1 Electroweak Measurements at NuTeV: Are Neutrinos Different? Mike Shaevitz Columbia University for the NuTeV Collaboration Introduction to Electroweak Measurements NuTeV Experiment and Technique Experimental and Theoretical Simulation Data Sample and Checks Electroweak Fits Interpretations and Summary

2 Electroweak Theory Standard Model –Charged Current mediated by W  with (V-A) Neutral Current mediated by Z 0 with couplings below –One parameter to measure! Weak / electromagnetic mixing parameter sin 2  W (Related to weak/EM coupling ratio g’=g tan  W ) Neutrinos are special in SM –Only have left-handed weak interactions  W  and Z boson exchange

3 History of EW Measurements Discovery of the Weak Neutral Current (1973 CERN) First Generation EW Experiments (late 1970’s) – Precision at the 10% level –Tested basic structure of SM  M W,M Z Gargamelle HPWF CIT-F CCFR, CDHS, CHARM, CHARM II UA1, UA2, Petra, Tristan, APV, SLAC eD NuTeV, D0, CDF, LEP1 SLD LEPII, APV, SLAC-E158 Gargamelle Second Generation EW Experiments (late 1980’s) –Discovery of W,Z boson in –Precision at the 1-5% level –Radiative corrections become important –First limits on the M top Current Generation Experiments – Precision below 1% level – Discovery of the top quark – Constrain M Higgs  Predict light Higgs boson (and possibly SUSY) – Use consistency to search for new physics !

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5 Current Era of Precision EW Measurements Precision parameters define the SM: –  EM   45ppb Z ) – G     GeV -2 10ppm – M Z  23ppm Comparisons test the SM and probe for new physics –LEP/SLD (e+e-), CDF/D0 (p-pbar), N, HERA (ep), APV Radiative corrections are large and sensitive to M top and M Higgs –M Higgs constrained in SM to be less than 196 GeV at 95%CL

6 Are There Cracks? All data suggest a light Higgs except A FB b Global fit has large   =23/15 (9%) –A FB b is off about 3   inv also off by  –N     g R b too low Higgs Mass Constraint Leptons Quarks Other anomalies: - g-2 at BNL (2.6  ) - Atomic Parity Violation (0 – 2.2  ) - |V ud | (2.3 – 3.0  )

7 From Bruce Knuteson’s Columbia Colloquium Precision Electroweak measurements X Added to his list

8 NuTeV Adds Another Arena Precision comparable to collider measurements of M W Sensitive to different new physics –Different radiative corrections Measurement off the Z pole –Exchange is not guaranteed to be a Z Measures neutrino neutral current coupling –LEP 1 invisible line width is only other precise  measurement Sensitive to light quark (u,d) couplings –Overlap with APV, Tevatron Z production Tests universality of EW theory over large range of momentum scales

9 For an isoscalar target composed of u,d quarks: NC/CC ratio easiest to measure experimentally but... –Need to correct for non-isoscalar target, radiative corrections, heavy quark effects, higher twists –Many SF dependencies and systematic uncertainties cancel –Major theoretical uncertainty m c  Suppress CC wrt NC Neutrino EW Measurement Technique Charged-Current (CC) Neutral-Current (NC)

10 Charm Mass Effects CC is suppressed due to final state c-quark  Need to know s-quark sea and m c –Modeled with leading-order slow-rescaling –Measured by NuTeV/CCFR using dimuon events ( N  cX  X) (NuTeV: M. Goncharov et al., Phys. Rev. D64: ,2001 and CCFR: A.O. Bazarko et al., Z.Phys.C65: ,1995) Charged-Current Production Neutral-Current of Charm

11 Before NuTEV N experiments had hit a brick wall in precision  Due to systematic uncertainties (i.e. m c.... ) (All experiments corrected to NuTeV/CCFR m c and to large M top > M W )

12 NuTeV’s Technique R  manifestly insensitive to sea quarks –Charm and strange sea error negligible –Charm production small since only enters from d V quarks only which is Cabbibo suppressed and at high-x But R  requires separate  and   beams  NuTeV SSQT (Sign-selected Quad Train) Cross section differences remove sea quark contributions  Reduce uncertainties from charm production and sea

13 Beam is almost pure or    in mode 3     in  mode 4     Beam only has  electron neutrinos  Important background for isolating true NC event NuTeV Sign-Selected Beamline Dipoles make sign selection - Set   type - Remove e from K long (Bkgnd in previous exps.)

