The Strategy of Discrimination between Flavors for Detection of Cosmogenic Neutrinos
TThe Strategy of Discrimination between Flavors for Detection of CosmogenicNeutrinos
Kwang-Chang Lai ∗ Center for General Education, Chang Gung University, Kwei-Shan, Taoyuan, 333, TaiwanLeung Center for Cosmology and Particle Astrophysics (LeCosPA), National Taiwan University, Taipei,106, Taiwan
Chih-Ching Chen † Graduate Institute of Astrophysics, National Taiwan University, Taipei,106, Taiwan.Leung Center for Cosmology and Particle Astrophysics (LeCosPA), Taipei, 106, Taiwan.
Pisin Chen ‡ Graduate Institute of Astrophysics, National Taiwan University, Taipei,106, Taiwan.Department of Physics, National Taiwan University, Taipei, 106, Taiwan.Leung Center for Cosmology and Particle Astrophysics (LeCosPA), Taipei, 106, Taiwan.Kavli Institute for Particle Astrophysics and Cosmology,SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA (Dated: November 3, 2018)We propose a new method to identify flavors of ultra high energy cosmic neutrinos. Energyloss of leptons in matter provides important informations for the detection of neutrinos originatedfrom high energy astrophysical sources. 50 years ago, Askaryan proposed to detect Cherenkovsignals by radio wave from the negative charge excess of particle showers. The theory of Cherenkovpulses with Fraunhofer approximation was widely studied in the past two decades. However, athigh energies or for high density materials, electromagnetic shower should be elongated due tothe Landau-Pomeranchuck-Migdal (LPM) effect. As such the standard Fraunhofer approximationceases to be valid when the distance between the shower and the detector becomes comparable withthe shower length. We have performed Monte Carlo simulations recently to investigate this regimebased on the finite-difference time-domain (FDTD) method, and modified time domain integrationmethod. In this work, we adopt the deduced relationship between the radio signal and the cascadedevelopment profile to investigate its implication to lepton signatures. Our method provides astraightforward technique to identify the neutrino flavor through the detected Cherenkov signals.
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I. INTRODUCTION
The nature and origin of ultra-high energy cosmic rays(UHECRs) have remained a mystery. These amazinglyenergetic events have been observed beyond ≈ . eV,the so-called Greisen-Zatsepin-Kuzmin(GZK) [1] cut-off.The GZK feature on the UHECR spectrum has beenfirst observed by the High Resolution Fly’s Eye Exper-iment [2] and later confirmed by the Pierre Auger Ob-servatory [3]. Above this energy scale, UHECRs interactwith CMB photons through the GZK processes [1], pro-ducing cosmogenic neutrinos. The GZK feature on thecosmic ray energy spectrum guarantees the existence ofthe cosmogenic neutrinos. However, none of these havebeen observed so far. Detecting these ultra high energy(UHE) neutrinos provides critical informations for un-raveling the mystery of the origin and evolution of thecosmic accelerators and will be one of the utmost tasksin the coming decade [4]. ∗ Electronic address: [email protected] † Electronic address: [email protected] ‡ Electronic address: [email protected]
One promising way of detecting UHE neutrinos is theradio approach. When an ultra-high energy cosmic neu-trino interacts with ordinary matters on the Earth, itwould lead to a hadronic debris, either by charged cur-rent or neutral current. The former also produces a lep-ton with corresponding flavor. Both the high energy lep-tons and the hadronic debris induce particle showers. Asproposed by Askaryan in the 1960’s [5], the high energyparticle shower develops in a dense medium would havenet negative charges. This charge imbalance appears asa result of the knocked-off electrons being part of theshower, as well as the positrons in the shower annihilat-ing with the electrons of the medium. The net charges ofthe showers, typically 20% of total shower particles, serveas a source emitting the Cherenkov radiations when theytravel in the medium. The sizes of the showers are quitelocalized (tens of cm in radial and few meters in longi-tudinal development) compared to those develop in theair (km scale), and therefore result in coherent radiationsfor wavelengths longer than the shower sizes. The cor-responding coherent wavelength turns out to be in theradio band, from hundreds of MHz to few GHz.In this article, we discuss the possibility to identify theflavors of the cosmogenic neutrinos detected by the radioneutrino telescope, such as ANITA [6], Askaryan Radio a r X i v : . [ h e p - ph ] M a r Array (ARA) [7], and ARIANA [8].
