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Electroweak Measurements at NuTeV: Are Neutrinos Different?

Electroweak Measurements at NuTeV: Are Neutrinos Different?. Introduction to Electroweak Measurements NuTeV Experiment and Technique Experimental and Theoretical Simulation Data Sample and Checks Electroweak Fits Interpretations and Summary. Mike Shaevitz

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Electroweak Measurements at NuTeV: Are Neutrinos Different?

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

  2. Electroweak Theory • Standard Model • Charged Current mediated by W with (V-A) Neutral Current mediated by Z0 with couplings below • One parameter to measure! • Weak / electromagnetic mixing parameter sin2qW(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 Gargamelle CCFR, CDHS, CHARM, CHARM IIUA1 , UA2 , Petra , Tristan , APV, SLAC eD • 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  MW,MZ • Second Generation EW Experiments (late 1980’s) • Discovery of W,Z boson in 1982-83 • Precision at the 1-5% level • Radiative corrections become important • First limits on the Mtop • Current Generation Experiments • Precision below 1% level • Discovery of the top quark • Constrain MHiggs •  Predict light Higgs boson • (and possibly SUSY) • Use consistency to search for new physics ! GargamelleHPWF CIT-F NuTeV, D0, CDF, LEP1 SLDLEPII, APV , SLAC-E158

  4. Current Era of Precision EW Measurements • Precision parameters define the SM: • aEM-1 = 137.03599959(40) 45ppb (200ppm@MZ) • Gm = 1.16637(1)10-5GeV-2 10ppm • MZ= 91.1871(21) 23ppm • Comparisons test the SM and probe for new physics • LEP/SLD (e+e-), CDF/D0 (p-pbar), nN , HERA (ep) , APV • Radiative corrections are large and sensitive to Mtop and MHiggs • MHiggs constrained in SM to be less than196 GeV at 95%CL

  5. Are There Cracks? • All data suggest a light Higgs except AFBb • Global fit has large c2=23/15 (9%) • AFBb is off about 3s • Ginv also off by ~2s • Nn = 2.9841  0.0083 Higgs Mass ConstraintLeptonsQuarks gRb too low Other anomalies:- g-2 at BNL (2.6s) - Atomic Parity Violation (0 – 2.2s) - |Vud| (2.3 – 3.0s) c2=10.2/5

  6. From Bruce Knuteson’s Columbia Colloquium Added tohis list X Precision Electroweak measurements

  7. NuTeV Adds Another Arena • Precision comparable to collider measurements of MW • 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 n 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

  8. Neutrino EW Measurement Technique Charged-Current(CC) Neutral-Current(NC) • 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 mc Suppress CC wrt NC

  9. Charm Mass Effects Charged-Current Production Neutral-Current of Charm • CC is suppressed due to final state c-quark  Need to know s-quark sea and mc • Modeled with leading-order slow-rescaling • Measured by NuTeV/CCFR using dimuon events (nN m cX  mmX) (NuTeV: M. Goncharov et al., Phys. Rev. D64: 112006,2001 and CCFR: A.O. Bazarko et al., Z.Phys.C65:189-198,1995)

  10. Before NuTEV • nN experiments had hit a brick wall in precision Due to systematic uncertainties (i.e. mc ....) (All experiments corrected to NuTeV/CCFR mcand to large Mtop > MW )

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

  12. NuTeV Sign-Selected Beamline • Beam is almost pure n or n(n in n mode 310-4, n in n mode 410-3) • Beam only has ~1.6% electron neutrinos  Important background for isolating true NC event Dipoles make sign selection - Set n /n type - Remove ne from Klong (Bkgnd in previous exps.)

  13. 168 Fe plates (3m3m 5.1cm) 84 liquid scintillation counters Trigger the detector Measure: Visible energyn interaction point Event length 42 drift chambers Localize transverse vertex Solid Fe magnet Measures mmomentum/charge PT = 2.4 GeV for dP/P  10% NuTeV Lab E Neutrino Detector 690 ton n-target Toroid Spectrometer Target / Calorimeter Continuous Test Beamsimultaneous with n runs- Hadron, muon, electron beams- Map toroid and calorimeter response

  14. NuTeV Detector Picture from 1998 - Detector is now dismantled

  15. NuTeV Collaboration K. S. McFarland Cincinnati1, Columbia2, Fermilab3, Kansas State4, Northwestern5, Oregon6, Pittsburgh7, Rochester8(Co-spokepersons: B.Bernstein, M.Shaevitz)

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

  17. NuTeV Data Sample n toroid spectrometer target - calorimeter Events Events Data/MC Data/MC Ehad (GeV) • Events selections: • Require Hadronic Energy, EHad > 20 GeV • Require Event Vertex with fiducial volume • Data with these cuts: 1.62million n events 351 thousand n events Ehad (GeV)

  18. Determine Rexp: The Short to Long Ratio: Event Length DistributionNeutrinos NC CC Event Length DistributionAntineutrinos Cuts:- Ehad > 20 GeV- Fiducial Vol.

