top quark physics outline
TRANSCRIPT
Top Quark Physics
E. Boos
High Energy Theory Division
Skobeltsyn Institute of Nuclear Physics, Moscow State University
Outline
• Introduction. Discovery in RUN1. Puzzles
• Basic production processes at colliders. (NLO generator SingleTop)
• Decays and spin correlations
• Top mass, Vtb, Top Yuakawa coupling
• ”New Physics” via top quark (few examples)
• ConclusionsLHC/LC Study Group,“Physics interplay of the LHC and the ILC,” arXiv:hep-ph/0410364
M. Beneke et al.,“Top quark physics,”arXiv:hep-ph/0003033
S. Willenbrock,“The standard model and the top quark,” arXiv:hep-ph/0211067
D.Chakraborty, J.Konigsberg, D.Rainwater,“Review of Top Quark Physics,” arXiv:hep-ph/0303092
S.Dawson, “The Top Quark, QCD, and New Physics,” arXiv:hep-ph/0303191
E. Boos, “Top quarks at photon colliders,” arXiv:hep-ph/0009100
Top quark
• Qtem = 2
3 | e |
• Weak isospin partner of b quark: T t3 = 1
2
• Color triplet
• spin- 12
SU(3) SU(2) U(1)Y
QiL =
0
@
uL
dL
1
A
0
@
cL
sL
1
A
0
@
tL
bL
1
A 3 2 1
6
uiR = uR cR tR 3 1 2
3
diR = dR sR bR 3 1 − 1
3
In the Standard Model top quark couplings are uniquely fixed by the principle
of gauge invariance, the structure of the quark generations, and a requirement
of including the lowest dimension interaction Lagrangian.
Top quark has been found by the Fermilab CDF and D0 collaborations.
RUN1 results:
• Mt = 174.3 ± 3.2(stat) ± 4.0(syst)
• σtt(CDF Mt = 175GeV ) = 6.5+1.7−1.4pb
σtt(D0 Mt = 172GeV ) = 5.9 ± 1.7pb
• λt(Mt) = 1.00 ± 0.03
• |Vtb| > 0.78 ( 90% CL)
• The 95% Confidence Level Limit on single top production cross section :
13.5 pb by CDF
39 pb (17 pb Neural Network) (s-channel) and
58 pb (22pb Neural Network) (W-gluon fusion) by D0
SM prediction: σSM = 2.43 ± 0.32 pb
• FCNC coupling limits
Br(t → Zq) < 33%(95%CL) Br(t → γq) < 3.2%(95%CL)
Top quark is the heaviest elementary particle found so far with a mass slightly
less than the mass of the gold nucleus.
• Top decays (τt ∼ 5 × 10−25 sec) much faster than a typical time-scale for
a formation of the strong bound states (τQCD ∼ 3 × 10−24 sec). So, top
provides, in principle, a very clean source for a fundamental information.
• Top is so heavy and point like at the same time. So, one might expect a
possible deviations from the SM predictions more likely in the top sector.
• Top Yukawa coupling λt = 23/4G1/2F mt is very close to unit. Studies of
top may shed a light on an origin of the mechanism of the EW symmetry
breaking.
Top quark physics will be a very important part of research programs for all
future hadron and lepton colliders.
