Calibration of centre-of-mass energies at LEP2 for a precise measurement of the W boson mass
W mass and widthEmily Nurse0. W mass and widthEmily Nurse1 Overview Standard Model Precision...
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Transcript of W mass and widthEmily Nurse0. W mass and widthEmily Nurse1 Overview Standard Model Precision...
W mass and width Emily Nurse 1
W mass and width Emily Nurse 2
Overview
• Standard Model Precision Measurements– Motivation for W mass and width measurements
• The Tevatron and CDF– W and Z production– W and Z reconstruction at CDF
• Analysis Strategy and Measurement steps• Results and implications
W mass and width Emily Nurse 3
Standard Model
The Standard Model (SM) describes the Universe’s fundamental building blocks and their interactions.
Comparisons of predictions with experimental data have successfully tested the theory to a high precision but some questions remain un-answered.
What’s the origin of particle mass? (SM Higgs?)
Is the SM the full story? (SUSY?, extra-dimensions?, …??)
W mass and width Emily Nurse 4
Discovering new physics
Direct Discovery of New Particles
Precision Measurementsof SM Parameters
80 180 280
Reconstructed Mass (GeV)
W mass and width Emily Nurse 5
Testing the SM - W and Z bosons
• The W and Z bosons were predicted by Glashow, Salam and Weinberg’s electroweak theory in the 1960s discovered by the UA1/UA2 experiments in 1983, with masses (MW
and MZ) consistent with the tree level predictions. • Current SM calculations make very accurate predictions of
MW and MZ and the widths (W and Z) including higher order radiative corrections (i.e. through remormalisation of SM parameters). LEP experiments measure MZ=91187.6 2.1 MeV (0.002%) and
Z=2495.2 2.3 MeV (0.09%). LEP2 and Tevatron experiments measure MW=80403 29 MeV
(0.04%) and W=2141 41 MeV (1.9%).
prior to these results
W mass and width Emily Nurse 6
Testing the SM - W mass
rW: radiative corrections dominated by tb and Higgs loops
we can constrain MH by precisely measuring MW and Mt
known to 0.015%
known to 0.0009% MZ known to 0.002%
MW known to 0.036%
GF is found from muon lifetime measurements and can be predicted in terms of MW
(tree level)
Write g in terms of and cosw=MW / MZ and rearrange:
rW could also have contributions from new particle loops
+e+e
W+
g
g
W mass and width Emily Nurse 7
Testing the SM - W width
Within the SM W is predicted by summing leptonic and hadronic partial widths:
(Note: Most higher order corrections are absorbed in the experimental values of MW and GF.)
€
W = 3ΓW0 + 3KQCD Vqq '
2
|no top|
∑ ΓW0
W0 = (We) is precisely predicted in terms of MW and GF :
PDG: J. Phys. G 33, 1
• Measuring W tests this accurate SM prediction (deviations of which suggest
non-SM decay modes).
• W is an input to the MW measurement: MW~W / 7.
W = 2091 2 MeV
predominantly from MW
W mass and width Emily Nurse 8
The Tevatron
The Tevatron currently has ~2.5 fb-1 on tape (6-8fb-1 expected by the end of Run II).The Tevatron is a W/Z factory (as well
as many other things!) : (Wl) ~ 2700 pb (currently ~7 million created, ~0.9 million to analyse). (Zll ) ~ 250 pb (currently 0.7 million created, ~40 thousand to analyse).
But : precision measurements are hard! We need a “precision level” calibration of our detector to keep systematics low. These analyses are based on 200/350pb-1 of CDF data.
W mass and width Emily Nurse 9
W and Z production at the Tevatron
The large masses (~100 GeV ) of W and Z bosons gives their decay products large pT.The electron and muon channels are used to measure W properties, due to their clean experimental signature.
LEADING ORDER
Similar for Z production (decays into two charged leptons)
W events: Charged lepton is detected and momentum directly measured. Neutrino cannot be detected! Transverse momentum (pT) is inferred by a vector sum of the total “transverse energy (Esin)” in the detector. The “missing ET
(ETmiss)” is found by constraining the sum to zero interpreted as the neutrino pT.
