physics - CUWiP 2014

ν physicsCUWiP 2014

January 19, 2014

Lauren [email protected]

Caltech

1The Standard Model

I Standard Model is a theory of “almost everything"

I incredibly experimentally successful – top quark (1995), tauneutrino (2000), Higgs boson (2013) all as expected

1The Standard Model

I Standard Model is a theory of “almost everything"I incredibly experimentally successful – top quark (1995), tau

neutrino (2000), Higgs boson (2013) all as expected

1The Standard Model

I Standard Model is a theory of “almost everything"I incredibly experimentally successful – top quark (1995), tau

neutrino (2000), Higgs boson (2013) all as expected

NeutrinosThe Standard Model

I three "flavors" of neutrinos – electron (e), muon (µ), tau (τ )

I match the three flavors of leptonsI interact with weak force and gravity only; no QCDI electron neutrino at least 200,000 times less massive than

electronI consequently, nearly impossible to detect

I neutrino first theorized in 1930, not detected until 1956

I about 1011 neutrinos pass through you every secondI mean free path of a neutrino in water = 1600 light-years

I three "flavors" of neutrinos – electron (e), muon (µ), tau (τ )I match the three flavors of leptons

I interact with weak force and gravity only; no QCDI electron neutrino at least 200,000 times less massive than

I neutrino first theorized in 1930, not detected until 1956I about 1011 neutrinos pass through you every second

I mean free path of a neutrino in water = 1600 light-years

I three "flavors" of neutrinos – electron (e), muon (µ), tau (τ )I match the three flavors of leptonsI interact with weak force and gravity only; no QCD

I electron neutrino at least 200,000 times less massive thanelectron

I consequently, nearly impossible to detectI neutrino first theorized in 1930, not detected until 1956I about 1011 neutrinos pass through you every secondI mean free path of a neutrino in water = 1600 light-years

I three "flavors" of neutrinos – electron (e), muon (µ), tau (τ )I match the three flavors of leptonsI interact with weak force and gravity only; no QCDI electron neutrino at least 200,000 times less massive than

electron

I consequently, nearly impossible to detectI neutrino first theorized in 1930, not detected until 1956I about 1011 neutrinos pass through you every secondI mean free path of a neutrino in water = 1600 light-years

I three "flavors" of neutrinos – electron (e), muon (µ), tau (τ )I match the three flavors of leptonsI interact with weak force and gravity only; no QCDI electron neutrino at least 200,000 times less massive than

I neutrino first theorized in 1930, not detected until 1956I about 1011 neutrinos pass through you every secondI mean free path of a neutrino in water = 1600 light-years

NeutrinosBeyond the Standard Model

However, neutrinos are not terribly well-explained by theStandard Model.

I Neutrinos shouldn’t have mass! However, we’ve experimentallyverified that they do.

I How, if they’re so light? We have observed oscillations betweenflavor states – only possible if there are different masseigenstates.

I Solar neutrino problem – we only observe 1/3 of the expectednumber of electron neutrinos from the sun.

I Solution: while all of the neutrinos from the sun are produced aselectron neutrinos, only 1/3 of them are still electron neutrinos bythe time they reach earth.

I Neutrinos are a superposition of flavor eigenstates, but thesuperposition varies with time.

The Standard ModelOther outstanding issues with the Standard Model

I CP violation – what little CP violation we observe is not nearlyenough to explain the matter/anti-matter asymmetry of theuniverse

I What is dark matter? Cosmological data says dark matter shouldbe far more prevalent than regular matter.

I What is dark energy? Standard Model predictions of the energydensity of vacuum are 120 orders of magnitude off cosmologicalmeasurements.

It makes sense to use particles the Standard Modeldoesn’t explain to probe questions the Standard Modelcan’t answer.

Sterile neutrinosMotivations

There are yet more problems with the Standard Model forneutrinos.

I MiniBooNE and LSND detected an excess of νµ → νeappearances (Dib, Helo, Kovalenko, and Schmidt 2011).

I Reactor-based experiments (Gosgen, SRP, ILL, Bugey, etc.)have seen fewer than expected antineutrinos (Mention, Fechner,Lasserre, Mueller, Lhuillier, Cribier, and Letourneau 2011).

Solution?Add a fourth type of neutrino – one that does not interact weakly (is"sterile"). Such a particle would evade detection and could solvethese anomalies.

Is this relevant to anyone but high energy theorists?Yes! Neutrino physics has profound implications for astrophysics.

Solution?

Add a fourth type of neutrino – one that does not interact weakly (is"sterile"). Such a particle would evade detection and could solvethese anomalies.

Is this relevant to anyone but high energy theorists?

Yes! Neutrino physics has profound implications for astrophysics.

Why are neutrinos relevant?The Early Universe

Neutrinos determine elemental abundances in the earlyuniverse.

I the creation of all elements involves neutrinos!

I Shortly after the Big Bang, protons and neutrons are freelyconverted from one to the other.

I However, as the universe cools, the conversion betweenneutrons and protons slows down:

n + νe → p + e− (1)

I the rate of this reaction depends on the flux of electron neutrinosI this means the reaction rate is effectively determined by the rate

of oscillations into and out of electron flavorI all flavors of neutrinos can influence the final proton/neutron ratio

of the universe (roughly 1:7 in standard cosmology)

I the creation of all elements involves neutrinos!I Shortly after the Big Bang, protons and neutrons are freely

converted from one to the other.

I However, as the universe cools, the conversion betweenneutrons and protons slows down:

n + νe → p + e− (1)

converted from one to the other.I However, as the universe cools, the conversion between

neutrons and protons slows down:

n + νe → p + e− (1)

I the rate of this reaction depends on the flux of electron neutrinos

I this means the reaction rate is effectively determined by the rateof oscillations into and out of electron flavor

I all flavors of neutrinos can influence the final proton/neutron ratioof the universe (roughly 1:7 in standard cosmology)

n + νe → p + e− (1)

of oscillations into and out of electron flavor

I all flavors of neutrinos can influence the final proton/neutron ratioof the universe (roughly 1:7 in standard cosmology)

n + νe → p + e− (1)

Why are neutrinos relevant?Big Bang Nucleosynthesis

I During BBN, the vast majority of the neutrons are incorporatedinto alpha particles (helium nuclei).

I About one out of every 10,000 neutrons is incorporated intodeuterium nuclei.

I Lithium, tritium and beryllium are also formed (but much moreinfrequently).

I Primordial abundances determine what could be formed in theearly universe (e.g. the abundances in very old stars).

I Standard BBN (Wagoner, Fowler, Hoyle 1967) incrediblysuccessful – can use experimental bounds on primordialabundances to put bounds on new physics (see my poster!)

I All heavier elements were formed during r process, which is alsoa neutrino-related process.

Why are neutrinos relevant?Supernovae

I SNe are the other major source of nucleosynthesis – allelements heavier than beryllium are formed during core-collapsesupernovae.

I A single type II SNe emits 1058 neutrinos, which carry away 99%of the gravitational potential energy from the star’s collapse. Tounderstand SNe, we must understand neutrinos.

I We know significantly less about r process than BBN – neutrinosmay be a key part.

I Neutrinos were detected from SN1987a. Modern detectors arefar better and should detect far more. (And can serve as an earlywarning system for optical astronomers – neutrinos pass throughthe outer layers of the dying star while optical photons scatter.)

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