中微子振荡 —大亚湾实验及其未来发展

王贻芳中国科学院高能物理研究所

物质世界最基本的单元之一：中微子中微子是构成物质世界的最基本单元之一：

中微子质量极轻，不带电荷，与物质的相互作用十分微弱 , 极难探测 需要用体积庞大的探测器宇宙中的中微子与光子一样多， ~ 100/cm3

大多数粒子物理和核物理反应都有中微子产生：粒子和原子核衰变 , 核反应堆 ,太阳的热核反应 , 超新星爆发 ,加速器， g-暴，宇宙线 ……奇怪的中微子：只有左旋中微子 弱作用宇称不守恒的原因

e

e

bsd

tcu

中微子研究的中心议题：质量• 由于数目巨大，中微子质量对宇宙的形成与演化有重要影响• 卫星实验测得： Σi mvi

< 0.7 eV

• 粒子物理标准模型认为中微子质量为零– 中微子质量不为零 == 》超出标准模型的新物理– 如何在标准模型中赋予中微子质量 ？

• Option 1: Dirac 中微子• Option 2: Majorana中微子

• 中微子是 Dirac或 Majorana，即中微子与反中微子是否同一个粒子是粒子物理的一个根本问题

• 中微子与宇宙学关系密切：– 构成宇宙中的暗物质– 中微子振荡与宇宙中物质与反物质不对称有关– 中微子与大尺度宇宙结构的形成有关

中微子是粒子物理，天体物理与宇宙学研究中的热点与交叉

中微子振荡• 1962 年，因信仰共产主义而逃到前苏联的

Bruno Pontecorvo 提出如果中微子质量不严格为零，且中微子的质量本征态与弱作用本征态不同，根据量子力学，不同的中微子之间可以相互转换

• 判断中微子质量是否为零的方法

Rome, Cimitero Acattolico Dubna ， Pontecorvo’s ofice

两种中微子之间的振荡e e

振荡几率 振荡频率

三种中微子振荡

• A mixing matrix:

• Unknown parameters in neutrino oscillation:– 13 , mass hierarchy, CP phase + Majorana phase

Atmospheric Solar Majorana phase

CP phase & 13

Atmosphericaccelerator

solarreactor

Double beta decays

reactoraccelerator

大气中微子振荡60 年代，印度的宇宙线实验中发现大气中微子反常1985 年，美国 IMB和日本神岗实验证实大气中微子反常1998年 , 日本超级神岗实验证实大气中微子振荡

小柴昌俊 小柴昌俊 20020011 Nobel priceNobel price

太阳中微子振荡6060 年代，年代， R. DavisR. Davis 发现到达地球的太阳中微子数只有理发现到达地球的太阳中微子数只有理论预期值的论预期值的 1/3 1/3

Theoretical Predictions

e + 37Cl 37Ar + e-

15001500 米深的矿井中米深的矿井中 615615 吨四氯乙烯吨四氯乙烯 在在 3030 年中一共探测年中一共探测

到约到约 20002000 个中微子个中微子

R. Davis2001 Nobel price

太阳中微子振荡 : SNO 实验• 2001 年 , 加拿大的 SNO 实验发现太阳中微子 (e) 在

飞向地球的过程中变为另一种中微子 ( 或 )

太阳中微子振荡 : KamLAND 实验

• 2001 年 , 日本的 KamLAND 实验发现反应堆中微子 (e) 与太阳中微子(e) 有相同的消失性能 , 证明太阳和反应堆中微子确实是发生了振荡

中微子振荡及其未解决的问题中微子振荡：

中微子振荡与中微子质量相关联，是中微子研究中的核心问题中微子振荡于 1998 年被证实，获 2001 年 诺贝尔奖已发现的中微子振荡有两种：大气中微子振荡和太阳中微子振荡

中微子振荡三个未解决的问题：寻找第三种振荡，用 13表示 大亚湾中微子实验质量顺序问题 大亚湾中微子实验二期 ？( 类似于夸克 ) 对称性破缺？ 未来的加速器实验

1

2

3

12 太阳中微子振荡

23 大气中微子振荡

13 ? mi ?

