# Presentation: 679812

## Search for Neutrino Oscillation with MiniBooNE Detector

- Hai-Jun Yang
- University of Michigan
- University of Nebraska
- Lincoln, Nov. 29, 2007

## Outline

- Brief introduction of neutrino
- Physics Motivation of MiniBooNE
- MiniBooNE Neutrino Beam
- Events in the Detector
- Two Independent Analyses
- MiniBooNE Results

## The Standard Model

## About Neutrino

- Wolfgang Pauli postulated existence of neutrino (“little neutral ones”) in order to explain the missing energy in nuclear  decay in 1930.
- Enrico Fermi presented theory of beta decay in 1934.

## Brief History of Neutrino

- 1930

- 1962

- 1934

- 1956

- 1968

- 1988

- 1995

- 2002

- 1991

**1995 ******

- 2001

- 1987

- 1998

- 2002

- 2001

## Neutrino Oscillations (2 flavors)

********

******e**

**=**

**Flavor eigenstates**

**Mass eigenstate****s**

******1**

******2**

**cos ******

**cos ******

**-sin ******

**sin ******

**|************(t)> = -sin ******** |********1****> + cos**** ******** |********2****> **

**e****-iE1t**

**e****-iE2t**

- (For 3  flavors mixing, it needs 3×3 unitary matrix with CP-violating phase.)

## Neutrino Oscillations (2 flavors)

-     Neutrino flavor states are comprised of mass states

**ELECTRON**

- 

- e

**m****1**

**m****2**

- 

- e

## Neutrino Oscillations (2 flavors)

**P****osc**** =****sin****2****2******** sin****2**** 1.27 ********m****2**** ****L**
**	****	****	****	****	****	****E**

- Distance from neutrino beam creation point to detection point (m)

-  is the mixing angle

- m2  is the difference of the squared masses of the two neutrino states (eV2)

- E is the energy of the neutrino (MeV)

## Neutrino Oscillation Parameters

- Chooz (reactor beam)
- future exp., Double Chooz, Daya Bay(reactor), NOvA, T2K(accelerator)

- 0.12 (10o)

- Solar Neutrino Oscillation (Homestake, GALLEX, SAGE,
- Kamiokande-II, Super-K, SNO etc.), confirmed by KamLAND (reactor beam)

- Atmospheric Neutrino Oscillation (IMB,MARCO,Soudan,
- Kamiokande-II, Super-K etc.), confirmed by K2K, MINOS (accelerator beam)

## The LSND Experiment

***Oscillations?***

- Signal:    p  e+ n
- 			 n p  d (2.2MeV)

- LSND took data from 1993-98
- Nearly 49000 Coulombs of
- protons on target
- Baseline: 30 meters
- Neutrino Energy: 20-55 MeV
- LSND Detector:
-   -- 1280 phototubes
-   -- 167 tons Liquid Scintillator
- Observe an excess ofe :  -- 87.9 ± 22.4 ± 6.0 events.
*LSND Collab, PRD 64, 112007*

## The LSND Experiment

## Physics Motivation

-  If the LSND signal does exist, it may imply new physics beyond SM.

******m****2****atm**** + ********m****2****sol**** ≠ ********m****2****lsnd**

**K2K, MINOS**

-  Simplest model has three
-      Neutrino mass eigenstates,
-      ******m****2****21**** + ********m****2****32**** = ********m****2****31**
-  Data indicates 3 mass differences

- m2atm  ~  2.4  10-3 eV2
- m2sol   ~  8 10-5 eV2
- m2lsnd  ~  0.1 ~ 2 eV2

- LSND Signal: Yes or NO ?

## The MiniBooNE Experiment

- Proposed in 1998,operating since 2002
- The goal of the MiniBooNE Experiment:
- 	to confirm or exclude the LSND result and
- 	extend the explored oscillation parameter space

- An order of magnitude
- longer baseline (~500 m)
- than LSND (~30 m)

- An order of magnitude
- higher energy (~500 MeV)
- than LSND (~30 MeV)

- MiniBooNE and LSND have similar L/E, but have
- different signal, background and systematics.

