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[已翻译,待校对] 中微子超光速,1109.4897试验报告中文版

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发表于 2011-9-30 22:33:44 | 显示全部楼层 |阅读模式
这个话题很多朋友感兴趣,我帮忙翻译一下,水平有限,错误难免,希望各位不吝赐教
原内容为http://arxiv.org/ftp/arxiv/papers/1109/1109.4897.pdf
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 楼主| 发表于 2011-9-30 22:36:06 | 显示全部楼层
Abstract

The OPERA neutrino experiment at the underground Gran Sasso Laboratory has measured the velocity of neutrinos from the CERN CNGS beam over a baseline of about 730 km with much higher accuracy than previous studies conducted with accelerator neutrinos. The measurement is based on highstatistics data taken by OPERA in the years 2009, 2010 and 2011. Dedicated upgrades of the CNGS timing system and of the OPERA detector, as well as a high precision geodesy campaign for the measurement of the neutrino baseline, allowed reaching comparable systematic and statistical accuracies. An early arrival time of CNGS muon neutrinos with respect to the one computed assuming the speed of light in vacuum of (60.7 ± 6.9 (stat.) ± 7.4 (sys.)) ns was measured. This anomaly corresponds to a relative difference of the muon neutrino velocity with respect to the speed of light (v-c)/c = (2.48 ± 0.28 (stat.) ± 0.30 (sys.)) ×10-5.

摘要

Gran Sasso实验室的OPERA中微子实验旨在测量CERN CNGS中微子束通过一段730千米长基线的速度,实验的精确度较之前实验有很大提高(之前实验使用加速器产生的中微子)。测量是基于OPERA实验于2009,2010,和2011年获得的高统计数据。我们对CNGS测时系统和OPERA检测器做了针对性的升级,并且引入了高精度大地测量系统用以测量基线长度,藉此实现大致相当的系统和统计精度。CNGS实验中测量结果显示μ子中微子完成全程所用时间比真空中光的理论耗时短了(60.7 ± 6.9 (统计) ± 7.4 (系统))ns。此异常结果意味着μ子中微子的速度比真空中光速快(v-c)/c = (2.48 ± 0.28 (统计) ± 0.30 (系统)) ×10-5(约十万分之2.48)。

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 楼主| 发表于 2011-9-30 23:17:38 | 显示全部楼层
本帖最后由 lch=gta 于 2011-9-30 23:18 编辑

1. Introduction
The OPERA neutrino experiment [1] at the underground Gran Sasso Laboratory (LNGS)
was designed to perform the first detection of neutrino oscillations in direct appearance mode in
the νμ→ντ channel, the signature being the identification of the τ− lepton created by its charged
current (CC) interaction [2].
In addition to its main goal, the experiment is well suited to determine the neutrino
velocity with high accuracy through the measurement of the time of flight and the distance
between the source of the CNGS neutrino beam at CERN (CERN Neutrino beam to Gran Sasso)
[3] and the OPERA detector at LNGS. For CNGS neutrino energies, <Eν> = 17 GeV, the relative
deviation from the speed of light c of the neutrino velocity due to its finite rest mass is expected
to be smaller than 10-19, even assuming the mass of the heaviest neutrino eigenstate to be as large
as 2 eV [4]. Hence, a larger deviation of the neutrino velocity from c would be a striking result
pointing to new physics in the neutrino sector. So far, no established deviation has been observed
by any experiment.
In the past, a high energy (Eν > 30 GeV) and short baseline experiment has been able to
test deviations down to |v-c|/c < 4×10-5 [5]. With a baseline analogous to that of OPERA but at
lower neutrino energies (Eν peaking at ~3 GeV with a tail extending above 100 GeV), the MINOS
experiment reported a measurement of (v-c)/c = 5.1 ± 2.9×10-5 [6]. At much lower energy, in the
10 MeV range, a stringent limit of |v-c|/c < 2×10-9 was set by the observation of (anti) neutrinos
emitted by the SN1987A supernova [7].
In this paper we report on the precision determination of the neutrino velocity, defined as
the ratio of the precisely measured distance from CERN to OPERA to the time of flight of
neutrinos travelling through the Earth’s crust. We used the high-statistics data taken by OPERA
in the years 2009, 2010 and 2011. Dedicated upgrades of the timing systems for the time tagging
of the CNGS beam at CERN and of the OPERA detector at LNGS resulted in a reduction of the
systematic uncertainties down to the level of the statistical error. The measurement also relies on
a high-accuracy geodesy campaign that allowed measuring the 730 km CNGS baseline with a
precision of 20 cm.


