MINOS 1 and e Physics in MINOS and e Physics in MINOS Antineutrinos Overview Oscillations ...
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Transcript of MINOS 1 and e Physics in MINOS and e Physics in MINOS Antineutrinos Overview Oscillations ...
1
MINOS
and and ee Physics in MINOS Physics in MINOS
Antineutrinos Overview Oscillations Systematics
e AnalysisNearest neighbors selectionBackground estimations
Summary
Alex Himmel, Pedro Ochoa
2
MINOS
Approx. 6% of our beam is made of muon antineutrinos.
Unique advantage: both MINOS detectors are magnetized.Allows us to separate neutrinos and antineutrinos
on an event-by-event basis.
1x1020 POT
Amplified spectrum
v
Difficulty: not many events in osc. peak region
MC
Antineutrinos in MINOS
Difficulty: not many events in osc. peak region
ELmvvP
4
22sin22sin1)( Δ−=→ θ
with SK parameters
15 30 0 E (GeV)0
0.5
1
3
MINOS
Very interesting physics can be done with antineutrinos:
Very strong involvement of Caltech group in these areas.
oscillation analysis: A large CPT-violating region still unexplored
90%, 95%, 99% and 3σ CPT violating regions still allowed by global fit (except LSND)
M.C. Gonzalez-Garcia, M. Maltoni and T. Schwetz (hep-ph/0306226)
→ transitions: have never been looked for before in atmos sector.Some models beyond the SM predict them
(i.e. Langacker and Wang, Phys. Rev. D 58 093004). Could fully explain the atmospheric neutrino results
(Alexeyev and Volkova, hep ex/0504282) If 10% or more of neutrinos that disappear transition to
antineutrinos then we will see them.
Antineutrino physics
3) Measurement of Beam e’s: important for e analysis
4
MINOS
MINOS could distinguish between Δm223 and Δm2
23 at 90% C.L. if Δm2
23 > 0.004 eV2 in ~1 more year, during normal “neutrino” running
But if CPT is conserved, the reach of antineutrino oscillation analysis would be relatively small:
~3 first years of data (6x1020 POT)(no systematics included)
)2(sin 232 θ
223mΔ
Preliminary MC
90% Tentative exclusion limit
only valid for high mixing angle
90% 95% 99% 3σ
Antineutrino oscillations
5
MINOS
These difficulties can be overcome with a small amount of reversed horn current running (RHC).
In this case negative particles from the target are focused thus yielding an antineutrino beam:
Forward horn current (FHC)
Reversed horn current (RHC)
Peak reduction due primarily to cross-section
difference ()
1x1020 POT 1x1020 POT
Antineutrino running
6
MINOS
Combining FHC with RHC can obtain a measurement of Δm2
23 that rivals the first MINOS measurement of Δm2
23:
90% C.L. 68.3% C.L.
6x1020 POT (FHC) +1x1020 POT (RHC)(no systematics)
223mΔ
Only ~4 months of antineutrino running (plus ~1 more year of normal running)
are needed !
This data would considerably reduce our best current limits on neutrino CPT
Effort led by Caltech
90% Tentative exclusion limit
90% 95% 99% 3σ
Preliminary MC
Antineutrino running
)2(sin 232 θ
7
MINOSAntineutrino systematics
Systematic errors are a crucial question in combining FHC and RHC data.
~30% of antineutrinos produced outside of the target region While neutrinos are also produced outside the target, they are
negligible compared to those focused from the target. A large fraction of the difference between the near and far
detectors comes from decay pipe antineutrinos.
8
MINOSAntineutrino systematics
Uncertainties in the decay pipe modeling could affect the far/near ratio creating a false signal.
Toy systematic model: 50% Scaling of the decay pipe componentThe other components of the flux unchanged
Preliminary results suggest that the effect is small compared to the expected statistical error at 1x1021 POT.
9
MINOS
FluggNew Monte CarloOld Monte Carlo
Beam systematics
Working to update the beam Monte Carlo from Geant3 to Geant4.
Use Flugg to run the new geometry in Fluka, a more trusted physics simulation
Geant-Fluka Physics
Geant 4 Physics
FluggGeant 3
GeometryGeant 4
Geometry
Fluka Geometry
Fluka Physics
10
MINOSMuon Scattering
Study shows Geant4 greatly overestimates the data, especially at lower muon momenta.
How well does Geant4 model multiple scattering? Compared Geant4 and some IHEP data (1986) of muons on
a Cu target.
11
MINOSMuon Chopper
Another technique for assessing systematics associated with charge separation was developed at Caltech:
1) Identify stopping muons at the ND:
2) Remove everything but the last x GeV’s of energy:
3) Run reconstruction over muon chopped data and MC
4) Calculate ID efficiency & purity in data & MC for different values of x.
12
MINOSMuon Chopper
The following sources of systematics are addressed by the Muon Chopper:
1) Magnetic field2) Multiple scattering3) Reconstruction / Backgrounds
Preliminary results indicate charge separation is reasonably well modeled by the MC:
@900 MeV
Data MC
P(-|μ-) %70.1±0.
265.8±0.
5
P(+|μ-) %
3.0±0.1 3.4±0.2
P(?|μ-) %25.5±0.
229.6±0.
5< 10% systematic in purity !
