4/24/2002ulrich heintz - neppsr 20021 the standard model ulrich heintz boston university new england...
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4/24/2002 Ulrich Heintz - NEPPSR 2002 1
The Standard Model
Ulrich HeintzBoston University
NEW ENGLAND PARTICLE PHYSICS STUDENT RETREAT
North Woodstock, New Hampshire August 18-23, 2002
4/24/2002 Ulrich Heintz - NEPPSR 2002 2
wanteda unified theory to explain
the fundamental constituents of matterquarks & leptons
their interactionsgravityelectromagnetismweak interactionstrong interaction
4/24/2002 Ulrich Heintz - NEPPSR 2002 3
Outlinethe standard modelexperimental confirmationprecision measurementsdoes it fit?future prospectsare we done?summary
4/24/2002 Ulrich Heintz - NEPPSR 2002 4
internal symmetriesall particles have three internal symmetries
U(1) invarianceweak hypercharge Y=2(Q-T3)
gauge boson B - coupling = g1
SU(2) invariance weak isospin T3
gauge bosons W+ W0 W- - coupling = g2
SU(3) invariancecolor
gauge bosons G1...8 (gluons) - coupling = g3
4/24/2002 Ulrich Heintz - NEPPSR 2002 5
principle of local gauge invarianceglobal gauge transformations
observables can only depend on ||2
theory is invariant under ’= e-i U(1) and ’ satisfy the Schrödinger equation
local gauge transformations to get invariance under (x,t) ~(x,t) = e-i(x,t) (x,t) replace
electromagnetic interaction spin 1 particle photon (gauge boson)
ti
m
2
2
1
ieVtt
Aie
4/24/2002 Ulrich Heintz - NEPPSR 2002 6
electroweak mixingneutrinos have no electromagnetic
interactions U(1) cannot be the electromagnetic U(1) define electromagnetic field A / g2 B + g1 W0
the orthogonal combination is Z0 / -g1 B + g2 W0
or with g1 / sinw and g2 / cosw
there must be a neutral current interaction mediated by the Z0 boson
wwe
ww
ww
ggg
W
B
Z
A
sincos4
cossin
sincos
21
00
4/24/2002 Ulrich Heintz - NEPPSR 2002 7
particle massesexplicit mass terms break SU(2) symmetryHiggs mechanism
add complex scalar field
SU(2) doublet, SU(3) singlet
3 components absorbed by W§ and Z0
1 physical particle Higgs boson H0
couplings to fermions fermion masses
43
210 2
1
i
i
4/24/2002 Ulrich Heintz - NEPPSR 2002 8
Higgs potential ground state degenerate at finite
carries SU(2) and U(1) quantum numbers SU(2) and U(1) symmetries spontaneously broken invariant under U(1)EM photon massless
V()
3
1,2,4
spontaneous symmetry breaking
v
0
2
10
4/24/2002 Ulrich Heintz - NEPPSR 2002 9
Glashow-Weinberg-Salam modelthree massive bosons: W+,Z0,W-
one massless boson:
unifies weak and electromagnetic forces
W
e
e
W
e
eZ0
e,e
e,e
22Wg
22Wg
2Zg
e
e
eg
wZWewwZwW MMggg 222 cos4cossinsin
4/24/2002 Ulrich Heintz - NEPPSR 2002 10
parity violation in weak interactionshandedness
W couples only to lefthanded particles
lefthanded
righthanded
W-e e-
W- ee-
4/24/2002 Ulrich Heintz - NEPPSR 2002 11
Glashow-Weinberg-Salam modelW couples only to lefthanded fermion doublets
Z couples differently to left/righthanded fermions
couples equally to left/righthanded fermions
LLLeLLL
e
b
t
s
c
d
u
wRwL
wRwL
wRwL
RL
ccd
ccu
cc
cc
22
22
22
sin3
2sin
3
21
sin3
4sin
3
41
sin2sin21
01
all left-handed fermions are SU(2) doublets
all right-handed fermions are SU(2) singlets
all left-handed fermions are SU(2) doublets
all right-handed fermions are SU(2) singlets
4/24/2002 Ulrich Heintz - NEPPSR 2002 12
