PRECISION MEASUREMENTS

Transcription

1 PRECISION MEASUREMENTS AT Z RESONANCE Z Lineshape and number of neutrinos Lecture 2 1 October 212 Shahram Rahatlou Fisica Nucleare e Subnucleare III, Anno Accademico

2 EVENT SELECTION e + e - pairs reconstructed in electromagnetic calorimeter ALEPH μ + μ - pairs in muon chambers typically in the outer layer quarks produce jets with multiple tracks and energy deposits in both electromagnetic and hadronic calorimeters τ + τ - events have missing energy and multiple topologies 1 prong decays with 1 e/mu/pi 3 prong decays require visible energy to be below total center-of-mass energy 1 E 8 6 (GeV) ch N ch 1 Figure 2.1: Experimental separation of the final states using onlytwovariables,thesu the track momenta, E ch,andthetrackmultiplicity,n ch,inthecentraldetectoroftheal experiment. 22

3 SOURCES OF BACKGROUND Non-resonant ee ϒϒ q anti-q interaction uniform cross section across spectrum Data driven method to estimate background in signal sample Lower momentum spectrum compared to signal Event not balanced along the beam line Negligible machine background τ + τ - nisi FNSN II 8-29 because of missing neutrino energy can mimic ee/μμ final states selection based on total visible energy helps reducing mis-identification Mode Fraction (Γ i /Γ) Co Modes with one charged particle Γ 1 particle neutrals K ν τ ( 1-prong ) (85.35±.7) % Γ 2 particle neutrals K L ν τ (84.72±.7) % Γ 3 µ ν µ ν τ [a] (17.36±.6) % Γ 4 µ ν µ ν τ γ [b] ( 3.6 ±.4 ) 1 3 Γ 5 e ν e ν τ [a] (17.84±.6) % Γ 6 e ν e ν τ γ [b] ( 1.75±.18) % Γ 7 h K L ν τ (12.3±.11) % Γ 8 h ν τ (11.75±.11) % Γ 9 π ν τ [a] (11.6±.11) % Γ 1 K ν τ [a] ( 6.86±.23) 1 3 Γ 11 h 1 neutralsν τ (36.92±.14) % Γ 12 h π ν τ (25.87±.13) % Γ 13 π π ν τ [a] (25.41±.13) % Γ 14 π π non-ρ(77)ν τ ( 3. ±3.2 ) 1 3 Γ 15 K π ν τ [a] ( 4.54±.27) 1 3 Γ 16 h 2π ν τ (1.76±.15) % Γ 17 h 2π ν τ ( 9.39±.14) % Γ 18 h 2π ν τ (ex.k ) ( 9.23±.14) % Γ 19 π 2π ν τ (ex.k ) [a] ( 9.17±.14) % Γ 2 π 2π ν τ (ex.k ), < scalar Γ 21 π 2π ν τ (ex.k ), vector < Γ 22 K 2π ν τ (ex.k ) [a] ( 5.8 ±2.3 ) 1 4 Γ 23 h 3π ν τ ( 1.37±.11) % Γ 24 h 3π ν τ ( 1.21±.1) % Γ 25 π 3π ν τ (ex.k ) [a] ( 1.8±.1) % 23

4 SAMPLE PURITY ALEPH DELPHI L3 OPAL qq finalstate acceptance s /s >.1 s /s >.1 s /s >.1 s /s >.1 efficiency [%] background [%] e + e final state acceptance.9 < cos θ <.7 cos θ <.72 cos θ <.72 cos θ <.7 s > 4m 2 τ η < 1 η < 25 η < 1 efficiency [%] background [%] µ + µ final state acceptance cos θ <.9 cos θ <.94 cos θ <.8 cos θ <.95 s > 4m 2 τ η < 2 η < 9 m 2 /s >.1 ff efficiency [%] background [%] τ + τ final state acceptance cos θ <.9.35 < cos θ <.94 cos θ <.92 cos θ <.9 s > 4m 2 τ s > 4m 2 τ η < 1 m 2 /s >.1 ff efficiency [%] background [%] Monte Carlo simulations used to estimate detector acceptance and signal efficiency Reject initial state radiation with s /s requirement 24

