Search for heavy right-handed gauge bosons decaying into boosted heavy neutrinos with the ATLAS detector at √ s = 13 TeV Debarati Roy on behalf of the ATLAS Collaboration ILHC-ICTP2019 1
Standard Model (SM) and beyond • Left Right Symmetric Model • SM : (LRSM) : => Extremely successful theory. => Restores parity by introducing => Guided through new particle right-handed gauge bosons (W R ) discoveries (Higgs boson glorifies its & right-handed neutrinos (N R ). success in 2012!) => Small neutrino mass can be • Couple of experimental observations explained via its coupling to N R SM cannot explain direct towards new via mass mixing matrix. Physics ✨✨👼 ✨✨ q ¯ m WR ~ TeV => Neutrino oscillation concludes m NR ~ GeV neutrino has a very small mass. W ∗ R q q ¯ N R => Several searches performed in LHC ` W R to explain origin of a very small q ` neutrino mass! Final state => 2 jets + 2 leptons 2 (resolved topology)
Extending phase space with boosted topology Several searches performed in ATLAS (& in CMS) with resolved topology. Latest ATLAS result in resolved scenario m NR << m WR : 4 [TeV] • Less explored phase space ATLAS Majorana N , ee channel R 3.5 -1 s =13 TeV, 36.1 fb g = g with limited discovery R L Obs. 95% CL limit R 3 N Exp. 95% CL limit potential estimation => m Exp. limit 1 ± σ 2.5 Sensitivity drops with Exp. limit 2 ± σ 2 resolved topology. • More efficient to consider 1.5 boosted scenario! 1 0.5 m NR << m WR 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 m [TeV] W R • First time we looked at possibility for boosted heavy neutrinos in ATLAS with 80 fb -1 of data at 13 TeV. arXiv:1904.12679 3
Boosted heavy neutrino search : Introduction • Final state consists of a large radius jet & 3 10 Events / 40 GeV ATLAS Simulation two leptons. -1 s = 13 TeV, L = 80 fb SR (ee) 2 Signal: M , M 10 W • Electron (e) & muon ( μ ) final states looked N R R [GeV], [GeV] 300, 3000 10 at separately with no flavour mixing. 400, 4000 500, 5000 600, 6000 • A balancing topology between hardest e (e1) 1 or μ ( μ 1) & highest mass large radius jet (j) 1 − 10 along with 2 nd hardest e (e2) or μ ( μ 2) inside 2 − 10 0 200 400 600 800 1000 that large radius jet gives well shaped j m [GeV] detector level variables. 3 10 Events / 40 GeV • Different N R mass computation performed ATLAS Simulation -1 s = 13 TeV, L = 80 fb SR ( ) µ µ 2 Signal: M , M 10 between e & μ final states due to nature of W N R R [GeV], [GeV] 300, 3000 jet reconstruction : 10 400, 4000 500, 5000 600, 6000 e channel : Mass of large radius jet (e energy 1 part of j energy, a distinguishing feature of 1 − 10 this search ) 2 − 10 0 200 400 600 800 1000 μ channel : Mass of large radius jet & μ 2. j, 2 µ m [GeV] arXiv:1904.12679 4
Boosted heavy neutrino search : Analysis Selection Object Selection : • Exactly 2 leptons & at least 1 large radius trimmed jet. • Isolated e1/ μ 1 & non-isolated e2/ μ 2 (2 nd hardest leptons allowed to be close to large radius jet). • Highest mass large radius (R = 1.0) jet (j) used with p T > 200 GeV, | η | < 2.0 (m j > 50 GeV in e final state). • p T,e1/e2 > 26 GeV, | η | < 2.47 excluding crack region. p T, μ 1/ μ 2 > 28 GeV, | η | < 2.5. [GeV] 800 Topological Cuts : ATLAS Simulation R N 700 m Signal selection efficiency • Azimuthal separation (d Φ ) 0.20 0.27 600 between e1/ μ 1 & j > 2.0. Muon channel 0.32 0.20 Electron channel 500 0.38 0.26 • ∆ R between e2/ μ 2 & j < 1.0. More boosted 0.39 0.31 0.20 400 0.46 0.37 0.24 Further Background Reduction Cuts : 0.44 0.41 0.30 0.19 300 • Dilepton invariant mass (m ll ) > 0.48 0.44 0.31 0.20 200 200 GeV. 0.40 0.34 0.23 0.14 0.34 0.21 0.12 0.08 More boosted 100 • d Φ between e1( μ 1) & e2( μ 2) > 1.5. 2000 3000 4000 5000 6000 arXiv:1904.12679 m [GeV] 5 W R
