1Caius Howcroft

Atmospheric Neutrinos at

MINOSCaius Howcroft

Caltech, 17/May/2005

✦Atmospheric Neutrinos

✦MINOS

✦MINOS Far Detector

✦Type of Atmospheric Neutrinos at MINOS

✦Two MINOS Atmospheric Analyses

2Caius Howcroft

Atmospheric Neutrinos

• Produced in the atmosphere from interactions of primary cosmic rays.

-( )!

µ-( )!

Down-!

Up-!

µ

"

p,He

#,$

µ

Detector

ZenithZenith

Detector

%

%

"

Earth

#&"

-( )!e

p + N → π± + X

π± → µ± + νµ(ν̄µ)

µ± → e± + νe(ν̄e) + νµ(ν̄µ).

1

νe + ν̄e

νµ + ν̄µ

= 1/2

νµ/ν̄µ ≈ 1

L

3Caius Howcroft

Atmospheric Neutrinos (2)

• First evidence of neutrino oscillations came from Super Kamiokande (1998).

0

100

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500

600

-1 -0.5 0 0.5 1

Num

ber

of E

vents

Sub-GeV e-like

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-1 -0.5 0 0.5 1

Sub-GeV µ-like

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-1 -0.5 0 0.5 1

cos!

Num

ber

of E

vents

Multi-GeV e-like

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350

-1 -0.5 0 0.5 1

cos!

Multi-GeV µ-like + PC

(νµ

ντ

)=

(cos θ sin θ

− sin θ cos θ

) (ν1

ν2

)

P (νµ → ντ ) = sin2 2θ sin2(δm2L/4E)

downup

νμ↔ντ OSCILLATIONS

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MINOS Goals

• Demonstrate the oscillation behaviour

• Measure Δm

• Search for sub dominate ν appearance

223

e

Provide very high statistics discrimination against other models for neutrino disappearance , e.g. decoherence, neutrino decay, ...?

to better than 10%

θ13 3σ discovery limit is factor 2 improvement on CHOOZ current limit.

In addition: MINOS Far Detector capable of first direct observation of atmospheric ν and ν oscillations

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MINOS: Basic Idea(Main Injector Neutrino Oscillation Search)

Position of minimum Δm2

Depth of minimum sin22θ

Near(unosc)

Far(oscillated)

735km

νμ

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MINOS Taking Data Now

MINOS Far Detector beam event

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MINOS Far Detector Site

• Located in former iron mine in northern Mn, USA.

• Also the home of Soudan1&2 (ret.) and CDMS.

• MINOS is 2341 ft. (2070 mwe) below the surface.

MINOS

Soudan 2/CDMS IIshaft

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The Far Detector

• 8m octagonal steel scintillator tracking calorimeter.

• Two 15m sections (supermodules)

• 5.23 kton total mass.

• Each supermodule has a 1.5T toroidal magnetic field

• Hadronic energy resolution ~55%/√E

• Completed in Aug 2003.A single supermodule

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The Far Detector (2)

• Steel-Scintillator sandwich, each layer (plane) consists of a 2.54 cm steel +1 cm scintillator

• Each scintillator plane divided into 192 x 4cm wide strips

• Alternate planes have orthogonal strip orientations, U and V ⇒ 3D trackingU V U V U V U V

steel

scintillator

orthogonal orientations of strips

Scintillation light collected by fibres glued into each strip and read out by multi pixel PMTs

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The Far Detector (3)

The Far Detector during construction.

v uy

x

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The Far Detector (4)

The completed Far Detector.

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Live Time• Since completion in August 2003 the

Far Det has been taking cosmics data.

• Beam switched on 1st March 2005 at which point the Far Detector had collected 420 days of physics quality data, excellent for a detector still in commissioning stage.

• Total of 5.99 kton-years of data suitable for atmospheric neutrino studies, c.f. Soudan 2's 7.36 kton-years

• Will still take cosmics data during beam running.