Z events: Both charged leptons are detected and their momenta measured.
W mass and width Emily Nurse 10
W and Z production at the Tevatron
Initial state gluon radiation from incoming quarks gives the W a boost in the transverse direction W pT
The recoiling gluons form hadrons that are detected in the calorimeter Hadronic recoil
HIGHER ORDER CORRECTIONS
Final state radiation affects the kinematics of the charged lepton
Goes into ETmiss = pT
measurement!
W mass and width Emily Nurse 11
e
Detecting particles at CDF
SILICON
DRIFT CHAMBER SOLENOID
EM HADRONIC MUONCHAMBERS
Electrons: detected in central trackers (drift chamber provides p measurement) and EM calorimeter (provides energy measurement).
Muons: detected in central trackers (drift chamber provides p measurement), calorimeter (MIP signal) and muon chambers.
ETmiss : Hadronic recoil found by summing the EM
and HADRONIC calorimeter energy.
TRACKERS CALORIMETERS
€
δpT / pT ≈ 0.0005 × pT [GeV/c; beam constrained]; η <1
€
δET / ET ≈ 13.5%/ ET ⊕ 1% η <1.1
W mass and width Emily Nurse 12
Analysis strategy: measuring MW and W
• Ideal world: MW and W would be reconstructed from from the invariant mass of the W decay products (Breit-Wigner lineshape of propagator peaks at the mass and has an intrinsic width).• Reality: The neutrino is not detected thus the invariant mass cannot be reconstructed. Instead we reconstruct the transverse mass.
€
mT = 2pTl pT
ν (1− cosφlν )
-channel: central trackere-channel: EM calorimeter
inferred from missingtransverse energy
MW
W
Breit-Wigner:
W mass and width Emily Nurse 13
Analysis strategy: measuring MW and W
• MW/W found from MT MC template fits data.
• Simulate MT distribution with a dedicated fast parameterised MC.• Utilise well understood data samples (Z events used extensively) to calibrate detector simulation to high precision - we need an excellent description of the lineshape! - W fit range: 90 -200 GeV
- MW fit range:65 - 90 GeV
W templates
MW templates
W mass and width Emily Nurse 14
MW vs W
• The MW and W analyses are very similar - with different
dominant uncertainties.• They are performed independently using 200(350)pb-1 of
data for the MW (W) analyses .
• As I describe the the measurement steps I will discuss the method used in the analysis for which the effect is more important:
MW =
W =
W mass and width Emily Nurse 15
Measurement Steps
Muon momentum measurement:
Electron energy measurement: ppTT
= E = ETTmissmiss = -(U + p = -(U + pTT
leplep))
Hadronic recoil measurement:
Generator effects:PDFs, QCD, QED corrections.
Backgrounds:
€
mT = 2pTl pT
ν (1− cosφlν )
W mass and width Emily Nurse 16
Measurement Steps : 1
Muon momentum measurement:
Electron energy measurement: ppTT
= E = ETTmissmiss = -(U + p = -(U + pTT
leplep))
Hadronic recoil measurement:
Generator effects:PDFs, QCD, QED corrections.
Backgrounds:
€
mT = 2pTl pT
ν (1− cosφlν )
W mass and width Emily Nurse 17
Generator effects : PDFs
€
x p
€
x p
€
l
€
Parton Distribution Functions (PDFs) are parameterised functions that
describe the momentum distribution of quarks in the (anti)proton.
Different PDFs result in different acceptance and spectra:
Use CTEQ6M and the CTEQ6 ensemble of
2x20 error PDFs (20 orthogonal
parameters varied up and down within
their errors).
MW = 11 MeV, W = 17 MeV
W mass and width Emily Nurse 18
Generator effects : QCD/QED corrections
• Simulate QCD corrections (initial state gluon radiation) using RESBOS [Balazs et.al.
PRD56, 5558]: NLO QCD + resummation + non-perturabtive.
