- Email: [email protected]
KEKB and the BELLE experiment
Nuclear Instrumentsand Methods in PhysicsResearchA 368 (1995) 74-80
Section A
EISEVIER
KEKB and the BELLE experiment Junji Haba * Department
of Physics,
Osaka Universiry Osaka. Japan
Abstract
KEKB, the Japanese asymmetric e+e- collider B-factory project, and BELLE, the international experiment at KEKB, are reviewed. Emphasis is placed on the differences between KEKB and BELLE and the other competing projects, SLAC-B (PEPII) and the BaBar experiment.
1. Introduction
2.I. The existing facilities
B-factory projects are advancing at several places in the world. Among them, asymmetric e+e- colliders, tuned to run on the Y (4s)) are at the forefront of the competition. These projects are KEKB in Japan and SLAC-B (PEPII) in the U.S. The design and the status of the KEKB project and the BELLE experiment are reviewed in this paper. Since the physics goals and the requirements for the detectors have been reviewed elsewhere [ l-31, this paper discusses the features of the experiments and the differences between them.
KEXE3 has a 2.5 GeV linac and a storage ring with a circumference of 3016 m, formerly used as the TRISTAN injector and its main ring respectively. PEP11 has a powerful 50 GeV linac and a storage ring of 2219 m circumference as existing facilities. To enable direct injection to the 8 GeV electron high energy ring (HER) and to get a greater positron production yield, the KEKB linac will be extended to 8 GeV as illustrated in Fig. 2. Even with this new linac, the injection time in KEKB will still be longer than in PEPII. This is because the PEP11 linac is more powerful and there are more bunches in the larger ring of KEKB. The circumference of the storage ring may affect the dumping time of the beam, especially for the 3.5 GeV (low energy) positron ring (LER). This is important for stable operation of the accelerator. The larger circumference of the KEKB ring is not advantageous in this sense, although its longer straight section is useful for installing the components of the local chromaticity correction, newly applied in KEKB. This refers to a scheme where the large chromaticity produced by the final quadrupole magnets is corrected within the interaction region [ 41.
2. The KEKB accelerator An asymmetric B factory copiously produces B mesons from the decay of the Y (4S), boosted in asymmetric collisions of electrons and positrons. Each beam has its own independent beam pipe and different beam energy and optics (i.e. a double ring), and collides at a single interaction point. Each beam should have an unprecedented high current, resulting in a luminosity as high as 10” cm-‘s-l. This will meet the required production rate of more than lo8 BB pairs per year. Fig. 1 illustrates the layout of the two rings of KEKB. It has an interaction point in the Tsukuba experimental hall where the BELLE detector will be installed. Details of the KEKB accelerator can be found in the KEKB B-Factory Design Report [4]. The major differences between the KEKB and PEP11 accelerators are summarized in Table 1. In the following subsections, the major items listed in the table are briefly discussed.
* Present address: KEK, National Laboratory for High Energy Physics, Japan, e-mail [email protected]
0168~9002/95/$)9.50 @ 1995 Elsevier Science B.V. All rights reserved SSDfOl68-9002(95)00889-6
2.2. Beam crossing scheme The most significant and important difference between PEP11 and KEKB is in the beam crossing arrangement. In PEPII, electron and positron beams collide at the interaction point (IP) at zero crossing angle (i.e. head-on), while in KEKB the beams cross at an angle of 11 mrad. This is demonstrated in Fig. 3. Since an unusually high beam current needs to be stored in a B-factory, as many RF buckets as possible should be filled with particles. Due to the resulting small spacing of the beam bunches, the two beams should be transversely separated immediately after crossing so as to avoid parasitic
J. Haba/Nucl.
Instr. and Meth. in Phys. Res. A 368 (1995)
TSU,KUBA
74-80
,
Fig. 1. Schematic illustration of the KEKB accelerator complex.
