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A long baseline ν τ appearance experiment

in the CNGS beam from CERN to Gran Sasso

A.Ereditato, LNGS SC, September

13, 1999


A long baseline ν τ appearance experiment

in the CNGS beam from CERN to Gran Sasso

A.Ereditato, SPSC, September 14,

1999


COLLABORATION

K.Kodama, N.Ushida

Aichi University, Aichi, Japan

M.Guler, E.Pesen, M.Serin-Zeyrek, R.Sever, P.Tolun, M.T.Zeyrek

METU, Ankara, Turkey

D.Duchesneau, J.Favier, H.Pessard

LAPP, IN2P3-CNRS and Universite’ de Savoie, Annecy, France

M.T.Muciaccia, P.Righini, S.Simone

Bari University and INFN, Bari, Italy

C.Haeberli, M.Hess, R.Klingenberg, U.Moser, K.Pretzl, M.Weber

Bern University, Bern, Switzerland

L.Benussi, G.Van Beek, P.Vilain, G.Wilquet

IIHE(ULB-VUB), Brussels, Belgium

M.Spinetti, L.Votano

LNF, Frascati, Italy

T.Kawamura, S.Ogawa, H.Shibuya

Toho University, Funabashi, Japan

J.Dupraz, J.P.Fabre, P.Schilly, U.Stiegler

CERN, Geneva, Switzerland

J.Goldberg, M.Khalifa

Israeli group c/o Technion, Haifa, Israel

F.W.Buesser, A.Geiser, K.Hoepfner, B.Koppitz, B.Naroska, W.Schmidt-Parzefall, R.van Staa

Hamburg University, Hamburg, Germany

S.Aoki, T.Hara

Kobe University, Kobe, Japan

L.Chaussard, M.Chemarin, Y.Declais, P.Jonnsson, S.Katsanevas, I.Laktineh, J.Marteau, G.Moret

IPLN, IN2P3-CNRS and Universite’ C. Bernard Lyon I, Villeurbanne, France

A.Artamonov, P.Gorbounov, V.Khovansky

ITEP, Moscow, Russia

N.D’Ambrosio, P.Boschan, D.Frekers, D.Rondeshagen, J.Schmand, H.J.Wortche, T.Wolff

Muenster University, Muenster, Germany

K.Hoshino, M.Komatsu, M.Miyanishi, M.Nakamura, T.Nakano, K.Niwa, O.Sato

Nagoya University, Nagoya, Japan

N.Bruski , S.Buontempo, F.Carbonara, A.Cocco, V.Cuomo, G.De Lellis, A.Ereditato, G.Fiorillo,

R.Listone, M.Messina, P.Migliozzi, S.Sorrentino, P.Strolin, V.Tioukov

“Federico II” University and INFN, Naples, Italy

J.E.Campagne, B.Merkel, J.P.Repellin, J.J.Veillet

Laboratoire de l’Accelerateur Lineaire (LAL), IN2P3-CNRS and Universite’ Paris-Sud, Orsay, France

G.Rosa

“La Sapienza” University and INFN, Rome, Italy

E.Barbuto, C.Bozza, G.Grella, G.Romano

Salerno University and INFN, Salerno, Italy

Y.Sato, I.Tezuka

Utsunomiya University, Utsunomyia, Japan


The physics aim

search for ν τ appearance

in the SK parameter region

central Δm 2 value: 3.5 x 10 -3

(From 2 to 6 x 10 - 3

eV 2 @ 90% CL)


The approach of the experiment

• CNGS long baseline beam optimal for ν μ −ν τ appearance

• Direct detection of τs by decay topology (γcτ ~ 1mm)

• Vertex detector high resolution (~ 1μm)

large mass (~ 1kton)

• Re-birth of old technique Emulsion Cloud Chamber (ECC)

• Feasibility - today’s nuclear emulsion:

high density data storage device

- Machine coated Emulsion Sheets (MES)

- high-speed automatic scanning systems


Microscope event view

Emulsion


to beam

Good tracks

appear as

dots

~100 μm

Tracking

implies

connection of

dots

in different

layers


Automatic scanning

Computer controlled microscope

video processor

CCD

Digitised tomographic images

~ 100 x 100 μm 2 views



x 50


a few μm focal depth


Emulsion

(on movable stage)

thin

optical slices


frame = Σ adjacent views

matching grains → track


Track Selector (Nagoya)

