FrascatiErascati Physics Series Vol. XXXVI (2004), pp. 327–3343277334 DADANEΦNE 2004: PhysicsPHYSICS atAT mesonMESON factoriesFACTORIES –7 Frascati,Erascati, June 7-11,7—11, 2004 Selected Contribution in Plenary Session

FIRST MEASUREMENT OF KAONIC HYDROGEN AND NITROGEN X-RAYS AT DADANEΦNE

J. Zmeskal ∗* Institute for Medium Energy Physics, Austrian Academy ofof ScSciencesiences Vienna, Austria

Abstract The DEAR experiment (DAΦNE(DAQNE Research) is a powerfpowerfulul efforteffort to study low-energylow—energy physics. DEAR uses the unique beam ooff almost monochromatic negative produced by the Frascati Φ-Fa(I3—Factoryctory complex at LNF (Laboratori(Laboratori Nazionali di Frascati), to perform a precisprecisee measurement of the energies emitted in the transitions to the ground state ooff kaonic hydrogen. The shift and the widthWidth of the 1s state are sensitive quantitiquantitieses to test the current understanding of low-energylow—energy antikaon-nucleonantikaon— intinteraction.eraction.

1 Introduction

The objective of DEAR is the precise determination of the isoisospinspin dependent antikaon-nucleonantikaon—nucleon scattering lengths, through an eV-leveleV—level measurement of the

∗* On behalf of the DEAR Collaboration 1s shift and width in kaonic hydrogen and kaonic deuterium 1)1).. DEAR in-in— vestigates the characteristic properties of the strong intinteractioneraction of antikaons with at almost zero energy, whereby the light quarkquarkss (u,(u, d, s) are involved. The masses of the nucleons are considerable largelargerr than the sum of their constituent quark masses. This phenomenon is proposeproposedd to originate from spontaneous breaking of chiral symmetry of massless quarks due to strong in-in— teraction 2)2).. In the world in which we live, quarks are massive and their mamassessses are different:different: m(s)m(s)>>m(d)~m(u).m(d) m(u). Therefore, chiral symmetry must be broken ≫ ∼ and the SU(3) symmetry is not exact to the extent required to oobtainbtain the experimentally observed mass spectrum. We do not know, at least on a fundamental level, the origin of tthehe sym-sym— metry breaking, its nature, to what extent it is broken, whicwhichh are the breaking mechanisms and therefore, which model must be used to descridescribebe it. In this work a measurement of energies and widths of the X-rayX—ray transitransitionstions to the ground state in kaonic hydrogen atoms, which will allow to exextracttract the KK’p−p s-waves—wave interaction at low-energy,low—energy, is presented. Further onon,, this information will give an important contribution to the understanding of the cchiralhiral symmetry breaking scenario in the strangeness sector. In contradiction to the analysis of the low-energylow—energy scatteriscatteringng experiments (extrapolated(extrapolated down to threshold, a negative energy shift wawass extracted) three out of four kaonic hydrogen X-rayX—ray experiments performed in tthehe last twenty-twenty— fivefive years claimed to observe a positive energy shift and therthereforeefore an attractive between the kaon and the (only(only the KpX measure-measure— ment gives also a negative shift) 3)3).. Our analysis of the kaonic hydrogen data gives a negative shift and therefore a repulsive contributicontributionon of the strong inter-inter— action, which confirmsconfirms the result of the KpX experiment 4)4),, but with a much better precision. A repulsive contribution of the strong interaction can be trtracedaced back to the presence of the Λ(1405)A(1405) resonance, which leads to the pospossiblesible existence of strongly bound kaonic states in light nuclei 5).5).

2 The DEAR experimental setup

DEAR makes use of low momentum negative kaons, produced by ththee decay of Φ-mesons—mesons at DAΦNE.DANE. The kaons are degraded in energy to a few MeMeV,V, enter a gaseous hydrogen target through a thin window and are finallfinallyy stopped in the gas. The kaon entrance window has a diameter of 100 mm and iiss made of Kapton with a thickness of 125 µum.m. The distance from the entrance window to the center of the beam pipe is approximately 110110 mm.

APD CryoCryo-- Cooler

TMP

CCD Electronics CryoTiger CCD Cooling F Vacuum Chamber CCD Target CellCe _.< PrePre-Amplifier-Amplifier

CCD55CCD55-Chips-Chips

e+ e-

Figure 1: TheThe DEAR setup: A cryogeniccryogenic lightweightlightweight targettarget cellcell surrounsurroundedded byby aa CCD-detectorCOD—detector inside aa commoncommon vacuum housing.

