The Large Hadron ColliderMachine, Experiments, PhysicsTop Physics (at the LHC)

Johannes HallerThomas Schörner-Sadenius

Hamburg UniversitySummer Term 2009

JH/TSS UHH SS09: LHC 2

– The top quark decays as a quasi-free quark no hadronisation phase chance to study free quark and its properties (spin!).

Hadronisation time from QCD considerations: Hadronisation takes place at energy scale ΛQCD of the order of 250 MeV. Using Heisenberg this cor- responds to about 3x10-24 s:

– Top mass might decay to other, yet undiscovered particles.– Top quark allows for very precise tests of SM via its connection to the W and Higgs masses.

Remember: The top quark was discovered in 1994in proton-antiproton collisions at the Tevatron. It proved to be very heavy:

This makes the top quark unique in many respects:– Mass: New physics phenomena might couple to high masses. Remember for example the Higgs coupling to fermions (Yukawa couplings):

TOP PHYSICS

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ehVe

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LRLR

L

LRYukawa

2

22

0

2

ss QCDt2424 1031044.0

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sin48

11

sin16

cos3,

1

1

sin

1

2

2

2

22

2

4

2

21

21

W

H

WW

t

W

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WF

W

M

M

M

mMmr

rGM

JH/TSS 3

Remember direct and indirect measurements of W, t, H masses at LEP and Tevatron:

– In addition, top constitutes an important background for new physics like Higgs and SUSY!

Many good reasons to measure and understand top quark production and top properties!

– Top quark currently only produced at Tevatron (extremely small cross-section at HERA). It is pro- duced via strong interactions:

Cross-sections Tevatron: ~pb. LHC: ~nb !!!

More gluon-induced processes at LHC: centre-of-mass energy, parton luminosities and PDFs. LHC is a top factory precision measurements!Current status top mass from Tevatron:

TOP PHYSICS

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mt =173.1±1.3GeV

UHH SS09: LHC

JH/TSS 4

From full events like the following one would liketo extract a number of things:

– The ttbar production cross-section, spin correlations and charge asymmetries.– Top mass and charge.– Cross-section for single top production (FCNC?).– Top decays (W helicity, FCNC, tH+b, …)

Top quark decays are weak processes! In cross sectionsquare of CKM matrix element:

Therefore decays tWs and twd that are allowed byall quantum numbers are practically not realised! BR(tWb)~100%!

TOP PHYSICS: GOALS

bWVt

igtb

5122

9991.0039.0009.0

040.09747.0221.0

003.0221.09753.0

tbtstd

cbcscd

ubusud

VVV

VVV

VVV

UHH SS09: LHC

JH/TSS 5

All tops decay into Wb. The W can then decay leptonically or hadronically – this allows a simpleclassification of top-antitop events:

– Dileptons:- easy to identify (at least e,μ channels)- small cross-sections for e,μ. - missing energy of two neutrinos difficult!

– Lepton + Jets:- cross-section about 30%.- only one neutrino.

– Purely hadronic:- difficult to separate from QCD background – will be ignored here!

Note: All events contain 2 b jets possible identification using “b tagging” – mostly via secondary vertex reconstruction!

Define strategies to enrich and refine clean samples:

TOP EVENTS: CLASSIFICATION

- Exploit kinematic and topological variables!- b jet identification with lifetime information.- b jet identification with leptons from blνX.

UHH SS09: LHC

JH/TSS 6

B hadrons (here from tWb) have typically largerlifetimes (of the order of some picoseconds) than for example D mesons or other decaying hadrons Will decay in flight after ~mm (cτ~500 μm) distance from vertex. Try to reconstruct this “secondary vertex” b jet identification!

“B TAGGING”

Combination of various observables like decay length,decay length significance, lepton pT, invariant mass etc. allows efficient and pure b tag.

UHH SS09: LHC

JH/TSS 7

Typical selection: – >1 central hard jets (pT>20 GeV) – 2 charged central leptons with high transverse momentum > 15 GeV and opposite charge.– missing ET > 35 GeV (accounts for neutrinos).– specific cuts against background.

TOP PAIRS: DILEPTON CHANNEL

pbtt 3.2

7.26.8

Distribution of selected events: data versus signal and background Monte Carlo simulations.

Before 2-jet cut: Low jet multiplicity well de-scribed by MC without top!At high jet multiplicities topcontribution needed to de-scribe the data “Top discovery” !!!

After 2-jet cut: Adjust top signal MC con-tribution until data are described. Best descriptiongives cross-section for top-antitop production!

Dominated by statistics: only28 events selected here!

UHH SS09: LHC

JH/TSS 8

– >2 central hard jets (pT>20 GeV) – 1 hard charged central lepton.– large missing ET.– specific cuts against background.

TOP PAIRS: LEPTON + JETS CHANNEL

Distribution of selected events: data versus signal and background Monte Carlo simulations as function of transverse momentum of all objects in event.

B tag improves purity of top sample!

