Transcript Gauge Extension of the MSSM

Single Top
(And the Search for New Physics)
Tim M.P. Tait
Fermi National Accelerator Laboratory
CTEQ Summer School
Madison, Wisconsin
6/30/2004
Outline
•
•
•
•
•
•
Introduction: Why is single top important?
Production modes in the SM
Tools
Beyond the SM
Polarization
Summary
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The King of Fermions!
• In the SM, top is superficially
much like other fermions.
• What really distinguishes it is
the huge mass, roughly 40x
larger than the next lighter
quark, bottom.
• This may be a strong clue that
top is special in some way.
• It also implies a special role
for top within the Standard
model itself.
• Top is the only fermion for
which the coupling to the
Higgs is important: it is a
laboratory in which we can
study EWSB.
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SM Fermions
Tim Tait
3
Top in the Standard Model
• In the SM, top is the marriage between a left-handed
quark doublet and a right-handed quark singlet.
• This marriage is consummated by EWSB, with the mass
(mt) determined by the coupling to the Higgs (yt).
• This structure fixes all of the renormalizable interactions
of top, and determines what is needed for a complete
description of top in the SM.
• Mass: linked to the Yukawa coupling (at tree level)
through: mt = yt v.
• Couplings: gS and e are fixed by gauge invariance. The
weak interaction has NC couplings, fixed in addition by
s2W. CC couplings are described by Vtb, Vts, and Vtd.
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Why measure single top?
• Single top is our primary means to measure top’s CC interactions.
• If top indeed plays a special role in EWSB, we would expect its
weak interactions would be the place in which we could realize that
it is special. Thus, there is interest beyond t t production.
• We know that top has a weak interaction, but not much beyond that.
• This information comes from the decay, t W b.
Wm-t-b vertex:
gVtb m
 1  5 
2
GF mt3
2
Gt 
Vtb  ...
8 2
Left-handed!
• However, because Gt is much smaller than experimental resolutions,
it is very difficult to use the decay to measure the magnitude of the
weak interaction.
• Single top will be visible sometime in the next year(s) at run II!
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SM: Vtb, Vts, Vtd
2
• In the SM, the CC
Vtb
BR(t  Wb)

 0.940.31
CDF:
0.24
2
2
2
interactions are described
BR(t  Wq) Vtd  Vts  Vtb
by Vtb, Vts, and Vtd.
CDF PRL86, 3233 (2001)
V
>>
V
,
V
tb
ts
td
• Vts and Vtd are measured
indirectly from b physics.  0.9739  0.9751 0.221  0.227 0.0029  0.0045 
 0.221  0.227 0.9730  0.9744 0.039  0.044 

• Vtb can be constrained 
 0.0048  0.014
0.037  0.043 0.9990  0.9992 
using unitarity.
• This assumes the SM, with V †V  1  Vub 2  Vcb 2  Vtb 2  1
3 generations.
• Physics beyond the SM  0.9730  0.9746 0.2174  0.2241 0.003  0.0044 
0.213  0.226
0.968  0.975
0.039  0.044


can easily modify these   0.08

 0.11
0.07  0.9993


results (in a big way).


