Electroweak Physics Precision Experiments: Historical Perspective LEP/SLC Physics Probing the Standard Model Beyond the Standard Model
The Z, the W, and the Weak Neutral Current Primary prediction and test of electroweak unification WNC discovered 1973 (Gargamelle, HPW) 70 s, 80 s: weak neutral current experiments (few %) Pure weak: νn, νe scattering Weak-elm interference in ed, e + e, atomic parity violation W, Z discovered directly 1983 (UA1, UA2)
90 s: Z pole (LEP, SLD), 0.1%; lineshape, modes, asymmetries LEP 2: M W, Higgs, gauge self-interactions Tevatron: m t, M W 4th generation weak neutral current experiments Implications SM correct and unique to zeroth approx. (gauge principle, group, representations) SM correct at loop level (renorm gauge theory; m t, α s, M H ) TeV physics severely constrained (unification vs compositeness) Precise gauge couplings (gauge unification)
Results before the LEP/SLD era Global Analysis more information than individual experiments caveat: exp./theor. systematics, correlations Model-independent fits Unique νq, νe, eq SM correct to first approximation Contrived imitators out
0.4 0.2 0.2 0.1 0.3 ε L (d) (u) 0.0-0.2 1.0 0.7 ε R (d) (u) 0.0-0.1 0.0-0.4 0.4 0.0-0.2-0.4-0.2 0.0 0.2 0.4 ε L (u) -0.2-0.1 0.0 0.1 0.2 ε R (u)
g V -1.0-0.8-0.6-0.4-0.2 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.3 0.6 ν e µ reactor ν e (LANL) e all -1.0-0.8-0.6-0.4-0.2 0.0 0.2 0.4 0.6 0.8 1.0 g A
QCD evolved structure functions Radiative corrections necessary (sin 2 θ W defns) sin 2 θ W =.230 ±0.007, m t < 200 GeV Unique fermion reps. (wnc + wcc) t and ν τ exist Grand unification Ordinary SU(5) excluded; consistent with SUSY GUTS and perhaps even the first harbinger of supersymmetry Stringent limits on new physics Z, exotic fermions, exotic Higgs, leptoquarks, 4-F operators
The LEP/SLC Era Z Pole: e + e Z l + l, q q, ν ν LEP (CERN), 2 10 7 Z s, unpolarized (ALEPH, DELPHI, L3, OPAL); SLC (SLAC), 5 10 5, P e 75 % (SLD) Z pole observables lineshape: M Z, Γ Z, σ branching ratios e + e, µ + µ, τ + τ q q, c c, b b, s s ν ν N ν = 2.986 ± 0.007 if m ν < M Z /2 asymmetries: FB, polarization, P τ, mixed lepton family universality
The Z Lineshape Basic Observables: e + e f f (f = e, µ, τ, s, b, c, hadrons) (s = E 2 CM ) σ f (s) σ f sγ 2 Z (s M 2 Z )2 + s2 Γ 2 Z M 2 Z (plus initial state rad. corrections) Peak Cross Section: σ f = 12π M 2 Z Γ(e + e )Γ(f f) Γ 2 Z
Partial Widths: Γ(f f) C fg F M 3 Z 6 2π [ ḡv f 2 + ḡ Af 2] (plus mass, QED, QCD corrections; C l = 1, C q = 3; ḡ V,Af = effective coupling (includes ew)). At tree level: ḡ Af = ± 1 2, ḡ V f = ± 1 2 2sin2 θ W q f where sin 2 θ W 1 M W 2 is the weak angle, ± 1 is the weak M Z 2 2 isospin (+ for (u, ν), for (d, e )), and q f is the electric charge
LEP averages of leptonic widths Γ e 83.92 ± 0.12 MeV Γ µ 83.99 ± 0.18 MeV Γ τ 84.08 ± 0.22 MeV Γ l 83.98 ± 0.09 MeV 1000 800 m H [GeV] 600 400 m Z = 91 188 ± 2 MeV m t = 174.3 ± 5.1 GeV 200 83.5 84 84.5 [MeV] Γ l
