We present a measurement of the charge asymmetry $\delta_L$ in the mode $K_L
\to \pi^{\pm}e^{\mp}\nu$ based on 298 million analyzed decays. We measure a
value of $\delta_L = (3322 \pm 58(stat) \pm 47(sys))\cdot 10^{-6}$, in good
agreement with previous measurements and 2.4 times more precise than the
current best published result. The result is used to place more stringent
limits on CPT and $\Delta S = \Delta Q$ violation in the neutral kaon system.Comment: Submitted to Physical Review Letters, Dec 31, 2001. 4 pages, 4
  figure

Alavi-Harati, A

Alexopoulos, T

Arenton, M

Arisaka, K

Averitte, S

Barbosa, R

Barker, A

Barrio, M

Bellantoni, L

Bellavance, A

Belz, J

Ben-David, R

Bergman, D

Blucher, E

Bock, G

Bown, C

Bright, S

Cheu, E

Childress, S

Coleman, R

Corcoran, M

Corti, G

Cox, B

Crisler, M

Erwin, A

Ford, R

Glazov, A

Golossanov, A

Graham, G

Graham, J

Hagan, K

Halkiadakis, E

Hamm, J

Hanagaki, K

Hidaka, S

Hsiung, Y

Jejer, V

Jensen, D

Kessler, R

Kobrak, H

LaDue, J

Lath, A

Ledovskoy, A

McBride, P

Mikelsons, P

Monnier, E

Nakaya, T

Nelson, K

Nguyen, H

O'Dell, V

Pang, M

Pordes, R

Prasad, V

Qi, X

Quinn, B

Ramberg, E

Ray, R

Roodman, A

Sadamoto, M

Schnetzer, S

Senyo, K

Shanahan, P

Shawhan, P

Shields, J

Slater, W

Solomey, N

Somalwar, S

Stone, R

Swallow, E

Taegar, S

Tesarek, R

Thomson, G

Toale, P

Tripathi, A

Tschirhart, R

Turner, S

Wah, Y

Wang, J

White, H

Whitmore, J

Winstein, B

Winston, R

Yamanaka, T

Zimmerman, E

English

arXiv

DSpace at NTUA

Measurement of the KL Charge Asymmetry

Alavi-Harati, A.

Alexopoulos, T.

Arenton, M.

Arisaka, K.

Averitte, S.

Barbosa, R.F.

Barker, A.R.

Barrio, M.

Bellantoni, L.

Bellavance, A.

Belz, J.

Ben-David, R.

Bergman, D.R.

Blucher, E.

Bock, G.J.

Bown, C.

Bright, S.

Cheu, E.

Childress, S.

Coleman, R.

Corcoran, M.D.

Corti, G.

Cox, B.

Crisler, M.B.

Erwin, A.R.

Ford, R.

Glazov, A.

Golossanov, A.

Graham, G.

Graham, J.

Hagan, K.

Halkiadakis, E.

Hamm, J.

Hanagaki, K.

Hidaka, S.

Hsiung, Y.B.

Jejer, V.

Jensen, D.A.

Kessler, R.

Ladue, J.

Lath, A.

Ledovskoy, A.

Mcbride, P.L.

Mikelsons, P.

Monnier, E.

Nakaya, T.

Nelson, K.S.

Nguyen, H.

O'Dell, V.

Pang, M.

Pordes, R.

Prasad, V.

Qi, X.R.

Quinn, B.

Ramberg, E.J.

Ray, R.E.

Roodman, A.

Sadamoto, M.

Schnetzer, S.

Senyo, K.

Shanahan, P.

Shawhan, P.S.

Shields, J.

Slater, W.

Solomey, N.

Somalwar, S.V.

Stone, R.L.

Swallow, E.C.

Taegar, S.A.

Tesarek, R.J.

Thomson, G.B.

Toale, P.A.

Tripathi, A.

Tschirhart, R.

Turner, S.E.

Wah, Y.W.

Wang, J.

White, H.B.

Whitmore, J.

Winstein, B.

Winston, R.

Yamanaka, T.

Zimmerman, E.D.