Current Knowledge of 13

Direct search PRD 62, 072002

Allowed region

G.L.Fogli et al., J.Phys.Conf.Ser.203:012103

M.C. Gonzalez-Garcia et al., JHEP1004:056,2010

• No good reason(symmetry) for sin2213 =0

• Even if sin2213 =0 at tree level, sin2213 will not vanish at low energies with radiative corrections

• Theoretical models predict sin2213 ~ 0.1-10 %

An experiment with a precision for sin2213 less than 1% is desired

model prediction of sin2213

Experimentally allowedat 3 level

大亚湾反应堆中微子实验• 用大亚湾反应堆测量 13 是我国粒子物理发展的一个重大机遇：

– 功率高（世界第二）– 周围有山，便于建设地下实验室以屏蔽宇宙线本底– 造价较低，没有根本的技术困难

• 世界各国共有 8 个实验建议，三个正在进行中• 难点：精度较过去提高一个量级• 我们的突破：独特设计 + 环境优势 达到精度要求

Reactor neutrino exp.• Clean signal, no cross talk with and matter effects• Relatively cheap compare to accelerator based experiments • Can be very quick

• Provides the direction to the future of neutrino physics

Reactor experiments:

Pee 1 sin22sin2 (1.27m2L/E)

cos4sin22sin2 (1.27m2L/E)

Long baseline accelerator experiments:

Pe ≈ sin2sin22sin2(1.27m2L/E) +

cos2sin22sin2(1.27m212L/E)

A()cos213sinsin()

How Neutrinos are produced in reactors ?

The most likely fissionproducts have a total of 98 protons and 136 neutrons, hence on average there are 6 n which will decay to 6p, producing 6 neutrinos

Neutrino flux of a commercial reactor with 3 GWthermal : 6 1020 s

nepe

10-40 keV

Neutrino energy:

Neutrino Event: coincidence in time, space and energy

epnnemMMTTE )(

Neutrino detection: Inverse-β reaction in liquid scintillator

1.8 MeV: Threshold

s(0.1% Gd)

n + p d + (2.2 MeV)n + Gd Gd* + (8 MeV)

Reactor Experiment: comparing observed/expected neutrinos:

Precision of past exp.

• Reactor power: ~ 1%• Spectrum: ~ 0.3%• Fission rate: 2%

• Backgrounds: ~1-3%

• Target mass: ~1-2%• Efficiency: ~ 2-3%

Typical precision: 3-6%

We need a precision of ~ 0.4%

Currently Proposed sites/experiments

Site

(proposal)

Power

(GW)

Baseline

Near/Far (m)

Detector

Near/Far(t)

Overburden

Near/Far (MWE)

Sensitivity

(90%CL)

Angra dos Reis (Brazil)

6.0 300/1500 50/500 200/1700 ~0.006

Double Chooz (France)

8.7 150/1067 10/10 60/300 ~ 0.03

Daya Bay (China)

17.4 360//500/1800

40//40/80 260/260/910 ~ 0.008

Reno

(S. Korea)

17.3 150/1500 20/20 230/675 ~ 0.03

How to reach 1% precision ?• Increase statistics:

– Powerful nuclear reactors(1 GWth: 6 x 1020 e/s)

– Larger target mass

• Reduce systematic uncertainties:– Reactor-related:

• Optimize baseline for best sensitivity and smaller residual errors

• Near and far detectors to minimize reactor-related errors

– Detector-related:• Use “Identical” pairs of detectors to do relative measurement

• Comprehensive program in calibration/monitoring of detectors

• Interchange near and far detectors (optional)

– Background-related• Go deep to reduce cosmic-induced backgrounds• Enough active and passive shielding

Layout• Near-far

cancellation• Two near sites

and one far sites connected by ~3000 tunnel

• Event rate ：~1200/day near~350/day far

• backgrounds ：B/S ~0.4% nearB/S ~0.2% far

Ling-Ao near ：2 modules, 40 t target500m to Ling-Aooverburden: 112m

far: 4 modules, 80t target1600m to Ling-Ao1900m to Daya Bayoverburden ： 350m

Daya Bay near ：2 modules, 40 t target360m to Daya Bayoverburden: 98m

entrance

8% slope

0% slope

0% slope

0% slope

entrance

Detector design: multiple detector modules and multiple VETO

• Multiple detector modules to reuce errors and cross check

• Multiple muon veto:– Two-layers of cerenkov detector– RPC at the top – Total efficiency > (99.5 0.25) %

Redundancy is a key for the success of this experiment

Central Detector modules• Three zones modular structure:

I. target: Gd-loaded scintillator

-catcher: normal scintillator

III. Buffer shielding: oil

• 192 8”PMT/module

• Reflector at top and bottom to ease engineering difficulties and save cost:

Photocathode coverage 5.6 % 12%(with reflector)

