- Email: [email protected]
Search for massive eνγ and μνγ final states at the CERN super proton synchrotron collider
Volume 135, number 1,2,3
PHYSICS LETTERS
2 February 1984
SEARCH FOR MASSIVE e v ' / A N D Itv¥ FINAL STATES AT THE CERN SUPER PROTON SYNCHROTRON COLLIDER
UA1 Collaboration, CERN, Geneva, Switzerland G. ARNISON J, A. ASTBURY J, B. AUBERT b, C. BACCI i, G. BAUER 1, A. BI~ZAGUET d, R.K. BOCK d, T.J.V. BOWCOCK f M. CALVETTI d, p. CATZ b, p. CENNINI d, S. CENTRO d F. CERADINI d,i, S. CITTOLIN d, D. CLINE 1, C. COCHET k, j. COLAS b, M. CORDEN c, D. DALLMAN d,~/, D. DAU 2, M. DeBEER k, M. DELLA NEGRA b,d, M. DEMOULIN d, D. DENEGRI k, A. Di CIACCIO i, D. DiBITONTO d, L. DOBRZYNSKI g, J.D. DOWELL c, K. EGGERT a, E. EISENHANDLER f N. ELLIS d p. ERHARD a, H. FAISSNER a, M. FINCKE 2 G. FONTAINE g, R. FREY h, R. FRUHWIRTH ~, J. GARVEY c, S. GEER a, C. GHESQUII~RE g, P. GHEZ b, K.L. GIBONI a, W.R. GIBSON f, Y. GIRAUD-Ht~RAUD g, A. GIVERNAUD k A. GONIDEC b G. GRAYER J, T. HANSL-KOZANECKA a, W.J. HAYNES J, L.O. HERTZBERGER 4, C. HODGES h, D. HOFFMANN a, H. HOFFMANN d, DA. HOLTHUIZEN 4, R.J. HOMER c, A. HONMA f, W. JANK d, G. JORAT d, P.I.P. KALMUS f, V. KARIM.g,KI e, R. KEELER f, I. KENYON c, A. KERNAN h, R. KINNUNEN e, W. KOZANECKI h, D. KRYN d, g F. LACAVA i, j._p. LAUGIER k, j._p. LEES b, H. LEHMANN a, R. LEUCHS a A. LI~VI~QUE k, d, D. LINGLIN b, E. LOCCI k j..j. MALOSSE k, T. MARKIEWICZ d, G. MAURIN d, T. McMAHON c, j._p. MENDIBURU g, M.-N. MINARD b M. MOHAMMADI 1, M. MORICCA i, K. MORGAN h, H. MUIRHEAD s, F. MULLER d, A.K. NANDI J, L. NAUMANN d, A. NORTON d A. ORKIN-LECOURTOIS g, L. PAOLUZI i, F. PAUSS d, G. PIANO MORTARI i E. PIETARINEN e, M. PIML~ e, A. PLACCI d, j.p. PORTE d, E. RADERMACHER a, j. RANSDELL h, H. REITHLER a, J.-P. REVOL d, j. RICH k, M. RIJSSENBEEK d, C. ROBERTS J, J. ROHLF d, 3, p. ROSSI d, C. RUBBIA d, B. SADOULET d, G. SAJOT g, G. SALVI f, G. SALVINI i j. SASS k, j. SAUDRAIX k, A. SAVOY-NAVARRO k, D. SCHINZEL d, W. SCOTT J, T.P. SHAH J, D. SMITH h, M. SPIRO k, J. STRAUSS ~, J. STREETS c, K. SUMOROK d, F. SZONCSO Q, C. TAO 4, G. THOMPSON f, J. TIMMER d, E. TSCHESLOG a, j. TUOMINIEMI e, B. Van EIJK 4, j..p. VIALLE d, j. VRANA g, V. VUILLEMIN d, H.D. WAHL ~, P. WATKINS c, j. WILSON c R. WILSON 3, C.-E. WULZ ~, Y.G. XIE d M. YVERT b and E. ZURFLUH d Aachen a-Annecy(LAPPJ b-Birmingham C-CERN d-Helsinki e-Queen Mary College, London fParis(Coll. de France) g-Riverside h-Rome J-Rutherford Appleton Lab. J-Saclay(CEN) k_ Vienna 9~Collaboration
Received 19 October 1983 The observation of an apparent excess of radiative Z° decays into e+e-7 and ~+#-3' has prompted the search for massive eu~,and ~3' final states containing an energetic photon. No events were found other than those consistent with QED radiative effects in leptonic W decays. The data sample corresponds to an integrated luminosity of 0.136 pb -1 produced at the CERN super proton synchrotron (SPS) collider. An upper limit on the occurrence of such events is given. r University of Wisconsin, Madison, Wl, USA. 2 University of Kiel, Fed. Rep. Germany. 3 Harvard University, Cambridge, MA, USA. 250
4 NIKHEF, Amsterdam, The Netherlands. 5 Visitor from the University of Liverpool, England. 0.370-2693/84/$ 03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 135, number 1,2,3
PHYSICS LETTERS
