Tau physics at future machines

ELSEVIER

UCLEAR PHYSICS

lquclear Physics B (Proc. Suppl.) 55C (1997) 453-460

PROCEEDINGS SUPPLEMENTS

Tau Physics at Future Machines

B. Spaan *

Institut fiir Kern- und Teilchenphysik, Technische Universit~it Dresden 01062 Dresden, Germany

The prospects on Tau physics at future machines will be discussed, with future machines under consideration are B-factories and a r/charm factory. Based on the experiences maded at existing experiments, the focus of this talk will be the issue of precision measurements and the need for the reduction of systematic errors.

1. I n t r o d u c t i o n

Since the discovery of the r lepton in 1975 [1], the knowledge about its properties and interac- tions has improved dramatically.

Despite new discoveries of rare 7- decay modes such as the first obervation of 7- decays involv- ing f l mesons reported on this conference by CLEO [2], the major focus of the 7- physics is shifting towards high precision measurements. These high precision measurements involve determination of the 7- static properties, the inves- tigation of the lepton universality and tests of the Lorentz structure of 7- decays. In addition, the analysis of the hadronic final states provides a unique environment to test the validity of the CVC hypothesis and to determine the properties of the resonances produced in 7- decays. Contrary to almost all other resonance production mechanisms, those resonances are produced in a well defined initial state with no additional particles being produced.

Although predicting the future of 7- physics is basically impossible, it is obvious that precision measurements will continue to be a major issue in the future. However, in many areas, the precision of the measurements of not limited anymore by the statistical error by from the systematic error. When comparing the crucial branching ratio of the decay 7-- --+ e-C'eur (Be) as measured by ALEPH [3] and CLEO [4], the need for improv- ing systematic error becomes evident:

*Supported by the Bundesministerium filr Bildung, Wis- senschaft, Forschung und Technologic, under contract number 056DDllP.

0920-5632/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII: S0920-5632(97)00243-0

ALEPH: Be -- (17.79 5= 0.12 :k 0.06) % CLEO: Be = (17.76-4- 0.064- 0.17) %

CLEO, having the world largest 7- data sample available for the analysis, is unable to compete with the systematic error as given by ALEPH. ALEPH however, is unable to compete with CLEO's data sample size, resulting in a much larger statistical error. Unfortunately, with the termination of the LEP I programme, the 7- physics of the four LEP experiments is limited to the analysis of their present data sample. Con- siderable improvements of the statistical error by those experiments cannot be expected anymore.

The abovementioned comparison demonstrates that reducing the systematics error will be a key issue for future machines and experiments. Note that a major fraction of CLEO's systematic error arises from uncertainties in the knowledge of the v-pair cross section and of the luminosity. CLEO determines the luminosity by means of large angle Bhabha scattering events. Considerable theoretical effort has gone into the calculation of the small angle Bhabha cross section, which is used at LEP to determine the luminosity with an incred- ibly high precision. The cross section for large angle Bhabha scattering is less well known, resulting in a ~ 1% error for the luminosity determination. Since CLEO has to rely on the number of 7- pairs produced, the theoretical uncertainties in 7- and large angle Bhabha production account for the dominant fraction of CLEO's systematic error. Since many future machines, such as the B-factories, will most probably rely on the

454 B. Spaan/Nuclear Physics B (Proc. Suppl.) 55C 0997) 453-460

knowledge of the number of r pairs produced, it is highly desirable that some more theoretical effort will be devoted to the calulations of those cross sections.

In the following, future machines will be asso- ciated with B-factories and a r / c h a r m factory.

2. Partial Shopping List

With the future machines, the r da ta sample sizes will increase significantly compared to the present da ta samples. In addition to the issue of precision measurements , there is large number of topics which can be addressed by the future machines. A part ial list of tasks, including precision measurements , is listed in the following to illus- t ra te how rich the field of r physics is and will be.

• Precision measurements

Lepton Universality

Measurements of the lepton universality require the determination of a variety of branching ratios of the decays r - ---+ ev~,,~ (B~), r - ~ h-,,~ (Bh), and the measurement of mass and life- t ime of the r. From all measurements, the mass of the r is known with the highest precision (o ' (mT) /m~) ~_ 0.02%), whereas the other quantities are known with a precision at the 0.5% level, limited by the systematic error given by the contributing experiments. Clearly, there is room for improvements to test the lepton universality with a higher precision.

