The 11th Symposium on Accelerator Science ~nd Technology, H arima Science (Jarden City, 1997

B, 'tCharm, and+Factories

Shin-ichi KUROKAWA
KEK, High Energy Accelerator Research Organization
1-1 Oho, Tsukuba-shi, lbaraki-ken, 305 JAPAN

Abstract
Electron-positron collider type particle factories, BFactory, 'tCharm Factory, and q, Factory, aim at achieving a. h 1ummos1ty. . m. a range of 5 x 1032em-2s-1 to 1034em-2s-1 .
h1gThis goal requires storing large currents, 0.5 A to 5 A, inrings by distributing them into large number of bunches. To
avoid unnecessary collisions, all factories adopt two-ring
scheme, where electrons and positrons are stored in differentrings. One q, Factory, DA<PNE is now being commissioned
and two asymmetric-energy B-Factories, KEKB and PEP-II,
now under construction, will be completed by the end of1998. IHEP, Beijing, expects R&D of its .Charm Factory,
BTCF, to be approved soon.

1 Introduction
A factory is an accelerator facility that is optimized toproduce some kind of particles copiously for high-energy
physics studies. All factories now being constructed or-C\1-1o·29Q)

10·31

E(.)

planned are electron-positron colliders and named after theparticles they produce. B-Factory is tuned to produce Bmesons, .Charm Factory t-leptons and J/'lf particles, and c1>
Factory q, mesons. The energy of beams in these factories
are mainly determined by the mass of particles they produce.
Figure 1 shows the hadronic cross section and expected
number of events per year for factories with a luminosity of1033cm·2s· 1• The event rate at B-Factory is two orders ofmagnitude lower than that of 'tCharm and q, Factories;
therefore, B-Factory demands the highest luminosity amongfactories, namely, 3 x 1033cm·2s·' to 1034cm-2s· 1, whereas thatof .Charm and q, Factories is around 5 x 1032cm·2s· 1 to1033cm·2s· 1• In this paper two B-Factories, KEKB atKEK[1], PEP-II at SLAC[2], .Charm Factory, BTCF at
IHEP, Beijing[3], and q, Factory, DA<PNE at Frascati[4], arediscussed. Figures 2,3,4, and 5 show layouts of thesefactories. Table 1 summarizes basic parameters of thesefactories.

c:·u0

q == u,d,s,c,b,t(/)
(/)
(/)

ez(.)

-~

II·.....J

10·33......

c:0e"0......
a3a3..cc:

Q)>-

10·35......

Q)

0c.

~:.c·c:c:....c:(/)

Q)

10·37>

w

a3IQ)

+Q)
10

10·39

110Ecm (GeV)
Fig. 1 Cross section and events per year for factories.

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The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997
TSUKUBA

ORO

Accumulation Ring)

Fig.

2 Schematic layout of KEKB.

Fig. 3 Schematic layout of PEP-II.

Fig. 4 Schematic diagram of BTCF.

Crab Crossing Scheme
AF Deflector(Crab Cavity)[Non-Cr-schemeJ

Fig. 6 Principle of crab crossing.

Fig. 5 Layout of DA<I>NE and its injector.

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Table 1Main parameters of factories
KEKB
10
2
number of rings
1
number of interaction points
symmetric/asymmetric energy collision asymmetric3016
circumference (m)8 (e-)/3.5(e+)
beam energy (GeV)
l.l(e-)/2.6(e+)
total current per beam (A)5000
number of bunches
bunch spacing (m)0.59
2 X 11crossing angle (mrad)0.039/0.052
beam-beam tuneshift 'E,..(Ew3311~-function at IP ~x */~v * (em)luminosity (I0 33 cm-2 s- 1)

RF frequency (MHz)type of cavity 1ldetector

508.887
NCC,SCCBELLE

PEP-II
32
1
asymmetric2199
9(e-)/3.1(e+)
l.O(e-)/2.14(e+)
1658
1.14
0
0.03/0.03
50/1.5(e+)
67/2.0(e-)
476
NCC
BaBar

BTCF(high luminosity)
12
1
symmetric385
1.5-2.5
0.57
86
3.78
2x 2.60.04/0.04
65/1

DA<I>NE0.522symmetric
97.7
0.51
5.24
1200.812 X 12.5
0.04/0.04
450/4.5476

368.25NCC
KLOE, FINUDA

sec1) NCC and SCC stand for normal conducting cavity and superconducting cavity, respectively.

2collide at one interaction point (IP) at KEKB, PEP-II, andBTCF, and at two IPs at DA<I>NE.