Quantum Chromo Dynamicsgauge theory of the strong
interactions 8 massless gauge bosons (gluons)
g
q
q
3gg
g
g
all leptons are SU(3) singlets
all quarks are SU(3) triplets
all leptons are SU(3) singlets
all quarks are SU(3) triplets
4/24/2002 Ulrich Heintz - NEPPSR 2002 13
standard modelinput parameters of the standard model
3 gauge couplings: gU(1) gSU(2) gSU(3)
1 Higgs vev: v or MZ or MW
9 fermion masses: me m m mu md ms mc mb mt
4 CKM parameters: 1 2 3
1 Higgs mass: MH or 18 free parametersneutrino masses and mixing parameters
4/24/2002 Ulrich Heintz - NEPPSR 2002 14
Outlinethe standard modelexperimental confirmationprecision measurementsdoes it fit?future prospectsare we done?summary
4/24/2002 Ulrich Heintz - NEPPSR 2002 15
experimental confirmationweak neutral currents
observed at CERN in 1973
ratio of rates depends on
“charged current”
W§
e e
“neutral current”
Z0
e e
4.0sin
cos
2
w
wW
Z
g
g
4/24/2002 Ulrich Heintz - NEPPSR 2002 16
experimental confirmation
e+e-
e- e+e-
4/24/2002 Ulrich Heintz - NEPPSR 2002 17
experimental confirmationvector boson masses
prediction
observed at CERN in 1983
GeV 93sin
1
24sin
8
2
2
2
1
2
2
wF
W
wW
W
WF
GM
g
M
gG
GeV 590
GeV 580
Z
W
M
M
4/24/2002 Ulrich Heintz - NEPPSR 2002 18
Outlinethe standard modelexperimental confirmationprecision measurements
properties of the Z boson ( e+e- colliders) properties of the W boson ( pp colliders) the top quark neutrino couplings ( fixed target experiment)
does it fit?future prospectsare we done?summary
4/24/2002 Ulrich Heintz - NEPPSR 2002 19
LEP/SLDcolliding e+ and e- beamsbeam energy ¼ 46 GeVproduce Z0 bosons
LEP 4 detectors: ALEPH, DELPHI, L3, OPAL recorded 17£ 106 Z0 decays (1990 – 1995)
SLC SLD detector recorded 550,000 Z0 decays (1993 – 1998)
Z0
e
e
4/24/2002 Ulrich Heintz - NEPPSR 2002 20
Large Electron-Positron collider at CERN
8.5 km
L3
Aleph
Delphi
Opal
CERN
4/24/2002 Ulrich Heintz - NEPPSR 2002 21
Stanford Linear Collider
¼ 1.5 km
SLD
4/24/2002 Ulrich Heintz - NEPPSR 2002 22
Z0 lineshape measurementsZ0 lineshape
MZ=91.1876±0.0021 GeV
Z = 2.4952±0.0023 GeV
h = 41.541±0.037 nb
Re=had/ee=20.804±0.050
R=had/=20.785±0.033
R=had/=20.764±0.045
consistent with lepton universality
4/24/2002 Ulrich Heintz - NEPPSR 2002 23
partial widths of Z0
visible decay modes had = 1744.4±2.0 MeV
ℓℓ = 83.984±0.086 MeV
invisible width inv = Z -had -3£ℓℓ =499.0±1.5 MeV
inv/ℓℓ = 5.942±0.016
standard model prediction /ℓℓ = 1.9912±0.0012
3 light neutrinos
4/24/2002 Ulrich Heintz - NEPPSR 2002 24
many ways to measure sin2 w
left/right asymmetry ALR
Z couples differently to left/right handed fermions expect parity violating asymmetry
couplings depend on sin2w ALR depends on sin2w
only possible at SLC (e- polarization ¼ 75%)
0
RL
RLLRA
Z0 e-e+ Z0 e-e+
L R
4/24/2002 Ulrich Heintz - NEPPSR 2002 25
many ways to measure sin2 w
forward/backward asymmetries
requires no beam polarization
depends on sin2wBF
BFFBA
Z0 e-e+
Z0 e-e+
forward backward
L L
L R
R R
R L
F B
4/24/2002 Ulrich Heintz - NEPPSR 2002 26
many ways to measure sin2 w
sin2eff
differs from sin2w by radiative corrections
agreement between all measurements
4/24/2002 Ulrich Heintz - NEPPSR 2002 27
radiative correctionsthe gauge sector of the standard model
is determined (at tree level) by sin2 w , MZ, ) gW, gZ, MW, AFB,...
