Search for Neutrino Tau in the Long Baseline Appearance Experiment Feng Zhou 8.811 Term Paper 2005-12-16
Search for Neutrino Tau in the Long Baseline Appearance Experiment Feng Zhou 8.811 Term Paper 2005-12-16
ABSTRACT This experiment is deigned for the appearance search of v>v oscillation in the parameter region indicated by Super-Kamiokande,as the explanation of the zenith dependence of the atmospheric neutrino deficit.The detection is mainly based on the nuclear emulsion technology for the direct observation of the decay of tau leptons produced in v.charged current interactions.The performance of this experiment is compared with the CHORUS experiment 1
ABSTRACT This experiment is deigned for the appearance search of ν µ − >ν τ oscillation in the parameter region indicated by Super-Kamiokande, as the explanation of the zenith dependence of the atmospheric neutrino deficit. The detection is mainly based on the nuclear emulsion technology for the direct observation of the decay of tau leptons produced in ν τ charged current interactions. The performance of this experiment is compared with the CHORUS experiment. 1
Contents 1.Introduction 1.1 Physics motivations.………………………- 3 12Hist0 y Backgr0und………… 5 12.1 Neutrions.……………………… 5 1.2.2 Neutrino Oscillation Theory....................................... 6 1.2.3 Indication of neutrino oscillation................................ 2.Detector 2.1 Physics requirements and conceptual design..................14 2.2 The neutrino beam........... 15 2.3 ECC technique............. 17 2.4 Target.………… 18 2.5 Electronic trackers................................................... 19 2.6 Muon spectrometers. 20 3.Analysis 3.1 Signal detection...............21 3.2 Selection Criteria................................................ 22 3.3 Signal estimation............ 23 3.4 Backgre0und.…………………………… 25 4.Conclusions and outlook.............................................. 28 Reference.-……… 29 Appendix.… 31
Contents 1. Introduction 1.1 Physics motivations………………….………………………… 3 1.2 History Background…………………………………………… 5 1.2.1 Neutrions………………………..………………………….... 5 1.2.2 Neutrino Oscillation Theory…………………….…….…….. 6 1.2.3 Indication of neutrino oscillation.…………………………… 7 2. Detector 2.1 Physics requirements and conceptual design………………….14 2.2 The neutrino beam……………………………………………. 15 2.3 ECC technique…………………………………………………17 2.4 Target…………………………………………………………..18 2.5 Electronic trackers…………………………………………….. 19 2.6 Muon spectrometers……………………………………………20 3. Analysis 3.1 Signal detection………………………………………………...21 3.2 Selection Criteria……………………………………………… 22 3.3 Signal estimation……………………………………………… 23 3.4 Background…………………………………………………… 25 4. Conclusions and outlook……………..………….……………. 28 Reference……………………………………………………………...29 Appendix……………………………………………………..……. 31 2
Chapter 1 Introduction 1.1 Physics motivations The neutrinos were introduced in 1932 by W.Pauli to explain the continuous energy spectrum of the electrons emitted in B decay.Since then by means of a large amount of theoretical and experimental work many properties of neutrinos have been determined. They culminated in the neutrino representation inside the Standard Model frame which has been very successful in describing all known neutrino interactions. Despite of the very positive results many questions remain still unsolved about neutrinos nature.In fact the neutrinos are so weakly interacting particles that it is difficult to obtain measurements with large statistics and high precision.Among all the unsolved questions the more fundamental one is whether or not neutrinos are massive particles. The Standard Model was constructed to give massless neutrino in according to the failure of observing experimentally the right-handed neutrinos Anyhow it is generally believed that,despite of its spectacular success,the Standard Model is incomplete.Many of it generalizations predict massive neutrino.Also the cosmology strengthens the hypothesis of massive neutrinos.In fact the problem of the nature of the dark matter has.as a possible particle solution,neutrinos with mass value of the order of the electronvolt. An indirect but very sensitive method to search for non-zero