CMS-EXO-16-049

CMS-EXO-16-049 ; CERN-EP-2018-183
Search for dark matter particles produced in association with a top quark pair at $\sqrt{s} = $ 13 TeV
CMS Collaboration
18 July 2018
Phys. Rev. Lett. 122 (2019) 011803
Abstract: A search is performed for dark matter particles produced in association with a top quark pair in proton-proton collisions at $\sqrt{s} = $ 13 TeV. The data correspond to an integrated luminosity of 35.9 fb$^{-1}$ recorded by the CMS detector at the LHC. No significant excess over the standard model expectation is observed. The results are interpreted using simplified models of dark matter production via spin-0 mediators that couple to dark matter particles and to standard model quarks, providing constraints on the coupling strength between the mediator and the quarks. These are the most stringent collider limits to date for scalar mediators, and the most stringent for pseudoscalar mediators at low masses.
Links: e-print arXiv:1807.06522 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Selected $ {{p_{\mathrm {T}}} ^\text {miss}} $ distributions in SRs: 2RTT SR for the all-hadronic (upper left), the $\ell $+jets (upper right), and the different-flavor, $ {{m_{\mathrm {T2}}} ^{\ell \ell}} > $ 110 GeV SR in the dileptonic channel (lower). The solid red line shows the expectation for a signal with $m_ {\mathrm {a}} = $ 100 GeV and $m_{\chi} = $ 1 GeV. The last bin contains the overflow events. The lower panel shows the ratio of the observed to the fitted distribution (points), and the ratio of the background expectation before the fit to the fitted distribution (dashed magenta line). The vertical bars indicate the statistical uncertainty on the data. The horizontal bars on the rightmost plot indicate the bin width. The uncertainty bands in both panels include the statistical and systematic uncertainties on the total background.
png pdf	Figure 1-a: Selected $ {{p_{\mathrm {T}}} ^\text {miss}} $ distributions in SRs: 2RTT SR for the all-hadronic. The solid red line shows the expectation for a signal with $m_ {\mathrm {a}} = $ 100 GeV and $m_{\chi} = $ 1 GeV. The last bin contains the overflow events. The lower panel shows the ratio of the observed to the fitted distribution (points), and the ratio of the background expectation before the fit to the fitted distribution (dashed magenta line). The vertical bars indicate the statistical uncertainty on the data. The horizontal bars on the rightmost plot indicate the bin width. The uncertainty bands in both panels include the statistical and systematic uncertainties on the total background.
png pdf	Figure 1-b: Selected $ {{p_{\mathrm {T}}} ^\text {miss}} $ distributions in SRs: 2RTT SR for the $\ell $+jets. The solid red line shows the expectation for a signal with $m_ {\mathrm {a}} = $ 100 GeV and $m_{\chi} = $ 1 GeV. The last bin contains the overflow events. The lower panel shows the ratio of the observed to the fitted distribution (points), and the ratio of the background expectation before the fit to the fitted distribution (dashed magenta line). The vertical bars indicate the statistical uncertainty on the data. The horizontal bars on the rightmost plot indicate the bin width. The uncertainty bands in both panels include the statistical and systematic uncertainties on the total background.
png pdf	Figure 1-c: Selected $ {{p_{\mathrm {T}}} ^\text {miss}} $ distributions in SRs: 2RTT SR for the different-flavor, $ {{m_{\mathrm {T2}}} ^{\ell \ell}} > $ 110 GeV SR in the dileptonic channel. The solid red line shows the expectation for a signal with $m_ {\mathrm {a}} = $ 100 GeV and $m_{\chi} = $ 1 GeV. The last bin contains the overflow events. The lower panel shows the ratio of the observed to the fitted distribution (points), and the ratio of the background expectation before the fit to the fitted distribution (dashed magenta line). The vertical bars indicate the statistical uncertainty on the data. The horizontal bars on the rightmost plot indicate the bin width. The uncertainty bands in both panels include the statistical and systematic uncertainties on the total background.
png pdf	Figure 2: The exclusion limits at 95% CL on the signal strength $\mu =\sigma /\sigma _{\text {th}}$ computed as a function of the mediator and dark matter mass, assuming a scalar (left) and pseudoscalar (right) mediator. The mediator couplings are assumed to be $ {{\mathrm {g}} _{{\mathrm {q}}}} = {\mathrm {g}} _{\chi}=$ 1. The dashed magenta lines represent the 68% probability interval around the expected limit. The observed limit contour is almost coincident with the boundary of the 68% probability interval.
