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Jet energy measurement with the ALEPH detector at LEP2

Jet energy measurement with the ALEPH detector at LEP2. Precise M w measurement with 4 & 2 quarks final state d (M w stat ) 4lep = 22Mev - Calorimeter calibration - Jet reconstruction (E jet , Q jet ) - Detector simulation - Jet hadronization modelling Also important for :

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Jet energy measurement with the ALEPH detector at LEP2

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  1. Jet energy measurement with the ALEPH detector at LEP2 Precise Mw measurement with 4 & 2 quarks final state d(Mwstat)4lep = 22Mev - Calorimeter calibration - Jet reconstruction (Ejet, Qjet) - Detector simulation - Jet hadronization modelling Also important for : ZZ, Zh, ha, Zg Calor 2002 Jet session

  2. From LEP1 to LEP2 At LEP1 : Jets contents studies - t physics Flavor tagging (lepton-id, pt, gp0 id) QCD through event topology At LEP2 reconstructed jets = partons quadrivector Jet energy , Jet angle, Jet mass 2 jet l n W+W-> qqqq Z g Calor 2002 Jet session

  3. Aleph electromagnetic calorimeter calibration 36 modules : 2 endcaps (12), Barrel (12) - Proportionnal tubes & lead (23X0) - Transverse segmentation 0.9°x0.9° Calibration : - gain monitored by an Fe55 source - shower leakage & saturation from test beam - intercalibration from e= 2-15 GeV from gg->ee * d(E)/E ~ 0.3% - absolute calibration from : Z->ee , e+e-> e+e- s =183->207 GeV * d(E)/E ~ 0.7% * s(E)/E = 0.18/E + 0.009 Module intercalibration Calor 2002 Jet session

  4. Hadronic calorimeter calibration 36 modules with analog & digital read-out: - Iron slates & streamer tubes - 3.7°x3.7° granularity s(E)/E = 0.85/E Calibration - gaz gain monitoring (up to 10%) - intercalibration from muon (0.4%) - absolute calibration: * minimum ionizing particles * d(E)/E ~ 1.4% For systematics: Detector response is: - shaked according to resolution - MC is rescaled to the data Calor 2002 Jet session

  5. E-flow algorithm - Good track resolution - Calorimeter segmentation is used to identify differents contribution: * Charged tracks & identified lepton * g ( and p0) * hadronic neutral * Residual from g, charged hadron Energy resolution from Z->qqg events s(E)/E = (0.59+-0.03)/E + (0.6+-0.3) GeV Expected resolution at high energy is derived Calor 2002 Jet session

  6. Eflow performances Track component (60%) g & neutral hadrons Calor 2002 Jet session

  7. Jet reconstruction Algorithm used Durham: - E-flow object with yij = 2 min(Ei2,Ej2)(1-cosqij)/Ecm2 < ycut associated in the same jet - WW-> q1q2q ’1q ’2 forced into 4 jets Jet performances studied from Z data - Energy response and resolution d(Ejet)/Ejet = 0.67/Ejet (10% at 45 GeV- perfect detector:6-7%) - Angular resolution 0.9° with Eflow charged track only : 1.6°, calorimeters : 1.4° Calor 2002 Jet session

  8. Mw from multi jets events Mw fitting procedure: Rescaling : i Ei = Ecm ,  ui i pi = 0 Jets axis are kept 4C fit : Jet direction varies within errors recursive procedure 5C fit : As 4C with mass equal conditions raw rescaled 4C Fitting procedure implies large use of MC 2-jet mass fitted by changing underlying W mass parameter Mandatory : MC should match data for Ejet, qij, mjet Bias - systematic determination from MC experiments Calor 2002 Jet session

  9. Systematic on Mw from jets Jet energy response - from Z events Angular dependence - Z event compare track / calorimeter axis d(Mw4q) = 5Mev d(Mw4q) = 7Mev Calor 2002 Jet session

  10. Jet mass dependence Data MC For massless partons : Mij = 2EiEj (1-cosqij) But pij # Eij Fragmentation of jets - Detector effect - Particles shared between jets - modelisation (CR) and d(Mw)  mi/Mwd(midata_miMC)qij Massjet(GeV) Current Studies - new methods to evaluate corresponding systematics - detailed MC/Data comparison - additional constraints - alternative jet algorithm Calor 2002 Jet session

  11. Jet mass ongoing studies Jet masses : - multiplicity Charged energy Photon energy Neutral hadron & residual - Jet built cutting low energy residual/neutral - Expected Mw stable statistical loss - coherent with mjet=0 constraint (21% loss) mjet=0 Calor 2002 Jet session

  12. Jets systematics shared between experiments At LEP all experiments uses same hadronization scheme d(Mw) = 48 MeV fragmentation & color reconnection baryon content Common systematic Colour reconnection in WW->4q channel / WW-> lnqq ’ Jet fragmentation comparing several MC ( Ariadne-Herwig- Jetset) affect jet masses = angular opening of jets affect jets direction qjet Mw from 4q low weight in the combination (27%) Calor 2002 Jet session

  13. Alternative Jet algorithm R Design analysis to be less sensitive to FSI - Interjet particles association - fragmentation scheme - Color reconnection - BE effect ( 5MeV) Algorithm excluding particle from interjet region Loss in statistic Calor 2002 Jet session

  14. Jets algorithm comparison pdur Dp q pcone R=0.75 Dp - Reduced sensitivity to CR - CR related systematic down by factor 2 Calor 2002 Jet session

  15. Conclusions -Detailed studies made to improve systematic on Mw estimation robustness for summer 4q lnqq Calorimeter calibration 4 Mev 5MeV Jet calibration 7 MeV 10MeV Jet angle 5 Mev 4 MeV Calorimeter simulation 10MeV 15MeV FSI( CR,BE, Fragm) 48MeV 20MeV - At LEP2 the fitting method relies upon the agreement between Data/MC - The shower simulation in calorimeter has to be understood in detail to fully exploit the kinematic constraints Calor 2002 Jet session

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