Four-jet events from LEP

UCLEAR PHYSIC~

ELSEVIER

Nuclear Physics B (Proc. Suppl.) 52A (1997) 19-23

PROCEEDINGS SUPPLEMENTS

Four-Jet Events from LEP John Carr ~ a Centre de Physique des Particles de Marseille, IN2P3-CNRS, 163 Ave. de Luminy, Case 907, 13288 Marseille, C E D E X 09. In the data collected during November 1995 at 130/136 GeV with a total integrated luminosity of 5.7 pb -1, ALEPH has observed an excess of four-jet events compared to the expectations from the standard model processes. The properties of these events are presented together with the observations of the OPAL, DELPHI and L3 experiments.

1. I n t r o d u c t i o n During November 1995 the LEP collider increased energy above the Z ° resonance for the first time opening a new window for discoveries. D a t a corresponding to an integrated luminosity of 5.7 pb -1 was recorded in the A L E P H detector with similar amounts of d a t a being collected in the OPAL, D E L P H I and L3 detectors. An analysis performed with the A L E P H d a t a [1], which was designed as a search for the reaction e+e - --+ h A --+ bbbb, shows an excess of events containing four jets compared to the expectations from standard model processes. The QCD background of e+e - --+ qq(7) has a total cross-section of around 300 pb at 133 GeV with approximately 80% of this cross-section being radiative returns to the Z ° while the four-fermion background e+e - --+ Z(7*)Z(7* ) --+ qqqq h a s a cross-section of around 3 pb. When the A L E P H four-jet events are analysed by forming the masses of pairs of jets, an enhancement appears in the sum of di-jet masses around 105 GeV. This paper describes the A L E P H analysis in sections 2 and the searches from the other LEP experiments for similar events in section 3. Details of the exotic properties of the A L E P H events are given in section 4.

2. A L E P H

data analysis

The A L E P H events are selected from a sample of hadronic events which contain at least eight charged particles with the scalar sum of their mo0920-5632(97)/$17.00 © 1997 ElsevierScience B.V, All rights reserved. PlI: S0920-5632(96)00527-0

m e n t a being 10% of the centre-of-mass energy. Cuts, described in detail in reference [1], are used to reject events which contain a real Z ° and large initial state radiation where the photon either escapes detection or is observed in the detector. The charged and neutral particles in the events are clustered into jets using first the D u r h a m algorithm with a value of Yc~,t = 0.008 and four or five jet events are selected. The J A D E algorithm with a value of Yc~,t = 0.022 is then applied on the remaining events to increase the efficiency for finding four jets. These cuts, as all others, were optimised using a sample of h A Monte-Carlo events. For events which are clustered into five jets, the pair of jets having the lowest invariant mass is combined into one jet making a four-jet event. Further cuts, again described in detail in reference [1], are applied to reduce the backgrounds from standard model processes. After all cuts the selection efficiency is 42% for h A Monte-Carlo and 16 events are selected in the data. The expected background is 8.6 events with 8.3 coming from qq and 0.3 events from e+e - --+ ZZ,y*Z,q,*7* and W W processes. In order to form the jet pair masses a rescaling algorithm is applied to the jet energies. This algorithm assumes the jet directions are perfectly measured and calculates the jet energies using e n e r g y - m o m e n t u m conservation with the b e a m energies. From the three possible pairings of the four jets, the combination with m i n i m u m A M = M i j - M k l is chosen. With this the resolution on E M = Mij + Mkl is 1.6 G e V / c 2 and

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J. Carr/Nuclear Physics B (Proc. Suppl.) 52A (1997) 19-23

on A M is 5.4 G e V / c 2. Tile distribution of £ M from the 16 selected events is shown in figure 1. In this plot an accumulation of events is seen in P,M near 105 G e V / c 2.

event rate and ~2M distribution at 91 GeV.

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Figure 2. Distribution of the sum of the di-jet masses for 91 GeV d a t a and J E T S E T MonteCarlo with jet energies rescaled to 133 GeV. Figure 1. Distribution of the sum of the di-jet masses.

The events in this mass peak have been studied in detail to ensure that the effect is not caused by any bias in the detector. The jets are found to be uniformly distributed in the apparatus. Further, the jet energy corrections from the rescaling algorithm are found to have a normal distribution with the expected errors. A check of the background normalisation and shape in the E M distribution of figure 1 can be made using the data taken at the Z ° peak during 1994. Figure 2 shows this d a t a taken at 91 GeV and the J E T S E T Monte-Carlo where both the jet energies have been rescaled using 133 GeV as the center-of-mass energy in order to compare directly with the high energy data. There are 1803 d a t a events and 1861 Monte-Carlo events in this plot showing good agreement in the four-jet

3. A n a l y s e s o f O P A L , D E L P H I

a n d L3

The OPAL, D E L P H I and L3 experiments have performed searches for four-jet events motivated by searches for h A and H + H - . All three experiments have also performed modified analyses to be similar to the A L E P H analysis. As an example figure 3 shows the ~ M distribution from the D E L P H I analysis [21 . This distribution shows a slight excess in the mass region around 105 G e V / c 2. The analyses by OPAL and L3 show no accumulation of events near this mass but all experiments show more four-jet events in the d a t a than in the Monte-Carlo. Table 1 summarises the number of four-jet events for the four experiments after 'ALEPH-like' cuts in the whole mass range. When normalized to the A L E P H luminosity x efficiency the Monte-Carlo prediction

Carr/Nuclear Physics B (Proc. Suppl.) 52A (1997) 1~23

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Table 1 Comparison of number of observed 4-jet events for the four LEP experiments in the whole E M range. The column labelled O + D + L is the total for OPAL, D E L P H I and L3 normalised to the A L E P H £ x efficiency. OPAL DELPHI L3 O+D+L ALEPH Four-jet events in d a t a 9 13 11 12 16 MC expectation from SM 7.9 9.8 7.5 9.2 8.6 £ x efficiency ( p b - 1 ) 2.2 2.6 1.8 2.4 2.4 Resolution on •M G e V / c 2 2 1.9 2 1.6

from O P A L + D E L P H I + L 3 agrees with that from A L E P H and the event total is about 1.4 ~r in excess of the Monte-Carlo. The A L E P H excess is larger in absolute numbers and is about 1.9 ~r Using the event selection efficiency for the h A signal the A L E P H excess of 16-8.6 events corresponds to a cross-section of 3.1 -I- 1.7 pb.

