited space between the existing electromagnetic CsI calorimeter and the tracking volume, for which a large outer radius is mandatory for good momentum resolution. Therefore, the CLEO III RICH follows the &proximity focusing' design [10], pioneered by the Colle`ge de France/Strasbourg group [11]. It consists of solid radiators, followed by a gas volume in which the Cherenkov cone can expand and thin multi-wire proportional chambers to detect the Cherenkov photons. To allow for very thin (4.5 mm) MWPCs the photosensitive gas in the chambers is a CH / triethylamine (TEA) mixture which has an absorption length of about 0.5 mm. However, this gas only converts photons with wavelengths between 135 and 165 nm to single photoelectrons, with a maximum quantum e$ciency of 33% at 150 nm. This wavelength regime, also referred to as the vacuum ultraviolet (VUV), strongly restricts the choice of transparent materials for the RICH. The only solid materials with a band gap su$ciently large to transport VUV photons are alkali-halide crystals, of which LiF was chosen for radiators and CaF for the MWPC entry window. In the wavelength range of interest they both have a refractive index of about 1.5 with LiF showing smaller dispersion ((5%). These polycrystals can be polished such that their transmission reaches 80% at 150 nm. The expansion volume on the other hand has to be free
G. Viehhauser / Nuclear Instruments and Methods in Physics Research A 446 (2000) 97}105
of oxygen, water and large organic molecules, because all these substances are opaque (p "10 Mb at 150 nm) in the VUV. For the CLEO III RICH this volume will be #ushed by pure nitrogen, which except for several vibrational and rotational absorption lines below 145 nm, is transparent in the required wavelength range. Preliminary measurements show that a transmission of better than 98% for the 16 cm deep expansion volume could be achieved in the CLEO III RICH. No mirrors are needed in the &proximity focusing' design. However, the resulting images are not circular rings, but conical intersections, distorted and truncated by refraction at the radiator/expansion volume boundary. This refraction, due to the dispersion of the refractive index, together with the unknown photon emission point due to the "nite radiator thickness, are the dominant e!ects for the
Fig. 7. CLEO III RICH &sawtooth' radiator design (lower) as opposed to planar radiator. Photon paths are shown by dashed lines.
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single photon Cherenkov angle resolution. Refraction also prohibits photons from charged particles traversing the radiator normally from escaping the crystal. The CLEO III RICH therefore introduces a new radiator shape in the central portion of the detector. There the outer surfaces of the crystals have a pattern of &sawtooth'-shaped 4 mm deep grooves, the surfaces of which can be crossed by the Cherenkov photons almost perpendicularly, thus also reducing the distorting e!ects of refraction (Fig. 7). For the refractive index of LiF the di!erence of the Cherenkov angle for kaons and pions at the benchmark momentum of 2.65 GeV/c is about 13 mrad. Thus, the opening angle of the Cherenkov cone has to be measured with an accuracy of about 4 mrad for each particle. The errors on the singlephoton Cherenkov angle measurements of all the photons emitted by one charged particle are largely uncorrelated, and therefore this goal is achieved by detecting at least 12 photoelectrons per ring with a single-photon Cherenkov angle resolution of about 14 mrad. In the design of the CLEO III RICH great emphasis has been put on achieving a large photoelectron yield. The RICH consists of two cylinders, an inner cylinder of radiator crystals glued on a carbon "ber support tube and an outer cylinder of 30 MWPCs in r which extend over the whole length (2.5 m) of the RICH (Fig. 8). No support elements are protruding into the expansion volume and chambers are only separated by thin aluminum
Fig. 8. Azimuthal cross-section of sector of the CLEO III RICH.
