Measurement of the W boson polarisation in tbar{t} events from pp collisions at sqrt{s} = 8 TeV in the lepton\xa0+\xa0jets channel with ATLAS

M Aaboud,M Primavera,A C Daniells,K M Black,V D Peshekhonov,J Smiesko,G Jarlskog,K Iordanidou,J B Hansen,C C Ohm,Z Zhao,A A Komar,S Calvet,A Baroncelli,A Farbin,D Hayakawa,F Rubbo,S Meehan,M Melo,F Lacava,K Motohashi,V Garonne,M Barbero,J P Ochoa-Ricoux,J Maeda,E Gorini,B K Wosiek,T Harenberg,S Grinstein,Z Zhang,I A Connelly,G H Herbert,S González De La Hoz,M Sahinsoy,T Lecompte,J Faltova,H Jiang,D Varouchas,G Aielli,C Valderanis,K Liu,P Tornambe,W A Cribbs,S A Tupputi,B J Gallop,J Liebal,A Kravchenko,J Barreiro Guimarães Da Costa,J Ye,S Kido,A J Beddall,P Pralavorio,K Lantzsch,T C Petersen,I A Minashvili,O Abdinov,L S Kaplan,O V Solovyanov,A Tricoli,J Poveda,M Nagel,D Whiteson,X Lei,Y Makida,E Lançon,D V Perepelitsa,E Etzion,D Lynn,A Ashkenazi,M Kagan,U Schäfer,M S Neubauer,L Fabbri,T H Lin,M Vreeswijk,O L Fedin,F Cormier,A Pingel,O Ricken,M Wessels,V A Mitsou,S Simion,V Veeraraghavan,B A Petersen,P Wagner,P Ferrari,A Milov,S Stapnes,S S Mortensen,K Tackmann,S Vlachos,A M Nairz,R Castelijn,O Bessidskaia Bylund,N Benekos,F Ito,R Nagai,A Aloisio,J Lacey,L J Levinson,N Kanaya,Y Hasegawa,K Augsten,G T Forcolin,J Glatzer,J T Baines,F Cerutti,G A Vasquez,L Adamczyk,E J Gallas,M Hance,M H Genest,G L Alberghi,C Kozakai,P Bussey,E Busato,P Laurelli,P Vankov,O S Abouzeid,R Moles-Valls,T J Jones,K Mönig,J Henderson,L Merola,N Rompotis,L Serin,A G Foster,I Gorelov,A Chafaq,D Pallin,D Britzger,R Zhang,J Janssen,S Snyder,F Lanni,H Stenzel,P Rieck,T Varol,S Veneziano,J J Kempster,L Dell’Asta,V Tsiskaridze,D Tsybychev,A Negri,W Buttinger,G Callea,J Dassoulas,C Malone,L Pacheco Rodriguez,S Shimizu,G A Popeneciu,D G Charlton,F Buehrer,I Maznas,A Coccaro,L P Rossi,S Terada,R Hertenberger,L Zhou,A Klimentov,M Frate,W Fedorko,J A Stillings,A S Bell,A Gershon,C Vittori,B Burghgrave,R Abreu,F Fabbri,H Oberlack,C Guyot,S Lammers,K P Oussoren,E Lipeles,S Roe,S Rave,P O J Gradin,M Shapiro,E Petrolo,R Tanioka,S J Mcmahon,C Farina,N P Hessey,V S Martoiu,C Diaconu,A White,N Javadov,S Giagu,S Schmitt,I Yusuff,M Ishitsuka,Y Nakahama,R M Fakhrutdinov,F Derue,G Mchedlidze,K Hanagaki,R W Stanek,A V Anisenkov,M Schott,E Duchovni,J Goncalves Pinto Firmino Da Costa,F Seifert,J C-L Tseng,G Darbo,B Åsman,I Ueda,A Krasznahorkay,T Cornelissen,H Iwasaki,Y Fang,M Geisen,A Cueto,F Paige,K Hamacher,S W Weber,S M Mazza,F Petrucci,M Mantoani,D Melini,Y Abulaiti,P Butti,T Kharlamova,N Barlow,P Seema,T Z Kowalski,A J Chuinard,J Schaeffer,H Chen,T Bisanz,N Anjos,M J White,M Romano,M V Chizhov,F Cirotto,S Wenig,G Chiarelli,S Williams,A S Boldyrev,J Godlewski,A Sidoti,L Fiorini,J Su,D Hirschbuehl,L Živković,Y Kataoka,G Aad,O Bulekov,M Vanadia,A Alonso,T Berry,L Chytka,D Zerwas,D Iliadis,M Bedognetti,C Bini,A Robson,B Axen,A Sapronov,J R Dandoy,J Cochran,T Dado,S Mehlhase,M Boehler,N B Atlay,A D Pilkington,D Mori,L Asquith,M Ishino,A Dell’Acqua,M Pagáčová,L Y Shan,T Kishimoto,P Jackson,M Stoebe,A Catinaccio,T Wengler,S Chen,F Rühr,C Escobar,A Shcherbakova,J H Broughton,A Ouraou,L Schoeffel,S Adachi,K Yoshihara,B Simmons,N Hod,M Casolino,G Brooijmans,U Landgraf,K M Tsui,S Valentinetti,J J Chwastowski,M M Perego,R Slovak,S Majewski,C Fukunaga,Q Li,T Alexopoulos,R G Reed,M Elsing,B P Nachman,T Colombo,A Lipniacka,D Lewis,K Kroeninger,J M Butterworth,H Schulz,J W Smith,V Chiarella,A R Stradling,D C O’Neil,G Poulard,A Poley,F Ceradini,C Li,I Caprini,C Meroni,G D Hallewell,C Sandoval,A M Soukharev,A Lucotte,A Valero,D Sosa,V J Martin,A A Maier,E Vilucchi,N A Styles,I Dawson,K Nagata,C Doglioni,S