Results from IceCube

Nuclear Instruments and Methods in Physics Research A 725 (2013) 1–6

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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima

Results from IceCube E. Resconi Technische Universität München, Boltzmannstrasse 2, D-85748 Garching, Germany

For the IceCube Collaboration art ic l e i nf o

a b s t r a c t

Available online 29 April 2013

The construction of IceCube neutrino telescope and IceTop surface array was successfully completed at the South Pole during December, 2010. IceCube is the most sensitive telescope to date for observing high energy neutrino sources. The performance of the IceCube detector and a summary of results will be reported from earlier years as the detector increased in size from 40, 59 and 79 strings. New analysis methods developed for the study of the Southern Hemisphere as well as for the extended regions in the sky will be emphasized. The long term experience with AMANDA and IceCube has proven that the South Pole ice is an ideal site for astroparticle physics. New ideas and possible future projects beyond IceCube will also be presented. & 2013 Elsevier B.V. All rights reserved.

Keywords: Neutrino astronomy Cosmic rays

In contrast to photons, high-energy (HE) neutrinos (Eν 4 100 GeV) carry an unambiguous signature for both acceleration and interaction of protons at cosmic sites. The long standing problem of the origin and production mechanisms of cosmic rays motivates the use of HE neutrinos as probe for a deep investigation of the non-thermal universe [1]. High energy neutrinos are expected to be produced in the decay of mesons, created through the interaction of an accelerated proton with ambient protons or photons. Candidate cosmic accelerators are both extra-galactic objects, like Active Galactic Nuclei (AGN) and Gamma Ray Bursts (GRBs), as well as galactic sources, like micro-quasars and supernovae remnants (SNRs) present in star forming regions. For a review of candidate astronomical sources, we refer to [2]. On the other hand, HE neutrinos may also be produced by the annihilation of dark matter particles gravitationally trapped at the center of the Sun, the Earth and our galaxy. For a review about indirect dark matter search with neutrino telescopes we refer to [3]. Moreover, HE cosmic neutrinos present a unique opportunity to study the interactions of elementary particles at energies comparable and beyond those obtained in current or planned colliders. Sterile neutrinos, Lorentz symmetry violation (LV), neutrino decay, monopole and double Sleptons are examples of possible phenomena that can be probed with HE neutrinos. For a description of the phenomenology involved we refer to [4,5]. The broad discovery potential of HE neutrino astronomy, combined with the weak interaction of neutrinos with matter, motivates the construction of cubic-kilometer observatories. Natural transparent materials are used to meet the gigaton detector

E-mail address: [email protected] 0168-9002/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.04.002

mass scale deep underground needs. A recently completed project of this scale is IceCube. AMANDA, operated for 12 years between 1997 and 2009, was the pathfinder detector for IceCube. Neutrino telescopes are versatile detectors. In addition to the physics goals summarized above, IceCube has the capability to detect neutrino bursts from nearby supernovae by exploiting the low photomultiplier noise in the Antarctic ice (on average 286 Hz for IceCube) [8,9]; to measure the primary composition of cosmic rays by analyzing events seen in coincidence by the air shower array IceTop [10] and the deep strings of IceCube [11]; and to explore energies below 100 GeV with the use of the sub-detector DeepCore [12].

1. Detector operation, calibration, simulation IceCube [6,7], the largest neutrino telescope in history, is now a reality. The construction, spanning 7 years, was completed on December 18, 2010. The 2800 m thick glacial ice sheet at the South Pole is used by IceCube as a Cherenkov radiator for charged particles. The Cherenkov light produced by the collision of cosmic neutrinos with subatomic particles in the ice or nearby rock is detected by an embedded array of Digital Optical Modules (DOMs). Each DOM incorporates a 10 in. diameter R7081-02 photomultiplier tube (PMT) made by Hamamatsu Photonics. The completed array consists of 5160 DOMs deployed at depths of 1450–2450 m attached to 86 vertical strings. Moreover, 80 IceTop surface stations provide a unique air shower detector for the study of primary cosmic rays. The layout of the array is shown in Fig. 1.

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E. Resconi / Nuclear Instruments and Methods in Physics Research A 725 (2013) 1–6

Fig. 1. (Upper) Top view of the IceCube geometry as deployed in its final stage. (Lower) Conceptual three dimensional view of the IceCube observatory and DeepCore.

