Search for Dark Matter Produced in Association with a Higgs Boson Decaying to bb̄ Using 36 fb − 1 of pp Collisions at ffiffi s p = 13 TeV with the ATLAS Detector

Several extensions of the standard model predict associated production of dark-matter particles with a Higgs boson. Such processes are searched for in final states with missing transverse momentum and a Higgs boson decaying to a bb̄ pair with the ATLAS detector using 36.1 fb−1 of pp collisions at a center-ofmass energy of 13 TeVat the LHC. The observed data are in agreement with the standard model predictions and limits are placed on the associated production of dark-matter particles and a Higgs boson.

The signal is characterized by high E miss T , no isolated leptons, and an invariant mass of the h candidate m h compatible with the observed Higgs boson mass of 125 GeV [73]. In the signal region (SR) described below, the dominant backgrounds from Z(νν) + jets, W + jets, and tt production contribute respectively 30-60%, 10-25%, and 15-50% of the total background, depending on E miss T and the b-tag multiplicity. The models for V + jets and tt are constrained using two control regions (CR): the single-muon control region (1µ-CR) is designed to constrain the tt and W + jets backgrounds, while the two-lepton control region (2 -CR) constrains the Z +jets background contribution.
The SR requires E miss T > 150 GeV, and no isolated e or µ. The multijet background contributes due to mismeasured jet momenta. To suppress it, additional selections are required: min ∆φ E miss T , p j T > π/9 for the three highest-p T (leading) small-R jets, ∆φ E miss T , p miss,trk T < π/2, and p miss,trk T > 30 GeV for events with fewer than two central b-tagged small-R jets. The requirements using p miss,trk T also reduce non-collision backgrounds.
In the "resolved" regime, defined by E miss T < 500 GeV, the h candidate is reconstructed from two leading b-tagged central small-R jets, or, if only one b-tag is present in the event, from the b-tagged central small-R jet and the leading non-b-tagged central small-R jet. At least one of the jets comprising the h candidate must satisfy p T > 45 GeV. A separation in ∆φ between the h candidate and E miss T of more than 2π/3 is required following the back-to-back configuration of the Higgs boson recoiling against DM. To improve the trigger efficiency modeling, events are retained only if the scalar sum H T of the p T of the two (three) leading jets fulfills H T,2 j > 120 GeV (H T,3 j > 150 GeV) if two (more than two) central jets are present. Further optimization of the event selection described below provides an additional background reduction of up to 60% relative to Ref. [31], for a small signal loss. Events with a hadronic τ-lepton candidate, identified either by an algorithm based on a boosted decision tree [74] or as small-R jets containing one to four tracks within the jet core and ∆φ E miss T , p j T < π/8, are rejected to reduce the tt background, which can enter the SR if at least one top quark decays as t → Wb → τνb. This background is further reduced by removing events with more than two b-tagged central jets, which typically happens for tt events with t → Wb → csb decays. Since most of the hadronic activity in a signal event is expected from the h → bb decay, the scalar sum of the p T of the two jets forming the h candidate and, if present, the highest-p T additional jet must be larger than 0.63 × H T,all jets . Finally, ∆R p j 1 h , p j 2 h < 1.8 is required for the two jets forming the h candidate.
In the "merged" regime, defined by E miss T > 500 GeV, the leading large-R jet represents the h candidate. Further selection optimization reduces backgrounds, primarily tt production, by up to 30% relative to Ref.
[31], for a small signal loss: events containing τ-lepton candidates with ∆R p τ , p J > 1.0 are vetoed; no b-tagged central small-R jets with ∆R p j,b-tag , p J > 1.0 are allowed in the event; and the scalar sum of p T of the small-R jets with ∆R p j , p J > 1.0 is required to be smaller than 0.57 times that sum added to p J T . The resolution in m h is improved using muons associated with small-R jets in the resolved regime or with track-jets matched to large-R jets in the merged regime [68, 75].
The event selection in the 1µ-CR is identical to the SR, except that exactly one isolated µ candidate with p µ T > 27 GeV is required, and that p µ T is added to E miss T to mimic the behavior of events contaminating the SR when the charged lepton is not detected.
Events in the 2 -CR are collected using a single-e or single-µ trigger, and selected by requiring one pair of isolated e or µ, one of which must have p T > 27 GeV. Events with a Z boson candidate are retained, Table 1: Dominant sources of uncertainty for three representative Z -2HDM scenarios after the fit to data: (a) with (m Z , m A ) = (0.6 TeV, 0.3 TeV), (b) with (m Z , m A ) = (1.4 TeV, 0.6 TeV), and (c) with (m Z , m A ) = (2.6 TeV, 0.3 TeV). The effect is expressed as the fractional uncertainty on the signal yield. Total is the quadrature sum of statistical and total systematic uncertainties. The impact of the luminosity uncertainty, which does not affect backgrounds with free normalizations, varies due to the changing background composition with increasing E miss T .
