Fully Hadronic Diboson Resonance Search:
Overview and Run II Background Studies
Matthew Epland
Duke University
June 25, 2015
Matthew Epland
Fully Hadronic Diboson Resonance Search
June 25, 2015
1 / 13
Outline
Introduction to the Diboson Resonance Search
Run I Search: Methods
Run I Search: Results
Recent Theory Papers
Run II Search: Changes from Run I
Run II Search: Background Studies and First Data
Conclusions
Adapted from a talk given at the June 2015 US ATLAS Workshop
at the University of Illinois. All slides with non–public plots have
been redacted!
Matthew Epland
Fully Hadronic Diboson Resonance Search
June 25, 2015
2 / 13
Introduction to the Diboson Resonance Search
Goal: Search for new physics decaying to SM W and Z vector bosons
at O(TeV) scales
Examples: Extended gauge sector W 0 → W Z
Extra–dimensional spin–2 graviton GRS → W W or ZZ
Expect low signal cross section → Turn to hadronic decays with their
higher branching ratios, in exchange for accepting large QCD
backgrounds
lν, ll
τ ν, τ τ
νν
qq
W
22%
11%
–
67%
Z
7%
3%
20%
70%
Branching Ratios [1]
Matthew Epland
Fully Hadronic Diboson Resonance Search
June 25, 2015
3 / 13
Run I Search: Methods
Looking for resonances of mass O(TeV), SM W and Z have masses
O(100 GeV) → final state jets are highly boosted
Jets for each quark pair are so boosted the pair appear to be one fat
jet in the detector
Can use jet substructure to discriminate between background QCD jets
and diboson jets
q
V1
q
W0
Fat Jets
q
V2
q
Diboson Fat Jets [2]
Matthew Epland
QCD Fat Jets [2]
Fully Hadronic Diboson Resonance Search
June 25, 2015
4 / 13
Run I Search: Methods – Jet Tagging
Jets are reconstructed with the Cambridge–Aachen (C/A) algorithm
Hard substructure is reconstructed with the BDRS–A algorithm
Filtering requires a minimum of three subjets
De–Clustering Algorithm Parameters [3]
1 ,m2 )
Mass–drop: µ12 ≡ max(m
, µ≤1
m12
√
√
12
Subjet momentum–balance: y ≡ min (pt1 , pt2 ) ∆R
y > 0.2
mj ,
Matthew Epland
Fully Hadronic Diboson Resonance Search
June 25, 2015
5 / 13
Run I Search: Methods – Trigger and Cuts
Trigger
EF j360 a10tcem – High pt large radius jet trigger, selecting > 99% of
C/A R = 1.2 jets with |η| < 2.0 and ungroomed pt > 540 GeV [4]
Boson Jet Tagging
µf = 1 (no mass–drop requirement)
√
√
yf = 0.2 then cut to y ≥ 0.45
ntrk < 30
mj within 26 GeV of mW or mZ
Event Selection
|η| < 2.0
Rapidity Difference: |∆y12 | < 1.2
pt asym ≡ (pt1 − pt2 ) / (pt1 + pt2 ) < 0.15
mjj > 1.05 TeV
Matthew Epland
Fully Hadronic Diboson Resonance Search
June 25, 2015
6 / 13
Run I Search: Results
Data
Background model
1.5 TeV EGM W', c = 1
2.0 TeV EGM W', c = 1
2.5 TeV EGM W', c = 1
Significance (stat)
Significance (stat + syst)
ATLAS
3
s = 8 TeV, 20.3 fb-1
10
102
104
Data
Background model
1.5 TeV Bulk GRS, k/MPI = 1
2.0 TeV Bulk GRS, k/MPI = 1
Significance (stat)
Significance (stat + syst)
ATLAS
3
10
Events / 100 GeV
104
Events / 100 GeV
Events / 100 GeV
After cuts we observe an excess around 2 TeV with a global significance of
2.5 σ, not a discovery but exciting!