  19. Cross Section Model LO pdfs for Iron (CCFR) Radiative corrections Isoscalar corrections Heavy quark corrections RLong Higher twist corrections Detector Response CC  NC cross-talk Beam contamination Muon simulation Calibrations Event vertex effects Neutrino Flux nm and ne flux From Rexp to Rn Need detailed Monte Carlo to relate Rexp to Rn and sin2qW Analysis goal is use data directly to set and check the Monte Carlo simulation

  20. Monte Carlo also Corrects for NC/CC Confusion A few CC events look like NC events • Short nm CC’s(20% n , 10%n) • muon exits, range outat high y • Short ne CC’s (5%) • ne N  e X • Cosmic Rays (Correction) (0.9%/4.7%) A few NC events look like CC events • Long nm NC’s (0.7%) • punch-through effects

  21. Key Elements of Monte Carlo • Parton Distribution Model • Needed to correct for details of the PDF model • Needed to model cross over from short nm 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

  22. Use Enhanced LO Cross-Section • PDFs extracted from CCFR data exploiting symmetries: • Isospin symmetry: up=dn , dp=uu , 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 nNm+m-X data) • RL , F2 higher twist (from fits to SLAC, BCDMS) • d/u constraints from NMC, NUSEA(E866) data • Charm sea from EMC F2cc This “tuning” of model is crucial for the analysis Neutrino xsec vs y at 190 GeV Antineutrino xsec vs y at 190 GeV CCFR Data

  23. NuTeV Neutrino Flux • Use beam Monte Carlo simulation tuned to match the observed nm 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 ~0.3 -3% level for Ep , EK , K/p Events Events

  24. Charged-Current Control Sample 20 50 100 180 20 50 100 180EHad (GeV) • 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.

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

  26. Direct Measurments of ne Flux DataMC 20 50 100 180 20 50 100 180EHad (GeV) 1. nmCC (wrong-sign) events in antineutrino running constrain charm and KL production 2. Shower shape analysis can statistically pick out n events (80 < En < 180 GeV) 3. ne from very short events (En > 180 GeV) • Precise measurement of ne in tail region of flux • Observe ~35% more ne than predicted above 180 GeV,and a smaller excess inn beam • Conclude that we should require Ehad < 180 GeV NuTeV preliminaryresult did not havethis cut shifts sin2qW by +0.002

  27. Rexp Stability Tests vs. Experimental Parameters Verify systematic uncertainties with data to Monte Carlo comparisons as a function of exp. variables. • Longitudinal Vertex: checks detector uniformity c2 = 50 / 62 dof c2 = 49 / 62 dof Note: Shift from zero is because NuTeV result differs from Standard Model

  28. Stability Tests (cont’d) Yellow band is stat error • Rexp vs. length cut: Check NC  CC separation syst. • “16,17,18” Lcut is default: tighten  loosen selection • Rexp vs. radial bin: Check corrections for ne and short CC which change with radius.

  29. Distributions vs. Ehad Short Events(NC Cand.) vs Ehad Long Events(CC Cand.) vs Ehad

  30. Stability Test: Rexp vs EHad exp exp 20 50 100 180 20 50 100 180 EHad (GeV) 20 50 100 180 20 50 100 180 EHad (GeV) • Short/Long Ratio vs EHad checks stability of final measurement over full kinematic region • Checks almost everything: backgrounds, flux, detector modeling, cross section model, .....

  31. Fit for sin2qW (From NuTeV and CCFR) This combined fit is equivalent to using R- in reducing systematic uncertainty

  32. Uncertainties in Measurement • sin2qWerror statistically dominated  R- technique • Rn uncertainty dominated by theory model

  33. NuTeV Technique Gives Reduced Uncertainties 4 better 6 better

  34. The Result • NuTeV result: • Error is statistics dominated • Is 2.3 more precise than previous nN experimentswhere sin2qW = 0.22770.0036 and syst. dominated • Standard model fit (LEPEWWG): 0.2227  0.00037A 3s discrepancy ........... Phys.Rev.Lett. 88,91802 (2002)

  35. Result from Fit to Rn and Rn Discrepancy is neutrinos not antineutrinos Discrepancy is left-handed coupling to u and d quarks n NC coupling is too small

  36. Comparison to Other EW Measurements MW = 80.136  0.084 GeV from QCD and electroweak radiative corrections are small • Precision comparable to collider measurements but value is smaller Each electroweak measurement also indicates a range of Higgs masses

  37. SM Global Fit with NuTeV sin2qW • Without NuTeV: • c2/dof = 19.6/14, probability of 14% • With NuTeV: • c2/dof = 28.8/15, probability of 1.7% • Upper mHiggs limit only weakens slightly NuTeV

  38. Possible Interpretations • Changes in Standard Model Fits • Change PDF sets • Change MHiggs • “Old Physics” Interpretations: QCD • Violations of “isospin” symmetry • Strange vs anti-strange quark asymmetry • Shadowing and other nuclear effects • Are n’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