Z-Pole Asymmetries Effective axial and vector couplings of Z to fermion f ḡ Af = ρ f t 3f ḡ V f = ρ f [t 3f 2 s 2 f q f ] where s 2 f the effective weak angle, s 2 f = κ f s 2 W (on shell) = ˆκ f ŝ 2 Z ŝ2 Z + 0.00029 (f = e) (MS ), ρ f, κ f, and ˆκ f are electroweak corrections, q f = electric charge, t 3f = weak isospin
A 0 = Born asymmetry (after removing γ, off-pole, box (small), P e ) forward backward : A 0f F B 3 4 A ea f (A 0e F B = A0µ F B = A0τ F B A0l F B universality) τ polarization : P 0 τ = A τ + A e 2z 1+z 2 1 + A τ A e 2z 1+z 2 (z = cos θ, θ = scattering angle) e polarization (SLD) : A 0 LR = A e mixed (SLD) : A 0F B LR = 3 4 A f A f 2ḡ V F ḡ Af ḡ 2 V F + ḡ2 AF
The Z Pole Observables: LEP and SLC (01/03) Quantity Group(s) Value Standard Model pull M Z [GeV] LEP 91.1876 ± 0.0021 91.1874 ± 0.0021 0.1 Γ Z [GeV] LEP 2.4952 ± 0.0023 2.4972 ± 0.0011 0.9 Γ(had) [GeV] LEP 1.7444 ± 0.0020 1.7436 ± 0.0011 Γ(inv) [MeV] LEP 499.0 ± 1.5 501.74 ± 0.15 Γ(l + l ) [MeV] LEP 83.984 ± 0.086 84.015 ± 0.027 σ had [nb] LEP 41.541 ± 0.037 41.470 ± 0.010 1.9 R e LEP 20.804 ± 0.050 20.753 ± 0.012 1.0 R µ LEP 20.785 ± 0.033 20.753 ± 0.012 1.0 R τ LEP 20.764 ± 0.045 20.799 ± 0.012 0.8 A F B (e) LEP 0.0145 ± 0.0025 0.01639 ± 0.00026 0.8 A F B (µ) LEP 0.0169 ± 0.0013 0.4 A F B (τ ) LEP 0.0188 ± 0.0017 1.4
Quantity Group(s) Value Standard Model pull R b LEP/SLD 0.21664 ± 0.00065 0.21572 ± 0.00015 1.1 R c LEP/SLD 0.1718 ± 0.0031 0.17231 ± 0.00006 0.2 R s,d /R (d+u+s) OPAL 0.371 ± 0.023 0.35918 ± 0.00004 0.5 A F B (b) LEP 0.0995 ± 0.0017 0.1036 ± 0.0008 2.4 A F B (c) LEP 0.0713 ± 0.0036 0.0741 ± 0.0007 0.8 A F B (s) DELPHI/OPAL 0.0976 ± 0.0114 0.1037 ± 0.0008 0.5 A b SLD 0.922 ± 0.020 0.93476 ± 0.00012 0.6 A c SLD 0.670 ± 0.026 0.6681 ± 0.0005 0.1 A s SLD 0.895 ± 0.091 0.93571 ± 0.00010 0.4 A LR (hadrons) SLD 0.15138 ± 0.00216 0.1478 ± 0.0012 1.7 A LR (leptons) SLD 0.1544 ± 0.0060 1.1 A µ SLD 0.142 ± 0.015 0.4 A τ SLD 0.136 ± 0.015 0.8 A e (Q LR ) SLD 0.162 ± 0.043 0.3 A τ (P τ ) LEP 0.1439 ± 0.0043 0.9 A e (P τ ) LEP 0.1498 ± 0.0048 0.4 Q F B LEP 0.0403 ± 0.0026 0.0424 ± 0.0003 0.8
LEP 2 M W, Γ W, B (also hadron colliders) M H limits (hint?) W W production (triple gauge vertex) Quartic vertex SUSY/exotics searches
Other: atomic parity (Boulder); νe; νn (NuTeV); M W, m t (Tevatron)
Non-Z Pole Precision Observables (1/03) Quantity Group(s) Value Standard Model pull m t [GeV] Tevatron 174.3 ± 5.1 174.4 ± 4.4 0.0 M W [GeV] LEP 80.447 ± 0.042 80.391 ± 0.018 1.3 M W [GeV] Tevatron /UA2 80.454 ± 0.059 1.1 g 2 L NuTeV 0.30005 ± 0.00137 0.30396 ± 0.00023 2.9 g 2 R NuTeV 0.03076 ± 0.00110 0.03005 ± 0.00004 0.6 R ν CCFR 0.5820 ± 0.0027 ± 0.0031 0.5833 ± 0.0004 0.3 R ν CDHS 0.3096 ± 0.0033 ± 0.0028 0.3092 ± 0.0002 0.1 R ν CHARM 0.3021 ± 0.0031 ± 0.0026 1.7 R ν CDHS 0.384 ± 0.016 ± 0.007 0.3862 ± 0.0002 0.1 R ν CHARM 0.403 ± 0.014 ± 0.007 1.0 R ν CDHS 1979 0.365 ± 0.015 ± 0.007 0.3816 ± 0.0002 1.0
