PoS(ICRC2015)1171 eV). 18 10 ∼ http://pos.sissa.it/ PeV) is an important step 5 10 − eV or 1 20 10 − 15 1 PeV) and the predicted cosmogenic flux at higher energies ( ∼ ∗ [email protected] Radio detector arrays such as RICE,and ANITA, the ARA, radio and transparency ARIANNA of exploitmonitored glacial the with ice, Askaryan sparse effect which instrumentation. together We describe enablewould here enormous lower the volumes the design of energy for threshold ice a of phased to radiothe radio be techniques astrophysical array to flux that the observed PeV with scale, IceCubeoverlap allowing over measurement an with of extended optical energy range. Cherenkov experiments Meaningfulradio energy could array be design would used also for provide efficient energydiscover coverage cosmogenic calibration. of . the The large effective phased volume required to Speaker. The detection of high energy neutrinos (10 toward understanding the most energeticmental cosmic accelerators, at and energy would scales enablethere tests that are of cannot two funda- easily expected be populationsat achieved of lower on neutrinos: energies Earth. ( the In astrophysical this flux energy observed range, with IceCube ∗ Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. c

Abigail Vieregg Department of Physics, Enrico Fermi Institute,Chicago, University IL of 60637, Chicago, USA 5640 SKavli Ellis Institute Avenue, for Cosmological Physics, University60637, of USA Chicago, 933 E 56th Street, Chicago, IL Andres Romero-Wolf Jet Propulsion Laboratory, California Institute ofCA Technology, 4800 91109, Oak USA Grove Drive, Pasadena, Stephanie Wissel Department of Physics and Astronomy, University90095, of USA California, Los Angeles, Los Angeles, CA Keith Bechtol Wisconsin IceCube Particle Physics Center (WIPAC), Madison,Department WI of 53703, Physics, USA University of Wisconsin–Madison,E-mail: Madison, WI 53706, USA The 34th International Cosmic Ray Conference, 30 July- 6 August, 2015 The Hague, The Netherlands A New Trigger for Detection ofUsing PeV a to Phased EeV Radio Neutrinos Array PoS(ICRC2015)1171 Keith Bechtol 2 1 PeV. This lower energy threshold would provide ∼ 100 PeV) cosmic neutrinos are anticipated to reveal the most 100 TeV (3; 4), and searches for Lorentz invariance violation (5). > ∼ 50 PeV. Here, we describe a trigger configuration using phased radio arrays & 1 km (11; 12) which allows large volumes to be monitored with sparse instrumentation. ∼ α L Ultra-high energy (UHE, One promising way to detect the highest energy neutrinos is through the coherent, impulsive Current radio detector arrays have energy thresholds matched to the energy scale of cosmo- The ARA (14) and ARIANNA (15) experiments are ground-based radio arrays in early stages In current and previously-deployed experiments, a threshold-crossing trigger is used to deter- The key to lowering the energy threshold of a radio experiment and increasing sensitivity at A New Trigger for Detection of PeV to EeV Neutrinos Using a Phased Radio Array 1. Introduction radio emission from electromagnetic showersthe induced Askaryan by effect (6). interactionsscales in as Beam a the test dielectric square — measurements ofexpected (7; the angular 8; particle emission cascade 9) pattern energyGlacial confirm and ice at that frequency is frequencies an the dependence below excellent Cherenkov from 1length medium power GHz numerical for detection and simulations of validate (10). high the energy neutrinos with a radio attenuation distant, most obscured, and highestare energy expected particle from accelerators interactions in ofand the UHE additional contributions Universe. cosmic may rays UHE arise with fromof neutrinos the prompt UHE cosmic emission neutrinos at microwave is background individual sources. awell (1), sensitive The as spectrum discriminator the of composition theof of cosmological fundamental UHE evolution physics, cosmic of including rays constraints UHEcenter-of-momentum (2). on sources, energies the as neutrino-nucleon UHE interaction neutrinos cross may section also at enable several tests to detect neutrinos at an energy scale of genic neutrinos, of development with aboth small use number a similar of fundamental stations experimentaland deployed design: a in an data array acquisition Antarctica. of system antennas compriseeach ARA (16 a individual in single and station the station. ARIANNA can case The reconstruct of stationsstations ARA) a are can neutrino quasi-independent be in event. positioned that several For kilometersevent ground-based rate apart experiments, increases to multiple linearly cover with large the volumes number of of ice, stations. and the neutrino mine when individual antennas receive an excesssufficient in power number above of typical coincident thermal noise antenna-level (16; triggerslevel 17). occur trigger If within a a is