Measurement of the η mass by the NA48 Experiment

Transcription

1 Measurement of the η mass by the NA48 Experiment Rainer Wanke Institut für Physik, Universität Mainz Crystal Ball Meeting Mainz, March 20, 2004 Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.1/26

2 Overview Introduction The NA48 Experiment Liquid Krypton Calorimeter Analysis Method Systematic Uncertainties Cross-Checks Result and Conclusion Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.2/26

3 g g n Previous η mass measurements ω Long history of η mass measurements: Old bubble chamber measurements: 1 MeV/c 2 above more recent measurements. Since 1992: No bubble chamber data in PDG fit any more. Krusche et al., Plouin et al., Duane et al., "!#"! Average γ d π p p p η mass [MeV/c ] η η p η η 3 He Λ neutrals PDG 2002 τ m m Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.3/26

4 The NA48 Experiment NA48: Fixed-target experiment with proton beam from CERN SPS. Goal of the experiment: Measurement of direct CP violation in K L decays. Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.4/26

5 NA48 Beam-line (η Runs) η targets π beam ~126 m Final Collimator ~ 114 m Detector η runs: Taken for calorimeter calibration = π beams along the K L beam axis ( 10 6 per pulse, average energy 100 GeV) η targets: Two polyethylen targets (2 cm thick) within decay region. = π 0, η production by charge exchange. Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.5/26

6 NA48 Detector Muon veto sytem Hadron calorimeter Liquid krypton calorimeter Hodoscope Drift chamber 4 Anti counter 7 Helium tank Drift chamber 3 Magnet Drift chamber 2 Anti counter 6 Main detector components: Magnet spectrometer Drift chamber 1 Kevlar window Anti-counters for photons, muons Liquid Krypton Calorimeter = Neutral kaon decays and η mass measurement. Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.6/26

7 Liquid Krypton Calorimeter Photons, Electrons create electromagnetic showers im liquid krypton (ρ = 2.7 g/cm 3, radiation length = 4.7 cm, T = 121 K). Ionization products drift to electrodes. = Collected charge particle energy DETAIL ON RIBBONS AND SPACER-PLATE +/ rad 2 cm x 2 cm cell cathodes anodes cells of 2 2 cm 2 along beam axis in 10 m 3 liquid krypton. Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.7/26

8 Liquid Krypton Calorimeter (Shower Reconstruction) Shower energy: Total energy deposition of all cells within 11 cm radius. Corrections: Dependence from impact point in a cell (< 1%). Energy losses at the calorimeter borders. Energy losses in dead cells ( 0.4% of all). Energy losses on material before the LKr (15 MeV for γ s). Energy transfers between neighboring showers. Shower position: Energy centre-of-gravity in 3 3 inner cells. (Corrected for bias from missing cells.) Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.8/26

9 Liquid Krypton Calorimeter (Energy and Position Resolution) Energy resolution: E E = 3.2% E[GeV] 90 MeV E 0.42% = E/E 1% for 20 GeV electrons/photons. Position resolution: 1 mm for 20 GeV photons. Resolution σ(e/p) σ(e)/e Energy (GeV) Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.9/26

10 Liquid Krypton Calorimeter (Energy Scale) Determination of the energy scale: E/p K 0 π e + ν e decays (K e3 ): Measure p(electron) in spectrometer and compare with energy in LKr. K S π 0 π 0 decays: Beginning of decay region defined by veto counter. = Fit of the decay vertex distribution. Non-linearities: < % Electron Energy (GeV) Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.10/26

11 Determination of the Decay Vertex Decays in two photons: (z.b. π 0 γγ) m 2 = 2 E 1 E 2 (1 cos θ) = E 1 E 2 d 2 12 d 2 LKR E 1 d 12 If mass m known: θ d LKr E 2 d LKR = 1 m E1 E 2 d 2 12 m Decays in many photons: (z.b. K L /η π 0 π 0 π 0 ) d LKR = 1 m K/η i,j;i>j E i E j d 2 ij E i Turning it around: d ij If vertex position d LKR is known = mass m can be measured! m K d LKr E j Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.11/26

