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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

New Physics with top events at LHC

M. Cobal, University of Udine

IFAE, Pavia, 19-21 April 2006

studying the top
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

beyond the sm
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

resonances

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

resonances1
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

new physics in t bw

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

new physics in t bw a fb
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%]
new physics in t bw w polarization
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

new physics in t bw1

AFB

A+

A-

New Physics in tbW

L = 10 fb-1

Limits on the anomalous couplings:

Mb taken into account

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

t q z
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

t q g
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

t q g1
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

likelihood
Likelihood

L = 10 fb-1

  • Discriminant variable: LR = ln(Ls/LB)

qZ channel 

qg channel 

qg channel 

results
Results
  • BR 5s sensitivity
  • Expected 95% CL limits on BR (no signal)
  • Dominant systematics: MT and etag < 20%
t q z t q g

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
present and future limits
Present and future limits

ATLAS/CMS combination

will improve the limit

slide19

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
search strategies for h tb
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
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)
significance and reach
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 reach1
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
slide24
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

susy virtual effects
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!
if susy is light
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

few details
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”
effect
% 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?
experimental study
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)

higher order qcd effects
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.
higher order qcd effects1
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)

systematic uncertainties
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.
systematic uncertainties1
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%.
slide34
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

higgs boson reconstruction
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

conclusions
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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