II. THE STRATEGY OF FLAVORIDENTIFICATION
As neutrinos interact with matters to produce observ-able signals, the major channel is the changed-current(CC) interaction. The electron produced through ν e CCinteraction has a large interaction cross section with themedium and produces a shower within a short distancefrom its production point. Contrary to the electron, themuon produced through ν µ CC interaction can travel along distance in the medium before it loses all its energyor decays. However, a muon does emit dim light alongits propagation so that only those detectors near to themuon track can be triggered.As for ν τ detection, the ν τ -induced tau leptons be-have differently at different energies for a fixed detec-tor design. For a neutrino telescope such as IceCube,the observable energy range for the double bang eventis 3 . < E ν < ν τ appears like a track event.Note that the dim lights emitted from the track canonly trigger the nearby optical detectors but cannot bereceived by radio detectors. A different strategy is takento construct track events for radio detectors. For cosmo-genic neutrinos, the energy of the CC-induced muon ortau lepton is so high that a muon or tau lepton not onlyemits dim lights but also produce mini-showers along itspropagation through the detector fiducial volume. Bydetecting the radio emissions from these mini-showers,a track event is reconstructed for a muon or tau leptontraveling through the detector. By observing a singleshower, a ν e signal is identified from a track event for a ν µ or ν τ .It is challenging to distinguish between ν µ and ν τ sig-nals because both muons and tau leptons produce similartrack-like events. Simulation of lepton propagation in iceshows that the compositions of the mini-showers are dif-ferent for muon and tau lepton track events. The mini-showers that consist of the track events are composedof two categories, electromagnetic (EM) and hadronicshowers. The energy loss distribution between EM andhadronic showers is different for muon and tau leptontrack events. A muon track event loses more energythrough EM showers than through hadronic ones whilea tau track loses more energy through hadronic showersthan through EM ones. By collecting mini-showers, mea-suring their attributes and evaluating energy losses, onecan distinguish between muon and tau track events. III. SIMULATION FOR NEUTRINO EVENTS
ARA detectors receive radio emissions from showerparticles created by cosmogenic neutrinos in ice. Theseradio signals are Cherenkov radiations produced bynet charges of showers particles. We adopt COSIKA-IW [10, 11] code, a modification of COSIKA [12] programfor dense-target simulation, to simulate EM and hadronicshowers. In Fig. 1 and 2, longitudinal developments ofcharges are shown for EM and hadronic showers respec-tively. The hadronic shower is simulated by a protonhitting a dense medium and evolves as a typical profilewith one peak at the shower maximum. But,f for theEM shower, the case is different. At energies higher than10 eV, bremstrahlung and pair-production processes aresuppressed by Landau-Pomeranchuck-Migdal [13] effect.As a result, cascades are stretched, shower developmentis elongated and several peaks appear on the profile. FIG. 1: Longitudinal profile of a 10 eV electron shower inice. As LPM suppression increases with the square root ofthe energy, multiple peaks occur during the elongated showerdevelopment. The profile is sensitive to the initial interactionsof the cascade. With the shower profile, Cherenkov radiations can besimulated and evaluated with the time-domain finite-difference (FDTD) method[14]. Between the shower pro-file ρ ( x ) and the radio signal E ( t ) exists a one-on-onecorrespondence [15]. The electric field of Cherenkovradiation can be calculated by solving the inhomoge-neous Maxwell equations, as it has been demonstratedby Alvarez-Muniz et al [16]. EM and hadronic show-ers produce signals received in different patterns in radiodetectors. By measuring radio signals, showers are iden-tified and their energies are inferred.To study muon and tau tracks, we adopt the muonMonte Carlo package to simulate muon and tau leptonpropagations in ice. In Fig. 3 and 4, energy loss from EMand hadronic showers are shown for muon and tau tracksrespectively. For muon propagation, EM processes aredominant mechanism to lose energy and hadronic pro-cesses are subdominant. For tau lepton propagation, thecase is reversed. Moreover, the energy loss distribution FIG. 2: Longitudinal profile of a 10 eV proton shower in ice.The hadronic shower is initiated by the cascade of mesons.The decay length of neutral pions is longer than the LPM in-teraction length in ice for electrons at energy 10 eV. Mean-while the secondary mesons produce very few electrons dueto LPM suppression.