1.序言
  OPERA中微子实验和Gran Sasso Laboratory (LNGS)的目标是对μ子中微子向τ子中微子转化的中微子振荡做史上第一次探测。实验特征现象为电相互作用生成τ-轻子。
  此实验还有一个次要目标,即精确测量中微子从生成处CERN(CERN Neutrino beam to Gran Sasso)运动至LNGS的OPERA检测器所需的时间。对于能量为17GeV的CNGS中微子而言,由于其有一定质量,理论速度应慢于光速。但即使对于最重的中微子(2eV)速度之差也应小于10-19c。因此,如果实验数据与之不符,那我们可能需要新的理论来解释中微子行为。目前为止,没有任何实验得出过和理论不符的结果。
  过去的实验使用能量大于30GeV的高能中微子和较短的基线,得到了|v-c|/c < 4×10-5的结果。MINOS实验使用低能中微子(能量众数3GeV,最高达100GeV)和于OPERA长度相当的基线, 得到了(v-c)/c = 5.1 ± 2.9×10-5的结果。当中微子能量低至10MeV时,速度范围变的更小(|v-c|/c < 2×10-9),此结果由观测SN1987A超新星爆发释放的中微子得到。
  本报告中通过CERN至OPERA的距离/中微子穿过地壳从CERN到达OPERA的时间对中微子速度做出精确测定。测量是基于OPERA实验于2009,2010,和2011年获得的高统计数据。我们对CERN的CNGS测时系统和LNGS的OPERA检测器做了针对性的升级,藉此减小系统误差,使其接近统计误差。并且引入了高精度大地测量系统用以精确测量基线长度,对于730km长的基线,测量误差小于20cm。

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 楼主| 发表于 2011-10-1 00:24:13 | 显示全部楼层
2. The OPERA detector and the CNGS neutrino beam
The OPERA neutrino detector at LNGS is composed of two identical Super Modules,
each consisting of an instrumented target section with a mass of about 625 tons followed by a
magnetic muon spectrometer. Each section is a succession of walls filled with emulsion film/lead
units interleaved with pairs of 6.7 × 6.7 m2 planes of 256 horizontal and vertical scintillator strips
composing the Target Tracker (TT). The TT allows the location of neutrino interactions in the
target. This detector is also used to measure the arrival time of neutrinos. The scintillating strips
are read out on both sides through WLS Kuraray Y11 fibres coupled to 64-channel Hamamatsu
H7546 photomultipliers [8]. Extensive information on the OPERA experiment is given in [1] and
in particular for the TT in [9].
The CNGS beam is produced by accelerating protons to 400 GeV/c with the CERN Super
Proton Synchrotron (SPS). These protons are ejected with a kicker magnet towards a 2 m long
graphite neutrino production target in two extractions, each lasting 10.5 μs and separated by 50
ms. Each CNGS cycle in the SPS is 6 s long. Secondary charged mesons are focused by two
magnetic horns, each followed by a helium bag to minimise the interaction probability of the
5
mesons. Mesons decay in flight into neutrinos in a 1000 m long vacuum tunnel. The SPS/CNGS
layout is shown in Fig. 1. The different components of the CNGS beam are shown in Fig. 2.
The distance between the neutrino target and the OPERA detector is about 730 km. The
CNGS beam is an almost pure νμ beam with an average energy of 17 GeV, optimised for νμ→ντ
appearance oscillation studies. In terms of interactions in the detector, the νμ contamination is
2.1%, while νe and νe contaminations are together smaller than 1%. The FWHM of the neutrino
beam at the OPERA location is 2.8 km.
The kicker magnet trigger-signal for the proton extraction from the SPS is UTC
(Coordinated Universal Time) time-stamped with a Symmetricom Xli GPS receiver [10]. The
schematic of the SPS/CNGS timing system is shown in Fig. 3. The determination of the delays
shown in Fig. 3 is described in Section 6.
The proton beam time-structure is accurately measured by a fast Beam Current
Transformer (BCT) detector [11] (BFCTI400344) located (743.391 ± 0.002) m upstream of the
centre of the graphite target and read out by a 1 GS/s Wave Form Digitizer (WFD) Acqiris
DP110 [12]. The BCT consists of toroidal transformers coaxial to the proton beam providing a
signal proportional to the beam current instantaneously transiting through it, with a few hundred
MHz bandwidth. The start of the digitisation window of the WFD is triggered as well by the
magnet kicker signal. The waveforms recorded for each extraction by the WFD are stamped with
the UTC and stored in the CNGS database.
The proton beam has a coarse bunch structure corresponding to the 500 kHz of the CERN
Proton Synchrotron (PS) (left part of Fig. 4), on which the fine structure due to the 200 MHz SPS
radiofrequency is superimposed, which is actually resolved by the BCT measurement, as seen in
the right part of Fig. 4.

