F(\t(o3 i SJ ix LAL 86/11 DES* B6-050

A SEARCH FOR SINGLE PHOTONS AT FETRA

by

CELLO Collaboration

Suimitted to Phyiia LeiteAi

U.E.R Inlilal Nttnnal 4t Pkyaeat Nudtairi •I I'Univifiitt Palis-Sud tt PhyiitjM in P>rlkiihs

Bitlment 200 - 9M05 ORSAY Cedex LAL 06/11 OESY 86-050 A search for single photons at PETRA

by CELLO Collaboration

H.-J. Behrend, J. Burger, L. Criegee, H. Fenner, J.H- Field, G. Franke, J. Fuster , Y. Holler, J. Meyer, V. Schroder, H. Sindt, U. Timm, G.G. Winter, W. Zimmermann Deutsches Elektronen-Synchrotron, DESY, Haaburg, Germany P.J. Bussey, C. Buttar, A.J. Campbell, J.B. Dainton, D. Hendry, G. HcCurrach, J.M. Scarr, 1.0. Skillicorn, K.M. Smith University of Glasgow, United Kingdoa V. Blobel, M. Feindt, M. Poppe, H. Spitzes II. Institut fur Experinentalphysik, Universitat Haaburg, Geraany W.-O. Anel, A. Bohrer, J. Engler, G. Fliigge, D.C. Fries, W. Fues ', K. Gamerdinger, P. Grosse-Wiesmann , J. Hansmeyer, Th. Menkes, G. Hopp, H. Jung, J. Knapp, H. Krù'ger, H. KUster, P. Mayer, H. Miiller, K.H. Ranitzsch, H. Schneider, J. Wolf Kernforschungszentrua Karlsruhe and Universitat KarlsrKhe, Geraany W. de Boer, G. Buschhorn, W. Christiansen, G. Grindhammer, B. Gundtrson, Ch. Kiesling, R. Kotthaus, H. Kroha, D. Ltiers, H. Oberlack, B. Sack, P. Schacht, G. Shooshtari, W. Wiedenmann Max-Planck-lnstitut fiir Physik und Astrophysik, NUnchen, Geraany A. Cordier, H. Davier, 0. Fournier, M. Gai1 Iard, .J.F. Grivaz, J. Haissinski, V. Journé, F. Le Diberder, E. Ros ', A. Spadafora, J.-J. Veillet Laboratoire de l'Accélérateur Linéaire, Orsay, France S * B. Fatah , R. George, M. Goldberg, 0. Hamon, F. Kapusta, F. Kovacs, L. Poggioli, M. Rivoal Laboratoire de Physique Nucléaire et Hautes Energies, Université de Paris, France G. d'Agostini, F. Ferrarotto, H. Gaspero, B. Stella University of Rome and INFN, Italy R. Aleksan, G. Cozzika. V. Ducros, Y. Lavagne, F. Ould Saada, J. Pamela, F. Pierre, J. Zacek ' Centre d'Etudes Nucléaires, Saclay, France G. Alexander, G. Bella, Y. Gnat, J. Grunhaus, A. Levy Tel Aviv University, Israel

1) Or leave of absence from inst. de Fisica Corpuscular, Universidad de Valencia, Spain 2) Now at SCS, Hamburg, Germany 3) No* at Stanford Linear Accelerator Center, USA 4} Nuw at Universidad -Tutônoma de Madrid, Spain 5) Now at Univ'sity oi Sebha, Physics Department, Libya 6) On leave of absence from Nuclear Center, Charles University, Prague, Czechoslovakia A search for single photons at PETRA

CELLO Collaboration

Abstract :

A search for single photons, produced in e'e" collisions together vith particles interacting only weakly with matter, has been performed using the CELLO detector operating at the PETRA 3torage ring. From the absence of any signal, an upper limit i3 set at 15 (90V. CD on the number of light neutrino species, and lower limits on various super3ummetric particle masses are derived. For massle3s photinos, mass degenerate scalar partners of the left and right handed electrons are excluded below 37.7 Gev/c2 (90V. CD. 2

