PoS(ICRC2015)1136 a and e S. Yoshida a http://pos.sissa.it/ , B. K. Shin d , S. Ueyama a , T. Shibata b , M. Relich a ∗ , H. Sagawa c , K. Mase a , J. N. Matthews b , T. Kuwabara a c , D. Ikeda b , A. Ishihara a [email protected] Speaker. Askaryan Radio Array (ARA) iscosmogenic neutrinos being above built 10 PeV. at The ARA the detectorthe South aims excess to Pole, charge observe aiming the in radio a to emissions particlewas from observe shower first high induced predicted energy by by a Askaryan in neutrinothe 1962 SLAC interaction. accelerator and in Such experimentally 2000. a confirmed We radio also byTelescope emission performed Saltzberg Array an Electron et experiment Light al. of Source the using (ARAcalTA) ARA toradiation calibration verify with and the the understanding the of detector the response Askaryan usedwe in irradiated an the ice ARA target experiment. with 40 InSource MeV the electron (TA ARAcalTA beams ELS) experiment, using located the Telescope in Arraysignals Electron a would Light include radio two quiet kind of open-air backgrounds:air environment transition and of radiations ice, from the the and Utah boundary radiopolarizations between desert. emissions and from Observed angular the distributions sudden ofThese beam the first appearance. observational radio results signals We from measured ARAcalTA to are coherences, presented understand in the these observed proceedings. signals. ∗ Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. E-mail: Department of Physics, Hanyang University, 222 Wangsimni-ro Seongdung-gu Seoul, 133-791, Physics And Astronomy, University of Utah,High 201 Energy South, Accelerator Salt Research Lake Organization City, (KEK), UT 2-4 84112, Shirakata-Shirane, U.S. Tokai-mura, ICRR, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8522, Japan Department of Physics, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan c ⃝ The 34th International Cosmic Ray Conference, 30 July- 6 August, 2015 The Hague, The Netherlands for the ARA collaboration, M. Fukushima Korea ∗ R. Gaïor Naka-gun, Ibaraki, 319-1195, Japan Experimental calibration of the ARA radiotelescope neutrino with an electron beam in ice G. B. Thomson e c b a d PoS(ICRC2015)1136 eV has K. Mase ] (ARA- 19 8 10 × 5 ]. Neutrinos rarely interact with matter, and they 2 3 , ]. 2 5 , 1 ]. The last feature is the polarization. The radiation is expected to be highly 9 ] and this Askaryan effect was confirmed experimentally by D. Saltzberg et al. at ]. Although the expected event rate depends on the evolution models of cosmic-ray 6 4 ]. Since the power of Askaryan radiation is quadratically proportional to the amount of 7 We performed the ARAcalTA experiment using the TA-ELS 40 MeV electron beams at the TA The ARA detector aims to observe coherent radio emissions from a net negative charge ex- The understanding of Askaryan radiation as well as the detectors is crucial for the ARA ex- IceCube has performed a search for cosmogenic neutrinos with energies above 100 PeV by ob- The origin of ultra high energy cosmic rays (UHECRs) with the energies above calTA). polarized in a radial direction with regard to the beam axis. The ARAcalTA experiment was con- site in the middle of the desertseveral in features Utah had in January, to 2015. be Indiation measured. order power to One is confirm important the expected Askaryan to feature radiation, be isbeam quadratically coherence bunch proportional of train. to the the radiation. Another electronpeaking number feature The in around is ra- an almost the accelerator horizontal angular direction distribution.because with the regard A interference to wide the condition Gaussian-like beam is distribution length direction weak of is observation due [ expected. to the This is short electron track compared to the wave- cess produced in aA. cascade Askaryan shower [ after a neutrino interaction. This was first predicted by G. 2. The ARAcalTA experiment periment. In order to verifywe the performed understanding an experiment of of Askaryan the radiation ARA and calibration the with the ARA TA detector Electron units, Light Source [ the charge excess in an electro-magneticat (EM) energies shower, above the 10 Askaryan PeV. radiation In becomes addition,quencies prominent the of attenuation our length interest in (200–800 the MHz) icelight is for (about about radio 100 one waves m). kilometer, with Therefore, and the the longer fre- neutrinos. Askaryan than radiation the is one the for optimal visible tool to search for cosmogenic serving Cherenkov light derived from secondaryinteractions relativistic [ charged particles initiated by neutrino SLAC [ come from the deepof universe, cosmic-ray so source the evolution neutrinoobservation from flux of cosmogenic the we neutrinos beginning observe can elucidate of on the the the origin of Earth Universe UHECRs. reflects up the to history now. Therefore, the sources and the composition, the ratecomposition. is about IceCube one event has per not(ARA) year detectors in yet the are observed optimistic being cosmogenic case built ofneutrinos neutrinos. at a by about proton the The one South order Askaryan Pole of Radio to magnitude Array [ extend the ability to search for cosmogenic Calibration of the ARA telescope with an electron beam 1. Introduction not yet been revealed completely,(TA) although and ground-base Auger large have detectorsinteract such been with as observing Telescope cosmic UHECRs Array microwave moregenerate background so-called precisely photons cosmogenic with during neutrinos large [ their statistics. propagation to UHECRs the Earth and PoS(ICRC2015)1136 K. Mase by a radiation safety 9 ) due to the short electron track ◦ s to enhance a signal amplitude as µ target angle: 30 deg target angle: 45 deg. target angle: 60 deg Observation elevation angle [deg.] 3 Emission peak Cherenkov angle 0 10 20 30 40 50 to measure these Askaryan radiation features, intending to recreate the

