Recent results of the CPLEAR experiment

Session 5

CP, T VIOLATION

NUCLEAR PHYSfCS A

The CPLEAR Collaboration

R. Adler”, T, Alhalel”, A. Angelopoulos’ ‘I A, Apostolakisl s E. Aslanides’~“,

G. Backenstossr, C.F. Beer, J. Bennets? V, Bertin Ill J.K. Bienlein’r*, F. Blocl?,

Ch. Bulars, P_ Garlsonr4, J. C~~YE.&EO*~ E. Cawley”, S. Gha~~ambous~s~ M, Gh~d~as~s~

G Chardin’s, M Danielsson*4, S Dedoussis 15, M. Dejardin’, J. Derre’s, M. Dodgsons,“,

J.:G. Dousse6, J.‘Duclo@, A, Ealetr’, B, Eckart2, C. Eleftheriadis”, I. Evangelou*, L. Faravel’, P. Fassnacht”, J.L. Faure13, G. Felder2, R. Ferteira-Marques*, W. Fetscher”, M. Fidecaror, A. FilipciE”, D. Francis13, J. Fry’, C. Fuglesangr4@, E. Gabathulers, R. Garnet’, D, Garreta7s1s, T. Gersdisf6, B.-J. Gerberx7, A. Gas,

P. ~urnp~~~er rrbrIt C. Guyotm, F.F. %rrisons~“, P-J_ Haymans, W.G. Beyes7*f,

R.W. Hollanderr’, K. Jamsor++, H-3. Johner 6*, K. Jon-And’*, A. Kerek’*, J. Kerns,

P R Kettl.er6 C Kochowski r3, P. Kokkass, R. Kreuger 5, T, Lawry3yh, R. Le

d. Lioliosrs, E. hachado”, P. Maleyg, I. MandiC”, N. Manthoss, G. Marel’s,

GaP,

M. Mikui’c, J. Miller3, F, Montanetr’, T. Nakada”, A. Qnofre4, B. Pagels’, P- Pavlopoulos” t F. Pelucchi”, J. Pinto da Gunha4, A. Policarpci’, G. Polivka~, B. For&ma’, R. Rickenbach2, B.L. Roberts3, E. Ram&i’, T. Rnf”‘, L. Sack@, L. Sakeliour, P. Sanders”, C. Santonis, K. Sarigiannisr, M, Schgfer”, LA. Schallers,

A. Schoppe?, P. Schun@, A. Soares13, S. Szilagyi’41, L. Tauscher’, C. Thibault12, F. Touchard la, C. Touramanis 15, F. Triantiss, D.A. Tr6ster2j, E. Van Beveren4, M. Van den Putt@, C.W.E. Van Eijk”, G. Varner’, S. Vlachos’, P, Weber17, 0. Wigger”, C. Witzi~*~~, M. Wolterxr, G. Yechex3, D. ~~v~~~kl~ and D. ~~~erman3~

