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NuTeV Revisited: Neutrino Electroweak Physics Insights

Explore NuTeV measurements revisited by Kevin McFarland, University of Rochester, highlighting precision challenges in neutrino electroweak couplings and unresolved puzzles in neutrino physics. Discover the NuTeV measurement technique, sign-selected beamline advancements, NLO corrections, QCD and QED calculations, and investigations on QCD symmetry violations and asymmetric strange sea effects. Uncover the implications of these findings on sin2θW and exploring the complex interplay of neutrinos in electroweak physics.

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NuTeV Revisited: Neutrino Electroweak Physics Insights

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  1. Revisiting NuTeV Kevin McFarland University of Rochester DIS 2008, UC-London, 8 April 2008

  2. The Big Picture • Neutrinos are important in electroweak physics • there is a glorious history, of course… • … but precision today in neutrino electroweak couplings lags behind other sectors • neutrino couplings are the most difficult couplings to measure precisely at the Z0 pole • matter effects in ν oscillations are sensitive to only flavor non-diagonal couplings • Some outstanding puzzles in neutrino physics • ~3σ NuTeV result σ(νq→νq)/σ(νq→μq’) • ~2σ deficit in “Nν” LEP measurement of Γ(Z0→νν) • To date, no precise measurement of σ(νe→νe)

  3. NuTeV Measurement Technique • Measure nNC/CC ratio to extract ratio of weak couplings • ratio is experimentally and theoretically robust • largest uncertainty: suppression of charm production in CC (mc) • can extract sin2qW. NuTeV measurement often quoted this way. • With neutrino and anti-neutrino beams, can form Charged-Current(CC) Neutral-Current(NC) NuTeV Revisited, K. McFarland

  4. Dipoles make sign selection - Set n /n type - Remove ne from KL (Bkgnd in previous exps.) NuTeV Sign-Selected Beamline • Beam identifies neutral currents as 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 NC events since no final state muon NuTeV Revisited, K. McFarland

  5. Paschos-Wolfenstein à la NuTeV NuTeV fit for sin2θWand mc given external constraint from strange sea analysis. (More later) • NuTeV result: • Statistics dominate uncertainty • EWK fit (LEPEWWG 2001): • 0.2227  0.00037, a 3s discrepancy NuTeV Revisited, K. McFarland

  6. NLO Corrections NLO QED calculation NLO QCD corrections

  7. EW Radiative Corrections • Effective weak couplings well known • EM radiative corrections are large • Bremsstrahlung from final state lepton in CC is a big correction. • Not present in NC; promotes CC events to higher y so they pass energy cut. • {dRn, dRn, dsin2qW} ≈ {+.0074,+.0109,-.0030} • Only one calculation used (or usable) for NuTeV result. Vulnerable? • Better to have independent confirmation since the effect is not trivial • Also, there is a physics concern with the Bardin and Dokuchaeva calculation… D. Yu. Bardin and V. A. Dokuchaeva, JINR-E2-86-260, (1986) NuTeV Revisited, K. McFarland

  8. nμ nμ Z q q EW Radiative Corrections (cont’d) • This diagram has a colinear singularity • The correct approach is to explicitlyfactorize QED corrections between PDFevolution and the hard scattering process • Bardin and Doukachaeva calculation regularized this colinear singularity by assigning the incoming quark a mass of xmN • Martin-Roberts-Stirling-Thorne (EPJ C39 155, 2005) have calculated NLO QED PDF evolution • Diener-Dittmaier-Hollik (Phys. Rev. D69 (2004) 073005) & Arbuzov, Bardin and Kalinovskaya (JHEP 0506:078, 2005) have improved regularization. But… • DDH code cannot generated needed differential cross-sections • was used (painfully) to evaluate scheme dependence, however • ABK did their calculation in unobservable variables (combined μ+γ !) • Baur-Wackeroth calculation in process. • Have promised to address these problems. NuTeV Revisited, K. McFarland

