The τ lifetime was measured with the Mark II vertex detector at the storage ring PEP. ττ=(3.20±0.41±0.35)×10-13 sec was found, which agrees well with e-μ-τ university. © 1983 The American Physical Society

Abrams, GS

Amidei, D

Blocker, CA

Boyarski, AM

Breidenbach, M

Burke, DL

de la Vaissiere, C

Dorfan, JM

Feldman, GJ

Gidal, G

Gladney, L

Gold, MS

Goldhaber, G

Golding, L

Hanson, G

Hoard, C

Hollebeek, RJ

Innes, WR

Jaros, JA

Kadyk, JA

Lankford, AJ

Larsen, RR

LeClaire, B

Levi, M

Lockyer, N

Lüth, V

Matteuzzi, C

Nelson, ME

Ong, RA

Perl, ML

Richter, B

Rowson, PC

Schaad, T

Schellman, H

Schlatter, D

Sheldon, PD

Strait, J

Trilling, GH

Yelton, JM

Zaiser, C

English

J. A. Jaros

D. Amidei

G. H. Trilling

G. S. Abrams

C. A. Blocker

A. M. Boyarski

M. Breidenbach

D. L. Burke

J. M. Dorfan

G. J. Feldman

G. Gidal

L. Gladney

M. S. Gold

G. Goldhaber

L. Golding

G. Hanson

C. Hoard

R. J. Hollebeek

W. R. Innes

J. A. Kadyk

A. J. Lankford

R. R. Larsen

B. LeClaire

M. Levi

N. Lockyer

V. Lüth

C. Matteuzzi

M. E. Nelson

R. A. Ong

M. L. Perl

B. Richter

P. C. Rowson

T. Schaad

H. Schellman

D. Schlatter

P. D. Sheldon

J. Strait

C. de la Vaissiere

J. M. Yelton

C. Zaiser

Crossref

Physical Review Letters

Precise Measurement of the Tau Lifetime

eScholarship - University of California

UC IrvineUC Irvine Previously Published WorksTitlePrecise Measurement of the Tau LifetimePermalinkhttps://escholarship.org/uc/item/02d0k6cvJournalPhysical Review Letters, 51(11)ISSN0031-9007AuthorsJaros, JAAmidei, DTrilling, GHet al.Publication Date1983-09-12DOI10.1103/physrevlett.51.955Copyright InformationThis work is made available under the terms of a Creative Commons Attribution License, availalbe at https://creativecommons.org/licenses/by/4.0/ Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaVOLUME 51, NUMBER ll PHYSICAL REVIEW LETTERS 12 SEPTEMBER 198$Precise Measurement of the Tau LifetimeJ. A. Jaros, D. Amidei, G. H. Trilling, G. S. Abrams, C. A. Blocker, A. M. Boyarski,M. Breidenbach, D. L. Burke, J. M. Dorfan, G. J. Feldman, G. Gidal, L. Gladney,M. S. Gold, G. Goldhaber, L. Golding, G. Hanson, C. Hoard, R. J. Hollebeek,W. R. Innes, J. A. Kadyk, A. J. La,nkford, R. R. Larsen, B. LeClaire,M. Levi, N. Lockyer, V. Luth, C. Matteuzzi, M. E. Nelson, R. A. Ong,M. L. Perl, B. Richter, P. C. Rowson, T. Schaad, H. Schellman,D. Schlatter, '~ P. D. Sheldon, J. Strait, C. de la Vaissiere, 'J. M. Yelton, and C. ZaiserStanford Linear Accelerator Center, Stanford University, Stanford, California 94305, andLawrence Berkeley Laboratory and Department of Physics, University of California,Berkeley, California 94720, and Department of Physics, Harvard University,Cambridge, Massachusetts 02138(Received 2 August 1983)The z lifetime was measured with the Mark II vertex detector at the storage ringPEP. y~ = (3.20+0.41+ 0.35) x 10 ~3 sec was found, which agrees well with e-p-Tuniversality.PACS numbers: 14.60.Jj, 13.35.+sThe electron and muon couple to the chargedweak current with identical strengths to a highdegree of accuracy. ' A measurement of the 7lifetime tests whether it too couples with theuniversal Fermi strength. Several experiments' 'have measured the average decay length of 7leptons which have been pair produced in e 'einteractions and find the lifetime to be compati-ble with e pTun-iv. -ersality within large statisti-cal and systematic errors. Since the 7 lifetimeprovides an exacting test of the standard-modelassumptions, ' it is important to measure it tohigh accuracy. In this Letter we describe a newmeasurement' of the T lifetime made with a high-precision vertex detector in conjunction with theMark II detector at the Stanford Linear Accelera-tor Center e'e storage ring PEP. This devicemeasures the decay length with a resolutioncomparable to the expected decay length (680 pmat the PEP center-of-mass energy 29 GeV), incontrast to the previous measurements whichhad resolutions about 5 times worse.The Mark II vertex detector' is a high-preci-sion, cylindrical drift chamber located betweena beryllium beam pipe and the central Mark IItracking chamber. The beam pipe, which is 0.6/pof a radiation length thick, also serves as theinner wall of the chamber so as to minimizemultiple Coulomb scattering. The chamber is120 cm long and has been seven axial layers ofdrift cells, four of which are just outside thebeam