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Top Quark Physics
E. Boos
High Energy Theory Division Skobeltsyn Institute of Nuclear Physics, Moscow State University

Outline
· Introduction. Discovery and Puzzles · Basic production processes at colliders · Decays and spin correlations · Top mass, Vtb , Top Yuakawa coupling · "New Physics" via top quark
LHC/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

· Conclusions


Top quark ·Q
t em

=

2 3

|e|
1 2

t · Weak isospin partner of b quark: T3 =

· Color triplet · spin1 2

S U (3) Qi = L u d
i R i R

S U (2) 2 1 1

U (1)
1 6 2 3 1 -3

Y

0 @

u d

L

L R

1 A

0 @

c

L L

s c

1 A

0 @

t

L L

b t

1 A

3 3 3

= =

u d

R R

R R

R

s

b

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(sy st) · tt (C DF Mt = 175GeV ) = 6.5+1.7 pb ¯ -1.4 tt (D0 Mt = 172GeV ) = 5.9 ± 1.7pb ¯ · t (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:
SM

= 2.43 ± 0.32 pb


Top quark is the heaviest elementary particle found so far with a mass slightly less than the mass of the gold nucleus. · Top decays (t 5 â 10-25 sec) much faster than a typical time-scale for a formation of the strong bound states (QC D 3 â 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 t = 23/4 GF 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.
1/2


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 t
¡ ¢¡ ¡ ¸ ¤ ¸ ¸

q

t
¥

¤¥

¤ ©¨ §¦ ¤

¥

©¨ ¦

g

t

g

t

g

t



¤

+
g t g t

+
¸ ¥

g

t
¨! "% §$ # " ¤ ¸ ¨ !

Three mechanisms of the single top production: t-channel (Q2 < 0) W s-channel (Q2 > 0) W 2 associated tW (Q2 = MW ) W Q2 - W-boson virtuality W


Basic production processes cross sections Tevatron ( s = 1.8 TeV pp) ¯ ¯ Tevatron ( s = 2.0 TeV pp) LHC ( s = 14 TeV pp)
NLO

(pb)

¯ q q tt ¯ 90% 85% 10%

¯ g g tt 10% 15% 90%

4.87 ± 10% 6.70 ± 10% 833 ± 15%

s channel Tevatron ( s = 2.0 TeV pp) ¯ LHC ( s = 14 TeV pp) 0.90 ± 5% 10.6 ± 5%

t channel 250 ± 5% 2.1 ± 5%

Wt 0.1 ± 10% 75 ± 10%

LHC will the Top factory: about 10 mln top quarks per year (or 1 top per second) with 10 f b-1 luminosity


hep-ex/0506005
DO Run II Preliminary
11
Cacciari et al. JHEP 0404:068 (2004) 2 m t =175 GeV/c

CDF Run 2 Preliminary 7.0 ± 8.6 ± 4.7 ± 6.7 ± 6.0 ± 5.6 ± 5.0 ± 5.8 ± 5.2 ± 7.8 ±
2.4 2.1 2.5 2.4 1.6 1.6 1.1 1.1 1.6 1.6 1.2 1.1 2.4 1.9 1.3 1.2 2.9 1.9 2.5 2.5

dilepton
L=146 pb-1

14.3 7.2 11.4 11.1 7.2 8.2 7.7
2

+5.1 +2.6 -4.3 -1.9 +2.6 +1.6 -2.4 -1.7 +4.1 +2.0 -3.5 -1.8 +5.8 +1.4 -4.3 -1.4 +1.3 +1.9 -1.2 -1.4 +1.3 +1.9 -1.3 -1.6 +3.4 +4.7 -3.3 -3.8

pb pb pb pb pb pb pb

Dilepton: Combined -1
(L= 200pb )

± ± ± ± ± ± ± ± ± ±

1.7 1.2 1.1 1.1 1.8 1.8 1.6 1.6 1.2 1.2 0.9 0.6 1.1 0.8 1.3 1.3 1.3 1.0 4.7 2.3

l+jets (topological)
L=143 pb-1

Dilepton: MET, # jets -1
(L= 193pb )

Lepton+Jets: Kinematic -1
(L= 193pb )

l+jets (soft µ tag)
L=93 pb-1

Lepton+Jets: Kinematic NN -1
(L= 193pb )

eµ (Vertex tag)
L=158 pb-1

Lepton+Jets: Vertex Tag+Kinematic -1
(L= 162pb )

