revisiting nutev

Revisiting NuTeVRevisiting NuTeV

Kevin McFarlandKevin McFarlandUniversity of RochesterUniversity of Rochester

DIS 2008, UC-London, 8 April 2008DIS 2008, UC-London, 8 April 2008

The Big PictureThe Big Picture

Neutrinos are important in electroweak physicsNeutrinos are important in electroweak physics– there is a glorious history, of course…there is a glorious history, of course…– … … but precision today in neutrino electroweak but precision today in neutrino electroweak

couplings lags behind other sectorscouplings lags behind other sectorsneutrino couplings are the most difficult couplings to measure neutrino couplings are the most difficult couplings to measure precisely at the Zprecisely at the Z00 pole pole

matter effects in matter effects in νν oscillations are sensitive to only flavor oscillations are sensitive to only flavor non-diagonal couplingsnon-diagonal couplings

Some outstanding puzzles in neutrino physicsSome outstanding puzzles in neutrino physics– ~3~3σσ NuTeV result NuTeV result σσ((ννq→q→ννq)/q)/σσ((ννq→q→μμqq’’))

– ~2~2σσ deficit in “N deficit in “Nνν” LEP measurement of ” LEP measurement of ΓΓ(Z(Z00→→νννν))

– To date, no precise measurement of To date, no precise measurement of σσ((ννe→e→ννe) e)

8 April 20088 April 2008 NuTeV Revisited, K. McFarlandNuTeV Revisited, K. McFarland 33

Measure Measure NC/CCNC/CC ratio to extract ratio of weak couplings ratio to extract ratio of weak couplings– ratio is experimentally and theoretically robustratio is experimentally and theoretically robust– largest uncertainty: suppression of charm production in CC (mlargest uncertainty: suppression of charm production in CC (mcc))– can extract sincan extract sin22WW. NuTeV measurement often quoted this way.. NuTeV measurement often quoted this way.

With neutrino and anti-neutrino beams, can formWith neutrino and anti-neutrino beams, can form

NuTeV Measurement TechniqueNuTeV Measurement Technique

(3) 2Coupling sinweak em WJ Q (3)Coupling weakJ

Charged-Current(CC)

Neutral-Current(NC)

0

Only valence quarks contribute

(in particular, cancels )

sea seaq q

s c s c

2 2 2, , ,L R L R L Rg u d


Beam identifies neutral Beam identifies neutral currents as currents as or or in in mode 3 mode 3 in in mode 4 mode 4Beam only has Beam only has electron neutrinoselectron neutrinos Important background for NC Important background for NC

events since no final state muonevents since no final state muon

NuTeV Sign-Selected BeamlineNuTeV Sign-Selected Beamline

Dipoles make sign selection - Set type - Remove e from KL (Bkgnd in previous exps.)


exp exp

2 2

2exp exp

large smallsin sin

sin systematics (i.e. )W W

W c

dR dR

d d

R R m

Paschos-Wolfenstein à la NuTeVPaschos-Wolfenstein à la NuTeV( )

( )

( )( ) 2 2 4

0( )

1 5sin sin (1 )

2 9CC

CC

NCW W

CC

R

2 ( ) sin 0.22770.0013( .)0.0009( .)

on shellW

statsyst

NuTeV result:NuTeV result:– Statistics dominate uncertaintyStatistics dominate uncertainty

EWK fit (LEPEWWG 2001): EWK fit (LEPEWWG 2001): – 0.2227 0.2227 0.00037, a0.00037, a 3 3 discrepancy discrepancy

exp

exp

0.3916 0.0013 ( : 0.3950) 3

0.4050 0.0027 ( : 0.4066)

R SM difference

R SM Good agreement

NuTeV fit for sin2θW

and mc given external constraint from strange sea

analysis. (More later)

NLO CorrectionsNLO Corrections

1.1. NLO QED calculationNLO QED calculation

2.2. NLO QCD correctionsNLO QCD corrections


EW Radiative CorrectionsEW Radiative CorrectionsEffective weak couplings well knownEffective weak couplings well known

EM radiative corrections are largeEM radiative corrections are large– Bremsstrahlung from final state lepton in CC is a big Bremsstrahlung from final state lepton in CC is a big

correction.correction.

Not present in NC; promotes CC events to higher y Not present in NC; promotes CC events to higher y so they pass energy cut.so they pass energy cut.

