Search for Dark Matter Candidates and Large Extra Dimensions In
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Search for Dark Matter Candidates and Large Extra Dimensions in Events with a Photon and Missing Transverse Momentum in pp Collision Data at √s=7TeV with the ATLAS Detector The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Aad, G. et al. “Search for Dark Matter Candidates and Large Extra Dimensions in Events with a Photon and Missing Transverse Momentum in Pp Collision Data at Sqrt[s]=7TeV with the ATLAS Detector.” Physical Review Letters 110.1 (2013). As Published http://dx.doi.org/10.1103/PhysRevLett.110.011802 Publisher American Physical Society Version Final published version Citable link http://hdl.handle.net/1721.1/77158 Terms of Use Creative Commons Attribution 3.0 Detailed Terms http://creativecommons.org/licenses/by/3.0/ week ending PRL 110, 011802 (2013) PHYSICAL REVIEW LETTERS 4 JANUARY 2013 Search for Dark Matter Candidates and Large Extra Dimensionspffiffiffi in Events with a Photon and Missing Transverse Momentum in pp Collision Data at s ¼ 7 TeV with the ATLAS Detector G. Aad et al.* (ATLAS Collaboration) (Received 20 September 2012; published 3 January 2013) Results of a search for new phenomena inp eventsffiffiffi with an energetic photon and large missing transverse momentum in proton-proton collisions at s ¼ 7 TeV are reported. Data collected by the ATLAS experiment at the LHC corresponding to an integrated luminosity of 4:6fbÀ1 are used. Good agreement is observed between the data and the standard model predictions. The results are translated into exclusion limits on models with large extra spatial dimensions and on pair production of weakly interacting dark matter candidates. DOI: 10.1103/PhysRevLett.110.011802 PACS numbers: 13.85.Rm, 13.85.Qk, 14.70.Kv, 14.80.Rt Events with an energetic photon and large missing mo- the presence of an energetic photon or jet from initial-state mentum in the final state constitute a clean and distinctive radiation. The interaction of WIMPs with standard signature in searches for new physics at colliders. In par- model (SM) particles is assumed to be driven by a mediator ticular, monophoton, and monojet final states have been with mass at the TeV scale and described using a studied [1–8] in the context of searches for supersymmetry nonrenormalizable effective theory [12] with several and large extra spatial dimensions (LED), aiming to pro- operators. The vertex coupling is suppressedpffiffiffiffiffiffiffiffiffiffi by an vide a solution to the mass hierarchy problem, and the effective cutoff mass scale Mà M= g1g2, where M search for weakly interacting massive particles (WIMPs) denotes the mass of the mediator and g1 and g2 are as candidates for dark matter (DM). the couplings of the mediator to the WIMP and SM The Arkani-Hamed, Dimopoulos, and Dvali (ADD) particles. model for LED [9] explains the large difference between This Letter reports results of the search for new phe- 2 pffiffiffi the electroweak unification scale Oð10 Þ GeV and the nomena in the monophoton final state, based on s ¼ 19 Planck scale MPl Oð10 Þ GeV by postulating the pres- 7 TeV proton-proton collision data corresponding to an ence of n extra spatial dimensions of size R, and defining a integrated luminosity of 4:6fbÀ1 collected with the fundamental Planck scale in 4 þ n dimensions, MD, given ATLAS detector at the LHC during 2011. The ATLAS 2 2þn n by MPl MD R . The extra spatial dimensions are com- detector is described in detail elsewhere [13]. The data pactified, resulting in a Kaluza-Klein tower of massive are collected using a three-level trigger system that selects graviton modes. At hadron colliders, these graviton modes events with missing transverse momentum greater than may escape detection and can be produced in association 70 GeV. In the analysis, events are required to have a miss 150 GeV with an energetic photon or a jet, leading to a monophoton reconstructed primary vertex and ET > , where miss or monojet signature. ET is computed as the magnitude of the vector sum of the The presence of a nonbaryonic DM component in the transverse momentum of all noise-suppressed calorimeter Universe is inferred from the observation of its gravita- topological clusters with jj < 4:9 [14,15]. A photon is tional interactions [10], although its nature is otherwise also required with transverse momentum pT > 150 GeV unknown. A WIMP with mass m in the range between and jj < 2:37, excluding the calorimeter barrel or end- 1 GeV and a few TeV is a plausible candidate for DM. It cap transition regions 1:37 < jj < 1:52 [13]. With these could be detected via its scattering with heavy nuclei [11], criteria, the trigger selection is more than 98% efficient, as the detection of cosmic rays (energetic