s = 8 TeV, 20.3 fb-1
102
10
WZ Selection
1
10−1
s = 8 TeV, 20.3 fb-1
102
10
ZZ Selection
1
1
−1
−1
10
10
10−2
10−2
−3
−3
2
2.5
3
1.5
2
2.5
3
3.5
3.5
mjj [TeV]
3
2
1
0
−1
−2
10
1.5
2
2.5
3
1.5
2
2.5
3
3.5
3.5
Significance
1.5
Significance
10
Significance
Data
Background model
1.5 TeV Bulk GRS, k/MPI = 1
2.0 TeV Bulk GRS, k/MPI = 1
Significance (stat)
Significance (stat + syst)
ATLAS
3
10
WW Selection
10
3
2
1
0
−1
−2
104
3
2
1
0
−1
−2
1.5
2
2.5
3
1.5
2
2.5
3
mjj [TeV]
3.5
3.5
mjj [TeV]
Background-only fits to the dijet mass distributions in data after tagging
with the W Z (L), W W (C), and ZZ (R) selection [4]
Matthew Epland
Fully Hadronic Diboson Resonance Search
June 25, 2015
7 / 13
Recent Theory Papers
The Run I result was publicly released on June 2nd, quite a few theory
papers are now out in response:
Interpretations of the ATLAS Diboson Anomaly [5]
A W 0 boson near 2 TeV: predictions for Run 2 of the LHC [6]
Interpretations of the ATLAS Diboson Resonances [7]
2 TeV Walking Technirho at LHC? [8]
Diboson Signals via Fermi Scale Spin–One States [9]
There are many proposed underlying phenomena, see references for full
details
Matthew Epland
Fully Hadronic Diboson Resonance Search
June 25, 2015
8 / 13
Conclusions
The Run I search gave us an intriguing 2.5 σ result
Work on Run II analysis framework is ongoing
Run II will (hopefully) provide a definitive answer on the 2 TeV excess
Matthew Epland
Fully Hadronic Diboson Resonance Search
June 25, 2015
9 / 13
Backup
Matthew Epland
Fully Hadronic Diboson Resonance Search
June 25, 2015
10 / 13
± 2σ unceirtainty
102
RS
EGM W', c = 1
10
1
10−1
1.4
1.6
1.8
2
2.2
2.4
2.6
2.8
3
mW' [TeV]
Observed 95% CL
s = 8 TeV, 20.3 fb-1
Expected 95% CL
± 1σ uncertainty
± 2σ uncertainty
102
→ ZZ) [fb]
ATLAS
103
RS
± 1σ uncertainty
104
Bulk GRS k/ MPI = 1
104
ATLAS
Observed 95% CL
s = 8 TeV, 20.3 fb-1
Expected 95% CL
103
± 1σ uncertainty
± 2σ uncertainty
102
Bulk GRS k/ MPI = 1
RS
Expected 95% CL
σ(pp → G ) × BR(G
Observed 95% CL
s = 8 TeV, 20.3 fb-1
→ WW) [fb]
ATLAS
103
RS
104
σ(pp → G ) × BR(G
σ(pp → W') × BR(W' → WZ) [fb]
Run I Search: σ Results
10
1
10−1
1.4
1.6
1.8
2
2.2
2.4
2.6
2.8
3
mG [TeV]
10
1
10−1
1.4
1.6
1.8
2
2.2
2.4
2.6
RS
2.8
3
mG [TeV]
RS
Upper limits, at 95% C.L., on the section times branching ratio limits for
the W Z (L), W W (C) ZZ (R) window selection. The solid red line
displays the predicted cross section for the appropriate model as a function
of the resonance mass [4]
Matthew Epland
Fully Hadronic Diboson Resonance Search
June 25, 2015
11 / 13
Fraction of jets / 5.0 GeV
Run I Search: Channel Overlap
0.22
0.2
0.18
bulk GRS → WW (m = 1.8 TeV)
ATLAS Simulation
s = 8 TeV
G
bulk GRS → ZZ (m = 1.8 TeV)
G
0.16
Pythia QCD dijet
0.14
0.12
|∆ y | < 1.2
jj
|η | < 2
j
1.62 ≤ mjj < 1.98 TeV
y > 0.45
0.1
0.08
0.06
0.04
0.02
0
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
mj [TeV]
“The distribution of. . . mass mj of the groomed jet, in simulated signal and background
events. The signal and background distributions are normalised to unit area, and the last
bin of each histogram includes the fraction of events falling outside of the displayed range.
Requirements are placed on the events used to ensure that the kinematics of the signal
and background events are comparable.” [4]
Matthew Epland
Fully Hadronic Diboson Resonance Search
June 25, 2015
12 / 13
References
[1] Particle Data Group Collaboration, K. Olive et al., Chin.Phys. C38 (2014) 090001.
[2] E. Kajomovitz, Searching for new particles decaying to W and Z bosons with ATLAS,
https://indico.cern.ch/event/399003, 2015.
[3] Performance of Boosted W Boson Identification with the ATLAS Detector, Tech. Rep.
ATL-PHYS-PUB-2014-004, CERN, Geneva, Mar, 2014.
https://cds.cern.ch/record/1690048.
[4] ATLAS Collaboration, G. Aad et al., arXiv:1506.00962 [hep-ex].
[5] K. Cheung, W.-Y. Keung, P.-Y. Tseng, and T.-C. Yuan, arXiv:1506.06064
[hep-ph].
[6] B. A. Dobrescu and Z. Liu, arXiv:1506.06736 [hep-ph].
[7] J. Hisano, N. Nagata, and Y. Omura, arXiv:1506.03931 [hep-ph].
[8] H. S. Fukano, M. Kurachi, S. Matsuzaki, K. Terashi, and K. Yamawaki,
arXiv:1506.03751 [hep-ph].
[9] D. B. Franzosi, M. T. Frandsen, and F. Sannino, arXiv:1506.04392 [hep-ph].
Matthew Epland
Fully Hadronic Diboson Resonance Search
June 25, 2015
13 / 13