m WR : The discriminating variable for region definition Events / 100 GeV 4 10 -1 • m WR Computation in e final state : ATLAS s = 13 TeV, 80 fb CR (ee) Data t t 3 10 Invariant mass of j + e1. Z+jet(s) Single-t W+jet(s) • m WR Computation in μ final state : 2 Diboson 10 MC stat. unc. Invariant mass of j + μ 1 + μ 2. 10 • Control Region (CR : m WR < 2 TeV) 1 Data / Pred. 2 shows reasonable data-mc 1.5 1 agreement including statistical 0.5 400 600 800 1000 1200 1400 1600 1800 2000 j,e1 m [GeV] uncertainty. • Signal Region (SR : m WR > 2 TeV). Events / 100 GeV 3 10 -1 ATLAS s = 13 TeV, 80 fb VR ( e) µ Data • A Validation Region (VR) studied t t 2 10 Single-t with a hard e inside j balanced by Diboson MC stat. unc. a μ to conclude that data can be 10 well predicted by mc (when a hard 1 e inside j). Data / Pred. 2 ↑ ↑↑ ↑ 1.5 1 arXiv:1904.12679 0.5 6 500 1000 1500 2000 2500 3000 j, µ 1 m [GeV]
Performance of large radius jet with a hard e inside • Large radius jet reconstruction in ATLAS based on energy clusters calibrated at hadronic scale. • Effect of a non-negligible fraction of JMS ATLAS Simulation s = 13 TeV 1.05 EM clusters in j investigated in terms m (4 TeV), m (400 GeV) m (3 TeV), m (150 GeV) W W N N R R 1.04 R R m (5 TeV), m (500 GeV) m (3 TeV), m (300 GeV) W W of jet energy scale (JES) & jet mass N N R R R R 1.03 scale (JMS) as a function of ratio of 1.02 energy of e to the energy of j. 1.01 1 0.99 0 0.2 0.4 0.6 0.8 1 • A weak dependence j e2 E / E (within scale expected uncertainty range) concludes no additional correction factor needs to be implemented. arXiv:1904.12679 7
Performance of large radius jet with a hard e inside • Large radius jet reconstruction in ATLAS based on energy clusters calibrated at hadronic scale. • Effect of a non-negligible fraction of JES ATLAS Simulation s = 13 TeV 1.05 EM clusters in j investigated in terms m (4 TeV), m (400 GeV) m (3 TeV), m (150 GeV) W W N N R R 1.04 R R m (5 TeV), m (500 GeV) m (3 TeV), m (300 GeV) W W of jet energy scale (JES) & jet mass N N R R R R 1.03 scale (JMS) as a function of ratio of 1.02 energy of e to the energy of j. 1.01 1 0.99 0 0.2 0.4 0.6 0.8 1 • A weak dependence j e2 E / E (within scale expected uncertainty range) concludes no additional correction factor needs to be implemented. arXiv:1904.12679 8
Performance of large radius jet with a hard e inside • JMS as a function of generator level large radius jet mass shows reasonable behaviour : • Effect of a non-negligible fraction of 1 JMS-1 JES Events normalised to bin entries -1 0.4 ATLAS Simulation s = 13 TeV ATLAS Simulation s = 13 TeV, 80 fb 1.05 EM clusters in j investigated in terms 0.8 m (4 TeV), m (400 GeV) m (3 TeV), m (150 GeV) m (3 TeV), m (300 GeV) W W 0.35 N N R R 1.04 R R W 0.6 N R of jet energy & jet mass scales as a R m (5 TeV), m (500 GeV) m (3 TeV), m (300 GeV) W W N N R R 0.3 R R 0.4 1.03 function of ratio of energy of e 0.2 0.25 1.02 to the energy of j. 0 0.2 1.01 0.2 − 0.15 1 0.4 − 0.1 • A weak dependence 0.6 − 0.99 0 0.2 0.4 0.6 0.8 1 j e2 0.05 E / E 0.8 − (within scale expected uncertainty 3 10 × 1 0 − 100 200 300 400 500 600 range) concludes no additional j m [GeV] gen correction factor needs to be implemented. Events mostly concentrated at the JMS expected value equal to unity. arXiv:1904.12679 9
Overlap Removal (OR) Strategy for e close to hadronic activity • In signal topology e2 always close to a real jet => Standard OR in ATLAS removes jet or e if within ∆ R < 0.4 : Thus signal efficiency drops off ! => A modified OR approach followed for e2 : Within ∆ R ~ 0.04 of e & jet, Events / 98 GeV 4 10 events dominated with a true e -1 ATLAS s = 13 TeV, 80 fb VR ( e) µ j ,e Data small 0.04 < R < 0.4 mis-reconstructed as a jet. Thus events ∆ 3 y 10 t t with ∆ R > 0.04 selected. Single-t 2 10 MC stat. unc. => Further standard e efficiency 10 correction factor cannot be used. 1 Data / Pred. 2 Thus in VR additional criterion 1.5 1 ↓ applied : a b-tagged jet & data-mc 0.5 100 200 300 400 500 600 700 800 e p [GeV] comparison done within 0.04 < ∆ R <0.4. T Residual disagreement in addition to statistical, theory & b-tagging uncertainties quantified as an additional efficiency correction factor uncertainty. arXiv:1904.12679 10
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