High luminosity and large stored current

The luminosity of an electron-positron collider is given
by3L~2.2xiO"S(l+r{ E/J
where L stands for luminosity in cm-2 s- 1, £, beam-beamtuneshift, r, the ratio of vertical beam size to horizontal beam
size at the collision point, /, beam current in A, E, beamenergy in Ge V, and {3y *, beta-value at the interaction point.
All factory designs assume that the beam-beamtune shift sis between 0.03 and 0.05 and {3y * between 1 and 5
em. They also adopt a flat-beam scheme where r is of theorder of a few %. Since the luminosity is inversely
proportional to the beam energy, lower-energy factories

require higher currents. If, we assume, for example,
L=l0 33cm- 2 s- 1, £=0.03, and {3y*=l em, and use typical energyof B (5 GeV), 'tCharm (1.5 GeV) and <1> (0.5 GeV) Factories,
we find that we need to store 0.30 A forB-Factory, 1.0 A for'tCharm Factory, and 3.0 A for <1> Factory. High-energy
colliders can achieve higher luminosity with relatively lowercurrents.
These large currents should be achieved with multibunch beam storage in rings. If we store n bunches ofelectrons and positrons in a ring, these bunches collide at 2 xn points along the ring. Almost all of these collisionsexcept at interaction points are unnecessary. The simplest
way to avoid these unnecessary collisions is to use two rings:
one for electrons and the other for positrons. All factories
Electrons and positrons
adopt this two-ring scheme.

Coupled-bunch instabilities

Under high-current and large number of bunches
stored in factories, coupled-bunch oscillation grows quickly,
since bunches couple to each other through wakes
produced by bunch-environment (RF cavities and vacuum
In order to prevent thesecomponents) interactions.
instabilities, HOM(higher-order mode)-free RF cavities andsmooth vacuum components should be used. All factories
use single-cell, single-mode cavities, either normalorand DA<I>NE)PEP-II,(KEKB,conducting
superconducting (KEKB, and BTCF) [5]. KEKB uses
normal conducting cavities called ARES [6] for its positron
ring, and both ARES and single-cell, single-mode
superconducting cavities[?] for its electron ring. In Octoberand November 1996 a prototype superconducting cavity for
KEKB was beam-tested at the accumulation ring ofTRISTAN[8]. The cavity could store 573 rnA at 2.5 GeV.
This current is about a half of the KEKB electron ring andthe same as that of BTCF.
At KEKB and PEP-II, high stored current in a ring
excites coupled-bunch instabilities due to not only ROMs
but also fundamental mode of RF cavities. The coupledbunch instability becomes extremely strong, and the growthtime of which is of the order of a few 10 J.lSec. TheAn RF
mechanism of this instability is the following.
cavity should be matched to its power sotrrce (klystron) toprevent reflection of RF power. The beam through thecavity excites RF field which is not in phase with the RF-11-

The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

field from the power source and destroys the matchingcondition. By changing the resonant frequency of thecavity by a small amount, ilf, this matching is restoredagain.
This M is called the detuning frequency and isinversely proportional to the stored energy of the cavity.
At the positFon ring of PEP-II, this detuning frequency isnearly equal to the revolution frequency. If the same typeof cavities were used also at KEKB, the detuning frequency
would become twice as large as the revolution frequency at
the positron ring of KEKB. When we increase the stored
current in a ring, the peak of the RF impedance approaches
to and passes the mode frequency of anti-dampimg
coupled-bunch modes; these modes are strongly excited.
To circumvent the instability, KEKB and PEP-II
adopt different approaches: KEKB uses large-stored energy
cavities ARES to make Af small compared to the revolution
frequency, whereas PEP-II's method is to use direct RF
feedback and feedback through a comb filter to lower theimpedance of the cavity at the anti-damping coupled-bunchmode frequencies.
The difference of the circumferences of rings offactories make this situation differ from each other.
Larger rings (KEKB and PEP-II) have severe problems,
whereas this instability is of no concern for BTCF andDA<l>NE that has only shorter circumferences, where thedetuning frequency is small compared to the revolution
frequency.
4