14 NuTeV Lab E Neutrino Detector –168 Fe plates (3m  3m  5.1cm) –84 liquid scintillation counters Trigger the detector Measure: Visible energy interaction point Event length –42 drift chambers Localize transverse vertex – Solid Fe magnet Measures  momentum/charge P T = 2.4 GeV for  P/P  690 ton  target Target / Calorimeter Toroid Spectrometer Continuous Test Beam simultaneous with runs - Hadron, muon, electron beams - Map toroid and calorimeter response

15 NuTeV Detector Picture from Detector is now dismantled

16 NuTeV Collaboration Cincinnati 1, Columbia 2, Fermilab 3, Kansas State 4, Northwestern 5, Oregon 6, Pittsburgh 7, Rochester 8 (Co-spokepersons: B.Bernstein, M.Shaevitz) K. S. McFarland

17 Neutral Current / Charged Current Event Separation Separate NC and CC events statistically based on the “event length”defined in terms of # counters traversed

18 Events selections: –Require Hadronic Energy, E Had > 20 GeV –Require Event Vertex with fiducial volume Data with these cuts: 1.62  million events 351 thousand  events target - calorimeter toroid spectrometer Ehad (GeV) Events Data/MC Events Ehad (GeV) Data/MC NuTeV Data Sample

19 Determine R exp : The Short to Long Ratio: NC CC Cuts: - Ehad > 20 GeV - Fiducial Vol. Event Length Distribution Neutrinos Event Length Distribution Antineutrinos

20 From R exp to R Cross Section Model –LO pdfs for Iron (CCFR) –Radiative corrections –Isoscalar corrections –Heavy quark corrections –R Long –Higher twist corrections Detector Response – CC  NC cross-talk – Beam contamination – Muon simulation – Calibrations – Event vertex effects Neutrino Flux –  and e flux Need detailed Monte Carlo to relate R exp to R and sin 2  W Analysis goal is use data directly to set and check the Monte Carlo simulation

21 Monte Carlo also Corrects for NC/CC Confusion A few CC events look like NC events Short  CC’s (20% , 10%  ) –muon exits, range out at high y Short e CC’s (5%) – e N  e X Cosmic Rays (Correction) (0.9%/4.7%) Long  NC’s (0.7%) –punch-through effects A few NC events look like CC events

22 Key Elements of Monte Carlo Parton Distribution Model –Needed to correct for details of the PDF model –Needed to model cross over from short  CC events Neutrino fluxes – –Electron neutrino CC events always look short Shower Length Modeling –Needed to correct for short events that look long Detector response vs energy, position, and time –Test beam running throughout experiment crucial Top Five Largest Corrections

23 PDFs extracted from CCFR data exploiting symmetries: –Isospin symmetry: u p =d n, d p =u u, and strange = anti-strange Data-driven: uncertainties come from measurements LO quark-parton model tuned to agree with data: –Heavy quark production suppression and strange sea (CCFR/NuTeV N     X data) –R L, F 2 higher twist (from fits to SLAC, BCDMS) –d/u constraints from NMC, NUSEA(E866) data –Charm sea from EMC F 2 cc This “tuning” of model is crucial for the analysis Neutrino xsec vs y at 190 GeVAntineutrino xsec vs y at 190 GeV CCFR Data Use Enhanced LO Cross-Section

24 Use beam Monte Carlo simulation tuned to match the observed  spectrum –Tuning needed to correct for uncertainties in SSQT alignment and particle production at primary target –Simulation is very good but needs small tweaks at the  level for E , E K, K/  Events NuTeV Neutrino Flux

25 Charged-Current Control Sample Medium length events (L>30 cntrs) check modeling and simulation of Short charged-currents sample –Similar kinematics and hadronic energy distribution to short CC events Good agreement between data and MC for the medium length events E Had (GeV)

26 Approximately 5% of short events are e CC events (It would take a 20% mistake in e to move NuTeV value to SM ) –Main e source is K  decay (93% / 70%) –Others include K L, ’s (4%/18%) reduced by SSQT and Charm (2%/9%) –Main uncertainty is K  e3 branching ratio (known to 1.4%) !! But also have direct e measurement techniques. 98% 1.7% 98% 1.6% NuTeV Electron Neutrino Flux

