Determination of αs from hadronic event shapes in e+e− annihilation at  GeV

Abstract

Results are presented from a study of the structure of high energy hadronic events recorded by the L3 detector at √ s ≥ 192 GeV. The distributions of several event shape variables are compared to resummed O(α 2 s ) QCD calculations. We determine the strong coupling constant at three average centre-of-mass energies: 194.4, 200.2 and 206.2 GeV. These measurements, combined with previous L3 measurements at lower energies, demonstrate the running of α s as expected in QCD and yield α s (m Z ) = 0.1227 ± 0.0012 ± 0.0058, where the first uncertainty is experimental and the second is theoretical.Submitted to Phys. Lett. B IntroductionThe measurement of the energy dependence of the strong coupling constant, α s , is an important test of Quantum Chromo-Dynamics (QCD). Hadronic events produced in e + e − annihilation offer a clean environment to perform such measurements. The high energy phase of the LEP collider gives a unique opportunity to measure QCD observables over a wide energy range and to perform a precise test of the energy dependence of the strong coupling constant. In addition, the study of hadronic events allows to check the validity of the QCD models used for background modelling in other studies, such as new particle searches.In its last two years, the LEP collider operated at various centre-of-mass energies, √ s, between 192 and 208 GeV. These are grouped in three samples of average centre-of-mass energies 194.4, 200.2 and 206.2 GeV, corresponding to the ranges 192 ≤ √ s < 196 GeV, 200 < √ s < 202 GeV and √ s > 202 GeV respectively.We report on measurements of five event shape distributions using 436.8 pb −1 of data collected with the L3 detector [1] at these centre-of-mass energies, as detailed in Table 1. To allow a direct comparison with our earlier QCD studies at lower energies [2-6], we follow an identical analysis procedure. The value of α s is extracted in each energy range by comparing the measured event shape distributions with predictions of second order QCD calculations [7] supplemented by resummed leading and next-to-leading order terms [8][9][10][11]. These values are used, together with previous L3 measurements at lower effective centre-of-mass energies, from 30 GeV to 189 GeV, to study the energy evolution of α s . Event SelectionThe criteria for the selection of e + e − → qq → hadrons events are identical to those used in our previous QCD study at √ s = 189 GeV [6]. They are based on the measured total visible energy, E vis , the energy imbalances parallel, E , and perpendicular, E ⊥ , to the beam direction and on the cluster multiplicity, N cl . These variables are constructed using energy clusters in the electromagnetic and hadronic calorimeters with a minimum energy of 100 MeV.The efficiency of the selection criteria and the purity of the data sample are estimated using Monte Carlo events for the process e + e − → qq(γ) generated by the KK2F [12] program, interfaced with JETSET PS [13] routines to describe the QCD parton shower evolution and hadronisation. The events are then p...

Inclusive-jet photoproduction at HERA and determination of

“…Table 8, indicates that the performance of the three jet algorithms is similar. These determinations are consistent with previous determinations in NC DIS [5][6][7] at HERA, with the results obtained in pp collisions [70] and have a precision comparable to those obtained in individual determinations from e + e − experiments [71]. These values are also con- sistent with the world average [63], as well as with the HERA 2004 average [72] and the HERA 2007 combined value [73].…”

Event shapes in e+e annihilation and deep inelastic scattering

“…One important result from such studies is an overall value for α s , which in this case is α s (M Z ) = 0.1227 ± 0.0012 ± 0.0058 [98], where the first error is experimental and the second is theoretical. Similar results have been given by other collaborations, e.g.…”

NLO QCD corrections to five-jet production at LEP and the extraction of α s (M Z )

“…At LEP experiments, exclusive processes with up to five jets in the final state were studied in detail; inclusive measurements are available for up to six jets [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Theoretically, e + e − annihilation cross sections into two [17,18] and three [19][20][21] jets are known at next-to-next-to-leading order (NNLO) in perturbative QCD, the production of four jets is known at next-to-leading order (NLO) [22][23][24][25][26], while only leading order (LO) predictions were available so far for five-jet observables.…”