A Fluctuation Probe of Disoriented Chiral Condensates

We show that an event-by-event fluctuation of the ratio of neutral pions or resulting photons to charged pions can be used as an effective probe for the formation of disoriented chiral condensates. The fact that the neutral pion fraction produced in case of disoriented chiral condensate formation has a characteristic extended non gaussian shape, is shown to be the key factor which forms the basis of the present analysis. PACS numbers: 25.75.+r,13.40.-f,24.90.+p Apart from studying the dynamics of nuclear matter at extreme conditions of temperature and density, the main goals of relativistic heavy-ion collision experiments have been to probe for the possible existence of quarkgluon plasma (QGP) and chiral symmetry restoration. It is also believed that the hot and dense nuclear matter formed in the collision, while going through the expansion and cooling stage, may relax into a vacuum state oriented quite differently from the normal ground state. This results in the formation of what are known as Disoriented Chiral Condensates (DCC) which finally decay producing an imbalance in pion production. In such a case the probability distribution of the neutral pion fraction is very different from what one would expect in the absence of any DCC production [1, 2]. It has also been suggested that DCC formation can be taken as a signal of chiral symmetry restoration at high temperature and density [3, 4]. Theoretical calculations on lifetime of DCC gives encouraging results on its detection in actual experiments [5]. Several experiments, starting from cosmic ray studies [6], hadron-hadron collision [7], to the present day heavy-ion collision experiments [8, 9] in which attempts have been made to look for DCC signal, have either produced null results or have managed to put an upper limit in DCC production. Recently fluctuations in the ratio of positive to negative charge and baryon to anti-baryon numbers have been shown to be effective probes for the possible existence of QGP [10, 11, 12]. Fluctuations in baryon number or charge asymmetry have been shown to be very much dependent on whether the system goes through a QGP phase or a purely hadronic gas phase. In the present letter we show that a similar event-by-event fluctuation study of the neutral to charged pion ratio can be an effective probe for DCC production. The basic difference between the events with DCC and those without any DCC formation lies in the probability distribution of the neutral pion fraction. For events with DCC the probability distribution of neutral pion fraction, f, is given by P(f) = 1/2 p f where f = N�0/N�, (1)