Perspectives on singletopquark production - PowerPoint PPT Presentation

Title: Perspectives on singletopquark production


1
Perspectives on single-top-quark production

• Zack Sullivan
• Aspen Winter Conference 2007

2
What is single-top-quark production?
Tevatron
LHC (s-chan?)
t-channel
s-channel
Wt associated
For details see ZS, PRD70, 114012 (04)
1 Harris, Laenen, Phaf, ZS, Weinzierl, PRD 66
(02) 054024 2 Tait, PRD 61 (00) 034001 Belyaev,
Boos, PRD 63 (01) 034012
3
Learning about the Wtq vertex

• Measuring B(t?Wb) only tells us Vtb Vtd, Vts
• 
  CDF, PRL 86 (01) 3233
• In the SM Vtb 0.9991 0.0001 (PDG 2004)
• Relax Unitarity of CKM matrix
• Single-top cross section proportional to Vtb2
• Measure B(t?Wb) in t t, extract ?Vtb ??t/2
• in each channel

See polarization of top at production (5 fb-1)
Vtb
Stelzer, ZS, Willenbrock, PRD 58 (98) 094021
4
Current evidence
CDF results from Note 8588 Note 8585
D0 results from hep-ex/0612052
Twice as much data on tape. We will know better
in summer.
Vtb1.3?0.2
New NN
Vtb0.3
5
New phenomena affects- and t-channel separately
Resonances
New q-t-X verticies
Vtb1.3?0.2
Z?
KK-modes etc.
New NN
BR(t?Zc)lt0.33 CDF, PRL80,2525(98) will soon change
4th generation? t-T mixing? Suppress t not s?
Vtb0.3
6
Model independent searches forW ? at Tevatron
and LHC

• LHC can test SM-like W? bosons up to 5.5 TeV
• Couplings of 10-1?gSM up to 3 TeV

7
The lasting importance ofsingle-top-quark
production
Theory
Experiment
A decade of intense collaboration and inspiration
has led to a new or modified understanding of
every aspect of the observable cross section.
8
How to see the initial state
W-gluon fusion (circa 1996)
A mathematical trick let us resum large logs
into a b PDF.
Gained new nomenclature t-channel production
1st testable need for a heavy-quark PDF

• Heavy-quark (b, c) PDFs are more than a trick
• b c are full-fledged components of the proton
  structure.
• HERA is beginning to see evidence of intrinsic
  charm stay tuned!
• Leads to other processes ,
  Zb/Zc, Zbj/Zcj, Wbj

9
Fully exclusive NLO calculations
Required new methods to calculate fully exclusive
cross sections with massive states

• Worked out analytically in
• Harris, Laenen, Phaf, ZS, Weinzierl, PRD 66,
  054024 (02)
• Numerically studied using ZTOP
• ZS, PRD 70, 114012 (04)
• Now in MCFM 5.1
• Campbell, K. Ellis

New baseline NLO standard
Cuts ETjgt15 GeV, ?jlt2.5, no cuts on t
10
Paradigm of jet calculations

• How do we interpret fully differential NLO?
• We are calculating jets not partons
• Calculations are not well defined
• w/o a jet definition
• Bad things happen if you
• treat jets as partons

dNLO jets/hadrons ? better
partons Our language should evolve to recognize
this
ZTOP
11
NLO matching to event generators required for
t-channel single-top

• HERWIG/PYTHIA Wbb 1/3 of LO prediction
• Both used
• IS radiation is too soft and forward, miss hard b
• Background to W-Higgs 3 times larger as well
• Feed LO events from MadEvent/CompHEP into
  HERWIG/PYTHIA and normalize to NLO tj, tb, tjj,
  tbj samples.
• Other prescriptions MLM, CKKW, etc.

bad shapes
ZTOP
ZS, PRD 70, 114012 (04)
12
Lessons of top-quark polarization
d

• The top quark decays before it hadronizes, and
  the pure V-A induce a 100 correlation between
  the d-jet in the event and the e/m
• The angular variable is a nice
  discriminant

V-A
V-A
13
Angular correlations are important
Many analyses do this (e.g. SUSY, H?WW,
etc.) Getting these right will be a big concern
in a few years
14
Use the correlated information
ZS, PRD 72, 094034 (05)
What is used now
Why use these, when the primary information is in
the well-measured angular correlations?
15
Conclusions

• Things not covered
• 1st PDF uncertainties
• Modifed Tolerance Method (what you use for PDF
  errors)
• Kinematic uncertainties
• Push for NN b-tags and clever uses

• Single-top-quark production forces us to
  reconsider our intuitions and develop new
  technologies that push the frontiers of
  perturbative QCD
• We will have precision measurements of weak
  interaction structure.
• Single-top has changed how we think about the
  cross section.