masses is to look at the occurrence of neutrino oscillation,which can happen if neutrinos are massive.The essence of neutrino oscillation is very simple.If neutrinos are massive,it is possible that the mass eigenstates are different from the weak-eigenstatesie..The neutrino mass matrix in the flavor basis is not diagonal.If this condition is satisfied the weak-cigenstats neutrino produced by the charged current weak-interactions is a linear superposition of different mass eigenstates.During its propagation the different mass states evolve differently so the composition of the initial state in terms of weak states changes with time.This means that the detection of the neutrino through the weak
Chapter 1 Introduction 1.1 Physics motivations The neutrinos were introduced in 1932 by W. Pauli to explain the continuous energy spectrum of the electrons emitted in β decay. Since then by means of a large amount of theoretical and experimental work many properties of neutrinos have been determined. They culminated in the neutrino representation inside the Standard Model frame which has been very successful in describing all known neutrino interactions. Despite of the very positive results many questions remain still unsolved about neutrinos nature. In fact the neutrinos are so weakly interacting particles that it is difficult to obtain measurements with large statistics and high precision. Among all the unsolved questions the more fundamental one is whether or not neutrinos are massive particles. The Standard Model was constructed to give massless neutrino in according to the failure of observing experimentally the right-handed neutrinos. Anyhow it is generally believed that, despite of its spectacular success, the Standard Model is incomplete. Many of it generalizations predict massive neutrino. Also the cosmology strengthens the hypothesis of massive neutrinos. In fact the problem of the nature of the dark matter has, as a possible particle solution, neutrinos with mass value of the order of the electronvolt. An indirect but very sensitive method to search for non-zero masses is to look at the occurrence of neutrino oscillation, which can happen if neutrinos are massive. The essence of neutrino oscillation is very simple. If neutrinos are massive, it is possible that the mass eigenstates are different from the weak-eigenstates ν e ,ν µ ,ν τ , i.e.. The neutrino mass matrix in the flavor basis is not diagonal. If this condition is satisfied the weak-eigenstats neutrino produced by the charged current weak-interactions is a linear superposition of different mass eigenstates. During its propagation the different mass states evolve differently so the composition of the initial state in terms of weak states changes with time. This means that the detection of the neutrino through the weak 3
interactions has non zero probability to reveal a weak state different from the one in which the neutrino has been produced. In the last 30 years some unexpected experimental observations of the solar and atmosphere neutrinos were interpreted as hints in favor of neutrino oscillation.Moreover in the last a few years.two collaborations.LSND and Superkamiokande.claimed evidences of neutrino oscillation but it is fair to say that the problem is still open My experiment is designed to search for v>v,oscillation with the aim to reach high sensitivity in the Am for large mixing angle.It is a long baseline appearance experiment which searches for v,in the almost pure v.beam.The v,presence is searched through the charged current reaction v.N->tY.Thus the v.detection essentially consists in the r detection.The r short lifetime has suggested to uses a hybrid detector to distinguish the occurrence of v,CC interactions from the much more frequent v CC or NC interactions
