Direct Measurement of the NuMI Flux with Neutrino-Electron Scattering in MINERvA Jaewon Park...

Direct Measurement of the NuMI Flux with Neutrino-Electron Scattering in

MINERvA

Jaewon Park

University of Rochester

On behalf of MINERvA Collaboration

December 20, 2013

Fermilab Joint Experimental-Theoretical Seminar

2

Outline

• Neutrino experiments and their fluxes• ν-e scattering: signal • Event reconstruction• Backgrounds and how to remove them• Background Prediction• Systematic uncertainty• Result and Conclusions

20 December 2013 Jaewon Park, U. of Rochester FNAL JETP

3

Oscillation Experiment Strategy

• In fact the flux doesn’t just decrease like 1/L2 – Oscillations

– Near detector sees line source, far detector sees point source

• Far detector sample is always very different from near detector sample

1111 AN 2222 PANbeam-

Near Detector (ND)Far Detector (FD)

2

1

2

1

2

1

1

2

222

2

A

A

N

N

A

NP

or ?


Isn’t this just 1/L2?

N: events in dataB: Background: EfficiencyA: Acceptance: Cross section

4

Needs of Precision Oscillation Experiments


• Precise measurement of oscillation parameters is the key to answer important questions like neutrino mass hierarchy and CP violation

• To achieve the highest precision, we need:– (High intensity beam) × (big detector) × (long operation) – Low uncertainties on flux prediction– Better understanding of neutrino interactions

• More accurate cross-section (May 10 JETP by MINERvA) • Understanding nuclear effects (October 11 JETP by MINERvA) • Detailed understanding of background interactions

• This talk is about a method to constrain or measure the neutrino flux using neutrino-electron scattering– This helps to reduce flux normalization uncertainties on MINERvA’s

absolute cross-section measurements– This technique can be used in future high intensity beam

experiments to measure the flux

5

Neutrinos from an Accelerator


NearDetector

FarDetector

proton

K,Horn 1

Horn 2Target

Decay pipe Rock

• Neutrino beam is generated from a decay of secondary or tertiary particles Hard to control beam itself, too hot to measure in situ

• Flux has large uncertainties due to poor knowledge of hadron production• Non-perturbative QCD governs it Difficult to calculate from basic

principles• ~15-30% normalization uncertainties on flux

6

Neutrinos from an Accelerator


NearDetector

FarDetector

proton

K,Horn 1

Horn 2Target

Decay pipe Rock

• Kaon and muon decays are main source of electron neutrinos

K

e

e0

e

e

7

Constraining flux with Hadron Production Data

p πn

target

ν

decay pipe


• Hadron production primarily function of xF=pion/proton momentum ratio and ptransverse– Use NA49

measurements – Scale to 120 GeV using

FLUKA (simulation)– Check by comparing to

NA61 data at 31 GeV/c [Phys.Rev. C84 (2011)034604]

• Use MIPP (120GeV protons) for K/π ratio

Particle production xF Reference

NA49 pC

@158 GeV

π± <0.5 Eur.Phys.J. C49 (2007) 897

K± <0.2 G. Tinti Ph.D. thesis

p <0.9 Eur.Phys.J. C73 (2013) 2364

MIPP pC @

120 GeV K/π ratio A. Lebedev Ph.D. thesis

8

NA49: pC → π,K,p @ 158 GeV

NA49 datavs. GEANT4

Uncertainties7.5% systematic2-10% statistical

π+ which makea νμ in MINERvA

focusingpeak

f(xF,p

T) = E d3σ/dp3 = invariant production cross-section

high energytail


9

Need more than Hadron Production Measurements

• Hadron Production measurements don’t tell the whole story, only 70%– Some pion production is out of

range of Hadron Production data– Tertiary production of neutrinos

also important (n, , KL,S)

• Beamline geometry and focusing elements contribute uncertainties


10

Special Runs to Understand Flux• MINERvA integrated 10% of our total neutrino

beam exposure in alternate focusing geometries: – Changed horn current– Changed Target Position

• Purpose is to disentangle focusing uncertainties from hadron production uncertainties

– Different geometry focuses different parts of xF pT space, but same horn geometry and current

