ÇIL'ISO H 2'J
LAL 87-53 Nov. 1987
Recent Results from the UA2 Experiment
Jean-Paul REPELLIN
Invited talk presented at the International Conference on Physics in Collision VII Tsukuba, Japan, August 25-27,1987
U.E.R raâ^ Institut National de Physique Nucléaire de et l'Université Peris-Sud de Physique des Particules
Bâtiment 200 - 91105 ORSAY Cedex LAL 87-53 Nov. 1987
RECENT RESULTS FROM THE UA2 EXPERIMENT
Presenied by J.P. REPELLIN Laboratoire de l'Accélérateur Linéaire, Bât 200,91405 Orsay - France
Abstract Recent results from the UA2 experiment at the CERN pp collider are presented. The full statistics is used to measure the properties of the Intermediate Vector Bosons and to test the Standard Mode: Searchet for exotic processes are also described.
1. INTRODUCTION
This review presents results obtained by the UA2 experiment at the CERN pp collider. The ex periment took data from 1981 until 1985 and the total integrated luminosity amounts to about 900 nb_1.
A selection bas been made among the results obtained in the UA2 experiment
In section 2, we first recall the main caracteristics of the detection of electrons and jets in the UA2 detector. Tests of the standard model are then presented. Results using the full statistics are given on the measurement of the masses of the Inlermediaie Vector Bosons (TVB) and on their production properties.
The third section collects a few results on hadron physics. It describes a search for the decay of the IVB into quark-antiquark pairs. A study of four-jet events is presenied and a preliminary measurement of the direct photo* production cross section is given.
In section 4, we report on searches for various exotic processes.
2. TEST OF THE STANDARD MODEL The W and die Z are observed in the UA2 detector by their decays
W -> ev <0
Z ->eV <2>
The apparatus is instrumented1) to delect electrons in the central and forward backward regions. The central region covers the full azimuth and polar angle between 40° and 140°, corresponding lo about 2 units of pseudorapidily. The forward (backward) region covers 90 % of the azimuth and polar angle between 20° and 37.5° (142.5° to 160°). Both regions are equipped with electromagnetic calorimeters preceeded by an active preshower detector. The resolution on the photon or electron energy is - 2 -
°E/E ~ ^ % AI E (GeV) and the energy scale is known with an accuracy of 1.5 %. These properties have been maintained for the whole operation of the apparatus and a fraction of the calorimeters have been recalibrated with an electron beam at the end of the experiment In the central region the electromagnetic calorimeter ( 17 radiation length) is followed by a hadronic calorimeter (4 absorption lengths). The calorimeters arc segmented into 240 cells built in a tower structure pointing to the center of the interaction region. The response or the calorimeters was also studied with hadron beams. The energy resolution for single pions varies from 32 % at 1 Gc V to 11 % at 70 GeV and is about 12 % Tor 40 GeV hadron-jets.
Jets of particles arc defined in the analysis of the calorimeters as clusters of adjacent cells with an energy exceeding a threshold set at 0.4 Gc V.
Electrons are defined as jets which satisfy further requirements2'3). The energy distribution in the calorimeter cells which belong to the cluster must be compatible with that of a single electromagnetic shower. A charge particle track which paints to the cluster must be reconstructed, and aligned with a hit observed in the preshowcr counter. The efficiency of the electron signature for isolated electrons is 75 %
and the rejection against hadron jets is about 104.
2.1 Measurements of the W boson mass
A sample of 251 events is selected4) to study the decay mode W-» e v by requiring the presence in the event of an electron candidate, and of a missing transverse momentum (identified as the possible emission of a neutrino). The following cuts are applied ; pj > 20 GeV/c and ituf > 50 GeV (ml is the mass calculated from the transverse momenta of the electron and the neutrino).
The background contamination to this sample is estimated. It comprises 11.2 hadronic QCD contribution ; 5.0 W~»tv,T->evv decays and 9.3 Z->e+e~ decays where one lepton is undetected.