A summary of the deployment seasons and IceCube configurations is reported in Table 1. The AMANDA telescope, consisting of 687 optical modules on 19 strings between depths of 1500 m and 2000 m, operated as an independent instrument from 2000 to 2006. It was integrated into IceCube operation from 2007 to 2008. In 2009, AMANDA was decommissioned. IceTop stations are instrumented with DOMs identical to those in the deep ice. Here the signals arise from muons, electrons and gamma rays in cosmic ray air showers [10]. These particles deposit energy in the ice tanks housing the DOMs, resulting in light pulse width up to several hundred nanoseconds long. The arrival times and amplitudes in the surface array are used to reconstruct the shower core position, direction, and energy. An overall timing resolution of 10 ns provides a pointing accuracy of about 11. The PMT pulses range from single photoelectrons at the periphery of

the showers to 105 photoelectrons for an 1 EeV shower that occurs within the array. To achieve the implied dynamic range, each tank contains two DOMs operating at gains differing by a factor 50. A νμ with energy greater than 100 GeV interacting in or around the IceCube instrumented volume creates a muon that generates Cherenkov light along its path as it traverses kilometers of ice. Above 1 TeV, the muon loses energy stochastically to produce multiple showers of secondary particles, resulting in an overall light yield proportional to the muon energy [17,18]. For DOMs close to the incoming track, most photons arrive in a pulse less than 50 ns wide, in which the earliest photons have traveled straight from the muon track without scattering. Significant scattering [19,20] lengthens the light pulses with increasing distance; ultimately reaching 1 μs (FWHM) for DOMs 160 m away from a muon track. The parameters for IceCube are DOM time response, optical sensitivity, time dispersion and optical attenuation introduced by the ice. PMT waveforms are extracted and correlated to reconstruct the incoming direction and energy of the muon. Design studies for IceCube physics goals [7] have shown that sufficient reconstruction quality is achieved for a PMT timing resolution of 5 ns, low-temperature noise rate below 500 Hz, and effective dynamic range of 200 photoelectrons per 15 ns. The description of the characterization of IceCube PMTs is reported in [21]. The analog signal produced by the PMT is digitalized in the DOM by a custom made Analog Transient Waveform Digitizer (ATWD) and an fADC [22]. The data from a single trigger consists of at least one ATWD waveform and one fADC waveform, plus a time stamp and the local coincidence signal from the adjacent DOMs. The number and time distribution of photons registered in calibrated waveforms are determined via a feature extractor algorithm [24]. Pulse shape functions from the PMT and the fADC must be deconvolved by efficient and robust algorithms. The hardware in the ice as well as the DAQ system performs very well in the full 86-strings IceCube. Very few DOMs, about 1%, have shown any problematic behavior. The overall data taking live time is on an average 98% in each physics run. Part of the data taking is devoted to calibration and maintenance of the detector. As a consequence, the live time effectively used for data analysis is on an average 93%. Each single isolated hit is acquired from the entire detector. Events reconstruction and on-line filtering are performed at the South Pole. A neutrino sample with a purity of ∼70% is obtained on-line at a rate of 0.3 Hz. A view of the data rate is reported in Fig. 2. The experiment control system for the IceCube detector is realized via an interface called IceCube Live [23]. The Live system allows IceCube members world-wide to control the operation of the detector, to define alerts based on on-line data analysis and to follow ambient quantities (temperature, wind speed, etc.). Examples of subsystems hosted by IceCube Live are the monitoring system of the detector and the Data Acquisition system (DAQ) dedicated to alerts triggered by a supernovae core collapse. The IceCube software is built within a highly modular framework called IceTray [25]. Software environments for specialized tasks such as online-filtering, simulation (IceSim), reconstruction (IceRec), or analysis are bundled to form different meta-projects. The simulation needs of IceCube are very demanding and for this reason a software package in the Python language has been written to manage, run, control and monitor the generation of the IceCube detector simulation data and the related filtering and reconstruction analyses (IceProd [26]). Simulation production is distributed among university computer clusters.

2. Verification of IceCube reconstructions To understand and monitor the performance of the IceCube detector, a series of low level quantities are used. Typical variables

E. Resconi / Nuclear Instruments and Methods in Physics Research A 725 (2013) 1–6

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Table 1 A summary of the following quantities: number of strings deployed vs. deployment seasons of IceCube; start and stop date of the physics runs and live time used in analysis; status of AMANDA; number of DeepCore strings present in the detector configuration; number of atmospheric neutrinos measured at the final level of data analysis. No. strings

Deploy. season

Physics run (start–stop)

AMANDA status

DeepCore

No. νatm

1 9

04-05 05-06

– Start: 2006-06 Stop: 2006-11 Total: 137.4 days

Running Running

– –

– 234 [13] (based on 6 months)

22

06-07

Start: 2007-02-16 Stop: 2008-04-5 Total: 275.70 days

Running (integrated mode)

Design study [16]

5,114 [14]

40

07-08

Start: 2008-04-05 Stop: 2009-05-20 Total: 375.5 days

Running (integrated mode)

