New Upper Limit on Strange Quark Matter Abundance in Cosmic Rays with the PAMELA Space Experiment

In this work we present results of a direct search for strange quark matter (SQM) in cosmic rays with the PAMELA space spectrometer. If this state of matter exists it may be present in cosmic rays as particles, called strangelets, having a high density and an anomalously high mass-to-charge ($A/Z$) ratio. A direct search in space is complementary to those from ground-based spectrometers. Furthermore, it has the advantage of being potentially capable of directly identifying these particles, without any assumption on their interaction model with Earth’s atmosphere and the long-term stability in terrestrial and lunar rocks. In the rigidity range from 1.0 to $\sim 1.0\times {10}^3\mathrm{GV}$, no such particles were found in the data collected by PAMELA between 2006 and 2009. An upper limit on the strangelet flux in cosmic rays was therefore set for particles with charge $1\le Z\le 8$ and mass $4\le A\le 1.2\times {10}^5$. This limit as a function of mass and as a function of magnetic rigidity allows us to constrain models of SQM production and propagation in the Galaxy.

Figure 1

Top panel: $A/Z$ distribution for H (left peak), He (right peak), considering events with $R<5\mathrm{GV}$. The bumps in the distribution of H and He indicate the presence of isotopes such as deuterium, tritium, and ${}^3\mathrm{He}$ (region a in text). Bottom panel: $1/\beta$ distribution for $R\ge 5\mathrm{GV}$ H (wider distribution), He (narrower distribution). Note the better resolution for helium due to the higher charge released in the scintillators (region b in text).

Figure 2

Integral upper limit in terms of rigidity, as measured by PAMELA, for nuclei up to $Z=8$.

Figure 3

Integral upper limits in terms of baryon number ($A$) as measured by PAMELA, for nuclei up to $Z=8$ compared with previous results. The dotted line is the predicted flux of strangelets [3]. The solid black lines represent previous experimental limits for strangelets which are translated into flux limits. The curves labeled (a) [34], (b) [35], (d) [36], (e) [37], and (f) [38] come from relic searches in terrestrial material. The curve labeled (c) [39] comes from searches which bombard materials with slow-moving heavy ions. The curves labeled with (h) [15, 16, 17, 18] represent satellite-based searches, obtained from the exposure of the single experiments assuming Poissonian statistics. The curves labeled with (i) [14], (j) [19], and (k) [40] represent previous detections of events consistent with strangelet signals by balloon-borne cosmic-ray detectors.