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a!& __ __ !!I3 ELSEVIER Nuclear Physics B (Proc. Suppl.) 98 (2001) 125-129 SUPPLEMENTS www.elsevier.nl/locate/npe Tau polarization at DELPHI Igor Boyko a YJERN/EP CH-1211 Geneva 23 Switzerland This report describes a precise measurement of tau lepton polarization using about 110 pb-’ of data collected by DELPHI experiment in 1993-95 at and near the Z pole. In addition, a measurement of tau polarization well above the Z peak is presented for the first time. The results are based on 430 pb-’ of DELPHI data collected in 1997-99 at anaverage LEP energy of about 197 GeV. 1. Introduction Due to the parity violating nature of the elec- troweak interaction the fermions produced near Z” resonance are polarized. The decay of tau lep- tons within a detector provides a possibility to measure the tau polarization from its decay spec- tra. Due to the polarization of Z bosons, the tau polarization P, depends on the angle 0 between the electron beam and the produced r-. At the Born level this dependence has the form PT(0) = - A,(1 + ms20) + 2A,cosO 1+ cos20 + ‘LA,A,cosO (1) where the constants AL axe related to the vector and axial-vector effective couplings to the Z: A = hat V1 L 2 vu( + a: _ = 1 _ 4sin241ept (2) at eff * Therefore the study of the tau polarization angu- lar dependence allows one to extract simultane+ ously A, and A, and to test the lepton univer- sality in the neutral current sector or, assuming the lepton universality, to determine the effective electroweak angle with improved precision. The analysis starts from the standard tau pair preselection. Since the tau polarization is mea- sured from the tau decay spectra, the decay mode of each tau candidate must be identified. DEL- PHI has used three different approaches for the decay identification. In the first one, “exclusive identification”, five different decay modes (elec- tron; muon; pion; rho; al) are identified and analysed independently and the extracted values of tau polarization are then combined. In the “neural network” approach, all one prong decays were classified into five categories (same as for the exclusive identification) by means of a neu- ral network and the tau polarization was fitted simultaneously in all categories. In the third approach, “inclusive hadronic selection”, all one prong hadronic tau decays were selected with- out further separation between different hadronic modes. The inclusive hadronic selection has lower sensitivity to the polarization but it gains from higher selection efliciency and smaller systematic uncertainty. The results obtained with three dif- ferent methods were combined taking into ac- count statistical and systematic correlations. The measurement of tau polarization using 38 pb-’ of data collected by DELPHI experiment in 1990-92 was published in [l]. In this report we present the final results obtained with the rest of LEPl data (about 110 pb-’ collected in 1993-95) which were recently published in [2]. In this anal- ysis the angular acceptance was extended from 1 cosO1 < 0.73 (barrel only) to ] COST] < 0.94 for tau decays to muons and for the inclusive hadronic analysis. After 1995 LEP was operated at energies above 160 GeV. The statistics of tau pair events is ex- tremely low at these energies and the relative background is high. However, the measurement of the tau polarization at the high energies, even with a large uncertainty, is important to constrain some specific models; for example models with contact interactions between fermions. In the last part of this report we present for the first time a measurement of tau lepton polar- 0920-5632/01/$ see hmt matter 0 2001 Elsevier Science B.V. All rights reserved Pll SO920-S632(01)0121 l-7

Tau polarization at DELPHI

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Page 1: Tau polarization at DELPHI

a!& __ __ !!I3 ELSEVIER Nuclear Physics B (Proc. Suppl.) 98 (2001) 125-129

SUPPLEMENTS www.elsevier.nl/locate/npe

Tau polarization at DELPHI

Igor Boyko a

YJERN/EP CH-1211 Geneva 23 Switzerland

This report describes a precise measurement of tau lepton polarization using about 110 pb-’ of data collected by DELPHI experiment in 1993-95 at and near the Z pole. In addition, a measurement of tau polarization well above the Z peak is presented for the first time. The results are based on 430 pb-’ of DELPHI data collected in 1997-99 at anaverage LEP energy of about 197 GeV.

1. Introduction

Due to the parity violating nature of the elec- troweak interaction the fermions produced near Z” resonance are polarized. The decay of tau lep- tons within a detector provides a possibility to measure the tau polarization from its decay spec- tra.

Due to the polarization of Z bosons, the tau polarization P, depends on the angle 0 between the electron beam and the produced r-. At the Born level this dependence has the form

PT(0) = - A,(1 + ms20) + 2A,cosO

1+ cos20 + ‘LA,A,cosO ’ (1)

where the constants AL axe related to the vector and axial-vector effective couplings to the Z:

A = hat V1 L 2

vu( + a: ’ _ = 1 _ 4sin241ept (2) at eff *

Therefore the study of the tau polarization angu- lar dependence allows one to extract simultane+ ously A, and A, and to test the lepton univer- sality in the neutral current sector or, assuming the lepton universality, to determine the effective electroweak angle with improved precision.