1. Introduction. We have recently reported the observation of four Z 0 -+ e+e- events [ 1]. In one of these events, the momentum measurement by curvature of the negative track showed a value of (9 ,+ 1) GeV/c, much smaller than the corresponding calorimeter deposition of (48 -+ 1) GeV. We interpreted this event as the likely emission of a hard " p h o t o n " * 1 accompanying the electron. Subsequent calibrations of the electromagnetic (EM) counters indicated that the centroid of the energy deposition in the calorimeters was significantly displaced with respect to the incident electron track, indicating an angle of Aq5 = (14 + 4) degrees between the charged and neutral components. This made the possibility of an external bremsstrahlung in the vacuum pipe and detector window seem unlikely. The estimated probability of an internal bremsstrahlung exceeding the angle and energy observed is, according to Berends [2], about 0.005, or 2% for the sample of four events. A rather similar event has been reported by the UA2 Collaboration [3]. In this case the photon and electron hit separate cells, thus directly indicating a finite e-3, separation. Characteristics of these two events are given in table 1. More recently, an event of the type/1+/J-3 , has been observed in our study of Z 0 -+/a+/~- decays [4]. Also in this event, a small but finite angle (~8 °) is observed between the muon and the hard photon (E ~ 30 GeV). It is certainly premature to draw any conclusion about the origin of such events. The present paper deals with a search for an analogous phenomenon in the charged intermediate vector boson channels, namely for processes leading to final states of the type eu3, and ~uT, where 3' is a hard photon of transverse energy E T in excess of 10 GeV. The UA1 detector has been designed to study the large-angle phenomenology of p~ collisions at x/F = 540 GeV. The apparatus and the data-taking conditions are essentially unchanged from those previously reported, and we refer the reader to ref. [1] for more details. The main features of the UA1 detector that are relevant for the present investigation are (i) electron detection, (ii) neutrino detection, (iii) momentum analysis of charged particles, and (iv) muon detection. 4:1 We define a " p h o t o n " candidate as a neutral electromagnetic transverse energy deposition in excess of 10 GeV (see text in section 3 for details).
2 February 1984
Table 1 Energy, angle, and mass properties of the ee,y events.
E7 Ee 1 Ee2 A¢(e27) m(ele2) m(ele27)
m(euD m(e2~ )
(GeV) (GeV) (GeV) (°) (GeV/c 2) (GeV/c 2) (GeV/c 2) (GeV/c 2)
UA1 a)
UA2 b)
38.8 61.0 9 14.4 42.7 98.7 88.8 4.6
24.4 68.5 11.4 31.8 49.8 89.7 74.1 9.1
-+ 1.5 -+ 1.2 -+ 1 -+ 4.0 -+ 2.4 -+ 5.0 -+ 2.5 +- 1.0
+- 1.4 -+ 1.6 +_0.9
+- 2.8
a) For the UA1 event e 1 = e ÷ and e 2 = e-. b) The UA2 numbers are calculated from ref. [3].
Details of the features (i) to (iii) of the UA1 detector can be found in ref. [5], and (iv) can be found in ref. [ 1]. 2. Event selection. The event selection follows very closely the one used in our previous analysis to search for ordinary W ~ eu and W -+/au decays, except that the procedure has been modified so as not to exclude the radiative events on the basis of track isolation or event topology. 2.1. Search for eu7 events. Results are based on an integrated luminosity of 0.136 pb-1, which is corrected for dead-time and similar losses. This exposure resulted in the sample of 52 charged intermediate vector boson decays in the (Uee) channel previously reported [5]. The trigger selection required the presence of an EM cluster at angles larger than 5 ° with respect to the beam direction, and with transverse energy in excess of 10 GeV. After on-line filtering and complete offline reconstruction, about 1.5 × 105 events had at least one EM cluster (two adjacent EM calorimeter cells) with E T > 15 GeV. To search for the process We -+ e-+Ue3`,events have been required to survive at least one of two complementary sets of cuts: (i) Missing-energy selection: In the hermetic UA1 detector an energetic neutrino manifests itself as a transverse energy imbalance. We require a transverse energy imbalance that is in excess of 15 GeV and, in order to ensure the best accuracy, the missing energy vector must not point vertically (-+20°). Finally we impose a topology cut * 2 to remove spurious back#2 See footnote on next page.
