Minutes of the LEP WG on ALEPH-4jet events on December 18th, 1996 Present: Aleph: Dieter Schlatter (Chairman) Patrick Janot, Shan Jin, Eric Lanson Delphi: Wim de Boer, Christof Kreuter, Pierre Lutz, Jesus Marco L3: Riccardo Faccini, Marco Pieri, Mario Campanelli, Marta Felcini, Andre Kounine Opal: Satoru Yamashita, Peter Igo-Kemenes, Junichi Kanzaki TOPIC 1. Minutes ======= The chairman asked to prevent the spreading of rumours from our meetings. In order to minimize the difference in interpretations from the results during the meetings it was decided to distribute minutes, which should be accessible to the 4 collaborations. The minutes will be written in turn by representatives from the different collaborations. Wim de Boer accepts to start for the first round. TOPIC 2. DELPHI Results. ============== Jesus Marco presented a) homework from the previous meetings on detector resolutions and efficiencies after passing the ALEPH 4-vectors 10 times though the DELPHI detector simulation. He finds a resolution for the sum of jetmasses in the hA channel of 1.9 GeV and a Z-mass resolution for radiative return events of 2.8 GeV. b) The DELPHI data at 172 GeV has been analysed. The invariant mass cuts from ALEPH have been relaxed by 10% in order to take the smaller acceptance of the vertex detector in 1995 into account, which was upgraded only in 1996. Combining all data at 130, 161 and 172 GeV, DELPHI finds 44 events in good agreement with the MC expectation of 46. In the peak region (95-115 GeV) 15 events are found with 11 expected, assuming the smallest mass difference between pairs is the right combination. If the 2 best combinations are plotted, 28 entries in the data against 22 in the MC. In conclusion, good agreement between data and MC with at most a slight excess if both mass combinations are used. c) Excercise with the ALEPH 4-vectors. DELPHI reproduces the peak with an efficiency of about 65 %. Some questions concerning ALEPH 4-jet events after passing through the DELPHI simulation: -quite a few forward jets, but may be not unexpected for 4-jet events -invariant masses between pairs tend to cluster near limit -electromagnetic jets? He showed the M_13+M_24 vs M_14+M_23 plot for the ALEPH 4-vectors, where the labeling is according to energy (jet 1 most energetic etc.) It does not cluster like the QCD background. On the other hand the energy in a 1 degree cone around the most energetic charged particle shows a distribution in agreement with a DELPHI Monte Carlo simulation, which shows a slight increase near 35 GeV from initial state radiation. TOPIC 3. OPAL Results. ============ a) Satora Yamashita presented the results on resolution with the OPAL detector. He finds for hA a 2 GeV resolution, for the Z-mass 4.2 GeV and an efficiency for reconstructing the ALEPH events of 45%. b) Junichi Kanzaki studied the effects of jetfinding algorithms. He finds that some of the ALEPH events vary in mass with the algorithm in constrast to the hA signal. During the discussion: 1) the C-parameter analysis was wrong. 2) for the particles labeled "neutral hadronic jets" in the ALEPH 4-jet events one can change the mass hypothesis, since this is not known. However, one should only change the momentum accordingly, not the energy, since this is what is measured in the experiment. Note that some ALEPH events have many particles labeled "neutral hadronic jets", so the effect of mass assignment has to be studied. TOPIC 4. L3 Results. ========== Riccardo Faccini presented the L3 resolutions: for hA sigma is 2.5-2.9 GeV (at 161 GeV, at 135 GeV about 2 GeV) and for the Z0: sigma = 3 GeV. He brought up the question if one should use different models to compare with, e.g. not an equal mass hypothesis, but e.g. a mass difference of 10 GeV and look again for the best combinations. This was considered something to be pursued, but one should first focus on a comparison of the experiments. A study of qq-gamma events showed a peak after a constrained fit. TOPIC 4. ALEPH Results. ============= Patrick Janot gave the resolution for ALEPH. He finds for hA a 3 GeV resolution and an efficiency for reconstructing the ALEPH events of 71%. Topic 5 Future work. =========== The following ideas were considered of interest: 1) Make a scatter plot of the mass sums for the ALEPH 4-vectors WITHOUT any detector simulation in order to make sure everyone uses comparable analysis tools (rescaling algorithm, jet algorithm). In order to make a comparison more easily, the following is proposed (not discussed during the meeting): 2 separate plots with DURHAM and JADE as jetfinder, respectively M_13+M_24 on y-axis in 2 GeV bins from 50 to 130 GeV M_14+M_23 on x-axis in 2 GeV bins from 50 to 130 GeV. 2) hA 4-vectors will be distributed by P. Janot in order to do the same excercise with a known signal in order to study the stability 3) Look at the comparison of data from the different experiments with looser cuts 4) Add data from all experiments. In order to do so, it was suggested that all experiments give all invariant mass combinations ( 6 per event) for all their candidates after their cuts, including the background events 5) All experiments make their data available as 4-vectors. Since this implies a discussion within each collaboration, it will probably take somewhat more time to get at this stage. ============================================================================= From: Wim de Boer Tel: Karlsruhe +49 (721) 608 3593 Institut fuer Fax: Karlsruhe +49 (721) 262 607 Experimentelle Kernphysik Tel: CERN +41 (22) 76 73529 Universitaet Karlsruhe Physikhochhaus Postfach 6980 email: wim.de.boer@cern.ch 76128 Karlsruhe Germany ==============================================================================