Collider aspects of flavour physics at high Q

2.7 MB·English

by  T. Lari

Collider aspects of flavour physics at high Q
∗
F.delAguila1,J.A.Aguilar–Saavedra1,∗,B.C.Allanach2,∗,J.Alwall3,Yu.Andreev4,D.Aristizabal
Sierra5,A.Bartl6,M.Beccaria7,8,S.Be´jar9,10,L.Benucci11,S.Bityukov4,I.Borjanovic´8,G.Bozzi12,
G.Burdman13,∗,J.Carvalho14,N.Castro14,∗,B.Clerbaux15,F.deCampos16,A.deGouveˆa17,
C.Dennis18,A.Djouadi19,O.J.P.E´boli13,U.Ellwanger19,D.Fassouliotis20,P.M.Ferreira21,
R.Frederix3,B.Fuks22,J.-M.Gerard3,A.Giammanco3,S.Gopalakrishna17,T.Goto23,
B.Grzadkowski24,J.Guasch25,T.Hahn26,S.Heinemeyer27,A.Hektor28,M.Herquet3,B.Herrmann22,
8
K.Hidaka29,M.K.Hirsch5,K.Hohenwarter-Sodek6,W.Hollik26,G.W.S.Hou30,T.Hurth31,32,
0
0 A.Ibarra33,J.Illana1,M.Kadastik28,S.Kalinin3,C.Karafasoulis34,M.Karago¨zU¨nel18,
2 T.Kernreiter6,M.M.Kirsanov35,M.Klasen22,∗,E.Kou3,C.Kourkoumelis20,S.Kraml22,31,

n N.Krasnikov4,∗,F.Krauss36,∗,A.Kyriakis34,T.Lari37,∗,V.Lemaitre3,G.Macorini38,39,
a M.B.Magro40,W.Majerotto41,F.Maltoni3,R.Mehdiyev42,43,M.Misiak24,31,F.Moortgat44,∗,
J
G.Moreau19,M.Mu¨hlleitner31,M.Mu¨ntel28,A.Onofre14,E.O¨zcan45,F.Palla11,L.Panizzi38,39,
1
1 L.Pape44,∗ S.Pen˜aranda5,R.Pittau46,G.Polesello47,∗,W.Porod48,∗,A.Pukhov49,M.Raidal28
A.R.Raklev2,L.Rebane28,F.M.Renard50,D.Restrepo51,Z.Roupas20,R.Santos21,S.Schumann52,
]
h G.Servant53,54 F.Siegert52,P.Skands55,P.Slavich56,31,J.Sola`10,57,M.Spira58,S.Sultansoy59,43,
p A.Toropin4,A.Tricomi60,∗,J.Tseng18,G.U¨nel53,61,∗,J.W.F.Valle5,F.Veloso14,A.Ventura7,8,
-
p G.Vermisoglou34,C.Verzegnassi38,39,A.Villanova delMoral5,G.Weiglein36,M.Yılmaz62
e 1 Departamento deF´ısicaTeo´ricaydelCosmosandCAFPE,Universidad deGranada, E-18071
h
[ Granada,Spain;

2 DAMTP,CMS,UniversityofCambridge,Cambridge, CB30WA,UnitedKingdom,
1
v 3 CentreforParticlePhysicsandPhenomenology (CP3),Universite´ Catholique deLouvain, B-1348
0 Louvain-la-Neuve, Belgium;
0
4 Institute forNuclearResearchRAS,Moscow,117312, Russia
8
1 5 AHEPGroup,Instituto deF´ısicaCorpuscular (CSIC,UniversitatdeValencia),E–46071Vale`ncia,
.