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ground associated with two-jet events. After this selection we are left with 199 candidates for further analysis. (ii) Charged-particle selection: In addition to an EM cluster with E T > 15 GeV, we require the presence o f an associated, isolated [5] track with PT > 7 GeV/c in the central detector (CD), less than 600 MeV deposited in the hadron calorimeter cells immediately behind the electromagnetic cluster, less than 3 GeV transverse energy deposited in the hadron calorimeter within a larger specified ,3 cone, and a missing transverse energy in excess o f 10 GeV. No further cut on the topology o f the event has been made. We are left with the 52 published W_+ -~ e-*ve decays, plus an additional 27 events for further analysis. 2.2. Search for la~' events. Results are based on an integrated luminosity o f 0.111 p b - 1 , which is corrected for dead-time and similar losses. The trigger selection required the presence of at least one penetrating track detected in the UA1 muon chambers with pseudorapidity Ir/I ~< 1.3, and pointing in both projections to the interaction vertex within a specified cone of aperture e l 50 mrad. After off-line reconstruction we require a charged track in the CD with PT 2> 15 GeV/c, or p > 30 GeV/c, which matches, in both projections, the reconstructed track in the muon chambers. Removing cosmic rays and identified light meson decays, we are left with 144 muon candidate events. This event sample has been examined by physicists on Megatek, the visual scanning and interactive facility. Excluding those events containing hadronic jets with E T > 10 GeV (mostly two-jet events) we are left with 18 events having a single muon candidate.
3. Results. The three event samples have been examined by physicists on the Megatek facility. In searching for an energetic p h o t o n the following definition of a photon candidate was used: (i) We require a cluster in the electromagnetic calorimeters with E-r > 10 GeV. ,2 Events having two coplanar (_+30°) calorimeter clusters with E T > 15 GeV are excluded if the missing transverse energy vector is in the direction (_+20°) of one of the clusters. A genuine eve3, event having this topology would be recovered by the high-PT track selection. ,3 A cone in (rapidity, azimuthal angle) space was used, centred on the high-PT track, and described by the expression (r/2 + ~2)1/2 < 0.7, where r~ is in units of rapidity and ~ is in radians. 252
2 February 1984
(ii) The corresponding shower profile and shower size must be consistent with test beam data for single electromagnetic showers [5]. (iii) The photon must be isolated such that (a) no tracks with PT ) 500 MeV/c are associated with the shower (apart from a collinear muon or electron candidate), and (b) there is less than 600 MeV present in the hadronic calorimeter immediately behind the shower. 3.1. Search for ev7 events. In the two event samples of section 2.1, we have looked for the simultaneous presence o f a photon candidate (defined above) and an electron candidate. An electron candidate is defined in an identical way to our photon definition but with the additional requirement that there should be a charged track with PT > 7 GeV/c associated with the electromagnetic shower. The photon and electron may be resolved if they are separated by more than 3 standard deviations in either the ~ (azimuthal) or 0 (polar) coordinate (typically 12 ° in ~bor 2 ° in 0). No ew)' events satisfying these requirements were found. Amongst the 52 published W events [5] which are contained in these samples, we note that there is one W event in which the track m o m e n t u m is more than 4 standard deviations less than the shower energy (the track m o m e n t u m is 7 GeV/c and the shower energy is 40 GeV [5]). We interpret this as evidence for the presence of an energetic photon; however, the electron and photon are collinear and therefore not resolved by the above cuts. Furthermore, the observation of one such event out o f the sample o f 52 W events is consistent with the expected rate from QED radiative corrections (internal and external bremsstrahlung). 3.2. Search for laY7 events. In the event sample o f section 2.2, we look for the presence o f a photon candidate (defined previously), and find no events. There is no restriction on the angle o f the photon with respect to the muon. The 18 muon events in which we have a single muon and no hadronic jet activity provide us with a preliminary sample of W e -+ ll+-vu candidates [4]. The energy deposited by the muon in the EM calorimeters is consistent with that expected from radiative and ionization losses, together with the contribution from the minimum-bias background (figs. 1 and 2). To consolidate these results, the UA1 jet algorithm [6] has been used to search for energetic photons in the three event samples ,4. Once again, after requiring ,4 See footnote on next page.
PHYSICS LETTERS
Volume 135, number 1,2,3 I
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the l e p t o n to lie outside a hadronic jet, we find no evidence for an energetic p h o t o n in any of the events.
i
6 [-
18 EVENTS
I
>~
W --~ ~v
0 0
2
4
ENERGY {GeV)
Fig. 1. Energy deposited by the muon in the EM calorimeters for a preliminary sample of 18 We ~ V±vU candidates. The data are consistent with the curve, which is a prediction including radiative [2] plus ionization losses, and a contribution from the minimum-bias background.