png pdf	Figure 2-a: The exclusion limits at 95% CL on the signal strength $\mu =\sigma /\sigma _{\text {th}}$ computed as a function of the mediator and dark matter mass, assuming a scalar mediator. The mediator couplings are assumed to be $ {{\mathrm {g}} _{{\mathrm {q}}}} = {\mathrm {g}} _{\chi}=$ 1. The dashed magenta lines represent the 68% probability interval around the expected limit. The observed limit contour is almost coincident with the boundary of the 68% probability interval.
png pdf	Figure 2-b: The exclusion limits at 95% CL on the signal strength $\mu =\sigma /\sigma _{\text {th}}$ computed as a function of the mediator and dark matter mass, assuming a pseudoscalar mediator. The mediator couplings are assumed to be $ {{\mathrm {g}} _{{\mathrm {q}}}} = {\mathrm {g}} _{\chi}=$ 1. The dashed magenta lines represent the 68% probability interval around the expected limit. The observed limit contour is almost coincident with the boundary of the 68% probability interval.
png pdf	Figure 3: The 95% observed and median expected CL upper limits on the coupling strength of the mediator to the standard model quarks under the assumption that $ {\mathrm {g}} _{\chi}= $ 1. A dark matter particle with a mass of 1 GeV is assumed. The green and yellow bands indicate respectively the 68% and 95% probability intervals around the expected limit. The interpretations for a scalar (left) and a pseudoscalar (right) mediator are shown.
png pdf	Figure 3-a: The 95% observed and median expected CL upper limits on the coupling strength of the mediator to the standard model quarks under the assumption that $ {\mathrm {g}} _{\chi}= $ 1. A dark matter particle with a mass of 1 GeV is assumed. The green and yellow bands indicate respectively the 68% and 95% probability intervals around the expected limit. The interpretations for a scalar mediator are shown.
png pdf	Figure 3-b: The 95% observed and median expected CL upper limits on the coupling strength of the mediator to the standard model quarks under the assumption that $ {\mathrm {g}} _{\chi}= $ 1. A dark matter particle with a mass of 1 GeV is assumed. The green and yellow bands indicate respectively the 68% and 95% probability intervals around the expected limit. The interpretations for a pseudoscalar mediator are shown.

Summary

In summary, a comprehensive search for dark matter particles produced in association with a top quark pair yields no significant excess over the predicted background. The results presented in this Letter provide 30--60% better cross section limits compared to earlier searches targeting the same signature [57,59]. The analysis offers stronger constraints than direct and indirect experiments for dark matter masses of O(10 GeV) and below. Over much of the parameter space, the $\mathrm{ t \bar{t} } + \chi \overline{\chi}$ signature has better sensitivity for spin-0 mediators than dark matter production in association with a jet [14] -- previously considered to be the most sensitive signature. For the pseudoscalar model, the $\mathrm{ t \bar{t} } + \chi \overline{\chi}$ signature provides the most stringent cross section constraints for mediator masses of around 200 GeV and below. The observed (expected) limits exclude a pseudoscalar mediator with mass below 220 (320) GeV under the $\mathrm{g}_{\mathrm{q}}=\mathrm{g}_{\chi}=$ 1 benchmark scenario. The $\mathrm{ t \bar{t} } + \chi \overline{\chi}$ signature provides the best sensitivity for the scalar mediator model and is currently the only collider signature that is sufficiently sensitive to exclude regions of parameter space with these values of the couplings. The observed exclusion of a mediator with mass below 160 GeV (240 GeV expected) provides the most stringent constraint to date on this model.

