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New Physics with top events at LHC

New Physics with top events at LHC. M. Cobal, University of Udine IFAE, Pavia, 19-21 April 2006. Studying the top. Is it ‘standard’ physics?  Discovered 10 years ago.. but still so little known about it… Large mass: unique features for investigation of EW symmetry

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New Physics with top events at LHC

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  1. New Physics with top events at LHC M. Cobal, University of Udine IFAE, Pavia, 19-21 April 2006

  2. Studying the top Is it ‘standard’ physics?  Discovered 10 years ago.. but still so little known about it… • Large mass: unique features for investigation of EW symmetry breaking and physics beyond SM  Key for revealing new physics at the LHC? LHC: top-factory. NLO production cross-section ~830 pb. at L=21033:  2 tt events per second !  more than 10 million tt events expected per year: perfect place for precision physics

  3. Beyond the SM  non-SM production (Xtt)  resonances in the tt system  MSSM production  unique missing ET signatures from  non-SM decay (tXb, Xq)  charged Higgs  change in the top BR, can be investigated via direct evidence or via deviations of R(ℓℓ/ℓ)=BR(Wℓ) from 2/9 (H+,cs).  FCNC t decays: tZq tq tgq  highly suppressed in SM, less in MSSM, enhanced in some sector of SEWSB and in theories with new exotic fermions  non-SM loop correction  precise measurement of the cross-section  ttNLO-ttLO/ ttLO <10% (SUSY EW), <4% (SUSY QCD) typical values, might be much bigger for certain regions of the parameter space  associated production of Higgs  ttH

  4. xBR required for a discovery σxBR [fb] 30 fb-1 830 fb 300 fb-1 mtt [GeV/c2] 1 TeV Resonances • ts< 10-23 s  no ttbar bound states within the SM • Many models include the existence of resonances decaying to ttbar • SM Higgs(but BR smaller with respect to the WW and ZZ decays) • MSSM Higgs(H/A, if mH,mA>2mt, BR(H/A→tt)≈1 for tanβ≈1) • Technicolor Models, strong ElectroWeak Symmetry Breaking, Topcolor Clear experimental signature and ability to reconstruct top also make it a useful “tool” for studying exotica  Shown sensitivity up to a few TeV ATLAS: study of a resonance Χ once known σΧ, ΓΧ and BR(Χ→tt) Reconstruction efficiency for semileptonic channel:  20% mtt=400 GeV  15% mtt=2 TeV

  5. Resonances • Re-do with full simulation testing:  sensitivity  mass resolution • X tt  WbWb  lnbjjb topology was studied (X being a `generic', narrow resonance) s(ppX)xBR(Xtt) [fb] 5s discovery potential Mx = 800 GeV

  6. L New Physics in tbW • Event selection • ≥ 4 jets with PT > 20 GeV and |h| < 2.5 • ≥ 1 lepton with PT > 25 GeV and |h| < 2.5 • 2 b-tagged jet • ETmiss > 20 GeV • |Mjj –MW| < 100 GeV • |Mjjb-MT| < 200 GeV Signal efficiency: 8.7% SM background ~ 40k evts (~30k from ttbar with a t and ~10k from single top L = 10 fb-1

  7. New Physics in tbW: AFB • New asymmetries defined: A± • AFB = 0.2234 ± 0.0035(stat) ± 0.0130(sys) [s/AFB = 6.0%] • A+ = -0.5472 ± 0.0032(stat) ± 0.0099(sys) [s/A+ = 1.9%] • A- = 0.8387 ± 0.0018(stat) ± 0.0028(sys) [s/A- = 0.4%]

  8. New Physics in tbW:W polarization AFB = a0(FL-FR) = 0.2226 (LO) A+ = a1Fl – a2F0 = -0.5482 (LO) A- = -a1FR +a2F0 = 0.8397 (LO) (FL,FR,F0 defined as in SN-ATLAS-2005-052 Fi = width for a certain W polarization

  9. AFB A+ A- New Physics in tbW L = 10 fb-1 Limits on the anomalous couplings: Mb taken into account

  10. Top quark FCNC decay • GIM suppressed in the SM • Higher BR in some SM extensions (2-Higgs doublet, SUSY, exotic fermions) • 3 channels studied:

  11. Probabilistic approach • Preselection • General criteria: • ≥ 1 lepton (pT > 25 GeV and |h| < 2.5) • ≥ 2 jets (pT > 20 GeV and |h| < 2.5) • Only 1 b-tagged jet • ETmiss > 20 GeV • Events classified into different channels (qZ, qg or qg) • Specific criteria for each channel • After the preselection, probabilistic analysis:

