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

Studying the top LHC: top-factory. NLO production cross-section ~830 pb. at L=2  :  2 tt events per second !  more than 10 million tt events expected per year: perfect place for precision physics 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?

Beyond the SM  non-SM production (X  tt)  resonances in the tt system  MSSM production  unique missing E T 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   tt NLO -  tt LO /  tt LO <10% (SUSY EW), <4% (SUSY QCD) typical values, might be much bigger for certain regions of the parameter space  associated production of Higgs  ttH

Resonances  s < 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 m H,m A >2m t, 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 ATLAS: study of a resonance Χ once known σ Χ, Γ Χ and BR(Χ→tt) Reconstruction efficiency for semileptonic channel:  20% m tt =400 GeV  15% m tt =2 TeV  xBR required for a discovery m tt [GeV/c 2 ] σxBR [fb] 30 fb fb -1 1 TeV 830 fb  Shown sensitivity up to a few TeV

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

New Physics in t  bW L Event selection ≥ 4 jets with P T > 20 GeV and |h| < 2.5 ≥ 1 lepton with P T > 25 GeV and |h| < b-tagged jet E T miss > 20 GeV |M jj –M W | < 100 GeV |M jjb -M T | < 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

New Physics in t  bW: A FB New asymmetries defined: A ± A FB = ± (stat) ± (sys) [  /A FB = 6.0%] A + = ± (stat) ± (sys) [  /A + = 1.9%] A - = ± (stat) ± (sys) [  /A - = 0.4%]

New Physics in t  bW:W polarization A FB = a 0 (F L -F R ) = (LO) A + = a 1 F l – a 2 F 0 = (LO) A- = -a 1 F R +a 2 F 0 = (LO) (F L,F R,F 0 defined as in SN-ATLAS F i = width for a certain W polarization

New Physics in t  bW A FB A+A+ A-A- L = 10 fb -1 Limits on the anomalous couplings: Mb taken into account 1  limits2  limits V R Є[-0.10,0.15]V R Є[-0.14,0.19] g L Є[-0.08,0.05]g L Є[-0.10,0.07] g R Є[-0.02,0.02]g R Є[-0.04,0.04]

Top quark FCNC decay GIM suppressed in the SM Higher BR in some SM extensions (2-Higgs doublet, SUSY, exotic fermions) 3 channels studied: BR in SM2HDMMSSMR SUSYQS t  qZ ~ ~10 -7 ~10 -6 ~10 -5 ~10 -4 tqtq ~ ~10 -6 ~10 -9 t  qg ~ ~10 -4 ~10 -5 ~10 -4 ~10 -7

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

tqZtqZ Specific criteria:  ≥ 3 leptons PTl2,l3 > 10 GeV and |h|<2.5 2 leptons same flavour and opposite charge PTj1 > 30 GeV backgnd evts,  x BR = 0.23% L = 10 fb -1 M jl+l- M l b

tqtq Specific criteria:  1 photon PT > 75 GeV and |  |<2.5  20 GeV < m  j < 270 GeV  < 3 leptons backgnd evts,  x BR = 1,88% MjMj L = 10 fb -1 PTPT

tqgtqg Specific criteria:  Only one lepton  No  with P T > 5 GeV  E vis > 300 GeV  3 jets (P T1 > 40 GeV, P T2,3 > 20 GeV and |h| < 2.5)  P Tg > 75 GeV  125 < mgq < 200 GeV backgnd evts,  x BR = 0,39% L = 10 fb -1 M l b M gq

Likelihood Discriminant variable: L R = ln(L s /L B ) qZ channel  q  channel  qg channel  L = 10 fb -1

Results BR 5  sensitivity Expected 95% CL limits on BR (no signal) Dominant systematics: M T and  tag < 20% t  qZtqtq t  qg L = 10 fb x x x10 -3 L = 100 fb x x x10 -3 t  qZtqtq t  qg L = 10 fb x x x10 -3 L = 100 fb x x x10 -4

t  q Z, t  q , Studying tt events with full sim  Reconstruct Z(  ) and then constrain the SM leg  Put together q-jet and Z(  ) to give a top Preliminary   /  STAT S  S/ S  SYST  L/ LBR MAX (FCNC) t  qZ4.3%6%17.252%4% t  q  2.0%7%22.523%4% M(qZ) M(q  )

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

H ±  tb Heavy charged Higgs in MSSM  m 2 H = m 2 A +m 2 W  Charged Higgs is considered heavy: m H > M t +m b  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 m A and tan  Preliminary

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

Search for 4 b-jets Signal properties  Exponential decrease with m A  Quadratic increase with tan  in interesting region tan  > 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)

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

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

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

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, , 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!

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

Few details.. A few details of the preliminary approximate treatment: 1) Assume M SuSY  400 GeV 2) Compute the real (i.e. With PDF..) d  /dM tt = usual stuff (see paper..) 3) Take qqbar  ttbar in Born approximation (  10%  ) and compute to 1 loop gg  ttbar for M ttbar  1 TeV (0.7 TeV  M ttbar  1.3 TeV) 4) Separate t L tbar L + t R tbar R = “parallel spin” from t L tbar R + t R tbar L = “anti-parallel spin”

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

Experimental study 10 6 tt events generated with Pythia, and processed through the ATLAS detector fast simulation (5 fb -1 ) Selection: –At least ONE lepton, p T >20 GeV/c, |  | > 2.5 –At least FOUR jets p T >40 GeV/c, |  | > 2.5 Two being tagged b-jets –Reconstruct Hadronic Top |M jj -M W |< 20 Gev/c ; |M jjb -M t |< 40 Gev/c –Reconstruct leptonic Top |M jj -M W |< 20 Gev/c ; |M jjb -M t |< 40 Gev/c Resulting efficiency: 1.5% M tt (TeV)

Higher order QCD effects NLO QCD effects (final state gluon radiation, virtual effects) spoil the equivalence of M tt 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 Monte Carlo (incorporates a full NLO treatment in  Mtt distributions generated in LO and NLO and compared  Mtt value obtained at parton level, as the invariant mass of the top and anti-top quark, after both ISR and FSR. The LO and NLO total cross sections are normalised to each other.

Higher order QCD effects  Deviations from unity entirely due to differences in M tt shape  Relative difference between √s and M tt 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) M tt (TeV) LO/NLO

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 M tt 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.

Systematic Uncertainties ISR and FSR  The M tt 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%.

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 0.7 pb (NLO) m H =120GeV  Very demanding selection in a high jet multiplicity final state ttjj: 507 pb ttZ: 0.7 pb ttbb: 3.3 pb

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 E T 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)  g ttH /g ttH ~16% for m H =120 GeV hep-ph/ results can be extrapolated to MSSM h

Conclusions ATLAS sensitivity to ttbar resonances:  5  discovery (m X = 1 TeV/c 2, L=30 fb -1 );  x BR ~ 10 3 fb) ATLAS sensitivity to new physics in the t  bW decay:  M b should be taken into account  g R Є [-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, 5  significance)  BR(t  qZ)~10 -4  BR(t  q  )~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 tan  and LARGE INVARIANT MASSES