Standard Model Physics at ATLAS/LHC S. Tokár Comenius Univ., Bratislava On behalf of the ATLAS collaboration 30 August 2004 S. Tokar, HS 2004, Smolenice 1
Outline • LHC and ATLAS performances (Parameters of LHC, ATLAS structure and performances) • Higgs boson physics (very briefly) • Prospects of QCD at 14 TeV Multijets, top production, p.d.f. sensitive processes • Measurements for precision SM physics W and Top mass, constraints on Higgs mass via EW physics • Some top physics topics Single top production, spin effects, anomalous top couplings 30 August 2004 S. Tokar, HS 2004, Smolenice 2
LHC (Large Hadron Collider) • pp collisions at √ s = 14 TeV • Bunch crossing: 25 ns • Low luminosity L= 20 fb -1 • High luminosity L= 100 fb -1 ( ≈ 10 34 cm -2 s -1 ) LHC is top,W,Z, … factory Process σ (mb) Ev./10fb -1 Large statistics for SM W → e ν 15 ~10 8 processes ⇒ Z → e + e − 1.5 ~10 7 • SM precision physics (EW, top-,b-physics, multijets…) 0.8 ~10 7 tt jets 100 ~10 9 • Big potential for new physics (Higgs,SuSy…) (p T >200 GeV) 30 August 2004 S. Tokar, HS 2004, Smolenice 3
ATLAS performance Multi purpose particle detector (coverage up to | η |=5, L=10 34 cm -2 s -1 ) • Inner Detector σ ≈ ⋅ ⊕ p 0 05 . % p GeV ( ) 0 1 . % T T Tracking range | η |< 2.5 • EM Calorimetry σ ≈ ⊕ E 10 % E GeV ( ) 1 % η < Fine granularity up to 2 5 . • Hadronic Calorimetry σ ≈ ⊕ E 50 % E GeV ( ) 3 % η < Range: 4 9 . • Muon System σ − η < p ∼ 2 7 %, range: 2 7 . T Precision physics in | η |<2.5 Magnetic field : Lepton energy scale: 0.02% (Z → ll) 2T Solenoid + 3 air core toroids Jet energy scale: 1.0 % (W → jj) Luminosity precision ≤ 5% 30 August 2004 S. Tokar, HS 2004, Smolenice 4
Higgs boson Production Production of SM Higgs: • Gluon fusion gg → H • Weak boson (W,Z) fusion qq q q H ′ ′ WBF: → • Top-quark associated gg, qq ttH → production • Weak boson associated production qq q q H ′ ′ → Channels for detection: H Z (*) Z (*) 4l, H γγ → → → H W +(*) W -(*) l l +p + - miss → → The cross sections for different H boson T production processes vs M H H τ τ + - ttH, H bb → → 30 August 2004 S. Tokar, HS 2004, Smolenice 5
Higgs couplings To verify Higgs mechanism experimentally: • Higgs mass(es), spin, CP • Higgs widths and couplings to different particles: Hbb, Htt, H , HW W + - + − τ τ , HZZ, Hgg, H , HHH, γγ … Typical accuracies for couplings and widths : 20-30% 10% accuracy for HZZ, HWW couplings over W threshold Systematic errors contribute up to half the total error 5 σ discrepancy from SM up to Precision of Higgs boson couplings m A ≈ 300 GeV (MSSM) determination vs Higgs mass 30 August 2004 S. Tokar, HS 2004, Smolenice 6
QCD measurements • The LHC physics is based on the interactions of quarks and gluons • Factorization : a convolution of partonic x-section and PDF’s: = ∑∫ σ µ µ σ µ µ ( ) 1 ( ) 2 ˆ dx dx F ( x , ) F ( x , ) ( ; s , ) 1 2 i 1 F j 2 F ij F R f • PDF’s are obtained from a global fit of DIS and DY data + DGLAP evolution to higher scales Q 2 → DGLAP splitting functions: theory is at NNLO. • Partonic x-section: perturbative expansion in α S ( LO, NLO, NNLO, …) • Scale choice : µ F = µ R = Q ⇐ typical process scale (usually set by invariant mass or p T of hard probe) � Problems: if two (or more) scales present in the hard scattering -expansion contains: ( α S L 2 ) n and ( α S L) n (L=ln(Q/Q 1 ) process → σ ˆ � Tools: DGLAP + BFKL evolution equations → resummations 30 August 2004 S. Tokar, HS 2004, Smolenice 7
LHC Parton Kinematics 10 9 � Accurate measurements of QCD x 1,2 = (M/14 TeV) exp( ± y) 10 8 Q = M M = 10 TeV related processes at LHC will constrain the PDF’s. 10 7 10 6 M = 1 TeV � The kinematic acceptance of the LHC detectors allows a large range of 10 5 Q 2 (GeV 2 ) x and Q 2 to be probed 10 4 M = 100 GeV Processes to be studied: 10 3 � Multijet physics – test of pQCD, y = 6 4 2 0 2 4 6 10 2 M = 10 GeV dijet physics: constraints on PDF’s fixed 10 1 HERA � Drel-Yan processes , pp γ + − → Z , l l target ( ) q and q densities 10 0 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 10 0 qq γ g → � Direct photon production x Values of x and Q 2 probed in the qg →γ q (sensitive to gluon density) production of an object (mass M, � Top and heavy quark (c,b) production rapidity y) at √ s=14 GeV 30 August 2004 S. Tokar, HS 2004, Smolenice 8