E/E = 12%/E r = 13 cm

Target: 20 t, 1.6m-catcher: 20t, 45cmBuffer: 40t, 45cm

Total weight: ~100 t

Water Buffer & VETO• At least 2m water buffer to shield backgrounds from

neutrons and ’s from lab walls

• Cosmic-muon VETO Requirement: – Inefficiency < 0.5%

– known to <0.25%

• Solution: Two active vetos– Active water buffer, Eff.>95%

– Muon tracker, Eff. > 90%• RPC

– total ineff. = 10%*5% = 0.5%

• Two vetos to cross check each

other and control uncertainty

Background related error • Uncorrelated backgrounds: U/Th/K/Rn/neutron

Single gamma rate @ 0.9MeV < 50Hz

Single neutron rate < 1000/day 2m water + 50 cm oil shielding

• Correlated backgrounds: n E0.75

Neutrons: >100 MWE + 2m water Y.F. Wang et al., PRD64(2001)0013012 8He/9Li: > 250 MWE(near), >1000 MWE(far)

T. Hagner et al., Astroparticle. Phys. 14(2000) 33

大亚湾实验测量 Sin2213 的灵敏度

sources Uncertainty

Reactors 0.087% (4 cores)

0.13% (6 cores)

Detector

(per module)

0.38% (baseline)

0.18% (goal)

Backgrounds 0.32% (Daya Bay near)

0.22% (Ling Ao near)

0.22% (far)

Signal statistics 0.2%

其他物理目标– 首次测量 M2

13

– 测量超新星中微子– 寻找不活跃的中微子

2

22 2

1 1,3

2 2 22 2 2

2 2 2 2 2 21 1,3

(1 )

min

rAA A A A Aii i D c d i r iANbin

r iA A As

i A i i b i

A ANbinc i dD r

r i AD c r shape d B

TM T c b B

T

T T B

c b

Sensitivity: important to constraint models

• A very interesting seesaw model: almost model independent

S.F. Ge, H.J. He, F.R. Yin, JCAP05(2010)017

Civil construction

•Tunnel length: ~ 3100m

•three experimental halls •One assembly hall •Water purification hall

大亚湾土建进展• 3 号厅（远厅）

– 隧道开挖完成，开始大厅挖掘• 2 号厅（岭澳近厅）

– 隧道与实验大厅开挖完成，正在进行水池建造与大厅装修

• 4 号厅（纯水厅）已完工• 5 号厅（液闪厅）已完工，

交付使用• 1 号厅（大亚湾近厅）

– 已完成，即将交付使用。

地面安装大厅投入使用，正地面安装大厅投入使用，正在进行中心探测器装配在进行中心探测器装配

隧道入口隧道入口 隧道内隧道内

控制室投入使用控制室投入使用

实验大厅

探测器部件生产：大部分已完成

反射板生产

完成了满载与负压测试、激光形位测量、喷涂、真空检漏后的钢罐运抵现场

支撑平台已完工，进行了满载测试、测量、喷涂

RPC 裸室 完成制造与测试

探测器装配：完成了试安装，通过了检漏。两个正式探测器的安装正在进行，即将完成。

反射板与洁净间

中心探测器装配，吊装 3米罐 中心探测器装配，吊装反射板

中心探测器顶部

液闪混制

200200吨液袋检漏吨液袋检漏

200200吨存储池，完成安装吨存储池，完成安装

液闪厅及混制设备液闪厅及混制设备

液闪的寿命与稳定性是最大的技术挑战液闪的寿命与稳定性是最大的技术挑战完成了设备工艺实验完成了设备工艺实验液闪寿命经过了长期考验液闪寿命经过了长期考验

PMT readout and triggerPMT readout

Fan-out

Trigger boardPrototype system test

FADC

清华大学负责的触发板，已与前端电子学完成了联调最近在大亚湾现场完成了 Mini Dry Run ，成功实现了探测器 --> 前端电子学 -->触发 -->数据获取 -->数据因特网传输 -->离线分析的全流程

North America (14)

BNL, Caltech, George Mason Univ., LBNL,

Iowa state Univ. Illinois Inst. Tech., Princeton,

RPI, UC-Berkeley, UCLA, Univ. of Houston,

Univ. of Wisconsin, Virginia Tech.,

Univ. of Illinois-Urbana-Champaign,

Asia (15) IHEP, Beijing Normal Univ., Chengdu Univ. of Sci. and Tech., CGNPG, CIAE, Dongguan Polytech. Univ., Nanjing Univ.,Nankai Univ.,

Shenzhen Univ., Tsinghua Univ., USTC, Zhongshan Univ., Hong Kong Univ.