collisions by the following bunches. In the head-on collision scheme, this requires a bending magnet with a very strong field just outside the IP However, this initiates very intense synchrotron radiation (SR) from the beam. This SR is harmful for the experiment and must be prevented from illuminating the IP components by using a masking system (although complete masking of the intense SR is very difficult). Furthermore, the existence of a separation bending magnet so close to the IP puts serious constraints on the deTable 1 Major differences between the KEKB and PEP11 accelerators. LER and HER are the low and high energy ring parameters respectively Parameter LER Beam energy (GeV) Circumference (ml Damping time (ms) Ring shape Injector Crossing angle (mrad) Number of bunches Bunch spacing (ml Bunch length (cm) Target luminosity (cm-*se’) Coupled bunch instability
KEKB HER
8.0 3016 35.3 80.5 rounded square 2.5 - 8 GeV 3.5
+I1 5120 0.59 0.4
-11 5120 0.59 0.4
LER
PEP11 HER
9.000 2219 36.8 54.0 hexagonal existing powerful linac
sign of the detector components around the IP. In the PEP11 design, only every second bunch is filled to avoid parasitic collisions. The bunch spacing is therefore longer ( 1.26 m) than that of KERR, where all bunches are filled. Table 2 Possible choices of the crossing angle, and their implications to the hardware design and beam dynamics Crossing angle
Hardware
Beam dynamics
Omrad
Very compact separation bending magnets (such as permanent magnets) are necessary
Rapid beam separation is critical for avoiding parasitic crossing effects
2 mrad
Superconducting separation bend is feasible with a reasonable field strength (< 0.7 T)
3.109
0.0 1658 1.26
0.0 1658 I .26
1.o
1.o
1 x 1034
3 x 1033
choke mode cavity ARES cavity
superconducting cavity
Acceptable regarding parasitic crossing effects with a bunch spacing of ~0.6 m
Smrad
8 mrad
Separation bending magnets are no longer necessary Synchro-betauon resonance uncertainty
10 mrad
20 mrad
Use of common quadrupole magnet for two opposing beams per side becomes difficult
increased need for crab-crossing
II. e+e-
EXPERIMENTS
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in Phys.
Res.
A 368 (1995)
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The finite crossing angle of KEKB is chosen to enable operation without a separation bending magnet. It is also possible to have a common quadrupole magnet for the two opposing beams per side. This makes the components of the interaction region very simple. Therefore, the impact on the detector design is minimum. Possible choices of crossing angle and the implications associated with this are summarized in Table 2. With a 11 mrad crossing, we are not concerned about parasitic collisions with the following bunches, even for the shortest bunch spacing (0.6 m) in full-bunch operation. Another effect of this crossing scheme appears in the length of each bunch. In the finite angle crossing scheme, there is possibly a significant coupling between synchrotron oscillation (along the bunch axis) and betatron oscillation (perpendicular to the bunch axis). This may be harmful for stable operation. To reduce the coupling, the length of a bunch in KEKB is chosen to be shorter (0.4 cm) than in PEPII. There have been discussions on whether the finite crossing angle scheme will give high enough luminosities ( 10” cm-*s-l) to meet the requirements of the experiment. The KEKB accelerator team have chosen the finite crossing angle scheme after feasibility studies with realistic beam-beam simulations. Fig. 4 shows a contour plot of the expected luminosity in the plane of beam tuning parameters. Several resonances which induce a luminosity reduction can be seen in the figure. However, a sizable amount of area appears free of resonances. Thus it is believed that, using the results of the simulation studies conducted so far, the design luminosity can be achieved with a crossing angle of 2 x 11 mrad. 2.3. Coupled bunch instability For very high beam currents at full-bunch operation, coupled bunch instability (CBI) is one of the most serious problems in RF cavities. CBI occurs when the wake of the preceding bunch disturbs the RF accelerating field for the following bunches. CBI can be considered to consist of two components, the higher order mode (HOM) and the fundamental mode. In KEKB, the HOM wake will be absorbed by a choke mode cavity, newly devised in KEK. In this cavity, only the HOM is transported to a choke structure attached to the cavity and then absorbed by sintered Sic in the shape of a bullet. For the fundamental mode, the ARES cavity is being developed at KEK, in which the wake will be dumped by the energy stored in the large storage cavity attached to the accelerating cavity. A schematic illustration of the ARES cavity is shown in Fig. 5. A superconducting cavity, which
Fig. 2. The KEKB linac upgrade plan. The present linac (upper diagram) will be upgraded (lower diagram). In thenewlinac.theelectrons emitted in the gun are accelerated towards the left and then bent and accelerated towards the right. The positrons will be produced by incident electrons of 3.7 GeV instead of the present 0.25 GeV.
J. HabdNucl.
Instr. and Meth. in Phys. Res. A 368 (1995)
74-80
AcceleratingCavity
StorageCavity Fig. 3. The layout of the accelerator components in the interaction region of KEKB.
Fig. 4. Simulation result for the expected luminosity in the plane of tbe tsansverse tune, vx and vy.
will be applied in PEPII, is also one of the other candidates in KEKB to protect against CBI.