At hadron and lepton colliders, top quarks may be produced either in pairs or
singly. At the Tevatron and LHC: Top pair (left), Single top (right)
q
q
t
t
g
g
t
t
+
g
g
t
t
+
g
g
t
t
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Three mechanisms of the single top production:
t-channel (Q2W < 0)
s-channel (Q2W > 0)
associated tW (Q2W = M2
W )
Q2W - W-boson virtuality
Basic production processes cross sections
σNLO (pb) qq → tt gg → tt
Tevatron (√
s = 1.8 TeV pp) 4.87 ± 10% 90% 10%
Tevatron (√
s = 2.0 TeV pp) 6.70 ± 10% 85% 15%
LHC (√
s = 14 TeV pp) 833 ± 15% 10% 90%
s channel t channel Wt
Tevatron (√
s = 2.0 TeV pp) 0.90 ± 5% 2.1 ± 5% 0.1 ± 10%
LHC (√
s = 14 TeV pp) 10.6 ± 5% 250 ± 5% 75 ± 10%
LHC will the Top factory: about 10 mln top quarks per year
(or 1 top per second) with 10 fb−1 luminosity
) (pb)t t→ p(pσ0 2 4 6 8 10 12 14
0
11Cacciari et al. JHEP 0404:068 (2004)
2=175 GeV/ctm CDF Run 2 Preliminary
All Hadronic: Vertex Tag 2.3 4.7± 2.5
2.5± 7.8 )-1(L= 165pb
Lepton+Jets: Soft Muon Tag 1.0 1.3± 1.9
2.9± 5.2 )-1(L= 193pb
Lepton+Jets: Jet Prob Tag 1.3 1.3± 1.2
1.3± 5.8 )-1(L= 162pb
Lepton+Jets: Double Vertex Tag 0.8 1.1± 1.9
2.4± 5.0 )-1(L= 162pb
Lepton+Jets: Vertex Tag 0.6 0.9± 1.1
1.2± 5.6 )-1(L= 162pb
Lepton+Jets: Vertex Tag+Kinematic 1.2 1.2± 1.6
1.6± 6.0 )-1(L= 162pb
Lepton+Jets: Kinematic NN 1.6 1.6± 1.1
1.1± 6.7 )-1(L= 193pb
Lepton+Jets: Kinematic 1.8 1.8± 1.6
1.6± 4.7 )-1(L= 193pb
Dilepton: MET, # jets 1.1 1.1± 2.4
2.5± 8.6 )-1(L= 193pb
Dilepton: Combined 1.2 1.7± 2.1
2.4± 7.0 )-1(L= 200pb
0 2.5 5 7.5 10 12.5 15 17.5 20
DO Run II Preliminary
σ(pp− tt−) (pb)0 2.5 5 7.5 10 12.5 15 17.5 20
dilepton
l+jets (topological)
l+jets (soft µ tag)
eµ (Vertex tag)
l+jets (Impact parameter)
l+jets (Vertex tag)
all hadronic
Cacciari et al. JHEP 0404:068(2004), mt = 175 GeV/c2
L=146 pb−1
14.3 +5.1−4.3
+2.6−1.9
pb
L=143 pb−1
7.2 +2.6−2.4
+1.6−1.7
pb
L=93 pb−1
11.4 +4.1−3.5
+2.0−1.8
pb
L=158 pb−1
11.1 +5.8−4.3
+1.4−1.4
pb
L=164 pb−1
7.2 +1.3−1.2
+1.9−1.4
pb
L=164 pb−1
8.2 +1.3−1.3
+1.9−1.6
pb
L=162 pb−1
7.7 +3.4−3.3
+4.7−3.8
pb
The first 95% confidence level upper limits on single top
production cross sections in RUN2 by D0 collaboration are
PLB 622, 265 (2005)
5.0 pb in the s-channel and
4.4 pb in the t-channel
New CDF and D0 are very similar:
NN 3.2 pb s-channel, 3.1 t-channel with 700 pb−1
The first Single Top observation is expected at the Tevatron RUN2 rather soon
when accumulated integrated luminosity will be about 1.5 fb−1
Main problem is large backgrounds (W + jets, Wbb, tt etc.) and complicated
analysis to extract the signal
Problems and requirements for a generator for the single top signal:
• Double counting and negative weights
• Matching of various NLO contributions at the generator level. One should
have the correct NLO rate and correct shapes of the NLO distributions
• Matching to showering programs
• Correct spin correlations
