Search For Fourth

Generation Quarks in

Lepton plus Jets decay

Channel

(b’ l + jets)

Natalee Raymond

Macalester College

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Outline

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ATLAS

Standard Model

Lepton + Jets Channel

Signal Properties

W-tags Object Selection

Lepton + Jets Cuts

Sensitivity

Conclusion

ATLAS

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• ATLAS

• Beam direction: Z-axis

• x-y plane is transverse to the beam direction

• The azimuthal angle, φ , is measured around the beam axis

• The pseudo-rapidity is defined as η = -ln tan(θ/2)

Hadronic Calorimeter

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Overview of Hadronic Calorimeter

The Standard Model

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4th generation Why not?

• 6 quarks

• 6 leptons

• Force carriers

+23

−13

Charge:

W Boson

• Force carrier that mediates the Weak interaction

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Meghan mostly talked about the Z boson but I will talk about the W

boson. They are quite similar:

Lepton + Jets Channel • Search for fourth-generation quark, b’, in the lepton+jets decay channel

• Lepton+jets decay mode: One W decays leptonically and the others hadronically

Pair production and decay of b’ quarks to WWbWW𝑏

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Missing Energy

One Lepton

8 jets

Final State Production

of the b’ pair:

*Heavy b' decays will

produce high pT products!

Standard Model Backgrounds

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• Searching for something

new that has 1 lepton

(electron) and a lot of

jets

• What already exists that

have this property?

• SM Backgrounds (MC):

• W+Jets

• ttbar

• Z+Jets

• Diboson: WW, WZ, ZZ

N jets in SM Backgrounds with 1 electron +

missing energy

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Signal vs. MC

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• Signal: 500 GeV b’

• b’ Signal scaled by

100

• All samples have a

missing energy

(MET) cut > 10

GeV

• Use the number of

hadronic W bosons

N jets in events with 1 electron + missing energy

Eve

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Delta R

• ∆R = ∆η𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛2 + ∆φelectron

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Think of ∆R as a cone that tells

us how close particles are to each

other:

* d = ∆R Different

Cones

None taken

W-tags • Making W-tags:

• Reconstruct a jet with a cone size of 0.4 (AntiKt4LCTopoJets)

• Find 2 jets with pT > 25 GeV each

• Within ∆R < 1 of each other

• Mass of two jets together close to the mass of the W: 70-100 GeV

• Select high pT hadronic W’s in an event

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11 W

Cone

of 0.4

Cone

of 1

W-tag Plot • Implement lepton + jets selection and look at high energy W

tags there for the b' search

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Number of W-tags (NW) in events with 1 lepton + missing energy

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Lepton + Jets Cuts

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13 Electron pT of Single-Lepton Skims in GeV

DATA:

• skimmed single-electron sample: plots shown with luminosity = 1 / fb, data

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• Cuts to be

made:

• Missing

Energy

(MET)

• Transverse

W Mass

MET CUT ( >35 GeV)

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Missing Energy of Single-Lepton Skims after Cut (GeV)

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• Missing Transverse Energy:

• Transverse Energy of the neutrino

Transverse W Mass Cut ( >25 GeV)

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Transverse W Mass of Single-Lepton Skims after Cut (GeV)

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• mT2 (l,v) = 2 Et

l Etv (1-cosφl,v)

Number of Jets

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Eve

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Njets with pT > 25 GeV in events with 1 electron + missing energy

Results: Number of W-tags (NW)

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Njets = 6, = 7, ≥ 8 and NW = 0, = 1, ≥ 2

Njets = 6 Njets = 7

Njets ≥ 8

NW

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NW

NW

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Yes??

Maybe…

Sensitivity

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• Previous result:

• >372 GeV (CDF,

l+jets)

(Mass of b’)

Conclusion

• Examined the Lepton + Jets Decay Channel for the down-type

fourth generation quark, b’ .

• W-tags were made

• The b’ quark as not been found as yet:

• Bummer …

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Acknowledgements

• Katherine Copic:

• My Mentor who is thankfully still sane after dealing with me this

summer

• John Parsons:

• The opportunity to do this REU and the wealth of information

provided

• Evan Wulf:

• Not ignoring me when I called you for help 6 times in a row

• Tim Andeen

• For helping me fix the computer whenever I broke it

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Questions??

Back-Up Slides

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Invariant Mass of Jet Pairs

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Eve

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Invariant Mass of Jet Pair in Single Lepton Sample in GeV

• Jet Pairs:

• Jet pT > 25 GeV each

• Within ∆R < 1 of each other

W-tag: Jet pairs

between 70-100

GeV

OLD INV MASS

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