pipe at an average radius of 11.2 cm, withthe remaining three at 31.2 cm. The drift cellradius is 0.53 cm throughout the chamber. Theaverage spatial resolution in hadronic events is110 p.m per layer. Tracks which have beenextrapolated to the interaction point are meas-ured with an accuracy of or ( pm) = ((95)'+ [95/P(GeV/c)]'] '~' in the plane perpendicular to thebeams, the second term arising from multipleCoulomb scattering in the beam pipe. The ver-tex detector is used in conjunction with the cen-tral drift chamber to measure charged-particlemomenta. The central chamber has sixteencylindrical layers of sense wires at radii be-tween 41 and 145 cm. Six layers are parallel tothe incident beams and the other ten are +3 rela-tive to the incident beams. Both chambers op-erate inside a 2.3 kG solenoidal magnetic field.The momentum resolution of the vertex-detector-central-chamber combination is hP/P = [(0.02)'+(0.011P)']' ' when tracks are not constrainedto pass through the luminous region. Just outsidethe solenoid coil there are eight lea, d-liquid-a, r-gon shower counters which detect photons over65/p of the solid angle. The Mark II detector isdiscussed more thoroughly by Schindler et al. 'The data presented here correspond to an inte-grated luminosity of 41 pb '. All the data. weretaken at a center-of-mass energy of 29.0 GeV.We measure the 7 lifetime by measuring thedisplacement of its three-prong decay verticesfrom the point where the e ' and e- beams collide.Since the 7 is produced with the known beam en-ergy, the average displacement is proportionalto the lifetime.1983 The American Physical Society 955VOLUME 51, NUMBER 11 PHYSICAL REVIEW LETTERS 12 SEPTEMBER 1983Tau production is so distinctive at high ener-gies that events can be selected with very littlebackground. We find three-prong T decays bychoosing charge-balanced two-jet events with1+3 or 3+3 charged-prong topologies. We re-quire that there be at least one three-particlecombination with unit charge and an invariantmass between 0.7 GeV/c2 and 1.5 GeV jc' calcu-lated assuming the prongs to be pions. To elim-inate beam-gas backgrounds and suppress con-tamination from two-photon ~-pair production,we demand that the total charged plus neutral en-ergy in the event exceed 25% of the center-of-mass energy and that the energy of the three-particle combination exceed 3.0 GeV. We sup-press hadronie backgrounds by requiring the in-variant mass of the charged and neutral particlesin each jet be less than 2.0 GeV/c'. To guardagainst radiative Bhabha scattering, we requirethe total charged energy in the event to be lessthan 85% of the center-of-mass energy, and thethree-particle invariant mass calculated underthe assumption that each prong is an electron tobe greater than 300 MeV/c'. Using Monte Carlomethods, we estimate that (4+2) /o of the selecteddecays are misidentified hadrons, and (4+ 1)%%uoof the events are ~ pairs produced via the bvo-photon mechanism. Backgrounds from radiativeQED events and beam-gas processes are negligi-ble.A total of 621 three-prong decays are selectedwith the above criteria. We next impose severaltracking-quality cuts. Each of the three tracksmust extrapolate to within 5 mm of the beamposition inthe plane (x-y) perpendicular to thebeams and to within 5 cm of the vertex formedfrom all the tracks in the event in the directionalong the beams. To minimize both scatteringand decay probabilities, we require each trackto have momentum greater than 400 MeV jc. Wefurther require that each track has been meas-ured in at least two of the inner vertex-detectorlayers, and at least one of the outer layers.Finally, each track fit must have an overall y'per degree of freedom less than 5, and the y'per degree of freedom from the vertex detectormust be less than 4. This leaves 185 decays suit-able for vertexing.The most probable decay length is determinedfrom the position of the decay vertex, the beamposition, the v direction, and the errors in thesequantities.We find the decay-vertex position and its er-rors in the x-y plane from the parameters of thethree particle trajectories with a y minimiza-tion procedure. Requiring the vertex y' to beless than 15 for three degrees of freedom leaves180 decays. This g' distribution is somewhatbroader than we expect from our known trackingerrors, and we have therefore increased ourestimate of the vertex-position errors by 10%.The