interactions has non zero probability to reveal a weak state different from the one in which the neutrino has been produced. In the last 30 years some unexpected experimental observations of the solar and atmosphere neutrinos were interpreted as hints in favor of neutrino oscillation. Moreover in the last a few years, two collaborations, LSND and Superkamiokande, claimed evidences of neutrino oscillation but it is fair to say that the problem is still open. My experiment is designed to search for ν − >ν τ oscillation with the aim to reach µ high sensitivity in the ∆m 2 for large mixing angle. It is a long baseline appearance experiment which searches for ν τ in the almost pure ν beam. The ν τ presence is µ searched through the charged current reaction ν τN− > τX . Thus the ν τ detection essentially consists in the τ detection. The τ short lifetime has suggested to uses a hybrid detector to distinguish the occurrence of ν τ CC interactions from the much more frequent ν CC or NC interactions. µ 4
1.2 History Background 1.2.1 Neutrions The neutrino was introduced by W.Pauli in 1930 to conserve energy in nuclear B decay which required a final state electrically neutral particle with a spin of 1/2 [1]. Anti-neutrinos from reactors were discovered by F.Reines and C.Cowan in 1956 [2]by making use of the reaction+p->n+e*.The positron and electron annihilate,giving two simultaneous photons.The neutron is thermalized until it is eventually captured by a Cadmium nucleus,emitting photons some 15 micro seconds after the positron signal. These delayed coincidence signals were detected and the existence of the neutrino was confirmed.The neutrinos produced in association with muons were observed at the Brookhaven National Laboratory in 1962 [3]and were found not to be the same as those produced in association with electrons.This was the discovery of a second type of neutrino ()The tau particle was discovered at the Stanford Linear Accelerator Center in 1975 [4]and its associated tau neutrino was also discovered by DONUT Collaboration in 2000 [5].The precise measurement of the decay width of the Z boson in e'e'collider at LEP constrains the number of light neutrino families to three [6]. In the Standard Model,12 particles.6 quarks and 6 leptons,are the constituents of matter. All of these have been discovered experimentally.Each charged lepton (electron, muon,tau is associated with a neutral lepton or neutrino (vv.)The quarks are grouped by pairs according to the same rule and so the three generations of leptons and quarks can be written down as follows. leptons a quarks In the standard model.the neutrino has a zero mass,a zero charge and a spin 1/2 There is no compelling reason why neutrinos should have zero mass like the photon). and many experiments have tried to find the neutrino mass directly by measuring the
1.2 History Background 1.2.1 Neutrions The neutrino was introduced by W. Pauli in 1930 to conserve energy in nuclear β decay which required a final state electrically neutral particle with a spin of 1/2 [1]. Anti-neutrinos from reactors were discovered by F. Reines and C. Cowan in 1956 [2] by making use of the reactionν e + p−+ n +> e . The positron and electron annihilate, giving two simultaneous photons. The neutron is thermalized until it is eventually captured by a Cadmium nucleus, emitting photons some 15 micro seconds after the positron signal. These delayed coincidence signals were detected and the existence of the neutrino was confirmed. The neutrinos produced in association with muons were observed at the Brookhaven National Laboratory in 1962 [3] and were found not to be the same as those produced in association with electrons. This was the discovery of a second type of neutrino (ν ). The tau particle was discovered at the Stanford Linear Accelerator Center in 1975 µ [4] and its associated tau neutrino was also discovered by DONUT Collaboration in 2000 - [5]. The precise measurement of the decay width of the Z boson in e+ e collider at LEP constrains the number of light neutrino families to three [6]. In the Standard Model, 12 particles, 6 quarks and 6 leptons, are the constituents of matter. All of these have been discovered experimentally. Each charged lepton (electron, muon, tau ) is associated with a neutral lepton or neutrino (ν e ,ν ,ν τ ). The quarks are µ grouped by pairs according to the same rule and so the three generations of leptons and quarks can be written down as follows. ⎛ν e ⎞ ⎛ν µ ⎞ ⎛ν τ ⎞ leptons ⎜ ⎜ ⎝ ⎟ ⎟ ⎜ ⎜ ⎟ ⎟ ⎜ ⎜ ⎠ ⎝ µ ⎠ ⎝ τ ⎠ ⎟ ⎟ e ⎛ ⎜ ⎜ ⎝ u d ⎞ ⎛ ⎟ ⎟ ⎜ ⎜ ⎠ ⎝ c s ⎞ ⎛ ⎟ ⎟ ⎜ ⎜ ⎠ ⎝ t b ⎞ ⎟ ⎟ ⎠ quarks In the standard model, the neutrino has a zero mass, a zero charge and a spin 1/2. There is no compelling reason why neutrinos should have zero mass ( like the photon), and many experiments have tried to find the neutrino mass directly by measuring the 5