  39. Standard Model Fits to Quark Couplings Example variationswith LO/NLO PDF Sets(no NLO mc effects) • Difficult to explain discrepancy with SM using: • Parton distributions or LO vs NLO or • Electroweak radiative corrections: heavy mHiggs (S.Davidson et al. hep-ph/0112302)

  40. 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 • A  2Z due to neutron excess (corrected for in NuTeV) • Isospin violating PDF’s, up(x)  dn(x)(Sather; Rodinov, Thomas and Londergan; Cao and Signal) • Changes d/u of target  mean NC coupling • Asymmetric heavy-quark sea,s(x) = s (x)(Signal and Thomas; Burkhardt and Warr; Brodsky and Ma) • Strange sea doesn’t cancel in R- • Mechanisms for different shadowing in NC/CC • Effects Rn, R directly Bottom Line: R- technique is very robust

  41. Detailed Examination of Symmetry Violation Effects F(x;effect) New Publication: On the Effects of Asymmetric Strange Seas and Isospin-Violating Parton Distribution Functions on sin2qW Measured in the NuTeV Experiment. (G.P. Zeller et al., Phys.Rev.D65:111103,2002; hep-ex/0203004) • Parameterize the shifts from various asymmetries for the NuTeV sin2qW analysis technique

  42. Quantitative Results:Isopin Symmetry Violation • Isospin symmetry violation: up  dn and dp  un • Full “Bag Model” calculation: (Thomas et al., MPL A9 1799)  Dsin2qW = -0.0001 • Violation is x dependent; opposite sign at large and small x • Largest contribution from md – mu ~ 4 MeV and then mdd – muu mass differences. • “Meson Cloud Model”: (Cao et al., PhysRev C62 015203) Dsin2qW = +0.0002 • What is needed to explain the NuTeV data? • Can global PDF fits accommodate a large enough isospin violation to explain NuTeV?  CTEQ

  43. 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 n and dimuon data can measure this asymmetry: n s m c mmX • NuTeV separate n and beams important for reliable separation of s ands distributions(NuTeV: M. Goncharov et al.and CCFR: A.O. Bazarko et al.)

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

  45. 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 Q2 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/0204007) No evidence for predicted 1/Q2 dependence in the NuTeV kinematic region x > 0.01 (NMC) NuTeV sees no effect with increasing the Ehad cut. Low-x phenomena like VMD effects mainly sea quarks and the effect is canceled in R- Would increase both Rnand R To explain deviation from SM would require a very large Rnshift and make the discrepancy with SM 4.5s Nuclear Effects These nuclear effects will changeRnand R more than R-Making their deviation with the SM much more significant Hard to explain NuTeV discrepancy with nuclear effects

  46. Are n’s Different? • LEP 1 measures Z lineshape and partial decay widths to infer the “number of neutrinos” • Neutrino oscillations (Giunti et al. hep-ph/0202152) • ne ns oscillation make real ne background subtraction smaller(Requires high Dm2 and ~20% mixing; will be addressed by MiniBooNE) • NuTeV measures ne flux from shower shape analysis  hard to allow such large mixing

  47. “New Physics” Interpretations • Z’, LQ, ... exchange • NuTeV needs LL enhancedrelative to LR coupling • Gauge boson interactions • Allow generic couplings • Example: Extra Z’ boson • Mixing with E(6) Z’ • Z’=Zcosb + Zcosb • 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 Dc2 7.5 mZ’=600 GeV, mixing ~10-3, b1.2 • “Almost sequential” Z’ with opposite coupling • NuTeV would want mZ’ ~1.2 TeV • CDF/D0 Limits: mZ’ > 700 GeV (Cho et al., Nucl.Phys.B531, 65.; Zeppenfeld and Cheung, hep-ph/9810277; Langacker et al., Rev.Mod.Phys.64,87; Davidson et al., hep-ph/0112302 .) (S.Davidson et al. hep-ph/0112302)

  48. New Physics Radiative Corrections:S,T,U Formalism LEP Z NuTeV NuTeV • 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, c2 = 11.3 / 3 df cannot explain neutrino results Vertex and box corrections suppressed orwould make fermions heavy - May be different for b-quarks which are related to heavy top mass W. Loinaz, et al. (hep-ph/0210193)

  49. Recent Summary of Possible InterpretationsS.Davidson, S.Forte,P.Gambino,N.Rius,A.Strumia (hep-ph/0112302) • QCD effects: • Small asymmetry in momentum carried by strange vs antistrange quarks   CCFR/NuTeV n 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 nZn • MSSM: • Loop corrections wrong sign and small compared to NuTeV • Contact Interactions: • Left-handed quark-quark-lepton-lepton vertices, eLLnnqq , with strength ~0.01 of the weak interaction  Look Tevatron Run II • Leptoquarks: • SU(2)L triplet with non-degenerate masses can fit NuTeV and evade p-decay constraints • Extra U(1) vector bosons (“Designer Z”) : • An unmixed Z’ with B-3Lm symmetry can explain NuTeV • Mass: 600 < MZ’< 5000 GeV or 1 < MZ’ < 10 GeV

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