An estimate of the W mass, mw, is obtained from a comparison of the mF? distribution with that expected from W decays. A Monte-Carlo program is used which generates the dN/drru, distribution for
different values of mw. The W longitudinal momentum distribution is obtained from the quark (antiquark)
5 structure functions * of the proton (antiproton). The W transverse momentum. pT, is generated from the distribution of ref. 6, which agrees well with the data. The decay is described by standard (V-A) coupling with decay parameters given by the Standard Model
The best fit to die experimental distribution is :
mW = 80.2 ± 0.6(staL) ± 0J(syS]) ± 1.3(sys2> GeV, (3) and if die W width, r\y, is fitted as an additional free parameter, we obtain r\y < 7 GeV (90 * confidence level). The statistical uncertainty in (3) takes into account the resolution of die energy measurement, and also cell-to-cell uncertainties of the energy calibration. Systematic uncertainties of die mass measurement have two major contributions, which are quoted separately. The uncertainty (sysj) of Eqn.(3) is mainly due to possible systematic biases in die evaluation of pi, and consequently m_. The second systematic uncertainty (sys2) reflects the measurement uncertainty on the global energy scale of the calorimeter
calibration. The influence on the mass fit of the (dominandy low-pj) background contribution is small.
2.2 Measurement of the Z boson mass
The study of the Z -* e+c~ decay was done on a sample of events obtained by requiring the pre sence of 2 electromagnetic clusters, one of which at least had to satisfy loose electron criteria. A total of 39 events satisfy m^ a 76 GcV and arc attributed to the Z, with an esdmatcd background of 1,3 events.
The process of internal brcmsstrahlung can produce events of the type Z -4 e+c""y ; one such event has been observed and is included in the final event sample, This event consists of a 24 GeV photon separated in space by 31° from an electron of 11 GcV. The probability of seeing at least one event having an (cev) configuration less likely than that observed in die sample of 39 Z decays is estimated to be * 0.4,
Using a relativistic Brcit-Wigncr shape modified by the mass resolution, the Z mass is evaluated Tram a fit to the mass values of a sub-sample of 25 events for which both electron energies are accurately
measured and for which mcc > 76 GcV. The result is :
mz = 91.5 ± 1.2 (stat) ± 1.7 (syst) GeV (4)
where the systematic uncertainty results mainly from the calorimeter energy scale of the central and forward calorimeters. From (3) and (4), we measure :
ir^ - mw = 11.3 ± 1.3 (stat.) ± 0.5 (sys^ ± 0.8 (sys2) GeV (5)
where (sysj) again results from systematic uncertainties of the P^ evaluation in (3) and (sys2> reflects the differing global energy scale uncertainties of the forward and central calorimeters.
23 Width of the W and Z bosons. Limits on the number of neutrino types
A direct measurement of the width, r%, of the Z is difficult since a precise knowledge of die shape of die mass resolution is required. The average measurement error is estimated to be 3.1 GuV, which is of the same Ofder as the expected Z width. Following?), we obtain
Tz = 2.7 ± 2.0 (stat) ± 1.0 (sysl) GeV < 5.6 GeV (90 % confidence level), where the quoted systematic error reflects the uncertainty of the average measurement error.
As described in ref. 3), we can obtain an independent but model-dependent7) estimate of die ratio of
total widths, T^fTy ^rom
rz/Tw = RMP R01 RlcP*, where RexP = o?X / a^ is the experimentally determined cross-section ratio from this experiment, where R* is die ratio Rexp = 7.2^2 (stat), < 10.42 (95 % confidence level), (6) Both Rln and R'cPl depend on sin^w explicitly through the neutral current couplings and implicitly via the W and Z masses. However the product Rm R,cPt is constant to within 1 % over the range sin26u/ = 0.232 + 0.009. A more important uncertainty results from the use of different sets of structure functions in the theoretical calculation of Rtn. Recent estimates give values of R10 ranging between 0.285 and 0.325. Using RCXP as in (6) and the value R,h = 0.305 ± 0.020, we evaluate r#Tw - 0.82 $'H (slat) ± 0.06 (thcor), < 1,19 ± 0.08 (thcor) (95 % confidence level), (7) where the theoretical error reflects the uncertainly on R^ as quoted above. The ratio of lout) widths is sensitive to both the number of neutrino types and the mass of the top quark, mt. Assuming that the charged members of any new family and also any other unknown particle arc loo massive to contribute significantly to W or to Z decays, we obtain the results summarized in figure 1 which shows the expected variation of r^/Tyy with mt if the number of light neutrino types is respectively 3,4 or 7. The error bars at the end of each of these lines shows the effect of varying sin^O^ in the range 0.232 ± 0.009. Also shown in figure 1 are the experimental evaluations r^j/Tyy as given in (7). From the 95 % confidence limit and using the conservative upper estimate of R10 = 