  9. QCD Radiative Corrections (S.Davidson et al., KSM and S. Moch, , S. Kretzer and M-H. Reno, B. Dobrescu and K. Ellis) • NLO terms only enter multiplied by isovector valence quark distributions • highly suppressed. Calculate 1/5 s shifts in sin2qW • also have evaluated corrections individually for neutrino and anti-neutrino NC/CC ratios and effects of cuts (KSM and S. Moch) NuTeV Revisited, K. McFarland

  10. QCD SymmetryViolations What symmetry violations can affect the result? u≠d in target (neutron excess) asymmetric heavy seas

  11. Symmetry Violating QCD Effects • Paschos-Wolfenstein R- 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 • To get a rough idea offirst two effects, can calculate them for R- NuTeV Revisited, K. McFarland

  12. Asymmetric Strange Sea Why it might be so How it is measured at NuTeV This is what drives us to update the NuTeV measurement

  13. A Very Strange Asymmetry • Paschos-Wolfenstein relation assumes that strange sea is symmetric, i.e., no “valence” strange distribution • if there were on, this would be a big deal since it is an isovector component of the PDFs(charm sea is heavily suppressed) • ~30% more momentum in strange sea than in half of strange+anti-strange seas would “fix” NuTeV sin2θW • Why might one think that the strange and anti-strange seas would be different? • Perturbative strange sea is (roughly) momentum symmetric… • But “intrinsic” strange sea of the nucleon need not be! • so is a DIS probe of intrinsic strangeness! G.P. Zeller et al., Phys.Rev.D65:111103,2002) Brodsky and Ma, Phys. Let. B392 NuTeV Revisited, K. McFarland

  14. How Does NuTeV Measure This? • m± from semi-leptonic charm decay • Fits to NuTeV and CCFR n and dimuon data can measure the strange and antistrange seas separately • NuTeV separate n and beams important for reliable separation of s ands NuTeV Revisited, K. McFarland

  15. NEW NuTeV NLO Analysis • Have incorporated CTEQ strange “valence” evolution and CTEQ parameterizations • thanks esp. to Amundson, Kretzer, Olness & Tung • NuTeV NLO analysis (Phys.Rev.Lett.99:192001,2007) is near zero, but slightly positive • will shift central valuetowards standard modeland increase uncertainties • at NLO, with CTEQ6 as base PDF courtesy heroic efforts of D. Mason, P. Spentzouris NuTeV Revisited, K. McFarland

  16. Same Data: LO Analysis • For analysis of sin2θW want an analysis using the same LO cross-section model as NuTeV • published NuTeV result was based onLO cross-sections fit to CCFR data • S-/S+, if fit to NuTeV data, is 0.10±0.04 Neutrino Beam Anti-neutrino Beam NuTeV Revisited, K. McFarland

  17. NuTeV Update Effects to be incorporated Numerical Estimations

  18. What’s in the Update? • Three large effects • Strange Sea (just discussed), S-/S+=0.09±0.04 • External K+e3 branching ratio • Brookhaven E-865, famous for “fixing” the unitarity of the first row of the CKM matrix • this was a many standard deviation shift! • Strong effect on our electron neutrino background • d/u PDF uncertainties • pointed out by Kulagin and Alekhin that these were underestimated in published result • also corrected target neutron excess NuTeV Revisited, K. McFarland

  19. Changes in Prediction of Rν published: updated: NuTeV Revisited, K. McFarland

  20. NuTeV 99% Conf. Prediction Graphical Shifts in Rν mtop d/u νe Strange Sea NuTeV Revisited, K. McFarland

  21. NuTeV 99% Conf. Prediction Directions of Effects not Considered Shadowing (VMD) mc Valence Isospin Violation NuTeV Revisited, K. McFarland

  22. What’s Next? • Move NuTeV analysis to cross-sections based on NuTeV structure function results • Incorporate complete treatment of QED radiative corrections, including PDF evolution, if available • Refit data with external strange sea constraints NuTeV Revisited, K. McFarland

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