1 Spain.
0 6 Institut fu¨rTheoretische Physik,Universita¨tWien,A-1090Vienna,Austria
8
7 Dipartimento diFisica,Universita` diSalento,73100Lecce,Italy
0
: 8 INFN,SezionediLecce,Italy
v
9 GrupdeF´ısicaTeo`rica,UniversitatAuto`nomadeBarcelona, E-08193Barcelona, Spain
i
X
10 Institut deF´ısicad’AltesEnergies, UniversitatAuto`nomadeBarcelona, E-08193Barcelona,Spain
ar 11 INFNandUniversita` diPisa,Pisa,Italy
12 Institut fu¨rTheoretische Physik,Universita¨tKarlsruhe, D-76128Karlsruhe, Germany
13 Instituto deF´ısica,Universidade deSa˜oPaulo,Sa˜oPauloSP05508-900, Brazil
14 LIP-DepartamentodeF´ısica,Universidade deCoimbra,3004-516 Coimbra,Portugal
15 Universite´ LibredeBruxelles, Bruxelles, Belgique
16 Departamento deF´ısicaeQu´ımica,Universidade EstadualPaulista,Guaratingueta´ –SP,Brazil
17 Dept.ofPhysics&Astron.,Northwestern University, Evanston,IL60208,USA.
18 UniversityofOxford,DenysWilkinson Building, KebleRoad,Oxford,OX13RH,UK
19 Laboratoire dePhysiqueThe´orique, Univ. Paris-Sud,F-91405Orsay,France
20 Uni. Athens,PhysicsDept.,Panepistimiopolis, Zografou15784Athens,GREECE
21 CentrodeF´ısicaTeo´ricaeComputacional, FaculdadedeCieˆncias,Universidade deLisboa,
1649-003, Lisboa,Portugal
22 LPSC,Universite´ GrenobleI/CNRS-IN2P3,F-38026Grenoble, France
23 Theorygroup,IPNS,KEK,Tsukuba, 305-0801, Japan
24 Institute ofTheoreticalPhysics,WarsawUniversity, PL-00681Warsaw,Poland
∗Report of WorkingGroup 1of theCERNWorkshop“FlavourintheeraoftheLHC”,Geneva, Switzerland, November
2005–March2007.
