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Search for the Production of a Long-Lived Neutral Particle PHYSICAL REVIEW LETTERS 122, 151801 (2019) Search for the Production of a Long-Lived Neutral Particle Decaying withinp theffiffi ATLAS Hadronic Calorimeter in Association with a Z Boson from pp Collisions at s =13 TeV M. Aaboud et al.* (ATLAS Collaboration) (Received 7 November 2018; published 15 April 2019) This Letter presents a search for the production of a long-lived neutral particle (Zd) decaying within the ATLAS hadronic calorimeter, in association with a standard model (SM) Z boson produced via an intermediate scalar boson, where Z → lþl− (l ¼ e, μ). The data used were collected by the ATLAS pffiffiffi detector during 2015 and 2016 pp collisions with a center-of-mass energy of s ¼ 13 TeV at the Large Hadron Collider and correspond to an integrated luminosity of 36.1 Æ 0.8 fb−1. No significant excess of events is observed above the expected background. Limits on the production cross section of the scalar boson times its decay branching fraction into the long-lived neutral particle are derived as a function of the mass of the intermediate scalar boson, the mass of the long-lived neutral particle, and its cτ from a few centimeters to one hundred meters. In the case that the intermediate scalar boson is the SM Higgs boson, its decay branching fraction to a long-lived neutral particle with a cτ approximately between 0.1 and 7 m is m excluded with a 95% confidence level up to 10% for Zd between 5 and 15 GeV. DOI: 10.1103/PhysRevLett.122.151801 Many extensions to the standard model (SM) such as an additional Uð1Þd dark gauge symmetry [16,17].One supersymmetry [1,2], inelastic dark matter [3], and hidden such model has been tested by the ATLAS experiment in a valley scenarios [4,5] predict the existence of long-lived search for a new particle that is mediated by the Higgs neutral particles that can decay hadronically. Search for boson and decays promptly to a lepton pair [18,19]. This long-lived neutral particles is an emerging field of research analysis expands the search to a more general case to that has attracted significant theoretical and experimental include a possible new scalar (Φ) that couples to Z and Zd, interests. So far, only searches for the pair production of instead of only the Higgs boson, and considers the scenario such particles have been carried out by the ATLAS [6–9], in which the Zd decays hadronically with a cτ between a CMS [10,11], and LHCb [12,13] experiments at the Large few centimeters and 100 meters, where c is the speed of τ Z Hadron Collider (LHC), and the CDF [14], and D0 [15] light and is the d proper lifetime.pffiffiffi experiments at the Tevatron. The analysis uses data from s ¼ 13 TeV proton- This Letter reports a new way to look for new physics proton (pp) collisions at the LHC that were recorded by (NP) beyond the SM in a collider using singly produced the ATLAS detector in 2015 and 2016 with single-electron long-lived neutral particle, which is one potential scenario and single-muon triggers [20], corresponding to an inte- that NP can manifest itself but had never been considered grated luminosity of 36.1 Æ 0.8 fb−1. The ATLAS detector in theories or experiments. Among many possible single [21] is a multipurpose particle detector with a cylindrical production final states, this Letter focuses on search for geometry [22]. Thep distanceffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi between two objects in the a hadronically decaying long-lived neutral particle, η–ϕ space is ΔR ¼ ðΔηÞ2 þðΔϕÞ2. Transverse momen- denoted by Zd hereafter, produced in association with a tum is defined by pT ¼ p sin θ. It consists of an inner SM Z boson through an intermediate scalar Φ or Higgs detector (ID) [23] surrounded by a solenoid that produces a þ − boson, pp → Φ=H → ZZd, where Z → l l (l ¼ e, μ). 2 T magnetic field, electromagnetic and hadronic calorim- Production of a new particle in association with a Z boson eters, and a muon spectrometer in a magnetic field is a popular scenario in hidden- or dark-sector models with produced by a system of toroid magnets. The ID measures the trajectories of charged particles over the full azimuthal angle and in a pseudorapidity range of jηj < 2.5 using *Full author list given at the end of the article. silicon pixel, silicon microstrip, and straw-tube transition- radiation tracker detectors. Liquid-argon electromagnetic Published by the American Physical Society under the terms of calorimeters (LArCal) extend from 1.5 to 2.0 m in radius the Creative Commons Attribution 4.0 International license. z Further distribution of this work must maintain attribution to in the barrel and from 3.6 to 4.25 m in j j in the end caps. the author(s) and the published article’s title, journal