loops introduce additional dependencies
MW = MWtree (1 + O(mt
2) + O(logmH))similarly for other quantities
W W
t
b
W W
H0
4/24/2002 Ulrich Heintz - NEPPSR 2002 28
precision measurementsglobal electroweak fit
are all measurements consistent with one set of parameters?
all Z-pole data:
%)12(
10/3.15
GeV 033.0372.80
00063.022315.0sin
GeV 82
GeV 10171
2
2
10941 dof
M
m
m
W
w
H
t
4/24/2002 Ulrich Heintz - NEPPSR 2002 29
what about the W?W§ is charged
cannot produce (singly) in e+ e- collisions need beam energy > MW for e+e-! W+W-
hadron colliders...
4/24/2002 Ulrich Heintz - NEPPSR 2002 30
Tevatron at Fermilabcolliding p and p beamsbeam energy ¼ 900 GeVproduce W§ and Z0 bosons and more...2 detectors: CDF and DØbetween 1992 and 1995
recorded 105 W and 104 Z0 decays
4/24/2002 Ulrich Heintz - NEPPSR 2002 31
Tevatron at Fermilab
2 km
DØCDF
4/24/2002 Ulrich Heintz - NEPPSR 2002 32
CDF Detector
installation of silicon detector in CDF, 2001
4/24/2002 Ulrich Heintz - NEPPSR 2002 33
DØ Detector
DØ detector installed in the Collision Hall, January 2001
4/24/2002 Ulrich Heintz - NEPPSR 2002 34
DØ detector
4/24/2002 Ulrich Heintz - NEPPSR 2002 35
protons/antiprotons are not elementary de Broglie wavelength of 900 GeV
protons
quark-antiquark and quark-gluon scattering W
hadron colliders
m10 proton theof size m10 15-18
E
hc
u
u
d
u
u
d
4/24/2002 Ulrich Heintz - NEPPSR 2002 36
W and Z production in proton-antiproton collisions
W or Z are not the only particles produced underlying event
quarks have a distribution of momenta W or Z momentum not known
u and d quarks participate both W and Z can be produced
hadron colliders
Z0
u
uW
u
d
parton distribution functions
4/24/2002 Ulrich Heintz - NEPPSR 2002 37
W and Z decays
hadron colliders
W! ud, cs 68.5% swamped by pp! qq, gg
W! 10.5% !/e/+’s
W! 10.5% momentum resolution
W! e 10.5% best
Z! qq 69.9% swamped by pp! qq, gg
Z! 20.0% invisible
Z!+ - 3.4% !/e/+’s
Z!+ - 3.4% momentum resolution
Z! e+ e- 3.4% best
4/24/2002 Ulrich Heintz - NEPPSR 2002 38
anatomy of a W! e event
hadron colliders
e
p
pT
pz
proton beam
antiproton beam
pZ
precoil
Wmeasure
electronsneutrinosrecoil (everything else)
DØ detector
4/24/2002 Ulrich Heintz - NEPPSR 2002 39
neutrino signatureneutrino
measurenothing
momentum conservation
TrecoilT
eTT
recoilTT
eT
recoilzrecoilepp
pppp
ppp
pppppp
missing
0
knowt don'but 0
4/24/2002 Ulrich Heintz - NEPPSR 2002 40
W mass measurementW boson
measure
MT is not invariantdepends on pT
W and parton distribution functions
detector acceptances
need a detailed model ofW boson productiondetector
2222
WWT
WTT
TeT
WT
TeT
WT MpEM
ppE
ppp
Z mass measurement
why transverse mass?
