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. 2015 Feb 24;75(2):92. doi: 10.1140/epjc/s10052-015-3306-z

Search for dark matter in events with heavy quarks and missing transverse momentum in pp collisions with the ATLAS detector

ATLAS Collaboration180, G Aad 85, B Abbott 113, J Abdallah 152, S Abdel Khalek 117, O Abdinov 11, R Aben 107, B Abi 114, M Abolins 90, O S AbouZeid 159, H Abramowicz 154, H Abreu 153, R Abreu 30, Y Abulaiti 147, B S Acharya 165, L Adamczyk 38, D L Adams 25, J Adelman 108, S Adomeit 100, T Adye 131, T Agatonovic-Jovin 13, J A Aguilar-Saavedra 126, M Agustoni 17, S P Ahlen 22, F Ahmadov 65, G Aielli 134, H Akerstedt 147, T P A Åkesson 81, G Akimoto 156, A V Akimov 96, G L Alberghi 20, J Albert 170, S Albrand 55, M J Alconada Verzini 71, M Aleksa 30, I N Aleksandrov 65, C Alexa 26, G Alexander 154, G Alexandre 49, T Alexopoulos 10, M Alhroob 113, G Alimonti 91, L Alio 85, J Alison 31, B M M Allbrooke 18, L J Allison 72, P P Allport 74, A Aloisio 104, A Alonso 36, F Alonso 71, C Alpigiani 76, A Altheimer 35, B Alvarez Gonzalez 90, M G Alviggi 104, K Amako 66, Y Amaral Coutinho 24, C Amelung 23, D Amidei 89, S P Amor Dos Santos 126, A Amorim 126, S Amoroso 48, N Amram 154, G Amundsen 23, C Anastopoulos 140, L S Ancu 49, N Andari 30, T Andeen 35, C F Anders 58, G Anders 30, K J Anderson 31, A Andreazza 91, V Andrei 58, X S Anduaga 71, S Angelidakis 9, I Angelozzi 107, P Anger 44, A Angerami 35, F Anghinolfi 30, A V Anisenkov 109, N Anjos 12, A Annovi 47, M Antonaki 9, M Antonelli 47, A Antonov 98, J Antos 145, F Anulli 133, M Aoki 66, L Aperio Bella 18, R Apolle 120, G Arabidze 90, I Aracena 144, Y Arai 66, J P Araque 126, A T H Arce 45, F A Arduh 71, J-F Arguin 95, S Argyropoulos 42, M Arik 19, A J Armbruster 30, O Arnaez 30, V Arnal 82, H Arnold 48, M Arratia 28, O Arslan 21, A Artamonov 97, G Artoni 23, S Asai 156, N Asbah 42, A Ashkenazi 154, B Åsman 147, L Asquith 150, K Assamagan 25, R Astalos 145, M Atkinson 166, N B Atlay 142, B Auerbach 6, K Augsten 128, M Aurousseau 146, G Avolio 30, B Axen 15, G Azuelos 95, Y Azuma 156, M A Baak 30, A E Baas 58, C Bacci 135, H Bachacou 137, K Bachas 155, M Backes 30, M Backhaus 30, E Badescu 26, P Bagiacchi 133, P Bagnaia 133, Y Bai 33, T Bain 35, J T Baines 131, O K Baker 177, P Balek 129, F Balli 137, E Banas 39, Sw Banerjee 174, A A E Bannoura 176, H S Bansil 18, L Barak 173, S P Baranov 96, E L Barberio 88, D Barberis 50, M Barbero 85, T Barillari 101, M Barisonzi 176, T Barklow 144, N Barlow 28, S L Barnes 84, B M Barnett 131, R M Barnett 15, Z Barnovska 5, A Baroncelli 135, G Barone 49, A J Barr 120, F Barreiro 82, J Barreiro Guimarães da Costa 57, R Bartoldus 144, A E Barton 72, P Bartos 145, V Bartsch 150, A Bassalat 117, A Basye 166, R L Bates 53, S J Batista 159, J R Batley 28, M Battaglia 138, M Battistin 30, F Bauer 137, H S Bawa 144, J B Beacham 110, M D Beattie 72, T Beau 80, P H Beauchemin 162, R Beccherle 124, P Bechtle 21, H P Beck 17, K Becker 120, S Becker 100, M Beckingham 171, C Becot 117, A J Beddall 19, A Beddall 19, S Bedikian 177, V A Bednyakov 65, C P Bee 149, L J Beemster 107, T A Beermann 176, M Begel 25, K Behr 120, C Belanger-Champagne 87, P J Bell 49, W H Bell 49, G Bella 154, L Bellagamba 20, A Bellerive 29, M Bellomo 86, K Belotskiy 98, O Beltramello 30, O Benary 154, D Benchekroun 136, K Bendtz 147, N Benekos 166, Y Benhammou 154, E Benhar Noccioli 49, J A Benitez Garcia 160, D P Benjamin 45, J R Bensinger 23, S Bentvelsen 107, D Berge 107, E Bergeaas Kuutmann 167, N Berger 5, F Berghaus 170, J Beringer 15, C Bernard 22, P Bernat 78, C Bernius 110, F U Bernlochner 21, T Berry 77, P Berta 129, C Bertella 83, G Bertoli 147, F Bertolucci 124, C Bertsche 113, D Bertsche 113, M I Besana 91, G J Besjes 106, O Bessidskaia Bylund 147, M Bessner 42, N Besson 137, C Betancourt 48, S Bethke 101, W Bhimji 46, R M Bianchi 125, L Bianchini 23, M Bianco 30, O Biebel 100, S P Bieniek 78, K Bierwagen 54, J Biesiada 15, M Biglietti 135, J Bilbao De Mendizabal 49, H Bilokon 47, M Bindi 54, S Binet 117, A Bingul 19, C Bini 133, C W Black 151, J E Black 144, K M Black 22, D Blackburn 139, R E Blair 6, J-B Blanchard 137, T Blazek 145, I Bloch 42, C Blocker 23, W Blum 83, U Blumenschein 54, G J Bobbink 107, V S Bobrovnikov 109, S S Bocchetta 81, A Bocci 45, C Bock 100, C R Boddy 120, M Boehler 48, T T Boek 176, J A Bogaerts 30, A G Bogdanchikov 109, A Bogouch 92, C Bohm 147, V Boisvert 77, T Bold 38, V Boldea 26, A S Boldyrev 99, M Bomben 80, M Bona 76, M Boonekamp 137, A Borisov 130, G Borissov 72, M Borri 84, S Borroni 42, J Bortfeldt 100, V Bortolotto 60, K Bos 107, D Boscherini 20, M Bosman 12, H Boterenbrood 107, J Boudreau 125, J Bouffard 2, E V Bouhova-Thacker 72, D Boumediene 34, C Bourdarios 117, N Bousson 114, S Boutouil 136, A Boveia 31, J Boyd 30, I R Boyko 65, I Bozic 13, J Bracinik 18, A Brandt 8, G Brandt 15, O Brandt 58, U Bratzler 157, B Brau 86, J E Brau 116, H M Braun 176, S F Brazzale 165, B Brelier 159, K Brendlinger 122, A J Brennan 88, R Brenner 167, S Bressler 173, K Bristow 146, T M Bristow 46, D Britton 53, F M Brochu 28, I Brock 21, R Brock 90, J Bronner 101, G Brooijmans 35, T Brooks 77, W K Brooks 32, J Brosamer 15, E Brost 116, J Brown 55, P A Bruckman de Renstrom 39, D Bruncko 145, R Bruneliere 48, S Brunet 61, A Bruni 20, G Bruni 20, M Bruschi 20, L Bryngemark 81, T Buanes 14, Q Buat 143, F Bucci 49, P Buchholz 142, A G Buckley 53, S I Buda 26, I A Budagov 65, F Buehrer 48, L Bugge 119, M K Bugge 119, O Bulekov 98, A C Bundock 74, H Burckhart 30, S Burdin 74, B Burghgrave 108, S Burke 131, I Burmeister 43, E Busato 34, D Büscher 48, V Büscher 83, P Bussey 53, C P Buszello 167, B Butler 57, J M Butler 22, A I Butt 3, C M Buttar 53, J M Butterworth 78, P Butti 107, W Buttinger 