Standard Model at LHC

Transcription

1 Standard Model at LHC Eric COGNERAS, LPC Clermont-Ferrand On behalf of the ATLAS and CMS collaborations HEP-MAD 07, Antananarivo, Madagascar 2007, september 14

2 Motivation SM successfully tested at current energy LHC prospect physics at higher energy : Able to search directly beyond SM But also precise measurements of SM parameters Goal for SM physics: Deeper understand the SM with : Precision EW measurements (M W, sin 2 θ W, ) Top physics (mass,cross section, measurement) Indirect estimate the Higgs mass Ui Using M W, m Top, sin 2 θ W precise measurement Search for deviation from SM u c t d s b e µ τ ν e ν µ ν τ γ W Z g 14/09/2007 Eric COGNERAS - Standard LHC 2

3 The LHC Experimental Framework CERN (Geneva) pp s = 14 TeV (10 Tevatron) Luminosity : EW precision measurement Low luminosity phase L cm -2 s -1 ( ?) [10 fb -1 /year : 10 TV] High luminosity phase L cm -2 s -1 (2010?-X) [100 fb -1 /year : 100 TV] Experiments ATLAS and CMS general purpose physics LHCb b physics ALICE heavy ion physics 14/09/2007 Eric COGNERAS - Standard LHC 3

4 Experimental Framework Tracking ( η <2.5, B=2T) : Si pixels and strips Transition i Radiation i Detector (e/π separation) Calorimetry ( η <5) : EM : Pb/LAr with Accordeon shape HAD : Fe/scintillator (central), Cu-W/Lar (forward) MuonSpectrometer ( η <2.7) : air-core toroids with muon chambers Tracking ( η <2.5, B=4T) : Si pixel and strips Calorimetry ( η <5) : EM: PbWO 4 crystals HAD : brass/scintillator (central,end-cap), Fe/Quartz (forward) MuonSpectrometer ( η <2.5) : return yoke of solenoid instrumented with muon chambers 14/09/2007 Eric COGNERAS - Standard LHC 4

5 x 1 x 2 Hadron collider problem : PDF to be better known p-p collisions Cross section and Event rate Process σ (nb) Evt/y (Low L:10 fb -1 ) Minimum Bias Inclusive jets (p T >200 GeV) W lν (l=e,µ) Z e + e tt bb or 2 orders of magnitude larger than Tevatron LHC is a W, Z, top factory : small statistical errors in precision measurements large samples for studies of systematic effects (calibration and syst. controls) 14/09/2007 Eric COGNERAS - Standard LHC 5

6 SM LHC W/ Z physics (precision EW measurement) Parameters related to indirect M H measurement : W mass and width sin²θ W Constraints on the PDF W/Z inclusive cross section as well as W/Z+jets W rapidity Measurement of gauge boson pair production Triple Gauge Boson Coupling Top physics Parameters related to indirect M H measurement : Top mass/cross section Deeper understanding of Top quark : Top spin correlation, probe of the Wtb vertex, single Top cross section, Top charge QCD ( see Albert talk on tuesday) Higgs boson direct search B physics 14/09/ Eric COGNERAS - Standard LHC

7 Precision EW Measurement M W measurement LHC expects : δm W < 15 MeV W channel : same as the Tevatron using W lν decay Event selection: q 1 isolated lepton (e,µ) p T > 25 GeV in η <2.4 q missing E T > 25 GeV No jet with p T > 30 GeV Recoil u < 20 GeV But LHC statistics is higher (60M recons. W low Lumi [10 TV]) Several methods available: R=M T W / M T Z spectrum Small syst, sample /10 p Tl spectrum pile-up, theo. know. of P T W M T W high stat, pile-up 14/09/ Eric COGNERAS - Standard LHC

8 Precision EW Measurement M W measurement Use Transverse mass to cope with unmeasured p ν L M T W = T T = l υ l, 2 p p (1 cos Δ φ ν ), l = e, μ p Tν from p Tl & recoil Use the knowledge on the Z boson to constrain the W mass lz, M Z lw, PT = PT M l W p Tl spectrum shape is sensitive to M W p Tν not necessary W mass : fit exp. shape to MC sample with different Values of M W 14/09/ Eric COGNERAS - Standard LHC