References
1	G. Bertone, D. Hooper, and J. Silk	Particle dark matter: evidence, candidates and constraints	PR 405 (2005) 279	hep-ph/0404175
2	J. L. Feng	Dark matter candidates from particle physics and methods of detection	Ann. Rev. Astron. Astrophys. 48 (2010) 495	1003.0904
3	T. A. Porter, R. P. Johnson, and P. W. Graham	Dark matter searches with astroparticle data	Ann. Rev. Astron. Astrophys. 49 (2011) 155	1104.2836
4	G. Bertone et al.	Identifying WIMP dark matter from particle and astroparticle data	JCAP 03 (2018) 026	1712.04793
5	U. Haisch, F. Kahlhoefer, and J. Unwin	The impact of heavy-quark loops on LHC dark matter searches	JHEP 07 (2013) 125	1208.4605
6	T. Lin, E. W. Kolb, and L.-T. Wang	Probing dark matter couplings to top and bottom quarks at the LHC	PRD 88 (2013) 063510	1303.6638
7	M. R. Buckley, D. Feld, and D. Gon\ifmmode \mbox\cc\else \cc\fialves	Scalar simplified models for dark matter	PRD 91 (2015) 015017	1410.6497
8	U. Haisch and E. Re	Simplified dark matter top-quark interactions at the LHC	JHEP 06 (2015) 078	1503.00691
9	C. Arina et al.	A comprehensive approach to dark matter studies: exploration of simplified top-philic models	JHEP 11 (2016) 111	1605.09242
10	ATLAS Collaboration	Search for new phenomena in final states with an energetic jet and large missing transverse momentum in pp collisions at $ \sqrt{s}= $ 8 TeV with the ATLAS detector	EPJC 75 (2015) 299	1502.01518
11	CMS Collaboration	Search for dark matter in proton-proton collisions at 8 TeV with missing transverse momentum and vector boson tagged jets	JHEP 12 (2016) 083	CMS-EXO-12-055 1607.05764
12	ATLAS Collaboration	Search for new phenomena in final states with an energetic jet and large missing transverse momentum in pp collisions at $ \sqrt{s}= $ 13 TeV using the ATLAS detector	PRD 94 (2016) 032005	1604.07773
13	CMS Collaboration	Search for dark matter produced with an energetic jet or a hadronically decaying W or Z boson at $ \sqrt{s} = $ 13 TeV	JHEP 07 (2017) 14	CMS-EXO-16-037 1703.01651
14	CMS Collaboration	Search for new physics in final states with an energetic jet or a hadronically decaying W or Z boson and transverse momentum imbalance at $ \sqrt{s}= $ 13 TeV	PRD 97 (2018) 092005	CMS-EXO-16-048 1712.02345
15	CMS Collaboration	Search for the production of dark matter in association with top-quark pairs in the single-lepton final state in proton-proton collisions at $ \sqrt{s} = $ 8 TeV	JHEP 06 (2015) 121	CMS-B2G-14-004 1504.03198
16	ATLAS Collaboration	Search for dark matter in events with heavy quarks and missing transverse momentum in pp collisions with the ATLAS detector	EPJC 75 (2015) 92	1410.4031
17	CMS Collaboration	Search for dark matter produced in association with heavy-flavor quark pairs in proton-proton collisions at $ \sqrt{s}= $ 13 TeV	EPJC 77 (2017) 845	CMS-EXO-16-005 1706.02581
18	CMS Collaboration	The CMS trigger system	JINST 12 (2017) P01020	CMS-TRG-12-001 1609.02366
19	CMS Collaboration	The CMS experiment at the CERN LHC	JINST 3 (2008) S08004	CMS-00-001
20	CMS Collaboration	Particle-flow reconstruction and global event description with the CMS detector	JINST 12 (2017) P10003	CMS-PRF-14-001 1706.04965
21	M. Cacciari, G. P. Salam, and G. Soyez	The anti-$ {k_{\mathrm{T}}} $ jet clustering algorithm	JHEP 04 (2008) 063	0802.1189
22	M. Cacciari, G. P. Salam, and G. Soyez	FastJet user manual	EPJC 72 (2012) 1896	1111.6097
23	CMS Collaboration	Determination of jet energy calibration and transverse momentum resolution in CMS	JINST 6 (2011) 11002	CMS-JME-10-011 1107.4277
24	M. Cacciari, G. P. Salam, and G. Soyez	The catchment area of jets	JHEP 04 (2008) 005	0802.1188
25	CMS Collaboration	Jet algorithms performance in 13 TeV data	CMS-PAS-JME-16-003	CMS-PAS-JME-16-003
26	CMS Collaboration	Identification of heavy-flavour jets with the CMS detector in pp collisions at 13 TeV	JINST 13 (2018) P05011	CMS-BTV-16-002 1712.07158
27	CMS Collaboration	Performance of electron reconstruction and selection with the CMS Detector in proton-proton Collisions at $ \sqrt{s}= $ 8 TeV	JINST 10 (2015) P06005	CMS-EGM-13-001 1502.02701
28	P. Nason	A new method for combining NLO QCD with shower Monte Carlo algorithms	JHEP 11 (2004) 040	hep-ph/0409146
29	S. Frixione, P. Nason, and C. Oleari	Matching NLO QCD computations with parton shower simulations: the POWHEG method	JHEP 11 (2007) 070	0709.2092