Properties of the r Lepton

Static properties such as mass, life- t imes etc. are crucial ingredients in other measurements, such as the test of the lepton universality. There might be some improvement on the error on the r mass resulting from a measurement at a r / c h a r m factory, ex- ploiting a similar technique as used at

BES [5], however yielding smaller systematic errors. We have seen on this conference, that the determinat ion of the r lifetime (r~ = 290.21 4- 1.15 fs) is completely dominated by measurements performed at LEP [6]. Thus, any improvements on the lifetime measurements have to come from measurements at future machines. The comparison between ALEPH [6] and CLEO [7] demonstrates the current dominance of the LEP experiments:

ALEPH: r~ = (290.5 + 1.5 4- 1.2) fs CLEO: r~ = (289.0 4- 2.8 4- 4.0) fs.

Note that the CLEO measurement is not based on da ta from the newly in- stalled Silicon Vertex Detector. A v / cha rm factory wll not be able to contribute significantly to those measurement. The B-factories however, are equipped with high precision vertex detectors. With bet ter particle identification methods compared to e.g. CLEO-II , a significantly lower background level will help to achieve very small systematic errors. There is some hope that the precision of these measurements can indeed be improved.

Branching Ratio Measurements

The determinat ion of the major branching ratios provide the information necessary for sensitive tests of the s tandard model, such as lepton universality test and a test of the Conserved Vector Current hypothesis (CVC). In addition, the long standing question whether all branching ratios, not counting the very rare ones, have been observed can be addressed with a higher precision, which arises from the huge da ta samples accessible at future machines.

B. Spaan/Nuclear Physics B (Proc. Suppl.) 55C (1997) 453-460 455

- Lorentz Structure of r Decays

Compared to the #, the Lorentz structure in leptonic r decays is poorly known, as noted by several speakers at this conference (e.g. see the talk of A. Pich [8]). In order to get the full picture of leptonic r decays, the measurements of the Michel parameters p, r/, ~ and ~ , the u~ polarization, and the polarization of the charged daugh- ter particle ~ have to be known. The determinat ion of the latter could be done by equipping the instrumented flux return of a detector with a po- larimeter, allowing to detect and stop the ~ and to determine its polarization by the analysis of the subsequent decay p - --4 e-Pei ' u. The other param- eter can be determined with high precision as can be seen from the recent CLEO measurement , presented at this conference [9]. Future machines, having excellent lepton identification and high background suppression capabilities, will be able to increase the precision of those measurements considerably.

• Analysis of Hadronic Final States

Although hadronic final states have started to become analyzed shortly after the discovery of the r , comparably little is yet known. For example, the 31r final states exhibit several not yet understood problems. The branching ratios between charged and neu- tral final states disagree on the 2or level, as pointed out by Evans [10] at this workshop. In addition, the mass spectrum and the decay dynamics are poorly known. OPAL [11] and ARGUS [12] revealed discrepancies between measured decay dynamics and theoretical model descriptions. It was also shown tha t the mass spectrum of the 3~r final s tate cannot be described by assum- ing the presence of a single resonance, the al . Speculations on the contribution of an

excited a t resonance or non resonant contributions have been made. However, there are much more possible mechanisms leading to distortions of the 3~r mass spectrum. For example, the al can decay not only into a pn final state but also to K * K due to its high mass. The mass spectrum however, starts well below the K * K mass thresh- old. Thus, the opening of another decay channel will also cause distortions of the 3~r mass spect rum. In order to disentan- gle all contributions to the 37r decay mode, a coupled channel part ial wave analysis of all possible hadronic decay channels with J P = 1 + is highly desirable. Such analy- ses however, require a huge amount of da ta which will hopefully be available from future machines.

