Status of the KLOE Experiment

STATUS OF THE KLOE EXPERIMENT

V.V.KULIKOV

Institute for Theoretical and Experimental Physics, Moscow 117259, Russia

In 1999 the KLOE experiment started to take d ata at e+e−-collider DAPHNE at Frascati. Tothe end of 1999 an integrated luminosity of 2 pbn−1 which is equivalent to 107 φ-meson decays hasbeen acquired. This data sample has been used to study the detector performance and to startdata analysis in all directions of the wide physics program of the KLOE experiment.

Keywords: accelerator, calorimeter, chamber, meson, kaon, CP violation

1 INTRODUCTION

A φ-factory named DAPHNE has been built at the Laboratori Nazionali di Frascati,sponsored by the Istituto Nazionale di Fisica Nucleare of Italy. DAPHNE is a 2x510 MeVhigh luminosity e+e−- collider. At final target luminosity of 1033cm−2s−1 DAPHNE willproduce 5000 φ(1020) mesons per second. φ-mesons predominantly decay into pairs of slowmonochromatic (approx. 120 MeV/c) kaons (K+K−49%, KsKl34%). This rather high kaonflux and very small hadronic background give unique possibility to study various problems ofkaon physics (Maiani,1995). Three experiments are prepared at DAPHNE - KLOE, DEARand FINUDA. DEAR will measure with high precision a hadronic shift of the deepest en-ergy level in kaonic hydrogen and deuterium. FINUDA main topic is a study of Lambda-hypernuclear levels and lifetimes, and their non-mesonic decays. The KLOE experimentwhich is the subject of this report, has wide physics program which can be summarised asfo llows:

• Study of CP and a search for CPT violation (this item has been discussed in detail atprevious school by J.Lee-Franzini (Lee-Franzuni,1998)) by measurement of

– ε′/ε in Ks and Kl decays to 2π

– interference effects in a time evolution of K0K0 system

– charge asymmetries in semileptonic decays of Ks and Kl

• Kaon physics

– form factors in semileptoni c decays of Kl and K±

– rare Ks decays

– Kl → Ks regeneration cross section at low momentum

• Non-kaon physics

– radiative decays of φ-meson

– low energy σ(e+e− → hadrons) from e+e− → π+π−γ with γ emission in initialstate

The KLOE detector is designed primarily with the goal of precision CP violation studies atDAPHNE. The neutral K m eson pairs produced at DAPHNE are in a well-defined quantum-mechanical (odd under charge conjugation) and kinematics state. One of the mesons is a Ks,travelling at DAPHNE on average 6 mm, while the other is a Kl, travelling on average 3.5 m.This gives KLOE unique advantage for the study of the neutral K meson system not availableelsewhere. Because of quantum mechanical coherence and large DAPHNE luminosity, acomplete set of precision ’kaon-interferometry’ experiments can be performed at D APHNE,determining all 16 parameters which define the system, including the measurements of ε′/εwith two alternative methods, via counting or via interferometry. No other experiment atany other accelerator can do this.

A special, highly innovative detector is necessary to accomplish the above objectives. Theapparatus must be able to track charged particles of momenta between 50 and 270 MeV/c.It must also detect with very high efficiency photons with energy as low as 20 MeV, measure their energies with a resolution of 15% at 100 MeV, and provide the coordinates ofthe photon conversion point. The above requirements place extremely stringent mechanicaldemands on the tracking drift chamber (DC), which is placed in front of the electromagneticcalorimeter (EMC). The chamber should be essentially a largely empty space. It must alsobe huge, since the scale of KLOE is driven by the average flight path of the Kl mesons, 3.5m. A practical compromise is to detect the Kl decays in a big, cylindrical chamber of radius2 m and length 3.7 m, with spherical end plates and walls all made of carbon fibre.

2 The KLOE DETECTOR

The detector lay out is shown in Fig.1. The KLOE detector components, in order of in-creasing radius from the beam interaction point are as follows. A 10 cm diameter beam pipesurrounds the luminous point which has dimensions 1.5 mm(H)x20 mkm(V)x3 cm(L) . Thebeam pipe is made of 0.5 mm thick beryllium alloy to minimise multiple sc attering, energyloss for charged K mesons and Kl mesons to Ks mesons regeneration. The magnets of thelow beta insertion as well as the veto calorimeters surrounding them are mounted directly onthe beam pipe, inside KLOE. The next component is the largest in the world DC which is acylinder with inner(outer) diameter 0.5m (4m) and a length of 3.7 m. It is filled with 12582