~ 100 μm

(emulsion beam)

T


Track Selector

angular detection efficiency:

(CHORUS data)


Track Selector

angular and position

resolution:

100 μm segments

(CHORUS data)


Basic ECC concept:

Passive target material and emulsion tracking

large mass

high space resolution

Beam

Emulsion Sheet (ES)

Passive material plate


γ -ray

Electromagnetic shower

1956 Balloon experiment

P T = constant, over 10 TeV

Nuclear emulsion

(poured both-side)

Primary vertex

Carbon plate

Space

Pb plate

1 mm

Pb plate

5 mm

Pb plate

2.5 mm

1971 aircraft experiment

X particle (charm)

Example: charm discovery with an ECC detector

K.Niu, E.Mikumo and Y.Maeda, Progr.Theor.Phys. 46 (1971) 1644.

Nuclear emulsion

(poured one-side)

ECC history

experience of Japanese groups:


Industrial Emulsion Sheets (MES)

• joint R&D project between Fuji Co. and the Nagoya group

• diluted (x2) emulsion gel, good sensitivity (~30 grains/100 μm)

• large production capability (photographic films production lines)

• OPERA: ~150000 m 2 MES

• Test results (see later)

• To be studied: long term performance, temperature control, handling,

transportation, etc.

Surface protection Coating

Emulsion (50 micron)

Emulsion

Plastic Base (200 micron)

Emulsion

Emulsion (50 micron)

Plastic Base (200 micron)


OPERA ECC elementary cell: two structures under study

compact

ES

τ

Pb

1 mm

ν τ

spacer


τ detection long decay kinks

• Compact cell - direct kink detection (in space)

- cost effective use of ES

- experience exists

- e mode: ID by electron cascade showers

- μ mode: ID by muon spectrometer, range and

MS in emulsion

- h mode not used: reinteraction BG.

• Spacer cell - direct kink detection in low density material

- all decay modes studied

- twice ESs needed for a given target mass

- novel technique: test measurements needed

Short τ decays (IP):

charm & reinteraction BG; not used for discovery;

can contribute to determine the oscillation parameters


A sequence of cells (~15x15 cm 2 x-section) forms a brick:

building block of a massive target (walls and supermodules)

simulated ν μ CC event


Event finding and reconstruction:

• Electronic trackers behind each

target wall: identify fired brick

• Remove brick for scanning:

- general scanning on downstream ESs

- scan back, locate vertex region

- local general scanning

- decay search

• May remove other bricks (τ candidates):

particle ID and momentum measurement (MS)

Strategy:

emulsion scanning

event reconstruction

data taking from data storage

offline algorithms

• Present analyses (CHORUS, DONUT):

test bench for OPERA


DONUT ECC:

5 x 5 mm 2 times 8 cells:

100000 track segments;

900 stopping tracks;

180 small angle, high energy;

1 neutrino interaction

(require low IP tracks)


ν

Event 3024-30175

20

15

10

5

41

42

43

44

45

10

11

12

13

14

15

DONUT ECC: candidate ν τ event: ν τ τ e

(units are mm)


CHORUS data used to simulate

the OPERA cell (spacer)


Scanning speed is of great

importance for OPERA:

100

Predicted angle scan(view/sec)

General angle scan(view/sec) 30 30

• ~20 bricks/day

• ~50 cm 2 general scanning/brick

• ~1000 cm 2 /day

10

3 3 3

aim: ~10 cm 2 /hour/system

(UTS ~ 1 cm 2 /hour)

1

0.2

0.4

0.25

R&D efforts underway

in Japan and in Europe

0.1

1982

0.008

1994

1996

1998

2000

Track Selector road-map


The detector

• Modular detector structure:

cells bricks walls modules supermodules

electronic trackers

• Performance is given per supermodule

μ spectr.

