The NA48 Experiment: Lepton Universality Tests and New Prospects (P326) Cristina Biino INFN Torino On behalf of the NA48/NA62/P326 collaboration New Trends

The NA48 Experiment:

Lepton Universality Tests and New Prospects (P326)

Cristina Biino

INFN TorinoOn behalf of the NA48/NA62/P326 collaboration

New Trends in High-Energy Physics

Yalta, Crimea, Ukraine, September 15-22, 2007

Yalta 2007 2 Cristina Biino

IntroductionIntroduction

Violation of Lepton Universality is predicted by almost every new physics model.

Kaon Physics:

So far precision is poor, tests on levels of 1-2% (Kl3) or even 3-4% (Ke2/K2)

Kl2 decays Kl3 decaysKe2/K2Ke3/K3 , Vus


OverviewOverview

NA48 experiment: Recent results and prospectives on Lepton

Universality test in K l decays

Recent results from K 0l decays :– CKM matrix element |Vus| – Lepton Universality test

CERN P326: A proposal to measure the rare decay K+ +-


1. Lepton universality with Kl decays

– Physics motivations

– Results from NA48 data collected in 2003/2004

– Run 2007


Well predicted by the Standard Model, but must include radiative corrections: (Finkemeier, Phys. Lett. B387 (1996): RK = -(3.78 ± 0.04) % )

RK(SM) = (2.472 ± 0.001) ·10-5 0.04% precision!

The value of RK could be different in case of SUSY LFV. The difference wrt SM could be as large as ± 3% (Masiero, Paradisi,

Petronzio, Phys. Rev. D74 (2006) 011701). Measurement of RK test the /e universality and provides a

sensible test of the SM.

RK(PDG 2006) = (2.45 ± 0.11) ·10-5

From three experiments from the 70’s, with kaons at rest.

K

K

eKeK R

mm

mm

m

m

K

eKR

1

2

22

22

2

2

KKe2e2/K/K22 - Introduction - Introduction


A sample collected during 2003 data taking– 1 month of data taking using a downscaled trigger

A sample collected at the end of 2004 data taking in a special run– 56 hours of data taking with a very efficient minimum bias trigger

NA48 – Recent DataNA48 – Recent Data

NKe2raw - NKe2back Acc(K2) C

RK =

TrEff(Ke2) Acc(Ke2) Ce D NK2raw - NK2back

Where D is the downscaling for the K2 events and Cl represents the correction due to PID.

Most of the systematics cancel in the ratio of the partial widths.

The dominant contribution, the background subtraction error, scales with the statistics.


Similar kinematics allows to exploit systematic cancellations– Keep at maximum the same cuts

for both modes• 1 track topology• z of decay vertex• Geometric acceptance at

chambers and Lkr calorimeter• Track momentum

– Particle identification using ELKr/Pspectr

• Muons : E/P < 0.2• Electrons: E/P > 0.95

– Cut on the missing mass with the right lepton mass assignment

Ke2/KKe2/K2: Analysis2: Analysis


The dominant background is K2 – A muon loses almost

all its energy in the calorimeter, giving the E/p of the electron

– It is measured from the data in momentum bins

Ke3 contribution is obtained by MC

Data MC

Ke2/KKe2/K2: Background2: Background


• NA48 - 2004 data sample– 3407 ± 63st ± 54sy Ke2 events– The bigger systematics is the K2 background subtraction

RK = (2.455 ± 0.045 ± 0.041) • 10-5

• NA48 - 2003 data sample– 4670 ± 77st +29

-8 sy Ke2 events – Trigger efficiency is the bigger systematics

RK = (2.416 ± 0.043 ± 0.024) • 10-5

• KLOE – ~ 8000 Ke2 events from 1.7 fb-1

– Different systematics

RK = (2.55 ± 0.05 ± 0.05) • 10-5

Ke2/KKe2/K2: Recent Results2: Recent Results

Combine all preliminary results

and PDG2006: RK = (2.457 ± 0.032) • 10-5 2/ndof = 2.44/3


A dedicated run to measure RK has been approuved for 2007– 4 months of data taking

– Collect ~150,000 Ke2

– Goal to reach an error of ± 0.3%

Optimized beam parameters wrt 2004– Kaon momentum from 60 GeV/c to 75 GeV/c– Kaon momentum bite from 3 GeV/c to 2.5 GeV/c

– pT kick from the spectrometer magnet from 120 to 263 MeV/c

All these improve the kinematic separation of KAll these improve the kinematic separation of Ke2e2

