Tau-Charm Factory project at Beijing

N ELSEVIER Nuclear Physics B (Proc. Suppl.) 59 (1997) 316-323

PROCEEDINGS SUPPLEMENTS

Tau-Charm Factory Project at Beijing He Sheng Chen ~ *

qns t i tu te of High Energy Physics, Chinese Academy of Sciences, P.O. Box 918, Beijing 100039, China

The Tau-Charm Factory (TCF) is a proposed high luminosity e+e - collider at the threshold region of the production of r and charm. It could provide many precision measurements and search for new phenomena in the flavor physics. IHEP, Beijing, is an ideal cite to build a TCF. The status of the feasibility study of Beijing Tau-Charm Factory (BTCF) is reported.

1. B E P C a n d B E S

Beijing Electron-Positron Collider (BEPC) was built successfully in 1988 at the Institute of High Energy Physics (IHEP), Beijing. The construction of BEPC was on schedule, within the budget. BEPC reached the designed luminosity soon after its commissioning. BEijing Spectrometer (BES) is the only detector in BEPC. It started data tak- ing since the middle of 1989. So far, BES has collected 9M J / ~ events, 3.6M ~ ' events, 5 pb -1 at the r + r - threshold, and 22.3 pb -1 at 4.03 GeV to study D.,, D, and r . In addition to physicists and students from IHEP, many physicists and students from Chinese universities and 10 US universities work also on the BES collaboration.

Many interesting physics results have been obtained in BES, including:

• The precision measurement mass:

mr = 1776.96+0118 +0.~5_0.17 MeV;

of the Tau

* The observation of the decays of ~(2230) into pi6, 7r+Tr - , ~r°~r °, rp/;

* The measurement of fD8 and fD from the pure leptonic decays of Ds and D:

J/-)¢~s ~ Aqt3+150 +180 n t ° v - 1 3 0 - 40

¢~ ~ 2130+150 +80. vvv-- 150 --40,

*The work is suppor t ed by the National Natura l Science Foundat ion of China under contracts 19375047, 19355001, and 19555003.

0920-5632/97/$17.00 © Elsevier Science B.V. All rights reserved. PII S0920-5632(97)00457-X

• The observation of the suppression in both the vector-pseudoscalar decays and the vector-tensor decays of k 0~.

Since 1994, BEPC started a upgrading program. The machine upgrading includes two ma- jor improvements:

• The energy of the linac injector has been increased from 1.3 GeV to 1.55 Gev by upgrading the microwave power supply.

• The peak luminosity is increased by a factor of 1.5 on the base of 6 × 103°cm-2s -1 at the energy of 4 GeV by improving the magnet system, the RF system, the vacuum system, as well as the control and diagnostic systems.

BES II, the upgraded BES detector, has the following improvements: the main draft chamber, TOF and the luminosity monitors are re-built. The vertex chamber of Mark III replaced the old central drift chamber. The performance of the sub-detectors were improved. For example, the time resolution of the TOF in BES II has been improved from 330 ps to 200 ps. The DAQ time per event has been decreased from 20 ms to 10 m s .

The highlight of the physics program for the upgraded BEPC/BES II includes:

• Search for Glueballs: ~(2230), 0(1710), f0(1500)... More statistics may enable us to measure the widths and the branching ra- tios of their decay channels, as well as their spin-parities.

H.S. Chen/Nuclear Physics B (Proc, Suppl.) 59 (1997) 316-323 317

• The precision measurement of Rhad in the energy range between 2 to 5 GeV; This is important for many electroweak calcula- tions.

2. T a u - C h a r m F a c t o r y

The idea of the Tau-Charm Factory (TCF) was first proposed by Dr. Jasper Kirkby at 1987 [1]. TCF is a high luminosity e+e - collider near the threshold of the production of the tau lepton and Charm. The luminosity of TCF should be 1033cm-2s - a or higher. The physics goals of TCF are the precision tests of the Standard Model, and to search for new phenomena. The physics advantages of TC F are:

• the large production cross sections near the thresholds with the high luminosity provide very high statistics of event samples for many important physics;

• The lower systematic errors because near the threshold:

- Backgrounds are very small (e.g. below open the charm production);

- High efficiency to reject backgrounds by single tagging;

- B a c k g r o u n d s could be measured directly by running below the threshold;

- Small Lorentz boost;

- High statistics samples of J / ~ and ~ events for the detector calibration.

The Tau-Charm Factory, together with the two B-factories~ will provide leading experiments in the frontier of the high precision measurements for the high energy physics in the next century.