The target cell, surrounded by the CCD mounting devices, is pplacedlaced in the center of the vacuum chamber and is connected to the two-stwo—stagetage closed cycle helium refrigerator via a copper cylinder. The refrigrefrigeratorerator system (APD(APD Cryo-Cooler)Cryo—Cooler) provides a cooling power of about 10 W at 25 K. ThThee insulation vacuum is maintained to better than 1010’6−6 mbar using a wide-rangewide—range turbo molecular pump (TMP), see also figurefigure 1. A light-weightlight—weight target cell with a glass-fiberglass—fiber reinforced epoepoxyxy grid was con-con— structed to avoid fluorescencefluorescence X-raysX—rays in the region of intereinterestst and to minimize the bremsstrahlung background. The cell has a diameter of 121255 mm and a height of 140 mm. The hydrogen gas target cell (ultra pure hydhydrogenrogen gas is used, which is cleaned through a palladium diffusiondiffusion device) typically works at a gas pressure of 2 bar and has a temperature of 23 K, stabilizestabilizedd to better than 0.1 K. With these settings a gas density of 2.2 mg/cmmg/cm33 (3.1(3.1 % of liquid hydro-hydro— gen density) is achieved, corresponding to a gas pressure at room temperature of about 30 bar. 16 CCD detector chips (CCD55-30)(CCD55—30) with a total area of 116 cmcm22 were used. Each chip has 1242 x 1152 pixels with a pixel size of 22.5 µpmm x 22.5 µpmm and a depletion depth of 30 µpm.m. To minimize thermal noise, and thus reduce the overall noise, the CCDs are operated at a temperattemperatureure around 150 K, cooled by two one-stageone—stage closed-cycleclosed—cycle refrigerators (APD(APD CryoTiger), each with a refrigeration capacity of 20 W at 120 K. Thus, an energy resolution of 150 eV at 6 keV could be achieved. A sophisticated shielding of the DEAR target and detector wawass developed through differentdifferent test runs at DAΦNE.DAQNE. Finally, a graded shielshieldingding structure was used, starting with lead, followed by a copper and aluminum llayer,ayer, with an inner layer of polycarbonate. With this setup the bremsstrabremsstrahlunghlung background could be reduced drastically, which was demonstrated using nitrogen as target gas.

3 Kaonic nitrogen results

The series of kaonic nitrogen measurements performed at DAΦDAQ’NENE had multiple tasks and deliverables: a firstfirst measurement of kaonic nitrognitrogenen transitions; the study of the machine background and the DEAR setup performanperformance.ce. A refinedrefined analysis of the kaonic nitrogen spectrum taken in OcOctobertober 2002 was performed 6)6).. For the firstfirst time three lines of kaonic nitrogen transitiontransitionss were clearly identifiedidentified (fig.(fig. 2) and the corresponding X-rayX—ray yyieldsields could be determined (see table 1). Using these experimental yields as input for a cascade calcucalculationlation 7)7),, in particular, the residual K-shellK—shell population coulcouldd be extracted. A K-K— shell electron fraction of approximately 1-3%1—3% is found, usiusingng the present cascade approach 6)6).. Understanding the atomic cascade processes in kaonic nitrnitrogenogen is especially important to prove the feasibility for a preciprecisionsion measurement to determine the charged kaon mass –, still an open problem 8).. Table 1: Kaonic nitrogen transition energies,energies, measured events,events, andand extractedextracted yields transition energy [keV][keV] events yield [%][%] 7 H 6 4.5773 3310 :I: 690 41.5 :l: 8.7 :l: 4.1 → ± ± ± 6 H 5 7.5957 5280 :I: 380 55.0 :l: 3.9 :l: 5.5 → ± ± ± 5 H 4 13.996 1210 :I: 320 57.4 :I: 15.2 :I: 5.7 → ± ± ±

1““

7000 KN (6-5)(675)

eV 1 1000010000 eV 6500 /20 6000 l KN (7-6)(7-6)

Counts 5500 ll 8000 Counts / 20 5000 7.2 7.6 8.0 eV eV Energy [keV] 20 6000 , per

4000 , KN (6-5) Counts Counts per 20 KNKN(5-4)(5-4)

2000 , ,

0 2 4 6 8 10 12 14 16

Energy (keV)

Figure 2: Energy spectrum ofof kaonic nitrogen. TheThe arrowsarrows indicate the position ofof the kaonic nitrogen X-rayX—ray lines.lines. TheThe peaks atat 1.4,1.4, 1.7,1.7, 3.63.6 andand 15.715.7 keV areare thethe Al-KAl—Koz,α, Si-KSi—Ka,α, Ca-KCa—Koz,α, andand Zr-KZr—Kaα lines,lines, respectively.respectively. TheThe insertinsert shows thethe fifitt ofof the 66H55 kaonic nitrogen transition. →