## The MiniBooNE Collaboration

**University of Alabama****	****	****          Los Alamos National Laboratory**
**Bucknell University****	****	****          Louisiana State University**
**University of Cincinnati****	****	****          University of Michigan**
**University of Colorado****	****	****          Princeton University**
**Columbia University****	****	****          Saint Mary’s University of Minnesota**
**Embry Riddle University****	****	****          Virginia Polytechnic Institute**
**Fermi National Accelerator Laboratory      Western Illinois University**
**Indiana University****	****	****          Yale University**

- 2 National Laboratories, 14 Universities, 77 Researchers

## MiniBooNE Neutrino Beam

## Fermilab Proton Booster

**Booster**

- Target
- Hall

- 4 1012 protons per 1.6 s pulse
- delivered at up to 5 Hz.

**MiniBooNE** extracts beam
- from the 8 GeV Proton Booster

- Delivered to a  1.7  Be target

- within a magnetic horn
- (2.5 kV, 174 kA) that
- (increases the flux by 6)

- Results correspond to
- (5.580.12) 1020 POT

## The MiniBooNE Experiment

- The FNAL Booster delivers 8 GeV protons to the MiniBooNE beamline.
- The protons hit a 71cm beryllium target producing pions and kaons.
- The magnetic horn focuses the secondary particles towards the detector.
- The mesons decay into neutrinos, and the neutrinos fly to the detector, all other secondary particles are absorbed by absorber and 450 m dirt.
- 5.6E20 POT for neutrino mode since 2002.
- Switch horn polarity to run anti-neutrino mode since January 2006.

## MiniBooNE Flux (Geant 4 Simulation)

- The intrinsic *e* is ~0.5% of the
- neutrino Flux, it’s one of major
- backgrounds for   e search.

- “Intrinsic” e + e sources:
-   e+ e    (52%)
- K+  e+ e      (29%)
- K0  e e          (14%)
- Other	         (  5%)

-   e e
-                 K  e e

-  K 

-  

- Antineutrino content: 6%

-  e/ = 0.5%

## Modeling Production of Secondary Pions

- HARP @ CERN, 8.9 GeV Proton Beam
- 5% Be target to measure  production
- With E910 @ BNL + previous world data fits
- Basis of current MB  production model

- HARP collab., hep-ex/0702024

## Modeling Production of Secondary  Kaons

- Uses a Feynman Scaling
- Parameterization.

- data -- points
- dash --total error
-    (fit  parameterization)

- K0 data are also
- parameterized.

*In situ measurement*
*of K**+** from LMC*
*agrees within errors*
*with parameterization*

## Stability of Running

- Observed and
- expected events
- per minute

- Full  Run

- Stability of Running

## Events in the Detector

## The MiniBooNE Detector

- 12m diameter tank
- Filled with 800 tons of ultra
-       pure mineral oil
- Optically isolated inner region with 1280 PMTs
- Outer veto region with 240 PMTs.

## 10% PMT coverage

- Two types of Hamamatsu
- PMT Tubes:
-     R1408(79%, from LSND)
-     R5912(21%, new)
-   Charge Resolution:
-   1.4 PE,  0.5 PE
-   Time Resolution
-   1.7 ns,   1.1ns

## Optical Model

-     Detected photons from
-  Cherenkov (prompt, directional)
-  Scintillation (delayed, isotropic)
-  Ratio of  prompt/late light ~  3:1

- Attenuation length:  >20 m @ 400 nm

*We have developed *
*39-parameter “Optical Model” based on internal calibration and external measurement*

## Beam Window

- A 19.2 s beam trigger window encompasses the 1.6 s spill.
- Multiple hits within a ~100 ns window form “subevents”
- Most events are from  CC interactions (+n  +p) with
- characteristic  two “subevent” structure from stopped ee