• Finite top and W widths
• Separation Top and antiTop since the rates are different (for the LHC)
• Anomalous Wtb and FCNC couplings
Generators for the single top signal:
ONETOP
TopRex
generators based on MADGRAPH&MADEVENT
generators based on PYTHIA
SingleTop - generator based on CompHEP (PYTHIA and NLO computations)
LO order diagrams
q q
t
t
+W
+W
+W
t
t
b
b
b
b
g
g
tq
q
+W
b
(a)
(b)
(c)
(d)
Representative loop and tree NLO diagrams to the t- and s- channel single top
production
q q
t+W
+W
+Wt
t
bb
g
g
t
q
+Wb
(a)
(b)
(c)
(d)
q qg
g qq
b
q
q
q
t
+W +W
t
t
b
g
q+W
(a) (b)
(c)
q g
gq
q
b
bq
s-channel at NLO and LO times a K-factor of 1.54 (Zack Sullivan)
NLOLO � �� ��
� ! (GeV)
"$#&%'")(+*-
, (fb/G
eV)
200150100500
6
5
4
3
2
1
0
NLOLO . /0 12
34
57698:5<;'=
(fb)
543210-1-2-3-4-5
120
100
80
60
40
20
0
NLOLO > ?@ AB
C DFE G (GeV)
H7I9J:HLK+M-
NO (fb/G
eV)
200150100500
6
5
4
3
2
1
0
NLOLO P QR ST
UWV X
Y7Z9[:Y7\:]^
(fb)
543210-1-2-3-4-5
200
150
100
50
0
Transverse momentum and pseudorapidity of the top quark and b-jet
t-channel
Splitting on pt of the b-jet (b-jet not from top decay)
2 → 2 with ISR at ”small” pt region
(CompHEP + ISR from PYTHIA)
2 → 3 at ”large” pt region
(CompHEP)
( for both cases with spin correlated 1 → 3 top subsequent decay)
The separation parameter (P0)bt of ”small” and ”large” pt regions is turned such
that:
1. The total rate is normalized to the NLO rate
σ2→2 |P bt <(P0)b
t+σ(2→3) |P b
t >(P0)bt
= σNLO
2. The distributions are smooth
Matching CompHEP&PYTHIA(2 → 2) and CompHEP (2 → 3) distributions (P qT
> 20 GeV)
including deays 2 → 4 and 2 → 5 (LHC )
CompHEP (tqb+ISR) and Pythia (tq+ISR) processes, PTb cut = 20 GeV
0
5
10
0 50 100 150 200PT(t), [GeV]
dσ/dP
T(t), [p
b/GeV
]
0
2
4
6
-5 -2.5 0 2.5 5 yt
dσ/dy
t, [pb]
0
5
10
15
50 100 150 PT(q), [GeV]
dσ/dP
T(q), [
pb/G
eV]
0
2
4
6
-5 -2.5 0 2.5 5 yq
dσ/dy
q, [pb]
1
10
0 20 40 60 PT(b), [GeV]
dσ/dP
T(b), [
pb/G
eV]
0
2
4
-5 -2.5 0 2.5 5 yb
dσ/dy
b, [pb]
Matching CompHEP&PYTHIA(2 → 2) and CompHEP (2 → 3) distributions (P qT
> 10 GeV)
including deays 2 → 4 and 2 → 5 (LHC )
CompHEP (tqb+ISR) and Pythia (tq+ISR) processes, PTb cut = 10 GeV
0
5
10
0 50 100 150 200PT(t), [GeV]
dσ/dP
T(t), [p
b/GeV
]
0
2
4
6
-5 -2.5 0 2.5 5 yt
dσ/dy
t, [pb]
0
5
10
50 100 150 PT(q), [GeV]
dσ/dP
T(q), [
pb/G
eV]
0
2
4
6
-5 -2.5 0 2.5 5 yq
dσ/dy
q, [pb]
1
10
0 20 40 60 PT(b), [GeV]
dσ/dP
T(b), [
pb/G
eV]
0
2
4
6
-5 -2.5 0 2.5 5 yb
dσ/dy
b, [pb]
Comparisons with MCFM for top decay products (t-channel):
Transverse momentum and pseudorapidity of l and νl from top decayEta_e
Entries 69032Mean 0.2317RMS 1.108
-5 -4 -3 -2 -1 0 1 2 3 4 50
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4Eta_e
Entries 69032Mean 0.2317RMS 1.108
CompHEP
MCFM
of e+, top onlyη Pt_eEntries 137587Mean 38.02RMS 20.38
0 20 40 60 80 100 120 140 160 180 2000
0.005
0.01
0.015
0.02
0.025Pt_e
Entries 137587Mean 38.02RMS 20.38
CompHEP
