# Presentation: 707247

## Updates to the Low Energy Excess in MiniBooNE

- Chris Polly, Indiana University* *now at Urbana-Champaign

## The MiniBooNE Collaboration

- Recap of last year's neutrino oscillation result
- Analysis updates, emphasis on e-like excess at low energy
- Status of antineutrino running

- ~80 physicists from ~18 institutions

- OUTLINE

## MiniBooNE's Motivation: The LSND signal

- LSND found an excess of e in  beam
- Excess: 87.9 ± 22.4 ± 6.0 (3.8)
- Under a 2 mixing hypothesis:

- —

- —

## MiniBooNE's Motivation: The LSND signal

- m2 ~ 1 eV2 impossible with only 3
- Requires extraordinary physics!

- Sterile neutrinos *hep-ph/0305255*
- Neutrino decay *hep-ph/0602083*
- Lorentz/CPT viol. *PRD(2006)105009**                          *(T. Katori, A. Kostelecky, R. Tayloe)‏
- Extra dimensions h*ep-ph/0504096*

- Unlike atmos and solar...LSND unconfirmed

- LSND found an excess of e in  beam
- Excess: 87.9 ± 22.4 ± 6.0 (3.8)
- Under a 2 mixing hypothesis:

- —

- —

## The MiniBooNE design strategy...must make 

- Start with 8 GeV proton beam from FNAL Booster
- Add a 174 kA pulsed horn to gain a needed x 6
- Requires running  (not anti-) to get flux
- Pions decay to  with E in the 0.8 GeV range
- Place detector to preserve LSND L/E:	MiniBooNE: 	(0.5 km) / (0.8 GeV)	LSND:		(0.03 km) / (0.05 GeV)‏
- Detect  interactions in 800T pure mineral oil detector
- 1280 8” PMTs provide 10% coverage of fiducial volume
- 240 8” PMTs provide active veto in outer radial shell

## Key points about the signal

- LSND oscillation probability is < 0.3%
- After cuts, MiniBooNE has to be able to find ~300 e CCQE interactions in a sea of ~150,000  CCQE
- Intrinsic e background
- Actual e produced in the beamline from muons and kaons
- Irreducible at the event level
- E spectrum differs from signal
- Mis-identified events
- CCQE easy to identify, i.e. 2 “subevents” instead of 1.  However, lots of them.
- Neutral-current (NC) 0 and radiative  are more rare, but harder to separate
- Can be reduced with better PID
- Effectively, MiniBooNE is a ratio meas. with the  constraining flux X cross-section

## Analysis Chain: Flux Prediction

## Meson production at the target

- HARP collaboration,
- hep-ex/0702024

- Kaons:

- Pions:

- MiniBooNE members joined the HARP collaboration
- 8 GeV proton beam
- 5% Beryllium target
- Data were fit to Sanford-Wang parameterization

- Kaon data taken on multiple targets in 10-24 GeV range
- Fit to world data using Feynman scaling
- 30% overall uncertainty assessed

## Meson production at the target

- HARP collaboration,
- hep-ex/0702024

- Aside on relevance to Project X:

- Pions:

- MiniBooNE members joined the HARP collaboration
- 8 GeV proton beam
- 5% Beryllium target
- Data were fit to Sanford-Wang parameterization

- MiniBooNE flux carefully tuned and verified with  beam most robust MC available for predicting  and K fluxes at Booster energies.
- Muon g-2 example: MB provided flux prediction for very forward (<45mrad) 3 GeV pions.
- Very forward HARP data not yet published reliance on SW fit creates some extrap. error.
- Also used E910 production data.

## Final neutrino flux estimation

-                 K  e e

- Flux intersecting MB detector (not cross-section weighted)‏
- Intrinsic contamination e = 0.5%
-   e+ e     (52%)‏
- K+  e+ e    (29%)‏
- K0  e e         (14%)‏
- Other               (5%)
- Wrong-sign  content: 6%

**-**

**-**

**Notes:**
epanding “other”...mu- -> e- numu nuebar: 2%   pi+ -> e+ nue: 2% other: 1%

## Analysis Chain: X-Section Model

## Nuance Monte Carlo

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

- Comprehensive generator, covers entire E range
- Predicts rates and kinematics of specific  interactions from input flux
- Expected interaction rates in MiniBooNE (before cuts) shown below
- Based on world data,  CC shown below right