It will be vital to the success of the LHC to
develop close interactions between theory and
experiment of the type single-top-quark
production has enjoyed.
16
Extra slides
17
Uncertainties (PDFs, kinematic)
18
Exclusive t 1 jet at NLO
ZS, hep-ph/0408049
b-jets/non-b jets have different distributions
Cuts ETjgt15 GeV, ?jlt2.5, no cuts on t

• Cross sections uncertainties depend on cuts
• Shapes are stable, only normalizations vary
• s-channel, t bNLO 1.54?t bLO
• t-channel, t jNLO t jLO if using ?lQ2,
  ?hQ2mt2

ZTOP
19
Single-top-quark production
For details see ZS, hep-ph/0408049

• s-/t-channel now known fully differentially1
• If Mt 178.04.3 GeV ? ?t 1.880.27 pb ?s
  0.820.11 pb
• First honest PDF uncertainties included above
• ??t 11 8 ??s 4.7 3.9

1 Harris, Laenen, Phaf, ZS, Weinzierl, PRD 66
(02) 054024 2 Tait, PRD 61 (00) 034001 Belyaev,
Boos, PRD 63 (01) 034012
20
Experimental reach vs. theory

• Theory errors are smaller than experimental reach
• Not necessarily true with extreme cuts
• Tevatron is statistics limited until 30 fb-1
• LHC is completely systematics limited
• factor of 10 discrepancy in background estimates
  at LHC
• A new study is needed to resolve whether
  s-channel is observable at the LHC needs to use
  corrected NLO signal and backgrounds.

21
W ? at Tevatron

• Best way to look
• for W ? bosons.
• Simmons Tait, Yuan (97)
• Fully differential NLO for arbitrary V, A
  couplings ZS, PRD 66 (02) 075011
• First use modified tolerance method for PDF
  uncertainties
• CDF used MW ?gt550 GeV (was 420 GeV) PRL 90
  (03) 081802
• Look for resonant peak in Wbb invariant mass
  same rate for L/R-handed
• Use spin correlations to tell if W ? has left or
  right-handed interactions
• Run II can reach 800-900 GeV (2 fb-1)

Run I
ZS, PRD 66 (02) 075011
22
W ? at LHC

• Huge cross section at LHC
• 10 TeV W ? _at_ 50/yr (high lum.)
• Total rate less than s-channel single-top sample
  if MW ?gt2 TeV
• Must use invariant mass
• Only background gt 1 TeV t-channel single-top
• Completely missed by HERWIG/PYTHIA!

23
W ? at LHC

• Using Mbjl? can reach 5.5 TeV
• PDFs induce kinematic limit here
• Can reach couplings 10? smaller than gSM!
• Most perturbative theories predict couplings
  within factor of 2 of gSM

• Coupling limit is model independent (if ?W ? lt
  MW ?)
• Can use ZS, PRD 66 (02) 075011 to find limits in
  favorite model
• Littlest Higgs models can be ruled out in 1 year
  at LHC!

24
Topcolor-Assisted Technicolor

• Both ? and ?0 might appear in single-top-quark
  production
• Width is typically few ?100 GeV, so may not be
  clear resonance
• Cross section large enough to reach 1 TeV for ?
  at LHC
• ? is fraction of Mt due to TC
• Rb puts lower limit of M? gt 250 GeV

25
More on Top pions

• Topcolor can have charged heavy scalars ?
• Very similar to W ? production, but scalar
  resonance
• Can also produce neutral top pions ?0 that decay
  to tc

Tait, Yuan, PRD 63 (01) 014018
26
Phenomenological Observations

• b-tagging has room to grow 30-40/b ? 60/b
• 2.5x improvement in signal
• This is central to finding new physics H, SUSY,
  W ?, Z ?,
• Remember single-top is a large component of the
  Wbb background
• Jet-energy resolution limits cut on Mt(bl?),
  currently 35 GeV ? 20 GeV
• 1.8x drop in Wjj background
• Hurts MET from calorimeter
• A larger cone size may lower out-of-cone
  uncertainties, and increase signal.

• Lower ETj thresholds from 20-30 GeV ? 15 GeV.
• 1.5x improvement in signal
• ?e,? lt 1 ? ?e,? lt 2
• 1.5x improvement in signal
• Is there a clever way to improve range in ??
  for CDF?

27
Extra W ? plots