6 strings approved

14,121 [15] upward-going

59

08-09

Start: 2009-05-20 Stop: 2010-05-30 Total: 348 days

Decommissioned

1 string deployed

43,339 [15] upward-going

79

09-10

Start: 2010-05-31 Stop: 2011-05-12

–

6 strings deployed

∼70; 000 upward-going

86

10-11

Start: 2011-05-13 Still running

–

8 strings deployed

–

include the number of hit DOMs (NChannel), the number of hits that each DOM receives in a given run (occupancy), the center of gravity of all PMT hits of an event weighted by the charge per PMT (COG), and the number of direct hits. Moreover, reconstructed variables and relative quality parameters, like the zenith and azimuth distribution of events at different purity levels and the energy distribution based on different reconstruction algorithms, are used for the verification of the detector operation. Extensive comparisons with MonteCarlo simulation reveals the status of the understanding. There are two signatures of neutrinos used in IceCube for physics analysis of the track-like events and the cascades-like events. We describe here studies of the behavior of IceCube based on track-like events. For studies of cascade-like events we refer to Ref. [27]. 2.1. Track reconstruction A variety of track reconstructions and quality parameters have been developed for the IceCube telescope. An independent verification of the absolute pointing accuracy of the IceCube detector and the angular resolution of the reconstruction has been performed. These properties were verified using the signature left by the Moon, which shadows the incoming cosmic rays [28]. As reported in Ref. [29], an unbinned maximum likelihood method was developed to reconstruct the shape and the position of this shadow. The energy spectrum of the primary cosmic rays participating to this search and the significant shadow of the Moon detected in IceCube-59 strings configuration is reported in Fig. 3. The observation of the shadow is established with high statistical significance revealing no presence of an evident off-set and confirms in the downward sector, the expected angular resolution. In the complete IceCube configuration, the Moon will be followed on the monthly basis. 2.2. Energy estimation The energy of the muons detected in IceCube is estimated in two ways; for muons with energy above the TeV scale, the stochastic energy losses along the track are proportional to the energy of the muon; for lower energetic muons which are fully contained in the IceCube volume, the track length provides the estimation of the energy. Details about muon energy estimation

Fig. 2. Trigger rate, muon filter rate and on-line rate of IceCube in final configuration. The rates show a discontinuity between May and June 2011 due to the inclusion of the final seven strings deployed. The filters are processed on-line at the South Pole.

are reported in Ref. [30]. Four energy estimators have been compared with MonteCarlo data MuE, Muon-Energy (mean and median), Photorec, TruncEnergy. All these methods respond linearly in the regime between 10 TeV and 10 PeV, and are characterized by an energy resolution 0:2 ologðEÞ o 0:4.

3. Overview of IceCube results In this paper, we choose to not prioritize few single results but instead to summarize nearly the whole family of hypotheses IceCube as tested with the related references. The aim is to cover a large part of the IceCube physics and to provide here a starting point for more detailed papers. IceCube is a discovery instrument. In order to avoid practitioner biases, IceCube employs a blind analysis in most searches. A fairly extensive list of searches (multiple comparisons or multiple tests) is typically realized on IceCube data samples. This is due to the fact that it is not known a priori which category of source (AGNs, GRBs, galactic sources), region of the sky (galactic or extragalactic) or mechanism (cosmic

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E. Resconi / Nuclear Instruments and Methods in Physics Research A 725 (2013) 1–6

Fig. 4. Energy spectrum of atmospheric neutrinos measured in IceCube 40-strings configuration. Various components of the atmospheric neutrinos are shown as well as AMANDA, Frejus and SuperKamiokande data for comparison.

Fig. 3. Shadow of the Moon, figures taken from Ref. [29]. Upper plot: estimated energy spectrum of cosmic ray primaries participating in the shadow analysis; the dashed line represents all events, the solid lines represent the primaries with declination greater than −301. Bottom plot: image of the shadow of the Moon as measured with the IceCube-59 configuration.

ray interaction, dark matter annihilation, exotic) will produce a measurable extraterrestrial neutrino signal. The larger the number of IceCube tests involved, the higher the probability of a false discovery claim. To avoid this, the IceCube collaboration has the policy to: (1) correct final probabilities with a trial factor typically determined via MonteCarlo randomization and (2) fix a discovery threshold for all the tests which corresponds to 5s. In the case of a very high statistics analysis, such as one based on atmospheric muons, the blindness strategy is not applied. IceCube is a unique 3D detector for cosmic rays, opening the possibility to investigate fundamental properties of cosmic rays like the energy spectrum and composition in the energy region 1014 −1018 eV, as well as new features like small and large scale anisotropies [31]. Further, understanding atmospheric muons provides an independent way to test and reduce systematic uncertainties of the detector. The atmospheric muon rate is strongly influenced by the temperature in the atmosphere. It is known that the correlation coefficient depends on the relative contribution of mesons (pions and kaons) in the atmosphere. It is then possible to use the very high statistic of atmospheric muons and their seasonal variation to measure the K=π ratio (RK=π ) as reported in detail in Ref. [32]. RK=π represents an important input in the detector simulation, and therefore a more precise measurement reduces one of the sources of systematic uncertainties in IceCube. The reference value is RK=π ¼ 0:149 70:060 [33] and the latest measure by IceCube is

