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The NuTeV Anomaly:

This presentation discusses the NuTeV anomaly, its measurement technique, and the implications for new physics. It also explores other possibilities and the impact of future results.

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The NuTeV Anomaly:

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  1. A hint of New Physics? or Train Wreck of the Mundane? The NuTeV Anomaly: Kevin McFarland University of Rochester WIN03, Lake Geneva, WI 8 October 2003

  2. Introduction to NuTeVsin2qW To Follow: (1) three talks on QCD-related uncertainties (2) I return with a summary of these and other issues

  3. NuTeV Collaboration Cincinnati1, Columbia2, Fermilab3, Kansas State4, Northwestern5, Oregon6, Pittsburgh7, Rochester8 K. McFarland, Rochester

  4. Measurement Technique • For an isoscalar target composed of u,d quarks: • NC/CC ratio easiest to measure experimentally but ... • Many SF dependencies and systematic uncertainties cancel, BUT • Must correct for up-down quark difference in target, EW radiative corrections, heavy quark effects, non-QPM parts of the cross-section, etc. • Here is where QCD and QED enter (constrained by data where available) Charged-Current(CC) Neutral-Current(NC) K. McFarland, Rochester

  5. 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+CCFR: M. Goncharov et al., Phys. Rev. D64: 112006,2001 and D. Mason presentation at ICHEP ’02. CCFR: A.O. Bazarko et al., Z.Phys.C65:189-198,1995.) asymptote from s(x), Vcd threshold set by mc K. McFarland, Rochester

  6. Before NuTeV mc was Limit • nN experiments had hit a brick wall in precisionDue to systematic uncertainties (i.e. mc ....) (All experiments corrected to NuTeV/CCFR mc and to large Mtop > MW ) K. McFarland, Rochester

  7. 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 uncertainty small • dVquarks only: Cabbibo suppressed and at high-x • But R- requires separate n andn beams NuTeV SSQT (Sign-selected Quad Train) beamline • Realized n in n mode 310-4, n in n mode 410-3, 1.6% nene K. McFarland, Rochester

  8. 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 isolating true NC event K. McFarland, Rochester

  9. Neutral Current / Charged Current Event Separation • Separate NC and CC events statistically based on the “event length”defined in terms of # counters traversed • n.b., electron neutrinos will be NC candidates K. McFarland, Rochester

  10. n toroid spectrometer target - calorimeter Events Events Data/MC Data/MC Visible E (GeV) Visible E (GeV) NuTeV Data Sample • Event selections: • Require energy in calorimeterEvisible > 20 GeV • Require Event Vertex within fiducial volume • Data with these cuts: 1.62million n events 351 thousand n events K. McFarland, Rochester

  11. NC CC Determine Rexp: Event Length DistributionNeutrinos Event Length DistributionAntineutrinos K. McFarland, Rochester

  12. Paschos-Wolfenstein à la NuTeV • NuTeV result: • Statistics dominate uncertainty • Standard model fit (LEPEWWG): • 0.2227  0.00037, a 3s discrepancy K. McFarland, Rochester

  13. Measurement Uncertainties • sin2qWerror statistically dominated (R- technique) • Rnuncertainty dominated by theory model K. McFarland, Rochester

  14. MW = 80.136  0.084 GeV from Compared to Other Measurements Given other inputs to SM, what Higgs Mass is implied? K. McFarland, Rochester

  15. NuTeV SM 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 weakens slightly K. McFarland, Rochester

  16. InterpretingNuTeV sin2qW Report from the NuTeV collaboration Kevin McFarland University of Rochester WIN03, Lake Geneva, WI 8 October 2003

  17. New Physics? A brief comment on the possibilities.

  18. New Physics Summary • The cause of NuTeV’s anomaly is highly unclear • Beyond SM effects explaining NuTeV are strained • It’s not SUSY loops or RPV SUSY • Hard to fit with leptoquarks • “Designer” Z’ is possible • Heavy-light n mixing + more miracles • So the community focuses on mundane explanations • I would argue that none of these are outstanding candidates either • c.f. (g-2)m. “Everyone knows” it is SUSY but result is theoretically shaky due to e+e- and t differences in HVP. • g-2 has the opposite problem: too many explanations! S. Davidson et al. hep-ph/0112302 Li & Ma, Takeuchi et al K. McFarland, Rochester

  19. Corroborating Evidence and Impact of Future Results Is there other evidence of BSM physics or Mundane Physics? What can we learn in the future?