Supermodule structure

SPACER

COMPACT

DIMENSIONS (m 2 ) ~ 6.3 x 6.3 x 3.5 ~ 6.3 x 6.3 x 3

CELLS/BRICK 30 56

BRICK THICKNESS (cm) 14.2 7.7

BRICK THICKNESS (X 0

) 5.3 10

BRICK WEIGHT (kg) 7.6 14.2

BRICKS/WALL 1340 1340

MODULES/SUPERMODULE 18 24

BRICKS/SUPERMODULE 24120 32160

ES SURFACE/SUPERMODULE (m 2 ) 32500 40500

SUPERMODULE WEIGHT (ton) 185 455


Supermodule side view

spacer

compact


Supermodule assembly: vertical wagon


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Wagon

(Package of 3 to 10 bricks)

IBEAM

For Hang-up

Wall

Slot

Supermodule assembly: horizontal wagon


Supermodule assembly: harp

I beams for

wall hang-up

Brick

extraction/insertion

electronic

trackers

bricks

harps


Electronic trackers

• Task: identify fired brick (shower axis),

tracking and shower energy

• Require: σ ~ 1 cm, high efficiency, long term reliability

large area (~ 3500 m 2 /proj.) cost effective, mature, well proven technique

• Options: scintillator strips or liquid scint. tubes (WLS fiber r/o)

prototype studies underway (see later)

Events

compact

spacer

Events

Event profile (cm)

MC: 2cm wide strips

Event profile (cm)

Events

Events

Center of gravity (cm)

Center of gravity (cm)


Muon detection and spectrometers

• Need: - identify muons for charm BG rejection

- require ε μ (charm) ~ 95%

- measure μ charge reduce BG for muonic decays

- knowledge of the beam

• Spectrometer options: iron toroids (baseline) or dipoles

• Require: Δp/p ~30% @ 10 GeV/c

• Muon ID: crossing the spectrometer (~ 85 %)

stopping in the spectrometer (~ 10 %)

range out in the target (~ 5 %)

• In addition: exploit p measurement by MS (low momenta)


Dipolar magnet design


Detector layout with

dipolar magnets


Detector layout: number and types of Supermodules:

• Physics requirements: τ detection efficiency, background, sensitivity

•Test results

• Studies and simulations

• Cost effectiveness

• Technical considerations

Measurements with prototypes

• test technical solutions (ES, brick material, packing,…)

• angular resolution

• momentum measurement by multiple scattering

• electron ID

• brick finding efficiency

• vertex finding efficiency


Angular resolution measurement

determines kink angle acceptance

ES1

ES2

A

b

s

o

r

b

e

r

θ1

Spacer

Δθ

θ 2

Beam

Particle


PS test beam:

brick 2, 3, 4, 10 GeV/c π

(at different angles)


efore

after

ES alignment through pattern association: few μm accuracy


Measured angular resolution with MES:

~5 mrad in projection cut on τ kink space angle: 25 mrad


Electron identification

• Compact bricks: - need to reject hadronic interactions

- identify τ e decays

• Exploit electron energy loss in the lead/ES shower detector

• Test beam electrons: sample EM showers at 3X 0 (~17 cells)

(MC predicts: ~10 e - in ±25 mrad and ±500 μm)

• Full shower reconstruction: determine e - detection efficiency


8 GeV e event (test beam):

transverse projection at 3X 0

11 e tracks in ±500 mm

(0.3 BG tracks)


Electron ID: simulated

shower electrons at 3X 0

position and angle

distributions

800

700

600

500

400

300

200

100

0

1000

800

600

400

200

0

1000

0.3

800

600

0.2

400

200

0.1

5001000

-200 0

0

-400

-0.1

-600 -5000

-800 -1000

-0.2

-1000

-0.3

position

μm

-0.2

angle

0

0.2

rad


Test beam

electron event:

shower reconstruction

0.5 mm

10 mm


Momentum measurement by MS in the cells

• Momentum measurement:

tool for BG reduction and

candidate event kinematics

Dense material

• Spectrometers (muons)

MS (hadrons and muons)

X1

X2

Emulsion

Y

Y1

2

Cell

• Detect track deviations in the

ECC structure (lead/ES cells)

X3

Y3

• Exploit the

Relative Scattering Method

X4

X5

Y

Y5

4

• Test beam pions (results)