presented by Andreas &hopper

‘University of Athens, GR-16680 Athens, Greece

*University of Basle, CR-4056 Basle, Switzerland

3University of Boston, Boston, MA 52215, USA

4LIP and University of Goimbra, P-3000 Coimbra, Portugal

‘Technical University of Delft, 2629 JB D&t, Netherlands

“University of Fribourg, CB-1709 Friboug, Switzerland

‘CERN, CK-1211 Geneva 23, Switzerland

“University of Ioannina, GR-45110 Ioanniaa, Greece

‘University of Liverpool, Liverpool L69 3BX, UK

“J. Stefan Institute and Department of Physics, University of Ljubljana,

438c R. Adler et al. I Recent results of the CPLEAR experiment

YU-61111 Ljubljana, Slovenia

“CPPM, IN2P3-CNRS et Universite d’Aix-Marseille II, F-13288 Marseille, France

“CSNSM, INZP3-CNRS, F-91405 Orsay, France

13DAPNIA/SPP, CE Saclay, F-91191 Gif-sur-Yvette, France

14MSI, S-10405 Stockholm, Sweden

15University of Thessaloniki, GR-54006 Thessaloniki, Greece

“Paul-Scherrer-Institut(PSI), CH-5232 Villigen, Switzerland

“ETH-IMP Ziirich, CH-8093 Ziirich, Switzerland

“Permanent address: DESY, Hamburg, Germany

‘Now at ECP/SA, CERN, Switzerland

‘Now at ESA/EAC, K5ln, Germany

‘Now at TRIUMF, Vancouver, Canada

eN~w at QMW, University of London, England

fNow at CEBAF, Newport News, Virginia, USA

SNOW at SUeR, Fribourg University, Switzerland

“Now at University of Virginia, USA

‘Permanent address: Atomki, Debrecen, Hungary

jNow at SBS, Basle, Switzerland

‘Now at Brookhaven National Laboratory, LI, NY, USA

Abstract The CPLEAR experiment at CERN has been designed to measure CP and T violating

effects in the neutral kaon system by observing particle-antiparticle asymmetries. The motivation and the physics program of CPLEAR is p resented and first analyses of the

K”,F to A+?T-, ?y”rro and x+rr-?r’ decays are discussed. Special emphasis is given to the recent results of the measurement of the CP violating parameters cp+_ and v+_ based on first data taken in 1990/1991.

1. INTRODUCTION

Since the discovery of CP violation in the neutral kaon system more than 25 years ago, it has been the aim of several experiments to measure CP violating effects in the decay of the long lived neutral kaon KL into two pions. The CPLEAR experiment has a different

and unique approach. It measures time dependent asymmetries by tagging KO's and p’s produced in proton-antiproton annihilations at rest and detecting their decays into two- pion, three-pion and semileptonic final states. This allows the determination of several

R. Adler et al. I Recent results of the CPLEAR experiment 439c

CP as well as T violating parameters and provides tests of CPT invariance. To allow a discussion on how various measurable quantities are sensitive to the possible violation of these discrete symmetries, a short summary of the phenomenological parameters used to describe CP, T and CPT violation effects is given in the following without assuming CPT invariance from the very beginning [1,2].

In the neutral kaon system, one distinguishes between the eigenstates of the whole

interaction KL,Ks and the eigenstates of the strangeness conserving strong interaction K’,p. Due to the weak interaction the latter are mixed. Solving the time dependent

SchrZdinger equation with pertubation theory and allowing for a possible T and/or CPT violation, one finds

I& > = 1 + ET + &PT)IK" > +(l - ET - &PT)(F > 1 (1)

- IKL> = 1 + ET - 6CPT)IK" > -(I - ET + &PT)lK' >

1 >

where ET and &PT are small compared to 1. The parameters ET and &PT can be expressed as functions of the elements of the K” and p mixing matrix as follows:

ET = < KOIHIK’O > - < KOJHpFJ >

i(rs - YL) + 2(W - ms) (3)

-

‘%PT = < KcJH]= > - < KOJH]KO >

qrs - 7L) + 2(% - ms) ’ (4

where mL, ms are the masses and 7~, 7s the decay widths of KL and KS. From equation 3 one can show that eT would vanish if T invariance was assumed (the transition proba- - bility of K” going to K” is equal to that of p going to K’). If CPT is a good symmetry, &pT (eq. 4) becomes zero (the mass and lifetime of K” and p are the same). One can therefore conclude that a non vanishing value of ET is a measure of T violation in the mixing of the neutral kaons, while a value of 6cj7T different from zero would be a signal of CPT violation. If both T and CPT were good symmetries, CP symmetry would be respected as well.

Until today, the only symmetry violation observed in the mixing of neutral kaons was in the decays of KL into two pions, and in the charge asymmetry of the Kr, semileptonic decays [3]. These measurements show that the particle KL is not identical to the pure CP odd eigenstate K2 but that it has a small admixture EL of the CP even eigenstate K1:

~KL, >= IKz > SELIKI > .