Kuehn,R L Bates,R Narayan,T Wenaus,B M M Allbrooke,B M Barnett,E Higón-Rodriguez,N F Castro,R Lafaye,R Hanna,J Taenzer,Z Jiang,S Jiggins,I Korolkov,J Jongmanns,F Guescini,J Dopke,L E Pedersen,C J Potter,M Atkinson,R D St Denis,S Spagnolo,S Manzoni,A Limosani,G Iakovidis,Ch Geich-Gimbel,S M Gibson,G Cree,G Salamanna,Y Oren,P S Miyagawa,T A Beermann,J P Araque,W C Fisher,F Scuri,M G Alviggi,E Ritsch,S Demers,M A Chelstowska,R Hauser,M Gignac,G Ernis,W Lukas,M Kretz,E Rossi,E L 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Aleksa,T Brooks,K Belotskiy,N Valencic,T Masubuchi,Q Hu,G Jones,J R Batley,P Laycock,S P Marsden,J Saxon,A Bassalat,C David,P J Lösel,N A Rusakovich,R D Harrington,G Piacquadio,G F Tartarelli,G Bella,H Kolanoski,G Watts,J Chudoba,P Fassnacht,C Kourkoumelis,J W Hetherly,G Sciolla,G Bertoli,M Ernst,I Deigaard,M Biglietti,N Besson,J Meyer,M Swiatlowski,S Nowak,M Spousta,D Bortoletto,C E Pandini,H Abreu,S Pataraia,D Schaile,T Lazovich,S Palestini,M G Kurth,R Ströhmer,M K Ayoub,C Ferretti,T Kunigo,S A Weber,A Nisati,A Basalaev,C Gumpert,G Stoicea,G Sekhniaidze,H Arnold,T Carli,O Zenin,C Schiavi,T Barillari,R Soualah,V Radescu,F Dallaire,V Gratchev,N Bruscino,D Kyriazopoulos,M Purohit,R Gonçalo,R Vanguri,G Crosetti,D Álvarez Piqueras,R M Bianchi,P Hanke,R Staszewski,M Hirose,D Benchekroun,F Djama,G Brandt,H F-W Sadrozinski,J Mamuzic,N L Belyaev,T Naumann,A Straessner,S Koperny,M Talby,K Jakobs,D Emeliyanov,R Rosten,V Hedberg,C M Macdonald,O Lundberg,O A Ducu,U Bratzler,G Avolio,R Brock,M 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Von Der Schmitt,R E Ticse Torres,M C N Fiolhais,J C Hill,D A Soh,C Da Via,P C F Glaysher,A C Dudder ,J R Bensinger,B Di Micco,I Vukotić ,F Muêller ,A A Affolder,D P C Sankey,A Strübig ,S Yamamoto,F Rauscher,P Farthouat,M Leyton,P Chang,U Mallik,C Stănescu ,M Przybycień ,A Kobayashi,B Esposito,M Weber ,K Bos,M A B Do Vale,M Taševský ,C A Solans Sanchez,D Ferrère ,E Le Guirriec,P Kodyš ,M I Alstaty ,H Kagan,J A Murillo Quijada,T Bołd ,D Pohl,K Wong ,A Canepa,E V Bouhova-Thacker,F De Lorenzi,J A Valls Ferrer,D E Ferreira De Lima,A Pin ,K W Woźniak ,Ø Dale ,G Marchiori,T Löhse ,Yuan Li ,B R Mellado Garcia,D Simon,J-Y Hostachy,J E Pilcher,L P Calôba ,M Arık ,S Schaetzel,K Sliwa,E Piccaro,C N P Gee,S M Romano Saez,J Terrón ,J Rothberg,D I Narrias Villar,J Muêller ,A A Nepomuceno,T Agatonović-Jovin ,A De Salvo,R Goncalves Gama,I A Cioară ,R De Asmundis,B Di Girolamo,L M Mir,J T P Burr,Y Amaral Coutinho,Y Ilchenko,А Антонов ,M Myška ,R Santonico,A Gascon Bravo,H Pirumov,D M Gingrich,Y F Ryabov,S Henrot-Versillé ,S P Amor Dos Santos,N Giokaris,M Marčišovský ,M A L Leite,S Passaggio,J Silva,M P Giordani,R E Long,J C Vermeulen,M Medinnis,P J Clark,J E Salazar Loyola,Yu A Tikhonov,H Duran Yildiz,M Zur Nedden,L R Flores Castillo,R Di Nardo,H Öhman ,B Lund‐Jensen ,Š Valkár ,В Ф Казанин ,J M Iturbe Ponce,D Boscherini,G Y Jeng ,S De Cecco,M Ziolkowski,A De Maria,H A Neal,L Hervás ,P Bartoš ,S Jin,M Rescigno,P Némethy ,J Tanaka,R C W Henderson,M Dumančić ,A Di Simone,P Giromini,L Cerdá Alberich ,F Friedrich,P Johansson,F Pastore ,J Mašík ,M Svatoš ,T Kobayashi,S Youssef,E Magradze,A La Rosa,D Fassouliotis,R P Middleton,C Rogan ,B D Cooper,E J W Moyse,Κ Παπαγεωργίου ,K K Gan,P Pétroff ,O M Kind,S Malyukov,D Strom ,J Krstić ,R Piegaia,A Filipčǐc ,G Polesello,T Moa,M N K Smith,L A M Wiik-Fuchs,M El Kacimi,C Böhm ,J Große-Knetter ,S Tanaka,F K Loebinger,E Fullana Torregrosa,D Della Volpe,J Braciník ,S P Ahlen,W J Murray,D Amidei,S Albrand,A Andreazza,E Torrence,A 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doi:10.1140/epjc/s10052-017-4819-4