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G. Viehhauser / Nuclear Instruments and Methods in Physics Research A 446 (2000) 97}105
support rails. These two cylinders are only connected at the ends and structural stability is based on the sti!ness of the cylinders, not its individual elements. The photon detectors are asymmetric MWPCs with 72 wires running parallel to the CLEO z-axis. Each chamber is divided longitudinally into 8 segments, such that each segment matches the largest size of polished detector entry window crystal available (20;30 cm) and the distance between ceramic wire support spacers. The cathodes are vacuum-deposited Ag traces on the detector entry window crystals and printed circuit boards with a pad segmentation of 7.5;8 cm on the outer radius of the RICH at a distance of 1 mm from the wires. Readout of the charge induced on these pads provides the spatial information for each signal. E$cient photoelectron reconstruction also puts several demands on the RICH readout electronics. The exponential single photoelectron pulse-height spectrum requires very low-noise electronics for a low pulse-height cut-o! in the reconstruction. In addition, e$cient declustering asks for a high segmentation of the readout. For preampli"cation the CLEO III RICH uses VA}RICH chips, a develop-
ment of the VLSI Viking chip, which are mounted directly on the back of each chamber. It provides an ENC"130 e\#9 e\/pF for 2 ls shaping time. Its intrinsic multiplexing capability allows to segment the readout into 230 400 channels. The signals from these chips are then digitized by a VME-based ADC/sparsi"er system. A common mode subtracted noise of better than 400 e\ has routinely be achieved with the whole readout chain and real chambers. Two of the 30 photon detectors have been tested together with three radiator crystals in a muon halo at Fermilab [12,13]. The testbeam setup consisted of a MWPC reference tracking system and an aluminum expansion gap box with the chambers and radiators, which could be rotated to simulate di!erent track incidence angles as in CLEO III. In the testbeam Cherenkov angle measurement resolutions of about 5 mrad have been obtained (Fig. 9). Based on these testbeam results Monte Carlo extrapolations to CLEO III acceptance and the better tracking there predict a separation power well within the requirements except for the very end of the detector. The RICH has been installed as the "rst major new component into CLEO III in August 1999.
Fig. 9. CLEO III RICH testbeam results. The left plot shows the results from the testbeam for the Cherenkov angle resolution per track. The right plot gives the extrapolation to CLEO III acceptance and tracking resolution.
G. Viehhauser / Nuclear Instruments and Methods in Physics Research A 446 (2000) 97}105
7. CLEO III installation CLEO II has been disassembled in March/June 1999. Since then the RICH has been installed and the general DAQ and trigger have been replaced and tested with the CsI calorimeter. The drift chamber is completed and ready for installation at the beginning of September. The Silicon vertex detector is partially assembled but has seen an extension of its installation schedule. The start-up date for CESR and its B-physics detector CLEO III is foreseen to be November 8, 1999.
References [1] N. Cabibbo, Phys. Rev. Lett. 10 (1963) 531.
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[2] M. Kobayashi, T. Maskawa, Prog. Theor. Phys. 49 (1973) 652. [3] L. Wolfenstein, Phys. Rev. Lett. 51 (1984) 1945. [4] See the review article by C. Jarlskog, in: C. Jarlskog (Ed.), CP Violation, World Scienti"c Press, Singapore, 1987. [5] D. Rubin, Proceedings of IEEE/APS Particle Accelerator Conference, 1995. [6] Y. Kubota et al., Nucl. Instr. and Meth. A 320 (1992) 66. [7] G. Brandenburg et al., Proceedings of the Third International Symposium on the Development and Application of Semiconductor Tracking Detectors, Melbourne, December 1997. [8] H. Kagan et al., Nucl. Instr. and Meth. A 383 (1996) 189. [9] K.K. Gan et al., Nucl. Instr. and Meth. A 374 (1996) 27. [10] J. Seguinot, T. Ypsilantis, Nucl. Instr. and Meth. 142 (1977) 377. [11] J.L. Guyonnet et al., Nucl. Instr. and Meth. A 350 (1994) 430. [12] M. Artuso et al., Nucl. Instr. and Meth. A 441 (2000) 374. [13] M. Artuso et al., Nucl. Instr. and Meth. A 419 (1998) 577.
SECTION II.
The CLEO III upgrade Georg Viehhauser Physics Department, Syracuse University, 201 Physics Building, Syracuse, NY 13244-1130, USA
Abstract The electron storage ring CESR at Cornell University is presently undergoing a major upgrade together with its B-physics detector CLEO. Improved focusing and superconducting cavities will increase the luminosity of the collider to more than 2;10 cm s\. To accommodate the necessary accelerator elements the tracking region of CLEO has to be rebuilt. The new tracking system consists of a four-layer silicon vertex detector and a large volume drift chamber, which also provides dE/dx particle identi"cation information. The CLEO III upgrade also includes a new particle identi"cation system which allows full exploitation of the possibilities provided by the upgraded accelerator. This PID system is a proximity focusing RICH with lithium #uoride radiators and multi-wire chambers "lled with a methane/triethylamine mixture to detect UV Cherenkov photons. Installation of the new detector elements in the CLEO detector is taking place during summer 1999. 2000 Elsevier Science B.V. All rights reserved.