HAL-IN2P3

A Measurement of the KL Charge AsymmetryA. Alavi-Harati, T. Alexopoulos, M. Arenton, K. Arisaka, S. Averitte, R.F.Barbosa, A.R. Barker, M. Barrio, L. Bellantoni, A. Bellavance, et al.To cite this version:A. Alavi-Harati, T. Alexopoulos, M. Arenton, K. Arisaka, S. Averitte, et al.. A Measurementof the KL Charge Asymmetry. Physical Review Letters, American Physical Society, 2002, 88,pp.181601. <in2p3-00012724>HAL Id: in2p3-00012724http://hal.in2p3.fr/in2p3-00012724Submitted on 8 Apr 2003HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.arXiv:hep-ex/0202016 v1   6 Feb 20021A Measurement of the KL Charge Asymmetry(Submitted to Physical Review Letters, December 31st, 2001)A. Alavi-Harati12, T. Alexopoulos12, M. Arenton11, K. Arisaka2, S. Averitte10, R.F. Barbosa7,∗∗, A.R. Barker5,M. Barrio4, L. Bellantoni7, A. Bellavance9, J. Belz10, R. Ben-David7, D.R. Bergman10, E. Blucher4, G.J. Bock7,C. Bown4, S. Bright4, E. Cheu1, S. Childress7, R. Coleman7, M.D. Corcoran9, G. Corti11, B. Cox11, M.B. Crisler7,A.R. Erwin12, R. Ford7, A. Glazov4, A. Golossanov11, G. Graham4, J. Graham4, K. Hagan11, E. Halkiadakis10,J. Hamm1, K. Hanagaki8, S. Hidaka8, Y.B. Hsiung7, V. Jejer11, D.A. Jensen7 , R. Kessler4, H.G.E. Kobrak3, J. LaDue5,A. Lath10, A. Ledovskoy11, P.L. McBride7, P. Mikelsons5, E. Monnier4,∗, T. Nakaya7, K.S. Nelson11, H. Nguyen7,†,V. O’Dell7, M. Pang7, R. Pordes7, V. Prasad4, X.R. Qi7, B. Quinn4,§, E.J. Ramberg7, R.E. Ray7, A. Roodman4,M. Sadamoto8, S. Schnetzer10, K. Senyo8, P. Shanahan7, P.S. Shawhan4, J. Shields11, W. Slater2, N. Solomey4,S.V. Somalwar10, R.L. Stone10, E.C. Swallow4,6, S.A. Taegar1, R.J. Tesarek10, G.B. Thomson10, P.A. Toale5,A. Tripathi2, R. Tschirhart7, S.E. Turner2, Y.W. Wah4, J. Wang1, H.B. White7, J. Whitmore7, B. Winstein4,R. Winston4, T. Yamanaka8, E.D. Zimmerman4The KTeV Collaboration1 University of Arizona, Tucson, Arizona 857212 University of California at Los Angeles, Los Angeles, California 900953 University of California at San Diego, La Jolla, California 920934 The Enrico Fermi Institute, The University of Chicago, Chicago, Illinois 606375 University of Colorado, Boulder, Colorado 803096 Elmhurst College, Elmhurst, Illinois 601267 Fermi National Accelerator Laboratory, Batavia, Illinois 605108 Osaka University, Toyonaka, Osaka 560-0043 Japan9 Rice University, Houston, Texas 7700510 Rutgers University, Piscataway, New Jersey 0885411 The Department of Physics and Institute of Nuclear and Particle Physics,University of Virginia, Charlottesville, Virginia 2290112 University of Wisconsin, Madison, Wisconsin 53706We present a measurement of the charge asymmetry δL in the mode KL → pi±e∓ν based on 298 million analyzeddecays. We measure a value of δL = (3322 ± 58(stat) ± 47(sys)) · 10−6, in good agreement with previousmeasurements and 2.4 times more precise than the current best published result. The result is used to place morestringent limits on CPT and ∆S = ∆Q violation in the neutral kaon system.PACS numbers: 11.30.Er, 11.30.-j, 13.25.Es, 13.20.-vThe charge asymmetry in KL semileptonic decays isdeeply related to CP violation through neutral kaonmixing. The KL wavefunction is proportional to (1 +L)K0 − (1 − L)K0, where L parameterizes CP vio-lation in mixing. This parameter can be determined inthe decay modes KL → pi±e∓ν (Ke3) and KL → pi±µ∓ν(Kµ3). The charge asymmetry (δL) is defined asδL =BR(e+pi−)− BR(e−pi+)BR(e+pi−) + BR(e−pi+).Assuming ∆S = ∆Q and no CPT violation in the decayK0(K0)→ e+pi−ν(e−pi+ν¯), δL is simply 2Re L. To firstorder in the parameters that violate CP and CPT,δL = 2Re L − 2Re Y −Re(x − x¯).The terms Y and Re(x − x¯) parameterize CPT