uncertainties in this procedure are includedin the systematic errors discussed below.The & direction is approximated quite closelyby the direction of the three-pion system sincethat system is slow in the T rest frame, and thev. is highly relativistic in the laboratory.We measure the beam position on a run-by-runbasis by finding the position which minimizes thedistance of closest approach for an ensemble oftracks. In a typical 2-h storage-ring fill, wemeasure the average beam position within 20p,m vertically and 50 p,m horizontally. We meas-ure the rms beam size to be 65+ 15 pm vertical-ly and 480+ 10 pm horizontally. The beam posi-tion is stable during a fill, and is usually con-stant for large blocks of data. . Beam-positionmonitors sensitive to 20- pm shifts in the beamposition flagged the few runs where steeringchanges occurred midrun, and runs with exces-sive beam motion were ignored. We have checkedthe beam-position determination by vertexinghadronic events, and we find good agreement be-tween the average vertex position and the beamposition.The projected decay length is given in terms ofthe decay-vertex position relative to the beamposition (x„,y„), the sum of the beam and vertexerror matrices o... and the two-dimensional 7direction cosines t; by the expression&v &yytx +yv Oxxfy —&xy(&v ty +yvfx)The decay length is then l =ly/sinG, where & isthe angle between the three-pion direction andthe beam direction. The resolution in the decaylength depends on the opening angles, track mo-menta, and orientation of each decay. . It variesbetween 500 and 1800 jUm for most decays andaverages 1100 pm. After requiring the resolu-tion to be less than 1700 jLf.m, we are left with156 decays for study. Figure 1 shows their de-cay-length distribution. The mean of the distribu-tion is 679 pm, and its shape is asymmetric,showing the expected long-lifetime tail.We use a maximum-likelihood fit to determinethe average decay length. The fitting function isthe convolution of an exponential decay distribu-VOLUME 5 1, NUMBER 11 PHYSICAL REVIEW LETTERS 12 SEPTEMBER 198$~20cu I5&- IOw 5I I I I IIIIII~ 80O& eo—Cf)I— 40LLJ20I I I I I(a)-2 0 2DECAY LENGTH (mm 1FIG. 1. Measured & decay lengths. The solid curveis the best fit to the data described in the text.L ~l-2 —I- 0 I 2DECAY LENGTH {rnm)50Ition with a Gaussian resolution function whosewidth o, is calculated event by event. The resultof the fit depends on both the width and the po-tential offset of the resolution function, and sowe have checked those quantities with two differ-ent methods. The first is from a study of "pseudo-tau decays" in hadronic events. For this study,we select three-track combinations in hadronicevents which pass our tracking criteria andwhich have kinematic properties like the three-pion 7 decays. We systematically exclude anytrack which may come from K,' decay and thehighest-momentum track in a jet to suppress con-tamination from events with secondary vertices.Figure 2(a) shows the decay-length distributionfor the pseudo-tau decays. This distribution ispeaked near zero decay length with some excessof events at positive decay lengths. The fittedmean decay length is 72 + 38 p.m. We have simu-lated this measurement with Monte Carlo tech-niques and find that the presence of charm andbottom decays give decay lengths in the range 50to 150 pm, depending on the D'/D' ratio and thelifetime of bottom hadrons. This confirms thatthere is little if any offset in our resolution func-tion. Figure 2(b) shows the distribution of decaylengths of the pseudo-tao events divided by theirerrors. The distribution is represented nicelyby a unit-width Gaussian function, which is alsoshown in the figure. A fit confirms that our cal-culation of the width of the resolution function iscorrect to the 5'fo level. Our second method forchecking the resolution function is to fit the ob-served 7. decay-length distribution for the widthor offset of the resolution function in addition tothe decay length. We find the offset to be consis-tent with zero, 83 + 128 pm; the factor whichscales the width of the resolution function is 1.00+0.085. So the data are consistent with ourchoice of resolution function.40CUo 30—V)20LdIo—0 I I-3 —2 —I 0 I 2DECAY LENGTH/ERRORFIG. 