Overview of Dark Matter Searches at the ATLAS Experiment

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1 1th International Symposium on Cosmoloy and Particle Astrophysics (CosPA 015) International Journal of Modern Physics: Conference Series Vol. 43 (016) (8 paes) c The Author(s) DOI:.114/S Overview of Dark Matter Searches at the Experiment Int. J. Mod. Phys. Conf. Ser Downloaded from by on 11/17/17. For personal use only. Bernhard Meirose On behalf of the Collaboration Department of Physics, University of Texas at Dallas, 800 W Campbell Rd, Richardson Dallas, TX 75080, United States of America bernhard.meirose@utdallas.edu Published 7 July 016 After a brief introduction a concise summary of some of experiment s straies to search for dark matter is presented. Emphasis is iven to searches in channels involvin the His boson. Keywords: Dark Matter; His boson; LHC;. PACS numbers: Bn, 1.60.Fr, Ec, d 1. Introduction A non-baryonic dark matter (DM) component in the universe is commonly used to explain a rane of astrophysical measurements (see, for example, Ref. 1 for a review). Since none of the known Standard Model (SM) particles are adequate DM candidates, new particle types have been hypothesized. Weakly interactin massive particles (WIMPs) are one such class of particle candidates that miht be detectable by LHC experiments. They are expected to couple to SM particles throuh a eneric weak interaction, which could be the weak interaction of the SM or a new type of interaction. Such a new particle could account for the correct relic density values for non-relativistic matter in the early universe as measured by the Planck 3 and WMAP 4 satellites if its mass were between a few GeV and a TeV and if it had electroweak-scale interaction cross sections. Other new particle physics models such as Supersymmetry also predict WIMPs. Since the discovery of a His boson with a mass of approximately 15 GeV, 5, 6 the properties of this new particle have been studied extensively. All results obtained This is an Open Access article published by World Scientific Publishin Company. It is distributed under the terms of the Creative Commons Attribution 4.0 (CC-BY) License. Further distribution of this work is permitted, provided the oriinal work is properly cited

2 B. Meirose Int. J. Mod. Phys. Conf. Ser Downloaded from by on 11/17/17. For personal use only. so far are consistent with the expectations of the lon-souht SM His boson. Sizeable deviations from the SM expectation cannot, however, be yet excluded; the total branchin ratio of beyond-the-sm decays of the His boson is only weakly constrained, and its value could be as hih as 40%. 7 In the so-called Hisportal models 8 the His boson can decay to a pair of WIMP dark-matter particles if the decay is kinematically allowed. Decays to WIMPS are enerally invisible to detectors, because WIMPs interact so weakly. They do not deposit enery in the calorimeter, hence their production leads to sinatures with missin transverse momentum ( p T miss ), the manitude of which is called ET miss. Collider data can be used to directly constrain the branchin ratio of the His boson to invisible particles. Similarly, limits can be placed on the cross section times branchin ratio of any additional His bosons decayin predominantly to invisible particles.. Invisible decays of a His boson usin vector-boson fusion The Collaboration searches for invisible decays of a His boson produced via the vector-boson fusion (VBF) (see Ref. 9 and references therein) process as seen in Fi. 1. In the SM, the process H ZZ 4ν is an invisible decay of the His boson, but the branchin fraction (BF) is 0.1%, which is below the sensitivity of the search performed by. The sinature of the VBF process is two jets with a lare separation in pseudorapidity and lare ET miss.theobserved data rate a is consistent with backround estimates. Assumin the SM production cross section, acceptance and efficiency for invisible decays of a His boson with a mass of 15 GeV, an upper bound is set on the BF(H invisible) < 0.8 at the 95% confidence level (CL). The results are interpreted in the His-portal darkmatter model where the 90% CL limit on the BF(H invisible) leads to an upper bound on the DM nucleon scatterin cross section as a function of the dark-matter particle mass, which is presented Fi.. These limits are complementary to the results from direct dark-matter detection experiments. q q q W ± /Z H W ± /Z q χ 0 χ 0 Fi. 1. Feynman diaram for the VBF H( invisible) process from Ref. 9. a Unless otherwise specified, all searches summarized in this paper were performed with a dataset correspondin to an interated luminosity of 0.3 fb 1 of proton proton collisions at s =8TeV, which were recorded by the detector at the LHC