1. Introduction CP violation can be accommodated into the Standard Model through the Cabbibo} Kobayashi}Maskawa (CKM) mixing matrix [1,2], which describes the mixing of strong interaction quark eigenstates in weak processes. In the popular parametrization by Wolfenstein [3] < !)+
"
1!j/2
j
Aj(o!ig)
!j
1!j/2
Aj
Aj(1!o!ig)
!Aj
1
(1)
where A&1, the Cabibbo angle j"0.22. This parameterization is valid to O(j). CP violation is
E-mail address: [email protected] (G. Viehhauser).
introduced by the complex phase o#ig. Fig. 1 shows the current experimental limits on this parameter. Contributing measurements are studies of B mixing, measurements in the B system of < /< and limits from B mixing as well as measurements of e in K decays. The "gure also ) includes a triangle derived from the requirement that the CKM matrix is unitary [4]: < < H # < < H #< < H +< #j< #< H "0. The angles a, b, and c are all, in principle, measurable from decays of B mesons and provide an important proving ground for the Standard Model. The Cornell e>e\ Storage Ring CESR is a symmetric collider operating at the B(4S)PBBM resonance. Therefore, the Lorentz boosts for the emerging B mesons are small, making measurements of the unitarity triangle via time-dependent BBM mixing unlikely. However, the CLEO detector at CESR will be able to extend its valuable contribution to B physics research in its third phase of
0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 0 3 9 - 5
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G. Viehhauser / Nuclear Instruments and Methods in Physics Research A 446 (2000) 97}105
(like e.g. KHc and qc) calls for good particle identi"cation capability of the detector.
2. Cornell e>e\ storage ring upgrade
Fig. 1. The 1p experimental bounds on CP violating parameter o#ig.
operation by measuring some parameters of the CKM triangle using rare decay rates which are proportional to CKM matrix elements and by possible observation of &direct' CP violation in charged B decays. Some of the most promising measurements are E The error in the < can be reduced by exclusive measurements such as BPpll, BPoll and BPu ll over the whole q range. Previous measurements of these decays su!er from errors on form factors. E The dominant error in the e band is due to < . This parameter can be measured better through dC(B\PDHll)/dq, since as qP0 many hadronic form factor uncertainties are reduced. This measurement requires a high statistics sample, as the rate falls sharply at low q. E The ratio < /< can be measured using BPqc and BPKHc &penguin' decays. CLEO has already observed the latter mode at about 10\. If more than one amplitude does contribute to BPqc and if these amplitudes have di!erent weak (CKM) phases, then direct CP violation might be observable in decay rate asymmetries in B>Pq>c and B\Pq\c decays. These measurements require a signi"cant increase in the number of observed B decays from the 9 fb\ collected by CLEO so far. In addition, similarity in the event topology of some CP conserving modes to potentially CP violating modes
Of primary importance for CLEO's future physics potential is the luminosity increase of CESR. This goal will be pursued in Phase III by multibunch operation, superconducting RF cavities and a redesigned interaction region [5]. The "rst two parts of this plan have been introduced gradually during previous CESR operation. So far 9 trains of 4 bunches each have been successfully stored in the ring under CLEO II running conditions. A horizontal crossing angle at the interaction point of 2.3 mrad has been used. In parallel CESR's four 5-cell room-temperature copper RF cavities have gradually been replaced by superconducting Nb cavities. At the high beam currents projected in Phase III the coupling of the beam to the walls of the small aperture copper cavities introduces unwanted high-order modes in the stored beams. The new cavities have a larger bore and will deliver 325 kW/cavity as compared to 150 kW/Cavity for the previous design. With the present bunch operation and 2 out of 4 cavities swapped, CESR delivered an instantaneous luminosity of up to 8;10 cm s\ during CLEO II.V operation in early 1999. The third cavity was replaced after the CLEO II.V shutdown and successfully tested in a machine development run. Installation of the last superconducting RF cavity is foreseen for September 1999. The "nal design for the CLEO III interaction region includes a set of two superconducting Nb quadrupole magnets at 65 cm and one rare earth NdFeB quadrupole magnet at 35 cm from the interaction point on both sides of the detector. The new magnets provide higher "eld gradients, made necessary by the wider aperture required by the crossing angle, and should improve focusing at the IP, while reducing the e!ects of secondary crossings at $2.1 m from the IP. With this magnet layout bH is expected to reduce from 18 to 7 mm at the T collision point. The superconducting quadrupole magnets will not be available for the CLEO III startup expected in Fall 1999 and will be installed
G. Viehhauser / Nuclear Instruments and Methods in Physics Research A 446 (2000) 97}105
during the "rst half of 2000. Until then the present quadrupole focusing system will be used. With all the above improvements, CESR anticipates storing 500 mA/beam and luminosities in excess of 2;10 cm s\, which is just 2.5 times higher than achieved during operation in early 1999. During regular operation CLEO III hopes to accumulate 20}30 fb\ yr\.
