Physics at Hadron Colliders Physics Tevatron Partat III

Physics at Hadron Colliders Physics Tevatron: Partat. III: the W and Z Lecture I Marina Cobal Università di Udine 1

Overview of production rates at LHC 2

Event kinematics: pseudorapidity 3

Isolation of physics objects 4

Muons • Because of it’s long lifetime, the muon is basically a stable particle for us (c ~ 700 m) • It does not feel the strong interaction – Therefore, they are very penetrating • It‘s a minimum ionising particle (MIP) – Only little energy deposit in calorimeter • However, at high energies (E>0. 2 Te. V) muons can sometimes behave more like electrons! – At high energies radiative losses begin to dominate and muons can undergo bremsstrahlung • Muons are identified as a track in the muon chambers and in the inner tracking detectors • Both measurements are combined for the best track results 5

Electrons and Photons • Energy deposit in calorimeter – “Narrow“ shower shape in EM calorimeter – Energy nearly completely deposited in EM calorimeter • Little or no energy in had calorimeter (hadronic leakage) • Electrons have an associated track in inner detector • If there is no track found in front of calorimeter: photon – But be careful, photon might have converted before reaching the calorimeter 6

Cluster reconstruction Losses between PS and S 1 e with energy E strips Middle Back Longitudinal Leakage Upstream Losses Upstream Material • Presampler LAr Calorimeter Input to clustering: – Cells calibrated at the EM scale • Sum energy in EM calo, correct for losses in upstream material, longitudinal leakage and possible other lossses between calo layers (if applicable) – e. g. • Typically need to find best compromise between best resolution and best linearity 7

e/jet and /jet separation • Leakage into 1 st layer of hadronic calorimeter • Analyse shape of the cluster in the different Transverse shower shape in 2 nd EM layer (ATLAS) Electron or photon cut layers of the EM calo – “narrow“ e/ shape vs “broad“ one from jet mainly jets • Look for sub-structures within one cluster – Preshower in CMS, 1 st EM layer with very fine granularity in ATLAS – Very useful for 0 / separation, 2 photons from 0 tend to end up in the same cluster at LHC energies ATLAS 8

Bremsstrahlung • • • Electrons can emit photons in the presence of material At LHC energies: – electron and photon (typically) end up in the same cluster – Electron momentum is reduced – E/p distribution will show large tails Methods for bremsstrahlung recovery – Gaussian Sum Filter, Dynamic Noise Adjustment – Use of calorimeter position to correct for brem – Kink reconstruction, use track measurement before kink 9

Material in Tracker CMS • CMS Silicon detectors at hadron colliders constitute significant amounts of material, e. g. for R<0. 4 m – CDF: ~20% X 0 – ATLAS: ~20 -90% X 0 – CMS: ~20 -100% X 0 10

Effects of Material on Analysis • Causes difficulties for e/ identification: – Bremsstrahlung – Photon conversions • Constrained with data: – Photon conversions – E/p distribution – Number of e±e± events 11

Conversion reconstruction • Photons can produce electron pairs in the presence of material • Find 2 tracks in the inner detector from the same secondary vertex – Need for outside-in tracking • However, can be useful: – Can use conversions to x-ray detector and find material before calorimeter (i. e. tracker) ATLAS CDF 12

W/Z: lepton ID For electrons we need: High efficiency for isolated electrons Low misidentification of jets Measured using Z’s Electron reconstruction improved • Track, calorimeter cluster matching • Recover bremsstrahlung losses Muon Performance: Efficiency: 98% depending on |η| Measured using Z’s ETmiss measured in Z→μμ events (Data vs. MC) Very good agreement over 6 orders of magnitude with overall Pythia 6 normalization to data. ATLAS-CONF-2012 -101 13

Electron Identification • • • Desire: – High efficiency for (isolated) el – Low misidentification of jets Cuts: – Shower shape – Low hadronic energy – Track requirement – Isolation Performance: – Efficiency measured from Z’s using “tag and probe” method – Usually measure “scale factor”: • SF= Data/ MC (=1 for perfect MC) • Easily applied to MC CDF ATLAS Loose cuts 85% 88% Tight cuts 60 -80% ~65% 14

