Physics at Hadron Colliders Physics at the Tevatron: Part ...cobal/marina_W_3.pdf · "2.04 GeV Per...
Transcript of Physics at Hadron Colliders Physics at the Tevatron: Part ...cobal/marina_W_3.pdf · "2.04 GeV Per...
Physics at the Tevatron: Lecture I
MarinaCobal
UniversitàdiUdine
1
Physics at Hadron Colliders Part III: W
• Because of it’s long lifetime, the muon is basically a stable particle for us (cτ ~ 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 TeV) 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
2
Muons
3
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
• 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 4
( )33210 EWEEEWbE presrec ++++= λ
Cluster reconstruction Losses between PS and S1
strips
e± with energy E
Middle Back
Presampler LAr Calorimeter Upstream Material
Upstream Losses
Longitudinal Leakage
5
e/jet and γ/jet separation
• Leakage into 1st layer of hadronic
calorimeter
• Analyse shape of the cluster in the different
layers of the EM calo
– “narrow“ e/γ shape vs “broad“ one from
mainly jets
• Look for sub-structures within one cluster
– Preshower in CMS, 1st EM layer with
very fine granularity in ATLAS
– Very useful for π0→γγ / γ separation, 2
photons from π0 tend to end up in the
same cluster at LHC energies
jet
e or γ
cut
Transverse shower shape in 2nd EM layer (ATLAS)
ATLAS
6
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
7
Material in Tracker
• Silicon detectors at hadron colliders constitute significant amounts of material, e.g. for R<0.4m – CDF: ~20% X0 – ATLAS: ~20-90% X0 – CMS: ~20-100% X0
CMS CMS
8
Effects of Material on Analysis
• Causes difficulties for e/γ identification:
– Bremsstrahlung
– Photon conversions
• Constrained with data:
– Photon conversions
– E/p distribution
– Number of e±e± events
9
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
W/Z: lepton ID
10
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:MW 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 Pythia6 normalization to data.
ATLAS-CONF-2012-101
11
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=εData/εMC (=1 for perfect MC) • Easily applied to MC
CDF ATLAS
Loosecuts 85% 88%
Tightcuts 60‐80% ~65%
12
Lepton Momentum Scale
• Momentum scale: – Cosmic ray data used for detailed cell-
by-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
13
Momentum/Energy Scale and Resolution
• Systematic uncertainty on momentum scale: 0.04%
Υ→µµ
Z→µµ
Z→ee
14
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
• Decays
– 17% in muons
– 17% in electrons
– ~65% of τ’s decay hadronically in 1- or 3-prongs
(τ±→π±ν, τ±→π±ν+nπ0 or τ±→3π±ν, τ±→3π±ν+nπ0)
• For reconstruct hadronic taus
– Look for “narrow“ jets in calorimeter (EM + had)
• i.e. measure EM and hadronic radius
(measurement of shower size in η-ϕ):
∑Ecell⋅R2cell/∑Ecell
– Form ΔR cones around tracks
• tau cone
• isolation cone
– associate tracks (1 or 3) 15
Taus
16
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
• 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, γ, µ, jet…)
– Pile-up effects will need to be corrected for17
Missing Transverse Energy
18
Missing Transverse Energy
• Difficulttounderstandquantity
16/12/12 C.8 A. Bettini 19
Larghezze leptoniche W
MW =g
22
8GF
!
"#
$
%&
1/2
=!"
2GF
1
sin#W=
37.3
sin#W GeV
MW
MZ
= cos!W
MW= 80 GeV M
Z! 91 GeV
Le masse (approssimativamente)
Da valore misurato di θW
W. Larghezze leptoniche (uguali per universalità). Per calcolo serve teoria
!e" = !µ" = !#" =g
2
$%&
'()
2
MW
24*=
1
2
GFMW
3
3 2*! 225 MeV
NB. In generale le larghezze dei BI sono proporzionali al cubo della massa
W: Larghezze adroniche
!cd" ! W # cd( ) = 3$ V
cd
2
!e% = 3$ 0.22
2$ !
e% & 33 MeV
mt> m
W! "
td= "
ts= "
tb= 0
Vub<<1 ! "
ub# 0 V
cb<<1 ! "
cb# 0
!us" ! W # us( ) = 3! V
us
2
"e! = 3!0.224
2!"