viola-tion in the ∆S = ∆Q and ∆S = −∆Q transitionsrespectively[1, 2]:< e+pi−ν|K0 >< e−pi+ν¯|K0 >∗=1− Y1 + Yx =< e+pi−ν|K0 >< e+pi−ν|K0 >x¯ =< e−pi+ν¯|K0 >∗< e−pi+ν¯|K0 >∗.In the Standard Model, the ∆S = ∆Q violation is CPT-conserving and occurs in 2nd order weak decays. Esti-mates for |x| are in the range 10−7[3]. δL is modified toorder |x|2 and will not be considered in this letter.The measurement of δL can be compared to expec-tations derived from the KL → 2pi (Kpi2) amplitudes(η+−, η00)[2, 4]:Re (Y +x− x¯2+ a) = Re(23η+− +13η00)−δL2. (1)This comparison is sensitive to CPT violation in Ke3 andKpi2 decays, where the latter is parameterized by Re a.The current world average on δL comes mainly froma measurement by the CERN-Heidelberg collaborationin 1974. Using 34 million Ke3 decays, they foundδL = (3409 ± 171(stat) ± 50(sys)) · 10−6[5]. Wedescribe in this letter a measurement of δL in the Ke3mode based on approximately 298 million Ke3 decays.220 cm100 120 140 160 180Distance from Target (m)TriggerHodoscopesMuonFilterCollimator2 KL beamsVacuum TankVacuum WindowSpectrometerMagnetSweepingMagnetRegeneratorDriftChambersm  HodoscopesCsIHadron VetoBackAntiPhoton Veto DetectorsFIG. 1: The KTeV detector configured for measuringRe(′/).The KTeV beamline and detector at Fermilab is de-scribed elsewhere[6]. For this analysis, the detector isconfigured for the measurement of Re(′/). As shownin Fig. 1, two approximately parallel neutralKL beamsenter a long vacuum tank, which defines the fiducial vol-ume for accepted decays. One of the beams strikes anactive absorber (regenerator), which serves to tag the co-herent regeneration of KS . The regenerator moves tothe other beam in between Tevatron spill cycles. Follow-ing the vacuum tank, there are 4 planar drift chambersand an analysis magnet that imparts a 411 MeV/c hor-izontal transverse kick to the charged particles. A highprecision 3100-element pure Cesium Iodide calorimeter(CsI) is used primarily to measure the energy of e± andphotons. Photon veto detectors surrounding the vacuumtank, drift chambers, and CsI serve to reject events withparticles escaping the calorimeter.Only Ke3 decays in the beam opposite the regenera-tor were used in this analysis. They were triggered bythe presence of hits in the drift chambers and scintil-lators placed immediately upstream of the CsI, and bythe lack of activity in the regenerator or photon vetoes.To keep the event rate managable, the trigger rejectedbeam muons and Kµ3 events by requiring no activity inthe scintillation counters (muon veto) placed behind 4meters of Fe absorbers downstream of the CsI.Selecting Ke3 events is relatively straightforward, asthey are the most copious KL decay mode (BR(Ke3)≈ 0.39). We identify two-track vertices in the regionbetween 90 and 160 meters from the target with kine-matics inconsistent with KL → pi+pi−pi0 (Kpi3) andΛ(Λ¯) → ppi−(p¯pi+). The tracks are required to extrap-olate to CsI energy deposits while maintaining sufficientclearance from the inner and outer edges. A track is iden-tified as an electron if its momentum exceeds 5 GeV/cand its CsI energy deposit divided by its momentum(E/P) exceeded 0.925. Pions are identified as having mo-mentum exceeding 8 GeV/c and E/P less than 0.925.The tighter pion momentum requirement rejects Kµ3background, since 8 GeV/c is the threshold for minimum-ionizing particles to penetrate through to the muon vetocounters.To minimize background from target KS coherentlyinterfering with the KL, the event proper time (τ ) had toexceed 10.5 KS lifetimes (τS). The KL momentum (PK)reconstruction has ambiguities inherent to any 3-bodydecay with one unobserved