key: cord-0112740-tpk9athj
authors: Pich, Antonio
title: Workshop on Tau Lepton Physics: $mathbf{30^{th}}$ Anniversary
date: 2021-12-19
journal: nan
DOI: nan
sha: c499ada2a9a4dedf6462c2ff39de94bafbe9199e
doc_id: 112740
cord_uid: tpk9athj

The first Workshop on Tau Lepton Physics took place at Orsay in 1990. The evolution of the field and some physics highlights are briefly described, following the presentations discussed at the fifteen $tau$ workshops that have been held since then.

In 2020 we were supposed to celebrate the 30 th anniversary of the Workshop on Tau Lepton Physics, a very successful series of scientific meetings [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] that was initiated in 1990, at Orsay, by Michel Davier and Bernard Jean-Marie [1] . Owing to the covid pandemic, the 16 th Tau Workshop and this celebration have finally taken place on-line, after a one-year delay. Meanwhile, the τ community was shocked with the sad losses of our friends Simon Eidelman and Olga Igonkina, summary speaker and main organizer, respectively, of the TAU 2018 Workshop [15] , who are deeply missed by all of us.

In the following sections, I describe the 1990 status and the posterior evolution of the field, through a selection of physics highlights. Obviously, I cannot cover the large number of excellent § A much larger anomaly, the 1987 HRS claim [20] of a huge 5% τ + branching ratio into the G-parity violating ηπ +ν τ final state, had just been dismissed on experimental grounds. A workshop devoted to study the physics potential of a low-energy τ-charm factory (τcF) was organised in 1989 at SLAC [21] , triggering a renovated interest in this type of physics. This was followed by several other τcF meetings at different sites (Spain, USA, China. . . ) that would later culminate with the BEPCII project in Beijing. However, with the available luminosity, charm and charmonium were the clear physics priorities of this τcF.

The start of the LEP operation in August 1989 was the main motivation to organise a topical workshop fully devoted to the τ lepton. Although the initially planned scientific programme of LEP had not paid much attention to τ physics, it was soon realised that this e + e − collider was an excellent environment to perform precise measurements of the τ properties. An increasing interest in this third-generation particle was clearly manifested by the large number (117) of participants attending the Orsay meeting, which triggered many ideas, suggestions and lively discussions. First, very preliminary, analyses of the LEP data were already presented (H.-S. Chen, D.E. Klem, G.G.G. Massaro, S. Orteu, S. Snow, A. Stahl, P. Vaz, M. Winter, F. Zomer), showing the high physics potential of the new collider. In the Orsay proceedings [1] , one can already find first studies on topics that would later become common ingredients of the τ research: isospin relation with e + e − data (S. Eidelman and V. Ivanchenko), polarization analysers (A. Rougé), resonance studies (J.H. Kühn), etc. I was invited to discuss a possible determination of the strong coupling (Λ MS ) from the inclusive τ decay width, suggested by Stephan Narison and myself [22] . It was a quite bold and heterodox proposal at the time, and my talk was finally scheduled in the new-physics section.

The next workshops on Tau Lepton Physics (Columbus 1992 [2] , Montreux 1994 [3] , Estes Park 1996 [4] , Santander 1998 [5] , Victoria 2000 [6] ) witnessed a fast and drastic qualitative change on the status of τ physics, with lots of good data coming from CLEO, LEP and SLD, together with the last ARGUS analyses. In 1992, the disturbing τ anomalies were already solved (M. Davier, W.J. Marciano in [2] ). A tight (95% CL) limit on unmeasured decay modes was set with the LEP data, B unseen < 0.11% (M. Davier in [2] ), and BES released a very precise measurement of the τ mass (H. Marsiske in [2] ), slightly below the previous world average. It was followed by a precise ALEPH measurement of the τ lifetime, subsequently confirmed by the other LEP detectors and SLD, which shifted τ τ to smaller values (M. Davier in [3] ). Those measurements are compared with their 2014 values in Figs. 2 and 3. The current PDG averages are not much different: m τ = (1776.86 ± 0.12) MeV and τ τ = (290.3 ± 0.5) fs [23] . The combination of these two experimental inputs eliminated the previous discrepancy with the electronic branching ratio, as shown in Fig. 4 (A. Pich in [6] ).

