20th Lomonosov Conference on Elementary Particle Physics

22nd - Aug -2021

Searches for new phenomena with the ATLAS detector
Mohammed Faraj

- Despite its success, the Standard Model (SM) is notconsidered as a complete theory since several aspects are
not included within its prediction:Introduction❑ Hierarchy problem❑ Existence of dark matter and dark energy❑ Gravitational force❑ Neutrino Masses and oscillation❑ …etc- For those and other reasons, experimental searches for
new phenomena predicted by more advanced theories canhelp establish new theoretical frameworks to cover theSM weaknesses.
❑ SUSY❑ Extra Higgs doublets❑ TopColour❑ Randall-Sundrum Models❑ ..etc2

Introduction• Several Beyond Standard Model (BSM) analyses
are taking place in the ATLAS experiment toaddress the many questions left unanswered bySM.
• From a myriad of BSM scenarios, new phenomena
could manifest themselves via Resonant or nonResonant effects in different ranges of energy.
• This talk covers new results from the ATLAS
experiment, targeting BSM (non-)resonances andexotic decays into long-lived particles in channels:• Hadronic (allhad)• Single Lepton (1L)
• Dilepton (2L)
• Diboson (VV)3

Searchers for Resonances and exotic decays in Hadronic decay channel• Covered in heavy gauge bosons, heavy pseudo(scalar) Higgs boson, excited quark, KaluzaKlein excitations (gluon and Graviton excitations), Top-assisted-technicolour Z’, Top-downtheories (LLPs)..etc• Wide resonance mass ranges go from about 400 GeV up to 10 TeV.
• Highlighting several techniques and improvements:❑ New trigger technique for low mass acceptance.
❑ Improved b-tagging algorithm for jets with high pT.
❑ Dedicated techniques to identify the top-jets, in particular, boosted Tops.
❑ Algorithms to reconstruct tracks and vertices of particles with macroscopic lifetime.

4Vector Boson Resonances decaying
in top- and b-quark• Large-R jets are identified as containing a topquark decaying Hadronically (known as toptagged jets) using DNN.
• Small-R jets are reconstructed using the anti𝑘𝑡 algorithm and identified as b-jets using theDLr1 algorithm.
• Improved multi-jets BK estimation based ondata-driven method.
• Two signal regions “SR” are defined according
to the top- and b-tagging status.

• Limits set on the right-handed W’ boson“predicted by Chiral W’-boson model” withmass in the range 1.5-5TeV.

Full Run 2 Dataset

5ATLAS-CONF-2021-043

Top- and b- tagged jets

Resonances in 1L and 2L decay channels• They are the cleanest decay channels in particular the dilepton one.
• A probe for Z’ in Sequential Standard Model SSM, E6-motivated Grand Unificationmode, Top-assisted-technicolour, and excitations in Randall-Sundrum Model..etc/• Other variables sensitive to the presence of new physics might be used.
Highlighting several techniques and improvements:❑ Using the generated MC samples or data-driven methods toestimate the BK in Signal regions (SR).
❑ Reweighting techniques are used to get better Data/MCagreements.
❑ New tools to keep leptons with high pT and close to jets.
❑Advanced techniques can be used to reconstruct the top-antitop
quark Mass.

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Vector-like quarks production in2L/3L decay channel• Many BSM addresses the existence of VLQs that
couple preferentially to third-gen quark as well as
to the SM W/Z and Higgs boson.
• VLQ of the top (T) and bottom (B) quarks canexist with charges 2/3e and -1/3e respectively(they can have exotic charges).
• Deep neural network (DNN) is used to classify
the jets originated from either Z/W or Higgs
boson or top quark.
• In 3L channel, the scalar sum of jets and leptons
pT (HT) used as discriminating variable while in 2L
channel the m(Zb) is used.
• Limits in singlet (doublet) model are set at 𝑚 𝑇 >1.27 (1.46) TeV and 𝑚𝐵 > 1.20 (1.32) TeV.
Full Run 2 Dataset

ATLAS-CONF-2021-024
7

Single Vector-like quark
production in 1L decay channel• This search designed to target the production ofT-> t (lep)Z/H (had).
• Results are interpreted in several benchmark
scenarios to set limits on the mass and universal
coupling strength (κ).
• re-clustered Large-jet (RC) are used for tagging
the hadronic decay of top-quark, W/Z boson andHiggs boson.
• Events are divided based on the number of jetsand b-tagged jets “LJ and HJ”.
• Meff (scalar sum of lepton and jets pT + MET ) isused as a discriminating variable.
• All k values above the black line are excluded ateach mass point.
Full Run 2 Dataset

ATLAS-CONF-2021-040
8

non-resonant Dilepton• Several recent measurements give hints of possible lepton
flavour (LFU) violation in e.g. rare decay of B-meson into
K-meson and a pair of muons or electrons.
• Interpreted with four-fermion (two quarks “b-quark andS-quark” and two leptons: bs2L) ‘Contact’ interactions (CI)models.
• A fit-based extrapolation procedure is used to estimate
the tails of the Top mll distribution.
• Contract interactions with (energy scale Λ and coupling 𝑔∗ )Λ/𝑔∗ lower than 2 (2.4) TeV are excluded for electrons(muons) in regions with 0b- and 1b-tagged jets.

Full Run 2 Dataset

arXiv:2105.13847v1

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Heavy diboson resonances insemileptonic channel• Searches for heavy resonances, W’ and Z,’ in themass range 300 GeV to 5 TeV via gg fusion (ggF),
Drell-Yan (DY) or vector boson fusion (VBF).
• H → 𝑏𝑏ത reconstructed as 2 small-R jets or asingle large-R jet depending on pT(V).
• Variable radius (VR) track jets are used toidentify the b-jets from boosted Higgs bosondecay.
• Two signal regions are defined based on thenumber of b-tagged jets (1b or 2b).

