CP violation results on Kaons by the NA48 experiment

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ELSEV1ER Nuclear Physics B (Proc. Suppl.) 133 (2004) 227-232

i l ll[O] I ~':~ : i "-] -" i'k'~ [ ~ 1 :! P R O C E E D I N G S S U P P L E M E N T S

www.elsevierphysics.eom

CP violation results on Kaons by the NA48 experiment

M. L e n t P *

a I N F N Sezione di F i r e n z e ,

V i a G . S a n s o n e 1, 50019 S e s t o F . ( F i r e n z e ) , I t a l y

Recent results on CP violation on kaons by the C E R N NA48 experiment will be discussed. A preliminary measurement of the charge a s y m m e t r y in KL ~ 7r4-eTv has been made: ~eL = (3.317+O.070stat 4-0.072~y~t) x 10 -3.

A search for K s --+ 3~r ° decays has been performed with a preliminary estimation of the real and imaginary p a r t of the rlooo parameter: Re(rl000 ) = ( - 2 . 6 4- 1.0star 4- 0.5~v~t) x 10 -2 and Im(rlooo ) = ( - 3 . 4 4- 1.O~t~,t 4- 1.1~y~t) x 10 -2. The angular a s y m m e t r y in KL --+ ~r+Tr-e+e - has been measured to be A~ = (14.2 ± 3.0star -t- 1.9~y~t)%, while no a s y m m e t r y has been observed in K s --+ ~r+1r-e+e-: A~ = (0.5 4- 4.08tat ± 1.6syst)%. Finally, the direct CP violation in KS,L --+ 2fr has been clearly settled, with Re(e//e) = (14.7 ± 2.2) x 10 -4.

1. I N T R O D U C T I O N

Neutral K a o n s were the place w h e r e C P violation w a s discovered for the first time. N A 4 8 is an experiment dedicated to the precise study of C P violation in the neutral K a o n system: its b e a m setup is m a d e of t w o neutral b e a m s [I], one d o m - inated b y

K L

decays, the other by

K s

decays. 1 , 1 . T h e b e a m s e t u p

K S target

I

K,,.,oe, I ~o~, I I IOl I; ~ []

A

I

l _ k l

l

r - ~ • + ' . d Z - i " ~'~' ~

Bent e~s+al :_e .~_

o =o

~126m - 1 1 4 m

Figure I. T h e N A 4 8 b e a m setup.

A p r i m a r y proton b e a m f r o m the C E R N Su- *on behalf of the N A 4 8 collaboration

per P r o t o n Syncroton impinges, with a n o m i n a l flux of 1.5xl012 particles per spill, on a beryllium target ("far target") 40 c m long a n d 2 m m wide; after a m a g n e t sweeping sector a n d several stages of collimation, a neutral b e a m is formed. T h e exit face of the last collimator is located 126 m d o w n s t r e a m of the target, at the entrance of the fidncial k a o n decay region.

T h e p r i m a r y protons w h i c h have not interacted in the target are deflected towards a bent silicon crystal: a small fraction of these protons is chan- neled by the crystal a n d f o r m a secondary proton b e a m w h i c h is transported towards a second beryllium target ("near target" ); after only 6 m of m a g n e t sweeping a n d collimation another neutral b e a m is formed.

T h e t w o neutral b e a m s can be present at the s a m e time ("simultaneous b e a m runs" ) or one per time ("far target run" or "near target run") if only

K L

or

K s

decays have to be studied.

T h e secondary proton b e a m is usually at m u c h lower intensity (about 3 x 1 0 7 particles per spill) with respect to the p r i m a r y one. In s o m e special runs ("High Intensity K s runs") the p r i m a r y proton b e a m is sent directly to the second target, r e m o v i n g the first target a n d by-passing the silicon crystal; the proton b e a m is attenuated a n d collimated to the desired intensity far u p s t r e a m of the second target.

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228 M. Lenti/l"V~tclear Physics B (Proc. Suppl.) 133 (2004) 227-232

1.2. T h e D e t e c t o r s

~ L ~ r ~ t 217¸1 m

Figure 2. T h e NA48 detectors.

T h e detectors are located downstream of the kaon decay volume which lies inside a large, 90 m long, vacuum t a n k t e r m i n a t e d by a 0.3% radia- tion lenghts thick Kevlar window; starting at the center of the Kevlar window, a 16 cm diameter vacuum beam pipe traverses all the detectors to let the neutral beam pass through vacuum.

