Measurement of the Running of the Electromagnetic Coupling at Large Momentum-Transfer at LEP

Achard, P.; L3 Collaboration

doi:10.1016/j.physletb.2005.07.052

Published 2005 | Version v1

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Measurement of the Running of the Electromagnetic Coupling at Large Momentum-Transfer at LEP

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The evolution of the electromagnetic coupling, alpha, in the momentum-transfer range 1800GeV^2 < -Q^2 < 21600GeV^2 is studied with about 40000 Bhabha-scattering events collected with the L3 detector at LEP at centre-of-mass energies 189-209GeV. The running of alpha is parametrised as: alpha(Q^2) = alpha_0/(1-C Delta alpha(Q^2)), where alpha_0=\alpha(Q^2=0) is the fine-structure constant and C=1 corresponds to the evolution expected in QED. A fit to the differential cross section of the e+e- ->e+e- process for scattering angles in the range |cos theta|<0.9 excludes the hypothesis of a constant value of alpha, C=0, and validates the QED prediction with the result: C = 1.05 +/- 0.07 +/- 0.14, where the first uncertainty is statistical and the second systematic.

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The evolution of the electromagnetic coupling, α , in the momentum-transfer range 1800 GeV2<−Q2<21600 GeV2 is studied with about 40 000 Bhabha-scattering events collected with the L3 detector at LEP at centre-of-mass energies s=189–209 GeV . The running of α is parametrised as: α(Q2)=α01−CΔα(Q2), where α0≡α(Q2=0) is the fine-structure constant and C=1 corresponds to the evolution expected in QED. A fit to the differential cross section of the e+e−→e+e− process for scattering angles in the range |cosθ|<0.9 excludes the hypothesis of a constant value of α , C=0 , and validates the QED prediction with the result: C=1.05±0.07±0.14, where the first uncertainty is statistical and the second systematic.

Other

The evolution of the electromagnetic coupling, alpha, in the momentum-transfer range 1800GeV^2 < -Q^2 < 21600GeV^2 is studied with about 40000 Bhabha-scattering events collected with the L3 detector at LEP at centre-of-mass energies 189-209GeV. The running of alpha is parametrised as: alpha(Q^2) = alpha_0/(1-C Delta alpha(Q^2)), where alpha_0=\alpha(Q^2=0) is the fine-structure constant and C=1 corresponds to the evolution expected in QED. A fit to the differential cross section of the e+e- ->e+e- process for scattering angles in the range |cos theta|<0.9 excludes the hypothesis of a constant value of alpha, C=0, and validates the QED prediction with the result: C = 1.05 +/- 0.07 +/- 0.14, where the first uncertainty is statistical and the second systematic.