energy spectrum of tritium B decays.There was no evidence of finite mass,and the upper limit for the v mass is now 3 eV at 95%C.L [7]. There are two minimal modifications of the field content of the Standard Model that can lead to massive neutrinos.First it is possible to add a Higgs triplet,A,to the usual doublet to get a term of the form v CMv A.The non-vanishing vacuum expectation value of A gives a Majorana mass and breaks L-conservation.Another possible modification is to assume the existence of it would lead to Dirac mass term from the usual Higgs mechanism. 1.2.2 Neutrino Oscillation Theory Neutrino oscillations take place when physical mass eigenstates v,differ from weak eigenstates v.,where v is defined to the neutrino state which connects to a charged lepton a via charged current interactions.The weak-eigenstate v can be expressed as a linear superposition of the mass eigenstateswhere Uis a unitary mixing matrix.With three neutrino flavors the mixing matrix can be parameterized by three complex rotations in the following way: 1 0 0 c13 0 sl3e e12 s12 01 0 0 U=0c23 323 0 0 -s12 cl2 00e 0 0-s23c23人-sl3e0cl3 00 1八00e% where sij=sin(6 and cij cos(0).The phases andare Majorana-phases and do not enter into the oscillation probabilities,however they can have phenomenological consequences,e.g.in neutrino-less double beta decay [8].The CP-phase 6 is related to the CP violation effects.The most general parameterization of neutrino mixings can be found in [9]. A mass eigenstate of generation i after a time interval t is given by g0》=ey0) 6
energy spectrum of tritium β decays. There was no evidence of finite mass, and the upper limit for the ν mass is now 3 eV at 95%C.L [7]. e There are two minimal modifications of the field content of the Standard Model that can lead to massive neutrinos. First it is possible to add a Higgs triplet, ∆ , to the usual doublet to get a term of the form ν LCMν L∆ . The non –vanishing vacuum expectation value of ∆ gives a Majorana mass and breaks L-conservation. Another possible modification is to assume the existence of ν : it would lead to Dirac mass term from the R usual Higgs mechanism. 1.2.2 Neutrino Oscillation Theory ν differ from Neutrino oscillations take place when physical mass eigenstates i weak eigenstates ν α , where ν α is defined to the neutrino state which connects to a charged lepton α via charged current interactions. The weak-eigenstate ν α can be ∑U αν i i i a unitary mixing matrix. With three neutrino flavors the mixing matrix can be parameterized by three complex rotations in the following way: expressed as a linear superposition of the mass eigenstates ν ν α = , where U is i , i δ ⎛ c12 s12 0 ⎜ ⎜ ⎜ ⎞ ⎟ ⎟ ⎟ ⎛1 0 0 c13 0 s13e 1 0 0 ⎜ ⎜ ⎜ ⎞ ⎟ ⎟ ⎟ ⎛ ⎜ ⎜ ⎜ ⎛ ⎜ ⎜ ⎜ ⎞ ⎟ ⎟ ⎟ ⎞ ⎟ ⎟ i φ2 ⎟ 0 i φ1 U = 0 23 23 c s 0 1 0 − s12 c12 0 e 0 s13e i δ 0 − s 23 23 c − 0 c13 0 0 1 0 0 e ⎝ ⎠⎝ ⎠⎝ ⎠⎝ ⎠ sin( θij cos( θij φ1 2 φ Majorana-phases and do not enter into the oscillation probabilities , however they can have phenomenological consequences, e.g. in neutrino-less double beta decay [8]. The CP-phase δ is related to the CP violation effects. The most general parameterization of neutrino mixings can be found in [9]. A mass eigenstate of generation i after a time interval t is given by where sij = ) and cij = ). The phases and are − t iE i ν i ν i ( ) t = e (0) . 6