27 Direct Measurments of e Flux 1.   CC (wrong-sign) events in antineutrino running constrain charm and K L production 2. Shower shape analysis can statistically pick out events (  E  GeV) 3. e from very short events (E  GeV) –Precise measurement of e in tail region of flux –Observe  more e than predicted above 180 GeV, and a smaller excess in  beam –Conclude that we should require E had < 180 GeV NuTeV preliminary result did not have this cut  shifts sin 2  W by E Had (GeV) Data MC

28    dof    dof Verify systematic uncertainties with data to Monte Carlo comparisons as a function of exp. variables. Longitudinal Vertex: checks detector uniformity R exp Stability Tests vs. Experimental Parameters Note: Shift from zero is because NuTeV result differs from Standard Model

29 R exp vs. length cut: Check NC  CC separation syst. –“16,17,18” L cut is default: tighten  loosen selection R exp vs. radial bin: Check corrections for e and short CC which change with radius. Stability Tests (cont’d) Yellow band is stat error

30 Short Events (NC Cand.) vs E had Long Events (CC Cand.) vs E had Distributions vs. E had

31 Stability Test: R exp vs E Had Short/Long Ratio vs E Had checks stability of final measurement over full kinematic region –Checks almost everything: backgrounds, flux, detector modeling, cross section model,..... exp E Had (GeV)

32 Fit for sin 2  W This combined fit is equivalent to using R  in reducing systematic uncertainty (From NuTeV and CCFR)

33 Uncertainties in Measurement sin 2  W error statistically dominated  R  technique R uncertainty dominated by theory model

34 NuTeV Technique Gives Reduced Uncertainties  4 better  6 better

35 NuTeV result: –Error is statistics dominated –Is  2.3 more precise than previous N experiments where sin 2  W =  and syst. dominated Standard model fit (LEPEWWG):  A 3  discrepancy The Result Phys.Rev.Lett. 88,91802 (2002)

36 Result from Fit to R and R  Discrepancy is left-handed coupling to u and d quarks Discrepancy is neutrinos not antineutrinos  NC coupling is too small

37 Comparison to Other EW Measurements Each electroweak measurement also indicates a range of Higgs masses M W =  GeV from QCD and electroweak radiative corrections are small Precision comparable to collider measurements but value is smaller

38 SM Global Fit with NuTeV sin 2  W Without NuTeV: –  2 /dof = 19.6/14, probability of 14% With NuTeV: –  2 /dof = 28.8/15, probability of 1.7% Upper m Higgs limit only weakens slightly NuTeV

39 Possible Interpretations Changes in Standard Model Fits –Change PDF sets –Change M Higgs “Old Physics” Interpretations: QCD –Violations of “isospin” symmetry –Strange vs anti-strange quark asymmetry –Shadowing and other nuclear effects Are ’s Different? –Special couplings to new particles –Mixing and Oscillation Effects “New Physics” Interpretations –New Z ’ or lepto-quark exchanges –New particle loop corrections –Mixtures of new physics

40 Standard Model Fits to Quark Couplings Difficult to explain discrepancy with SM using: –Parton distributions or LO vs NLO or –Electroweak radiative corrections: heavy m Higgs Example variations with LO/NLO PDF Sets (no NLO m c effects) (S.Davidson et al. hep-ph/ )

41 Symmetry Violating QCD Effects Paschos-Wolfenstein Relation Assumptions: –Assumes total u and d momenta equal in target –Assumes sea momentum symmetry, s =  s and c =  c –Assumes nuclear effects common in W/Z exchange Violations of these symmetries can arise from 1.A  2Z due to neutron excess (corrected for in NuTeV) 2.Isospin violating PDF’s, u p (x)  d n (x) (Sather; Rodinov, Thomas and Londergan; Cao and Signal) –Changes d/u of target  mean NC coupling 3.Asymmetric heavy-quark sea, s(x) =  s (x) (Signal and Thomas; Burkhardt and Warr; Brodsky and Ma) –Strange sea doesn’t cancel in R  4. Mechanisms for different shadowing in NC/CC –Effects R  R  directly Bottom Line: R  technique is very robust

42 Detailed Examination of Symmetry Violation Effects New Publication: On the Effects of Asymmetric Strange Seas and Isospin-Violating Parton Distribution Functions on sin 2  W Measured in the NuTeV Experiment. ( G.P. Zeller et al., Phys.Rev.D65:111103,2002; hep- ex/ ) Parameterize the shifts from various asymmetries for the NuTeV sin 2  W analysis technique F(x;effect)