9 Precision EW Measurement M W measurement : systematics LHC goals : W lν : one lepton species, lowl L, per experiment, after 1 year source CDF RunII, combined channel LHC 200 pb -1 60Mevts, 10fb -1 stat. error : negligible Statistics 0 MeV <2 MeV Lepton scale 17 MeV 15 MeV Energy resolution 3 MeV 5 MeV Recoil model 12 MeV 5 MeV syst. error : MC modelling of phys. detector responses physics Lepton id MeV P T W 3 MeV 5 MeV W PDF 11 MeV 10 MeV W width MeV Radiative decays 11 MeV <10 MEV P T W spectrum PDF W width W rad. decays Background Z ll LHC data Z ll,r,tevatron Theo. calculation MC detector Background 0 MeV 5 MeV TOTAL 26 MeV <25 MeV lepton E&p scale Lepton resolution Recoil Z ll, E/p for e ± Z ll, E/p for e ± Z ll Combining channels and ATLAS/CMS exp., should reach ΔM W 15 MeV 14/09/ Eric COGNERAS - Standard LHC

10 Precision EW Measurement Determination of sin²θ W Use A FB in p-p Z/γ* l + l - Parity violation in neutral current Asymmetry in the angular distribution of leptons from Z decay (θ-dependence of cross-section) Test of the Standard Model (universality) At the Z pole, A FB comes from the interference of vector and axial component of the coupling All events Events with quark direction 2 A ( sin ) correctly estimated FB = b a θw Where a, b calculated to NLO QED and QCD Assumption for p-p p colliders : the quark direction is the same as the boost of the Z Correct for large di-lepton rapidities Only EM calorimeters provide the required large η-coverage (Z e + e-) 14/09/ Eric COGNERAS - Standard LHC

11 Precision EW Measurement Determination of sin²θ W Event selection: p Te > 20 GeV 85.2 < M ee < 97.2 GeV Results L = 100 fb -1 y cuts e + e - A FB sin 2 θ W ( y(z) > 1) (Stat) (Stat) y( l 1,2 ) < x x 10-4 y( l 1 ) < y( l 2 ) < x x 10-4 Sensitivity increases with forward electrons Only Forward Electrons All Events Current error on world average 1.6x10-4 need small systematic error : PDF uncertainty, precise knowledge of lepton acceptance and efficiency effects of higher order QCD 14/09/ Eric COGNERAS - Standard LHC

12 Precision EW Measurement Triple gauge boson coupling Probe b the non abelian structure t of fsm q q s-channel LO Feynman diagram : V1, V2, V3 = Z, W, γ WW, ZW, Wγ Diboson final states have predictable σ production and manifest the gauge boson coupling In SM, only charged coupling WW γ and WWZ are allowed 14 possible WWγ and WWZ coupling Use 5 independent, CP conserving, EM gauge invariance preserving couplings : g 1Z, κ γ, κ Z, λ γ, λ Z TGC manifest in : At SM tree level, g 1Z =κ γ =κ Z =1 and λ γ =λ Z =0 λ γ and λ Z grow with s big advantage for LHC Δκ Z γ = κ γ -1, Δg 1Z = g Z 1-1, Δκ Z = κ Z -1growwith s with Cross section enhancement High pt(v=w, Z, γ) Production angle 14/09/ Eric COGNERAS - Standard LHC

13 Precision EW Measurement Triple gauge boson coupling Exemple of WZ production Use only leptonic final state Event selection: Exactly 3 leptons with pt>25 GeV At least one pair of leptons with same flavour and opposite charge and mll-mz <10 GeV SM Δg 1Z = 0.05 Coupling Present Value LHC Sensitivity (95% CL, 30 fb-1, 1 exp) Δg Z Δκ γ γ Δκ Z λ γ λ Z /09/ Eric COGNERAS - Standard LHC

14 QCD-oriented Measurement Measurement of W/Z cross sections Estimation of the cross section N σ ( pp Z / W + X μμ) = Candidates (1 - fbackground ) Event selection: Z : 2 isolated µ, p µ T > 20 GeV, η <2, 84<M µµ <99 GeV,... W :1isolated µ, p µ Miss T > 25 GeV, η <2, 40<M T (µ,e T )<200GeV,... ε total Ldt Results (L=1 fb-1 [~600k Z μμ, ~6M W μν]) σ(z μμ + X) = 1160 ± 1.5 (stat) ± 27 (syst) ± 116 (lumi) pb σ(w μν + X) = ± 6 (stat) ± 485 (syst) ± 1470 (lumi) pb Already dominated by systematics Systematics come mainly from theory (acceptance+pdf uncertainty). At a later stage these processes can be used as luminosity it monitor (Error on luminosity: it ~5%) Z μμ CERN/LHCC CMS TDR 8.2 W μν 14/09/ Eric COGNERAS - Standard LHC