• MINERvA does this by using low hadron energy charged current events, where energy dependence of cross section is very well understood


Normal Running

Target Moved

upstream

Pion Phase Space

Neu

trin

os a

t M

INE

RvA

xF

xF

Pt (

GeV

/c)

Pt (

GeV

/c)

Inclusive Event

Spectra

11

Flux constraint using Near Detector

Cross-section uncertainty goes intoflux uncertainty

MINERvA

Flux uncertainty goes intocross-section uncertainty

Neutrino Flux and Cross-section Measurement

A

N

AN


• Flux and cross-section are anti-correlated with given Near Detector constraint

σ (Cross Section)

Φ (

Flu

x)

1111 AN

N: Events: Efficiency

A: Acceptance: signal cross section

Measurement uncertainty

12

Known Interaction (Standard Candle)

• ν-e scattering is well known interaction we can use to constrain the neutrino flux

AN

Flux constraint using ND

Cross-section uncertainty goes intoflux uncertainty

ν-e Scattering20 December 2013 Jaewon Park, U. of Rochester FNAL JETP

e e

0Z 1111 AN

σ (Cross Section)

Φ (

Flu

x)

13

Outline

• Neutrino experiments and their fluxes• ν-e scattering: signal• Event reconstruction• Event selection• Background Prediction• Systematic uncertainty• Result and Conclusions


14

ν-e Scattering History

e

First unambiguousneutral current(Gargamelle)

Solar ν oscillation measurementusing ν-e scattering (SNO)

e

e

e

e

x

epn ,, epn ,,

x

,,ex

ZW

Coherent forward scatterings

Additional interaction for e

All flavor

Matter effect is due tocharged current νe scattering

on electrons, only for νe

Electroweaktheory


15

Neutrino Scattering on Nucleon

• Let’s use well-known reaction to measure the flux• Standard electroweak theory predicts it precisely

– Point-like scattering• Very small cross section (~1/2000 of ν-nucleon scattering)

– Low center of mass energy due to light electron• Very forward electron final state (Experimental signature)• Good angular resolution is important to isolate the signal

ee ee

e e

0Z

e

Very forward single electron final state

νe→ νe candidate event

ElectronElectron


16

ν-e Scattering

• E > 0.8 GeV– High background rate and tough reconstruction at low energy

• Predict 147 signal events for 3.43×1020 Protons On Target (POT) – ~100 events when you fold in (reconstruction + selection) efficiency of ~ 70%

• Not a large sample in low energy run but still useful to constrain absolute flux

Ee )( dyd

energy) (neutrino

KE)(electron y


FLUX e Scattering Events

e Scattering Events

24

22

2

)1(sinsin2

1

2

)(y

EmG

dy

eedWW

veF

GF and θW: well-known electroweak parameters

17

Signal Events


• Signal is mixture of in LE-FHC (neutrino beam)• ~100 signal events for 3.43E20 POT• Can’t distinguish neutrino type

• Still useful to constrain the flux– Total events: Constraint for integrated flux– Electron spectrum: Constraint for flux shape

eeee ee and ,,,

%9: and

%91: and

ee

ee

ee

E>0.8 GeV

E<0.8 GeV is not used•Large background•Tough reconstruction

For remainder of talk, means and

18

Outline

• Neutrino experiments and their fluxes• ν-e scattering: signal • Event reconstruction• Backgrounds and how to remove them• Background Prediction• Systematic uncertainty• Result and Conclusions


19

Data and Simulation Samples

• All Low Energy neutrino data is used for the analysis: more than previous analyses shown to date (3.43 × 1020 Protons on Target)

• Time-dependent effects (calibrations, accidental activity) included in the simulation

MINERvA ran in three kinds of beam:

Low Energy neutrino

Low energy anti-neutrino

“Special Runs”: higher energy runs to constrain flux model


Thank you!for the excellent ν beam

20

MINERvA Detector


Inner Detector

5m

3.5m

4m

Outer Detector(steel + scintillator)

Nuclear Targets(C, Pb, Fe, H2O)

Tracker(Active target)

Electromagnetic Calorimetry

Hadronic Calorimetry

21

MINOSNear Detector

(muon spectrometer)