t t Production Cross Section Test of QCD predictions: top pair production (inclusive and differential x-sections) is an effective tool: • big m t ⇒ α S (m t )~0.1 ⇒ pExpansion converges rapidly • top decays before hadronization ⇒ spin of top is not diluted Theory for top X-section: NNLO-NNNLL ( Kidonakis et al., PRD68,114014 (2003) ) • Partonic Xsection: α 2 µ µ ∞ 2 ( ) n s ( ) ∑ ∑ n σ = πα µ η ˆ ( , ) n k k η = − S 4 ( ) f ( )ln 1 i j S ij 2 2 m m 2 4m = = n 0 k 0 Usual scale choice: µ F = µ R = µ ∈ (m t /2, 2m t ) or top p T A discrepancy may indicate a new physics! Progress at MC: radiative gluon corrections included: MCatNLO (Frixione et al, hep-ph/0311223) ATLAS: Statistical uncertainties < 1% → Systematics (Exp.& Theo.) will be dominant 30 August 2004 S. Tokar, HS 2004, Smolenice 9
t t Cross Section at 14TeV σ d dp total T NNLO: uncertainty from scale (m t /2, 2m t ) < 3% !!! (N.Kidonakis, hep-ph/0401147) 30 August 2004 S. Tokar, HS 2004, Smolenice 10
Top Mass Measurement m top ≡ fundamental SM param. ( with m W consistency check of SM Higgs) Top samples (t → Wb): Dileton (W → l ν and W → l ν ), 4.9% Lepton+jets (W → l ν and W → jj ), 29.6% All jets (W → jj and W → jj ), 44.6% Lepton +jets channel Borjanovic et al., SN-ATLAS-2004-040 At prod. level: S/B=10 -5 Sel. cuts : ≥ ≥ miss p ( ) l 20GeV E , 20GeV T T ( ) ≥ > η ≤ ∆ = 4 jets p 40 GeV , 2 5 . , R 0 4 . T ⇒ S/B~78, 8700 tt events /10fb -1 ,2b-tag Invariant mass of jjb ⇒ (b-jets calibtrated using Z+b events, M W window used: ± 20 GeV ) 30 August 2004 S. Tokar, HS 2004, Smolenice 11
Top mass in lepton + jets channel Mass uncertainty Source ∆ m(t)[GeV] light jet energy scale 0.2 • Statistics ~ 0.1 GeV b-jet energy scale 0.7 • Systematics ~ 1.3 GeV ISR 0.1 FSR 1.0 ⇒ b-fragmentation 0.1 Reduction of systematics (FSR, Combinatorial bkgd 0.1 b-jet energy scale) possible via Leptonic part - mass reconstructed via : full tt-bar reconstruction using ν = • missing transverse energy ( ) kinematic fit ⇒ miss E E T T • constraint m(l ν )=M W for neutrino p z Promising: l +J/ ψ channel •Strong correlation between m t & m(l,J/ ψ ) • BR=3.2 × 10 -5 (2700 ev/100fb -1 , sel. ε≈ 16%) • non-sensitive to jet energy, S/B ≈ 55 t → W + b, W + → l ν , b → J/ ψ X 30 August 2004 S. Tokar, HS 2004, Smolenice 12
Top mass in other channels Dilepton channel Selection: 2 isolated leptons (p T >35,25 GeV) High missing E T ( > 40 GeV) 2 b-jets with p T > 25 GeV Neutrino momenta from: conservation laws, kinematic constraints Mass uncertainty: ∆ m stat+rec (t) = 0.3 GeV/c 2 , ∆ m sys (t) = 1.7 GeV/c 2 (pdf, b-fragmentation, b-jet scale, FSR) All jets channel Selection: ≥ 6jets with p T >40 GeV , ≥ 2 b-jets, Small missing E T S/B = 1/19 ⇒ Kinematic fit ( W + top mass constraints used) S/B = 6/1 ⇒ High PT subsample ( p T (t) > 200 GeV) → 3300 evts/10 fb -1 ⇒ S/B = 18/1 Mass uncertainty: ∆ m stat (t) = 0.2 GeV/c 2 , ∆ m sys (t) = 3 GeV/c 2 30 August 2004 S. Tokar, HS 2004, Smolenice 13
W boson mass Present Status: ∆ M W = 0.033 GeV Selection: pp → W+X with W → l ν , l ≡ e, µ • Isolated charge lepton: p T > 25 GeV • missing transverse Energy: E > 25 GeV / T • Rejection of high p T W bosons Method: transverse mass is constructed: M T = 2p p (1-cos φ ) l ν ∆ W T T ∆ϕ ≡ angle (l, ν ) in transverse plane Position of Jacobian falling edge is sensitive to M W Sensitivity is reduced by detector smearing The process x-section at LHC is 30 nb (10 4 × LEP) → after selection and reconstruction 60 M W bosons are expected/ 10fb -1 . Precision of M W is limited by systematics ! ⇒ 30 August 2004 S. Tokar, HS 2004, Smolenice 14
W mass precision Systematics comes mainly from MC modeling (physics, detector performance) Source ∆ M W /channel comments statistics < 2 MeV 60M W’s/year W width 7 MeV pdf < 10 MeV Recoil model 5 MeV Radiative decays < 10 MeV W p T spectrum 5 MeV Backgrounds 5 MeV Lepton identification 5 MeV Lepton E-p scale < 15 MeV Lepton E-p resolution 5 MeV Total < 25 MeV Per channel Combining channels (e, µ ) ⇒ ∆ M W ≈ 20 MeV Combining with CMS ∆ M W ≈ 15 MeV ⇒ Sample size: 10 fb -1 30 August 2004 S. Tokar, HS 2004, Smolenice 15
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