– I.e. a Fourth generation
PDG: http://pdg.lbl.gov/pdg.html
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Overview: Single Top in the SM
• Single top quarks are (dominantly) produced at hadron colliders
through interactions involving a W boson and b quark.
2
• Thus, rates are directly proportional to Vtb
“time”
• At tree level there are three modes:
• S-channel W exchange
– Large rates at Tevatron run II, small at LHC.
• T-channel W exchange
– Dominant mode at Tevatron run II and LHC.
• T W associated production
– Very tiny at Tevatron run II, large rate at LHC.
• At higher orders, these processes mix with each other and with
QCD (t t) production combined with top decay.
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S-channel Mode: Basics
•
•
The s-channel mode proceeds through a virtual W boson, which “decays”
into t b. The W-boson has time-like momentum.
Thus, it looks quite a bit like high mass e+ n production.
  gV  ud  PLuu  
*
ud
•
•
m
 gmn  pWm pnW / MW2
s  MW2
2 g 2VudVtb
u P u  u P u 
 gVtb  ut  PLub  
2  t R d  u L b 
s  MW
m
The initial state is dominantly u d. This is why it is reasonably large at the
Tevatron, but small at LHC.
Experimental Signature: W b b
Stelzer, Willenbrock PLB357, 125 (1995)
– Top decay:
• W boson: Leptonic decay is very helpful with QCD backgrounds.
• b jet: together with W, “reconstructs” mt.
– b: Quite high pt. Very useful to tag it and
thus remove backgrounds, mostly from
t-channel mode.
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S-channel Mode: Beyond LO
•
•
At NLO in aS, corrections look a lot like W production. (+ final state corrections).
The inclusive s has been known at NLO for some time.
Smith, Willenbrock PRD54,6696 (1996)
Mrenna, Yuan PLB416,200 (1998)
•
•
Differential cross sections are also known at NLO.
Dominant (theoretical) uncertainties:
Harris, Laenen, Phaf, Sullivan,
Weinzierl, PRD 66 (02) 054024
– Top mass: dmt ~ ±5 GeV leads to: ds ~ ±6%
– Scale variation: mtb/2 < m < 2 mtb leads to: ds ~ ±5%
– PDFs are predominantly valence quarks; reasonably well known, ds ~ ±5%
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S-channel Mode: Polarization
• Strong polarization between top spin and “d” quark direction:
– This is a consequence of the vector particle exchange
2 g 2VudVtb
u P u  u P u 

2  t R d  u L b 
s  MW
Mahlon, Parke PLB476 323 (2000); PRD55 7249 (1997)
– At Tevatron, most d’s come from the anti-proton, implying the top spin
correlates at almost 100% with the beam axis.
– The helicity basis (or polarization along the direction of motion) is
something like 80% in the SM.
– This result doesn’t depend on the vector exchange, making the helicity
basis an interesting means to study physics beyond the SM.
– At the LHC, with no initial anti-proton, the helicity basis is thus still
interesting.
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T-channel Mode: Basics
•
•
The t-channel mode also proceeds through a virtual W boson, exchanged
between a light quark line and a b. The W has space-like momentum.
Thus, it looks something like (“double”) deeply inelastic scattering.
  gV  ud  PLuu  
*
ud
•
•
m
 gmn  pWm pnW / MW2
t  MW2
2 g 2VudVtb
 ut PRud   uu PLub 
 gVtb  ut  PLub  
2


t  MW 
m
The initial state is dominantly u b. This is why it is reasonably large at both
Tevatron and LHC.
Dawson NPB249, 42 (1985)
Dicus, Willenbrock PRD34,155 (1986)
Experimental Signature: W b + forward jet
Yuan PRD41, 42 (1990)
– Top decay:
• W boson: Leptonic decay is very
helpful with QCD backgrounds.
• b jet: together with W, “reconstructs” mt.
– jet: Moderately high pt. It can be used as
a tag to remove backgrounds.
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T-channel Mode: Beyond LO
•
•
At NLO in aS, corrections look like DIS (times two).
The inclusive s has been known at NLO for some time.
Sullivan, Stelzer, Willenbrock PRD56, 5919 (1997)
•
•
•
Harris, Laenen, Phaf, Sullivan,
Weinzierl, PRD 66 (02) 054024
Differential cross sections are also known at NLO.
Inclusive rate has resummed “W-gluon fusion” into “W-b fusion”.
Dominant (theoretical) uncertainties:
– Top mass: dmt ~ ±5 GeV leads to: ds ~ ±3%
– Scale variation: mt/2 < m < 2 mt leads to: ds ~ ±4%
– PDFs include gluon/sea; not so well known, ds ~ ±7%
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T-channel Mode: Polarization
• Strong polarization between top spin and “d” quark direction:
– This is again a consequence of the vector particle exchange
2 g 2VudVtb
 ut PRud   uu PLub 