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Figure 3. D E L P H I distribution of the sum of the di-jet masses.

4. P r o p e r t i e s o f A L E P H

events

In this section the properties of the A L E P H events are studied, considering various hypotheses for the nature of the events. The first possibility explored is that of a statistical fluctuation of the QCD background. Other possibilities are the h A and H + H - signals which originally motivated the analysis. It will be shown that the events are

unlike any of these processes and further unusual properties of the events will be presented.

4.1. Compatibility with QCD Two variables have been chosen to compare the observed four-jet events with the QCD background. For each event a QCD matrix element squared is evaluated using the measured four parton kinematics. From the possible parton orderings the one is chosen which gives the most QCDlike matrix element squared. Exotic processes give lower average values for this quantity. For e+e - -+ q~] the average value of the logarithm of this QCD matrix element squared is 2.80 with a distribution which has an RMS of 0.46, while for h A the average is 2.36 with RMS 0.37. The second discriminating variable used is the rapidity weighted jet charge defined as Qjet = E y i Q i / E y i where the sum is over the particles in the jet, Qi is the electric charge and gi is the rapidity relative to the jet axis. Exotic events might have higher jet charge than QCD events. From these variables two 'rarity' quantities, rl, r2, are defined for each event as the fractions of QCD simulated events with matrix element squared less than its own and jet charge larger than its own. Figure 4 shows the distribution of rl and r2 for the 9 events in the peak of figure 1, plus 3 events from outside the peak which have a mass close to 105 G e V / c 2 when the second best A M combination is taken. QCD events appear uniformly distributed in such a plot, while for the d a t a 8/12 events have rare IMoc,)l 2, 7/12 have rare 1(2yetl with 5 events simultaneously rare in both. The probability to find QCD events as rare or rarer than these events is about 0.02%.

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J Carr/Nuclear Physics B (Proc. Suppl.) 52A (1997) 19-23

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4.2. C o m p a t i b i l i t y w i t h h A a n d H + H The four-jet analysis was originally designed to search for h A --+ bbbb with M h = M A . The 4 b jet final state is about 80% of the total for large tan ft. The mass peak shown above was observed in the analysis before applying the final lifetimebased b jet tag. Of the 12 peak events only one event has at least two jets that pass the b tag and no events pass the predetermined cut for the four b jet tag. No lepton with high transverse momentum to its jet is observed when 3.2 =t= 1.4 would be expected for 12 bbbb events. Further the expected cross-section for h A production with M h = M A ---- 53GeV/c 2 is 0.5 pb corresponding to 1.2 events expected in the data sample. Hence h A production is unable to explain the excess. For H + H - production the cross-section is again about 0.5 pb with the major decay channel being c~6s. The large cs content leads to an expectation of 5.3 ± 1.6 K ° mesons in the 12 events while only 2.7 +1.4 are observed. In a separate H + H - search [3] no events of the type v + u 6 s or

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Figure 5. Distribution of di-jet mass difference A M for the data (points), standard processes (shaded histogram) and various particle pair hypotheses.

In order to investigate the value of the electric charge of a hypothetical particle and antiparticle decaying each to two jets, a charge separation variable is used. This variable is defined as AQ = IQ1 + Q2 - Q3 - Q4i where the jet pairing according to the di-jet mass difference is 1-2 and 3-4, and Qi is the rapidity weighted jet charge

J. Carr/Nuclear Physics B (Proc. SuppL) 52A (1997) 19-23

defined earlier. The distribution of AQ is shown in figure 6. The mean value of A Q for the d a t a is 0.64 + 0.09 compared with the expectation of 0.38 :t: 0.01 from standard processes. Thus if the observed events are interpreted as particle pair production the particle must have large electric charge.

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Figure 6. Distribution of the event charge separation A Q for data, standard processes and various particle pair hypotheses.

5. C o n c l u s i o n All the four LEP experiments observe an excess of four-jet events over the prediction of the standard model, amounting to a 2.2 cr effect. A L E P H sees an accumulation of these events with the summed di-jet mass near 105 G e V / c 2, DELPHI sees a slight excess in this mass region but OPAL and L3 see no clustering in mass. A detailed study of the A L E P H events demonstrates that they are unlike the events of the QCD background. The events are not compatible with h A or H + H - production.

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With the next LEP run at 161 GeV starting in June 1996 more d a t a will soon be available to understand the observations reported in this paper. REFERENCES 1.

'Four-jet final state production in e+e collisions at center-of-mass energies of 130 and 136 GeV', A L E P H Collaboration, D. Buskulic et al., submitted to Z. Phys C. 2. 'Search for Pair Production of Heavy Objects in 4-Jet Events at V~ -- 130-136 GeV', DELPHI Collaboration, D E L P H I 96-91, contribution to ICHEP96 conference, Warsaw July 1996. 3. 'Search for supersymmetric particles in e+e collisions of center-of-mass energies of 130 and 136 GeV', A L E P H Collaboration, D. Buskulic et el., submitted to Phys. Lett. B.