e! ! 35 MeV
Tre colori
!ud" ! W # ud( ) = 3$ V
ud
2
!e% = 3$ 0.974
2$ !
e% = 2.84 $ !e% & 640 MeV
!cs" ! W # cs( ) = 3$ V
cs
2
!e% = 3$ 0.99
2$ !
e% & 660 MeV
!W" 2.04 GeV
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
Formazione risonante di W Sia W sia Z si possono produrre in formazione con un collisore quark-(anti)quark
⇒ UA1 (CERN). Scoperta nel 1983
s = xqxqsEnergia nel CM dei quark
u + d! e"+!
e
Devono avere lo stesso colore
Devono avere la giusta chiralità
Processo principale da osservare
u + d ! e++"
e
Formazione risonante di W
u + d! e"+#
e
Vicino a risonanza ⇒ Breit e Wigner (come per e+e–)
! ud! e""
e( ) =1
9
3!
s
#ud#e"
s "MW( )
2
+ #W/ 2( )
2
Probabilità che i colori siano uguali
Piccola σmax<<< σtot≈100 mb. Le interazioni deboli sono deboli!
!max ud! e""
e( ) =!max ud ! e+"
e( )
=4!
3
1
MW
2
#ud#e"
#W
2=
4!
3
1
812
0.640$0.225
2.042
GeV-2%& '($388 µb/GeV
-2%& '( ) 8.8 nb
Sezioni d’urto Fascio di p = fascio a larga banda di partoni (q, g, e qualche q) Fascio di p = fascio a larga banda di partoni (q, g, e qualche q)
< x >!M
W
s!M
Z
s! 0.15
OK. Ce ne sono parecchi
Consideriamo l’annichilazione di un quark e un antiquark di valenza se √s=630 GeV, la frazione di quantità di moto che serve per essere in risonanza
Produzione di W da pp La larghezza della banda delle energie dei partoni >> larghezze delle risonanze W e Z Il riferimento del lab. è il cm di pp, non di qq; questa coppia, e la W o Z cui dà origine, hanno un moto longitudinale diverso da caso a caso
s = xdxus
Calcolo di sez. d’urto (incertezze di QCD e di funzioni di struttura) prevedeva a √s=630 GeV:
! pp"W " e#e( ) = 530
+170
$90pb
@ √s=630 GeV <x> = MW/√s≈0.15, quark di valenza dominano sul mare
più analogo da ud
Le sezioni d’urto crescono rapidamente con l’energia e con essa le possibilità di momento longitudinale del bosone
Formazione risonante di W e Z Nel 1978 Cline, McIntire e Rubbia proposero di trasformare l’acceleratore di protoni SpS del CERN in un anello di accumulazione pp nel quale protoni e antiprotoni potevano circolare in versi opposti, nella stessa struttura magnetica (esistente), sfruttando la simmetria CPT
Il grande problema che Rubbia e Van der Meer risolsero fu il “raffreddamento” dei pacchetti di particelle dei fasci a dimensioni abbastanza piccole nel punto di collisione
Nel 1983 si raggiunse la luminosità L=1032 m–2 s–1, sufficiente a scoprire W e Z.
Nel 1983 W e Z furono scoperte
Esercizio. Quanti eventi W→eν e Z →e+e– si rivelano in un anno con luminosità L=1032 m–2 s–1 ed efficienza di rivelazione del 50%?
I segnali La produzione di IVB è un processo raro 10–8 -- 10–9 (σtot(pp)≈ 70 mb = 7×1010 pb ) [l’interazione debole, è debole] Il potere di reiezione del rivelatore deve essere > 1010 Stati finali più frequenti sono qq
!
