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A Proposal to Study Rare Kaon Decays at the CERN SPS Augusto Ceccucci/CERN. Physics Introduction Rare Kaon Decays in the SM…. …and Beyond Flavour as a probe of New Physics complementary to the high energy frontier Experimental state-of-the-art Recent results and world-wide perspectives

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A Proposal to Study Rare Kaon Decays

at the CERN SPS

Augusto Ceccucci/CERN

  • Physics Introduction
    • Rare Kaon Decays in the SM….
    • …and Beyond
      • Flavour as a probe of New Physics complementary to the high energy frontier
  • Experimental state-of-the-art
    • Recent results and world-wide perspectives
  • Description of the CERN proposal P-326
    • Technique
    • Status

Munich MPI

quark mixing and cp violation
Quark Mixing and CP-Violation
  • Cabibbo-Kobayashi-Maskawa (CKM) matrix:
  • Non-diagonal (e.g. Vus≠0)
  •  Flavour Violation
  • 3 or more quark generations
  •  CP-Violation in SM (KM)

Ng=2 Nphase=0  No CP-Violation

Ng=3 Nphase=1  CP-Violation Possible

e.g., Im lt= Im Vts*Vtd ≠ 0  CPV

Munich MPI

ckm unitarity and rare kaon decays
CKM Unitarity and Rare Kaon Decays

The unitarity of the CKM matrix can be expressed by triangles

in a complex plane.

There are six triangles but one is more “triangular”:


It is customary to employ the Wolfenstein parameterization:

Vus ~lVcb ~ l2 A Vub ~ l3 A(r- ih) Vtd ~ l3 A(1-r- ih)

Sensitive to |Vtd|


Im lt = A2l5h

Re lt = A2l5r

Munich MPI

status of unitarity triangle
Status of Unitarity Triangle

Sides vs. CPV


Rare kaon decays are loop-dominated. They are a unique

probe of the sd transitions and provide independent CKM tests

Munich MPI

the four golden modes of kaon physics
The four golden modes of Kaon Physics
  • Short distance dynamics:
    • W-top quark loops constitute the
    • dominant contribution:
  • The EW short-distance amplitude is common in the SM…
  • …but potentially different beyond SM
  • Important to address all these decays

Adapted from G. Isidori @ Flavour in the LHC era, 5-7 Nov 05, CERN

Munich MPI

k p nn theory in standard model
K→pnn : Theory in Standard Model

NLO Calculation:

Buchalla & Buras, 1993





The Hadronic Matrix Element is measured and isospin rotated

Munich MPI

predictions in sm
Predictions in SM

This used to be the largest theoretical error

(+/- 0.037). It was reduced by a NNLO calculation

A. Buras, M. Gorbahn, U. Haisch, U. Nierstehep-ph/0508165)

  • 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

The errors are mostly due to the uncertainty of the CKM parameters and not to the hadronic uncertainties

Munich MPI

theory vs experiment
Theory vs. Experiment

Adapted from U. Haisch @ Flavour in the LHC era, 6-8 Feb 06, CERN

Munich MPI

intrinsic theory error
Intrinsic theory error

Combining information from BR(K+→p+nn) and BR(K0→p0nn) one obtains:

(Buras et al. hep-ph/0508165)

So for a 10% uncertainty on Pc,

one can extract, in priciple,

a 3.4%exp. determination of

sin2b from kaon decays.

It is currently 4.6% from B decays

Munich MPI

beyond standard model
Beyond Standard Model
  • Compare two scenarios:
    • Minimal Flavour Violation
      • All mixing governed by universal CKM matrix
        • No Extra Complex Phases
      • Same operators as in SM
      • Different coefficients
      • Stringent correlation with B rare decays
    • New sources of Flavour Symmetry Breaking ~ TeV scale
      • Extra phases can lead to large deviations from SM predictions, especially for the CP-Violating modes

Munich MPI

k p nn

hep-ex/0403036 PRL93 (2004)


Stopped K+

~0.1 % acceptance

  • BR(K+→ p+ nn ) = 1.47+1.30-0.89 × 10-10
  • Compatible with SM within errors

Munich MPI

setting the bar for the next generation of k p nn experiments
Setting the bar for the next generation of K+→p+nn experiments

