t th production at the LHC

Transcription

1 t th production at the LHC Laura Reina Spring Institute 04: High-energy physics after LHC Run I March 04

2 Outline Motivations for t th studies. First results coming from LHC s Run I data, most expected from Run II data. Difficult channel: Run I results are encouraging and show how theoretical systematic uncertainty could become a limitation. Review of recent theoretical progress and ongoing studies. Outlook and conclusions.

3 Motivations [GeV] MH pp ZH (NNLO QCD +NLO EW) pp tth (NLO QCD) pp WH (NNLO QCD + NLO EW) σ(pp H+X) [pb] pp qqh (NNLO QCD + NLO EW) 0 0 pp H (NNLO+NNLL QCD + NLO EW) s= 8 TeV s [TeV] LHC HIGGS XS WG 0 σ(pp H+X) [pb] 0 0 Small cross section that grows substantially from 7 8 to 4 TeV. For MH = GeV (and including NLO corrections): s = 7 TeV: σ(t th) 86 fb s = 8 TeV: σ(t th) 30 fb s = 4 TeV: σ(t th) 6 fb pp tth (NLO QCD) pp WH (NNLO QCD + NLO EW) pp ZH (NNLO QCD + NLO EW) pp H (NNLO+NNLL QCD + NLO EW) pp qqh (NNLO QCD + NLO EW) LHC HIGGS XS WG 03

4 Motivations After the discovery of a Higgs-boson at GeV, the focus is on precision measurements of its couplings. g t g h σ t th σ SM t th t = κ t t th gives direct access to the top-higgs Yukawa coupling: crucial to disentangle new physics from measurements of the ggh and γγh couplings. g γ,z γ,z t, X H H H g γ γ σ ggh σ SM ggh = κ g(κ t,κ b,m H,X) Γ γγh Γ SM γγh = κ γ(κ t,κ b,κ W,m H,X)

5 κ g. ATLAS SM - s = 7 TeV Ldt = fb Best fit - s = 8 TeV Ldt = 0.7 fb 68% CL.8 9% CL Combined H γγ, ZZ*, WW* See studies in: ATLAS-CONF-0-7, and arxiv: CMS-PAS-HIG--00 κ γ Notice: hard to constrain κ t from (κ g,κ γ ) fit, direct κ t measurement is crucial sign of κ t cannot come from t th H +t production (see Biswah, Gabrielli, Mele, arxiv:.0499, pp tq +H tq +γγ)

6 New: study of spin correlation in t th Spin-correlation effects can be used to distinguish scalar vs pseudoscalar associated production, i.e. SM from non-sm effects Artoisenet, Frederix, Mattelaer, Rietkerk, arxiv:.3460 and can be very visible in decay product s kinematic distributions, Ellis, Hwang, Sakurai, Takeuchi, arxiv:3.736 and even more can be used to improve the separation of signal (t th) and some irreducible backgrounds (e.g. t tγγ) (Biswah,Frederix, Gabrielli, Mele, arxiv: ) See B. Mele s talk

7 Results for t th from LHC Run I ATLAS CMS t th,h b b : ATLAS-CONF-0-3 t th,h γγ : ATLAS-CONF t th,h b b,τ + τ : CMS-PAS HIG-3-09 t th,h γγ : CMS-PAS HIG-3-0 With a data set of about fb at s = 7 TeV and 0fb at s = 8 TeV, they observe: a 9% C.L. upper limit of.4 (CMS) and.3 (ATLAS) times σ SM in the H γγ channel; 9% C.L. upper limit of. (CMS) times σ SM in the H b b channel; first measurement of signal strength: σ/σ SM = (combining H b b,γγ,τ + τ, and assuming SM Br).

8 Events / GeV Data ATLAS preliminary SR+CR background fit SM signal (m = 6.8 GeV) H SR-only background fit Signal region Leptonic channel tth /σ SM 9% CL limit on σ tth Observed CL s limit Expected CL s limit ± σ H γγ tth channels comb. ATLAS preliminary ± σ Data 0 s = 8 TeV Ldt = 0.3 fb Control region - s = 8 TeV Ldt = 0.3 fb [GeV] m γγ m H [GeV] γγ bb CMS Preliminary - s = 7 TeV, L =.0 fb ; - s = 8 TeV, L = 9. fb κ f 3. - CMS Preliminary s = 8 TeV, L = 9. fb tth, H bb, ττ, γγ, WW, ZZ m H =.7 GeV/c ln L Hadronic ττ 6 4l 4 3l. 0 Same-Sign l Combination Best fit σ/σ SM at m H =.7 GeV κ V