MCFM
of e+, top onlyTP
Eta_nuEntries 69032Mean 0.1146RMS 1.106
-5 -4 -3 -2 -1 0 1 2 3 4 50
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4Eta_nu
Entries 69032Mean 0.1146RMS 1.106
CompHEP
MCFM
of neutrino, top onlyη Pt_nuEntries 137587Mean 50.99RMS 25.72
0 20 40 60 80 100 120 140 160 180 2000
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0.018
Pt_nuEntries 137587Mean 50.99RMS 25.72
CompHEP
MCFM
of neutrino, top onlyTP
Top pair and single top in e+e− collisions (ILC)
e+e− → tt → WWbb, W → f f ′,
where e.g. for W+
f = u, c, νe, νµ, ντνµ; f ′ = d, s, e, µ, τ
Gauge invariant s-channel subset of 10 diagrams
e
e
γ, Z t
t b
W+ e
νe
diagr.1,2
e
e
γ, Z e
e νe
W+ b
t
diagr.3,4
e
e
γ, Z b
b t
W+ e
νe
diagr.5,6
e
e
γ, Z
e
νe
W+ b
t
diagr.7,8
eW+ e
νeνe
eW+ b
t
diagr.9
e
eZ νe
νe e
W+ b
t
diagr.10
One should split top pair and single top contributions in the s-channel subset
Gauge invariant t-channel subset of 10 diagrams
ee
γ, Ze
e νe
W+ b
t
diagr.1,2
e eγ, Z
tt
bW+
e νe
diagr.3,4
e eγ, Z
bb
tW+
e νe
diagr.5,6
ee
γ, ZW+ b
tW+
eνe
diagr.7,8
eνe e
W+ b
tW+
eνe
diagr.9
ee
Z νe
νee
W+ b
t
diagr.10
All the diagrams contribute to Single Top
(at LEP2 the rate is too small, about 10−5 pb)
In case of γγ collisions there are no nontrivial gauge invariant subsets. A situ-
ation is similar to single top at the LHC in Wt mode.
γt W−
btγ
t
diagr.1
γt
tγ
t W−
b
diagr.2
γ tt
bW+
γ W−
diagr.3
γ tt
W−b
γ b
diagr.4
γb
bγ
b W−
t
diagr.5
γ bb
tW+
γ W−
diagr.6
γ bb
W−t
γ t
diagr.7
γb W−
tbγ
b
diagr.8
γ W−W+
tb
γ b
diagr.9
γ W−W+
bt
γ t
diagr.10
γW−
W+γ
W+ b
t
diagr.11
γW+ b
tW+γ
W−
diagr.12
γ
γ
W−
W+ b
t
diagr.13
The top pair rate has to be removed in order to get the correct single top rate.
Single Top Diagrams in γe Collisions
γ
e
e νe
W+ b
t
diagr.1
γ tt
bW+
e νe
diagr.2
γ bb
tW+
e νe
diagr.3
γW+ b
tW+
eνe
diagr.4
This is one of so called ”gold plated” processes in γe collision mode of ILC
Cross sections of Top production processes at LC
In SM top decays to W-boson and b-quark practically with 100% probability
t
b
W ν
l
dΓ ∼ |M|2 ∼ (t+ms) · `b · ν, where in the top-quark rest frame, the spin four-
vector is s = (0, s), and s is a unit vector that defines the spin quantization axis
of the top quark
In the top quark rest frame:1Γ
dΓd cos θ`
= 12(1 + cos θ`)
Hence the charged lepton tends to point along the direction of top spin.
Mahlon,Parke; E.B.,Sherstnev
Top quark mass.
In SM W-boson, Top quark and H boson masses are connected to each other
via loop contributions to W and Z propagators
W W
t
b
Z Z
t
t
h
+
h
M2W =
πα√2GF
s2W
(1−∆r)where ∆r contains the one-loop corrections.
(∆r)top ≈ − 3GF m2t
8√
2π2
1t2W
where t2W ≡ tan2 θW .
This one-loop correction depends quadratically on the top-quark mass.
(∆r)Higgs ≈ 11GF M2Zc2
W
24√
2π2ln
m2h
M2Z
This one-loop correction depends only logarithmically on the Higgs-boson mass,
so ∆r is not as sensitive to mh as it is to mt.