2. OPERA探测器和CNGS中微子束
  LNGS的OPERA中微子探测器由两个相同的超级模块组成,每个模块包含625t重的靶,靶上安装有测量仪器,靶之后是一个磁μ子能谱仪。每部分中都有一系列的感光乳剂底片,底片由256个6.7 × 6.7 m2纵横交错的闪烁器分隔开来,组成目标追踪器(TT)。TT可以测得中微子作用在靶中的位置。(?)
  这个探测器同时也能测量中微子到达时间,闪烁器的数据通过WLS Kuraray Y11光纤传输至64通道Hamamatsu H7546光电倍增管。详尽信息请参照[1]和[9]。CNGS中微子束由被CERN超级质子同步加速器(SPS)加速至400 GeV/c动量的质子产生。这些质子被冲击磁体发射至2m长的石墨靶上,每次脉冲10.5μs,脉冲之间间隔50ms。每个SPS周期为6s。带电介子被磁透镜汇聚,穿过氦气袋以减少发生相互作用的可能。介子继而穿过一个1000m长的真空管道,并在其中衰变成中微子。SPS/CNGS设置见图1,CNGS中微子束产生器见图2。
  石墨靶和OPERA探测器相距约为730km。CNGS中微子束几乎全部由μ子中微子组成,平均能量17GeV,此为研究μ子中微子向τ子中微子振荡的有利条件。检测器中探测到的反μ子中微子约占2.1%,电子和反电子中微子合计少于1%。OPERA处中微子束的半峰全宽为2.8km。
  SPS中冲击磁体的触发信号由Symmetricom Xli GPS接收器调校至世界标准时间。SPS/CNGS的时间机制见图3。本文第6部分会详细叙述图3中各项延迟的测定。
  质子束的时间结构由粒子束电流检测器(BFCTI400344)测量,其位于石墨靶中心(743.391 ± 0.002)m之前,信号被1 GS/s的波形数字转化器(WFD)Acqiris DP110读取。粒子束电流检测器由和质子束同心的环形线圈组成,可以根据穿过其中的电流输出数百MHz的信号。WFD和冲击磁体由同一信号激活。WFD收集到的数据被校准至世界标准时间并储存在CNGS的数据库中。
  图4为BCT探测到的结果,左图显示质子束的低频率周期对应500kHz的CERN的质子同步加速器之频率,这实际上是右图中更高频率的200MHz质子束叠加而成。

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