" #B W $ qq ( ) = 3" #B W $ l% l( ) 3 = numero di colori
Sperimentalmente q ⇒ jet Fondo enorme da
!
gg" gg, gq" gq, gq " gq , qq " qq
Gli stati finali leptonici hanno un S/N più favorevole
W → e νe e isolato, alto pT
W → µ νµ µ isolato, alto pT
Z → e– e+ 2e 2 isolati, alto pT
Z → µ–µ+ 2µ 2 isolati, alto pT
} + ν ad alto pT = grande pT mancante Rivelatore ermetico (UA1 misurava pT mancante con la precisione di qualche GeV)
Importante quantità cinematica misurata: il momento trasverso pT = componente del momento perpendicolare ai fasci
UA1. In costruzione
16/12/12
UA1. Il rivelatore centrale va al museo
16/12/12 C.8 A. Bettini 29
UA1. Prima W
W→eν
L’eliminazione delle tracce con pT< 1 GeV rende completamente pulito l’evento, sopravvivono solo elettrone e il “neutrino”
Nei calorimetri elettromagnetici le W appaiono come un deposito localizzato di energia in direzione opposta al momento mancante
Misura di MW
W→l νl pTe
e
νe
W
LAB
e
νe
W
CM. W
pTe
θ*
pe = mW/2
I momenti trasversi di q e q sono piccoli, quindi lo è anche quello della W.
pTe è il medesimo nei due riferimenti = (mW/2) sin θ*
Distribuzione angolare di decadimento nel CM nota
!
dn
d"*trasf .coordinate
# $ # # # # # dn
dpT=dn
d"*d"*
dpT
!
dn
dpT=
1
mW
2
"
# $
%
& ' 2
( pT2
dn
d)*
Picco “Jacobiano” per pTe = mW/2
Picco “Jacobiano” per pTmissing = mW/2
Il moto trasverso della W (pTW≠0) sbrodola il picco, ma non lo cancella. La misura di mW si
basa sulla misura dell’energia del picco o del fronte di discesa
W’s @ UA1
UA1 MW= 82.7±1.0(stat)±2.7(syst) GeV ΓW<5.4 GeV UA2 MW= 80.2±0.8(stat)±1.3(syst) GeV ΓW<7 GeV
33
W Boson mass • Real precision measurement:
– LEP: MW=80.367±0.033 GeV/c2
– Precision: 0.04%
• => Very challenging!
• Main measurement ingredients:
– Lepton pT
– 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)
34
How to Extract the W Boson Mass
• Uses “Template Method”:
– Templates created from MC simulation for different mW
– Fit to determine which template fits best
– Minimal χ2 ⇒ W mass!
• Transverse mass of lepton and Met
mW=80.4 GeV +1.6 GeV - 1.6 GeV
How to Extract the W Boson Mass
• Alternatively can fit to – Lepton pT or missing ET
• Sensitivity different to different systematics – Very powerful checks in this analysis:
• Electrons vs muons • Z mass • mT vs pT vs MET fits
– The redundancy is the strength of this difficult high precision analysis
36
Hadronic Recoil Model
• Hadronic recoil modeling
– Tune data based on Z’s
– Check on W’s
37
LHC signals of W’s
• 0.2-0.3 pb-1 yield clean signals of W’s and Z’s
[GeV]T m40 50 60 70 80 90 100 110 120
Eve
nts
/ 2.5
GeV
2000
4000
6000
8000
10000
12000
14000
[GeV]T m40 50 60 70 80 90 100 110 120
Eve
nts
/ 2.5
GeV
2000
4000
6000
8000
10000
12000
14000 = 7 TeV)sData 2010 (
! e"W
QCD
!# "W
-1 L dt = 36 pb$ATLAS
Systematic Uncertainties
• Overall uncertainty 60 MeV for both analyses – Careful treatment of correlations between them
• Dominated by stat. error (50 MeV) vs syst. (33 MeV)
Limited by data statistics
Limited by data and theoretical understanding
Wμν
MTWµ
PTZ
µ
Mee
W signal @ LHC
40
W Boson Mass
• World average:
MW=80399 ± 23 MeV • Ultimate Run 2 precision:
~15 MeV
(GeV)Wm80 80.2 80.4 80.6
LEP2 average 0.033±80.376
Tevatron 2009 0.031±80.420
D0 Run II 0.043±80.402
D0 Run I 0.083±80.478
Tevatron 2007 0.039±80.432
CDF Run II 0.048±80.413
CDF Run 0/I 0.081±80.436
World average 0.023±80.399
July 09
W Boson Width
1600 2000 2400
Width of the W Boson
[MeV]W! February 2010
Measurement [MeV]W!