28, A Buzatu 53, M Byszewski 10, S Cabrera Urbán 168, D Caforio 20, O Cakir 4, P Calafiura 15, A Calandri 137, G Calderini 80, P Calfayan 100, L P Caloba 24, D Calvet 34, S Calvet 34, R Camacho Toro 49, S Camarda 42, D Cameron 119, L M Caminada 15, R Caminal Armadans 12, S Campana 30, M Campanelli 78, A Campoverde 149, V Canale 104, A Canepa 160, M Cano Bret 76, J Cantero 82, R Cantrill 126, T Cao 40, M D M Capeans Garrido 30, I Caprini 26, M Caprini 26, M Capua 37, R Caputo 83, R Cardarelli 134, T Carli 30, G Carlino 104, L Carminati 91, S Caron 106, E Carquin 32, G D Carrillo-Montoya 146, J R Carter 28, J Carvalho 126, D Casadei 78, M P Casado 12, M Casolino 12, E Castaneda-Miranda 146, A Castelli 107, V Castillo Gimenez 168, N F Castro 126, P Catastini 57, A Catinaccio 30, J R Catmore 119, A Cattai 30, G Cattani 134, J Caudron 83, V Cavaliere 166, D Cavalli 91, M Cavalli-Sforza 12, V Cavasinni 124, F Ceradini 135, B C Cerio 45, K Cerny 129, A S Cerqueira 24, A Cerri 150, L Cerrito 76, F Cerutti 15, M Cerv 30, A Cervelli 17, S A Cetin 19, A Chafaq 136, D Chakraborty 108, I Chalupkova 129, P Chang 166, B Chapleau 87, J D Chapman 28, D Charfeddine 117, D G Charlton 18, C C Chau 159, C A Chavez Barajas 150, S Cheatham 153, A Chegwidden 90, S Chekanov 6, S V Chekulaev 160, G A Chelkov 65, M A Chelstowska 89, C Chen 64, H Chen 25, K Chen 149, L Chen 33, S Chen 33, X Chen 33, Y Chen 67, H C Cheng 89, Y Cheng 31, A Cheplakov 65, E Cheremushkina 130, R Cherkaoui El Moursli 136, V Chernyatin 25, E Cheu 7, L Chevalier 137, V Chiarella 47, G Chiefari 104, J T Childers 6, A Chilingarov 72, G Chiodini 73, A S Chisholm 18, R T Chislett 78, A Chitan 26, M V Chizhov 65, S Chouridou 9, B K B Chow 100, D Chromek-Burckhart 30, M L Chu 152, J Chudoba 127, J J Chwastowski 39, L Chytka 115, G Ciapetti 133, A K Ciftci 4, R Ciftci 4, D Cinca 53, V Cindro 75, A Ciocio 15, Z H Citron 173, M Citterio 91, M Ciubancan 26, A Clark 49, P J Clark 46, R N Clarke 15, W Cleland 125, J C Clemens 85, C Clement 147, Y Coadou 85, M Cobal 165, A Coccaro 139, J Cochran 64, L Coffey 23, J G Cogan 144, B Cole 35, S Cole 108, A P Colijn 107, J Collot 55, T Colombo 58, G Compostella 101, P Conde Muiño 126, E Coniavitis 48, S H Connell 146, I A Connelly 77, S M Consonni 91, V Consorti 48, S Constantinescu 26, C Conta 121, G Conti 57, F Conventi 104, M Cooke 15, B D Cooper 78, A M Cooper-Sarkar 120, N J Cooper-Smith 77, K Copic 15, T Cornelissen 176, M Corradi 20, F Corriveau 87, A Corso-Radu 164, A Cortes-Gonzalez 12, G Cortiana 101, G Costa 91, M J Costa 168, D Costanzo 140, D Côté 8, G Cottin 28, G Cowan 77, B E Cox 84, K Cranmer 110, G Cree 29, S Crépé-Renaudin 55, F Crescioli 80, W A Cribbs 147, M Crispin Ortuzar 120, M Cristinziani 21, V Croft 106, G Crosetti 37, T Cuhadar Donszelmann 140, J Cummings 177, M Curatolo 47, C Cuthbert 151, H Czirr 142, P Czodrowski 3, S D’Auria 53, M D’Onofrio 74, M J Da Cunha Sargedas De Sousa 126, C Da Via 84, W Dabrowski 38, A Dafinca 120, T Dai 89, O Dale 14, F Dallaire 95, C Dallapiccola 86, M Dam 36, A C Daniells 18, M Danninger 169, M Dano Hoffmann 137, V Dao 48, G Darbo 50, S Darmora 8, J Dassoulas 74, A Dattagupta 61, W Davey 21, C David 170, T Davidek 129, E Davies 120, M Davies 154, O Davignon 80, A R Davison 78, P Davison 78, Y Davygora 58, E Dawe 143, I Dawson 140, R K Daya-Ishmukhametova 86, K De 8, R de Asmundis 104, S De Castro 20, S De Cecco 80, N De Groot 106, P de Jong 107, H De la Torre 82, F De Lorenzi 64, L De Nooij 107, D De Pedis 133, A De Salvo 133, U De Sanctis 150, A De Santo 150, J B De Vivie De Regie 117, W J Dearnaley 72, R Debbe 25, C Debenedetti 138, B Dechenaux 55, D V Dedovich 65, I Deigaard 107, J Del Peso 82, T Del Prete 124, F Deliot 137, C M Delitzsch 49, M Deliyergiyev 75, A Dell’Acqua 30, L Dell’Asta 22, M Dell’Orso 124, M Della Pietra 104, D della Volpe 49, M Delmastro 5, P A Delsart 55, C Deluca 107, D A DeMarco 159, S Demers 177, M Demichev 65, A Demilly 80, S P Denisov 130, D Derendarz 39, J E Derkaoui 136, F Derue 80, P Dervan 74, K Desch 21, C Deterre 42, P O Deviveiros 30, A Dewhurst 131, S Dhaliwal 107, A Di Ciaccio 134, L Di Ciaccio 5, A Di Domenico 133, C Di Donato 104, A Di Girolamo 30, B Di Girolamo 30, A Di Mattia 153, B Di Micco 135, R Di Nardo 47, A Di Simone 48, R Di Sipio 20, D Di Valentino 29, F A Dias 46, M A Diaz 32, E B Diehl 89, J Dietrich 16, T A Dietzsch 58, S Diglio 85, A Dimitrievska 13, J Dingfelder 21, P Dita 26, S Dita 26, F Dittus 30, F Djama 85, T Djobava 51, J I Djuvsland 58, M A B do Vale 24, D Dobos 30, C Doglioni 49, T Doherty 53, T Dohmae 156, J Dolejsi 129, Z Dolezal 129, B A Dolgoshein 98, M Donadelli 24, S Donati 124, P Dondero 121, J Donini 34, J Dopke 131, A Doria 104, M T Dova 71, A T Doyle 53, M Dris 10, J Dubbert 89, S Dube 15, E Dubreuil 34, E Duchovni 173, G Duckeck 100, O A Ducu 26, D Duda 176, A Dudarev 30, F Dudziak 64, L Duflot 117, L Duguid 77, M Dührssen 30, M Dunford 58, H Duran Yildiz 4, M Düren 52, A Durglishvili 51, D Duschinger 44, M Dwuznik 38, M Dyndal 38, J Ebke 100, W Edson 2, N C Edwards 46, W Ehrenfeld 21, T Eifert 30, G Eigen 14, K Einsweiler 15, T Ekelof 167, M El Kacimi 136, M Ellert 167, S Elles 5, F Ellinghaus 83, N Ellis 30, J Elmsheuser 100, M Elsing 30, D Emeliyanov 131, Y Enari 156, O C Endner 83, M Endo 118, R Engelmann 149, J Erdmann 177, A Ereditato 17, D Eriksson 147, G Ernis 176, J Ernst 2, M Ernst 25, J Ernwein 137, D Errede 166, S Errede 166, E Ertel 83, M Escalier 117, H Esch 43, C Escobar 125, B Esposito 47, A I Etienvre 137, E Etzion 154, H Evans 61, A Ezhilov 123, L Fabbri 20, G Facini 31, R M Fakhrutdinov 130, S Falciano 133, R J Falla 78, J Faltova 129, Y Fang 33, M Fanti 91, A Farbin 8, A Farilla 135, T Farooque 12, S Farrell 15, S M Farrington 171, P Farthouat 30, F Fassi 136, P