  20. Is NuTeV Result Confirmed?Are there corroborating data? • In short, nothing definite. • It is consistent with previous nN measurements, but even combined, these are low precision. • Other tests of neutrino neutral current also consistent, but not high precision… • An interesting hint? • Model building around this is a challenge. S. Davidson et al. hep-ph/0112302 W. Loinaz et al, hep-ph/0210193 X. Li and E. Ma, hep-ph/0212029 K. McFarland, Rochester

  21. Future Data and Interpretation • Strange Sea Asymmetry, up≠dp, nuclear effects • For the most part, I would argue the data in hand already constrains these possibilities well enough • Any continuing debate is over interpretation • Caveat: no independent check of Z0 exchange nuclear effects (by definition). Rely on n CC and ℓ± NC. • Isospin violation in PDFs, e.g., un≠dp • Almost completely unconstrained, even at levels that would appear a priori ludicrous. • FNAL-P906 p±p, p±d Drell-Yan can directly probe this • Re-analysis of old n bubble-chamber data? np vs. nn Martin et al, hep-ph/0308087 K. McFarland, Rochester

  22. Future Data (cont’d) • Other precision EW data with quarks or neutrinos • e-Baryon scattering is undergoing a re-emergence! • QWEAK at JLab (ep) • DISParity at SLAC (eD) to redo Prescott experiment • These experiments suffer from many of QCD uncertainties that are worries in interpreting NuTeV. Worse because lower Q2? • Future neutrino experiments will be very very tough • Is there any point to re-measuring this in n DIS? • More statistics would help, but NuTeV systematic floor is 0.0008 (c.f., total NuTeV error of 0.0016) • Same systematic concerns • Maybe worth doing if there were a 1 TeV n beam at LHC. • n-e scattering would be a great measurement, but it’s tough • Cross-section is down by factor of a few 103 • Normalization? Bigi et al, hep-ph/0106177 K. McFarland, Rochester

  23. Future Data (cont’d) • Always the possibility of a future discovery impacting the NuTeV interpretation • LHC or TeVatron finds a Z’ • Giga-Z confirms and strengthens small deficit in invisible width • … K. McFarland, Rochester

  24. Electron Neutrino Background Under Control? • How it is measured • The recent BNL-E865 Measurement K. McFarland, Rochester

  25. 98% 1.7% 98% 1.6% Electron Neutrino Background • Approximately 5% of NC candidates are ne CC events(It would take a 20% overestimate of ne to move NuTeV to SM) • Main ne source is K decay (93% / 70% of total in n /n beams) • Others include KL’s (4%/18%) and Charm (2%/9%) • Main uncertainty is Ke3 branching ratio (known to 1.4%) ! • Unless BNL-E865 iscorrect. They claim Ke3BR is 6% higher thanPDG, fixing Vus problembut exacerbating NuTeVby +0.7s • Also have directne measurement. K. McFarland, Rochester

  26. Direct Measurments of ne Flux • nmCC (wrong-sign) events in anti-neutrino beam constrain charm and KL production • Shower shape analysis can statistically pick out n events (En> 80 GeV) • Most precise at highest energies • Good agreement in peak flux region (80< En <180 GeV) • Poor agreement with simulation on high energy tail (expected from inability to measure high E nmCC, smearing) • Remove events from analysis with En> 180 GeV. Concern? • Observe ~35% more ne than predicted above 180 GeV,and a smaller excess inn beam • Conclude that we should require Ehad < 180 GeV K. McFarland, Rochester

  27. NC Candidates vs. visible Energy Systematic error bands Feel-Good Checks of ne • Rexp vs. radial binElectron neutrino background much higher near edge of detector. Statistical error bands • Rexp vs. visible energyElectron neutrino background dominant at high visible energy K. McFarland, Rochester

  28. Stability Checks Is NuTeV Result robust if data is sub-divided to highlight different kinematic regions, backgrounds?

  29. Why We (NuTeV) Believe the Experimental Analysis:“Stability Tests” Verify systematic uncertainties with data to Monte Carlo comparisons as a function of exp. variables. • Longitudinal Vertex: checks detector uniformity c2 = 50 / 62 dof Note: Shift from zero is because NuTeV result differs from Standard Model c2 = 49 / 62 dof K. McFarland, Rochester

  30. 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. K. McFarland, Rochester

  31. Distributions vs. Ehad NC Candidates vs. visible Energy CC Candidates vs. visible Energy Systematic error bands K. McFarland, Rochester

  32. exp exp 20 50 100 180 20 50 100 180 Evis (GeV) 20 50 100 180 20 50 100 180 Evis (GeV) Stability Test: Rexp vs. Energy • Modeling of NC/CC Ratio vs. visible energy checks • backgrounds • cross-section model • detector effects • Bottom line: no obvious causes for concern K. McFarland, Rochester