0.325. the data exclude more than seven neutrino types when no requirement is made on mt. This limit decreases to three neutrino types in the case mt > 74 GeV. Figure 1 : The value of rjir^f measured, with the expected variation with mt ~"H ti to to ™ •, (G.VI 2,4 Measurement of the Standard Model parameters The masses of the weak bosons, m\y and mz, are two essential parameters of the Standard Model. In its minimal expression, it relates them to the fine structure constant a, the Fermi constant Gp and the weak mixing angle 0\v via dw following relations : 2 2 m^ = A /f(l-Ar)sin 0w] - (8) m| = A2 / [ (1 - Ar) sin^yv cos^w 1 (9) 1/2 where A = (na /J 2GF) = 37.2810 ± 0.0003 GeV. In relations (8) and (9), the quantity Ar accounts for the effects of one-loop radiative corrections on the W and 2 masses and has been computed to be Ar = 0.0711 + 0.0013 (references can be found in 4>) assuming that mt= 35 GeV and that the mass of the Higgs boson is mu = 100 GeV. This quantity is insensitive to m^ but would change if the top quark were more massive (Ar = 0 for mt ^ 270 GeV with sin2ew =0.232). From a measurement of the ratio mw/tnz, which is free from the common systematic uncertainty of calorimeter energy calibration on the W and Z mass scale, a direct measurement of sin28\y is provided via the relation : sin26w * l " (mw^z)2 0°) From cqn. (10), wc evaluate : sin2ew - 0.232 ±*).025 (stat) ± 0.010 (syst) where the quoted unceruinty includes the contribution of a ± 0.5 GeV systematic error on the value of myy. This result is indépendant of other experiments, and of theoretical uncertainties. By using accurate existing measurements of A, and the value of Ar, a more precise measurement of sin2©^ is obtainable from a best Tit to Eqns. (8) and (9). We obtain : sin2ew = 0.232 + 0.003 (slat) ± 0.008 (syst) (11) These results are in excellent agreement with those obtained in low energy neutrino experiments8), which average to : sin2^ = 0.232 ± 0.004 (stai) ± 0.003 (theor) (12) The results are summarized in figure 2, where correlations between uncertainties of the my/ and m£ measurements are shown in the [ m^, (m^ - m\y) ] plane. Figure 2 : Confidence contours (68 % level) in the mZ " mW, "*zp/a/i* (i) statistical error only (ii) including systematic errors. Region (a) is allowed by low energy measurements. Curve are from standard model prediction with (b) and without (c) radiative corrections. Any departure from the minimal Standard Model will induce modifications to the above formalism. In particular values of sirAty deduced from Eqn. (8), from Eqn. (10), or from low energy neutrino experiments, will generally differ. All existing measurements are in excellent agreement with the predictions of the minimal Standard Model ; nevertheless, they can be used to place limits on possible deviations from the minimal Standard Model. In particular the quantity : P=- m^cos2^ is sensitive to the Higgs sector. Assuming the value of <0,0711 for Ar, wc measure from (12) p » 1.001 ± 0.028 (stat) ± 0.006 (sysl), in good agreement with the minimal Standard Model prediction of p = 1. The relations (8) and (9) may also be used to measure the radiative correction parameter Ar. Eliminating sin2^,wc obtain ; from which we deduce : AT = 0.068 ± 0 087 (sut) ± 0.030 (syst), in agreement with the minimal Standard Model prédiction. The value of sin20^ in (12), from low energy experiments, can be used to provide a more accurate measurement of Ar. We measure : AT - 0.068 ± 0 022 (stat) ± 0.032 (sysl). We therefore conclude that the existing data are consistent with, but barely sensitive to, the existence of radiative corrections from known processes (see Frg. 2). 2J Measurements of the cross-sections From the number of W -»• cv and Z" -> e+e~ decays observed in the experiment as reported in sections 2.1 and 2.2, we extract cross sections9?. The product £ of the geometrical acceptance and the effects of the Irinematical cuts is obtained from a Monte-Carlo simulation. The overall efficiency r\ of the electron identification criteria are 0.72 and 0.94 for W and Z respectively. The cross sections are measured at the two energies : J s = 546 GeV and >J s = 630 GcV at which the collider has been working and wiih a total luminosity & of 142 nb"* and 768 nb"'respectively. The measured W cross-sections are : o^(546 GeV) = 0.61 ± 0.10 (stat) ± 0.07 (syst.) nb o^r(630 GeV) * 0.57 ± 0.04 (stat.) ± 0.07 (syst) nb where the quoted systematic errors include contributions from uncertainties on e\y, ?IW m^ *& -Tne uncertainly on % is estimated to be ± 8 %. 