The Elusive but Essential Neutrino - Indian Institute of ... neutrinos detected via the reaction:...

The Elusive but Essential Neutrino

Jim Strait, [email protected]

XXI DAE-BRNS High Energy Physics Symposium9 December 2014

Outline

• A quick introduction to Particle Physics

• Introducing Neutrinos

• A brief history of Neutrinos

• Neutrino Oscillations

• Survey of current and future Neutrino Experiments

• The Long-Baseline Neutrino Experiment / Facility

• Conclusions

9 Dec 2014Jim Strait | XXI DAE-BRNS HEP Symposium2

What Are We Trying to Learn in Particle Physics?

Particle physics is the study of matter and energy at the most fundamental level:• What is matter made out of?• What are the fundamental forces?• How do the fundamental forces and the structure of matter shape

the universe?As we learn about these questions, new questions arise:• Why is there matter but no anti-matter?• What is “dark matter”? …and “dark energy”???• Why are some particles so massive – 100’s of times more massive

than a proton – and some are so light – a billion times less massive than a proton?

Can we understand all of this in one unified theory?


What Is Matter Made Of?


Four Fundamental Forces

• Gravity: Very weak, but affects all matter and energy. Affects the overall structure of the universe.

• Electromagnetism: Acts on charged particles – holds the electrons in place around the nucleus. Familiar in everyday life.

• Strong force: Acts on quarks and particles made from them –holds the quarks together inside protons and neutrons, and holds the nucleus together.

• Weak force: Acts on all particles, but only at very short distances. Too weak to hold anything together, but is crucial for reactions that make the sun shine.


Force Carrying Particles

At the quantum mechanical level, forces are carried by particles:• Gravity – graviton (massless, one type)• Electromagnetic force – photon (massless, one type)• Strong force – gluon (massless, 8 types)• Weak force – W+ W-, Z (~80 x the mass of the proton)


Decay of a neutron into a proton, electron, and neutrino.

neut

ron

proton

The Standard Model of Particle Physics


H

Higgs B

oson

What are Neutrinos?

• Neutrinos are particles with no electric charge and almost no mass. They feel only the Weak Interaction (and Gravity)

• They are among the most abundant particles in the universe.

• They are produced in great quantities by the sun and other stars, in the earth, by cosmic ray interactions in the atmosphere. Trillions of neutrinos pass through your body each second.

• Neutrinos hardly ever interact – a typical neutrino could travel through more than 100 million miles of lead unscathed.

• There are three (known) types (“flavors”) of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos. Once produced, they can change (“oscillate”) from one type to another and then back again.


Why are Neutrinos Important?

Neutrinos play an important role in natural processes that are crucial to why we exist.• The reactions in the core of the sun.• The explosions of supernova stars in which the heavy

elements are created and expelled into space to form planets and provide the building blocks for life.

• Small differences between neutrinos and their anti-particle counterparts could help explain why more matter than anti-matter was produced in the Big Bang.

Because neutrinos hardly interact, they can tell us what happens in places we cannot “see” otherwise:• In the core of the sun• In the center of a supernova at the moment it explodes.


Where Did the Neutrino Come From?

Or more correctly, how did we come to know that it is there?Studying radioactive decays in the 1920’s...


Beta decay:• Apparently a 2‐body process• Energy of varies from event to event• Efinal < Einitial• Lfinal Linitial

Alpha decay:• 2‐body process• Energy of always the same• Energy and angular momentum conserved

The Problem with Beta Decay


Expected thisObserved that

14C spin = 0

14N spin = 1e‐ spin = 1/2

total spin = 1/2or 3/2

Dear Radioactive Ladies and Gentlemen,

As the bearer of these lines, to whom I graciously ask you to listen, will explain to you in more detail, because of the "wrong" statistics of the N- and Li-6 nuclei and the continuous beta spectrum, I have hit upon a desperate remedy to save the "exchange theorem" of statistics and the law of conservation of energy. Namely, the possibility that in the nuclei there could exist electrically neutral particles, which I will call neutrons, that have spin 1/2 and obey the exclusion principleand that further differ from light quanta in that they do not travel with the velocity of light. The mass of the neutrons should be of the same order of magnitude as the electron mass and in any event not larger than 0.01 proton mass. The continuous beta spectrum would then make sense with the assumption that in beta decay, in addition to the electron, a neutron is emitted such that the sum of the energies of neutron and electron is constant.