  12. tqZ • Specific criteria: • ≥ 3 leptons • PTl2,l3 > 10 GeV and |h|<2.5 • 2 leptons same flavour and opposite charge • PTj1 > 30 GeV • 453.8 backgnd evts,e x BR = 0.23% L = 10 fb-1 Mjl+l- Mlnb

  13. tqg • Specific criteria: • 1 photon • PT > 75 GeV and |h|<2.5 • 20 GeV < mgj < 270 GeV • < 3 leptons • 290.7 backgnd evts,e x BR = 1,88% L = 10 fb-1 Mgj PTg

  14. tqg • Specific criteria: • Only one lepton • No g with PT > 5 GeV • Evis > 300 GeV • 3 jets (PT1 > 40 GeV, PT2,3 > 20 GeV and |h| < 2.5) • PTg > 75 GeV • 125 < mgq < 200 GeV • 8166.1 backgnd evts,e x BR = 0,39% L = 10 fb-1 Mlnb Mgq

  15. Likelihood L = 10 fb-1 • Discriminant variable: LR = ln(Ls/LB) qZ channel  qg channel  qg channel 

  16. Results • BR 5s sensitivity • Expected 95% CL limits on BR (no signal) • Dominant systematics: MT and etag < 20%

  17. M(qZ) M(qg) tqZ, tqg, Preliminary • Studying tt events with full sim • Reconstruct Z(g) and then constrain the SM leg • Put together q-jet and Z(g) to give a top

  18. Present and future limits ATLAS/CMS combination will improve the limit

  19. Preliminary H±tb • Heavy charged Higgs in MSSM • m2H = m2A+m2W • Charged Higgs is considered heavy: mH > Mt+mb • MSSM in the heavy limit no decay into sparticles • No production through cascade sparticle decay considered • Decay mainly as H±tb • Difficult jet environment • BR depends on mA and tanb

  20. Search strategies for H±tb • Resolving 3 b-jets: inclusive mode • LO production through gb tH± • Large background from tt+jets • High combinatorics • Resolving 4 b-jets: exclusive mode • LO production through gg tH±b • Smaller background (from ttbb and ttjj+ 2 mistags) • Even higher combinatorics • Both processes simulated with Pythia; same cross section if calculated at all orders • gbtH±: massless b taken from b-pdf • gg tH±b: massive b from initial gluon splitting • Cross sections for both processes as the NLO gbtH±: cross section

  21. Search for 4 b-jets • Signal properties • Exponential decrease with mA • Quadratic increase with tanb in interesting region tanb > 20 • Final state: bbbbqq’ln • Isolated lepton to trigger on • Charged Higgs mass can be reconstructed • Only final state with muon investigated • Background simulation • ttbb • ttjj • (large mistag rates, large cross section) • b’s from gluon splitting passing theshold of ttbb generation)

  22. Significance and Reach • Kinematic fit in top system • Both W mass constraints • Both top mass constraints • Neutrino taken from fit • Event selection and efficiencies 4 4

  23. Significance and Reach • Significance as function of cut on signal-background • Due to low statistics interpolation of number of background events as function of number of signal events • Optimization performed at each mass point

  24. H±tb • Fast simulation • 4 b-jets analysis • No systematics (apart uncertainty on background cross sec) • Runninng mb • B-tagging e static L = 30 fb-1

  25. SUSY Virtual effects • It is possible to detect virtual Electroweak SUSY Signals (=VESS) at LHC (=ATLAS,CMS) ?? • Tentative answer from a theory-experiment collaboration (!) • M. Beccaria, S. Bentvelsen, M. Cobal, F.M. Renard, C. Verzegnassi Phys. Rev. D71, 073003, 2005. • Alternative (~equivalent) question: it is possible to perform a “reasonably high” precision test of e.g. the MSSM at LHC (assumed preliminary SuSY discovery…)? • Wise attitude: Learn from the past! • For precision (= 1 loop) tests, the top quark could be fundamental via its Yukawa coupling!

  26. If SUSY is light.. • Briefly: if e.g. All SuSY masses  MSuSY  M  400 GeV, from an investigation of ds/dMtt for Mttbar  1 TeV, “SuSY Yukawa” might be visible because of Sudakov logarithmic expansions • (~valid for Mttbar >> M, Mt that appear at 1-loop) Diagrams for ew Sudakov logarithmic corrections to gg ttbar

  27. Few details.. • A few details of the preliminary approximate treatment: • Assume MSuSY 400 GeV • Compute the real (i.e. With PDF..) ds/dMtt = usual stuff (see paper..) • Take qqbar  ttbar in Born approximation ( 10% s ) and compute to 1 loop gg ttbar for Mttbar  1 TeV (0.7 TeV  Mttbar  1.3 TeV) • Separate tLtbarL + tRtbarR = “parallel spin” from tLtbarR + tRtbarL = “anti-parallel spin”

  28. % Effect • 10-15% effect (for large tanb) in the  1 TeV region (“modulo” constant terms, that should not modify the shape) • What are the systematics uncertainty to be compared with?