6 The total cross section for W production is predicted to be ) a\y(546 GeV) * 4.7 *0'7 nb, and aw(630 GeV) * S.8*j Q nb, where the errors reflects theoretical uncertainties on the structure functions and ambiguities in the choice of the Q2 scale. Using these values and the results of our measurements, we obtain: B(W -» ev) = 0.10 ± 0.01 (exp.) $jj£ (thecc.) for the decay branching ratio, where the second error results from the theoretical uncertainty on Ow mentioned above. This value agrees with the Standard Model prediction, which, in the case of three fermion families, falls in the range 0.086 - 0.11 depending on the mass of the top quark. The measured Z cross-sections are : c£ (546 GcV) = 116 ± 39 (sut.) ± 11 (syst.) pb, c^ (630 GeV) = 73 ± 14 (sut.) ± 7 (syst.) pb, 6 nD m The total cross section for Z production is predicted to be ) 0"2(546GeV)* 1.3_02 > ^ +05 0£ (630 GeV) x 1.7 0 j nb. Using these values and the results of our measurements, we obtain : B(Z-»cV} = 0.046 ± 0.008 (cxp.) ^JJJ (theor.) for the decay branching ratio, where the second error results from the theoretical uncertainties on o*£- This value is consistent with the Standard Model prediction, which for three fcrmion families is in the range 0.031 to 0.035, depending on the value of the top quark mass. 2.6 Hadron jets associated widi IVB Production Non-leading QCD contributions to IVB production generate final slates in which one or more gluon or quark jet is produced in association with the IVB at a rale depending on the value of the strong coupling constant as. Most jets from initial slate gluon radiation tend to go in (he directions of (he beams, and therefore escape detection. Sometimes, however, they have large enough transverse momenta to be detected. Furthemore. the IVB can be produced in association with a quark from the interaction of an 2 incoming quark-gluon pair. The production cross sections have been calculated to 0(a) in perturbative QCD10). A comparison with our W event sample provides a quantitative test of the theory. In die present analysis the search for jets is restricted to the central UA2 calorimeter which detects electromagnetic and hadronic showers over the full azimuth range for pseudorapidities rql < 1. We only retain jets having a transverse energy exceeding 10 GeV. The W sample is found to contain 221 events without jet, 29 events with one jet, and 1 event with two jets. The distribution of the jet transverse energy is shown in figure 3. For the shaded event containing two jets the sum of their transverse energies is plotted. This event (having jet transverse energies of 41 and 24 GeV) and the event having the largest jet transverse energy (74 GeV) stand out from the rest of the sample. They were discussed in more detail in previous pobbcauons 2.3-'1). UAI l> rr f, ', „, r 1 ";-n-Fv,~.. , Fjgure 3: Transverse energy distribution of Figure 4 : Invariant mass distribution W-assoriatedjets. Dotted Une is from the of the W + jet(s) system QCD model i0> In particular, the two outstanding events mentioned earlier, which happen to have «cured in the early stages of data taking, do not require any particular mechanism other than that provided by the standard model of strong and electrowcak interactions. A further illustration of this fact is given in figure 4 where we plot the invariant mass distribution of the W + jct(s) system. Data and theory arc in good agreement and we find no evidence for an additional production mechanism. The jet transverse energy distribution obtained with as = 0.15 is compared in figure 3 to that observed in the data. The agreement is well within statistical uncertainties. In particular there are 26.1 W + t jet events (background subtracted) against an expectation of 28.1, one W + 2 jets event against an expectation of 2.9 and 2 events having a total jet transverse energy in excess of 50 GeV against an expectation of 3.3. This comparison implicilcly assumes that the factor Kn associated with final states containing n partons docs not depend on n. In fact the relative population of W + 1 jet events 12 provides essentially a measure ' of cts Kj/Kn» 0.13 ± 0.03, where the quoted uncertainly is only statistical. Tr-e Z sample contains 4 events with one jet and no event with two or more jets. Their configurations arc also in agreement with theoretical expectations. 3. RESULTS ON HADRON PHYSICS The observation of hadron jets in large transverse energy final states, and the study of their properties, are one of die main results obtained at the CERN p p collider. The dominant two-jet configuration has been extensively studied (see for example rcf. 13 and 14) and compares succesfully with the predictions of QCD to leading order in the strong coupling constant o^. In this section we present a search for decays of the IVB into quark-antiquark pairs and a study of four-jet events. 