Pauli’s Bold Hypothesis of a New Unobserved Particle


First Observation of Neutrinos, 26 Years Later


Anti‐neutrinos produced by a nuclear reactor at the Savannah River Plant:n p + e +

Anti‐neutrinos detected via the reaction: + p n + e+

Detector: 0.2 kt water with dissolved cadmium chloride surrounded by scintillation counters

e+ detected by the reactione+ + e

n detected by reaction

Not only did this experiment prove the existence of neutrinos, it also measured the cross-section for

+ p e+ + nto be

6.3 x 10-44 cm2 (10%) in agreement with the value of

6 x 10-44 cm2 (25%) predicted by the weak interaction theory developed by Fermi in 1934.

Nobel prize 1995


Two Types of Neutrinos

• In the mean time the pion was discovered in cosmic rays and later studied with accelerators.

• The pion decays into a muon plus a neutrino: +

• The neutrino is inferred by the missing energy and angular momentum. Since the energy of the muon from a stopped pion is always the same, there must be only one neutrino.

• The muon decays into an electron and two neutrinos e + +

• That there are two neutrinos is inferred since the electron energy varies from event to event.

• Are the neutrinos from and the same as the one from decay?


Two Types of Neutrinos


1962 Experiment at Brookhaven’s AGS:Neutrinos from pion decay only make muons when they interact but no electrons.

There are two types of neutrinos: e and

Nobel Prize 1988

• produced when a primary cosmic rayhits an air molecule

• Then • and + e+ e

or e e

Atmospheric Neutrinos


• First observed in 1965 in the Kolar Gold Field Neutrino Experiment at a depth of 7000 feet (2100 m)

Phys Lett 18, 196 (1965)

Solar Neutrinos


Solar Neutrino Experiment

• Detect neutrinos from the sun via the reactione + 37Cl 37Ar + e in a tank of 600 t of dry cleaning fluid (perchloroethylene) located 4850 feet (1480 m) underground in the Homestake Gold Mine in South Dakota

• A few 37Ar per day were produced, and detected by their radioactive decay

• First measurements in 1967 • Big surprise ... only ~1/3 the expected

rate was detected–Do we not understand how the sun shines?– Is there a mistake in the experiment?– Is there something “wrong” with neutrinos?

This was a big puzzle for many years!


A Third Neutrino

• A heavy muon-like particle called the was discovered in 1978 in an experiment at SLAC colliding electrons with positrons:e+ + e + +


e + missing energy + + missing energy

• The missing energy must be neutrinos, and there must be a third type of neutrino:e+ + e + +

e + e + + + +

• The was directly observed in 2000 in an experiment at Fermilab.

The Standard Model of Particle Physics


H

Higgs B

oson

Back to Solar Neutrinos

Over several decades after the first results, lots of work was done.Do we not understand how the sun shines?• Many checks of the theory and measurements of the relevant

nuclear physics reactions still resulted in the same predition.Is there a mistake in the experiment?• Many checks of the experiment found no mistakes.• Other experiments confirmed the deficit of events.Is there something “wrong” with neutrinos?• Experiments with atmospheric neutrinos also showed a deficit of

relative to what is expected.

=> Maybe there really is something wrong with (our understanding) of neutrinos!


Neutrino Decay or Neutrino Oscillations?

• Maybe something happens to the neutrinos between where they are produced and detected.– The controlled experiments with reactors and accelerators detect the

neutrinos near the source.– Solar and atmospheric neutrinos travel much farther

• Could neutrinos decay into something lighter that we cannot detect? ??

• Could neutrinos transform from one type to another (neutrino oscillations)?– The solar neutrino experiments only measure e

– The atmospheric neutrino experiments only measure e and


Atmospheric Neutrinos Again• 1998: The Super-Kamiokande results consistent with neutrino

oscillations


50 kt water detector 1000 m underground in the Kamioka Mine in Japan

Solar Neutrinos Again

• 2002: An experiment in Canada (SNO) found the expected number of neutrinos in neutral current reactions, in which the neutrino does not change into a charged lepton.

• + 2H + p + n

n + 2H 3H +

All the solar neutrinos are there!=> Neutrinos don’t decay=> Neutrinos must change “flavor”


1 kt heavy water (D2O) 2000 m underground in the Sudbury Mine in Canada

Neutrinos are Unusual Particles

• Only neutral fundamental matter particles (fermions)=> perhaps they are their own anti-particle? (Majorana particles)

• Dramatically lighter than other fermions => Is there a different mechanism for generating their mass?