Inside the Detector

−60°

+60°

xvxu

Nuclear Target

Tracker

EcalHcal

Number of channels: ~31kNumber of scintillator plane: 128

Scintillator plane

(X, U, V stereo angle)

Tracking Ecal (lead absorber + tracking plane)Tracking Hcal (steel absorber + tracking plane)

22

Detector Technology


• Extruded plastic scintillator with wavelength shifting fiber readout• 64 channel multi-anode PMT for photo-sensor

Wavelength shifting fiber

8×8 pixels

64 channel multi-anode PMT

Scintillator strip

17 mm

16 mm

Position resolution: ~3mm

2.1m127 strips into a plane

2.5 m

23

ν + e- → ν + e- candidate event

X-View U-View V-View

Data run: 2157/12/1270/2

ee

0Z

e

Fiducial volume


24

Single Electron ReconstructionNuclear Target Region

(He,C/H2O/Pb/Fe)HCALECAL

Track-like

Shower-like

Track-like part (beginning of electron shower) gives good direction


25

Single Electron ReconstructionNuclear Target Region

(He,C/H2O/Pb/Fe)HCALECAL

Track-like part (beginning of electron shower) gives good direction


Shower cone

26

Critical Variables for Signal

• Electron Identification– Must discriminate from photons

• Electron Energy Measurement

• Electron Angular Measurement


27Electron Photon Discrimination using dE/dx


• Electromagnetic shower process is stochastic– Electron and photon showers look very similar

• Photon shower has twice energy loss per length (dE/dx) at the beginning of shower than electron shower

– Photon shower starts with electron and positron

γe

ee

e

e e

γ

γγ

e

γe

γe

e

e

e

e

e

Electron-induced electromagnetic shower

Photon -induced electromagnetic shower

MINERvA Preliminary

28

Energy and Angle Reconstruction


• Energy resolution ~ 5%• Projected angle resolution ~ 0.3 degree (2 sigma truncated RMS)• Precise angle reconstruction is critical to separate ν e elastic scattering

from background– Lower energy angular resolution is worse due to multiple scattering

Using simulated signal

Using simulated signal

3.6% %2.5)(

EE

E

MINERvA PreliminaryMINERvA Preliminary

29

Outline

• Neutrino experiments and their fluxes• ν-e scattering: signal and backgrounds• Event reconstruction• Backgrounds and how to remove them• Background Prediction• Systematic uncertainty• Result and Conclusions


30

Initial Background Rejection• -e scattering is very rare, even for interactions:

• Simple cuts can eliminate most background events while keeping high fraction of signal events– Obvious muon-like event rejection– Upstream energy rejection

• Removes neutrino interactions upstream of detector that make


e e

n p

W

N N

Z

0

e Quasi-elastic (CCQE) Coherent 0

,

N X

ZW ,

Most Events

( Charged or neutral Current)

Rare but hard to reject:

31

Background Events


Use Eθ2 to selectvery forward signal

pene

nepe

e e

p n

WElectron neutrino fraction in flux is small ~ 1%.

electron

proton

z

x

0

2

4

6

8

MeVMCMC

•If recoil nucleon is not observed, it looks similar to signal•Angles of electron have wide spread while signal is very forward

0

0

NN

AA

NC-coherent π0

NC-resonant π0

Neutral current single π0

1. Small opening angle between two gammas

π0 (1.1 GeV)

γ (67 MeV )π0 (7.5 GeV)

0

0

2. One of gammas is not observed in the detector

Simulated event Simulated event

Also, photon has wide spread of angleIn addition, use dE/dx to reject

32Example: Neighborhood Energy

• Neighborhood energy = energy around shower cone• Small neighborhood energy means isolated shower