/ dof = 1.4 / 42"
SM* (Preliminary)
CDF-Ia 329±2,032
CDF-Ib 138±2,043
-I#D 172±2,242
CDF-II 72±2,033
-II#D 72±2,034
Tevatron Run-I/II 49±2,046
LEP-2* 83±2,196 42±World Av.* = 2,085
Spin e polarizzazione della W
Nel riferimento del c.m. della W l’energia dell’elettrone >> me. chiralità ≈ elicità
V–A ⇒W si accoppia solo a
fermioni con elicità – antifermioni con elicità +
Mom. ang. Tot. J=SW=1 Jz (iniz.) = λ = –1 Jz’ (fin.) = λ’ = –1
!
d"
d#$ d%1,%1
1[ ]2
=1
21+ cos&*( )
'
( ) *
+ ,
2
N.B. Se fosse stato V+A
!
d"
d#$ d1,1
1[ ]2
= –1
21+ cos%*( )
&
' ( )
* +
2
L’asimmetria avanti-indietro è conseguenza della violazione di P
Per distinguere V–A da V+A sono necessarie misure di polarizzazione dell’elettrone
W!" e
!#e
W Cross Section Results σTh,NNLO=2687±54pb W
• Uncertainties: – Experimental: 2% – Theoretical: 2% – Luminosity: 6%
• Can we use these processes to normalize luminosity? – Is theory reliable enough?
LHC W Cross Sections
• Data agree well with theoretical
expectation
– Uncertainties: 13% (W), 17% (Z)
– Luminosity uncertainty 10%
44
45
WW Cross Section at Tevatron
• WW cross section
– Use W→µν and W→eν
– 2 leptons and missing ET
• Result:
– Data: σ=13.6+-3.1 pb
– Theory: σ=12.4+-0.8 pb
• Campbell, Ellis
Total W production x-section
• Inclusive cross sections:
• Experimental precision at the 1% level, especially for ratio-observables
• Excellent agreement with NNLO QCD, both at 7 and 8 TeV
• Many diff. distributions measured
• Confidence in background predictions for many searches
W charge asymmetries
47
Phys. Rev. D85 (2012) 072004 CMS-PAS-EWK-11-005 arXiv:1206.2598
Sensi&vetochoicesofPDF
|l!|
0 0.5 1 1.5 2 2.5
lA
0.1
0.15
0.2
0.25
0.3
0.35 = 7 TeV)sData 2010 (
MSTW08HERAPDF1.5ABKM09JR09
-1 L dt = 33-36 pb"
Stat. uncertainty
Total uncertainty
ATLAS
W→lν
LHC is a pp-collider: 2 valence up quarks, 1 valence down quark Sea quark Valence quark Since valence quarks have high x ➡ more W+ (u dbar) than W-(ubar d) ( remember that the proton is uud) Here the PDF’ are giving very different predictions
48
(W)
! n
-jets
)"
(W +
!
-310
-210
-110
data energy scale unfolding MadGraph Z2 MadGraph D6T Pythia Z2
CMS
= 7 TeVs at -136 pb
# e$W > 30 GeVjet
TE
inclusive jet multiplicity, n
(n-1
)-jet
s)"
(W +
!
n-je
ts)
"(W
+
! 0
0.1
0.2
1 2 3 4
W/Z + jets : jet multiplicities
jets
) [pb
]je
tN!
(W +
"
1
10
210
310
410 + jets#l$W
=7 TeVsData 2010, ALPGENSHERPAPYTHIABLACKHAT-SHERPA
-1Ldt=36 pb% jets, R=0.4Tanti-k
|<4.4jet y>30 GeV, |Tjetp
ATLAS
jets
) [pb
]je
tN!
(W +
"
1
10
210
310
410
jetNInclusive Jet Multiplicity,
0! 1! 2! 3! 4!
Theo
ry/D
ata
0
1
jetNInclusive Jet Multiplicity,
0! 1! 2! 3! 4!
Theo
ry/D
ata
0
1
jetNInclusive Jet Multiplicity,
0! 1! 2! 3! 4!
Theo
ry/D
ata
0
1
Inclusive Jet Multiplicity Ratio
0!1/! 1!2/! 2!3/! 3!4/!
- 1
jets
)je
tN!
(W +
"
jets
) /
jet
N!(W
+
"
0.1
0.2
0.3
0.4 + jets#l$W
=7 TeVsData 2010, ALPGENSHERPAPYTHIABLACKHAT-SHERPA
-1Ldt=36 pb%
jets, R=0.4Tanti-k|<4.4jet y>30 GeV, |
Tjetp
ATLAS
Inclusive Jet Multiplicity Ratio
0!1/! 1!2/! 2!3/! 3!4/!