• Neutrino flavors seem to be strongly mixed => Why so different from the quarks?

• Do neutrinos violate matter – antimatter symmetry (CP violation)?=> Could they be the key to understanding why the universe is

made of matter and no antimatter (baryon asymmetry of the universe)?


Neutrino Masses are Very Small

• Neutrino masses are much smaller than other fundamental fermions, but at least a factor of 106.

• Is there a different mechanism for generating neutrino masses than for other particles, which are generated by the Higgs mechanism?


E.Lissi, International Meeting for Large Neutrino Infrastructures, 23 June 2013

Neutrino Flavor Mixing


Neutrino production and detection determined by flavor eigenstates e, , of the weak interaction

but propagation through space (and matter) is determined by mass eigenstates 1, , of the Hamiltonian (with masses m1, m2, m3 ), these can be related by

Different masses will lead to interference between the propagating waves that affects the flavor probability at detection as a function of distance - “flavor mixing” or “neutrino oscillations”

Neutrino Flavor Mixing


Neutrino production and detection determined by flavor eigenstates e, , of the weak interaction

but propagation through space (and matter) is determined by mass eigenstates 1, , of the Hamiltonian (with masses m1, m2, m3 ), these can be related by

Different masses will lead to interference between the propagating waves that affects the flavor probability at detection as a function of distance - “flavor mixing” or “neutrino oscillations”

• Quantum interferometry on a continental scale

Neutrino Mixing Matrix


3

2

1

τ3τ2τ1

μ3μ2μ1

e3e2e1

τ

μ

e

ννν

UUU

UUU

UUU

ννν

sij = sinijcij = cosij

solar andreactor

atmosphericand accelerator

reactor andaccelerator

Mixing Quarks Leptons1‐2 12 13o 34o2‐3 23 2.3o ~43o1‐3 13 0.5o 9o

Phase for neutrinos is unknown – related to CP‐violation

Pontecorvo‐Maki‐Nakagawa‐Sakata

Neutrino vs. Quark Mixing


Strikingly different! Is this telling us something fundamental?

A different mechanism for mass generation?

Neutrinos Quarks

Neutrino Oscillations


• In a simplified 2-neutrino model, the probability of a neutrino produced as one flavor to be detected with a different flavor is:

• is the mixing angle• m2 = m2

2 - m12

• L = distance traveled• E = neutrino energy

Neutrino Mass Hierarchy


m221 = +7.6 x 10-5 eV2

|m232| = 2.4 x 10-3 eV2

m221 / |m2

32| = 0.03

Is third neutrino heavier or lighter than the other two? (MH question)

What is the overall mass scale?

e Appearance in a Beam


Approximation to 3‐flavor vacuum mixing with m212 << m31

2

Effectively 2‐flavor mixing since m212 0.03 m31

2

Wavelength in L/E ~ 1.2 km/GeV => maximum probability at 600 km for a 1 GeV neutrino

Amplitude proportional to sin2213

L – distance from source to detectionE – neutrino energy

e Appearance in a Beam


Full equation for 3-flavor mixing

Now includes dependence on phase manifest in interference terms between the two oscillation scales

Changing , => matter/antimatter asymmetry if 0 or 180°

Could this difference help explain why the Big Bang produced more matter than antimatter ... So that we can exist to ask this question?

J. Boehm, thesis 2009

Matter Effect


Additional e interaction mixing angles and mass differences modified by terms proportional to electron density

Sign of effect depends on mass ordering => method to determine the Mass Hierarchy

A complication in determining true matter-antimatter difference

Is the Three-Neutrino Model Complete?

• Hints of deviations implying a fourth “sterile” neutrino– Reactor anomaly => ~7% deficit at short distances– Short-baseline anomaly, a.k.a. “LSND anomaly”

=> small -> e appearance rate at small L/E

• These effects can be tested by– Direct searches for sterile neutrino signatures– Over-constraining to PMNS matrix to test it unitarity

• Many neutrino experiments are under way or planned to understand the nature of neutrinos.