3.1 Search for W and Z decays into quark-antiquark pairs We expect the IVB's to decay into quark-antiquark pairs with well defined branching fractions. Namely, excluding decay modes with a top quark in the final stale (if at all present they would have a very different configuration as a result of the targe top mass), we expect'5* : r(W-»qq)/r(W-+ev)*6, and T(Z -»qq )/r(Z -> ee) * (15-28s2 + 88s4V3)/(Ms2 + 8s4) a 20, where s = sinOu/ is the sine of the mixing angle of the electroweak theory. The observation of such decays is an important qualitative check of the Standard Model predictions. The experimental difficulties arise from the need lo detect a signal which appears as an excess of only a few percent over the copious background of two-jet events produced by the strong interaction between colliding partons. This requires both a very good two-jet mass resolution and a large integrated luminosity. We report here the results of a search for a signal from IVB decays into quark-antiquark pairs in the mass distribution of jet-pairs observed in the UA2 detector16'. The data were collected at a total ccntre-of- mass energy 4 s = 630 GeV. The total integrated luminosity associated with these measurements is ^=0.73pb'1. In this analysis, the emphasis is placed on the best possible mass resolution. We use selection criteria which reject events departing substantially from average behaviour, and consequently populating the tails of the mass resolution curve. High mass tails are particularly dangerous in the present case, since the expected signal is superimposed on a steeply falling background. The jel definition algorithm starts by reducing the measured cell energy pattern to a set of clusters, jet energies arc measured as (he sum of the energies of all calorimeter cells having their centre within a cone of anglco) around the jet axis. Wc adjust the value of u in order to minimize energy measurement errors. The selection criteria mainly retain events with a limited transverse momentum and in which the two jets have a good longitudinal containment in the calorimeter. r. _.,..., , , ,,,...,.., r,- Figure S shows the mass distribution of the events which pass the standard cuts. Each event has been given a weight (m/100)5 in order to attenuate the steep decrease of the mass spectrum while collecting data in relatively wide mass bins. A structure is clearly visible in qualitative agreement with that expected from W and Z decays. Figure 5 : The jet-pair mass distribution of the events. The smooth curves are the resut of the best fits to the background and the signal Two independent evaluations of the mass resolution 4m have been made. One is obtained from a direct comparison of the experimental p5 and pi! distributions where jà is the projection of the transverse momentum along the two-jet direction and Pi! along the perpendicular projection. The energy resolution affects mostly the distribution of p* while p)J, provides a direct measurement of the Pj of the jet pair. The other evaluation is obtained from a Monte-Carlo simulation of the experiment and allows for indépendant estimate of each of the relevant contributions. The two independent evaluation of Am are in excellent agreement, - S GeV in the V/ region. To evaluate the number of events in the observed signal and to obtain a measure of its significance, we fit the data to a form : A {nr" expt-pm) + ÇSf.m.mo) ) where die fust term gives a parametrization of the QCD background and the second is the sum of two gaussian distributions describing W and Z decays respectively, HIQ being the W mass. The result of the fit gives a signal of 632 ± 190 events with a significance of 3.3 standard deviations and a mass m0 = 82 + 3 GcV. The expected number of signal events is 340 ± 80. The - 10 - number of signal events obtained from the result of the Tit is larger than expectation by approximately 1.4 standard deviation. Stronger evidence for the signal and a significant quantitative measurement of the W,Z->q q branching fractions will require the collection ofasignificantly larger daiasanple. 3.2 Study of four-jet events To higher orders in o,, QCD predicts the occurence of final states containing hard bremsstrahlung gluon jets. In particular thrcc-ict events have been the subject of detailed studies by the UAl17>and UA218) Collaborations. Their relative yield and phase-space density are in good agreement with QCD predictions. We obtained12! a value of a,, K3/K2 * 0.23 ± .Ol(stat) ± .04(syst) for the strong coupling constant os where Kj and K3 represent the contributions arising from higher order corrections to the two and three-jet cross-sections. The agreement with QCD is also observed in four-jet events. Here, however, a new production mechanism which is absent in the case of two-jet and three-jet events is expected, at least in principle, to play a role : the production of two independent jet pairs from two independent pairs of incoming partons6* to which we refer as multiparton mechanism. The cross-section to produce two jet-pairs, (13) and (2,4), via the multiparton mechanism, can be written do(U ; 2,4) _ 1 doq.3) do(2,4) d^dE^dEfdE? °efT dE^dE? dE^dE4 where ocff = itRrfj is an effective cross-section defined by (13). In this naive picture, a complete independence between the two jet-pairs is assumed. This relation suggests a comparison between real four- jet events and artificial four-jet events obtained by combining pairs of real two-jet events. A comparison is made of die transverse energy and angular balance distributions of the data events and the two possible sources : gluon bremsstrahlung in a simple QCD model and multiparton process. It 18 allows to place