Implications of NuTeV Standard Model Test

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Implications of NuTeV Standard Model Test

• Beyond the Standard Model?? QCD Effects??

NuTeV charged, neutral currents induced by
neutrinos New measurement of Weinberg angle
Possible New Physics Beyond Standard
Model? Normal (QCD) Explanations of NuTeV
Anomaly? radiative corrections parton Charge
Symmetry Violation strange quark momentum
asymmetry nuclear, shadowing corrections
Tim Londergan Nuclear Theory Center, Indiana
University PAVI06 Milos, Greece May 16-20,
2006
Supported by NSF PHY- 0244822 In collaboration
with Tony Thomas (Adelaide/JLab)
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The NuTeV Experiment charged, neutral currents
from neutrino DIS
800 GeV p at FNAL produce pi, K from
interactions in BeO target Decay of charged pi,
K produces neutrinos, antineutrinos Almost
pure muon neutrinos (small ?e contamination
from Ke3 decay) Only neutrinos penetrate
shielding

• Dipoles select sign of charged meson
• Determine nu/nubar type

NuTeV Rochester/Columbia/FNAL/Cincinnati/Kansas
State/Northwestern/ Oregon/Pittsburgh neutrino
collaboration G. Zeller etal, PRL 88, 091802
(02) PR D65, 111103 (02)
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Separate Neutral, Charged-Current Events
NuTeV Detector 18 m long, 690-ton steel
scintillator Steel plates interspersed with liq
scintillator, drift chambers
Charged current Track through several
plates Large visible energy deposit
Neutral current Short visible track Large
missing energy

• NuTeV Events
• 1.62 million n
• 351,000

• NuTeV event selection
• Large E in calorimeter
• event vertex in fiducial volume

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The Paschos-Wolfenstein Ratio
Neutrino Total Cross Sections on Isoscalar
Target
Paschos Wolfenstein ( PR D7, 91 (73))
Independent measurement of Weinberg angle, using
ratio of total X-sections for neutrinos,
antineutrinos on isoscalar target PW ratio ?
minimizes sensitivity to PDFs, higher-order
corrections
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The Paschos-Wolfenstein Ratio

• PW Ratio depends on the following assumptions
• Isoscalar target (NZ)
• include only light (u, d) quarks
• neglect heavy quark masses
• assume isospin symmetry for PDFs
• no nuclear effects (parton shadowing, EMC, .)
• no contributions outside Standard Model

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NuTeV Determination of Weinberg Angle

• Construct ratios
• Individual ratios less dependent on overall
  normalization
• Very precise charged/neutral current ratios
• Different cuts, acceptance dont construct PW
  ratio directly
• depends strongly on Weinberg angle
• weak dependence on Weinberg angle

3s below SM
agree with SM
These ratios lead to a NuTeV value for the
Weinberg angle
The NuTeV result is 3 s above very precise
value (from EW processes at LEP)
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NuTeV Result 3s Discrepancy from LEP value
NuTeV work at LO in QCD (with improvements) and
find s2w(NuTeV)0.22760.0013stat 0.0006syst
0.0006th
-0.00003(Mt/GeV-175)0.00032 lnMH/100GeV where
s2w1-M2w/M2z (on-shell) Global fit s2w
0.2229 0.0004 2.8s discrepancy
SM
Nuclear effects and ? oscillation explanations
very unlikely
Corresponds to 1.2 decrease in gL2 This is
large compared to precision of EW
measurements!
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Running of Weinberg Angle

• Effective Weinberg angle depends on Q2

Warning gauge and process dependent parameter
E158 Q dependence of Weinberg angle
confirmed at 6s level NuTeV discrepancy of 3s
with SM prediction Czarnecki-Marciano, PRD53,
1066 (96)
Qweak
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New Physics explanation for NuTeV?

• The problem extremely precise
• EW data confirms SM!
• Mass, width of Z, W
• X-sections, branching ratios
• at Z peak LEP, SLD
• LR and FB asymmetries in ee-
• scattering
• new particles must satisfy all
• these constraints
• EW constraints 0.1 level
• NuTeV 1.2
• new particles have heavy masses
• ? Q2 dependence must be slow
• new physics hard to satisfy
• EW constraints!