2


t  MW 
Mahlon, Parke PLB476 323 (2000); PRD55 7249 (1997)
– For this process, the d’s are the forward ‘spectator’ jets, implying the top
spin correlates at almost 100% with the jet direction.
– The process b d t u pollutes this slightly.
– The helicity basis is also very highly polarized in the SM: around 83%.
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T W Mode: Basics
•
The third mode has an on-shell W boson.
– Like the other two modes, it is
proportional to |Vtb|2.
– The fact that the W is real and observable
makes it interesting as a direct probe of
the W-t-b vertex, with less worry that new
physics may be contributing.
•
•
Tait PRD61, 034001 (2000)
Belyaev, Boos, PRD63, 034012 (2001)
The initial state is dominantly g b. This, and the heavy final state, is why it
so tiny at Tevatron, but considerable at LHC.
Experimental Signature: W+ W- b
– Top decay:
• W+ boson: Leptonic decay is very helpful with QCD backgrounds.
• b jet: together with W, “reconstructs” mt.
– W-: It can be used to remove some QCD backgrounds, but makes the
events overall look a lot more like t t, which is huge at the LHC.
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T W: Beyond LO
Zhu, hep-ph/0109269
• Total rate “known” at NLO.
– Missing q q initial states.
– At NLO, this process mixes with t t
followed by top decay.
NLO
• Uncertainties:
– Scale (mt + mW)/2 < m < 2(mt+mw):
ds ~ ±5%
– PDFs: ds ~ ±10%
LO
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• Polarization is very complicated,
with no known basis resulting in
high top polarization.
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Single Top in the SM
t-channel
t-channel
s-channel
LHC
tW
Run II
s-channel
tW
s
Run I Limits
Tevatron
Run II
LHC
st (NLO)
< 13.5 pb
1.98±0.13 pb
247±12 pb
ss (NLO)
< 12.9 pb
0.88±0.09 pb
10.7±0.9 pb
stW (LL)
0.09±0.02 pb
56±8 pb
Total
2.95±0.16 pb
314±15 pb
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Sum of top and anti-top.
Tim Tait
Any day at Run II!
CDF PRD65, 091102 (2002)
DØ PLB517, 282 (2001)
16
W Polarization
• W Polarization
– This is a direct test of the left-handed nature of the W-t-b vertex.
– SM: Left-handed interaction implies that W’s are all left-handed or
longitudinal.
DØ: f0  0.56  0.31  0.04
– SM: Depends on mt & mW:
CDF: f0  0.91  0.37  0.13
mt2
# longitudinal W's
f0 

Total # W's
2MW2  mt2
70%
CDF PRL84, 216 (2000)
DØ hep-ex/0404040
– lW correlated with the direction of pe compared with the direction of pb
in the top rest frame.
– The W polarization is independent of the parent polarization. Thus, it is a
good test of W-t-b and can be measured with large statistics from QCD
production of top pairs.
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Top Polarization
• Top Polarization
– Single tops have close to 100% polarization for the correct
choice of basis. Even the helicity basis makes an interesting
prediction.
– t polarization correlates with pe:
top
M 
 g 2Vtb
m
 un  PL ue  2