the following limit ! on the contribution of (he multiparton mechanism : Reff à 0.6 fm. The four-jet event sample is in remarkable agreement with the simulation from the QCD model. • * Examples are given in figure 6 which compares some cos (Oy distributions (a>- is the angle between jets i aod j). For this purpose jets are sorted in order of decreasing energy in the centre of mass system, with the results that jets 3 and 4 are in general radiated gluons. one from jet 2. the other from jet 1 or again from jet 2. This is visible from the enhancements observed in the vicinity of cos to. ± > 1 with respect to phase space distributions obtained by setting the subprocess matrix elements equal to unity in the QCD model calculation. Figure (j ; Cos û> ,7 distri butions of four-jet events compared with the leading- order QCD model (solid line) and four-body phase space (dashed line) 3 J Direct photon production We have previously presented results on direct photon production at the CERN pp collider1'). Here, we present new results using a larger data sample corresponding to a total integrated of 768 nb'1 collected at J s = 630GcV. Single photon candidates are associated with clusters of cells in the central calorimeter. In order to suppress the background from high-pf hadrons, we require the photon candidate to be well isolated from other particles : no charged track and no other shower detected in the presliower counter in a cone of about 15e around the photon direction. The residual contamination of the sample is mainly caused by unresolved 1 1 1 1 1 1 x° and r\ decays. This background is estimated from the 50 fraction of events for which the photon candidate has an V§,630GeV 20 _ - associated conversion signal. 10 - \ 0 = 90" - 5 - - After subtracting the background, the direct photon production cross section varies with pj ss shown on % 2 - \\ - CD 1 figure 7. Only the Py-dependant errors are shown on tiie ^. 1 1 0.5 - - figure. The systematic uncertainty on the normalisation 0.2 - of the cross-section is 25 % in total. 0.1 - The measured direct photon cross section is compared £0.05 - 2 Co.02 - with an 0 1 1 1 1 1 depends, however, only little on the definition of Q2. 10 20 30 40 50 60 P,T Figure 7 : Inclusive photon cross-section - 12 - The prediction, shown as the upper curve on figure 7, includes contributions from the bremsstranluitg of the final stale quarks. The isolation criteria partiy suppress such processes in the data, and this has not been taken into account when evaluating the cut efficiency. In the tower curve, the contribution from bremsstrahlung photons with angles smaller than 20° to the original quark direction has been removed. The difference between the curves corresponds to an additional uncertainty caused by our ignorance about the contribution of final state bremsstrahlung to the data. Reasonable agreement is observed between the measuremeats and the calculation. 4. SEARCH FOR EXOTIC PROCESSES The UA2 detector can be used to search for various hypothetical processes which would manifest through the production of electrons, photons, jets or even non interacting particles. The latter will manisfest themselves by an imbalance in the transverse momentum in the event so that a missing transverse momentum provides a good signature even in a detector with limited coverage like TJA2. We win report in turn on these searches22). 4.1. Search for excited elections Excited lepton states are expected in models in which leptons are composite particles. We report on a search for excited electrons through the decay sequence leading to an election and a photon : W-»e*v->evv (14) The analysis starts with the full data sample, which consists ot S340 events containing at least one electron candidate with transverse momentum above 11 GeV4). Of these, only 24 contain also a photon candidate with transverse momentum above 10 GeV, These events are compatible with the expected background from jet pairs, where one jet fakes an electron and the other jet fakes a photon. In addition, if we require that the missing transverse momentum be larger than (Mu/ - M*)/4 (M* is the invariant electron-photon mass), none of these events survives, whereas more than 95 % of the events originating from process (14) survive (the exact number depending upon the mass of (he excited electron). To interpret this result, we have performed a Monte-Carlo simulation of this process in the detector. We have assumed that the W couples to the transition magnetic moment of the e* - v current23), thus producing an effective gauge-invariant Lagrangian, which can be written as : e^Ce^KVM^Wae'oûPqpvL + hx (15) where g is the SU(2) coupling constant, X represents the coupling strength, and M* is the mass of the excited election. From the absence of candidates for process (14) we extrvct a 95 % confidence region in the \ - M* space for which the existence of excited electrons is excluded. This region is shown in figure 8 with recent and most stringent limits from e+e* data ^ - 13 - Figifrc 8 : Limits on excited electrons shown as 95 % confidence level curves in the X- 94* plane * tl *t II «1 IS l« II || |V II 11 4.2. Search for t and Win Z decays In this section, we study low-mass electron pairs as a possible signal for supersymmetric particles. More specifically, we consider the following processes : pp -» Z +... -»e"e + ...