3. CLEO III overview Fig. 2 shows a quadrant of the longitudinal cross-section of CLEO III. One of the key considerations in the CLEO III design is to keep the proven components of the successful CLEO II detector [6] wherever possible. This is possible for the whole outer section of CLEO consisting of the superconducting solenoid magnet, the CsI electromagnetic calorimeter and the muon chamber system. In addition, CLEO III has to provide space for the focusing accelerator elements protruding towards the IP. This requires a new drift chamber
99
with a new silicon vertex detector. The redesign of the tracking system also allows at the same time for changes in the material distribution resulting in less material inside the tracking volume and before the endcap calorimeters. The CLEO III tracking system covers 93% of the solid angle. Furthermore, the particle identi"cation capability of CLEO III will be improved by a RICH detector replacing the TOF system just outside of the drift chamber. Its design is driven by the limited space available inside the existing CsI calorimeter. It therefore uses a proximity focusing design with triethylamine (TEA) gas as the photosensitive material. In addition the trigger and DAQ of the experiment have to be replaced to prepare CLEO III for the higher luminosity.
4. Silicon vertex detector Tracking in CLEO III should achieve comparable quality to CLEO II.V (p /p"0.6% at 2 GeV).
Fig. 2. CLEO III quarter section. Only components inside the barrel electromagnetic calorimeter are shown.
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G. Viehhauser / Nuclear Instruments and Methods in Physics Research A 446 (2000) 97}105
To maintain this performance even with a reduced outer radius of the tracking volume the new silicon vertex detector for CLEO III will be of increased importance for precision tracking. In addition it will provide e$cient stand-alone reconstruction for low momentum particles such as slow pions in DHPDp decays. Reconstruction of secondary vertices will be of less importance as the Lorentz boosts at a symmetric e>e\ collider operating at the B(4S) do not provide su$cient separation from the primary vertex (j&30 lm for B mesons). An additional new task for the CLEO III silicon vertex detector will be to provide information on the zcoordinate needed for the reconstruction in the RICH detector. Table 1 and Fig. 3 give the mechanical layout of the CLEO III silicon vertex detector. Its design Table 1 CLEO III silicon vertex detector parameters Layer number
1
2
3
4
Radius (cm) Detectors in azimuth Detectors along z Channel noise (e\) (no irradiation) Collected signal (10e\) S/N ratio (no irradiation)
2.50 7 3 360 1.8 48
3.75 10 4 470 1.7 36
7.00 18 7 600 1.6 27
10.00 26 10 740 1.5 24
facilitates construction and operation by the use of only one kind of detector. Each detector is double sided with an active area of 5.12;2.56 cm and a thickness of 0.3 mm. Each has 511 channels in both directions, r and z, resulting in strip widths of 50 lm in r and 100 lm in z. These silicon vertex detectors are chained in z both mechanically and electronically. Up to 5 detectors form a mechanical entity called ladder held together by v-shaped beams of CVD diamond. This beam structure provides a thin (0.1% X ) yet sti! support with a sag of less than 25 lm and a resonance frequency larger than 100 Hz, su$cient protection from any excitation related to line frequency. In total there are 447 identical detectors combined into 61 ladders. Ladders will be mounted at the ends on support cones made of copper to e$ciently remove the 0.5 kW of heat generated by the readout electronics. Much e!ort has gone into reducing the noise in all layers of the silicon vertex detector. Each detector ladder is coupled by a #ex circuit to a hybrid card which holds RC, preampli"er/shaper and digitizer/sparsi"er chips. The #ex circuit decouples the readout chips from the detectors mechanically as well as thermally, allowing the detector to run at lower temperature thus reducing noise. Detector capacitances have been kept below 10 pF/strip for r and z strips by choosing the detector p-side for
Fig. 3. CLEO III silicon vertex detector layout.