• Constrain non-perturbative parameter using our own Z data :
• QED bremsstrahlung reduces l pT
• Simulated at NLO (one-) using Berends&Kleiss [Berends et.al. ZPhys. C27,
155] / WGRAD [Baur et.al. PRD59, 013002].• PHOTOS [Barberio et.al. Comput. Phys. Comm., 66, 115] used to establish systematic due to neglecting NNLO (two-) terms.
MW = 3 MeV, W = 7 MeV
MW () = 12 MeV, W () = 1 MeV MW (e) = 11 MeV, W (e) = 8 MeV
W mass and width Emily Nurse 19
Measurement Steps : 2
Muon momentum measurement:
Electron energy measurement: ppTT
= E = ETTmissmiss = -(U + p = -(U + pTT
leplep))
Hadronic recoil measurement:
Generator effects:PDFs, QCD, QED corrections.
Backgrounds:
€
mT = 2pTl pT
ν (1− cosφlν )
W mass and width Emily Nurse 20
Momentum scale set with di-muon
resonance
peaks in data, using well known particle
masses: J/ ; 1S Z
Lepton momentum calibration (pT)
M (GeV)
M (GeV)
DataMC
DataMC
MW () = 17 MeV, W () = 17 MeV
p scale known to 0.021%
1/<pT>(GeV-1)
W mass and width Emily Nurse 21
Lepton momentum resolution (pT)
fullMC = (q/pT)meas - (q/pT )gen taken from full GEANT MC.
• Sample this histogram and multiply by a constant parameter: fastMC = SresfullMC
• Sres found by tuning to M in
Z data.
MW () = 3 MeV, W () = 26 MeV
W mass and width Emily Nurse 22
Measurement Steps : 3
Muon momentum measurement:
Electron energy measurement: ppTT
= E = ETTmissmiss = -(U + p = -(U + pTT
leplep))
Hadronic recoil measurement:
Generator effects:PDFs, QCD, QED corrections.
Backgrounds:
€
mT = 2pTl pT
ν (1− cosφlν )
W mass and width Emily Nurse 23
The electron’s journey through CDF
energy leakage “out the back” of the EM calorimeter
energy loss in solenoid
bremsstrahlung in silicon
track momentum measurement in COT
energy measurement in EM calorimeter
W mass and width Emily Nurse 24
Scale and found in two independent ways:
1) Fit to Mee peak in Zee data using well known Z mass/width.
2) Fit to E/p in We data (since p has already been well
calibrated.)
Electron energy calibration/resolution (pTe)
electron energy measured in EM calorimeter
electron momentum measured in central tracker
Fundamentally E = p (electron mass is negligible).
Photons are emitted from electron (bremsstrahlung) which reduces p.
The photons usually end up in the same calorimeter tower as theelectron thus E doesn’t decrease.
€
E
€
p
Calorimeter scale: Emeas = scale Etrue
Calorimeter resolution: (E) / E = 13.5% / √ET
W mass and width Emily Nurse 25
Bremsstrahlung in
tracker
E
p
MW (e) = 30 MeV, W (e) = 17 MeV scale: resolution:
Mee (GeV) E/p
DataMC
DataMC
E scale known to 0.034%
Leakage “out the
back” of the EM
calorimeter
MW (e) = 11 MeV, W (e) = 31 MeV
Electron energy calibration/resolution (pTe)
W mass and width Emily Nurse 26
Measurement Steps : 4
Muon momentum measurement:
Electron energy measurement: ppTT
= E = ETTmissmiss = -(U + p = -(U + pTT
leplep))
Hadronic recoil measurement:
Generator effects:PDFs, QCD, QED corrections.
Backgrounds:
€
mT = 2pTl pT
ν (1− cosφlν )
W mass and width Emily Nurse 27
Hadronic Recoil: U
• To get pT we need a good model of the total
energy in W events.• U=(Ux, Uy)=towersEsin (cos, sin)• Vector sum over calorimeter towers
Excluding those surrounding lepton
• Recoil has 3 components:
ppTT = E = ETT
missmiss = -(U + p = -(U + pTTleplep))
(3) Underlying energy Multiple interactions and remnants from collision.