R R , , RR, , sinsin22WW} ≈ } ≈

{+.0074,+.0109,-.0030}{+.0074,+.0109,-.0030}

Only one calculation used (or usable) for NuTeV Only one calculation used (or usable) for NuTeV result. Vulnerable?result. Vulnerable?– Better to have independent confirmation since the Better to have independent confirmation since the

effect is not trivialeffect is not trivial– Also, there is a physics concern with the Bardin and Also, there is a physics concern with the Bardin and

Dokuchaeva calculation…Dokuchaeva calculation…

D. Yu. Bardin and V. A. Dokuchaeva, JINR-E2-86-260, (1986)


EW Radiative Corrections EW Radiative Corrections (cont’d)(cont’d)This diagram has a colinear singularityThis diagram has a colinear singularity

The correct approach is to explicitlyThe correct approach is to explicitlyfactorize QED corrections between PDFfactorize QED corrections between PDFevolution and the hard scattering processevolution and the hard scattering process– Bardin and Doukachaeva calculation regularized this colinear singularity by Bardin and Doukachaeva calculation regularized this colinear singularity by

assigning the incoming quark a mass of xmassigning the incoming quark a mass of xmNN

Martin-Roberts-Stirling-Thorne Martin-Roberts-Stirling-Thorne (EPJ (EPJ C39 155, 2005) C39 155, 2005) have calculated NLO QED PDF have calculated NLO QED PDF evolution evolution

Diener-Dittmaier-Hollik Diener-Dittmaier-Hollik (Phys. Rev. D69 (2004) 073005)(Phys. Rev. D69 (2004) 073005) & Arbuzov, Bardin and Kalinovskaya & Arbuzov, Bardin and Kalinovskaya (JHEP 0506:078, 2005)(JHEP 0506:078, 2005) have improved regularization. But… have improved regularization. But…– DDH code cannot generated needed differential cross-sectionsDDH code cannot generated needed differential cross-sections

was used (painfully) to evaluate scheme dependence, howeverwas used (painfully) to evaluate scheme dependence, however

– ABK did their calculation in unobservable variables (combined ABK did their calculation in unobservable variables (combined μμ++γγ !) !)

Baur-Wackeroth calculation in process.Baur-Wackeroth calculation in process.– Have promised to address these problems.Have promised to address these problems.

μ

qZ

q

μ


QCD Radiative CorrectionsQCD Radiative Corrections

NLO terms only enter multiplied by isovector valence quark NLO terms only enter multiplied by isovector valence quark distributionsdistributions– highly suppressed. Calculate 1/5 highly suppressed. Calculate 1/5 shifts in sin shifts in sin22WW – also have evaluated corrections individually for neutrino and anti-neutrino also have evaluated corrections individually for neutrino and anti-neutrino

NC/CC ratios and effects of cuts NC/CC ratios and effects of cuts (KSM and S. Moch)(KSM and S. Moch)

2 2

1 22 2 2 2 31 3

32 2 4 4

u d

Su d u d

R

U D C S C CC

U D

2 22 , ,,

where ( ) in target, etc.

NLO coefficient fcns. in SF ii

u d u du d L R

U x u u dx

C F

(S.Davidson et al., KSM and S. Moch, , S. Kretzer and M-H. Reno, B. Dobrescu and K. Ellis)

QCD SymmetryQCD SymmetryViolationsViolations

What symmetry violations can What symmetry violations can affect the result?affect the result?

1.1. u≠d in target (neutron excess)u≠d in target (neutron excess)

2.2. asymmetric heavy seasasymmetric heavy seas


Symmetry Violating QCD EffectsSymmetry Violating QCD EffectsPaschos-Wolfenstein RPaschos-Wolfenstein R-- assumptions: assumptions:– Assumes total u and d momenta equal in targetAssumes total u and d momenta equal in target– Assumes sea momentum symmetry, s =Assumes sea momentum symmetry, s =s and c =s and c =cc– Assumes nuclear effects common in W/Z exchangeAssumes nuclear effects common in W/Z exchange

To get a rough idea ofTo get a rough idea offirst two effects, can first two effects, can calculate them for Rcalculate them for R--

2 2

2 2

2 2

2 2 2

3

13

2

2 (3 )

u d

v vu d

v v

v vu d

v v

d u d cv v

R

U DN

U D

U D

U D

S

U D

2 22 , ,,

( )where

( ) , etc.

( ) , etc.

( )

kinematic charm CC suppression

p pv v v

p nv v v

u d u du d L R

c

N ZN

AU x u d dx

U x u d dx

S x s s dx

Asymmetric Strange SeaAsymmetric Strange Sea

1.1. Why it might be soWhy it might be so

2.2. How it is measured at NuTeVHow it is measured at NuTeV This is what drives us to update the This is what drives us to update the

NuTeV measurementNuTeV measurement


A Very Strange Asymmetry A Very Strange Asymmetry

Perturbative strange sea is (roughly) Perturbative strange sea is (roughly) momentum symmetric…momentum symmetric…

But “intrinsic” strange sea of the nucleon But “intrinsic” strange sea of the nucleon need not be!need not be!– soso is a DIS probe of intrinsic is a DIS probe of intrinsic

strangeness!strangeness!