Source of uncert. identified as having 83 GeV < m ee < 99 GeV or 71 GeV < m µµ < 106 GeV with an opposite-charge requirement in the µµ case. In addition, a measure of the E miss T significance given by the ratio of the E miss T to the square root of the scalar sum of p T of all leptons and small-R jets in the event must be less than 3.5 GeV 1/2 . This requirement separates Z( )+jets processes from tt production, as E miss T originates from finite detector resolution for the former and mainly from neutrinos for the latter. To mimic Z → νν decays in the SR, the E miss T is set to the p T of the dilepton system, which is then ignored in the subsequent analysis. All other selection requirements are identical between the 2 -CR and the SR.
Subdominant backgrounds, including diboson, Vh, single top quark, and multijet production, contribute less than 10% of the total background in the SR. Multijet production is negligible for E miss T > 350 GeV. Its m h distribution is determined from data in a dedicated multijet-enriched sideband, defined by inverting the min ∆φ E miss T , p j T requirement. Dominant sources of experimental systematic uncertainty arise from the number of background MC events, the calibration of the b-tagging efficiency and integrated luminosity, as well as the scale and resolution of the energy and the mass of jets. Uncertainties associated with the τ vetoes are found to be negligible. Dominant sources of theoretical systematic uncertainty originate from the modeling of the signal and background processes such as tt, V +jets, Vh, diboson, and multijet production. The few relevant changes in the estimation of systematic uncertainties relative to Ref.
[31] encompass: the improved calibrations of the b-tagging efficiency using tt events [68, 70] as well as of the jet energy and mass scales using various in situ methods [69, 76]; the reduced uncertainty from the new jet-mass observable [68,69]; and the uncertainty of 3.4% on the integrated luminosity of data collected in 2016. Table 1 quantifies dominant sources of uncertainty after the fit to data assuming three representative Z -2HDM-scenarios. This search is statistically limited for E miss T 300 GeV.
A fit to the m h observable based on a binned likelihood approach [77, 78] is used to search for a signal. Systematic uncertainties are included in the likelihood function as nuisance parameters with Gaussian or , which are each split into two subregions with one and two b-tags. In the 1µ-CR, the electric charge of the µ is used to separate tt from V + jets since the former provides an equal number of µ + and µ − , while a prevalence of µ + is expected from the latter process due to PDFs [79]. Only the total event yield is considered in the 2 -CR due to limited data statistics. The normalizations of tt, W+HF, and Z+HF processes are free parameters in the fit, where HF represents jets containing b-or c-quarks. In the SR, the contribution from Z +jets is increased by about 50% by the fit relative to theory predictions, staying within uncertainties, while tt is reduced by up to 30% at high E miss T . The normalizations of other backgrounds modeled using MC simulations are constrained to theory predictions within uncertainties, as detailed in Ref.  bin at detector level, after all SR selections except the requirements on b-tag multiplicity and m h range as used in the fit. The A × ε term quantifies the probability for an event to be reconstructed in the same E miss T bin as generated and to pass all σ vis,h(bb)+DM selections, where A represents the kinematic acceptance and ε accounts for the experimental efficiency. The results are shown in Table 2. To minimize the dependence on the E miss T distribution of a potential h + DM signal, the standard fit approach is modified to analyze one E miss T range at a time in the SR. The Z -2HDM model is used to evaluate the dependence of the σ vis,h(bb)+DM limits and of A × ε on the event kinematics within a given E miss T bin. A range of (m Z , m A ) parameters that yield a sizable contribution of 10% × σ h+DM × B(h → bb) in a given E miss T bin is considered. Corresponding variations of 25% (70%) in the expected limits and of 50% (25%) in A × ε are found in the resolved (merged) regime. Table 2 quotes the least stringent limit and the lowest A × ε value in a given E miss T bin after rounding. The limits are valid for p T,h 1.5 TeV.
In summary, a search for DM produced in association with a Higgs boson in final states with E miss T and a bb pair from the h → bb decay was conducted using 36.1 fb −1 of pp collisions at √ s = 13 TeV recorded by the ATLAS detector at the LHC. The results are in agreement with SM predictions, and a substantial region of the parameter space of a representative Z -2HDM model is excluded, significantly improving upon previous results. Stringent limits are also placed on the production cross-section of non-SM events with large E miss T and a Higgs boson without extra model assumptions.
We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently.