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 neutrino couplings are the most difficult couplings to measure precisely at the Z 0 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 “N ν ” LEP measurement of Γ(Z 0 →νν) –To date, no precise measurement of σ(νe→νe)

8 April 2008 NuTeV Revisited, K. McFarland 3 Measure NC/CC ratio to extract ratio of weak couplings –ratio is experimentally and theoretically robust –largest uncertainty: suppression of charm production in CC (m c ) –can extract sin 2  W. NuTeV measurement often quoted this way. With neutrino and anti-neutrino beams, can form NuTeV Measurement Technique Charged-Current (CC) Neutral-Current (NC)

8 April 2008 NuTeV Revisited, K. McFarland 4 Beam identifies neutral currents as or    in mode 3     in  mode 4     Beam only has  electron neutrinos  Important background for NC events since no final state muon NuTeV Sign-Selected Beamline Dipoles make sign selection - Set   type - Remove e from K L (Bkgnd in previous exps.)

8 April 2008 NuTeV Revisited, K. McFarland 5 Paschos-Wolfenstein à la NuTeV NuTeV result: –Statistics dominate uncertainty EWK fit (LEPEWWG 2001): –  , a 3  discrepancy NuTeV fit for sin 2 θ W and m c given external constraint from strange sea analysis. (More later)

NLO Corrections 1. NLO QED calculation 2. NLO QCD corrections

8 April 2008 NuTeV Revisited, K. McFarland 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.  R,  R ,  sin 2  W } ≈ {+.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-E , (1986)

8 April 2008 NuTeV Revisited, K. McFarland 8 EW Radiative Corrections (cont’d) This diagram has a colinear singularity The correct approach is to explicitly factorize QED corrections between PDF evolution and the hard scattering process –Bardin and Doukachaeva calculation regularized this colinear singularity by assigning the incoming quark a mass of xm N Martin-Roberts-Stirling-Thorne (EPJ C39 155, 2005) have calculated NLO QED PDF evolution Diener-Dittmaier-Hollik (Phys. Rev. D69 (2004) ) & 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. μ q Z q μ

8 April 2008 NuTeV Revisited, K. McFarland 9 QCD Radiative Corrections NLO terms only enter multiplied by isovector valence quark distributions –highly suppressed. Calculate 1/5  shifts in sin 2  W –also have evaluated corrections individually for neutrino and anti-neutrino NC/CC ratios and effects of cuts (KSM and S. Moch) (S.Davidson et al., KSM and S. Moch,, S. Kretzer and M-H. Reno, B. Dobrescu and K. Ellis)

QCD Symmetry Violations What symmetry violations can affect the result? 1. u≠d in target (neutron excess) 2. asymmetric heavy seas

8 April 2008 NuTeV Revisited, K. McFarland 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 of first two effects, can calculate them for R -

Asymmetric Strange Sea 1. Why it might be so 2. How it is measured at NuTeV This is what drives us to update the NuTeV measurement This is what drives us to update the NuTeV measurement

8 April 2008 NuTeV Revisited, K. McFarland 13 A Very Strange Asymmetry 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! Brodsky and Ma, Phys. Let. B392 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 sin 2 θ W Why might one think that the strange and anti-strange seas would be different? G.P. Zeller et al., Phys.Rev.D65:111103,2002)

8 April 2008 NuTeV Revisited, K. McFarland 14 How Does NuTeV Measure This?  ± from semi-leptonic charm decay Fits to NuTeV and CCFR  and  dimuon data can measure the strange and antistrange seas separately –NuTeV separate and   beams important for reliable separation of s and  s

8 April 2008 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 value towards standard model and increase uncertainties –at NLO, with CTEQ6 as base PDF courtesy heroic efforts of D. Mason, P. Spentzouris

8 April 2008 NuTeV Revisited, K. McFarland 16 Same Data: LO Analysis For analysis of sin 2 θ W want an analysis using the same LO cross-section model as NuTeV –published NuTeV result was based on LO cross-sections fit to CCFR data –S - /S +, if fit to NuTeV data, is 0.10±0.04 Neutrino Beam Anti-neutrino Beam

NuTeV Update 1. Effects to be incorporated 2. Numerical Estimations

8 April 2008 NuTeV Revisited, K. McFarland 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

8 April 2008 NuTeV Revisited, K. McFarland 19 Changes in Prediction of R ν published: updated:

8 April 2008 NuTeV Revisited, K. McFarland 20 NuTeV 99% Conf. Prediction Graphical Shifts in R ν d/u νeνe m top Strange Sea

8 April 2008 NuTeV Revisited, K. McFarland 21 NuTeV 99% Conf. Prediction Directions of Effects not Considered mcmc Shadowing (VMD) Valence Isospin Violation

8 April 2008 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