SEARCHES FOR NEW PARTICLES IN UA2
The UA2 Collaboration Bern - Cambridge - CERN - Heidelberg - Milano Orsay (LAL) - Pavia - Perngia - Pisa - Saclay (CEN)
presented by Laura Perini University Palermo and J.N.F.N. Milano, Italy
Abstract During the 1988-1989 runs at the CERN pp Collider (v^ = 630 GeV) the UA2 experiment has collected data corresponding to an integrated luminosity of 7.8 pb~l. On this sample searches have been performed for top quark and supersymmetric particles. No evidence for such particles has been found and the lower limits provided for their masses are: top mass greater than 69 GeV (95 % C.L.); squark (gluino) mass greater than 74 (79) GeV (90 % C.L.); selectron (Wino) mass greater than 38 (45) GeV.
Introduction The search for the top quark
Data have been collected by the upgraded UA2 A sixth quark flavour forming an SU(2) doublet detector during the 8 months of the 1988-1989 runs with the b quark is predicted by the standard model of the CERN pp Collider at peak luminosities of up (SM). If the top mass is smaller than the W mass, to 3 x I0,,0crrr2s~1. The event sample collected is the top quark can be produced at the pp Collider about ten times as large as the one collected dur from two main processes ing the previous Collider runs. The upgraded UA2 PP-+W + X; W -+tboTtb (1) (Fig.l) is described in detail elsewhere [1]: the main PP ft + X (2) areas of improvement are electron identification, At y/s = 630 GeV, process (1) is dominant. The crucial for the top, ë, W searches and hermeticity SM predicts thatthe most abundant final state from of the calorimeter coverage, crucial also for q and g top decay is t -> bqg} but this hadronic decay is very search, which is performed on rnultijet events with difficult to observe because of the QCD rnultijet background: so the search has been focused on the large transverse momentum imbalance (fr). The enhanced hermeticity was obtained by adding to semileptonic decay t -> bev (branching ratio 1/9 in the central calorimeter f2] two endcap calorimeters, SM). covering the full azimut rial angle down to ± 3 units The expected event configuration is a high PT in pseudorapidity r) [3]. electron, some ^from the undetected neutrino and 1 or more hadronic jets. The selections applied to the data sample are:
- Pt(e) > 12 GeV where the electron is defined using the standard UA2 identification criteria [4] in the central calorimeter; -£>15GcV;
- Et{jetx) >10 GeV where jeti is the highest Et cluster in the pscudorapidity range < 2.2 except ing the electron; - Angle A(£((ï - jeti) < 160°; in order to reject the background of two-jets where a hadronic jet was rriisi den tilled as an electron. A total of 137 events passed the above selection criteria. The expected background comes mainly from the process W + jet(s); W -.c. ^ (3) A simulation performed using the E.KS Monte Carlo [5] predicts 148.5 ±14.5 events. À detailed eval uation of other background sources, such as b-quark decay, jets misidentified as an electron, and Z decay, gives 5.9 ± 1.7 events. We define the transverse mass Mr as
Fig. I. Partial and schematic longitudinal view of the UA2 detector. Mr = sj2PTP^{\~cos^fV)
451 range 30 < mtop < 69 (71) GeV is excluded at 95 (90) % CL. By only considering the it production process and using the correct matrix elements for b' decay, a lower bound is obtained on the mass of a possible V (fourth generation sequential quark): the
result is mb, > 54 (57) GeV at 95 (90 %) CL. Note that this limit on the mass of the top quark relics on the S.M. assumption that no charged Higgs (H*)
exists with a mass smaller than mtop\ if such a //* would exist the top preferred decay would be b and the semileptonic decay searched for in this analysis would be strongly suppressed.
Search for squarks and gluinos
In supersymmetric (SUSY) theories the squark (q) and the gluino (g) are the partners of the quark
Mr IGeV/c*) and the gluon. Their production is expected to take place through standard QCD processes, resulting in Fig. 2. Best lit (full curve to Mr distribution of data with abundant production in ppcollisions, for sufficiently no top signal. The lowest expected contribution from top 2 small masses. A multiplicative quantum number (m — 65 GeV/c ) is added (dashed curve). iop (R-parity) is associated to each superpartner and is conserved in all interactions: this implies that spar- ticles are produced in pairs (qq, gg, qg), and decay The measured MT distribution (Fig. 2) peaks around 80 GeV while most of the top signal for the into other SUSY particles, to the Lightest SUSY top masses relevant to this analysis is expected to Particle (LSP), assumed to be electrically neutral, which would escape detection. By identifying the 7 be in the range 15 < MT < 50 GeV; for this reason the above EKS prediction was obtained normalizing to with the LSP the simplest decays are:
the data in the range MT > 60 GeV. Restricting the g - q q 7 (4) analysis to the range 15 < Mr < 50 GeV we observe q -> qg and q q 7 (5) 17 events against an estimated total background of These processes give the signature of a high
26.2 ± 3.4 events. In order to extract limits on the tfT accompanied by two or more hadronic jets. The
top mass miop) the expected number of observed top main background to the SUSY in this analysis
events Niop and their MT distribution is computed comes from beam halo particles, which are rejected
as a function of mtop. The acceptance for top is es using the dedicated VETO counters and strict tim timated using the EURO JET Monte Carlo J6]. The ing cuts, and from multijet events where some trans production cross section (r(tb) via W decay (process verse energy escapes detection, either because of the 1) is normalized to the W -* ev signal observed by presence of hard muons or neutrinos, or because of UA2; for the cross section o-(tt) via ft production detection inefficiency (cracks). The selection crite (process 2) the lowest value given in [7] is taken, ria applied to the data are: based on the complete order (r*J) QCD calculation - fr> 20 GeV, corresponding to the full effi ciency for the calorimeter trigger specifically aimed oft8]- at selecting events with a large ^,[9]. The main residual uncertainties on the pre - Presence of at least two hadronic jets with dicted Niop come from the error on the W mass (0.4 E\ > 25 GeV and Ef, > 15 GeV, both in the pseudo- GeV) which translates into a 5 % error on (r(tb)) and from the systematic effects on the acceptance, rapidity range \r}\ < 0.85.
