Search for nuclearites with the ANTARES detector
aa r X i v : . [ a s t r o - ph . H E ] O c t Search for nuclearites with the ANTARESdetector
G.E. Pˇavˇala¸s
Institute for Space Sciences, Bucharest-Magurele,Romania
Abstract.
ANTARES is an underwater detector located in the Mediterranean Sea, near the Frenchcity of Toulon, dedicated to the search for cosmic neutrinos. ANTARES is optimized to detectthe Cherenkov signal from up-going relativistic particles, but could also observe massive exoticobjects, such as magnetic monopoles and nuclearites. In this article we present a search strategyfor nuclearites and determine the sensitivity to nuclearites of ANTARES detector in completeconfiguration, using a set of data taken in 2008.
Keywords: quark matter, nuclearites, neutrino telescopes
PACS:
INTRODUCTION
Strange quark matter, composed of nearly equal number of up, down and strange quarks,could be the ground state of nuclear matter [1]. Massive lumps of strange quark matter,named "nuclearites" by Glashow and De Rujula (1984), could be present in cosmicradiation and reach the Earth [2]. Nuclearites may be produced in the Early Universeor as debris from supernovae and strange stars collisions [3]. They could be detectedwhile traveling through transparent media (water, air), where they transfer some of theirenergy as visible light.ANTARES is a neutrino telescope aimed to detect high energy neutrinos from galacticand extragalactic sources, such as supernovae, binary systems and gamma ray bursts.The detector is optimized to collect the Cerenkov light emitted by up-going relativisticmuons, produced in neutrino interactions below the detector. ANTARES could also besensitive to the signal of down-going nuclearites [4, 5, 6].The goal of this study is to develop a search strategy for nuclearites with the fullyinstalled ANTARES telescope.
THE ANTARES DETECTOR
General description.
ANTARES is located on the floor of the Mediterranean Sea,at a depth of 2.5 km. It consists of a three dimensional array of 885 photomultipliertubes (PMTs) distributed on 12 lines anchored on the seabed. The detector is operatedfrom a control room on shore. The sensitive element of the ANTARES telescope is ahemispherical 10” Hamamatsu photomultiplier tube, housed in a pressure resistant glasssphere [7]. A triplet of PMTs forms a storey, together with the read-out and controlelectronics. A detector line has a length of 450 m and contains 25 storeys, distributed atvery 14.5 m, starting 100 m above the seabed. Every line is connected to the JunctionBox, itself connected to the shore station at La Seyne-sur-Mer through a 40 km longelectro-optical cable. The detector instruments a surface area of about 0.1 km . Data acquisition system.
The PMT signals larger than a preset threshold of 0.3photoelectrons (pe) are digitized and the corresponding time and charge informations,referred to as L0 hits, are sent to shore. A L1 hit is defined either as a local coincidenceof L0 hits on the same storey within 20 ns, or as a single hit with a large amplitude,typically 3 pe. The raw data are processed by a computer farm in the shore station, usingdifferent triggers for physics signals [8].Two muon triggers operated during data acquisition in 2008: the directional triggerand the cluster trigger. The directional trigger requires five local coincidences causallyconnected, within a time window of 2.2 m s, and the cluster trigger requires two coinci-dences between two L1 hits in adjacent or next-to-adjacent storeys. When a muon eventis triggered, all PMT pulses are recorded over 4 m s in a snapshot .ANTARES was taking data in partial configurations since March 2006 and wascompleted in May 2008. Measurements of the atmospheric muon flux with the firstANTARES line and the 5-line detector are presented in [9], and respectively [10]. NUCLEARITES
Nuclearites are hypothetical massive particles, composed of nearly equal numbers of up,down and strange quarks [2]. They are believed to be neutralized by electrons inside thequark core or by an external electron cloud, forming a sort of an atom. Their velocityis non-relativistic ( b = − ) and interact with the surrounding medium via elastic andquasi elastic collisions, with an energy loss: dE / dx = − sr v , where r is the density ofthe medium, v is the nuclearite velocity and s its geometrical cross section: s = (cid:26) p ( M N / pr N ) / for M N ≥ . ∗ GeV; p × − cm for lower masses , where r N = . × g cm − is the density of a nuclearite. Nuclearites movingslowly through water would produce a thermal shock emitting blackbody radiation. Theluminous efficiency was estimated to be h ≃ × − by [2] and the number of visiblephotons emitted per unit path length can be computed from: dN g / dx = h dE / dx p ( eV ) , assuminga mean energy of visible photons of p eV. DATA AND MONTE CARLO SIMULATIONS
Monte Carlo simulations.
The Monte Carlo simulations of nuclearite events inANTARES use a hemispherical generation volume of 548 m radius, that surroundssymmetrically the detector [11]. The initial point of the trajectory and the direction of thenuclearite are randomly generated.The algorithm proceeds in steps of 2 ns, evaluatinghe position and b of nuclearites, as well as the number of hits on the PMTs. We havesimulated a sample of about 350000 down-going nuclearites in 12 line configuration,with an initial velocity (before entering the atmosphere) of b = − , in the mass rangefrom 10 to 10 GeV . A typical nuclearite would cross the ANTARES detector in acharacteristic time of 1 ms, producing a large light signal.The main background for nuclearites is represented by the down-going atmosphericmuons. The atmospheric muon sample used in this analysis was generated with theMUPAGE code [9], considering an energy range from 20 GeV to 500 TeV. Other sourcesof background are the decays of K40 (with a constant rate of about 27 kHz), and thebioluminiscence (characterized by large fluctuations in time).The MC samples are processed with the same triggers used in data acquisition, and thebackground is added from selected runs. Processing the nuclearite events with triggersdesigned for relativistic particles results in a sequence of snapshots, distributed on adetector crossing timescale up to three orders of magnitude larger than for muons.