3 Overview of Dark Matter Searches at the Experiment WIMP-nucleon cross section [cm DAMA/LIBRA, 99.7%CL 90% CL exclusion: CoGeNT, 99%CL CRESST-II 36 CRESST-II, 95%CL CDMSlite CDMS-Si, 68%CL SuperCDMS 37 CDMS-Si, 95%CL XENON0 LUX 38 VBF H(inv.) 39 His portal model scalar 40 fermion vector s = 8 TeV, 0.3 fb 49 BF (H invisible) < 0.3 at 90% CL 50 1 WIMP mass [GeV Int. J. Mod. Phys. Conf. Ser Downloaded from by on 11/17/17. For personal use only. Fi.. The WIMP nucleon cross section as a function of the WIMP mass. The exclusion limits and sinals observed by the direct detection experiments are compared to the results from the BF(H invisible) limit in the His-portal scenario, translated into the WIMP nucleon cross section. The exclusion limits are shown at 90% CL. Fiure from Ref Invisible decays of the His in association with a hadronically decayin vector boson searches for His boson decays to particles that are invisible to the detector in the process where a His boson is produced in association with a vector boson (V = W or Z) that decays hadronically. This results in events with two or more jets and lare missin transverse momentum (see Ref. 1 and references therein). No excess of candidates is observed over the expected backround. The null results are used to set 95% CL upper limits on the product of the VH cross sections and the V jj and H inv. decay branchin ratio, σ VH BR(H inv.), as a function of the His boson mass in the rane 115 <m H < 300 GeV as shown in Fi. 3. The observed upper limit on σ VH BR(H inv.) varies from 1.6 pb at a His mass of 115 GeV to 0.13 pb at 300 GeV. Assumin SM production and includin the H contribution as sinal the results lead to an observed upper limit of 78% at the 95% CL on the branchin ratio of a 15 GeV Hiss boson to invisible particles. 4. Invisible decays of the His in association with a Z Boson The Collaboration searches for evidence of invisible-particle decay modes of a His boson produced in association with a Z boson (see Ref. 13 and references therein). The search uses 4.5 fb 1 of data recorded with the detector in 011 at s = 7 TeV and 0.3 fb 1 of data recorded in 01 at s = 8 TeV. No deviation from the SM expectation is observed. Assumin the SM rate for ZH production, an upper limit of 75% at the 95% CL is set on the branchin ratio to invisible-particle decay modes of the His boson at a mass of 15.5 GeV. The limit on BR(H inv.) for the 15.5 GeV His boson is also interpreted in terms of an upper limit on the DM nucleon scatterin cross section. Fi. 4 shows 90% CL upper limits on the DM nucleon scatterin cross section for three model variants in which