Charged particles are detected using a high resolution magnetic spectrometer which consists of a dipole magnet with a horizontal transverse mo- m e n t u m kick of 265 M e V / c and a set of four drift chambers, two of them located upstream of the magnet and two downstream. T h e magnetic spec- t r o m e t e r is contained inside a t a n k filled with he- lium in order to reduce multiple scattering. T h e m o m e n t u m resolution is:

ap(%) = 0.48 @ 0.009p (p in G e V / c ) (1) P

A quasi-homogeneous liquid k r y p t o n (LKr) calorimeter is located downstream of the spectrometer. This detector has a 127 cm long pro- jective tower structure which is made of copper- beryllium ribbons extending between the front and the back of the calorimeter with a 4-48 mrad accordion geometry. The 13212 readout cells each have a cross-section of 2 × 2 cm ~. The energy resolution is:

3.2 9.0

(%) = ~ • -~- G 0.42 (E in GeV) (2)

Two planes of scintillators, segmented in horizontal and vertical slabs, form the charged ho- doscope, located in between the magnetic spectrometer and the LKr calorimeter; it is used for triggering and measuring the time of charged particles.

Behind the LKr electromagnetic calorimeter, a 6.7 nuclear interaction lenght thick hadronic calorimeter is located, followed by a set of three planes of muon veto counters.

1.3. D a t a s a m p l e s

Section 2 is based on d a t a collected with the "simultaneous beams" setup in the year 1999.

Section 3 is based on d a t a collected in the year 2000 with a 40 days long "near target high intensity run" and a 40 days long "far target run". The magnetic spectrometer was not present in these two runs.

Section 4 is based on d a t a collected with the "simultaneous beams" setup in the year 1998 and 1999 and with the "near target" setup in a 2 days long high intensity run in 1999.

Section 5 is based on d a t a collected with the "simultaneous beams" setup in the year 1997, 1998, 1999 and 2001.

2. C H A R G E A S Y M M E T R Y I N /(L --~

~ e T y

The charge a s y m m e t r y in K~3 decays is defined

as

F(KL --+ ~-e+u~) - F(KL --+ 7r+e-#e)

5~ - r(KL -~ ~-e+.~) ¥ F(/~L -~ ~ + e - ~ ) (3)

If C P T s y m m e t r y holds, this observable is equal to 2Re(e).

During the 2001 run about 2 × l0 s K~3 decays have been collected, along with n~ samples for Re(et/~) measurement. T h e systematic effects due to detector a s y m m e t r y are reduced by regular changes of the spectrometer magnet polarity. The backgrounds could be reduced to a negligible level by selection cuts. T h e main source of systematic uncertainties arises from charge dependent inter- actions of ~ i in the electromagnetic calorimeter which are reflected in asymmetric trigger and par- ticle identification efficiency (Table 1).

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M. Lenti/Nuclear Physics B (Ptvc. Suppl.) 133 (2004) 227-232 229

Table 1: Systematic corrections to 6~ in 10 - s Trigger +26.2 4- 6.0 Punch t r o u g h - 1 . 4 4- 3.5 Pion ID - 1 7 . 1 4- 2.4 Acceptance 4-0.5 Background 4-0.5

T h e corrections are calculated using various control samples involving charged pious, like K s -~ ~r+Tr - and K L ~ 7r+~r-~r °. 5 • 10 "3 5.0 4,0 3.5 1,5 ' ' ' , 10 20 30 40 50 50 70 80 90 100 110 P~ (GeV/c)

Figure 3. 6~ as a function of pion m o m e n t u m .

T h e d a t a for this m e a s u r e m e n t has been collected during the last period of 2000 d a t a taking in special conditions with high intensity near target b e a m and without the spectrometer. Almost 6.5 x 106 K ° -+ 7r%r°zr ° events have been collected. This d a t a is normalized to pure 1425 -+

7r°lr°Tr° sample from far-target b e a m run (about 155 x 106 events) i m m e d i a t e l y preceding the near- t a r g e t run in practically the same detector conditions, which lead to cancellation of the acceptance in the first order. T h e result is o b t a i n e d from a fit to a function f n e a r f ( E , t ) - 3.0 = A(E)[1 + Irloool2e tt~L-tl~s T f a r ~37ro + 2D ( E)e t l 2 ~ - t t2~s (Re(~0oo) c o s ( A m t ) -Irn(r]ooo) sin(Amt))]

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in kaon energy E and proper t i m e t bins. T h e

D ( E ) denotes K ° - / ~ o production a s y m m e t r y

(dilution) at the t a r g e t and is fixed to the values m e a s u r e d by NA31 [3].

T h e charge a s y m m e t r y in Ke3 decays, after all the corrections, is shown in Fig.3 in bins of pion m o m e n t u m .

T h e preliminary result is

5; = (3.317 ± 0.070~t~t + 0.072~y~t) × 10 -~ (4) which is in good a g r e e m e n t with previous mea- surements [2].

