Status of Ag measurement in NA48/2Status of Ag measurement in NA48/2
DANE 2004Luca Fiorini
Scuola Normale Superioreand INFN Pisa
on behalf of NA48/2 collaboration:Cambridge, CERN, Chicago, Dubna, Edinburgh, Ferrara, Firenze, Mainz, Northwestern, Perugia, Pisa, Saclay, Siegen, Torino, Vienna
Status of Direct CP violation measurement
in K±3 @NA48/2
– Search for direct CP violation in:
K± ±+-
K± ±00
– Data for present analysis:● collected from August 6 to September 7, 2003● divided in 3 supersamples (self-complete data set ~ 2
weeks period where all magnet exchange permutations have been done)
– Additional data (not included in present analysis):● 2 tracks events (K3 with lost pion)● June-July 2003 data with different systematic effects
Main Goal and Data samples
Direct CP violation @ NA48/2 2
Measurement PrinciplesKinematics Dynamics
Lorentz-invariantssi = (PK-Pi)2, i=1,2,3
s0 = (s1+s2+s3)/3u = (s3-s0)/m
2
v = (s2-s1)/m2
-1.2 < u < 1.2
Mk
p1
p2
p3
Matrix Element (K± 3)|M(u,v)|2 ~ 1 + g±u + hu2 + kv2
CP Violation:Ag = (g+-g-)/(g++g-)≠0
Charged mode is statistically favored (~4:1)but neutral mode has a larger absolute value of g:• gc = -0.2154 ± 0.0035 (K± ±+-)
• gn = 0.652 ± 0.031 (K± ±)
If acceptances are equal for K+ and K-, Ag can be extracted by a linear fit
N(u,K+,Bup) = R(u) = N(1+g+u)/(1+g-u) ~ N(1+2gAgu)
N(u,K-,Bdown)
If acceptances are equal for K+ and K-, Ag can be extracted by a linear fit
N(u,K+,Bup) = R(u) = N(1+g+u)/(1+g-u) ~ N(1+2gAgu)
N(u,K-,Bdown) Direct CP violation @ NA48/2 3
Asymmertry: Theory Vs. Experiment
Theoretical predictions for Ag:
(2~4) ·10-4 Belkov(1989-1995)(2.3± 0.6) · 10-6 Maiani (1995)~ 10-5 Scimemi (2003)A
gc=A
gn · [1+O( )] = A
½/A
3/2 ~ 1/20
Experimental Results:BNL(1970): A
gc= (-7.0±5.3) · 10-3
FNAL, HyperCP (2000): Ag
c= (2.2±1.5±3.7) · 10-3 (Choong, PhD
Thesis)
Protvino, ISTRA+: Ag
n= (-0.3±2.5) · 10-3 (Denisov, Frontier Science
2002)
NA48/2 goalA
g < 2·10-4
Direct CP violation @ NA48/2 4
NA48/2 Beam Line
114m decay volume
Beams steered to coincide within <1mm at DCH1
Simultaneous K+ and K- focused beams with momentum 60±3 GeV/c
z
y
1.3·1011p/s
Direct CP violation @ NA48/2 5
NA48/2 Detectors
Main detector components:
•Kabes (MicroMega TPC)1mm stripsMax rate: 2MHz/strip
X = 50 m (drift) Y = 80 m (strips) t = 0.7 ns
•Magnetic spectrometer (4 DCHs): p/p(KBS) p/p(DCH) ~ 1% (60 GeV K)
•Liquid Krypton EM calorimeter (LKr) E/E = 3.2%/E 9%/E 0.42%
•Hadron calorimeter, photon vetos, muon veto counters
Direct CP violation @ NA48/2 6
NA48 Strategy for Ag
Spectrometer magnet current inversion: every 24h (every ~3h in 2004)Achromat magnet current inversoin: every 7 days
BASIC PRINCIPLE OF THE MEASUREMENT(K+,B up/down) and (K-,B down/up) acceptances are equal and cancel out in the
ratio.
Deviations from the principle
● Bup ≠B
down Imperfect field inversion Hall probes to measure magnet field
● Non inversable mag. field (Earth field and res. magn. of the beam iron pipe) Measured in spring 2003 (10-4 p
kick), corrected at software level.
● Imperfect K+ K- beams coaxiality Acceptance cut around COG position.● Since UP and DOWN conditions are realized at different time, time instability right-left asymetric of detectors or trigger is dangereus. Direct CP violation @ NA48/2 7
Track Momentum Correction● and corrections applied:
P=p(1+)(1+qbp)P – corrected track momentum
p – observed track momentum
q – charge of the track
b – sign of magnetic field
● : corrects the magnetic field by putting the value (M+
3+M3)/2 to PDG value of K mass
● : corrects the misalignment (DCHs relative movement) by putting the difference between M+
3 and M3 to zero
Day-sample
b(M3-M+
3), keV
70 mDCH shift
K±±+-
Direct CP violation @ NA48/2 8
Kaon spectra and StatisticsK±±+-
Events
Events K+
K-
M3π, GeV/c2
Mass Resolution: 1.7 MeV
ALLK-K+
720
120
290
310
1120400ALL
19070SS3
450160SS2
480170SS1
Events per supersample (in 106)
Direct CP violation @ NA48/2 9
Acceptance CancellationsK±±+-
Physical asymmetries:● AS → slope of ratio N(K+,B
up)/N(K-,B
down)
● AJ → slope of ratio N(K+,B
down)/N(K-,B
up)
Apparatus-induced asymmetries: ● A+ → slope of ratio N(K+,B
up)/N(K+,B
down)
● A- → slope of ratio N(K-,B
up)/N(K-,B
down) Z
axis
(beam
dir
ect
ion)
Sale
ve
Jura
X axis
Top view of the setup
ASJ = (AS+AJ)/2 physics asymmetry
A± = (A++A-)/2 = (AS-AJ)/2 asymmetry induced by experimental setup
[ many of the effects observed in A± cancel in ASJ ]
Direct CP violation @ NA48/2 10
Asymmetry Stability Vs. K momentum
K±±+-
Statistical error of A : 2,7.10-4 Statistical error of Ag: 2,7.10-4
2/ndf 6/11
ASJ + offset = 0 ± 0,117.10-3
2/ndf 6,7/11
A = (0,131 ± 0,117).10-3
PK, GeV/c PK, GeV/c
AS+offsetAJ+offset
A+A-
(AS+AJ)/2 + offset
(A++A-)/2
Direct CP violation @ NA48/2 11
Asymmetry Stability Vs. TimeK±±+-
2/ndf 5,6/12
2/ndf 13,5/12AS+offsetAJ+offset
A+A-
(AS+AJ)/2 + offset
(A++A-)/2
Day-sample pair Day-sample pair
Day-sample pairDay-sample pairDirect CP violation @ NA48/2 12
Conclusions
● More than109 K±±+- decays in 1 month of 2003 data taking● The statistical error of Ag at this level of analysis is 2.7·10-4
● Major sources of systematics identified, up to now the error dominated by statistics.
● About 4·107 K±±00 decays have been collected. Very early stage of analysis of the CP-violation in this mode
● The statistical error of Ag is about 5·10-4
● RUN 2004, 60 days of data taking are on progress.
● NA48/2 is NOT only direct CP violation, many other interesting analysis on progress
Ke2
, Ke3
, Ke4
,
K±±+-
K±±
Direct CP violation @ NA48/2 13