The 4~r final states also deserves more at- tention. Here, it is known tha t pTrTr and 0aTr contribute. Again, the intermediate resonances and their possible interferences are not fully understood. From e+e - data, additional information can be provided for 47r masses above the r mass, which is useful to understand and constrain the contribution of higher mass resonances. Below the r mass, the future machines will have much larger 4~r da ta samples. The analysis of the 47r mass spectrum might also help to understand the 27r mass spectrum. CLEO and ALEPH presented new, interesting results on the 2~r mass spect rum at this conference [13,14]. In the higher mass region, a distortion appears which is commonly interpreted as the presence of an excited p resonance, the p~. The reality however, might be more complicated. Don- nachie and Clegg [15] interpreted the wTr mass spectrum, observed in 47r final states of 7- and e+e - data, as originating from the interference of two resonances, the p and the p(1450). Since the p can decay into wr , the opening of this decay channel will lead to a distortion in the region of the wTr thresh- old. Kiihn and Santamar ia [16] were able to describe the pion form factor IF1~=112,

456 t?. Spaan /Nuclear Physics B (Proc. Suppl.) 55C (1997) 453-460

measured from e+e - ~ 7r+Tr - , by means of a p~ or by the assumption of a distortion by the opening of the ¢07r channel. In order to achieve a bet ter understanding of the hadronic final states with j R = 1- , a complete coupled channel partial wave analysis seems to be necessary, a task which will hopefully be performed at future machines.

Search for CP-Violation in r Production and Decay

On this conference, we have seen several presentations on non Standard Model CP- violation effects in 7. production and decay. For the CP-violat ion in 7. decays, see the talks of Mirkes, Nelson and Tsai [17], given at this workshop. The search for CP- violation in 7- production has been pursued at LEP, leading to limits on the weak elec- tric d ipolmoment of the r in the order of d z < O10-1Secm [18].

CP violation in 7- pair production manifests itself as as function of charge and spin, these are the only properties which allow to distinguish pointlike particles of the same type. In 7- pair production, 4 different spin configurations can occur, denoted as:

~= • . I

T

¢:: • . _ - I t + t - > ,

T

. - i t + t - > , + • T--

, = I t + t - > , ~+ " 7"-

where the arrows correspond to the helicity of the r ' s and the superscripts represent the charges of the individual 7- leptons. The state I t + t - > transforms under CP as follows: PI t + t - > = I t + t - > yielding

I t+$ - > when subjected to charge conju- gation. Thus CPI 1"+4-> = +11 t + t - > and CP[ t + j ' - > = +11 t + j ' - > are CP +1 eigenstates. For the other two possible states, it is easy to show that they are not CP eigenstates but linear combinations of those states are CP eigenstates:

1 (I T+t-> +1 t+*->) . c e = +1

1 (I t + t - > - I t+ t -> ) : c P = - 1 .

The three CP = 1 eigenstates correspond to the P-wave states which can be produced by a photon or a Z °, both being C P = +1 eigenstates. The observation of the C P = - 1 eigenstate would proove the presence of CP violation in the 7. pair production. In e+e - --4 r+7. - events, the helicity state

1 : ~ (I t + t - > + $ + t - > ) is supressed with re-

spect to I t + t - > and I ] ' + t - > by a factor 7 2, with 3' = Ecms/2m(7.). This s tate and the CP = - 1 state show very similar decay product correlations. Therefore, the sensi- t ivity to CP-violat ing effects is diluted by the presence of the CP = +1 state, resulting in a close to zero sensitivity for a 7./charm factory and very high sensitivities for LEP experiments, where the 7- spins pro- jected onto the production axis is almost purely Jz = 4-1. B-factories will achieve similar sensitivities due to their large da ta samples.

Upper Limit on the ur Mass

Currently, the best upper limit on the mass of the 7. neutrino has been obtained from the combination of two ALEPH measurements, yielding rn(u,.) < 18 MeV/c 2 at 95% CL [19]. From cosmology however, it seems to be unlikely that unstable, heavy neutri- nos can be found in this mass region, since their decay is either visible or their decay products would interfere with the primor- dial nucleosynthesis. Some models exclude masses to well below 1 MeV/c 2, However,


there is reason to believe that an O(MeV) mass neutrino is not absolutely excluded, demonstra t ing the need for a much higher sensitvity for the uT mass determination. The B-factories and the v / cha rm factory claim to be able to reach limits on the u~ mass of m(uT) < 2 - 3 MeV/c 2, thus being sensitive to parts of the allowed mass region.