u

PL ut 
b

 p M2 
W
W
m
2 g 2Vtb
u P u  u P u 
 2
2  b R n  e L t 
pW  M W
anti-top
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Tools
• Pythia (Herwig)
– Leading order, no polarization.
– S-channel in Pythia: kludged together
– Probably best used in tandem with MADevent or COMPhep
• ONETOP
– Leading Order; interfaced with Pythia
– All processes, including polarization
• ZTOP
– Next to leading order (differential) s- and t-channels, no
polarization coded.
– Publicly available soon.
• MCFM
– Next to leading order (s- and t-), leading order tW.
– Version coming soon including single top processes.
– Will include final state radiation off of top.
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How to Make Single Tops
BEYOND
the Standard Model
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New Interactions
•
•
•
A model independent way to study new physics is provided by effective
Lagrangians, adding interactions beyond those in the SM.
The SM already contains all renormalizable interactions (with couplings of
mass dimension 4 or less); we must include non-renormalizable terms.
Couplings for ‘higher dimensional’ operators have negative dimension so
that the Lagrangian stays at dimension 4:
Counting Dimension

H, Vm
m
•
•
•
: 3/2
: 1
: 1
dimension 0
g
dimension 4
   m V m
3/ 2
3/ 2
1
dimension -2
1
L2
dimension 6
m   m 
3/ 2
3/ 2 3 / 2
3/ 2
This theory makes sense as an expansion in energy. Observables depend
on En / Ln, so provided E << L, the expansion makes sense.
Gauge symmetries of the Standard Model such as SU(3) invariance, etc.
are still respected by the new interactions.
They can be understood as residual effects from very heavy particles.
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Nonstandard Top Interactions
• Top may couple in a funny way to strange, down, or bottom:


g
Wtdi
Wtdi
m

t

P
d


t

P
d
W

R
m R i
L
m L i
  h.c.
2 i
– All of these modify all three single top rates.
– But aren’t these operators dimension 4?
• Yes, but their SU(2)xU(1) description
was dimension 6!
1
H † H  Q3 m  Dm Q2   h.c.
2 
LWts
v 


t
2
L
• Top may have FCNC’s with up or charm and Z/g/:
g
cos W

Ztui
R
m
PL  Wm s   ...
 LWts

t m PR ui   LZtui t m PLui Z m  h.c.
i
 1

1
 g S   gtui ts mn PR ui  gtui ts mn PLui G mn  h.c.
LL
i  LR

 1
 mn
2
1
 e   tui ts mn PR ui   tui ts mn PLui F  h.c
3 i  LR
LL

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2
Wts
Tim Tait
These new interactions
can arise in many models.
They lead to new single
top modes, top decays,
and more exotic
processes …
22
New Charged Interactions
Wts
• As my first case, I turn on the W-t-s coupling:  L  0.41
• To be perverse, at the same time I turn on a negative W-t-b:  LWtb  0.164
• I chose this because it looks like the SM with a funny CKM matrix:
 0.9745 0.224 0.0037 
 0.9745 0.224 0.0037 
 0.224 0.9737 0.042    0.224 0.9737 0.042 




 0.008 0.040 0.9991  SM
 0.008
0.55
0.835  Effective
• Clearly, all three single top cross sections change:
s
s
s
s-channel: over-all rate
unchanged, but now we
produce t s 1/3 of the time.
CTEQ, 6/30/04
s
t-channel and tW: The rates themselves change, because now there
is significant production from an initial state strange quark, with a
larger probability than bottom to be found at high x in the proton.
Tim Tait
But we needed to tag the b quark to see the s-channel at all!
23
FCNC Interactions
• As a second example, consider a FCNC interaction of Z-t-c:


