Recent CP violation measurements at CERN-NA48 experiment For the NA48 and NA48/2 collaborations:...
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Transcript of Recent CP violation measurements at CERN-NA48 experiment For the NA48 and NA48/2 collaborations:...
Recent CP violation measurements at
CERN-NA48 experiment
For the NA48 and NA48/2 collaborations: Cagliari, Cambridge, CERN, Chicago, Dubna, Edinburgh, Ferrara, Firenze, Mainz, Northwestern, Orsay,
Perugia, Pisa, Saclay, Siegen, Torino, Vienna, Warsaw
Cristina BiinoINFN, Torino
ICHEP 2008Philadelphia, Jul.29-Aug.5, 2008
ICHEP08 Cristina Biino 2
A brief history of NA48
2007 Ke2/K
2 runNA62
• NA48 (1997-2001):Direct CP-Violation in neutral K
Re(ε’/ε) = (14.7 ± 2.2)·10-4
• NA48/1 (2002):Rare KS decays and hyperons
• NA48/2 (2003-2004):Direct CP-Violation in charged K
• NA62 (2007-2008) and P326 (2008…): R(Ke2/K2), and proposal K+ +
KTeV (2008) 19.2 ± 2.1 10-4
ICHEP08 Cristina Biino 3
NA48 detector
1 m
Decay productsFrom K decays
• Magnetic spectrometer
4 high-resolution DCHs
• Scintillator hodoscope
150 ps time resolution - fast trigger
• LKr electromagnetic calorimeter Quasi-homogeneous, high granularity,
excellent e/ discrimination
• Hadron calorimeter, veto counters, vetos
p/p = (1.0 0.044 p)% (p in GeV/c)
E/E = (3.2/√E 9.0/E 0.42)% (E in GeV/)
ICHEP08 Cristina Biino 4
CP violation in K0 decays:
Measurement of | | with KL → decays
ICHEP08 Cristina Biino 5
The CP violation parameter |
|
= + ’ = A(KL +–)
A(KS +–)
is a fundamental observable of CP violation
Indirect CPV Direct CPV
Experimental method:
Measure precisely the ratio R = BR(KL +–) / BR(KL e). Extract | | computed as: =
using the best single KS and KL lifetimes measurements and normalisation BR(KL e) and BR(KS +–) from KLOE, KTeV and NA48.
BR(KL +–)BR(KS +–)
KS
KL
KL ~ K2 + K1
CP = +1CP = 1
CP(+–) = + 1
KS ~ K1 + K2
ICHEP08 Cristina Biino 6
| |: event selection
KL →
CP-violating process need to suppress dominant background channels (Ke3, K3, K3 ) by 4-5 orders of magnitude small background achieved: 0.5% data well described by MC about 47k events selected
selection based on electron identification in the LKr calorimeter ( E/p ≈ 1) about 5 x 106 Ke3 events selected with 0.5% background
KL e
Dedicated 2-day run with a pure, low intensity KL beam: ~80 x 106 2-tracks events recorded.
ICHEP08 Cristina Biino 7
Results on | |
Uncertainty source Correction Uncertainty
Particle ID +1.34% 0.05%
K2 background –0.49% 0.03%
Muon ID +0.48% 0.18%
Trigger –1.29% 0.11%
Energy spectrum 0.20%
Radiative corrections
0.10%
MC statistics 0.10%
Total +0.04% 0.33%
Experimental value:
R = BR(KL +–) / BR(KL e ) = (4.835 0.022stat. 0.016syst.) 10–3
BR(KL+–)=(1.941 0.019) 10–3
Published in Phys. Lett. B645 (2007) 26
Radiative corrections: - +– Inner Bremsstrahlung (IB) component included
- direct emission (DE) component subtracted (CP conserving)
Corrections and systematics on R
CP violation parameter:
= (2.223 0.012) 10–3 BR(KL+–)
BR(KS+–)|+-| = KL
KS
ICHEP08 Cristina Biino
| |: comparison of results
The NA48 result is in very good agreement with KTeV and KLOE experiments
These new measurements contradict the PDG’ 04 value !
BR(KL+–) / BR(KLe) [10–3] |+–| [10–3]
ICHEP08 Cristina Biino 9
Search for direct CP violation:
Measurement of the Ag and Ag
0 asymmetries in K and K 0 0 decays
ICHEP08 Cristina Biino 10
CP Violation in K± 3 decays
-
-g
g g ΔgA
g g 2g
+
+
-= =
+
|M(u,v)|2 1 + gu + hu2 + kv2 + …
Dalitz plot forK± 3
Only direct CP violation is involved in charged kaon decays!