2. (a) Measured decay lengths for ' pseudo-7"decays; (b) decay length divided by its calculated errorfor pseudo-z decays. The solid curve is a unit-vridthGaussian function, normalized to the data between -2.5~l /0 &2.5.The result of the maximum-likelihood fit to the~ decay-length distribution is 705+87 pm, wherethe error is statistical only. The fit is shownsuperimposed on the data in Fig. 1, and describesthe data adequately. Correcting the decay lengthfor backgrounds (+30 jm) and noting that initial-state radiation lowers the average & energy from14.5 to 13.8, we find 7, =(3.20+0.41) &&10 "sec.We have investigated several potential sourcesof systematic error. The first is a measure-ment bias which comes about because the open-ing angles of the decay are differentially largerwhen the vertex position has fluctuated towardlong decay lengths than when it has fluctuated to-ward short decay lengths. Larger opening anglesresult in smaller vertex errors, and consequent-ly more weight in the fit for the longer decaylengths. In previous determinations of ~ decaylengths, -250- pm corrections were made to ac-count for the bias. It has been reduced to the 25-pm level in this experiment by the improvementsin tracking resolution and the reduction of multi-ple Coulomb scattering. Secondly, we havechecked the accuracy of the analysis techniquewith Monte Carlo-simulated raw data. The simu-957VOLUME 51, NUMBER ll PHYSICAL REVIEW LETTERS 12 SEPTEMBER 1983lation included pion and kaon decay, photon con-versions, nuclear absorption and scattering, andmultiple Coulomb scattering in the beam pipeand wires of the vertex detector, and gives agood representation of the data. About 1000 de-cays were generated for each of three lifetimes,and then analyzed with the same programs usedto analyze the actual data. The average decaylengths generated were 0, 629, and 1327 p,m; therespective fitted values were 48+ 36, 609+35,and 1252+ 56 pm. Finally, we have consideredhow errors in the various quantities essentialto the analysis induce errors in the decay length.The uncertainties in the width (+7/q) and offset(+50 pm) of the resolution function each lead touncertainties in the fitted decay length of +35pm. Reasonable variations in the shape of theresolution function can change the result by +30pm. The uncertainty in the background correc-tion is +15 p,m. The result is insensitive tosmall changes in the estimated beam positionand width. Altogether, including these and theother uncertainties in the analysis, we estimatethe systematic error to be 80 pm.In conclusion, we have measured the T lifetimeto be r„=(3~ 20+0.41+0.35) X10 "sec, wherethe first error is statistical and the second issystematic. This value is consistent with previ-ous measurements, and is in good agreementwith the lifetime expected from e-p-T universal-17, = T &(m &/m, )sB, = 2.8 + 0.2 x 10 's s,where T„(m„) is the muon lifetime (mass) andB, is the T branching ratio into evv. 'Ke thank R. Baggs, V. Golubev, R. Leonard,T. Nakashima, R, Stickley, and Y. Y. Wang fortheir help with the design and construction of theMark II vertex detector. We are especially grate-ful to the PEP operations group for providingstable beams and record luminosities during therunning of this experiment. This work was sup-ported in part by the U. S. Department of Energy,under Contracts No. DE-AC03-76SF00515, No. %-7405-ENG-48, and No. DE-AC03-76SF00098.&'&Present address: CERN, CH-1211 Geneva 22,Switzerland.~ ~Present address: University of Massachusetts,Amherst, Mass. 01002.Present address: Laboratoire de Physique Nucle-aire et Hautes Energies, Universite Pierre et MarieCurie, Paris F-75230, France.'D. A. Bryman et al. , Phys. Rev. Lett. 50, 7 (1983).'G. J. Feldman et al. , Phys. Rev. Lett. .48, 66 (1982).3%. T. Ford et al. , Phys. Rev. Lett. 49, 106 (1982).4H. J. Behrend et al. , Nucl. Phys. B211, 369 (1983).5See, for example, M. Wirbel, Phys. Lett. 121B,252 (1983).6A preliminary analysis of half of the data reportedhere was given by J. A. Jaros, J. Phys. (Paris), Colloq.43, C3-106 (1982).~J. A. Jaros, in Proceedings of the InternationalConference on Instrumentation for Colliding BeamPhysics, edited by W. Ash, SLAC-Report No. 250(SLAC, Stanford, California, 1982).B. H. Schindler etal. , Phys. Bev. D 24, 78 (1981).~C. A. Blocker et al. , Phys. Lett. 109B, 119 (1982).The uncertainty in z~ comes primarily from the errorin g~.

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Precise Measurement of the Tau Lifetime

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