g
 RZtc t m PR c   LZtc t m PL c Z m  h.c.
cos W
Note left- and right-handed
versions – influence polarization!
• We could have chosen Z-t-u, instead (or as well).
• New s-channel and tZ modes:
– …which won’t be counted by the
usual single top analyses,
because there is no extra b or W.
• T-channel mode:
tZ
s-channel
– Like the W-t-s story, takes
advantage of larger c content of
proton compared to b.
t-channel
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Charged Resonances
• A charged resonance (which couples to t and b) can mediate single
top production in the same way the W boson does in the SM.
• In many theories (I’ll show a couple in a moment), such objects
prefer to couple to the third generation, which makes top a
particularly good place to look for them.
• Generically, I will refer to a scalar of this type as a “charged Higgs”
and a vector of this type as a W’.
• These clearly affect the s- and t-channel rates, and turns on new
processes (t W’ and t H-) analogous to t W.
• First let’s run through some models which contain these objects,
then see what they do to single top.
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W’ : “ Topflavor ”
SU(2)1 x SU(2)2 x U(1)Y
•
•
•
•
Generically, W’ bosons come
from extending the EW gauge
sector to include new forces.
The usual SU(2)L is the diagonal
combination.
The SU(2) x SU(2) breaking
occurs through a Higgs S, which
is a bi-doublet under both
SU(2)’s.
This model has been called
“Topflavor”: a separate weak
interaction for the 3rd family.
Chivukula, Simmons, Terning PRD53, 5258 (1996)
Muller, Nandi PLB383, 345 (1996)
Malkawi, Tait, Yuan PLB385, 304 (1996)
CTEQ, 6/30/04
Extra SU(2) group contains
additional W and Z bosons!
Recently proposed to increase
mh in the MSSM!
Batra, Delgado, Kaplan, Tait, JHEP 0402,043 (2004)
Tim Tait
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Charged Higgs: H+
• In the SM, the Higgs doublet contains a pair of charged scalars, and
two (real) neutral scalars.
• However, after EWSB, the charged and one of the neutral scalars
are “eaten”: they come become the longitudinal W and Z bosons.
• The one remaining boson is the Higgs particle.
• In a theory with extra Higgs doublets, there will be more “left-overs”
which become physical Higgses.
• For example, in a model with two Higgs doublets (as minimal SUSY
models for example), there will be a pair of charged Higgses, and
three neutral Higgs after EWSB.
• Because the fermion masses come from interactions with the Higgs,
the 3rd generation (and top particularly) generically couples much
more strongly. For example in SUSY:
mb tan  
 mt cot 
H -t-b coupling : 
PR 
PL 
v
v


+
CTEQ, 6/30/04
Tim Tait
Right-handed coupling!
27
Top Pion: +
• Charged Higgs-like objects also occur in theories with dynamical
electroweak symmetry-breaking.
• As an example, let’s consider Topcolor-assisted-Technicolor (TC2).
– Technicolor works pretty well to generate W/Z masses, but has
problems with the large top mass. Generic solutions aren’t consistent
with precision EW data.
– To help technicolor out with the top mass, Chris Hill introduced a new
force which was an SU(3) ‘color’ interaction which only top feels.
– This force adds some extra EWSB by forming a Higgs doublet as a
bound state of top quarks. This extra EWSB couples strongly to top,
and provides a large mass.
– This again looks something like a two Higgs doublet model. The extra
scalars are expected to be among the lightest of the new states.
– They couple strongly to top by construction, and very weakly to other
fermions.
– Their phenomenology is very similar to the charged Higgs of SUSY.
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±