Ke2/KKe2/K2: NA62 2: NA62 (ex NA48)(ex NA48) 2007 run 2007 run


It is mainly K2

• Below 35 GeV/c use kinematic separation– Build M2

miss with electron assumption

equal samples ofKe2 and K2

• Above 35 GeV/c use ELKr/ptr

to identify the electron- But 5•10-6 of muons experience a catastrophic energy-loss in the LKr and are mis-identified as electrons

Montecarlo simulation

Ke2/KKe2/K2:2: Background to Ke2Background to Ke2


Measure this background during the run

Put a 9 X0 lead bar between the hodoscope planes in front of the LKr– Lose 18 % of acceptance– All electrons are killed– Only muons pass– The E/p distribution of

the muons is measured

Ke2/KKe2/K2:2: Background to Ke2Background to Ke2


Ke2/KKe2/K2:2: ExpectationsExpectations

Expected precision on Rk

– Statistical 0.28% with 150,000 Ke2 decays

– Systematic 0.2%• Thanks to the measurement of the K2 background and to

the improved beam and spectrometer conditions

The 2007 dedicated Run:– It is going on since ~ the end of June– The detector has restarted well after 3 years– Data taking is quite smooth– As a reference, a good shift (8 hours) is giving

around 800 Ke2


2. Lepton universality and CKM matrix element |Vus| with the

K0l decays

– Measurements of BR(K0e) and of BR(K0)

– Extraction of |Vus|

– |Vus| and Lepton Universality re


NA48: Extraction of |Vus| from NA48: Extraction of |Vus| from KKl3l3

(Kl3) = BR(Kl3)/K =

GF2

3843mK5 SEW |Vus|2 |f+(0)|2 IK (1+K)

Short-distance radiative correction

The measured quantity

Phase space integral containingform-factor parameterization

Long-distance correction(isospin symmetry breaking)

NA48 Data sample: ~3 days of dedicated running (very soft trigger) in 2003

Analysis method: measurement of BR(Ke3)/BR(K0), BR(K3)/BR(K0) normalization to a large and well-know mode with a similar signature.

Decay channel K+ K– Background level

K0e 56,195 30,898 ~10–4

K0 49,364 27,525 ~0.2%

K0 461,837 256,619

Event samples collected


NA48:NA48: KKl3 and |Vus|: resultsl3 and |Vus|: results

BR(Ke3)/BR(K0) = 0.24700.0009stat.0.0004syst.

BR(K3)/BR(K0) = 0.16370.0006stat.0.0003syst.

Both measurements above the PDG’06 average, but agree with other experiments normalizing to 0; Comparison to KLOE: a hint that the PDG’06 value of BR(0) is biased (waiting for KLOE 0!)

BR(Ke3) = (5.1680.019stat.0.008syst.0.030norm.)%BR(K3) = (3.4250.013stat.0.006syst.0.020norm.)%

BR(Ke3) measurements

PDG’06

BNL E8652003

KLOE(2007 absolutemeasurement)

ISTRA+2007

NA48/22007

%

PDG’06

KLOE(2007 absolutemeasurement)

NA48/22007

BR(K3) measurements%

Eur. Phys. J. C50 (2007) 329 + erratum

Assuming Br(K2)

from PDG


NA48:NA48: KKl3 and |Vus|: resultsl3 and |Vus|: results

Errors dominated by those of long-distance corr.

|Vus| = 0.2277 0.0013other 0.0019th. == 0.2277 0.0023

Combined Ke3 and K3 result: compatible to CKM unitarity

Error dominated by the theoretical uncertainty of f+(0)

|Vus|f+(0) = 0.2193 0 .0012 [Ke3]

= 0.2177 0.0013 [K3] = 0.2188 0.0012 [Kl3]

|Vus|unitarityf+(0) = 0.2185 0.0022

|Vud| = 0.9738 0.0003|Vub| = (3.60 0.7) 10-3

f+(0) = 0.961(8)

Marciano & Roos ‘84

Leutwyler & Roos ‘06


|Vus| and Lepton Universality: |Vus| and Lepton Universality: resultsresults

One can evaluate :

for the K mode:

re = 1.0059(87)

using 2004 BRs

re = 1.019(13)

(Re)obs 3 Ie3 (1+e3) [ |Vus| f+(0) ]23,obs ( G

F )2

re = = =

(Re)SM e3 I3 (1+3) [ |Vus| f+(0) ]2e3,obs ( Ge

F )2

I : integral of form factors over phase space

(1+) : form factor corrections for SU(2) breaking and long distance EM interactions

for the KLS mode:

re = 1.0039(56)

using 2004 BRs

re = 1.054(15)

for the l mode:

re = 0.9966(30)

