Search for Centauro Events at CDF

Abstract:

Introduction

In the early 1970's, unusual cosmic-ray events were detected and named ``Centauro" because they appeared to be different ``beasts" in the upper (very little electromagnetic activity) and lower chambers (large hadronic multiplicity)[1,2]. The characteristics of these observed Centauro events are:

• Hadron multiplicities between 63-90, with a mean of 75
• multiplicities consistent with 0
• Average hadron transverse momentum GeV
• Mean of hadrons centered around 2.2 with
• Average total energy is 1740 TeV or TeV

Because these events made up a significant fraction (%) of the cosmic-ray events observed in these mountain-top emulsion experiments, the rate is expected to be significant. If they occur at 1% of the inelastic rate, then we would expect a cross-section of at TeV. The CDF data set analyzed contains 167,000 1-vertex events and, using a 50 mb minimum bias cross-section, we estimate the integrated luminosity to be . We would therefore expect to contain Centauro's in this data set.

Although there were many speculations as to why such a large hadronic to electromagnetic ( had-em) asymmetry would be observed, none are particularly satisfying and the fact remains that this asymmetry is extremely unusual within both cosmic-ray and accelerator data. Theory predicts chiral symmetry during particle production, and most hadronization data appear to be statistically consistent with equal production of .

One proposed mechanism for producing large had-em particle asymmetry has seen great popularity recently. It assumes that, within very energetic hadron-hadron or heavy-ion collisions, a region of vacuum is produced that has no preferred chiral orientation, i.e. it is chirally disoriented. When this vacuum expands and cools below some critical temperature, it adjusts (condenses) to the chiral state of the event vacuum by emitting pions. This cluster of emitted pions is therefore called a ``Disoriented Chiral Condensate" (DC), and can provide a large hadronic () to electromagnetic ('s from ) asymmetry[3].

The unique experimental signature for both Centauro and DC events is a large had-em particle asymmetry. In this analysis, individual particles are detected using calorimeter towers and identified as either hadronic or EM using additional information (tracking, hadronic fraction of , and longitudinal energy deposit).

The asymmetry variable used is:

which weights any asymmetry by the number of particles participating.

The Centauro analysis determines the -region with maximum particle multiplicity within a window of summed over for each event. Centauro candidates are selected based on the total multiplicity, mean particle , and mean within the window. The distribution of particle multiplicity versus mean does not show evidence for Centauro events. (Minimum multiplicity and mean cuts are shown that accept 83% of simulated Centauro events.) The Centauro candidates are greatly reduced by requiring . The events at high multiplicity and mean tend to be associated with jets. When events with a diffractive signature are selected, there is still no evidence for Centauro's in the distribution.

The had-em asymmetry for the minimum bias sample also shows no evidence for Centauro's in the tails of the distribution. The expected asymmetry distributions for simulated Centauro's, assuming either all- or all-, are shown superimposed.

The DC analysis uses a window of by radians and explores all phase space for the -window with the maximum had-em asymmetry. DC candidates are then determined on the basis of their maximum hadronic or EM asymmetry.

Conclusions

A minimum bias data sample representing an integrated luminosity of approximately has been analyzed for evidence of Centauro events. A simulation of Centauro events, including detector response, has shown that they should be observable and fairly well separated from the continuum. Although there are events that satisfy one or more of the various kinematic or topological requirements (multiplicity , GeV, , large asymmetry, and diffractive topology), these events are mostly jet-like and appear to be tails of the continuum distributions. There is no evidence for a distinct class of events that satisfy the Centauro kinematics or had-em asymmetry presented in the cosmic ray literature.

The 95% confidence level upper limits for various Centauro selections are determined by making cuts on the kinematics or asymmetry, assuming that all events passing the cuts are Centauro candidates, and using the acceptance for simulated Centauro's passing these same cuts. In most cases the upper limits are b while the predictions discussed in this note range between 330 to 500 b. Although this search should have detected Centauro events produced in collisions at TeV, uncertainties in the Centauro assumptions, such as the cross-section and energy threshold, allow for the possibility that higher energy is required to produce these exotic events. There is also the possibility that these events are unique to cosmic-rays and cannot be produced in collisions.

Using the same data sample we have searched for evidence of DC events by looking for unusual hadronic-electromagnetic particle asymmetry within clusters of by . A few DC configurations have been simulated, indicating that the all-em DC should be discernable from the continuum distribution, but the all-hadronic DC would be hidden. Because of the vague nature of the predictions, the phase space of possible DC configurations is very large and simulations will not help interpret the result. However the candidate events with large had-em asymmetry appear to be statistically consistent with the continuum and tend to have jet-like characteristics.

References

1

M. Tamada, ``Evidences for new type of cosmic ray nuclear interactions named 'Centauro' ", Nuovo Cimento 41B, 245 (1977);
C.M.G. Lattes, Y. Fujimoto and S. Hasegawa, ``Hadronic interactions of high energy cosmic-rays observed by emulsion chambers", Phys. Rep. 65, 151 (1980).

2

S. Hasegawa, et al. (Brazil-Japan Collab.), ``Centauro species in cosmic ray observation", ICR-151-87-5, 120pp (1987);
L. T. Beradzei et al. (Chacaltaya and Pamir Collab.), Nucl. Phys. B370, 365 (1992).

3

There are numerous papers on DC, and new preprints appear every other week. The following is a list of the more recently published articles, which in turn reference previous works.
R.D. Amado et al., Phys. Rev. Lett. 72, 970 (1994); A.A. Anselm and M. Bander, JETP Lett. 59, 503 (1994); S. Barshay, Particle World 3, 180 (1994); J.P. Blaizot and A. Krzvwicki, Phys. Rev. D50, 441 (1994); H.T. Eize, Nucl. Phys. A566, 571c (1994); S. Gavin et al., Phys. Rev. Lett. 72, 2143 (1994); Z. Huang and X.N. Wang, Phys. Rev. D49, R4335 (1994); E.M. Igentritz et al., Phys. Lett. B325, 263 (1994); I.I. Kogan, Phys. Rev. D48, R3971 (1994); D. Metzger et al., Phys. Lett. B321, 66 (1994); S. Pratt and V. Zelevinsky, Phys. Rev. Lett. 72, 816 (1994); K. Rajagopal, Nucl. Phys. A566, 567c (1994); F. Wilczek, Nucl. Phys. A566, 123c (1994);