Using the relations between K1, K2 and K”, p

(5)

IKI > = $[lK" > +IF>]

I&> = $[lK’>-IF>] ,

(6)

(7) one can show with equation 2 that

et = ET - &PT , (8)

44oc R. Adler et al. I Recent results of the CPLEAR experiment

which means, that the measured CP violation (EL # 0) requires either T non invariance (ET # 0) or CPT violation (&p~ # 0) or both. The question of whether T and/or CPT are violated can be answered by introducing the quantity t r< Ks(KL >E 2[Re(cT) - ;Im(&p~)] and by looking at the phase of the measurable quantitity T+_

,

which is related to the phenomenological parameters Ed and e’ by

11+- M eL + e’ ) (10)

where E’ is a measure of direct CP violation in the decay of the neutral kaon into two pions. Applying unitarity and assuming that E’/Q is small compared to unity, one can show that

In the case of CPT invariance t is real and ‘p+- is expected to be [3]:

(Cm-) = tan-l 2(mr. - ms) ‘p+-

7s = (43.7 f 0.1)”

(11)

(12)

If one assumes T invariance, t would be imaginary and cp!T_) would have to be ‘p!C_PT)+900. Since p+_ is measured to be [3]

Gsured = (46.5 f 1.2)” (13)

one can conclude, that the observed CP violation is mainly related to T violation in the mixing of neutral kaons. However, T violation has never directly been detected. The CPLEAR experiment allows for the first time a direct measurement of Ed. This is discussed in another contribution to this conference [5]. In addition, the CP violating parameter ES, which can be measured in the decay of the KS into three pions, has never been determined directly, but will be measured as part of the CPLEAR program. The experimental quantity to be measured is defined as:

(14)

where 713~ is expected to differ from es only by a small term of the order of e’ and hence be a good measure of the CP odd admixture in the KS:

JKS > = ]K1> +cs]Kz > . (15)

Since from equation 1 we may write

ES = ET + bT , (16)

then under the assumption of CPT invariance one expects ES = EL = cT (eq. 8, 16).

2. THE CPLEAR EXPERIMENT

Through the decay of Ka and K’l into different final states, the CPLEAR experiment will measure the following symmetry violating parameters:

R. Adler et al. 1 Recent results of the CPLEAR experiment 441c

In the x+x- final state: lg+_ 1 which is sensitive to EL, (eq. 10) and cp+- which allows a test of CPT invariance (eq. 12).

In the x+x-x0 final state: )q+_ol which is sensitive to es (eq. 14, 15).

In the semileptonic final state: ET which is a direct measure of T violation (eq. 3).

In the x0x0 final state: the CP violating parameter ~00.

In addition to these symmetry violating parameters, CPLEAR will measure in the semileptonic decay of the neutral kaon:

l The mass difference between KS and KL which is an important quantity for the determination of cp+- (eq. 20).

l The parameter 2, which provides a test of the AS = AQ rule.

The method of CPLEAR is to measure symmetry violating effects through the obser- vation of particle-antiparticle decay asymmetries in detecting initial K” and p decays into various final states. This method takes advantage of the particularity of the neutral

kaon system, where the mass difference between KS and KL is approximately equal to half the decay width of the short lived kaon. Therefore the interference between KS and KL is observable in the decay of initial K”‘s and F’s before the short lived kaon has disappeared. The decay rate of K” and p into, for example, a two pion final state f as a function of its decay time is given by

‘R(t)) oc eWrst f (7jf12e-rLt ?z 2\7fle-~(7S+rL)t cos(Amt - cpf) (17)

and allows the determination of 1vfl and pf, where nf = A(KL --) f)/A(Ks + f). It is advantageous to measure the ratio and difference of rates, which leads to the time

dependent asymmetry Af(t) defined as

,Q(t) _ R(t) - R(t) R(i) + R(t) ’

since CP violation appears in the rate differences and since systematic errors are mini- mized in the ratio of rates.

