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1. Search for a light pseudoscalar Higgs boson in the boosted μμττ final state in proton-proton collisions at $$ \sqrt{s} $$ = 13 TeV

   https://doi.org/10.1007/JHEP08(2020)139  

   Sirunyan, A. M. ; Tumasyan, A. ; Adam, W. ; Ambrogi, F. ; Bergauer, T. ; Dragicevic, M. ; Erö, J. ; Escalante Del Valle, A. ; Frühwirth, R. ; Jeitler, M. ; et al ( August 2020 , Journal of High Energy Physics)

   A bstract A search for a light pseudoscalar Higgs boson (a) decaying from the 125 GeV (or a heavier) scalar Higgs boson (H) is performed using the 2016 LHC proton-proton collision data at $$ \sqrt{s} $$ s = 13 TeV, corresponding to an integrated luminosity of 35 . 9 fb − 1 , collected by the CMS experiment. The analysis considers gluon fusion and vector boson fusion production of the H, followed by the decay H → aa → μμττ , and considers pseudoscalar masses in the range 3 . 6 < m a < 21 GeV. Because of themore »large mass difference between the H and the a bosons and the small masses of the a boson decay products, both the μμ and the ττ pairs have high Lorentz boost and are collimated. The ττ reconstruction efficiency is increased by modifying the standard technique for hadronic τ lepton decay reconstruction to account for a nearby muon. No significant signal is observed. Model-independent limits are set at 95% confidence level, as a function of m a , on the branching fraction (ℬ) for H → aa → μμττ , down to 1 . 5 (2 . 0) × 10 − 4 for m H = 125 (300) GeV. Model-dependent limits on ℬ(H → aa) are set within the context of two Higgs doublets plus singlet models, with the most stringent results obtained for Type-III models. These results extend current LHC searches for heavier a bosons that decay to resolved lepton pairs and provide the first such bounds for an H boson with a mass above 125 GeV.« less
2. Search for massive long-lived particles decaying semileptonically at $${\sqrt{s}}=13\,\hbox {TeV}$$

   https://doi.org/10.1140/epjc/s10052-022-10186-3  

   Aaij, R. ; Abdelmotteleb, A. S. ; Beteta, C. Abellán ; Gallego, F. J. ; Ackernley, T. ; Adeva, B. ; Adinolfi, M. ; Afsharnia, H. ; Agapopoulou, C. ; Aidala, C. A. ; et al ( April 2022 , The European Physical Journal C)

   Abstract A search is performed for massive long-lived particles (LLPs) decaying semileptonically into a muon and two quarks. Two kinds of LLP production processes were considered. In the first, a Higgs-like boson with mass from 30 to 200 $$\text {\,GeV\!/}c^2$$ \,GeV\!/ c 2 is produced by gluon fusion and decays into two LLPs. The analysis covers LLP mass values from 10 $$\text {\,GeV\!/}c^2$$ \,GeV\!/ c 2 up to about one half the Higgs-like boson mass. The second LLP production mode is directly from quark interactions, with LLP masses from 10 to 90 $$\text {\,GeV\!/}c^2$$ \,GeV\!/ c 2 . The LLPmore »lifetimes considered range from 5 to 200 ps. This study uses LHCb data collected from proton-proton collisions at $$\sqrt{s} = 13\text {\,TeV} $$ s = 13 \,TeV , corresponding to an integrated luminosity of 5.4 $$\text {\,fb} ^{-1}$$ \,fb - 1 . No evidence of these long-lived states has been observed, and upper limits on the production cross-section times branching ratio have been set for each model considered.« less
3. Search for exotic decays of the Higgs boson into long-lived particles in pp collisions at $$ \sqrt{s} $$ = 13 TeV using displaced vertices in the ATLAS inner detector

   https://doi.org/10.1007/JHEP11(2021)229  

   Aad, G. ; Abbott, B. ; Abbott, D. C. ; Abed Abud, A. ; Abeling, K. ; Abhayasinghe, D. K. ; Abidi, S. H. ; Abramowicz, H. ; Abreu, H. ; Abulaiti, Y. ; et al ( November 2021 , Journal of High Energy Physics)