Lepton Momentum Scale • Momentum scale: – Cosmic ray data used for detailed cellby-cell calibration of CDF drift chamber – E/p of e+ and e- used to make further small corrections to p measurement – Peak position of overall E/p used to set electron energy scale • Tail sensitive to passive material 15

Momentum/Energy Scale and Resolution Z ee Z • Systematic uncertainty on momentum scale: 0. 04% 16

Beware of Environment • Efficiency of e. g. isolation cut depends on environment – Number of jets in the event • Check for dependence on distance to closest jet 17

Taus • Decays – 17% in muons – 17% in electrons – ~65% of ’s decay hadronically in 1 - or 3 -prongs ( , +n 0 or 3 , 3 +n 0) • For reconstruct hadronic taus – Look for “narrow“ jets in calorimeter (EM + had) • i. e. measure EM and hadronic radius (measurement of shower size in - ): Ecell R 2 cell/ Ecell – Form ΔR cones around tracks • tau cone • isolation cone – associate tracks (1 or 3) 18

Missing Transverse Energy • But it‘s not that easy. . . – Electronic noise might bias your ET measurement – Particles might have ended in cracks / insensitive regions – Dead calorimeter cells – Effects from beamhalo events • Corrections needed to calorimeter missing ET – Correction for muons • Recall: muons are MIPs – Correct for known leakage effects (cracks etc) – Particle type dependent corrections • Each cell contributes to missing ET according to the final calibration of the reconstructed object (e, , , jet…) – Pile-up effects will need to be corrected for 19

Missing Transverse Energy • Difficult to understand quantity 20

Missing Transverse Energy • Missing energy is not a good quantity in a hadron collider as much energy from the proton remnants are lost near the beampipe • Missing transverse energy (ET) much better quantity – Measure of the loss of energy due to neutrinos • Definition: – • Best missing ET reconstruction – Use all calorimeter cells with true signal – Use all calibrated calorimeter cells – Use all reconstructed particles not fully reconstructed in the calorimeter • e. g. muons from the muon spectrometer 21

EW Sector

Snapshot of the EW Standard Model

Snapshot of the EW Standard Model

Fermion Boson vertices

Standard Model relationship

Gauge bosons couplings

A closer look at W-boson production: @LHC Same colour Right chirality

A closer look at W-boson production: @LHC

A closer look at W-boson production: @UA 1/2 Proton beam= Wide band of partons (q, g and few antiquarks) Antiproton beam= Wide band of antipartons (antiquarks, g, and some q) Lets consider a valence quark-antiquark annihilation If √s=630 Ge. V, the fraction of momentum needed to create the resonance is” Ok: there are many

The W resonance Close to the resonance Breit e Wigner cross section Probability to have the same colour Small smax<<< stot 100 mb. Cross section raise rapidly with energy , together with the possibility for the W boson to be created with a longitudinal momentum.

W and Z signatures Their production is a rare process: 10– 8 -- 10– 9 (stot(pp) 70 mb = 7 1010 pb ) [The detector rejection power against the background must be > 1010 More frequent final states are: qq But: q jet Huge background from: Leptonic final states have a more favourable S/N W e ne e W µ nµ high p. T Z e– e+ high p. T Isolated, high p. T µ Isolated, 2 e 2 Isolated, }+ High PT n Hermetic detector

W/Z analysis: selection Focus on leptonic decays: Hadronic impossible due to huge QCD dijet background Z Z- boson -Two leptons p. T>20 Ge. V Electron, muon, tau W-boson -One lepton p. T>20 Ge. V -Missing ET>20 Ge. V: Signature of undetected particle (neutrino) Necessary ingredients: Electron energy scale Track momentum scale Lepton ID and trigger efficiencies Missing ET resolution Luminosity … Leptons and MET are crucial! W 33

W/Z analysis: Lepton Identification Electrons: compact EM cluster in calorimeter Matched to track Muons: Track in the muon chambers Matched to track Taus: Narrow jet Matched to one or three tracks Neutrinos: Imbalance in transverse momentum Inferred from total transverse energy measured in detector 34

The W and Z discovery • In 1978 Cline, Mc. Intire e Rubbia proposed to transform the CERN Sp. S accelerator ina storage ring wherre protons and antiprotons could travel in opposite direction, within the same existing magnetic structure. • The big problem solved by Rubbia and Van der Meer was the cooling of the particle bunches close to the collision points. • In 1983 the luminosity of L=1032 m– 2 s– 1 was reached, sufficient to discover the W and Z