43 Quantitative Results: Isopin Symmetry Violation Isospin symmetry violation: u p  d n and d p  u n –Full “Bag Model” calculation: (Thomas et al., MPL A9 1799)   sin 2  W =  Violation is x dependent; opposite sign at large and small x Largest contribution from m d – m u ~ 4 MeV and then m dd – m uu mass differences. –“Meson Cloud Model”: (Cao et al., PhysRev C )   sin 2  W = –What is needed to explain the NuTeV data? –Can global PDF fits accommodate a large enough isospin violation to explain NuTeV?  CTEQ

44 Quantitative Results: Strange vs. Anti-strange asymmetry Non-perturbative QCD effects can generate a strange vs. anti-strange momentum asymmetry –Fits to NuTeV and CCFR  and  dimuon data can measure this asymmetry:  s  c  X –NuTeV separate  and   beams important for reliable separation of s and  s distributions (NuTeV: M. Goncharov et al.and CCFR: A.O. Bazarko et al.)

45 Preliminary NLO Analysis of NuTeV Dimuon Data (Dave Mason et al.(NuTeV Collab.), ICHEP02 Proceedings) s =  s s (black)  s (blue)

46 Nuclear Effects Use NuTeV CC data to fit parton distributions ? Need to worry about nuclear effects that could be different for W and Z exchange? Most nuclear effects are only large at small Q 2 and would effect W and Z the same –EMC effect –Nuclear shadowing –Vector meson dominance (VMD) VMD could be different for W and Z exchange (Miller and Thomas, hep-ex/ ) –No evidence for predicted 1/Q 2 dependence in the NuTeV kinematic region x > 0.01 (NMC) –NuTeV sees no effect with increasing the E had cut. –Low-x phenomena like VMD effects mainly sea quarks and the effect is canceled in R - Would increase both R  and R  To explain deviation from SM would require a very large R  shift and make the discrepancy with SM 4.5  These nuclear effects will change R  and R  more than R - Making their deviation with the SM much more significant  Hard to explain NuTeV discrepancy with nuclear effects

47  LEP 1 measures Z lineshape and partial decay widths to infer the “number of neutrinos”  Neutrino oscillations (Giunti et al. hep-ph/ )  e  s oscillation make real e background subtraction smaller (Requires high  m 2 and ~20% mixing; will be addressed by MiniBooNE)  NuTeV measures e flux from shower shape analysis  hard to allow such large mixing Are ’s Different?

48 “New Physics” Interpretations Z’, LQ,... exchange –NuTeV needs LL enhanced relative to LR coupling Gauge boson interactions –Allow generic couplings –Example: Extra Z’ boson Mixing with E(6) Z’ Z’=Z  cos  + Z  cos  LEP/SLC mix<10 -3 Hard to accommodate entire NuTeV discrepancy. –Global fits somewhat better with E(6) Z’ included Example: Erler and Langacker: SM    7.5 m Z’ =600 GeV, mixing ~10 -3,  –“Almost sequential” Z’ with opposite coupling NuTeV would want m Z’ ~1.2 TeV CDF/D0 Limits: m Z’ > 700 GeV (Cho et al., Nucl.Phys.B531, 65.; Zeppenfeld and Cheung, hep-ph/ ; Langacker et al., Rev.Mod.Phys.64,87; Davidson et al., hep-ph/ ) (S.Davidson et al. hep-ph/ )

49 New Physics Radiative Corrections: S,T,U Formalism Radiative corrections –1) Vertex corrections –2) Box corrections –3) Vacuum polarization or oblique (propagator) corrections Use S,T,U formalism to parameterize these type of corrections S and T only,   = 11.3 / 3 df  cannot explain neutrino results Vertex and box corrections suppressed or would make fermions heavy - May be different for b-quarks which are related to heavy top mass NuTeV LEP Z  NuTeV W. Loinaz, et al. (hep-ph/ )