  29. Experimental study • 106 tt events generated with Pythia, and processed through the ATLAS detector fast simulation (5 fb-1) • Selection: • At least ONE lepton, pT >20 GeV/c , || > 2.5 • At least FOUR jets pT >40 GeV/c , || > 2.5 Two being tagged b-jets • Reconstruct Hadronic Top |Mjj-MW |< 20 Gev/c ; |Mjjb-Mt |< 40 Gev/c • Reconstruct leptonic Top |Mjj-MW |< 20 Gev/c ; |Mjjb-Mt |< 40 Gev/c • Resulting efficiency: 1.5% Mtt (TeV)

  30. Higher order QCD effects • NLO QCD effects (final state gluonradiation, virtual effects) spoil the equivalence of Mtt with √s • The tt cross section increases from 590 to 830 pb from LO to NLO • Also the shape gets distorted by NLO effects • Effects of NLO QCD has been investigated using MC@NLO Monte Carlo (incorporates a full NLO treatment in Herwig) • Mtt distributions generated in LO and NLO and compared • Mttvalue obtained at parton level,as the invariant mass of the top and anti-top quark, after both ISR and FSR. The LOand NLO total cross sections are normalised to each other.

  31. Higher order QCD effects • Deviations from unity entirely due to differences in Mtt shape • Relative difference between √s and Mtt remains bounded (below roughly 5%) when √s varies between 700 GeV and 1 TeV (chosen energy range). • For larger √s, the difference raises up to a 10 % limit when √s approaches what we consider a realistic limit (√s =1,3 TeV) LO/NLO Mtt (TeV)

  32. Systematic Uncertainties • Main sources: • Jet energy scale uncertainty • Uncertainties of jet energy development due to initial and final state showering • Uncertainty on luminosity • Jet energy scale: • A 5% miscalibration energy applied to jets, produces a bin-by-bin distorsion of the Mtt distribution smaller than 20%. • Overestimate of error, since ATLAS claims a precision of 1% • Luminosity • Introduces an experimental error of about 5%. At the startup this will be much larger.

  33. Systematic Uncertainties • ISR and FSR • The Mtt distribution has been compared with the same distribution determined with ISR switched off. Same for FSR. • Knowledge of ISR and FSR: order of 10%, so systematic uncertainty on each bin of the tt mass was taken to be 20% of the corresponding difference in number of evts obtained comparing the standard mass distribution with the one obtained by switching off ISR and FSR • This results in an error < 20% • Overall error • An overall error of about 20-25% appears realistically achievable • Does not exclude that further theoretical and experimental efforts might reduce this value to a final limit of 15-10%.

  34. ttH The Yukawa coupling of top to Higgs is the largest.  It is a discovery mode of the Higgs boson for masses less than 130 GeV  Measuring the coupling of top to Higgs can test the presence of new physics in the Higgs sector  Very demanding selection in a high jet multiplicity final state 0.7 pb (NLO) mH=120GeV ttjj: 507 pb ttZ: 0.7 pb ttbb: 3.3 pb

  35. Higgs boson reconstruction  Reconstruct ttH(h)  WWbbbb  (l)(jj)bbbb  Isolated lepton selection using a likelihood method  Jet reconstruction: 6 jets at least, 4 of which b-tagged  Reconstruct missing ET from four-momentum conservation in the event (+W mass constraint in z)  Complete kinematic fit to associate the two bs to the Higgs (can improve the pairing efficiency to 36%, under investigation) results can be extrapolated to MSSM h gttH/gttH~16% for mH=120 GeV hep-ph/0003033

  36. Conclusions • ATLAS sensitivity to ttbar resonances: • 5s discovery (mX = 1 TeV/c2, L=30 fb-1); s x BR ~ 103 fb) • ATLAS sensitivity to new physics in the t  bW decay: • Mb should be taken into account • gRЄ [-0.02,0.02]  factor 2-3 better than the present limits • Improvements expected combining semi and dileptonic channels • LHC sensitivity to FCNC decays (L=100 fb-1, 5s significance) • BR(t  qZ)~10-4 • BR(t  qg)~10-5 • BR(t  qg)~10-3 • Improvement combining ATLAS and CMS • Sensitivities at the level of SUSY and Quark Singlets models predictions • Top production at LHC might be sensitive to ew SUSY effects, particularly for “light SuSY”, large tanb and LARGE INVARIANT MASSES

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