In aupersymmetric theories!)), there is no general prediction for the mass spectrum of the partners of the ordinary particles. Because of the expected R-parity conservation^!, it is commonly believed that the lightest supersummetric particle (LSP) must be stable, and cosmological arguments lead to the conjecture that it is neutral and colorless|31. Candidate LSP's are therefore the photino y, the zino z or the higgsina h, and in some models a îcalar neutrino ~> ; the gravitino G can also be considered as the LSP. The couplings of these particles are completely fixed, except for possible mixings in the gaugino sector. In Ihis letter, ve will 033ume that the J Is unmixed. The LSP is in all cases expected to interact only weakly with ordinary matter. For instance, >;e scattering has a cross-section orders of magnitude smaller than that of Compton scattering because of the large mass of the exchanged scalar electron, making the photino observable only indirectly (typically by missing momentum) if stable. As a consequence, pair production of LSP's in e'e" collisions can be detected only in association vith an initial state radiated photont4l. This tagging procedure was first suggested[5] in order to count the number of light neutrino species via the reaction e'e'-»)jvv. The cross-section for this reaction readsl6]:

dole'e'-^îs) 2 a I

dxdy n x(l-y2) where x=2EAJs~, y=cos8 , s=s(l-x) . Here X is any v, v, (i runs from I to N, with N the number of light neutrino species). E is the energy of the tagged photon, and 6 i3 its polar angle with respect to the electron beam direction. From (1), it is evident that an experiment aiming at the study of such a reaction needs good photon defection at low x and at high lyl. In addition, to ensure that only weakly interacting particles are produced in association with the tagged photon, the veto capability of the detector must be extended to the s

largest possible solid angle. However, in e'e" storage rings, it is unavoidable

that a hole be left for the beam pipe. This causes the reaction e*e'-»e*e";j to

be the major experimental background, when both electrons are scattered at

small polar angles. If B., is the minimum veto angle, this background is kinemoticolly totally eliminated as soon as Xi>2B,vith x, =xjl-yj ; but in practice, due to the dominance of the virtual Compton scattering configuration where only one electron is deflected, this background remains negligible down to Xj=6,.. For the results reported here, the minimum veto angle is 50 mrad.

Thic means that, for a typical CM energy of 44 GeV, no yt cut will be needed for single photons with ly|<.83 beyond that implied by a trigger threshold of-Z.5 GeV.

The data entering the present analysis were collected in 1984-85 with the CELLO detector [71 operating at the PETRA e'e" storage ring. At fixed CM energies of 38.28, 43.45, 43.60, 44.20 and 46.57 GeV, integrated luminosities of 8.9, 1.4, 17.0, 9.2 and I.I pb*1 respectively were obtained i for a cross-section behaving like (I), this is equivalent to 37.6 pb"1 at

J<3>" = 42.6 GeV.

Charged particles are detected down to |cos8l=.98 by a system of cylindrical proportional and drift chambers, completed by two planes of proportional chambers placed perpendicularly to the beam in the forward and backward directions (end cap chambers). Electrons and photons are detected

and measured in the 20 radiation length (Xa) deep barrel '|co3BI<.86) and end c jp (.92f|eos8l<.99) lead-liquid argon calorimeters. The energy resolution is iE/E=5'/.*l0y..',jEGeV for electromagnetic showers. The fine lateral segmentation of the seven longitudinal samplings provides a resolution of~60mrad on the shower direction, measured in the calorimeter alone without vertex constraint, both in ajimuth and polar angle. This angular resolution is only weakly energy dependent in the domain of interest for thi3 analysis. The acceptance gap between the barrel and end cap calorimeters vas filled m I9S4 by a crude lead-scintillatar sandwich orrou read by 4