The analysis starts from the standard tau pair preselection. Since the tau polarization is mea- sured from the tau decay spectra, the decay mode of each tau candidate must be identified. DEL- PHI has used three different approaches for the decay identification. In the first one, “exclusive identification”, five different decay modes (elec- tron; muon; pion; rho; al) are identified and analysed independently and the extracted values

of tau polarization are then combined. In the “neural network” approach, all one prong decays were classified into five categories (same as for the exclusive identification) by means of a neu- ral network and the tau polarization was fitted simultaneously in all categories. In the third approach, “inclusive hadronic selection”, all one prong hadronic tau decays were selected with- out further separation between different hadronic modes. The inclusive hadronic selection has lower sensitivity to the polarization but it gains from higher selection efliciency and smaller systematic uncertainty. The results obtained with three dif- ferent methods were combined taking into ac- count statistical and systematic correlations.

The measurement of tau polarization using 38 pb-’ of data collected by DELPHI experiment in 1990-92 was published in [l]. In this report we present the final results obtained with the rest of LEPl data (about 110 pb-’ collected in 1993-95) which were recently published in [2]. In this anal- ysis the angular acceptance was extended from 1 cosO1 < 0.73 (barrel only) to ] COST] < 0.94 for tau decays to muons and for the inclusive hadronic analysis.

After 1995 LEP was operated at energies above 160 GeV. The statistics of tau pair events is ex- tremely low at these energies and the relative background is high. However, the measurement of the tau polarization at the high energies, even with a large uncertainty, is important to constrain some specific models; for example models with contact interactions between fermions.

In the last part of this report we present for the first time a measurement of tau lepton polar-

0920-5632/01/$ see hmt matter 0 2001 Elsevier Science B.V. All rights reserved

Pll SO920-S632(01)0121 l-7

Page 2: Tau polarization at DELPHI

Figure 1. Distribution used to extract tau polar- ization in different decay modes (electron, muon, pion and rho). Crosses are data, gray histograms are the background and solid lines are the fit re- sults. Dotted (dashed) lines show the simulation with left (right) polarized taus.

ization at collision energy well above the Z pole. The results are based on about 430 pb-’ of data collected by DELPHI in 1997-99. The method of inclusive hadronic decays was used as it is most robust against systematic uncertainties and back- ground contamination. The average collision en- ergy corresponding to this data sample was about 197 GeV.

2. Tau polarization at LEPl

After the initial tau pair preselection the tau decay modes were separated using DELPHI par- ticle identification. Electrons were identified us- ing the dE/dx measurement and the deposition in the electromagnetic calorimeter. Muons were

SuppI.) 98 (2001) IZS--129

identified using muon chambers and the hadron calorimeter (HCAL) response. Hadrons were identified using the dE/dx information and the energy deposition in the first four (out of nine) layers of the electromagnetic calorimeter.

DELPHI

Figure 2. Distribution used to extract tau polar- ization in al decay mode. Crosses are data, gray histograms are the background and solid lined are the fit results. Dotted (dashed) lines show the simulation with left (right) polarized taus.

After the separation of the decay modes the tau polarization was extracted fitting the experimen- tal distributions by linear combinations of simu- lated events with left- and right-polarized taus. The fits are illustrated in Fig.1. For the elec- tron decay mode the sensitive variable was a “mo- mentum estimator” constructed from the mea- sured track momentum and the electromagnetic calorimeter deposition. For muons and pions the track momentum was used. In the case of the rho channel the situation is more complicated because the decay of rho itself also provides information on tau polarization. Therefore the polarization was extracted from the distribution of the vari- able E (see [2]) which contains all the available

Page 3: Tau polarization at DELPHI

I. Boyko /Nuclear Plysics B (Proc. Suppl.) 98 (2001) 125-129 127

polarization information. In the case of al, the correlations of three angles 8, /? and 7 character- ising the decay chain were used.

For each decay mode the tau polarization was measured in bins of cos 0 (polar angle of tau pro- duction). The barrel region was divided into six equal bins and two other bins corresponded to the DELPHI forward and backward regions. The re- sults from exclusive modes, neural network anal- ysis and inclusive hadronic selection were com- bined bin-by-bin taking into account correlations. The results of the combination are presented in Fig.3.

DELPHI

0 0.05 0.1 0.15 0.2 0.25 0.3

I I I I I I I

ew AT

Al?

WV __l;=__c_

xv ---a---

pv ---=s--

ap -_

Inclusive +!$_..

N.N. Z$Z,. -:::.>,, ‘“,S? i,’

0 0.05 0.1 0.15 0.2 0.25 0.3

Figure 4. The measured parameters A, (upper point in each pair) and A, (lower points). The gray band is the DELPHI AL measurement.