To measure such asymmetries, the CPLEAR experiment has to produce and tag K”, F and to detect their decay products. The K” and F are produced in proton-antiproton annihilation at rest:

PF -+ K-?r+K”

K+7&:

BR = 2 x 1O-3

PF + BR = 2 x 1O-3 OQ)

The antiprotons are provided by LEAR. Because of flavour conservation in the strong interaction, the strangeness of the neutral kaon can be tagged by measuring the charge of the accompanying charged kaon. The missing momentum and missing mass of the K” and F are then reconstructed by determining the vector momenta of K* and 6. The decay

442~ R. Adler et al. 1 Recent results of the CPLEAR experiment

channels are identified by detecting the decay products **, Y’, e* and by reconstructing the decay vertex of the neutral kaon. This requires precision tracking of the charged particles and measurement of the position and energy of photons from the r” decay. The full event is finally reconstructed by measuring the vector momenta of the secondary particles and by applying an overall constraint fit.

Figure 1 shows a cross-section of the CPLEAR detector, which has been designed to perform these measurements. It also shows a proton-antiproton annihilation into K-r+K’ with the neutral kaon decaying into two charged pions. The detector, which is contained in a solenoid providing a magnetic field of 0.44 Tesla, has a cylindrical shape, is 3.6 m long and has a radius of 1 m. The antiprotons stop and annihilate in a spherical hydrogen gas target at 16 atm pressure. Two multiwire proportional chambers (PC) and a set of six drift chambers (DC) perform the precision tracking of charged particles. In addition, two layers of streamer tubes (ST) all ow a fast online reconstruction of the coordinates along the axis of the cylinder. The charged kaons and pions are identified and separated by the particle identification detector (PID) consisting of a scintillator-Cherenkov-scintillator sandwich. The Cherenkov (C) is a threshold counter producing light for pions above 190 MeV/c but giving no signal for kaons below 670 MeV/c. The scintillators (Sl, S2) are used for dE/dx and for time of flight measurements. A high granularity gas-sampling electromagnetic calorimeter (CAL) made of small high-gain tubes allows the detection of photons and the identification of electrons with a spatial resolution on a single photon conversion point of 5 mm. The small branching ratio of g&en channels (eq. 19) and the high pionic background from pp annihilation requires a dedicated high level trigger system to provide an efficient online event selection. The initial antiproton rate of 1 MHz is reduced to a final trigger rate of less than 1 KHz by the different trigger levels which include X+ and K- identification, track parametrisation, kinematic cuts at the primary vertex and pattern recognition of clusters found in the electromagnetic calorimeter.

3. RESULTS

The results discussed here are based on data taken in four short running periods in 1990 and 1991 where only part of the trigger processors and part of the calorimeter electronics were installed. These periods were mainly devoted to detector studies and trigger development. Results with part of this statistics were were presented elsewhere

[41* The data were collected with two different kind of triggers. The CPLEAR trigger

requires two or four tracks with at least two tracks hitting the proportional chambers, to ensure a primary vertex inside the target. This trigger selects all possible K’,?!?’ final states (?r+%-, rr”?yo, 37r, semileptonic) but at that time suffered from a too high rate due to the fact that the trigger electronics was not completed. The long distance trigger requires at least four tracks with only two of them being inside the proportional chambers volume. This low rate trigger selects specifically neutral kaon decays into two charged pions in the lifetime region above 57s where the interference of KS and KL is maximal. Below we will discuss the two-pion (charged and neutral) and the three-pion final states. Further


results on measurements for the semileptonic decay channel are presented elsewhere [5].

3.1. The two charged pion final state The parameters q+_ and ‘p+- are measured in the decay of K”,p into rr+rr- through

the time dependent asymmetry A+- (eq. 18):

A+-(t) = 2Re(c) - 2~9+_~ef(‘S-‘L)* COS(b?tt - Q+_)

1 + [7j+_1*e(~s-7Llf

From data collected with the long distance trigger (4~10~ events) and with the CPLEAR trigger (2.10’ events), four track events (Fig.1) are selected offline with one kaon candidate of at least 350 MeV/c transverse momentum. On these events a kinematic and geometric constrained fit is applied to reduce the residual background from the three-pion and

semileptonic decay channels. The decay time distribution for all selected neutral kaon decays into two charged pions, corrected for acceptance, is displayed in Figure 2. The measured decay time resolution is O.lrs, where rs is the KS lifetime [3]. In the region below 10~ the data are clearly dominated by the two-pion decays. At large lifetimes the