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the measurement of the z coordinate. The p-stops have an &atoll' shape surrounding the strips with an insulating barrier between p-stops for di!erent strips [7]. Reverse biasing the p-stops by about 10 V reduces the detector capacitance, allows for a lower detector bias voltage and widens the charge distribution, which improves the position measurement using the charge center of gravity. The custom preampli"er/shaper chip (FEMME) [8] based on the Viking design uses shaping with a shaping time of 2 ls to achieve the low noise of ENC"149 e\#5.5 e\/pF. Even though the shaping time corresponds to 36 beam crossings, we expect only about 10\ occupancy per strip. The last chip in the readout chain is the SVX}CLEO digitizer/sparsi"er based on the FNAL/LBL SVX}II(b) which digitizes the signal with an 8-bit ADC. Taking into account measured parameters wherever available and assuming reasonable noise contributions from other sources the signal-to-noise ratio is expected to be better than 20 even after irradiation with an integrated dose of 300 krad, more than the expected dose for 10 yr of CLEO III operation. Silicon vertex detectors of the CLEO III &atoll' design have been tested in the RD42 beam at CERN with a readout consisting of VA2 hybrids and a VME-based ADC system. Track residuals of 8 lm have been observed after reconstruction of the charge center-of-gravity (Fig. 4). At present layers 1 and 2 of the CLEO III silicon vertex detector are completed and mounted to the beampipe. All support elements are at hand and ladders for layers 3 and 4 are being assembled.
5. Drift chamber The goal for the CLEO III drift chamber is to provide the same or better momentum resolution and dE/dx performance as the CLEO II drift chamber while giving up more than 10 cm in outer radius to the RICH. During Phase II the momentum resolution was dominated by multiple scattering. Consequently, to recover the loss in lever arm, the design for the CLEO III drift chamber reduces the amount of scattering material in the tracking volume.
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Fig. 4. Testbeam track residuals of the CLEO III silicon vertex detector. The Gaussian "t yields 8.4 lm position resolution.
Fig. 5. CLEO III drift chamber.
As shown in Fig. 5, the mechanical design for the CLEO III drift chamber features conical endplates with a tapered inner section (&wedding cake') to accommodate the focusing elements of the accelerator. The conical shape of the endplates transfers the wire tension load to the outer surface of the drift chamber, allowing the inner support tube closing the detector volume towards the silicon vertex detector to be made of lightweight Rohacell. This shape also allows for a reduced thickness, lowering the amount of material in front of the endcap electromagnetic calorimeter. The inner tapered section of the drift chamber contains 16 axial super-layers, while the conical
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outer section houses 8 stereo u and v super-layers. Each of the 14;14 mm square cells contains 3 "eld wires per sense wire. In addition to reducing the amount of support material in the tracking volume the drift chamber gas was also switched from Ar/C H 50/50 to He/C H 60/40 as the argon-based gas accounted for half of the scattering material in the chamber during CLEO II operation. While having a much longer radiation length, helium-based gas mixtures also have a smaller Lorentz angle, which increases the detection e$ciency at the cell edges inside the magnetic "eld (Fig. 6). He/C H 60/40 was chosen for its primary ionization yield which matches the one for the CLEO II gas, maintaining the dE/dx performance of the chamber. The new helium-based gas mixture was introduced already during the last period of CLEO II operation after its properties were discovered in test chamber studies [9].