3. The BELLE experiment Both at BELLE and at BaBar, CP asymmetry will be measured in the time distribution of the B mesons decaying into specific CP eigenstates. The second B meson decay will be “tagged”, specifying the flavour of the first. Many decay modes of CP eigenstates, such as B --+ J/$K or are proposed to determine the phase in the B -+ r+n-, Kobayashi-Maskawa matrix and the angles of the unitarity triangle. With these aims in mind, the requirements for the detector are as follows: - Wide angular acceptance, especially in the forward region where many particles are boosted. - Vertex detection to measure the decay time with a spatial resolution of better than 100 pm. - Tracking with good momentum resolution even for low momentum particles such as the rr from D’ + Drr. - Good particle identification for r, K and leptons to enable flavour tagging of inclusive or exclusive channels. - Electromagnetic calorimetry with good energy resolution, even for low energy photons to enable efficient r” detection. - Capability to run at a high event rate with high background conditions.
\
I
Fig. 5. Conceptual illusaation of the ARES cavity.
Fig. 6 shows the BELLE detector, designed to meet the above requirements. The main features of the detector are as follows: A vertex detector (SVD) consisting of two octagonal superlayers, each of which is composed of two closely-packed double-sided silicon microstrip detectors (DSSD). A cylindrical drift chamber (CDC) extending between 8.5 < r < 90 cm in radius and capable of dE/dX measurement. An aerogel Cherenkov counter (ACC) to distinguish r and K in the momentum range between 1.2 and 3.5 GeVlc. A time of flight counter (TOF) with a timing resolution of better than 100 ps. A CsI ( Tl) calorimeter. A superconducting solenoid with its inner radius at 170 cm, generating a 1.5 T magnetic field. A return yoke instrumented with fine sampling resistive plate counters (RPC) for the detection of KL and CL. 3.1. Major di$erences in detector design The basic concepts of the BELLE and the BaBar detectors are very similar. However, there is a significant difference in the cross section of the detectors: BELLE is octagonal while BaBar is hexagonal. The other major differences are summarized in Table 3. Table 3 Major differences between tbe BELLE and BaBar detectors BELLE
BaBat
octagonal 2 superlayers of 2 DSSD with double metal layers [I]; slam r-4 strips
hexagonal (3+2) DSSD with read &ace on kapton film:
(cm) No. of layers
8.5l90.0 52
Particle ID
aerogel Chrenkov ‘l-OF, dE/dX 125 1168+6624+ 1024
24.24h5.18 40 support tube t = 20 cm DIRC dE/dX 90 5880 + 900
crosssection Vertex detector
intermediate hacker
Tracking ‘ia/‘wt
CSI, ‘in (cm) No. of crystals
11. e+e-
EXPERIMENTS
J. Hahu/Nucl.
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.i iii..
Instr. and Meth. in Phys. Res. A 368 (1995)
-:
74-80
;,,
I ,
0
:_I
1 1
I,
Fig. 6. Side view of the BELLE detector.
3.2. Silicon vertex detector The BELLE vertex detector (SVD) has two superlayers arranged in an octagon, each of which consists of two closely packed DSSD ladders as shown in Fig. 7. The DSSDs in each superlayer have their strips slanted by f20 mrad in the z direction. Thus each superlayer can resolve a stereo ambiguity and can distinguish an accidental noise hit, due to electronics or soft X-ray background. The vertex detector of BaBar (SVT) [ 51 consists of three cylindrical inner layers surrounded by two outer layers of a novel arch structure. This becomes possible with the use of flexible kapton fan-out circuits. The function of the inner layers is the same as the BELLE SVD, while the outer layers will be used as an intermediate tracker between the inner layers and the.centml tracker (which starts at r = 24 cm, outside of the support tube inserted at r = 20 cm). This complicated structure and the unprecedented length of the outer layers may make their construction more difficult. The expected vertex resolution for the B --+ J/(/IK%decay is plotted in Fig. 8 for the BELLE SVD. By fitting the distribution with two Gaussians, the effective resolution, i.e. the
average rms, ( flui + fso$) ‘I’, is evaluated to be 36 pm. This is slightly better than the corresponding number for the BaBar system (70 ,um) because BaBar has a significant amount of gold plate inside its beam pipe to attenuate synchrotron light produced at the separation bending magnet. 3.3. The main tracking system The BELLE main tracker (CDC) is a cylindrical drift chamber extending from r = 8.5 cm to r = 90 cm. To accommodate the final quadrupole magnet placed inside the detector system, the inner part of the endplate of the chamber must have a conical shape. The BaBar main tracker has a relatively short tracking length due to the support tube which holds the vertex detector and the accelerator components such as the separation bending magnet. To compensate for this BaBar has outer silicon layers to serve as an intermediate tracker. Fig. 9 shows the expected transverse momentum resolutions for the BELLE tracker with (solid curve) and without (dashed curve) the SVD. Also shown is the resolution for a hypothetical tracker with a five layer silicon detector (such
J. Haba/Nucl.
Instr. and Meth. in Phys. Res. A 368 (1995)
19
74-80
10
1
0.1 0:1
i
Fig. 7. Cross sectionalview of the BELLE SVD.