UA 1 under construction

UA 1 goes to the museum! 6/5/2021

UA 1. The first W C. 8 A. Bettini 38 6/5/2021

W e Nei calorimetri elettromagnetici le W appaiono come un deposito localizzato di energia in direzione opposta al momento mancante L’eliminazione delle tracce con p. T< 1 Ge. V rende completamente pulito l’evento, sopravvivono solo elettrone e il “neutrino”

W’s @ UA 1 UA 2 MW= 82. 7± 1. 0(stat)± 2. 7(syst) Ge. V MW= 80. 2± 0. 8(stat)± 1. 3(syst) Ge. V GW<5. 4 Ge. V GW<7 Ge. V

Misura di MW W l nl p. T e e e p Te W q* W I momenti trasversi di q e q sono piccoli, quindi lo è anche quello della W. ne LAB pe = m. W/2 ne CM. W p. Te è il medesimo nei due riferimenti = (m. W/2) sin q* Distribuzione angolare di decadimento nel CM nota Picco “Jacobiano” per p. Te = m. W/2 Picco “Jacobiano” per p. Tmissing = m. W/2 Il moto trasverso della W (p. TW≠ 0) sbrodola il picco, ma non lo cancella. La misura di m. W si basa sulla misura dell’energia del picco o del fronte di discesa

W cross section measurement

+ W and W differential cross section

Measured + W and W cross section

Larghezze leptoniche W Le masse (approssimativamente) Da valore misurato di q. W W. Larghezze leptoniche (uguali per universalità). Per calcolo serve teoria NB. In generale le larghezze dei BI sono proporzionali al cubo della massa C. 8 A. Bettini 45 6/5/2021

W: Larghezze adroniche Per calcolare le larghezze in qq bisogna tener conto di • un fattore 3 perché ci sono 3 colori • la matrice di mescolamento Due tipi di decadimento • nella stessa famiglia • in diverse famiglie (piccola larghezza) Tutti gli elementi non diagonali sono piccoli, quindi W decade poco in quark di diverse famiglie Tre colori

Spin e polarizzazione della W Nel riferimento del c. m. della W l’energia dell’elettrone >> me. chiralità elicità V–A W si accoppia solo a fermioni con elicità – antifermioni con elicità + Mom. ang. Tot. J=SW=1 Jz (iniz. ) = l = – 1 Jz’ (fin. ) = l’ = – 1 N. B. Se fosse stato V+A L’asimmetria avanti-indietro è conseguenza della violazione di P Per distinguere V–A da V+A sono necessarie misure di polarizzazione dell’elettrone

Measurement of Z-boson production x-section

Total W and Z production x-section Phys. Rev. D 85 (2012) 072004 CMS-PAS-12 -011 Measurements already limited by sys and lumi uncertainties Good agreement with NNLO prediction ATLAS points overlap with CMS X. Wu, SUSY 2012, 13/08/12 Discriminating power against different PDF sets 49

W Boson mass • Real precision measurement: – LEP: MW=80. 367± 0. 033 Ge. V/c 2 – Precision: 0. 04% • => Very challenging! • Main measurement ingredients: – Lepton p. T – Hadronic recoil parallel to lepton: u|| • Z ll superb calibration sample: – but statistically limited: • About a factor 10 less Z’s than W’s • Most systematic uncertainties are related to size of Z sample – Will scale with 1/√NZ (=1/√L) 50

Measurement of the W Boson Mass 51

How to Extract the W Boson Mass • • Alternatively can fit to – Lepton p. T or missing ET Sensitivity different to different systematics – Very powerful checks in this analysis: • Electrons vs muons • Z mass • m. T vs p. T vs MET fits – The redundancy is the strength of this difficult high precision analysis

Systematic Uncertainties Limited by data statistics Limited by data and theoretical understanding • • Overall uncertainty 60 Me. V for both analyses – Careful treatment of correlations between them Dominated by stat. error (50 Me. V) vs syst. (33 Me. V)

LHC signals of W’s with ~0. 3 pb-1 • 0. 2 -0. 3 pb-1 yield clean signals of W’s and Z’s 54

LHC W Cross Sections • Data agree well with theoretical expectation – Uncertainties: 13% (W), 17% (Z) – Luminosity uncertainty 10% 55