9 Main limitations All signal and background samples used in experimental analyses still do not include NLO QCD corrections; Irreducible backgrounds introduce large uncertainty (e.g. t tb b, t tjj for H b b) 4 jets 0 b tags jets 0 b tags 6 jets 0 b tags 4 jets b tags jets b tags 6 jets b tags 4 jets b tags jets b tags 6 jets b tags 4 jets 3 b tags jets 3 b tags 6 jets 3 b tags 4 jets 4 b tags ATLAS Preliminary (Simulation) m H = GeV jets tt+hf jets 4 b tags tt+light jets ttv W+jets Z+jets Diboson Single top Multijet 6 jets 4 b tags Events Data/MC CMS Preliminary - s = 8 TeV, L = 9. fb Lepton + 6 jets + 4 b-tags tth() x 30 tt + lf tt + cc tt + b tt + bb Single t tt + W,Z EWK Bkg. Unc. Data best Higgs mass t th: PYTHIA t t+jets: ALPGEN+HERWIG (ATLAS), MADGRAPH+PYTHIA (CMS) t tw/z: MADGRAPH+PYTHIA

10 t th: towards more accurate theoretical predictions NLO QCD corrections to pp t th from: Beenakker et al. (arxiv:hep-ph/00708, arxiv:hep-ph/03) Dawson et al. (arxiv:hep-ph/0070, arxiv:hep-ph/0438) used to estimate the theoretical uncertainties currently used in Higgs searches Higgs Cross Section Working Group (HXSWG-t th) (First Yellow Report, arxiv:0.09) σ(pp tth) [fb] µ =m + M H / 0 t s=4 TeV s=7 TeV M H [GeV] LHC HIGGS XS WG 00 m H GeV, s = 4 TeV δσ NLO scale (%) [+.9, 3.3] δσ NLO PDF+αs ±8.9 where scale: µ 0 / < µ < µ 0 PDF:MSTW08, CTEQ6.6, NNPDF.0

11 Matched at NLO to Parton Shower Monte Carlo generators NLO calculation (by Dawson et al.) interfaced with Parton Shower Monte Carlo generators (PYTHIA/HERWIG) within POWHEG-BOX Sherpa and successfully compared to (HELAC-NLO+POWHEG-BOX) Garzelli, Kardos, Trócsányi ; Jäger, Hartanto, Reina, Wackeroth Les Houches Higgs Working Group (03) for a standard choice of selection cuts, and assuming H γγ (all decays implemented through the PS MC), p jet T > 0 GeV, yjet < 4. p l T > 0 GeV, y l <. R l,jet > 0.4

12 dσ dp [fb],h ratio to POWHEG-Box SHERPA p,h [GeV] dσ dy [fb] H ratio to POWHEG-Box SHERPA y H dσ dp [fb],t ratio to POWHEG-Box SHERPA p,t [GeV] dσ [fb] dyt ratio to POWHEG-Box SHERPA y t

13 dσ dp [fb],j 0 0 POWHEG-Box dσ dp [fb],l 0 0 POWHEG-Box POWHEG Box p,j [GeV] POWHEG Box p,l [GeV] dσ [fb] d Rj j POWHEG Box POWHEG-Box R j j dσ dn [fb] POWHEG Box POWHEG-Box N (# of jets)

14 Independent calculation from also successfully compared with (both t th and t ta) Garzelli, Kardos, Trócsányi ; Frederix (HXSWG-t th, Yellow Report II, arxiv:0.3084)

15 Background: t tb b NLO QCD corrections to pp t tb b calculated in: Bredenstein et al. (arxiv: , arxiv:090.00, arxiv: ) Bevilacqua et al. (arxiv: ) Bevilacqua et al. (arxiv: ): ratio t tb b/t tjj See Bevilacqua s talk updated in the context of HXSWG-t th ( s = 7,8 GeV) (Yellow Report 3, arxiv: ) dσ/dm [fb/gev] dσ NLO /dσ LO 0 0 LO, s = 8 TeV NLO, s = 8 TeV Sherpa+OpenLoops m b b [GeV] LHC HIGGS XS WG 03 dσ/d R[fb] LO, s = 8 TeV NLO, s = 8 TeV Sherpa+OpenLoops R b b Now interfaced with PS Monte Carlo (Sherpa) in the context of OPENLOOP+Sherpa Cascioli, et al (arxiv:309.9) dσ NLO /dσ LO LHC HIGGS XS WG 03

16 Powhel: t th vs t tb b HELAC-NLO calculation (Bevilacqua et al.) interfaced with PS Monte Carlo using POWHEG Kardos, et al.(arxix:303.69) Garzelli, et al. (HXSWG, Yellow Report 3, arxix: ) dσ dm [fb/gev] b b (a) +Pythia t th +Pythia t tb b LHC Higgs XS WG 03 dσ dm [fb/gev] b b (b) +Pythia t th +Pythia t tb b LHC Higgs XS WG s = 4TeV s = 4TeV m b b [GeV] m b b4 [GeV]

17 Outlook and Conclusions t th crucial player in precision measurement of Higgs couplings: rich potential when all properties are considered See B. Mele s talk today First measurements from Run I of the LHC very promising. Awaiting Run II for much larger statistics. Great progress in implementing existing NLO QCD calculations into PS Monte Carlos, available to the experiments for future analyses. Complex backgrounds like t tb b now thoroughly under examination as benchmark process for several cutting-edge multijet automated NLO calculators. See G. Bevilacqua s talk on Friday