- 

- e

- Tank
- Hits

- Example
- Event

## Cuts to Select Neutrino Events

- Veto Hits < 6  removes
- through-going cosmics
-  This leaves
- “ Michel electrons”
- (ee) from cosmics

- Tank Hits > 200
- (equivalent to energy)
- removes Michel electrons,
- which have
- 52.8 MeV endpoint

## Calibration Sources

## Nuance MC Event Rates

- D. Casper, NPS, 112 (2002) 161

- Event neutrino energy (GeV)

## CCQE Events

- 	39% of total
-  Events are “clean” (few particles)
-  Energy of the neutrino
- 	can be reconstructed

*****or e*

- 

- or e

- n

- p

- Reconstructed from:
-     Scattering angle
-     Visible energy (Evisible)

## Events Producing Pions

- N

- 

- 0

- N

- 

- 

- NC0
- The 0 decays to 2 photons,
- which can look “electron-like”
- mimicking the signal...
- 	<1% of 0 contribute
- 	     to background.

- N

- 

- 

- N

- 

- 

- 25%

- 8%

- CC+
- Easy to tag due to 3 subevents.
- Not a substantial background to
- the oscillation analysis.

- (also decays to a single photon
-      with 0.56% probability)

- Events Producing Pions

## External Sources of Background

- EnhancedBackgroundCuts

-  interactions outside of the detector Ndata/NMC = 0.99 ± 0.15

- Cosmic Rays: measured from out-of-beam data: 2.1 ± 0.5 events

**Tank**

## MiniBooNE Event Types

- Produced in most CC events.
- Usually 2 subevent.
- Electrons:
- Tag for e CCQE signal.
- 1 subevent, fuzzy ring.
- 0s:
- Can form a background if one
- photon is weak or exits tank.
- In NC 0  case, 1 subevent.

## Two Independent Analyses

## Blindness Analysis

- MiniBooNE is searching
- for a small but distinctive
- event signature (electron)

- In order to maintain blindness,
- Electron-like events were sequestered,
- Leaving ~99% of the in-beam events available for study.
- Rule for cuts to sequester events:  <1 signal outside of the box
- Low level information which did not allow particle-id was
- available for all events.

## Pre-selection Cuts

- Both Algorithms and all analyses
- presented here share
- “hit-level pre-cuts”:

- Number of Veto hits < 6

- Number of Tank hits > 200

- Only 1 subevent

- And a track radius precut:
-     R < 500 cm

-  Data
-  MC

## Track-Based (TB)  Analysis

- Uses detailed, direct reconstruction of particle tracks,
-     and ratio of fit likelihoods to identify particles.

- Philosophy:

- Each event is characterized by 7 reconstructed variables:
- vertex *(x,y,z)*, time, energy, and direction *(****)**(U**x**, U**y**, U**z**)*.
- Resolutions:
-     vertex: 22 cm
-     direction:  2.8
-     energy: 11%

## e /  Separation

- log(Le/L)>0 favors electron-like hypothesis

- Separation is clean at
- high energies where
- muon-like events have
- long tracks.
- Analysis cut was chosen
- to maximize the
- e sensitivity

- e /  Separation

## e /  Separation

- e /  Separation

**Notes:**
In developing L_pi an inv mass of 135 MeV is forced (its a constraint)

## Testing e /  Separation using data

- BLIND

- 1 subevent
- log(Le/L)>0 (e-like)
- log(Le/L)<0 (-like)
- M>50  (high mass)

- log(Le/L)

- invariant mass

- signal

- Testing e /  Separation using data

## 1 subevent

- 2 Prob for mass<50 MeV
- (“most signal-like”): 69%

- Next: look
-           here....