5 CROSS SECTION MEASUREMENT Nsel: selected number of events σ = N sel N bg ɛ sel L Nbg: estimated background very small at LEP and well measured efficiency & acceptance: well under control through detailed detector simulation Luminosity:.5% uncertainty Very precise measurement of luminosity with bhabha scattering cross section known exactly biggest limitation: detector acceptance at very small angles (2-6 mrad) 25

6 HADRONIC CROSS SECTION $ had [nb] 4 3 ALEPH DELPHI L3 OPAL $ 2 % Z 1 measurements (error bars increased by factor 1) $ from fit QED corrected.4 M Z E cm [GeV] Similar energy dependence for muon and tau cross sections 26

7 INTEREFERENCE OF t- CHANNEL IN ee 1 L3 e + e! # e + e! (") 44 o < + < 136 o $ [nb].5 s-channel t-channel interference 1.5 ratio *s [GeV] no peaking structure in cross section due to t-channel 27

8 Sh. Rahatlou 28 INVISIBLE WIDTH AND # OF NEUTRINOS Γ inv = Γ Z (Γ had + Γ ee + Γ µµ + Γ ττ ) R inv = N ν ( Γνν Γ ll R inv = ( 12πR l σ had m2 Z ) SM ) 1 2 R l (3 + δ τ ) Parameter Average Correlations [MeV] Γ ff $ had [nb] Without Lepton Universality Γ had Γ ee Γ µµ Γ ττ Γ bb Γ cc Γ inv Γ had ± Γ ee ± Γ µµ ± Γ ττ 84.8 ± Γ bb ± Lepton Universality! Γ cc 3.5 ± Γ inv ± ALEPH DELPHI L3 OPAL average measurements, error bars increased by factor 1 2& 3& 4& E cm [GeV] N ν = ±.82 δn ν 1.5 δn had n had 3. δn lep n lep 7.5 δl L Largest uncertainty due to luminosity: ~.46 on N ν

9 Sh. Rahatlou 29 DIRECT MEASUREMENT OF # NEUTRINOS Nucl.Phys.Proc.Suppl.85:67-71,2 d 2 σ de γ d cos θ γ = H(E γ, cos θ γ,s)σ (s ) Experimental signature: empty detector with 1 photon only! Main background: radiahve Bhabha scajering electron and positrons lost in the beam line Requirements σ (s) = 12π M 2 Z sγ e N ν Γ ν (s M 2 Z )2 + s 2 Γ 2 Z /M 2 Z Good trigger capability for low energy photons, s is the square of th s = s(1 2E γ / s). n by: +W terms( Good hermihcity at very low angles to discriminate background few % of dominant term at Z pole

10 RESULTS OF DIRECT MEASUREMENT LEP1 results on N ν with the neutrino counting method Acceptance Y ear L dt Ndata N back Γ inv N ν N ν E γ in GeV pb 1 in MeV Aleph cosθ < E γ > 1.5 ±34 ± 34 ±.2 ±.2 Delphi cosθ < E γ > 3. ±.32 ±.19 L3 cosθ < E γ > 1. ±12 ± 12 ±.7 ±.7 Opal cosθ < E γ > 1.75 ±26 ± 17 ±.16 ±.1 N ν =2.98 ±.7(stat) ±.7(syst) number of events / (.5 GeV) Data $$% signal ee% backgr. %%%+2% backgr. L3 ' (pb) L N $ =4 N $ =3 N $ =2 data E % (GeV) Sh. Rahatlou &s (GeV) 3

11 Sh. Rahatlou 31 WHY TWO METHODS? Different stapspcs indirect method relies on radiahve correchons - - > low stahshcs If New Physics exists we should measure same value with both methods Remember: indirect method measures what we DO NOT see If parhcle X exists indirect method would measure N < 3 exohc Z decays to undetectable parhcles accounted for as invisible Direct method measures explicitly decays to neutrinos Two dispncpve signatures in direct method photon energy spectrum cross sechon vs s: peak cross sechon at slightly higher energy than Z mass New exohc parhcles could result in peaks beyond Z