K L  K S Regeneration in KLOE

KLKS Regeneration in KLOE Simona S. Bocchetta XII Frascati Spring School “Bruno Touschek” May 17th, 2007

About strange particles… Neutral K mesons: see P. Franzini lectures The quark eigenstates are: The CP eigenstates are: M. Gell-Mann A. Pais

A particle mixture? Pure K0: K1 and K2 mixture, with mean lifetimes t2>>t1 K2(t) K1(t) Suggestion for experimental check: Pais and Piccioni 1955 O. Piccioni Regeneration prediction!

The Regeneration Phenomenon Consider a KL beam impinging on a thickness of material: For regeneration: e~ 10-3 « 1 (indirect CP), suppose e negligible: The state out of the material will be: f(q), f(q): scattering amplitudes of K0 and K0 q: scattering angle If f(q)≠f(q), the state out of the material will have a regeneration component. regeneration coherent incoherent Depends on: density and size of the material momentum of the incident K in KLOE the incoherent regeneration has the main effect in the detector!

The KLOE experiment decay length lS = 6 mm lL= 3.4 m pL,S = 110 MeV bL,S = 0.22 s(e+e-f) = 3.1 mb BR(fKS KL) = 34 % BR(KS p+p-) = 69.2 % Our DATA sample: 328 pb-1 KLOE 2001-2002: 1,21 108 events

Regenerating surfaces in KLOE Drift Chamber (DC) Beam Pipe (BP) 10 cm 4.3 cm e+ e- Z axis Beryllium (Be) • DCcylinder-shape • - 750 mm of C • - 150 mm of Al • BPsphere-shape • - 62% Be • - 38% Al • thickness: 500 mm • Becylinder-shape • thickness: 50 mm 25 cm beam line

The analysis guidelines • KL tag: after the KS identification cutting on kinematic variables, look for the KL in the other side of the detector: • Reconstruction: evaluation of tracking and vertexing efficiencies for the region of the regenerating surface (DC and BP-Be) • KL decays: identification via kinematic variables • Signal selection: sample enriched of regeneration events cutting on kinematic variables • Fit in the vertex coordinates: extraction of the events number and the cross section • Comparison with expectations and existing measurements

KL reconstruction • using from KS and the interaction • point, we get the line of flight of KL • search 2 tracks of opposite sign which • originates near the KL line of flight • request of vertex reconstruction • with 2 tracks DC: 21<r<30 cm erec = 71.0 ± 0.5 % BP-Be: 0<r<15 cm erec = 70.0 ± 0.7% p0, n p+ p, m, e KL vertex coordinates: KS line of flight KL line of flight p, m, e p-

Regeneration kinematic variables To select a regeneration-enriched sample we need 2 kinematic variables: INVARIANT MASS: For regeneration events: Minv ≈ MKL So we select: 492.5<Minv<502.5 MeV

Regeneration kinematic variables DELTA P: KL momentum (from tagging) tracks momenta from KL For regeneration events: D|p| ≈ 0 So we select: -10 < D|p| < 20 MeV

Y versus X r versus Z Transverse radius r (cm) Radius r (cm) Detector x-ray, extraction of Nreg Spatial distribution of KL vertex after the selections in Minv and in D|p|: DC BP Be extract the number of regeneration events by fitting the distributions in r, r for each regenerating surface.

Fit shapes BP-Be: 0 < r < 15 cm |z| < 15 cm DC: 21 < r < 30 cm |z| < 120 cm Selection of the regeneration region: • Fit in r variable for DC: MC shapes for background; 2 gaussians for the peak. • Matched fit in r & r variables for BP-Be: MC shapes for background; • 2 gaussians for the peak in the orthogonal coordinate to the surface; change of • variables r = r sinq for the other peak, including angular distribution of KL~ sin2q.

Regeneration cross sections The cross section depends on the probability of regeneration and on the thickness of the regenerating surfaces: target density = target thickness where: target atomic weight average length covered from theKL until the regenerating surface Main systematic error source: surfaces thickness ~10% DC: Still to do… Be BP:

Regeneration cross section (mbarn) He Be C Al Comparison with expectations & measurements All the results as a function of the atomic weight A. For DC & BP average atomic weight: where: Comparison with the calculation of R. Baldini - A. Michetti (‘96) and the Novosibirsk CMD-2 result (‘99), only existing measurement at this momentum value: THANK YOU!