wavelength shifters, the 30-called 'hole tagger'. With only two samplings

after 4 and 3>!0, the energy resolution is rather poor, but efficient veto capability i3 ensured in this angular range. A rough position measurement is also provided by the eight-fold azimuihal segmentation of this device. This upgrade of the detector was crucial for this analysis, 3ince it allowed the elimination of the large potential background coming from th^ reaction e"e""*SX!J • witn one photon remaining in3ide the beam pipe, another one going into the hole tagger region, and only one detected. Finally, a lead glass arratj closes the acceptance from 130 mrad (lower limit of the angular coverage of the end cap calorimeter) down to 50 mrad. High energy muons can be identified o.i/r 927. of the solid angle in the large drift chambers which surround the detector beyond an 80 cm thick iron absorber. Single photon events were triggered by a total energy deposit in excess of ai^ adjustable threshold in any of the 16 modules of the barrel calorimeter. This threshold (position at 507. of the maximum efficiency) was set ai -3 GeV for the first half of the data (3et A), and decreased to ~2 GeV afterwards (set B). This improvement was made possible by a better rejection of the out of time signals (cosmic3 and electronic noi3e) at the trigger level. This rejection is achieved by sampling the 1.5 ^is-long liquid argon signal at a set of times fixed with respect to the beam crossing (twice in A, four times in e). In the subsequent analysis the known pulse shape is adjusted to this set of measurements, from which one derives the time of initiation of the signal with a resolution of 25 ns for shower3 above ~4 GeV, degrading slowly to 45 ns for 2 GeV showers. A timing cut at '. 2.5 0 is then applied. The trigger efficiency for single photons was determined using events from QED reactions which lead to a single electromagnetic shower in the barrel calorimeter, and which were triggered independently by a large energy deposit in an end cap calorimeter module (QED-I sample). The result is shown in Fig. I for sets A and B separately. The 103s induced by the timing cut wa3 calculated in the 3ame vau. 5

The data sample initially consisted of 200K events with only one electromagnetic shower with lyl<.83, i.e. in the barrel calorimeter, and with no reconstructed charged track coming from the interaction vertex. The large co3mic background still surviving the timing cut was drastically reduced by requiring : 0 that the 3hower longitudinal and lateral developments be compatible with those of a photon coming from the interaction region, and ift that the direction of the photon shower point toward the interaction vertex within ! 2o, both in polar angle and in azimuth. The losses of good events introduced by this procedure were determined using the photons of ee^ final 3tate events with all particles measured in the barre) calorimeter (QED-II sample). Finally, a specific algorithm va3 developed to search for strings of hits in the central detector Qligned with the direction of the photon showers, thus allowing further rejection of cosmic particles. At this stage, the remaining sample of 4.5K event3 consisted mostly of photons from QED reactions leading to additional electromagnetic showers. These were vetoed by a signal in the hole tagger, in the end cap calorimeter, or in the lead glass array. An example of a single photon event vetoed in the hole tagger is shown in Fig. 2. This procedure was slightly complicated by the

presence of a 20 X0long conical copper absorber in the 100-130 mrad angular range, installed at 50 cm from the interaction point to shield the central detector against synchrotron radiation. Most of the energy of a beam electron scattered in this direction is absorbed, but some lateral leakage is always expected. In the wor3t cases, only low energy particles emerge, but they can still be detected through hits in the end cap chambers. Thi3 problem

however only affect3 electrons accompanying low xr photons (XT<.I5). For such photons, the veto energy threshold in the end cap calorimeter was decreased, and the hit pattern in the end cap chambers wa3 U3ed in additional rejection criteria. The lo3se3 introduced by these various vetos were determined using events triggered at random beam crossings ('random' sample). b

The 30 events surviving at this 3tage were scanned. All could be

unambiguously rejected

i) by a characteristic hit pattern in the central detector : beam-gas,

beam-wall, or beam-beam interactions, vith no track found by the pattern

recognition program, or

ii) as cosmics, identified by o 3patial correlation between at least two of

the following : a) a hit pattern in the central detector, b) a minimum ionizing particle pattern in a calorimeter module, c) a hit in a muon chamber.

The overall detection efficiency for îingle photon events vas determined as a function of the photon energy and polar angle, taking into account ;he effects of finite resolution. The inefficiencies associated with photon conversion in the beam pipe or in further material up to the barrel calorimeter, vith the boundaries betveen calorimeter modules, vith the trigger, the timing cut, the shower shape cuts, the pointing test, the cosmic rejection algorithm and with the various veto3 (hole tagger, end cap calorimeter and chambers, lead glass array) were all calculated using the appropriate previously defined control samples (QED-I, QED-II, and 'random') .

The resulting efficiency is shown in Fig.3, together with the expected density distribution of single photon events produced according to (1). In this case, the overall efficiency is 35V. for x>.l and iyl<. 83.

The visible cross section expected from the three known neutrino species is 18 fb. With our integrated luminosity the corresponding number of events is 0.68, which gives a SIX probability of not observing any event. The

90'/. and 95". CL upper limit on the total number of light neutrino species set by this experiment are 14.9 and 20.1 respectively. Similar results have been obtained recently using the same method [8]/F1/ and from studies of the production and decay of the intermediate vector bosons at the CERN pp collider[9]. 7

We now turn to the interpretation of this result in terms of bounds on

masses of super symmetric particles.