3 . S E A R C H F O R K s -+ 3~r °

T h e CP violating p a r a m e t e r U00o is defined as

A( K s -+

~°~°Tr°)

~ o o o = A ( K L -+ 7r°Tr°zr °) (5)

If C P T s y m m e t r y is conserved, the real p a r t of r/ooo is given by CP violation in mixing and is equal to Re(e) while the i m a g i n a r y p a r t can be sensitive to direct CP violation.

0.06 ¢:.- ~.~.,~0.04 0,02 -0,02 -0.04 -0.06 -0.08 -OA -OA

F

R . . . ~ ~ 6 ± ] . O t . . . . . . . + 0 . 5 . J.)x 1 ~ q . " 000 ={.-q 4 ± l.O~ + I N . . . ~ a~ -- s.,. ) X X ~ I ndf = 4 i 51405 t,-2 iO 99 % C L ~ _{Fixim [e 11ooo = Re a:} Im qo~l = (-1.2 _+ 0.7 +11 ,)xl0 -2 _~tat- " sys -0.08 - 0 . 0 6 -9.04 -0.02 0.02 0.04 0,06 Re qooo

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230 M. Lenti/Nuclear Physics B (Proc. Suppl.) 133 (2004) 227-232

T h e fit results in (Fig.4)

Re(7ooo) = ( - 2 . 6 ± 1.Ostat ± 0.5syst) X 10 -2 (7)

Im(qooo) = (--3.4 ± 1.Ostat ± 1.1syst) × 10 -2 (8)

T h e systematic uncertainties are summarised in Table 2.

Table 2: uncertainties in 7000 in 10 - 2

Re(7ooo) r'~(7ooo)

Accidentals ±0.I ±0.6 Energy scale ±0.i ±0.i K o _ / ~ 0 dilution ±0.3 ±0.4 Acceptance ±0.3 ±0.8 Binning ±0.i ±0.2

Total ±0.5 ±i.i

T h e uncertainty of this preliminary measurement is ten times smaller t h a n the best previously published result by C P L E A R [4].

Assuming C P T conservation and fixing the real part of 70oo to Re(c) = 1.6 × 10 -3, the imaginary

part of 7ooo can be constrained with b e t t e r preci- sion to

Im(7ooo) = ( - 1 . 2 ± 0.7star ± 1.1syst) × 10 -2 (9)

This corresponds to B R ( K s --+ ~r%r%r °) < 3.0 ×

10 -7 at 90% CL. This result improves by two or- ders of magnitude the best limit obtained by the SND collaboration [5].

4. A N G U L A R A S Y M M E T R Y I N KL -+

7 C + T r - e + e -

T h e K L -~ 7r+Tr-e+e - decay is d o m i n a t e d

by the CP-violating inner bremsstrahlung (IB) t e r m and by the CP-conserving direct M1 emis- sion (DE) graph. The interference between the IB ( C P = + I ) and the DE ( C P = - I ) amplitudes produces a CP-violating circular polarization of the 7"- If ¢ is the angle between the e+e - and ~c+~v - planes, the following a s y m m e t r y can be measured:

Nsin ¢ cos ¢>o - Nsin ¢ cos ¢<0 (10) A~ = Nsin ¢ cos ¢>o T Nsin ¢ cos ¢<o

The prediction by the Heiliger and Sehgal[6] model is A¢ ~ 14%. z

J

.% 4 KL --+ ~+~e+e" (a 2 1 0 i i i I i i , ~ , , , t , , , t , , , i -0.5 -0,3 -0.1 0,1 0,3 0.5 ~ = s i n ~ c o s q ~ 4 5 21 I < A ~ r ~ > = 3 . 4 9 % O F t i L I i i i I L i i I i i i I i t ~ -0.5 -0.3 -0,1 0,1 0.3 0,5 ¢=sln~cos~ 5 5 [ - . A c c e p t a n c e c o r r e c t e d [ I 2 = • - ° 1 0 -0.5 -0,3 -0.1 0.1 0,3 0.5 ¢=sin~cos~

Figure 5. Distribution of KL --+ 7r+~-e+e -

events in the angular variable sin ¢ cos ¢ before(a) and after(c) acceptance corrections. T h e his- tograms are Monte Carlo predictions. Accep- t a n c e as a function of sin ¢ cos ¢ (b); the solid line is a polynomial fit to the Monte Carlo calcu- lation.

