The prospects for tau physics at future high-luminosity facilities are
briefly discussed. Although important (and often complementary) contributions
will be made from other machines, the unique experimental environment near
threshold makes the Tau-Charm Factory the best experimental tool for $\tau$
physics.Comment: 9 pages, LaTeX (figures not included), CERN-TH.7065/9

Pich, A.

English

arXiv

7 páginas, 3 tablas.-- Comunicación presentada al 3º Workshop On The Tau-Charm Factory celebrado en Marbella (España) del 1 al 6 de Junio de 1993.The prospects for tau physics at future high-luminosity facilities are briefly discussed. Although important (and often complementary) contributions will be made from other machines, the unique experimental environment near threshold makes the Tau-Charm Factory the best experimental tool for $\tau$ physics.Work supported in part by CICYT (Spain), under grant No. AEN93-0234.Peer reviewe

Pich, Antonio

Digital.CSIC

arXiv:hep-ph/9312273v1  13 Dec 1993CERN-TH.7065/93TAU PHYSICS PROSPECTSAT THE TAU-CHARM FACTORYAND AT OTHER MACHINESA. Pich∗†Theory Division, CERN, CH-1211 Geneva 23ABSTRACTThe prospects for tau physics at future high-luminosity facilities are briefly discussed. Although important(and often complementary) contributions will be made from other machines, the unique experimentalenvironment near threshold makes the Tau-Charm Factory the best experimental tool for τ physics.Talk given at theMarbella Workshop on the Tau-Charm FactoryMarbella, Spain, June 1993∗ On leave of absence from Departament de F´ısica Teo`rica, Universitat de Vale`ncia, and IFIC, CentreMixte Universitat de Vale`ncia–CSIC, E-46100 Burjassot, Vale`ncia, Spain.† Work supported in part by CICYT (Spain), under grant No. AEN93-0234.CERN-TH.7065/93November 1993TAU PHYSICS PROSPECTS AT THE TAU-CHARMFACTORY AND AT OTHER MACHINESA. Pich∗†Theory Division, CERN, CH-1211 Geneva 23AbstractThe prospects for tau physics at future high-luminosity facilities are briefly dis-cussed. Although important (and often complementary) contributions will bemade from other machines, the unique experimental environment near thresh-old makes the Tau-Charm Factory the best experimental tool for τ physics.The physics programme of the Tau-Charm Factory (τcF) was established in the 1989 SLAC work-shop [1]. At that time, the experimental knowledge on the τ was quite poor [2], and the main part ofthe physics community was not giving much attention to that (still exotic) lepton. The situation hascompletely changed since then. ARGUS and CLEO have substantially increased their τ+τ− data sam-ple, and the four LEP collaborations have demonstrated the potential of this machine for making cleanτ physics. Moreover, a small, but very important, contribution has been accomplished at the Beijingcollider. The growing interest on the τ particle has been reflected in several specialized reviews [3, 4],which have emphasized the unique properties of this heavy lepton for testing the Standard Model, andin the first two workshops [5, 6] devoted entirely to the τ . The qualitative change of τ physics can beappreciated in Table 1, which compares the status of several τ measurements in the 1990 compilation ofthe Particle Data Group [7] with the more recent world averages [6,8]; the main experimental sources ofthe improvements are also indicated.Obviously, our knowledge on the τ lepton properties is going to be further improved in the nextfew years. To get a proper feeling on the quantitative impact of the τcF in this field, one should analyzethe precisions that can be reached with present facilities. Moreover, the possible contributions of otherfuture machines should also be considered. Figure 1 shows the energy and luminosity of present andfuture e+e− colliders in the energy range 1 GeV ≤ Ec.m. ≤ 100 GeV. All machines above√s = 2mτproduce τ ’s and can make significant contributions to τ physics.Figure 1: Luminosity/Energy plot of present and future e+e− colliders, in the energy range 1 GeV≤ Ec.m. ≤ 100 GeV.The different running energies have their own advantages and problems; thus, a given collider canbe very good for measuring some parameter and totaly insensitive