short formed, time andand window, a the recorded. station- antenna Essentially waveforms this of typethe the power of seen candidate combinatoric by threshold-crossing neutrino individual trigger event antennas is as are a only digitized function sensitive of to time. higher energies is the ability tonoise. distinguish For weak antennas neutrino-induced triggering impulsive independently, signals the from amplitude thermal of the thermal noise is determined by meaningful overlap with optical Cherenkov detectorsprovide for an energy efficient calibration, means and to importantly, achieveneutrino may the flux large reported effective volume by needed IceCube,proposed to trigger which study configuration the extends would astrophysical increase to the at acceptance least for UHE 2 neutrinos. PeV (13).2. At the Phased same Array Concept time, the PoS(ICRC2015)1171 (2.1) Keith Bechtol 100 m below the ∼ , and the station-level trigger . 200 m below the surface and the ice ) 10 ∼ of the system is determined by the gain / G eff 10 . A trigger array is constructed of 16 dipole × 1 N ( 3 10 averages down the uncorrelated thermal noise from 10log = eff G 0 km deep. . 3 ∼ phased array configuration 8 km deep, and Summit Station in Greenland, where the array would be each trigger channel corresponds to a single effective beam . 2 The trigger threshold of radio detectors is typically set by the maximum rate at which antenna Further sensitivity gains may be possible by recognizing that the antennas that form the trigger We describe here the example of a 16-channel station that uses dipole-like antennas deployed One possible station layout is shown in Figure ∼ waveforms can be digitized and recordedarray, while maintaining a high livetime fraction. For a phased do not have to beto the construct same two distinct, antennas co-located usedpacked antenna for as arrays. possible detailed and The event provides first the analysis. most is sensitivearray the trigger Indeed, is possible phased it (the a array “trigger is set array”). that advantageous of The isenergy second antennas as resolution closely that possible are for spaced neutrino furtherdesign events (the represents apart a “pointing to departure array”). provide from the This all best two-component previous radio station pointing detector resolution neutrino and experiments. 2.1 Example: a 16-channel station down boreholes beneath the firnstations, layer include of the a South deep Pole, glacier. where Possible the sites array for would such be a station, or set of Such interferometric techniques have been extensively used in radio astronomy (reviewed by 18). is the union of simplearray, threshold-crossing rather than triggers individual on antennas. the In individual thisbe effective configuration, reduced beams the by threshold of on roughly the the phased the electricmaintaining field square the could root same of overall trigger thethe rate number antenna per of from trigger antennas Askaryan channel. that emissionreduction are Since scales in the phased linearly the electric together effective with field while electric the producedfor field energy at finding threshold in neutrinos. directly the Equivalently, translates the particletrino into effective cascade, energy volume a since this of lower events the energy could detector threshold bechannels, is detected the increased from phased at farther array fixed away. that neu- To providesthat minimize the each the trigger effective number beam needs of covers to a trigger be large as solid closely angle. packed as possible such is surface and the ice is antennas strung vertically downwould one naturally borehole be as sensitive close to together vertically-polarized as signals, possible. although one This could configuration combine signals of each individual antenna, G, and the number of antennas, N, by: each antenna while maintaining the same signalthe strength signals for real from plane-wave multiple signals. antennas If with weantennas, the combine we proper can time effectively delays increase to the accountfrom gain for a of the given the distance direction. system between of Manyeffective antennas antenna different for beam sets incoming patterns plane of waves that delays wouldantenna with together but the with cover same much the antennas higher same gain. can solid The create angle effective multiple as gain each G individual A New Trigger for Detection of PeV to EeV Neutrinos Using a Phased Radio Array the temperature of the icemany antennas and in a the noise temperature of the amplifiers. Combining signals from PoS(ICRC2015)1171 4 ∼ CPU, Data Archive DAQ Unit Digitizer Trigger in elevation Keith Bechtol ◦ 4 . Beam 1 Beam 2 Beam 4 20 compared to Beam 3 ∼

Filter Filter Filter Filter Beam Former v

Pointing Array RF Front End Antennas Antenna 1 Antenna 3 Antenna 2 Antenna 4 80 dB 2 stage ampliier,