Long-Baseline, Accelerator-Based Experiments


CNGS:ICARUS & OPERA

Short-Baseline, Accelerator-Based Experiments


MicroBooNE

Reactor-Based Experiments


Short Baseline (~1 km)

JUNO

Long Baseline (~50 km)Mass Hierarchy

Very Short Baseline (~10 m)Reactor Source Anamolies

Atmospheric and Ultra-High Energy Neutrino Experiments



Unique feature is magnetic field => distinguish from

Mass Hierarchy is major goal

Solar Neutrino Experiments


The Next-Generation Long-Baseline Experiment: LBNF

5 Nov 14J.Strait| Future Plans in the Americas44

Far Detector1475 m Underground

Near Detector

• Comprehensive experiment to determine CP violation, Mass Hierarchy, and make precision measurements of oscillation parameters

• Astrophysical neutrinos and proton decay with Far Detector• Precision neutrino scattering measurements with Near Detector


Fermilab Accelerator Complex


Beamline for a New Long-Baseline Neutrino Facility

How to Make a Neutrino Beam


How to Make a Neutrino Beam


Focusing Horns

Decay Pipe: Diameter = 4 mLength = 200~250 m

Target Hall

Decay Pipe

Monitoring the Beam with Muons• Measure muons from the same pion decays that make the neutrinos:

+


Prototypes in the existing NuMI beam

• Make high‐precisions measurements ofthe neutrinos at the source ... beforethey have started to oscillate. This is crucial to achieve high‐precision oscillation measurements!

• Proposed to be designed and built in India

• Main elements:– High precision straw‐tube

tracker with embedded high‐pressure argon gas targets

– 4 electromagnetic calorimeter and muon identification systems

– Large‐aperture dipole magnet

• Enables a diverse physics program of its own

The Near Detector


Measuring a Neutrino Event in the Near Detector


The Far Detector: A Massive Liquid Argon Time-Projection Chamber

• 1300 km fromthe source.

• Located under-ground to protect against cosmic ray backgrounds

• Able to distinguish e from • Provides unprecedented

precision for measuring neutrinos in such a massive detector.

• Sensitive to atmospheric neutrinos, neutrinos from a supernova, and proton decay.


LBNE Liquid Argon TPC50,000 tonnes of LAr

How Does a LAr TPC Work?


A Neutrino Event in a LAr TPC


LBNE Collaboration


>550 (~25% non‐US) members, 90 (35 non‐US) institutions,

9 countries

Michigan StateMilano

Milano/BicoccaMinnesota

MITNapoliNGA

New MexicoNorthwesternNotre Dame

OxfordPadovaPanjabPavia

PennsylvaniaPittsburghPrincetonRensselaerRochester

Rutherfod LabSanford Lab

SheffieldSLAC

South CarolinaSouth Dakota

South Dakota StateSDSMT

Southern MethodisStonybrookt

SussexSyracuse

TennesseeTexas, Arlington

Texas, AustinTuftsUCLAUEFS

UNICAMPUNIFAL

Virginia TechWarwick

WashingtonWilliam and Mary

WisconsinYale

Yerevan

UFABCAlabamaArgonneBHUBostonBrookhavenCambridgeCatania/INFNCBPFCharles UChicagoCincinnatiColorado Colorado StateColumbiaCzech Technical UDakota StateDelhiDavisDrexelDukeDuluthFermilabFZUGoiasGran SassoGSSIHRIHawaiiHoustonIIT GuwahatiIndianaINRIowa StateIrvineKansas StateKavli/IPMU‐TokyoLancasterLawrence Berkeley NLLivermore NLLiverpoolLondon UCLLos Alamos NLLouisiana StateManchesterMaryland

Indian Institutions are important members of this collaboration

A New ELBNF Collaboration

• The experiment design shown has been developed by the LBNE Collaboration

• A new, broader, more international collaboration is in the process of being formed to carry out this experiment. – It is currently called the “Experimental program at the Long-Baseline

Neutrino Facility” or “ELBNF.”– It will marshal resources from around to world to make the best

possible experiment.

• Next steps– Open meeting at Fermilab this Friday, the 12th. Remote participation

is possible and welcome. – Initial collaboration meeting in the 2nd half of January, tentatively 22-24

January at Fermilab ... Details soon.

• Please see me if you are interested and would like more information: [email protected]


Conclusions

• Neutrinos are rather mysterious particles– They are nearly massless– They hardly ever interact with anything

• Yet they are essential to how the Universe works– Key element of the nuclear reactions in the Sun– Crucial to how Supernovas explode– May help explain why the Big Bang made more matter than

antimatter

• We have learned a lot about neutrinos since they were postulated >80 years ago ... many surprises along the way.

• There is still a lot to learn, and new experiments are poised to make new (and perhaps surprising) discoveries.


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