SAMPLE t
SAMPLE 2
12 8
8
4
o
Ol 0
,
1
2
3
I
2
3
ENERGY {OeV) i
4. Conclusions. We have f o u n d no evidence for events o f the type ev7 o r / a v 7 where the EM neutral particle or cluster (7, 7r0, or 77) has a transverse energy in excess of 10 GeV. Therefore, the radiative effect observed in the Z 0 candidates is not present in the m u c h larger sample o f W decays. We can also calculate an upper limit for R, the fraction o f l e p t o n i c W decays which are a c c o m p a n i e d by an energetic p h o t o n , provided we a d o p t a m o d e l of the decay kinematics o f the process. Our evaluation is based on the electron samples. This enables us to use our published sample o f 52 W ~ eu decays in the calculation o f the limit on R. (i) T h r e e - b o d y phase space: R - BR(W ± ~ £± u~7)/BR(W ± ~ £±u~). Allowing for selection efficiency and geometrical acceptance, we find that R < 0.1 (90% confidence level). (ii) P r o d u c t i o n o f an excited state o f the charged lepton: W ± ~ £*vQ(£* ~ £-+7) •
161
16 12
2 February 1984
i
T h e n R - BR(W ± -~ £*v~ ~ £±Tv~)/BR(W ± ~ £± v~), and the experimental sensitivity depends u p o n the 1 mass o f the excited spin ~ leptonic state £*. In fig. 3a the result is shown as a f u n c t i o n o f this mass. In the mass range 10 G e V / c 2 ~< mQ. ~< 70 G e V / c 2 we place an upper limit on R falling f r o m 0.2 at low masses to 0.1 at high masses (90% confidence level). (iii) Radiative decay o f a heavy W into a lighter W: -~ WT(W --+ £v~)
16 SAMPLE 3
SAMPLE 4
12
12
8
8
and W -~ WT(W ~ £vQ). T h e n R - ( o B R ) w / ( o B R ) w. An u p p e r limit on R depends u p o n the masses o f the W and the W, and their spin assignments. Fixing the W mass at the measured
4 0
0 0
I
2
3
I
ENERGY
2
3
(GeV)
Fig. 2. The same as fig. 1 for the four longitudinal samplings of the electromagnetic shower calorimeter.
4:4 In the search for a photon using the UA1 jet algorithm, a photon candidate was defined as a calorimeter cluster in which (i) there is no associated CD jet, and (ii) there are two adjacent EM cells having between them E T > 10 GeV and 80% of the cluster energy. All events from samples (1) to (3) having such a "photon candidate" were found, on closer inspection, to be two-jet events. 253
Volume 135, number 1,2,3 i
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An inclusive search for additional e+e - pairs in this mass range has also given a negative result.
I
a
10 W ---. e* v e
0.8
o~ o
06
04
L
0.2 o
I
o
i
I
20
I
I
I
60
40
80
e* MASS (5eV/c 2)
i
i
i
'I I I
b W---*W T IC
i
i
W--~W
i
o~ o~
'
06
v
0.4 02 20
40
60
80
L 100
10
I
14o
2 February 1984
/
160
180
200
MASS (GeV/c 2)
Fig. 3. Upper limit (90% CL) for R, the fraction of leptonic W decays accompanied by a photon with transverse energy in excess of 10 GeV. The electron samples have been used to extract the limits on R, which are model-dependent. Results are shown for the following two models: (a) the process W ~ e're(e* e3,) with mw = 80.9 GeV/c2 [5], and (b) W ~ W'r(W ~ eve) where one of the boson masses has been taken to be 80.9 GeV/c2 , and the shaded band shows the dependence of the limit on the bosons spin assignments (0 or 1).
The continued sucess of the Collider and the steady increase in luminosity which has made this result possible depend critically upon the superlative performance of the whole CERN accelerator complex, which was magnificently operated by its staff. We thank W. Kienzle who, as coordinator, balanced very effectively the sometimes conflicting interests of the physicists and accelerator staff. We have received enthusiastic support from the Director General, H. Schopper, and his Directorate, for the results emerging from the SPS Collider programme. We are grateful to the management and staff of CERN and of all participating Institutes who have vigorously supported the experiment. The following funding Agencies have contributed to this programme: Fonds zur F6rderung der Wissenschaftlichen Forschung Austria. Valtion luonnontieteellinen toimikunta, Finland. Institut National de Physique Nucl~aire et de Physique des Particules and Institut de Recherche Fondamentale (CEA), France. Bundesministerium fCir Forschung und Technologie, Germany. Istituto Nazionale di Fisica Nucleare, Italy. Science and Engineering Research Council, United Kingdom. Department of Energy, USA. Thanks are also due to the following people who have worked with the Collaboration in the preparation of and data collection on the runs described here: O.C. Allkofer, F. Bernasconi, F. Cataneo, R. Del Fabbro, L. Dumps, D. Gregel and G. Stefanini.