   A bstract A novel search for exotic decays of the Higgs boson into pairs of long-lived neutral particles, each decaying into a bottom quark pair, is performed using 139 fb − 1 of $$ \sqrt{s} $$ s = 13 TeV proton-proton collision data collected with the ATLAS detector at the LHC. Events consistent with the production of a Higgs boson in association with a leptonically decaying Z boson are analysed. Long-lived particle (LLP) decays are reconstructed from inner-detector tracks as displaced vertices with high mass and track multiplicity relative to Standard Model processes. The analysis selection requires the presence ofmore »at least two displaced vertices, effectively suppressing Standard Model backgrounds. The residual background contribution is estimated using a data-driven technique. No excess over Standard Model predictions is observed, and upper limits are set on the branching ratio of the Higgs boson to LLPs. Branching ratios above 10% are excluded at 95% confidence level for LLP mean proper lifetimes cτ as small as 4 mm and as large as 100 mm. For LLP masses below 40 GeV, these results represent the most stringent constraint in this lifetime regime.« less
4. The second Higgs at the lifetime frontier

   https://doi.org/10.1007/JHEP07(2020)029  

   Alipour-Fard, Samuel ; Craig, Nathaniel ; Gori, Stefania ; Koren, Seth ; Redigolo, Diego ( July 2020 , Journal of High Energy Physics)

   A bstract We assess the current coverage and the future discovery potential of LHC searches for heavy Higgs bosons decaying into long-lived particles (LLPs), focusing primarily on the production of pairs of LLPs with hadronic final states. These signatures are generic in dark sectors where a heavy scalar decays into pairs of lighter states which subsequently mix with the Standard Model Higgs. We show that a handful of existing analyses provide broad coverage of LLP decay lengths ranging from millimeters to tens of meters, and explore the complementarity between searches for displaced and prompt final states in several simplified models.more »For both heavy singlet and heavy doublet scalars, LLP searches typically provide the leading sensitivity in current data and exhibit the strongest discovery potential in future LHC runs. We further translate the impact of these searches into the parameter space of various Twin Higgs models, demonstrating that LLP searches are a promising avenue for discovering a Twin Higgs with displaced decays. Finally, we propose a variety of additional search channels that would improve coverage of the second Higgs at the lifetime frontier.« less
5. Inclusive search for highly boosted Higgs bosons decaying to bottom quark-antiquark pairs in proton-proton collisions at $$ \sqrt{s} $$ = 13 TeV

   https://doi.org/10.1007/JHEP12(2020)085  

   Sirunyan, A. M. ; Tumasyan, A. ; Adam, W. ; Ambrogi, F. ; Bergauer, T. ; Dragicevic, M. ; Erö, J. ; Escalante Del Valle, A. ; Frühwirth, R. ; Jeitler, M. ; et al ( December 2020 , Journal of High Energy Physics)

   A bstract A search for standard model Higgs bosons (H) produced with transverse momentum ( p T ) greater than 450 GeV and decaying to bottom quark-antiquark pairs ( $$ \mathrm{b}\overline{\mathrm{b}} $$ b b ¯ ) is performed using proton-proton collision data collected by the CMS experiment at the LHC at $$ \sqrt{s} $$ s = 13 TeV. The data sample corresponds to an integrated luminosity of 137 fb − 1 . The search is inclusive in the Higgs boson production mode. Highly Lorentz-boosted Higgs bosons decaying to $$ \mathrm{b}\overline{\mathrm{b}} $$ b b ¯ are reconstructed as single large-radius jets,more »and are identified using jet substructure and a dedicated b tagging technique based on a deep neural network. The method is validated with Z → $$ \mathrm{b}\overline{\mathrm{b}} $$ b b ¯ decays. For a Higgs boson mass of 125 GeV, an excess of events above the background assuming no Higgs boson production is observed with a local significance of 2.5 standard deviations ( σ ), while the expectation is 0.7. The corresponding signal strength and local significance with respect to the standard model expectation are μ H = 3 . 7 ± 1 . 2(stat) $$ {}_{-0.7}^{+0.8} $$ − 0.7 + 0.8 (syst) $$ {}_{-0.5}^{+0.8} $$ − 0.5 + 0.8 (theo) and 1 . 9 σ . Additionally, an unfolded differential cross section as a function of Higgs boson p T for the gluon fusion production mode is presented, assuming the other production modes occur at the expected rates.« less