The high energy physics community shows great interests on the idea of TCF. Many workshops were held dedicated to discuss the physics, the machine and the detector of the TCF:

• The SLAC workshop at 198912];

• The LAL-Orsay workshop at 1990;

• The CERN workshop about the TCF detector at Sept. 199013];

• The Sevilla workshop at 199114];

• The Marbella workshop at 199315];

• The SLAC workshop at 1994;

• The Argonne workshop at 1995;

• The IHEP Beijing workshop at Feb. 199616].

The physics at the tau-charm energy region, the design of the machine and the detector were discussed extensively in these workshops. They en- courage more physicists to work on the idea of TCF.

In addition, many HEP laboratories and orga- nizations around world had seriously discuss the cite and the feasibility to build a TCF, including SLAC, Orsay, Sevilla of Spain, CERN (using the ISR tunnel), Novosibirsk, Argonne, and BEPC.

Dr. J. Kirkby summarized the various interesting physics topics for high-luminosity e+e - ma- chines near the tau charm threshold[5] in the table 1.

In the following discussion, only some selected topics related to the flavor physics are discussed.

3. F l a v o r P h y s i c s in T C F

3.1. M e a s u r e m e n t o f m ~ The measurement of m.~ is very interesting for

the flavor physics[7]. Actually, there is no any known reason to believe that m.~ is exactly zero. In another hand, a massive uT has a large im- pact to the u-oscillation and the lepton mixing, as well as to the cosmology. A massive ur is also a candidate of the dark-matter.

The typical method to obtain the up limit of m ~ is to fit the end point of the invariant mass spectrum in the decays of r -+ multiple hadrons:

• r --+ 5~r+u, 37r+27r°u, 57r±~°u;

• r -+ K + l f - r r ± v , which has a small Q- value, but requires 7r/K separation. Since TCF runs at the threshold, the momentum of Ir and K are smaller. It is much easy to separate them at an experiment at TCF than the experiments in the higher energy regions.

318 H.S. Chen /Nuclear Physics B (Proc. Suppl.) 59 (1997) 316-323

Table 1 Physics-reach versus luminosit

Luminosity [cm-'%- 1]

1032

1033

1034

r of the 7.-charm Factory

Physics-reach per 1 year's data glueballs, hybrid exotic and hybrid charmonium excited ¢ and D states semi-leptonic D decays to O(1%) precision 7. decay Br's to O(0.1%) precision A~, Ec, Ec, ~ , etc. decays to 0(5%) precision V-A structure in 7. decays comparable to precision in/~ decays doubly Cabibbo suppressed D °, D ±, D~ decays to 0(3%) precision pure leptonic D decays, fD and fO, to 0(2%) precision 7. ~ eX limit ~_ 10-5; constraints on Ur masses below 1MeV/c 2 D°/9 ° mixing at SM level, 10 .5 rare 7./D/J - ¢ decays (LFV, FCNC, etc.) to limits ~ _ 10 .7 - 10 - s direct u, mass limit ___ 1MeV/c 2 CP violation in D decays a SM level CP violation in 7. decays at milli-weak level(10 -3) CP violation in A, E decays at SM level ? ?

? 9 ? ?

The current up limit on m ~ is 24 MeV at 95% C.L.. If the up limit on m~, could be reduced to a few MeV, it will give a tight restriction to many models of the cosmology.

TC F has many advantages in the measurement of rnu~. The optimal CMS energy for the measurement is 3.68 GeV. It has the largest cross section of the tau pair production: 2.3 nb. One year of running could provide 3 x 1077. + 7.- events. A Monte Carlo study shows that the event select efficiency for these channels is about 15%. Thus one will obtain several thousands high- multiplicity hadron decay events for the measurement. This data sample has very low background, and small systematic errors, as well as the advan- tage in the decay channel 7. --+ K + K-7r + u.

A Monte Carlo study predicts that the up limit on rn~, from one year of TCF running at 3.68 GeV is about 1 MeV at 95% C.L. from the channel of 7. --~ K+K-rr:t:u. The limited factors on the measurement are the measurement error on mr and the beam energy spread. A monochro- matic beam could further decrease the up limit to 0.8 MeV.