80.3
80.4
80.5
150 175 200
mH [GeV]114 300 1000
mt [GeV]
mW
[Ge
V]
68% CL
∆α
LEP1, SLD dataLEP2 (prel.), pp− data
Mtop [GeV/c2]
Mass of the Top Quark (*Preliminary)Measurement Mtop [GeV/c2]
CDF-I di-l 167.4 ± 11.4D∅-I di-l 168.4 ± 12.8CDF-II di-l* 165.3 ± 7.3CDF-I l+j 176.1 ± 7.3D∅-I l+j 180.1 ± 5.3CDF-II l+j* 173.5 ± 4.1D∅-II l+j* 169.5 ± 4.7CDF-I all-j 186.0 ± 11.5
χ2 / dof = 6.5 / 7
Tevatron Run-I/II* 172.7 ± 2.9
150 170 190
CDF and D0 combined
(see A.Hoang et. al)
[GeV]s330 335 340 345 350 355 360
[GeV]s330 335 340 345 350 355 360
[p
b]
σ
0
0.2
0.4
0.6
0.8
1default+beam spread+beamstrahlung+ISR
γ, Z
e−
e+
b
W−
W+
b
(b)
γ, Z
e−
e+
b
W−
W+
b
γ, Z
e−
e+
W−
b
b
W+
γ, Z
e−
e+
W−
b
b
W+
νe
e−
e+
W−
b
b
W+
γ, Z
e−
e+
b
W−
W+
b
γ, Z
e−
e+
W−
b
b
W+
γ, Z
e−
e+
W−
b
b
W+
νe
e−
e+
W−
b
b
W+
Accuracies: top mass - to 100 MeV, top width - to 50 MeV
at ILC with 300 fb−1
Delicate computational problems at NLO and NNLO level:
The width Γt ∼ mtα and Ekin ∼ mtα2s are of the same order
At the Tevatron Run II with 2fb−1 one expects:
δMW ∼ 27 MeV
δMt ∼ 3 GeV
yielding a prediction for the Higgs mass with an uncertainty of
δMh
Mh∼ 40%
At the LHC with 10 fb−1
δMt ∼ 0.7 GeV
At ILC with 500 fb−1 from the top pair threshold scan one can get
δMt ∼ 0.1 GeV
|Vtb| measurements
At LHC and Tevatron Run2 via single top
_
_�`a
b
c
b�d ef gh h ij_
_�`a
b
k
l c_
_�` a
bl c
m d ef gh h ij
ck
ab
a b gn no ep gq ir s�t o ru eq po h
V 2tb could be measured with an accuracy of 10% dominated by systematics
At ILC (1 TeV, 500 fb−1) in eγ collisions -
2-3 % accuracy dominated by statistics
Top Yukawa coupling ttH measurements
For the LHC complete NLO computations have been performed(LO diagrams are shown)
W. Beenakker et al. hep-ph/0211352; S.Dawson et al. hep-ph/0211438
vv
v
wxx y
xv
v
v xx y
wxv
wx
xv x y
xv wx
xy
x
v x
v x yxx
v wxv x ywxx
v
xv
x
xv x ywx
v xx
yx
v wx
Top Yukawa could be measured with an accuracy from 16% at low Lumi to 11%
at high Lumi regime
New Physics via Top (examples):
• Wtb anomalous couplings
• FCNC
• Various SUSY effects without and with R-parity violation
• Charged Higgs in top decays
• New strong dynamics (W ′, Z ′, πT , ρT , topgluon, WLWL → tt ...)
• Kaluza-Klein graviton excitations and radion in ADD and RS scenarious
• ...
Maximal value of the CP even light Higgs in MSSM is about 135-140 GeV
(not MZ) due to large top quark mass
Mmaxh =
√
M2Z + ε
ε =3GF m4
t√2π2 sin2 β
[
f(t)
]
, where t = log
(
M2S
m2t
)
Anomalous Top Couplings
The top quark interactions of dimension 4:
L4 = −gs tγµTatGaµ − g√
2
X
q=d,s,b
tγµ(vWtq − aW
tq γ5)qW+µ
−2
3etγµtAµ − g
2 cos θW
X
q=u,c,t
tγµ(vZtq − aZ
tqγ5)qZµ
The dimension 5 couplings have the generic form:
L5 = −gs
X
q=u,c,t
κgtq
ΛtσµνTa(fg
tq + ihgtqγ5)qGa
µν − g√2
X
q=d,s,b
κWtq
Λtσµν(fW
tq + ihWtq γ5)qW+
µν
−eX
q=u,c,t
κγtq
Λtσµν(fγ
tq + ihγtqγ5)qAµν − g
2 cos θW
X
q=u,c,t
κZtq
Λtσµν(fZ
tq + ihZtqγ5)qZµν
where |f |2 + |h|2 = 1.