The most commonly considered LSP is the photino, which could be

produced by the reaction e'e"-»^,^ . Since e*e"-*yy goes by t-channel e

exchange (e is either one of eL or eR, the scalar partners of the left and right

handed electrons), the number of single photon event3 expected in this ca3e

will depend on both the photino and scalar electron masses. We haved used

the exact cross-section for radiative production of photino pairs as given in

Ref. [11] /F2/. The domain we exclude is presented in Fig.4, for the two cases

where ii etand ëK ars degenerate in mass, ii) one is much heavier than the

other. In Fig.4 we also combine the results obtained here and in an

investigation by CELLO of single and pair productions of scalar elec*rons[l3î

to obtain the largest domain excluded by this experiment. For massles3 photinos, a 90V. CL lover limit of 37. ? GeV/c2 is set on the ma33 of degenerate scalar electrons.

The case where the L5P is ihe zino can be readily deduced from the former one, by replacement of the ee^ coupling by the eez coupling. However, even in the most favorable case (equal mass scalar electrons, ma33less zino3, and no z-h mixing!, the 90V. CL lower limit which can be set on the mas3 of the scalar electron 13 far less stringent than that provided by ê pair production.

In the case where the L5P is an electron-type scalar neutrino ~>. the t-channel exchanged particle is now a wino v. We haved used (1) with o(e'e"-»wi as given m Réf. [H] to obtain the excluded domain shown in Fig.5.

For the sake of simplicity, we have assumed no vino-charged higgsino mixing, and we did not tahe into account other scalar neutrino species (which in any ca3e contribute only by s-channel Z* exchange). Also shovn m Fig.5 is the additional domain excluded by CELLO in a search for single and pair production of <'inos|13|. For mo33le3s ï, a 90V. CL lower limit of 40.2 GeV/c2 is set on the mass of the wtno. 8

Finally, we consider the case of radiative photino-gravitino production.

Here the LSP is the gravitino/F3/'. The photino should be sufficiently light not

to conflict yth the standard model of cosmology|15l, in which case its

lifetime is such that it can be considered as stable in thi3 experiment.

Photino-gravitino production from e'e" is dominated by s-channel one photon

e(Change where a JJJJG vertex is involved vith a coupling proportional to 1/d.

Here d is the scale parameter of supersymmetry breaking which i3 directly

related to the gravitino mass. The cross-section for this process is given in

Ref.[18|. and leads to a 90V. CL lower limit of 0.8 I0'$ eV for the mass of the

gravitino.

Our results improve all comparable previous PFTRA Iitnit3t16,17], and

the most recent PEP limits[Ç,l9) for high photino or tca^ar neutrino masses.

Our limit on the gravitino mass 13 two orders of magnitude better than that

obtained from J/T decay3[20|.

To summarize, we have searched for single photons produced in e'e'

collisions at J =42.6 GeV, No event wa3 observed, which is compatible with

the expectation from e*e"->;<;*••.> for the three known neutrino species. This

result sets a limit of 15 (90V. CL) on the total number of light neutrino species,

and restricts the allowed mass range for various supersymrnetric particles.

Acknowledgements:

«le are indebted to the PETRA machine group and 'he OESï computer

center for their support during this experiment, «le acknowledge the

invaluable efforts of all engineers and technicians of the collaborating institutions 111 th? construction and maintenance of the apparatus, in particular the operation of the magnet system by M.Clausen, P.Ropnack and the cryogenics group. We thank M.Bcurdinauc1 for his advice during tne design phase of the hole tagger, and P.Fayet for comments and suggestions during this analysis. The visiting groups wish to thank the DESV directorate for the hospitality experienced at QESV. 9

Footnotes: /FI/ ! Combining our result vith the most recent one3 from the ASP and MAC Collaborations!?,!!}), one obtamsllOj a 90X (?5» CL limit of 8 (10) on the total number of light neutrino species. /F2/ : We have checked that !U, vith X=jj;j and ote'e"-»!;^ a3 Qiven in Ref.[l2], gives results indistinguishable from those of Ref. 111] for any practical purpose. / F3/ •" In the case of broken global supersymmetry, the L3P i3 the gold3tino which is absorbed by the super-Higgs mechanism of local super3ymmetry breaKing while the gravitino acquires n ma3s. The phenomenological analysis presented here is equally valid in both casesHl. lu