On the other hand the kaonic nitrogen measurement is essentiessentialal to tune the machine and to optimize the apparatus for the kaonic hydrhydrogenogen experiment. Improvements of the detector shielding (signal(signal to backgroubackground)nd) as well as an optimization of the kaon stopping distribution in the gaseogaseousus target cell could be measured directly with kaonic nitrogen X-raysX—rays within a fefeww days. 4 Kaonic hydrogen firstfirst (preliminary)(preliminary) results

The experimental challenge of DEAR is the extraction of a smasmallll signal in the presence of a large low-energylow—energy X-rayX—ray background mainly from electron gamma showers resulting from lost and due to eeitherither Touschek scattering or interaction with residual gas. The careful opoptimizationtimization of the shielding of our experimental setup and the improvements in the beam optics achieved by the machine crew made the goal of DEAR possible, nnamely,amely, to perform a firstfirst measurement of kaonic hydrogen X-raysX—rays at DAΦNDAQ’NEE in the last two months of 2002. eV /60 Counts

X—ray energy (c)

Figure 3: Background subtracted kaonickaonz'e hydrogen spectrumfor anan inteintegratedgrated luminosityluminosity ofof 6060 pbpb’l.−1. Kaonic hydrogen lineslines KKa,α, Kβfl andand KK7γ areare fi fitted.tted.

Although, all materials used for the target cell and the mounmountingting devices for the CCDs were carefully checked, still it was not possiblpossiblee to avoid iron impurities completely. The iron fluorescencefluorescence line coming frfromom iron impurities overlaps partly with the kaonic hydrogen KKa—line.α-line. Due to background mea-mea— surements with nitrogen gas in the target cell and with hydrohydrogen,gen, but without collisions in the DAΦNEDAQ’NE interaction zone (no(no kaons), the iroironn fluorescencefluorescence line could be determined and therefore subtracted. The linearity as well as the energy stability of the CCD detecdetectortor were measured in-situin—situ using titanium and zirconiumZirconium lines (the(the fofoilsils were placed on top of the target cell). The CCD detector system was extremelextremelyy stable (better than 0.1%) during beam time. Stability was checked by fixingfixing tthehe energy position of the Ti-Ti— and Zr-linesZr—lines and then the position of the CCa—linea-line (originating(originating from the glass-fiberglass—fiber epoxy grid of the target cell) was fittedfitted wwithith an accuracy better than 1 eV. In addition the Ti KKa—lineα-line width for the sum of all CCDs was better than 150 eV for the whole beam time. Two completely independent analyses starting from the set ooff raw-dataraw—data were performed. The main differencesdifferences were the treatment of ththee subtraction of the continuous background and the determination of the flufluorescenceorescence X-X— ray lines. Figure 3 shows the resulting kaonic hydrogen specspectrumtrum with the continuous background as well as the fluorescencefluorescence X-rayX—ray lineliness subtracted. The

KKa—lineα-line together with the X-rayX—ray lines of the K-complexK—complex are cleaclearlyrly visible. Both analysis methods gave a compatible (preliminary) result:

ε6 = —193193 :1: 37 (stat.) :I: 6 (syst.)(syst.) eV, − ± ± ΓF = 249 :1: 111 (stat.)(stat.) :1: 30 (syst.)(syst.) eV. ± ± In summary, the analysis of the firstfirst measurement of kaonic hyhydrogendrogen at DAΦNEDAQ’NE already leads to an improved accuracy in the determinadeterminationtion of the shift and width of the ground state of kaonic hydrogen and confirmsconfirms tthehe repulsive contribution of the strong interaction found in the KpX expeexperimentriment at KEK, Japan. For the firstfirst time the Kβfl and KK7γ lines could be disentangled. Theoretical predictions based on chiral perturbation theotheoryry and quantum fieldfield theoretical approach are confronted with our new resulresultt 9)9) 10)10)..

5 Future program

DEAR has performed the most precise measurement on the kaonikaonicc hydrogen at present, disentangling the line of the K-complexK—complex for the firsfirstt time. Essential for a future eV level measurement of the shift and width for kaonikaonicc hydrogen and for a firstfirst measurement of kaonic deuterium is an upgrade of ththee setup, which is in progress 11) .. A large area Silicon-Drift-DetectorSilicon—Drift—Detector is under constructioconstruction.n. A drastic improvement in the signal-to-backgroundsignal—to—background ratio is eexpected.xpected. Studies of other light exotic atoms (mainly(mainly 44HeHe and 33He)He) are in discussion. A high-precisionhigh—precision experiment to determine the charged kaon mmassass 12) seems to be feasible in the future.

Acknowledgments

The DAΦNEDA<1>NE group is thankfully acknowledged for the excellenexcellentt cooperation and team-work.team—work. Part of the work was supported by “Transnatio“Transnationalnal Access to Research Infrastructure”Infrastructure77 (TARI), Contract No. HPRI-CT-19HPRI—CT—1999—00088.99-00088.

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