- HIGH ENERGY PHYSICS AT IHEP by Weiren Zhao

from IHEP(Institute of High Energy Physics)

CAS( Chinese Academy of Sciences)

P.0.Box 918, Beijing(100039) P.R.China

-- Aug. 1994 -

1. General Introduction

1). The BEPC

Contents

2). The Synchrotron Radiation Facility

3). FEL (Free Electron Laser) Project

4). The Proton Linac

5). The Cosmic Ray and Astrophysics

6). Theoretical Physics

7). International Collaboration

8). The Future Plan of Tau-Charm factory

2. BES Physics

1 ). Tau Mass Measurement

2). Branch Ratio of 1/;'-+ T+T-

3). J /1/; Decay

4). ~(2230)

5). Ds Physics

3. Conclusion

1. General Introduction

The IHEP (Institute of High Energy Physics) of CAS (Chinese Academy of Sciences), founded in 1973, is a comprehensive research institute. Among its various scientific research fields, IHEP mainly devotes to high energy ex­perimental physics on BEPC/BES.

1). The BEPC

BEPC (Beijing Electron Positron Collider), the first HEP experimental base in China, was constructed during the years from 1984 to 1988. It mainly consists of a linac and a storage ring. Around the ring, there are synchrotron radiation facilities. BES (Beijing Spectrometer) is the single detector on the interaction point of the collider.

BEPC Parameters: 86

Ecm = 3 - 5.6 Ge V Peak Luminosity= 6 X 1030 /crn 2 /s (at Ecm=4GeV)

The BEPC runs about 5000 hours per year. Among them, about 2500 hours are used for BES physics experiments, and about 1000 hours for the synchrotron radiation facility running in the dedicated mode. Other hours are for the machine study and linac injection.

The following data have been collected on BEPC/BES:

9 x 106 J /1/J events, 1.4 x 106 'iti' events, and 22.3 pb-1 (at Ecm =4.03Ge V) Ds & T data.

The team on BES experimental physics :

There are more than 200 Chinese scientists working on BES. About 150 of them are from IHEP. Others are from about 13 Chinese universities and institutes.

Since the Spring of 1991, about 40 American scientists from 10 institutes and universities of USA have participated the BES collaboration.

From 1994 to early 1996, BEPC is undergoing its upgrade, after which the luminosity will be 3 times as it is now.

2). The Synchrotron Radiation Facility

The BSRF (Beijing Synchrotron Radiation Facility) is around the stor­age ring of BEPC. It has altogether 7 beam lines and 10 experimental sta­tions. They are topography station, EXAFS (Extensive X-ray Absorb Fine Structure) station, small angle x-ray scaterring station & diffraction station, photoelectron spectrometer station, lithography station, fluorescence analysis station, diffuse scattering station, and others.

BSRF users distributed all of China. There are hundreds of scientists from 56 institutions doing various experiments and interesting results have been obtained.

The research and application subjects are in the field of condensed solid physics, electronics, super-conduct & laser materials, medicine, resource & environment sciences.

3). FEL (Free Electron Laser) Project

Based on a 30 MeV electron linac accelerator, the FEL project in IHEP, a Compton infra-red mode FEL facility made big progress in the past two years. And the stimulated emission was firstly observed in April 1993.

4). The Proton Linac

Another accelerator in IHEP is the BPL (Beijing Proton Linac ). Its per­formance parameters, and the experiment & application on it are as following:

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Energy: 35MeV, Current: 60rnA, frequency: 12.5MHz and running 4000 hours/year

Experiments and applications: a) Low energy nuclear physics experiments (new nuclide 19 Na) b) Neutron therapy for cancer c) Radio isotope production for hospitals

5). Cosmic Ray Physics and Astrophysics

In China, the research on cosmic ray physics started in 1950's. Now there are about 70 .scientists of IHEP working in the field. In 1960's, a large cloud chamber was built on the mountain in YunNan Province, the south west China, where a charged, stable particle with its mass> 20 GeV was found in the exper­iments. After that, an emulsion chamber was built on the Kanbala mountain in Tibet, China. And an EAS (Extensive Air Shower) array and a VHE (Very High Energy) gamma ray astronomy observation station were built in 1980's, both of them were in the suburb of Beijing. The space high energy astronomy observation loaded on balloon flights and the high energy astrophysics data analysis on computer are also features of IHEP in the field.