15 QCD-oriented Measurement Constraints on PDF using W rapidity distributions At LHC, experimental uncertainty t is dominated by systematics (large event production) The theoritical uncertainties are dominated by PDFs Exp. uncertainty sufficiently small to distinguish between different PDF sets PDF error sensitive to W eν rapidity distribution e ± rapidity spectrum shape sensitive to gluon shape parameter (valence quark density) Probe low-x gluon PDF at Q²=M 2 W PDF uncertainties only slightly degraded after detector simulation and selection cuts dσ(w W eν)/d y dσ(w W eν)/dy e - rapidity Generated CTEQ61 MRST02 ZEUS02 Reconstructed ud W du W + + e ν e ν e + rapidity Generated CTEQ61 MRST02 ZEUS02 Reconstructed y y 14/09/ Eric COGNERAS - Standard LHC

16 Top Quark Physics Direct discovery in 1995 (Fermilab) Completes the 3 family structure of the SM Very high h mass 175 GeV Production cross-section Resonance production Production kinematics Top Spin Top Mass Top Width Top Charge τ had = Λ -1 QCD >> τ decay No Top Hadron : Opportunity to t b measure parameters of a free quark Top Spin _ Polarization t Y Production at LHC: tt production Rare/non SM Decays Branching Ratios W + X W helicity l + ν Anomalous Couplings CP violation + (90%) (10%) V tb σ ~ 14TeV (NLO) 8M Low Lum single top production t-channel σ ~ 250 pb Wt-channel σ ~70pb W* (s-channel) σ ~ 10 pb 14/09/2007 Eric COGNERAS - Standard LHC 16

17 tt Physics Decay : Determined by decay of Ws Fully hadronic channel (44 %) Di-leptonic channel (5 %) Semi-leptonic channel (30 % no τ) Golden Channel Selection : Only e/µ events Trigger large event yield small backbround 4 jets (ΔR=0.4) p T >40 GeV p T >20 (25) GeV 2 b-jets W(h (had) t (had) p T >20 GeV t (lep) 14/09/2007 Eric COGNERAS - Standard LHC 17

18 tt Physics Top quark mass measurement (Invariant mass spectrum of reconstructed Had. Top : most straightforward technique) Without b-tagging (early data) With b-tagging Event topology: 3 jets with highest p T Event s Selection efficiency : ε=5.3 53% Numb ber of Event ts (ATL-PHYS-PUB ) 40 L=100 pb -1 ( cm -2 s -1 ) Full simulation Signal (MC@NLO) W+n jets (Alpgen) + combinatorial M jjj (GeV) S/B=O(100) Very small SM Bck L=O(100) pb -1 (few cm -2 s -1 ) Top signal W+jets background Selection efficiency : ε=1-2% Top mass (GeV) In this way, Δm(top) ~1.3 GeV When Kinematical Fit (using the leptonic side) is used, Δm(top) ~1 GeV 14/09/2007 Eric COGNERAS - Standard LHC 18

19 tt Physics tt cross section CERN/LHCC Semi-leptonic channel (tt bbqq μν l ): L=10fb -1 CMS TDR 8.2 Δσ tt /σ tt = 9.7%(syst)±0.4%(stat)±3%(lum) ε reco = 6.3% Di-leptonic channel (tt bb lν l lν l ): L=10fb -1 Δσ tt /σ tt = 11%(syst)±0.9%(stat)±3%(lum) ε reco = 5% (S/B=5.5) With Tau leptons (tt bb τν τ lν l, τ hadrons): Δσ tt /σ tt = 16%(syst)±1.3%(stat)±3%(lum) [ ε reco =2%, S/B~1, ε τ-tag =30% ] Fully Hadronic channel: L=1fb -1 Δσ tt/σ tt = 20%(syst)±3%(stat)±5%(lum) ( ) ( ) ε reco =2%, S/B<1/9 (QCD) 14/09/2007 Eric COGNERAS - Standard LHC 19

20 Top polarization Test the t bw decay vertex tt Physics Measure W polarization (F 0, F L, F R ) through lepton angular distribution in W cm system: (1/Γ)dΓ Γ/dcos(θ l *) cos(θ l *) L=10fb -1 Semilep. + Dileptonic (Eur.Phys.J.C44S ) F 0 F L F R SM (M t =175 GeV) (m b =0) Error (±stat ±syst) ± ± ± ± ± ± Syst ( E b-jet,m top,fsr ) δf 0 / F 0 ~ 2% ; δf R ~ /09/2007 Eric COGNERAS - Standard LHC 20

