Electroweak Physics. Precision Experiments: Historical Perspective. LEP/SLC Physics. Probing the Standard Model. Beyond the Standard Model

Transcription

1 Electroweak Physics Precision Experiments: Historical Perspective LEP/SLC Physics Probing the Standard Model Beyond the Standard Model

2 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)

3 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)

4 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

5 ε L (d) (u) ε R (d) (u) ε L (u) ε R (u)

6 g V ν e µ reactor ν e (LANL) e all g A

7 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

8 The LEP/SLC Era Z Pole: e + e Z l + l, q q, ν ν LEP (CERN), Z s, unpolarized (ALEPH, DELPHI, L3, OPAL); SLC (SLAC), , 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 = ± if m ν < M Z /2 asymmetries: FB, polarization, P τ, mixed lepton family universality

9 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

10 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

11 LEP averages of leptonic widths Γ e ± 0.12 MeV Γ µ ± 0.18 MeV Γ τ ± 0.22 MeV Γ l ± 0.09 MeV m H [GeV] m Z = ± 2 MeV m t = ± 5.1 GeV [MeV] Γ l

12 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 (f = e) (MS ), ρ f, κ f, and ˆκ f are electroweak corrections, q f = electric charge, t 3f = weak isospin

13 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 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

14 The Z Pole Observables: LEP and SLC (01/03) Quantity Group(s) Value Standard Model pull M Z [GeV] LEP ± ± Γ Z [GeV] LEP ± ± Γ(had) [GeV] LEP ± ± Γ(inv) [MeV] LEP ± ± 0.15 Γ(l + l ) [MeV] LEP ± ± σ had [nb] LEP ± ± R e LEP ± ± R µ LEP ± ± R τ LEP ± ± A F B (e) LEP ± ± A F B (µ) LEP ± A F B (τ ) LEP ±

15 Quantity Group(s) Value Standard Model pull R b LEP/SLD ± ± R c LEP/SLD ± ± R s,d /R (d+u+s) OPAL ± ± A F B (b) LEP ± ± A F B (c) LEP ± ± A F B (s) DELPHI/OPAL ± ± A b SLD ± ± A c SLD ± ± A s SLD ± ± A LR (hadrons) SLD ± ± A LR (leptons) SLD ± A µ SLD ± A τ SLD ± A e (Q LR ) SLD ± A τ (P τ ) LEP ± A e (P τ ) LEP ± Q F B LEP ± ±

16 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

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18 Other: atomic parity (Boulder); νe; νn (NuTeV); M W, m t (Tevatron)

19 Non-Z Pole Precision Observables (1/03) Quantity Group(s) Value Standard Model pull m t [GeV] Tevatron ± ± M W [GeV] LEP ± ± M W [GeV] Tevatron /UA ± g 2 L NuTeV ± ± g 2 R NuTeV ± ± R ν CCFR ± ± ± R ν CDHS ± ± ± R ν CHARM ± ± R ν CDHS ± ± ± R ν CHARM ± ± R ν CDHS ± ± ±

20 Quantity Group(s) Value Standard Model pull g νe V CHARM II ± ± g νe V all ± g νe A CHARM II ± ± g νe A all ± Q W (Cs) Boulder ± ± Q W (Tl) Oxford/Seattle ± ± Γ(b sγ) Γ SL BaBar/Belle/CLEO ± τ τ [fs] direct/b e /B µ ± 0.59 ± ± α (3) had e + e /τ decays ± 0.83 ± ± (a µ α 2π ) BNL/CERN ± 0.79 ± ±

21 New Inputs, Anomalies, Things to Watch The W Mass CDF, DØ, and UA2: (59) GeV LEP2 (3/03): (42) GeV LEP2 (12/02): (42) GeV (used in our table) SM: (18) GeV - Will slightly increase M H

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23 Higgs limits/hints Direct limit: M H > GeV (95% cl) (Hints around 116 GeV weakened in final analysis)

24 A b F B = (17) is 2.4σ below expectation of (8)

25 R b = ± (SM: ± , agrees at 1.1σ) A b = ± (SM: ± 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

26 a µ = (g µ 2)/2 More sensitive than a e to new physics BNL (2002) + other: a exp µ = (8) Hadronic light by light has settled down, but considerable uncertaintly from a Had µ a exp µ asm µ = (26 ± 11) (2.6σ) (using e + e data for a Had 1.1σ (using τ decay data) New physics? Supersymmetry: ( m 55 GeV tan β) µ )

27 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

28 NuTeV ( ) ν µ N ( ) ν µ X ( ) ν µ N µ X Little c threshold uncertaintly s 2 W = (16), 3.0σ above SM value (4) g 2 L g 2 R = (14) is 2.9σ below expected (2) = (11) is 0.7σ above expected (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?)

29 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))

30 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 ( ): V ud = (5) = (14) (2.3σ) Neutron decay lifetime/asymmetry: V ud = (13) = (28) (3σ) Problem with K e3 branching ratio or K L lifetime? W mixing? Problem aggravated by light-heavy neutrino mixing

31 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?

32 0.25 weak mixing angle scale dependence in MS bar scheme SM NuTeV sin 2 θ W old Q (APV) W QWEAK E158 MSSM new Q W (APV) Z pole Q [GeV]

33 Fit Results (06/02) (Erler, PL) M H = GeV, m t = ± 4.4 GeV, α s = ± , ˆα(M Z ) 1 = ± ŝ 2 Z = ± , χ 2 /d.o.f. = 49.0/40(15%)

34 m t = ± 4.4 GeV GeV from indirect (loops) only (direct: ± 5.1) Z t, b t, b Z t W + b W +

35 α s = ± Higher than world average α s = (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

36 Higgs mass M H = GeV direct limit (LEP 2): M H > (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

37 Γ, σ, R, R Z had l q asymmetries ν scattering M W m t 200 all data 90% CL M H [GeV] excluded m t [GeV]

38 80.6 direct (1σ) indirect (1σ) 80.5 all (90% CL) M W [GeV] m t [GeV] M H [GeV]

39 Beyond the standard model ρ 0 ; S, T, U: Higgs triplets, nondegenerate fermions or scalars; chiral families (ETC) for M H = (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 = (M H = GeV) for S = U = 0 For T = U = 0: S = , M H < 570 GeV at 95%; N fam = 2.81 ± 0.29 (cf. lineshape: N ν = ± 0.007) N fam = 2.79 ± 0.43 for T = 0.01 ± 0.11

40 T Γ, σ, R, R Z had l q asymmetries ν scattering 1 all: M all: M all: M S H H H M W Q W = 115 GeV = 340 GeV = 1000 GeV

41 Supersymmetry decoupling limit (M new > GeV): only precision effect is light SM-like Higgs little improvement on SM fit SUSY parameters constrained

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43 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 > GeV (Tevatron, LEP 2, WNC), θ Z Z < few 10 3 (Z-pole)

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45 Other Exotic fermion mixings Large extra dimensions New four-fermi operator Leptoquark bosons Gauge unification: GUTs, string theories α + ŝ 2 Z α s = ± M G GeV Perturbative string: GeV (10% in ln M G ). Exotics: O(1) corrections.

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47 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)

48 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