References value [5] and varying both the W mass and the spin assignments, the results are shown in fig. 3b. Outside the region where the W mass is within 10 GeV/c 2 of the W mass we put an upper limit on R. This varies from 0.2 at low W masses (>~30 GeV/c 2) to 0.1 at high W masses (90% confidence level). Finally, we remark that the e+e invariant mass of both events in table 1 is in the interval 4 0 - 5 0 GeV/c 2.
254
[1] UA1 Collab., G. Arnison et al., Phys. Lett. 126B (1983) 398. [2] F. Berends et al., Nucl. Phys. B202 (1982) 63, and private communications. [3] UA2 Collab., P. Bagnaia et al., Phys. Lett. 129B (1983) 130. [4] UA1 Collab., G. Arnison et al., in preparation. [5] UA1 Collab., G. Arnison et al., Phys. Lett. 122B (1983) 103; 129B (1983) 273. [6] UA1 Collab., G. Arnison et al., Phys. Lett. 132B (1983) 214,224.
PHYSICS LETTERS
2 February 1984
SEARCH FOR MASSIVE e v ' / A N D Itv¥ FINAL STATES AT THE CERN SUPER PROTON SYNCHROTRON COLLIDER
UA1 Collaboration, CERN, Geneva, Switzerland G. ARNISON J, A. ASTBURY J, B. AUBERT b, C. BACCI i, G. BAUER 1, A. BI~ZAGUET d, R.K. BOCK d, T.J.V. BOWCOCK f M. CALVETTI d, p. CATZ b, p. CENNINI d, S. CENTRO d F. CERADINI d,i, S. CITTOLIN d, D. CLINE 1, C. COCHET k, j. COLAS b, M. CORDEN c, D. DALLMAN d,~/, D. DAU 2, M. DeBEER k, M. DELLA NEGRA b,d, M. DEMOULIN d, D. DENEGRI k, A. Di CIACCIO i, D. DiBITONTO d, L. DOBRZYNSKI g, J.D. DOWELL c, K. EGGERT a, E. EISENHANDLER f N. ELLIS d p. ERHARD a, H. FAISSNER a, M. FINCKE 2 G. FONTAINE g, R. FREY h, R. FRUHWIRTH ~, J. GARVEY c, S. GEER a, C. GHESQUII~RE g, P. GHEZ b, K.L. GIBONI a, W.R. GIBSON f, Y. GIRAUD-Ht~RAUD g, A. GIVERNAUD k A. GONIDEC b G. GRAYER J, T. HANSL-KOZANECKA a, W.J. HAYNES J, L.O. HERTZBERGER 4, C. HODGES h, D. HOFFMANN a, H. HOFFMANN d, DA. HOLTHUIZEN 4, R.J. HOMER c, A. HONMA f, W. JANK d, G. JORAT d, P.I.P. KALMUS f, V. KARIM.g,KI e, R. KEELER f, I. KENYON c, A. KERNAN h, R. KINNUNEN e, W. KOZANECKI h, D. KRYN d, g F. LACAVA i, j._p. LAUGIER k, j._p. LEES b, H. LEHMANN a, R. LEUCHS a A. LI~VI~QUE k, d, D. LINGLIN b, E. LOCCI k j..j. MALOSSE k, T. MARKIEWICZ d, G. MAURIN d, T. McMAHON c, j._p. MENDIBURU g, M.-N. MINARD b M. MOHAMMADI 1, M. MORICCA i, K. MORGAN h, H. MUIRHEAD s, F. MULLER d, A.K. NANDI J, L. NAUMANN d, A. NORTON d A. ORKIN-LECOURTOIS g, L. PAOLUZI i, F. PAUSS d, G. PIANO MORTARI i E. PIETARINEN e, M. PIML~ e, A. PLACCI d, j.p. PORTE d, E. RADERMACHER a, j. RANSDELL h, H. REITHLER a, J.-P. REVOL d, j. RICH k, M. RIJSSENBEEK d, C. ROBERTS J, J. ROHLF d, 3, p. ROSSI d, C. RUBBIA d, B. SADOULET d, G. SAJOT g, G. SALVI f, G. SALVINI i j. SASS k, j. SAUDRAIX k, A. SAVOY-NAVARRO k, D. SCHINZEL d, W. SCOTT J, T.P. SHAH J, D. SMITH h, M. SPIRO k, J. STRAUSS ~, J. STREETS c, K. SUMOROK d, F. SZONCSO Q, C. TAO 4, G. THOMPSON f, J. TIMMER d, E. TSCHESLOG a, j. TUOMINIEMI e, B. Van EIJK 4, j..p. VIALLE d, j. VRANA g, V. VUILLEMIN d, H.D. WAHL ~, P. WATKINS c, j. WILSON c R. WILSON 3, C.-E. WULZ ~, Y.G. XIE d M. YVERT b and E. ZURFLUH d Aachen a-Annecy(LAPPJ b-Birmingham C-CERN d-Helsinki e-Queen Mary College, London fParis(Coll. de France) g-Riverside h-Rome J-Rutherford Appleton Lab. J-Saclay(CEN) k_ Vienna 9~Collaboration
Received 19 October 1983 The observation of an apparent excess of radiative Z° decays into e+e-7 and ~+#-3' has prompted the search for massive eu~,and ~3' final states containing an energetic photon. No events were found other than those consistent with QED radiative effects in leptonic W decays. The data sample corresponds to an integrated luminosity of 0.136 pb -1 produced at the CERN super proton synchrotron (SPS) collider. An upper limit on the occurrence of such events is given. r University of Wisconsin, Madison, Wl, USA. 2 University of Kiel, Fed. Rep. Germany. 3 Harvard University, Cambridge, MA, USA. 250
4 NIKHEF, Amsterdam, The Netherlands. 5 Visitor from the University of Liverpool, England. 0.370-2693/84/$ 03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 135, number 1,2,3
PHYSICS LETTERS