4 B. Meirose BR(H inv.) [pb s = 8 TeV 0.3 fb W/Z( jj) H( inv.) Observed (CLs) Expected (CLs) ± 1σ ± σ 95% CL limit on σ VH 1 Int. J. Mod. Phys. Conf. Ser Downloaded from by on 11/17/17. For personal use only m H [GeV Fi. 3. Upper limits on σ VH BR(H inv.) at 95% CL for a His boson with 115 <m H < 300 GeV. The full and dashed lines show the observed and expected limits, respectively. Fiure from Ref. 1. DM Nucleon cross section [cm His-portal Model 1 DAMA/LIBRA 3σ CDMS 95% CL XENON LUX, vector DM s = 7 TeV, Ldt=4.5 fb s = 8 TeV, Ldt=0.3 fb ZH ll + inv. CRESST σ CoGeNT XENON0, scalar DM, fermion DM 3 DM Mass [GeV Fi. 4. Limits on the DM nucleon scatterin cross section at 90% CL, extracted from the BR(H inv.) limit in a His-portal scenario, compared to results from direct-search experiments. Cross-section limits and favored reions correspond to a 90% CL, unless stated otherwise in the leend. The results from the direct-search experiments do not depend on the assumptions of the His-portal scenario. Fiure from Ref. 13. a sinle DM candidate is either a scalar, a vector or a Majorana fermion. Spinindependent results from direct-search experiments are also shown. Direct search results do not depend on the assumptions of the His-portal scenario. Within the constraints of a of a His-portal scenario however, these results provide the stronest available limits on low-mass DM candidates. There is no sensitivity to these models once the mass of the DM candidate exceeds half of the SM His boson s mass. Limits are set on an additional neutral His boson with a mass between 0 GeV and 400 GeV that is produced in in association with a Z boson and decayin to invisible particles. Fi. 5 shows the 95% CL upper limits on σ ZH BR(H inv.) in the mass rane 1 <m H < 400 GeV for the combined 7 and 8 TeV data

5 Overview of Dark Matter Searches at the Experiment BR(H inv.) [fb s = 7 TeV, L dt = 4.5 fb s = 8 TeV, L dt = 0.3 fb ZH ll +inv. σ ZH,SM Observed 95% CL limit Expected 95% CL limit ±1σ σ ZH 300 ±σ m H [GeV Int. J. Mod. Phys. Conf. Ser Downloaded from by on 11/17/17. For personal use only. Fi. 5. Upper limits on σ ZH BR(H inv.) at 95% CL for a His boson with 1 <m H < 400 GeV, for the combined 7 and 8 TeV data. The full and dashed lines show the observed and expected limits, respectively. Fiure from Ref. 13. The expectation for a His boson with a production cross section equal to that expected for a SM His boson and BR(H inv.) = 1 is also shown. 5. Dark matter in events with a monojet plus missin transverse enery Events with an eneretic jet and lare missin transverse momentum in the final state constitute a clean and distinctive monojet sinature in searches for new physics beyond the SM at colliders. uses monojet-like sinatures in the search for WIMPs (see Ref. 14 and references therein). Here, WIMPs are assumed to be produced in pairs, and events are identified via the presence of an eneretic jet from initial-state radiation (ISR). The interaction of WIMPs with SM particles is described as a contact interaction usin an effective field theory (EFT) approach, mediated by a new heavy particle or particles havin a mass that is too lare to be produced directly at the LHC (see Fi. 6(a)). The validity of the effective field assumption depends on the momentum transfer of the process modeled, which should be below the enery scale of the underlyin interactions. Besides the EFT operators, the pair production of WIMPs is investiated within a so-called simplified model, where a pair of WIMPs couples to a pair of quarks explicitly via a new Z vector boson mediator particle (see Fi. 6(b)). Monojet plus ET miss candidates are selected by requirin at least one jet with transverse momentum p T > 10 GeV and no leptons. Nine sinal reions are considered with increasin missin transverse momentum requirements between ET miss > 150 GeV and ET miss > 700 GeV. Good areement is observed between the number of observed events and SM expectations. The results are presented as exclusion limits on pair production of weakly interactin dark matter candidates. Fiure 7 shows the upper limits for the WIMP nucleon scatterin and WIMP annihilation cross sections. The result depends stronly on the EFT operator and the values for