3.2 . C P - v i o l a t i o n i n 7. s e c t o r

It is very interesting to search for the pos- sible CP-violation in 7- sector[7]. In the Stan- dard Model, the CP-violation exists only in the charged current couplings among quarks and de- scribed by a phase in the CKM-matrix. The Stan- dard Model does not predict any measurable effect from the CP-violation in the leptons. How- ever, many models beyond the Standard Model, such as the multiple-Higgs model, the lepto- quark model, and SUSY, predict higher values of the CP=violation, which maybe measurable. In the neutral current, the electric dipole moment (EDM) or the weak dipole moment (WDM) of participating particles could introduce the CP- violation. At the Tau-charm energy region, the effect of WDM is negligible compared to EDM. The dipole moment of a lepton is proportional to m~. Thus the dipole moment of the tau lepton should be bigger and easier to be measured if the lepton has a dipole moment. The electric dipole moment of r , could be measured at TCF. e+e - ~ 7.+r - near the threshold is a good reac- tion to test the CP-invariance. One could deter- mine the tau spin from the measurements of the

H.S. Chen/Nuclear Physics B (Proc. Suppl.) 59 (1997) 316-323 319

tau decay products. However, so far these is no any reliable theoret-

ical prediction on the CP-violation in the lepton sectors. The present estimation on the leptonic CP-violat ion is in the level of 10 -3. Thus at least 10 6 to 10 7 tau events are necessary to study the CP-violation.

One has to find some sensitive observables to make a model-independent search for the CP-

violation in the production-decay sequence of v + r - . The observable could be some spin-energy correlations, or some decay properties of r - vs. its corresponding CP conjugation.

Y.S. Tsai suggested to search for the CP- violation at the r + v - threshold with polarized beam(s)[ l l ] . V " t ' - V - a r e produced mostly in S- wave, and are highly polarized iff the direction of the beam longitudinal polarization, independent on the outgoing angle of e + or e - . The polarization of the outgoing r e is

P1 - P2 P _

I-PIP2

where /)1 and P2 are the polarization of e + and e - . Actually, to simplify the requirements to the machine design, only the e - beam has to be polarized for the measurement. The polarization of the outgoing r e at T C F is a factor of 7.7 higher than the one at the B-factory, because near the threshold.

Tsai suggested to use a variable of #7

#7 = (#xql)-~

in v - ( i f ) --+ rr-(q~) + rr°(q~) + u, and its CP

conjugated process. If A is different from the corresponding CP conjugated process, CP is violated in the r sector.

3 .3 . D ° - / )o M i x i n g The mass eigenstates of D ° - D ° are different

from CP eigenstates DI and D2:

1 D, = - ~ ( D ° +1i) ° )

1 o D2 = - - ~ ( D -19 °)

where CPID°> = IDo).

The mixing rate rD is defined

# o f D ° decay as D o

rD = # o f D o decay as D °

where x = drn/[' , y = dP/[ ' .

x 2 + y2

2

The Standard Model predicts that rD is in the order of 10 - l ° - 10 -9 . The current up limit on ru is 3.7 × 10 -3 at 90% C.L. from E691. If r u >> 10 -9, it would indicate some new physics.

The promising channels to study the D ° D °

mixing are :

tP(3.77) -+ D°[? °

0/(4.14) -+ D - D * + - + r r + D - D °

O1(4.14) ~ D ° D * ° ~ 7/Tr°D°Z) o

For the leptonic decays of D ° D ° -+ K e u K e u ,

one expected to reconstruct 20 K events in one year running. The statistics could be improved by including K p u channel of D ° decays, as well as D ° ~ KsTr+e-v , or K+Tr°e -u .

For the hadronic decays of D°[? ° --+

K - r r + K + I r - , one expected to reconstruct 35 K events in one year running. The statistics could be improved by including the decay channels of K-r r+r r - and K-rr+~r+rr - .

The Monte Carlo study shows tha t the measurement in TCF could reach the sensitivity on rD at a up limit of 5 x 10 -a at 95% C.L..

3 .4 . C P - v i o l a t i o n in C h a r m D e c a y s The CP-violation is observed only in K ° - f ~ "° so

far. The two B-factory experiments are going to measure the CP-violation in B-mesons. It is interested to search for the CP-violation on the Charm meson decays. The observation the CP-violation on the Charm meson decays would indicate a new physics. There are many discussions about the CP-violation on the Charm meson decays. Only the search for the CP-violation in the decay amplitudes of D :k, and in the interference of mixing and decay of D ° D ° are discussed here[5].