Photo-electron and fast-ion instabilities

Two new types of coupled-bunch instabilities may beserious to B-Factories and 1:Charm Factory: one is thephoto-electron instability(PEI) and the other fast-ioninstability(FII).
PEl is a serious concern for positron rings.
Positrons stored in a ring emit synchrotron lights, whichthen hit the inner wall of vacuum ducts and produce
photoelectrons. These electrons are attracted by beam
potentials and move toward the beam. Although theelectrons stay in a vacuum pipe only short period ( a few 10nsec) and are absorbed quickly, continuous production andabsorption result in somewhat equilibrium distribution ofelectrons around the beam. This electron cloud acts as asource of wake and excites a coupled-bunch instability.
Vertical oscillation of bunches is excited. The growthtime of the instability may be shorter than 1 msec at KEKB.
PEl was first observed at Photon Factory ring of KEK[9]and confirmed by an experiment at BEPC by IHEP-KEK
collaboration[ I 0].
FII[ll, 12,13] is a serious concern for electron rings.
A bunch train in a ring ionizes residual gas and creates ions.
These ions are accumulated rapidly within a bunch train.
If a bunch in the head of the train start to oscillate due tosome disturbances, the oscillation couples resonantly tothat of trailing bunches through ions and grows along thebunch train. In a flat beam case, vertical oscillation ofbunches are excited. Linear theory predicts that thegrowth time of this instability at KEKB and PEP-II is of theorder of 1 msec. Note that this instability is different fromion trapping. In factories, ions created by a bunch train
are cleared in a bunch gap. Ions are not continuously
trapped and the first bunch in the train always sees freshgas.
Two experiments done at LBL[14] and KEK[15]detected this instability.
Bunch-by-bunch beam feedback systems have acapability of damping transverse coupled-bunch
instabilities with 1 msec or faster damping time and are
expected to cure the instabilities.
5Asymmetric energy collisions at B-Factories

KEKB and PEP-II, are asymmetric energy electronpositron colliders. The energies of electrons and positrons
are different (3.5 GeV positrons and 8 GeV electrons atKEKB and 3.1 GeV positrons and 9 GeV electrons at PEPII). Electrons have higher energy than positrons in order toavoid ion trapping, which becomes much serious at lowenergies. Positron ring, therefore, is called low-energy ring(LER) and electron ring high-energy ring (HER).
In symmetric energy collision of 5.3 GeV electronsand positrons, such as at CESR, B-mesons and anti B-mesons
are produced at rest and decay at the point where they arecreated. Identification of B or anti B-mesons is impossible.
B-mesons and anti B-mesons produced in an asymmetricenergy collider move toward the direction of incoming
electrons and travel a few 100 f..Lm before decaying into
secondary particles. By detecting these secondary particles,
we can identify B and anti B-mesons. This is of vital
importance to KEKB and PEP-II that aim at detecting CPviolation effect, which is a subtle difference of behavior
between particles and anti-particles. BELLE detector[16]surrounds the IP of KEKB and BaBar detector[ 17] that ofPEP-II.
BTCF and DA<l>NE are symmetric energy colliders:the energy of electrons and positrons is the same.
6Interaction regions

Electrons and positrons are made to collide at IP andshould be separated into different rings after the collision.
Finite-angle crossing scheme is attractive, since it separates
beams naturally. KEKB, BTCF and DA<l>NE adopt thisscheme. Crossing angles ofKEKB, BTCF and DA<PNE are±11 rnrad, ±2.3 mrad, and ±12.5 rnrad, respectively.
Without crossing at angle, electric separators should be used
to separate beams at symmetric energy colliders, andseparation dipole magnets at asymmetric energy colliders.
PEP-II uses separation dipole magnets and beams collide
head-on.
Synchrobetatron resonances may be excited by anfinite-angle collision, where beam-beam force felt by aparticle in a bunch differs according to not only its transverse
position but also its longitudinal position. This dependence
on particle's longitudinal position makes transverse andlongitudinal motion of bunches couple to each other andexcites synchrobetatron resonances.

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At KEKB, extensive simulation study showed that
there still remained ample areas in a Vx-Vy diagram where no
reduction of luminosity took place. This is encouraging;
however, KEKB people are now developing crab cavities asa fall-back option. Figure 6 illustrates principle of crab
crossing[18, 19]. Incoming bunches are tilted by a crab
cavity and collide head-on in a center-of-mass frame at IP.
Outgoing bunches are tilted-back by another crab cavity.
Crab cavities should be superconducting to supply enoughvoltage necessary for these kicks.
KEKB, PEP-II and BTCF have small ~Y * of 1 - 2 em
at IP. To achieve this value, final focus quads should belocated inside the detector under the detector solenoid filed.
The use of ordinary iron magnets are precluded; either
superconducting or permanent magnets should be used.
Permanent separation magnets and fmal focus quads areused in PEP-II, where as superconducting anti-solenoids
and final focus quads are installed in KEKB. Antisolenoids effectively cancel out the solenoid field of thedetector and simplify coupling correction. BTCF will use
superconducting doublets as its final focus magnets.