Lepton+Jets: Vertex Tag -1
(L= 162pb )

l+jets (Impact parameter)
L=164 pb-1

Lepton+Jets: Double Vertex Tag -1
(L= 162pb )

l+jets (Vertex tag)
L=164 pb-1

Lepton+Jets: Jet Prob Tag -1
(L= 162pb )

Lepton+Jets: Soft Muon Tag -1
(L= 193pb )

all hadronic
L=162 pb-1

0

All Hadronic: Vertex Tag -1
(L= 165pb )

Cacciari et al. JHEP 0404:068(2004), m = 175 GeV/c t

0

2

4

6 8 10 (pp t t ) (pb)

12

14

0

2.5

5

7.5

10 12.5 15 17.5 20

(pp

-

tt) (pb)

-


The best 95% confidence level upper limits on single top production cross sections in RUN2 by D0 collaboration are hep-ex/0505063 6.4 pb in the s-channel and 5.0 pb in the t-channel The first Single Top observation is expected at the Tevatron RUN2 rather soon when accumulated integrated luminosity will be about 1-1.5 f b-1 Main problem is large backgrounds (W + j ets, W b¯, tt etc.) and complicated b¯ analysis to extract the signal


Top pair and single top in e+ e

-

collisions (ILC)

¯ ¯ e+ e- tt W W b¯, b W ff , where e.g. for W + f = u, c, e , µ , µ ; f = d, s, e, µ, Gauge invariant s-channel subset of 10 diagrams
e e ¯ , Z t W
+

t ¯ b e e ¯ e ¯ e ¯ b t

e e ¯

, Z e W
+

e e ¯ ¯ b t

e e ¯

, Z b W
+

¯ b t e e ¯

e

, Z
+

eW ¯

e e ¯ ¯ b t

e e ¯
e

W W

+

+

e e ¯ ¯ b t

diagr.1,2 e e ¯ Z e W
+

diagr.3,4

diagr.5,6

diagr.7,8

diagr.9

diagr.10

One should split top pair and single top contributions in the s-channel subset


Gauge invariant t-channel subset of 10 diagrams
e , Z e e ¯ W+ diagr.1,2 e e ¯ Z e W e e ¯ ¯ b t diagr.10 e e ¯ ¯ b t e , Z t W
+

e t ¯ b e ¯

e

, Z b W
+

e ¯ b t e ¯

e ¯

e ¯

e , ZW W+ e ¯

+

e ¯ b t e ¯

e W e ¯

W +

e +

e ¯ b t e ¯

diagr.3,4

diagr.5,6

diagr.7,8

diagr.9

+

All the diagrams contribute to Single Top (at LEP2 the rate is too small, about 10-5 pb)


In case of collisions there are no nontrivial gauge invariant subsets. A situation is similar to single top at the LHC in W t mode.
t t W- ¯ b t t tt t W+ diagr.3 W- t ¯ b W+ t diagr.10 t ¯ b W- t b diagr.4 W- ¯ b W +W + t diagr.11 t W- ¯ b W- ¯ b bb ¯ b W- t diagr.5 W- W+ ¯ b W+ t diagr.12 ¯ b t W- diagr.13 W+ b W+ diagr.6 W- ¯ b t ¯ b t W- b t diagr.7 ¯ b W- t

diagr.1 b b W- t ¯ b diagr.8

diagr.2 W+ b diagr.9

The top pair rate has to be removed in order to get the correct single top rate.


Single Top Diagrams in e Collisions
e e W
+

b

e

t
+

¯ t b

b
+

b ¯ t
e

¯ t diagr.1

e

W



e

e

W

W e

W
+

+

b ¯ t
e

diagr.2

diagr.3

diagr.4

This is one of so called "gold plated" processes in e 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

b

t

W

l

d |M|2 (t + ms) · b · , where in the top-quark rest frame, the spin fourvector 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: 1 d = 1 (1 + cos ) d cos 2 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
t W b W Z t t Z

h +
2GF

h

M

2 W

=
top

s

2 W

(1-r )

where r contains the one-loop corrections. where t
m2 h 2 MZ 2 W

(r)

-

3GF m2 1 t 2 t2 8 2 W
2 11GF MZ c 24 2 2 2 W

tan2 W .

This one-loop correction depends quadratically on the top-quark mass. (r)
Higgs

ln

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 .