50 Recent Summary of Possible Interpretations S.Davidson, S.Forte,P.Gambino,N.Rius,A.Strumia (hep-ph/ ) QCD effects: –Small asymmetry in momentum carried by strange vs antistrange quarks   CCFR/NuTeV dimuons limits –Small isospin violation in PDFs  expected to be small Propagator and coupling corrections to SM gauge bosons: –Small compared to effect –Hard to change only Z MSSM: –Loop corrections wrong sign and small compared to NuTeV Contact Interactions: –Left-handed quark-quark-lepton-lepton vertices,  LL  qq, with strength  of the weak interaction  Look Tevatron Run II Leptoquarks: –SU(2) L triplet with non-degenerate masses can fit NuTeV and evade  decay constraints Extra U(1) vector bosons (“Designer Z”) : –An unmixed Z ’ with B-3L  symmetry can explain NuTeV –Mass: 600 < M Z ’ < 5000 GeV or 1 < M Z ’ < 10 GeV

51 “The NuTeV Anomaly, Neutrino Mixing and a Heavy Higgs” W. Loinaz, N. Okamura, T. Takeuchi, and L. Wijewardhana (hep-ph/ ) Suppression of the Z coupling occur naturally in models which mix the neutrinos with heavy gauge singlet states –i.e. L = cos  light + sin  heavy Z reduced by cos 2  –Models would then reduce: sin 2  NuTeV by  Z invisible width by  –But this changes G F extracted from  -decay by  and messes up dozens of Z-pole agreements Can fix this if G F is compensated by a shift in the  parameter (T)  allowed with “new physics” Fits to Z-pole and NuTeV data of “new physics” (S and T parameters) plus this Z  Need heavy Higgs (T < 0) with an  mixing) Apparent NC coupling to the Z is too small  Reduced coupling due to neutrino mixing LEP Z 

52 W. Loinaz et al. Model Cont’d: Trying to Fit It All Together Including S, T, and  Mixing (  )  Good fit with   = 1.2 / 2 df Best T<0 “new physics” is big M higgs But this fit contradicts M W-boson so need (Note: M W off ~1.5  with SM M Higgs = 85 GeV and off > 2  with M Higgs = 115 GeV) Conclusion: Everything fits with –Mixed neutrinos:  –Heavy Higgs: M higgs  GeV –New physics (U-parameter type)  New heavy bound states???

53 Lose-Lose vs. Win-Win (M. Chanowitz, Phys.Rev.D66:073002, hep-ph/ ) M Higgs prediction from the SM are only valid if SM is valid which doesn’t seem to be the case –Global fit to all data give poor confidence level of CL = 1% From NuTeV and A b FB vs the leptonic measures –Global fit removing NuTeV and A b FB gives good confidence level of CL = 65% But this fit give an M hHggs = 43 GeV which has a CL of 3.5% to be consistent with the LEP M Higgs >114 GeV Lose - Lose The SM is disfavored and need new physics whether one includes NuTeV and A b FB or not. – If NuTeV and A b FB are correct, then need new physics to explain their 3  discrepancies. – If NuTeV and A b FB are due to systematic uncertainties, then need new physics to explain why M Higgs is so low Win - Win NuTeV + A had FB A lep + M W

54 Future Measurements Unfortunately, the high-energy neutrino beam at Fermilab has been terminated  Need to rely on other experiments for progress Upcoming new measurements: –Low Q 2 measurements: (New physics could shift NuTeV/APV different from Z-pole LEP/SLD) SLAC E-158 Pol. electron-electron scattering JLab QWEAK Pol. elastic ep (But these test e’s and not ’s) Other measurements need theory input (g-2, APV, |V ud |) –High energy measurements: Fermilab Tevatron CDF/D0 Run 2 and LHC searches for Z’ and leptoquarks –Neutrino mixing and oscillations? (Plot from J.Erler and P.Langacker) Tevatron (D0,CDF) LHC (Atlas,CMS) Future

55 Summary NuTeV measurement has the precision to be important for SM electroweak test Many experimental checks and the R  technique is robust with respect to systematic uncertainties For NuTeV the SM predicts  but we measure sin 2  W (on-shell)  (stat.)  (syst.) (Previous neutrino measurements gave  In comparison to the Standard Model The NuTeV data prefers lower effective left-handed quark couplings Or neutral current coupling that is ~1.1% smaller than expected The discrepancy with the Standard Model could be related to: Quark model uncertainties but unlikely or only partially and / or Possibly new physics that is associated with neutrinos and interactions with left-handed quarks or neutrino mixing/oscillations Jan/Feb 2002 CERN Courier article: “Is the latest NuTeV result a blip or another neutrino surprise? Only time will tell.” Concluding remark from Tatsu Takeuchi at Fermilab Wine-n-Cheese last Friday: “The Standard Model is finally showing cracks and water is starting to rush into the Titanic.”