1
25 Departament deF´ısicaFonamental, UniversitatdeBarcelona, E-08028Barcelona, Spain
26 Max-Planck-Institut fu¨rPhysik,D-80805Munich,Germany
27 Instituto deFisicadeCantabriaIFCA(CSIC–UC),E-39005Santander, Spain
28 NationalInstitute ofChemicalPhysicsandBiophysics, Ravala10,Tallinn10143, Estonia
29 DepartmentofPhysics,TokyoGakugeiUniversity, Koganei,Tokyo184-8501, Japan
30 DepartmentofPhysics,NationalTaiwanUniversity,Taipei,Taiwan10617
31 TheoryDivision,PhysicsDepartment, CERN,CH-1211Geneva,Switzerland.
32 SLAC,StanfordUniversity, Stanford, CA94309,USA.
33 Deutsches Elektronen-Synchrotron DESY,D-22603Hamburg, Germany
34 Institute ofNuclearPhysics,NCSR“Demokritos”, Athens,Greece
35 INRMoscow,Russia
36 IPPPDurham,DepartmentofPhysics,UniversityofDurham,DurhamDH13LE,UnitedKingdom
37 Universita´ degliStudidiMilanoandINFN,I-20133Milano,Italy.
38 Dipartimento diFisicaTeorica,Universita` diTrieste,Miramare,Trieste,Italy
39 INFN,SezionediTrieste,I-34014Trieste,Italy
40 FaculdadedeEngenharia, CentroUniversita´rioFundac¸a˜oSantoAndre´,SantoAndre´ –SP,Brazil
41 Institut fu¨rHochenergiephysik derO¨sterreichischen AkademiederWissenschaften, A-1050Wien,
Austria
42 Universite´ deMontre´al, De´partementdePhysique, Montre´al,Canada.
43 Institute ofPhysics,AcademyofSciences, Baku,Azerbaijan.
44 ETHZurich,CH-8093Zurich,Switzerland
45 Univ. CollegeLondon,PhysicsandAstronomyDept.,London, UK
46 Dipartimento diFisicaTeorica,Universita` diTorinoandINFN,SezionediTorino,Italy,
47 INFN,SezionediPavia,I-27100 Pavia,Italy.
48 Institut fu¨rTheoretische PhysikundAstrophysik, Universita¨tWu¨rzburg, 97074Wu¨rzburg, Germany
49 SINPMSU,Russia,
50 Laboratoire dePhysiqueThe´orique etAstroparticules, UMR5207,Universite´ Montpellier II,
F-34095Montpellier Cedex5.,France
51 Instituto deF´ısica,Universidad deAntioquia -Colombia
52 Institute forTheoretical Physics,TUDresden, Dresden,01062, Germany,
53 CERN,PhysicsDept.,CH-1211Geneva23,Switzerland
54 ServicedePhysiqueThe´orique, CEASaclay,F91191Gif–sur–Yvette, France
55 Theoretical Physics,FermiNationalAcceleratorLaboratory, Batavia,IL60510, USA,
56 LAPTH,CNRS,UMR5108,ChemindeBellevueBP110,F-74941,Annecy-le-Vieuy, France
57 HEPGroup,Dept. Estructura iConstituents delaMate`ria,UniversitatdeBarcelona, E-08028
Barcelona, Spain.
58 PaulScherrerInstitute, CH-5232VilligenPSI,Switzerland,
59 TOBBUniversityofEconomicsandTechnology, PhysicsDepartment,Ankara,Turkey,
60 Diparimento diFisicaeAstronomia, UniversitadiCatania,I-95123 Catania,Italy.
61 Univ. CaliforniaatIrvine,PhysicsandAstronomyDept.,Irvine, USA,
62 GaziUniversity, PhysicsDepartment,Ankara,Turkey.
∗ editor
Abstract
This chapter of the report of the “Flavour in ther era of LHC” workshop dis-
cusses flavour related issues in the production and decays of heavy states at
LHC, both from the experimental side and from the theoretical side. We re-
viewtopquarkphysicsanddiscussflavouraspectsofseveralextensionsofthe
Standard Model, such as supersymmetry, little Higgs model or models with
extra dimensions. This includes discovery aspects as well as measurement of
severalproperties oftheseheavystates. Wealsopresentpublicavailablecom-
putational toolsrelatedtothistopic.
3
Contents
1 Introduction 1
1 TasksofWG1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 TheATLASandCMSexperiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2 Flavourphenomenaintopquarkphysics 6
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2 Wtbvertex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1 Wtbanomalous couplings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 Measurement ofV insingletopproduction . . . . . . . . . . . . . . . . . . . . . . . . 11
tb
3 FCNCinteractions ofthetopquark . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.1 Topquarkproduction intheeffectivelagrangian approach . . . . . . . . . . . . . . . . 15
3.2 HiggsbosonFCNCdecaysintotopquarkinageneraltwo-Higgsdoubletmodel . . . . . 17
3.3 Singletopproduction bydirectSUSYFCNCinteractions . . . . . . . . . . . . . . . . . 18
3.4 ATLASandCMSsensitivity toFCNCtopdecays . . . . . . . . . . . . . . . . . . . . . 22
4 Newphysicscorrections totopquarkproduction . . . . . . . . . . . . . . . . . . . . . . 25
4.1 Potentialcomplementary MSSMtestinsingletopproduction . . . . . . . . . . . . . . . 26
4.2 Anomaloussingle-top production inwarpedextradimensions . . . . . . . . . . . . . . . 27
4.3 Non-standard contributions tott¯production . . . . . . . . . . . . . . . . . . . . . . . . 31
3 Flavourviolation insupersymmetricmodels 34
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
1.1 TheMSSMwithR-parityconservation . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
1.2 TheMSSMwithbroken R-parity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
1.3 Spontaneous R-parityviolation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
1.4 Studyofsupersymmetry attheLHC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
1.5 Inclusive searches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