• Rare Decay Modes

With future machines, it seems possible to detect r decays with branching ratios in the < 10 -6 range. A prominent represen- tat ive of the rare decay modes is the decay r - --+ rpr -ur which can occur due to isospin symmet ry breaking effects (allowed) and via second class currents, which are for- bidden in the s tandard model. Theoreti- cal predictions for the branching ratio of this decay mode give B ( r - - 4 rlTr-1]r ) ,~ 0(10-~) . The most sensitive upper limit for this decay mode up to now has been reported by CLEO on this conference to be B ~ - ~ r < 1.4.10 -4 [20]. It was also shown, that the detection of this decay will be tech- nically challenging due to the presence of background from the decays r - --+ rjK-uT and r - --+ r/rr-rr°Ur.

A variety of other rare decay modes can be investigated, e.g. r decays to t,, plus 4 oth- ers leptons. Here, the predictions for the branching ratios of some of those modes are in the 10 .7 to 10 -5 region, which can be easily reached by future machines. In fact, the decay with the largest predicted branching ratio, r -+ ee+e-urue , has already been observed by CLEO [21].

• Many other Analysis Topics

A long list of other analysis topics exists, by far to long to be listed here completely. It contains QCD tests, determinat ion of as, search for the anamalous magnetic moment of the r (aT 1 -- 7(g~ - 2)), measurement of the tauonium states and much more.

3. r P h y s i c s a t F u t u r e M a c h i n e s

On this conference, M. Tigner gave an excellent overview over the properties of future machines [22]. In this talk, the the focus is the r physics at B-factories and at a r / c h a r m factories.

In the near future, there will be detectors at three B-factories taking data. Two of them, BELLE and BABAR, are located at asymmetr ic e+e - machines, operating in the Ec,~ region of the T(4S) resonance. Both will have very similar design luminosities of t: = 3- 1033cm-2s -1 , which will be increased to ~; = 1.1034cm-2s -1 at a later stage. Thus, one can expect that both detectors will each be provided with 30.106 r pairs per year at design luminosity. Both machines and detectors are on a similar schedule, expecting to s tar t da ta taking in the summer of 1999. Although these detectors are not optimized for r physics but for the measurement of CP violation in the B ° system, the demands for their pr imary goals result in excellent r physics capabilities. Their main features are: Excellent vertexing, tracking, photon detection, and particle, i.e. lepton, identification capabilities.

In addition, CESR and CLEO will be upgraded to become a symmetr ic B-factory. The CESR upgrade will result in increasing the luminosity of the machine to /: > 1 . 1033cm-2s -1, within a factor of three comparable to those of the asym- metric machines. The CLEO II detector will be upgraded to become CLEO III. The upgrade programme will provide CLEO III with a RICH counter and a new drif tehamber as well as a new vertexdetector. Like the detectors at asymmetr ic machines, CLEO III is optimized for B-physics, but will provide excellent r physics capabilities.

All B-factories are approved and are under con- struction. For a couple of years now, plans exist to build a r / c h a r m factory. Several sites have been proposed: Argonne, Beijing, Dubna and Novosibirsk. If approved, the da ta taking might start shortly after the year 2002. The current design calls for luminosities of £ = 1. 1033cm-2s -1, Contrary to the B-factories, a r / c h a r m factory is optimized for r and charm physics. In ad-

458 B. Spaan/Nuclear Physics B (Proc. Suppl.) 55C (1997) 453-460

dition, the machine will allow to operate at different centre-of-mass energies for the r physics programme. As it will be shown later, the determinat ion of branching ratios at the r / C h a r m factory does not require the knowledge of the number of r pairs produced. Therefore, the major systematic error a t t r ibuted to the branching ratio measurements for machines operating in the centre-of-mass region of the T(4S) resonance will not be present. This feature will enable the v / c h a r m factory to lower the systematic error of branching ratio measurements considerably. A r / c h a r m factory will exhibit additional features which are specifically designed for the r physics programme. It is planned, to install a longitudinal beam polarization scheme, resulting in a the polarization of the r leptons. With polarized r leptons available, the determination of the Lorentz structure of the decays does not rely on r spin correlations anymore. Note that due to the low energy of the r leptons produced the v / cha rm factory, the spins of both r ' s are only marginMly correlated. Measurements of the Michel-Parameters ~ and ~5 which rely on the knowledge of the r spin orientation, would hardly be possible without having polarized r pairs available.