NuTeV
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Designer Particles I delicately adjust to fit
all existing data

• oblique corrections high mass
• scale, couples only to vector
• bosons parameters constrained
• by EW data cant fit NuTeV
• extra Z (unmixed) possible
• runs into trouble with
• latest muon g-2

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Designer Particles II More Attempts to fit
NuTeV

• minimal SUSY loops No most have wrong sign
  others violate
• existing constraints
• Leptoquarks (bosons that couple to leptons
  quarks) models with
• very carefully tuned mass splittings still
  possible could be tested at
• Tevatron, LHC Davidson, Gambino etal, J High E
  Phys 2, 37 (2002)

MSSM, light sleptinos, gauginos
Leptoquark (solid) extra gauge bosons (red)
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EW Radiative Corrections

• EM radiative corrections, process specific to
  NuTeV, are large
• Bremsstrahlung from final state lepton in CC ?
  significant correction.
• Not present in NC (promotes CC events to higher y
  so they pass energy cut)
• dR n, dR?n, dsin2qW .0074,.0109,-.0030
• New calculation of this effect (Diener,
  Dittmaier, Hollik PR D69, 073005 (04) )
• not yet implemented in analysis
• Corrections likely larger than NuTeV estimate

D. Yu. Bardin and V. A. Dokuchaeva,
JINR-E2-86-260, (1986)
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Isospin Violating Corrections to PW Ratio
Changes in PW ratio from isospin violating PDFs
PW Correction ? valence parton charge symmetry
violation (CSV) parton charge symmetry
CS (rotation of 180o about 2 axis in isospin
space)

• We know the origins of parton CSV
• quark mass difference

• Electromagnetic contributions most important EM
  effect

n-p mass difference
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CSV Contribution to NuTeV Result
Sather Analytic Quark Model Approximation for
Valence Parton CSV.
Leads to analytic results (model-dependent)
PW Ratio CSV Corrn using Sather Rodionov
Sather CTEQ4LQ
40 decrease in anomaly!
NuTeV Dont evaluate PW Ratio! CSV Contribn to
NuTeV result

• Calculate parton CSV at low (quark model)
  momentum scale
• Evolve up to Q2 of NuTeV expt (20 GeV2)
• Evaluate with NuTeV functional

30 decrease in anomaly
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Phenomenological Parton CSV PDFs
MRST PDFs from global fits include CSV for 1st
time Martin, Roberts, Stirling, Thorne Eur Phys
J C35, 325 (04) Choose restricted form for
parton CSV
90 conf limit (?)
f(x) 0 integral matches to valence PDFs at
small, large x
Best fit ? -0.2, large uncertainty ! Best fit
remarkably similar to quark model CSV
calculations
ADEL (1994)
MRST (2004)
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QED Splitting a New Source of Isospin Violation
MRST, Eur.Phys.J. 39, 155 (05) Glueck,
Jimenez-Delgado, Reya, PRL95, 022002 (05)
QED evolution, quark radiates photon Evolve in
Q2

• qualitatively similar to quark model CSV
• QED varied while quarks frozen
• contributes even if mu md and Mn Mp
• add to quark model CSV term ?
• increase CSV factor 2
• MRST incorporate QED splitting with PDFs
• in global fit to high energy data

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Summary, CSV Effects on Weinberg angle

• Phenomenology
• MRST global fit ? limits valence CSV
• -0.8 ? 0.65
• ? -0.6 ? remove NuTeV anomaly!
• ? 0.6 ? anomaly twice as large!
• Theoretical estimates
• quark model remove 1/3 anomaly
• QED splitting remove 1/3
• ? at current limits, CSV could
• produce observable effects
• ( 5-6) in certain reactions

90 confidence limits
MRST global fit to valence CSV
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Strange Quark Contributions to PW Ratio
Contribution from strange quarks
s-sbar momentum asymmetry
Strange quark normalization constrained (no net
strangeness in nucleon)
If s quarks carry more momentum than sbar
decrease anomaly
Determination of s, sbar quark PDFs Opposite
sign dimuons from neutrinos

• CCFR charge of faster muon determines neutrino
  or antineutrino
• most precise way to determine s, sbar PDFs?
  CCFR, NuTeV

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Analyses of s quark momentum asymmetry

• NuTeV analyzed s, sbar for small x
• Best fit
• Do not enforce normalization condition
• NLO treatment of dimuon prodn

• But, from normalization
• If s lt sbar for small x, requires s gt sbar for
  large x

CTEQ Kretzer etal, PRL 93, 041802 (04), Olness
etal, Eur Phys J C40, 145 (05)

• Global analysis of parton PDFs ? CTEQ6
• Includes CCFR, NuTeV dimuon data
• (includes exptl cuts on dimuons)
• Extract best fit for s, sbar distns
• enforce s normalization condn

• Catani et al. hep-ph/0404240 0406338
• in NNLO, can generate nonzero momentum asymmetry
• find small mom asymmetry lt 0 from 3-loop
  contribns

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Results CTEQ Global fit vs. Bjorken x
µ other

• CTEQ S- gt 0, strange asymmetry
• decreases NuTeV anomaly
• dimuon data most sensitive for s PDFs
• CTEQ s contribn removes
• 0 ? 25 of anomaly

-.001 lt S- lt .004
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NuTeV Analysis of s, sbar Quark Distn

• Re-analyzed dimuon data
• preliminary analysis S- 0
• appears negative
• strangeness not conserved?
• NLO treatment of dimuon prodn

• NuTeV group
• now using CTEQ PDFs (u,d)
• very similar analysis method
• acceptance corrections?
• fragmentation functions?
• charm mass (CTEQ uses HERA data)?