Days since 2003-07-01

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To

tal live t

ime

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100% live time

live time

MINOS PRELIMINARY

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What's in an Event?

DATA

UZ

VZ

Timing

Calorimetic

➪Two Views (U-Z and V-Z) are combined to give 3D tracks and showers.➪Event timing information ➪Calorimeteric information➪Event charge and muon momentum from curvature of tracks in B-field.

(2.4 ns)

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Atmos ν Pros and Cons• MINOS Far Detector has a lot going for it:

• Deep (2070 mwe) 100,000 reduction in cosmic muon and large.

• Magnetic field, event charge and muon momentum, even muons that leave the detector.

• Potential problem, 80% of the detector surface is uninstrumented.

Event appears to start 1m from detector edge

Hit in VetoShield

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Event Classesν

µ

ν

µ

ν

µ

ν

µ

Fully Contained

Partially ContainedDownward-going

Partially Containedupward-going

neutrino induced rock muon

FC

PCDN

PCUP

νinducedµ

Sneaky Stopping Cosmic Muons

Sneaky Thru-going Cosmic Muons

StoppingCosmic Muons (Direction Wrong)

Thru-going Cosmic Muons (Direction wrong)

Cut: Containment, topology

Cut: Containment, topology

Cut: timing

Cut: timing

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Neutrino Induced Muons• Look for events coming from below the horizon. The flat

overburden at Soudan means can also look for events slightly above horizon.

• Event selection is based on event timing, timing resolution 2.4ns per channel. Require 20 planes and 2m tracks, look at event timing 1/β (c/v).

Downward going cosmic ray muons

Upward going events

Select 91 events.

B. Rebel (FNAL)

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Neutrino Induced Muons(2)

UZ

VZ

Timing

Timing

µ+

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Neutrino Induced Muon(3)

NUANCE Monte Carlo: ✦Bartol'96 flux ✦MC normalised to data

Event Charge ID ✦Use the charge of the muon to separate ν and ν events ✦Efficiency is a function of, muon momentum, track length and vxB

Charge ID is a work in process

B. Rebel (FNAL)

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FC PC-Down Selection

• FC and PC-Down have same background: sneaky cosmic muons. Therefore, use same cuts.

• Rejection factor of 1 in 107 required to achieve a signal to background ratio of 10:1.

• Use Monte Carlo signal (Barr et al.) and cosmic muon background to develop cuts based analysis to reduce signal:background to 1:1, then apply veto shield (~97% efficient).

• Use veto shield to measure remaining background in sample.

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FC/PCDN:Containment

• Define a fiducial volume:

• 0.5m from detector edge

• 5 planes from each supermodule end

• Cuts:

• FC - require both ends be 'contained'

• PCDN - require upper end 'contained' and lower end non-contained.

Fiducial Distance / m 0 0.5 1 1.5 2 2.5 3 3.5 4

Eve

nts

10

210

310

410

510

610

Data (102 livedays)

Cosmic Muon MC

1000!CC MC µ"

Muons entering through coil hole. Removed with 40cm radial cut.

Signal:Cosmic ~ 1:300

MINOS PRELIMINARY

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FC/PCDN: Δz• Background dominated by steep cosmic ray muons

• Cut out sneaky muons

Δz

/ m Z

! 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Even

ts

-110

1

10

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310

data

µ MC cosmic

" MC atmos

Signal:Cosmic ~ 1:15

MINOS PRELIMINARY

22Caius Howcroft

FC/PCDN: Q/Direction• Remaining background is steep, highly curving cosmic ray muons

• Deposit lots of charge in first plane (Qvtx)

|z!|cos

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

/ P

Ev

txQ

0

50

100

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200

250

300

"MC atmos

|z!|cos

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

/ P

Ev

txQ

0

50

100

150

200

250

300

µMC cosmic

zenith!cos

-1 -0.8-0.6-0.4-0.2 0 0.2 0.4 0.6 0.8 1

/ P

Ev

txQ

0

50

100

150

200

250

300

"MC atmos

zenith!cos

-1 -0.8-0.6-0.4-0.2 0 0.2 0.4 0.6 0.8 1

/ P

Ev

txQ

0

50

100

150

200

250

300

µMC cosmic

z

y (zenith)