BACK-UP SLIDES

Effetto coerente ed incoerente KS KS KL q d 2 1 Si definisce: ampiezza di rigenerazione nella direzione q. • Mezzo rigeneratore = distribuzione uniforme di centri scatteratori, l’azione complessiva di • questi centri potrà risultare in un effetto coerente o incoerente, ciò dipende da: • densità e dimensioni del materiale • impulso dei K incidenti Consideriamo due centri scatteratori 1 e 2 distanti d. Le due onde uscenti di KS si possono scrivere così: |1>S=exp(ipSd cosq) freg(q) |KS> |2>S=exp(ipLd) freg(q) |KS> La probabilità di rigenerazione per il sistema dei due centri scatteratori è: |<KS|1+2>S|2 = 2 |freg(q)|2 {1 + cos[d (pL - pS cosq)]} • I casi sono due: • Se d(pL-pScosq)≤1 si ha un’addizione coerente delle ampiezze delle due onde di KS • Se d(pL-pScosq)»1 l’intensità del KS risulta in un contributo medio nullo: • si ha la rigenerazione incoerente In KLOE la rigenerazione incoerente è l’effetto di rigenerazione dominante nel rivelatore.

Data and MC samples, KL tag • 2001/2002 sample for Data & Monte Carlo (328 pb-1) • KL tag: same selection as for KL BR measurements • Requests: • the vertex reconstructed with two tracks of opposite • charge must stay in the fiducial volume centered in the • nominal position of f: • the invariant mass of two tracks (in the hypothesis m=mp) • within 5 MeV from the KS mass: • the KS momentum within 10 MeV of the nominal value r = (x2+y2)1/2 < 10 cm |z| < 20 cm 492.7 < Minv < 502.7 MeV After this selection we have: NKLtag ~ 1.2 · 108

Regenerating surfaces in KLOE Drift Chamber (DC) 25 cm Beam Pipe (BP) 10 cm e+ e- 4.3 cm Z axis Beryllium (Be) • DC cylinder-shape • transverse radius 25 cm • made of: • -750 mm of Carbon A=12 • - 60% carbon fibers • - 40% epoxy • -150 mm of Aluminium A=27 • BP sphere-shape • radius 10 cm • made of: • - 62% Beryllium A=9 • - 38% Aluminium A=27 • thickness 500 mm • Be cylinder-shape • transverse radius 4.3 cm • thickness 50 mm • A=9

Reconstruction efficiency • The reconstruction efficiency depends on: • the tracking efficiency • the vertex reconstruction efficiency Both these efficiencies were calculated from MC and corrected with check measurements using data; the efficiencies depend on: • tracks momentum • decay region Pions from KLsemileptonic decays have the same momentum spectra of pions from regenerated KS Selection of a pure sample (95%) of Ke3decays using calorimeter variables. Reconstruction efficiency values: erec = 71.0 ± 0.5 % erec= 70.0 ± 0.7% • DC: 21 < r < 30 cm, |z| < 160 cm • BP-Be: 0 < r < 15 cm, |z| < 15 cm

KL charged decays analysis semileptonic: CPV: p+p-p0: regeneration: • Study of kinematic variables: • missing momentum: • squared missing mass: • hypotesis: pion mass MeV MeV Ke3 Km3 reg p+p-p0 Pmiss Pmiss Monte Carlo data CPV MeV2 MeV2

Regeneration kinematic variables KL momentum (from tagging) tracks momenta from KL To select a regeneration-enriched sample we need 2 kinematic variables: INVARIANT MASS: DELTA P: Regeneration event features: KL→p+p-, too • Minv ≈ MKL • D|p|≈0 • angular distribution (more study in future) KL→p+p-, too

Signal selection: Minv below the peak: semileptonic background + regeneration + CPV if we choose: 559,023 evs survive

Signal selection: D|p| below the peak: semileptonic background + regeneration + CPV symmetric peak asymmetric peak (the KL gives a small fraction of its momentum to the target nucleus) if we choose: 272,958 evs survive

Y versus X r versus Z Detector x-ray, extraction of Nreg Spatial distribution of KL vertex after the selections in Minv and in D|p|: DC BP Be extract the number of regeneration events by fitting the distributions in r, r foreach regenerating surface. Transverse radius r (cm) Radius r (cm)

Regeneration cross section The cross section depends on the probability of regeneration and on the thickness of the regenerating surfaces: target density where: target atomic weight = target thickness to take out from fit already evaluated to estimate average length covered from theKL until the regenerating surface

Fit shapes BP-Be: 0 < r < 15 cm |z| < 15 cm DC: 21 < r < 30 cm |z| < 120 cm Selection of the regeneration region • Fit in r variable for DC: MC shapes for background; 2 gaussians for the peak. • Matched fit in r & r variables for BP-Be: MC shapes for background; • 2 gaussians for the peak in the orthogonal coordinate to the surface; change of • variables r = r sinq for the other peak, including angular distribution of KL~ sin2q.

