John Womersley

The Top Quark: 2006 and BeyondThe Top Quark: 2006 and Beyond

An (updated) summary of TOP2006: International Workshop on Top Quark Physics

Coimbra, Portugal, January 2006

John WomersleyDirector of Particle Physics

CCLRC – Rutherford Appleton Laboratory

John Womersley

The Standard ModelThe Standard Model

• Decades of experimentation with accelerators, and theoretical synthesis, have culminated in what we call the “Standard Model”

– A theory of matter and forces

– A quantum field theory describing point-like fermions (quarks and leptons)

• matter particles

– which interact byexchanging vector bosons (photons, W± and Z, gluons)

• Force carriers

If this was all we were talking about, I don’t think we’d be here

John Womersley

a revolução está vindoa revolução está vindo! *! *

John Womersley

* revolution is coming* revolution is coming

• The standard model makes precise and accurate predictions• It provides an understanding of what nucleons, atoms, stars,

you and me are made of

• Its spectacular success in describing phenomena at energy scales below 1 TeV is based on– At least one unobserved ingredient

• the SM Higgs– Whose mass is unstable to loop corrections

• requires something like supersymmetry to fix– And which has an energy density 1060 times too great to

exist in the universe we live in

• The way forward is through experiment (and only experiment)– tantalizing – we know the answers are accessible– and also a bit frustrating – we have known this for 20

years…

But (like capitalism!) it contains the

seeds of its own destruction

But (like capitalism!) it contains the

seeds of its own destruction

John Womersley

Quarks and lepton

s4%

Meanwhile, back in the universe …Meanwhile, back in the universe …

• What shapes the cosmos?– Old answer: the mass it

contains, through gravity• But we now know

– There is much more mass than we’d expect from the stars we see, or from the amount of helium formed in the early universe

• Dark matter– The velocity of distant

galaxies shows there is some kind of energy driving the expansion of the universe, as well as mass slowing it down

• Dark Energy• We do not know what 96% of

the universe is made of!

John Womersley

These questions seem to come together at the TeV scale:

With TeV scale accelerators we are exploring what

the universe contained ~ 1ps after the big bang!

WIMPMass and cross section

EW symmetrybreaking

John Womersley

What does any of this have to do with top?What does any of this have to do with top?

• We know there’s new physics at the electroweak scale• We really don’t know what it is• Right now, the top quark is our only window on this physics

– Couples strongly to the Higgs field: what is this telling us?– A window on fermion mass generation: does it really

happen through Yukawa couplings?– Offers a unique physics laboratory:

QCDElectroweak

physics

Higgsor

new physics

top

John Womersley

Top and new physics Top and new physics

Solutions to EWSB• Supersymmetry

– Top Yukawa coupling is modified w.r.t. its SM value– Mass scale of top partners must be low (not true of other

superpartners)– New physics associated with top may be first to be seen

• Little Higgs models– New vector-like top-partner T, m ~ 1–2 TeV– mixes with top, decays to th, tZ, bW

• Strongly-coupled models: Technicolor and its descendents– If mass dynamically generated, top is special because of

its large mass: extra interactions (topcolor…)– Resonances in tt, tb (single top in s-channel)

• Modified spacetime: Extra dimensions– Not such a special role for top, but can havett production

through KK resonances

John Womersley

tt final statestt final states

• Standard Model: t Wb dominates

21%

15%

15%

1%3%1%

44%

tau+X

mu+jets

e+jets

e+e

e+mu

mu+mu

all hadronic

bbbbb

qqqql-l-W-

t

bbbbb

qql+qql+W+

t

30% e/ + jets 5% ee/e/

John Womersley

How to catch a Top quark

tt

W

b

W

b

Muon

Neutrino

John Womersley

So…So…

• Top requires an excellent understanding of the whole detector and of QCD

– Triggering, tracking, b-tags, electrons, muons, jets, missing ET

– Performance must be understood and modelled

In particular:

• Early effort to understand Jet Energy Scale – for event kinematics and top quark mass

• b-tagging– To reduce backgrounds– To reduce combinatorics in measurements of top quark

properties

• Sophisticated techniques – To maximise sensitivity to rare processes – To maximise sensitivity to deviations from SM

• Team work and efficient tools

John Womersley

Jet energy scaleJet energy scale

• The jet energy scale is the dominant uncertainty in many measurements of the top quark.