related to the uncertainty on the jet fragmentation - If' E$, < 10 GeV AJ)(jctx - jct2) < 160° in order and on the calorimeter response to jets, expccially to reject the dominant two-jet events, where a fake
fris generated by the mismcasure of E\. to low energy ones. The value of Niop used for set
ting the limits on miop was obtained by adjusting - Events with electrons (purely calorimetric cri all parameters in their allowed range to the value teria are used with efficiency ~ 100 %) are rejected
that minimize Ntop . The limits on the mass were in order to eliminate W ev decays.
then obtained by comparing the MT distribution of - Isolation of the ^vector is required: the observed events with that expected from back A^jefj - fT) < 140° ancf ground sources alone or in the presence of a top &
452 section, we can extract lower limits on the squark Search for selectron and Wino and gluino masses, knowing the q and g produc tion cross section and decay modes and the accep A full account of the UA2 search for ë, W is tances for the selected processes. The production given in ref. [14]; here only a brief summary of the cross section computation is based on ref. [10], us analysis and the results are given. The processes ing the EHLQ I [11] structure functions: five squark searched for are: flavours degenerate in mass are assumed. The g and pp-+ Z + X; Z -+ ë ë; each ë -> e 7 (6) q decays (4) and (5) are assumed to account for 100 PP-+W + W; eachW->eî> (7) % of the braching ratio (BR) and to be isotropic; the gluinos are fragmented according to ref. [12]. A lull The LSP is assumed to be 7 in (6) and v in (7): Monte Carlo simulation of the calorimeter response it escapes detection giving as signature for both of to the final state particles is used for the acceptance the above processes the presence of an electron pair computation; the acceptance is low (~ 0.1 %) for q, associated to high tfT. The SM background for reac g masses below 50 GeV and reaches typically 5 % tions (6) and (7) is given by e-pairs from the Drell for the largest accessible masses (~ 100 GeV). The Yan process; the background of fake electrons, from systematic uncertainties coming from the choice of QCD processes where hadronic jets are misidenti- the a, and Q7 scale, structure functions, fragmen fied as electrons, is estimated using the data. The tation functions, and from the systematic error on mass limits obtained at the 95 % CL are:
the energy scale and on the calorimeter response to - mP: > 38 GeV under the assumption m^L = m-tR jets have been studied and taken into account in the and my = 0 (the result does not significantly change computation of the mass limits. The mass regions for photino masses up to 20 GeV). excluded at 90 % CL in the m$ - m$ plane by this - m^r > 45 GeV under the assumptions that the analysis are shown in Fig. 3, which display also pre branching ratio W e v is at least 20 % and = viously published results from UA1 and CDF (CDF 0 (the result does not significantly change for sneu- has presented higher mass bounds at this Confer trino masses up to 25 GeV). ence). The mass limits thus obtained are indepen dent of the 7 (LSP) mass up to 20 GeV, whereas References they strongly depend on the assumption of 100 % BR for the direct decay (4) (5): cascade decays as [I] UA2 Collaboration, C.N. Booth, Proc 6th Top the ones envisaged in réf. [1 3] would give a softer tf , r ical Workshop on Proton-Antiproton Collider Phy resulting in lower mass limits. The resulting mass sics, Aachen, 1986, eds. K.Eggert et al.: (World limit, shown in Fig. 3, can be summarized as fol Scientific, Singapore, 1987), p. 381. lowing: at 90 % CL, we exclude 50 < m$ < 74 GeV [2] A. Beer et al.: Nucl Instrnm.. Methods 224 (1984) independent of m$, 50 < m- < 79 GeV independent g 360. of mq, and 50 < m < 106 GeV for m~ = nig = m. g [3] F. Alberio et al.: The Electron, Jet, and Miss ing Transvers Energy Calorimetry of the Upgraded UA2 Experiment at the CERN Collider, in prepa ration for Nucl. Instrum. Methods. [4] T. Akesson et al: (UA2 Collaboration) Z. Phys. C46 (1990) 179. [5] S.D. Ellis, R. Kleiss, W.J. Stirling: Phys. Lett. 154B (1985) 435. 6] A. Ali et al.: Nnd. Phys. B292 (1987) 1. 7 G. Altarelli et al.: Nnd. Phys. B308 (1988) 724. 8] P. Nason, S. Dawson, and R.K. Ellis: Nucl. Phys. B303 (1988) 607. [9] P. Moreira, G. Polesello: Nnd. lustrum. Methods 287 (1990) 417. [10] S. Dawson, E. Eichten and C. Quigg: Phys. Rev. D31 (1985) 1581. [II] E. Eichten et al.: Rev. Mod. Phys. 56 (1984) 579 and 58 (1986) 1065. [12] A. De Rujula and R. Petronzio: Nucl Phys. B261 (1985) 1587. [13] H. Baer et al.: Phys. Lett. 161B (1985) 175 and Fig. 3. Mass region (m^, m^) excluded by the present ex Phys. Rev. Lett 63 (1989) 352. periment (shaded area) with the SUSY model described in [14] T. Akesson et al.: (UA2 Collaboration) Phys. the text. The dashed curves indicate the previous mass limits Lett. 238B (1990) 442. from UA1 and the dash-dotted curve the one from CDF. All limits are for 90 % CL.
453