90 80 70 60 50 40 30

1 Emission zenith angle in ice [deg.] ice in angle zenith Emission Top: a schematic of the ARAcalTA experiment. Bottom: the relation between the emission zenith ]. 9 The 40 MeV electron beam from TA-ELS consists of bunch trains that includes about 10 bunches with a time separation1.9 of ns (FWHM) 350 which ps. was changed The from duration the TA time original of of 1 a bunch train was configured to actual condition in the ARA experiment as close as possible. figured as shown in Fig. Figure 1: angle in ice and the observationof elevation the angle ice (n=1.78) from into the account. targetactual Each to point measurement the represents antenna was an by performed. observation elevation taking angle TheEach the of elevation ice refractive the target angle index antenna inclination when is angle an limited covers a byis different the covered emission maximum by angle range, antenna changing so tower the thatthe height. target a expected wide inclination Askaryan emission angle. angle radiation range is Note shifted also from that the the peak Cherenkov angle of (56 the angular distribution for length [ much as possible by avoiding interference caused0.5 by Hz the and bunch the structure. maximum Theregulation. rate electron of number The the in beam electron was a number bunch was train changed is during limited measurements to to 10 confirm the coherence of Calibration of the ARA telescope with an electron beam PoS(ICRC2015)1136 . ◦ K. Mase 30 cm placed × 30 cm , so, the corresponding × 8 10 × 2 to access different emission angles in ice ◦ and 60 4 ◦ , 45 ◦ . 1 The radio emissions were observed by the ARA antennas installed on top of an antenna tower Askaryan radiation is expected to be produced by the electron beam inside the ice target in We performed the experiment over two weeks using the accelerator in 8 days in total. The Electrons run about 20 cm and stop inside the target, simply losing the energy by ionizations. The electron beam was injected to an ice target with the size of 100 cm ) by GEANT4 simulation. The ice target was supported by a plastic box that has a slit at the σ as shown in Fig. using an antenna mast.away The from tower the was beam fixed injection towere point the used to ground and satisfy with placed the the horizontallylarizations far-field horizontal condition. and of distance radio vertically, Two of being emissions ARA 7.3 hung with biconesignals m the from antennas were separation the then of mast, filtered 126 by tofiers cm band-pass measure with filters between the the (230–430 gain two po- of MHz) centers. 35 andto dB. The amplified This filter observed by filter environmental is low not noise noise used at ampli- fast for oscilloscope the ARA but (10 TA only site. GSa/s) for to theantenna ARAcalTA The be experiment tower digitized. signals up to were The 12.2 transferred height mprobe with of from the 40 the the angular antennas m ground. distribution could cables Radio ofpoint be emissions to the is changed were a Askaryan 4.6 by measured radiation. about the m, every the 1 Since accessible m the elevation to height angle of from the the beam target injection to the antenna is limited up to 45 the ARAcalTA experiment. The radiationa is wide expected to angular show distribution coherence, for a our high experiment. polarization and The expected waveforms and the signal powers EM shower energy is abouttherefore, the 30 radiation PeV. can This be energy observed is in the high ARAcalTA enough experiment. to produce Askaryan radiation, The ice target was inclined to angles of 30 3. Expected Askaryan radiation and backgrounds measurements were performed during thetook day not data to for disturb each thewithout TA configuration fluorescence any of observation. target. the Each We measurement ice contains inclination typically 200 angle events. and each height as well as the one above the beam end pipe.(1 The lateral beam spread at the injection point is estimated to be 4.5 cm Those electrons are used toAskaryan emulate radiation, a even high though energylower EM a than shower cascade in the is an critical ice notcan energy target generated be of that estimated in 69 will by ice MeV generate and the in since a energy ice. the negative of charge electron excess each The energy inelectron electron, corresponding energy a is the is energy shower 40 for which total MeV is an number and about the of EM 25%. typical electrons shower electron In number in the is a ARAcalTA about experiment, bunch the train bottom with the width of 25 cm to directly inject beams to the target. Calibration of the ARA telescope with an electron beam radiations by controlling the supplywas voltage directly for measured by the a accelerator Faraday heater.used Cup when (FC). The electron Since total beams the are electron FC shot number absorbsthe to all current the the on target. electrons, the A it beam Wall can Current piperadio not Monitor induced be emissions (WCM), by which the were measures beam, observed. was usedfrom A to the clear estimate WCM the correlation was electron between found, number and the while the charge uncertainty from of the the FC electron number and was the estimated one to be 0.2%. PoS(ICRC2015)1136 and ◦ ]. The K. Mase 11 ]. ]. The radio 9 12 , where the effect 2 . 