• CDF and DØ use different approaches to determine the jet energy scale and uncertainty:– CDF: Scale mainly from single particle response + jet

fragmentation model. Cross-checked with photon/Z-jet pT balance etc.

• ~3% uncertainty in Run II. Further improvements in progress.

– DØ: Scale mainly from photon-jet pT balance. Cross-checked with the closure tests in photon/Z+jet events etc.

• new Run II calibration (uncertainty ~ 2%) will come out soon.

• In-situ mW calibration has been successfully used to improve JES by both CDF and DØ in top mass measurements.

• Expect result on the b-jet energy scale from photon/b-jet pT balance & Z bb soon.

John Womersley

B-taggingB-tagging

• A significant body of experience has been gained at the Tevatron experiments– both have developed multiple b-

tagging tools

• Many issues deserve attention for the LHC:– Alignment of the silicon tracking

detector– Understanding charge deposition– Understanding material in the

tracking volume– Tracking simulation and its relation to

reality• Monte Carlo scale factors

– Determination of efficiencies from data – calibration data must be collected at appropriate ET and η

John Womersley

Event GeneratorsEvent Generators

Significant progress on event generators:• top-quark production with spin correlations• single top production including 2 2 + 2 3 with proper

matching• tree level generators with additional multi-jets in the final

state • prescriptions to match tree-level + showering without double

counting• generators with full NLO corrections to top production

processes• event generators for top production and decays due to

interactions beyond SM

MC@NLO

TopReXAlpgen

NLO calculation

Additional fin

al state jetsPolarised top decays

Full 2 6 process

Pythia

+ work on b-quark fragmentation in top decays

John Womersley

Top productionTop production

• If the top is “just” a very heavy quark, its production cross section can be calculated in QCD

L=230 pb-1

1 secondary vertex tag

PLB 626, 35 (2005)

Dileptons:Cleanest channel

Lepton + jets, b-tagged:Higher yields

John Womersley

• All channels consistent with each other and with QCD• Ongoing effort to combine measurements within and among experiments.

Cross section measurementsCross section measurements

John Womersley

Extracting the top massExtracting the top mass

Two basic techniques• Template method:

– extract a quantity from each event, e.g. a reconstructed top mass

– find the best fit for the distribution of this quantity to “templates”

• Matrix element (or dynamic likelihood) method– Calculate a likelihood distribution from each event as a

function of hypothesised top mass– Multiply these distributions to get the overall likelihood

John Womersley

Top massTop mass

• Both experiments are now simultaneously calibrating the jet energy scale in situ using the W jj decay within top events

Combined fit to

top mass …

… and shift in overall jet scale from nominal value

But …no information on ET or dependence, or on b-jet scaleDØ lepton + jets

matrix element

John Womersley

Mass in dilepton eventsMass in dilepton events

• Reduced statistics, but less sensitivity to JES– On the other hand, can’t incorporate W jets calibration

in the same way

• With the full statistics, these channels are starting to become competitive with lepton + jets channel

Feb 2006 Update750 pb-1

John Womersley

Top mass status January 2006Top mass status January 2006

CL 95% @ GeV 186 GeV;91 4532

HH MM

Most precise measurements come from lepton + jets

Use of W jets calibration is an important recent improvement

John Womersley

ProspectsProspects

• With plausible (but not easy to achieve) assumptions about evolution of systematic errors

hep-ex/0510048

= ~1.1 GeV

John Womersley

Improvements in progressImprovements in progress

• The Tevatron experiments have quantified the improvements in sensitivity needed to reach their Higgs projections– EM coverage, efficiency; dijet mass resolution; b-tagging...– Will also improve performance for top

Improvement in b-taggingusing neural networks

Improvement in dijet mass resolutionusing track momentum as well as calorimetry

John Womersley

W massW mass

• Tevatron goal: improve on LEP2 – will require ~ 1fb-1 or more

• Strategy: extract mass from kinematic quantities– Transverse mass

– (Lepton pT)

– (Missing ET)

• Overall scale is set by Z (using LEP’s mass measurement)

• CDF analysis with ~ 200pb-1 mW = 76 MeV mW still blinded

John Womersley

W mass prospectsW mass prospects

John Womersley

What this would meanWhat this would mean

• A 25-30 % measurement of the Higgs mass

John Womersley

How does top decay?How does top decay?

