Precision Measurement of sin 2 W from NuTeV Jae Yu (for NuTeV Collaboration) University of Texas at...

Precision Measurement of sin2W from NuTeV

Jae Yu (for NuTeV Collaboration)University of Texas at Arlington

SSI 2002, Aug. 14, 2002

• Introduction• Past measurements• Current Improvements• What’s so new about the results?• Conclusions

Aug. 14, 2002 J. Yu: sin2W at NuTeVSSI 2002

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NuTeV CollaborationT. Adams4, A. Alton4, S. Avvakumov7, L.de Babaro5, P. de Babaro7, R.H. Bernstein3, A. Bodek7, T. Bolton4, J. Brau6, D. Buchholz5, H. Budd7, L. Bugel3, J.

Conrad2, R.B. Drucker6, B.T.Fleming2, J.A.Formaggio2, R. Frey6, J. Goldman4, M. Goncharov4, D.A. Harris3, R.A. Johnson1, J.H.Kim2, S.Kutsoliotas9, M.J. Lamm3, W. Marsh3, D. Mason6, J. McDornald8, K.S.McFarland7, C. McNulty2, Voica Radescu8, W.K. Sakumoto7, H.

Schellman5, M.H. Shaevitz2,3, P. Spentzouris3, E.G.Stern2, M. Vakili1, A. Vaitaitis2, U.K. Yang7, J. Yu3*, G.P. Zeller5, and E.D. Zimmerman2

1. University of Cincinnati, Cincinnati, OH45221, USA2. Columbia University, New York, NY 10027 3. Fermi National Accelerator Laboratory, Batavia, IL 60510 4. Kansas State University, Manhattan, KS 66506 5. Northwestern University, Evanston, IL 60208 6. University of Oregon, Eugene, OR 97403 7. University of Rochester, Rochester, NY 14627 8. University of Pittsburgh, Pittsburgh, PA 15260 9. Bucknell University, Lewisburg, PA 17837*: Current affiliation at University of Texas at Arlington


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• Standard Model unifies Weak and EM to SU(2)xU(1) gauge theory – Weak neutral current interaction– Measured physical parameters related to mixing parameters

for the couplings

• Neutrinos in this picture are unique because they only interact through left-handed weak interactions Probe weak sector only– Less complication in some measurements, such as proton

structure

Electroweak Threory

WZ

W2W

2

FWW cosθMM

8M2g

G ,gsinθe ,gtanθg' ,


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sin2W and -N scattering

• In the electroweak sector of the Standard Model, it is not known a priori what the mixture of electrically neutral electomagnetic and weak mediator is This fractional mixture is given by the mixing angle

• Within the on-shell renormalization scheme, sin2W is:

• Provides independent measurement of MW & information to pin down MHiggs

• Comparable size of uncertainty to direct measurements• Measures light quark couplings Sensitive to other types (anomalous) of

couplings • In other words, sensitive to physics beyond SM New vector bosons,

compositeness,-oscillations, etc

2

22

1sinZ

WShellOn

w M

M


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How do we measure?

• Cross section ratios between NC and CC proportional to sin2W

• Llewellyn Smith Formula:

)(CC

)(CC

W4

W22

)(CC

)(NC)(

σ

σ1θsin

95

θsin21

ρσ

σR

WEMweak QIcoupling 2)3( sin)3(weakIcoupling

Some corrections are needed to extract sin2W from measured ratios (radiative corrections, heavy quark effects, isovector target corrections, HT, RL)


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Previous Experiment

• Conventional neutrino beam from /k decays • Focus all signs of /k for neutrinos and antineutrinos• Only in the beam (NC events are mixed)

• Very small cross section Heavy neutrino target e are the killers (CC events look the same as NC events)


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Charged Current Events

Neutral Current Events

How Do We Separate Events?

x-view

y-view

x-view

y-viewNothing is coming in!!!

Nothing is coming in!!!

Nothing is going out!!!

Event Length


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Event LengthDefine an Experimental Length variable Distinguishes CC from NC experimentally in statistical manner

Compare experimentally measured ratio

to theoretical prediction of R

Candidates CC

Candidates NC

Cut

Cut

Long

ShortExp N

N

LL

LL

N

NR


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Past Experimental Results

• Significant correlated error from CC production of charm quark (mc) modeled by slow rescaling, in addition to e error

2ShellOnW

Z

WShellOnW

2

0.19GeV/c80.14M

0.00360.2277MM

1θsin


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The NuTeV Experiment• Suggestion by Paschos-Wolfenstein formula by separating andbeams:

Reduce charm CC production error by subtracting sea quark contributionsOnly valence u, d, and s contributes while sea quark contributions cancel outMassive quark production through Cabbio suppressed dv quarks only