12 Principle of the η Mass Measurement Measure π 0 vertex positions of the π 0 s from a η 3π 0 decay. d π0 LKR = 1 m π 0 E i E j d 2 ij Measure η vertex from all γ s: d η LKR = 1 m η i,j;i>j E i E j d 2 ij π 0 lifetime negligible = η vertex d η LKR = π0 vertices d π0 LKR = m η = m π 0 i,j;i>j E i E j d 2 ij ( 1 3 E1 E 2 d E 3 E 4 d ) E 5 E 6 d 2 56 Conclusions: Normalization to π 0 mass m π 0 = ± MeV/c 2. No dependence from the energy scale. Only Non-linearities in the LKr have to be considered. (Not for symmetric events with E any shower = 1 6 E tot!) Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.12/26

13 Event Selection Only events with exactly 6 showers in-time in the LKr calorimeter. Shower energy fully reconstructed: Distance between 2 showers > 10 cm. Sufficient distance to calorimeter edges. Distance to dead cells > 2 cm. Total energy: 70 GeV < E tot < 180 GeV Reconstructed vertex position: Within 4 m from η target. (vertex resolution: 50 cm) Minimization of dependence from non-linearities: Only symmetric events with (Rejects 99.9% of all data!) 0.7 < E shower 1 6 E tot < 1.3 = 8 GeV < E shower < 37 GeV (ɛ /ɛ measurement: 3 < E < 100 GeV) Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.13/26

14 Identification of η/k 0 π 0 π 0 π 0 Rellipse = constant: Ellipsoids around correct π 0 masses. Two-dimensional case: Ellipse (e.g. KL π 0 π 0 ) Mass γ3 γ4 [GeV/c2] χ2 -like variable for agreement of 2γ masses m1, m2, m3 with mπ0 : m +m +m m2 +m m (m ) (m m ) π Rellipse = σ1 3 σ2 3 σ KL π0 π0 candidates σ1, σ2, σ3 fitted to data Assignment of photons to π s: Take pairings with smallest Rellipse (99.75% correct) Mass γ1 γ2 [GeV/c ] 2 0 Background suppression (e.g. Dalitz decays πdalitz e+ e γ): Require Rellipse < 1.5 (= Background negligible) Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.14/26

15 Data Taking Data taking period in 2000 (June September): Accident end of 1999 No drift chambers = Helium tank evacuated, no window between decay region and helium tank. = No material between collimator and LKr calorimeter (Except: Hodoscope 1 m before LKr.) = Ideal environment for measurement of neutral decays! First part of the run period: K L run for ɛ systematics + η run Second part: High-intensity K S run. Trigger: Horizontal + vertical projections of LKr signals. = Total energy, energy centre-of-gravity, decay vertex (Very loose selection criteria.) Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.15/26

16 Measurement of the η Mass In total 1134 selected η π 0 π 0 π 0 events. Mass resolution of single events: 1 MeV/c 2. Result: m η = ± stat MeV/c 2 events per 0.1 MeV/c 2 M(η) M(PDG) (GeV/c 2 ) Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.16/26

17 Systematic Uncertainties (Non-Linearities) Determination of non-linearities: M/M Use π 0 γγ and known η target position. = π 0 mass can be determined from γ energies M/M R(γ) (cm) E(π 0 ) (GeV) Look for dependencies of the π 0 mass from: shower energies shower positions Emax(γ) (GeV) Emin(γ) (GeV) Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.17/26

18 Systematic Uncertainties (Non-Linearities) Parametrization of non-linearities of the energy measurement: E E = α E + β E + γ r LKr Determination of the parameters from π 0 γγ: η mass: α = ± 10 MeV = ± 2 kev β = ± 0.02 MeV 1 = ± 2 kev γ = ± 10 5 cm 1 = ± 23 kev If not requiring symmetric events: m η (α) ± 35 kev m η (β) ± 25 kev Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.18/26

19 Systematic Uncertainties (Other Contributions) Energy transfer between two showers ( shower profile): Compare GEANT with data from monochromatisc electron beams. = ± 33 kev Uncertainties in the shower position: Difference with uncorrected positions = ± 9 kev Non-gaussian contributions to the energy measurement: (e.g. Hadron production in showers) Turning on/off the corresponding parametrization from K e3 and π 0 data = ± 2 kev Overlaid events: negligible Possible bias from fit method: Monte Carlo simulation = 7 ± 5 kev Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.19/26

20 Cross-Check: η Mass from η/π 0 γγ Mass measurement using η γγ decays: Principle: Build ratio of η and π 0 mass from γγ decays from η target. Two possibilities: d LKr = 1 m π0 /η E1 E 2 d 2 12 Equal photon shower distances: = Energies different by factor of m η /m π 0 4 Sensitivity to non-linearities. = m η = ± 0.35 MeV/c 2 Equal energies of η and π 0 : = Shower separation different by factor of 4 Sensitivity to energy transfers. = m η = ± 0.14 MeV/c 2 Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.20/26

