New physics with top events at lhc
This presentation is the property of its rightful owner.
Sponsored Links
1 / 36

New Physics with top events at LHC PowerPoint PPT Presentation


  • 103 Views
  • Uploaded on
  • Presentation posted in: General

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

Download Presentation

New Physics with top events at LHC

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


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


New physics with top events at lhc

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


New physics with top events at lhc

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 [email protected] 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%.


New physics with top events at lhc

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


  • Login