Fig. 6. He/C H and Ar/C H in the CLEO II drift chamber. The upper plot shows the improved resolution of the DH!D mass di!erence. The lower plot shows the improved e$ciency at cell edges.
At present the drift chamber has passed "nal acceptance tests and is ready for installation into the CLEO III detector.
6. RICH detector The goal of the CLEO III particle identi"cation system is to achieve 4pp/K separation at 2.65 GeV/c, the average maximum momentum for B daughter particles at a symmetric collider operating at the B(4S) resonance. At such momenta the dE/dx measurement by the drift chamber will provide better than 2p separation, so that an additional 3p will have to be provided by the CLEO III RICH. For momenta between 0.9 and 2 GeV where dE/dx will provide almost no particle separation, the RICH should achieve in excess of 4p by itself. The design of the CLEO III RICH is driven by the limited space between the existing electromagnetic CsI calorimeter and the tracking volume, for which a large outer radius is mandatory for good momentum resolution. Therefore, the CLEO III RICH follows the &proximity focusing' design [10], pioneered by the Colle`ge de France/Strasbourg group [11]. It consists of solid radiators, followed by a gas volume in which the Cherenkov cone can expand and thin multi-wire proportional chambers to detect the Cherenkov photons. To allow for very thin (4.5 mm) MWPCs the photosensitive gas in the chambers is a CH / triethylamine (TEA) mixture which has an absorption length of about 0.5 mm. However, this gas only converts photons with wavelengths between 135 and 165 nm to single photoelectrons, with a maximum quantum e$ciency of 33% at 150 nm. This wavelength regime, also referred to as the vacuum ultraviolet (VUV), strongly restricts the choice of transparent materials for the RICH. The only solid materials with a band gap su$ciently large to transport VUV photons are alkali-halide crystals, of which LiF was chosen for radiators and CaF for the MWPC entry window. In the wavelength range of interest they both have a refractive index of about 1.5 with LiF showing smaller dispersion ((5%). These polycrystals can be polished such that their transmission reaches 80% at 150 nm. The expansion volume on the other hand has to be free
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of oxygen, water and large organic molecules, because all these substances are opaque (p "10 Mb at 150 nm) in the VUV. For the CLEO III RICH this volume will be #ushed by pure nitrogen, which except for several vibrational and rotational absorption lines below 145 nm, is transparent in the required wavelength range. Preliminary measurements show that a transmission of better than 98% for the 16 cm deep expansion volume could be achieved in the CLEO III RICH. No mirrors are needed in the &proximity focusing' design. However, the resulting images are not circular rings, but conical intersections, distorted and truncated by refraction at the radiator/expansion volume boundary. This refraction, due to the dispersion of the refractive index, together with the unknown photon emission point due to the "nite radiator thickness, are the dominant e!ects for the
Fig. 7. CLEO III RICH &sawtooth' radiator design (lower) as opposed to planar radiator. Photon paths are shown by dashed lines.
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single photon Cherenkov angle resolution. Refraction also prohibits photons from charged particles traversing the radiator normally from escaping the crystal. The CLEO III RICH therefore introduces a new radiator shape in the central portion of the detector. There the outer surfaces of the crystals have a pattern of &sawtooth'-shaped 4 mm deep grooves, the surfaces of which can be crossed by the Cherenkov photons almost perpendicularly, thus also reducing the distorting e!ects of refraction (Fig. 7). For the refractive index of LiF the di!erence of the Cherenkov angle for kaons and pions at the benchmark momentum of 2.65 GeV/c is about 13 mrad. Thus, the opening angle of the Cherenkov cone has to be measured with an accuracy of about 4 mrad for each particle. The errors on the singlephoton Cherenkov angle measurements of all the photons emitted by one charged particle are largely uncorrelated, and therefore this goal is achieved by detecting at least 12 photoelectrons per ring with a single-photon Cherenkov angle resolution of about 14 mrad. In the design of the CLEO III RICH great emphasis has been put on achieving a large photoelectron yield. The RICH consists of two cylinders, an inner cylinder of radiator crystals glued on a carbon "ber support tube and an outer cylinder of 30 MWPCs in r which extend over the whole length (2.5 m) of the RICH (Fig. 8). No support elements are protruding into the expansion volume and chambers are only separated by thin aluminum
Fig. 8. Azimuthal cross-section of sector of the CLEO III RICH.