We have presented three new measurements of the mass of the T lepton. The fit, we extract PID efficiencies and mis-ID rates from selected data control samples of radiative Bhabha events, J/ψ → ρπ, and cosmic ray events, correct the selection efficiencies of the different τ pair final states and propagate these changes to the event selection efficiencies ǫ i . We then refit our data with these modified efficiencies. The difference between the fitted τ mass from these two fits, 0.048 MeV/c 2 , is taken as the systematic error due to misidentification between different channels.

h. Background Shape In this analysis, the background cross section σ B is assumed to be constant for different τ scan points. The background cross sections have also been estimated at the last three scan points by applying their selection criteria on the first scan point data, where the τ pair production is zero. After fixing σ B to these values, the fitted τ mass becomes:

The fitted τ mass changed by 0.04 MeV/c 2 compared to the nominal result.

i. Fitted Efficiency Parameter The systematic uncertainties associated with the fitted efficiency parameter are obtained by setting R Data/MC at its ±1σ value and maximizing the likelihood with respect to m τ with σ B FIG. 7: Comparison of measured τ mass from this paper with those from the PDG. The green band corresponds to the 1 σ limit of the measurement of this paper Figure 7 shows the comparison of measured τ mass in this paper with values from the PDG [7] ; our result is consistent with all of them, but with the smallest uncertainty.

Using our τ mass value, together with the values of B(τ → eνν) and τ τ from the PDG [7] , we can calculate g τ through Eq. 1:

which can be used to test the SM. Similarly, inserting our τ mass value into Eq. 2 , together with the values of τ µ , τ τ , m µ , m τ , B(τ → eνν) and B(µ → eνν) from the PDG [7] and using the values of F W (-0.0003) and F γ (0.0001) calculated from reference [1] , the ratio of squared coupling constants is determined to be: The excellent quality of the LEP data brought a new era of precision physics, which was complemented with many theory contributions, allowing us to perform accurate tests of the SM, in both the electroweak and QCD sectors. Around 2000, lepton universality was tested with a 0.2% precision for charged currents (A. Pich in [6] ), and the measured leptonic Z couplings already indicated that low values of the Higgs mass were favoured (D.W. Reid in [6] ). Thanks to the τ polarization emerging from the decay Z → τ + τ − , it was possible to analyse the Lorentz structure of the leptonic τ decays and put relevant bounds on hypothetical right-handed charged-current couplings (A. Stahl in [5] ).

Detailed experimental analyses of the inclusive hadronic τ decay width and its invariant-mass distribution, performed by ALEPH (L. Duflot in [2] ), CLEO and OPAL (S. Menke in [5] ), put on very firm grounds the determination of the strong coupling at the τ mass (E. Braaten in [4] ), providing a beautiful test of its QCD running, shown in Fig. 5 , and a very accurate measurement of α s (M Z ) τ = 0.1202 ± 0.0027 (M. Davier in [6] ), in excellent agreement with the direct determination at the Z peak, α s (M Z ) Z = 0.1183 ± 0.0027. To put in perspective the importance of these analyses, we should recall that the advocated pre-LEP value was α s (M Z ) = 0.11 ± 0.01 [24] and the first (1992) precise lattice determination, α s (M Z ) = 0.105 ± 0.004 [25] , was substantially The early world averages were dominated by measurements from PEP, but soon other facilities were able to make important contributions. However, the advantages of LEP for tau lifetime measurements are so large that the total weight of all other measurements is now very small.

N ew precise measurements have greatly reduced the uncertainty on many tau branching fractions. Fi gu re 5 plots the RPP summary table value for the branching fraction error of 7 large tau branching fractions beginning with the first edition they were listed. These branching fractions include more than 85% of all tau decays. Note the si gn ificant decrease in the errors in the 1996 edition. The errors on many branching fractions are now near the 0.1 % level, and this sets the current goal for the interna! consistency of the tau listings.

The error on the 3-prong topological branching fraction (B3) follows unusual behaviour relative to the other branching fractions. Except for B3, the leptonic branching fractions have the smallest absolute error of the large branching fractions.

The error on B3 droped dramatically in 1986, and then remained aproximately constant until the 1996 edition where it once again decreased, but not by as much as the other branching fractions. This behaviour suggests that sorne measurements first included in the 1986 edition underestimated their errors on B3, and it has taken almost 10 years for the weight of more recent experiments to dominate the world average. Note that the only remaining problem observed in tau branching fraction data is a significant disagreement between the fit and average values for the charged prong topological lower than the currently accepted value.