• Smarter beamline Removes all neutral secondaries to eliminate e content

r1

RRθsin

2

1ρ

σσ

σσR W

22

CCCC

NCNC


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• Calorimeter– 168 FE plates & 690tons– 84 Liquid Scintillator– 42 Drift chambers interspersed

The NuTeV Detector

• Solid Iron Toroid• Measures Muon momentum p/p~10%

Continuous test beam for in-situ calibration


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The NuTeV Detector

A picture from 1998. The detector has been dismantled to make room for other experiments, such as DØ


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• Ehad > 20GeV– To ensure vertex finding efficiency– To reduce cosmic ray contamination

• Xvert and Yvert within the central 2/3– Full hadronic shower and muon containment– Further reduce e contamination

• Longitudinal vertex, Zvert, cut– To ensure neutrino induced interaction– Better discriminate CC and NC

NuTeV Event Selection


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Remaining number of events: 1.62M 350k

Events and Flux After Selection


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NuTeV Event Length Distributions

250k101k

1167k457k

R=Nshort/NLongNlongNshortMode

t)0.0007(sta0.3916 t)0.0016(sta0.4050

Energy Dependent Length cut implemented to improve statistics and reduce systematic uncertainties.

Good Data-MC agreement in the cut region


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Event Contamination and Backgrounds

•SHORT CC’s (20% ) exit and rangeout•SHORT e CC’s (5%) eNeX•Cosmic Rays (0.9%)

•LONG NC’s (0.7%)hadron shower punch-through effects

•Hard Brem(0.2%)Deep events


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Sources of experimental uncertainties kept small, through modeling using and TB data

Other Detector Effects

Effect Size(sin2W) Tools

Zvert 0.001/inch +- events

Xvert & Yvert 0.001 MC

Counter Noise 0.00035 TB ’s

Counter Efficiency 0.0002 events

Counter active area 0.0025/inch CC, TB

Hadron shower length 0.0015/cntr TB ’s and k’s

Energy scale 0.001/1% TB

Muon Energy Deposit 0.004 CC


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• Neutrino events in anti-neutrino running constraint charm and KL induced production (Ke3) in the medium energy range (80<E<180GeV)

• Shower Shape Analysis can provide direct measurement e events, though less precise

Measurements of e Flux

e from very short events (E>180 GeV)• Precise measurement of e flux in the tail region of flux ~35% more e

in than predicted• Had to require (Ehad<180 GeV)

due to ADC saturation

e

e

MCmeas

0.041.01

0.031.05

/NN

Results in sin2w shifts by +0.002

Weighted average used for e R

exp~0.0005


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MC to Relate Rexp to Rand sin2W

• Parton Distribution Model– Correct for details of PDF model Used CCFR data for PDF– Model cross over from short CC events

• Neutrino Fluxes ,e,,e in the two running modes e CC events always look short

• Shower length modeling– Correct for short events that look long

• Detector response vs energy, position, and time– Continuous testbeam running minimizes systematics


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Rexp Stability Check

• Crucial to verify the Rexp

comparison to MC is consistent under changes in cuts and event variables– Longitudinal vertex Detector

uniformity– Length cut Check CC to NC

cross over– Transverse vertex NC

background at the detector edge– Visible energy (EHad)Checks

detector energy scale and other factors

Green bands represent 1 uncertainty.


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• Thanks to the separate beam Measure R’s separately• Use MC to simultaneously fit and to sin2W and mc, and sin2W and

• R Sensitive to sin2W while R isn’t, so Ris used to extract sin2W and R to control systematics• Single parameter fit, using SM values for EW parameters (=1)

sin2W Fit to Rexp and R

exp

input as GeV/c 0.141.38c

m w/ (syst) 0.06(stat) 0.091.32cm2

(syst) 0.0009(stat) 0.00130.2277θsin W2

expRexpR

•Two parameter fit for sin2W and yields

0.0400.9983ρ

0.00310.2265θsin

0

W2

)(CC

)(CC

W4

W22

)(CC

)(NC)(

σ

σ1θsin

9

5θsin

2

1ρ

σ

σR

Syst. Error dominated since we cannot take advantage of sea quark cancellation


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NuTeV sin2W Uncertainties

150GeVM

ln0.00032

50GeV

175GeVM0.00022θδsin

H

2

22tshell)(On

W2

1-Loop Electroweak Radiative Corrections based on Bardin, Dokuchaeva JINR-E2-86-2 60 (1986)

0.00032RL

0.00022

0.08MW (GeV/c2)

0.00162Total Systematic Uncertainty

0.00064Total Physics Model Systmatics

0.00011RadiativeCorrection

0.00005Non-isoscalar target

0.00014Higher Twist

0.00047CC Charm production, sea quarks

0.00063Total Experimental Systematics

0.00018Energy Measurements

0.00046Event Length

0.00039e flux

0.00135Statistical

sin2WSource of Uncertainty

/

Dominant uncertainty


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NuTeV vs CCFR Uncertainty Comparisons

Technique worked!