7 'tCharm Factory, BTCF
Although a few .Charm Factories were ever planned inthe world, only one 'tCharm Factory project, BTCF, Beijing
TauCharm Factory at IHEP, Beijing, is alive. Feasibility
study of this factory has been completed and the instituteexpects its R&D to be approved soon. BTCF is planned tohave three modes of operation: (1) high-luminosity mode, {2)longitudinal- polarization mode, and (3) monochromatormode. High-luminosity mode tries to reach 1033cm-2s- 1 at 2
Ge V, whereas the longitudinal polarization mode aims atstoring longitudinally polarized beams at 1.8-2.0 GeV withsomewhat lower luminosity for the search of the CP violationin 't-decays. The monochromator modes aims at increasing
the event rate at J/'\jf energy (1.55 GeV) by reducing thespread in the center of mass energy. This is obtained byusing opposite large dispersion values of two beams at the IP.
The luminosity of this mode will be 1032cm-2s- 1•

8 t Factory DA~NE
DA<I>NE has two IPs. One IP is surrounded byKLOE[20], the main physics motivation of which is theobservation of direct CP-violation in KL decays, i.e. themeasurement of E'/E with accuracy in w-4 range. Toachieve this goal, a luminosity of 5 X 1032cm-2s-l is required.
The other IP of DA<I>NE is assigned to a smaller size
detector, FINUDA[21], for study of hypernuclei formationand decay.

9 Perspectives

BTCF needs a 3-4 year R&D period and the construction will
start early next century at the earliest. BTCF will be able tolearn experiences gained by DA<I>NE, KEKB, and PEP-IIand construction will be done on the basis of thesefactories.

References[1 ]KEKB B-Factory Design Report, KEK Report 95-7,
August 1995.
[2]PEP-II An Asymmetric B Factory, SLAC-PUB-5379,
June 1993.
[3]Feasibility Study Report on Beijing Tau-Charm Factory,
IHEP-BTCF Report 03, October 1996.
[4]G. Vignola and the DA<I>NE Project Team, DA<I>NE, theFirst <!>-Factory, Proceedings OfEPAC96, pp.22-26.
[5]K. Akai, RF Issues for High Intensity Factories,
Proceedings ofEPAC96, pp.205-209.
[6]Y. Yamazaki and T. Kageyama, Particle Accel., 44
pp.107-127 (1994).
[7]T. Furuya, et al., Proceedings ofEPAC96, pp.2127-29.
[8]S. Mitsunobu, et al., Status and Development ofSuperconducting Cavity for KEKB, to be published inProceedings of 1997 Particle Accelerator Conference,
Vancouver, May 1997.
[9]M. Izawa, et al., Phys. Rev. Lett., 74 5044 (1995).
[10]Z.Y. Guo, et al., The Experimental Study on BeamPhotoelectron Instability in BEPC; to be published inProceedings of 1997 Particle Accelerator Conference,
Vancouver, May 1997.
[ 11]K. Yokoya, private communication.
[12]T.O. Raubenheimer and F. Zimmerman, SLAC PUB95-6740 (1995).
[13]G.V. Stupakov, et al., SLAC PUB-95-6805 (1995).
[14]J. Byrd, et al., First Observation of Fast Ion Instability,
Phys. Rev. Lett., June 1997.
[ 15]H. Fukuma, et al., Experimental Observation of the
Ion-Related Coupled Bunch Instability in a Bunch
Train in TRISTAN AR, to be published in Proceedingsof 1997 Particle Accelerator Conference, Vancouver,May 1997.
[16]Study ofCP Violation in B Meson Decays, KEK
Report 95-1, April1995.
[17]BaBar Technical Design Report, SLAC-R-95-457,
March 1995.
[18]R.B. Palmer, SLAC-PUB 4707 (1988).
[19]K. Oide and K. Yokoya, Phys. Rev. A40 315 (1989).
[20]The KLOE collaboration, KLOE, a General Purpose
Detector for DA<I>NE, Frascati Internal Note LNF92/109, April1992.
[21]The FINUDA collaboration, FINUDA; a Detector for
Nuclear Physics at DA<I>NE, Frascati Internal Note
LNF-93/021, May 1993.

DA<I>NE has just started its commissioning and two BFactories, KEKB and PEP-II, will be commissioned in 1998.

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