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Revisiting NuTeV

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

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Revisiting NuTeV

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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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