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support rails. These two cylinders are only connected at the ends and structural stability is based on the sti!ness of the cylinders, not its individual elements. The photon detectors are asymmetric MWPCs with 72 wires running parallel to the CLEO z-axis. Each chamber is divided longitudinally into 8 segments, such that each segment matches the largest size of polished detector entry window crystal available (20;30 cm) and the distance between ceramic wire support spacers. The cathodes are vacuum-deposited Ag traces on the detector entry window crystals and printed circuit boards with a pad segmentation of 7.5;8 cm on the outer radius of the RICH at a distance of 1 mm from the wires. Readout of the charge induced on these pads provides the spatial information for each signal. E$cient photoelectron reconstruction also puts several demands on the RICH readout electronics. The exponential single photoelectron pulse-height spectrum requires very low-noise electronics for a low pulse-height cut-o! in the reconstruction. In addition, e$cient declustering asks for a high segmentation of the readout. For preampli"cation the CLEO III RICH uses VA}RICH chips, a develop-
ment of the VLSI Viking chip, which are mounted directly on the back of each chamber. It provides an ENC"130 e\#9 e\/pF for 2 ls shaping time. Its intrinsic multiplexing capability allows to segment the readout into 230 400 channels. The signals from these chips are then digitized by a VME-based ADC/sparsi"er system. A common mode subtracted noise of better than 400 e\ has routinely be achieved with the whole readout chain and real chambers. Two of the 30 photon detectors have been tested together with three radiator crystals in a muon halo at Fermilab [12,13]. The testbeam setup consisted of a MWPC reference tracking system and an aluminum expansion gap box with the chambers and radiators, which could be rotated to simulate di!erent track incidence angles as in CLEO III. In the testbeam Cherenkov angle measurement resolutions of about 5 mrad have been obtained (Fig. 9). Based on these testbeam results Monte Carlo extrapolations to CLEO III acceptance and the better tracking there predict a separation power well within the requirements except for the very end of the detector. The RICH has been installed as the "rst major new component into CLEO III in August 1999.
Fig. 9. CLEO III RICH testbeam results. The left plot shows the results from the testbeam for the Cherenkov angle resolution per track. The right plot gives the extrapolation to CLEO III acceptance and tracking resolution.
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7. CLEO III installation CLEO II has been disassembled in March/June 1999. Since then the RICH has been installed and the general DAQ and trigger have been replaced and tested with the CsI calorimeter. The drift chamber is completed and ready for installation at the beginning of September. The Silicon vertex detector is partially assembled but has seen an extension of its installation schedule. The start-up date for CESR and its B-physics detector CLEO III is foreseen to be November 8, 1999.
References [1] N. Cabibbo, Phys. Rev. Lett. 10 (1963) 531.
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[2] M. Kobayashi, T. Maskawa, Prog. Theor. Phys. 49 (1973) 652. [3] L. Wolfenstein, Phys. Rev. Lett. 51 (1984) 1945. [4] See the review article by C. Jarlskog, in: C. Jarlskog (Ed.), CP Violation, World Scienti"c Press, Singapore, 1987. [5] D. Rubin, Proceedings of IEEE/APS Particle Accelerator Conference, 1995. [6] Y. Kubota et al., Nucl. Instr. and Meth. A 320 (1992) 66. [7] G. Brandenburg et al., Proceedings of the Third International Symposium on the Development and Application of Semiconductor Tracking Detectors, Melbourne, December 1997. [8] H. Kagan et al., Nucl. Instr. and Meth. A 383 (1996) 189. [9] K.K. Gan et al., Nucl. Instr. and Meth. A 374 (1996) 27. [10] J. Seguinot, T. Ypsilantis, Nucl. Instr. and Meth. 142 (1977) 377. [11] J.L. Guyonnet et al., Nucl. Instr. and Meth. A 350 (1994) 430. [12] M. Artuso et al., Nucl. Instr. and Meth. A 441 (2000) 374. [13] M. Artuso et al., Nucl. Instr. and Meth. A 419 (1998) 577.
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