2 5 for a configuration of the ice inclination angle of 30 ]. We would also expect significant TR in this experiment 2 10 . The corresponding simulated waveform is compared with the ◦ ], showing the Askaryan radiation is expected to be larger than thermal noises. 9 . The waveform shapes changed slightly for the Hpol antenna due to the angular ◦ Several data were taken with same configurations to check the reproducibility. The observed Two other backgrounds would also be included for our experiment. The first background Clear signals of radio emissions were observed especially for the vertically placed ARA bicone signals were found to be stable, thesignal waveform having amplitude the same of shape. the The stability averagedobserved of signal waveforms the time maximum was from the found start to timeto further of be assess the less the beam stability than was of also 5%. the foundangle system, to the by be The antenna less tower 15 stability than was intentionally of 100 rotated ps. the in In azimuthal order dependence of the antenna gain,hand, which the shape is was correctable unchanged takingto for the the the gain Vpol target into antenna account. because afterThe the the On amount antenna rotation. the kept of the other the same decrease Thethe direction was signal waveform proportional time amplitude to delay. decreased thetime geometrical by This was distance 11% used result change to after confirms calculated monitor that the with the our direction rotation. antennas of the was antenna at mast far-field during region. the measurements. The The delay spread could be transition radiation (TR)significant due in to previous the experiments transition [ from air to the ice target,second which background have would been be radiation duethe to expected a coherent sudden radio appearance emission of from the the electron beam, cosmic similar ray to air shower sudden death [ (Vpol) antenna for all the configurationshorizontally with placed the (Hpol) ice antenna. target. The waveforms Smallershow of signals Vpol the were and observed high Hpol for antenna polarizations the informationover of already the observed 200 radio events emissions. isthe shown antenna An in elevation example Fig. angle of ofdata. a 0 The waveform large averaged difference of thesignal amplitude amplitude between is data about and 6 simulation timesindicates was larger another found. than emission. the The The expected observed waveform signalsfirst shapes from part, between Askaryan data though radiation, and there which simulation iscomponent. are a similar The difference for frequency for the spectra the for later each part, waveform which are also also indicates shown another in background Fig. 4. Results Calibration of the ARA telescope with an electron beam were simulated for each measurement configurationthe of ice different inclination elevation angle angle of using the thethese antenna actual proceedings and [ accelerator configurations. These results are presented in based on the short track length in the ice block compared to the frequencies ofsignals from interest the [ sudden appearance wasthe experimentally beam measured line, by however, removing the the contribution iceused from target the the from ice sudden target appearance and would the be electrons were different stopped when inside we the ice. due to the band-pass filter to filterexpected environmental to noise come is clearly from seen. the lowest The frequencies dominantcase around contribution of is 230 the MHz. Askaryan radiation, This where is more differentThis contribution from is is the due expected standard from to higher the frequency bunch signals. structure of the accelerator beam and our detector responses [ The simulated waveform and the frequency spectrum are shown in Fig. PoS(ICRC2015)1136 ] ◦ 9 K. Mase Frequency [MHz] . The emission is thought ◦ (black) and a simulated corresponding ◦ ). The overall systematic uncertainties on ◦ 150 200 250 300 350 400 450 500 -16 -17 -18 -12 -13 -14 -15