− 2 jets − 2 b jets− lepton

− ETmiss

pbtt 6.1

7.16.8

1 b tag 2 b tags

UHH SS09: LHC

JH/TSS 9

– Theoretical predictions are derived in the usual way: factorised ansatz:

TOP PAIRS: SUMMARY TEVATRON

Comparison of different cross-section measurements from D0 and CDF and theory prediction:

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- Lepton+jets best!- Agreement theory-data.- No indications for BSM.- Large errors!

UHH SS09: LHC

JH/TSS 10

– Can derive mass information from all three event classes (dilepton, lepton+jets, hadronic), but - dileptons difficult because of two neutrinos. - hadronic channel suffers from large background lepton+jets channel most promising!Again selection: - 1 hard charged lepton. - ETmiss > 20 GeV - at least 4 hard jets!Basic assumption for these event candidates: ttbar!– 24 possibilities to attribute jets to quarks (com- binatorics!!).– Perform kinematic fit for each possibility:

TOP MASS MEASUREMENTS

– First term: Variation of momenta within their errors (otherwise χ2 becomes big). – Other terms constrain masses of W and top quark, ensure that momentum conservation is obeyed!– For each attribution of jets to quarks one obtains one χ2 and one mt,reco. Choose the one with the smallest χ2.– Use MC-simulated events to check validity of method: How well do we reconstruct the mass?

- More correct attributions with 2 b tags (less combi- natorics).- For 2 b tags 50% correct!- Correct combinations have better mass resolution!

UHH SS09: LHC

JH/TSS 11

TOP MASS MEASUREMENTS

UHH SS09: LHC

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Summary of measurements:

Prospects of top mass measurements at the LHC:

– Determination of top mass from comparison of distributions of mt,reco in MC and data (“template method”).

Best single top mass measurement:

But also other methods are used in the lepton+jetschannel.

TOP MASS MEASUREMENTS

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mt =172.1±1.7GeV

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mt =173.1±1.3GeV

GeVmt 0.1

UHH SS09: LHC

JH/TSS 13

Development of to mass measurements (direct and indirect) over the times:

Development of top mass uncertainty with time:

TOP MASS MEASUREMENTS

– Consistency of direct and indirect determinations:

Data suggest low mass Higgs boson of 80 GeV (at least less than 144 GeV!)

UHH SS09: LHC

JH/TSS 14

– Top quarks decay as fermions in V-A theory:

– V-A coupling at tbW+ vertex requires left-handed b quark. Neglecting b quark mass, b quarks must also have negative helicity.– But then also W+ boson only with negative helicity (numbers f: SM predictions for relative fractions):

– SM test: Measurement of f+. Value different from 0 would mean new physics at tbW+ vertex.

W helicity experimentally accessible! Angle θ* between negative top direction and direction of charged lepton in W rest system:

TOP: W HELICITY IN TOP EVENTS

bWVt

igtb

5122

Assuming F0=0.70 (SM):

Limit on F+ < 0.10!

UHH SS09: LHC

JH/TSS 15

– Top quark pairs from pp collisions are basically unpolarized, but the two top spins are correlated: Either parallel (qq events) or antiparallel (gg). – Test of the top quark spin: - Search for new physics, i.e. CP violating interactions, Higgs with undefined parity, properties of s-channel resonance - Test of top decays as a quasi free quark - precise test of production and decay mechanism - Top spins affect the angular distributions of decay products important for event selections.– Define observables C and D (a and b are quantisation axes, for example reconstructed top flight directions).

… but how to measure the spin of the top quark?

Spin of top quark from angular distribution of decay products:

TOP: SPIN CORRELATIONS

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tttt

SSD

SbSaC

4

cos12

1

cos

1fd

d

l+,d νl,u b W+ soft jet from qqbar

κf 1 -0.31 -0.41 0.41 0.51

Studies at ATLAS of observables A, AD that are closely correlated to C, D (in various channels):

precision 4-5% !Currently achieved at TEVATRON:

15-20% large potential at LHC!

UHH SS09: LHC

JH/TSS 16

TOP: SPIN CORRELATIONS: REAL LIFE

Dilepton analysis:- For parton and detector level.- With and without correlation of top quark spins.

Experimentally extremely difficult to achieve precision!

UHH SS09: LHC

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– Possibility to produce single top quarks in pp:

Small cross-sections, and also experimentally more demanding (fewer b jets and kinematic constraints, …) Use multi-variate methods to select events: - likelihoods - neural networks - decision trees … one number between 0 and 1 for each event. Latest result from CDF:

SINGLE TOP PRODUCTION

SM: 1.98±0.25 pb

SM: 0.88±0.11 pb

CDF: σ = 2.3 ± 0.5 pb5σ statistical significance discovery !

UHH SS09: LHC

JH/TSS 18

– Search for flavour changing neutral currents.– Search for fourth-generation quarks:

Additional generations might effectively reduce Vtb (which is only indirectly known!) and thus the single-top cross-section. Do we observe that? Also: Are the unitarity relations fulfilled?– Measurement of tt+jets events test of SM couplings.– Measurement of tt+photon events measurement of top quark electric charge.– …

Lots to do for LHC!

TOP: FURTHER TOPICS

UHH SS09: LHC