Erler, Ramsey-Musolf ‘06

Results from Flavianet Working Group on Kaon Decays


3. P326 : a proposed experiment to measure the rare decay K+ at the CERN SPS

– Theoretical prediction and experimental situation

– P326 proposal: • beam and detector challenges

• experimental techniques and background rejection


CP

Standard Model predictions

BR(K++) (1.6×10-5)|Vcb|4[2+(c-)2] (8.0 ± 1.1)×10-11

BR(KL0) (7.6×10-5)|Vcb|42 ± 0.6×10-11

Golden modes

FCNC loop processes

Short distance dynamics dominated

One semileptonic operator, hadronic matrix element related to measured quantities

Present measurement (E787/949): BR(K+) = 1.47 × 1010 (3 events)+1.30

-0.89

KK++→→++ : a theoretical clean : a theoretical clean environment sensitive to new environment sensitive to new

physicsphysics


P326P326 a 10% new measurement (100 a 10% new measurement (100 eventsevents))

CERN-SPSC-2005-013 SPSC-P-326

September 2005: presented at CERN SPSCDecember 2005: R&D endorsed by CERN Research BoardStart of test beams at CERN in 20062007: prototypes construction and test beams at CERN and Frascati2008 – 2010: Technical design and constructionStart of data taking 2011

Schedule

Located in the same hall as NA48

CERN, Dubna, Ferrara, Firenze, Frascati, Mainz, Merced,

Moscow, Napoli, Perugia, Pisa, Protvino, Roma,

Saclay, San Luis Potosi, Sofia, Torino, Triumf

Proposal to Measure the Rare Decay K at the CERN SPS


P326 guidance P326 guidance principlesprinciples

O(100) K++events ~ 10% background

Kaon decay in flight techniqueIntense proton beam from SPSHigh energy K (PK = 75 GeV/c) Kaon ID (CEDAR)

Kinematical rejection

Kaon 3-momentum: beam trackerPion 3-momentum: spectrometer

KK+

m2miss=(PKP)2

Veto and particle ID/ detection: calorimetersCharged veto: spectrometer//e separation: RICH

BR(SM) = 8×10-11

Acceptance 10%~ 1013 K+ decays in 2 years


Kinematics and Kinematics and BackgroundsBackgrounds

92% of total background

Allows us to define a signal region K+ +0 forces us to split it into

two parts (Region I and Region II)

Span across the signal region Rejection must rely on vetoes

Kinematically constrained Not kinematically constrained

8% of total background


The Beam: narrow band, high The Beam: narrow band, high intensityintensity

• P Kaon = 75 GeV/c (P/P ~ 1.1%)• Fraction of K+ ~ 6.6%• Negligible amount of e+

• Beam acceptance = 12 str (×25 NA48/2)• Area @ beam tracker = 58×24 mm2

• Integrated average rate = 760 MHz• K+ decays / year = 4.8 × 1012

• P proton = 400 GeV/c• Proton/pulse 3.3×1012 (×3.3 NA48/2)• Duty cycle 4.8/16.8 s

Primary beam Secondary beam

Vertical

viewDECAYDECAY

VOLUME VOLUME (~100 m)(~100 m)

K+ decay rate: ~11 MHz

p

Achromat 1

Quadrupoles

Radiator

Achromat 2Cedar

ScraperGigatracker

20 mm

5 m

Target


The P326 layoutThe P326 layout

Technical Design Report in preparation

VACUUM10-6 mbar

50 MHz

800 MHz 11 MHz

K+ ~ 75 GeV

+


The tracking systemThe tracking system

The Gigatracker (i.e. the beam spectrometer)

The Spectrometer (i.e. the downstream tracker)

3 Si pixel stations across the 2nd achromat: (60 × 27 mm2)

4 chambers with 4 double layers of straw tubes each ( 9.6 mm) Rate: ~ 45 KHz per tube (max 0.8 MHz)

Low X/X0 0.1% X0 per view in vacuum

Good hit space resolution 130 m per view

Moment p measurement 1 magnets (270 pt kick)

Veto for charged particles 5cm radius beam hole displaced in the bending plane according to beam path

m2miss resolution ~1.1×103 GeV2/c4

main contribution from θK measurement

θK

p

pK

θK

θ

m2miss

resolution

8.8

m

7.2

m2.1

m

Time resolution (ps)