• Limits on the W’->Wh production are set as wellas the exclusion contours in {gH,gf} HVT space.
• Limits on the production of Z’->Zh is set in the0L/2L decay channel excluding masses below 2.9(3.2)TeV for Model A (B) with gv=1 (3).
Full Run 2 Dataset

ATLAS-CONF-2020-043 (Z’)
ATLAS-CONF-2021-026 (W’)

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Full Run 2 Dataset

arXiv:2107.06092v1

Exotic decay of Higgs boson “allHad”• Predicted by Top-down (bottom-up) theories e.g.
theories based on Neutral Naturalness concept,
Folded SUSY ..etc.

• Higgs boson decays into pair of electrically neutral
pseudoscalar “a” boson and “a” boson subsequently
ത
decaying into 𝑏𝑏.
• Data-driven technique is used to estimate thecontributions from BK.

• Displaced vertices DV are reconstructed from a
combined collection of standard and LRT tracks.
• The number of observed events in SR = 0.

Prompt• Branching ratios above 10% are excluded.

CR+(MS1+MS2)
11

Full Run 2 Dataset

ATLAS-CONF-2021-032

Long-lived particles decaying into
hadronic jets in the ATLAS Muon System• Sensitive to a large variety of models. However, in thisanalysis, the results were interpreted in terms ofscalar portal model, where an SM-like Higgs, Φ ,decaying into two Long-lived scalars, s.

s• “ ” decaying mainly to the heaviest fermion pair e.g.
ത 𝑐𝑐ҧ and 𝜏𝜏.ҧ𝑏𝑏,
• Improved BK estimation in MS and more advanced
modelling and algorithm used for trigger and vertex
reconstruction.

• Branching ratios above 10% are excluded for Φ withmass =125GeV.
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• Highlighting the new results from (non-)resonant BSM searches in the ATLAS experiment.
Besides, the results from exotic decays into long-lived particles (LLP).
• In these analyses, several techniques have been used and introduced to increase thesensitivity of New physics.
• No significant deviations from the SM have been seen. Observations are in agreement with
the SM on a wide range of cross-sections.

Backup

Full Run 2 Dataset

JHEP 10 (2020) 61𝑡𝑡ҧ quark resonances in allhad
decay channel• A Spin-1, 𝑍` 𝑇𝐶2 , particle predicted from the Topcolour-assistedtechnicolour IV model is considered:❑ For top-antitop invariant mass greater than 1.4TeV.
❑ Couples only to the first- and third-generation quarks.
• In this analysis, the IV model parameters are chosen to maximize
branching fraction for 𝑍` 𝑇𝐶2 → 𝑡𝑡ҧ decay.
• Fully-hadronic top decays are captured in a large-radius jet.
• Two signal regions are defined based on the number of b-tagged
jets: SR1b and SR2b.
• BK from multijets and top-quark pair production are obtained fromfit to data.
• 𝑍` 𝑇𝐶2 for masses < 3.9 (4.7) TeV are excluded for ΓΤ𝑚 = 1% (3%).

1736.1 fb-1 Dataset

arXiv:1902.10077v2

𝑡𝑡ҧ quark resonances in allhad
decay channel• Spin-1 and Spin-2, gKK and Gkk respectively,
excitations
predicted in Randall-Sundrum (RS)model
❑ For top-antitop invariant mass 0.5-3TeV and0.5-5TeV for Gkk and gKK respectively.
• Fully-hadronic top decays are captured in a largeradius jet.
• Two signal regions are defined based on the numberof b-tagged jets: SR1b and SR2b.
• BK from multijets and top-quark pair production areobtained from fit to data.
• gKK for masses < 3.4TeV are excluded for ΓΤ𝑚 =30%.
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Resonances in dijet mass
distributions• Signal dijets close in |Δy| while large for BKs.
• Mjj scanned in regions with at least one btagged jet or with exactly two b-jets.
• Limits set on several benchmarks with mjj inrange 1.1-8TeV: e.g. excited quarks, chiral
excitation of the W, Heavy Z` and W` gaugebosons.
• For reinterpretation: limits on genericGaussian-shaped of various widths as a functionof the mass.
Full Run 2 Dataset

JHEP 03 (2020) 145

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Dilepton resonances• Dielectron and dimuon resonances in the mass
range of 250 GeV to 6TeV.
• Generic signal shapes are constructed from nonrelativistic Breit–Wigner functions of variouswidths convolved with the detector resolution.
• Exclusions on several benchmarks: e.g. Z` boson inSSM, HVT and E6 GUT Model.

Full Run 2 Dataset

Phys. Lett. B 796 (2019)20

non-resonant Dilepton• Search for enhanced rate of dilepton (ee and 𝜇𝜇) eventwith masses above 2TeV.
• Interpreted with four-fermion (two quarks and twoleptons: 2q2L) ‘Contact’ interactions (CI) models.
• Data-driven method used to estimate the BKs in the low
mass region then extrapolate to higher mass single-bin
signal regions (SR).
• Lower limits on the CI scale for different chiral structure
and interference sign. The strongest Lower limits is on
the LL constructive model.

Full Run 2 Dataset

JHEP 11 (2020) 005
21

Vector-like quarks production in 2L/3L decay channel

Full Run 2 Dataset

ATLAS-CONF-2021-024
22