- log(Le/L)>0 (e-like)
- log(Le/L)<0 (-like)
- mass<200  (low mass)

## Summary of Track-Based Cuts

-  Log(Le/L)
-  Log(Le/L)
-  invariant mass

- Backgrounds after cuts

- “Precuts”   +

- Summary of Track-Based Cuts

**Notes:**
cut to rject low energy events near wall is:   .not.(R>426 cm .and. Evisible<280)

## Boosted Decision Trees (BDT) Analysis

-     which are used to make a series of cuts to
-     classify the events – decision tree.
- Boosted Decision Trees combine many trees
-     (weak classifiers) to build a powerful committee
-     to improve signal efficiency.

- Philosophy:

- Boosted Decision Trees (BDT) Analysis

## Resolutions:

-    vertex: 24 cm
-    direction: 3.8o
-    energy: 14%

- Examples of  “Analysis Variables”

- Reconstructed quantities which are inputs to EQE

-  CCQE

-  CCQE

- UZ = cos**z

- Evisible

## Many Variables   A Single PID Variable 

**“****A procedure that combines many weak classifiers**
**to form a powerful committee”**

**Boosted Decision Trees**

- hit level
- (charge, time,
- position)

- analysis
- variables

- One single
- PID “score”

- Byron P. Roe, Hai-Jun Yang, Ji Zhu *et al*., NIM A543 (2005) 577, physics/0408124
- Hai-Jun Yang, Byron P. Roe, Ji Zhu, NIM A555 (2005) 370, physics/0508045

## A Decision Tree

-     based on MC study)

- (Nsignal/Nbkgd)
- 40000/40000

- 30,245/16,305

- 9755/23695

- 20455/3417

- 9790/12888

- 1906/11828

- 7849/11867

- signal-like

- bkgd-like

- bkgd-like

- bkgd-like

- signal-like

- bkgd-like

- etc.

*This tree is one of many possibilities...*

- Variable 1

- Variable 2

- Variable 3

## A set of decision trees can be developed,

- 	each re-weighting the events to enhance
- 	identification of backgrounds misidentified
- 	by earlier trees    (“boosting”)
- For each tree, the data event is assigned
- 	+1 if it is identified as signal,
- 	-1 if it is identified as background.
- The total for all trees is combined into a “score”

- negative

- positive

- Background-like

- signal-like

## BDT Efficiency and backgrounds after cuts:

- Analysis cuts on PID score as a function of Energy

- signal

- background

- Efficiency after precuts

## Sources of Uncertainty

-  Flux from K+ decay		3.3 / 1.0		      √	    	    √
-  Flux from K0 decay 		1.5 / 0.4		      √		    √
-  Target and beam models		2.8 / 1.3		      √
-  -cross section 		            12.3 / 10.5	      √		   √
-  NC 0 yield			1.8 / 1.5 		      √
-  External interactions (“Dirt”) 	0.8 / 3.4 	     	      √
-  Optical model			6.1 / 10.5 	      √	   	   √
-  DAQ electronics model		7.5 / 10.8 	      √

- Source of
- Uncertainty
- On e background

- Checked or
- Constrained
- by MB data

- Further
- reduced by
- tying
- e to 

- Track Based
- /Boosted
- Decision Trees
- error in %

## Main Backgrounds	

- Main Backgrounds

********** ****mis-id**** (36%) **

**intrinsic ********e**** ****(64%)**

- Predictions of the backgrounds are among the
- nine sources of significant error in the analysis

## Summary of predicted backgrounds for

- the final MiniBooNE result

## The Neutrino Oscillation Results

## Track-based Analysis Results

- Counting Experiment:    475<EQE<1250 MeV
- data:   380 events
- expectation: 358 19 (stat)  35 (sys) events

- significance:
-  0.55 

- Track-based Analysis Results

## Track Based energy dependent fit results:

*Error bars are*
*diagonals of*
*error matrix.*
*Fit errors *
*for >475 MeV:*
- Normalization 9.6%
- Energy scale: 2.3%

- Best Fit (dashed):	 (sin22, m2) = (0.001, 4 eV2)

- Data are in good agreement with background prediction.