1162 KL -+ 7~+Tc-e+e - candidates have been selected in data collected in 1998 and 1999. T h e angular a s y m m e t r y before any acceptance correction is (24.9 ± 2.9~tat)%. T h e measured A~, obtained in a model-dependent w a y by taking into account the acceptance correction is['/]:

A~ = (14.2 ± 3.0~tat ± 1.9~yst)% (11) and it is a clear signature of indirect CP violation. T h e K s --+ 7c+Tr-e+e - decay is dominated by the CP-conserving inner bremsstrahlung (IB) term only and no angular a s y m m e t r y is expected:

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M. Lenti/N2tclear Physics B (Proc. Suppl.) 133 (2004) 227-232 231 .e- 4 " O < 3 K s --> n+~z-e+e" (a) A ¢ = (0.5 + 4.0st~t)% 0 , . . I , . , I , , . I . . , I . , . - 0 . 5 - 0 . 3 -0,1 0,1 0,3 0.5 ~=sin~ocos~o 5 E (b) K s ~ x+~z'e+e" ..+- _- ~ 4-4-4_ + 4 _ ~ -+- 4 - , . _+_4 <A~=ee> = 3.51% 2 i i , i , i i I i , i I , i i I i a i - 0 . 5 - 0 . 3 -0.1 0.1 0.3 0.5 ¢=s[n~cose#

Figure 6. Distribution of events(a) and acceptance(b) for K s -+ 7r+Tc-e+e - decays as a function of s i n ¢ c o s ¢. The histogram in (a) is the Monte Carlo prediction. The solid line in (b) rep- resents the average value of the acceptance.

it is an i m p o r t a n t cross-check of t h e / ( L analysis. Since no interference term is present in the ampli- tude of this decay mode, the acceptance correction is uniform over the entire sin ¢ cos ¢ domain. Based on the d a t a collected in 1999 the asymme- t r y is[7]:

A~ = (0.5 4-

4.08tat 4-

1.68y~t)% (12)

which is compatible with a null a s y m m e t r y and gives confidence that A~ is not generated by de- t e c t o r effects.

5. D I R E C T C P V I O L A T I O N I N

KS,L -->

27r

The double ratio of decay widths of K s and

KL

into 2 pions is given by:

F ( K L --~ ~r%r 0 )

r(Ks ~r%r0) _ (el

R - r ( g c - ~ + ~ - ) 1 --

6Re --2J

(13) r(Ks-~rr+rr-)

If the four modes are taken in the same decay region and simultaneously, R reduces to the double ratio of the number of decays. The data have been collected in the years 1997, 1998, 1999 and 2001. T h e main aim of the 2001 run was not only to add more statistics to the already published

Re(el~e)

result [8] but also to perform additional systematic checks. This was possible due to several changes in the experimental conditions (Ta- ble 3).

Table 3: Change in conditions of 2001 run

98-99 2001

proton energy 450 GeV 400 GeV SPS cycle time 14.4 s 16.8 s spill lenght 2.4 s 5.2 s effective s.1. 1.7 s 3.6 s duty cycle 0.17 0.31 /£L beam intensity (ppp) 1.5 x 1012 2.4 x 1012

K s

beam intensity (ppp) 3 x 107 5 x 107

Besides rebuilt drift chambers and change in the beam energy, a significant increase of the duty cycle allowed to collect 29% of the total statistics with reduced instantaneous beam intensity. The fact that the result is fully compatible with the one obtained from 1997-1999 period confirms the robustness of the

Re(erie)

measurement with respect to the accidental beam activity.

The result from 2001 d a t a only is [9]

Re(ez/e)

= (13.7 4-3.1) x 10 -4 (14) the result from 97-98-99 d a t a is [8]

Re(el/e) = (15.3 + 2.6) × 10 -4 (15) and the combined result is

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232 M. Lenti/Nuclear Physics B (Proc. Suppl.) 133 (2004) 227-232 I~C 1.1 1.o5 Z 2/ndf = 2 7 / 1 9

4-,,-++,----'- •

~.++ +4.-'~

, .- f. # 4 p , "r ~ T 0.95 0,970 810 ' 910 'l/O'lrO ' 0 1 _{120 130 140 150 160}i , i , i , i , i _{' 1 7 0} K a o n E n e r g y ( G e V )

7. A.Lai et al., CERN-EP/2003-006, submitted to EPJ.

8. A.Lai et al., Eur. Phys. J. C 22 (2001) 231. 9. R.Batley et al., Phys. Lett. B 544 (2002) 97.

Figure 7. The Double Ratio versus kaon energy.

This final result reaches the design accuracy and leads to a world average of (16.6 ± 1.6) × 10 -4.

6. C O N C L U S I O N S

The NA48 experiment provided several measurement of CP violation effects in the neutral kaon system. In the year 2003 a search for CP violation in the charged kaons sector will be performed.

R E F E R E N C E S

1. N.Doble et al., Nucl. Instr. Meth. B 119 (1996) 181.

2. A.Alavi-Harati et al., Phys. Rev. Lett. 88 (2002) 181601.

3. G.Barr et al., Phys. Lett. B 317 (1993) 233. 4. A.Angelopoulos et al., Phys. Lett. B 425

(1998) 391.

5. M.N.Achasov et al., Phys. Lett. B 459 (1999) 674.

6. P.Heiliger and L.M.Sehgal, Phys. Rev. D 48 (1993) 4146;