INTRODUCTION TO EXPERIMENTAL PARTICLE PHYSICS: 2psi.petnica.rs/2018/notes/lecture2.pdf · tt = pb...

INTRODUCTION TO EXPERIMENTAL PARTICLE PHYSICS: 2

KATE SHAW

UNIVERSITY OF SUSSEX

INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS

INTRODUCTION TO TOP PHYSICS

TOP QUARK ▸  Most massive of known

fundamental particles ~ 173 GeV (~ tungsten atom!!)

▸  Mass is of order of the electroweak symmetry breaking scale – probe for new physics

▸  Weak isospin partner of the b-quark

▸  What is its electric charge?

▸  What is its spin?

▸  Which forces does it interact with?

▸  What is its antiparticle?


TOP QUARK

▸  Large mass suggests it may play a special role in the SM and also in many beyond the Standard Model (BSM) theories


TOP QUARK ▸  Top has a large production

cross-section at the LHC

▸  σtt = 830 pb @ 13TeV (∼ 500 tt pairs/min,∼ 30 million tt in 36fb−1)

▸  Because the top is so heavy its lifetime is very short (as we shall see…) and decays before it can hadronise

▸  Thus providing unique opportunity to study a bare quark!!


DISCOVERY : 2 MARCH 1995


DISCOVERY ▸  1973 top and bottom quarks

predicted by Makoto Kobayshi and Toshihide Maskawa to explain CP violations in kaon decay

▸  1995 discovered at the Tevatron at Fermilab by CDF and D0

▸  2008 Nobel prize in physics awarded!!

Tevatron

Proton-antiproton collider

Run II ended operation in 2011 at a center of mass energy of 1.96 TeV

Experiments CDF and D0 each collected 10 fb-1 of data.


TOP PAIR PRODUCTION Ø  Top quark pair production governed by strong interactions (gg fusion dominant

(~80%))

Ø  NNLO + NNLL with mt = 172.5 GeV at 8TeV CM Energy σtt = pb [4]

Ø  Gluon scattering dominant at the LHC (~85%)

Ø  Quark scattering (~15%) ( but dominant at Tevatron)

[4] Phys. Lett. B710 (2012) 612, arXiv:1111.5869

Problem:

Why is qq scattering dominant at the Tevatron and gg at the LHC?


SINGLE TOP PRODUCTION ▸  Single top production proceeds via electroweak interaction involving

a tWb vertex

▸  s-channel proceeds via a time-like off-shell W boson

▸  t-channel involves exchange of space-like W-boson

▸  associated production of a top quark and on-shell W-boson

s-channel t-channel (dominant) top in association with a W


SINGLE TOP PRODUCTION Ø  3 modes sensitive to different manifestations of models of new physics

NLO+NNLO with mt = 173.3 GeV at 8TeV @LHC

[1]

[2]

[3]

[1] Phys. Rev. D 81 (2010) 054028, arXiv:1001.5034.

[2] Phys. Rev. D 83 (2011) 091503, arXiv:1103.2792

[3] Phys. Rev. D 81 (2010) 054028, arXiv:1001.5034.


TOP DECAY The top quark can only decay through the weak interaction almost 100% into Wb

Lifetime of the top (5 x 10-25s) is shorter than the timescale of hadronisation

Top-quarks decay almost 100% to W-boson and Bottom-quark |Vtb| ~1

Final state topology is dictated by the leptonic or hadronic decay of the W-boson

vtb


TOP DECAY The top quark can only decay through the weak interaction almost 100% into Wb

vtb

The W can decay:

W-> lv (l=e,µ,τ) BR ~ 1/9 per lepton

W -> qq, BR ~ ?

vtb


TOP DECAY TOPOLOGIES What are the final state particles in single top and top pair production?

