search for resonant wz production at the lhc
TRANSCRIPT
Institute for Nuclear and Particle Physics - TU Dresden
Abhishek Nag
Search for resonant WZ production at the LHC
July 8, 2021
July 8, 2021
Introduction: Standard Model
The Standard Model does not adequately explain:
● Gravity● Dark Matter● Dark energy● Neutrino Masses● Matter- antimatter asymmetry
Problems:
● Hierarchy problem → couldn’t explain the low mass of Higgs
● CP violation → weak interactions violate not only the charge-conjugation symmetry C between particles and antiparticles and the P or parity, but also their combination.
2
● Need beyond SM physics to explain such problems.
July 8, 2021
Large Hadron Collider
3
Accelerates proton bunches on 27 km ring and collides them at √s = 13 TeV
July 8, 2021
ATLAS DetectorElectrons:
● tracks in Tracker● Energy deposits in EC
Quarks & Gluons (as jets):
● tracks in Tracker● Showers in EC + HC
4
Muons:
● tracks in Tracker & MS● Energy deposits in EC + HC
Neutrinos: reconstructed missing transverse energy (ET
miss)
The goal is a precise measurement of the charge, position and energy of the particle
July 8, 2021
Resonances
● If (pA+pB)2≈ m2C, amplitude 𝑴 and consequently the cross
section becomes large → resonances○ extremely short lived
● In the past, resonances have been helpful in discoveries, e.g. Z boson
5
s-channel resonance diagram Amplitude M
Invariant mass of A+B: s = (pA + pB)2
[1]
July 8, 2021
How to look for new physics?
● Direct Search○ Enough energy at LHC to produce the particle on shell.○ Looking for a bump in the invariant mass distribution.
● Indirect Search○ New particles produced at much higher energy than that of LHC.○ Deviations from the SM observables like cross-sections, mass,
couplings showing up in precision measurements■ Effective Field theory approach
6
New physics may be beyond our reach
July 8, 2021
Vector Boson pairs
7
July 8, 2021
Vector Boson pair production
8
July 8, 2021
New physics in tail
9
July 8, 2021
Effective Field Theory (EFT)
● An EFT is a low-energy approximation for a more fundamental theory involving interactions at a mass scale Λ
● Deviations are parameterized by higher order operators from the SM fields
● dim-6 terms are generally better constrained in Triple Gauge Couplings --> Look at dim-8 for Vector Boson Scattering
10
operators of odd dimension violate baryon or lepton number4
July 8, 2021
EFT example
● If tails show discrepancies with the SM → can be used to look for new physics
● Recent CMS result published [3]
12
[2]
WZjj → lvlljj from CMS
cj/Λ4
July 8, 2021
New physics in bump
13
July 8, 2021
● Resonance searches: the simplest way to discover new particles.● Observe a statistically significant bump above a smooth background● Model-independent probe to new physics.
14
Fig: Feynman diagram for W’ -> WZ process at the LHC
● Resonance benchmark:○ Heavy vector Triplets (simplified Lagrangian) produced either by qqF or VBF○ Georgi - Machacek (GM) Higgs Triplet Model produced via VBF
Resonance Search: Looking for a bump
July 8, 2021
GM Model
15
● GM model[4] is minimal extension of the SM Higgs sector
● Extended by two isospin triplets: one complex and one real scalar
● A custodial SU(2) symmetry is imposed.● 10 physical Higgs scalars: a fiveplet (H5), a triplet (H3)
and two singlets (h, H).● The fiveplet H5 is fermiophobic and couples to vector
bosons.● Use H5Plane benchmark[5]
○ The mass of H3 is larger than the mass of H5
Cheng-Wei Chang, CEPC 2017
July 8, 2021
Heavy Vector Triplets
● Parameterized Lagrangians[6] incorporating heavy vector triplet (HVT) permit interpretation of searches for vector resonances.