80.5

LEP1, SLD data - LEP2 (prel.), pp data 68% CL

[GeV]
W

80.4

m

80.3



mH [GeV] 114 300 150

1000 175

200

mt [GeV]


Mass of the Top Quark (*Preliminary) 2 Measurement Mtop [GeV/c ] CDF-I di-l D-I di-l CDF-II di-l* CDF-I l+j D-I l+j CDF-II l+j* D-II l+j* CDF-I all-j
2

167.4 ± 11.4 168.4 ± 12.8 165.3 ± 7.3 176.1 ± 7.3 180.1 ± 5.3 173.5 ± 4.1 169.5 ± 4.7 186.0 ± 11.5 / dof = 6.5 / 7

Tevatron Run-I/II*

172.7 ± 2.9

150

Mtop

170 2 [GeV/c ]

190

hep-ex/0507091 CDF and D0 combined


At the Tevatron Run II with 2f b M

-1

one expects: 27 M eV 3 GeV

W t

M

yielding a prediction for the Higgs mass with an uncertainty of Mh 40% Mh At the LHC with 10 f b
-1

Mt 0.7 GeV At ILC with 500 f b
-1

from the top pair threshold scan one can get Mt 0.1 GeV


|Vtb | measurements At LHC and Tevatron Run2 via single top
' & ¢' & ' & & ( )

V

At ILC (1 TeV, 500 f b-1 ) in e collisions 2-3 % accuracy dominated by statistics

& ( ( )90 ) 8 90 & 0 76 554 32 1@ 76 554 32 §1 ) 8 ( 0 C2 ) ( 4AAB 4D C2DB EH §GB F 5 6E )
2 tb

could be measured with an accuracy of 10% dominated by systematics


¯ 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

PQ

PQ

PQ

Q

I I I Q Q I Q I

I I I I Q Q R Q I PQ R Q I PQ I Q I Q Q I Q

I Q

Top Yukawa could be measured with an accuracy from 16% at low Lumi to 11% at high Lumi regime

R Q Q R Q Q PQ R Q PQ PQ I R Q Q I Q I Q

R

R


New Physics via Top (examples):
·W
tb

anomalous couplings

· FCNC · Various SUSY effects without and with R-parity violation · Charged Higgs in top decays ¯ · New strong dynamics (W , Z , T , T , topgluon, WL WL 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 M
max h

=

2 MZ + 2 MS m2 t

3G F m 4 t f (t) , = 2 sin2 2

where

t = log


Anomalous Top Couplings The top quark interactions of dimension 4:
L
4

=

g ¯ -gs t µ T a tGa - µ 2

q =d,s,b

2 g ¯ - et µ tAµ - 3 2 cos

X

¯ t µ (v

W tq

-a
Z tq

W tq 5

)q W

+ µ

W q =u,c,t

X

¯ t µ (v

- aZ 5 )q Z tq

µ

The dimension 5 couplings have the generic form:
L
5

=

-g

s

q =u,c,t

X



g tq


µ

¯ t

µ

T a (f

g tq

+ ih

g tq 5

)q G

a µ

g - 2

q =d,s,b

-e

q =u,c,t

X

tq

X



W tq

¯ t

¯ t

µ

(f

W tq

+ ih

W tq 5

)q W

+ µ

¯ t

(f

tq

+ ih

tq 5

)q A

µ

g - 2 cos

W q =u,c,t

X



Z tq

µ



(f

Z tq

+ ihZ 5 )q Z tq

µ

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 s · LEP2 · Tevatron Run1 · HERA · Unitarity violation bounds


Anomalous Wtb Couplings · Lagrangian g L= V 2 with µ prop FL2 FR 2
tb - b W ¯ µ P- t -

1 2M

W

- Wµ ¯ b

µ

L R (F2 P- + F2 P+ )t + h. c.

± ± ± Wµ = Dµ W - D Wµ , Dµ = µ - ieAµ , L R = i/2[µ , ] and P± = (1 ± 5 )/2. The couplings F2 and F2 are ortional to the coefficients of the effective Lagrangian W = 2MW W (-ftb - ihW ), tb tb W = 2MW W (-ftb + ihW ), |FL2,R2 | < 0.6 from unitary bounds tb tb

· |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 s data to 3 â 10-3 level


W tb anomalous couplings limit on TEVATRON and LHC:
(E.Bo os,L.Dudko,T.Ohl,EPJ99)
0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1 -0.2 -0.4 0.03 0.025 0.02 0.015 0.01 0.005 0 -0.005 -0.01 -0.015 -0.06

FL2

-0.3

-0.2

-0.1

0

0.1

0.2

0.3 0.4 FR2

FL2

-0.04

-0.02

0

0.02

0.04

0.06 FR2


Uncorrelated limits on anomalous couplings from measurements at different machines. F Tevatron ( LHC ( e ( se e ( se
sy s.
+ e- + e-

L 2

F

R 2

sy s.