13
MINOS
E
LmvvP e 4
sinsin2sin)(2232
232
132 Δ
≅→ θθ At MINOS’ baseline of 735 km,
Expect ~14 e CC events (E<10
GeV) appearing in the MINOS Far Detector for every 1x1020
POT of data if θ13 is at CHOOZ
limit
e Appearance
At Caltech concentrating on:
developing the best possible e selection
measuring two of the main backgrounds.
Main challenge at MINOS consists in distinguishing between EM and hadronic showers.
14
MINOS
For analysis need to have as good e selection as possible to
maximize signal.
Have been working on a nearest neighbors selection in collaboration with Cambridge University.
Most available selections use multivariate techniques that rely on reconstructed quantities.
But this analysis is a special case:
Number of reco variables ~ number of strips in event
Compare each input event to large libraries of simulated
e CC and NC events. Select N best matches
Basic idea:
Why not perform event ID using strip information alone?
Construct discriminant from N best matches information
(e.g. fracCC=fraction of N best matches which are e CC)
Nearest Neighbors Selection
15
MINOS
Advantages:
Approach is in principle optimal. No loss of info from raw→reconstructed quantities
Largely reconstruction-free.
But only optimal if fully sample phase space
Need large libraries (~50-100 Million events of each type). So far have generated ~50 Million events at Caltech.
Determine how well two events match by asking:
( ) ( )∑∑∫∞
=pl st
dnPnP0
21 ,, λλλl
Poisson
“what is probability the two events come from same hit pattern at PMTs?”
Nearest Neighbors Selection
plane #
plane #plane #
plane #
Str
ip #
Str
ip #
Str
ip #
Str
ip #
16
MINOS
Example of discriminant:
fracCC(y<0.5)=fraction of 20 best matches that were e with y<0.5
Already provides the best significance !
Information of N best matches is very rich:
Plenty of room for further improvement !
Library size:~1M e
~1.5M NC
NCCC e
Good separation
Nearest Neighbors Selection
datamcncνe
νμcc
Currently working hard to get selection fully operational in the Near Detector:
Using different background estimating techniques to understand data-MC discrepancy.
17
MINOS
FD Performance
Selected events:
Sensitivity limited by statistical fluctuations of background.
Define figure of merit FOM=Signal/√Background
For 0 < Ereco < 6 GeV:
FOM=2.29
Our selection already has a FOM at least 15% higher than all the other selection methods.
sin2(2θ13) = 0.1, |Δm31|2 = 2.710-3eV2, sin2(2θ23) = 1, POT=4x1020
cut
Note: preselection includedPreliminary MC
CC e NC CC CC Beam e
7.98 6.98 2.20 0.97 2.00
antineutrinos muon removal
2 methods for addressing background have been developed at Caltech.
Library size:5M e
10M NC
~end of 2007
Selected events:
18
MINOS
Apply muon removal (MR) to both data and MC
Apply e selection on both. Use differences in both samples to reweight the NC expectation
in the e analysis.
Use Muon Removal (MR) to assess the NC Background:
ND databefore MR
after MR
(NN selected events)
MCMCMR
DATAMR NN
NN =
# of NC events in e analysis
# of e candidates in MR data
# of e candidates in MR MC
# of e candidates that are NC in MC
MR reweighting removed the ~60% overall normalization discrepancy
NC Background
19
MINOS
Need to tag antineutrinos coming from + decay. Use fact that antineutrino spectrum is practically the same independently of the beam configuration:
Irreducible background in e analysis: intrinsic beam e‘s
Nearly all come from +→ e+ + e +
Low energy (LE) pseudo-medium energy (pME) pseudo-high energy (pHE)
MC MCMC
Most antineutrino parents just go
through the center of both
horns
Beam e’s from antineutrinos
Work led by Caltech, in collaboration with BNL
20
MINOS
Only + component changes significantly when running in pME or pHE !The Technique:
Scale pME (or pHE) and LE data to same POT and take the difference
from +
pME
parME×
from +
LE
parLE×
(pME-LE)TRUE at 1e18 POT
Fit with using shapes from the MC:
Corrections due to differences in the
antineutrinos from and K-
Expected sensitivity:
Sensitivity to beam e’s
Using 2.5x1019 POT of pME data 27%
Using 1.6x1019 POT of pHE data 25 - 30%
pHE data already taken!
Beam e’s from antineutrinos
21
MINOS
The CHOOZ limit will be reached by end of 2007
Expect 1st MINOS e appearance result by
next year.
Summary & Ongoing Work
Very positive outlook. Working hard to:
Further improve selection Assess systematics.
Critical role played by Caltech group in these two areas.
e appearance:
Antineutrinos: Only ~4 months of antineutrino running are needed to make a
measurement of Δm223 with a precision that rivals the first MINOS
Δm223 result.
Will search for → transitions for the first time. Developing tools for beamline simulation.
22
MINOS
Backup
23
MINOS
steel thickness: 2.54cm | strip width: 4.12cm (Molière radius ~3.7cm)
short event, often diffuse
1.8m
νμ CC Event NC Event νe CC Event
long μ track & hadronic activity at vertex
3.5m
short, with typical EM shower profile
2.3m
(MC)
e Appearance in MINOS Challenge: At MINOS, we lack the granularity to fully resolve
hadronic vs. EM showers:
n p
-
p p
W WZ
n p
e e-