E787/E949: BR(K+→ p+ nn ) = 1.47+1.30-0.89 × 10-10

Current constraint on r,hplane


100 events


100 events


Munich MPI

k 0 l p 0 nn e391a upper limit
K0Lp0nn :E391a Upper Limit
  • 10% of RUN I
  • Pencil beam
  • Expected background
  • from K0Ldecays: 0.02
  • Acceptance: 0.73%
  • BR(K0Lp0nn)<2.8610-7 90%CL
  • Preliminary (Ken Sakashita@KAON2005)
    • 6 improvement over KTeV one day special run
    • 2 improvement over published limit (KTeV Dalitz technique)
  • For the future: JPARC LOI-05
  • Recently, J-PARC made a call for proposals

Munich MPI

k 0 s l p 0 e e and k 0 s l p 0 m m
K0S,L→p0 e+e-and K0S,L→p0m+m-

BR(KS→p0ee)  10-9 = 5.8 +2.8-2.3(stat) ± 0.8(syst)

PLB 576 (2003)

BR(KS→p0mm)  10-9 = 2.9 +1.4-1.2(stat) ± 0.2(syst)

PLB 599 (2004)




6 events, expected back. 0.22

7 events, expected back. 0.15

BR(KL→ p0 ee ) < 2.8 × 10-10 @90%CL KTeV PRL93, 021805 (2004)

BR(KL→ p0 mm ) < 3.8 × 10-10 @90%CL KTeV PRL86, 5425 (2001)

Munich MPI


K0L→p0ee (mm) in SM

With the KS measurements, the KLBR can be predicted

* Interference between short- and long-distance physics*

(Isidori, Unterdorfer, Smith,

EPJC36 (2004))

Constructivenow favored by two independent analyses*


*G. Buchalla, G. D’Ambrosio, G. Isidori, Nucl.Phys.B672,387 (2003)

*S. Friot, D. Greynat, E. de Rafael,

hep-ph/0404136, PL B 595


Munich MPI

  • K+p+nn
    • Already 3 clean events are published (E787/E949)
    • Experiment in agreement with SM within large errors
    • Next round of exp. need to collect O(100) events to be useful
    • Move from stopped to in flight technique (FNAL Proposal turned down by P5)
    • Proposal for in-flight decays: CERN P-326
    • Letter of Intent at J-PARC to continue the study with decays at rest
  • K0Lp0nn
    • Large window of opportunity exists.
    • Upper limit is 4 order of magnitude from the SM prediction
    • First results E391a (proposed SES~3 10-10)
    • Proposal being prepared to continue at J-PARC
  • K0Lp0ee(mm)
    • Long distance contributions under good control
    • Measurement of KSmodes has allowed SM prediction
    • KS rates to be better measured
    • Background limited (study time dep. Interference?)
    • 100-fold increase in kaon flux to be envisaged

Munich MPI

proposal to measure the rare decay k p n n at the cern sps



Proposal to Measure the Rare Decay K+p+ n n at the CERN SPS

CERN, Dubna, Ferrara, Florence, Frascati, Mainz, Merced, Moscow, Naples, Perugia, Protvino, Pisa, Rome, Saclay, San Luis Potosi, Sofia, Turin

Munich MPI

background rejection
Background rejection
  • Guidance: S/B = 10~10-12 rejection

1) Kinematical Rejection

2) Photon vetoes and PID (p-m)

Basic idea to reject K+ p+p0

P(K+)= 75 GeV/c

Require P(p+) < 35 GeV/c

P(p0) > 40GeV/c It cannot be missed in

the calorimeter/photon veto

Munich MPI

backgrounds kinematically constrained
Backgrounds kinematically constrained

Allows us to define the signal region

92% of K+ decays

K+p+p0forces us to split it into two parts

  • Region I: 0 < m2miss < 0.01 GeV2/c4
  • Region II: 0.026 < m2miss < 0.068 GeV2/c4

Munich MPI

backgrounds not kinematically constrained
Backgrounds not kinematically constrained

They span accross the signal regions

Must rely on Particle ID and veto

8% of K+ decays

Munich MPI

p 326 detector layout
P-326 Detector Layout

K+p+ n n




~11 MHz


75 GeV/c

800 MHz beam




Munich MPI

p 326 detector layout1
P-326 Detector Layout






~11 MHz


75 GeV/c

800 MHz beam



Munich MPI

  • Signal events expected per year@BR=8 10-11
    • 65 (16 Region I, 49 Region II)
  • Background events
    • ~9 (3 Region I, ~6 Region II)
  • Signal/Background ~ 8
    • S/B (Region I) ~5
    • S/B (Region II) ~ 9

For Comparison:In the written proposal we

quoted 40 events/year@BR=10-10to account for

some reconstruction and deadtime losses

Munich MPI


New high-intensity K+ beam for P-326



Munich MPI

decay tank
Decay Tank
  • Specification: 10-6 mbar
    • Study performed with Monte

Carlo using Fluka and Gheisha

to simulate the hadronic

interactions with the residual gas.