In The running of α s (s 0 ) obtained from the fit of the theoretical prediction to R τ,V +A (s 0 )below the τ mass in the top plot: the shaded band shows the data including experimental errors, while the curves give the four-loop RGE evolution for two and three flavours. The plot below shows the evolution of the strong coupling (measured at m 2 τ ) to M 2 Z predicted by QCD compared to the direct measurement. Flavour matching is accomplished at 3 loops at 2 m c and 2 m b thresholds. Spectral moments are again use ravel the different components o rate. Since we are mostly interes cific contributions from the us stra it is useful to form the difference

where the flavour-independent pe and gluon condensate cancel. [11] ; R.J. Sobie in [12] ; H. Hayashii, E. Tanaka, P. Roig in [13] ). Searches for processes violating lepton flavour or lepton number were highly benefited by the huge available statistics, reaching sensitivities of a few 10 −8 in many τ decay modes (A. Cervelli, M. Lewczuk, K. Inami in [11] ; Y. Jin, D.A. Epifanov, H. Aihara, S. Eidelman in [15] ). This has been complemented by the strong constraints coming from µ decays (B. Golden in [11] ; H. Natori in [12] ) and µN → eN conversion (T. Iwamoto in [15] )).

Many analyses of LEP data continued during this period. Worth mentioning are the complete list of ALEPH branching ratios (M. Davier in [8] ), and the inclusive strange spectral functions reported by ALEPH (M. Davier et al in [6] ) and OPAL (W. Mader in [8] ), which made it possible to extract values of the strange quark mass and the Cabibbo mixing (J. Prades in [8] ). I would like to stress here the comprehensive works on SU(3) breaking (and light-by-light contributions to g − 2) developed by my collaborator and friend Ximo Prades (Castellón 1963 -Granada 2010), who unfortunately is no longer with us.

A very important theoretical development was the impressive calculation of the O(α 4 s ) correction to the inclusive τ hadronic width (Baikov et al in [10] ), which would be later complemented with a 5-loop computation of the QCD β function (Baikov et al in [14] ) and an updated version of the Cabibbo-allowed ALEPH spectral functions (Z. Zhang in [13] ). This has made possible to determine the strong coupling at the N 3 LO, triggering a huge theoretical activity and many lively discussions at different τ meetings (moment analyses, OPE contributions, renormalons, duality vi- 

stent with the previous measure-EO [22] and ALEPH [20] .

ECTRUM obtain the true π − π 0 mass specst apply corrections for: (1) backearing due to finite resolution and ts, and (3) mass-dependent acceprect for these effects using an unure based on the singular value deethod [36] . d results are shown in terms of the or in Fig. 2 and Fig. 3(a) and (b) . factor F − π (s) is obtained from the spectrum dN ππ ds via the form, where B ππ is the branching fraction, (1/N ππ )(dN ππ /ds) is the normalized invariant mass-squared distribution for the τ − → π − π 0 ν τ decay, B e is the branching fraction for τ − → e − ν τνe and S EW = S ππ EW /S e EW is the short-distance radiative correction. In Eq. (4), we use the world average value (including our measurement) for the branching fraction B ππ = (25.24 ± 0.10)% and for the CKM matrix element V ud = 0.97377 ± 0.00027 [34] . For S EW , we take the value 1.0235±0.0003, to be consistent with the isospin breaking correction discussed in Ref. [37, 6] .

The relative systematic errors on the unfolded spectrum are 5.3% in the threshold region, 0.7% near the ρ(770) peak and 1.8% in the vicinity olations, etc). The current status has been recently reviewed in [26] , which contains an extensive list of relevant references.

The muon anomalous magnetic moment is another timely topic that has been discussed in detail at different meetings. The BNL E821 data was presented by B.L. Roberts in [7] and D. Hertzog in [8] , and the relevant SM contributions have been reviewed: QED (T. Kinoshita in [8] ; M. Hayakawa in [12] ), electroweak (A. Czarnecki in [7] ), hadronic vacuum polarization (A. Hoecker in [7] ) and light-by light (A. Vainshtein in [9] ; E. de Rafael in [12] ; H. Meyer in [15] ). In particular, there have been many experimental contributions from BaBar, Belle, BES, CLEO, CMD, KEDR, KLOE, SND, etc, providing the necessary input to the dispersive evaluation of the hadronic vacuum polarization contribution (M. Davier, H. Hagiwara et al in [14] ; B. Shwartz in [15] ). The radiative return method (J.H. Kühn in [8] ; G. Rodrigo in [9] ) and the complementary information from τ decay data were also discussed (S. Eidelman, M. Davier in [6] ; A. Hoecker in [7] ; Z. Zhang in [12] ). In the 2021 workshop, we have of course seen the new measurement of the Muon g − 2 experiment at Fermilab (J. Stapleton) and an overview of the theory status (G. Colangelo). The prospects to improve the poorly known electromagnetic dipole moments of the τ lepton have been also analysed (M. Fael et al in [12] ; M. Hernández-Ruiz et al in [15] ).

