Lecture 18. Neutrinos. Part IV Neutrino mass and Sterile Neutrinos

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1 Neutrinos Part IV Neutrino mass and Sterile Neutrinos

2 Measuring the Neutrino Mass Recall the beta spectrum You solved for massless neutrinos (in exam I) But neutrino mass carries away some energy Reduces maximum beta energy But beta decay produces electron neutrinos Which mass affects this? All of them! 2

events at end point Need extremely strong source Strongest tritium source in the world

3 KATRIN Experiment Measure beta energy from tritium decay Look at very end of spectrum Measure gap between end point and Q value Difference is the neutrino mass Very few events at end point Need extremely strong source Strongest tritium source in the world (25 g of tritium) Select end point energy with 10 m spectrometer Sensitive to 0.2 ev neutrino mass 3

4 KATRIN Spectrometer 4

KATRIN Experiment Tritium source Windowless gaseous source Extract betas without losing energy 10 m tube with 90 mm diameter column of tritium Tube held at 27 K Continuously inject and pump out

5 KATRIN Experiment Tritium source Windowless gaseous source Extract betas without losing energy 10 m tube with 90 mm diameter column of tritium Tube held at 27 K Continuously inject and pump out gaseous tritium Cycle through purifier in closed loop Beta electrons guided out of tube by strong B field (3.6 T) Separating the electrons from the tritium Electrons guided by B fields Guided around corners to block line of sight from source Series of pumping stations to remove tritium Reduce tritium by 12 orders of magnitude Saturates with tritium after 60 days Must be warmed to remove tritium Then repeat 5

6 KATRIN Spectrometer Filter off all but the end point energy betas MAC-E Filter (Magnetic Adiabatic Collimation with an Electrostatic Filter) Convert electron energy from cyclotron motion to linear motion Conservation of magnetic moment Include a retarding electric potential Only electrons above threshold will pass Measure electron energies at other end MAINZ experiment set limit of mv < 2 ev 6

7 How to determine the neutrino mass Direct mass measurement approaching oscillation measurements 7

8 How Many Types of Neutrinos? The SM includes three neutrinos One for each lepton flavor Of course, the mass eigenstates are different There are still only three How do we know this number? LEP measured width of Z resonance Width sensitive to number of light neutrinos Neutrinos broaden peak Data shows 3 light neutrinos What about heavy neutrinos? This measurement is sensitive up to half the Z mass 3 neutrinos below 45 GeV Says nothing about heavier neutrinos Only measures weakly interacting neutrinos Sterile neutrinos could exist Don't couple to Z 8

9 Sterile Neutrinos Non-interacting neutrinos Do not participate in SM interactions e.g don't couple to W/Z May mix with 3 known neutrinos Only accessible through oscillations Replace PMNS Matrix with 4X4 version 3 additional mixing angles One possibility: Right Handed Neutrinos No weak interactions Naturally expected because electrons are both left and right handed Seesaw mechanism would yield such a neutrino 9

Searches for Sterile Neutrinos Look for disappearance at short length scale Oscillations scale with Larger Δm2 gives smaller length scale Size of mixing angle gives size of

10 Searches for Sterile Neutrinos Look for disappearance at short length scale Oscillations scale with Larger Δm2 gives smaller length scale Size of mixing angle gives size of oscillation Daya Bay and Double Chooz see too few reactor neutrinos Could be due to oscillation to sterile neutrino Could also be due to uncertainty in reactor nuclear physics 10

Particle Physics WS 2012/13 ( )

Particle Physics WS 2012/13 (29.1.2013) Stephanie Hansmann-Menzemer Physikalisches Institut, INF 226, 3.101 Content Today Short Reminder on Neutrino Oszillation Solar Neutrino Oszillation and Matrial Effect