1.6 Massmeasurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
1.7 Flavourstudies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
2 Effectsofleptonflavourviolation ondi-lepton invariant massspectra . . . . . . . . . . . . 44
3 Leptonflavourviolation inthelong-lived stauNLSPscenario . . . . . . . . . . . . . . . . 46
4 Neutralinodecaysinmodelswithbroken R-parity . . . . . . . . . . . . . . . . . . . . . . 47
5 Reconstructing neutrino properties from collider experiments in a Higgs triplet neutrino
massmodel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
6 SUSY(s)lepton flavourstudieswithATLAS . . . . . . . . . . . . . . . . . . . . . . . . . 51
7 Usingthel+l− + E/ +jetvetosignature forsleptondetection . . . . . . . . . . . . . . 53
T
8 Using the e±µ∓ + E/ signature in the search for supersymmetry and lepton flavour
T
violation inneutralino decays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
9 Neutralinospinmeasurement withATLAS . . . . . . . . . . . . . . . . . . . . . . . . . . 57
10 SUSYHiggs-boson production anddecay . . . . . . . . . . . . . . . . . . . . . . . . . . 59
10.1 SUSYHiggs-boson flavour-changing neutralcurrents attheLHC . . . . . . . . . . . . . 60
4
10.2 H bs¯andB-physicsintheMSSMwithNMFV . . . . . . . . . . . . . . . . . . . . . 63
→
11 Squark/gaugino production anddecay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
11.1 Flavour-violating squark-andgaugino-production attheLHC . . . . . . . . . . . . . . . 65
11.2 Flavour-violating squarkandgluinodecays . . . . . . . . . . . . . . . . . . . . . . . . 67
12 Topsquark production anddecay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
12.1 Associated stop-chargino production atLHC:Alightstopscenario test . . . . . . . . . . 71
12.2 Exploiting gluino-to-stop decaysinthelightstopscenario . . . . . . . . . . . . . . . . . 73
12.3 Astudyonthedetection ofalightstopsquarkwiththeATLASdetector attheLHC . . . 75
12.4 Stopdecayintoright-handed sneutrino LSP . . . . . . . . . . . . . . . . . . . . . . . . 78
13 SUSYSearchesat√s = 14TeVwithCMS . . . . . . . . . . . . . . . . . . . . . . . . . 80
4 Non-supersymmetricStandardModelextensions 83
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
2 Newquarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
2.1 Singlets: charge2/3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
2.2 Singlets: charge 1/3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
−
2.3 Quarksfromextradimensions: charges 1/3and5/3 . . . . . . . . . . . . . . . . . . . 100
−
2.4 Fourthsequential generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
3 NewLeptons: heavyneutrinos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
3.1 Production ofheavyneutrinosinglets . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
3.2 Heavyneutrinoproduction fromW decays . . . . . . . . . . . . . . . . . . . . . . . . 109
R
3.3 Heavyneutrinopairproduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
4 Newneutralgaugebosons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
4.1 Z′ bosonsinthedilepton channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
4.2 Z′ inhadronic channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
5 Newchargedgaugebosons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
5.1 Discoverypotential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
6 Newscalars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
6.1 Scalartripletseesawmodels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
6.2 Thediscovery potential oftheBabu-Zeemodel . . . . . . . . . . . . . . . . . . . . . . 126
5 Tools 129
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
2 ABriefSummaryofTheSUSYLesHouchesAccord2 . . . . . . . . . . . . . . . . . . . 129
2.1 TheSLHA2Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
2.2 ExplicitProposalsforSLHA2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
3 SuSpect, HDECAY, SDECAYandSUSY-HIT . . . . . . . . . . . . . . . . . . . . . . . . 141
3.1 SuSpect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
3.2 HDECAY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
3.3 SDECAY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
3.4 SUSY-HIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
4 FeynHiggs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
5
5 FchDecay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
6 MSSMNMFVinFeynArtsandFormCalc . . . . . . . . . . . . . . . . . . . . . . . . . 146
7 SPheno . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
8 SOFTSUSY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
9 CalcHepforbeyondStandardModelPhysics . . . . . . . . . . . . . . . . . . . . . . . . 149
10 HvyN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
11 PYTHIAforFlavourPhysicsattheLHC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
12 SherpaforFlavourPhysics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
6
Chapter 1
Introduction
1 Tasks ofWG1
The origin of flavour structures and CPviolation remains as one of the big question in particle physics.