pt(GeV/c)
as the BaBar SVT) followed by the CDC which extends only from r = 20 cm to r = 90 cm (dash-dotted curve). The BELLE tracker system exhibits a slightly better resolution over the whole momentum range. Continuous and uniform tracking outwards from the hit closest to the interaction point is very important. Thanks to the finite crossing angle, the BELLE tracker system is free from structure such as the support-tube which constrains the present BaBar tracker
151. 3.4. Particle identi$cation (PID) The most distinct difference between the detector systems is the choice of technology for particle identification. The identification of rr and K mesons plays two important roles. The first is K identification to tag the flavour of the B meson, the one not decaying into the CP eigenstate. The momentum range required is up to 1.2 GeV/c. The second is n, K identification to distinguish decay modes such as B --+ r’n-
Fig. 8. Expectedvertex resolutionin BELLE for B + J/I&
decay.
Fig. 9. Expectedmomenmmresolutionfor severalsacker configurations.
and B + ?r+K-. The capability of identification should be as high as 4 GeVlc. The BELLE collaboration has selected an aerogel Cherenkov counter (ACC), while BaBar has selected a DIRC (Detection of Internally Reflected Cherenkov light) [ 51. The DIRC is a type of ring imaging Cherenkov detector where the light is emitted and transported in a rectangular quartz bar, and then projected from the end of the bar onto a photon detector array. At the design stage, the BELLE collaboration had three options for particle identification technology: a RICH with a CsI photocathode, a DIRC, and an ACC. The first was dropped since the production of a stable CsI photocathode is still not well established. The secondoption was seriously considered but it impacted on the design of the end yoke of the solenoid magnet (and thus accelerator components), since the quartz bars of the DIRC would need to penetrate the end yoke toward the outside of the magnet. We concluded that this impact was too serious to accept. The expected performance of the ACC detector is not superior in the highest momentum region to the other imaging Cherenkov devices. However a simulation study shows that degradation of less than 10% is expected in the final B asymmetry measurement error when comparing ACC with an imaginary perfect tiarticle identifier. The quality of the aerogel radiator produced in KEKB shows dramatic progress. The recent production batch exhibits a photoelectron yield as high as 17 with an aerogel radiator of refractive index n = 1.01 viewed by two 3-in. phototubes. Fig. 10 shows the detection efficiencies of the ACC with R = 1.015 for rr’s and K’s as a function of momentum, expected from test beam results. We assume that the dE/dX resolution is 5.5% in the CDC
II. e+e- EXPERIMENTS
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BELLE is significantly larger than in BaBar. This results in a larger cross section of a single crystal in the BELLE calorimeter and may have some effect on shower clustering.
pion
rk”” 4. Summary
1---_ /
-4-b-c
Ooosli
22
3
35
4
45
Momenhrm (GeVlc) Fig. 10. Expected performance of the ACC.
and the TOF resolution is 100 ps, both of which have already been accomplished in prototype detectors. 3.5. The electromagnetic
KEKB and PEP11 are competing asymmetric efe- collider projects to explore B physics and CP violation. The most significant difference between them is the crossing scheme of the colliding beams: a finite crossing angle in KEKB compared to a zero crossing angle in PEPII. This affects the design of the accelerator and the detector system. The scheme chosen by KEKB is ambitious and more advantageous for the detector: there is less synchrotron light and more flexibility in the detector design around the interaction point. KEKB and the BELLE detector are now in preparation and commissioning is expected in January 1999.
References calorimeter
There is no clear difference in performance between the calorimeters of the two experiments. They consist of more than 6000 CsI(T1) crystals, each read out with photodiodes. Due to the thicker PID device (ACC), the radius of the calorimeter system and thus the total amount of crystals in
[ I] M. T. Cheng [ 21 D. Boutigny [ 31 P. Hanison,
et al.. KBK Report 94-2; KEK Report 95-l. et al.. SLAGR-95-457. these Proceedings (3rd Int. Workshop on B-Physics at H&on Machines, Oxford, UK, 1995) Nucl. Iostr. aad Meth. A 368 (1995) 81. [4] N. Toge et al.. KBK Report 95-7. [S] BaBar Technical Design Report, SLAGR-95-457.