- 1
jets
)je
tN!
(W +
"
jets
) /
jet
N!(W
+
"
0.1
0.2
0.3
0.4
Goodagreementwithmul&‐partonmatrixelement+partonshowerpredic&ons
PurepartonshowerMCinadequatefordescribingmul&‐jetfinalstates
Phys. Rev. D85 (2012) 092002 JHEP 01 (2012) 010
normalizedtoσtot
!
" (W+ # n jets)
" (W+ # n -1 jets)
l l )
+ 1
-jet)
!(Z
("
) + 1
-jet)
# l
!(W
("
4
6
8
10
12
14
16 Channels combined!Data e-
Total syst. uncertainty stat. uncertainty$Total syst.
MCFM
-1 Ldt = 33 pb%
ATLAS > 20 GeV
T| < 2.5, p&|
l l )
+ 1
-jet)
!(Z
("
) + 1
-jet)
# l
!(W
("
4
6
8
10
12
14
16
Threshold [GeV]T
Jet p40 60 80 100 120 140 160 180 200
Theo
ry /
Dat
a ra
tio
0.60.8
11.21.4 PYTHIA
ALPGENMCFM
Threshold [GeV]T
Jet p40 60 80 100 120 140 160 180 200
Theo
ry /
Dat
a ra
tio
0.60.8
11.21.4
inclusive jet multiplicity, n
(W)
!(Z
)!
n-je
ts)
"(Z
+
! n
-jets
)"
(W +
!
0
0.5
1
1.5
2
data energy scale unfolding MadGraph Z2 Pythia Z2
CMS
= 7 TeVs at -136 pbe channel
> 30 GeVjetTE
1 2 3 4
X. Wu, SUSY2012, 13/08/12
inclusive jet multiplicity, n
(W)
!(Z
)!
n-je
ts)
"(Z
+
! n
-jets
)"
(W +
!
0
0.5
1
1.5
2
data energy scale unfolding MadGraph Z2 Pythia Z2
CMS
= 7 TeVs at -136 pb channelµ
> 30 GeVjetTE
1 2 3 449
Jetenergyscalesystema&cmostlycancelsout
Measurementswithsmallsystema&cuncertainty
Phys. Lett. B708 (2012) 221-240 JHEP 01 (2012) 010
W + jets/Z + jets : ratio and double ratio
!
" (W (# l$ )+1 jet)
" (Z(# ll)+1 jet)!
" (W+ # njets)
" (Z+ # njets)
" (Z)
" (W )echannel
µchannel
• Importance of diboson production • Sensitive to Triple Gauge Couplings (TGC): fundamental test of SM • WWγ and WWZ allowed; ZZγ, Zγγ 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 pT (short distances) • This is a region where the background from the right diagram is small
Diboson (WW, WZ, ZZ, Wγ, Zγ) production
50
Thisprocesshas… ..thisasabackground
Diboson (WW, WZ, ZZ)
51
ATLAS-CONF-2012-025
ATLAS-CONF-2012-090
arXiv:1208.1390
WW production
52
Measure cross section in WW→2l2n channel → Need good calibration for missing ET
)[GeV]Tmiss(llETm
50 100 150 200 250 300 350
Even
ts /
20G
eV
0
50
100
150
200
250
300
350
400
450DataDibosonW+jet/dijettopDrell-Yan
!l!l"WWstat+syst#
ATLAS Preliminary-1Ldt = 4.70fb$
= 7TeVs
8TeV,CMS(3.54T−1)7TeV,ATLAS(4.7T−1)/CMS(4.92T−1)
Alreadysystema&cslimited!
!
ATLAS:53.4 ± 2.1(stat)± 4.5(syst)± 2.1(lumi) pb
CMS: 52.4 ± 2.0(stat)± 4.5(syst)±1.2(lumi) pb
NLO : 45.1± 2.8 pb
69.9± 2.8(stat)± 5.6(syst)±3.1(lumi) pb
NLO: 57.3!1.6
+2.4 pb
ATLAS-CONF-2012-025 CMS PAS SMP-12-005 CMS PAS SMP-12-013