4/24/2002 Ulrich Heintz - NEPPSR 2002 41
W mass fitmaximum likelihood fit
MW=80.45§0.07 GeV
4/24/2002 Ulrich Heintz - NEPPSR 2002 42
systematic uncertaintiesinputs to MC model affect measurement
parton distribution functions pT distribution of W radiative corrections to W decay detector model
general procedure constrain input parameters vary parameters within the allowed range determine the effect on the mass measurement
(D0) GeV 084.0483.80
(CDF) GeV 079.0443.80
W
W
m
m(D0) GeV 084.0483.80
(CDF) GeV 079.0443.80
W
W
m
m
4/24/2002 Ulrich Heintz - NEPPSR 2002 43
LEP-II1996-2000 LEP operated above WW
threshold s = 161-209 GeVW+W- pair production
e
e
Z
W
W
e
e
W
W
e
e
W
W
4/24/2002 Ulrich Heintz - NEPPSR 2002 44
W mass at thresholdW mass
= 80.40 GeV § 0.20 (stat) § 0.07 (syst) § 0.03 (beam)
from WW at s = 161 GeV
4/24/2002 Ulrich Heintz - NEPPSR 2002 45
W mass from direct reconstructionWWqqqq
reconstruct 4 jets 5C fit (E, p, m1 = m2)
3 possible jet pairingscombinatoric
background
quarks are created so close together that their fragmentation is not independent
color-reconnectionBose-Einstein
correlations mW = 80.457 § 0.030(stat) § 0.054(syst) GeVmW = 80.457 § 0.030(stat) § 0.054(syst) GeV
4/24/2002 Ulrich Heintz - NEPPSR 2002 46
W mass from direct reconstructionWWqqℓ
neutrino not detected
2C fit (E, p, m1=m2)
mW = 80.448 GeV § 0.033(stat) GeV § 0.028(syst) GeV
DELPHI
4/24/2002 Ulrich Heintz - NEPPSR 2002 47
world average
4/24/2002 Ulrich Heintz - NEPPSR 2002 48
Tevatron tail of mT spectrum
W =2.115§0.105 GeV
LEP-II width of invariant
mass distributions W =2.150§0.091
GeV
direct measurement of W width
DØ
4/24/2002 Ulrich Heintz - NEPPSR 2002 49
Tevatron indirect measurement from ratio of B
W = 2.171§ 0.052 GeV
indirect measurement of W width
standard model LEP
eW
eWW
eeZW
ZeW
eeZZ
eWW
B
BRBB
BR
4/24/2002 Ulrich Heintz - NEPPSR 2002 50
W width
4/24/2002 Ulrich Heintz - NEPPSR 2002 51
gauge boson self couplingsthe SU(2)£U(1) theory predicts
seven Lorentz-invariant amplitudes each five constrained by electromagnetic
gauge invariance and neutron dipole moment
constrain two couplings, , Tevatron: W, WW, WZ production LEP-II: WW production
Z
W
W
W
W
4/24/2002 Ulrich Heintz - NEPPSR 2002 52
gauge boson self couplingsin standard
model = 1 ( =0) = 0
if W have only electromagnetic couplings = 0 ( = -1) = 0 excluded
Delphi, L3, Opal
95% cl
68% cl
standard model
4/24/2002 Ulrich Heintz - NEPPSR 2002 53
the top quarkdiscovered in 1995 by CDF and DØ
completes the 3rd generation mass is large!