Matrix element:
Asymmetry parameter:
K± : g = - 0.21134
0.00017
K± : g = 0.626 0.007
If Ag 0 Direct CP Violation
2even
1even
3oddK±
SM predictions: |Ag| : 10- 5 - 10- 6
If Ag ~ 10-4 New Physics !
u = 2mK(mK/3-E3*)/m
2
v = 2mK(E1*-E2
*)/m2|h|, |k| << |g|
If CP invariance holds: gK+ = gK-
ICHEP08 Cristina Biino 11
The NA48/2 approach
• NA48/2 method:- Use of intense simultaneous K+ and K- beams, superimposed in space, with narrow PK spectra
- Equalization of averaged K+ and K- acceptances by frequent alternations of magnet polarities (magnetic spectrometer, K beam lines)
- Measurement of asymmetry from slopes of normalized u-distribution ratios (no MC acceptance correction required! ) :
R(u) = N+(u) / N-(u) n ( 1+g u)
Ag = g / 2g
Induced instrumental asymmetry can only be due to charge-asymmetric and u-dependent effects that vary with time
(-70 53) 10-4 BNL AGS (1970)
Ag=
( 22 40) 10-4 HyperCP (2000)prelim.
{ (19 125) 10-4 CERN PS (1975)
Ag0 =
(2 19) 10-4 Protvino IHEP(2005)
{
• NA48/2 main goal: Measure linear slope (g) asymmetries for K
and K decays with accuracies Ag 2.2 10-4 and Ag0 3.5
10-4 respectively. significant improvement w.r.t. present measurements
Statistics required >2109 events in “charged” mode and >108 in “neutral” mode
Selection of K events
U
|V| even pion
in beam pipe
2.00x109 K+
events1.11x109 K– events
K+/K– ≈ 1.8
odd pionin beam pipe
Invariant mass
Based only on Hodoscope and Magnetic Spectrometer
Negligible
background
m = 1.7 MeV/c2
Events 3-tracks vertex in decay volume
Acceptance limited mostly by beam pipe through DCHs K±
ICHEP08 Cristina Biino 13
Acceptance equalization for K+ and K-
Fit of R(u) is sensitive to the time variation of asymmetries in experimental conditions with characteristic time smaller than corresponding field alternation periods (beam week,detector day (2003), 3 hours(2004 ) data taken in 9 SUPER-SAMPLES of ~2 weeks each
Cancellation of systematic biases: - Detector L-R asymmetries ( K+ Salève / K- Salève and K+ Jura / K- Jura ) - Beam line biases ( K+ beam Up / K- beam Up and K+ beam Down / K- beam Down) - Global time-variable biases (K+ and K- simultaneously recorded)
ZZ
XXYY
Achromats: KAchromats: K+ + UpUp
Achromats: KAchromats: K++ DownDown
B+B+
B-B-
Detector Left-Right asymmetry cancellation in the four K+/K ratios:
US
N ( , ,u)R (u)
N ( , ,
A
u)
B
BA
+
-
+ +-+
=
SD
N ( , ,u)R (u)
N ( , ,u)
A
B
B
A
+
-
+-- -
=
JU
N ( , ,u)R (u)
N ( , ,u)
A
B
B
A
+
-
-++ +
=
Jura Jura (Left)(Left)
SalèveSalève(Right)(Right)
Quadruple ratio:
R(u) = RUS(u)·RUJ(u)·RDS(u)·RDJ(u) ~ n· (1 + 4 Δg u) DJ
N ( , ,u)R (u)
N ( , ,
A
u)
B
BA
+
-
- -+-
=
♦
♦
♦
♦
ICHEP08 Cristina Biino 14
K± → ± : Ag final result
Δg = (0.6 ± 0.7stat. ± 0.4stat. (trig.) ± 0.5syst.) 10-4
K+/K-
Result compatible with SM predictions No evidence for New Physics
Published in Eur. Phys. J. C 52, 875 (2007)
Δg
9 super-samples give consistent results
Detector and beam line asymmetry effects at 10-4 level, reproduced by MC
Ag = (-1.5 ± 2.1) 10-
4A factor ~ 20 better precisionthan previous experiments
Systematic effects on Δg
Δg
(10-4)
Spectrometer alignment ± 0.1
Spectrometer magnetic field
± 0.3
Beam geometry and stray magnetic fields
± 0.2
Resolution and fitting ± 0.2
Accidental activity
(pile-up)± 0.2
Total systematics ± 0.5
L1 trigger efficiency ± 0.3
L2 trigger efficiency-0.1 ±
0.3
Time stability of result:
ICHEP08 Cristina Biino 15
Selection of K 0 0 events
Consider two 0 → decays which define
K± decay vertex position
- no geometrical information from the ± track to avoid charge-asymmetric biases - kaon momentum and invariant 3 cuts
Computation of u variable from the
0 0 invariant mass only:
charge-symmetric procedure
Based on Charged Spectrometer and LKr Electromagnetic Calorimeter
m = 0.9 MeV/c2
|v| Odd pion
in beam pipe
and wrong photon
pairings
M(0)2 EiEk (Dik)2/(Zik)2
9.13 107 events
K±
uEven
ts
M(3) [GeV/c2]
ICHEP08 Cristina Biino 16
K± → ± 0 0 : Ag0 final result
Statistical precision in Ag0 similar to K± → ±
mode despite 34 times less events: |g0 / g±| ~ 3.