Quantity Group(s) Value Standard Model pull g νe V CHARM II 0.035 ± 0.017 0.0398 ± 0.0003 g νe V all 0.041 ± 0.015 0.1 g νe A CHARM II 0.503 ± 0.017 0.5065 ± 0.0001 g νe A all 0.507 ± 0.014 0.0 Q W (Cs) Boulder 72.69 ± 0.44 73.10 ± 0.04 0.8 Q W (Tl) Oxford/Seattle 116.6 ± 3.7 116.7 ± 0.1 0.0 10 3 Γ(b sγ) Γ SL BaBar/Belle/CLEO 3.48 +0.65 0.54 3.20 ± 0.09 0.5 τ τ [fs] direct/b e /B µ 290.96 ± 0.59 ± 5.66 291.90 ± 1.81 0.4 10 4 α (3) had e + e /τ decays 56.53 ± 0.83 ± 0.64 57.52 ± 1.31 0.9 10 9 (a µ α 2π ) BNL/CERN 4510.64 ± 0.79 ± 0.51 4508.30 ± 0.33 2.5
New Inputs, Anomalies, Things to Watch The W Mass CDF, DØ, and UA2: 80.454(59) GeV LEP2 (3/03): 80.412(42) GeV LEP2 (12/02): 80.447(42) GeV (used in our table) SM: 80.391(18) GeV - Will slightly increase M H
Higgs limits/hints Direct limit: M H > 114.4 GeV (95% cl) (Hints around 116 GeV weakened in final analysis)
A b F B = 0.0995(17) is 2.4σ below expectation of 0.1036(8)
R b = 0.21664 ± 0.00065 (SM: 0.21572 ± 0.00015, agrees at 1.1σ) A b = 0.922 ± 0.020 (SM: 0.93476 ± 0.00012 agrees at 0.6σ) Compensation of L and R couplings (R b ) 5% effect, but 25% in κ probably tree level affecting third family New physics possibilities include Z with non-universal couplings, or b R mixing with B R in doublet with charge 4/3 New physics or fluctuation/systematics lead to smaller M H
a µ = (g µ 2)/2 More sensitive than a e to new physics BNL (2002) + other: a exp µ = 11659203(8) 10 10 Hadronic light by light has settled down, but considerable uncertaintly from a Had µ a exp µ asm µ = (26 ± 11) 10 10 (2.6σ) (using e + e data for a Had 1.1σ (using τ decay data) New physics? Supersymmetry: ( m 55 GeV tan β) µ )
α (5) had (M Z) Hadronic contribution to running of α up to Z-pole Largest theory uncertainty in M Z ŝ 2 Z Closely related to a had µ Recent progress using improved QCD calculations (high energy part) and precise BES data
NuTeV ( ) ν µ N ( ) ν µ X ( ) ν µ N µ X Little c threshold uncertaintly s 2 W = 0.2277(16), 3.0σ above SM value 0.2228(4) g 2 L g 2 R = 0.3001(14) is 2.9σ below expected 0.3040(2) = 0.0308(11) is 0.7σ above expected 0.0300(0) Possible QCD effects: Large isospin breaking is sea; large s s asymmetry; nuclear shadowing; NLQCD Possible exotic effects: designer Z ; ν mixing with heavy neutrino (CKM universality?)
Atomic Parity Violation Very precise measurement (0.4%) in cesium (single electron outside tightly bound core) Matrix element under control Previous hint (2.2σ) of discrepancy, but theory-dominated error Surprisingly large (O(1%)) radiative corrections: Breit, vacuum polarization, vertex, self-energy have now stabilized Now excellent agreement: Q W (Cs) = 72.69(44) 72.84(46) (SM: 73.10(4))
CKM Unitarity Expect 1 V ud 2 V us 2 V ub 2 = 0 V ud from β decay, V us from kaon and hyperon decays, V ub from b decays Superallowed (0 + 0 + ): V ud = 0.9740(5) = 0.0032(14) (2.3σ) Neutron decay lifetime/asymmetry: V ud = 0.9713(13) = 0.0083(28) (3σ) Problem with K e3 branching ratio or K L lifetime? W mixing? Problem aggravated by light-heavy neutrino mixing
Non-Z pole very important Z-pole most precise, but blind to many types of BSM (cf. NuTeV) Polarized Møller (SLAC E158), polarized ep (QWEAK at Jefferson Lab) Precise m t, M H (Tevatron, LHC, LC) Future: GigaZ option for LC?