The most important achievements in neutrino physics were also presented at the τ workshops, where a dedicated session has been always scheduled for this. Worth a mention for their direct connection with the τ lepton are the first direct observation of the τ neutrino by the DONUT experiment (B. Baller in [6] ), and the first ν µ → ν τ events registered ten years later with the OPERA detector (Y. Gornushkin in [11] ). The SNO measurement of solar neutrino fluxes (E.W. Beier in [7] ) and the SuperKamiokande atmospheric ν µ → ν τ signal (R. Svoboda in [6] ; J. Shirai in [8] ) were, of course, major milestones in neutrino oscillations. Regular updates of the oscillation data from solar, atmospheric, reactor and accelerator experiments have been discussed since then (J. Shirai in [8] ; C. Howcroft in [9] ; R.A. Johnson in [12] ; G.J. Barker in [13] ). 

With the start of operation of the LHC the main focus has moved to the energy frontier (Manchester 2010 [11] , Nagoya 2012 [12] , Aachen 2014 [13] , Beijing 2016 [14] , Amsterdam 2018 [15] , Indiana 2021). The high-momenta τ's produced at the LHC turn out to be an excellent signature to probe new physics. They have low multiplicity and good tagging efficiency. Moreover, their decay products are tightly collimated (mini-jet like) and momentum reconstruction is possible. Being a third-generation particle, the τ is also the lepton that couples more strongly to the Higgs; H → τ + τ − is in fact the 4 th largest branching ratio of the Higgs boson.

Thus, in the more recent τ workshops, we have seen a proliferation of Higgs-related measurements and exclusion plots from clever search analyses (D. Chakraborty, S. Knutzen in [13] ; A. Lusiani, Z. Mao, R. Reece in [14] ; C. Caputo, F. Lyu in [15] ). The detection of H → τ + τ − events and the corresponding measurement of the τ Yukawa coupling (T. Müller in [13] ; D. Zanzi in [14] ; L. Schildgen in [15] ) have been major milestones, together with the limits set on lepton-flavourviolating couplings of the Higgs (A. Nehrkorn in [14] ; B. Le in [15] ) and the Z boson (K. De Bruyn, A. Nehrkorn in [14] ; W.S. Chan in [15] ). It is remarkable that the LHC bounds on Z → with = are already better than the LEP ones. The LHC experiments have also provided relevant bounds on the τ → 3µ decay mode (K. De Bruyn in [14] ).

The strong improvement achieved in τ detection techniques has made possible to perform relevant tests of the SM itself. Worth mentioning are the first hadron-collider measurement of the τ polarization in W → τν decays (Z. Czyczula in [12] ) and the τ polarization asymmetry in Z → τ + τ − (V. Cherepanov in [14] ). More recently, the ATLAS and CMS experiments have been able to test lepton universality in W → ν decays, in good agreement with the SM, clarifying the puzzling 2.5 σ excess of τ events observed a long time ago in the LEP data.

The flavour anomalies identified in B decays have been one of the more recent highlights (E. Manoni in [12] ; A. Celis, T. Kuhr in [13] ; K. De Bruyn, S. Hirose, X.-Q. Li in [14] ; S. Benson, S. Fajfer in [15] ), since they indicate unexpected large violations of lepton universality in Another surprising result was reported by the BaBar collaboration (R. Sobie in [12] ), who observed a CP-violating rate asymmetry in the decay τ − → ν τ π − K 0 S (≥ 0π 0 ) that deviates by 2.8 σ from the SM expectation for K 0 −K 0 mixing. The BaBar signal has not been confirmed by Belle, which did not reach the required sensitivity (M. Hernández Villanueva in [15] ), and seems incompatible with other sets of flavour data (V. Cirigliano et al in [15] ). While future measurements should clarify this situation, the search for signatures of CP violation in τ decays remains an interesting goal (I. Bigi in [15] ).

