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

Transcription

1 Electroweak Physics Precision Experiments: Historical Perspective LEP/SLD Physics Probing the Standard Model Beyond the Standard Model P. Langacker Madrid 27/09/02

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: (few %) weak neutral current experiments 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 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)

3 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 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

4 The LEP/SLD Era Z Pole: e + e Z l + l, q q, ν ν LEP (CERN), Z s, unpolarized; SLC (SLAC), , P e 75 % 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 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)

5 The Z Lineshape e + e f f (f = e, µ, τ, s, b, c, hadrons); s = E 2 CM sγ 2 Z σ f (s) σ f ( s M 2 ) 2 s Z + 2 Γ 2 Z MZ 2 (plus initial state rad. corrections) σ f = 12π M 2 Z Γ(e + e )Γ(f f) Γ 2 Z Γ(inv) = Γ Z Γ(had) i N ν Γ(ν ν) R qi Γ(q i q i ) Γ(had), q i = b, c, s Γ(l i l i ) R li Γ(had) Γ(l i li ), l i = e, µ, τ (R e = R µ = R τ R l lepton universality) C f G F M Γ(f f) 3 Z 6 [ ḡv f 2 + ḡ Af 2] 2π (plus mass, QED, QCD corrections; C l = 1, C q = 3; ḡ V,Af = effective coupling (includes ew)) M Z, Γ Z, σ had, R l, R b, R c mainly weakly correlated

6 Z-Pole Asymmetries A 0 = Born asymmetry, after removing γ, offpole, box (small), P e forward backward : A 0f F B 3 4 A ea f (A 0e F B = A0µ F B = A 0τ 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 where s 2 f ḡ Af ḡ V f = ρ f t 3f = ρ f [ t3f 2 s 2 f q f the effective weak angle, s 2 f = κ fs 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 ]

7 bb _ A at s m Z FB ALEPH leptons DELPHI leptons L3 leptons OPAL leptons ALEPH jet-ch DELPHI jet-ch L3 jet-ch OPAL jet-ch ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± LEP Summer mt [GeV] ± Include Total Sys With Common Sys m H [GeV] 10 2 m t = ± 5.1 GeV α had = ± ,bb _ A FB

8 The Z Pole Observables: LEP and SLC (7/01) Quantity Group(s) Value Standard Model pull M Z [GeV] LEP ± ± Γ Z [GeV] LEP ± ± Γ(had) [GeV] LEP ± ± Γ(inv) [MeV] LEP ± ± 0.14 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 ± R b LEP + SLD ± ± R c LEP + SLD ± ± 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 τ (P τ ) LEP ± A e (P τ ) LEP ± s 2 l (Q F B) LEP ± ±

9 Non-Z Pole Precision Observables: Tevatron, LEP 2, νn, APV (7/01) y Group(s) Value Standard Model pull V] Tevatron ± ± ev] LEP ± ± ev] Tevatron,UA ± NuTeV ± ± ± CCFR ± ± ± CDHS ± ± ± CHARM ± ± CDHS ± ± ± CHARM ± ± CDHS ± ± ± CHARM II ± ± all ± CHARM II ± ± all ± ) Boulder ± 0.28 ± ± Oxford,Seattle ± 1.2 ± ± CLEO α ) π E ± 1.51 ± ±

10 20 LEP RacoonWW / YFSWW 1.14 Gentle 2.1 (±0.7%) 21/07/2000 Preliminary σ WW [pb] YFSWW 1.14 RacoonWW E cm [GeV]

11 A F B (b) A F B (b) = ± (SM: ±0.0008, 2.6σ) R b = ± (SM: ± , agrees at 1.4σ) A b = ± (SM: ± agrees at 0.7σ) 5% effect, but 25% in κ probably tree level affecting third family Compensation of L and R couplings (R b ) M H correlations, whether new physics or systematics

12 κ b % CL all data ρ b

13 LEP/SLD remarkably successful (beyond anticipation) Machine Detectors: ALEPH, DELPHI, L3, OPAL (LEP); SLD (SLC) Analysis: tides, water table/lake, trains LEPEWWG critical Theoretical inputs precise QED, EW, QCD, mixed radiative corrections sin 2 θ W definitions α had (running α) Packages: ZFITTER, TOPAZ0, ALIBABA, BHLUMI, GAPP, (LEP2) new physics parametrizations Global analyses: LEPEWWG, PDG, Polarization: SLC advantage; (Blondel scheme) Future: GigaZ?