Single top production

Top pair production


TOP PAIR DECAY CHANNELS


TOP PAIR DECAY CHANNELS All hadronic: BR~ 45% •  6 jets – 2 from b quarks, no neutrinos

Single lepton: BR~30% •  One lepton + 4 jets (2 from b quarks),

neutrino

Dilepton: BR ~5%

•  2 leptons, 2 b-jets, 2 neutrinos


TOP MEASUREMENTS

Production

•  Cross-section

•  Resonances

•  Fourth generation t’

•  Spin correlations

•  New physics (e.g. SUSY)

•  Flavour physics (FCNC)

Decay

•  Branching ratios

•  Charged Higgs (non-SM)

•  Anomalous couplings

•  Rare decays

•  CKM matrix elements

Properties

•  Mass

•  Kinematics

•  Charge

•  Lifetime and width

•  W helicity

•  Spin


TOP MEASUREMENTS – SELECTING EVENTS Top Pair production -> Dilepton channel


TOP MEASUREMENTS – SELECTING EVENTS

Ø Two leptons (pT > 20/30 GeV)

Ø Two jets – b-tagging is optional!

Ø Missing transverse energy (> ~ 40 GeV)

Top Pair production -> Dilepton channel


TOP MEASUREMENTS – SELECTING EVENTS Top Pair production -> single lepton channel

Jet Multiplicity

Even

ts

10

210

310

410

510

610 datatt

ZSingle topOthersUncertainty

ATLAS Preliminary-120.2 fb = 8 TeV,s

Jet Multiplicity0 1 2 3 4 5 6 7 8 9

Expe

cted

Dat

a

0.8

1

1.2

Ø One lepton (pT > 20/30 GeV)

Ø  four jets – b-tagging is optional!

Ø Missing transverse energy (> ~ 40 GeV)


TOP MEASUREMENTS – MEASURING CROSS-SECTION

Cross-section precise prediction is sensitive to the gluon parton distribution function (PDF) and the top quark mass – thus challenging for QCD calculation techniques!

BSM physics can lead to an enhancement of the ttbar production rate


TOP MEASUREMENTS – MEASURING CROSS-SECTION Measurement of the ttbar cross-section using eµ events with b-tagged jets at 13 TeV

Phys. Lett. B761 (2016) 136

How do we collect data and measure the cross-section?

1 GET THE DATA!

Ø  LHC collides protons, with 25 ns bunch spacing, at a s= 13 TeV, millions of time a second inside the center of ATLAS (~ 14 pp collision in each bunch crossing -pile up .

Ø Data corresponds to an integrated luminosity of 3.2 fb-1.

Ø  Events required to pass single-electron or single muon trigger (pT > 25 GeV)



Phys. Lett. B761 (2016) 136


2 SIMULATE MONTE CARLO EVENTS! Ø  This allows us to study what the Signal event and Background events look like!

Ø  Thus we can optimise the analysis and compare the data to background

Ø  Finally the signal and background efficiencies and uncertainities can be evaluated!

•  Wt single top Wt ->ev + µvb,

•  Z+jets ZZ->ττ->eµ,

•  Diboson - WW -> evµv, WZ->lveµ, ZZ->eµ



Phys. Lett. B761 (2016) 136


3 EVENT SELECTION ON THE DATA AND THE BACKGROUND

Ø  Exactly one isolated electron and muon, both with PT > 25 GeV

Ø  Two Jets PT > 25 GeV, tagged with one or two b-tags



Systematics:

Total relative uncertainty: 4.4%!

Ø Data statistics

Ø  Experimental and theorectial systematic effects

Ø  Integrated luminosity and LHC beam energy

Phys. Lett. B761 (2016) 136

MEASURE THE CROSS-SECTION, EVALUATE SYSTEMATICS!



Phys. Lett. B761 (2016) 136



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ATLAS+CMS Preliminary Nov 2017

* Preliminary

)-1 8.8 fb≤Tevatron combined 1.96 TeV (L )-1CMS dilepton,l+jets* 5.02 TeV (L = 27.4 pb