1. Introduction. We have recently reported the observation of four Z 0 -+ e+e- events [ 1]. In one of these events, the momentum measurement by curvature of the negative track showed a value of (9 ,+ 1) GeV/c, much smaller than the corresponding calorimeter deposition of (48 -+ 1) GeV. We interpreted this event as the likely emission of a hard " p h o t o n " * 1 accompanying the electron. Subsequent calibrations of the electromagnetic (EM) counters indicated that the centroid of the energy deposition in the calorimeters was significantly displaced with respect to the incident electron track, indicating an angle of Aq5 = (14 + 4) degrees between the charged and neutral components. This made the possibility of an external bremsstrahlung in the vacuum pipe and detector window seem unlikely. The estimated probability of an internal bremsstrahlung exceeding the angle and energy observed is, according to Berends [2], about 0.005, or 2% for the sample of four events. A rather similar event has been reported by the UA2 Collaboration [3]. In this case the photon and electron hit separate cells, thus directly indicating a finite e-3, separation. Characteristics of these two events are given in table 1. More recently, an event of the type/1+/J-3 , has been observed in our study of Z 0 -+/a+/~- decays [4]. Also in this event, a small but finite angle (~8 °) is observed between the muon and the hard photon (E ~ 30 GeV). It is certainly premature to draw any conclusion about the origin of such events. The present paper deals with a search for an analogous phenomenon in the charged intermediate vector boson channels, namely for processes leading to final states of the type eu3, and ~uT, where 3' is a hard photon of transverse energy E T in excess of 10 GeV. The UA1 detector has been designed to study the large-angle phenomenology of p~ collisions at x/F = 540 GeV. The apparatus and the data-taking conditions are essentially unchanged from those previously reported, and we refer the reader to ref. [1] for more details. The main features of the UA1 detector that are relevant for the present investigation are (i) electron detection, (ii) neutrino detection, (iii) momentum analysis of charged particles, and (iv) muon detection. 4:1 We define a " p h o t o n " candidate as a neutral electromagnetic transverse energy deposition in excess of 10 GeV (see text in section 3 for details).
2 February 1984
Table 1 Energy, angle, and mass properties of the ee,y events.
E7 Ee 1 Ee2 A¢(e27) m(ele2) m(ele27)
m(euD m(e2~ )
(GeV) (GeV) (GeV) (°) (GeV/c 2) (GeV/c 2) (GeV/c 2) (GeV/c 2)
UA1 a)
UA2 b)
38.8 61.0 9 14.4 42.7 98.7 88.8 4.6
24.4 68.5 11.4 31.8 49.8 89.7 74.1 9.1
-+ 1.5 -+ 1.2 -+ 1 -+ 4.0 -+ 2.4 -+ 5.0 -+ 2.5 +- 1.0
+- 1.4 -+ 1.6 +_0.9
+- 2.8
a) For the UA1 event e 1 = e ÷ and e 2 = e-. b) The UA2 numbers are calculated from ref. [3].
Details of the features (i) to (iii) of the UA1 detector can be found in ref. [5], and (iv) can be found in ref. [ 1]. 2. Event selection. The event selection follows very closely the one used in our previous analysis to search for ordinary W ~ eu and W -+/au decays, except that the procedure has been modified so as not to exclude the radiative events on the basis of track isolation or event topology. 2.1. Search for eu7 events. Results are based on an integrated luminosity of 0.136 pb-1, which is corrected for dead-time and similar losses. This exposure resulted in the sample of 52 charged intermediate vector boson decays in the (Uee) channel previously reported [5]. The trigger selection required the presence of an EM cluster at angles larger than 5 ° with respect to the beam direction, and with transverse energy in excess of 10 GeV. After on-line filtering and complete offline reconstruction, about 1.5 × 105 events had at least one EM cluster (two adjacent EM calorimeter cells) with E T > 15 GeV. To search for the process We -+ e-+Ue3`,events have been required to survive at least one of two complementary sets of cuts: (i) Missing-energy selection: In the hermetic UA1 detector an energetic neutrino manifests itself as a transverse energy imbalance. We require a transverse energy imbalance that is in excess of 15 GeV and, in order to ensure the best accuracy, the missing energy vector must not point vertically (-+20°). Finally we impose a topology cut * 2 to remove spurious back#2 See footnote on next page.