Qualitative disagreement between NuTeV/CTEQ,
using same data, techniques to extract s, sbar
??
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Nuclear Effects ??
NuTeV All data neutrino DIS on iron Nuclear
modification of PDFs Shadowing EMC
Effect Fermi motion . Miller/Thomas Int J
Mod Phys A20, 95 (05) Shadowing effect for
NuTeV??
charged leptons
Vector meson dominance
n CC events
Miller/Thomas n shadowing very different from
m
n NC events
Zo very small coupling to r ? NC shadowing 0
Different shadowing for
? account for anomaly??
NuTeV NO !

• Shadowing low Q2, data much higher Q2
• value very close to SM value
• (should be quite different in MT scenario)
  

Shadowing increase NuTeV anomaly??
Nuclear effects in n reactions Hirai, Kumano,
Nagai, PR D71, 113007 (05)
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Nuclear Effects (part II)
Brodsky, Schmidt, Yang (PR D70, 116003 (04))
Shadowing/Antishadowing ?-A processes

• include Pomeron, Odderon, Reggeon effects
• obtain both shadowing, antishadowing effects
• both constructive, destructive interference
• processes modified in different ways

Predict greater effect on
than
Remove 20 of Weinberg angle anomaly Partial
contribution along with CSV, s quarks ??
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Conclusions

• NuTeV Measured CC, NC X-sections for
  on Fe
• Large ( 3s), surprising discrepancy for
• New Physics difficult to fit LEP results,
  NuTeV
• designer particles unlikely ? very delicate
  
• QCD Corrections to NuTeV measurement??

• radiative corrns ? new calcn, not in analyses
• parton CSV ? could remove effect MRST
• strange quark asymmetry 1-2 s CTEQ
• nuclear effects 20, Brodsky etal

(model dependence)
NuTeV no!!

• CSV, strangeness at present, most plausible
• explanations for NuTeV anomaly
• small additive contribns from CSV,
  strangeness, nuclear
• effects ??

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Implications of NuTeV Standard Model Test

• Beyond the Standard Model?? QCD Effects??

NuTeV charged, neutral currents induced by
neutrinos New measurement of Weinberg angle
Possible New Physics Beyond Standard
Model? Normal (QCD) Explanations of NuTeV
Anomaly? radiative corrections parton Charge
Symmetry Violation strange quark momentum
asymmetry nuclear, shadowing corrections
Tim Londergan Nuclear Theory Center, Indiana
University PAVI06 Milos, Greece May 16-20,
2006
Supported by NSF PHY- 0244822 In collaboration
with Tony Thomas (Adelaide/JLab)
26
NuTeV/LEP/ PV MØller/ Qweak

• E158 at SLAC
• first measurement
• of PV in MØller sc.
• huge luminosity
• high polarization (80)

Warning gauge and process dependent parameter
At tree level, APV280 10-9
Qweak !!
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QCD Corrections to the NuTeV Result (
Something Old)
Isoscalar Effect (N ?Z for Fe)
Depends only on 2nd moment of light quark valence
PDFs (momentum carried by up, down valence
quarks) Isoscalar corrrection to PW ratio
NuTeV Isoscalar correction

• NuTeV expt doesnt evaluate PW ratio
• Isoscalar correction from Monte Carlo simulation
  of expt
• Although NuTeV significant difference from PW
  corrn, under control

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Qualitative Features of Parton Distributions
At low Q2, generate from quark models
Majority valence quark
Minority valence quark
residual (ud) diquark
residual (uu) diquark
Under CSV n? p,
residual (ud) diquark
residual (dd) diquark
only n-p mass difference
diquark mass difference
True for all intermediate states!
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Calculating CSV Corrections to PW Ratio

• Construct models for valence parton
  distributions
• Test how these models behave under CSV
  transformation

• Predict sign, magnitude of CSV effects in NuTeV
  expt

CSV Corrections to PW Ratio

• CSV correction to PW ratio independent of Q2

• Both numerator, denominator involve 2nd moment
  of valence distns
• numerator, denominator evolve identically with Q2

under LO evolution,

• Evaluate valence PDFs at low Q2

• Dominated by configurations with 3 valence
  quarks
• Easy to calculate, study behavior

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Quantitative Estimates of Parton CSV
Quark-model formula for parton PDFs
X qq 3qqbar 4q2 qbar. .
Large x dominated by Xdiquark states Sather
study variation with nucleon, quark mass
analytic expression
small shift in PDFs, due to quark, nucleon mass.
For large x, Sather predicts d gt u in both
cases. (From normalization, d lt u at small x)
work at low Q2 where 3-quark states dominate