23Caius Howcroft

FC/PCDN: Q/Direction(2)• Define 'steep' to be:

• Then require Qvtx<100

/PEvtxQ0 50 100 150 200 250 300

Even

ts

0

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40

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200

Data

µMC cosmic

10! "MC atmos

|<0.5z#>0.7, |coszenith#cos

cos θzenith > 0.7 or | cos θz| < 0.5

Signal:Cosmic ~ 1:1.5

MINOS PRELIMINARY

24Caius Howcroft

FC/PCDN:Veto Shield

• Added after detector construction, rather odd shape due to readout cables. Event so: 99.9% acceptance.

• Same scintillator strips used in detector, but parallel to Z axis. 4 sections (2 per supermodule) each 8 meters long.

X

Y

UV

Z out of page

WestEast

B-Field

Coil hole

Shield Layer

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FC/PCDN: Veto Shield (2)

• An event is 'vetoed' if energy is deposited in the shield with 100ns of the event.

• Measure veto shield efficiency using steep stopping cosmic ray muons.

• The signal rejection is dominated by shield noise:

Veto Efficiency = 97.1±0.2(sys) %

Signal Inefficiency = 2.5±0.2(sys) %

26Caius Howcroft

FC/PCDN: Veto Shield• Applying veto shield:

• Know veto shield efficiency and number of vetoed events, therefore can estimate cosmic ray muon background in selected sample.

•

Measured Cosmic Muon BG

MC Cosmic Muon BG

Events 4.4±0.4(stat)±0.3(sys) 4.9±0.5(stat)±0.7(sys)

DataTotal MC

(no oscillations)Total MC

(Δm2=0.0025eV2)

Total 94 120.1±24.0 94.3±18.9

PCDN 25 22.6±4.3 20.5±4.1

FC 69 98.5±19.7 73.8±14.8MC predictions include all backgrounds

27Caius Howcroft

Partially Contained - UP• Very different backgrounds to FC/PCDN. Stopping cosmic

ray muons with incorrect direction from timing, therefore timing cuts do the work here.

• However still need a containment and a Δz cut to remove upgoing neutrino induced muons (reduced to 1% of signal)

Signal:BG ~ 1:15

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PCUP:Timing • Direction of track is determined from timing of hits along

its length.

• The rms of the hits times about the track is calculated for the two hypotheses of +c or -c. The hypothesis with smallest rms is selected as track direction.

+c hypothesis+c hypothesis

29Caius Howcroft

PCUP:Timing• Use difference in RMS to selected events with good timing

• Remaining background is estimated from high statistics Monte Carlo studies <0.36 (68% c.f.). A x-check is made by extrapolating data distribution into signal region giving 0.5 events.

/ ns down

- RMSup RMS-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0

-110

1

10

210

data

µ MC cosmic

! MC atmos

DataTotal MC

(no oscillations)Total MC

(0.0025eV2)

Events 13 19.3±3.9 10.7±2.1

MINOS PRELIMINARY

30Caius Howcroft

Event Summary

DataMC νµ/νµCC

no oscillations

MC νµ/νµCC

0.0025eV2Cosmic Muon

MCOther

Backgrounds

Total 107 128.9±25.8 93.5±19 4.9±0.7 5.6±1.1

• Summary of selected events

Assumes no oscillations

Barr et al flux. Sys error dominated by (flux and

cross-section)

Other Backgrounds:ντCC

Rock muons νeCC

Neutral Current

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Example Events

FC PCDN

PCUP

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Event Direction

• Define a 'high resolution' sample for FC events, by applying similar timing cuts to PCUP.

• Place 30 events into a 'low resolution' sample which have an ambiguous direction.