Section A
EISEVIER
KEKB and the BELLE experiment Junji Haba * Department
of Physics,
Osaka Universiry Osaka. Japan
Abstract
KEKB, the Japanese asymmetric e+e- collider B-factory project, and BELLE, the international experiment at KEKB, are reviewed. Emphasis is placed on the differences between KEKB and BELLE and the other competing projects, SLAC-B (PEPII) and the BaBar experiment.
1. Introduction
2.I. The existing facilities
B-factory projects are advancing at several places in the world. Among them, asymmetric e+e- colliders, tuned to run on the Y (4s)) are at the forefront of the competition. These projects are KEKB in Japan and SLAC-B (PEPII) in the U.S. The design and the status of the KEKB project and the BELLE experiment are reviewed in this paper. Since the physics goals and the requirements for the detectors have been reviewed elsewhere [ l-31, this paper discusses the features of the experiments and the differences between them.
KEXE3 has a 2.5 GeV linac and a storage ring with a circumference of 3016 m, formerly used as the TRISTAN injector and its main ring respectively. PEP11 has a powerful 50 GeV linac and a storage ring of 2219 m circumference as existing facilities. To enable direct injection to the 8 GeV electron high energy ring (HER) and to get a greater positron production yield, the KEKB linac will be extended to 8 GeV as illustrated in Fig. 2. Even with this new linac, the injection time in KEKB will still be longer than in PEPII. This is because the PEP11 linac is more powerful and there are more bunches in the larger ring of KEKB. The circumference of the storage ring may affect the dumping time of the beam, especially for the 3.5 GeV (low energy) positron ring (LER). This is important for stable operation of the accelerator. The larger circumference of the KEKB ring is not advantageous in this sense, although its longer straight section is useful for installing the components of the local chromaticity correction, newly applied in KEKB. This refers to a scheme where the large chromaticity produced by the final quadrupole magnets is corrected within the interaction region [ 41.
2. The KEKB accelerator An asymmetric B factory copiously produces B mesons from the decay of the Y (4S), boosted in asymmetric collisions of electrons and positrons. Each beam has its own independent beam pipe and different beam energy and optics (i.e. a double ring), and collides at a single interaction point. Each beam should have an unprecedented high current, resulting in a luminosity as high as 10” cm-‘s-l. This will meet the required production rate of more than lo8 BB pairs per year. Fig. 1 illustrates the layout of the two rings of KEKB. It has an interaction point in the Tsukuba experimental hall where the BELLE detector will be installed. Details of the KEKB accelerator can be found in the KEKB B-Factory Design Report [4]. The major differences between the KEKB and PEP11 accelerators are summarized in Table 1. In the following subsections, the major items listed in the table are briefly discussed.
* Present address: KEK, National Laboratory for High Energy Physics, Japan, e-mail [email protected]
0168~9002/95/$)9.50 @ 1995 Elsevier Science B.V. All rights reserved SSDfOl68-9002(95)00889-6
2.2. Beam crossing scheme The most significant and important difference between PEP11 and KEKB is in the beam crossing arrangement. In PEPII, electron and positron beams collide at the interaction point (IP) at zero crossing angle (i.e. head-on), while in KEKB the beams cross at an angle of 11 mrad. This is demonstrated in Fig. 3. Since an unusually high beam current needs to be stored in a B-factory, as many RF buckets as possible should be filled with particles. Due to the resulting small spacing of the beam bunches, the two beams should be transversely separated immediately after crossing so as to avoid parasitic
J. Haba/Nucl.
Instr. and Meth. in Phys. Res. A 368 (1995)
TSU,KUBA
74-80
,
Fig. 1. Schematic illustration of the KEKB accelerator complex.