Filter Filter Filter Filter

Filter

Bandpass Trigger Array RF Front End Antennas antennas 16 dipole 4 15 beams. ∼ Array (Both Pointing Polarizations) Ice Surface Array Trigger 30m (Vertical Polarization) 0.5m : An example station layout for a 16-antenna phased trigger array and accompanying pointing 30m : An example layout of the radio frequency chain of a 16-antenna phased trigger array and Left 30m 30m is 14.2 dBi (compared to 2.15 dBi for a single dipole), which corresponds to a factor of Right The phased array design is scalable and can be configured to achieve an even lower energy We have developed a Monte Carlo simulation package to quantify the acceptance of various The effective gain of such a 16-channel trigger array of dipole antennas calculated using Equa- 2.1 in electric field threshold. Thewith FWHM complete of azimuthal the coverage beam for of thisin a configuration. different single trigger By trigger channels, adjusting channel we the is can delaysfrom 5 cover visible among the neutrino antennas relevant events range with of only solid angle for incoming emission threshold. For example, aof phased 28.2 array dBi, with and 400 would dipole push antennas the would electric have field an threshold effective down gain by a factor of 2.2 Achieving An Energy Threshold of 1 PeV radio detector configurations, and moredesign. specifically, investigate The the simulation advantages formalism offocusing and a on assumed phased specific physics array antenna input designs, arekept signal described the processing in simulations chains, (19). general and by Rather analysis defining than algorithms, signal we detection have thresholds in terms of the electric field currently-implemented techniques. For large numberstion of for antennas, the the trigger array single-borehole is configura- antennas no would longer be optimal. deployed To down keep multiple the size boreholes. of the trigger array compact, trigger 3. Expected Performance Figure 1: array. accompanying pointing array. For simplicity, not all channel paths are depicted. from orthogonally-polarized antennas to createbe an constructed unpolarized of trigger. additional Theand antennas, pointing would array with require would both at horizontal leasttiming, two and and additional polarization vertical of boreholes polarization the to radio sensitivity, would uniquely emission only determine and need the thus to the digitize incident incoming the direction, direction signals of from the the neutrino. pointing array We antennas. tion A New Trigger for Detection of PeV to EeV Neutrinos Using a Phased Radio Array PoS(ICRC2015)1171 power 3 . 2 − Keith Bechtol , is comparable 2 2.3 6.0 15.6 Pessimistic Cosmogenic 7.7 19.6 52.9 Optimistic Cosmogenic 0.0 0.1 2.2 5 20% due to a longer radio attenuation length in ice Power Law with Cutoff < , 10 stations of 16-antenna phased arrays have a larger 2 for a variety of astrophysical and cosmogenic models. 2 3.8 0.9 18.4 power law based on IceCube observations (13), an E Power Law 3 . 2 − eV the acceptance is an order of magnitude more than IceCube. 10 stations of the shows the number of events detected as a function of energy for the same three shows the volumetric acceptance of a 10-station detector as a function of energy for 18 3 2 Expectation values for the total number of events detected in 3 years for 10 stations in different 16-antenna 16-antenna, phased 400-antenna, phased Station Configuration summarizes the expected total number of events detected with each detector configuration 1 power law extrapolation of the observed IceCube spectrum from one that has a cutoff at the Figure By phasing the antennas in a 16-antenna array, the UHE neutrino event rate could be increased For this study, we have chosen to simulate the experiment at Summit Station in Greenland with Figure We have described a new trigger for an in-ice phased radio array that is designed to achieve 3 . 2 − to the approach of the ARA experiment but with only 10 stations. law with a 1 PeVTable exponential cutoff (13), and optimistic and pessimistic cosmogenic models (2). for each model. A harder spectrum for PeV-scale neutrinos would yield aby higher more event than rate. a factor ofThe two 16-antenna over the phased non-phased configuration case,E and achieves sensitivity a extended low to lower enough energies. energy threshold to distinguish an detector configurations shown in Figure We show event rates for an E PeV scale. Phasing more antennasments lowers in the event threshold rates.As is even evident further, in and Figure makes marked improve- three different station configurations. We alsoto two show IceCube for analyses comparison optimized the for acceptancefigure different corresponding include energy analysis ranges efficiency, whereas (20; the 21). results Thedirectly of IceCube our comparable. curves simulation The in do standard not, the so method, the shown curves with are the not yellow line in Figure acceptance than IceCube above 30 PeV, andso the that acceptance grows by faster 10 with energy than IceCube, antennas 100 m below the surface.the surface Moving would the increase detectors the to acceptance South by Pole at a depth of 200 m below 400-antenna phased arrays have a larger acceptance than IceCube above 1 PeV. 4. Conclusions Table 1: configurations for spectra based on IceCube observations (13) and for cosmogenic models (2). strength arriving at the antennas. Inent this electric case, field thresholds, different station as configurations describedoverall correspond above. results to Depending could differ- on shift the up particularbetween system or configurations deployed, down are the (by valid. less than a factor of two), but the relative comparisons at the South Pole (11).acceptance We changes have linearly chosen with to the simulate number 10 of stations stations, for since this stations study, trigger but independently. we note that the A New Trigger for Detection of PeV to EeV Neutrinos Using a Phased Radio Array PoS(ICRC2015)1171 Physics , Keith Bechtol 6 10 5 10 4 (Oct., 2010) 13. 10 10 3 10 2 10 IceCube, contained event analysis IceCube, extremely high energy search 10 stations, 16 unphased antennas 10 stations, 16-antenna phased arrays 10 stations, 400-antenna phased arrays 6 1 Cosmogenic neutrinos: parameter space and 10 Cosmic rays at ultra high energies (neutrino?) Neutrino Energy (PeV) 0 J. Cosm. and Astropart. Phys. 10 , 1 − 10 2 − 4 3 2 1 0 1 2 3 5 4 (Jan., 1969) 423–424. 10 − − − −