-45

0.03 were found in all observed signals with Energy [J/MHz] Energy ◦ 10 10 10 10 10 10 10  6 0.07 from all the configurations with Vpol antenna. Time [ns]  and the antenna elevation angle of 0 0.03 was found. The distribution of the polarization and the angle are ◦  . The high coherence was also found even in the case of no target. The slope 4 . The expected energies from Askaryan radiation calculated in a way in Ref. [ 5 . 3 Left: Averaged waveform measured by the vertically polarized antenna with a configuration of 40 50 60 70 80 90 100 110 120 Clear signals were also observed even without any target. The signal amplitude is about one The polarizations of signals were first investigated using the Vpol and Hpol antenna signal To investigate the angular distribution of measured emissions, the measured radio emission The coherence of the measured signals were next investigated to understand the property of 0

0.1

0.4 0.3 0.2 -0.1 -0.2 -0.3 -0.4 Voltage [V] Voltage the ice target.polarization The value polarization of 0.82 for theshown in case Fig. without the target was also checked, and a lower third of the signals with the ice target at an antenna elevation angle of 0 information. High vertical polarizations of 0.92 the measured radio energy was estimated basedon on the those angle. measurements and it is 17-23% depending to be due to a suddenobserved appearance with of the ice the target beam and electron would charge. be This the emission background is for thought our to measurements. be also energies are plotted as a function ofas the shown elevation in angle Fig. of the antenna for each ice inclination angle the measured signals. Theamplitude signal for power all is the calculated waveform.ration by All show integrating observed a the signals, high both square coherence.run for of The is Vpol the relation shown and between waveform in Hpol, the Fig. with electronindex any number for configu- and coherence radio is derived power as for 1.78 one waveform for the Askaryan radiation normalizedfrequency to spectrum. the observed The waveform sharp (red). cutoffs Right: are The due corresponding to the band-pass filter to filter environmental noise. rotation angle and the distance changeThe by uncertainty the of rotation will the change antenna thechamber, power and waveform gain amplitude it by was is 6%. 5-17% estimated depending based on on the data angle measured (0 in an anechoic are also plotted for each configuration.magnitude As larger seen emissions from compared the to plot, expected we Askaryan observed radiation. more than one order of (rms) of the delay time from the expectation was found out to be 1.9 ns. This corresponds to a 9 Figure 2: the ice inclination angle of 30 Calibration of the ARA telescope with an electron beam PoS(ICRC2015)1136 K. Mase 0.01, showing a  Polarization angle [deg.] . The slope is 1.86 ◦ (electron number) 10 Log -80 -60 -40 -20 0 20 40 60 80