Signal × 200 Not Full Sample

MINERvA Preliminary

MINERvA Preliminary

5 cm

Neighborhood

Shower cone

33

Event Selection


Shower coneReconstruction

• Electron Energy>0.8GeV• Fiducial cut

Otherreconstruction quality cuts

Signalsample

• Eθ2

• dE/dx

Kinematic constraint on e scattering, using Mandelstam variables:

ee *cos12

s

t *cos12

1 y syt

)cos1(2 eEEu

2)1(2

)cos1(2)1(

ee

e

Eym

EEys

uts

ee mEy 2 ,10 Since 2

in CM frame

in lab frame

34

dE/dx Cut

• All cuts made on this sample except for the dE/dx cut• Neutrino interaction doesn’t always produce only

single electron or single photon (from π0)• Non-single particle activity affects dE/dx

tunedtuned

dE/dx<4.5MeV/1.7cm


MINERvA Preliminary

MINERvA Preliminary

35 Eθ2 Cut

• All cuts but E2 cut• Kinematic limit for signal

– Eθ2 < 2me

• Clean separation of signal

22 radGeV 0032.0 E

tuned tuned


MINERvA PreliminaryMINERvA Preliminary

36

Electron Spectrum after all cuts

tuned


True electron energy(signal only)

Reconstructed electron energy

MINERvA Preliminary

MINERvA Preliminary

37

Outline



38

Backgrounds after all Cuts


• Background prediction is affected by the flux and physics model• Cross-section of various neutrino reactions are uncertain

– That’s what MINERvA is trying to measure• Data-driven background prediction tuning is used to handle the

uncertainty of predicted background

0032.02 E SidebandSignal

22 radGeV 005.0 E

Need to know energy spectrum of background

MINERvA Preliminary

MINERvA Preliminary

39 4 Background Processes, 4 Sidebands


• Sideband = Outside of major Eθ2 and dE/dx cuts• (b) region is not used because there are not many events for tuning• Further, cut is slightly loosened on sideband so it gets some νμ CC for tuning

purpose

dE/dx(MeV/1.7cm)

0.00320.005

4.5

(a) Sideband

signal

20

(b) Unused

Eθ2 (GeV∙rad 2)

3 Min dE/dx

Energy

1.2

0.8

Sideband 1Sideband 3

Sideband 2

Sideband 4

Sideband 1, 2, 3(not sideband 4)

(Coherent π0 rich region)

• No side-exiting muon• Narrow shower at beginning• Eθ2<0.1

40

Sideband Populations


e e

n p

W

N N

Z

0

Coherent 0

,

N X

ZW ,

Most Events

( Charged or Neutral Current )

Rare but hard to reject:

e Charged Current

41

Sideband Tuning

• Scale three MC components to match to data• Minimize χ2 across 7 histograms

3 parameters tuned in this step• Minimize χ2 across 2 histograms

1 parameter tuned in this step

After tuning

Before tuning

Events with tracks in downstream Hadron Calorimeter are mostly νμ CC


Track Length in HCAL (modules)

Track Length in HCAL (modules)

Parameter Tuned value

0.83 ± 0.04

0.81 ± 0.03

0.94 ± 0.01

0.90 ± 0.08

νe

νμ NC

νμ CC

COH π0

MINERvA Preliminary

MINERvA Preliminary

42

dE/dx and in Sidebands after tuning


4.5dE/dx (MeV/1.7cm)

0.00320.005

Sideband

Signal

Eθ2

(GeV∙rad 2)

(a)

(b)

(c) Unused

• Both dE/dx and Eθ2 are well simulated in the sideband region after fitting

dE/dx (MeV/1.7cm)

Eθ2 (GeV∙radians 2)

# Events (Eθ2 < 0.2)

MINERvA Preliminary

MINERvA Preliminary

43

Outline



44

Systematic Uncertainties

• Error in background contribution– Flux uncertainties– Cross Section Uncertainties

• Error in efficiency and Acceptance


ABN

N: events in dataB: Background: Efficiency

A: Acceptance: signal cross section

45 Uncertainty in eCCQE extrapolation from sideband

• Previous MINERvA results on Quasi-elastic process shows that momentum transfer squared (Q2

QE) distribution is not what GENIE predicts Phys. Rev. Lett. 111, 022502 (2013), Phys. Rev. Lett. 111, 022501 (2013).