10 10 10 10 10 10

10 10 10 10

28 sr) (km VΩ 3 Effective Volume vs. Energy for 10 stations installed 100 m below the surface at Summit Station, Letters B detectabilty from PeV to ZeV It is worth noting the scalability of the radio technique for increasing acceptance at all energies. [1] V. S. Beresinsky and G. T. Zatsepin, [2] K. Kotera, D. Allard, and A. V. Olinto, The acceptance increases linearly with thephasing number of more stations, antennas and further in gainsstations each can each be station, with realized 400-antenna particularly by phased atand arrays above PeV could each neutrino detect year. energies. hundreds of neutrinos An at array PeV of energies 100 References Figure 2: Greenland. The yellow line is forbut with 16-channel phasing, stations and with the no red line phasing,the is the for volumetric orange 400-antenna acceptance phased line is array is stations. presented for For atfor similar each two the stations radio difference array trigger configuration, analyses level. with Black IceCube curves optimized indicate for the different volumetric energy acceptance ranges (20; 21). sensitivity to the astrophysicalIceCube neutrino for calibration, flux and discover at cosmogenic 1of neutrinos triggered in PeV events an with efficient and low way. signal-to-noise The above, inand reconstruction individual provide antennas may is energy limit a the topic overlap of gains with has further in investigation been sensitivity deployed that at can Summitchallenges be Station, of achieved Greenland the in in phased-array practice. design June (22). 2015 A to prototype explore phased this array and other potential A New Trigger for Detection of PeV to EeV Neutrinos Using a Phased Radio Array PoS(ICRC2015)1171 6 6 10 10 5 5 10 10 Keith Bechtol 4 4 10 power law, and the 10 3 . 2 3 − 3 Physical Review 10 10 , (Nov., 2012) 103006. 2 86 10 stations, 16 unphased antennas 10 stations, 16-antenna phased arrays 10 stations, 400-antenna phased arrays 10 stations, 16 unphased antennas 10 stations, 16-antenna phased arrays 10 stations, 400-antenna phased arrays 2 10 10 Neutrino Energy (PeV) 1 Neutrino Energy (PeV) 1 10 10 0 Kotera et al. 2010, Pessimistic Cosmogenic 0 10 (2011) 113009. 10 Phys. Rev. D , 1 IceCube 2014, Power Law w/ Exponential Cutoff at 1 PeV − 1 4 2 0 8 6 4 2 0 D83 −

10

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1 1 1 0 0 0 0 0 7 6 5 4 3 2 1 0 10 ubro eetdEet 3Years) (3 Events Detected of Number Years) (3 Events Detected of Number (1962) 441–443. 7 Calculation of High Energy Neutrino-Nucleon 6 6 10 10 14 (2) Phys.Rev. , 5 5 10 10 4 4 10 10 (May, 2002) 097303. 3 3 65 10 10 Implications of ultrahigh energy neutrino flux constraints for 2 10 stations, 400-antenna phased arrays 10 stations, 400-antenna phased arrays 10 stations, 16 unphased antennas 10 stations, 16-antenna phased arrays 10 stations, 16 unphased antennas 10 stations, 16-antenna phased arrays 2 Observation of the Askaryan Effect: Coherent Microwave Cherenkov power law with an exponential cutoff at 1 PeV. The bottom two panels show event 10 10 3 . 2 Neutrino Energy (PeV) − Neutrino Energy (PeV) 1 Excess Negative Charge of an Electron-Photon Shower and its Coherent Radio 1 IceCube 2014, Power Law 10 10 Measuring high energy neutrino-nucleon cross sections with future neutrino Phys. Rev. D (Mar., 2001) 2802. Soviet Physics JETP-USSR , , Kotera et al. 2010, Optimistic Cosmogenic 0 0 86 10 10 Event Rates vs. Energy for a variety of neutrino models. Event rates for three years of observation 1 1 − −

