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

New Physics with top events at LHC

M. Cobal, University of UdineM. Cobal, University of Udine

IFAE, Pavia, 19-21 April 2006IFAE, Pavia, 19-21 April 2006

Studying the top

LHC: top-factory. NLO production cross-section ~830 pb. at L=21033: 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 (Xtt)

resonances in the tt system MSSM production

unique missing ET signatures from

non-SM decay (tXb, Xq) charged Higgs

change in the top BR, can be investigated via direct evidence or via deviations of R(ℓℓ/ℓ)=BR(Wℓ) from 2/9 (H+,cs). FCNC t decays: tZq tq tgq

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-tt

LO/ 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

tbttttg ~

, ~ ,~ ~1

02,1

Resonances s< 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

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

xBR required for a discovery

mtt [GeV/c2]

σxB

R [

fb]

30 fb-1

300 fb-1

1 TeV

830 fb

Shown sensitivity up to a few TeV

Resonances

Mx = 800 GeV

(ppX)xBR(Xtt) [fb]

5 discovery potential

Re-do with full simulation testing: sensitivity mass resolution

X tt WbWb lbjjb topology was studied

(X being a `generic', narrow resonance)

New Physics in tbW: AFB

New asymmetries defined: A±

AFB = 0.2234 ± 0.0035(stat) ± 0.0130(sys) [/AFB = 6.0%]

A+ = -0.5472 ± 0.0032(stat) ± 0.0099(sys) [/A+ = 1.9%]

A- = 0.8387 ± 0.0018(stat) ± 0.0028(sys) [/A- = 0.4%]

)12()()(

)()( 3

2

xcon

xxNxxN

xxNxxNA

New Physics in tbW: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-2005-052Fi = width for a certain W polarization

New Physics in tbW

AFB

A+

A-

L = 10 fb-1

Limits on the anomalous couplings:

Mb taken into account

1 limits 2 limits

VRЄ[-0.10,0.15] VRЄ[-0.14,0.19]

gLЄ[-0.08,0.05] gLЄ[-0.10,0.07]

gRЄ[-0.02,0.02] gRЄ[-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 SM 2HDM MSSM R SUSY QS

tqZ ~10-14 ~10-7 ~10-6 ~10-5 ~10-4

tq ~10-14 ~10-6 ~10-6 ~10-6 ~10-9

tqg ~10-12 ~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:

N

i

backndiB

N

i

signalis

PL

PL

1

1

tqZ

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, x BR = 0.23% L = 10 fb-1

Mjl+l- Mlb

tq Specific criteria:

1 photon PT > 75 GeV and ||<2.5

20 GeV < mj < 270 GeV < 3 leptons

290.7 backgnd evts, x BR = 1,88%

Mj

L = 10 fb-1

PT

tqg Specific criteria:

Only one lepton No 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, x BR = 0,39%

L = 10 fb-1

MlbMgq

Likelihood

Discriminant variable: LR = ln(Ls/LB)

qZ channel

q channel

qg channel

L = 10 fb-1

Results

BR 5 sensitivity

Expected 95% CL limits on BR (no signal)

Dominant systematics: MT and tag < 20%

tqZ tq tqg

L = 10 fb-1 5.1x10-4 1.2x10-4 4.6x10-3

L = 100 fb-1 1.6x10-4 3.8x10-5 1.4x10-3

tqZ tq tqg

L = 10 fb-1 3.4x10-4 6.6x10-5 1.4x10-3

L = 100 fb-1 6.5x10-5 1.8x10-5 4.3x10-4

tqZ, tq,

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/ SSYST L/ L BRMAX(FCNC)

t qZ 4.3% 6% 17.2 52% 4% 26 10-4

t q 2.0% 7% 22.5 23% 4% 4.5 10-4

M(qZ) M(q)

Present and future limits

ATLAS/CMS combination

will improve the limit

H±tb

Heavy charged Higgs in MSSM m2

H = m2A+m2

W

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 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

gbtH±: massless b taken from b-pdf gg tH±b: massive b from initial gluon splitting Cross sections for both processes as the NLO gbtH±: cross section

Search for 4 b-jets

Signal properties Exponential decrease with mA

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

44

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 mb

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, 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..

Briefly: if e.g. All SuSY masses MSuSY M 400 GeV, from an investigation of d/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..

A few details of the preliminary approximate treatment:

1) Assume MSuSY 400 GeV

2) Compute the real (i.e. With PDF..) d/dMtt = usual stuff (see paper..)

3) Take qqbar ttbar in Born approximation ( 10% ) and compute to 1 loop gg ttbar for Mttbar 1 TeV (0.7 TeV Mttbar 1.3 TeV)

4) Separate tLtbarL + tRtbarR = “parallel spin” from tLtbarR + tRtbarL = “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

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

NLO QCD effects (final state gluon radiation, 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 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 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)

Mtt (TeV)

LO

/NL

O

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 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%.

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)mH=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 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)

gttH/gttH~16%for mH=120 GeV hep-ph/0003033

results can be

extrapolated

to MSSM h

Conclusions ATLAS sensitivity to ttbar resonances:

5 discovery (mX = 1 TeV/c2, L=30 fb-1); x BR ~ 103 fb)

ATLAS sensitivity to new physics in the t bW decay: Mb should be taken into account gR 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

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

Documents

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