A neutrino of the generation after a time interval oft is given by lv》=∑U.l=∑e() The time propagation equation of a weak eigenstate neutrino can be written, y.t0》=U.eUv.0) The general vacuum oscillation probability is then given by Rw={,e.-∑U,iUge-r 2E For example,the probability of the oscillation v>v is P.cos)sin()sin(167m(e)L(m) E(GeV) 1.2.3 Indication of neutrino oscillation The first experiment that obtained some hints in favor of neutrino oscillation was a solar neutrino search carried out in the Homestake mine in the early '70.During the last 30 years a large number of experiments strengthened this hypothesis measuring neutrino fluxes from different sources:solar atmospheric,reactor and accelerator.We will briefly discuss the existing experiments 1.Solar Neutrinos The major part of the energy of the sun (98%)is produced in the reaction of the thermonuclear pp cycles.Neutrinos are emitted in six reactions of the solar pp cycle: three of these give monochromatic energy lines,the others produce neutrino with continuum spectra (fig.1). The solar neutrino puzzle started to emerge in the late sixties and it took nearly forty years to resolve it The first data on solar neutrinos obtained in the Homestake experiment [11] already displayed a difference of roughly a factor of 2:5 between the measured flux and the prediction,albeit at a very low confidence level.The first detection of solar neutrinos was awarded with the Nobel prize in 2002.As more and more data were gathered,the
A neutrino of the generation _ after a time interval of t is given by − t iE ν (t) = ∑Uαi ν (t) = ∑ e U i ν (0) α i αi i i i The time propagation equation of a weak eigenstate neutrino can be written, = e U − t iE ν (t) i U + ν (0) α αi iα α The general vacuum oscillation probability is then given by 2 ∆ L m − t iE * * ij = e ) i P ν α = ∑ U U U U exp(−i ν α −>ν β ν β αj βj αi βi ij 2E For example, the probability of the oscillation ν µ − > ν τ is 4 2 2 2 ( Pν µ↔ντ = cos (θ )sin (2θ )sin ( 267.1 ⋅ ∆m23 ( 2 (eV ) ⋅ km L ) ) 13 23 GeV E ) 1.2.3 Indication of neutrino oscillation The first experiment that obtained some hints in favor of neutrino oscillation was a solar neutrino search carried out in the Homestake mine in the early ’70. During the last 30 years a large number of experiments strengthened this hypothesis measuring neutrino fluxes from different sources: solar atmospheric, reactor and accelerator. We will briefly discuss the existing experiments. 1. Solar Neutrinos The major part of the energy of the sun (98%) is produced in the reaction of the thermonuclear pp cycles. Neutrinos are emitted in six reactions of the solar pp cycle: three of these give monochromatic energy lines, the others produce neutrino with continuum spectra (fig.1). The solar neutrino puzzle started to emerge in the late sixties and it took nearly forty years to resolve it. The first data on solar neutrinos obtained in the Homestake experiment [11] already displayed a difference of roughly a factor of 2:5 between the measured flux and the prediction , albeit at a very low confidence level. The first detection of solar neutrinos was awarded with the Nobel prize in 2002. As more and more data were gathered, the 7
Figure removed for copyright reasons. [10]J.N.Bachall,Astrophys.J.467,475 (1996). Fig 1.1 Solar neutrino speetruns [10] discrepancy between the observed neutrino flux and the theoretically predicted flux increased as it can be seen in figure 1 2[12].With the advent of the first solar neutrino data from Kamiokande [13]in 1989 the evidence for the solar neutrino deficit was strengthened.This development continued with the first data from Sage [14]in 1991 and from Gallex [15]in 1994.The next major breakthrough in the observation of solar neutrinos was the high quality spectral data delivered by the successor experiment of Kamiokande.Super-K [16].in 1999.The culmination and solution to the solar neutrino puzzle then was achieved by the neutral current data of the SNO experiment in 2002.The neutral current data allow to precisely determine the overall flux of all active neutrino flavors from the Sun and they are found to be in excellent agreement with the predictions. Thus the only possible conclusion is that the electron neutrinos from the Sun undergo a transition to another active flavor.This conclusion can be drawn at the very high confidence level of 5.3,ie.the probability that this result is a statistical fluctuation is approximately I in 10'.A very detailed description of the impact of the various pieces of evidence and systematical errors is given in [17] The Kamland experiment,however,provides an independent check of the results of the solar neutrino experiments.Kamland measures the survival probability of electron 8