Ag0 = (1.8 ± 1.8) x 10-4
Systematic effects on Δg
Δg
(10-4)
Overlap of LKr showers ± 0.5
L1 HOD trigger efficiency ± 0.1
L1 LKr trigger efficiency ± 0.1
L2 trigger efficiency ± 0.3
Stray magnetic fields ± 0.1
Accidental activity(pile-up) ± 0.2
Total ± 0.6
Final result consistent with SM predictions:
Published in Eur. Phys. J. C 52, 875 (2007)
Δg = (2.2 ± 2.1stat. ± 0.6syst.) 10-4
Time stability of result:
K+/K-
Δg
7 super-samples give consistent results
Detector and beam line asymmetry effects at 10-4 level,reproduced by MC
ICHEP08 Cristina Biino 17
|Vus | Measurement from K±l3
ICHEP08 Cristina Biino 18
|Vus | Measurement from K ± l3
K± collected during a special low intensity minimum bias run
Measurement of leptonic and semileptonic decays.Method of Measurement:
• Normalize Ke3 and K3 to K2 very similar topologies and selection criteria• Select one track and two photons, consistent with a 0, from a common decay vertex• Distinction of Kl3 and K2 mainly through kinematics• Particle identification for electrons ( ~ 98.6%), pions ( ~ 99.5%) and muons ( ~ 99.8%)
ICHEP08 Cristina Biino 19
|Vus | Measurement from K ± l3
Channel Acc x P-Id K+ (K-)
Ke3: 56k (31k)
Kμ3: 49k (28k)
π± π0 462k (257k)
Acceptance x Particle ID K+
(K-)
6.98 ± 0.01 % (6.94 ± 0.01)%
9.27 ± 0.01 % (9.25 ± 0.01)%
14.18 ± 0.01 % (14.12 ± 0.01)%
Background
< 0.1%
~ 0.2%
~ 0.3%
ICHEP08 Cristina Biino 20
|Vus | Measurement from K ± l3
RKe3/K2 = 0.2470 0.0009stat 0.0004sys
RK3/K2 = 0.1636 0.0006stat 0.0003sys
• Quadratic expansion for f+(t) (’+ = 0.02485(163), ”+ = 0.00192(62)) and linear expansion for f0(t) (0 = 0.00196(34) from PDG2006 • Form factor variations within their errors and differences to pole model parametrization taken as systematic uncertainties
Results:
Sistematics:
Ke3/K2: Mainly Ke3, K2 acceptance, trigger efficiency
K3/K2: Mainly K3 form factors and Ke3, K2 acceptance
Accuracy 0.4%
Published in Eur. Phys. J. C 50, 329 (2007) & erratum
Form factors:
Using new KLOE measurement of
BR(K2()) = 0.2065(5)(8)
BR(K± e3) = 0.05104 0.00019stat 0.00008sys
BR(K±3) = 0.03380 0.00013stat
0.00006sys
ICHEP08 Cristina Biino 21
|Vus | Measurement from K ± l3
Ke3 : |Vus | f+(0) = 0.21794(43)exp (52)norm, (61)ext = 0.2179(9)
K3 : |Vus | f+(0) = 0.21818(46)exp (52)norm, (66)ext = 0.2182(10)
Use K = 12.370(19) ns (average PDG2006 & Kloe
2008), ......
Very good agreement between Ke3 and K3
Test of lepton universality:
Ratio(K3/Ke3) = 0.663 0.003stat 0.001sys
Most precise measurement so far and consistent with lepton
universality (SM prediction = 0.661(3))
Combination of Ke3 and K3 (taking into account correlation) :
| Vus | f+(0) = 0.2180 0.0008and finally using f+(0) = 0.964 0.005
(RBC-UKQCD’07) :
| Vus | = 0.2261 0.0014
Determination of |Vus |:
ICHEP08 Cristina Biino 22
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