particle. The longitudinalcomponent of the neutrino momentum in the kaon restframe can be either parallel or anti-parallel to the kaonflight direction in the lab frame. This introduces twopossible solutions for PK . We use the low PK solutionto calculate τ . Based on Monte Carlo (MC) simulations,this choice is at least 70% correct for τ < 22.5 τS . A smallcorrection, derived from MC, was made for errors in thePK solution and the residual target KS interference.The measurement of δL requires a careful control ofsystematics. The most important of these is to ensurean equal acceptance and efficiency for oppositely chargedparticles. We combine data sets of opposite analysis mag-net polarities, which was reversed about once per day, toensure that the detector is exposed to oppositely chargedparticles in the same manner. We also used data collectedduring rare decay running (E799), which included a tran-sition radiation detector (TRD)[7, 8] used for the studyof electron and pion identification.There are eight possible configurations for Ke3 decays:e+pi− or e−pi+, east (E) or west (W ) KL beams, and pos-itive (+) or negative (−) magnet polarities. The numberof Ke3 events in each configuration (Ni) depends on thebranching ratio (BR), the acceptance and efficiency (A),and the flux N (KL). We define R as:R4 =BR(e+pi−)A(e+pi−, E,+)N (KL, E,+)BR(e−pi+)A(e−pi+, E,−)N (KL, E,−)(2)×BR(e+pi−)A(e+pi−, E,−)N (KL, E,−)BR(e−pi+)A(e−pi+, E,+)N (KL, E,+)×BR(e+pi−)A(e+pi−,W,+)N (KL,W,+)BR(e−pi+)A(e−pi+,W,−)N (KL,W,−)×BR(e+pi−)A(e+pi−,W,−)N (KL,W,−)BR(e−pi+)A(e−pi+,W,+)N (KL,W,+),where the four numerators and denominators representNi, i = 1, 8. Since the fluxes cancel and A(e±pi∓) =A(e∓pi±) under magnetic field reversal, Eq. 2 reducesto:raw δL =R− 1R+ 1σ(raw δL) =18·√Σ8i=11Ni. (3)Equation 3 defines the “raw” δL, and does not take intoaccount several effects discussed below. Our data yieldsraw δL = (3417 ± 58) · 10−6 (ppm). Figure 2 shows itsdependence on the decay vertex distance from the target.In principle, the same flux and acceptance/efficiencycancellation occurs in the pairing of the first and second3-10-4281420100 120 140 160012345Distance from Target (meters)Raw Asymmetry (10-3 )Millions of Events / 0.5 meterFIG. 2: Raw δL and number of events versus vertex distancefrom target. A fit to a constant yields 3417 ± 58 ppm with aχ2 of 81.53 for 81 degrees of freedom.(third and fourth) factors of Eq. 2, such that δL can bemeasured within the east and west beams. Eq. 3 is thegeometric mean of the two δL measurements.Figure 3 shows the raw asymmetry in various datasubsets. In particular, we see the importance of com-bining data from opposite polarity runs. The differencebetween measurements made with only one magnet set-ting, δL(+) − δL(−), is significant and on the order ofδL itself. This difference is due to very small offsets ofthe inner apertures and is well-reproduced by our MC.The MC is not used to correct this large effect sinceEq. 2 naturally accounts for such geometrical asymme-tries. On the other hand, the values for δL agree betweendecays in the east and west beams. Other data divi-sions yield good agreement except for the sample with8 GeV/c < Ppi < 15 GeV/c (discussed below).Raw Asymmetry (10-6)30 GeV/c < Pe < 100 GeV/c15 GeV/c < Pe < 30 GeV/c5 GeV/c < Pe < 15 GeV/c30 GeV/c < Pp< 100 GeV/c15 GeV/c < Pp < 30 GeV/c8 GeV/c < Pp < 15 GeV/cOUTBENDING DECAYSINBENDING DECAYSWEST BEAMEAST BEAMPOS MAGNET POLARITYNEG MAGNET POLARITYALL EVENTS2000 2500 3000 3500 4000 4500 5000FIG. 3: Raw δL for all events and various data subsets. Theline indicates the raw value of 3417 ppm.We account for the different behavior of