to other properties of the τ . Theluminosity is clearly an important ingredient, but not always the decisive one. There are three energyregions worth while to be considered: the τ+τ− threshold (BEPC, τcF), the Υ region (DORIS, CESR,BF) and the Z peak (LEP, ZF).LEP has the great advantage of producing τ ’s with a sizeable boost and low backgrounds. It isobviously the best machine for lifetime measurements. Adding the 1990–1992 data sample of the fourLEP experiments (2× 105 τ pairs) an accuracy of about 3 fs has been obtained [9]: ττ |LEP = 293.5± 2.8fs. In spite of having accumulated a much larger statistics (2 × 106 τ pairs), the CLEO result [8],ττ |CLEO = 296 ± 10 fs, is less precise; a smaller average flight path and larger hadronic backgroundsmake difficult to achieve a better sensitivity [10]. At threshold, to measure the lifetime is clearly notpossible.The τ -mass measurement requires completely different experimental conditions. With only 7(τ+τ− → e±µ∓ + 4ν) events, taken at threshold, BES [11] has been able to achieve an unbelievable∗On leave of absence from Departament de F´ısica Teo`rica, Universitat de Vale`ncia, and IFIC, CentreMixte Universitat de Vale`ncia–CSIC, E-46100 Burjassot, Vale`ncia, Spain.†Work supported in part by CICYT (Spain), under grant No. AEN93-0234.1Table 1: Recent improvements in τ physics. [(a) 95% C.L.; (b) 90% C.L.]Parameter 1990 [7] 1993 [6, 8] Experimentmτ (MeV) 1784.1+2.7−3.6 1777.0± 0.3 BESmντ (MeV) < 35 (a) < 31 (a) ARGUS + BESττ (fs) 303± 8 294.7± 3.0 LEP, CLEOBe (%) 17.9± 0.4 17.88± 0.15 LEP, CLEO, ARGUSBµ (%) 17.8± 0.4 17.42± 0.17 LEP, ARGUSB(h−ντ ) (%) 12.6± 1.2 12.04± 0.24 LEP, ARGUSB1 (%) 86.13± 0.33 85.23± 0.19 ? LEP (84.80± 0.23)B(3pi±2pi0ντ ) (%) – 0.54± 0.10 CLEOB(pi−pi0ηντ ) (%) < 1.1 (a) 0.17± 0.02 CLEOB(µ−γ) < 5.5× 10−4 (b) < 4.2× 10−6 (b) CLEOB(e−e+e−) < 3.8× 10−5 (b) < 3.5× 10−6 (b) CLEOρe 0.64± 0.06 0.717± 0.038 ARGUSρµ 0.84± 0.11 0.762± 0.046 ARGUSξl – 0.90± 0.18 ARGUShντ – −1.25± 0.26 ARGUSd˜Zτ (MZ) (e cm) – < 3.7× 10−17 LEPµ(ντ ) (µB) < 4× 10−6 (b) < 5.4× 10−7 (b) BEBCB(Z → ττ) (%) 3.33± 0.13 3.36± 0.02 LEPB(Z → τe) – < 1.3× 10−5 (a) LEPB(Z → τµ) – < 1.9× 10−5 (a) LEPαs(mτ ) 0.12 – 0.41 0.35± 0.03 LEP, CLEO, ARGUS∑excl.Bi (%) 93.5± 2.4 100.3± 1.391.0± 3.3 ?ALEPHARGUSaccuracy of 0.5 MeV: mτ = 1776.9 ± 0.5 MeV. A recent update, including more channels, has furtherimproved the precision to 0.3 MeV [8]. ARGUS and CLEO, with many orders of magnitude more data,have only reached precisions of 2.8 and 1.8 MeV, respectively [8], using clever kinematic tricks.Between these two extreme cases (lifetime and mass), nearly all aspects of τ physics (except theZτ+τ− couplings) can be addressed in any of the three energy regions; with slightly different strategies,because of the different kinematics (and background) conditions. Nevertheless, the sensitivity to a givenmeasurement can change with the centre-of-mass energy (or with the symmetric/asymmetric configurationof the machine [10]). The potential information provided by different facilities should then be regardedas complementary. Tau physics can and should be done at any (existing or planned) e+e− collider abovethreshold.Present experiments can still increase their sensitivities and better (in some cases new) resultsshould be expected in the near future; however, they are soon going to reach their systematic limits[9, 10, 12–14]. Further improvements in τ physics require new high-precision machines working in thehigh-statistics regime. As shown in Fig. 1 three high-luminosity colliders have been proposed, one in eachof the three energy regions discussed before: the τcF, the B Factory (BF) and the Z Factory (ZF). Table2 shows the number of τ+τ− pairs that would be produced by these facilities. The highest productionrate corresponds to the τcF, but the differences are not very large. Note that these numbers would furtherincrease if higher luminosities are achieved. A next generation BF could reach L = 1034cm−2s−1 and ahigh-luminosity τcF with L = 5× 1033cm−2s−1 has already been considered at this workshop [15, 16].2Table 2: Comparison of τ yearly data samples at the Z, B and τ -charm factories.Collider ZF BF τcFLuminosity (cm−2s−1) 2× 1032 1033 1033Nτ+τ− 0.3× 107 0.9× 1070.5× 107 (3.57 GeV)2.4× 107 (3.67 GeV)3.5× 107 (4.25 GeV)The larger statistics is, however, not the main advantage of the τcF. With the sharp increase instatistics foreseen in these facilities, the attainable precision will be limited by backgrounds and systematicerrors. To achieve precise –O(0.1%)– measurements of τ branching ratios, for instance, the normalization,detection efficiencies and backgrounds must each be known at the 0.1% level. The decisive advantagethat makes the τcF the best experimental tool for τ physics, is its capability of tightly controlling thebackgrounds and systematic errors.The threshold region provides a unique environment, where backgrounds are both small and experi-mentally measurable. By adjusting the beam energy above or below the τ+τ− threshold, the backgroundscan be directly measured, avoiding any need for Monte Carlo simulations with their inevitable uncertain-ties. Moreover, the data samples are very pure because they are free of heavier flavour backgrounds. bcontaminations are completely absent and, running below the charm threshold, one can collect τ eventsthat are completely free of c backgrounds.Furthermore, at the τcF it would be possible –for the first time at any machine– to single-tag τ+τ−events; i.e. by observing the decay of one τ , its partner would be cleanly tagged, without any pre-selectionof its decay mode. This completely avoids normalization uncertainties, which turns out to be crucial toachieve very precise measurements of branching ratios. Single-tagging requires a clean signature from asingle τ decay. With the capability of the τcF to produce τ ’s without heavy flavours contamination, thereare several distinct signatures [13]: e + Emiss, µ + Emiss, and (at 3.57 GeV) monochromatic-pi + Emiss.Detailed studies indicate backgrounds of between 10−3 (at 3.57 GeV) and 10−4 (at 3.67 GeV) for thee + Emiss trigger, which has a τ+τ− event detection efficiency of 24%. For comparison, the typicalbackground contaminations in present samples of double-tagged τ+τ− events at B or Z Factory energiesare about 5%. The lowest backgrounds achieved in LEP data are about 1%, while retaining adequatedetection efficiency. This corresponds to a qq¯ rejection of ≈ 4 × 10−4, to be compared with ≈ 10−5 atthe τcF.The threshold region has also several kinematic advantages that result from the low particle ve-locities, such as monochromatic spectra for two-body decays. Due to the Coulomb interaction, the τ+τ−production cross-section has a finite value of 0.20 nb [17] at threshold, which makes feasible to collecta copious sample of τ+τ− events almost at rest. Running just above the τ+τ− threshold at 3.57 GeV,the one-prong decays τ− → l−ν¯lντ , pi−ντ ,K−ντ are kinematically separated (Fig. 2a). In contrast, athigher energies the distributions completely overlap (Fig. 2b) and the particle identification requirements–especially between pi and K– are more severe [18]. Measurements of τ branching ratios can then bedone much more precisely at threshold.Figure 2: Momentum spectra from one-prong τ decays at centre-of-mass energies of a) 3.57 GeV (1.5MeV above the τ+τ− threshold) and b) 10 GeV.Figure 3 clearly illustrates the power of this kinematic constraint to search for exotic two-bodydecays like τ− → l−X (l = e, µ; X = Majoron, familon, flavon, . . . ), which, at threshold, lead tothe distinctive signature of monochromatic leptons (Fig. 3a) [19]. In contrast, the sensitivity is muchweaker at higher energies since the lepton spectrum from the l−X decay is broad, spreading over thefull spectrum of the standard l−ν¯lντ decay (Fig. 3b). For τ− → e−X (τ− → µ−X), the expected3branching-ratio sensitivity at the τcF is better than 10−5 (10−3) for one-year’s data [19], to be comparedwith the present limits of 1-2%. The experimental sensitivity could be improved a further order-of-magnitude if the monochromator optics is successful. Sensitivity to still-lower branching ratios wouldrequire improvements in the pie rejection of the τcF detector, e.g. with a fast RICH.Figure 3: The combined electron spectra (solid histograms) from the standard decay τ− → e−ν¯eντ mixedwith