3 . 4 . 1 . C P - v i o l a t i o n in t h e D e c a y A m p l i - t u d e s of D +

It is interesting to search for the CP-violation directly in the decay amplitudes of D + (AC = 1)

320 H.S. Chen/Nuclear Physics B (Proc. Suppl.) 59 (1997) 316-323

The C P asymmetry in the D + decays is defined:

A c p = r ( D - --+ f) - r (D + -+ f) r ( D - -~ f) + r(D+ - + / )

The Standard Model predicts that A c p is in the order of 10 -3 . The promising decay channels for the measurement are D + ~ I f + I ( *° --4

K + I f - T r + and D + --4 7r+r/--4 7r+7" Y. One expects that with several years of running, 105 events of D + decays could be collected, and the sensitivity to the measurement of A c p is about 10 -3. The results from the Monte Carlo studies about the CP-violation in D + decay amplitudes for one year running are summarized in the table2.

3.4.2. CP-violat ion in Single D ° Decays The optimal CMS energy to study the C P -

violation in the decays of single D ° is at k0(4.14). One could select D - D *+ ~ D - T r + D °

and its charged conjugation, then look for the 7r+~r - , K + K - .... The C P asymmetry in the D ~- decays is defined:

d c p = P ( D ° ~ f ) - r ( D ° --+ ]) r (D o + f) + r (D ° + ])

The Standard Model predicts that A c p is in the order of 10 -3 for the channel. The table 3 sum- marizes the results from the Monte Carlo studies about the CP-violation in single D ° decays for 5.8 x 1 0 7 D ° D ° events. It will takes several years of running to collect the data sample.

4. TCF Project in Beijing

Chinese high energy physicists are interested very much on the idea of TCF. After the success of BEPC and BES, we believe that TCF is a natural continuation of BEPC. BTCF could be built in the current cite of BEPC. Many existing facilities in BEPC could be used or upgraded for BTCF to reduce the cost of the construction. More im- portantly, chinese physicists and engineers have many valuable experiences from the construction of BEPC and BES, as well as from the data anal- ysis and the physics in the tau-charm energy region. The high energy physics community around world gives a strong support to the idea of BTCF.

Table 3 C P violation in D ° decays

Channel

K + K -

7r + Tr-

reconst, events

160 000 63 000

~o~o 32 000 p~° 473 000

K ~ ° 218 000 K,¢ 45 000 K ~ 49 000 K , w 187 000

K + K - ~ ° 95 000 7r+ 7r+ 7r- 7r -

K + K - T r + Tr -

236 000 76 000

~A [10 -3 ]

2.5 4.0 5.6 1.5 2.1 4.7 4.5 3.4 3.3 2.1 3.6

The International Committee for Future Acceler- ator (ICFA) and the Asia Committee for Future Accelerator (ACFA) made strong statements to recommend the BTCF project.

4.1. B T C F Feasibil i ty S t u d y At the end of 1994, IHEP obtained a fund

for the feasibility study of the Beijing TCF from the Chinese Government. The study was started at the beginning of 1995. The study covers the physics potential in TCF, the design of the machine and the detector. The machine physicists and engineers, and physicists, from both IHEP and many chinese universities participated the study. The detail Monte Carlo studies optimized the design of the machine and the detector. Many important measurements and tests were carried out as well. The Beijing TCF workshop was held at Feb. 1996 to discuss the feasibility study of BTCF[6].

Oct. 1996 IHEP finished the Feasibility Study Report on Beijing Tau-Charm Factory[9]. The feasibility study suggested the following main features of the BTCF machine:

• The C.M.S. energy range: 3.0 to 5.0 GeV, optimized at 4.0 GeV;

• The luminosity: 1 × 1033crn-2s-1;

• Two storage rings with the same circumfer- ences of 385.4 m;

H.S. Chen/Nuclear Physics B (Proc. Suppl.) 59 (1997) 316--323 321

Table 2 CP violation in D + decay amplitudes

D + decay

~r+~ 71"+71- °

K * o K +

¢7r +

A(expected) [10 -3]

- 1 . 5 ± 0 . 4

2 .8±0.8

one year)

Efficiency

29% 73% 44% 29%

Events reconstructed in one year

1.9 x 105 1.3 x 105 1.9 x 105 1.8 x 10 5

O" A

[10 -3] 2.3 2.8 2.3 2.4

• Multiple bunches;

• One collision point;

• The injector: upgrade the existing BEPC Linac

• The lattice: flexible with different modes:

The high luminosity modes, including the cross angle collision and the head- on collision;

- T h e monochromator with the beam spread of 0.1 MeV for the measurements of J/q / , qd ~, m . . . .

- The polarization to search for the CP- violation in tau and charm decays.