particles andantiparticles in matter. For each cut that introduces abias, we measure f+ (f−), the inefficiency in the e+pi−(e−pi+) configuration. The correction to raw δL for thecut, summarized in table I, is simply (f+ − f−)/2.Many biases due to pi± interaction differences were con-sidered, and data were used to measure most of thesecorrections. These include the pi± energy deposition inthe CsI, the loss of pions due to interactions in the trig-ger scintillators, and pion punchthrough past the Fe ab-sorbers depositing energy in the muon veto. Events withE/P for pions exceeding 0.925 would be removed since theanalysis requires exactly one identified electron. This ef-fect, studied using pions in Kpi3 events, removes ≈ 0.6%of the pions, with the probability being slightly larger forpi+ than pi−. The asymmetry between pi+ and pi− hasa strong momentum dependence and explains the trendseen for the 8 GeV/c < Ppi < 15 GeV/c data sample (seeFig. 3). The correction to δL is −156± 10 ppm.A correction was estimated for possible biases dueto the trigger muon veto requirement, which removedapproximately 3% of pions due to decay-in-flight andpunchthrough. Good events can also be lost if accom-panied by accidental muons. Only pion punchthroughwould cause a bias due to the neutron excess in the Feabsorbers. The estimate used a Ke3 sample collected inE799, where the TRD gave additional electron purity.The sample is triggered by requiring muon counter hits,as these events are the ones that would be removed by theanalysis trigger. This sample had a charge asymmetry of(4.4± 1.3) · 10−3, consistent with no bias. Nevertheless,we use a correction of 34 ± 40 ppm, where the error isgiven by the sample statistics.Pion interactions in the trigger scintillator can confusethe reconstruction so that the track fails to match to aCsI cluster. This effect, studied using Ke3 events witha relaxed track-cluster matching requirement, removes ≈0.42% of all pions and was slightly more probable for thepi− than pi+, leading to a correction of 54± 10 ppm.Considering e± interaction differences, one has primar-ily e+ annihilation and δ-ray production. An e+ annihi-lation occuring near the upstream surface of the triggerscintillator would fail the trigger requirement due to thereduced energy deposition. δ-rays more than 10 MeV andemitted from the vacuum window or tracking system cancause losses since the momentum transfer would drasti-cally change the event kinematics. These losses causea bias due to the small difference between the Bhabha(e+e−) and Moller (e−e−) scattering cross section. Thesecorrections were derived by a Geant simulation of the de-tector material. They are of order ±10 ppm, mitigatedby the high momentum of the tracks and the minimalmaterial (2% X0) upstream of the CsI.Finally, a Ke3 sample was used to study potential bi-ases of the E/P cut on electrons. This sample was se-lected by a relaxed E/P cut and by the tagging of aminimum-ionizing pi in the CsI. The E/P removed re-4moved approximately 0.233% of e+ and e−. No bias isseen, and we assigned a correction of −19±18 ppm. Thisresult was confirmed with a similar Ke3 sample collectedin E799, where the TRD gave additional electron purity.We applied a small correction to account for pions ab-sorbed in the vacuum window, drift chambers, and he-lium bags. This was estimated assuming isospin con-servation in strong interactions, and so the pi+ and pi−absorption differences depended on the excess protons,which was predominantly in the form of hydrogen. Weaccounted for a small bias due to pi− charge exchangenear the upstream surface of the trigger scintillator (anal-ogous to