Hadronic events in e + e -

Transcription

1 Hadronic events in e + e - Hadronic cross-section, asymmetry (Very short on) Accelerators and detectors Events in the continuum; below, above and at the Z Event selection, ISR WW events Selection of heavy-uark initiated jets Gluon tag Unfolding Why e + e -? Point-like particles in the initial state Initial state centre-ofmass energy well determined Wide c.m. energy range experimentally explored A. De Angelis Int. PhD school on Strong Interactions and Multiparticle Dynamics, Bologna 003 A. De Angelis 003 Annihilation into hadrons e + e - -bar Phase : Phase : Phase 3: Phase 4: Electroweak (Z decay into -bar) Parton Shower (see the lectures by Calucci, Treleani) Fragmentation (models, see Abbiendi, Fabbri, Giovannini, Dremin, Kittel) Resonance decays (precision spectroscopy)

2 A. De Angelis 003 Cross-section 3 + σ ( e e α s R = Nc Q all uarks 3 π In the continuum (i.e., far from resonances), when the γ dominates R had bb cc 0.55 ; 0.0 ; 0.35 total had had + + ) = R σ ( e e µ µ ) σ ( ee + µ + µ ) 86.8 nb s (GeV ) Above the Υ threshold, care about Z threshold (σ had >> σ point ) WW, ZZ A. De Angelis 003 When the Z dominates... 4 had = ( a + v ) all uarks + v I3 = had 0.70 ; total a v where for d, s, b and bb had for u, c 0. ; = I cc had 3 a 4Q sin θ = I W B f e - θ e + F fbar σ had ~ 30 nb Forward-Backward asymmetry 0.0 ; A ( + cos θ ) ( f ) dσ ( f ) 3 ( f ) = σ cos cos + AFB θ total d θ 8 3 ( f ) vea v e f a f At tree level AFB (Z) = 4 v + a v + a A ( b) FB ( c) FB 0.06 e e f f

3 A. De Angelis VHE accelerators in the continuum e + e - Years C.m. energy Expts Lumi (pb - /exp.) PETRA GeV TASSO, JADE, CELLO PEP GeV MarkII,DELCO, MAC,TPC,HRS TRISTAN GeV AMY, TOPAZ SLC GeV SLD, (Mark II) 0 LEP GeV ALEPH, L3 50 at the Z (60) DELPHI,OPAL 700 above the Z The Z channel goes much beyond 9. GeV... A. De Angelis Detectors Typical structure of collider detector: central tracking, external calorimetry and then µ-chambers 3

4 A. De Angelis 003 Hadron identification: long-lived charged hadrons 7 For K +, p based on de/dx (mostly < ~ GeV) Ring Imaging CHerenkov where available Performance computed from data p from Λ, π from K 0 s 0 p (GeV/c) A. De Angelis 003 Hadron identification: weakly decaying, resonances 8 For strange particles (cτ ~ cm), based on secondary vertices well separated from the primary Example: V0 (Λ and K 0 s ) eff. ~30%, pur. ~90% For resonances, based on invariant mass plots => large combinatorial background 4

5 A. De Angelis 003 The LEP detectors 9 A. De Angelis 003 How to select hadronic events? At the Z 0 Very easy (and minimum bias ) Ennemies are leptonics (low multiplicity) and γγ, machine background (Tranverse) charged energy > ~0% E CM Number of charged particles > ~5 4 with momentum (or P T ) > ~00 MeV 4 well measured 4 coming from the primary vertex Typical purities > 99.7% losses ~ 30% >4 MZ/experiment (σ had ~ 30 nb) 5

6 A. De Angelis 003 Above the Z, the paradise is lost... Low cross-section (~0. GeV) ISR reduces E cm (80% of the events are below 0.85 E cm ) Background from WW, ZZ, γγ E cm (GeV) L/exp (pb - ) Events >.85E cm A. De Angelis 003 How to select hadronic events above the Z? s ' < s Effective (i.e., after ISR) c.m. energy computed from momentum conservation in the hypothesis that the ISR photon is lost in the beam pipe => possible bias 6

7 A. De Angelis 003 Contamination from WW, ZZ 3 Contamination from WW: ~7 pb ZZ: ~pb Forward region: γγ: dominant, but easier to cut misunderstood Bhabhas Based on topology => possible bias Typical eff. ~ 80%, pur. ~80% A. De Angelis 003 WW events 4 X-section ~ 4/9 decay into 4, ~ 4/9 decay into lν Experimental detection: For 4, eff. ~80%, pur. ~75% For, eff. ~60%, pur. ~90% 7

8 A. De Angelis 003 Experimental selection of b-bbar events 5 Main experimental techniue: based on VD Si VD resolution: ~0 µm << cτ b Cut on the b-tag probability (probability that none of the tracks in a jet/event comes from a vertex displaced form the primary) Efficiency can be computed directly from data (from tracks at impact parameter < 0) Typical performance on a b jet: eff. 40%, pur. 90% Can also select c-jets (worse performance) To avoid bias, measure the jet opposite to the tagged one! Another techniue: high-p, p T lepton b -> c W - -> c l - ν gives also the uark charge from the lepton sign, but lower efficiency A. De Angelis 003 Gluon jets - experimental selection 6 Q and g: different topology, different energy distribution (gluons come from bremsstrahlung => lower E) To get rid of possible bias select symmetric topologies Y -fold Mercedes 3-fold anti-tag the jets based on long lifetime OR leptons Typical performance (on M Z) 50,000 Y ( 0,000 g jets with purity ~ 80%), E ~ 0 GeV 5,000 Mercedes (,000 g jets with purity ~ 80%), E ~ 30 GeV OPAL proposes a techniue with even smaller bias: Study events with gluon and jets ~ // recoiling against it Only ~ 00 g jets/mz, but Very good QCD objects with E ~ 4 GeV 8

9 A. De Angelis The data correction procedure Truth Measured value How to unfold for detector effects and extract the true values for the observables? Truth gen. level Measured value Measured value in MC If we can neglect migration Truth = Measured value generator level Measured value in MC Correction factor A. De Angelis Unfolding can be more complicated... Subtracting the remaining background from the data distributions can be non-trivial Sometimes we can t neglect migration effects Example: We measure n charged tracks; how many where the charged tracks in reality? It must be an even number, and not always the same => Correction matrices, with regularization problems n n- n n+ n+4 9