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发表于 2011-9-30 23:17:38
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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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