W μν m Mee M TW m P TZ

W/Z: Drell-Yan W and Z production at LHC proceeds at the hard scattering level and first order via collisions of a valence quark (u, d) and a sea antiquark (Q≈100 Ge. V): • LHC parton density fractions in this process are typically 10 -4 < x < 10 -1. ATLAS + CMS • Cross sections at LHC are a factor of 3 higher than at the Tevatron. • At LHC: > 106 W→lν events and ~ 105 Z → l+l- events per experiment and per lepton channel in 2011 data !! 57

Importance of DY • Process: production of two leptons at high PT • It allows the measurement of few important parameters of the SM the forward backward asymmetry AFB. measurement of sinϑW: via the measurement of asymmetry AFB measurement of the MW mass • W± production: - W+ x-section larger than W- PDF: quark and antiquark density in protons [ud(bar)W+; u(bar) d W-] • W+jets production • Production of (WW, ZZ, WZ): to study Triple Gauge coupling constants All of this constitues background for new physics 58

Drell-Yan production cross sections ds/d. M CMS-PAS-EWK-11 -007 • Good agreement with NNLO theoretical prediction – NLO significantly undershoot the data in low M region 59

W/Z + jets : jet multiplicities normalized to stot Good agreement with multi-parton matrix element + parton shower predictions Pure parton shower MC inadequate for describing multi-jet final states X. Wu, SUSY 2012, 13/08/12 Phys. Rev. D 85 (2012) 092002 JHEP 01 (2012) 010 60

W + jets/Z + jets : ratio and double ratio JHEP 01 (2012) 010 Phys. Lett. B 708 (2012) 221 -240 e channel m channel Measurements with small systematic uncertainty Jet energy scale systematic mostly cancels out X. Wu, SUSY 2012, 13/08/12 61

Couplings between gauge bosons

Diboson (WW, WZ, ZZ, W , Z ) production This process has… . . this as a background • Importance of diboson production • Sensitive to Triple Gauge Couplings (TGC): fundamental test of SM • WW and WWZ allowed; ZZ , Z and ZZZ forbidden • Search for beyond SM → anomalous coupling and resonances • Background to Higgs and other new physics searched • If the TGC’s have non SM values, this shows up at large p. T (short distances) • This is a region where the background from the right diagram is small 63

Dibosons production

WW Cross Section at Tevatron • WW cross section – Use W and W e – 2 leptons and missing ET • Result: – Data: =13. 6+-3. 1 pb – Theory: =12. 4+-0. 8 pb • Campbell, Ellis 65

WW production ATLAS-CONF-2012 -025 CMS PAS SMP-12 -005 CMS PAS SMP-12 -013 Measure cross section in WW→ 2 l 2 n channel → Need good calibration for missing ET 7 Te. V, ATLAS(4. 7 fb− 1)/CMS(4. 92 fb− 1) 8 Te. V, CMS(3. 54 fb− 1) Already systematics limited! 66

WZ production Measure cross section in WZ->3 l 1 channel → very small bkg, lower statistics ATLAS new (ICHEP 2012) CMS PAS EWK-11 -010 normalized distribution (fiducial) ATLAS (4. 6 fb− 1) CMS (1. 09 fb− 1) X. Wu, SUSY 2012, 13/08/12 67

ZZ production at 8 Te. V ATLAS-CONF-2012 -090 CMS-PAS-SMP-12 -014 ATLAS (5. 8 fb− 1) CMS (5. 26 fb− 1) (include 2 l 2 t)

Limits on Triple Gauge Coupling WWZ • Set limits to the anomalous couplings assuming an effective Lagrangian with EW gauge and CP Phys. Lett. B 712 (2012) 289 -308 Phys. Lett. B 709 (2012) 341– 555 invariance • WWZ coupling probed by WW and WZ ICHEP, 4. 6 fb− 1

Limits on Triple Gauge Coupling ZZZ and ZZ • Set limits to the anomalous couplings assuming an effective Lagrangian – Use total number of events (ATLAS) or the shape of the ZZ invariant mass (CMS) CMS-PAS-SMP-12 -007 Phys. Rev. Lett. 108 (2012) 041804

Quartic couplings of EW vector bosons

Standard Model cross sections