Fig. 5. Results of the neutrino searches in connection with GRBs obtained on IceCube data in 40-strings and 59-strings configuration. The combined 40+59 upper limit is also shown with a high constraining power [52]. The flux lines from the predictions from Guetta et al. [43] and Waxman 2003 [44] are shown as well.

RK=π ¼ 0:09 7 0:04.1 The arrival direction distribution of cosmic rays at median energies of 20 and 400 TeV reveal small but statistically significant anisotropies at various scales. Details are reported in Refs. [34,35]. The origin of these anisotropies is still not understood and may be connected to the propagation of galactic cosmic rays. 3.1. Summary tables The IceCube 22-strings data sample corresponds to a total live time of 275.70 days. A total of 27 hypotheses have been tested in blind analyses on the sample. Among these searches, four have presented a p-value of the order of 1%. Following the IceCube analysis policy, no discovery was claimed and 90% CL upper limits were calculated. None of the 1% under fluctuations have been confirmed or strengthened in the successive samples. The IceCube 40-strings data sample corresponds to a total live time of 375.5 days. A total of 31 hypotheses have been tested in blind analyses on sample. Preliminary results from some of the performed searches are reported in Table 2 with the related references. 1

This value is still preliminary.

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Table 2 Partial list of IceCube searches realized on the 40 strings and AMANDA sample. A total of 31 hypothesis tests have been realized in a blinded fashion. All upper limits quoted are at 90% confidence level. Physics analysis

Results

Reference

Steady point source search—all-sky

First all-sky point source search

[39]

Upper limit northern sky E2 Φνμ ∼2−200  10−12 TeV cm−2 s−1 [TeV–PeV] Upper limit southern sky E2 Φνμ ∼3−700  10−12 TeV cm−2 s−1 ½ 4PeV No evidence of a point source. Transient point source search

Blazar multi-wavelength flares search Microquasar periodicity test Model independent flare search No evidence of a point source

[40] [41]

GRBs search

117 Gamma-ray bursts (GRBs) in the northern sky First analysis sensitive to the flux predicted by fireball phenomenology, upper limit on the fluence:

[42]

1:1  10−3 erg cm−2 [37 TeV–2.4 PeV] Guetta et al. flux [43] excluded at 90% confidence in the region 7 2248 s see Fig. 5 Atmospheric neutrinos spectrum Extra-terrestrial diffuse neutrino flux search

see Fig. 4 2

E ϕνμ o 8:9  10

[45] −12

−1

TeV s

cm

−2

−1

sr

valid in the range [34.7 TeV - 6.9 PeV]

[46]

First upper limit below the Waxman Bachall bound Models from Becker et al. [47], Stecker et al. [48] excluded at 90% confidence Extremely high energy (EHE) cosmogenic neutrinos search

No evidence for an astrophysical flux.

[49]

Upper limit E2 ϕνe þνμ þντ o 4:23  10−11 TeV cm−2 s−1 sr−1 [106:3 o Eν o 109:8 GeV] Astrophysical neutrino-induced cascades

Upper limit E2 ϕνe þνμ þντ o 9:5  10−8 GeV cm−2 s−1 sr−1 ½89 TeV o Eν o 21 PeV

[50]

Table 3 Partial list of IceCube searches realized on the 59 strings sample. Results reported are still preliminary. A total of 12 hypothesis tests have been realized in a blinded fashion. All upper limits quoted are at 90% confidence level. Physics analysis

Results

Reference

Steady point source search—all-sky

All-sky point source search based on the combined sample IC40+IC59 [TeV–PeV] No evidence of a point source.

[15]

Transient Point Source Search

Multi-flares search No evidence of a point source

[51]

GRBs Search

Upper limit of IC40+IC59: 0.22 times the flux calculated according to [43]. See Fig. 5

[52]

The IceCube 59-strings data sample corresponds to a total live time of 348 days. A total of 12 hypotheses have been tested in blind analyses on the sample. Preliminary results from some of the performed searches are reported in Table 3 with the related references. The IceCube 79-strings data sample corresponds to a total data taking time of 337 days. Two hypotheses tests have been performed: a first on a possible neutrino emission in coincidence with the anomalous flares observed from the Crab Nebula [53] and a second on the measurement of low energy neutrino induced cascades in DeepCore [54].

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