2 0 8 6 4 10

1 0 14 12 10 5 4 3 2 16

10 ubro eetdEet 3Years) (3 Events Detected of Number ubro eetdEet 3Years) (3 Events Detected of Number Lorentz-invariance violating cosmogenic neutrinos Cross Sections and Uncertainties Using theImplications MSTW for Parton Future Distribution Experiments Functions and Emission from Charge Asymmetry in High-EnergyLetters Particle Cascades Emission telescopes [5] P. W. Gorham et al., [7] D. Saltzberg et al., [3] D. Hooper, [4] A. Connolly, R. S. Thorne, and D. Waters, [6] G. Askaryan, rates based on optimistic and pessimistic cosmogenic fluxes (2). A New Trigger for Detection of PeV to EeV Neutrinos Using a Phased Radio Array Figure 3: for 10 stations installed 100no m phasing below are the shown surface in400 at phased yellow, Summit antennas 16-channel are Station, stations shown Greenland. with inspectra phasing red. based 16-channel are on The stations the top shown with IceCube two in observedtop panels orange, right neutrino show panel and flux event is rates (13). stations an based The with E on top two left possible panel neutrino is an E PoS(ICRC2015)1171 . , (Feb., (Dec., Keith Bechtol 35 88 (July, 2005) (Sept., 2014) 72 Journal of Glaciology arXiv:1404.5285 , 113 , Phys. Rev. D , . Phys. Rev. D Astroparticle Physics , , Interferometry and Synthesis in (Jan., 1992) 362–376. . A New Technique for Detection of PeV 45 Observations of the Askaryan Effect in Ice Probing the origin of cosmic rays with Observation of High-Energy Astrophysical The Antarctic Impulsive Transient Antenna , 2015. Physical Review Letters 8 , arXiv:1504.0800 . (Aug., 2009) 10–41. , First Constraints on the Ultra-High Energy Neutrino Design and initial performance of the Askaryan Radio 32 Phys. Rev. D , Electromagnetic pulses from high-energy showers: arXiv:1409.5413 (Oct., 2007) 171101. , These proceedings 99 Evidence for High-Energy Extraterrestrial Neutrinos at the IceCube (Nov., 2013) 1. , in Design and Performance of the ARIANNA Hexagonal Radio Array Accelerator measurements of the Askaryan effect in rock salt: A Site Characterization and Detector Development for the Greenland 342 South Polar in situ radio-frequency ice attenuation arXiv:1410.7369 . Wiley and Sons, 2 ed., 2001. , Astroparticle Physics An in situ measurement of the radio-frequency attenuation in ice at Summit , Science , (2005) 231–238. Flux from a Prototype Station of the (2014), ultra-high energy neutrino detector: Design, performance,balloon and flight sensitivity for the 2006-2007 Detector extremely high energy neutrinos using the IceCube Observatory Neutrino Observatory Radio Astronomy 2013) 112008. Station, Greenland (2014), Neutrinos Using a Phased Radio Array Systems (2014), roadmap toward teraton underground neutrino detectors 023002. Physical Review Letters Implications for neutrino detection 101101. Array prototype EeV neutrino detector at2012) the 457–477. South Pole Neutrinos in Three Years of IceCube Data 51 [8] P. W. Gorham et al., [9] The ANITA Collaboration, P. W. Gorham, et al., [17] ARA Collaboration, P. Allison, et al., [20] IceCube Collaboration, [21] IceCube Collaboration, M. G. Aartsen, et al., [16] ANITA Collaboration, P. W. Gorham, et al., [19] A. G. Vieregg, K. Bechtol, and A. Romero-Wolf, [22] S. A. Wissel et al., [18] A. R. Thompson, J. M. Moran, and G. W. Swenson Jr., [11] J. Avva et al., [12] S. Barwick et al., [14] ARA Collaboration, P. Allison, et al., [15] S. W. Barwick et al., A New Trigger for Detection of PeV to EeV Neutrinos Using a Phased Radio Array [10] E. Zas, F. Halzen, and T. Stanev, [13] IceCube Collaboration, M. G. Aartsen, et al.,