4/24/2002 Ulrich Heintz - NEPPSR 2002 54
top quark signature
b
b
q
q
W Wt t
4/24/2002 Ulrich Heintz - NEPPSR 2002 55
top mass measurementmeasure lepton, 4 jets, missing pT
1 unknown 3 constraints 1C kinematic fit compare to MC
mtop = 176.0§6.5 GeV (CDF)mtop = 172.1§7.1 GeV (DØ)mtop = 174.3§5.1 GeV
mtop = 176.0§6.5 GeV (CDF)mtop = 172.1§7.1 GeV (DØ)mtop = 174.3§5.1 GeV
CDF
4/24/2002 Ulrich Heintz - NEPPSR 2002 56
does it all fit?global electroweak fit
include W mass, width and top mass measurements
consistent with standard model
%)14(
14/6.19
GeV 019.0403.80
00036.022255.0sin
GeV 81
GeV 3.48.175
2
2
4932 dof
M
m
m
W
w
H
t
4/24/2002 Ulrich Heintz - NEPPSR 2002 57
yet another way to measure sin2w
charged/neutral current cross section
is a function of sin2 w
but sensitive to sea quark contributions
sea quark contributions cancel but measure and
separately
“charged current”
W§
d,s u,c
“neutral current”
Z0
u,d,s u,d,s
CCCC
NCNC
CC
NC
R
R
4/24/2002 Ulrich Heintz - NEPPSR 2002 58
NuTeV
toroidal muon spectrometer
target/calorimeteriron
liquid scintillatordrift chambers
“Nu”trinos at the “Tev”atron
4/24/2002 Ulrich Heintz - NEPPSR 2002 59
NuTeVfixed target experiment p,K
two beams: and charged current events
neutral current events
4/24/2002 Ulrich Heintz - NEPPSR 2002 60
NuTeVresult
NuTeVsin2 w = 0.2277 § 0.0013 (stat) §
0.0009 (syst)
global electroweak fit: sin2 w = 0.22255 § 0.00036
3 standard deviation discrepancy
4/24/2002 Ulrich Heintz - NEPPSR 2002 61
does it still all fit?global electroweak fit
include sin2w from NuTeV
still consistent with standard model?
%)7.1(
15/8.28
GeV 018.0394.80
00036.022272.0sin
GeV 85
GeV 4.47.174
2
2
5434 dof
M
m
m
W
w
H
t
4/24/2002 Ulrich Heintz - NEPPSR 2002 62
precision measurements
4/24/2002 Ulrich Heintz - NEPPSR 2002 63
Outlinethe standard modelexperimental confirmationprecision measurementsdoes it fit?future prospects
Tevatron Run 2 LHC linear collider
are we done?summary
4/24/2002 Ulrich Heintz - NEPPSR 2002 64
Tevatron upgrade Main Injector
2r = 3.3 km 8150 GeV p synchrotron 8120 GeV to p, fixed
target 1508 GeV for p recovery 344 dipoles, 208 quads
p Source upgrades to target, Li lens debuncher and
accumulator stochastic cooling systems
Recycler 8 GeV p storage ring 416 permanent magnets
4/24/2002 Ulrich Heintz - NEPPSR 2002 65
Tevatron run 2now until LHC is competitive...s = 1.96 TeV
Run 2A (<2005)L≈1032 cm-2s-1, ∫Ldt≈2 fb-1
Run 2B (>2005)L≈5x1032 cm-2s-1, ∫Ldt≈13 fb-1
W mass to 27-17 MeVtop mass to 2.8-1.3 GeVHiggs see next lecture
4/24/2002 Ulrich Heintz - NEPPSR 2002 66
Tevatron run 2
4/24/2002 Ulrich Heintz - NEPPSR 2002 67
can we exclude the standard model?
global electroweak fit¼ 2008summer 2001
Grünewald, Heintz, Narain, Schmitt, hep-ph/0111217
4/24/2002 Ulrich Heintz - NEPPSR 2002 68
beyond Run 2LHC (starts in 2006?)
pp collisions at s = 14 TeV MW, mtop
Higgs phenomena beyond standard model
linear collider (starts before I retire?) e+e- collisions at s ¼ 500 GeV Giga-Z option MW
mtop
precision study of Higgs...?