  33. LO Cross-Section Model How does the model work?How is it used? NLO Corrections

  34. Enhanced LO Cross-Section • “Enhanced” means: include RL and higher twist terms • PDFs extracted from CCFR data exploiting symmetries: • Isospin symmetry: up=dn , dp=uu , and s(x) =s(x) • 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 Model is fit directly to this data; uncertainties come from data. Antineutrino xsec vs y at 190 GeV Neutrino xsec vs y at 190 GeV CCFR Data high y events are background to the neutral current sample K. McFarland, Rochester

  35. 20 50 100 180 20 50 100 180EHad (GeV) Charged-Current Control Sample • Medium length events, clearly CC but with similar kinematics to NC candidates from CC events, check modeling • Excellent agreement with prediction K. McFarland, Rochester

  36. PDF changes have little effect • Illustrates (relative) independence of R- from (most) PDF details, even s(x) ! • But this does not prove NLO effects are small • Also, this is R-, not the full NuTeV analysis. Extreme variationswith LO/NLO PDF Sets(no NLO mc effects). No attempt to make cross-section model + PDFs fit n data! (S.Davidson et al. hep-ph/0112302) K. McFarland, Rochester

  37. How is R- Changed in NLO QCD? • So NLO terms only enter multiplied by isovector valence quark distributions • for NuTeV this is a numerically negligible correction • however, again, NuTeV does not measure precisely R- (S.Davidson et al. hep-ph/0112302, K. McFarland and S. Moch hep-ph/0306052, S. Kretzer and M-H. Reno unpublished, B. Dobrescu and K. Ellis to appear) K. McFarland, Rochester

  38. Numerical Evaluation of NLO QCD Effects K. McFarland, Rochester

  39. Final Word on NLO QCD Effects • None of these analyses • account for fitting CCFR CC cross-sections and dimuon data at same order • check background predictions at NLO • Full NLO Analysis • Ellis and Dobrescu have written a generator • invaluable assistance! thank you! • too slow by 1-2 orders of magnitude to use. Working to improve. • need additions to associated evolution code for strange sea • Work in progress… K. McFarland, Rochester

  40. NuTeV’s Use of Neutrino and Anti-Neutrino Data What’s the difference between NuTeV and R- ?

  41. Paschos-Wolfenstein at NuTeV K. McFarland, Rochester

  42. How is NuTeV’s AnalysisDifferent from R- ? (Details) • Backgrounds (cosmic rays, non-neutrino events) • Taken from data. Only increase statistical errors. • Cross-talk (including ne background) • Dilute statistical significance of the result  • In the case of nmCC, cross-talk occurs for particular kinematics • High y, large qm • Different NC, CC acceptance • Very small effects from muon (energy, vertex). Likely negligible? • Use of external dimuon constraint on charm suppression (“mc”) reduces role of anti-neutrino data • Sensitive to charm model • And to non-QPM cross-section, e.g. RL K. McFarland, Rochester

  43. How is NuTeV’s AnalysisDifferent from R- ? (cont’d) • If this charm mass constrained fit were the problem, we should see a big difference when extracting sin2qW without constraint… • See very smalldifference if charmmass constraintdropped. • This is equivalent tosaying that Rn is in agreement with expectations. • Statistical and experimental systematics increaseModel errors, of course, decrease… K. McFarland, Rochester

  44. Electroweak Radiative Corrections Are they a concern?

  45. EW Radiative Corrections • I see no serious reason to believe effective coupling calculations are inadequate. Comments? • 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} • These should be checked.(Baur, Dittmaier, Hollik) D. Yu. Bardin and V. A. Dokuchaeva, JINR-E2-86-260, (1986) K. McFarland, Rochester

  46. Dividing NuTeV Data into Regions in Q2 What can we check to mitigate concerns about low Q2 data?

  47. Low Q2 Contributionsto NuTeV Analysis • Bulk of NuTeV data is high Q2, but some data with Q2<1 GeV2 is in analysis • visible energy cut limits this tovery low x since • sea region, common ton and anti-n • Unfortunately, for neutral currents, we don’t reconstruct Q2 • However, Evisible and Q2 are correlated by q(x) K. McFarland, Rochester

  48. Distributions vs. Evisible NC Candidates vs. visible Energy CC Candidates vs. visible Energy Systematic error bands K. McFarland, Rochester

  49. exp exp 20 50 100 180 20 50 100 180 Evis (GeV) 20 50 100 180 20 50 100 180 Evis (GeV) Rexp vs. Energy • A Q2 dependence would show up as a trend in this graph • Bottom line: no obvious causes for concern K. McFarland, Rochester

  50. QCD SymmetryViolations in R- What symmetry violations can affect the result? u≠d in target (neutron excess) asymmetric heavy seas process dependent nuclear effects

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