(2) Bremsstrahlung Photons emitted by lepton that do not end up in the excluded region
(1) QCD Gluons recoiling off
the boson
W mass and width Emily Nurse 28
Hadronic Recoil: U
U1U2
• Accurate predictions of U is difficult (and slow) from first principles.
• U simulated with ad-hoc parameterised model, tuned on Zll data.
• U split into components parallel (U1) and
perpendicular (U2) to Z pT
• 7 parameter model describes the response and resolution in the U1 and U2 directions as a function of the Z pT.
• Systematic comes from parameter uncertainties (limited Z stats).
Zee Zee
response resolution
DataMC
MW () = 12 MeV
W () = 49 MeV
MW (e) = 14 MeV
W (e) = 54 MeV
W mass and width Emily Nurse 29
Measurement Steps : 5
Muon momentum measurement:
Electron energy measurement: ppTT
= E = ETTmissmiss = -(U + p = -(U + pTT
leplep))
Hadronic recoil measurement:
Generator effects:PDFs, QCD, QED corrections.
Backgrounds:
€
mT = 2pTl pT
ν (1− cosφlν )
W mass and width Emily Nurse 30
multijet
muon channel only:
• Jet fakes/contains a lepton• ET
miss from misconstruction
electron and muon channel:
Backgrounds
Zll• One lepton lost• ET
miss from missing lepton
W• decays to e/• intrinsic ET
miss
Decay In-Flight
xxx
x
xx
x
x
K,fake
high-pT track
• Kaon/pion decays “In-Flight” to a .• Kink in track gives high-pT measurement. • ET
miss from mis-measured track pT.
Need the mT distributions and the
normalisations!
W mass and width Emily Nurse 31
• Dominant background is Z (it’s easy to lose a muon leg!) - but we can estimate this background very reliably (using full MC).
• Decay In-Flight (DIF) has large mT
tails: problematic for the width!
Backgrounds: Muon channel
fin
al
cu
t v
alu
e
/NDF
The handles we have on DIF are track quality 2
track and impact parameter.
Fractional background found from a template fit to the 2
track distribution. Z provides the signal template
High impact parameter cuts provide the DIF templateMW () = 9 MeV, W () = 33 MeV
W mass and width Emily Nurse 32
Backgrounds: Electron channel
fin
al
cu
t v
alu
e
• Multijet has large mT tails: problematic for
the width!
MW (e) = 8 MeV, W (e) = 32 MeV
Fractional multijet background found from a template fit to the ET
miss distribution.
“Anti-electron” sample provides the mulitjet template
W mass and width Emily Nurse 33
Measurement Steps
Muon momentum measurement:
Electron energy measurement: ppTT
= E = ETTmissmiss = -(U + p = -(U + pTT
leplep))
Hadronic recoil measurement:
Generator effects:PDFs, QCD, QED corrections.
Backgrounds:
€
mT = 2pTl pT
ν (1− cosφlν )
W mass and width Emily Nurse 34
Results: MW fits
DataMC
DataMC
Also includes fits to pTl and pT
:
MW = 80413 48 (stat + syst) MeV
MT() (GeV) MT(e) (GeV)
W mass and width Emily Nurse 35
MW systematic uncertainties
W mass and width Emily Nurse 36
MW : world average
Central value increases by 6 MeV:
80392 80398 MeV
Uncertainty reduced by 15%:
29 25 MeV
World’s most precise single measurement!
W mass and width Emily Nurse 37
MW : Implications
€
mH = 85−28+39 GeV
mH <166 GeV @ 95% C.L.
€
mH = 80−26+36 GeV
mH <153 GeV @ 95% C.L.
Previous World Data :
Including New MW :
Direct search from LEP II :
mH > 114.4 GeV @ 95% C.L.
Including New Mt :
W mass and width Emily Nurse 38
Results: W fits
W = 2032 71 (stat + syst) MeV
W mass and width Emily Nurse 39
W systematic uncertainties
W mass and width Emily Nurse 40
W : world average
Central value decreases by 44 MeV:
2139 2095 MeV
Uncertainty reduced by 22%:
60 47 MeV
World’s most precise single measurement!