## Low E Excess

- 96 ± 17 ± 20 events
- above background,
- for 300<EQE<475MeV
- Deviation: 3.7

- Report the full range:
- 300<EQE<3000 MeV

- to E>475 MeV

- Background-subtracted:

## Fit to the > 300 MeV range

- Best Fit (dashed):	 (sin22, m2) = (1.0, 0.03 eV2)
- 2 Probability: 18%

- }

**Ex****.**** ****in LSND**** ****allowed**** ****range**

-  Low E excess cannot be explained with e oscillation.

## Boosted Decision Trees Analysis

- Boosted Decision Trees Analysis

- Counting Experiment:    300<EQE<1600 MeV
- data:   971 events
- expectation: 1070 33 (stat)  225 (sys) events
- significance:   0.38 

## MiniBooNE First Results show no evidence 

- for e appearance-only oscillations

- Energy-fit analysis:
- solid:  TB
- dashed:  BDT
- Independent analyses
- are in good agreement.

- MiniBooNE first results
-     arXiv:0704.1500
- Phys. Rev. Lett. 98, 231801

## Exclusion Limits (90% CL)

## LSND signal: Yes or NO ?     NO

MiniBooNE low E excess: Yes or NO ?

- MiniBooNE low E excess: Yes or NO ?

*"As in many particle physics experiments, we have a result that *
*answers some questions and raises others," said MiniBooNE *
*co-spokesperson William Louis**.*

*"It clears one mystery but it leaves us with a puzzle that is important *
*to understand**,**" said Fermilab Director Pier Oddone. *

## Backup Slides

## Future Plans:

- Many more papers  supporting this analysis will follow,
*in the very near future:*
- 		 CCQE production (arXiv:0706.0926)
- 		0 production
- 		MiniBooNE-LSND-Karmen joint analysis
- We are pursuing further analyses of the neutrino data,
- 	including...
- 		an analysis which combines TB and BDT,
- 		more exotic models for the LSND effect.
- We are working hard to understand the low E excess.
- MiniBooNE is presently taking data in antineutrino mode.

## Low E Excess (current status)

- ? Instrumental background: NO
- ? Study the excess with both Track and Boosting analysis
-    Are they consistent in energy and numbers: YES
-    Are there any reco issues (sidebands, etc): TB NO
-    Excess down to 200 MeV with systematic errors.
-    Is the excess electron/gamma-ray like: YES
- ? Is it a source of background
-    Dirt/Delta rays: NO
-    Pion or muon mis-id (including brem): NO
-    Evis and UZ (low stats) shapes favor numu background(?)
-    Photonuclear: ~20% of excess. Needs systematic errors.
-    Other source of backgrounds? Still Investigating
- ? Do other data sets have low E excess
-    NuMI -- different source, energy, baseline, backgrounds:
-    appears consistent in energy and scales with nue rate.

## Low E Excess

- Future Work
- ? Continue checking reconstruction/PID.
- ? Study PN contribution to excess, confirm in data.
- ? Study new sources of backgrounds.
- ? Understand systematics below ~200 MeV
-    Apply to other distributions.
- ? Continue investigating forward excess.
- ? NuMI analysis matured, study correlations with
-    MB excess, i.e. excess/numu, excess/nue,
-    excess/pi0, etc. Does it scale with anything?
- ? Check excess with CCPi+ sample
- ? Check excess in Horn-off and anti-neutrino data.
-    Look for POT or (flux*xsec) scaling.

## One, Two or Three Sterile Neutrinos ?

- Michael Maltoni, arXiv:0711.2018
- Parameter goodness of fit (PG) test to appearance and disappearance datasets from MiniBooNE,  LSND, KARMEN and NOMAD experiments.
- 	      PG = 4.0  10-6  for (3+1 sterile ) model
- 	      PG = 4.8  10-5  for (3+2 sterile ) model
- Severe tension between different datasets.  With present experimental results, (3+1), (3+2) and (3+3) neutrino oscillation schemes is NOT possible to explain the LSND signal in terms of sterile neutrinos.