Sig

nal

/Bac

kgr

oun

d

(PK)/PK ~ 0.22%(K) ~ 16 rad(

(

Rate 760 MHz (charged particles) ~ 50 MHz / cm2

300×300 m pixels

200 Si m sensor + 100 Si m chip Low X/X0

Readout chip bump-bonded on the sensor (0.13 m technology)

Excellent time resolution needed for K+/+ association (t)~200 ps / station

Prototype Test Beam in 2007


The The particle ID systemparticle ID system

18 m long tube ( 2.5 m ), 17 focal lentgh mirrors

CEDAR: existing Cerenkov counter at CERN The CEDAR (i.e. the kaon ID)

The RICH (i.e. the pion ID)

Beam Composition

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

1650 1700 1750 1800 1850 1900 1950 2000 2050

Pressure

%

p

K

Beam composition

Adapted to P326 need:

Vary gas pressure and diaphragm aperture to select Kaons

• H2 instead of Ne

• New PMs and electronics

Ne @ 1 atm (thr = 13 GeV/c)

>3 / separation up to 35 GeV/c

High granularity (2000 PMTs)

Small pixel size (18 mm PMT)

Disentangle pileup in Gigatracker (t)~100 ps

PMTs tested in 2006

Prototype Test Beam in 2007


The Veto systemThe Veto system

Large angle (10-50 mrad): 13 ANTISTwo options under test: Lead-scintillator tiles or blocks or Lead-scintillator fibers (KLOE-like)

Photon vetoes

• Rings calorimeters (in vacuum)

• Rate: ~4.5 MHz () + ~0.5 MHz () (OR 12)

• 10-4 inefficiency for 0.05 < E< 1 GeV

• 10-5 inefficiency for E> 10 GeV Medium angle (1-10 mrad): NA48 LKr Calorimeter

New Readout• Rate: ~8.7 MHz () + ~4 MHz () + ~4 MHz ()

• 10-4 inefficiency for 1 < E< 5 GeV

• 10-5 inefficiency for E> 1 GeV

Inefficiency for E> 10 GeV tested on NA48/2 data (K+→)

Small angle (< 1 mrad): Shashlik technology

• Rate: ~0.5 MHz ()

• 10-5 inefficiency for high energy photons

Sampling calorimeter + Magnet for beam deflection

Muon veto and sweeper

• Rate: ~7 MHz () + ~3 MHz ()

• 10-5 inefficiency for detection

• Sensitivity to the MIP• em/hadronic cluster separation• 5Tm B field in a 30×20cm2 beam hole: deviate the beam out from the SAC


Preliminary sensitivity Preliminary sensitivity studiesstudies

Acceptance (60 m fiducial volume):

Region I: 4% Region II: 13% Total: 17%

To be reduced after analysis cuts(losses due to dead time, reconstruction inefficiencies…)

Simulation of the P-326 apparatus

Acceptance ~ 10% is achievable

Region I and II Momentum range: 15 < P < 35

GeV/c Against muons RICH operational reasons Plenty of energy in photon vetoes

Remind:

K m2miss = 0.0182 GeV2/c4

K m2miss

< 0


Signal and BackgroundsSignal and Backgrounds

Events/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 ~2Other 3 – track decays

~1 negligible ~1

K++0 1.3 negligible 1.3

K++ 0.5 0.2 0.2

K+e+(+)0, others

negligible

Total bckg. 9 3.0 6 S/B ~ 8 (Region I ~5, Region II ~9)


ConclusionsConclusions

P326 experiment: Search for new physics using the rare Kaon decay K+→+ A new experiment (~10-12 sensitivity per event) at an existing machine and employing the infrastructures of an existing experiment General design mostly defined. Overall simulation and performances under review R&D program: construction of detector prototypes and test in progress. Important results by end of 2007

An accurate measure of Kl2 decays can provide a sensitive test on SM and e/ universality. NA48 experiment: has presented two measurements compatible with SM predictions. NA62 experiment: a dedicated run is under way to collect 150,000 Ke2 and measure RK with 0.3% accuracy.

NA48 experiment: measured ratios of decay rates of Ke3 and K3 to K2 are presented. Both measurements are found to be larger than the current values given by PDG and lead to a larger magnitude for |Vus|. When combined with the latest PDG value of |Vud| the result is in agreement with unitarity of the CKM matrix. A lepton universality test is computed and a comparison to other kaon channels is done.

Documents

The NA48 Experiment: Lepton Universality Tests and New Prospects (P326) Cristina Biino INFN Torino On behalf of the NA48/NA62/P326 collaboration New Trends