collisions by the following bunches. In the head-on collision scheme, this requires a bending magnet with a very strong field just outside the IP However, this initiates very intense synchrotron radiation (SR) from the beam. This SR is harmful for the experiment and must be prevented from illuminating the IP components by using a masking system (although complete masking of the intense SR is very difficult). Furthermore, the existence of a separation bending magnet so close to the IP puts serious constraints on the deTable 1 Major differences between the KEKB and PEP11 accelerators. LER and HER are the low and high energy ring parameters respectively Parameter LER Beam energy (GeV) Circumference (ml Damping time (ms) Ring shape Injector Crossing angle (mrad) Number of bunches Bunch spacing (ml Bunch length (cm) Target luminosity (cm-*se’) Coupled bunch instability
KEKB HER
8.0 3016 35.3 80.5 rounded square 2.5 - 8 GeV 3.5
+I1 5120 0.59 0.4
-11 5120 0.59 0.4
LER
PEP11 HER
9.000 2219 36.8 54.0 hexagonal existing powerful linac
sign of the detector components around the IP. In the PEP11 design, only every second bunch is filled to avoid parasitic collisions. The bunch spacing is therefore longer ( 1.26 m) than that of KERR, where all bunches are filled. Table 2 Possible choices of the crossing angle, and their implications to the hardware design and beam dynamics Crossing angle
Hardware
Beam dynamics
Omrad
Very compact separation bending magnets (such as permanent magnets) are necessary
Rapid beam separation is critical for avoiding parasitic crossing effects
2 mrad
Superconducting separation bend is feasible with a reasonable field strength (< 0.7 T)
3.109
0.0 1658 1.26
0.0 1658 I .26
1.o
1.o
1 x 1034
3 x 1033
choke mode cavity ARES cavity
superconducting cavity
Acceptable regarding parasitic crossing effects with a bunch spacing of ~0.6 m
Smrad
8 mrad
Separation bending magnets are no longer necessary Synchro-betauon resonance uncertainty
10 mrad
20 mrad
Use of common quadrupole magnet for two opposing beams per side becomes difficult
increased need for crab-crossing
II. e+e-
EXPERIMENTS
76
J. Haba/Nucl. Instr. and Meth.
in Phys.
Res.
A 368 (1995)
74-80
The finite crossing angle of KEKB is chosen to enable operation without a separation bending magnet. It is also possible to have a common quadrupole magnet for the two opposing beams per side. This makes the components of the interaction region very simple. Therefore, the impact on the detector design is minimum. Possible choices of crossing angle and the implications associated with this are summarized in Table 2. With a 11 mrad crossing, we are not concerned about parasitic collisions with the following bunches, even for the shortest bunch spacing (0.6 m) in full-bunch operation. Another effect of this crossing scheme appears in the length of each bunch. In the finite angle crossing scheme, there is possibly a significant coupling between synchrotron oscillation (along the bunch axis) and betatron oscillation (perpendicular to the bunch axis). This may be harmful for stable operation. To reduce the coupling, the length of a bunch in KEKB is chosen to be shorter (0.4 cm) than in PEPII. There have been discussions on whether the finite crossing angle scheme will give high enough luminosities ( 10” cm-*s-l) to meet the requirements of the experiment. The KEKB accelerator team have chosen the finite crossing angle scheme after feasibility studies with realistic beam-beam simulations. Fig. 4 shows a contour plot of the expected luminosity in the plane of beam tuning parameters. Several resonances which induce a luminosity reduction can be seen in the figure. However, a sizable amount of area appears free of resonances. Thus it is believed that, using the results of the simulation studies conducted so far, the design luminosity can be achieved with a crossing angle of 2 x 11 mrad. 2.3. Coupled bunch instability For very high beam currents at full-bunch operation, coupled bunch instability (CBI) is one of the most serious problems in RF cavities. CBI occurs when the wake of the preceding bunch disturbs the RF accelerating field for the following bunches. CBI can be considered to consist of two components, the higher order mode (HOM) and the fundamental mode. In KEKB, the HOM wake will be absorbed by a choke mode cavity, newly devised in KEK. In this cavity, only the HOM is transported to a choke structure attached to the cavity and then absorbed by sintered Sic in the shape of a bullet. For the fundamental mode, the ARES cavity is being developed at KEK, in which the wake will be dumped by the energy stored in the large storage cavity attached to the accelerating cavity. A schematic illustration of the ARES cavity is shown in Fig. 5. A superconducting cavity, which
Fig. 2. The KEKB linac upgrade plan. The present linac (upper diagram) will be upgraded (lower diagram). In thenewlinac.theelectrons emitted in the gun are accelerated towards the left and then bent and accelerated towards the right. The positrons will be produced by incident electrons of 3.7 GeV instead of the present 0.25 GeV.
J. HabdNucl.
Instr. and Meth. in Phys. Res. A 368 (1995)
74-80
AcceleratingCavity
StorageCavity Fig. 3. The layout of the accelerator components in the interaction region of KEKB.
Fig. 4. Simulation result for the expected luminosity in the plane of tbe tsansverse tune, vx and vy.
will be applied in PEPII, is also one of the other candidates in KEKB to protect against CBI.