Data.
A period of 42.8 days of data taken in 12 line configuration, from Juneto December 2008, is considered in the analysis. Both the directional and the clustertriggers were operated during the data acquisition, with a high threshold of 3 pe. Theselected data satisfies certain quality criteria, such as a low biological activity.
ANALYSIS AND RESULTS
Analysis method.
The analysis uses a blinding policy, that proceeds according to thefollowing steps:1. look for the best discriminative variables, then define selection cuts for nuclearites.At this step we use simulated nuclearites and muons;2. comparison between a sample of 15% of data and Monte Carlo down-going muons,in order to validate the simulations;3. calculation of the ANTARES sensitivity to nuclearites for 42.8 days of 12 line data.
Definition of the selection conditions.
In the first step, we studied the distributions ofseveral parameters for the Monte Carlo nuclearite and atmospheric muon samples, suchas the number of L0 hits and L1 hits per snapshot and the duration of the cluster of L1hits, defined as the time difference between the last and the first L1 hit in the snapshot,see Figs. 1-2. The distributions are normalized to unity.The distributions in Fig. 1 are showing an excess of L0 and L1 signals per snapshotfor nuclearites (represented with dash-dotted line) with respect to the simulated atmo-spheric muons (dotted line). This fact allowed us to define a first selection criterion fornuclearites as a linear combination the number of L0 hits and L1 hits. This cut is shownin Fig. 2 (right), that separates the nuclearite snapshots (represented as points) from thesimulated muons (crosses). A small percentage of the actual simulated events is used,for the legibility of the figure. of L0 hits s n a p s ho t s -6 -5 -4 -3 -2 -1 MC nuclearitesMC muonsdata s n a p s ho t s -6 -5 -4 -3 -2 -1 MC nuclearitesMC muonsdata
FIGURE 1.
Normalized distributions of the number of L0 hits (left) and L1 hits (right) per snapshot forsimulated nuclearites (dash-dot line), muons (dotted line) and 15% of data (continuous line). dt (ns) s n a p s ho t s -6 -5 -4 -3 -2 -1 MC nuclearitesMC muonsdata o f L h i t s FIGURE 2.
Left: Normalized distributions of the L1 cluster duration (dt) per snapshot for simulatednuclearites and atmospheric muons. The distribution from 15% of data is also shown. Right: Linearselection cut in the distribution of the number of L1 hits as a function of L0 hits for nuclearite snapshots(points) and atmospheric muons (crosses).
Data - Monte Carlo comparison.
The comparison between data and Monte Carloatmospheric muons is also shown in Fig. 1 and Fig. 2 (left), using 15% of the data takenwith 12 lines in 2008, that is equivalent to 6.4 days of active time. The data and MonteCarlo distributions are in relatively good agreement, with some discrepancies in the tailsof the distributions.The efficiency of the first cut on simulated nuclearites considering all masses isrelatively high, about 74%, while the rejection efficiency for simulated down-goingmuons is 100%. After applying this cut on the data sample, two events remain.In order to optimize the rejection of the two remaining events in data, a second cut isapplied, that requires two or more snapshots within 1 millisecond, with the duration ofthe L1 hits cluster larger than 3000 ns. The rejection efficiency of the two combined cutsis 100% on the 15% data sample, while the overall efficiency for nuclearites, computedwith all masses, drops to 73%. Nuclearite events with 10 GeV mass do not pass theselection cuts. xpected sensitivity.
In the final step, we computed the sensitivity to nuclearites.The sensitivity is defined as the 90% C.L. flux upper limit that we expect for a givenbackground prediction, and no true signal [12]. The obtained sensitivity to down-goingnuclearites for a period of 42.8 days of 12 line ANTARES data and no background eventsvaries between 4 . × − cm − s − sr − for 10 GeV nuclearites and 5 . × − cm − s − sr − for 10 GeV.The sensitivity obtained for down-going nuclearites of lower masses is comparablewith the best upper flux limit established by the MACRO experiment, of 2 . × − cm − s − sr − [13]. The result obtained for 10 GeV nuclearite mass improves theMACRO result by about a factor 5.
CONCLUSIONS
ANTARES has the instrumental capability to search for massive exotic particles. In thisstudy, we have determined the sensitivity of ANTARES to nuclearites. These prelimi-nary results show that the sensitivity at lower masses is comparable with the MACROupper limit, and is improved by a factor 5 at large masses. ANTARES sensitivity tonuclearites will improve significantly with more data.
ACKNOWLEDGMENTS
The author would like to thank to Prof. Giorgio Giacomelli and Dr. Vlad Popa for usefuldiscussions and support.
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