• Q2QE and Eθ2 are highly correlated

• Compare ebackground prediction E2 extrapolation with two different models: one is GENIE, the other is one inspired by MINERvA data: systematic uncertainty: 3.3%


46 Flux and Cross Section Systematic Uncertainties on MC Background

• Sideband tuning reduced systematic uncertainty on predicted background– Predicted background (before tuning): 38.9 ± 6.2 (stat) ± 10.3 (sys)– Predicted background (after tuning): 32.9 ± 5.3 (stat) ± 5.7 (sys)

• The tuning didn’t eliminate systematic uncertainty but it gives confidence on background prediction

Uncertainty SourcesMC background uncertainty [events]

Before tuning After tuning

MC background events 38.9 32.9

MC bkg statistical 6.2 5.3

Total systematic 10.3 5.7

Flux_BeamFocus 1.1 0.2

Flux_NA49 1.8 0.3

Flux_Tertiary 7.0 1.1

GENIE 6.3 4.5

CCQE Shape 3.7 3.3

Total 12.1 7.8


47

Reconstruction Systematic Uncertainties

• Electromagnetic Energy Scale: look at electrons from stoppeddecays (Michel): see agreement at 4.2% level, add as systematic uncertainty


• Angular Alignment: look at data-simulation differences in angles for CC events with low hadron energy

– 3 (1) mrad correction in y (x)

– uncertainty is ±1mrad

MINERvA Preliminary

MINERvA Preliminary

Charged Current Events

with hadron energy<100MeV

48

Reconstruction Uncertainties

SourceUncertainty on Source

Systematic Uncertainty

Beam angle uncertainty x and y : ± 1 mrad 1.1% and 1.3%

Energy scale 4.2% 1.9%

EM calorimeter energy smearing

Additional energy smearing 0.0%

Absolute Electron Reconstruction Efficiency

2% based on muon studies 2.8%

All Reconstruction Uncertainties 5.4%

Simulation statistics (Bckgd) 6.0%

Flux (Bkgd) Beam focusing, Beam tuning 1.3%

Cross Section (Bkgd) GENIE, CCQE Shape 6.3%


49

Outline



50

Result• Found: 121 events before

background subtraction

• -e scattering events after background subtraction and efficiency correction:

123.8 ± 17.0 (stat) ± 9.1 (sys) total uncertainty: 15%

• Prediction from Simulation: 147.5 ± 22.9 (flux)– Flux uncertainty: 15.5%


Observed ν-e scattering events give a constraint on flux with similar uncertainty as current flux

uncertainty, consistent with prediction

51 Future Flux Measurement at NuMI

• Assuming similar signal/background ratio as in Low Energy Run:– Can expect statistical uncertainty = ~2% – Total systematic uncertainty on this measurement: 7% – Total uncertainty = ~7.3%

Medium Energy Flux: Now being produced in the NuMI Beamline, as of September 2013

~20 times thelow energy signal sample

MINERvA Preliminary

MINERvA Preliminary

52

Conclusions• -e scattering provides an independent flux measurement for

ν-nucleon cross-section normalization• Uncertainty on ν-e based flux measurement in Low Energy

beam is 15%– It is similar size as current flux prediction uncertainty– Will be used as constraint in future MINERvA cross section

measurements

• In Medium Energy run, estimate a 7% uncertainty on total flux – Currently dominant uncertainty is statistical uncertainty

• This technique could be used in future higher intensity experiments like NOvA and LBNE to provide a precise flux measurement


53

(Backup)

54

Neutrino Beam Spread

• Diameter of decay pipe: ~ 2m• 0.5 m transverse position at 500 m distance 1 mrad angle

MINERvA Preliminary MINERvA Preliminary

55

Data Energy Stability (Michel Electron)

• Some odd behavior is found between minerva1 and rest of playlists– 1 or 2% energy variation

• Use 4.2% as the systematic uncertainty on energy scale

MINERvA Preliminary MINERvA Preliminary

56 Sideband 4 (After Tuning)

MINERvA Preliminary

MINERvA Preliminary

MINERvA Preliminary

MINERvA Preliminary

57

Background Subtraction

MINERvA Preliminary

MINERvA Preliminary

58

Comparing Observed and Predicted Spectra

MINERvA Preliminary

MINERvA Preliminary

Direct Measurement of the NuMI Flux with Neutrino-Electron Scattering in MINERvA Jaewon Park...

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