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|l5]:N.Cabibbo, G.R.Forrar, andl.Maiani, Phys. Lett. I05B098I) 155. [16]: CELLO Coll., H.J.Behrendet al., Phys. Lett. 1MB (1982) 287, JADE Coll., W.Bartel et al., Phys. Lett. 1525(1985)385, MARK J Coll., B. Adevo et al., Phys. Lett. 152B (1985) 439, TASSOColl., R.Brandelik et al., Phys. Lett. 117B (1982) 365. [17|: JADE Coll., W.Bartel et al., Z. Phys. C29 (1985) 505, MARK J Coll., B.Adeva et al., Phys. Rev. Lett. 53(1984) 1806. [18]: P.Fayet, Preprint LPTENS 86-9. [19]: MAC Coll., E.Fernandez et al., Phys. Rev. Lett. 52(1984)22, MARK II Coll., L.Gladney et al., Phys. Rev. Lett. 51 (1983) 2253. 120]: MARK I Coll., unpublished, quoted in Réf. (4|. Figure captions:

Figure 1 : Single photon trigger efficiency as a function of energy, for the two periods of dota taMng (A and B ; see text)

Figures: This event vas triggered by the 10 GeV photon which can be ?een in the barrel calorimeter. The final state can be interpret?! as x^;, vith the second photon (indicated by a viggled line) detected in the hole tagger, and the third one remaining inside the beam pipe. The hits in *he central detector are due to synchrotron radiation. 2a and 2b are projections :pto planes containing the beam axis and transverse to it respectively. The various components of the calorimetric coverage are indicated : barrel (B) and end cap (EC) lead-liguid argon calorimeter, hole tagger (HT) and forward lead glass array (FW).The data of the calorimeter module containing the shower are displayed in 2c. The lead strips of the 7 layers in the U direction run parallel to the beam, those of the 7 layers in the V direction transverse to it, and those of the 5 layers in the W direction at -45° with respect to U and V. The magnification varies with the depth 30 that a shower pointing to the interaction vert-.- -' always appears perpendicular to tne layers.

Figure 3 : a) Detection efficiency for single photon events as a function of lyl, for various x values.The hatched area is excluded by the fiducial volume cuts, b) Lines of equal signal intensity, decreasing by steps of 20V. relative to the maximum, expected for v.vv production with the above efficiency function.

Figure 4 : In the J-£ mass plane, boundaries of the 90'/. CL (full lines) and 957. CL (dotted line) domains excluded by thi3 analysis for mass degenerate scalar electrons (A), and if one of the scalar electrons is very heavy (B). The dash-dot'ed contour indicates the additional domain excluded at 90"/. CL by another ctLLO analysis(13l, for mass degenerate scalar electrons.

Figure 5 : In the v-w mass plane, boundaries of the 90V. CL (full line) and 952 rl (dotted line) domains excluded by this analysis. The dashed-dotted contour indicates the additional domain excluded ot 90'/. CL by another CELLO analysi5[l3). 4.0 6.0 8.0 10. E(GeV)

4.0 6.0 8.0 10. E(GeV) Fig 1 ®

®

Fig. 2 0.6 V^» s x».300 -' > ^y

' ^ >. ' ^^M 0.5 y s xz.200 \ !-- ~~—-^^^ \ • --',- ^\x=.150 \ ,' ^v \ . • / y >. \ / 0 e 0.3 -s s >> \ .- ' - \ \ '.. ' . ^ 'y - ' / • L — \ \ \\ S ^x=.125 \ \ . ' // ; *•' 0.2 • ;>^ ^v_ \ y' ./-./ ' s s s 0.1 - .-• s* ^ ï x = .100 \

g xz.075 ^"~~^ 0.25 0.5 0.75

® X

FiQ 3 \ i i 20 • i /' -O /y' ' /^^-^^^*^^ ji ^*^**>^•«^^-^ ' ^ ^^~-•v^-%^ . v. ^-^ •v V.^v ^N. . \ ^w ta %\ \>s . 2 10 \ \. N \ y \ \ / / \ \ X \ / \ \ / \ \ / \ \ / excluded \ \ \ \ / . 1 1 10 20 30 40

2 Ms(GeV/c )

Fig. 5

V.