Within the Standard Model (SM) the related phenomena are successfully parametrised with the help of
theCKMmatrixinthequark sector andthePMNSmatrixinthelepton sector. Inbothsectors intensive
studiesofflavouraspectshavebeencarriedoutandarestillgoingonasdiscussedinthereportsbyWG2
and WG3. Following the unification idea originally proposed by Einstein it is strongly believed that
eventually both sectors can be explained by a common underlying theory of flavour. Although current
SMextensions rarelyinclude atheoryofflavour, manyofthemtackletheflavourquestion withthehelp
ofsomespecialansatzleading tointeresting predictions forfuturecolliderexperiments astheLHC.
This chapter of the ”Flavour in the era of LHC” report gives a comprehensive overview of the
theoretical and experimental status on: (i) How flavour physics can be explored in the production of
heavy particles like the top quark or new states predicted in extensions of the SM. (ii) How flavour
aspectsimpactthediscoveryandthestudyoftheproperties ofthesenewstates. Wediscussindetailthe
physics of the top quark, supersymmetric models, Little Higgs models, extra dimensions, grand unified
modelsandmodelsexplaining neutrino data.
Section2discussesflavouraspectsrelatedtothetopquarkwhichisexpectedtoplayanimportant
roleduetoitsheavymass. TheLHCwillbeatopquarkfactoryallowingtostudyseveralofitsproperties
ingreatdetail. TheWtbcouplingisanimportantquantitywhichintheSMisdirectlyrelatedtotheCKM
element V . In SM extensions new couplings can be presented which can be studied with the help of
tb
the angular distribution of the top decay products and/or in single top production. In extensions of the
SM also sizable flavour changing neutral currents decays can be induced, such as t qZ, t qγ or
→ →
t qg. The SM expectations for the corresponding branching ratios are of the order 10−14 for the
→
electroweak decays and order 10−12 for the strong one. In extensions like two-Higgs doublet models,
supersymmetry or additional exotic quarks they can be up to order 10−4. The anticipated sensitivity of
ATLASand CMS for these branching ratio is of order 10−5. New physics contribution will also affect
single and pair production of top quarks at LHC either via loop effects or due to resonances which is
discussed inthethirdpartofthissection.
In section 3 weconsider flavour aspects of supersymmetric models. Thisclass of models predict
partners for the SM particles which differ in spin by 1/2. In a supersymmetric world flavour would
be described by the usual Yukawa couplings. However, we know that supersymmetry (SUSY)must be
broken which is most commonly parameterized in terms of soft SUSY breaking terms. After a brief
overview oftheadditional flavourstructures inthesoftSUSYbreaking sectorwefirstdiscuss theeffect
of lepton flavour violation in models with conserved R-parity. They can significantly modify di-lepton
spectra,whichplayanimportantroleinthedetermination oftheSUSYparameters,despitethestringent
constraints from low energy data such as µ eγ. We also discuss the possibilities to discover super-
→
symmetryusingthee±,µ∓ +missingenergysignature. Leptonflavourviolationplaysalsoanimportant
role inlong lived stau scenarios withthe gravitino aslightest supersymmetric particle (LSP).Inmodels
with broken R-parity neutrino physics predicts certain ratios of branching ratios of the LSP in terms of
neutrino mixing angle (in case of a gravitino LSP the prediction will be for the next to lightest SUSY
particle). Here LHC will be important to establish several consistency checks of the model. Flavour
aspects affect the squark sector in several ways. Firstly one expects that the lightest squark will be the
lighteststopduetoeffectsofthelargetopYukawacoupling. Variousaspectsofitspropertiesarestudied
1
hereindifferentscenarios. Secondlyitleadstoflavourviolatingsquarkproductionandflavourviolating
decaysofsquarksandgluinosdespite thestringentconstraints fromlowenergy datasuchasb sγ.