4/24/2002 Ulrich Heintz - NEPPSR 2002 69
are we done? shortcomings of the
standard model too many free
parameters Higgs mechanism put in
by hand light fundamental scalar
requires fine tuning (hierarchy problem)
keeping the Higgs coupling finite as Q21, drives it to zero at low Q2 (triviality problem)
does the Higgs exist? only e&m and weak force
truly unified does not describe gravity
4/24/2002 Ulrich Heintz - NEPPSR 2002 70
summarystandard model
unifies electroweak interaction into one gauge theory
strong interaction many predictions confirmed precisely by
experiment some hints of discrepancies at the 3
sigma level theoretically unsatisfying
your charge find physics beyond the standard model
4/24/2002 Ulrich Heintz - NEPPSR 2002 71
weak interactionsmuon
decay
25
6
5
3
2
GeV10)1(16639.1
s102.2
)1(1921
F
F
G
cmG
e
e-
-
4/24/2002 Ulrich Heintz - NEPPSR 2002 72
weak interactionsmuon
decay
2
2
5
3
4
4
3
5
4
4
8
2
)8(121
)8(12
W
WF
W
W
W
W
M
gG
mg
M
m
M
g
W-
e
e-
-Wg
Wg
2WM
4/24/2002 Ulrich Heintz - NEPPSR 2002 73
Large Electron-Positron collider at CERN
4/24/2002 Ulrich Heintz - NEPPSR 2002 74
DØ detectorcalorimeter
measure energy and position
drift chambersmeasure direction
4/24/2002 Ulrich Heintz - NEPPSR 2002 75
electron signatureelectron
measureenergy Edirection ,
calculatemomentum
p p
e
Cal
orim
eter
EM
CH
FH
Dri
ft c
ham
ber
sin
cos
sinsin
sincos
c
Ep
c
Ep
T
4/24/2002 Ulrich Heintz - NEPPSR 2002 76
detector
4/24/2002 Ulrich Heintz - NEPPSR 2002 77
measuring the Wrecoil
measureenergy in cell i Ei
direction i, i
cannot measureparticles along beam
calculate
p p
electroncells
iT
recoilT pp
4/24/2002 Ulrich Heintz - NEPPSR 2002 78
neutrino signatureneutrino
measurenothing
momentum conservation
TrecoilT
eTT
recoilTT
eT
recoilzrecoilepp
pppp
ppp
pppppp
missing
0
knowt don'but 0
4/24/2002 Ulrich Heintz - NEPPSR 2002 79
detector
4/24/2002 Ulrich Heintz - NEPPSR 2002 80
Z mass measurementZ boson
measure
MZ is invariantno dependence on pT
Z
fit to Breit-Wigner) done (sort of...)
222
ZZZ
eeZ
eeZ
pEM
EEE
ppp
W mass measurement
4/24/2002 Ulrich Heintz - NEPPSR 2002 81
W mass measurementW boson
measure
MT is not invariantdepends on pT
W and parton distribution functions
detector acceptances
need a detailed model ofW boson productiondetector
2222
WWT
WTT
TeT
WT
TeT
WT MpEM
ppE
ppp
4/24/2002 Ulrich Heintz - NEPPSR 2002 82
why transverse mass?W boson W mass measurement
4/24/2002 Ulrich Heintz - NEPPSR 2002 83
Monte Carlo modelcomputer simulation
pick random pW, MW from distribution
pick random decay angles from distribution
simulate detector response
pe and pTrecoil
do this billions of times....
W mass fit
4/24/2002 Ulrich Heintz - NEPPSR 2002 84
References Thomas Junk, CERN (ITP 4/18/01) Searches for the Higgs Boson at
LEP Llewellyn Smith, on the determination of sin2 in semileptonic
neutrino interactions (KEK scan 83-4-300) K. McFarland “A Departure From Prediction: ewk Physics at NuTeV”,
FNAL Wine & Cheese October 26, 2001 D0 “Limits on anomalous WW couplings from...”, March 14, 1997 S. Schmidt-Kärst “W mass measurements using fully hadronic
events at LEPII”, ICHEP 2000, Osaka R. Ströhmer “W measurements using semileptonic and fully leptonic
events at LEPII”. ICHEP 2000, Osaka LEPEWWG/MASS/2000-01 “Combined preliminary results on mass
and with of W boson by LEP” Glenzinski, Heintz, “Precision measurements of the W boson mass” LEP electroweak working group summer 2002 plots LEPEWWG/2002-01 “Combination of preliminary ewk measurements
and constraints on the sm” Heintz “Electroweak Measurements from Fermilab”, APS centennial
meeting 1999
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