14 The Anomalous Magnetic Moment of the Muon a µ g µ 2 2 a SM µ = a QED µ + a Had µ + a EW µ = (7) µ = α 2π (27) α 2 π α 3 α (44) (41) π π α (170) = (0.29) π a QED a Had µ = a Had µ (vp) a Had µ (ll) = (692(6) 10.0(0.6) + 8.6(3.2)) a EW µ (2 loop) = 15.1(0.4) BNL (2002) + other: a exp µ = (8) a exp µ a SM µ = (26 ± 11) (2.6σ) Statistics? (will improve); a Had µ? New physics? Supersymmetry: ( m 55 GeV tan β)

15 Radiative Corrections Dominant two-loop electroweak (α 2 m 4 t, α2 m 2 t ) Dominant 3 loop QCD (4 loop estimate) Dominant 3 loop mixed QCD-EW (αα s vertex) Definitions of renormalized sin 2 θ W On shell: s 2 W 1 M W 2 MZ 2 Z mass: s 2 ( ) M Z 1 s 2 MZ πα(m Z ) 2GF MZ 2 MS : ŝ 2 Z ĝ 2 (M Z ) ĝ 2 (M Z )+ĝ 2 (M Z ) Effective (Z-pole): s 2 f 1 4 ( πα/ M 2 W = 2GF = (1 r) s 2 W ) 1 ḡ V f ḡ Af ( πα/ 2GF ) ŝ 2 Z (1 ˆr W ) M 2 Z = M 2 W c 2 W = M 2 W ˆρĉ 2 Z s 2 f = κ fs 2 W = ˆκ fŝ 2 Z s 2 e ŝ2 Z (κ f, ˆκ f depend on m t, M H ) ˆρ 1 + 3G F ˆm 2 t 8 2π 2 + ˆr W α

16 Definitions of sin 2 θ W On-shell : s 2 W = 1 M W 2 = (36) MZ 2 + most familiar + simple conceptually large m t, M H dependence from Z-pole observables depends on SSB mechanism awkward for new physics Z-mass : s 2 M Z = (8) + most precise (no m t, M H dependence) + simple conceptually m t, M H reenter when predicting other observables depends on SSB mechanism awkward for new physics MS : ŝ 2 Z = (15) + based on coupling constants + convenient for GUTs + usually insensitive to new physics + Z asymmetries independent of m t, M H theorists definition; not simple conceptually usually determined by global fit some sensitivity to m t, M H variant forms (m t cannot be decoupled in all processes; ŝ 2 ND larger by ) effective : s 2 l = (15) + simple + Z asymmetry independent of m t + Z widths: m t in ρ f only phenomenological; exact definition in computer code different for each f hard to relate to non Z-pole observables

17 Running of α Largest theory uncertainty in M Z ŝ 2 Z α(m 2 Z ) = α 1 α α = α l + α t + α (5) had α (5) had α α 1 (M Z ) ˆα 1 (M Z ) ( ) πα/ 2GF (cf. ahad µ ) M 2 Z = ˆρĉ 2 Zŝ2 Z (1 ˆr W ) Calculation of α (5) had ˆρ 1 + 3G F ˆm 2 t 8 2π 2 + ˆr W α + Data driven: R had up to 40 GeV; PQCD above Theory driven: PQCD + NPQCD (OPE, sum rules) above 2 GeV smaller uncertainties New BES-II data convergence Measurements of running α: TOPAZ (e + e µ + µ ); VENUS, L3 (Bhabha), OPAL (high Q 2 )

18 Recent evaluations of on-shell α (5) had(m Z ) (Adjusted to fixed α s (M Z ) = 0.120) Author(s) Result Comment Martin & Zeppenfeld ± PQCD for s > 3 GeV Eidelman & Jegerlehner ± PQCD for s > 40 GeV Geshkenbein & Morgunov ± Burkhardt & Pietrzyk ± O(α s ) resonance model PQCD for s > 40 GeV Swartz ± use of fitting function Alemany, Davier, Höcker ± includes τ decay data Krasnikov & Rodenberg ± PQCD for s > 2.3 GeV Davier & Höcker ± PQCD for s > 1.8 GeV Kühn & Steinhauser ± complete O(α 2 s ) Erler ± converted from MS scheme Davier & Höcker ± use of QCD sum rules Groote et al ± use of QCD sum rules Jegerlehner ± converted from MOM Martin, Outhwaite, Ryskin ± includes new BES data Pietrzyk ± details not published Trocóniz, Ynduráin ± BES, α s = 0.117(3)

19 Z pole + LEP 2 + WNC + Tevatron SM tested at 0.1% level, including EW loops (gauge principle, group, representations, renorm. field theory) sin 2 θ W ; m t, α s (loops; agree with direct) determined; α had, M H constrained M H < 205 GeV (direct: M H > 112 GeV) SUSY however, sensitive to A F B (b) severe constraint on TeV physics unification (decoupling): expect 0.1% TeV compositeness: expect several % precise gauge coupling constants (unification)

20 Global Electroweak Fits much more information than individual experiments caveat: experimental/theoretical systematics, correlations PDG 00 review + spring 01 (J. Erler and PL) Complete Z-pole and WNC (important beyond SM) New radiative correction program (Erler) GAPP: Global Analysis of Particle Properties Fully MS (ZFITTER on-shell) New α had, correlated with α s Good agreement with LEPEWWG up to wellunderstood effects (WNC, HOT, α had ) despite different renormalization schemes erler/electroweak/

