1. Motivation: The Standard Model (SM) of elementary particle physics has been tremendously successful at describing high energy physics phenomena, but it remains incomplete. Several beyond-standard-model (BSM) theories
introduce extra gauge bosons, many with generation-dependent lepton couplings. It is worthwhile to study the ditau decay channels of these additional gauge bosons as it could lead to new physics and tests of lepton universality.
This poster outlines a search for two such heavy gauge boson candidates as well as one candidate motivated by a model of large extra dimensions.
Search for Z’ → ττ → eμ and Large Extra Dimensions with Two Taus at 𝑠 = 8 TeV
The CMS Collaboration
F. Romeo, A. Santocchia, A. Gurrola, W. Johns, P. Sheldon, A. Johnson, J. Cumalat, E. Luiggi, T. Kamon, A. Florez, A. Kaur Kalsi, J.B. Singh, V. Bhatnagar, and K. Mazumdar
Z’-like resonances
E6 Model
E6 → SO(10)xU(1)ψ → SU(5)xU(1)χxU(1)ψ → SMxU(1)θE6
U(1)θE6
→ Z’(θE6
) : cos(θE6
)Z’ψ - sin(θE6
)Z’χ
θE6
ϵ (-90°, 90°) : θE6
= 0 → Z’ψ , θE6
= -90° → Z’χ
Exclude on Z’ψ
Sequential Standard Model
Z’SSM identical to SM Z, only heavier
Not gauge invariant
Exclude on Z’ SSM
Arkani-Hamed, Dimopoulos and Divali (ADD) model
Introduced to solve hierarchy problem of SM
𝑀 𝑝𝑙
2
= 𝑀 𝐷
2+𝑛
× 𝑅 𝑛
𝑀 𝑝𝑙 = Plank scale, 𝑀 𝐷 = effective Plank scale, 𝑅 = size of extra dimensions
If 𝑀 𝐷 ~ O(1) TeV →𝑅 ≤ O(1) mm for n ≥ 2
ADD model includes infinite tower of Kaluza-Klein (𝑮 𝑲𝑲) excitations of gravitons
Direct graviton emission results in apparent non-conservation of momentum
(Suppressed by 𝑀 𝐷
2+𝑛
)
Virtual graviton couplings result in anomalous production of fermion-antifermion,
diboson pairs
Couplings depend weakly on n, higher sensitivity
Relevant parameters of ADD
𝜎𝑆𝑀+𝐿𝐸𝐷 = 𝜎𝑆𝑀 + 𝜎𝑖𝑛𝑡 𝜂 𝐺 + 𝜎 𝐺 𝜂 𝐺
2
int = interference, G = graviton, 𝜂 𝐺 =
1
Λ 𝑇
4 =
𝐹
𝑀 𝑆
4 = parameters regulating ADD
effects
𝑀𝑆 = UV cutoff on the sum of Kaluza-Klein excitations of virtual graviton exchange
Exclude on Λ 𝑇 in GRW, 𝑀𝑆 and n in HLZ conventions
Theory Analysis
Event Selection
Muons
Identified based on HighPt ID
convention
Global, small IP, small
∆𝑝𝑡
𝑝𝑡
𝐼 𝑃𝐹 =
𝑝 𝑇
𝑐ℎ𝑎𝑟𝑔𝑒𝑑
+max( 𝑝 𝑇
𝑛𝑒𝑢𝑡𝑟𝑎𝑙
+𝑝 𝑇
𝛾
−∆𝛽,0)
𝜇 𝑝 𝑇
Electrons
Identified using HEEP 4.1 selections
Optimized for high-pT electrons
Combination of track, ECAL, HCAL
requirements ensure clean electrons
Topological Selections
Require back to back electrons, muons
Opposite charge
0 b-tagged jets
ET > 20 GeV
Cuts designed to exploit collinearity of τ
decay products
Analysis Strategy
Blind signal region (SR) until understand BG mass shapes
Estimate BG contributions to SR via data-driven analysis (TTJets, QCD) or take
from MC with scale factor (diboson, DYJets, WJets)
Study visible mass (e, μ, ET) distributions for signal and BG
Unblind once we understand shapes, set limits on signal mass
Data-Driven Background
Estimation
MC models not perfect descriptors of
physics/detector, lack statistics
Would like to estimate BGs from data
Strategy: define controls regions (CRs) to
enhance BGs, examine cut performance
for data in CRs
Use cut efficiencies in CRs to estimate BG
contribution to data in SR
Other Backgrounds
For diboson, DYJets: define CR for BG,
measure scale factor (SF) between data
and MC in CR
Apply SF to MC in signal region to get BG
estimation for data in SR
For every BG, run a closure test:
compare estimated SR contribution to
MC SR contribution
Results
τ physics
Z’SSM expected to couple equally to pairs of electrons, muons,
taus (similar to SM Z)
Other beyond-standard-model theories predict generation-
dependent couplings favoring ditau decays
Worthwhile to study ditau decays as it could lead to new
physics, test lepton universality
eμ decay channel has small BR, but is the “cleanest”, and has the
highest reconstruction efficiency
Hadronic decay channels also being studied
ee, μμ channels not explored due to large DY background
Mass shape in Signal Region
Data observed in SR in agreement with SM
expectations for SR
No excesses observed
Set mass limit to exclude Z’-like resonances
below certain visible mass
Mass Limits on Z’-like resonances
Use Bayesian method for limit
extraction with flat prior for signal
cross section and log-normal priors
for systematic uncertainties
All tau decay channels combined for
final limits
Limits taken at 95% confidence level
Z’SSM excluded below 1.4 TeV
Z’ψ excluded below 1.1 TeV
Data and Monte Carlo
Data
Data were collected by CMS during the 2012 run of the Large Hadron Collider and
total 19.7 fb-1 of integrated luminosity
Background MC
Signal MC
Selection Cuts
All signal samples generated with PYTHIA. For Z’SSM and Z’ψ, tau decays simulated
with TAUOLA, and independent samples generated for different Z’ mass values (500
to 2500 GeV in 250 GeV intervals). For ADD model, independent samples generated
for different values of ΛT (1600 to 4400 GeV in 400 GeV intervals.)
SM background MC generated with POWHEG, MADGRAPH, and PYTHIA and
interfaced with TAUOLA to simulate tau decays. Significant backgrounds include:
𝒕 𝒕+Jets: typically has one or two b-tagged jets, produces real isolated leptons from W±
bosons from top decays
QCD: produces non-isolated jets that can fake leptons
Drell-Yan: can produce taus that mimic Z’ → ττ events. Low mass compared to signal
W+Jets: can produce clean muon from W ± decay, and jets can fake electrons
WW, WZ, ZZ: can produce real, isolated e’s and μ’s when bosons decay leptonically
tW: t can decay into W ±, then both W ± can decay leptonically
High-pT μ ID
HEEP v4.1
Results for the 8 TeV analysis are forthcoming, but not yet published .Below are the
results from the 7 TeV analysis, which was published in Phys. Lett. B 716 (2012) 82-
102. Note that it did not include searches for extra dimensions.
For more information, please see CMS-EXO-11-031 and CMS-EXO-12-046
Systematic Uncertainties
Statistical uncertainty of data used in CRs for
data driven BGs contributes large systematic
uncertainty to mass shape
Other systematics include those from object
energy scale and resolution, misidentified b-jets
pileup, luminosity, and theoretical sources (PDFs)