→
Also other non-supersymmetric extensions of the SM, such as grand unification and little Higgs
or extra dimensional models, predict new flavour phenomena which are presented in section 4. Such
SMextensions introduce newfermions(quarks andleptons), gaugebosons (charged and/or neutral)and
scalars. We study the LHCcapabilities to discover these new mass states, paying a special attention on
how to distinguish among different theoretical models. We start with the phenomenology of additional
quarks and leptons, studying in detail their production at LHC and decay channels available. It turns
out that particles up to a mass of 1-2 TeV can be discovered and studied. Besides the discovery reach
we discuss the possibilities to measure their mixing with SM fermions. They are also sources of Higgs
bosons(producedintheirdecay)andhencetheycansignificantly enhancetheHiggsdiscoverypotential
of LHC. Extended gauge structures predict additional heavy gauge bosons and, depending on the mass
hierarchy,theycaneitherdecaytonewfermionsorbeproducedinttheirdecay. Inparticular,theproduc-
tion of heavy neutrinos can be enhanced when the SM gauge group is extended with an extra SU(2) ,
R
whichpredicts additional W bosons. Ealsodiscuss flavouraspects forthediscovery ofthenewgauge
R
bosons. This is specially important for the case of an extra Z′, which appears in any extension of the
SMgaugegroup, andforwhichmodeldiscrimination iscrucial. Thepresence ornotofnewW′ bosons
alsohelpsidentifyadditionalSU(2)gaugestructures. Finally,severalSMextensionspredictanewscalar
particles. Insomecasesthenewscalarsareinvolved intheneutrino massgeneration mechanism, e.g.in
someLittleHiggsmodelsandintheBabu-Zeemodel,whicharerealisationsofthetypeIIseesawmech-
anism (involving ascalar triplet). In these twocases, high energy observables, such asdecay branching
ratios ofdoubly charged scalars, can be related tothe neutrino mixing parameters measured in neutrino
oscillations.
Last but not least computational tools play an important role in the study of flavour aspects at
LHC. In section 5 we give an overview of the public available tools ranging from spectrum calculators
overdecaypackagestoMonteCarloprograms. InadditionwebrieflydiscussthelatestversionofSUSY
Les Houches Accord which serves as an interface between various programs and now includes flavour
aspects.
2 The ATLAS andCMS experiments
TheCERNLargeHadron Collider (LHC)iscurrently being installed inthe27-km ringpreviously used
fortheLEPe+e− collider. Thismachinewillpushbackthehighenergyfrontierbyoneorderofmagni-
tude,providing ppcollisions atacenter-of-mass energyof√s= 14TeV.
Fourmainexperiments willbenefit from thisaccelerator: twogeneral-purpose detectors, ATLAS
(Fig.1.1)andCMS(Fig.1.2),designed toexplore thephysicsattheTeVscale; oneexperiment, LHCb,
dedicated to the study of B-hadrons and CP violation; and one experiment, ALICE, which will study
heavy ion collisions. Here only the ATLAS and CMS experiments and their physics programs are dis-
cussedinsomedetail.
ThemaingoaloftheseexperimentsistheverificationoftheHiggsmechanismfortheelectroweak
symmetry breaking and the study of the “new” (i.e. non-Standard Model) physics which is expected to
manifest itselfattheTeVscaletosolvethehierarchy problem. Thedesignluminosity of1034 cm−2s−1
ofthenewaccelerator willalsoallowtocollectverylargesamplesofBhadrons, WandZgaugebosons
andtopquarks, allowingstringent testsoftheStandardModelpredictions.
Sincethisprogrammeimpliesthesensitivity toaverybroadrangeofsignatures andsinceitisnot
known how new physics may manifest itself, the detectors have been designed to be able to detect as
manyparticles andsignatures aspossible, withthebestpossibleprecision.
In both experiments the instrumentation is placed around the interaction point over the whole
solidangle, exceptfortheLHCbeampipe. Astheparticles leavetheinteraction point, theytraversethe
2
Fig.1.1:AnexplodedviewoftheATLASdetector.
InnerTracker,whichreconstructsthetrajectoriesofchargedparticles,theElectromagneticandHadronic
calorimeterswhichabsorbeandmeasurethetotalenergyofallparticlesexceptneutrinosandmuons,and
the Muon Spectrometer which is used to identify and measure the momentum of muons. The presence
of neutrinos (and other hypothetic weakly interacting particles) is revealed as a non-zero vector sum of
theparticlemomentaintheplanetransverse tothebeamaxis.