Fassnacht 30, D Fassouliotis 9, A Favareto 50, L Fayard 117, P Federic 145, O L Fedin 123, W Fedorko 169, S Feigl 30, L Feligioni 85, C Feng 33, E J Feng 6, H Feng 89, A B Fenyuk 130, S Fernandez Perez 30, S Ferrag 53, J Ferrando 53, A Ferrari 167, P Ferrari 107, R Ferrari 121, D E Ferreira de Lima 53, A Ferrer 168, D Ferrere 49, C Ferretti 89, A Ferretto Parodi 50, M Fiascaris 31, F Fiedler 83, A Filipčič 75, M Filipuzzi 42, F Filthaut 106, M Fincke-Keeler 170, K D Finelli 151, M C N Fiolhais 126, L Fiorini 168, A Firan 40, A Fischer 2, J Fischer 176, W C Fisher 90, E A Fitzgerald 23, M Flechl 48, I Fleck 142, P Fleischmann 89, S Fleischmann 176, G T Fletcher 140, G Fletcher 76, T Flick 176, A Floderus 81, L R Flores Castillo 60, M J Flowerdew 101, A Formica 137, A Forti 84, D Fortin 160, D Fournier 117, H Fox 72, S Fracchia 12, P Francavilla 80, M Franchini 20, S Franchino 30, D Francis 30, L Franconi 119, M Franklin 57, M Fraternali 121, S T French 28, C Friedrich 42, F Friedrich 44, D Froidevaux 30, J A Frost 120, C Fukunaga 157, E Fullana Torregrosa 83, B G Fulsom 144, J Fuster 168, C Gabaldon 55, O Gabizon 176, A Gabrielli 20, A Gabrielli 133, S Gadatsch 107, S Gadomski 49, G Gagliardi 50, P Gagnon 61, C Galea 106, B Galhardo 126, E J Gallas 120, B J Gallop 131, P Gallus 128, G Galster 36, K K Gan 111, J Gao 33, Y S Gao 144, F M Garay Walls 46, F Garberson 177, C García 168, J E García Navarro 168, M Garcia-Sciveres 15, R W Gardner 31, N Garelli 144, V Garonne 30, C Gatti 47, G Gaudio 121, B Gaur 142, L Gauthier 95, P Gauzzi 133, I L Gavrilenko 96, C Gay 169, G Gaycken 21, E N Gazis 10, P Ge 33, Z Gecse 169, C N P Gee 131, D A A Geerts 107, Ch Geich-Gimbel 21, K Gellerstedt 147, C Gemme 50, A Gemmell 53, M H Genest 55, S Gentile 133, M George 54, S George 77, D Gerbaudo 164, A Gershon 154, H Ghazlane 136, N Ghodbane 34, B Giacobbe 20, S Giagu 133, V Giangiobbe 12, P Giannetti 124, F Gianotti 30, B Gibbard 25, S M Gibson 77, M Gilchriese 15, T P S Gillam 28, D Gillberg 30, G Gilles 34, D M Gingrich 3, N Giokaris 9, M P Giordani 165, R Giordano 104, F M Giorgi 20, F M Giorgi 16, P F Giraud 137, D Giugni 91, C Giuliani 48, M Giulini 58, B K Gjelsten 119, S Gkaitatzis 155, I Gkialas 155, E L Gkougkousis 117, L K Gladilin 99, C Glasman 82, J Glatzer 30, P C F Glaysher 46, A Glazov 42, G L Glonti 62, M Goblirsch-Kolb 101, J R Goddard 76, J Godlewski 30, C Goeringer 83, S Goldfarb 89, T Golling 177, D Golubkov 130, A Gomes 126, L S Gomez Fajardo 42, R Gonçalo 126, J Goncalves Pinto Firmino Da Costa 137, L Gonella 21, S González de la Hoz 168, G Gonzalez Parra 12, S Gonzalez-Sevilla 49, L Goossens 30, P A Gorbounov 97, H A Gordon 25, I Gorelov 105, B Gorini 30, E Gorini 73, A Gorišek 75, E Gornicki 39, A T Goshaw 45, C Gössling 43, M I Gostkin 65, M Gouighri 136, D Goujdami 136, M P Goulette 49, A G Goussiou 139, C Goy 5, E Gozani 153, H M X Grabas 138, L Graber 54, I Grabowska-Bold 38, P Grafström 20, K-J Grahn 42, J Gramling 49, E Gramstad 119, S Grancagnolo 16, V Grassi 149, V Gratchev 123, H M Gray 30, E Graziani 135, O G Grebenyuk 123, Z D Greenwood 79, K Gregersen 78, I M Gregor 42, P Grenier 144, J Griffiths 8, A A Grillo 138, K Grimm 72, S Grinstein 12, Ph Gris 34, Y V Grishkevich 99, J-F Grivaz 117, J P Grohs 44, A Grohsjean 42, E Gross 173, J Grosse-Knetter 54, G C Grossi 134, Z J Grout 150, L Guan 33, J Guenther 128, F Guescini 49, D Guest 177, O Gueta 154, C Guicheney 34, E Guido 50, T Guillemin 117, S Guindon 2, U Gul 53, C Gumpert 44, J Guo 35, S Gupta 120, P Gutierrez 113, N G Gutierrez Ortiz 53, C Gutschow 78, N Guttman 154, C Guyot 137, C Gwenlan 120, C B Gwilliam 74, A Haas 110, C Haber 15, H K Hadavand 8, N Haddad 136, P Haefner 21, S Hageböeck 21, Z Hajduk 39, H Hakobyan 178, M Haleem 42, D Hall 120, G Halladjian 90, G D Hallewell 85, K Hamacher 176, P Hamal 115, K Hamano 170, M Hamer 54, A Hamilton 146, S Hamilton 162, G N Hamity 146, P G Hamnett 42, L Han 33, K Hanagaki 118, K Hanawa 156, M Hance 15, P Hanke 58, R Hanna 137, J B Hansen 36, J D Hansen 36, P H Hansen 36, K Hara 161, A S Hard 174, T Harenberg 176, F Hariri 117, S Harkusha 92, D Harper 89, R D Harrington 46, O M Harris 139, P F Harrison 171, F Hartjes 107, M Hasegawa 67, S Hasegawa 103, Y Hasegawa 141, A Hasib 113, S Hassani 137, S Haug 17, M Hauschild 30, R Hauser 90, M Havranek 127, C M Hawkes 18, R J Hawkings 30, A D Hawkins 81, T Hayashi 161, D Hayden 90, C P Hays 120, J M Hays 76, H S Hayward 74, S J Haywood 131, S J Head 18, T Heck 83, V Hedberg 81, L Heelan 8, S Heim 122, T Heim 176, B Heinemann 15, L Heinrich 110, J Hejbal 127, L Helary 22, C Heller 100, M Heller 30, S Hellman 147, D Hellmich 21, C Helsens 30, J Henderson 120, Y Heng 174, R C W Henderson 72, C Hengler 42, A Henrichs 177, A M Henriques Correia 30, S Henrot-Versille 117, G H Herbert 16, Y Hernández Jiménez 168, R Herrberg-Schubert 16, G Herten 48, R Hertenberger 100, L Hervas 30, G G Hesketh 78, N P Hessey 107, R Hickling 76, E Higón-Rodriguez 168, E Hill 170, J C Hill 28, K H Hiller 42, S J Hillier 18, I Hinchliffe 15, E Hines 122, M Hirose 158, D Hirschbuehl 176, J Hobbs 149, N Hod 107, M C Hodgkinson 140, P Hodgson 140, A Hoecker 30, M R Hoeferkamp 105, F Hoenig 100, D Hoffmann 85, M Hohlfeld 83, T R Holmes 15, T M Hong 122, L Hooft van Huysduynen 110, W H Hopkins 116, Y Horii 103, A J Horton 143, J-Y Hostachy 55, S Hou 152, A Hoummada 136, J Howard 120, J Howarth 42, M Hrabovsky 115, I Hristova 16, J Hrivnac 117, T Hryn’ova 5, A Hrynevich 93, C Hsu 146, P J Hsu 152, S-C Hsu 139, D Hu 35, X Hu 89, Y Huang 42, Z Hubacek 30, F Hubaut 85, F Huegging 21, T B Huffman 120, E W Hughes 35, G Hughes 72, M Huhtinen 30, T A Hülsing 83, M Hurwitz 15, N Huseynov 65, J Huston 90, J Huth 57, G Iacobucci 49, G Iakovidis 10, I Ibragimov 142, L