Present constrains come from
• Low energy data via loop contributions
KL → µ+µ−, KL − KS mass difference, b → l+l−X, b → sγ
• LEP2
• Tevatron Run1
• HERA
• Unitarity violation bounds
Anomalous Wtb Couplings
• Lagrangian
L =g√2Vtb
[
W−ν bγµP−t − 1
2MWW−
µν bσµν(FL2 P− + FR
2 P+)t
]
+ h. c.
with W±µν = DµW±
ν − DνW±µ , Dµ = ∂µ − ieAµ,
σµν = i/2[γµ, γν ] and P± = (1 ± γ5)/2. The couplings F L2 and F R
2 are
proportional to the coefficients of the effective Lagrangian
FL2 = 2MW
Λ κWtb (−fW
tb − ihWtb ),
FR2 = 2MW
Λ κWtb (−fW
tb + ihWtb ), |FL2,R2| < 0.6 from unitary bounds
• |Vtb| is very close to 1 in SM with 3 generations. (|Vtb| is very weakly
constrained in case of 4 generations, e.g.)
• A possible V + A form factor is severely constrained by the CLEO b → sγ
data to 3 × 10−3 level
Wtb anomalous couplings limit on TEVATRON and LHC:
(E.Boos,L.Dudko,T.Ohl,EPJ99)
-0.2-0.1
00.10.20.30.40.50.6
-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4
Test
FR2
FL2
-0.015-0.01
-0.0050
0.0050.01
0.0150.02
0.0250.03
-0.06 -0.04 -0.02 0 0.02 0.04 0.06
Test
FR2
FL2
Uncorrelated limits on anomalous couplings from measurements at different
machines.
FL2 FR
2
Tevatron (∆sys. ≈ 10%) −0.18 ÷+0.55 −0.24 ÷+0.25
LHC (∆sys. ≈ 5%) −0.052÷+0.097 −0.12 ÷+0.13
γe (√
se+e− = 0.5 TeV) −0.1 ÷+0.1 −0.1 ÷+0.1
γe (√
se+e− = 2.0 TeV) −0.008÷+0.035 −0.016÷+0.016
FCNC couplings
• Couplings: tqg, tqγ, tqZ, where q = u, c
∆Leff =1
Λ[κγ,Z
tq etσµνqFµνγ,Z + κg
tqgstσµνλi
2qGiµν ] + h.c.
Information on FCNC couplings come from either top pair production with sub-
sequent decays to rear modes t → q V , where V = γ, Z, g
or from additional contributions to the single top production
t
cq → tq
t
qq → tc
t
cg → tg
t
gg → tc
All present and expected limits are presented in terms of Br fractions:
Γ(t → qg) =
κgtq
Λ
!28
3αsm3
t , Γ(t → qγ) =
κγtq
Λ
!2
2αm3t ,
Γ(t → qZ)γ =“
|vZtq|2 + |aZ
tq|2”
α m3t
1
4M2Z
sin2 2θW
1 − M2Z
m2t
!2
1 + 2M2
Z
m2t
!
,
Γ(t → qZ)σ =
κZtq
Λ
!2
α m3t
1
sin2 2θW
1 − M2Z
m2t
!2
2 +M2
Z
m2t
!