/
±
H
• How does H± affect single top?
– S-channel mode: the
intermediate particle is time-like,
and can go on-shell. Large
enhancements are possible,
provided there is enough energy.
M 
He, Yuan PRL83,28 (1999)
g H2 
s  M H2   iM H  G H 
s > 0!
– T-channel mode: the particle is
space-like and never goes onshell. The extra contribution to
the cross section is always very
tiny.
2
M 
t < 0!
CTEQ, 6/30/04
gH 
t  M H2 
Tim Tait
29
W’
• We can repeat a similar
analysis for the W’.
• The s-channel process can
show a large enhancement if
there is enough energy for the
W’ to be produced on-shell.
• The t-channel mode shows no
large enhancement, because
the additional cross section is
suppressed by the heavy mass.
• The topflavor W’ has lefthanded couplings, and thus
does not alter the expectations
for top polarization compared to
the SM.
CTEQ, 6/30/04
Sullivan hep-ph/0306266
Simmons, PRD55, 5494 (1997)
Tim Tait
30
s- Versus t-Channels
• s-channel Mode
– Smaller rate
– Extra b quark final state
– ss a |Vtb|2 in SM
• Sensitive to resonances
– Possibility of on-shell
production.
– Need final state b tag to
discriminate from
background: no FCNCs.
CTEQ, 6/30/04
• t-channel Mode
– Dominant rate
– Forward jet in final state
– st a |Vtb|2 in SM
• Sensitive to FCNCs
– New production modes.
– t-channel exchange of
heavy states always
suppressed.
Tim Tait
31
All Together
• We have seen how the s-channel mode is sensitive to charged
resonances.
• The t-channel mode is more sensitive to FCNCs and new
interactions.
• The tW mode is a more direct measure of top’s coupling to W and a
down-type quark (down, strange, bottom).
• From a theoretical point of view, they teach us different things.
• From an experimental point of view, they have different signatures
and different systematics.
• Even in the SM, they can be used together in a helpful way: Vtb
– Each rate is a different quantity proportional to |Vtb|2
– They provide an important cross-check on Vtb even in the SM.
– Of course, if there is new physics in single top production, the fact that
each mode responds differently can already give us a hint as to what
form the new physics takes, even before we see it manifest clearly.
CTEQ, 6/30/04
Tim Tait
32
ss-st Plane
Theory + statistical (2 fb-1)
3s deviation curves
Run II
LHC
Tait, Yuan PRD63, 014018 (2001)
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Tim Tait
33
More Exotic Stuff
CTEQ, 6/30/04
Tim Tait
34
Single Top + Higgs
SM like
No W coupling
No t coupling
yt = -1 x ytSM
• Very small in the SM
because of an efficient
cancellation between two
Feynman graphs.
• Thus, a sensitive probe
of new physics.
• Observable at LHC?
Tait, Yuan PRD63, 014018 (2001)
CTEQ, 6/30/04
Tim Tait
35
R-parity Violating SUSY
•
•
•
•
In SUSY theories, if R-parity is
violated, super-partners can
contribute at tree level to SM
processes such as single top.
Such interactions generally lead to
p decay, constraining their size.
However, for the 3rd family such
bounds are much weaker.
In this example, there is s-channel
stop ‘production’ followed by decay
into top through R-conserving
interactions into neutralino and top.
Berger, Harris, Sullivan PRD63,115001 (2001)
CTEQ, 6/30/04
Tim Tait
36
R-parity II: Slepton Exchange
R-parity violating interactions which
Violate lepton number can produce
Single tops through exchange of the
Super-partners of leptons (sleptons)
In either the s- or t- channels.
Oakes, Whisnant, Yang, Young, Zhang PRD57, 534 (1998)
CTEQ, 6/30/04
Tim Tait
37
Summary
• Top is unique as a laboratory for EWSB and fermion masses.
• Its huge mass may be a clue that it is special, and it plays an
important role in the SM and beyond.
• Single top production will most likely be observed within a year. This
will be the first direct measurement of top’s weak interactions.
• There are three modes: s-channel, t-channel, and associated
production with a W. All three are a measure of top’s CC weak
interactions.
• S-channel mode: appreciable at run II, sensitive to new charged
resonances.
• T-channel mode: dominant at run II and LHC, sensitive to nonstandard couplings of top.
• tW mode: only visible at LHC, largely sensitive only to top’s CC
weak interactions.
• SM makes definite predictions for spin, and they can be tested.
• It will be exciting to learn the TRUTH about top!
CTEQ, 6/30/04
Tim Tait
38
Supplementary Slides
Measurements
• How well are these quantities known?
• gS, e, and s2W are well known (gS at per cent level, EW
couplings at per mil level) from other sectors.
• mt is reconstructed kinematically at the Tevatron:
– Run I: mt = 178 ± 4.3 GeV
– Run IIb: prospects to a precision of ± 2 GeV (systematic).
• Vtd, Vts, and Vtb are (currently) determined indirectly:
–
–
–
–
Vtd: 0.004 – 0.014
(< 0.09)
PDG: http://pdg.lbl.gov/pdg.html
Vts: 0.037 – 0.044
(< 0.12)
Vtb: 0.9990 – 0.9993 (0.08 – 0.9993)
These limits assume the 3 (4+?) generation SM, reconstructing
the values using the unitarity of the CKM matrix.
• Vtb can be measured directly from single top production.
CTEQ, 6/30/04
Tim Tait
40
Top Sector and SUSY
Top plays an important role in the minimal supersymmetric standard model.
•
Most importantly, the MSSM only
survives the LEP-II bound on mh
because of the large yt:
•
The large top Yukawa leads to the
attractive scenario of radiative
electroweak symmetry-breaking:
SUGRA report, hep-ph/0003154
Radiative EWSB
Heinemeyer et al, JHEP 0309,075 (2003)
•
•
This mechanism is also essential
in many little Higgs theories.
(mt < 160 GeV rules out MSSM!)
CTEQ, 6/30/04
Tim Tait
41
Hints from b Couplings?
2.5 s deviation
• If top is special, b, its EW
partner, must be as well.
• Right-handed b couplings
measured at LEP deviate from
the SM at the ~ 3 s level.
• Left-handed at ~ 2 s.
• It has been argued that this
goes beyond the statistical
significance, because of the
role of AbFB in mH fit.
Chanowitz PRL87, 231802 (2001)
Choudhury, TT, Wagner, PRD65, 053002 (2002)
CTEQ, 6/30/04
• The “beautiful mirror” solution
requires an extra top-like quark
with mass < 300 GeV.
Tim Tait
42
More Phenomenology of H+
Marcela Carena + ~ 1 billion friends, hep-ph/0010338
• Rare Top Decay: t
H+ b
– Tevatron Run II (2 fb-1) : tan   1, M H  120 GeV
– LHC (100 fb-1):