the case of e+ trigger loss). For this, we usedthe measurements of [9] and [10] and extrapolated themto our pi− momentum range.Other small corrections, derived from MC, accountedfor the inexact reversal of the analysis magnet polarityand the residual coherent KS from the target, absorber,and KL scatters with the final collimator and regenera-tor. Table I shows the summary of all systematic cor-TABLE I: Corrections in ppm for this analysis.pi+pi− difference in CsI −156 ± 10pi± interaction in trigger scintillator +54± 10pi decay and punchthrough +34 ± 40e+e− difference in CsI −19 ± 18target/absorber KS −KL interference −12 ± 1e+ annihilation in spectrometer +11 ± 1Delta ray production −8.5± 4.3pi absorption in spectrometer +5.0± 3.2inexact analysis magnet polarity reversal −3.1± 1.6final collimator and regenerator scatters −1.2± 2.3pi+pi−pi0 background +0.5± 0.7Kµ3 background 0 ± 0Λppi and Λβ background 0 ± 0Total correction −95.3± 46.5rections. The uncertainties are uncorrelated since theyare mainly statistical errors of the Ke3 and Kpi3 controlsamples. Combining the corrections with raw δL, we find:δL = 3322± 58 (stat) ± 47 (syst) ppm= 3322± 74 (combined) ppm.This result is in excellent agreement with previous mea-surements and 2.4 times more precise than the currentbest result (see Fig. 4). A combination of all resultsincluding ours yields:δL = 3307± 63 ppm χ2 = 4.2/6 d.o.f..Substituting η+− and η00 values from [11] and the com-bined δL into Eq. 1, we find:Re (Y +x− x¯2+ a) = (1650± 16)− (1653± 32)= −3± 35 ppm.The result is consistent with no CPT violation and islimited by the charge asymmetry uncertainty. It limits|Re (Y + (x − x¯)/2 + a)| < 61 ppm at the 90% C.L..Barring fortuitous cancellations, this is thus far the moststringent limit on Y, x, and a.2500 3000 3500 4000Columbia 69Columbia-Harvard-Cern 70SLAC 72Princeton 73Cern-Heidelberg 74Cern-Heidelberg 74KTeV(10-6) Ke3 Km 3FIG. 4: Compilation of δL measurements including this result.Lines indicate the average and its uncertainty.We gratefully acknowledge the support and effort ofthe Fermilab staff and the technical staffs of the par-ticipating institutions for their vital contributions. Thiswork was supported in part by the DOE, the NSF andthe Ministry of Education and Science of Japan. A.R.B.,E.B. and S.V.S. acknowledge support from the NYI pro-gram of the NSF; A.R.B. and E.B. from the Alfred P.Sloan Foundation; E.B. from the OJI program of theDOE; K.H., T.N. and M.S. from the Japan Society forthe Promotion of Science; and R.F.B from the Fundac¸a˜ode Amparo a` Pesquisa do Estado de Sa˜o Paulo. P.S.S.acknowledges receipt of a Grainger Fellowship.† To whom correspondence should be addressed:hogann@fnal.gov.∗ Perm. address C.P.P. Marseille/C.N.R.S., France∗∗Perm. address Univ. of Sa˜o Paulo, Sa˜o Paulo, Brazil[1] C.D. Buchanan et al, Phys. Rev. D 45, 4088 (1992).[2] The Daphne Physics Handbook, Volume I, editors L. Ma-iani, G. Pancheri, N. Paver, INFN, 1992.[3] C. Dib and B. Guberina, Phys. Lett. B 25, (1991).[4] V. V. Barmin et al, Nucl. Phys. B 247, (1984).[5] Geweniger et al, Phys. Lett. B 48, 483 (1974).[6] P. Shawhan, Ph. D. Thesis, Univ. of Chicago (1999).[7] G. Graham, Ph.D. Thesis, Univ. of Chicago (1999).[8] N. Solomey et al, Nucl. Inst. Meth. A419, (1998).[9] T. Inagaki et al, Nucl. Inst. Meth.A359, 478-484 (1995).[10] V. Flaminio et al, CERN-HERA 83-01.[11] Particle Data Group, Review of Particle Physics, Eur.Phys. J. C 15, 1-878, (2000).

A Measurement of the KL Charge Asymmetry

Abstract

Similar works

Full text

Available Versions

DSpace at NTUA

HAL-IN2P3

The University of Manchester - Institutional Repository

HAL AMU