3. The BELLE experiment Both at BELLE and at BaBar, CP asymmetry will be measured in the time distribution of the B mesons decaying into specific CP eigenstates. The second B meson decay will be “tagged”, specifying the flavour of the first. Many decay modes of CP eigenstates, such as B --+ J/$K or are proposed to determine the phase in the B -+ r+n-, Kobayashi-Maskawa matrix and the angles of the unitarity triangle. With these aims in mind, the requirements for the detector are as follows: - Wide angular acceptance, especially in the forward region where many particles are boosted. - Vertex detection to measure the decay time with a spatial resolution of better than 100 pm. - Tracking with good momentum resolution even for low momentum particles such as the rr from D’ + Drr. - Good particle identification for r, K and leptons to enable flavour tagging of inclusive or exclusive channels. - Electromagnetic calorimetry with good energy resolution, even for low energy photons to enable efficient r” detection. - Capability to run at a high event rate with high background conditions.
\
I
Fig. 5. Conceptual illusaation of the ARES cavity.
Fig. 6 shows the BELLE detector, designed to meet the above requirements. The main features of the detector are as follows: A vertex detector (SVD) consisting of two octagonal superlayers, each of which is composed of two closely-packed double-sided silicon microstrip detectors (DSSD). A cylindrical drift chamber (CDC) extending between 8.5 < r < 90 cm in radius and capable of dE/dX measurement. An aerogel Cherenkov counter (ACC) to distinguish r and K in the momentum range between 1.2 and 3.5 GeVlc. A time of flight counter (TOF) with a timing resolution of better than 100 ps. A CsI ( Tl) calorimeter. A superconducting solenoid with its inner radius at 170 cm, generating a 1.5 T magnetic field. A return yoke instrumented with fine sampling resistive plate counters (RPC) for the detection of KL and CL. 3.1. Major di$erences in detector design The basic concepts of the BELLE and the BaBar detectors are very similar. However, there is a significant difference in the cross section of the detectors: BELLE is octagonal while BaBar is hexagonal. The other major differences are summarized in Table 3. Table 3 Major differences between tbe BELLE and BaBar detectors BELLE
BaBat
octagonal 2 superlayers of 2 DSSD with double metal layers [I]; slam r-4 strips
hexagonal (3+2) DSSD with read &ace on kapton film:
(cm) No. of layers
8.5l90.0 52
Particle ID
aerogel Chrenkov ‘l-OF, dE/dX 125 1168+6624+ 1024
24.24h5.18 40 support tube t = 20 cm DIRC dE/dX 90 5880 + 900
crosssection Vertex detector
intermediate hacker
Tracking ‘ia/‘wt
CSI, ‘in (cm) No. of crystals
11. e+e-
EXPERIMENTS
J. Hahu/Nucl.
78
.i iii..
Instr. and Meth. in Phys. Res. A 368 (1995)
-:
74-80
;,,
I ,
0
:_I
1 1
I,
Fig. 6. Side view of the BELLE detector.
3.2. Silicon vertex detector The BELLE vertex detector (SVD) has two superlayers arranged in an octagon, each of which consists of two closely packed DSSD ladders as shown in Fig. 7. The DSSDs in each superlayer have their strips slanted by f20 mrad in the z direction. Thus each superlayer can resolve a stereo ambiguity and can distinguish an accidental noise hit, due to electronics or soft X-ray background. The vertex detector of BaBar (SVT) [ 51 consists of three cylindrical inner layers surrounded by two outer layers of a novel arch structure. This becomes possible with the use of flexible kapton fan-out circuits. The function of the inner layers is the same as the BELLE SVD, while the outer layers will be used as an intermediate tracker between the inner layers and the.centml tracker (which starts at r = 24 cm, outside of the support tube inserted at r = 20 cm). This complicated structure and the unprecedented length of the outer layers may make their construction more difficult. The expected vertex resolution for the B --+ J/(/IK%decay is plotted in Fig. 8 for the BELLE SVD. By fitting the distribution with two Gaussians, the effective resolution, i.e. the
average rms, ( flui + fso$) ‘I’, is evaluated to be 36 pm. This is slightly better than the corresponding number for the BaBar system (70 ,um) because BaBar has a significant amount of gold plate inside its beam pipe to attenuate synchrotron light produced at the separation bending magnet. 3.3. The main tracking system The BELLE main tracker (CDC) is a cylindrical drift chamber extending from r = 8.5 cm to r = 90 cm. To accommodate the final quadrupole magnet placed inside the detector system, the inner part of the endplate of the chamber must have a conical shape. The BaBar main tracker has a relatively short tracking length due to the support tube which holds the vertex detector and the accelerator components such as the separation bending magnet. To compensate for this BaBar has outer silicon layers to serve as an intermediate tracker. Fig. 9 shows the expected transverse momentum resolutions for the BELLE tracker with (solid curve) and without (dashed curve) the SVD. Also shown is the resolution for a hypothetical tracker with a five layer silicon detector (such
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Fig. 7. Cross sectionalview of the BELLE SVD.