● Simplified phenomenological Lagrangian used.● Two benchmark models used:
○ Model A: heavy vectors emerging from an underlying weakly-coupled extensions of the SM gauge group (gv =1)
○ Model B: strongly coupled Composite Higgs scenario (gv =3)
16
July 8, 2021
● We look into fully leptonic decay mode:○ Small branching fractions○ Clean signature, low background○ Good sensitivity to low mass
● Experimental signature:○ 3 hight pT isolated leptons○ Missing transverse energy (ET
miss)
17
Selected events are separated into two categories targeting 2 production modes:
(a) qq̄ fusion (b) vector boson fusion
VBF category is defined exploiting the typical VBF topology:● two ”tagging” jets in forward region in opposite
direction● high invariant mass of tagging jets mjj.● vector bosons between tagging jets
WZ Resonance search analysis strategy
July 8, 2021
WZ inclusive Selection
18
Designed to select good W and Z pairs decaying leptonically
July 8, 2021
Signal Optimization
● Improve sensitivity to resonant signals with additional selections● Assuming the resonant particle is heavy and at rest
○ W and Z are produced back to back○ Decay products of W and Z will be boosted → boost proportional to mass of resonant particle
19
July 8, 2021
Signal Optimization
● 𝚫y(W,Z) < 1.5 → remove SM background● 𝚫φ(l, ET
miss) used to define 2 regions:○ Low mass : 𝚫φ(l, ET
miss) >1.5○ High mass: 𝚫φ(l, ET
miss) <1.5
20
arXiv:1406.4456
July 8, 2021
Signal Optimization
● 𝚫y(W,Z) < 1.5 → remove SM background● 𝚫φ(l, ET
miss) used to define 2 regions:○ Low mass : 𝚫φ(l, ET
miss) <1.5○ High mass: 𝚫φ(l, ET
miss) >1.5
● LT the scalar sum of the leptons pT○ Cut value depending on mWZ
21
arXiv:1407.3476
July 8, 2021
Signal Optimization
● 𝚫y(W,Z) < 1.5 → remove SM background● 𝚫φ(l, ET
miss) used to define 2 regions:○ Low mass : 𝚫φ(l, ET
miss) <1.5○ High mass: 𝚫φ(l, ET
miss) >1.5
● LT the scalar sum of the leptons pT○ Cut value depending on mWZ
● The ratio between the boson pT and the mWZ mass
○ pTW/mWZ and pT
Z/mWZ
22
arXiv:1806.01532
July 8, 2021
Signal Optimization
● 𝚫y(W,Z) < 1.5 → remove SM background● 𝚫φ(l, ET
miss) used to define 2 regions:○ Low mass : 𝚫φ(l, ET
miss) <1.5○ High mass: 𝚫φ(l, ET
miss) >1.5
● LT the scalar sum of the leptons pT○ Cut value depending on mWZ
● The ratio between the boson pT and the mWZ mass○ pT
W/mWZ and pTZ/mWZ
● Other variables: Mjj, 𝚫yjj, 𝚫φjj, Centrality → for VBF like events which have 2 jets
23
July 8, 2021
Signal Region definitions
24
● Starts from the inclusive WZ selection● Events are separated into two categories → qq fusion and vector boson fusion● Two possibilities of analysis under study:
1. Cut based analysis: ■ Two orthogonal SR used to evaluate the HVT qqF, HVT VBF and GM VBF models
2. GM Neural Network analysis:
■ Deep Neural Network trained using events at the inclusive selection level + Mjj>100 GeV + Njet>=2■ Training aiming to discriminate GM signals from backgrounds■ GM VBF signal region defined with a cut on the DNN output
VBF signal region:● At least two VBF jets, choosing the 2
pt-leading ones as candidates● mjj>500 GeV and |∆Yjj|> 3.5
qqF signal region:● pZ
T /mWZ > 0.35 and pWT /mWZ > 0.35
● Fail VBF signal selection
July 8, 2021
1. SM WZ background:○ Dominant background○ Shape are estimated from theory ○ Normalization fitted in dedicated control regions
(orthogonal to SR)i. WZ VBF control region ii. WZ qq̄ control region
Background overview
1.2. Fake/Non prompt background:
○ Z+jets, Zγ, Wγ, ttbar, single top or WW → where jets or photons were misidentified as leptons○ Estimated using data
3. Other background○ ZZ production: normalization fitted in corresponding 4 lepton control region (ZZCR). ○ Others including ttbarV and triple boson production: Modeled using theory.
25
WZ invariant mass
VBF SR
July 8, 2021
Control Regions for prompt background estimation
26
WZ VBF control region● At least two VBF jets, choosing the 2 pt-leading ones
as candidates● Low mjj or low |∆Yjj| → orthogonal to VBF SR● Fail qq signal region
Need to check if the theory prediction matches well with data
July 8, 2021
Other Control regions
27
WZ qqF control region ZZ control region
Let’s look at the invariant mass
28
July 8, 2021
WZ Invariant mass distribution
29
qq region vbf region
arXiv:1806.01532
July 8, 2021
Published Limits
30
● Expected limits: fit model with background only hypothesis● Observed limits: fit data with model
arXiv:1806.01532
qq region vbf region
July 8, 2021
Expected full RunII limits: qq Signal Region
31
published 36 fb-1 limits
● Limit on model A improved from ~2.2 to ~2.6 TeV
Work in Progress
July 8, 2021
Expected full RunII limits: HVT VBF Signal Region
32
published 36 fb-1 limits
Work in Progress
● Improvement in the limits
July 8, 2021
Expected full RunII limits: GM VBF Signal Region
33
published 36 fb-1 limits
Work in Progress
● Improvement in the limits
July 8, 2021
Combined LimitsIn different decay channels
34
● Fully leptonic limits are better for low mass
● Limits with hadronic decay channel are better at high mass
arxiv
July 8, 2021
Take home message
● Resonances offer an easy way to look for new physics● Leptonically decaying W and Z bosons give low statistics but cleaner signature● Fully leptonic decay channel offer best limits for low mass resonances● We see improvements with the expected full RunII limits
35
We are actively looking for physics beyond SM and there is room for surprises
36
Thank you!
BACKUP
37
July 8, 2021
Recent publication from CMS (link):
● Cut based analysis with additional cuts:
○ τh veto: hadronic decay of τ with pT > 18 GeV & |η|<2.3
○ max(Z*l)<1.0, where Z*
l is Zeppenfeld variable defined as
|ηl -(ηj1 + ηj2)/2|/|Δηjj|
● 2 dimensional fit with 7 bins in mT_WZ and 2 bins in Mjj
38
linkCMS full RunII limits