Iconomidou-Fayard 117, E Ideal 177, Z Idrissi 136, P Iengo 104, O Igonkina 107, T Iizawa 172, Y Ikegami 66, K Ikematsu 142, M Ikeno 66, Y Ilchenko 31, D Iliadis 155, N Ilic 159, Y Inamaru 67, T Ince 101, P Ioannou 9, M Iodice 135, K Iordanidou 9, V Ippolito 57, A Irles Quiles 168, C Isaksson 167, M Ishino 68, M Ishitsuka 158, R Ishmukhametov 111, C Issever 120, S Istin 19, J M Iturbe Ponce 84, R Iuppa 134, J Ivarsson 81, W Iwanski 39, H Iwasaki 66, J M Izen 41, V Izzo 104, B Jackson 122, M Jackson 74, P Jackson 1, M R Jaekel 30, V Jain 2, K Jakobs 48, S Jakobsen 30, T Jakoubek 127, J Jakubek 128, D O Jamin 152, D K Jana 79, E Jansen 78, H Jansen 30, J Janssen 21, M Janus 171, G Jarlskog 81, N Javadov 65, T Javůrek 48, L Jeanty 15, J Jejelava 51, G-Y Jeng 151, D Jennens 88, P Jenni 48, J Jentzsch 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R Kehoe 40, M Keil 54, J S Keller 42, J J Kempster 77, H Keoshkerian 5, O Kepka 127, B P Kerševan 75, S Kersten 176, K Kessoku 156, J Keung 159, R A Keyes 87, F Khalil-zada 11, H Khandanyan 147, A Khanov 114, A Kharlamov 109, A Khodinov 98, A Khomich 58, T J Khoo 28, G Khoriauli 21, V Khovanskiy 97, E Khramov 65, J Khubua 51, H Y Kim 8, H Kim 147, S H Kim 161, N Kimura 155, O Kind 16, B T King 74, M King 168, R S B King 120, S B King 169, J Kirk 131, A E Kiryunin 101, T Kishimoto 67, D Kisielewska 38, F Kiss 48, K Kiuchi 161, E Kladiva 145, M Klein 74, U Klein 74, K Kleinknecht 83, P Klimek 147, A Klimentov 25, R Klingenberg 43, J A Klinger 84, T Klioutchnikova 30, P F Klok 106, E-E Kluge 58, P Kluit 107, S Kluth 101, E Kneringer 62, E B F G Knoops 85, A Knue 53, D Kobayashi 158, T Kobayashi 156, M Kobel 44, M Kocian 144, P Kodys 129, T Koffas 29, E Koffeman 107, L A Kogan 120, S Kohlmann 176, Z Kohout 128, T Kohriki 66, T Koi 144, H Kolanoski 16, I Koletsou 5, J Koll 90, A A Komar 96, 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Tsirintanis 9, S Tsiskaridze 12, V Tsiskaridze 48, E G Tskhadadze 51, I I Tsukerman 97, V Tsulaia 15, S Tsuno 66, D Tsybychev 149, A Tudorache 26, V Tudorache 26, A N Tuna 122, S A Tupputi 20, S Turchikhin 99, D Turecek 128, I Turk Cakir 4, R Turra 91, A J Turvey 40, P M Tuts 35, A Tykhonov 49, M Tylmad 147, M Tyndel 131, K Uchida 21, I Ueda 156, R Ueno 29, M Ughetto 85, M Ugland 14, M Uhlenbrock 21, F Ukegawa 161, G Unal 30, A Undrus 25, G Unel 164, F C Ungaro 48, Y Unno 66, C Unverdorben 100, J Urban 145, D Urbaniec 35, P Urquijo 88, G Usai 8, A Usanova 62, L Vacavant 85, V Vacek 128, B Vachon 87, N Valencic 107, S Valentinetti 20, A Valero 168, L Valery 34, S Valkar 129, E Valladolid Gallego 168, S Vallecorsa 49, J A Valls Ferrer 168, W Van Den Wollenberg 107, P C Van Der Deijl 107, R van der Geer 107, H van der Graaf 107, R Van Der Leeuw 107, D van der Ster 30, N van Eldik 30, P van Gemmeren 6, J Van Nieuwkoop 143, I van Vulpen 107, M C van Woerden 30, M Vanadia 133, W Vandelli 30, R Vanguri 122, A Vaniachine 6, P Vankov 42, F Vannucci 80, G Vardanyan 178, R Vari 133, E W Varnes 7, T Varol 86, D Varouchas 80, A Vartapetian 8, K E Varvell 151, F Vazeille 34, T Vazquez Schroeder 54, J Veatch 7, F Veloso 125, T Velz 21, S Veneziano 133, A Ventura 73, D Ventura 86, M Venturi 170, N Venturi 159, A Venturini 23, V Vercesi 121, M Verducci 133, W Verkerke 107, J C Vermeulen 107, A Vest 44, M C Vetterli 143, O Viazlo 81, I Vichou 166, T Vickey 146, O E Vickey Boeriu 146, G H A Viehhauser 120, S Viel 169, R Vigne 30, M Villa 20, M Villaplana Perez 91, E Vilucchi 47, M G Vincter 29, V B Vinogradov 65, J Virzi 15, I Vivarelli 150, F Vives Vaque 3, S Vlachos 10, D Vladoiu 100, M Vlasak 128, A Vogel 21, M Vogel 32, P Vokac 128, G Volpi 124, M Volpi 88, H von der Schmitt 101, H von Radziewski 48, E von Toerne 21, V Vorobel 129, K Vorobev 98, M Vos 168, R Voss 30, J H Vossebeld 74, N Vranjes 137, M Vranjes Milosavljevic 13, V Vrba 127, M Vreeswijk 107, T Vu Anh 48, R Vuillermet 30, I Vukotic 31, Z Vykydal 128, P Wagner 21, W Wagner 176, H Wahlberg 71, S Wahrmund 44, J Wakabayashi 103, J Walder 72, R Walker 100, W Walkowiak 142, R Wall 177, P Waller 74, B Walsh 177, C Wang 33, C Wang 45, F Wang 174, H Wang 15, H Wang 40, J Wang 42, J Wang 33, K Wang 87, R Wang 105, S M Wang 152, T Wang 21, X Wang 177, C Wanotayaroj 116, A Warburton 87, C P Ward 28, D R Wardrope 78, M Warsinsky 48, A Washbrook 46, C Wasicki 42, P M Watkins 18, A T Watson 18, I J Watson 151, M F Watson 18, G Watts 139, S Watts 84, B M Waugh 78, S Webb 84, M S Weber 17, S W Weber 175, J S Webster 31, A R Weidberg 120, B Weinert 61, J Weingarten 54, C Weiser 48, H Weits 107, P S Wells 30, T Wenaus 25, D Wendland 16, Z Weng 152, T Wengler 30, S Wenig 30, N Wermes 21, M Werner 48, P Werner 30, M Wessels 58, J Wetter 162, K Whalen 29, A White 8, M J White 1, R White 32, S White 124, D Whiteson 164, D Wicke 176, F J Wickens 131, W Wiedenmann 174, M Wielers 131, P Wienemann 21, C Wiglesworth 36, L A M 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Yu 15, J Yu 8, J M Yu 89, J Yu 114, L Yuan 67, A Yurkewicz 108, I Yusuff 28, B Zabinski 39, R Zaidan 63, A M Zaitsev 130, A Zaman 149, S Zambito 23, L Zanello 133, D Zanzi 88, C Zeitnitz 176, M Zeman 128, A Zemla 38, K Zengel 23, O Zenin 130, T Ženiš 145, D Zerwas 117, G Zevi della Porta 57, D Zhang 89, F Zhang 174, H Zhang 90, J Zhang 6, L Zhang 152, R Zhang 33, X Zhang 33, Z Zhang 117, X Zhao 40, Y Zhao 33, Z Zhao 33, A Zhemchugov 65, J Zhong 120, B Zhou 89, L Zhou 35, L Zhou 40, N Zhou 164, C G Zhu 33, H Zhu 33, J Zhu 89, Y Zhu 33, X Zhuang 33, K Zhukov 96, A Zibell 175, D Zieminska 61, N I Zimine 65, C Zimmermann 83, R Zimmermann 21, S Zimmermann 21, S Zimmermann 48, Z Zinonos 54, M Ziolkowski 142, G Zobernig 174, A Zoccoli 20, M zur Nedden 16, G Zurzolo 104, V Zutshi 108, L Zwalinski 30
PMCID: PMC4376411  PMID: 25838796