8 6 4 2 0

10 # 7 Polarization and the antenna elevation angle of 0 ◦ 7.6 7.8 8 8.2 8.4 8.6

-11

-10.2 -10.4 -10.6 -10.8 10 -11.2 -11.4 -11.6 -11.8

(radio energy [J]) energy (radio Log Measured radio emission power as a function of electron number in the beam for a configuration Left: Polarization of measured radio emissions for all the configurations with the ice target. Right:

We have clearly observed radio signals that show high coherence and high vertical polarization 0 0.2 0.4 0.6 0.8 1 1.2

1 6 5 4 3 2 0 Counts high coherence. from the ice target with thetimes larger TA-ELS electron than the beam. expectation The for the observedradio Askaryan waveform signals radiation. amplitude from We is the have also about beam observed 6 itself.all relatively the large Since measured this radio sudden signals, the appearance observed backgroundfocus signals is does would to not be explain very account the likely for excess to bythe contain a Askaryan TR. Our full signals. current simulation The which mechanism takes ofdifficult into the to account Askaryan separate all radiation those the and two backgrounds TR radiations. and is Theseof closely radiations the related, could electron and also track it be in is reduced the becauseinseparable target the radiations, is length we comparable are to developing the a wavelength simulation of based our on interest. the To endpoints estimate these method described in of the ice inclination angle of 30 5. Summary Figure 4: Figure 3: the polarization angle. These results show a high vertical polarization. Calibration of the ARA telescope with an electron beam PoS(ICRC2015)1136 ◦ K. Mase (green filled circle), 60 ◦ , 61 (2008). , 083009 (2009). , 112008 (2013). 597 79 88 , 457 (2012). 35 (red filled circle), 45 ◦ , 833 (1993). Observation elevation angle [deg.] 89 8 , 423 (1969). 28B , 13 (2000). , 093105 (2005). , 6 (2000). 86 12 , 056602 (2011). 62 84 . Note that the antenna gain is corrected and the energy is normalized to 8 10 , 441 (1962). × 14 2 0 5 10 15 20 25 30 35 40 45 9

11 10 12 −

− − − Radio Energy [J] Energy 10 Radio 10 10 10 Observed radio emission energy as a function of the observation elevation angle of the antenna ] which treats all radiations in a consistent way. 13 C. W. James et al., Phys. Rev. E J. Zheng et al., Physics of Plasmas B. Revenu and V. Marin, paper 398, ICRC 2013 proceedings. P. Allison et al. (ARA Collaboration), Astropart. Phys. G. A. Askaryan, JETP D. Saltzberg et al., Phys. Rev. Lett. T. Shibata et al., Nucl. Instrum. Methods Phys.R. Res., Gaïor Sect. et A al., PoS(ICRC2015)1135 these proceedings. P. W. Gorham et al., Phys. Rev. E M. Ahlers, L. A. Anchordoqui, and S. Sarkar,M. Phys. G. Rev. D Aartsen et al. (IceCube Collaboration), Phys. Rev. D V. Berezinsky and G. Zatsepin, Phys. Lett. S. Yoshida and M. Teshima, Prog. Theor. Phys. [7] [8] [9] [4] [5] [6] [1] [2] [3] [11] [12] [13] [10] References (blue filled circle) andtypical without electron any number target of (black filledthe one circle). at 1 The m measuredAskaryan from energy radiation the is from source normalized to the to correct samethe the the open amount geometrical circle of (The factor. electron color The numbers represents expected the for radio ice each emission inclination configuration energy angle from is as same also as plotted the with measurements). Ref. [ Figure 5: for the cases with ice target of the inclination angles of 30 Calibration of the ARA telescope with an electron beam