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as the BaBar SVT) followed by the CDC which extends only from r = 20 cm to r = 90 cm (dash-dotted curve). The BELLE tracker system exhibits a slightly better resolution over the whole momentum range. Continuous and uniform tracking outwards from the hit closest to the interaction point is very important. Thanks to the finite crossing angle, the BELLE tracker system is free from structure such as the support-tube which constrains the present BaBar tracker
151. 3.4. Particle identi$cation (PID) The most distinct difference between the detector systems is the choice of technology for particle identification. The identification of rr and K mesons plays two important roles. The first is K identification to tag the flavour of the B meson, the one not decaying into the CP eigenstate. The momentum range required is up to 1.2 GeV/c. The second is n, K identification to distinguish decay modes such as B --+ r’n-
Fig. 8. Expectedvertex resolutionin BELLE for B + J/I&
decay.
Fig. 9. Expectedmomenmmresolutionfor severalsacker configurations.
and B + ?r+K-. The capability of identification should be as high as 4 GeVlc. The BELLE collaboration has selected an aerogel Cherenkov counter (ACC), while BaBar has selected a DIRC (Detection of Internally Reflected Cherenkov light) [ 51. The DIRC is a type of ring imaging Cherenkov detector where the light is emitted and transported in a rectangular quartz bar, and then projected from the end of the bar onto a photon detector array. At the design stage, the BELLE collaboration had three options for particle identification technology: a RICH with a CsI photocathode, a DIRC, and an ACC. The first was dropped since the production of a stable CsI photocathode is still not well established. The secondoption was seriously considered but it impacted on the design of the end yoke of the solenoid magnet (and thus accelerator components), since the quartz bars of the DIRC would need to penetrate the end yoke toward the outside of the magnet. We concluded that this impact was too serious to accept. The expected performance of the ACC detector is not superior in the highest momentum region to the other imaging Cherenkov devices. However a simulation study shows that degradation of less than 10% is expected in the final B asymmetry measurement error when comparing ACC with an imaginary perfect tiarticle identifier. The quality of the aerogel radiator produced in KEKB shows dramatic progress. The recent production batch exhibits a photoelectron yield as high as 17 with an aerogel radiator of refractive index n = 1.01 viewed by two 3-in. phototubes. Fig. 10 shows the detection efficiencies of the ACC with R = 1.015 for rr’s and K’s as a function of momentum, expected from test beam results. We assume that the dE/dX resolution is 5.5% in the CDC
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BELLE is significantly larger than in BaBar. This results in a larger cross section of a single crystal in the BELLE calorimeter and may have some effect on shower clustering.
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Momenhrm (GeVlc) Fig. 10. Expected performance of the ACC.
and the TOF resolution is 100 ps, both of which have already been accomplished in prototype detectors. 3.5. The electromagnetic
KEKB and PEP11 are competing asymmetric efe- collider projects to explore B physics and CP violation. The most significant difference between them is the crossing scheme of the colliding beams: a finite crossing angle in KEKB compared to a zero crossing angle in PEPII. This affects the design of the accelerator and the detector system. The scheme chosen by KEKB is ambitious and more advantageous for the detector: there is less synchrotron light and more flexibility in the detector design around the interaction point. KEKB and the BELLE detector are now in preparation and commissioning is expected in January 1999.
References calorimeter
There is no clear difference in performance between the calorimeters of the two experiments. They consist of more than 6000 CsI(T1) crystals, each read out with photodiodes. Due to the thicker PID device (ACC), the radius of the calorimeter system and thus the total amount of crystals in
[ I] M. T. Cheng [ 21 D. Boutigny [ 31 P. Hanison,
et al.. KBK Report 94-2; KEK Report 95-l. et al.. SLAGR-95-457. these Proceedings (3rd Int. Workshop on B-Physics at H&on Machines, Oxford, UK, 1995) Nucl. Iostr. aad Meth. A 368 (1995) 81. [4] N. Toge et al.. KBK Report 95-7. [S] BaBar Technical Design Report, SLAGR-95-457.