Both the Inner Tracker and the Muon spectrometer need to be placed inside a magnetic field in
ordertomeasurethemomentaofchargedparticlesusingtheradiusofcurvatureoftheirtrajectories. The
two experiments are very different in the layout they have chosen for the magnet system. In ATLAS,
a solenoid provide the magnetic field for the Inner Tracker, while a system of air-core toroids outside
the calorimeters provide the field for the Muon Spectrometer. In CMS, the magnetic field is provided
by a single very large solenoid which contains both the Inner Tracker and the calorimeters; the muon
chambers areembedded intheiron ofthesolenoid return yoke. Themagnet layout determines thesize,
theweight(ATLASislargerbutlighter) andeventhenameofthetwoexperiments.
The CMS Inner Detector consists of Silicon Pixel and Strip detectors, placed in a 4 T magnetic
field. The ATLAS Inner Tracker is composed by a smaller number of Silicon Pixel and Strip detectors
and a Transition Radiation detector (TRT) at larger radii, inside a 2 T magnetic field. Thanks mainly
to the larger magnetic field, the CMS tracker has a better momentum resolution, but the ATLAS TRT
contributes totheelectron/pion identification capabilities ofthedetector.
The CMS electromagnetic calorimeter is composed by PbWO with excellent intrinsic energy
4
resolution (σ(E)/E 2 5%/ E(GeV)). The ATLAS electromagnetic calorimeter is a lead/liquid
∼ −
p
3
Fig.1.2:AnexplodedviewoftheCMSdetector.
argonsamplingcalorimeter. Whiletheenergyresolutionisworse(σ(E)/E 10%/ E(GeV)),thanks
∼
toaveryfinelateralandlongitudinalsegmentationtheATLAScalorimeterprovidesmorerobustparticle
p
identification capabilities thantheCMScalorimeter.
In both detectors the hadronic calorimetry is provided by sampling detectors with scintillator
or liquid argon as the active medium. The ATLAS calorimeter has a better energy resolution for jets
(σ(E)/E 50%/ E(GeV) 0.03) than CMS(σ(E)/E 100%/ E(GeV) 0.05) because it is
∼ ⊕ ∼ ⊕
thickerandhasafinersamplingfrequency.
p p
The chamber stations of the CMSmuon spectrometer are embedded into the iron of the solenoid
return yoke, while those of ATLAS are in air. Because of multiple scattering in the spectrometer, and
the larger field in the Inner Tracker the CMS muon reconstruction relies on the combination of the
informationsfromthetwosystems;theATLASmuonspectrometercaninsteadreconstructthemuonsin
standalonemode,thoughcombinationwiththeInnerdetectorimprovesthemomentumresolutionatlow
momenta. Themomentumresolution for1TeVmuonsisabout7%forATLASand5%forCMS.
Muons can be unambiguously identified as they are the only particles which are capable to reach
the detectors outside the calorimeters. Both detectors have also an excellent capability to identify elec-
trons that are isolated (that is, they are outside hadronic jets). Forexample, ATLASexpects an electron
identification efficiency of about 70% with a probability to misidentify a jet as an electron of the order
of10−5 [1]. Thetau identification relies onthe hadronic decay modes, since leptonically decaying taus
cannotbeseparatedfromelectronsandmuons. Thejetsproducedbyhadronicallydecayingtausaresep-
aratedfromthoseproducedbyquarkandgluonssincetheyproducenarrowerjetswithasmallernumber
oftracks. Thecapability oftheATLASdetectortoseparateτ-jetsfromQCDjetsisshowninFig.1.3.
Theidentification ofthe flavour ofajetproduced byaquark ismoredifficult and itispractically
limited to the identification of b jets, which are tagged by the vertex detectors using the relatively long
lifetime of B mesons; the presence of soft electron and muon inside a jet is also used to improve the
b-tagging performances. In Fig. 1.4 the probability of mis-tagging a light jet as a b jet is plotted as a
functionoftheb-taggingefficiencyfortheCMSdetector[1];comparableperformances areexpectedfor
ATLAS.
4