almost square (2x2 and 3x3 cm) cells arranged in 58 circular layers. To minimise multiplescattering of charged p articles and Kl regeneration the field wires were made of 80 mkmaluminium and He-isobutane (90:10) gas mixture was used. DC radiation length is 900 m.A lead scintillating fibre sampling EMC of superior timing abilities surrounds the DC. EMC Fig.1has 24 barrel and 2x32 end-cap modules. The modules are made of interleaved layers of 0.5mm thick Pb foil and 1 mm diameter scintillating fibres. The barrel module is of 52.5c min width, 23 cm (13 radiation lengths) in thickness and 4.5 m in length. ¿From both endsfibres from 4.4x4.4 cm2 area are viewed by R5946 PMTs with mesh dynodes, which work itmagnetic field. Total number of PMTs is 4880, total weight is near 100 T. DC and EMCare inside of a superconducting solenoid providing a magnetic field of 0.6 Tesla. Iron returnyoke covers the detector from all sides. Total weight of the detector is 1000 tons. All theelectronics was specially designed by the KL OE collaboration for the experiment, togetherwith all the software necessary for transmission, data flow control and data reduction. TDCand ADC for DC and EMC have on board buffer memories which guarantee zero dead time.VME based data acquisition system has a high throughput of 50 Mbytes per second andcan take 10000 events per second. This data flow is filtered, processed and put to 40 GbDLT tapes by a scaleable farm of UNIX computers which now has near 50 processors. 5500tapes can be dealt with available robot tape library with access time of only few minutes.Processed data flow is divided in five streams for further physics analysis, among them arepar example KsKl-stream and K+K−-stream.

3 DETECTOR PERFORMANCE

At the end of DAPHNE commissioning in November 1998 a luminosity of 1.6×1030cm−2s−1

in single bunch mode and 1 × 1031cm−2s−1 with 13 bunches was reached. It was a goodstart supposing linear extrapolation to a target value of 120 bunches in each ring. But afterthe KLOE detector was rolled-in and magnetic field was on in March 1999 the luminositybecame much lower. DAPHNE performance gradually improved and to the end of 1999 aluminosity of 3.5× 1030cm−2s−1 was achieved with 20-40 bunches. During the second half ofthe year the KLOE had physics runs. Fig.2 shows the integrated l uminosity acquired by the Fig.2detector versus the day of a year. To the end of 1999 the KLOE had an integrated luminosityof 2 inverse picobarns which is equivalent to 107 produced φ-mesons. This data sample islarge enough to check the detector performance. The best process to study performance ofEMC in all energy range of the detector from 510 to 20 MeV is e+e− → e+e−γ. It gives thefollowing EMC parameters:

• energy resolution σE/E = 5.7%/√E(GeV ),

• time resolution σt = 110ps/√E(GeV ),

• position resolution for electromagnetic showers σ⊥ = 1.2 cm perpendicular to the EMC

module axis, σ‖ = 1.2cm/√E(GeV ) along module axis.

Bhabha eventa, cosmic rays and Ks decay to π+π− are the best to study performance ofDC. They give the following DC parameters:

• hardware (software) efficiency - 99% (97%),

• resolution igmarφ = 150 mkm, σz = 2 mm

• momentum resolution σpt/pt = 0.5% for Bhabha events

• vertex resolution σxy = 0.5 mm for Bhabha events

Resolutions in effective masses which define a particle identification capability of the detectorare

• for π0 from 2γ decay in Kl → π+π−π0 σ = 20 MeV

• for Ks from decay to π+π− σ = 1 MeV

• for Ks from decay to 2π0 σ = 27 MeV

• for Kl from decay to 3π0 σ = 45 MeV

These values are in reasonable agreement with expectations based on previous study withcosmic rays and on prototypes. Some improvement is expected in a near future due to bettercalibration and more sophisticated treatment of a raw data.

4 A START OF PHYSICS ANALYSIS

The data sample acquired in 1999 is large enough to start physics analysis in all directions of a wide physics program of the KLOE experiment. It is also comparable with thedata collected in experiments on VEPP-2M in phi-resonance at Novosibirsk during all itslifetime. Taking into account a superior tracking capability of the KLOE DC some studies inkaon physics and radiative φ-decays are already possible on the best in a world level. Someachievements have been already made on this way. I mention here only two of them:

• Identification of the firs t few tens of CP violation events in KsKl-stream.