Abstract

This article reports on a search for dark matter pair production in association with bottom or top quarks in 20.3fb-1 of pp collisions collected at s=8 TeV by the ATLAS detector at the LHC. Events with large missing transverse momentum are selected when produced in association with high-momentum jets of which one or more are identified as jets containing b-quarks. Final states with top quarks are selected by requiring a high jet multiplicity and in some cases a single lepton. The data are found to be consistent with the Standard Model expectations and limits are set on the mass scale of effective field theories that describe scalar and tensor interactions between dark matter and Standard Model particles. Limits on the dark-matter–nucleon cross-section for spin-independent and spin-dependent interactions are also provided. These limits are particularly strong for low-mass dark matter. Using a simplified model, constraints are set on the mass of dark matter and of a coloured mediator suitable to explain a possible signal of annihilating dark matter.

Introduction

The existence of dark matter (DM) in the Universe is highly motivated by many astrophysical and cosmological observations [14]. However, its nature remains a mystery. One of the best motivated candidates for a DM particle is a weakly interacting massive particle (WIMP) [5]. At the Large Hadron Collider (LHC), one can search for DM particles (χ) that are pair produced in pp collisions. These studies are sensitive to low DM masses (mχ 10 GeV), and therefore provide information complementary to direct DM searches, which are most sensitive to larger DM masses  [69].

If the particles that mediate the interactions between DM and Standard Model (SM) particles are too heavy to be produced directly in the experiment, their interactions can be described by contact operators in the framework of an effective field theory [1012]. For each operator considered, the reach is expressed in terms of the effective mass scale of the interaction, M, and of the χ–nucleon cross-section, σχ-N, as a function of mχ.

Since DM particles do not interact in the detector, the main signature of DM pair production at colliders is large missing transverse momentum. Initial-state radiation (ISR) of jets, photons, Z, or W bosons, was used to tag DM pair production at colliders in several searches at the Tevatron [13] and the LHC [1422].

A new search for DM pair production in association with one b-quark or a pair of heavy quarks (b or t) was proposed in Ref. [23]. The dominant Feynman diagrams for these processes are shown in Fig. 1. To search for these processes, dedicated selections are defined to reconstruct the various production and decay modes of these heavy-quark final states. For final states containing a semileptonic decay of a top quark, the results of the search for a supersymmetric partner of the top quark are used [24].

Fig. 1.

Fig. 1

Dominant Feynman diagrams for DM production in conjunction with a a single b-quark and b a heavy quark (bottom or top) pair using an effective field theory approach

The analysis presented in this article is particularly sensitive to effective scalar interactions between DM and quarks described by the operator [12]

Oscalar=qmqMNq¯qχ¯χ, 1

where N=3 for Dirac DM (D1 operator) and N=2 for complex scalar DM (C1 operator). The quark and DM fields are denoted by q and χ, respectively. The scalar operators are normalized by mq, which mitigates contributions to flavour-changing processes, strongly constrained by flavour physics observables [25, 26], through the framework of minimal flavour violation (MFV). The dependence on the quark mass makes final states with bottom and top quarks the most sensitive to these operators.

This search is also sensitive to tensor couplings between DM and quarks. The tensor operator (D9), which describes a magnetic moment coupling, is parameterized as [12]:

Otensor=q1M2χ¯σμνχq¯σμνq. 2

MFV suggests that the D9 operator should have a mass dependence from Yukawa couplings although canonically this is not parametrised as such.

The results are also interpreted in light of a bottom-Flavoured Dark Matter model (b-FDM) [27]. The b-FDM model was proposed to explain the excess of gamma rays from the galactic centre, recently observed by the Fermi Gamma-ray Space Telescope, and interpreted as a signal for DM annihilation [28]. This analysis of the data recorded by the Fermi-LAT collaboration favours DM with a mass of approximately 35 GeV annihilating into b-quarks via a coloured mediator. In this model, a new scalar field, ϕ, mediates the interactions between DM and quarks as shown in Fig. 2. DM is assumed to be a Dirac fermion that couples to right-handed, down-type quarks. The lightest DM particle, which constitutes cosmic DM, preferentially couples to b-quarks. The collider signature of this model is b-quarks produced in association with missing transverse momentum. This analysis sets constraints on the mass of the mediator and DM particle in the framework of the b-FDM model.

Fig. 2.

Fig. 2

Example of DM production in the b-FDM model

Detector description and physics objects

The ATLAS detector [34] at the LHC covers the pseudorapidity1 range of |η|<4.9 and is hermetic in azimuth ϕ. It consists of an inner tracking detector surrounded by a superconducting solenoid, electromagnetic and hadronic calorimeters, and an external muon spectrometer incorporating large superconducting toroidal magnets. A three-level trigger system is used to select events for subsequent offline analysis. The data set used in this analysis consists of 20.3fb-1 of pp collision data recorded at a centre-of-mass energy of s=8 TeV with stable beam conditions [35] during the 2012 LHC run. All subsystems listed above were required to be operational.

This analysis requires the reconstruction of muons, electrons, jets, and missing transverse momentum. Muon candidates are identified from tracks that are well reconstructed inside both the inner detector and the muon spectrometer [36]. To reject cosmic-ray muons, muon candidates are required to be consistent with production at the primary vertex, defined as the vertex with the highest Σ(pTtrack)2, where pTtrack refers to the transverse momentum of each track.

Electrons are identified as tracks that are matched to a well-reconstructed cluster in the electromagnetic calorimeter. Electron candidates must satisfy the tight electron shower shape and track selection criteria of Ref. [37]. Both electrons and muons are required to have transverse momenta pT>20 GeV and |η|<2.5. Potential ambiguities between overlapping candidate objects are resolved based on their angular separation. If an electron candidate and a jet overlap within ΔR<0.2, then the object is considered to be an electron and the jet is discarded. If an electron candidate and any jet overlap within 0.2<ΔR<0.4, or if an electron candidate and a b-tagged jet overlap within ΔR<0.2 of each other, then the electron is discarded and the jet is retained.

Photon candidates must satisfy the tight quality criteria and |η|<2.37 [38].

Jet candidates are reconstructed using the anti-kt clustering algorithm [39] with a radius parameter of 0.4. The inputs to this algorithm are three-dimensional topological clusters [40]. The four-momentum of the jet is defined as the vector sum of the four-momenta of the topological clusters, assuming that each cluster originates from a particle defined to be massless and to come from the interaction point.

To calibrate the reconstructed energy, jets are corrected for the effects of calorimeter response and inhomogeneities using energy- and η-dependent calibration factors based on simulation and validated with extensive test-beam and collision-data studies [40]. In the simulation, this procedure calibrates the jet energies to those of the corresponding jets constructed from stable simulated particles. In-situ measurements are used to further correct the data to match the energy scale in simulated events. Effects due to additional pp interactions in the same and preceding bunch crossings (pile-up effects) are corrected [41]. Only jets with pT>20(25) GeV and |η|<4.5(2.5) are considered in this analysis for final states involving b (t) quarks.