-  CC World data

- Input flux

## Nuance Monte Carlo

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

- Comprehensive generator, covers entire E range
- Predicts relative rate and kinematics of specific  interactions from input flux
- Expected interaction rates in MiniBooNE (before cuts) shown below
- Based on world data,  CC shown below right
- Also tuned on internal data

-  CC World data

- Input flux

## Tuning Nuance on internal  CCQE data

- across all
- angle vs.energy
- after fit

- Tuning Nuance on internal  CCQE data

- Poor agreement in Q2
- From Q2 fits to MB  CCQE data extract:
- MAeff -- effective axial mass
-   -- Pauli Blocking parameter
- Beautiful agreement after Q2 fit, even in 2D
- Ability to make these 2D plots is unique due to MiniBooNE's high statistics

- Before correction

- After correction

## Tuning Nuance on internal NC data

- NC π0 important background
- 97% pure π0 sample (mainly Δ→Nπ0)‏
- Measure rate as functionof momentum
- Default MC underpredicts rate at low momentum
- Δ→Nγ also constrained

- Tuning Nuance on internal NC data

- Invariant massdistributions in
- momentum bins

## Analysis Chain: Track-Based Likelihood Reconstruction and Particle ID

- Analysis Chain: Track-Based Likelihood Reconstruction and Particle ID

## TBL Analysis: Separating e from 

- data
- MC

- Analysis pre-cuts
- Only 1 subevent
- Veto hits < 6
- Tank hits > 200
- Radius < 500 cm

-  CCQE events (2 subevent)‏

- Event is a collection of PMT-level info (q,t,x)‏
- Form sophisticated Q and T pdfs, and fit for 7 track parameters under 2 hypotheses
- The track is due to an electron
- The track is coming from a muon

## Separating e from 0

- blind

- Extend fit to include two e-like tracks
- Very tenacious fit...8 minutes per event
- Nearly 500k CPU hours used (thanks OSG!)‏

## TBL Analysis:  Expected event totals

## Data/fit result after blind analysis complete...

- TBL shows no sign of an excess in the analysis region (where the LSND signal is expected for the 2 mixing hypothesis)‏
- Visible excess at low E

- What does it all mean?  There are a few possibilities...
- Some problem with LSND, e.g. mis-estimated background?
- Difference between neutrinos and antineutrinos?
- The physics causing the excess in LSND doesn't scale with L/E?
- Low E excess in MB related?

## Exploring the Low E Excess

## The low E excess has fueled much speculation...

- Commonplace

- SM, but odd

- Beyond the SM

- Muon bremsstrahlung                   (Bodek, 0709.4004)‏

- Anomaly-mediated         (Harvey, Hill, Hill, 0708.1281)‏

- New gauge boson            (Nelson, Walsh,0711.1363)‏

- Easy to study in MB with much larger stats from events with a Michel tag
- Proved negligible with MB data in 0710.3897

- Still under study, nuc. effects neglected, g
- Has to contribute...how much?

- Can accommodate LSND and MiniBooNE
- Firm prediction for anti-neutrinos

## Extending the analysis to lower energies

- Original excess quoted in initial oscillation PRL 98, 231801 (2007)‏
- 475-1250 MeV,  22 ± 40, 0.6
- 300-475 MeV,  96 ± 26, 3.7
- In summer 2007 extended analysis down to 200 MeV
- 200-300 MeV,  92 ± 37, 2.5
- Combined significance with proper systematic correlations
- 200-475 MeV,  188 ± 54, 3.5

## Extending the analysis to lower energies

- Original excess quoted in initial oscillation PRL 98, 231801 (2007)‏
- 475-1250 MeV,  22 ± 40, 0.6
- 300-475 MeV,  96 ± 26, 3.7
- In summer 2007 extended analysis down to 200 MeV
- 200-300 MeV,  92 ± 37, 2.5
- Combined significance with proper systematic correlations
- 200-475 MeV,  188 ± 54, 3.5

- Might have seen this presented in past with some caveats...
- Work was underway for a comprehensive review bkgs/errors (emphasis at low E), but also wanted to rapidly respond to inquiries about excess below 300 MeV.
- Starting with this talk...no more disclaimers.  PRL draft already circulating that covers ~1 year of very careful follow-up work.