CERN top Yellow Book, hep-ph/0003033
• M H  mt ? g b

t H- at LHC! (100 fb-1): M H
NLO: Berger, Han, Jiang, Plehn hep-ph/0312286

 400 GeV (low tan  )
Les Houches Higgs Report, hep-ph/0203056
• Breakdown of MSSM Higgs mass relation:
M H2   M A2  MW2
• Testable through pp
M H   300 GeV
CTEQ, 6/30/04
A0 H+ at LHC (100 fb-1):
Cao, Kanemura, Yuan hep-ph/0311083
Tim Tait
43
t t Production
• At a hadron collider, the largest
production mechanism is pairs of top
quarks through the strong interaction.
• (Production through a virtual Z boson
is much smaller).
• At leading order, there are gluon-gluon
and quark-anti-quark initial states.
• At Tevatron, qq dominates (~85%).
• At LHC, gg is much more important.
CTEQ, 6/30/04
Tim Tait
44
t t Production Rates
P. Azzi, hep-ex/0312052
LHC: stt ~ 850 ± 100 pb
tt is a major background to many new
physics searches (i.e. Higgs).
CTEQ, 6/30/04
• NNLO-NNNLL+: NLO + soft
gluon corrections, reexpanded to NNNLL & some
NNLO pieces.
• “Pure” NLO curve includes
PDF uncertainties.
• At Tevatron, uncertainties in
threshold kinematics
dominate. PDF uncertainties
are also important.
• At the LHC, uncertainties are
of the order of 10% are from
the gluon PDFs and variation
with the scale m.
Tim Tait
45
tt Resonances
•
•
A neutral boson can contribute to
tt production in the s-channel.
Many theories predict such exotic
bosons with preferential coupling
to top:
– TC2, Top Seesaw: top gluons
Hill PLB345,483 (1995)
Dobrescu, Hill PRL81, 2634 (1998)
– TC2, Topflavor: Z’
Hill PLB345,483 (1995)
Chivukula, Simmons, Terning PRD53, 5258 (1996)
Nandi, Muller PLB383, 345 (1996)
Malkawi, Tait, Yuan PLB385, 304 (1996)
Future EW Physics at the Tevatron, TeV-2000 Study Group
CTEQ, 6/30/04
•
•
•
Search strategy: resonance in tt.
Tevatron: up to ~ 850 GeV.
LHC: up to ~ 4.5 TeV.
Tim Tait
46
Top Yukawa Coupling
•
•
MS
M
SM prediction for the t coupling to the Higgs:
ytMS  t  1
v
We’d like to directly verify the relation to roughly the same precision
as mt itself: a few %.
– Higgs radiated from tt pair is probably the best bet.
• LHC: yt to about 10-15% for mh < 200 GeV.
H  WW
H  bb
Maltoni, Rainwater, Willenbrock, PRD66, 034022 (2002)
NLO: Dawson, Jackson, Orr, Reina, Wackeroth, PRD 68, 034022 (2003)
CTEQ, 6/30/04
Tim Tait
47
Top Decay
•
•
SM: BR into W+b ~ 100%.
Top decay represents our first
glimpse into top’s weak
interactions.
• In the SM, W-t-b is a left-handed
interaction: m (1 - 5).
2
Vtb
BR(t  Wb)