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ground associated with two-jet events. After this selection we are left with 199 candidates for further analysis. (ii) Charged-particle selection: In addition to an EM cluster with E T > 15 GeV, we require the presence o f an associated, isolated [5] track with PT > 7 GeV/c in the central detector (CD), less than 600 MeV deposited in the hadron calorimeter cells immediately behind the electromagnetic cluster, less than 3 GeV transverse energy deposited in the hadron calorimeter within a larger specified ,3 cone, and a missing transverse energy in excess o f 10 GeV. No further cut on the topology o f the event has been made. We are left with the 52 published W_+ -~ e-*ve decays, plus an additional 27 events for further analysis. 2.2. Search for la~' events. Results are based on an integrated luminosity o f 0.111 p b - 1 , which is corrected for dead-time and similar losses. The trigger selection required the presence of at least one penetrating track detected in the UA1 muon chambers with pseudorapidity Ir/I ~< 1.3, and pointing in both projections to the interaction vertex within a specified cone of aperture e l 50 mrad. After off-line reconstruction we require a charged track in the CD with PT 2> 15 GeV/c, or p > 30 GeV/c, which matches, in both projections, the reconstructed track in the muon chambers. Removing cosmic rays and identified light meson decays, we are left with 144 muon candidate events. This event sample has been examined by physicists on Megatek, the visual scanning and interactive facility. Excluding those events containing hadronic jets with E T > 10 GeV (mostly two-jet events) we are left with 18 events having a single muon candidate.
3. Results. The three event samples have been examined by physicists on the Megatek facility. In searching for an energetic p h o t o n the following definition of a photon candidate was used: (i) We require a cluster in the electromagnetic calorimeters with E-r > 10 GeV. ,2 Events having two coplanar (_+30°) calorimeter clusters with E T > 15 GeV are excluded if the missing transverse energy vector is in the direction (_+20°) of one of the clusters. A genuine eve3, event having this topology would be recovered by the high-PT track selection. ,3 A cone in (rapidity, azimuthal angle) space was used, centred on the high-PT track, and described by the expression (r/2 + ~2)1/2 < 0.7, where r~ is in units of rapidity and ~ is in radians. 252
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(ii) The corresponding shower profile and shower size must be consistent with test beam data for single electromagnetic showers [5]. (iii) The photon must be isolated such that (a) no tracks with PT ) 500 MeV/c are associated with the shower (apart from a collinear muon or electron candidate), and (b) there is less than 600 MeV present in the hadronic calorimeter immediately behind the shower. 3.1. Search for ev7 events. In the two event samples of section 2.1, we have looked for the simultaneous presence o f a photon candidate (defined above) and an electron candidate. An electron candidate is defined in an identical way to our photon definition but with the additional requirement that there should be a charged track with PT > 7 GeV/c associated with the electromagnetic shower. The photon and electron may be resolved if they are separated by more than 3 standard deviations in either the ~ (azimuthal) or 0 (polar) coordinate (typically 12 ° in ~bor 2 ° in 0). No ew)' events satisfying these requirements were found. Amongst the 52 published W events [5] which are contained in these samples, we note that there is one W event in which the track m o m e n t u m is more than 4 standard deviations less than the shower energy (the track m o m e n t u m is 7 GeV/c and the shower energy is 40 GeV [5]). We interpret this as evidence for the presence of an energetic photon; however, the electron and photon are collinear and therefore not resolved by the above cuts. Furthermore, the observation of one such event out o f the sample o f 52 W events is consistent with the expected rate from QED radiative corrections (internal and external bremsstrahlung). 3.2. Search for laY7 events. In the event sample o f section 2.2, we look for the presence o f a photon candidate (defined previously), and find no events. There is no restriction on the angle o f the photon with respect to the muon. The 18 muon events in which we have a single muon and no hadronic jet activity provide us with a preliminary sample of W e -+ ll+-vu candidates [4]. The energy deposited by the muon in the EM calorimeters is consistent with that expected from radiative and ionization losses, together with the contribution from the minimum-bias background (figs. 1 and 2). To consolidate these results, the UA1 jet algorithm [6] has been used to search for energetic photons in the three event samples ,4. Once again, after requiring ,4 See footnote on next page.