21 Cross-Check: K 0 Mass Measurement Selection of K L π 0 π 0 π 0 events: Data from 2000 K L run period Maximum lifetime < 6.5 τ KS (corresponds to 60 m from collimator) Otherwise: identical selection to η measurement. Main differences to η mass measurement: Collimation: K L beam much better collimated. = More narrow distribution of energy centre-of-gravity for K L. Lifetimes, masses (almost the same!), energy spectra = Slightly different decay topologies. = Only small differences. Taken into account in systematic uncertainties. Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.21/26

22 Cross-Check: K 0 Mass Measurement In total selected K L π 0 π 0 π 0 events. Result: m K 0 = ± stat MeV/c 2 events per 0.1 MeV/c 2 M(K 0 ) M(PDG) (GeV/c 2 ) Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.22/26

23 Further Cross-Checks Further Cross-Checks (a.o.): Data from for K 0 and η mass. = Consistent results. K 0 mass from K S π 0 π 0 decays from ɛ and η runs. = Consistent results. Kinematic fit of K L π 0 π 0 π 0 decays. = Change in result: ±10 kev/c 2 (cut dependent), with different systematics. Relaxation of cuts on event symmetry, photon energies and separation: = Change in result: 9 kev/c 2, but doubling the systematics. Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.23/26

24 Summary of Systematic Uncertainties m(η) m(k 0 ) [kev/c 2 ] [kev/c 2 ] Non-linearities: α = ± 10 MeV ±2 ±2 β = ± MeV 1 ±2 ±2 γ = ± 10 5 cm 1 ±23 ±23 Energy transfer between showers ±33 ±20 Measurement of shower positions ±9 ±5 Non-gaussian energy response ±2 ±2 Accidentally overlaying events - Monte Carlo statistics ±5 ±3 Total systematic uncertainty ±41 ±31 Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.24/26

25 Measurement of the η mass: m η = ± stat ± syst MeV/c 2 Previous world average: m η = ± 0.12 MeV/c 2 = Most precise measurement. More than 4 σ (540 kev) above old PDG value. We have omitted some results that have been superseded by later experiments. The omitted results may be found in our 1988 edition Physics Letters B204 (1988). η MASS Result We no longer use the bubble-chamber measurements from the 1960 s which seem to have been systematically high by about 1 MeV. Some early results have been omitted altogether. VALUE (MeV) EVTS DOCUMENT ID TECN COMMENT ±0.12 OUR NEW AVERAGE Error includes scale factor of 2.6. See th gram below. [ ± 0.12 MeV OUR 2002 AVERAGE] ±0.030± LAI 02 NA48 η 3π ±0.06 ±0.25 KRUSCHE 95D SPEC γ p ηp, thre ±0.15 PLOUIN 92 SPEC d p η 3 He ±0.25 DUANE 74 SPEC π p n neu We do not use the following data for averages, fits, limits, etc ±0.65 FOSTER 65C HBC ± FOELSCHE 64 HBC ± ALFF HBC ± BASTIEN 62 HBC WEIGHTED AVERAGE ±0.12 (Error scaled by 2.6) χ 2 LAI 02 NA KRUSCHE 95D SPEC 6.1 PLOUIN 92 SPEC 9.1 DUANE 74 SPEC (Confidence Level 0.001) Measurement of the K 0 mass: η mass (MeV) m K 0 = ± stat ± syst MeV/c 2 = Agreement within 1.1 σ (47 kev) with world average m K 0 = ± MeV/c 2. Page 1 Created: 6/5/200 Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.25/26

26 Conclusion NA48 Experiment at CERN: Measurement of the η mass to ±51 kev/c 2 by normalizing to the π 0 mass in η π 0 π 0 π 0 decays. Event reconstruction in liquid krypton calorimeter. Independence of calorimeter energy scale. Effect of non-linearity of energy measurement small. Many cross-checks, in particular: K 0 mass measurement with same method. Result significantly ( 0.5 MeV/c 2 ) higher than measurement at production threshold, about 0.5 MeV/c 2 below bubble chamber measurements. Eagerly awaiting new MAMI measurment! Rainer Wanke, Crystal Ball Meeting, Mainz, March 20, 2004 p.26/26