## A MB-LSND-KARMEN-Bugey Compatibility Test

-  For each m2, determine the MB and LSND measurement:
- 	zMB  zMB,      zLSND  zLSND  ,   zK  zK ,    zB  zB
- 	where z = sin2(2) and z is the 1 error
-  For each m2, form 2 between MB and LSND measurement
-  Find z*0** *that minimizes 2
- 	(weighted average of two measurements) and this gives 2min
-  Find probability of 2min for 1 dof;
- 	this is the joint compatibility probability for this m2

-  The combined compatible is at 8.6% C.L.

## Boosted Decision Tree   EQE data/MC comparison:

- Boosted Decision Tree   EQE data/MC comparison:

## Error Matrix Elements: 

-  N is number of events passing cuts
- MC is standard monte carlo
-  represents a given multisim
-  M is the total number of multisims
-  i,j are EQE bins

- Error Matrix Elements:

- Total error matrix
- is sum from each source.

- TB: e-only total error matrix
- BDT: -e total error matrix

- BDT

##  MAQE, elosf      6%, 2% (stat + bkg only)

-  QE  norm      10%
-  QE  shape     function of E
- e/ QE        function of E
-  NC 0 rate          function of 0 mom
-  MAcoh, coh ±25%
-    Nrate    function of  mom + 7% BF
-  EB, pF              9 MeV, 30 MeV
- s                    10%
-  MA1                 25%
-  MAN                40%
-  DIS                25%

- determined from
- MiniBooNE
-  QE data

- determined from
- MiniBooNE
-  NC 0 data

- Example:   Cross Section Uncertainties

- determined
- from other
- experiments

- (Many are common to  and e and cancel in the fit)

## Example:

- Optical Model Uncertainties

- 39 parameters must be varied
- Allowed variations are set by
- the Michel calibration sample

- To understand allowed variations,
- we ran 70 hit-level simulations,
- with differing parameters.
- 	“Multisims”

## K+ and K0 Decay Backgrounds

- K+ and K0 Decay Backgrounds

- At high energies > 1.5 GeV,
- above “signal range”
-  and “e -like” events are
- largely due to kaon decay
- By measuring high energy box events (>1.5 GeV) to estimate K+ & K0 production rate.

- signal
- range

## Nuance Parameters ( CCQE )

-  data well (arXiv:0706.0926)

**Kinetic Energy of muon**

- From Q2 fits to MB  CCQE data:
-      MAeff -- effective axial mass
-      EloSF -- Pauli Blocking parameter
- From electron scattering data:
-      Eb -- binding energy
-      pf  -- Fermi momentum

- data/MC~1
- across all
- angle vs.energy
- after fit

- Nuance Parameters ( CCQE )

## We constrain 0 production using data from our detector

- We constrain 0 production using data from our detector

*Because this constrains the **** resonance rate, *
*it also constrains the rate of ****N***

- Reweighting improves
- agreement in other
- variables, e.g.

- This reduces the error
- on predicted
- mis-identified 0s

## Other Single Photon Sources

- From Efrosinin, hep-ph/0609169,
- calculation checked by Goldman, LANL

- Neutral Current:  + N   + N + 

- Charged Current

-  + N   + N’ + 

- negligible

- where the presence of the  leads to mis-identification

- Use events where the  is tagged by the michel e-,
- study misidentification using BDT algorithm.

- < 6 events @ 95% CL

## Neutrino Oscillations

- Maki-Nakagawa-Sakata matrix (MNS matrix – neutrino mixing matrix)

- is a neutrino with definite flavor. **= e, 

- is a neutrino with definite mass. *i* = 1, 2, 3.

*s*ij = sinθij, *c*ij = cosθij

- phase factor δ is non-zero if neutrino oscillation violates CP symmetry

- phase factors α1 and α2 are non-zero if neutrinos are Majorana particles