Chinese Hong Kong Univ., Taiwan Univ., Chiao Tung Univ., National United Univ.

Europe (3)

JINR, Dubna, Russia

Kurchatov Institute, Russia

Charles University, Czech Republic

Daya Bay collaboration

~ 200 collaborators

What we can do after Daya Bay ?

下一代中微子实验：大亚湾二期• 中微子振荡三个未解决的问题：

寻找第三种振荡，用 13表示 大亚湾中微子实验质量顺序问题 大亚湾中微子实验二期 CP 对称破缺角 未来的加速器实验 1

2

3

12 太阳中微子振荡

23 大气中微子振荡

13 ? mi ?

为什么反应堆中微子：1 ）加速器中微子实验：造价昂贵 （探测器 + 加速器） 2 ）双 β 实验：造价昂贵，技术困难，科学风险大3 ）中微子绝对质量测量：造价昂贵，技术困难4 ）反应堆中微子实验：意义重大、风险小、条件优越、造价低、技术可行

测量磁矩：可能的未来，科学风险大精确测量混合参数：大亚湾、大亚湾二期

Neutrino Mass hierarchy• Mass Hierarchy:

• Fundamental to the Standard Model

• Fundamental to models beyond SM • Most GUTs predict a normal mass hierarchy a discriminator of

different GUTs and/or neutrino mass models

• Fundamental to many issues: – Matter-antimatter asymmetry via leptogenesis in specific seesaw models:

hierarchy related

– Supernova neutrinos: collective flavor transitions due to inverted mass hierarchy

– Radiative corrections to m and ij are more sensitive to the inverted hierarchy

But even more important…

Dirac or Majorana ? • Neutrino oscillation:

beyond SM in a way of Dirac or Majorana mode ?

• exp. may never give positive results

• If mass hierarchy is known, together with next generation exp., the neutrino Dirac or Majorana nature can be determined.

next gen. exp.

Measuring mass hierarchy• Long baseline accelerator neutrinos

– Through Matter effects– Expensive, project-X/LBNE in Fermilab/BNL

• Atmospheric neutrinos– Very weak signal– Huge detector, Expensive

• Reactor neutrinos– Method: distortion of energy spectrum– Enhance signature: Transform reactor neutrino

L/E spectrum to frequency regime using Fourier formalism

• need Sin2(213) > 0.02

• Need to know M223

S.T. Petcov et al., PLB533(2002)94S.Choubey et al., PRD68(2003)113006

J. Learned, PRD 78(2008)071302

42

Fourier transformation of L/E spectrum

• Frequency regime is in fact the M2 regime enhance the visible features in M2 regime

• Take M2 32 as reference

– NH: M2 31 > M2 32 , M2 31 peak at the right of M2 32

– IH: M2 31 < M2 32 , M2 31 peak at the left of M2 32 L/E spectrum

Our efforts

• Clear distinctive features: – FCT:

• NH: peak before valley

• IH: valley before peak

– FST: • NH: prominent peak

• IH: prominent valley

• Better than power spectrum

• No pre-condition of m223

43L. Zhan et al., PRD78:111103,2008

Quantify Features of FCT and FST• To quantify the symmetry

breaking, we define:

RV/LV: amplitude of the right/left valley in FCT

P/V: amplitude of the peak/valley in FST

• For asymmetric Pee

– NH: RL>0 and PV>0

– IH: RL<0 and PV<0

2008-07-17 44L. Zhan et al., PRD78:111103,2008

Baseline: 46-72 kmSin2(213): 0.005-0.05Others from global fit

Two clusters of RL and PV values show the sensitivity of mass hierarchy determination

In reality

L. Zhan, et. al., Phys.Rev.D79:073007,2009

Unfortunately, M2

21 /M223 ~ 3%

Requirement• To determine mass hierarchy at > 90% CL:

– Baseline: ~ 58 km, determined by 12

– Reactor power > 24 GWth

– Flux and detector size: ~ (250-700) ktyear

– Ideally, sin2213 > 0.02 & energy resolution < 2%

• IF sin2213=0.01, energy resolution < 2% & 700 ktyear

• For sin2213=0.02 , energy resolution < 3% & 700 ktyear

• Overburden > 1000 MWE• ~ 60 km from Daya Bay

• A huge e detector with mass >20kt– currently the largest on is 1kt (KamLAND & LVD)

