Revisiting NuTeV

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 ? oscillations are sensitive to
  only flavor non-diagonal couplings
• Some outstanding puzzles in neutrino physics
• 3s NuTeV result s(?q??q)/s(?q?µq)
• 2s deficit in N? LEP measurement of G(Z0???)
• To date, no precise measurement of s(?e??e)

3
NuTeV Measurement Technique
Charged-Current(CC)
Neutral-Current(NC)

• Measure n NC/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

4
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

5
Paschos-Wolfenstein à la NuTeV
NuTeV fit for sin2?Wand 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

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NLO Corrections

• NLO QED calculation
• NLO QCD corrections

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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.
• dR n, 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)
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EW Radiative Corrections (contd)

• 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
  0506078, 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? !)
• Baur-Wackeroth calculation in process.
• Have promised to address these problems.

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)

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QCD SymmetryViolations

• What symmetry violations can affect the result?
• u?d in target (neutron excess)
• asymmetric heavy seas

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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-

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Asymmetric Strange Sea

• Why it might be so
• How it is measured at NuTeV
• This is what drives us to update the NuTeV
  measurement

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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
  strangeanti-strange seas would fix NuTeV
  sin2?W
• Why might one think that the strange and
  anti-strange seas would be different?

G.P. Zeller et al., Phys.Rev.D65111103,2002)

• 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
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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

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.99192001,2007)
  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
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Same Data LO Analysis

• For analysis of sin2?W 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 0.100.04

Neutrino Beam
Anti-neutrino Beam
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NuTeV Update

• Effects to be incorporated
• Numerical Estimations

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Whats in the Update?

• Three large effects
• Strange Sea (just discussed), S-/S0.090.04
• External Ke3 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

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Changes in Prediction of R?
published
updated
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Graphical Shifts in R?
mtop
d/u
?e
Strange Sea
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Directions of Effects not Considered
mc
Shadowing (VMD)
Valence Isospin Violation
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Whats 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