+ distribution shows the expected KL to T A - decays due to CP violation and in addition a significant fraction of three-body background. A fit to the curve, leaving the value for the KS lifetime and the amount of background from the KL decays as free parameters, results in a value for the KS lifetime:

7 = (0.98 f O.Ol)rs . (21)

Fitted lifetime = (0.98tO.01)~~

Figure 1. The CPLEAR detector Figure 2. Lifetime distribution of the K” and p decays into z+rr-

Before extracting the asymmetry between the tagged K” and %? decays, a normaliza- tion has to be applied due to the different interactions of K+ and K- with the detector

444c R. Adler et al. ! Recent results of the CPLEAR experiment

material. The normalisation factor cx = N(p)/N(K’) = 1.14 f 0.01 has been determined using data at short decay times where the interference effect is negligible. Within the present statistics there is no variation of a as function of the kinematic variables such as the kaon momentum, as can be seen from Figure 3.

The time dependent asymmetry A+_, shown in Figure 4, is formed from the normalized

rates without any acceptance correction, since this cancels out in the ratio of equation 18. The dashed line is a fit to the data using equation 20, leaving II+_( and ‘p+_ as free parameters. In the region below 10~ the fit agrees well with the data whereas for larger lifetimes the measured asymmetry is diluted with the contamination of the three-body KL decays. The fit shown by the solid line takes this background into account and gives final values for the fitted parameters of:

)7+-j = (2.30 f 0.10 f 0.04) x 1O-3

‘p+- = 44.9” f 2.5” f 0.5” . (22)

The values have been corrected for regeneration effects by (-0.08 f 0.03) . 10e3 on T+_ and ($1 f 0.4)” on ‘p+_. The first error gives the statistical significance including the normahsation and the second error the systematic uncertanties. The error from the normahsation Q is dominated by the limited statistics of data from the short lifetime region. The main source of systematic errors is the correction for regeneration effects and the magnitude of the three-body background. The limited knowledge of the mass difference between KS and KL [3] t d in ro uces an additional error on cp+- of f0.8” which has not been included in the results.

Oo 100 200 300 400 500 600 700 so0 K” Momentum in MeV/c

100 200 300 400 5Qo 600 700 &Jo K” Momentum in ~eV/c

,,.,,.,,.,,,,,,..,,I,,.,,.,.,,,,.,

0 100 200 300 400 500 600 700 800 K” Momentum in M&/c

Figure 3. Number and ratio of tagged K” and p as function of their momentum

-0.6 assuming no background

‘._,’

, 0 2 4 6 6 IO 12 14 16 18

Neutral kaon Eigentime rT

Figure 4. The time dependent asymmetry A+_


3.2. The two neutral pion final state The determination of 700 and 900 from the neutral kaon decay into x0x0 from the

time-dependent asymmetry ADo requires the reconstruction of the Ko,%$ decay vertex

which cannot be directly measured, as in the case of the charged pions, by fitting the secondary tracks. Here the secondary vertex is reconstructed in a global constrained fit using the information of the electromagnetic calorimeter on the photon conversion point and the photon energy from neutral pion decays. The data for this analysis were taken in only one running period at the end of 1991 with the CPLEAR trigger, when the

electromagnetic calorimeter was for the first time fully operational. In the offline analysis, events with one charged kaon candidate of more than 350 MeV/c transverse momentum and two tracks with opposite charge were selected, demanding four photon showers in the calorimeter. The global fit requires energy and momentum conservation, the missing mass of the charged tracks to be equal to the kaon mass and the invariant mass of each

photon pair to be in agreement with the K’ mass. An event passing this fit is displayed in Figure 5.