Scientific goal: a l0-50kt underground LS detector 60km from reactor

1. Neutrino Mass hierarchy2. Precision mixing para. measurement: 12, M2

12, M231

Unitarity of the mixing matrix3. Supernova neutrinos == 〉 better than SuperK4. Geo-neutrinos == 〉 10 better than KamLAND5. Atmospheric neutrinos == 〉 SuperK6. Solar neutrinos ? 7. High energy neutrinos

1. Point source: GRB, AGN, BH, …2. Diffused neutrinos

8. High energy cosmic-muons1. Point source: GRB, AGN, BH, …2. Dark matter

9. Exotics 1. Sterile neutrinos2. Monopoles, Fractional charged particles, ….

LVD+MACRO+KamLAND+SuperK

Precision measurement of mixing parameter• Fundamental to the Standard Model and beyond

• Similarities point to a Grand unification of leptons and quarks

• Constrain all PMNS matrix elements to < 1% ! Probing Unitarity of UPMNS to <1% level !

If we can spend (0.1-0.5)B$ for each B/C/superB factories to understand UCKM (~ 1-2 elements for each factory), why not a super-reactor neutrino experiment(~ 3 elements) to understand UPMNS ?

Current BESIII

Vub 25% 5%

Vcd 7% 1%

Vcs 16% 1%

Vcb 5% 3%

Vtd 36% 5%

Vts 39% 5%

Current Daya Bay II

m212 5% < 1%

m223 12% < 1%

sin212 10% < 1%

Sin223 20% -

sin213 -

Supernova neutrinos• Less than 20 events observed so far (2001 Noble prize)• Assumptions:

– Distance: 10 kpc (our Galaxy center) – Energy: 31053 erg

– L the same for all types

– Tem. & energy

• Many types of events: e + p n + e+, ~ 3000 correlated events

e + 12C 13B* + e+, ~ 10-100 correlated events

e + 12C 11N* + e-, ~ 10-100 correlated events

x + 12C ｘ + 12C*, ~ 600 correlated events

x + p ｘ + p, single events

e + e- e + e-, single events

x + e- ｘ + e-, single events

T(e) = 3.5 MeV, <E(e)> = 11 MeVT(e) = 5 MeV, <E(e)> = 16 MeVT(x) = 8 MeV, <E(x)> = 25 MeV

SuperK can not see these correlated events

What to do with Supernova neutrinos

• Energy spectra & fluxes of all types of neutrinos – tem. and average energy of neutrinos

– Understand Supernovae

– neutrino properties: mass, mixing, …

– Earth tomography

– Neutrino models

– …

• Arrival time of all types of neutrinos absolute neutrino mass

探测器的概念设计

• Neutrino target: ~20kt LS, LAB based

30m(D)30m(H)• Oil buffer: 6kt• Water buffer: 10kt • PMT: 15000 20”• Cost: ~1.5 B RMB

可能的地点：惠州或海上据大亚湾 /海丰 60公里热功率 > 40 GW

Technical challenges• Requirements:

– Large detector: >10 kt LS

– Energy resolution: 2%/E 2500 p.e./MeV

• Ongoing R&D:– Low cost, high QE “PMT”

• A new design exist, patent pending,

• R&D contract to be signed with manufacture

– transparent LS: 15m >25m• Find out traces which absorb light, remove it from production

R&D program: ~ 3 yearsUseful for many future projectsSupport already from IHEP

Now: 1kt250 p.e./MeV

1 ）采用透射式光电阴极与反射式光电阴极相结合 == 〉 提高光阴极的有效面积 ==〉提高量子效率 :2 ）采用微通道板作为电子倍增 == 〉 不阻挡光电子 ==〉提高收集效率

新型光电倍增管的设计 : 提高光量子效率

Hammamatzu 的 SBA/UBA 光阴极可以得到 ~40% 光量子效率 ; 已基本满足要求估计可以提高光量子效率一倍以上

与工业界合作完成的 5”MCP-PMT

该设计已申请全球发明专利

Summary

• Knowing Sin2213 to 1% level is crucial for the future of the neutrino physics, particularly for the leptonic CP violation

• The Daya Bay experiment, located at an ideal site, will reach a sensitivity of <0.01 for sin2213

• The construction of the Daya Bay experiment is going on well, data taking will start at the end of 2012

• Daya bay experiment is only the start of major neutrino physics programs in China