The most important achievements in Astroparticle physics have been also reviewed at the τ workshops. Two recent IceCube highlights are the discovery of an astrophysical neutrino flux in 2013, which marked the birth of neutrino astronomy (D. Xu in [14] ), and the evidence for the identification of a blazar as an astrophysical neutrino source reported in 2018 (D. van Eijk in [15] ).

The forthcoming high-statistics data samples that will soon be accumulated by the Belle-II detector will give a new boost to precision τ physics [28] . In addition to much larger sensitivities to decays violating the lepton flavour or the lepton number, one expects significant improvements on the τ lifetime and branching ratios, decay distributions, CP asymmetries, Michel parameters, etc (M. Hernández Villanueva in [15] ). This superb physics potential will be complemented with more precise measurements of the τ mass at BES-III (J. Zhang in [15] ), and a new generation of muon experiments (C. Wu in [14] ; R. Bonventre, A. Bravar, A. Driutti, T. Iwamoto, A.-K. Perrevoort, N. Teshima in [15] ), neutrinoless double-beta-decay searches (L. Cardani in [15] ) and neutrino detectors (M. Komatsu, Z. Li, H. Lu in [14] ; D. van Eijk, I. Esteban, P. Fernández, A. Pocar et al, H. Seo, C. Timmermans, A. Tonazzo, M. Trini in [15] ) at different laboratories.

The LHC is also going to start its new Run 3, aiming to a sizeable increment of the integrated luminosity. This will be followed later by a much more significant improvement of the instantaneous luminosity at the HL-LHC, which will increase the potential for new discoveries [29] . In the long term, several linear and circular high-energy colliders are being discussed. Huge and clean τ + τ − data samples could be provided by an electron-positron TeraZ facility, running at the Z peak (M. Dam in [15] ). The projects to build a high-luminosity super-τcF (S. Eidelman in [13] ) are also in a quite advanced stage.

Thus, there is a bright future ahead of us with lots of interesting physics to be explored. We can look forward to many relevant experimental discoveries to be celebrated at the 40 th Tau Lepton Physics anniversary in 2030.

Proceedings, Tau Lepton Physics (TAU 90)

Proceedings, 2nd Workshop on Tau Lepton Physics (TAU 92

Proceedings, 3rd Workshop on Tau Lepton Physics (TAU 94

Proceedings, 4th Workshop on Tau Lepton Physics (TAU 96

Proceedings, 5th Workshop on Tau Lepton Physics (TAU 98

Proceedings, 6th International Workshop on Tau Lepton Physics

Proceedings, 7th International Workshop on Tau Lepton Physics

Proceedings, 8th International Workshop on Tau Lepton Physics

Proceedings, 9th International Workshop on Tau Lepton Physics (TAU 06

Proceedings, 10th International Workshop on Tau Lepton Physics

Proceedings, 11th International Workshop on Tau Lepton Physics

Proceedings, 12th International Workshop on Tau Lepton Physics

Proceedings, 13th International Workshop on Tau Lepton Physics

Proceedings, 14th International Workshop on Tau Lepton Physics (TAU 16

Proceedings, 15th International Workshop on Tau Lepton Physics (TAU 18)

Precision Tau Physics

Evidence for Anomalous Lepton Production in e + − e − Annihilation

Decay Correlations of Heavy Leptons in e +

The Physics of the tau Lepton

Evidence for the Decay τ + → π + ην τ

Proceedings, Tau -Charm Factory Workshop: Study of Tau, Charm and J/ψ Physics, Development of High Luminosity e + e − Rings, Design of e + e − Detectors for Tau Charm Physics

QCD Formulation of the tau Decay and Determination of Λ MS

QCD and experiment

A Determination of the strong coupling constant from the charmonium spectrum

Precision physics with inclusive QCD processes

Averages of b-hadron, c-hadron, and τ-lepton properties as

The Belle II Physics Book

Opportunities in Flavour Physics at the HL-LHC and HE-LHC

I would like to thank Michel Davier for his continuous support to the τ workshops, and the local organizers for making possible this 16 th meeting, in spite of the difficult circumstances. This work has been supported by MCIN/AEI/10.13039/501100011033, Grant No. PID2020-114473GB-I00, and by the Generalitat Valenciana, Grant No. Prometeo/2021/071.