Correct Track DirectionBefore Cut = 96.0 %After Cut = 99.9 %

FC events

33Caius Howcroft

Up/Down Ratio

• For high-resolution events can calculate the up-down ratio (Robs) and compare it to MC expectation in the absence of oscillations (RMC).

zenith!cos

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Ev

en

ts

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15

20

25

30DataMC null oscillations

2 = 0.0025 eV23

2 m"MC

MC Background

DataMC null oscillations

2 = 0.0025 eV23

2 m"MC

MC Background

Robs/RMC = 0.62±0.15(stat)±0.03(sys)

MINOS PRELIMINARY

34Caius Howcroft

Charge Separation• Measure charge from curvature of track in magnetic field.

• Require that |Q/P|/ σQ/P > 2.0

✦Track length✦Muon momentum✦vxB

Q/P!Q/P/

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5000

10000

15000

20000

25000

Data

+µMC

-µMC

+µ+ -µMC

Stopping cosmic ray muons

μ- μ+Good data/MC

agreement.

MINOS PRELIMINARY

35Caius Howcroft

Charge Separation (2)

Q/P!Q/P/

-10 -8 -6 -4 -2 0 2 4 6 8 10

Events

0

2

4

6

8

10

12

14

16

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20

22

24

Data

MC Expectation (no oscillations)

)2

=0.0025 eV2m"MC Expectation (

MC background

ν ν ν/ν

Data 34 18 55

MC (no oscillations)

45.6±8.9 24.9±4.7 68.5±14

MC

(0.0025 eV2)33.8±6.6 18.7±3.4 51.9±10

N+/N- = 0.53±0.15(stat)±0.02(sys)Expected N+/N- = 0.55

Charge Purity = 98.7%

MINOS PRELIMINARY

36Caius Howcroft

Q Separated Up-Down

zenith!cos

-1 -0.8-0.6-0.4-0.2 0 0.2 0.4 0.6 0.8 1

Even

tsµ"

0

5

10

15

20

25

30

zenith!cos

-1 -0.8-0.6-0.4-0.2 0 0.2 0.4 0.6 0.8 1

Even

tsµ"

0

5

10

15

20

25

30

Data

MC nill oscil

)2

=0.0025 eV2m#MC (

MC background

Robs/RMC

ν 0.72±0.35(stat)±0.03(sys)

ν 0.83±0.29(stat)±0.03(sys)

MINOS PRELIMINARY

37Caius Howcroft

Summary

• MINOS has collected 5.99 kton-years of cosmic data before beam switched on in March 2005.

• Have observed a total of 198 atmospheric neutrino events, 107 contained vertex and 91 upward-going muons.

• Contained event up-down ratio excludes no oscillations to 2.5σ.

• Contained events can be charged id'ed with 98% purity.

• Measured N+/N- = 0.53±0.15(stat)±0.02(sys) , c.f. expectation 0.55.

• No enough statistics yet to make any statement about charge separated oscillation parameters.

Acknowledgements: A. Blake, J. Chapman, B. Rebel and M. Thomson

`Thus I descended from the first circle down into the second, which girdles less space, and so much more woe that it goads to wailing. There abides Minos horribly, and snarls; he examines the sins at the entrance; he judges, and he sends according as he entwines himself.'

Dante Alighieri - Divina Commedia

Gustave Dore

39Caius Howcroft

backup1

2 / eV2

23 m

-510

-410

-310

-210

-110

2 /

eV

2 23

m

-510

-410

-310

-210

-110

! 2

= 0.0025 eV2

m ,2

= 0.0025 eV2 m !

!

!

90% C.L.95% C.L.Input Value

)2

(eV-!2m!

-410

-310

-210

-110

)2 (

eV

+!2

m!

-410

-310

-210

68%

90%

99%

FC Expected upmu Expected

MINOS PRELIMINARY MINOS PRELIMINARY

40Caius Howcroft

but why?

• Can accommodate LSND, solar and atmos results with CPT violation giving different masses for neutrino and anti-neutrino.

• Use a none zero theta 13 component to measure sign of deltam2

• "Because it's there." - George Mallory, 1924