## Update #1: Treatment of  flux errors

- Fit HARP/E910 data to SW parameterization.
- Use SW fit as central value (CV) MC
- Use covariance matrix governing SW parameters in 2 fit to assess error
- Problem: poor 2 due to SW parameterization not fully describing data at HARP's precision
- Old Solution: uniformly inflate HARP error until 2 is acceptable.
- Inflates HARP's ~7% error to closer to ~15%

- OLD METHOD:

- xsec (mb) vs p (GeV)‏

## Update #1: Treatment of  flux errors

- Fit HARP/E910 data to SW parameterization.
- Use SW fit as central value (CV) MC
- Use covariance matrix governing SW parameters in 2 fit to assess error
- Problem: poor 2 due to SW parameterization not fully describing data at HARP's precision
- Old Solution: uniformly inflate HARP error until 2 is acceptable.
- Inflates HARP's ~7% error to closer to ~16%
- Sounds dumb, but...
- Getting a good 2-dim parameterization in (p, not as easy as you might think
- Totally unimportant for e appearance where the  flux is heavily constrained from the *in situ*  measurement

- OLD METHOD:

- xsec (mb) vs p (GeV)‏

## Update #1: Treatment of  flux errors

- NEW METHOD:

- xsec (mb) vs p (GeV)‏

- Forget SW, use HARP data and fit with spline interpolation
- Vary HARP data with their own covariance matrix to produce flux systematic error
- Update #1 bottom line: No impact on e appearance
- Largest diff at low p ,not much  flux hitting det
- Still have additional 5% in errors from horn modeling and secondary interactions

- 81% of  flux crossing
- MB covered by HARP

## Update #2:  Improved 0/radiative  analysis

- Complete re-extraction of 0 weights
- Independent code, improved unsmearing technique, 11 bins, consistent with old result
- Fit over 9 bins in p to smooth reweighting function

- Z

- 

- p,n

- p,n

- 

- 

- 

- 

## Update #2:  Improved 0/radiative  analysis

- Applied *in situ* measurement of the  coherent/resonant production rate
- Coherent event kinematics more forward
- Resonant production increased by 5%
- Improvements to ->N bkg prediction
-  Resonant 0 fraction measured more accurately
- Old analysis,  created in struck nucleus not allowed to reinteract to make new 
- Complete combinatorial derivation based on branching ratios (, 0) and the pion escape probability ()
- Error on ->N bkg increased from 9 to 12%
- Update #2 bottom line:  Overall, produces a small change in e appearance bkgs

- Z

- 

- p,n

- p,n

- 

- 

- 

- C

- Z

- C

- 

- 

- 

## Update #3: Hadronic bkgs/errors in  interactions

- Mainly due to charged  absorption and charge exchange in the mineral oil, analogous to the same processes in the struck nucleus
- Use GEANT3 MC with GCALOR instead of GFLUKA default
- better  abs/cex handling (error=max{Ashery error,Ashery-GCALOR})
- better neutron scattering
- Cross-check:  Accounting for cex/abs differences GCALOR & GFLUKA give same result for e appearance bkgs

- OLD HADRONIC PROCESSES/ERRORS:

## Update #3: Hadronic bkgs/errors in  interactions

- Charged C elastic scattering
- Found  elastic scattering to be nearly absent in GCALOR
- Possibility that NC  have more scattering  making Cerenkov ring look more e-like
- Radiative  capture
-  capture is in GCALOR, but missing radiative branching fraction (40%, ~100MeV gamma)‏
-  induced ->N
- Abs/cex allowed in GCALOR, but radiative  branch missing
- Not as dangerous as in struck nucleus, since  propagates for some time and can give multiple rings
- None of these processes contributed a significant number of bkg events.

- ADDITIONAL HADRONIC PROCESSES:

## Update #3: Hadronic bkgs/errors in  interactions

- Update #3: Hadronic bkgs/errors in  interactions

- Photonuclear interactions
- Absent in GEANT3
- Can delete a  in a NC pi0 interactions, thus creating a single e-like ring
- 40,000 NC pi0 interactions
- Well-known cross-section, in fact in GEANT4 which allowed for cross-check
- Uncertainties enter via final states
- Only hadronic process found to contribute significantly

- ADDITIONAL HADRONIC PROCESSES:

- Z

- 

- p,n

- p,n

- 

- 

- 

- 

- 

- 

## Update #3: Hadronic bkgs/errors in  interactions

- ADDITIONAL HADRONIC PROCESSES:

- Update #3 bottom line:
- Additional p0 mis-id due to all modified hadronic processes (dominated by PN)‏
- 200-300 MeV, 40 events
- 300-475 MeV, 25 events
- 475-1250 MeV, 1 event
- Additional systematic error negligible relative to other errors

- e-like backgrounds

- EQE)‏

## Update #4: Additional cut to remove dirt events

- Dirt backgrounds tend to come from  that sneak through the veto and convert in tank  pile up at high radius
- Don't carry full  energy pile up at low visible energy
- Define R-to-wall cut, distance back to wall along reconstructed track direction
- Apply 2d cut as shown

## Update #4: Additional cut to remove dirt events

- Update #4 bottom line:  Removes ~85% of the dirt backgrounds at low energy

- No DIRT cuts

- With DIRT Cuts

## Update #4: Additional cut to remove dirt events

- Consistency-check: look at radial distribution after dirt cut applied
- Uniform excess throughout tank

## Update #5: New data

- Extra 0.83E20 POT during combined MiniBooNE/SciBooNE  running
- e-like events per POT evenly distributed throughout duration of run
- Update #5 bottom line: e-like event rate slightly higher for new data, but perfectly acceptable

## Final Results: Background event breakdown

- Above 475 MeV still dominated by intrinsic nue
- At low E transitions to NC 0 and ->N dominated bkgs

## Final Results: Impact on oscillation analysis

- No impact on primary oscillation analysis!

## Final Results: Extend 2 fit to low E

-                          E>475 MeV  E>200 MeV
- Null fit 2 (prob.):   9.1(91%)     22(28%)‏
- Best fit 2 (prob.):  7.2(93%)   18.3(37%)‏

- Adding 3 bins to fit causes chi^2 to increase by 11 (expected 3)‏
- Can see the problem...the best 2 fit that can be found does not describe the low E excess.

## Final Results: Compare update stages

- In 475-1250 MeV, excess is small/stable through all updates
- In 200-475 MeV, excess significance reduced due to additional hadronic bkgs, compensated by reduction in dirt background
- Excess at low E remains > 3 sigma after the comprehensive review

- FINAL

## Visible energy interesting to look at in case excess is not really due to e CCQE
Excess piles up below 400 MeV, analysis threshold set at 140 MeV Evis

- Visible energy interesting to look at in case excess is not really due to e CCQE
- Excess piles up below 400 MeV, analysis threshold set at 140 MeV Evis

- Final Results: Visible energy distribution

## Anti-neutrinos in MiniBooNE

## Antineutrinos in MiniBooNE

- Event rates after fiducial volume cuts:

- 6x1020 POT
-  mode

- 2x1020 POT
-  mode

**-**

- Have collected 3.3E20 POT in antineutrino mode
- Direct check of LSND result
- Cross-sections measurements
- Understanding low E excess

- PRELIMINARY
- SENSITIVITY

## Conclusions and references

- Summary
- A comprehensive review of all backgrounds and errors (with a particular emphasis at low E) has been completed
- No change to the analysis above 475 MeV
- The excess at low E energy is still >3.0 significant, and remains a mystery
- Next step: pulling together additional information from NuMI events and antineutrinos (still blind) into a global picture.
- For more info on MiniBooNE see
*Measurement of Muon Neutrino Quasi-Elastic Scattering on Carbon,* **PRL 100, 032310 (2008)****‏**
*First Observation of Coherent ****0** Production in Neutrino Nucleus Interactions with En<2 GeV, ***Phys Lett B. 664, 41 (2008)****‏**
*Compatibility of High ****m**2** ****e** and Anti-****e** Neutrino Oscillations Searches, ***Phys. Rev D 78, 012007 (2008)****‏**
*The Neutrino Flux Prediction at MiniBooNE, ***Accepted by PRD [arXiv:0806:1449]**
*The MiniBooNE Detector, ***Submitted to NIM A [arXiv:0806.4201]**
- Papers on the immediate horizon
- NuMI events in MiniBooNE
- BDT/TBL combination technique and result
- Analyzing the low E events in MiniBooNE (this work)‏
- CCpi+/CCQE ratio measurement
-  disappearance in MiniBoone

## Extra slides

## Antineutrino