0.25 weak mixing angle scale dependence in MS bar scheme SM 0.245 0.24 NuTeV sin 2 θ W 0.235 old Q (APV) W QWEAK E158 MSSM new Q W (APV) Z pole 0.23 0.225 0.001 0.01 0.1 1 Q [GeV] 10 100 1000
Fit Results (06/02) (Erler, PL) M H = 86 +49 32 GeV, m t = 174.2 ± 4.4 GeV, α s = 0.1210 ± 0.0018, ˆα(M Z ) 1 = 127.922 ± 0.020 ŝ 2 Z = 0.23110 ± 0.00015, χ 2 /d.o.f. = 49.0/40(15%)
m t = 174.2 ± 4.4 GeV 174.0 +9.9 7.4 GeV from indirect (loops) only (direct: 174.3 ± 5.1) Z t, b t, b Z t W + b W +
α s = 0.1210 ± 0.0018 Higher than world average α s = 0.1172(20) (Hinchliffe (PDG) 2001), because of τ lifetime insensitive to oblique new physics very sensitive to non-universal new physics (e.g., Zb b vertex) Z q G q
Higgs mass M H = 86 +49 32 GeV direct limit (LEP 2): M H > 114.4 (95%) GeV SM: 115 (vac. stab.) < M H < 750 (triviality) MSSM: M H < 130 GeV (150 in extensions) Z H Z indirect: ln M H but significant fairly robust to new physics (except S < 0, T > 0) however, strong A F B (b) effect M H < 215 GeV at 95%, including direct
1000 500 Γ, σ, R, R Z had l q asymmetries ν scattering M W m t 200 all data 90% CL M H [GeV] 100 50 20 excluded 10 100 120 140 160 180 200 m t [GeV]
80.6 direct (1σ) indirect (1σ) 80.5 all (90% CL) M W [GeV] 80.4 80.3 100 200 400 800 80.2 150 160 170 180 190 200 m t [GeV] M H [GeV]
Beyond the standard model ρ 0 ; S, T, U: Higgs triplets, nondegenerate fermions or scalars; chiral families (ETC) for M H = 115.6 (300) GeV S = 0.14 ± 0.10( 0.08) T = 0.15 ± 0.12(+0.09) U = 0.32 ± 0.12(+0.01) (2.6σ) ρ 0 = 1 + αt = 0.9997 +0.0011 0.0008 (M H = 73 +106 34 GeV) for S = U = 0 For T = U = 0: S = 0.10 +0.12 0.30, M H < 570 GeV at 95%; N fam = 2.81 ± 0.29 (cf. lineshape: N ν = 2.986 ± 0.007) N fam = 2.79 ± 0.43 for T = 0.01 ± 0.11
1.5 1 0.5 T 0 0.5 Γ, σ, R, R Z had l q asymmetries ν scattering 1 all: M all: M all: M 1.5 1.5 1 0.5 0 0.5 1 1.5 S H H H M W Q W = 115 GeV = 340 GeV = 1000 GeV
Supersymmetry decoupling limit (M new > 200 300 GeV): only precision effect is light SM-like Higgs little improvement on SM fit SUSY parameters constrained
A TeV scale Z? Expected in many string theories, grand unification, dynamical symmetry breaking Natural solution to µ problem Implications Exotics FCNC (especially in string models) Non-standard Higgs masses, couplings (doublet-singlet mixing) Non-standard sparticle spectrum Neutrino mass, BBN, structure Enhanced possibility of EW baryogenesis Typically M Z > 500 800 GeV (Tevatron, LEP 2, WNC), θ Z Z < few 10 3 (Z-pole)
Other Exotic fermion mixings Large extra dimensions New four-fermi operator Leptoquark bosons Gauge unification: GUTs, string theories α + ŝ 2 Z α s = 0.130 ± 0.010 M G 3 10 16 GeV Perturbative string: 5 10 17 GeV (10% in ln M G ). Exotics: O(1) corrections.
Conclusions WNC, Z, W unification are primary predictions and test of electroweak SM correct and unique to first approx. representations) (gauge principle, group, SM correct at loop level (renorm gauge theory; m t, α s, M H ) Watershed: TeV physics severely constrained (unification vs compositeness) unification (decoupling): expect 0.1% TeV compositeness: expect several % Precise gauge couplings (gauge unification)
Future Polarized Møller, polarized ep Precise m t, M H Direct production of new particles at hadron colliders CP violation in B system Electric dipole moments New precision era in future linear collider