Jets containing particles from the hadronisation of a b-quark (b-jets) are tagged using a multivariate algorithm [42, 43]. The b-tagging algorithm combines the measurement of several quantities distinguishing heavy quarks from light quarks based on their longer lifetime and heavier mass. These quantities include the distance of closest approach of tracks in the jet to the primary event vertex, the number and position of secondary vertices formed by tracks within the jet, as well as the invariant mass associated with such vertices. The algorithm is trained on Monte Carlo (MC) simulations and its performance is calibrated using data. To optimize the sensitivity of this analysis, a requirement on the output of the b-tagging algorithm which provides a 60 % (70 %) b-jet efficiency operating point is used in signal regions (SR) 1 and 2 (3 and 4) defined below. The corresponding misidentification probability is 15 % (20 %) for c-jets, and less than 1 % for light-quark jets. The aforementioned b-tagging efficiencies and misidentification probabilities were derived in a simulated tt¯ sample with jet transverse momenta of pT>20 GeV and |η|<2.5.

The missing transverse momentum, with magnitude ETmiss, is defined as the negative vector sum of the transverse momenta of jets, muons, electrons, photons, and topological clusters not assigned to any reconstructed objects [44].

Event selection

Candidate signal events containing at least one high-pT jet and large ETmiss are assigned to one of four orthogonal signal regions. The first two signal regions focus on events with DM produced in conjunction with one (SR1) or two (SR2) b-quarks in the final state. SR3 and SR4 target events in which DM is produced in conjunction with a tt¯ pair, where either both top quarks decay hadronically (SR3) or one top quark decays hadronically and the other semileptonically (SR4). SR4 was developed for a top squark search by the ATLAS Collaboration and coincides with the “tNbC_mix” signal region described in Ref. [24]. The four signal regions provide the complementary information needed in case of observation of a signal.

Events assigned to SR1 and SR2 are required to pass a calorimeter-based ETmiss trigger with a threshold of 80 GeV. To enrich the sample in ppχχ¯+b(b¯), events are required to have a low jet multiplicity (njets<5), ETmiss > 300 GeV, and the most energetic b-tagged jet must have a pT > 100 GeV. The azimuthal separation between the directions of the jets and the missing transverse momentum is required to be more than 1.0 radian. Events with at least one identified muon or electron are discarded to reject leptonic decays of W and Z bosons. Events satisfying these selection criteria are assigned to SR1 provided that the jet multiplicity does not exceed two. Events are assigned to SR2 when at least three jets are reconstructed in the event and the second most energetic jet has pT>100 GeV. If there is a second b-tagged jet it has to satisfy pT>60 GeV.

Events assigned to SR3 are required to pass triggers specifically designed to select hadronic decays of top quark pairs. Such triggers require either five jets with pT 55 GeV each or four jets with pT 45 GeV, of which one is tagged as a b-jet. To select ppχχ¯+tt¯ events, at least five reconstructed jets are required, of which at least two are b-tagged, and ETmiss>200 GeV. Furthermore, the azimuthal separation between the most energetic b-jet and the missing transverse momentum is required to be at least 1.6 radians. To reduce W/Z leptonic decays and leptonic top quark decays, events with at least one identified muon or electron are discarded. To maximize the rejection of the abundant tt¯ background, the Razor variable R [33] is used. This variable utilizes both transverse and longitudinal information about the event to fully exploit the kinematics of the decay. To separate signal and background, R>0.75 is required.

To enrich the sample in ppχχ¯+tt¯ with one semileptonic decay of the t quark, events assigned to SR4 use single-lepton or ETmiss triggers, and require exactly one isolated lepton (electron or muon) with pT>25 GeV, at least four high-pT jets, where one jet is b-tagged with pT>60 GeV. Events with ETmiss>270 GeV are selected when the transverse mass2 formed by the lepton and ETmiss, mT(,ETmiss), exceeds 130 GeV and ETmiss/HT4j>9GeV, with HT4j=i=14pT(jeti) and where the jets are ordered by decreasing pT. The azimuthal angle between the missing transverse momentum and the two most energetic jets is required to be greater than 0.6 radians.

Special variables, such as the asymmetric transverse mass amT2 [2931] and the topness variable [32], are used to reject the dileptonic tt¯ component of the background. Details can be found in Ref. [24]. The diboson background is suppressed by a requirement on the three-jet invariant mass (mjjj<360 GeV) [24]. A τ veto rejects tt¯ events with hadronically decaying τ leptons in the final state. Additional selection criteria [24] on the angles between the lepton and the various jets are imposed to further reduce the tt¯ background. Table 1 provides an overview of the selections applied in all four signal regions.

Table 1.

Selections for signal regions 1–4. Variables pTji (pTbi) represent the transverse momentum of the ith jet (b-tagged jet). The asymmetric transverse mass amT2 [2931], topness [32], mjjj and Razor R [33] are used to reject the abundant top quark background

SR1 SR2 SR3 SR4
Trigger ETmiss ETmiss 5jets ||4jets(1b) ETmiss||1 lepton (no τ)
Jet multiplicity nj 1–2 3–4 5 4
b-Jet multiplicity nb >0 (60 % eff.) >0 (60 % eff.) >1 (70 % eff.) >0 (70 % eff.)
Lepton multiplicity n 0 0 0 1 (=e,μ)
ETmiss >300 GeV >300 GeV >200 GeV >270 GeV
Jet kinematics pTb1>100 GeV pTb1>100 GeV pTj>25 GeV pTb1>60 GeV
pTj2>100 (60) GeV pT1-4>80,70,50,25 GeV
Three-jet invariant mass mjjj<360 GeV
Δϕ(ji,ETmiss) >1.0,i=1,2 >1.0,i=1-4 >0.6,i=1,2
Angular selections Δϕ(b1,ETmiss)1.6 Δϕ(,ETmiss)>0.6
ΔR(,j1)<2.75
ΔR(,b)<3.0
Event shape Razor R>0.75 topness>2
amT2 >190  GeV
mT+ETmiss >130 GeV
ETmiss/HT4j >9 GeV

The product of the detector acceptance A and the reconstruction efficiency ϵ for the selections described above varies between 0.1 and 8 % depending on the signal region, operator, and specific channel considered. SR1 and SR2 have the highest efficiencies (A×ϵ>2%) for the D9 operator, while SR3 and SR4 are most efficient for the D1 and C1 operators (A×ϵ>1%).

The dominant background for SR1 and SR2 is due to Zνν¯ events produced in conjunction with one or more jets. This irreducible background is estimated from data using two control regions (CRs). The first CR exploits Z+jets events with Zμ+μ-, while the second uses γ+jets events for which the production at high transverse momentum (pTγ>MZ) mimics that of Z+jets [45]. The γ+jets control region substantially increases the number of events at large missing transverse momentum. The transverse momentum of the dimuon pair or photon is added vectorially to the ETmiss of the event to simulate the Zνν¯ background. Corrections to compensate for the differences in efficiency and acceptance between the Z(νν¯)+jets and Z(μ+μ-)+jets or γ+jets are derived from data using control regions without b-tagged jets before applying any requirements on the missing transverse momentum. Remaining kinematic selections correspond to the ones described in Table 1. A muon control region is chosen because the energy loss of muons in the detector is comparatively small. The systematic uncertainties introduced by this data-driven procedure on the Z(νν¯)+jets background are approximately 10 %, mainly from the flavour composition of background processes, kinematic differences between the control and signal regions and relative normalizations of backgrounds.

