Search For Fourth
Generation Quarks in
Lepton plus Jets decay
Channel
(b’ l + jets)
Natalee Raymond
Macalester College
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
1
Outline
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
2
ATLAS
Standard Model
Lepton + Jets Channel
Signal Properties
W-tags Object Selection
Lepton + Jets Cuts
Sensitivity
Conclusion
ATLAS
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
3
• 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
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
4
Overview of Hadronic Calorimeter
The Standard Model
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
5
4th generation Why not?
• 6 quarks
• 6 leptons
• Force carriers
+23
−13
Charge:
W Boson
• Force carrier that mediates the Weak interaction
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
6
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
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
7
Missing Energy
One Lepton
8 jets
Final State Production
of the b’ pair:
*Heavy b' decays will
produce high pT products!
Standard Model Backgrounds
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
8
• 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
Eve
nts
Signal vs. MC
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
9
• 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
nts
Delta R
• ∆R = ∆η𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛2 + ∆φelectron
2
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
10
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
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
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
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
12
Number of W-tags (NW) in events with 1 lepton + missing energy
Eve
nts
Lepton + Jets Cuts
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
13 Electron pT of Single-Lepton Skims in GeV
DATA:
• skimmed single-electron sample: plots shown with luminosity = 1 / fb, data
Eve
nts
• Cuts to be
made:
• Missing
Energy
(MET)
• Transverse
W Mass
MET CUT ( >35 GeV)
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
14
Missing Energy of Single-Lepton Skims after Cut (GeV)
Eve
nts
• Missing Transverse Energy:
• Transverse Energy of the neutrino
Transverse W Mass Cut ( >25 GeV)
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
15
Transverse W Mass of Single-Lepton Skims after Cut (GeV)
Eve
nts
• mT2 (l,v) = 2 Et
l Etv (1-cosφl,v)
Number of Jets
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
16
Eve
nts
Njets with pT > 25 GeV in events with 1 electron + missing energy
Results: Number of W-tags (NW)
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
17
Njets = 6, = 7, ≥ 8 and NW = 0, = 1, ≥ 2
Njets = 6 Njets = 7
Njets ≥ 8
NW
Eve
nts
NW
NW
Eve
nts
Eve
nts
Yes??
Maybe…
Sensitivity
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
18
• 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 …
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
19
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
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
20
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
21
Questions??
Back-Up Slides
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
22
Invariant Mass of Jet Pairs
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
23
Eve
nts
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
Fou
rth
Gen
erat
ion
Qu
arks
in
Lep
ton
+ J
ets
Dec
ay C
han
nel
24