photons, electrons, determined using events selected with a muon trigger. The positrons, protons, antiprotons, or neutrinos) from cluster energies are corrected for the different response of annihilation in astrophysical sources [10], or via the calorimeters to hadronic jets, leptons, electrons or pair production at colliders where the WIMPs do not photons, as well as dead material and out-of-cluster energy interact with the detector and the event is identified by losses. The photon candidate must pass tight identification criteria [16] and is required to be isolated: the energy not *Full author list given at the end of the article. associatedpffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi with the photon cluster in a cone of radius ÁR ¼ ðÁÞ2 þðÁÞ2 ¼ 0:4 around the candidate is Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distri- required to be less than 5 GeV. Jets are defined using the bution of this work must maintain attribution to the author(s) and anti-kt jet algorithm [17] with the distance parameter set to the published article’s title, journal citation, and DOI. R ¼ 0:4. The measured jet pT is corrected for detector 0031-9007=13=110(1)=011802(18) 011802-1 Ó 2013 CERN, for the ATLAS Collaboration week ending PRL 110, 011802 (2013) PHYSICAL REVIEW LETTERS 4 JANUARY 2013 effects and for contributions from multiple proton-proton Simulated events corresponding to the þ process interactions per beam bunch crossing (pileup) [18]. with a minimum photon pT of 80 GeV are generated using Events with more than one jet with pT > 30 GeV and LO matrix elements from MADGRAPH [33] interfaced to j j 4 5 < : are rejected. Events with one jet are retained to PYTHIA 6.426 using CTEQ6L1 PDFs. Values for m increase the signal acceptance and reduce systematic between 1 GeV and 1.3 TeV are considered. In this analy- uncertainties related to the modeling of initial-state radia- sis, WIMPs are assumed to be Dirac fermions and the miss tion. The reconstructed photon, ET vector, and jets (if vertex operator is taken to have the structure of a scalar, found) are required to be well separated in the transverse vector, axial-vector or tensor, corresponding, respectively, Á ð missÞ 0 4 Á ð Þ 0 4 plane with ; ET > : , R ; jet > : , and to the operators D1, D5, D8, and D9 in Refs. [12,34]. Á ð missÞ 0 4 jet;ET > : . Additional quality criteria [19] are These operators correspond to spin-independent (D1 and applied to ensure that jets and photons are not produced D5) and spin-dependent (D8 and D9) interactions. The by noisy calorimeter cells, and to avoid problematic detec- MC samples are passed through a full simulation [35]of tor regions. Events with identified electrons or muons are the ATLAS detector and trigger system, based on GEANT4 vetoed to reject mainly W=Z þ jets and W=Z þ back- [36]. The simulated events are reconstructed and analyzed ground processes with charged leptons in the final state. as the data. Electron (muon) candidates are required to have pT > The normalization of the MC predictions for the domi- 20 GeV and jj < 2:47 (pT > 10 GeV and jj < 2:4), nant W=Z þ background processes are set using scale and to pass the medium (combined) criteria [20]. The final factors determined in a data control sample, resulting in a data sample contains 116 events, where 88 and 28 events significant reduction of the background uncertainties. A þ þ miss have zero and one jet, respectively. ET control sample with an identified muon is The SM background to the monophoton signal is domi- defined by inverting the muon veto in the nominal event nated by the irreducible Zð! Þþ process, and selection criteria discussed above. According to the simu- receives contributions from W=Z þ events with uniden- lation, the sample contains a 71% (19%) contribution from tified electrons, muons or hadronic decays, and W=Z þ W þ (Z þ ) processes. This control sample is used to jets events with an electron or jet misreconstructed as a normalize separately the W þ and Z þ MC predic- photon. In addition, the monophoton sample receives small tions determined by ALPGEN and SHERPA, respectively. In contributions from top-quark, , diboson (WW, ZZ, each case, the scale factor is defined as the ratio of the data WZ), þ jets, and multijet processes. to the given MC prediction, after the contributions from the Background samples of simulated W=Z þ events are rest of the background processes are subtracted. The scale generated using ALPGEN 2.13 [21], interfaced to HERWIG factors, extracted simultaneously to take into account cor- 6.510 [22] with JIMMY 4.31 [23], and SHERPA 1.2.3 [24], relations, are kðW þ Þ¼1:0 Æ 0:2 and kðZ þ Þ¼ using CTEQ6L1 [25] parton distribution functions (PDFs) 1:1 Æ 0:2, where statistical and systematic uncertainties and requiring a minimum photon pT of 40 GeV.