  • Measurements:
    • Vacuum test performed on the

existing tube of NA48.

    • A 10-5 mbar level reached

with only 1 pump.

    • With a few 50000 l/s diffusion or

cryogenics pumps the requested

vacuum level can be achieved

  • Conclusions:
    • The existingdecay tankcan be used

Munich MPI







Provide precise measurements on all beam tracks (out of which only ~6% are K+)

Provide very good time resolution

Minimise mass (multiple scattering and beam interactions)

Sustain high, non-uniform rate ( 800 MHz total)

  • Two Silicon micro-pixel detectors (SPIBES)
    • Timing
    • Pattern Recognition
  • Improved KABES (micromegas TPC)
    • To minimise scattering in the last station


X/X0 << 1%

Pixel size ~ 300 x 300 mm

s(p)/p ~ 0.4%

excellent time resolution

to select the right kaon track

Dependence of the signal to

background (from K+p+p0) ratio

as a function of the gigatracker

time resolution

Munich MPI


spibes hybrid pixel





Station 1(pixels) 2(pixels) 3(FTPC)

SPIBES (Hybrid Pixel)

G. Anelli, M. Scarpa, S. Tiuraniemi

  • 200 mm Silicon sensor (>11 000 e/h mip)
    • Following Alice SPD
    • Bump-bonding
  • Read-out chip
    • Pixel 300 mm x 300 mm
    • Thinned down to ~100 mm (Alice SPD 150 mm)
  • Beam surface ~ 14 cm2
    • Adapted to the size of the SPIBES

r-o chips

  • ~125 mm Cfibre for cooling & support

Front End and R/O

considerations based on

the experience of the

CERN-PH/MIC and PH/ED Groups with the


Munich MPI


ftpc kabes

KABES principle: TPC + micromegas



Gap 25 μm


Gap 25 μm



Pioneered in NA48/2

Tested in 2004 at high

intensity with 1 GHz FADC

  • In NA48/2 KABES has achieved:
    • Position resolution ~ 70 micron
    • Time resolution ~ 0.6 ns
    • Rate per micro-strip ~ 2 MHz
  • New electronic + 25µm mesh
  • strip signal occupancy divided by 3

Munich MPI

straw tracker
Straw Tracker


  • can (in principle) operate in vacuum decay volume
  • can be designed without internal frames and flanges
  • can work in high rate of hits
  • good space resolution (~130 m/hit for 9.6 straw)
  • small amount of material (~0.1% X0 per view)


no previous large straw system has been

operated in high vacuum

Munich MPI

downstream straw tracker
Downstream straw tracker
  • 6 chambers with 4 double layers of straw tubes each ( 9.6 mm)
  • Rate: ~45 KHz per tube (max 0.5 MHz) (m+p)

2.3 m

Operate in high vacuum

Low X/X0



7.2 m

X/X0 ~ 0.1% per view


130 mm / hit

s(P)/P = 0.23%  0.005%P

s(q) ~ 50  20 mrad

Good space


7.2 m




2 magnets:

270 and 360 MeV Ptkick

5.4 m

8.8 m

5 cm radius beam holes

displaced in the bending

plane according to the

75 GeV/c beam path

Veto for charged

negative particles

up to 60 GeV/c

Munich MPI

rich layout
RICH Layout

Munich MPI

rich as velocity spectrometer
RICH as velocity spectrometer….

Resolution of a 17m P-326 RICH


Munich MPI

na48 lkr as photon veto

Energy of photons

from K+ p+p0

hitting LKr: > 1 GeV


NA48 LKr as Photon Veto

Consolidation of the

safety/control system and

read-out under way

Munich MPI

lkr efficiency measured with data


E=11 GeV


P=42 GeV/c

Cluster not


Eg = 22 GeV



LKr efficiency measured with data

K+p+ p0 collected by NA48 in 2004

Events are kinematically selected.

p+ track and lower energy g are use to

predict the position of the other g


Munich MPI

example hadronic cluster of a photon
Example: “hadronic” cluster of a photon

Expected energy: ~29 GeV

Deposited energy: ~9 GeV

Maximum energy ~300 MeV

Expected g position

  • Measured LKr inefficiency per photon (Eg > 10 GeV):
  • h = (2.8 ± 1.1stat ± 2.3syst) × 10-5 (preliminary)

Munich MPI

beam test 2006
Beam test 2006
  • Idea for measuring inefficiency in the range 2 GeV < Eg< 10 GeV
    • Use of the NA48 set-up.
    • Photons produced by bremsstrahlung.
    • SPS can provide a suitable electron beam.

