Upload
isabel-miles
View
217
Download
0
Embed Size (px)
Citation preview
Ke2/Kmu2 and LFV tests
Crimea2008 Yalta 27/09/08 – 04/10/08
Andrey Zinchenko JINR, Dubna
On behalf of NA62 Collaboration:Bern, Birmingham, CERN, Dubna, Fairfax, Ferrara,
Florence, Frascati, Mainz, Menlo Park, Merced, Moscow, Naples, Perugia, Protvino, Pisa, Rome,
Saclay, San Luis Potosi, Sofia, Triumf, Turin
K+→e+νe in the Standard Model
In the Standard Model(SM), the decay K+→e+νe (Ke2) is strongly helicity-suppressed compared to the muonic channel K+→μ+νμ(Kμ2). The positron e+ must be left-handed due to angular momentum conservation – thus the coupling of the W+-Boson is suppressed by (1– β). In the SM, the ratio of the decay rates can be predicted very precisely and is given as
5
2
22
222
10)001.0477.2()1()(
)(
QEDK
eKeeK R
mm
mm
m
m
K
eKR
where δRQED= -3.6% is a correction due to Kl2γ(IB) and virtual photon processes (Cirigliano, Rosell, JHEP 0710:005, 2007).
K+→e+νe in Supersymmetry
In Supersymmetry (SUSY), K+→e+νe can proceed via exchange of a charged Higgs H+ instead of W+, but in this case, at tree level, the ratio Γ(K+→e+νe)/Γ(K+→μ+νμ) remains unchanged.
However, loop effects are predicted to lead to lepton flavour violating (LFV) couplings lH±ντ, which noticeably change only the rate of K+→e+νe
(Masiero, Paradisi, Petronzio, PRD 74, 2006):
The LFV parameter Δ13 should be of o(10-4 – 10-3) (similar to LFV in the neutrino sector). For given values of MH
± and tan β, deviations up to a few percent on RK are possible.
As a result of the helicity suppression of the SM contribution, the Ke2 decay rate is sensitive to New Physics.
Tree-level diagram for K+→l+ν
Loop diagram for K+→e+ντ
62
132
2
4
4
tan1eH
KSMK
SUSYK M
m
M
mRR
Entering the precision realm for RK
Main actors (experiments) in the challenge to push down precision on RK:KLOE
Preliminary result with 2001 – 2005 data: RK = (2.55 ± 5stat ± 5syst)×10-5
from 8000 Ke2 candidates (3% accuracy), perspectives to reach 1% errorafter analysis completion.NA48/2
Preliminary result with 2003 data: RK = (2.416 ± 43stat ± 24syst)×10-5, from~ 4000 Ke2 candidates, statistical error dominating (2% accuracy).
Preliminary result with 2004 data: RK = (2.455 ± 45stat ± 41syst)×10-5, from~ 4000 Ke2 candidates from special minimum bias run (3% accuracy).NA62Collected ~ 100,000 Ke2 events in dedicated 2007 run, aims at breakingthe 1% precision wall, possibly reaching < ~ 0.5 %.
Experimental Status
PDG2006 + three new preliminary measurements by NA48/2 and KLOE give 1.3% total accuracy and are in agreement with SM prediction:
RK=(2.475±0.032)×10-5
SUSY limits by RK measurements
(MH,GeV)
NA48/2 Experiment
Unseparated, simultaneous K± highly collimated beams , designed to precisely measure K±→π+π-π± (π0 π0 π±) dalitz-plot density:· pK ~ 60 GeV/c, σp ~ 3 Gev/c (3.8% p-bite)· spot of ~ 5 mm width DCH1 entranceTrack decay products with 4 DCH’s:· PT kick of 121 MeV/c after DCH2· σp/p ~ 1.02% 0.044% p [GeV/c]
NA48/2 Experiment
Scintillator hodoscope:
• establish event time (σ~150 ps), initiate trigger
LKr calorimeter: efficient vetoing, e.m. energy resolution
– σE/E = 3.2%/ √E[GeV] 9%/E[GeV] 0.42%
– σx,y = 4.2mm/√E[GeV] 0.6 mm, granularity of ~13,000 2×2 cm2 cells
Hadron calorimeter, Muon veto system
Analysis starting samples:
– 1-track minimum bias trigger, acquired during 12-hour run in 2003
– 1-track triggers, acquired during 56-hour special run in 2004
Ke2 trigger: 1 track (hodoscope) + ELKr > 10 GeV
Kμ2 trigger: 1 track (hodoscope), possibly downscaled
Analysis of Ke2/Kμ2 at NA48/2
Main analysis steps for Ke2, Kμ2 selection:
One track with 15 < p < 50 GeV, passing acceptance + quality cuts