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4. Conclusions. We have f o u n d no evidence for events o f the type ev7 o r / a v 7 where the EM neutral particle or cluster (7, 7r0, or 77) has a transverse energy in excess of 10 GeV. Therefore, the radiative effect observed in the Z 0 candidates is not present in the m u c h larger sample o f W decays. We can also calculate an upper limit for R, the fraction o f l e p t o n i c W decays which are a c c o m p a n i e d by an energetic p h o t o n , provided we a d o p t a m o d e l of the decay kinematics o f the process. Our evaluation is based on the electron samples. This enables us to use our published sample o f 52 W ~ eu decays in the calculation o f the limit on R. (i) T h r e e - b o d y phase space: R - BR(W ± ~ £± u~7)/BR(W ± ~ £±u~). Allowing for selection efficiency and geometrical acceptance, we find that R < 0.1 (90% confidence level). (ii) P r o d u c t i o n o f an excited state o f the charged lepton: W ± ~ £*vQ(£* ~ £-+7) •
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T h e n R - BR(W ± -~ £*v~ ~ £±Tv~)/BR(W ± ~ £± v~), and the experimental sensitivity depends u p o n the 1 mass o f the excited spin ~ leptonic state £*. In fig. 3a the result is shown as a f u n c t i o n o f this mass. In the mass range 10 G e V / c 2 ~< mQ. ~< 70 G e V / c 2 we place an upper limit on R falling f r o m 0.2 at low masses to 0.1 at high masses (90% confidence level). (iii) Radiative decay o f a heavy W into a lighter W: -~ WT(W --+ £v~)
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and W -~ WT(W ~ £vQ). T h e n R - ( o B R ) w / ( o B R ) w. An u p p e r limit on R depends u p o n the masses o f the W and the W, and their spin assignments. Fixing the W mass at the measured
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4:4 In the search for a photon using the UA1 jet algorithm, a photon candidate was defined as a calorimeter cluster in which (i) there is no associated CD jet, and (ii) there are two adjacent EM cells having between them E T > 10 GeV and 80% of the cluster energy. All events from samples (1) to (3) having such a "photon candidate" were found, on closer inspection, to be two-jet events. 253
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An inclusive search for additional e+e - pairs in this mass range has also given a negative result.
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Fig. 3. Upper limit (90% CL) for R, the fraction of leptonic W decays accompanied by a photon with transverse energy in excess of 10 GeV. The electron samples have been used to extract the limits on R, which are model-dependent. Results are shown for the following two models: (a) the process W ~ e're(e* e3,) with mw = 80.9 GeV/c2 [5], and (b) W ~ W'r(W ~ eve) where one of the boson masses has been taken to be 80.9 GeV/c2 , and the shaded band shows the dependence of the limit on the bosons spin assignments (0 or 1).
The continued sucess of the Collider and the steady increase in luminosity which has made this result possible depend critically upon the superlative performance of the whole CERN accelerator complex, which was magnificently operated by its staff. We thank W. Kienzle who, as coordinator, balanced very effectively the sometimes conflicting interests of the physicists and accelerator staff. We have received enthusiastic support from the Director General, H. Schopper, and his Directorate, for the results emerging from the SPS Collider programme. We are grateful to the management and staff of CERN and of all participating Institutes who have vigorously supported the experiment. The following funding Agencies have contributed to this programme: Fonds zur F6rderung der Wissenschaftlichen Forschung Austria. Valtion luonnontieteellinen toimikunta, Finland. Institut National de Physique Nucl~aire et de Physique des Particules and Institut de Recherche Fondamentale (CEA), France. Bundesministerium fCir Forschung und Technologie, Germany. Istituto Nazionale di Fisica Nucleare, Italy. Science and Engineering Research Council, United Kingdom. Department of Energy, USA. Thanks are also due to the following people who have worked with the Collaboration in the preparation of and data collection on the runs described here: O.C. Allkofer, F. Bernasconi, F. Cataneo, R. Del Fabbro, L. Dumps, D. Gregel and G. Stefanini.
References value [5] and varying both the W mass and the spin assignments, the results are shown in fig. 3b. Outside the region where the W mass is within 10 GeV/c 2 of the W mass we put an upper limit on R. This varies from 0.2 at low W masses (>~30 GeV/c 2) to 0.1 at high W masses (90% confidence level). Finally, we remark that the e+e invariant mass of both events in table 1 is in the interval 4 0 - 5 0 GeV/c 2.
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[1] UA1 Collab., G. Arnison et al., Phys. Lett. 126B (1983) 398. [2] F. Berends et al., Nucl. Phys. B202 (1982) 63, and private communications. [3] UA2 Collab., P. Bagnaia et al., Phys. Lett. 129B (1983) 130. [4] UA1 Collab., G. Arnison et al., in preparation. [5] UA1 Collab., G. Arnison et al., Phys. Lett. 122B (1983) 103; 129B (1983) 273. [6] UA1 Collab., G. Arnison et al., Phys. Lett. 132B (1983) 214,224.