-» e ef y + ... (16) pp-*Z + ...-» WW + ... -*eev~v + ... (17) where the f and the v escape detection (stable photino and slablev' or v -» vf). These processes are characterized by final states containing two electrons and missing transverse momentum. Toe data analysis is therefore performed using the total data sample from the Z triggers. The electron cuts of the W -» ev analysis are relaxed but are applied to both electron candidates. We obtain a total of 57 candidate electron pairs with Mee > 10 GeV2. Of the 39 evsnts from the Z sample (section 2.2), 21 satisfy the above criteria. Most of the 36 remaining events are concentrated at mass values near the kinematic threshold, in a region sull dominated by background. We estimate the background to be 20 ± 1 events. The signal from genuine electron pairs is therefore 16+6 events. We have sims iated processes (16) and (17) in order to estimate their contribution. We have then performed a likelihood fit to the event distribution, lairing into account the expected contributions from background, from >oe Drell-Yan continuum, which is the dominant standard process illowed by our kinematical cuts ami electron selection criteria, and from processes (16) and (17). The limits obtained are shown in figures 9a and 9b as 90 % confidence level contours in M£, MÇ and Mç&, My space respectively. The limits of figure 9a are valid for a stable photino and assume ej,, and êpi to be degenerate in mass. The limits of figure 9b assume that the W is a pure gaogino and that the branching ratio for W -* ev is 33 %. The proper helicity-conserving amplitudes were used for the W decay 25). The limits in figure 9a assume conservatively that Mi# > Mz/2, and similarly, the limits in figure 9b assume that Mf > M^. - u - " Figff* $a •' Limits on the eand g masses at90 % confidence level. The " ^ dashed area illustrâtes theoretical » uncertainty (k factor on Drell-Yan n process) e n * Ftftire 9b • Same for W and v masses I W 1* H « I * Il n M « 1% IttV/c1! HJ IGtV/c1! The hatched areas of figures 9a and 9b show how the limits vary if we increase the expected Drell- Yan contribution by SO % to take into account the larger higher-order QCD corrections (K factors) expected for M^ around 20 GeV (K - 2) as compared to Mw - Mz (K ~ 13). Recent results from searches for e~and W at cV machines (PETRA and PEP) are also shown. In the W case, the UA2 results improve significantly on the eV limits. 4.3. Search for q and g Inhadronkmachirtes.lhedcninanlsciircesofsupersymmetik q g~and£g production 26). In most models, the phottno is expected to be much lighter than the g~and i«oqY(ifM(1"' We have performed a Monte-Carlo simulation using the cross-sections of ref.26, a standard set of structure function parametrisations with AQCD = 200 MeV29) and the gluino fragmentation distributions suggested in30). We assume that there are five flavours ofq~ all degenerate in mass and that all decays are isotropic The experimental cuts and the geometrical acceptance result in an overall efficiency ranging from 10r5 to 1(H. when the q or g* masses increase from 10 to 60 GeV. The results are shown in figure 10a as 90 % confidence level contours in the Mg - M q~ space. We exclude q" masses between 9 and 46 GeV and g~ masses between IS and SO GeV. Fragmentation effects and higher-order terms result in an uncertainty of a few GeV on the limits in the regions of small g and q masses. The limits in the high-mass region are intensitivc to the value of the photino mass, provided that it remains smaller than - 30 GeV. The limit on the q" mass goes down from 60 GeV to 46 GeV if the gluino mass increases from 100 GeV lo 1000 GeV, whereas the limit of SO GeV on the g" mass is un-changed if the q~ mass becomes very large. Finally, we note that these results are not sensitive to the exact value of the photino lifetime if the average decay path in UA2 remains smaller than - 1 cm, which is the case, in most models, for photino masses larger than I GeV. Figure 10a : Limits on the ? versus q obtained from photon pairs accompanied by jets in the case of an unstable photino Figure 10b : same obtained from three-jet events with missing P, in the case of a stable photino • 2D II 40 H HO H| ICtV/t1) We return now to the case where die photino is stable. In this case, hadronic production of q" and g results in final states with two to six jets and missing transverse momentum, P^. to contrast to the previous case, where the requirement of two photons considerably reduces the hadronic background from multijet events, we now have to deal directly with this background, particularly since large P^ can be faked in UA2 by a jet escaping at polar angles 6 smaller than 20° with respect