Revisiting NuTeV -or- What Kevin Did Last Week

Revisiting NuTeV-or-

What Kevin Did Last Week

Kevin McFarlandRochester Neutrino Group Meeting

21 February 2011

The Big PictureNeutrinos are important in electroweak physics– there is a glorious history, of course…– … but precision today in neutrino electroweak

couplings lags behind other sectorsneutrino couplings are the most difficult couplings to measure precisely at the Z0 polematter 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 “Nν” LEP measurement of Γ(Z0→νν)– To date, only ~5% precision on σ(νe→νe)

21 February 2011

1984• Walter Mondale asks “where’s the beef”?• KSM takes driver’s education• FNAL-E744 (CCFRW) takes data

1993• William Jefferson Clinton becomes President• KSM plays minor role in production of his first offspring, who today wants to take driver’s ed• NuTeV Experiment proposed to recycle CCFR detector with a new beamline

1996• William Jefferson Clinton invites an intern back to see his cigar collection• KSM analyzes E744 (and E770) data to measure NC/CC• NuTeV takes first beam

1997• Father of one of KSM’s pothead buddies from college with cartoon last name finds himself looking blue dress DNA• NuTeV data taking complete

2001• Men with box cutters kill thousands. US begins self-destructive response• KSM presents NuTeV results “A departure from prediction” at Fermilab Wine and Cheese

2002

• Last “Saddam [yes]” election• MINERvA and T2K proposed (leaving to later immitation “Saddam [yes]” style elections)• “The NuTeV-a result-a, she ees-a sheet-a” – Quote from Theorist, whose last name begins with one of “DGLAP”

NuTeV Revisited, K. McFarland 421 February 2011

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

NuTeV Measurement Technique

(3) 2Coupling sinweak em WJ Q 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

n n

n n

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

2 2 2 212

Paschos - Wolfenstein Relation

sinNC NCW L R

CC CC

R g gn n

n n

q


Beam identifies neutral currents as n or nn in n mode 3104, n in n mode 4103 Beam only has ~1.6% electron neutrinos Important background for NC

events since no final state muon

NuTeV Sign-Selected Beamline

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

2 212

Paschos - Wolfenstein Relation

sinNC NCW

CC CC

Rn n

n n

q


exp exp2 2

2exp exp

large smallsin sin

sin systematics (i.e. )W W

W c

dR dRd dR R m

n n

n n

q qq

Paschos-Wolfenstein à la NuTeV( )

( )

( )( ) 2 2 4

0( )

1 5sin sin (1 )2 9

CC

CC

NCW W

CC

Rn n

n n

n nn n

n n

q q

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

on shellW

statsyst

q

NuTeV result:– Statistics dominate uncertainty

EWK fit (LEPEWWG 2001): – 0.2227 0.00037, a 3

discrepancyexp

exp

0.3916 0.0013 ( : 0.3950) 30.4050 0.0027 ( : 0.4066)

R SM differenceR SM Good agreement

n

n

NuTeV fit for sin2θW

and mc given external constraint from strange sea

analysis. (More later)

QCD SymmetryViolations

What symmetry violations can affect the result?

1. u≠d in target (neutron excess)2. asymmetric heavy seas


Symmetry Violating QCD EffectsPaschos-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-

2 2

2 2

2 2

2 2 2

3

1 32

2 (3 )

u d

v vu d

v v

v vu d

v v

d u d cv v

RU DNU DU DU DS

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 ZNA

U x u d dx

U x u d dx

S x s s dx

Asymmetric Strange Sea

1. Why it might be so2. How it is measured at NuTeV

This is what drives us to update the NuTeV measurement


A Very Strange Asymmetry

Perturbative strange sea is (roughly) momentum symmetric…But “intrinsic” strange sea need not be!– so is a DIS probe of intrinsic

strangeness!– Brodsky first suggested it! (though this

model at right is excluded by NuTeV data) 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 sin2θW

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?

± from semi-leptonic charm decay

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

and n beams important for reliable separation of s ands

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

( )

N X n


NEW NuTeV NLO AnalysisHave 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 modeland increase uncertainties

– at 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

NuTeV Update

1. Effects incorporated– Numerical Estimations

2. To do


What’s Done?Three large effects– Estimate for Strange Sea, S-/S+~0.09±0.04– External K+

e3 branching ratioBrookhaven E-865, famous for “fixing” the unitarity of the first row of the CKM matrix– also improves agreement with our own (less

precise) measurement of νe fluxStrong effect on our electron neutrino background

– d/u PDF uncertaintiespointed out by Kulagin and Alekhin that these were underestimated in published resultalso corrected target neutron excess


Changes in Prediction of Rν

exp

exp

0.0034 0.00130.0016 0.0027

pred

pred

R RR R

n n

n n

published:

exp

exp

0.0038 0.00130.0026 0.0029

pred

pred

R RR R

n n

n n

updated:

Rν Rνbar

Source σpub σnew Δ σpub σnew Δmtop n/a n/a +0.00001 n/a n/a -0.00015

d/u 0.00004 0.00013 +0.00000 0.00004 0.00018 +0.00000

νe 0.00028 0.00029 +0.00078 0.00062 0.00062 +0.00085

Strange PDFs 0.00023 0.00036 -0.00038 0.00052 0.00078 +0.00029

NLO QED 0 0.00013 n/a 0 0.00061 n/a

NLO QCD 0 0.00004 n/a 0 0.00043 n/a


NuTeV 99% Conf.

Prediction

Graphical Shifts in Rν

d/u νemtop Strange Sea, S-/S+=0.09


NuTeV 99% Conf.

Prediction

Directions of Effects not Considered(length of arrows are arbitrary)

mcShadowing (VMD) Valence Isospin Violation


What Are you Working on Now?

Current Work is on Improving Cross-Section Model– LO to NLO QCD cross-section– Higher twist (non-perturbative) effects smaller– Can build on better fits (global!) to two muon

data to constrain charmBase data for cross-section comes from NuTeV (previous analysis used CCFR)It’s “the gift that keeps on giving”…

19

What if We Don’t “Fix” NuTeV?

My money is on the second symmetry violating term,We may have discovered large (several %) isospin violation in parton distributions

21 February 2011 NuTeV Revisited, K. McFarland

2 2

2 2

2 2

2 2 2

3

1 32

2 (3 )

u d

v vu d

v v

v vu d

v v

d u d cv v

RU DNU DU DU DS

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 ZNA

U x u d dx

U x u d dx

S x s s dx

( ) , etc.p nv v vU x u d dx

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Revisiting NuTeV -or- What Kevin Did Last Week