Abstract

This paper presents a measurement of the polarisation of W bosons from tbar{t} decays, reconstructed in events with one high-p_{text{ T }} lepton and at least four jets. Data from pp collisions at the LHC were collected at sqrt{s} = 8 TeV and correspond to an integrated luminosity of 20.2 fb^{-1}. The angle theta ^{*} between the b-quark from the top quark decay and a direct W boson decay product in the W boson rest frame is sensitive to the W boson polarisation. Two different W decay products are used as polarisation analysers: the charged lepton and the down-type quark for the leptonically and hadronically decaying W boson, respectively. The most precise measurement of the W boson polarisation via the distribution of cos {theta ^{*}} is obtained using the leptonic analyser and events in which at least two of the jets are tagged as b-quark jets. The fitted fractions of longitudinal, left- and right-handed polarisation states are F_{mathrm {0}}=~0.709~pm ~{0.019}, F_{mathrm {L}}=~0.299~pm ~{0.015} and F_{mathrm {R}}=~-0.008~pm ~{0.014}, and are the most precisely measured W boson polarisation fractions to date. Limits on anomalous couplings of the Wtb vertex are set.

Highlights

By measuring the polarisation of the W boson with high precision, the Standard Model (SM) prediction can be tested, and new physics processes which modify the structure of the W tb vertex can be probed
The longitudinal, left- and right-handed W boson helicity fractions are measured using the angle between the charged lepton and the reversed b-quark direction in the W boson rest frame for leptonically decaying W bosons from ttdecays
A data set corresponding to 20.2 fb−1 of √pp collisions at the LHC with a centre-of-mass energy of s = 8 TeV, recorded by the ATLAS experiment, is analysed

Summary

Introduction

By measuring the polarisation of the W boson with high precision, the SM prediction can be tested, and new physics processes which modify the structure of the W tb vertex can be probed. The structure of the W tb vertex can be expressed in a general form using left- and right-handed vector (VL/R) and tensor (gL/R) couplings: LW t b = − √g bγ μ 2 (VL PL + VR PR) t Wμ−. PL/R refer to the left- and right-handed chirality projection operators, mW to the W boson mass, and g to the weak coupling constant. All of the vector and tensor couplings vanish in the SM, except VL, which corresponds to the CKM matrix element Vtb and has a value of approximately one. Dimension-six operators, introduced in effective field theories, can lead to anomalous couplings, represented by non-vanishing values of VR, gL and gR [4,5,6]

Methods

Results

Conclusion