PH599 Graduate Seminar presents: Discovery of Top Quark

Karen ChenStony Brook UniversityNovember 1, 2010

Karen Chen 2

Abstract

The third generation of quarks was predicted by Kobayashi and Maskawa to explain CP violation. When the bottom quark was discovered, the search to find its isospin partner began. The discovery of the top quark completes the family of six quarks in the Standard Model. Measurements of the top quark mass were conducted by the CDF and D0 experiments at the Fermilab Tevatron. The top quark mass was measured from events consistent with top pair production. The top pairs decay into a pair of bottom quarks and a pair of W bosons with a nearly 100% branching ratio. The experiments looked at events that result in either dilepton or lepton plus jets final states. It is possible for the W bosons to both decay into quarks but measurements based on events with all jets have low precision. More precise mass measurements were conducted after the top quark’s initial discovery. The experimental uncertainty associated with the top quark and W boson mass puts constraints on the mass of the Higgs boson. The high precision of the top quark mass may have important implications on the validity of the Standard Model.

Karen Chen 3

Outline

Discovery 3rd generation quarks to explain CP violation

Direct Measurement of Top Quark Mass Experiments at the Tevatron, detector basics Decay of top pairs

Possible final states: Dilepton, l+jets Event Selection

Signatures of signal and background processes Likelihood fits for mt

Top Quark Mass Relevance Today Constraints on Higgs mass

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Discovery of the top quark

1964 - CP violation found in kaons

1973 – (Cabibbo), Kobayashi and Maskawa Need a third generation of quarks to explain CP

violation

1977 – Bottom quark discovered! And thus begins the search for its isospin partner,

the top quark

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Discovery of the top quark

Tevatron – particle accelerator at Fermilab

Two experiments: Collider Detector at

Fermilab D0 (or DZero)

Proton, antiproton collisions

CDF and D0 measured the top quark mass

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Measurement of Top Quark Mass

Invariant Mass, m = m(E,p) E2 = (pc)2 + (mc2)2

Or more conveniently*: m2 = E2 – p2 = P2, where P is four vector momentum, P2 = E2 – p2 = E2 - px

2 - py2 - pz

2

Example of a two body decay A B+C mA

2 = (PB+ PC)2

To find top quark mass, we need the energy and momentum of the decay products.

Note: It is convenient to measure everything in units of GeV, so c is set to 1.

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Decays of top pairs

Top pair decay with branching ratio ~100%

W decay Branching ratios We ~1/9 W ~1/9 W ~1/9 Wqq ~2/3

WWbbttpp

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Decays of top pairs

Top pair decay with branching ratio ~100%

Final decay products Both W’s decay into leptons

tt->bb + ll “Dilepton final state”

One decays into a lepton, the other into quarks tt->bb + l + qq “Lepton + jets”

Both W’s decay into quarks tt->bb + qqqq “All Jets”, “fully hadronic”

WWbbttpp

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Decays of top pairs

What are “jets?” Why don’t you see a single quark? Quark confinement

Gravity, EM ~ 1/r2

Strong force increases with distance!

1. Collision produces quarks

2. Energy grows with distance

3. More quarks are created

4. Can combine to form hadrons

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Measurement of Top Quark Mass

Dilepton (~4.5%) Muons or electrons Pure signal, low yield

l+jets (~30%) Moderate yield and bg

All jets (~44.5%) Large backgrounds

Tau channels (~21%) At least one W decays into a Hard to identify decays Short lifetime, hadronize quickly

Tevatron Average:mt = 173.1 ± 0.6 (stat.) ± 1.1(syst.) GeV/c2

Hobbs et al.

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Background sources

Signal (Dilepton)

Both have final states of ee pair and two jets. What’s the difference?

Neutrinos appear in the signal process. Problem: Neutrinos are weakly interacting, we can’t

really see them!

Background (Diboson)

WWbbtt

ee ee eeqq

ZWpp

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Background discrimination: ET

Total ET = 0, Missing ET must be from neutrinos.

Dilepton: ET > 35GeV

l+jets: ET > 15GeVAbazov 2009

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Background sources

Signal

Both have W boson pair, so ET may be the same. What’s the difference?

The signal has two bottom quarks. You expect more jets in the signal than in the background.

Can you check if the jet is from a bottom quark or a lighter quark?

Background

WWbbtt WWpp

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b-tagging

b-tagging efficiency ~50% per jet

Misidentification P(b-tag|q) = 1% P(b-tag|c) = 15%

t

t

W+

W-

C

b ~10-12s

~10-13s

Vertex is fartherb-tagged jet

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Background discrimination: # jets

≥

http://www-d0.fnal.gov/

l + jets final statewith one b-tagged jet

Dilepton final state

Expect 2 jets Expect 4 jets

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Background discrimination: # jets

CDF detector crack at η = 1.1

http://www-cdf.fnal.gov/physics/new/top/2004/jets/cdfpublic.html

η = -ln(tan(θ/2))

θ: azimuthal angle from beam line

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Measurement of Top Quark Mass

At this point: Have candidate tt events (using ET, b-tagging, and

other cuts) with lowered background

Few unknowns Neutrino momentum Jet combinatorics

What you can do: A mix of template method and weighing methods that

depend on the kinematic observables to determine a best fit for mt.

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Measurement of Top Quark Mass

Example: l+jets Consider jet combinatorics under these constraints:

Likelihood function as a function of jet energy scale and Mt.

Energy = [1+ JES] f(s)JES = 0 -> perfect calibration

bqqbltt MMMM'

Hobbs et al.

'qqlWWMMMM

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Constraints on Higgs mass

Higgs boson explains why weak force carriers, W and Z, are massive.

The mass of the top quark is HUGE! compared to other elementary particles.

Higgs mass is related to W boson and top mass MW ~ log(MH)

MW ~ Mt2

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Constraints on Higgs mass

P. Renton 1995

Results from 1995 Results from 2006

Heinemeyer et al., 2006

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Summary

The main decay channel used for top quark mass measurement:

With appropriate cuts (ET, # of jets, b tagging), you can increase the purity of the tt signal.

Mass measurement of top quark was done with likelihood fits of mt using a combination of template and weighting methods.

Precision of top quark and W boson mass puts constraints on Higgs mass.

WWbbttpp

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References

M. Kobayashi, T. Maskawa (1973). "CP-Violation in the Renormalizable Theory of Weak Interaction". Progress of Theoretical Physics 49 (2): 652–657.

P Renton, arXiv:hep-ph/0206231v2 1 Aug 2002 P. Renton, Review of Experimental Results on Precision Tests of

Electroweak Theories, Lepton-Photon 95, p35 (1995), published by World Scientific.

P. Renton, arXiv:0809.4566v1 [hep-ph] 26 Sep 2008 S. Heinemeyer, W. Hollik, D. St ockinger, A.M. Weber, G. Weiglein,

arXiv:hep-ph/0604147v2 10 Oct 2006 V. Abazov et al. Measurement of the top quark mass in final states with two

leptons. Phys. Rev., D80:092006,2009. http://www-d0.fnal.gov/Run2Physics/WWW/results/summary.htm John D. Hobbs, Mark S. Neubauer, Scott Willenbrock, Tests of the Standard

Electroweak Model at the Energy Frontier, arXiv:1003.5733, 2010.