In the following sections, the methods how physics is pursued at the future machines is discussed.

4. r P h y s i c s at a B - F a c t o r y

In principle, asymmetr ic and symmetr ic B- Factories will exploit similar methods to analyze r events. Thus, the r physics p rogramme will we very similar to the current r physics p rogramme at CLEO II. The boost at the asymmetr ic machines is small compared to the boost which the r leptons experience in the centre-of-mass system. Contrary to the B-Meson system, the additional boost does not provide a significant lifetime information. However, the excellent vertexing capabilities of the B-factories will provide another possibility to discriminate r pair events from the background. Vertexing information can be ex- ploited in various ways. Some examples:

Distance of closest approach between two single prongs.

The average distance between those tracks is about dca "" 100pm. Since the impact pa- rameter resolution at the B-factories is expected to bet ter than tha t value, this infor- mat ion can be used to suppress background.

Distance of closest approach between a single prong and a 3 or 5 prong vertex.

• Distance between two 3 or 5 prong vertices.

Using the vertex information, a much cleaner r -event selection is possible, leading to a reduction in the systematic error for many measurements. In addition, a further improvement of the r-l ifetime measurement seems feasible.

5. v P h y s i c s at a r / C h a r m F a c t o r y

The physics at the v / cha rm factory will be quite different from those at B-factories. I t is fore- seen to operate at three different centre-of-mass energies:

1. E . . . . = 3.56 GeV

With ~r(e+e - --+ r + r - ) = 0.5 nb, the expected number of r pairs produced per year is N ~ = 0.5- 10 z.

2. Eecm~ = 3.67 GeV

Here, ~r(e+e - --+ r + r - ) = 2.4 nb, and Nrr = 2.4.107

3. Eecms = 4.25 GeV

Here, cr(e+e - --+ r + r - ) = 3.5 nb, and N~-T = 3.5. 10 7

When operating at Eecm~ = 3.56 GeV and Eec,~ = 3.67 GeV, the centre-of-mass energy is too low for the production of charmed mesons. Therefore, all leptons produced at these energies have to arise from r decays which enables the


determination of branching rations by means of tagging v pair events with one of the v's de- caying leptonically: v - ---+ C-Pet/r and subse- quently counting the individual v decay modes of the other v. Thus, the number of v pairs does not need to be known which removes the major source of systematic errors in e.g. CLEO branching ratio measurements. Due to the low centre-of-mass energies, muons and pions cannot be separated very well over a wide momentum range. The determination of the corresponding branching ratios and of the Michel parameters p and 77 would suffer. However, when running at E~cms = 3.56 GeV, the v 's are produced almost at rest, resulting in monoenergetic pions and kaons from the decay v - ~ h-vT, where h stands for ~- and K. Thus, muons can be easily separated from those decays when running at Eecm~ = 3.56 GeV , allowing a high precision determination of B , and of the Michel parameters p and 7/.

At Eecms = 4.25 GeV, above the charm thresh- old, the maximum v pair cross section is reached. Measurements, which do not rely heavily on the precision of NT¢ can be performed at this energy.

Detector specific systematic errors can be min- imized by cross checking the detector Monte Carlo simulation with events taken at frequent runs on the T(1S) or T(2S) resonances, where a very large number of events if collected. In addition, many other detector effects can be determined and understood from those calibration runs. Therefore, it is possible to further reduce the systematic errors.

For a more detailed description of the v physics potential of a r / c h a r m factory can be found in [23].

6. C o n c l u s i o n s

With the termination of the LEP-I programme, most of the future r physics will happen at B- factories and at a v /charm factory, if the latter will be build. The major objective of the future v physics programme will be to perform precision measurements, which requires the reduction of systematic errors well below the present level

at CLEO II. All future machines have the potential to yield considerably smaller systematic errors, thus allowing to significantly improve the current level of precision. In addition, the huge data samples give hope to observe rare v decays, to understand the resonance structure of hadronic r-decays, and they provide the basis for a search for non standard model physics in r production and decay. I believe, that with the new machines, v physics will continue to be exciting.

R E F E R E N C E S

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Tau physics at future machines