to the beams, or depositing only a small fraction of its energy in the forward calorimeters. We are thus led to look for multijet final states with large PC . We select an initial sample of 31075 two- or three- jet events, where each jet has a transverse energy above 12 GeV, and with at least one pair of jets with azimudial separation A^ between IS and 95° (a condition required by the on-line trigger). This number of events is reduced to 203 if one requires P™ > 30 GeV and a limited energy deposited in the forward calorimeters. - 16 - This large number of events docs not allow us to exclude possible q or g production. We therefore retain only the events with three jets. Among the thirteen such events, only twelve have similar configurations, where the third jet is the jet of largest transverse energy. Such configurations are not expected in supersymmetric processes, and we therefore require that the third jet has transverse energy less than 40 GeV. After this last cut one event remains, which is identified as a W -> ev decay accompanied by two jets (one of the three jeis is identified as an electron). From this analysis we extract limits in the M^ - Mq space, as shown in figure 10b. Because of the more stringent kinematic cuts applied than in the case of an unstable pholino, the limits of figure 10b are very «symmetric and not as good as the ones of figure 10a. Neither are they m good as the most recent preliminary limits from UA131), due to the better coverage in polar angle of the UA1 detector. We recall that, in the stable photino case, a lower limit M « > 23 GeV is obtained from experiments at PETRA24>- as shown in figure 10b. 4.4. Search for additional vector bosons Additional vector bosons arise naturally in the framework of many possible extensions of the minimal SU(2)L x U(l) standard model of electroweak interactions, whether U be through right-handed currents, composite models or various models derived from superstring theories25*. In the following, we extract general limits on additional charged or neutral vector bosons, as a function of their mass. Mw-, £•• their coupling to quarks, Xq, and their branching ratio to electrons, Bc, where XQ and Be are nomalized to die standard model values. We have simulated W' and Z1 production and decay in UA2, using the structure function parametrisation of reference 21. We have then performed a likelihood fit to our W and Z data samples, using the electron transverse momentum spectnim for the case of W, and the invariant mass spectrum of the two electrons for the case of Z\ One W -»ev candidate has PT * 77 GeV, which is unlikely for W -> ev decays not accompanied by jets4). We note that, for a W' of mass 160 GeV and with standard couplings, we should observe 15 events with P£ between 70 and 80 GeV. The results are shown in figures 1 la and 1 lb, for Wand Z' respectively, as 90 % confidence level 2 contours in the M^r, £- - ^n Be space. In principle, we would be sensitive to additional W' and Z' almost degenerate in mass with the standard vector bosons, because they would result in observed rates for W-» ev or Z -» e+e" larger than the theoretical predictions29). In practice, our cross-section measurement9) and the theoretical predictions are subject to large uncertainties, thus precluding any significant limit We also note that the high-mass limits of figures Ha and lib are affected by an overall uncertainty of ± S GeV, due to the structure function uncertainties and to the uncertainty on the absolute electron energy scale. An additional W with Myp < 25 GeV would have been seen at the ISR and at PETRA. Similarly, an additional Z" with My < 50 GeV would have been detected at PETRA. We therefore conclude from figures 1 la and 1 lb that additional vector bosons with masses smaller than the standard ones are excluded for couplings to quarks and Icptons similar to the standard model ones. In the high mass region, we exclude a W with Mw < 209 GeV and a Z' with My < 180 GeV, for standard couplings. Most models derived from superstring theories predict at least one extra Z boson with 2 a reduced coupling to quarks, *q - 0.2, and with B0 varying from 0.3 to 1.0 depending upon the masses of the new panicles predicted by these models. Our data do not exclude any region above M% in figure Sb 2 for Xq Be values between 0.0S and 0.20. Fjg ifft 11 : a) Limits on an additional charged vector boson W, the coupling X» and branching ratio to electron Be are normalized to the standard model values b) Same for an additional neutral vector boson Z' CONCLUSION We have shown that, within the present accuracy of the measurements of the intermediate vector bosons properties, there is very good agreement between the observations and the predictions of the Standard Model. Tests of QCD have been extended to the study of four-jet events and the measurement of the direct photon cross-section. The decay of the IVB into quark antiq laik pairs has been observed at 3 standard deviation level. 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