Observation of W tau-nu tau decays with the ATLAS experiment [Elektronische Ressource] / Guilherme Nunes Hanninger

Observation of W tau-nu tau decays with the ATLAS experiment [Elektronische Ressource] / Guilherme Nunes Hanninger

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Observ ation ofW!tn Deca ystwith the A TLAS Exp erimen tInaugural-DissertationzurErlangung der Doktorw urde¨derMathematisc h-Naturwissensc haftlic hen F akult at¨derRheinisc hen F riedric h-Wilhelms-Univ ersit at¨zu Bonnv orgelegt v onGuilherme Nunes HanningerausCuritiba - BrasilienBonn 2011Univ ersit at¨ BonnPh ysikalisc hes InstitutObserv ation of W !tn Deca ystwith the A TLAS Exp erimen tGuilherme Nunes HanningerDieser F orsc h ungsb eric h t wurde als Dissertation v on derMathematisc h-Naturwissensc haftlic hen-F akult at¨ der Univ ersit at¨ Bonn angenommenund ist auf der ULB Bonn http://hss.ulb.uni- bonn.de/diss_onlineelektronisc h publiziert.Referen t: Prof. Dr. Norb ert W ermesK oreferen t: Prof. Dr. Ian C. Bro c kAngenommen am: 22.02.2011T ag der Promotion: 29.04.2011AbstractPh ysics studies of pro cesses with t leptons in the final state, while c hallenging athadron colliders, are of great imp ortance at the LHC. Thet leptons pro vide imp ortan tsignatures in searc hes for the Higgs b oson as w ell as for new ph ysics in a wide rangeof theoretical mo dels. Deca ys of Standar d Mo del particles to t leptons, in particularZ!tt andW!tn , are imp ortan t bac kground pro cesses in those searc hes and theirtcross sections need to b e measured first. This thesis rep orts the first observ ation ofW!tn deca ys and of hadronically deca ying t leptons with the A TLAS exp erimen ttat the LHC.