Current constraints
CDF LEP-2 HERA
BR(t → gq) ≤ 29% – –
BR(t → γq) ≤ 3.2% – ≤ 0.7%
BR(t → Zq) ≤ 32% ≤ 7.0% –
Future expectations
Tevatron LHC e+e−
t → Run II decay production√
s > 500 GeV
g q 0.06% 1.6 × 10−3 1 × 10−5 –
γ q 0.28% 2.5 × 10−5 3 × 10−6 4 × 10−6
Z q 1.3% 1.6 × 10−4 1 × 10−4 2 × 10−4
Charged Higgs in Top Decay (impact of tau polarization)
t
b
W+ nl
l π+
Nl
diagr.1
t
b
H+ nl
l π+
Nl
diagr.2
In the rest frame of top t → bR → bτντ → bντ ντπwhere a resonance R is W boson or charged H
1Γ
dΓdyπ
= 1xmax−xmin
(1 − Pτ )log xmax
xmin+ 2Pτyπ( 1
xmin− 1
xmax), 0 < yπ < xmin
(1 − Pτ )log xmax
yπ+ 2Pτ (1 − yπ
xmax), xmin < yπ
where yπ =Etop
πMtop
, xmin =Emin
τMtop
, xmax =Emax
τMtop
, Eminτ =
M2R
2Mtop, Emax
τ =Mtop
2
Pτ = −1 for W boson and Pτ = 1 for charged Higgs
e+e− → tt → τντ bb + 2jets
Simulations are performed for e+e− collisions at 500 GeV cms
and for 500 fb−1 integrated luminosity
π-meson energy spectrum for the MSSM point
tan β = 50, µ = 500, MH± = 130 GeV with Br(t → H+b) = 9.1%
E.B., S.Bunichev, M.Carena, C.Wagner
GeV0 10 20 30 40 50 60 70 80 90 100
Even
ts /2
GeV
0
50
100
150
200
250
300
350
400
in Top r/f π E in Top r/f π E
From the signal+backgr fit MH± = 129.4 +/- 0.9 GeV
W ′ gauge boson in single topT.Tait, C.-P.Yuan; Z.Sullivan; E.B., V.Bunichev, L.Dudko, M.Perfilov
q
q′
t
b
W′
L =Vqiqj
2√
2gwqiγµ
(
aRqiqj
(1 + γ5) + aLqiqj
(1 − γ5))
W ′qj + H.c. , (1)
where aRqiqj
, aLqiqj
- left and right couplings of W ′ to quarks, gw = e/(sw) is
the SM weak coupling constant and Vqiqjis the SM CKM matrix element. The
notations are taken such that for so-called SM-like W ′ aLqiqj
= 1 and aRqiqj
= 0.
The interference between the SM and W ′ contributions (new)
|M |2 = V 2tbV
2ud(gW )4
[
(pupb)(pdpt)
(s − m2W )2 + γ2
W m2W
+ (2)
+2aLuda
Ltb(pupb)(pdpt)
(s − m2W )(s − M2
W ′) + γ2W Γ2
W ′
((s − m2W )2 + γ2
W m2W )((s − M2
W ′)2 + Γ2W ′M2
W ′)+
+(aL
ud
2aL
tb
2+ aR
ud
2aR
tb
2)(pupb)(pdpt) + (aL
ud
2aR
tb
2+ aR
ud
2aL
tb
2)(pupt)(pdpb)
(s − M2W ′)2 + Γ2
W ′M2W ′
]
Invariant mass of tb system for MW ′ 800 GeV
at the Tevatron (left) and LHC (right)
, GeV bMass t 100 200 300 400 500 600 700 800 900 1000
/dM
, f
b/9
Ge
Vσ
d
-1110
-1010
-910
-810
-710
-610
-510
-410
-310
-210
, GeV bMass t 100 200 300 400 500 600 700 800 900 1000
/dM
, f
b/9
Ge
Vσ
d
-1110
-1010
-910
-810
-710
-610
-510
-410
-310
-210SM+right W’SM+left W’SM W only
, GeV bMass t 100 200 300 400 500 600 700 800 900 1000
/dM
, f
b/9
Ge
Vσ
d
-810
-710
-610
-510
-410
-310
-210
-110
1
, GeV bMass t 100 200 300 400 500 600 700 800 900 1000
/dM
, f
b/9
Ge
Vσ
d
-810
-710
-610
-510
-410
-310
-210
-110
1SM+right W’SM+left W’SM W only
W ′ gauge boson in single top
Latest limits by the D0 collaboration
Generic search for a resonance in top pair production at the LHC
(MSSM Higgses H/A, Z ′, topgluon, RS-graviton, KK excitations in UED etc.)
Reconstructed mtt (GeV)
Entri
es/5
0 G
eV
0
200
400
1000 2000mtt (GeV)
σ*
Br (
fb
)
1
10
10 2
10 3
1000 2000 3000 4000 5000
Measured tt invariant mass distribution for reconstruction of a narrow resonance
of mass 1600 GeV decaying to tt and value of σ× BR required for a 5σ discovery
potential.
Conclusions
Discovery of the top quark has opened up many new avenues to interesting
physics
• Precision measurements of top quark characteristics such as mass, produc-
tion cross sections, decay width and branching fractions, spin correlations
are needed to test the SM
• Tests and understanding all possible deviations from the SM expectations
to check if top is exotic in some way
• Precise calculations and simulations, and measurements of the top event
kinematical characteristics to understand backgrounds to many other pos-
sible New physics processes
• Possible discovery and study of various New physics effects via top produc-
tion and/or decay
V. Bunichev