a hypothetical 1% two-body decay τ → eX at centre-of-mass energies of a) 3.57 GeV and b) 10GeV [19]. The particle X is a massless Goldstone boson. The dashed histograms show conventional(V-A) fits to the combined spectra. Each plot contains 200k τ− → e−ν¯eντ , corresponding to two months’data at 3.57 GeV.At threshold, the experimental data require small radiative corrections since the τ ’s are almost atrest and radiation can only bring them closer to rest. At higher-energies, the important role of initial-state QED bremsstrahlung results in large radiative corrections, which unavoidably introduce additionalcontributions into the final errors. This effect is particularly important for high-precision measurementsof the Michel parameters in leptonic τ decays. In the recent ARGUS measurement [20] of the ξ parameter,|ξ| = 0.90 ± 0.13 ± 0.10, initial-state radiation is the second main source of uncertainty with a ±0.06contribution to the final systematic error (background subtraction gives±0.08). Furthermore, the Lorentzboost smears the final charged-lepton energy spectrum and causes the parameters ρ and η to be stronglycorrelated. Clearly, the Lorentz structure of τ decays can be more accurately analyzed in the thresholdregion, where the τcF operates.Another important advantage of the τcF energy region is the existence of two precisely-knownenergy points, the J/ψ and ψ′, which provide a very high-rate (∼ 1 kHz) signal to calibrate and monitorthe detector performance. This allows for a tight control of the systematic errors, making possible, forinstance, to maintain the momentum-scale error of the τcF detector below 0.1% [15].In addition to excellent momentum measurements, there are advantages [15] for photon detectionand particle identification near threshold. Since particles are produced isotropically, the detection in-efficiency caused by charged and neutral pileup is minimized. This is especially important for decaysinvolving several neutral particles. The very good photon resolution of the τcF detector would increasethe sensitivity to the τ -neutrino mass, by allowing an optimal use of high-multiplicity τ decays containingneutrals, such as τ− → ντ2pi−pi+2pi0 [12,13]. Moreover, the kinematic limit of particles from τ decays atrest is 1.8 GeV, making the identification of pi, K and p easier than at higher-energy machines.The unique purity of the τcF data would offer the opportunity to perform a high-precision globalanalysis of (inclusive and exclusive) semi-leptonic τ -decay modes, including kinematical quantities moresubtle than the simple invariant-mass distribution of the hadronic final state [14]. The cleanness of the(bias-free) data, the excellent pi/K separation and the very good efficiency for neutral modes, wouldenourmously simplify the QCD analysis of current spectral functions, removing the Monte Carlo basedunfolding procedure which is needed, at higher energies, to correct for detector effects [14].Thus, the τcF experiment would benefit from a very high statistics, low and measurable back-grounds, and reduced systematic errors. The coincidence of all these features near threshold creates anideal facility for precision τ studies. Table 3 gives an illustrative list of expected sensitivities [13, 18,19, 21–23] at the τcF for some typical τ parameters. In all cases the improvements with respect to thepresent precisions are substantial. Note, that a BF (or ZF) with a similar data sample could obviouslyreach a comparable statistical precision for many of these observables. However, for most measurements,only the τcF is likely to achieve comparable systematic errors, thereby reaching the overall precisionsindicated in Table 3.AcknowledgementsBeing a theorist, I feel quite embarrassed talking about experimental acceptances, backgrounds,efficiencies, resolutions and so on. I am indebted to my experimental colleagues in the τ -physics workinggroup for their patience in helping me with this task. I would like to thank J.J. Go´mez-Cadenas and J.Kirkby for many discussions and useful comments on the manuscript.4Table 3: Examples of the expected precision at the τcF for some τ parameters. Some improved estimatesfrom preliminary analyses are indicated within brackets.Parameter Present accuracy τcF sensitivitymτ 0.3 MeV 0.1 MeVmντ < 31 MeV 2 MeVBe,µ 1% 0.1%Bpi 3% 0.1%BK 34% (12%) 0.5%|gτ/gµ| 0.6% 0.1%|gµ/ge| 0.6% 0.1%ρe,µ 6% 0.3% (0.03%)ξl 20% 3% (0.2%)hντ 22% 0.3%ηµ, δl – ±0.03 (±0.002)ξ′µ – 15%B(τ− → pi−ηντ ) < 3× 10−4 10−6B(τ− → l−X) < 2% 10−5B(τ− → 3l±) < 10−5 10−7B(τ− → µ−γ) < 4× 10−6 10−7aγτ < 0.1 0.001dγτ < 6× 10−6 e cm 10−7 e cmReferences[1] Proc. Tau-Charm Factory Workshop (SLAC, California, 1989), ed. L.V. Beers, SLAC-Report-343(1989).