Brodsky and Ma, Phys. Let. B392

Paschos-Wolfenstein relation assumes that strange sea is symmetric, i.e., no “valence” Paschos-Wolfenstein relation assumes that strange sea is symmetric, i.e., no “valence” strange distributionstrange distribution– if there were on, this would be a big deal since it is an isovector component of the PDFsif there were on, this would be a big deal since it is an isovector component of the PDFs

(charm sea is heavily suppressed)(charm sea is heavily suppressed)

~30% more momentum in strange sea than in half of strange+anti-strange seas would “fix” ~30% more momentum in strange sea than in half of strange+anti-strange seas would “fix” NuTeV sinNuTeV sin22θθWW

Why might one think that the strange and anti-strange seas would be different?Why might one think that the strange and anti-strange seas would be different?G.P. Zeller et al.,

Phys.Rev.D65:111103,2002)

( ) ( )s x s x


How Does NuTeV Measure This? How Does NuTeV Measure This?

±± from semi-leptonic charm decay from semi-leptonic charm decay

Fits to NuTeV and CCFR Fits to NuTeV and CCFR andand dimuon data dimuon data can measure the strange and antistrange seas separately can measure the strange and antistrange seas separately – NuTeV separate NuTeV separate

andand beams beams important for important for reliable separation reliable separation of s andof s andss

(Cabbibo supp.) beam: , cs d (Cabbibo supp.) beam: , cs d

( )

N X


NEWNEW NuTeV NLO Analysis NuTeV NLO AnalysisHave incorporated CTEQ strange “valence” Have incorporated CTEQ strange “valence” evolution and CTEQ parameterizationsevolution and CTEQ parameterizations– thanks esp. to Amundson, Kretzer, Olness & Tungthanks esp. to Amundson, Kretzer, Olness & Tung

NuTeV NLO analysis NuTeV NLO analysis (Phys.Rev.Lett.99:192001,2007)(Phys.Rev.Lett.99:192001,2007) is near is near zero, but slightly positivezero, but slightly positive– will shift central valuewill shift central value

towards standard modeltowards standard modeland increase uncertaintiesand increase uncertainties

– at NLO, with CTEQ6 as base PDFat NLO, with CTEQ6 as base PDF

courtesy heroic efforts of D. Mason, P. Spentzouris

(additional unc. of 0.00128 from external inputs, primarily B(c ))

( ) .00196 0.00065S x s s dx


Same Data: LO AnalysisSame Data: LO AnalysisFor analysis of sinFor analysis of sin22θθWW want an analysis using the same want an analysis using the same

LO cross-section model as NuTeVLO cross-section model as NuTeV– published NuTeV result was based onpublished NuTeV result was based on

LO cross-sections fit to CCFR dataLO cross-sections fit to CCFR data– SS--/S/S++, if fit to NuTeV data, is , if fit to NuTeV data, is 0.10±0.040.10±0.04

( )0.09 0.04

( )

x s s dxS

S x s s dx

Neu

trin

o B

eam

Anti-neutrino B

eam

NuTeV UpdateNuTeV Update

1.1. Effects to be incorporatedEffects to be incorporated

2.2. Numerical EstimationsNumerical Estimations


What’s in the Update?What’s in the Update?Three large effectsThree large effects– Strange Sea Strange Sea (just discussed), (just discussed), SS--/S/S++==0.09±0.040.09±0.04

– External KExternal K++e3e3 branching ratio branching ratio

Brookhaven E-865, famous for “fixing” the unitarity of the Brookhaven E-865, famous for “fixing” the unitarity of the first row of the CKM matrixfirst row of the CKM matrix

– this was a many standard deviation shift!this was a many standard deviation shift!

Strong effect on our electron neutrino backgroundStrong effect on our electron neutrino background

– d/u PDF uncertaintiesd/u PDF uncertaintiespointed out by Kulagin and Alekhin that these were pointed out by Kulagin and Alekhin that these were underestimated in published resultunderestimated in published result

also corrected target neutron excessalso corrected target neutron excess


Changes in Prediction of RChanges in Prediction of Rνν

exp

exp

0.0034 0.0013

0.0016 0.0027pred

pred

R R

R R

published:

exp

exp

0.0038 0.0013

0.0026 0.0029pred

pred

R R

R R

updated:


NuTeV 99% Conf.

Prediction

Graphical Shifts in RGraphical Shifts in Rνν

d/u νemtop Strange Sea


NuTeV 99% Conf.

Prediction

Directions of Effects not Considered Directions of Effects not Considered

mcShadowing (VMD) Valence Isospin Violation


What’s Next?What’s Next?

Move NuTeV analysis to cross-sections Move NuTeV analysis to cross-sections based on NuTeV structure function resultsbased on NuTeV structure function results

Incorporate complete treatment of QED Incorporate complete treatment of QED radiative corrections, including PDF radiative corrections, including PDF evolution, if availableevolution, if available

Refit data with external strange sea Refit data with external strange sea constraintsconstraints

revisiting nutev

Documents

n mode

neutrino physics

neutrino electroweak

nutev result qqqq

qed corrections

n lep measurement of

knownem radiative corrections

antineutrino beams