W mass and width Emily Nurse 41
Indirect Width Measurement
R = W
Z (Zll)
(Wl)
(W)
(Z) X X
Rexp = BR (Wl)
BR (Zll) Precision LEP Measurements
SM CalculationNNLO Calculation
CDF Run II INDIRECT width :
2092 ± 42 MeVPRL 94, 091803
CDF Run II DIRECT width :
2032 ± 71 MeVpreliminary
W mass and width Emily Nurse 42
Projections
20 MeV syst limit
2.5fb-1 : ~25 MeV ~35 MeV
Naïve statistical scaling, 20 MeV syst. limit
W mass and width Emily Nurse 43
Summary• Two new measurements from CDF:
– W mass : 80413 ± 48 MeV (stat + syst)
– W width : 2032 ± 71 MeV (stat + syst)• Both are the world’s most precise single measurements!!
• Getting to this point requires a “precision” level calibration of the detector.
• Together with direct Higgs searches we will continue to squeeze the phase space available to the SM Higgs.
• Analyses utilised 200 pb-1 and 350 pb-1 respectively, both CDF and DØ already have ~2.5 fb-1 on tape.
• Working on improved mass/width measurements to further test the SM and constrain mH
W mass and width Emily Nurse 44
Back-up slides…
W mass and width Emily Nurse 45
W mass
€
MW ∝ M top2
€
MW ∝ ln M H
e
e
GF (fermi-coupling constant) can be predicted in terms of MW
W width: W’ analysis excludes W’ < 788 GeV
W mass and width Emily Nurse 46
Generator effects : PDFs
9 - highx valence quarks.
∆XW = 0.5*√ (∑i ([∆i
up-∆idown) 2 ))/1.6
W mass and width Emily Nurse 47
Generator effects : QCD corrections
RESBOS: NLO QCD + resummation + non-perturbative.
Collins-Soper-Sterman (CSS) resummation formalism.Sums LL terms + sub-logs
Brock-Landry-Nadolsky-Yuan (BLNY) form:
exp [-g1 - g2ln(Q/2Q0) - g1g3ln(100x1x2)]b2
g1 =0.210.01; g1 =0.68+0.01-0.02; g3 =-0.6+0.05
-0.04; From fits to R209, E288, E605 fixed target Drell-Yan data (5< shat <18 GeV) + CDF RunI
€
g2 = 0.64 ± 0.05
b = −0.0014 ± 0.0010 GeV -1
€
dσ
dpTZ
~ (1+ B ⋅ pTZ ) × f (g1,g2,g3)RESBOS
W mass and width Emily Nurse 48
Simulated effects
• Bremsstrahlung in si (Bethe-Heitler equation)
– Migdal suppression
• Conversions (Bethe-Heitler equation)
• Compton scattering (for low energy photons: scattering off e ~ conversions)
• Ionisation energy loss
• Energy loss in coil
• Leakge into HAD calorimeter
• Acceptance
• Ionisation energy loss
• Multiple scattering
• Acceptance
Electrons Muons
W mass and width Emily Nurse 49
log10(incident electron energy)
visible EM energy fraction
Simulating Electrons () : CAL
superconducting coil
electromagnetic cal.
hadronic calorimeter
Soft electrons suffer absorption
in the coil
Energetic electrons leak into the
hadronic compartment
€
Ee ~ 100 MeV
€
Ee ~ 100 GeV
W mass and width Emily Nurse 50
We
Hadronic Recoil: U (pT)
U||
U • U split into components parallel (U||) and
perpendicular (U) to charged lepton.
• Many distributions used to cross check the model in Wl data:
DataMC
We
€
< u1
>= (P1
+ P2p
T) ∗(1 − e (−P3 ×pT ))
€
(u1) = (P
4+ P
5pT) ∗M
1∑E
T
M2
W mass and width Emily Nurse 51
Electron mT signed plot
W mass and width Emily Nurse 52
pT and ETmiss fits
W mass and width Emily Nurse 53
pT and ETmiss fits
W mass and width Emily Nurse 54
MSSM parameter range
Decoupling limit with SUSYmasses of order 2 TeV.
Moderate splitting between stop andsbottom doublets (m2/m2 <2.5)