18 18.5 19 19.5 20 20.5 21 21.5 2210 lepton energy E n e r g y l o ss ( e V / k m ) EM shower loss Hardonic shower lossTotal shower loss
FIG. 3: Energy loss for tau track. The dominant energy lossprocess of tau propagating in ice is photonuclear interaction.Taus energy loss rate through EM processes is suppressedsince the cross-sections of pair production and bremsstrahlungare inversely proportional to the lepton mass squared. among different types of showers depends upon track en-ergy. Once a track is sufficiently measured, its type isidentified and its energy can be inferred as well.
IV. SUMMARY
In this work, we propose our strategy to identify theneutrino flavor in observing cosmogenic neutrinos by ra- dio neutrino telescopes, such as ARA. We point out that ν e can be identified from ν µ and ν τ because ν e producesan EM shower in ice while ν µ and ν τ produce trackevents. We also propose to construct these tracks by
18 18.5 19 19.5 20 20.5 21 21.5 2210 lepton energy e n e r g y l o ss ( e V / k m ) EM shower lossHadronic shower lossTotal shower loss
FIG. 4: Energy loss for muon track. The dominant energyloss processes of muon propagating in ice are pair productionand bremsstrahlung. The secondary particles (e+, e?,) fromthose processes generate the electromagnetic shower. As canbe seen, the EM shower profile is distinguishable from thehadronic one. detecting those mini-showers emerged along the leptonpropagation. To distinguish between ν µ and ν τ , leptonpropagation in ice is simulated with MMC. We find thatenergy loss distribution among EM and hadronic showersdepends on both lepton identity and energy. Simulationsfor shower production with COSIKA-IW show differentparticle profiles for EM and hadronic showers so that theshower type and its energy can be inferred with FDTDmethod.In summary, neutrino flavors can be discriminated be-tween one another for cosmogenic neutrinos with radioneutrino telescopes. We will refine our method of flavordiscrimination with more detailed study on shower pro-duction, lepton propagation and radiation conversion. Acknowledgements
We would like to thank Albrecht Karle, David Besson,Peter Gorham and David Seckel for valuable discus-sions. This research is supported by Taiwan NationalScience Council (NSC) under Project No. NSC-100-2119-M-002-525, No. NSC-100-2112-M-182-001-MY3 and USDepartment of Energy under Contract No. DE-AC03-76SF00515. We would also like to thank Leung Centerfor Cosmology and Particle Astrophysics for its support. [1] K. Greisen, Phys. Rev. Lett. 16, 748 (1966). G. T. Zat-sepin and V. A. Kuzmin, JETP. , 114 (1966). [2] R. U. Abbasi et al., Phys. Rev. Lett. 100, 101101 (2008). [3] Yamamoto, T. 2008, International Cosmic Ray Confer-ence, 4, 335[4] P. Chen and K. D. Hoffman, Astronomy DecadalSurvey (2010-2020) Science White Paper, (2009),arXiv:0902.3288.[5] Askaryan, G. A., Zh. Eksp. Teor. Fiz. ,616 (1961) [So-viet Physics JETP , 441 (1962)].[6] ANITA Collab. (P. Gorham), Phys. Rev. Lett. ,051103 (2009).[7] ARA Collab. (P. Allison et al.), Astropart. Phys. , 457(2012).[8] S. W. Barwick, J. Phys. Conf. Ser. , 276 (2007).[9] U. F. Katz, Nucl. Inst. Meth. , 457 (2006).[10] J. Bolmont et al., Proceedings of the 30th ICRC, July, 2007.[11] S. Bevan et al., Astropart. Phys. 28, 366 (2007).[12] D. Heck et al., OCORSIKA: A Monte Carlo Code toSimulate 1 Extensive Air Showers, 2 Report FZKA, 6019,(1998).[13] L.D. Landau, I. Ya. Pomeranchuk, Dokl. Akad. NaukSSSR, 92, 535, 735 (1953); A.B. Migdal, Phys. Rev. 103,1811 (1956).[14] C.-Y. Hu, C.-C. Chen and P. Chen, Astropart. Phys.35