THE CERN Experiment P326for Rare Kaon Decays

Massimo Lenti

INFN Sezione di Firenze

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Outline of the presentation

• Physics motivations for K+→• The beam• The main background channels• The apparatus• The signal acceptance and S/B• Time schedule and Conclusions

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Physics motivations for K+→

CP

Standard Model predictions

BR(KBR(K++++) ) (1.6×10 (1.6×10-5-5)|V)|Vcbcb||44[[22+(+(cc--))22] ] (8.0 ± 1.1)×10 (8.0 ± 1.1)×10-11-11

BR(KBR(KLL00) ) (7.6×10 (7.6×10-5-5)|V)|Vcbcb||442 2 ± 0.6± 0.6×10×10-11-11

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Physics motivations, II• Theoretically very clean• Sensitive to Vtd

• Very sensitive to New Physics

Present (E787/949): BR(K+) = 1.47 ×1010

with 3 events

+1.30-0.89

Need a 10% measurement (100 events): P326

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How many K decays?• BR(K+→) ~ 8×10

• Look for ~100 signal events• Signal acceptance ~ 10%

~1013 K+ decays

NA48/P326

Use the existing CERN accelerators

Use the existing NA48 experimental hall

]

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The Beam

Primary Beam:• 400 GeV/c protons• 3×1012 protons/pulse (3×NA48/2)• 4.8/16.8 s duty cycle Secondary Beam:

• 75 GeV/c momentum (p/p~1%)• Beam acc.: 15 str (30×NA48/2)• Total rate: 800 MHz• K+ ~ 6%• 4.8×1012 K+decays/y

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The Beam purity

Keep vacuum at 106 mbar: use existing NA48 decay tank(already directly measured)

Tag the K+ in the beam: use a CEDAR

Only 6% K+ but:• protons and electrons don’t decay...• pions and muons decays cannot mimic K+ decays• but beam-gas interactions !!

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The CEDAR

The CEDAR is a Cherenkov counterUsed at CERN since long timeVary gas pressure and diaphragm aperture to select K+

November 2006: test beam with a CEDAR100 GeV/c beamFilled with Nitrogen

Adapt to P326 needs:Fill with HydrogenChange Phototubes and electronics

Beam Composition

00,10,20,30,40,50,60,70,8

1650 1700 1750 1800 1850 1900 1950 2000 2050

Pressure

%

PionsPions

KaonsKaonsProtonsProtons

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KinematicsK

K+

m2miss=(PKP)2

92% K+ decays 8% K+ decays

PK : beam spectrometerP : straw chambers spectrometer

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Beam Spectrometer (I)

CEDAR

Gigatracker

Achromat

Achromat

p

• 3 Silicon Pixels stations across the 2nd Achromat: 36(X) × 48(Y) mm per station• Beam rate: 800 MHz (“Gigatracker”), 50 MHz/cm2

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Beam Spectrometer (II)• 300×300 m pixels

• 200 m Si sensor + 100 m chip

• 0.13 m CMOS technology

(PK)/PK ~ 0.4%(K) ~ 16 rad

Low X/X0

(t) ~ 200 ps/station

Important for beam pile-up

Strong R&D ongoning...

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Straw Chambers Spectrometer

• 6 chambers with 4 double layers• Ø 9.6 mm straw tubes in vacuum• 0.1% X0 per view• 130 m hit resolution per view• 2 magnets (270 and 360 MeV/c pt kick)• holes follow beam path

8.8 m186.3 mfrom T0

5.4 m 5.4 m

7.2 m 7.2 m 3 coordinates

4 coordinates2 coordinates

1 coordinate

10 cm

Gas CF4-CO2-isoC4H10

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Kinematics Reco.

P

K

PK

K

Double spectrometer:almost independent momentummeasurement

Missing mass measurement: dominated by angle between Kaon and pion

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Background 1: K+→ (K)

Largest BR: 63.4%

Need ~ rejection factor• Kinematics: 10

• Muon Veto: 10• Particle ID: 5×10

MAMUD

RICH

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Muon Veto: MAMUD

MAgnetized MUon Detector

Sampling calorimeter + Magnet for beam deflection

Em/hadronic clusters separation

Sensitivity to MIP

10 detection inefficiency

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RICH

3 separation up to 35 GeV/c

• 18 m long• Neon at 1 atm ( thr.: 12 GeV/c)• 2000 PMT• 18 mm granularity• 100 ps resolution (to disentanglepileup in the Gigatracker)

PMTs tested in 2006 CEDAR testPrototype test beam in 2007

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Background 2: K+→ (K)2nd Largest BR: 20.9%

Large angle: 13 ANTIs (10 < acceptance < 50 mrad) Medium angle: NA48 LKr (1 < acceptance < 10 mrad) Small angle: IRC1,2 SAC (acceptance < 1 mrad)

Need ~ rejection factor• Kinematics: 5×10

• Photon Veto: 10per photon

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Large Angle Veto

Two options under test:• lead-scint. tiles read by WLS fibers• lead+scint.fibers (KLOE like)

• 13 ring em calorimeters in vacuum• 10ineff. 0.05<E<1 GeV• 10 ineff. E>1 GeV

Prototypes under constructionsTests at Frascati tagged photon beam

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Small Angle Veto

Tested in October 2006In the NA48 tagged photon beam(see later)

• shaslyk calorimeter on the beam axis• 10 ineff. High energy

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Liquid Kripton Calorimeter

Ineff. for E>10 GeV tested on data collected by NA48/2 (K+→)

Use the existing NA48 LKr calorimeter• 10 ineff. E>5 GeV• 10 ineff. 1<E<5 GeV

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Liquid Kripton Calorimeter (II)

October 2006 test:Tagged photon beamUsing the existing NA48 setup

vacuum

Electron beam

(25 GeV/c)Bremsstrahlung

Kevlarwindow

Driftchambers

MagnetCalorimeter

e-

X LKr cm

Energy deposition in LKr

electronelectron

En

erg

y G

eV

• 2×108 electrons collected• 10 ineff.sensitivity below 10 GeV

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The P326 Layout800 MHz

50 MHz

11 MHz

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Signal AcceptanceRegion I:0<mmiss

2<0.01 GeV2/c4

Region II:0.026<mmiss

2<0.068 GeV2/c4

Remind:K mmiss

2 < 0K mmiss

2 = 0.0182 GeV2/c4

Momentum range: 15 <p< 35 GeV/cFiducial decay region: 60 m

Acceptance: 4% (Region I), 13% (Region II): 17% (I+II)10% goal feasible (after analysis cuts, etc.)

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Signal/BackgroundEvents/year Total Region I Region

II

Signal (acc=17%) 65 16 49

K++0 2.7 1.7 1.0

K++ 1.2 1.1 <0.1

K+e++ ~2 negligible

~2

Other 3 – track decays

~1 negligible

~1

K++0 1.3 negligible

1.3

K++ 0.5 0.2 0.2

K+e+

(+)0,othersnegligibl

e

Total bckg. 9 3.0 6

S/B ~ 8 (Region I ~5, Region II ~9)

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Trigger Levels

• 11 MHz L0 trigger input

• 1track × × → 1 MHz L1 trigger input → PC farm

• Software trigger reduction ~ 40

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Conclusions

• 2006-2007: R&D, test beam• 2008-2010: Construction• 2011: start data-taking

• Full approval and funding expected end of 2007• Still need to strengthen the collaboration

• Clear Physics case• many other physics channels