Production of W/Z+jets with subsequent leptonic decays of W and to a much smaller degree Z is also a substantial source of background for SR1 and SR2 when the resulting charged leptons fail to be identified or if the W or Z bosons decay to τ leptons. These contributions are estimated from Z(+-)+jets and W(ν)+jets MC samples generated using ALPGEN2.3 [46] with the CTEQ6L1 [47] parton distribution function (PDF) set. The procedure used for the normalization of this sample is described in reference [48]. These samples are generated with up to five light partons (u,d,s) and one c quark or two heavy quarks (c, b) per event. W+b production is highly suppressed and therefore negligible. A control region enriched in W(ν)+jets events is selected by adding a lepton requirement to the selection and is used to validate the estimate of this background. The purity of W(ν)+jets in the control region for SR1 (SR2) is 67 % (47 %). After full selection the contribution of b(c)-quarks to the dominant W(ν)+jets background is approximately 39 % (38 %) for SR1 and 52 % (37 %) for SR2. The systematic uncertainty on this background is approximately 20 %. Finally, the small contribution from tt¯ is estimated using MC samples and validated in data control regions before applying signal selection requirements. The tt¯ process is selected with very high purity by requiring events with one lepton and large jet multiplicities.

The dominant source of background for SR3 and SR4 is tt¯ events. In SR3, this contribution is estimated from data using a control region not overlapping with SR4 and largely dominated by tt¯ events with one of the two top quarks decaying semileptonically. The five-jets requirement is relaxed to three jets. Additionally, the event is required to contain exactly one lepton with pTe(μ)>30(25) GeV and must fulfill ETmiss+mT>25(30) GeV for the electron (muon) channel. The potential signal contribution to this selection is less than 0.1 %. The uncertainties are small because the SR3 data control region uses a kinematic region similar to the signal region with the lepton veto and jet multiplicity being the main difference. These effects were studied and considered as systematic uncertainties. Dominant uncertainties are related to jets and the top quark momentum distribution. Corrections to compensate for the differences in efficiency and acceptance between hadronic and semi-leptonic top decays are derived from MC samples generated using the POWHEG BOX generator [49] interfaced with JIMMY4.31 [50] with the next-to-leading-order (NLO) PDF set CT10 [51]. The systematic uncertainty on the tt¯ background in SR3 of approximately 7% is derived by studying corrections for the top quark momentum distribution, and shower modelling by interfacing the same generator with PYTHIA6 [52, 53].

In SR4, the tt¯ background is estimated from data using a control region obtained by requiring 60GeV<mT<90 GeV and loosening the selection criteria on ETmiss, amT2, and ETmiss/HT4j. A similar selection, but applying an inverted b-tagging requirement, is used to estimate the W(ν)+jets background. The uncertainty on the tt¯ background is estimated to be approximately 20 % [24], which is larger than the uncertainty in SR3 due to the limited statistics. These uncertainties are evaluated by varying the renormalisation and factorisation scale of the simulations, comparing alternative PDF sets, and studying the effects of different shower generators and of ISR and final-state radiation.

Additional sources of background, which include single-top, tt¯+Z/W, and diboson production, are estimated in all signal regions using simulations and NLO cross sections [54, 55]. The single-top (s-channel) and Wt background is generated using the POWHEG generator. The single-top t-channel is generated with ACERMC3.8 [56] interfaced with PYTHIA6. Associated production of tt¯ and a vector boson (W, Z) are generated with MADGRAPH5 [57] with up to two additional partons interfaced with PYTHIA6. The cross-sections for tt¯ production in association with a W (Z) boson are determined using the MSTW2008 NLO (CTEQ6.6M) PDF sets. The diboson samples are generated using HERWIG6.520 [58, 59] and JIMMY4.31 with the CTEQ6L1 PDF set. The multijet background is estimated using data-driven methods [60] and is found to be negligible in all signal regions after full selection.

Object reconstruction efficiencies in simulated events are corrected to reproduce the performance measured in data. The systematic uncertainty of the background estimates derived from simulation combines the uncertainties on the efficiency of the b-tagging algorithm, the uncertainties on the determination of the energy scale and resolution of the jet energy and ETmiss, the theoretical uncertainty on the various cross-sections, changes in the shapes of distributions used to extrapolate event counts from control regions to the signal region, data driven corrections and the PDF uncertainties. Overall, the systematic uncertainty on the background estimated from simulation is calculated to be between 12 and 18 %, depending on the signal region.

The simulation of the signal samples of ppχχ¯+b(b¯), ppχχ¯+tt¯, and b-FDM employs the MADGRAPH5 generator interfaced with PYTHIA6 using the CTEQ6L1 PDF. Samples are generated for operators D1, C1, and D9, assuming M=1  TeV and mχ between 10 and 1300 GeV. Samples for the b-FDM model are generated for mχ values between 1 and 1300 GeV and mediator masses, mϕ, between 5 and 3000 GeV. The instrumental uncertainties on the simulated signal yields for D1, C1, and D9 operators are between 11 and 15 %, depending on the signal region. The equivalent uncertainties for the b-FDM model range between 6 and 16 % depending on mχ and the mediator mass. The uncertainties from the PDF are computed by comparing the rates obtained with the default PDF set (CTEQ6L1) with those obtained with two alternative sets (MSTW2008LO and NNPDF21LO [61, 62]). The uncertainties on the signal acceptance from PDF and scale variations are estimated to be approximately 10 % for the D1, C1, and D9 operators for mχ=10 GeV and approximately 6 % for b-FDM models.

The validity of the effective field theory assumption depends on the momentum transfer of the process modelled, which should be below the energy scale of the underlying interactions [63]. To account for this, the momentum transfer m(χχ)=Qtr in the events is required to be less than the energy scale probed. Specifically, Qtr must be smaller than the mass M of the heavy mediator. For an ultraviolet completion this implies M=M/gqgχ. Along with perturbativity of the couplings gqgχ<4π this leads to the following validity requirements on MC truth level: Qtr<4π(M3/mq)1/2 (D1), Qtr<4πM (D9), Qtr<(4π)2M2/mq (C1).

Results

Table 2 shows the expected background from various sources in the four signal regions as well as the observed yields in data. The expected signal yields for the operators D1, C1, and D9, as well as for the b-FDM model are also shown. The probabilities of the background-only hypothesis, p values, for the signal regions SR1, SR2, SR3, and SR4 are 0.09, 0.29, 0.24, and 0.18, respectively. As no significant excess is observed, limits on the signal yield are set using a profile likelihood ratio test following the CLs prescription [64]. Also given is the 95 % confidence level (CL) upper limit on the number of beyond-the-SM events. The yields for the b-FDM model are obtained assuming mχ=10 GeV and a mediator mass mϕ=600 GeV. The limit on M for a given assumption on mχ is determined by varying M and scaling the number of signal events predicted by the corresponding sample generated with M=1 TeV until it is equal to the observed upper limit on beyond-the-SM events. The corresponding production cross-section for DM produced via the D1 operator in association with b(t)-quarks and mχ=10 GeV is 38 (221) fb. The cross-section for b-FDM models with mϕ=600 and mχ=10 GeV is 134 fb. The signal efficiency is independent of M.

Table 2.

Expected background and signal yields for mχ=10 GeV compared with observed yields in data for the various signal regions. For the b-FDM model, mϕ is 600 GeV. The row labeled “total expected background” shows the sum of all background components. The quoted uncertainties include all statistical and systematic effects added in quadrature. The effective mass scale, M, is set to be 100/40/600 GeV for the D1/C1/D9 operators, approximately corresponding to the expected limit. The probabilities of the background-only hypothesis, p values, are also given. The last two lines show the observed and expected 95 % CL upper limits on the number of beyond-the-SM events

Background source SR1 SR2 SR3 SR4
Z(νν¯)+jets 190 ± 26 90 ± 25 1-1+6
W(ν)+jets 133 ± 23 75 ± 13 1.3 ± 0.3
tt¯ 39 ± 5 71 ± 9 87 ± 11 2.9 ± 0.6
Single top 8 ± 3 0.7 ± 0.3
tt¯+Z/W 1.4 ± 0.4
Diboson 22 ± 4 8 ± 1 0.8 ± 0.4
Total expected background 385 ± 35 245 ± 30 96 ± 13 7 ± 1
Data 440 264 107 10
Expected signal–D1 10 ± 2 49 ± 8 28 ± 2 35 ± 5
Expected signal–C1 17 ± 2 61 ± 9 45 ± 4 51 ± 12
Expected signal–D9 147 ± 25 69 ± 12 2 ± 1 2 ± 1
Expected signalb-FDM 192 ± 24 61 ± 8 1.0 ± 0.2
p value 0.09 0.29 0.24 0.18
Allowed non SM events–Obs. 124 79 41 10
Allowed non SM events–Exp. 81 67 33 7

Figure 3 shows the ETmiss distributions for (a) SR1, (b) SR2, and (d) SR4 and (c) the R variable for SR3.