Beamline worked!


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The NuTeV sin2W

2ShellOnW

2Z

2WshellOn

W2

ShellOnW

2

GeV/c 0.0880.14M

MM

1θsin

(syst) 0.0009(stat) 0.00130.2277θsin

Comparable precision but value smaller than other measurements


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SM Global Fits with NuTeV ResultWithout NuTeVdof=20.5/14: P=11.4%With NuTeVdof=29.7/15: P=1.3%Confidence level in upper Mhiggs limit weakens slightly.


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Tree-level Parameters: and sin2W(on-shell)

• Either sin2W(on-shell) or 0 could agree with SM but both agreeing

simultaneously is unlikely


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• Performed the fit to quark couplings (and gL and gR)– For isoscalar target, the N couplings are

– From two parameter fit to and

Model Independent Analysis

W42

02R

2R

2R

W4

W22

02L

2L

2L

θsin95

ρdug

θsin95

θsin21

ρdug

expR

0.00110.0310g

0.00140.3005g2R

2L

(SM: 0.3042 -2.6deviation)

(SM: 0.0301 Agreement)

Difficult to explain the disagreement with SM by:Parton Distribution Function or LO vs NLO or Electroweak Radiative Correction: large MHiggs

expR


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• R- technique is sensitive to q vsq differences and NLO effect– Difference in valence quark and anti-quark momentum fraction

• Isospin spin symmetry assumption might not be entirely correct– Expect violation about 1% NuTeV reduces this effect by using the ratio

of and cross sections Reducing dependence by a factor of 3• s vss quark asymmetry

– s and s needs to be the same but the momentum could differ• A value of s=s -s ~+0.002 could shift sin2W by -0.0026, explaining ½ the

discrepancy (S. Davison, et. al., hep-ph/0112302)• NuTeV di- measurement shows that s<<0.002

• NLO and PDF effects– PDF, mc, Higher Twist effect, etc, are small changes

• Heavy vs light target PDF effect (Kovalenko et al., hep-ph/0207158)– Using PDF from light target on Iron target could make up the difference

NuTeV result uses PDF extracted from CCFR (the same target)

What is the discrepancy due to (Old Physics)?


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• Heavy non-SM vector boson exchange: Z’, LQ, etc– LL coupling enhanced than LR needed for NuTeV

• Propagator and coupling corrections– Small compared to the effect

• MSSM : Loop corrections wrong sign and small for the effect• Gauge boson interactions

– Allow generic couplings Extra Z’ bosons???• LEP and SLAC results says < 10-3

• Many other attempts in progress but so far nothing seems to explain the NuTeV results – Lepto-quarks– Contact interactions with LL coupling (NuTeV wants mZ’~1.2TeV, CDF/D0: mZ’>700GeV)– Almost sequential Z’ with opposite coupling to

What other explanations (New Physics)?

Langacker et al, Rev. Mod. Phys. 64 87; Cho et al., Nucl. Phys. B531, 65; Zppenfeld and Cheung, hep-ph/9810277; Davidson et al., hep-ph/0112302


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Future???Muon storage ring can generate 106 times higher flux and well understood, high purity neutrino beam significant reduction in statistical uncertainty But e and from muon decays are in the beam at all times Deadly for traditional heavy target detectors


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Conclusions• NuTeV has measured sin2W:

– NuTeV result deviates from SM prediction by about +3(PRL 88, 091802, 2002)– Interpretations of this result implicates lower left-hand coupling (-2.6but good

agreement in right-hand coupling with SM• NuTeV discrepancy has generated a lot of interest in the community

• Still could be a large statistical fluctuation (5 has happened before) • Yet, many interpretations are being generated:

• Some could explain partially but not all• Asymmetric s-quark sea• Additional mediator, extra U(1) vector bosons, etc

• No single one can explain the discrepancy it still is a puzzle…• Could this be a signature of new physics?

• No other current experiment is equipped to redo this measurement• Muon storage ring seems to provide a promising future…

2ShellOnW

shellOnw

2

0.08GeV/c80.14M

(syst) 0.0009(stat) 0.00130.2277θsin

Precision Measurement of sin 2 W from NuTeV Jae Yu (for NuTeV Collaboration) University of Texas at...

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