[2] B.C. Barish and R. Stroynowski, Phys. Rep. 157 (1988) 1;K.K. Gan and M.L. Perl, Int. J. Mod. Phys. A3 (1988) 531;C. Kiesling, τ Physics, in High Energy Electron-Positron Physics, eds. A. Ali and P. So¨ding, AdvancedSeries on Directions in High Energy Physics, Vol. 1 (World Scientific, Singapore, 1988), p. 177.[3] A. Pich, Tau Physics: Present Status and Future Prospects, in Physics at LEP, eds. M. Aguilar-Ben´ıtez and M. Cerrada (World Scientific, Singapore, 1990) p. 323; Mod. Phys. Lett. A5 (1990) 1995;Tau Physics, in Heavy Flavours, eds. A.J. Buras and M. Lindner, Advanced Series on Directions inHigh Energy Physics, Vol. 10 (World Scientific, Singapore, 1992) p. 375; Tau Physics and Tau-CharmFactories, Nucl. Phys. B (Proc. Suppl.) 31 (1993) 213.[4] M. Perl, Rep. Prog. Phys. 55 (1992) 653;K. Riles, Int. J. Mod. Phys. A7 (1992) 7647;A.J. Weinstein and R. Stroynowski, Ann. Rev. Nucl. Part. Sci. 43 (1993) 3700.[5] Proc. Workshop on Tau Lepton Physics (Orsay, 1990), eds. M. Davier and B. Jean-Marie (EditionsFrontie`res, Gif-sur-Yvette, 1991).[6] Proc. Second Workshop on Tau Lepton Physics (Columbus, Ohio, 1992), ed. K.K. Gan, (WorldScientific, Singapore, 1993).[7] M. Aguilar-Ben´ıtez et al., Review of Particle Properties, Phys. Lett. B239 (1990) 1.5[8] E. Ferna´ndez, Tau Physics, plenary talk at the EPS-93 Conference (Marseille, 1993).[9] A. Kounine, Tau and Charm Physics at LEP, these proceedings.[10] A.J. Weinstein, Tau and Charm Physics Prospects at CESR/DORIS and B Factories, these proceed-ings.[11] J.Z. Bai et al. (BES), Phys. Rev. Lett. 69 (1992) 3021.[12] D.F. Cowen, The CLEO-II Limit on the Tau Neutrino Mass and Prospects for Its Improvement,these proceedings.[13] J.J. Go´mez-Cadenas, Sensitivity of Future High-Luminosity e+e− Colliders to a Massive τ Neutrino,these proceedings.[14] F. Le Diberder, QCD Tests in Tau Decays, these proceedings.[15] J. Kirkby, Detector and Computing Aspects, these proceedings.[16] J. Le Duff, Summary of the Machine Working Group, these proceedings.[17] M. Perrotet, An Improved Calculation of σ(e+e− → τ+τ−) Near Threshold, these proceedings;M.B. Voloshin, Topics in τ Physics at a Tau-Charm Factory, Univ. Minnesota preprint TPI-MINN-89/33-T (1989).[18] J.J. Gomez-Cadenas, Beautiful τ Physics in the Charm Land, in [1], p. 48[19] M.C. Gonza´lez-Garc´ıa, Exotic Tau Decays at Tau and B Factories, these proceedings;R. Alemany et al., Searching for Exotic τ Decays, CERN-PPE/93-49.[20] H. Albrecht et al. (ARGUS), A Determination of Two Michel Parameters in Purely Leptonic TauDecays, DESY 93-108.[21] A. Stahl, The Lorentz Structure of the Charged Weak Current in τ Decays, these proceedings.[22] W. Fetscher, Universality of the Charged Leptonic Weak Interaction, these proceedings; Phys. Rev.D42 (1990) 1544.[23] A. Pich, Perspectives on Tau-Charm Factory Physics, these proceedings.6

Tau Physics Prospects at the Tau-Charm Factory and at Other Machines

The prospects for tau physics at future high-luminosity facilities are briefly discussed. Although important (and often complementary) contributions will be made from other machines, the unique experimental environment near threshold makes the Tau-Charm Factory the best experimental tool for $\tau$ physics.The prospects for tau physics at future high-luminosity facilities are briefly discussed. Although important (and often complementary) contributions will be made from other machines, the unique experimental environment near threshold makes the Tau-Charm Factory the best experimental tool for $\tau$ physics

Tau Physics Prospects at the Tau-Charm Factory and at Other Machines

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