5%)

10%)

= 0.5 TeV) = 2.0 TeV)

-0.052Â+0.097 -0.008Â+0.035 -0.1 Â+0.1

-0.18 Â+0.55

-0.12 Â+0.13 -0.1 Â+0.1

-0.24 Â+0.25

-0.016Â+0.016


FCNC couplings · Couplings: tq g , tq , tq Z , where q = u, c L
ef f

1 = [

,Z tq

¯ etµ q F

µ ,Z

+

g tq gs

¯ t

µ

i qG 2

iµ

] + h .c .

Information on FCNC couplings come from either top pair production with subsequent decays to rear modes t q V , where V = , Z, g or from additional contributions to the single top production
t t

q q tc ¯ ¯ t

cg tg t

cq tq ¯ ¯

g g tc ¯


All present and expected limits are presented in terms of Br fractions:
(t q g ) (t q Z ) (t q Z )


= "



g tq



!2

8 s m 3
2

3 t

,

(t q ) = 1 2 4MZ sin2 2 1-
2 MZ m2 t



tq



!

2

2m3 , t
2 2 MZ m2 t

=

Z |vtq |2 + |aZ | tq



=



Z tq



!2

"

m

3 t

W

1- 2+

2 MZ m2 t 2 MZ m2 t

!

1+2

!

,

m

3 t

1 sin2 2

W

!2

!

Current constraints
CDF BR(t g q ) BR(t q ) BR(t Z q ) 29% 3.2% 32% LEP-2 ­ ­ 7.0% HERA ­ 0.7% ­


Future expectations
Tevatron t gq q Zq Run II 0.06% 0.28% 1.3% decay 1.6 â 10 2.5 â 10 1.6 â 10
-3 -5 -4

LHC production 1 â 10 3 â 10 1 â 10
-5 -6 -4

e + e- s > 500 GeV ­ 4 â 10 2 â 10
-6 -4


Charged Higgs in Top Decay (impact of tau polarization)
b t W+ l nl + Nl t H+ l nl + Nl b

diagr.1

diagr.2

In the rest frame of top t bR b b ¯
where a resonance R is W boson or charged H
1 d dy

=

x

max

1 -x

min

(1 - P )log (1 - P )log
where y =

xmax xmin xmax y

+ 2P y (

1 x
min

+ 2P (1 -
min

y x

-

1 x
max

), 0 < y < x x
min

min



max

),
=
max E Mtop



2 MR 2Mtop

top E Mtop

,

x

=

min E Mtop

,

x

max

,

E

min

=

,

E

max

=

Mtop 2

P = -1 for W boson and P = 1 for charged Higgs


¯ e+ e- tt b¯ + 2j ets b Simulations are performed for e+ e- collisions at 500 GeV cms and for 500 f b-1 integrated luminosity -meson energy spectrum for the MSSM point tan = 50, µ = 500, MH ± = 130 GeV with B r(t H + b) = 9.1% E.B., S.Bunichev, M.Carena, C.Wagner
E in Top r/f
Events /2GeV 400 350 300 250 200 150 100 50 0 0 10 20 30 40 50 60 70 80 90 100 GeV

From the signal+backgr fit M

H±

= 129.4 +/- 0.9 GeV


Extra Vector and Scalar Bosons
T.Tait, C.-P.Yuan hep-ph/0007298

q W

t

q

b

The NLO rate of q q W, W t ¯ (S ) in pb at the Tevatron (lower curves) and LHC ¯ b (upper curves) for various coupling parameters


c
+

t

b

b
+

The LO rate of single top production through the reaction c ¯ b M± in the top-color model.

t ¯ as a function of b


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.)
Entries/50 GeV
*Br (fb)

10

3

400

10
200

2

10

0

1000

Reconstructed mtt (GeV)

2000

1

1000

2000

3000

4000

mtt (GeV)

5000

¯ Measured tt invariant mass distribution for reconstruction of a narrow resonance ¯ of mass 1600 GeV decaying to tt and value of â BR required for a 5 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, production 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 possible New physics processes · Possible discovery and study of various New physics effects via top production and/or decay


V. Bunichev