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Single Intermediate Neutral Vector Boson Production at LEP 2

Paolo Spagnolo INFN-Pisa

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Zee Diagrams

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Introduction on Zee

The measurement of this cross section is an important test of the SM

The process is also called the electroweak Compton scattering

It is a mix between a Bhabha and a Z production

It is the dominant source of Z bosons at linear e+e- colliders with E>500 GeV

At LEP however the cross-section is nearly two orders of magnitude below the radiative return process Z:

e+e- Z e+e- ~ 1 pb

The cross-section is almost independent on the LEP energy ( <5%)

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Zee Signal Definition

only eeqq and eefinal states with the following cuts:

M(qq) or M() > 60 GeV

(e-) <12° and 12° < (e+) < 120° with E(e+) > 3 GeVor

(e+) >168° and 60° < (e-) < 168° with E(e-) > 3 GeV

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Zee Topology

The signature of the events is clear:

• one electron is lost in the beam-pipe

• the other electron is isolated and soft

• the Z recoils against the visible electron

Beam pipe direction

Soft electron

Missing electronZ qq/

e+/-

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Zee Signal Definition

With this definition the following MC events contribute to the signal:

• Zee • ee• • ZZ •Main source of background are:

qq, ZZ, WW, Zee

All the data collected at energies between 183 and 207 are taken into account in this analysis

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Other Diagrams

e-

e+

qq /

qq /

e-

e+

e+

e-

qq / Z

Ze+ e-

e+

e-

qq W

We

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Monte Carlo Generators

Channel

qqeeWWee

Monte Carlo

KK2F02KoralW03

Phot02 and Pyth05Pyth05 and KoralW03

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Monte Carlo

The signal simulation is performed using a dedicated 4-fermions final state MC generated by Koral produced to calculate the signal efficiency and MC measured cross-section THANKS TO BRIGITTE !!!

Koral has also the advantage to take properly into account of the interference of all diagrams contributing to the 4-f (ee) final state

The internal cuts of the Koral generator do not affect the analysis since they are beyond the cuts of signal definition

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Cross-section Measurement ()

± 2) % Efficiency

L pb-1 Integrated Luminosity

NDAT = 16 ± 4 events in the Data

NBKG = 1. 87 ± 0.22 estimated bkgnd

(NDAT - NBKGND) / L = 0.056 ± 0.014 ± 0.004 pb

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Selection Cuts

For the qq final state:

CLAS 16 preselection

One isolated electron

Qe cos Z < -0.85

Qe cos miss > 0.9

Emiss/ ELEP > 0.25

Pmiss > 20 GeV

All the definition cuts are also required

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13cos Z

0

BKGND

SIGNAL

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14cos MISS

BKGND

SIGNAL

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Isolated Electron

Only good tracks with the following requirements are considered:• Momentum p > 1GeV• d0 < 2 cm and z0 < 10 cm • At least 4 TPC hits• cos < 0.975

The most isolated track ID as electron is taken into accountFor the electron bremadd is applied The isolation cut on angle between the electron and the closest charged track is: cos < 0.8 (°)

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BKGND

SIGNAL

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Cross-section Measurement (qq)

± 1) % Efficiency

L pb-1 Integrated Luminosity

NDAT = 98 ± 10 events in the Data

NBKG = 21.7 ± 2.0 estimated bkgnd

(NDAT - NBKGND) / L = 0.50 ± 0.06 pb

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Cross-section Measurement (qq)

(GeV) NDATA NBKG L (pb-1) (pb) (pb) (pb) MC(pb)

s

183 4 0.7 21 56.8 0.28 0.14 0.20 0.20

189 17 4.5 21 174.3 0.34 0.10 0.12 0.11

192 5 0.6 22 28.9 0.69 0.31 0.41 0.29

196 11 1.8 21 79.9 0.53 0.18 0.22 0.18

200 13 2.1 22 80.3 0.58 0.18 0.21 0.17

202 9 1.3 23 41.9 0.78 0.27 0.34 0.24

205 10 2.4 25 81.7 0.37 0.14 0.17 0.16

207 29 8.3 23 133.7 0.65 0.15 0.17 0.14

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Other LEP MeasurementsL3 DELPHI

(pb) (pb) (pb) MC(pb) (pb) (pb) (pb) MC(pb)

s

183 0.51 0.19 0.16 0.16 0.56 0.27 0.22 0.24

189 0.55 0.10 0.09 0.09 0.64 0.15 0.14 0.14

192 0.60 0.26 0.21 0.21 0.63 0.40 0.30 0.33

196 0.40 0.13 0.11 0.13 0.66 0.22 0.18 0.19

200 0.33 0.12 0.10 0.14 0.57 0.20 0.17 0.18

202 0.81 0.26 0.23 0.19 0.19 0.19 0.21 0.20

205 0.56 0.16 0.14 0.14 0.37 0.18 0.15 0.19

207 0.59 0.12 0.10 0.11 0.68 0.15 0.14 0.14

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Next Steps

• Understand the small discrepancy at qq mass below the cut

• Compare KoralW with Pythia05 for the ZZ/Zee Background

• Possible review of thebackground estimation in the channel

• Calculate the systematic errors in the qq channel