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Observ ation ofW!tn Deca ys
t
with the A TLAS Exp erimen t
Inaugural-Dissertation
zur
Erlangung der Doktorw urde¨
der
Mathematisc h-Naturwissensc haftlic hen F akult at¨
der
Rheinisc hen F riedric h-Wilhelms-Univ ersit at¨
zu Bonn
v orgelegt v on
Guilherme Nunes Hanninger
aus
Curitiba - Brasilien
Bonn 2011Univ ersit at¨ Bonn
Ph ysikalisc hes Institut
Observ ation of W !tn Deca ys
t
with the A TLAS Exp erimen t
Guilherme Nunes Hanninger
Dieser F orsc h ungsb eric h t wurde als Dissertation v on der
Mathematisc h-Naturwissensc haftlic hen-F akult at¨ der Univ ersit at¨ Bonn angenommen
und ist auf der ULB Bonn http://hss.ulb.uni- bonn.de/diss_online
elektronisc h publiziert.
Referen t: Prof. Dr. Norb ert W ermes
K oreferen t: Prof. Dr. Ian C. Bro c k
Angenommen am: 22.02.2011
T ag der Promotion: 29.04.2011Abstract
Ph ysics studies of pro cesses with t leptons in the final state, while c hallenging at
hadron colliders, are of great imp ortance at the LHC. Thet leptons pro vide imp ortan t
signatures in searc hes for the Higgs b oson as w ell as for new ph ysics in a wide range
of theoretical mo dels. Deca ys of Standar d Mo del particles to t leptons, in particular
Z!tt andW!tn , are imp ortan t bac kground pro cesses in those searc hes and their
t
cross sections need to b e measured first. This thesis rep orts the first observ ation of
W!tn deca ys and of hadronically deca ying t leptons with the A TLAS exp erimen t
t
at the LHC. The analysis is based on a data sample corresp onding to an in tegrated
1luminosit y of 546 n b , whic h w as recorded at a proton-proton cen tre-of-mass energy
of 7TeV . A total of 78 data ev en ts are selected, with an estimated bac kground of
11.1 2.3 3:2 ev en ts from QCD pro cesses, and of 11.8 0.4 3:7(stat:) (syst:) (stat:) (syst:)
ev en ts from otherW and Z deca ys. The observ ed excess of data ev en ts o v er the total
bac kground is compatible with the SM exp ectation for W !tn deca ys, b oth in the
t
n um b er of ev en ts and in the shap es of distributions of c haracteristic v ariables.th
D isplay of the rst W!t n ev ent candidate obser v ed in A TL AS (ev ent collected on M ay 24 2010). e hadr onically decaying single-pr ong
h t
t candidate is clearly visible. I t has one w ell-identied track (r ed-orange) with a transv erse momentum of 10GeV and energy deposited in
the electr omagnetic and hadr onic calorimeters (20GeV and 10GeV , r espectiv ely). e energy depositions in the cells of hadr onic tile and
electr calorimeter form a narr o w sho w er typical of a hadr onict decay . I t is the only high-momentum object in the ev ent, no oth er
miss
jets, muons, electr ons or photons w er e found. E is indicated b y a r ed arr o w or dashed line.
TContents
Contents vii
1. Intro duction 1
2. The Standa rd Mo del and Bey ond 5
2.1. e S tandar d M odel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. e S tandar d M odel of electr o w eak interactions . . . . . . . . . . . . . . . . 7
2.2.1. S pontaneous br eaking ofSU(2) U(1) symmetr y . . . . . . . . . 9L Y
2.2.2. Bounds on the H iggs-boson mass . . . . . . . . . . . . . . . . . . . 12
2.2.3. H iggs sear ches at the A TL AS experiment . . . . . . . . . . . . . . . 14
2.3. S upersymmetr y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.3.1. SUSY sear ches at the A TL AS experiment . . . . . . . . . . . . . . . 16
3. The LHC and the A TLAS Detecto r 19
3.1. e Large H adr on Collider . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2. e A TL AS detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.3. I nner detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.3.1. P ix el detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.3.2. S emiconductor tracker . . . . . . . . . . . . . . . . . . . . . . . . 23
3.3.3. T ransition radiation tracker . . . . . . . . . . . . . . . . . . . . . . 24
3.4. Calorimetr y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.4.1. Liquid argon electr omagnetic calorimeter . . . . . . . . . . . . . . . 25
3.4.2. H adr onic calorimeter . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.5. M uon spectr ometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.6. M agnet system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.7. T rigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.8. L uminosity measur ement . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
vii4. Monte Ca rlo Event Generato r, Detecto r Simulation and Event Recon-
struction 33
4.1. E v ent generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.1.1. P ythia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.1.2. T auola and P hotos . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.2. D etector simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4.2.1. S imulation of pile-up effects . . . . . . . . . . . . . . . . . . . . . . 36
4.3. O bject r econstr uction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.3.1. M issing transv erse energy . . . . . . . . . . . . . . . . . . . . . . . 37
4.3.2. J et r econstr uction . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.3.3. E lectr on r econstr uction . . . . . . . . . . . . . . . . . . . . . . . . 39
4.3.4. M uon r econstr uction . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.4. S imulated data samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
5. Reconstruction and Identification of Hadronic t Deca ys 41
5.1. et lepton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
5.2. R econstr uction of hadr onict decays . . . . . . . . . . . . . . . . . . . . . . 43
5.2.1. T rack selection criteria fort r econstr uction . . . . . . . . . . . . . . 44h
5.3. H adr onict identication . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
miss5.4. H adr onict r e constr uction per formance int +E ev ents . . . . . . . . . . 50h T
5.4.1. D ata samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
5.4.2. E v ent selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
5.4.3. D istribution of kinematic and identication v ariables . . . . . . . . . 51
6. The W!tn Signal and the Background Pro cesses 57
t
6.1. W boson pr oduction at the LHC . . . . . . . . . . . . . . . . . . . . . . . 57
6.2. Kinematics ofW !tn ev ents . . . . . . . . . . . . . . . . . . . . . . . . 59
t
6.3. B ackgr ound pr ocesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
7. W !tn Event Selection 63
t
7.1. D ata samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
7.1.1. M onte Carlo samples . . . . . . . . . . . . . . . . . . . . . . . . . 63
7.2. T rigger r equir ements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
7.3. E v ent cleaning r equir ements . . . . . . . . . . . . . . . . . . . . . . . . . . 65
7.4. M issing transv erse energy . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
7.5. S election of hadr onict decays . . . . . . . . . . . . . . . . . . . . . . . . . 67
7.6. E lectr on and muon v eto . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
7.7. M issing transv erse energy signicance . . . . . . . . . . . . . . . . . . . . . 67