• In the SM, top decays almost exclusively to a W and a b-quark, but in principle it could decay to other down-type quarks too

• Can test by measuring R = B(t b)/B(t q)

• Compare number of double b-tagged to single b-tagged events

All consistent with R = 1 (SM)i.e. 100% top b

CL (Bayes) 95% @ 64.0

)(03.1 19.017.0

R

syststatR

DØ Run II PreliminaryLepton+jets (~230 pb-1)

Lepton+jets and dilepton (~160 pb-1)

CL C)&(F 95% @ 61.0

)(12.1 27.023.0

R

syststatR

John Womersley

Top Top charged Higgs charged Higgs

• If MH <mt - mb then t H+b competes with t W+b

• Sizeable B(t H+b) expected at– low tan:

H cs, Wbb dominate– high tan : H dominates

• different effect on cross section measurements in various channels.

• CDF used tt measurements in dileptons, lepton+jets and lepton+tau channels

• allowed for losses to t H+b decays

• Simultaneous fit to all channels assuming same tt

Still room for substantial BR t H+b (as high as 50%?)

John Womersley

Top chargeTop charge

• Using 21 double-tagged events, find 17 with convergent kinematic fit

• Apply jet-charge algorithm to the b-tagged jets– Expect b (q = 1/3) to fragment to a jet with leading negative

hadrons, butb (q = +1/3) to fragment to leading positive hadrons

– Jet charge is a pT weighted sum of track charges

– Allows to separate hypothesis of top W+b from Q W-b

• Data are consistent with q = ±2/3 and exclude q = ±4/3 (94%CL)

John Womersley

Spin in Top decaysSpin in Top decays

• Because its mass is so large, the top quark is expected to decay very rapidly (~ yoctoseconds)

• No time to form a top meson

• Top Wb decay then preserves the spin information

– reflected in decay angle and momentum of lepton in the W rest frame

• We find the fraction of RH W’s to be (95% CL)

F+ < 0.25 (DØ) ; 0.27 (CDF)

CDF finds the fraction of longitudinal W’s to be

F0 = 0.74 +0.22 –0.34 (lepton pT and cos * combined)

In the SM, F+ 0 and F0 ~ 0.7

All consistent with the SM

L=230 pb-1

PRD 72, 011104 (2005)

Left-handed Right-handed

Longitudinal

cos*

John Womersley

Spin correlations in top pair Spin correlations in top pair productionproduction

• DØ run I analysis, using only 6 events Phys. Rev. Lett. 85 256 (2000)

• CDF sensitivity studies… need a few fb-1 before correlations can be seen

• At LHC, precision measurements seem possible– Look at dilepton and l+jets events, various bases– Useful tool to study/look for nonstandard production

mechanisms (resonances, effects of extra dimensions)

John Womersley

New particles decaying to top?New particles decaying to top?

• One signal might be structure in thett invariant mass distribution from (e.g.) X tt

• Interesting features in both distributions, but are they consistent

?

?

John Womersley

• Alas, with about twice the data, the excess washes out

Feb 2006 Update682 pb-1

John Womersley

Single Top productionSingle Top production

• Probes the electroweak properties of top and measures CKM matrix element |Vtb|

• Good place to look for new physics connected with top• Desirable to separate s and t-channel production

• The s-channel mode is sensitive to charged resonances.• The t-channel mode is more sensitive to FCNCs and new

interactions.