21 Production cross section single Top Physics Common feature: (ATL-PHYS-PUB ) L=30fb -1 1 lepton, pt>25gev/c High Missing E T 2 jets (at least 1 b-jet) Separate Channels by (N j,n b ) in final state: t-channel: Stat: 7000 events (S/B=3) (N j =2,N b =1) Δσ/σ= 12%(syst)±1%(stat)±5%(lum) Wt-channel: Stat: ε~1% (S/B=15%) (N j =3,N b =1) Δσ/σ= 14%(syst)±1.5%(stat)±5%(lum) s-channel: Stat: 1200 events for tb (S/B=10%) (N j =2,N b =2) Δσ/σ= 16%(syst)±12%(stat)±5%(lum) L=10fb -1 (CERN/LHCC , CMS TDR 8.2) t-channel: Δσ/σ= 8%(syst)±2.7%(stat)±5%(lum) Wt-channel:Δσ/σ= 23.9%(syst)±8.8%(stat)±9.9%(MC) 8%(stat)±9 9%(MC) s-channel: Δσ/σ= 31%(syst)±18%(stat)±5%(lum) 14/09/2007 Eric COGNERAS - Standard LHC 21

22 Conclusion Precision measurements are possible with hadron collider, as demonstrated by the Tevatron LHC will prospect higher energy physics Detector to be understood with early data Higher luminosity it Better S/B ratio LHC goals are ambitious δm W < 15 MeV δm top < 1 GeV δsini 2 θ ~10 W -4 But reachable as soon as the detectors performances and the systematics will be understood 14/09/ Eric COGNERAS - Standard LHC

23 Back-up Slides

24 Precision EW Measurement M W measurement Simple and powerful in principle p : consider p l T spectrum correlation between Z and W decay CMS NOTE 2006/061 stat. error negligible (~2 MeV) BUT need to predict the spectrum precisely! 14/09/ Eric COGNERAS - Standard LHC

25 tt Physics Kinematical Fit (use leptonic side) Minimization of a χ² function with constraints on W and Top masses 2 χ = fit 2 PDG PDG fit 2 M jj M W M M l MW jjb M ν h Top M jjb M l Top σ W σ + W σ t σ t meas fit meas fit meas fit meas fit ηi η i ϕi ϕ i Ei E i Pi ν P i ν i i i i jets σ = η σ ϕ i= jets, lepton σe i= x, y, z σi ν Reduces FSR systematics Cleaner event sample (using a cut on χ²) measure m top as a function of χ² uncertainty Δm(top) [GeV] Hadronic Top Δm(top) [GeV] Kinematical Fit light-jet tnergy scale (1%) b-jet nergy scale (1%) FSR TOTAL ) 2 (GeV/c fit M top fit M top = χ ATLAS Preliminary χ 2 14/09/2007 Eric COGNERAS - Standard LHC 25

26 Top spin correlation tt Physics Testing the tt Production cross-sectionsection Although t and t are produced unpolarized their spins are correlated A= σ(t L t L ) + σ(t R t R ) - σ(t L t R ) - σ(t R t L ) σ(t L t L ) + σ(t R t R ) + σ(t L t R ) + σ(t R t L ) SM: A(LO) A(NLO) Other angular distributions: q t φ lq t l +,ν 1 dn 1 = (1 α N dcosφ 2 A Dα X α X cosφ ) A D (LO) SM: α X =spin analysing A D (NLO) power of fx 14/09/2007 Eric COGNERAS - Standard LHC 26

27 tt Physics Top spin correlation A) Spin correlations and angular distributions: L=10fb -1 SM M tt <550 GeV Error (±stat ±syst) A 0.42 ±0.014 ±0.023 A D ±0.008 ±0.010 (Eur.Phys.J.C44S ) Semileptonic + Dileptonic Syst (E b-jet,m top,fsr) ~4% precision (CERN/LHCC , 021 CMS TDR 8.2) B) Spin Asymmetries can also be used (X-check) t q θ l,ν l,νν Ã xx N(cosφ >0) - N(cosφ <0) 1 N(cosφ > 0) + N(cosφ < 0) 2 θ q t (for L=10fb -1 precision A,A D below 10% ) (hep-ex/ , subm. to Eur.Phys.J.C) A = - A D α X α x 14/09/2007 Eric COGNERAS - Standard LHC 27

28 tt Physics Probe the Wtb vertex B) Anomalous Couplings in the t bw decay (PRD67 (2003) , m b 0) Angular Asymmetries: A FB, A + and A - A FB [t=0] A ± [t= (2 2/3-1)] ± A - SM(LO): A + A FB cos(θ l *) 14/09/2007 Eric COGNERAS - Standard LHC 28

29 tt Physics Probe the Wtb vertex B) Anomalous Couplings in the t bw decay 1σ Results: SM(LO) : ρ L =0.423 ρ R = (m b 0) 14/09/2007 Eric COGNERAS - Standard LHC 29