Selection variation I Variation of cuts in the invariant mass M1: 495.0 < Minv < 500.0 MeV M2: 492.5 < Minv < 502.5 MeV M3: 490.0 < Minv < 505.0 MeV M4: 487.5 < Minv < 507.5 MeV M5: 485.0 < Minv < 510.0 MeV

Selection variation II Variation of cuts in D|p| -5 < D|p| < 10 MeV -10 < D|p| < 20 MeV -20 < D|p| < 30 MeV -30 < D|p| < 40 MeV -40 < D|p| < 50 MeV 25 fit foreach regenerating surface (DC & BP-Be) matching the cuts, we expect an asymptotic trend of the number of regeneration events which points to the true number. In the region 0<r<15 cm the matched fit on the Be gives not the same results of the fit in the only transverse radius r BUT: We need a further study for the layer of Beryllium up to now only DC & BP

Fit results, not yet corrected with e DRIFT CHAMBER BEAM PIPE 103 103 30 25 20 20 20 20 20 20 15 15 15 15 15 30 25 20 30 25 20 Tighter cut in D|p| N regeneration events N regeneration events 30 25 20 30 25 20 Tighter cut in Minv Tighter cut in Minv We can see the asymptotic trend, the results from fit must be corrected with the selection efficiencies, calculated in MC and corrected with data.

Our selection efficiency The total selection efficiency depends on the selection efficiency of the single cut: • To estimate e we’ve built the distributions in Minv and D|p| of regeneration events in data: • use a regeneration-enriched sample by selecting a region around the regenerating surfaces: • 23 < r < 28 cm for DC • 7 < r < 13 cm for BP • CP violating events are rejected by requesting • There is superimposition of peaks in both the distributions. • Request of the cut 492.5 < Minv < 502.5 MeV for the D|p| distribution, to reject the further semileptonic background. By subtracting to the fit results the semileptonic background, we can calculate the efficiencies for the regeneration events in data. We have applied the same method on the MC events. Finally, we correct the MC efficiencies with the ratio:

Fit in the invariant mass & D|p| INVARIANT MASS (DC) D|p| (DC) Data Fit Regen Ke3 Km3 bckg Data Fit Regen Ke3 Km3 CPV MeV MeV The invariant mass fit with a large cut in the regeneration regions provides us also a cross check for the number of regeneration events on the DC (not for BP): 38,043 ± 354 evs, compatible with the measurements obtained from the r fit. The D|p| peak shape is badly reproduced by the MC, but the efficiency calculation is not affected because only the fitted background shapes are taken from MC.

Number corrected for the efficiencies DRIFT CHAMBER BEAM PIPE 103 103 26 26 26 26 26 38 38 38 38 38 24 24 24 24 24 36 36 36 36 36 N regeneration events N regeneration events The measurement is reasonably stable, we choose a measurement foreach regenerating surface: DC: Nobs= (37,175 ± 469) events BP: Nobs = (24,388 ± 176) events

Probability & cross section DRIFT CHAMBER BEAM PIPE where:

Systematic errors • surfaces thickness 10% • error on the selection efficiencies: 2% BP • 1.5% DC • error on the reconstruction efficiencies: about 1% • nuclear interactions contamination: negligible • fit shapes: negligible • tails of the invariant mass distribution: about 2% ~10% We need further studies to find the right thickness of the regenerating surfaces, the idea is to use the energy loss of charged particles in the matter. We can directly measure the thickness of beam pipe!

Results DC: BP: Since the cross section on the Be is unknown, we can find variation bands for the cross sections on Be and C versus the cross section of the Aluminium. Beryllium Carbon Cross section on Be, C (mbarn) From fit preliminary results we find a cross section comparatively large for the Beryllium. So a value of about 75 mbarn for sBe would imply a small cross section on Aluminium as predicted from calculations of R. Baldini & A. Michetti (1996). Cross section on Aluminium (mbarn)

Regeneration cross section (mbarn) He Be C Al Comparison with expectations & measurements All the results as a function of the atomic weight A. For DC & BP average atomic weight: where: Comparison with the Novosibirsk CMD-2 result (‘99), only existing measurement at this momentum value:

K L  K S Regeneration in KLOE