21 Fit Results (06/02) (Erler, PL) M H = GeV, m t = ± 4.4 GeV, α s = ± , ˆα(M Z ) 1 = ± ŝ 2 Z = ± , s 2 l = ± , s 2 W = ± s 2 M Z = ± χ 2 /d.o.f. = 49.0/40(15%) LEPEWWG (on-shell, data set, H.O.T., α (5) had(m Z )) (05/02) M H = GeV α s = ± m t = GeV s 2 l = ± s 2 W = ± m t = ± 4.4 GeV GeV from indirect (loops) only (direct: ± 5.1)

22 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) Higgs mass M H = 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) 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

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24 Skeletons in the Closet Standard model (SU 3 SU 2 U 1 + general relativity) correct to cm, but 21 free parameters ( 28 with m ν 0). Gauge Problem complicated gauge group with 3 couplings charge quantization ( q e = q p ) unexplained Fermion problem Fermion masses, mixings, families unexplained Higgs/hierarchy problem Expect M 2 H = O(M 2 W ) higher order corrections: δm 2 H /M 2 W 1034 Strong CP problem Can add θ 32π 2 g 2 s F F to QCD (breaks, P, T, CP) d N θ < 10 9 but δθ weak 10 3 Graviton problem gravity not unified quantum gravity not renormalizable cosmological constant: Λ SSB = 8πG N V > Λ obs

25 The Two Paths: Unification or Compositeness The Bang unification of interactions grand desert to unification (GUT) or Planck scale elementary Higgs, supersymmetry (SUSY), GUTs, strings possibility of probing to M P and very early universe hint from coupling constant unification tests light (< GeV) Higgs (LEP 2, TeV, LHC) absence of deviations in precision tests (usually) supersymmetry (LHC) possible: m b, proton decay, ν mass, rare decays SUSY-safe: Z ; seq/mirror/exotic fermions; singlets The Whimper onion-like layers composite fermions, scalars (dynamical sym. breaking) not like to atom nucleus +e p + n quark at most one more layer accessible (LHC) rare decays (e.g., K µe) severe problem no realistic models effects (typically, few %) expected at LEP & other precision observables (4-f ops; Zb b; ρ 0 ; S, T, U) anomalous V V V, new particles, future W W W W

26 Beyond the standard model ρ 0 ; S, T, U: Higgs triplets, nondegenerate fermions or scalars; chiral families (ETC) S = 0.14 ± 0.10( 0.08) T = 0.15 ± 0.12(+0.09) U = 0.32 ± 0.12(+0.01) for M H = (300) GeV ρ 0 = 1 + αt = (M H = GeV) for S = U = 0 (2.6σ) 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

27 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

28 Supersymmetry decoupling limit (M new > GeV): only precision effect is light SM-like Higgs little improvement on SM fit SUSY parameters constrained Heavy Z : GUTs, string theories Typically M Z > GeV (Tevatron, LEP 2, WNC), θ Z Z < few 10 3 (Z-pole) Other Exotic fermion mixings Large extra dimensions New Four-fermi operators 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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31 A Heavy Z? Expected in many grand unified theories Expected in many string constructions Typically, M < Z 1 TeV Often family non-universal (FCNC) Natural solution to µ problem Nonstandard superpartner spectrum Typically M Z > GeV (Tevatron, LEP 2, WNC), θ Z Z < few 10 3 (Z-pole)

32 Possible hints of Z Atomic parity violation (controversial) SM: Q W (Cs) = ± 0.03 experiment: value has fluctuated depending on Breit correction (large), vac. pol. (large), and electron line (?) corrections. Recent: Q W = 72.12(28)(34) (2.2σ) (Johnson et al, 2001); 72.18(29)(36) (2σ) (Dzuba et al, 2002); 72.83(29)(39) (Kuchiev, 2002) Theory error? New NuTeV results ( ( ) ν µ N ( ) ν µ N) sin 2 θ W = (13)(9)(3.0σ) (SM: (4)) Z lineshape (σ had ) A F B (b) (if non-universal couplings) Should be observable at Tevatron if present Detailed studies of couplings at LHC and LC

33 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]

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35 Discovery of Pluto 1781 Uranus observed by (Sir William Herschel) 1846 Uranus orbit anomalies Neptune predicted (John Adams, Jean Leverrier) 1846 Neptune observed in predicted location 1900 s Further Uranus anomalies Pluto predicted by computers (several possible locations) 1930 Pluto discovered in one of predicted locations (Clyde Tombaugh) 1978 Charon discovered m Pluto too small to affect Uranus orbit

36 α 1 (α, sin 2 θ W ) α 2 (G F, M W ) α 3 me m µ mb M Z M unification

37 Conclusions WNC, Z, W are primary predictions and test of electroweak unification SM correct and unique to first approx. (gauge principle, group, representations) SM correct at loop level (renorm gauge theory; m t, α s, M H Watershed: TeV physics severely constrained (unification vs compositeness) implications for Tevatron (Run II) and LHC Precise gauge couplings (gauge unification) Future Direct production of new particles at hadron colliders CP violation in B system New precision era in future linear collider Polarized Møller, polarized ep, APV