What is more, the China-J apa.n cooperative project on the Yanbajing EAS array, Tibet, has been successfully working for years.

6). Theoretical Physics

About 20 physicists in IHEP devote to the theoretical particle physics. There are also about 20 physicists in IHEP working on the intermediate and high energy nuclear theory.

There are other divisions in IHEP, such as the Nuclear Tech. & Application Division, Computer Center etc. It's too much to introduce all of them here in detail.

7). International Collaboration

In 1950's and 60's, Chinese physicists participated the high energy physics collaboration in Dubna, former Soviet Union. Since 1970's hundreds of Chinese physicists, as the visiting scholar, were sent to J a.pan, USA and European countries to work and study. The experience they gained from abroad is very useful and helpful to the construction and running of BEPC & BES. Now IHEP keeps good cooperative relationships with KEK, CERN, SLAC and many other labs and univ in the world.

8). The Future Plan of TCF (Tau-Charm Factory):

The possible construction of TCF in Beijing is now under consideration. The first stage of the project is R & D, which would estimate the challenge from B-Factory to Tau-Charm physics, and research the key technology for

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the collider and detector.

The preliminary design of parameters: Energy : 3 - 5 GeV Luminosity : 1 x 1033

/ cm2 / s

Research field : Tau, Charm and Charmonium physics, the precise research and test of the standard model.

The cost for TCF construction is estimated to be 100 million USDs, which is too much money, if paid only by China. Now some sort of international cooperative project on it is under discussion. 1'he concern and support from world scientific community are welcomed.

Other large scientific projects, such as the third generation of synchrotron radiation light source, are also discussed in China.

2. BES Physics

BES is mainly composed of MDC (Main Drift Chamber), TOF (Time Of Flight), BSC & ESC (Barrel & Encl-cap Shower Counter), and MUC (Muon Counter).

Its performance parameters are as follows: MDC momentum resolution: o-p/p=l.73,/=-l-+_p.,..2

MDC dE/ dx resolution: 8.53 TOF time resolution: o-t = 330ps BSC energy resolution: o-E/E=223/ VE( Ge V) BSC spatial resolution: o-Z=3cm ESC energy resolution: o-E/E=2.13/VE(GeV) ESC spatial resolution: o-x=l.5cm o-y=l.7cm MUC spatial resolution: 0'¢=3cm aZ=5cm

1 ). Tau Mass Measurement

There were four experiments measuring r mass from 1978 to 1980. And the PDG (Particle Data Group) book of 1990 gave

mr = 1784.1~~:~ MeV.

Since precise measurements of r lifetime and its decay branch ratio were available in early 1990's, the r mass became more important in checking the lepton universality.

Mainly it refers a leptonic coupling constant Gr

a;= 19271'3 x Br(r -t ev11)/(LifetimeT x m~).

If the leptonic universality is correct, a; I G~ should be 1, which however couldn't be approved by old data.

In late 1991, the T mass was precisely re-measured on BEPC/BES. The procedure of the experiment was a data-driven scan around the energy region

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of e+e- --+ T+T- threshold.

The experiment gave a preliminary result of

AL,.= 1776.9:g:~ ± 0.2MeV,

which was based on 14 eµ events.

The final result, based on 64 events from more channels, was given as

AL,. = 1776.96~g:~~~g:igMeV,

which, along with the corrected T life time from LEP, solved the consistency problem of lepton universality, and gave:

(G,,./G1,)2 = 0.9997 ± 0.0122 (comparing the previous value of 0.935).

2). Branch Ratio of 'I/;'-+ T+T-

It's a.gain a check of lepton universality. '!jJ', with its energy above all of three leptons' pair production, provides unique chance to do the comparison. And the last branch ratio of 'I/;' ~ T+T- was firstly measured by BES.