X6

A Y6 B C


4 GeV/c pions:

momentum measurement

by Relative Scattering Method

(data and MC events)

45

40

35

30

25

20

15

10

5

0

0 1 2 3 4 5 6 7 8 9 10

Data 4GeV/c

M.C. 4GeV/c

Entries

Mean

RMS

301

3.867

1.150

8.404 / 16

P1 39.73

P2 3.702

P3 0.8354

P4 2.691

P5 5.336

P6 2.874

Entries

Mean

RMS

715

4.132

0.8824

11.96 / 16

120

100

80

Constant 111.6

Mean 4.102

60

Sigma 0.8384

40

20

0

0 1 2 3 4 5 6 7 8 9 10


4 GeV/c pions (star)

MC data points (circles)

Measured momentum (GeV/c)

10

9

8

7

6

5

4

3

2

1

0

0 1 2 3 4 5 6 7 8 9 10

Real momentum (GeV/c)


δp/p %

100

90

80

Momentum resolution by MS:

4 GeV data pions (star)

MC events (circles)

70

60

50

40

30

20

10

0

0 1 2 3 4 5 6 7 8 9 10

Real momentum (GeV/c)


Scintillator strip proto-plane

• 30 cm long strips (moulded)

• 1.2 mm double clad WLS fibres

• 4x4 pixel multianode readout


Setup for the scintillator strip proto-plane


Setup for the scintillator strip proto-plane

1m

π, μ

15 GeV/c

mini-walls 1.6 m iron + 2 m concrete

bricks

C1 C2 BPCs C3 C μ

1 m


Position resolution for

brick finding (spacer bricks)


τ detection efficiency

• Contributions: brick finding, geometry, vertex finding,

τ decay length, kink angle, p t cut, particle ID

function of the type of bricks/supermodules and of Δm 2

• Current estimates based on: test results,

experience gained with CHORUS/DONUT,

detailed simulations

Supermodule

Decay mode Det. Eff. x BR

spacer τ e 3.0 %

spacer τ μ 2.6 %

spacer τ h 7.3 %

spacer total 12.9 %

compact τ e 3.0 %

compact τ μ 2.6 %

compact total 5.6 %

Aim:

improve these figures by

• tuning CNGS beam spectrum

• test measurements

• optimised detector design


τ detection efficiency

LoI : spacer bricks ε ~ 0.29 x 0.85 (kinematics) = 0.25

In order to compare with the Progress Report:

• The beam energy went down from

22 to 17 GeV, lowering ε(gap)

0.25 x 0.93 = 0.23

• Industrial requirement: ES plastic base is now

200 μm m (100 in the LoI), i.e. more decays with lower ε 0.23 x 0.95 = 0.22

0.13 (Progress Report) to be compared with 0.22 (LoI)

This is due to more refined studies, measurements and experience gained

New estimates:

brick and vertex finding, particle ID and momentum measurement

fiducial volume, geometrical losses


Detection efficiency for the muonic decay channel

OPERA

CHORUS

ε brick x ε vert x ε geo 0.58 0.30

Kink efficiency

ε pt x ε θ x ε gap

= 0.29

0.37

total x BR ~ 0.03 ~ 0.02

CHORUS data from: Phys. Lett. B 424 (1998) 202; Phys. Lett. B 434 (1998) 205.


Background

Background sources: 1) prompt ν τ in the beam

(depend on the brick type) 2) cosmics and radioactivity

3) hadronic decays and re-interactions

4) muon scattering

5) charm decay

other sources being investigated (expected to be small)

1) Negligible: ~ 10 -7 x N cc ν τ events

2) Isolated ES track segments fake associations can complicate the analysis

- β-rays from lead lead selection, test measurements

- cosmics before assembly aim at ~1 segment/mm 2 ES packing at production,

controlled fading, ...