The proposed detector at BTCF is a gen- eral purposed magnetic spectrometer with high momentum and energy resolution, full solid angle coverage, and excellent particle identification. Based on the detail Monte Carlo studies for the physics potentials at T C F , the requirements on the performance of each subdetector were optimized. The figure 1 shows the concept design of the proposed detector, as well as the interaction region at BTCF. The main features of the detector are:

• The superconducting solenoid with the field of 1.2 Tesla;

The central drift chamber with small cell configuration and low-Z gas and field wires; The momentum resolution is 0.4% p ® 0.4%/fl. The minimum momentum for an efficient detection is 50 M e V . The geomet- ric acceptance is about 90%.

The particle identification detector;

The time-of-flight counters with a resolution of 80 - 100 ps;

The electromagnetic calorimeter, made of CsI(T1) crystals with PIN photodiode read- out; The energy resolution is 2 % / v ~ ® 1%. The angular separation for two gamma is 50 mr. The minimum photon energy for an efficient detection is 10 M e V .

The ha&on calorimeter, made of sandwich of resistive plate counters and iron plates (total of 26 layers) as the first choice. It provides the muon identification also.

• The forward detector, made of P b W 0 4 crystals and silicon strips.

The performance of the detector has large improvements compared with Mark III and BES. The detector fulfills the requirements from the physics in TCF.

The feasibility study was very successful. It is an important milestone for the TCF project in Beijing. It provides guidelines for the R & D of BTCF. Through the feasibility study, many R&D tasks has been identified. The aims of the R & D are to further explore the physics potential, to select the technical solutions among various options in the machine and detector design, to find solu- tion for most technical details through prototypes and to reach the final design of the machine and the detector.

5. S u m m a r y and O u t l o o k

At the threshold energy region of v-charm production, TCF could provide clean samples with

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H.S, Chen/Nuclear Physics B (Proc. Suppl.) 59 (1997) 316-323 323

very high statistics for many important precision measurements on

• Glueball studies: (2230), 0(1710), f0(1500)..

• Flavor physics:

- Precision measurement of rn,~ ;

- Search for CP-violation in r-sector and charm decays;

- Study rare and forbidden decays of r and charm mesons;

- Search for D ° - i5 ° mixing;

- Study for the structure of the weak charged current[7].

TCF is a natural continuation of the successful BEPC. IHEP Beijing is an ideal site for a TCF project. The feasibility study of BTCF is an important milestone for the construction of BTCF. An international review committee will meet in IHEP Beijing next week to review the feasibility study. IHEP is going to request the fund of the R~D of BTCF from the Chinese Government.

4. Proc. Meeting on Tau-Charm factory detector and Machine (Sevilla, Spain, 1991), eds. J. Kirkby and J.M. Quesada (univ. Sevilla 1991)

5. The third workshop on the Tau-Charm Fac- tory detector edited by Jasper Kirkby and Rita Kirkby, Edition Frontiers 1994

6. Beijing Tau-Charm Factory Workshop'96, I H E P - B T C F R e p o r t - 02 Feb. 1996.

7. H.S. Chen, Tau Physics, Proc. 17th Inter- national Symposium on lepton-photon inter- actions, eds. Zhi-Peng Zheng and He-Sheng Chen, World Scientific, Singapore, 1996.

8. Tau-Charm Factory detector Workshop r- charm factory notes: TCF-90-9

9. The Feasibility Study Report on Beijing Tau- Charm Factory, I H E P - B T C F R e p o r t - 03 October 1996.

10. Feasibility Study Report on Beijing Tau- Charm Factory IHEP-BTCF Report-01 Dec. 1995.

11. Y.S. Tsai, Phys. Rev. D51(1995) 3172.

6 . A c k n o w l e a d g e m e n t

I would like to thank the useful discussions with Prof. Z. P. Zheng, Prof. D.S. Du, Prof. G.M. Chen and Dr. D.H. Zhang.

R E F E R E N C E S

1. J. Kirkby, A r-charm factory at CERN, CERN-EP/87-210(1987); Proc. International School on Physics with Low Energy An- tiprotons: Spectroscopy of Light and Heavy Quarks (Eric, 1987), eds. U. Gastaldi and R. Klapisch (Plenum Press, New Yorl, 1989), p. 401.

2. Proc. Tau-Charm Factory workshop (SLAC, 1989), ed. L.V. Beers, SLAC report-343 (1989)

3. Tau-Charm Factory Detector Workshop, r- Charm Factory Note: TCF-90-9, 9-Oct. 1990 CERN.

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Tau-Charm Factory project at Beijing