1. THE CERN Experiment P326 for Rare Kaon Decays
Massimo Lenti
INFN Sezione di Firenze

2. Outline of the presentation
Physics motivations for K+→p+nn
The beam
The main background channels
The apparatus
The signal acceptance and S/B
Time schedule and Conclusions
P326
M. Lenti

3. Physics motivations for K+→p+nnCP
Standard Model predictions
BR(K+p+nn)  (1.6×10-5)|Vcb|4[sh2+(rc-r)2]  (8.0 ± 1.1)×10-11
BR(KLp0nn)  (7.6×10-5)|Vcb|4h2  (3.0 ± 0.6)×10-11
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4. Physics motivations, II
Theoretically very clean
Sensitive to Vtd
Very sensitive to New Physics
Present (E787/949): BR(K+p+nn) = ×10-10
with 3 events
+1.30
-0.89
Need a 10% measurement (100 events): P326
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5. ] How many K+ decays? BR(K+→p+nn) ~ 8×10-11
Look for ~100 signal events
Signal acceptance ~ 10%
~1013 K+ decays
NA48/P326
Use the existing
CERN accelerators
NA48 experimental hall
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6. 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 (Dp/p~1%)
Beam acc.: 15 mstr (30×NA48/2)
Total rate: 800 MHz
K+ ~ 6%
4.8×1012 K+decays/y
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7. The Beam purity Only 6% K+ but: protons and electrons don’t decay...
pions and muons decays cannot mimic K+ decays
but beam-gas interactions !!
Keep vacuum at 10-6 mbar:
use existing NA48 decay tank
(already directly measured)
Tag the K+ in the beam:
use a CEDAR
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8. The CEDAR The CEDAR is a Cherenkov counter
Used at CERN since long time
Vary gas pressure and diaphragm aperture to select K+
November 2006:
test beam with a CEDAR
100 GeV/c beam
Filled with Nitrogen
Adapt to P326 needs:
Fill with Hydrogen
Change Phototubes and electronics
Pions
Protons
Kaons
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9. Kinematics 92% K+ decays 8% K+ decays PK : beam spectrometer
qKp K+ n n
m2miss=(PK-Pp)2
92% K+ decays
8% K+ decays
PK : beam spectrometer
Pp : straw chambers spectrometer
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10. Beam Spectrometer (I)
Achromat
CEDAR
Achromat p
Gigatracker
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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11. Beam Spectrometer (II)
s(PK)/PK ~ 0.4%
s(qK) ~ 16 mrad
300×300 mm pixels
200 mm Si sensor mm chip
0.13 mm CMOS technology
Low X/X0
s(t) ~ 200 ps/station
Important for
beam pile-up
Strong R&D ongoning...
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12. Straw Chambers Spectrometer
from T0
5.4 m
7.2 m
Gas CF4-CO2-isoC4H10
3 coordinates
4 coordinates
2 coordinates
1 coordinate
10 cm
6 chambers with 4 double layers
Ø 9.6 mm straw tubes in vacuum
0.1% X0 per view
130 mm hit resolution per view
2 magnets (270 and 360 MeV/c pt kick)
holes follow beam path
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13. Kinematics Reco. Missing mass measurement: dominated by angle between
qpK qp PK qK Pp
Missing mass measurement:
dominated by angle between
Kaon and pion
Double spectrometer:
almost independent momentum
measurement
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14. Background 1: K+→m+n (Km2)
Largest BR: 63.4%
Need ~10-12 rejection factor
Kinematics: 10-5
Muon Veto: 10-5
Particle ID: 5×10-3
MAMUD
RICH
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15. Muon Veto: MAMUD MAgnetized MUon Detector Sampling calorimeter +
Magnet for beam deflection
Em/hadronic clusters separation
Sensitivity to MIP
10-5 m detection inefficiency
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16. RICH p-m 3s separation up to 35 GeV/c 18 m long
Neon at 1 atm (p thr.: 12 GeV/c)
2000 PMT
18 mm granularity
100 ps resolution (to disentangle
pileup in the Gigatracker)
PMTs tested in 2006 CEDAR test
Prototype test beam in 2007
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17. Background 2: K+→p+p0 (Kp2)
2nd Largest BR: 20.9%
Need ~10-12 rejection factor
Kinematics: 5×10-3
Photon Veto: 10-5 per photon
Large angle: ANTIs (10 < acceptance < 50 mrad)
Medium angle: NA48 LKr (1 < acceptance < 10 mrad)
Small angle: IRC1,2 SAC (acceptance < 1 mrad)
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18. Large Angle Veto 13 ring em calorimeters in vacuum
10-4 ineff. 0.05<Eg<1 GeV
10-5 ineff. Eg>1 GeV
Two options under test:
lead-scint. tiles read by WLS fibers
lead+scint.fibers (KLOE like)
Prototypes under constructions
Tests at Frascati tagged photon beam
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19. Small Angle Veto shaslyk calorimeter on the beam axis
10-5 ineff. High energy g
Tested in October 2006
In the NA48 tagged photon beam
(see later)
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20. Liquid Kripton Calorimeter
Use the existing NA48 LKr calorimeter
10-5 ineff. Eg>5 GeV
10-4 ineff. 1<Eg<5 GeV
Ineff. for Eg>10 GeV tested on data collected by NA48/2 (K+→p+p0) p g g
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21. Liquid Kripton Calorimeter (II)
Kevlar
window
Magnet
Calorimeter
October 2006 test:
Tagged photon beam
Using the existing
NA48 setup
vacuum e- g
Electron beam
(25 GeV/c)
Bremsstrahlung
Drift
chambers
Energy deposition in LKr
electron
2×108 electrons collected
10-5 ineff.sensitivity below 10 GeV
Energy GeV g
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X LKr cm

22. The P326 Layout
11 MHz
800 MHz
50 MHz
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23. Signal Acceptance Region I: 0<mmiss2<0.01 GeV2/c4 Region II:
Remind:
Km2 mmiss2 < 0
Kp2 mmiss2 = GeV2/c4
Momentum range: 15 <pp< 35 GeV/c
Fiducial 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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24. Signal/Background S/B ~ 8 (Region I ~5, Region II ~9) Events/year
Total
Region I
Region II
Signal (acc=17%) 65 16 49
K+p+p0
2.7
1.7
1.0
K+m+n
1.2
1.1
<0.1
K+e+p+p-n ~2
negligible
Other 3 – track decays ~1
K+p+p0g
1.3
K+m+ng
0.5
0.2
K+e+(m+) p0n,others -
Total bckg. 9
3.0 6
S/B ~ 8 (Region I ~5, Region II ~9)
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25. Trigger Levels 11 MHz L0 trigger input
1track × m! × g! → 1 MHz L1 trigger input → PC farm
Software trigger reduction ~ 40
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26. 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
P326
M. Lenti