citation, A scintillator-tile calorimeter (TileCal) provides hadronic and DOI. Funded by SCOAP3. calorimetery and covers the region 2.25 <r<4.25 m. 0031-9007=19=122(15)=151801(19) 151801-1 © 2019 CERN, for the ATLAS Collaboration PHYSICAL REVIEW LETTERS 122, 151801 (2019) The experimental signature searched for is the Zd decaying multiple pp interactions in the same and neighboring bunch within the TileCal, thus producing a jet that has little or no crossings (pileup) is included by overlaying minimum-bias energy deposited in the LArCal, and no charged tracks that events simulated with PYTHIA8.186 on each generated event point to the reconstructed location of the collision of in all samples. The generated samples were processed interest (hereafter called the primary vertex). through a GEANT4-based detector simulation [29,30] and Monte Carlo (MC) simulated events are used to optimize the standard ATLAS reconstruction software. the event selection and to help validate the analysis. Signal The selected events have a pair of oppositely charged and samples were generated using the PYTHIA 8.210 [24] isolated electrons [31] or muons [32] to form a Z boson generator with the NNPDF23LO parton distribution func- candidate. Electrons and muons are required to have jηj < 2 47 η < 2 4 p > 25 tions (PDFs) [25] and the A14 set of tuned parameters (A14 . and j j . , respectively, and T GeV tune) [26], with an assumption that the Zd decays only to (27 GeV) in data collected in 2015 (2016). The invariant ¯ the highest-mass heavy quark pair (bb or cc¯) that is mass of the Z candidate (mll) is required to be between 66 kinematically allowed. Nine samples were produced with and 116 GeV. Selected jets must have transverse energy E > 40 η < 2 0 three different Zd masses for each of three Φ masses T GeV and j j . to ensure the jets are com- (mZ ¼f5; 10; 15g; f10; 50; 100g, and f20; 100; 200g for pletely within the ID. They are reconstructed using the anti- d k R 0 4 mΦ ¼ 125, 250, and 500 GeV, respectively), where mΦ ¼ t algorithm [33,34] with a radius parameter ¼ . and 125 GeV corresponds to the SM Higgs boson. The cτ of calibrated to particle level [35]. Standard ATLAS jet- the Zd is a free parameter in this model. For each mass quality criteria [36] are applied, except the one for the hypothesis of Zd and Φ, its cτ is chosen to maximize the ratio of the energy deposited in the hadronic calorimeter to probability for Zd to decay inside the TileCal, which is the total energy since it removes signal jets. A jet is found to be around 20% for all samples, as shown in considered as a Zd candidate, referred to as a calorimeter- E =E > Fig. 1(a). The events were reweighted to produce samples ratio jet (CR jet) hereafter, if it satisfies log10ð Tile LArÞ 1 2 p > 1 with different cτðZdÞ [8] between 0.01 and 100 m. The . with no associated tracks [37] of T GeV origi- E E dominant SM background arises from events with a Z nating from the primary vertex, where Tile and LAr are the boson produced in association with jets (Z þ jets), where a jet energy deposited in the TileCal and LArCal, respec- E < 60 jet mimics the experimental signature of Zd decay inside the tively [6], as shown in Fig. 1(b). Jets with T GeV in 0 TileCal due to the presence of long-lived SM particles (KL, the transition region between the barrel and end cap Λ, etc), out-of-time pileup (additional pp collisions occur- cryostats (1.0 < jηj < 1.3) are not considered as CR-jet ring in bunch-crossings just before and after the collision of candidates due to noise in the gap scintillator of the TileCal interest), noise, detector inefficiencies, and beam-induced [38]. In addition, the timing of the CR jet is required to be background. Additional SM background processes include between −3 and 15 ns in order to suppress jets arising from the production of top quarks and W þ jets. The SM back- out-of-time pileup and beam-induced backgrounds [6]. The ground MC samples are generated with the configurations timing of a jet is obtained from its constituent calorimeter described in Ref. [27] for W þ jets and Z þ jets production, cells by calculating an average time over cells weighted by and Ref. [28] for tt¯ and single top production. The effect of cell energy squared where the cell time is measured 7 (a) 0.25 (b) 10 ATLAS (c) 0.18 (m ,m ) [GeV] -1 Φ Z 6 s=13 TeV, 36.1 fb ATLAS d ATLAS Simulation 10 0.16 125,10 Data s=13 TeV, 36.1 fb-1 0.2 5 → μ ν 250,50 10 W 0.14 top 500,20 4 10 Z+jets 0.12 0.15 500,200 103 Other MC 0.1 Decays in HCal d 102 0.08 Z 0.1 Number of Jets 10 Fraction of Jets 0.06 W± → e± ν 1 0.04 0.05 W± → μ± ν 3 - + Fraction of 0.02 2 Z → l l 0 1 0 − − 10 2 10 1 1 10 102 Data/MC 0 0123456789 −1 012345 c τ [m] Jet Track Multiplicity log (E /E ) 10 Tile LAr Z cτ m m m FIG.
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