Fig. 3.

Fig. 3

Comparison between data and expected SM background. a, b ETmiss variable for SR1 and SR2 and for an example signal with the operator D9. c R variable for SR3 excluding the selection on R and for an example signal with the operator D1. d ETmiss variable for SR4 excluding the selection on ETmissand for an example signal with the operator D1. Other backgrounds are composed of diboson and multijet production. The expected signal for χχ¯+b(b¯) (SR1, 2) and for χχ¯+tt¯ (SR3, 4) production for mχ= 10 GeV is given by the red line assuming M=100/40/600 GeV for the D1/C1/D9 operators, respectively. The final selection requirements are indicated by an arrow. The error bars represent the statistical uncertainty. The dashed area shows the systematic uncertainty on the background estimation. Events with values exceeding the range presented are included in the highest bin

Figure 4 shows the 90 % CL exclusion curves for the effective mass scale M as a function of mχ. The results for the operators D1, C1, and D9 are presented individually for all four signal regions. The best limits on the D1 and C1 operators are obtained using SR4, while SR1 provides the best limits on the D9 operator, as shown in Fig. 4. These limits are then converted into limits on the χ–nucleon cross-section [12]. Figures 5 and 6 show the corresponding 90 % CL exclusion curves for the spin-independent and spin-dependent χ–nucleon cross-section for the scalar (D1) and tensor (D9) operators as a function of mχ for the strongest results obtained in any signal region. The most stringent limits set by direct detection experiments [69] are also shown. Only mχ where more then 90% of the events fulfill the effective field theory validity constraints are shown in Figs. 5 and 6.

Fig. 4.

Fig. 4

Lower limits on M at 90 % CL for the SR1 (red), SR2 (black), SR3 (green), and SR4 (blue) as a function of mχ for the operators a D1, b C1, and c D9. Solid lines and markers indicate the validity range of the effective field theory assuming couplings gqgχ<4π, the dashed lines and hollow makers represent the full collider constraints

Fig. 5.

Fig. 5

Upper limits at 90 % CL on the spin-independent χ–nucleon cross-section (σχ-NSI) for the scalar operator D1 (red) as a function of mχ. The yellow and green curves represent the exclusion limits recently set by the LUX and Super-CDMS collaborations [6, 7, 65]. The coupling is assumed to be gqgχ=g=4π

Fig. 6.

Fig. 6

Upper limits at 90 % CL on the spin-dependent χ–nucleon cross-section (σχ-NSD) for the tensor operator D9 (red) as a function of mχ. The yellow and green curves represent the exclusion limits recently set by the COUPP and PICASSO collaborations [8, 9, 65]. The coupling is assumed to be gqgχ=g=4π

The limits shown are especially strong in the low-mass region where several collaborations [28, 6668] have recently claimed possible observations of DM. The results reported in this article represent the first ATLAS limits on the scalar operator C1 and they significantly improve the sensitivity to χ–nucleon interactions mediated by the scalar operator D1 compared to previous ATLAS results [14, 16, 18, 19].

Figure 7 shows the exclusion curves observed and expected for the b-FDM model as a function of the mediator and DM masses. For each point in (mχ, mϕ), the signal region with the best expected sensitivity is used, with SR1 dominating over the other signal regions. For a DM particle of approximately 35 GeV, as suggested by the interpretation of data recorded by the Fermi-LAT collaboration, mediator masses between approximately 300 and 500 GeV are excluded at 95 % CL.

Fig. 7.

Fig. 7

Exclusion contour at 95 % CL for the b-FDM model from combined results of SR1 and SR2. The expected limit is given by the dashed line, and the yellow band indicates the ±1σ uncertainty. The observed limit, largely dominated by SR1, is given by the solid red line. The region beneath the curve indicating the observed limit is excluded

Conclusions

In summary, this article reports a search for dark-matter pair production in association with bottom or top quarks. The analysis is performed using 20.3fb-1 of pp collisions collected at s=8 TeV by the ATLAS detector at the LHC. The results are interpreted in the framework of an effective field theory to set stringent limits on scalar and tensor interactions between Standard Model and DM particles. The data are found to be consistent with the Standard Model expectations, and limits are set on the mass scale of effective field theories that describe scalar and tensor interactions between DM and Standard Model particles. The exclusion limits are strongest at low DM masses. The limit on the χ–nucleon cross-section mediated by the D1 operator is improved significantly with respect to previously published ATLAS results by obtaining sensitivities of approximately σχ-NSI=10-42cm2 for mχ=10 GeV. Constraints on b-Flavoured Dark Matter models, suitable to explain a possible signal of annihilating DM, are also presented. The excluded regions depend on mχ and mϕ. For mχ=35 GeV, mediator particles with mϕ=300500 GeV are excluded.

Acknowledgments

We would like to thank Tongyan Lin (University of Chicago) for helpful discussions about the models presented and the interplay between collider DM constraints and direct and indirect DM experiments. 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. We acknowledge the support of ANPCyT, Argentina; YerPhI, Armenia; ARC, Australia; BMWFW and FWF, Austria; ANAS, Azerbaijan; SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI, Canada; CERN; CONICYT, Chile; CAS, MOST and NSFC, China; COLCIENCIAS, Colombia; MSMT CR, MPO CR and VSC CR, Czech Republic; DNRF, DNSRC and Lundbeck Foundation, Denmark; EPLANET, ERC and NSRF, European Union; IN2P3-CNRS, CEA-DSM/IRFU, France; GNSF, Georgia; BMBF, DFG, HGF, MPG and AvH Foundation, Germany; GSRT and NSRF, Greece; ISF, MINERVA, GIF, I-CORE and Benoziyo Center, Israel; INFN, Italy; MEXT and JSPS, Japan; CNRST, Morocco; FOM and NWO, Netherlands; BRF and RCN, Norway; MNiSW and NCN, Poland; GRICES and FCT, Portugal; MNE/IFA, Romania; MES of Russia and ROSATOM, Russian Federation; JINR; MSTD, Serbia; MSSR, Slovakia; ARRS and MIZŠ, Slovenia; DST/NRF, South Africa; MINECO, Spain; SRC and Wallenberg Foundation, Sweden; SER, SNSF and Cantons of Bern and Geneva, Switzerland; NSC, Taiwan; TAEK, Turkey; STFC, the Royal Society and Leverhulme Trust, United Kingdom; DOE and NSF, United States of America. The crucial computing support from all WLCG partners is acknowledged gratefully, in particular from CERN and the ATLAS Tier-1 facilities at TRIUMF (Canada), NDGF (Denmark, Norway, Sweden), CC-IN2P3 (France), KIT/GridKA (Germany), INFN-CNAF (Italy), NL-T1 (Netherlands), PIC (Spain), ASGC (Taiwan), RAL (UK) and BNL (USA) and in the Tier-2 facilities worldwide.

Footnotes

1

ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the centre of the detector, and the z-axis along the beam line. The x-axis points from the IP to the centre of the LHC ring, and the y-axis points upwards. Cylindrical coordinates (r,ϕ) are used in the transverse plane, ϕ being the azimuthal angle around the beam line. The pseudorapidity η is defined in terms of the polar angle θ as η=-lntan(θ/2). Observables labeled “transverse” are projected into the xy plane.

2

Since the longitudinal component of the momentum of the neutrinos is not measured, the measured properties of the W boson candidates are limited to their transverse momentum and transverse mass, defined as mT=(ETmiss+pT)2-(Exmiss+px)2-(Eymiss+py)2 where ETmiss is the magnitude of the missing transverse momentum vector, pT is the transverse momentum of the lepton and px and py (Exmiss and Eymiss) are the magnitude of the x and y components of the lepton momentum (missing transverse momentum) respectively.

References


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