To study the background in the selected events, a r”?yo invariant mass spectrum can be formed without constraining the K” missing mass. As can be seen from Figure 6 such an invariant mass spectrum shows a peak at the expected K” mass with a resolution of

B w 25 MeV/c*. Studies of the asymmetry A00 have recently started with high statistics using the first data taken in 1992.

1600 :

1400 I II’ after 5 C-fit

f 1200 -

I a=25MeV/c2

1000 : I ’ t

800 : 1

600 I t + t L ,

“Iz~--” n07ro invariant mass h4eV/c*l

IO

Figure 5. A K” -+ T’?T’ event in the the CPLEAR detector

3.3. The three-pion flnal state

Figure 6. The A’?T’ invariant mass

The CP violating parameter 73r (eq. 14) can be measured either in the a+n-?r’ or the K’A”IF’ decay of KS. In the former case the CP eigenvalue depends on the relative

446~ R. Adler et al. 1 Recent results of the CPL.EAR experiment

angular momentum I of ?T+?T- and 7r”:

cI+r+*-no >= (-1)‘+‘17r+*-7r0 > . (23)

A determination of CP violation from the KS --i ~+x-K’ decay would therefore imply a Dalitz plot analysis. However, the contribution from the CP conserving KS -+ &KA’ decay amplitude vanishes in the KS, KL interference if 7~+ and 7~~ are detected over all the phase space. Therefore, a non vanishing time dependent asymmetry A+_0 (eq. 18) is a clear evidence for CP violation in the KS decay without any angular momentum analysis

[61. The 37r” final state is a pure CP odd eigenstate and would therefore signal a clear CP

violation if detected in the KS decay. However, from the experimental point of view, the detection of two charged pions and of only one neutral pion from the ?~+K-K’ decay allows a better reconstruction of the neutral kaon decay than a six photon topology from the 3s’ decay.

The expected statistics from 1992 and 1993 should allow a statistically significant determination of q+-0. The 1991 data taken with the general but uncomplete CPLEAR trigger, as indicated earlier, show that the &A-A’ channel can be detected and distin- guished from the large two-pion background. Requiring four track events with at least one photon detected in the calorimeter and assigning to the missing momentum the ?Y’ mass, Figure 7 shows the invariant mass for the ?~+A-x’ final state. The mass resolution on the reconstruction of the neutral kaon is u x 14.0 MeV/c2.

3000 - lT+?T-I-?

tt +

N(y)>0 t t

+ t +

2000 - + +f

I + 4 4-e**

o

Sec. inv. mass tMeV/c*l

Figure 7. The invariant mass of the 7r+?r-7r” final state

R. Adler et al. I Recent results of the CPLEAR experiment 44lc

4. CONCLUSIONS

The results from early data taken by the CPLEAR experiment in 1990 and 1991 have

shown the validity of the CPLEAR method, using K”,p decay asymmetries to detect CP as well as T violation and to provide tests of CPT invariance. Our measurements of the CP violating parameters IT+- 1 and ‘p+- are in agreement with the world average values [3].

The improved 1992 and 1993 data collection with a fully operational trigger and electromagnetic calorimeter should ahow the improvement of the precision of x+x- CP-violation

parameters, the determination of 700 from the two neutral pion final state and, for the first time, the detection of CP violation in the &x-x0 KS decay.

5. REFERENCES

1 T. D. Lee and C. S. Wu, Annual Rev. of Nucl. Science 11 (1966) 511.

2 T. D. Lee, Particle Physics and Introduction to Field Theory, Harwood Academic Publishers, New York (1981).

3 Review of Particles Properties, Phys. Rev. D45, Part 2 (1992). 4 R. Adler et al., (CPLEAR Collaboration), Phys. Lett. B286 (1992) 180. 5 “A Study of T Violation via the Semileptonic Decays of Neutral Kaons in CPLEAR”,

CPLEAR Collaboration, presented by K. Jon-And at this conference. 6 L. M. SehgaI, L. Wolfenstein, Phys. Rev. 162 (1967) 1362.

Recent results of the CPLEAR experiment

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