 0.940.31
However, the decay does not offer
CDF:
0.24 •
2
2
2
BR(t  Wq) Vtd  Vts  Vtb
a chance to measure the
magnitude of the W-t-b coupling,
CDF PRL86, 3233 (2001)
but only its structure.
Vtb >> Vts, Vtd
• This is because the top width is
τ+X
well below the experimental
21%
resolutions.
jets
• Top is the only quark for which Gt
45%
>> LQCD. This makes top the only
quark which we see “bare” (in
μ+jets
15%
some sense).
 Top spin “survives” none+jets
μ+μ e+μ e+e
perturbative QCD (soft gluons).
1% 2% 1%
CTEQ, 6/30/04
15%
Tim Tait
48
Rare Decays
Solid lines: assume tc = 0.78
Dashed lines: assume no t-c-
• Many rare decays of top are possible.
• These can be searched for in large t t
samples, using one standard decay to
‘tag’ and verifying the second decay
as a rare one.
• One example is a FCNC: Z-t-c
1
g
mn
Z
m
W
(
H
Q
)
s
c


Z
t

c
mn
3
R
t
c
m
2
L
2 cos W
v mt
L2
• At LEP II, the same physics that
results in t Zq would lead to
e+ e Z*
tq.
• More possibilities, such as t c,
t
cg, etc…
 tcZ 
Top Physics, hep-ph/0003033
CTEQ, 6/30/04
Tim Tait
49
Single Top Production
•
Top’s EW interaction.
– Three modes:
• T-channel: q b q’ t
• S-channel: q q’ t b
• Associated: g b t W-
•
Any day at Run II!
Harris, Laenen, Phaf, Sullivan, Weinzierl, PRD 66 (02) 054024
Tait, PRD 61 (00) 034001; Belyaev, Boos, PRD 63 (01) 034012
Run I Limits
s
Tevatron
Run I
Tevatron
Run II
LHC
st (NLO)
1.45±0.08 pb
1.98±0.13 pb
247±12 pb
< 13.5 pb
ss (NLO)
0.75±0.07 pb
0.88±0.09 pb
10.7±0.9 pb
< 12.9 pb
stW (LL)
0.06±0.01 pb
0.09±0.02 pb
56±8 pb
Total
2.26±0.11 pb
CTEQ, 6/30/04
CDF PRD65, 091102 (2002)
DØ PLB517, 282 (2001)
2.95±0.16 pb
Tim Tait
314±15 pb
Now in MCFM!
50