30	S. Alioli, P. Nason, C. Oleari, and E. Re	A general framework for implementing NLO calculations in shower Monte Carlo programs: the POWHEG BOX	JHEP 06 (2010) 043	1002.2581
31	C. Oleari	The POWHEG-BOX	NPPS 205-206 (2010) 36	1007.3893
32	J. Alwall et al.	The automated computation of tree-level and next-to-leading order differential cross sections, and their matching to parton shower simulations	JHEP 07 (2014) 079	1405.0301
33	A. Denner, S. Dittmaier, T. Kasprzik, and A. Muck	Electroweak corrections to W+jet hadroproduction including leptonic W-boson decays	JHEP 08 (2009) 075	0906.1656
34	A. Denner, S. Dittmaier, T. Kasprzik, and A. Muck	Electroweak corrections to dilepton+jet production at hadron colliders	JHEP 06 (2011) 069	1103.0914
35	A. Denner, S. Dittmaier, T. Kasprzik, and A. Maeck	Electroweak corrections to monojet production at the LHC	EPJC 73 (2013) 2297	1211.5078
36	J. H. Kuhn, A. Kulesza, S. Pozzorini, and M. Schulze	Electroweak corrections to hadronic photon production at large transverse momenta	JHEP 03 (2006) 059	hep-ph/0508253
37	S. Kallweit et al.	NLO electroweak automation and precise predictions for W+multijet production at the LHC	JHEP 04 (2015) 012	1412.5157
38	S. Kallweit et al.	NLO QCD+EW predictions for V+jets including off-shell vector-boson decays and multijet merging	JHEP 04 (2016) 021	1511.08692
39	NNPDF Collaboration	Parton distributions for the LHC Run II	JHEP 04 (2015) 040	1410.8849
40	T. Sjostrand, S. Mrenna, and P. Z. Skands	A brief introduction to PYTHIA 8.1	CPC 178 (2008) 852	0710.3820
41	CMS Collaboration	Event generator tunes obtained from underlying event and multiparton scattering measurements	EPJC 76 (2016) 155	CMS-GEN-14-001 1512.00815
42	GEANT4 Collaboration	$ GEANT4--a $ simulation toolkit	NIMA 506 (2003) 250
43	D. Abercrombie et al.	Dark matter benchmark models for early LHC Run-2 searches: report of the ATLAS/CMS Dark Matter Forum		1507.00966
44	A. Albert et al.	Towards the next generation of simplified dark matter models	Phys. Dark Univ. 16 (2017) 49	1607.06680
45	P. Harris, V. V. Khoze, M. Spannowsky, and C. Williams	Constraining dark sectors at colliders: Beyond the effective theory approach	PRD 91 (2015) 055009	1411.0535
46	Y. Bai, H.-C. Cheng, J. Gallicchio, and J. Gu	Stop the top background of the stop search	JHEP 07 (2012) 110	1203.4813
47	C. G. Lester and D. J. Summers	Measuring masses of semi-invisibly decaying particles pair produced at hadron colliders	PLB 463 (1999) 99	hep-ph/9906349
48	M. Burns, K. Kong, K. T. Matchev, and M. Park	Using subsystem $ M_{T2} $ for complete mass determinations in decay chains with missing energy at hadron colliders	JHEP 03 (2009) 143	0810.5576
49	H.-C. Cheng and Z. Han	Minimal kinematic constraints and $ m_{T2} $	JHEP 12 (2008) 063	0810.5178
50	Particle Data Group	Review of particle physics	CPC 40 (2016) 100001 and 2017 update
51	L. Moneta et al.	The RooStats Project	in Proceedings, 13th International Workshop on Advanced computing and analysis techniques in physics research (ACAT2010), p. 057 Jaipur, India, February, 2010 [PoS(ACAT2010)057]	1009.1003
52	CMS Collaboration	CMS luminosity measurements for the 2016 data taking period	CMS-PAS-LUM-17-001	CMS-PAS-LUM-17-001
53	CMS Collaboration	Measurement of differential cross sections for top quark pair production using the lepton+jets final state in proton-proton collisions at 13 TeV	PRD 95 (2017) 092001	CMS-TOP-16-008 1610.04191
54	T. Junk	Confidence level computation for combining searches with small statistics	Nucl. Instrum. Methods A 434 (1999) 435	hep-ex/9902006
55	A. L. Read	Presentation of search results: the $ CL_s $ technique	JPG 28 (2002) 2693
56	G. Cowan, K. Cranmer, E. Gross, and O. Vitells	Asymptotic formulae for likelihood-based tests of new physics	EPJC 71 (2011) 1554	1007.1727
57	CMS Collaboration	Search for top squarks and dark matter particles in opposite-charge dilepton final states at $ \sqrt{s}= $ 13 TeV	PRD 97 (2018) 032009	CMS-SUS-17-001 1711.00752
58	ATLAS Collaboration	Search for top-squark pair production in final states with one lepton, jets, and missing transverse momentum using 36.1 fb$ ^{-1} $ of $ \sqrt{s}= $ 13 TeV pp collision data with the ATLAS detector	Submitted to JHEP	1711.11520
59	ATLAS Collaboration	Search for dark matter produced in association with bottom or top quarks in $ \sqrt{s}= $ 13 TeV pp collisions with the ATLAS detector	EPJC 78 (2018) 18	1710.11412