Data MC Ke2
p (GeV) p (GeV)
Analysis of Ke2/Kμ2 at NA48/2
Main analysis steps for Ke2, Kμ2 selection:
One track with 15 < p < 50 GeV, passing acceptance + quality cuts
Data MC Ke2
Zvtx (cm) Zvtx (cm)
Analysis of Ke2/Kμ2 at NA48/2
Main analysis steps for Ke2, Kμ2 selection:Associate electron track to LKr cluster, compute E/PVeto events with extra energy in LKr above 1.5 GeVCalculate missing mass squared, assuming electron
hypothesis
Data (2004)Data (2003)
Ke2 Ke2
E/P E/P
Analysis of Ke2/Kμ2 at NA48/2
Main bkg to Ke2 selection due to Kμ2 with μ faking e cluster, O(5×10-6):– At high momentum, p > ~30 GeV/c, can’t reject Kμ2 using kinematics– Evaluate probability of “catastrophic release” from selected μ sample– Statistics of μ sample induces a systematic error for bkg subtraction
For the analysis of 2004 data:– 1.59% systematic error due to subtraction of Kμ2 bkg for Ke2– 1.85% statistical error due to Ke2 counts
p (GeV) p (GeV)
Nμ bkg (Evts)MM2(e) (GeV/c2)
RK – Present experimental status
Recent (preliminary) results improved greatly with respect to 2006 PDGWorld average, RK= (2.457±32)×10-5, agrees with SM
NA62 Physics Run 2007
Beam + Detector:
• Charged kaons are produced by 400 GeV/c protons impinging on a Be target.
• An achromatic system of dipole magnets selects charged particles ( ~ 90 % π±, 10 % K±) with momentum of (75±2) GeV/c.
• The decay volume is housed in a ~ 110 m long vacuum tank. The decay products are measured in the detector of the former NA48/2 experiment.
Data taking:
• 120 days run period, June – October 2007.
• Minimum Bias-Trigger: Require minimum number of hits in drift chambers and hodoscope + energy deposition in the LKr calorimeter > 10 GeV.
> 120000 K± → e±ν decays recorded
Goal of NA62: Measurement of RK with accuracy < 0.5 %
Ke2 Identification and Background Rejection
Rejection of K±→μ±ν:
Events with ptrack < 35 Gev/c (40 %):
Kinematic separation by “Missing
mass” M2 = (PK – Ptrack)2
Events with ptrack > 35 GeV/c (60 %):
Separation by ratio E/p:
Electrons have E/p ~ 1,
Muons E/p << 1.
P
Most relevant background: Decays of K± → μ±ν (Br = 63 %).All other decays and processes are excluded kinematically or measured in data (e.g. beam halo background).
“Katastrophic Bremsstrahlung”
Problem: “Katastrophic Bremsstrahlung”A very small fraction of muons deposit all energy in the calorimeter by emission of a high energy bremsstrahlung photon. A few 10-6 of muons with E/p > 0.95 being mis-identified as electrons.
Solution: Cover part of the calorimeter with lead plate (see sketch), allowing only muons to pass. Direct measurement of muon background by explicit E/p measurement for
muons.
Background from bremsstrahlung photons
Number of events
Ptrack (GeV/c)
Ke2 Identification and Background Rejection
Use MC for checks of any bias due to the presence of Pb wallCan use Pb wall data to validate results of G4 simulationData-MC agreement looks good
Total Background Estimation
Will subtract background from beam halo directly measuring it:Use K-less data normalized in side-band of negative Mmiss
2
Expect ~ 7% total background to Ke2, dominated by Kμ2
Analysis of Ke2/Kμ2 at NA62
Data taking lasted 4 months:NA62 has the world largest data set, exceeding 100000 Ke2 eventsLargest homogeneus period (K+ data), lasted ~1 month: > 40000 Ke2 events
Conclusions
Golden LFV observable is RK:
• Preliminary mmt 2% from NA48/2, 3% from KLOE
• Results from preliminary measurement in agreement with SM
Huge effort to enter the precision realm for RK:
• KLOE expects to push error to ~1.3%, after analysis completion
• NA62 dedicated 2007 run aims at reaching <0.5% error