6 B. Meirose q χ q χ Z q χ q χ Int. J. Mod. Phys. Conf. Ser Downloaded from by on 11/17/17. For personal use only. Fi. 6. Feynman diarams for the production of weakly interactin massive particle pairs χ χ associated with a jet from initial-state radiation of a luon,. (a) A contact interaction described with effective operators. (b) A simplified model with a Z boson. Fiures from Ref. 14. WIMP-nucleon cross section [cm % CL D1: χχqq s=8 TeV, 0.3 fb µ D5: χγ χqγ q µ C1: χ χqq µν µν D11: χχg G C5: χ χg µν G µν truncated, couplin = 1 truncated, max couplin spin-independent 1 DAMA/LIBRA, 3σ CRESST II, σ CoGeNT, 99% CL CDMS, 1σ CDMS, σ CDMS, low mass LUX % CL Xenon0 90% CL CMS 8TeV D5 CMS 8TeV D11 WIMP mass m χ [GeV 3 WIMP-nucleon cross section [cm µ D8: χγ γ 5 χqγ γ 5 q µ µν D9: χσ χqσ µν q truncated, couplin = 1 truncated, max couplin spin-dependent 1 90% CL s=8 TeV, 0.3 fb COUPP 90% CL SIMPLE 90% CL PICASSO 90% CL Super-K 90% CL + - IceCube W W 90% CL CMS 8TeV D8 WIMP mass m χ [GeV Fi. 7. Inferred 90% CL limits on (a) the spin-independent and (b) spin-dependent WIMP nucleon scatterin cross section as a function of DM mass m χ for different operators. Results from direct-detection experiments for the spin-independent and spin-dependent cross section, and the CMS results are shown for comparison. Fiures from Ref. 14. the couplins considered. The limits remain enerally valid for WIMP masses up to O(0) GeV. 6. Dark matter in events with heavy quarks and missin transverse momentum searches for dark matter pair production in association with bottom or top quarks (see Ref. 15 and references therein). Events with lare missin transverse momentum are selected when produced in association with hih-momentum jets, one or more of which are identified as jets containin b-quarks. The dominant Feynman diarams for these processes are shown in Fi. 8. Candidate events with top quarks are selected by requirin a hih jet multiplicity and in some cases a sinle lepton. To search for these processes, selection criteria are defined to reconstruct the various production and decay modes of these heavy-quark final states. The data are consistent with the SM expectations, allow limits to be set on the χ nucleon cross-section. Fi. 9 show the correspondin 90% CL exclusion

7 Overview of Dark Matter Searches at the Experiment b b, t χ χ χ χ b b, t (a) (b) Fi. 8. Dominant Feynman diarams for DM production in conjunction with (a) a sinle b-quark and (b) a heavy quark (bottom or top) pair usin an effective field theory approach. Fiure from Ref. 15. Int. J. Mod. Phys. Conf. Ser Downloaded from by on 11/17/17. For personal use only. [cm χ-n σ SI fb, = 8 TeV s Scalar (D1) SuperCDMS (013) LUX (013) all limits at 90% CL, =4π m χ [GeV [cm χ-n σ SD fb, s = 8 TeV Tensor (D9) COUPP (01) PICASSO (01) all limits at 90% CL, =4π [GeV m χ Fi. 9. Upper limits at 90% CL on the spin-independent χ nucleon cross-section (σ SI )forthe χ-n EFT operator D1 (left) and on the spin-dependent χ nucleon cross-section (σ SD )fortheeft χ-n operator D9 (riht) as a function of m χ. The yellow and reen curves represent the exclusion limits set by direct detection experiments. Fiure from Ref. 15. curves for the spin-independent (left) and spin-dependent (riht) χ nucleon crosssection for EFT operators as a function of m χ. The most strinent limits set by direct detection experiments are also shown. Limits are especially stron in the low-mass reion. 7. Summary, other searches and preliminary Run prospects An overview of the experiment s DM searches is presented. None of the searches find deviations from the SM, and are presented as upper limits. Additional searches not described in this paper (Refs.: 16, 17, 18, 19, 0 and 1) find no deviations from the SM. The anticipated sensitivity to DM channels with the full Run data set can be found in Ref.. Acknowledments This work has been supported by US DOE Grant Number DE-SC00384 and by the oranizers of CosPA

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