1 Neutrino Physics 2 Pedro Ochoa May 22 nd 2006
2 What about solar neutrinos and the solar neutrino problem? KamLAND uses the entire Japanese nuclear power industry as a longbaseline source Kashiwazaki Takahama Ohi Kamland is an experiment which studies the disappearance of reactor neutrinos
3 In a fission reactor, there is a flux of associated with 235 U, 239 Pu, 241 Pu and 238 U that you can predict to a good accuracy.
4 You can detect these antineutrinos via inverse beta decay:
5 How the detector looks from the inside
6 So if there are oscillations, this spectrum will be distorted: At Kamland’s average L of about 180 km, the disappearance probability in the three neutrino model is, to a very good approximation: Notice similarity with 2 flavor approx.
7 Oscillations were observed indeed (2002)!! Kamland was actually the first experiment to observe the disappearance of “earthly” electron antineutrinos Other experiments hand’t seen anything (they were too close) Conclusive evidence of reactor disappearance: ( C.L.) Best fit values: The solar neutrino problem was finally solved !
8 III. Open Questions What are some of the unsolved problems in Neutrino Physics? Everything fits the model extremely well ! Things make sense in the light that neutrino mass eigenstates mix with the weak eigenstates creating the oscillation phenomenon measured in many experiments. We think that: Well, almost…. there is always a black sheep !! We know that neutrinos have mass !!!!!
9 Baseline ~30 m Neutrino Energy MeV, Let’s first discuss “The LSND Anomaly” (1995) detect prompt e track, 20<E e <60 MeV (+ scintillation) Oscillations? Search for through neutron capture: 2.2 MeV scintillation signal, 186µs later The Liquid Scintillator Neutrino Detector Experiment: Stop at Cu target Beam of protons on water produces π + mainly
10 The interior of the LSND detector:
11 Through they observed: excess of: oscillation probability: Do you see a problem with this picture? What the LSND experiment saw: Yes !! Only two independent m 2 if three neutrinos !!
12 How to explain the LSND anomaly? 1) There are more neutrinos : But LEP showed that there are three active (i.e. that interact with the Z) neutrino flavors only… the extra neutrinos would have to be sterile ! 2) CPT Violation (in other words, ): 3) Some weird combination (CPT + 1 sterile neutrino, sterile neutrino decay…) Before it could all nicely fit in a spectrum like But now it would have to be something like, which is unlikely.
13 Other experiments have ruled out parts of the LSND allowed region: green=unexplored The MiniBoone experiment at Fermilab will be able to put this issue to rest D=12m 800 tons of mineral oil If MiniBoone finds a signal new exciting physics !!
14 Let’s change topics now. Earlier I said that: What parameter am I leaving out? !!! Is it zero or just very small? Nobody knows… U is directly related to whether or not there is CP violation in the neutrino sector!
15 Best limit on θ 13 comes from a reactor experiment called CHOOZ: m 2 = eV 2 At their baseline (~1km): MINOS will actually expand that limit (or discover θ 13 ): At MINOS baseline (~735 km):
16 NOVA (NuMI Off-Axis Experiment) will be able to assess this much better: But remember: By going off-axis we can get more neutrinos in the energy region where we’re more interested. At 14mrad the spectrum peaks just above the first oscillation maximum. Use the same NuMI beam that used for MINOS !! Given the current limit of θ 13 set by CHOOZ, the oscillation probability cannot be larger than 5%. This is why to study these oscillations we need a monster detector.
17 What will be achieved: NOVA may also address one of the biggest puzzles in neutrino physics: What is the right hierarchy? (note: we know m 2 21 > 0 from solar neutrinos)
18 This would be achieved through something called Matter Effect: The basic concept is that electron neutrinos, besides oscillating in the usual way, can interact with the electrons in rock while they propagate: But this will not happen for This creates a small difference in the probability of seeing vs. The direction of this effect depends on the mass hierarchy. This effect must be disentangled from possible CP violation, which also implies that
19 Besides the hierarchy, there’s something about neutrino masses we still don’t know: ?? We don’t know the absolute scale of the neutrino masses !! There is actually a way of searching for this directly…
20 Endpoint energy E=18.57keV Neutrino mass should affect the spectrum of tritium decay: An experiment called KATRIN (Karlsruhe Tritium Neutrino Experiment) in Germany will look for this effect.
21 KATRIN will be able to measure the neutrino mass down to 0.2eV (90% CL). Observing this effect is a major technological challenge. The way they’ll do it is with a “MAC-E-Filter”: The beta electrons are transformed into a broad beam of electrons flying almost parallel to the magnetic field lines. Because of the electrostatic potential, all electrons with enough energy to pass the barrier will make it to the detector. Varying the E-field allows to measure the beta spectrum in a integrating mode.
22 The other way is through Neutrinoless Double Beta Decay (denoted 0vββ): 2 neutrino Double Beta Decay is actually known to exist and allowed by the Standard Model: But 0vββ has not been seen (convincingly at least): What is the condition for this to happen? That neutrinos are their own antiparticle !! (i.e. they are Majorana Particles) If 0vββ is observed then we’d know for sure that No piece of cake. First calculated in 1935 by M. Goeppert-Mayer, and first observed in 1987 (any ideas why so hard to observe?)
23 The 2vββ decay is a major background for the 0vββ search. It is a very hard measurement !!T 1/2 (2vββ) ~ years T 1/2 (0vββ) ~ years.
24 If 0vββ is observed, would that tell us something about the neutrino mass? Yes !! The effective neutrino mass (due to mixing) can be disentangled: Remember: electron neutrino has no definite mass Calculating the matrix elements is no picnic, and many authors disagree among themselves: Called the Matrix Element Several 0vββ candidates. Each experiment uses a different one (and different technique) Rodin et al, nucl-th/
25 We cannot go through all the proposed experiments that are going to try to measure 0vββ. However, you should know that somebody claims to have observed it already. Most sensitive experiments to date are based on germanium 76. This is one of them. Note that only a subset of the Heidelberg-Moscow collaboration claims the observation of 0vββ. The collaboration actually split over this.
26 Would you buy this? Other experiments will look in the same region to confirm/disprove it.
27 SUMMARY & CONCLUSIONS Neutrino physics is a field full of surprises, and with plenty of room for discovery !!