Beam test foreseen during

the 2006 SPS run








Electron beam

(25 GeV/c)




  • Calorimeter inefficiency below Eg < 5 GeV is not critical

Munich MPI

anti photon rings
ANTI-Photon Rings

From: Ajimura et al., NIMA 552 (2005)

  • Two designs under test:
    • spaghetti (KLOE)
    • lead/scintillator sandwich (CKM)
  • Extensive simulation under way
  • A tagged photon beam is available in Frascati to test existing prototypes

Munich MPI

other physics opportunities
Other Physics Opportunities
  • The situation is similar to NA48, which was designed to measure “only” e’/e but produced many more measurements
  • Accumulating ~100 times the flux of NA48/2 will allow us to address, for instance:
    • Cusp like effects (p-p scattering)
      • K+ p0 p0 e+n
    • Lepton Flavour Violation

K+ p+ m+ e- , K+p- m+ e+, (Ke2/Km2)

    • Search for new low mass particles
      • K+ p+ X
      • K+p+ p0 P (pseudoscalar sGoldstino)
    • Study rare p+ & p0 decays
    • Improve greatly on rare radiative kaon decays
    • Compare K+ and K- (alternating beam polarity)
      • K+/- p+/-p0g(CPV interference)
      • T-odd Correlations in Kl4
    • And possibly, given the quality of the detector, topics in hadron spectroscopy

Munich MPI

status of p 326 a k a na48 3
Status of P-326 (a.k.a. NA48/3)
  • Presented at the CERN SPSC in September 2005
    • Strong endorsement of the Physics Case
    • Review of the proposed technique
  • 2006 R&D plan endorsed by CERN RB on December 05
    • Resources being appropriated
  • Beam Test foreseen in Sept-Oct 2006
    • Measure LKr efficiency for 1-10 GeV photons
    • Equip a CEDAR counter with fast read-out
  • Collaboration still open to new groups
    • RICH responsibility
  • Seeking full approval by end of 2006….
    • Enter CERN Medium Term Plan
  • …to be able to start data taking some time in 2009-2010

Munich MPI

  • Clear physics case
    • The discovery of New Physics will dramatically increase the motivation for searches of new flavour phenomena
  • Healthy competition worldwide:
    • J-PARC   SPS
  • Exploit synergies and existing infrastructures

NA48 e’/e

 NA48/1 KS rare decays

 NA48/2 Dg/g in K  3p

P-326 K+p+nn

  • SPS
    • SPS used as LHC injector (so it will run in the future)
    • No flagrant time overlap with CNGS
    • P-326 fully compatible with the rest of CERN fixed target because P-326 needs only ~1/20 of the SPS protons
  • Join us!

Munich MPI

spare slides
Spare Slides

Munich MPI

direct cp violation in k p p p k p p 0 p 0
Direct CP-violation in K+/- p+/- p+p- K+/-p+/- p0p0


u = (s3-s0)/m2;

v = (s2-s1)/m2;

si = (PK-Pi)2, i=1,2,3 (3=odd );

s0 = (s1+s2+s3)/3.

|M(u,v)|2 ~ 1 + gu + hu2+ kv2

  • Measured quantity sensitive to direct CP violation:

Slope asymmetry:

Ag = (g+-g-)/(g++g-)≠0

Centre of mass frame

u = 2mK∙(mK/3-Eodd)/m2;

v = 2mK∙(E1-E2)/m2.

SM estimates vary within

an order of magnitude

(few 10-6…8x10-5). Models

beyond SM predict

substantial enhancement

Munich MPI

selected statistics 2003
Selected Statistics 2003

M=1.7 MeV/c2

Data-taking 2003:

1.61x109 events selected



even pion

in beam pipe


K+ : 1.03x109 events

odd pion

in beam pipe


K: 0.58x109 events

Munich MPI


stability and systematics









Achromats: K+ Up




Achromats: K+Down

Stability and Systematics

Control of



Control of



Munich MPI

na48 2 2003 data
NA48/2 (2003 data)

K+/-p+/- p+p-

Slope difference:

Δg = (-0.7±0.9stat.±0.6stat.(trig.)±0.6syst.)x10-4 =


Charge asymmetry:

Ag = (1.7±2.1stat.±1.4stat.(trig.)±1.4syst.)x10-4 =


K+/-p+/- p0p0

Slope difference:

Δg = (2.3 ± 2.8stat. ± 1.3trig.(stat.) ± 1.0syst. ± 0.3ext.)x10-4 =

(2.2 ± 3.1)x10-4

Charge asymmetry: [using g0=0.638 ]

A0g = (1.8 ± 2.2stat. ± 1.0trig.(stat.) ± 0.8syst. ± 0.2ext.)x10-4 =

(1.8 ± 2.6)x10-4

hep-ex/0602014; PLB 634 (2006)

Order of magnitude improvement

Munich MPI

observation of p p scattering effect in k 3 p decays
Observation of p-p scattering effect in K→3p decays

NA48/2 has made the first observation the of the charge exchange process+00in the K00decay.