Figure removed for copyright reasons. [10] J. N. Bachall, Astrophys. J. 467, 475 (1996). discrepancy between the observed neutrino flux and the theoretically predicted flux increased as it can be seen in figure 1.2[12]. With the advent of the first solar neutrino data from Kamiokande [13] in 1989 the evidence for the solar neutrino deficit was strengthened. This development continued with the first data from Sage [14] in 1991 and from Gallex [15] in 1994. The next major breakthrough in the observation of solar neutrinos was the high quality spectral data delivered by the successor experiment of Kamiokande, Super-K [16], in 1999. The culmination and solution to the solar neutrino puzzle then was achieved by the neutral current data of the SNO experiment in 2002. The neutral current data allow to precisely determine the overall flux of all active neutrino flavors from the Sun and they are found to be in excellent agreement with the predictions. Thus the only possible conclusion is that the electron neutrinos from the Sun undergo a transition to another active flavor. This conclusion can be drawn at the very high confidence level of 5.3σ , i.e. the probability that this result is a statistical fluctuation is approximately 1 in 107 . A very detailed description of the impact of the various pieces of evidence and systematical errors is given in [17]. The Kamland experiment, however, provides an independent check of the results of the solar neutrino experiments. Kamland measures the survival probability of electron 8 Fig 1.1 Solar neutrino spectrums [10]
anti-neutrinos produced in various nuclear power plants in Japan.This allows a precise test of the solar oscillation hypothesis without any astrophysical uncertainties.The first Kamland data were published in 2002 in [18].The Kamland result led to a flood of papers analyzing the Kamland data in combination with existing solar data.In all the papers basically the same result was found,namely that the so called MSW-LMA oscillation solution gives the best fit and that any other known mechanism can play at most a sub-leading role in the explanation of the solar neutrino deficit.The remaining parameter ranges derived by a fit to all existing data are at 3 given by [19] △mi=+7.105er2,sin(28.)-0.8 Figure removed for copyright reasons. [12]J.Bahcall.John Bahcall's homepage.http://www.sns.ias.edu/-jnb/. Fig 1.2 Comparison of the measured and predicted total rates of solar neutrinos[12] Atmosphere neutrinos In the Earth's atmosphere neutrinos are produced by reactions of cosmic radiation and the nuclei in the atmosphere.The main components of the cosmic radiation at the relevant energies are protons,which in turn produce via strong interactions mesons.Those mesons are mainly pions and undergo the following decay chain x产->+v(下】 2->e2+()+下(v) 9
anti-neutrinos produced in various nuclear power plants in Japan. This allows a precise test of the solar oscillation hypothesis without any astrophysical uncertainties. The first Kamland data were published in 2002 in [18]. The Kamland result led to a flood of papers analyzing the Kamland data in combination with existing solar data. In all the papers basically the same result was found, namely that the so called MSW-LMA oscillation solution gives the best fit and that any other known mechanism can play at most a sub-leading role in the explanation of the solar neutrino deficit. The remaining parameter ranges derived by a fit to all existing data are at 3σ given by [19] 2 +23 2 + 2. 0 ∆m21 = +7−3 ⋅10−5 eV 2 , sin (2θ ) = 8.0 12 − 2.0 Figure removed for copyright reasons. [12] J. Bahcall, John Bahcall's homepage, http://www.sns.ias.edu/~jnb/. Atmosphere neutrinos In the Earth's atmosphere neutrinos are produced by reactions of cosmic radiation and the nuclei in the atmosphere. The main components of the cosmic radiation at the relevant energies are protons, which in turn produce via strong interactions mesons. Those mesons are mainly pions and undergo the following decay chain ± ± π − > µ +ν (ν ) µ µ ± µ − > e ( ( ± +ν ν ) +ν ν ) e e µ µ 9 Fig 1.2 Comparison of the measured and predicted total rates of solar neutrinos[12]