Figure 3. Angular dependence of the measured tau polarization. The line shows the theoretical expectation.

Use of this formula causes a bias on AL of about 0.001, which is significant given the very high pre- cision of the combined LEP results. To avoid any possible bias from this source a more precise ap- proach was used. The fit was performed using as an estimation of P,(O) the ZFITTER [3] pre- diction averaged in cos 0 over the bin width and the centre-of-mass energies (weighted by the cor- responding luminosities). This automatically in- cluded QED and weak effects. The fitted values were the ratios of the couplings A, and A,.

The extracted values of A, and A, for different decay modes are presented in Fig.4. The results are in excellent agreement with the hypothesis of electron-tau universality. Combining with the previous results [l] one obtains the DELPHI final LEPl results:

A, = 0.1359 f 0.0079 f 0.0055

To extract the parameters A, and A, the me+ sured angular dependence of the polarization has to be fitted by a theoretical prediction. It was found that the formula (1) which was used by DELPHI in [l] is not precise enough. It is valid only at the Born level and only at the Z pole.

A, = 0.1382 f 0.0116 f 0.0005

A combination assuming lepton universality gives:

At = 0.1368 f 0.0065 f 0.0035

Page 4: Tau polarization at DELPHI

128 I. Boyko/Nucleur Physics B (Proc. Suppl.) 98 (2001) 125-129

DELPHI preliminary

Figure 5. Charged track momentum and recon- structed photon multiplicity in selected hadronic tau decays.

3. Tau polarization at LEP2

The tau pair preselection was performed us- ing the standard DELPHI procedure described in [4]. The reconstructed actual annihilation en- ergy (fi) was required to be more than 90% of the nominal LEP energy. In total, 737 tau pair candidates were selected from 430 pb-l of data collected by DELPHI in 1997-99 at LEP energies between 183 and 202 GeV.

The tau polarization was extracted using the inclusive one-prong hadronic selection method similar to the one described in [2]. Tau decays to electrons were rejected using the following condi- tions: the measured dE/dx had to be consistent with pion hypothesis and in addition, either the electromagnetic calorimeter deposition was low or there was a deposition deep inside the HCAL. Muons were rejected asking that the HCAL re- sponse was inconsistent with a MIP and that there were no hits in the muon chambers. Mul- tiprong tau decays were rejected by the require- ment that one and only one track in the hemi- sphere had hits in the Vertex Detector.

The analysis was restricted to the DELPHI bar- rel region 1 cosO1 < 0.74. 450 hadronic decay candidates were selected from the data with se- lection e%iciency of about 77% (within geomet-

rical acceptance). The non-tau background level was 4.5% and the internal background was 6.4%; mainly multiprong tau decays. The good agree- ment between data and Monte-Carlo in the se- lected sample is illustrated in Fig.4.

For the selected hadronic decays the visible in- variant mass was calculated using the charged track and all the photons reconstructed in a 30° cone around the track. The distribution of the invariant mass is presented in Fig.6. Tau po- larization was extracted by a simultaneous fit in three bins of invariant mass: 0 - 0.3GeV/2, 0.3-0.85GeV/c? and 0.85-l.gGeV/r?. The vari- ables sensitive to polarization were the two decay angles 0 and Q described in [2]. The fit is illus- trated in Fig.7.

DELPHI preliminary

Figure 6. Distribution of the invariant mass in tau hadronic decays.

The systematic error on the tau polarization was estimated to be 0.052, dominated by the statistics of test samples selected from data for calibrations. Averaging the results from three years of LEP running gives the DELPHI measure- ment of tau polarization at the average collision energy of 197 GeV:

P, = -0.16 f 0.13 f 0.05

Page 5: Tau polarization at DELPHI

I. Boylto/Nuclear Phy.Gcs B (Proc. Suppl.) 98 (2001) 125-129 129

DELPHI preliminary

Figure 7. Fit to the tau polarization in first (up- per plot), second (middle plots) and third (lower plots) bins of invariant mass. See Fig.1 caption for details.

DELPHI preliminary

The DELPHI measurements of the tau polar- ization at LEPl and LEP2 are presented in Fig.8.

REFERENCES

1. DELPHI toll., Zeit. Phys. C67 (1995) 183 2. DELPHI toll., Eur. Phys. J. Cl4 (2000) 557 3. D.Bardin et al., hep-ph/9908433 (1999) 4. DELPHI toll., Phys. Lett. B485 (2000) 45

Figure 8. Tau polarization measured by DEL- PHI at LEPl and LEPZ. Horizontal error bars represent the range of LEP energies in each mea- surement. The line is the ZFITTER prediction (assuming the fi cut at 0.9).