• Preliminary results on radiative decays of φ-meson ( φ → a0γ, φ → f 0γ, φ → η′γ)presented at EuroDAPHNE Collaboration meeting in Granada in February 2000.

I will illustrate a large potential of already taken data sample by an example of a precisemeasurement of the ratio R= BR(K± → π±π0)/ BR(K± → µ pmν) in which ITEP team ofthe KLOE collaboration is involved. In Fig.3 a typical event from K+K−-stream is shown Fig.3as presented by DIDONE event display. Only front view is given. K+ and K− mesons enterDC through inner wall from interaction point which is in a centre. Both kaons decay in DCvolume, one to ππ0 and another to µν.

Charged and neutral products of the kaon decays give hits in the barrel EMC. Fittedtracks are shown by solid lines. At the bottom the charged particle goes at large angle tothe middle plane of the DC. It can be seen by the large distance from the fitted track tothe hit signal wires in two stereo layers of DC in its outermost area. Using reconstructedmomentum of the charged particles the momentum of decay products in kaon rest frame canbe calculated. um part of a charge particle momentum distribution The high momentum Fig.4part of the distribution on this momentum is shown in Fig.4. Peaks from two body decaysK± → π±π0 (at pion momentum 205 MeV/c) and K± → µ±ν ( at muon momentum 235MeV/c) dominate the spectrum. Ratio of the events in these peaks gives the R-value afteracceptance corrections. A time evolution of this ratio and its statistical error is shown inFig.5 as a function of a run number (which is connected with the number of registered decays). Fig.5

Two horizontal lines give ± one σ deviation from PDG value (PDG,1998) for this ratio.It is seen that needed acceptance corrections are small as expected from small difference inkinematics of these two decays in nearly 4π acceptance of the KLOE detector. The changes ofthe ratio with a run number are connected with different trigger conditions used in differentruns. Corrections for this effect are yet to be done. But it is clear that statistical accuracyachieved to the end of 1999 is approximately three times smaller than world averaged valuewhich is mostly based on the data of an experiment (Usher,1992) at LEAR.

5 CONCLUSIONS

The first physics runs showed a good performance of the KLOE detector. Collected 107

φ decays are sufficient to start physics analysis in all directions of a wide physics programof the KLOE experiment. The detector is ready to take data at DAPHNE with increasedluminosity in 2000.

Acknowledgement sI am in debt to all my KLOE colleagues for their hard work which made the start of the datataking a real success. I also wish to thank Juliet Lee-Franzini for many helpful discussionsand Maxim Martemianov and Elena Turdakina for a help in a preparation of presentationmaterials.

References

Lee-Franzini, L. and Franzini, L.,”CP Violation in the K-sysmem”.Survey in High energyPhysics, Vol.13 ,1(1998)

Maiani, L.et al., ”The Second DAPHNE Physics Handbook”. S.I.S. INFN, LNF,Frascati,(1995)

Particle Data Group,”Review of Particle Physics”. Eur.Phys.J.C3, 445(1998)Usher, T et al.,”A Precision Measurement of the Branching Ratio K+ → π+π0/K+ → µ+ν”.

Phys.Rev. D45, 3961(1992)

Captions:

Fig.1 Cross section of the KLOE detector

Fig.2 Integrated luminosity acquired by the KLOE detector versus day of a year

Fig.3 An event e+e− → K+ +K−, K+ → µ+ ν,K− → π− + π0 in the KLOE detector

Fig.4 High momentum part of a charge particle momentum distribution from kaon decayin a rest frame. Peaks from two body decays K± → π±π0 (at pion momentum 205 MeV/c)and K± → µ±ν (at muon mome ntum 235 MeV/c) dominate the spectrum

Fig.5 Running ratio BR(K± → π±π0)/ BR(K± → µ±ν) vs run number. Straight linesgive ± one σ deviation from PDG world averaged value

Figure 1: Cross section of the KLOE detector

Figure 2: Integrated luminosity acquired by the KLOE detector versus day of a year

Figure 3: An event e+e− → K+ +K−, K+ → µ+ ν,K− → π− + π0 in the KLOE detector

Figure 4: High momentum part of a charge particle momentum distribution from kaon decayin a rest frame. Peaks from two body decays K± → π±π0 (at pion momentum 205 MeV/c)and K± → µ±ν (at muon momentum 235 MeV/c) dominate the spectrum

Figure 5: Running ratio BR(K± → π±π0)/ BR(K± → µ±ν) vs run number. Straight linesgive ± one σ deviation f rom PDG world averaged value