John Womersley

Single top searchesSingle top searches

• Much higher backgroundsthan tt production:

• Current results:

s (pb) t (pb) s+t (pb)

SM (NLO prediction) 0.88 ± 0.07 1.98 ± 0.21 ~2.86

95% CL upper limitsObserved(expected) 13.6(12.1) 10.1(11.2) 17.8(13.6)

MPV 68% CL 4.6 ± 3.8 0.0 +4.7-0.0 7.7 +5.1

–4.9

95% CL upper limitsObserved(expected) 6.4(5.8) 5.0(4.5)

MPV 68% CL 1.9+1.9-1.6 0.0 +2.4

-0.0

95% CL upper limitsObserved(expected) 5.0(3.3) 4.4(4.3)

World’s best limits

230 pb-1

370 pb-1

160 pb-1

John Womersley

Multivariate techniquesMultivariate techniques

• Reference case: DØ single top search– Last year, moved from

simple cuts to multivariate approach: roughly doubled sensitivity

• Likelihood discriminants• Neural networks• Decision trees

– less familiar in high energy physics

– Some attractive features

(“not a black box”)• Boosting algorithms

– Used in miniBooNE and GLAST

• Boosted DT results for single top soon

John Womersley

Single top searchesSingle top searches

• Not yet able to see SM rate, but starting to disfavour some models

e data only

data

only

John Womersley

Single top prospectsSingle top prospects

• with current sensitivity, statistically significant observation will happen in Run II – but improvements still desirable!

John Womersley

Tevatron PerformanceTevatron Performance

2002

2003

2004

2005 1.6 × 1032 cm-2 s-1

The highest luminosity hadron collider ever built

John Womersley

Tevatron StatusTevatron Status

Integrated Luminosity (fb-1)

0

1

2

3

4

5

6

7

8

9

9/29/03 9/29/04 9/30/05 10/1/06 10/2/07 10/2/08 10/3/09

Electron cooling in Recycler

8 fb -1

4 fb -1

2 fb -1

Champagne for 1 fb-1

John Womersley

Status of LHCStatus of LHC

• First collisions in Summer 2007• Initial measurements 2 years

from now?• First precision measurements 3

years from now with 1-10fb-1?

John Womersley

Top at LHCTop at LHC

• LHC has great potential for Top physics– Enormous cross sections

• 1 day at 1033 10 years at Tevatron for SM processes

– In many cases, negligible stat uncertainties

– Dramatic improvements over statistically limited Tevatron analyses (e.g. spin, polarisation, rare decays)

• Improved understanding of top and window on BSM physics

• LHC is on the road• Huge amount of work needed prior to measurements

– to understand the detectors & control systematics– Early top signals will also be critical to commissioning the

detectors• Some of the earliest LHC physics results, and earliest

sensitivity to new physics, should come from top physics• Top is also a background to discovery physics at LHC

– e.g. H WW, top, susy

LHC

Tevatron

John Womersley

Tools for top at LHCTools for top at LHC

• Enormous statistics– Even greater emphasis on control of systematics– Worry about issues at < 1% level that are not major

concerns at the Tevatron• Jet masses, calibrate parton energy or jet energy

– Can afford to talk about strategies like removing events with identified semileptonic b-decay jets

• What can be done at the start of the run? – Signal is large enough that clear lepton + jets signal can

be seen in 150 pb-1 with HT cuts, no b-tagging

• ATLAS studies + ongoing work for CMS Physics TDR

and W+jets for 150 pb-1tt

John Womersley

Top mass at LHCTop mass at LHC

• Lepton + jets - golden channel– S/B ~ 30, statistical uncertainty is tiny (100 MeV?)

• Can afford to select high-pT sample to reduce combinatorics, if desired

– would this help reconstructing t quark C of M for spin studies?

– Importance of kinematic fitting msys at the 1 GeV level (b-jet JES dominates)

• Dilepton channel msys at the 1.7 GeV level

• More exotic possibilities …– Measurements in all-jets channel

msys ~ 3 GeV– Leptonic final states with J/:

• statistics low, but msys 0.5 GeV??

• Methods have very different sensitivities to systematics• Combining all of the above: measure mt to ~ 1 GeV with 10 fb-1

John Womersley

mmtt and m and mWW at LHC at LHC

• Top: Measure mass to the 1 GeV level– Dominant systematic is b-jet energy scale

hep

-ph

/03

07

17

7

+ L

EPEW

WG

05

∆mt=1 GeV/c2

∆mW=15 MeV/c2

• W: LHC will have sufficient statistics to permit mW = 15 MeV– to reach this precision will be a challenging, multi-year

project– will there be a physics need – precision test of SUSY?