)-1 7 TeV (L = 4.6 fbµATLAS e)-1 7 TeV (L = 5 fbµCMS e

)-1 8 TeV (L = 20.2 fbµATLAS e)-1 8 TeV (L = 19.7 fbµCMS e

)-1 8 TeV (L = 5.3-20.3 fbµLHC combined e)-1 13 TeV (L = 3.2 fbµATLAS e

)-1 13 TeV (L = 2.2 fbµCMS e)-1* 13 TeV (L = 85 pbµµATLAS ee/

)-1ATLAS l+jets* 13 TeV (L = 85 pb)-1CMS l+jets 13 TeV (L = 2.2 fb

)-1CMS all-jets* 13 TeV (L = 2.53 fb

NNLO+NNLL (pp))pNNLO+NNLL (p

Czakon, Fiedler, Mitov, PRL 110 (2013) 252004 0.001±) = 0.118

Z(Msα = 172.5 GeV, topNNPDF3.0, m

[TeV]s13

700

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1

101

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105

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1011 Theory

LHC pp√s = 7 TeV

Data 4.5 − 4.9 fb−1

LHC pp√s = 8 TeV

Data 20.3 fb−1

LHC pp√s = 13 TeV

Data 0.08 − 14.8 fb−1

Standard Model Production Cross Section Measurements Status: August 2016

ATLAS Preliminary

Run 1,2√s = 7, 8, 13 TeV



Single top production cross-section in the t-channel at 8 TeV



Single top production cross-section

[TeV]s

6 7 8 9 10 11 12 13

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e to

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ark

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t-channel

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s-channel

MSTW2008 NNLO PDF = 172.5 GeVt NLO+NNLL at m

PRD 90 112006 (2014) -1t-channel 4.59 fbpaper in preparation -1t-channel 20.2 fb

arXiv:1609.03920 -1t-channel 3.2 fbPLB 716 (2012) 142 -1Wt 2.05 fbJHEP01 (2016) 064 -1Wt 20.3 fb

ATLAS-CONF-2016-065 -1Wt 3.2 fbATLAS-CONF-2011-118 -1 s-channel 95% CL limit 0.7 fb

arXiv:1511.05980 -1 s-channel 20.3 fb

ATLAS Preliminary September 2016

single top-quark production

stat total


SINGLE TOP PRODUCTION – MEASURING CKM MATRIX ELEMENT |VTB|

Ø  Measurement of single top quark cross-section determines CKM quark mixing matrix element Vtb

Ø  σsingletop proportional |Vtb|2, probes the electroweak Wtb vertex

vtb


CKM MATRIX •  Quark mixing described by unitary CKM matrix VCKM (Cabibbo-Kobayashi-Maskawa matrix)

▸  The matrix elements are determined from weak decays of the relevant quarks

|Vtb| govern the decay rate of the top and its decay width to Wb

Assuming there are three generations of quarks and applying the unitarity constraint |Vtb| approaches unity

|Vtb| = 0.9990 – 0.9992 at 90% C.L.

Indicates strength of flavour changing weak decays

CKM matrix describes probability of a transition for one quark to another quark j, proportional to |Vij|2


CKM MATRIX Weak interaction doublet partners of up-type quarks

CKM matrix

Mass eigenstates of d-type quarks

The single top cross-section is directly proportional to the square of the coupling at the production vertex, thus proportional to |Vtb|2

Thus |Vtb| is extracted by dividing the measured cross-section of single top production by SM expectation.


SINGLE TOP– MEASURING CKM MATRIX ELEMENT |VTB|

vtb


MEASURING TOP QUARK MASS Top quark mass is a fundamental parameter of the SM

Top is the only fermion with a mass of order of the electroweak symmetry breaking scale


MEASURING TOP QUARK SPIN Ø  Short lifetime of the top allows unique opportunity to study the bare

quark properties. Many quantum numbers such as its spin are transferred to the decay particles

Ø  The top – antitop spins are correlated to some degree, we want to measure this degree of correlation…

Ø  NEW PHYSICS could later polarization and spin correlation

One can measure the top quark pair spin structure using angular observables of their decay products which inherit the spins

Measure 15 observables, each sensitive to a different coefficient of the spin density matrix of tt ̄ production


ICHEP – INTERNATIONAL CONFERENCE FOR HEP


MEASURING TOP QUARK SPIN Ø  ATLAS NEW result shows the degree of correlation to be higher than

predicted by SM calculations! (3.2 s)

ATLAS-CONF-2018-027

The observable used to extract the spin correlation compared to different predictions

The slope in the data relative to the predictions indicates higher spin correlation


TOP AND ITS COUPLING TO THE HIGGS The Higgs boson (next lecture) couples to the top quark though its Yukawa coupling (~1, all other quark and lepton Yukawa couplings are small in comparison!)

INTRODUCTION TO EXPERIMENTAL PARTICLE PHYSICS: 2psi.petnica.rs/2018/notes/lecture2.pdf · tt = pb...

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