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00000 noimg: Luminosity-averaged centre-of-mass energies, $\langle \sqrt{s}\rangle$, and corresponding integrated luminosities, \lum, used in the analysis. The $\rts$ spread in each point is of the order of $1\GeV$. The numbers of observed events, $N_D$, are given together with the total Monte Carlo expectations, $N_{MC}$, and their breakdown into signal, $N_S$, and background, $N_B$, events. The last row lists the average centre-of-mass energy, the total integrated luminosity and the total numbers of events.: Measured, Meas., and expected, Exp., folded differential cross sections for the eight average centre-of-mass energies, $\langle \sqrt{s}\rangle$, and the ten $\|\cos\theta\|$ intervals, with expected average values $\langle\|\cos\theta\|\rangle$. The first uncertainty is statistical and the second systematic.: Combined differential cross sections for the luminosity-averaged centre-of-mass energy $\langle \sqrt{s}\rangle=198\GeV$, compared with the Standard Model expectations, ${\rm d}\,\sigma^{th}(1)/{\rm d}\, \|\cos\theta\|$, and the expectations for the case in which $\alpha$ does not change with $Q^2$, ${\rm d}\,\sigma^{th}(0)/{\rm d}\, \|\cos\theta\|$. The first uncertainties are statistical and the second systematic.: Sources of systematic uncertainty and their effect, $\Delta C$, on the determination of the $C$ parameter. md5:71c026e17d9c48fdd3072eed1f6853bd	9.6 kB	Preview Download
00001 $t$-channel Feynman diagrams contributing to Bhabha scattering. Diagrams with virtual-fermion vacuum-polarisation insertions generate an electromagnetic coupling $\alpha(Q^2)$. The sum of all diagrams including zero, one, two or more vacuum-polarisation insertions is denoted by the diagram to the left with the double-wavy photon propagator. md5:51eda6319668ff8930cc93fbc50b9d14	3.9 kB	Preview Download
00002 Measured Bhabha differential cross-sections for the ten $\|\cos\theta\|$ intervals used in the study as a function of the centre-of-mass energy $\rts$. The Standard Model predictions are represented by the solid lines. md5:792eb79dbb8351a9b4f1a5f591f84c4a	32.5 kB	Preview Download
00003 Measured differential cross-section as a function of $\|\cos\theta\|$. Data at different centre-of-mass energies are combined at the luminosity-averaged centre-of-mass energy $\langle\rts\rangle=198 \GeV$. The predictions in case of a running electromagnetic coupling and for a constant value $\alpha=\alpha_0$ are also shown. md5:d2f8f259b8317eb88e7424c246b46c58	21.9 kB	Preview Download
00004 Ratio between the measured Bhabha differential cross section as a function of $\|\cos\theta\|$ and the Standard Model expectations including a running electromagnetic coupling. Data at different centre-of-mass energies are combined at the luminosity-averaged centre-of-mass energy $\langle\rts\rangle=198 \GeV$. The inner error bars represent statistical uncertainties and the full error bars the sum in quadrature of the statistical and systematic uncertainties. The predictions for a constant value of $\alpha=\alpha_0$ are also shown. md5:446829e3fee01bf9b42fb81da73c737f	22.8 kB	Preview Download
00005 Evolution of the electromagnetic coupling with $Q^2$ determined from the present measurement of $C$ for $ 1800 \GeV^{2} < -Q^{2} < 21600 \GeV^{2}$, yellow band, and from previous data for Bhabha scattering at $2.10\GeV^2<-Q^2<6.25\GeV^2$ and $12.25\GeV^2<-Q^2<3434\GeV^2$~\protect\cite{l3-197}. The open symbols indicate the values of $Q^2$ where $\alpha(Q^2)$ was fixed to the QED predictions\protect\cite{ref:burkhardt_new} in order to infer the values of $\alpha(Q^2)$ shown by the full symbols. These QED predictions are shown by the solid line. md5:2eba8fb3fa58bfa8c329fd8dc2a7aece	20.1 kB	Preview Download
00006 Evolution of the electromagnetic coupling with $Q^2$ determined from the present measurement of $C$ for $ 1800 \GeV^{2} < -Q^{2} < 21600 \GeV^{2}$, yellow band, and from previous data for Bhabha scattering at $2.10\GeV^2<-Q^2<6.25\GeV^2$ and $12.25\GeV^2<-Q^2<3434\GeV^2$~\protect\cite{l3-197}, full symbols. The solid line represent the QED predictions\protect\cite{ref:burkhardt_new}. md5:46b45a1282b9e5e1bcf5474a45bd9943	19.5 kB	Preview Download
Fulltext md5:30f882c8d86300174f07969493b0ac5b	205.2 kB	Preview Download

Additional details

Translated title: MOMENTUM TRANSFER

Approval Report Number: CERN-PH-EP-2005-021
CDS: 855514
CDS Report Number: CERN-L3-301
Aleph number: 002538545CER

Is variant form of: Other: 687622 (Inspire); Other: arXiv:hep-ex/0507078 (arXiv); Other: CERN-PH-EP-2005-021 (CDS Report Number)
References: Other: http://weblib.cern.ch/abstract?CERN-PH-EP-2005-021 (URL)

Submitted: 2005-07-18

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Department: PH - Physics Department
Accelerator: CERN LEP
Experiment: L3

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Measurement of the Running of the Electromagnetic Coupling at Large Momentum-Transfer at LEP

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