John Womersley

… … and at ILCand at ILC

mt ~ 100 MeV claimed possible

• But requires further theoretical progress on higher order calculations– “The tools are there… (just) a lot of tedious work required”

John Womersley

Top + Higgs at LHCTop + Higgs at LHC

• tH+ and tbH+

– discovery modes for charged Higgs

• ttH– Verify top Yukawa

coupling– fermion mass generation

gt : 20 to 30 % @ 300 fb-1

John Womersley

Single top at LHCSingle top at LHC

• t-channel process– Cross section 120 times higher

than Tevatron

• ATLAS study

• s-channel process

– Direct extraction of |Vtb| from ratio of W* (single top) to real W

– Harder: cross section only 10 times higher than Tevatron

M(tb)

After preselection

S/B ~ 3

√(S+B)/S ~ 1.4% @ 30 fb-1

John Womersley

tW production at LHCtW production at LHC

• Only single top process where we directly observe the W– The t W mode is a more direct

measure of top’s coupling to W and a down-type quark (down, strange,bottom).

• Tiny cross section at Tevatron, significant at LHC (x 400)

• Theoretical definition is “delicate”; new work in progress

• Major background from tt

• ATLAS study:

S/B ~ 1/7

√(S+B)/S ~ 4% @ 30 fb-1

John Womersley

Top spin and polarisation at LHCTop spin and polarisation at LHC

• High statistics – can significantly improve on the Tevatron

• W helicity in top decay– Measure at the 1 – 7 % level– dominated by systematics

• Top spin in single top– 90% polarised– Measure at the few % level (fast

sim)– Search for CP violation!

• Top-antitop spin correlation

– A = 0.33 in SM– Measure A to ~ 10% (fast sim)

Full simulation

(preliminary)

1.5 fb-1

F0=0.677 ± 0.015

FL=0.309 ± 0.009

FR=0.014 ± 0.009

0)()()()(

)()()()tt(

LRRLRRLL

LRRLRRLL

tttttttt

ttttttA

John Womersley

FCNC decaysFCNC decays

• HERA sets limits on FCNC utZ, ut couplings– Still need to understand the origin of observed isolated

lepton events in e+p data in H1 (and not in Zeus)

• LHC sensitivities (100 fb-1, 5 significance)

John Womersley

More complementarity with HERAMore complementarity with HERA

• First experimental determination of b-quark distribution– Needed for single top

Z + b-jet

… and for backgrounds, like W/Z + heavy flavour

John Womersley

Top properties scorecardTop properties scorecard

• Is it a standard model up type quark?

Electric charge +2/3 Known not to be 4/3Colour triplet Yes? (production cross section)Spin ½ not really tested – spin

correlationsIsospin ½ Yes? (decay to W + down type quark)V – A decay at 20% levelBR to b quark ~ 100% at 20% levelFCNC probed at the 10% levelTop width ?? test with single top ??Yukawa coupling test at 20-30% level at LHC

• Top mass 172.7 ± 2.9 GeV• Higgs mass < 186 GeV

John Womersley

What do we know?What do we know?

• At the 20% level, top seems to behave like an up type quark which just happens to have an extraordinarily large mass

• That mass has been measured very precisely– Thus constraining the Higgs sector– We do not yet know if this mass arises (as in the SM) from

a Yukawa coupling or from something more interesting

• Searches for single top have sufficient sensitivity to see this process soon

• Starting to make interesting measurements of spin in top decays, searching for non-standard production and decay mechanisms

• Fascinating results from the Tevatron with much more to come

• Can look forward to a step-change in statistics at LHC, and sensitivity to new observables – CP violation in top?!

John Womersley

ConclusionConclusion

• Top is a unique window on particle physics

QCDElectroweak

physics

Higgsor

new physics

top

John Womersley

BBSS Mixing Mixing

• New DØ result at Moriond 2006, 1 fb-1 of data:

Likelihood minimum at mS = 19 ps-1

17 < mS < 21 ps-1 (90% CL)

– 3.8% probability for an oscillation with frequency above the region of sensitivity (> 22 ps-1) to give a minimum this significant anywhere in the range 16-22 ps-1