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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
L
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 ET
miss > 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 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
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)
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