1 bin = 0.00015 GeV2

30M events


PLB 633 (2006) hep-ex/0511056





N. Cabibbo, hep-ph/0405001 PRL 93121801 (2004)

N. Cabibbo and G. Isidori, hep-ph/0502130 JHEP 503 (2005)

M2(00) (GeV/c 2)2

Munich MPI

difference between p p scattering length in i 0 and i 2 states
Difference between p-p scattering length in I=0 and I=2 states


PLB 633 (2006)


(a0 – a2)m+ = 0.268 ± 0.010(stat) ± 0.004(syst) ± 0.013(theor)

In agreement with theory (a0 – a2)m+ = 0.265 ± 0.004 (Colangelo 2001)

Munich MPI


RK=G(K+ e+n) /G(K+m+n)




Munich MPI

  • To provide pion/muon separation and beam sweeping.
    • Iron is subdivided in 150 2 cm thick plates (260  260 cm2 )
  • Two coils magnetise the iron plates to provide a 5 Tm field integral in the beam region
  • Active detector:
    • Strips of extruded polystyrene scintillator (as in Opera)
    • Light is collected by WLS fibres with 1.2 mm diameter

Pole gap is 2 x 11 cm V x 30 cm H

Coils cross section 10 cm x 20cm

Munich MPI

trigger daq
Trigger & DAQ
  • Total input to L0: 11 MHz
  • L0 (example):
    • > 1 hit hodoscope  73%
    • muon veto  24%
    • Photon Veto  18%
    • <2 EM quadrants & E<50 GeV  3%
  • L0 output:
    • 3% x 11 MHz = 330 KHz

Keep: L0 + Control + Calibration + Spin-offs < 1 MHz

  • L1 in PC farm (à la LHCb) to keep as much flexibility as possible
  • Software trigger reduction ~40

Important synergies with LHC

to be exploited: for instance, the LHCb

TELL1 board

Munich MPI


NA48: ’/






no spectrometer



NA48/1 KS

’/lower inst. intensity



NA48/1: KS


NA48/2: K


Direct CP-Violation established


Re e’/e = 14.7 ± 2.2 10-4

Ave: Re e’/e = 16.7 ± 2.3 10-4

+ KLRare Decays

First observation of

K0S→p0 e+e-and K0S→p0m+m-

  • Search for Direct CP-Violation
  • in charged kaon decays
  • pp scattering: PLB 633 (2006)
  • (a0-a2)m+= 0.268 +/- 0.017

NA48/2: K


Munich MPI

straw elements and design

12.5 m

0.2 m Al

Glue – 5m

9.6 mm

25 m

Gold plated Tungsten wire 30 m

2300 mm

3 coordinates

4 coordinates

2 coordinates

1 coordinate

5.4 m

5.4 m

10 cm

186.3 m

from T0

8.8 m

7.2 m

7.2 m

Straw Elements and Design

12 ns rise time

100 ns total width


spacer, 25 mg

Two double layers form a view

Gas mixture: 20%Ar+80%CO2

To fit easily into decay volume

an octagonal shape is proposed

k12hika+ (Niels) 

About 2000 * 6 -> 12000 straws in total

Munich MPI



Munich MPI

physics motivation

: spin 0

: left-handed (in SM)

[2] Decay Form of

(A) Sensitive to any hypothetical weakly-interacting neutrals.

(B) Decay into different neutrino flavors :

[3] Cosmological Interests

Neutron star cooling model through pion pole mechanism :

: Physics Motivation

p0 copiously collected from K+ p+p0

[1] Helicity suppressed decay

(A) Neutrino mass : implies .

(B) Neutrino type : Majorana neutrino x2 larger branching ratio.

Munich MPI

branching ratio

1/3 sample

2/3 sample Saturation at 3.5x106

Branching Ratio

Conservative upper limit

# signal < 113 (90%CL) subtracting the non-Kp2 bkgnds;

New upper limit (E949) :

A factor of 3 improvement from the previous best result.

Munich MPI