Since the follovving value calculated from the branch ratio(B), lepton mass(m) and 'I/;' ma.ss(M) should be a constant,

Bu/ (1+4mf/M2 )(1-4mf/A12)

0·5

the branch ratios from 'I/;' to ee, µµ, and TT should follow the equation:

B,,.,,. / 0.3885 = Bµµ = Bee

The branch ratio is given by the tau pair event number, subtracting the QED process contribution, then over the 'I/;' event number,

B,,.,,. = (N,,.,,.-aQEDL)/ N"1,

where, the QED cross section of ee~ TT was calculated by theorists, the luminosity was measured by large angle dimuon events, the 1/;' number was collected at its resonance, and the TT number was collected on 4 channels, combined with acceptance, efficiency etc. and the weighed average value was above 9000 events.

Finally the bi·anch ratio was given a.S

Br('I/;' ~TT) = (3.69 ± 0.71±0.66) x 10-3.

This result is consistent with the above equation.

3). J /'I/; Decay

Motivation

The J / 1f; deca.y width and branch ratio fractions were mostly measured by fitting the cross section at its resonance region with ee, flf-l, and hadron products. The typical results reflected in PDG 90 values. However MARKIII re-did the work through another channel of 'I/;' decay and obtained higher accuracy. There were quite differences between the 1990 old data. and 1992 new data. So it seems still interesting to do it on BES in both methods.

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For method 1, the following results were given:

5.14 ± 0.38 KeV as the partial width to lepton,

74.9 ± 12.5 KeV as the partial width to hadron,

85.2 ± 13.2 KcV as the total width, and

(6.04±0.50)3 as the branch fraction to lepton.

For method 2, the channel is 1/1' firstly to rrrr.] / 'lj!, then J / 1p decays to leptons or anything else:

1p' -t 7f+7r- J /1/J, J ll/J -t z+ z- or anything

The branch ratio fractions can be given by the number of 7f7f lepton events, over the number of 1p' -t rrrr.] /1/1 events.

The big advantage of the method is the whole experiment is based on the pure data sample of 1jJ'. The events identified by 7r7r are highly effective. So we avoid all the uncertainty and errors from the QED background of e+ e- to leptons, the total number of triggered events, and the trigger efficiency etc.

The results and conclusion are as following:

B(Jj'lj)--+ ft+p-) = (5.94±0.19±0.18)%

B(J/1/J--+ e+e-) = (5.98±0.18±0.20)%

Be and Bµ are almost the same, which is consistent with e-p universality, and gives:

B(J/1/J -t z+z-) =,(5.96±0.14±0.20)%

Conclusion:

a) The widths of J/1/J decay have been given by two methods on BES. b) The results by two methods are consistent with each other. c) BES results are also in good agreement with MARKIII measurements. cl) This measurement significantly lower the uncertainty.

4). e(223o)

Brief review of previous experiment results: GAMS and LASS found some structure near 2.2 GeV. But only MARKIII observed a narrow resonance there, So it still needs confirming.

The e(2.2) was studied on BES by using the J/1/J radiative decay J/1/J-t 1J(+J(-,1](~](~, and 1PP

based on 7.8 million J/1p events.

The results gave following conclusions:

a) e does exist in J / 1/J radiative decay. b) The mass and width of e we measured are consistent with MARKIII

results. c) We found a new decay mode of e -t PP

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d) What is f? Is it ss, 4 quark state or glue ball? which is exciting but needs further study.

5). Ds Physics

a) 22.3 pb-1 D 11 data were collected at the energy of 4.03 GeV, which was firstly done by BES.

b) Direct branch ratio of D s ---+ <fJ7r was firstly measured by BES: B(D11 ---+ </nr) = (4.2~i:~~6:b ± 0.5)%.

c) Three pure leptonic D 11 decay events were found.

d) The decay constant fn. was measured:

f = (434+153+35) MeV D. -1~-~ ·

The main success in above was the data collected at 4.03 GeV, and from which, double tagged D 11 events were found from analyzing, although they were only three leptonic decay events and several others. Without them, the </nr branch ratio and Jn. can't be measured in the better way.

3. Conclusion

Owing to the continuous efforts of several generations of Chinese physi­cists, the high energy physics, cosmic ray physics and other researches have developed greatly in the IHEP. They have come into the international field, and significant results achieved. Chinese physicists expect more progress in the coming century. Since we all realize the science must be an international cause, it is hoped we would have more successful collaboration with the world scientific community.

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