(note: cosmics tracks will be needed for ES alignment)

Further studies are planned

keep this potential BG under control


3) Hadron decays and re-interactions

in the spacer (and base):

daughter p t cut: 250 MeV/c

BG < 10 -5 x N cc

4) μ MS in the lead (compact bricks):

pt(GeV/c)

combined p and p t cut

stars:BG

dots: signal

BG ~ 3 x 10 -5 x N cc

planned tests

measure this BG

p(GeV/c)


5) Charm induced background:

Need high (primary) muon detection efficiency

background

signal

μ - (undetected)

ν τ

τ -

h -

ν μ

D + neutrals

h +

(sign of daughter only measured if muon)


• single charm production:

Spacer bricks: e channel BG ~ 0.5 x 10 -5 x N DIS

μ “ “ 0.2 x 10 -5 “

h “ “ 2.0 x 10 -5 “

Compact bricks: e “ “ 0.4 x 10 -5 “

μ “ “ 0.2 x 10 -5 “

• associated charm production:

0.1 x 10 -5 x N DIS

(compact & spacer)

• charm BG evaluation will benefit from the ongoing CHORUS analysis


Sensitivity to ν μ -ν τ oscillations

Five supermodules

3600 CC and 1200 NC ν interactions/year

1

90% CL upper limit (full mixing):

10 -1

Δm 2 = 1.7 x 10 -3 eV 2 (2 years)

Δm 2 = 1.2 x 10 -3 eV 2 (4 years)

10 -2

10 -3

cover the SK parameter region

10 -4

10 -3 10 -2 10 -1 1


Discovery potential

OPERA is able to detect τ events in a 1-2 years run

Superm.

Decay

mode

a long run would then be justified:

2.25 x 10 20 pot (nominal 5 years’ run)

τ events per supermodule

Signal @

2 x10 -3

eV 2

Signal @

3.5 x10 -3

eV 2

Signal @

6 x10 -3

eV 2

Spacer Electron 0.26 0.81 2.38 0.010

BG

Spacer Muon 0.23 0.70 2.06 0.004

Spacer Hadron 0.64 1.97 5.79 0.036

Compact Electron 0.65 2.00 5.88 0.04

Compact Muon 0.56 1.73 5.08 0.16


• Five supermodules are required for ~ 20 events (@ 3.5 x 10 -3 eV 2 )

• Exercise to evaluate the physics reach 3 spacer + 2 compact SM

(~ same # of events for each decay channel)

• 2.25 x 10 20 pot (nominal 5 years’ run)

Decay mode

Signal @ Signal @ Signal @ BG

2 x10 -3 eV 2 3.5 x10 -3 eV 2 6 x10 -3 eV 2

Electron 2.1 6.4 18.9 0.11

Muon 1.8 5.6 16.3 0.32

Hadron 1.9 5.9 17.4 0.11

Total 5.8 17.9 52.6 0.54

OPERA

unambiguous oscillation signal in the SK region


4 σ discovery limit:

1-(6.3 x 10 -5 ) CL upper limit of the Poisson distribution with < > = BG

1

10 -1

10 -2

10 -3

10 -4

10 -3 10 -2 10 -1 1


Determination of the oscillation parameters (PDG, 90% CL)

10 -1

10 -2

10 -3

0 0.2 0.4 0.6 0.8 1


ν μ -ν e oscillations


Conclusions and outlook

• OPERA design based on: - the CNGS beam, optimal for ν τ appearance

- experience with the ECC detector technique

- the availability of industrial ES

- advances in the automatic emulsion scanning

- experience with CHORUS ν μ -ν τ

• Since the LoI: progress in the understanding of the experiment:

test measurements, simulations and studies to assess its feasibility

and performance. Detector options are being considered.

Started study of technical aspects.

• OPERA : will detect ~ 18 unambiguous τ events (< 1 BG) with

2.25 x 10 20 pot (3.5 x 10 -3 eV 2 ): covering the SK region.

Optimisation studies will be carried out with the aim to

improve the experiment performance.

• The Collaboration includes (already now) a broad international community:

the next milestone will be the submission of the Experiment Proposal

in approximately half a year.


Example of a brick assembly machine

Lead

Emulsion #1

Spacer

Emulsion #2

Empty box

input

Production

line #1

Box Cover

Humidity and temperature control

Ultrasound

welding machine

Brick

output

Production

line #2

Dark room

~ 8m

(production capability: ~1 brick/min)

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