Neutrinos in Minnesota

1. Peter Litchfield University of Minnesota Colloquium 12th October 2005 735 km Neutrinos in Minnesota

2. A Short History of the Neutrino • 1930 Pauli proposes the neutrino to explain  decay. • The first example of the particle physicist’s habit of proposing new particles to explain any new phenomenon. • e (1956), (1962), (1975) discovered • 1992 LEP says that there are only three light neutrinos with standard model interactions • 1975-1998 Neutrinos are boring, distinguished mainly by what the do not have • No mass, no strong interactions, no electromagnetic interactions, no right handed interactions • The standard model of particle physics is built with zero mass, left handed weakly interacting neutrinos.

3. A Short History of the Neutrino • 1957 Pontecorvo shows that if neutrinos have mass and more than one species (flavour) exists they may oscillate from one flavour to another as they travel through space. However there is no evidence that this happens. • 1968 The first fly in the ointment is Ray Davis’s observation of a deficit of neutrinos from the sun but nobody believes this is due to neutrinos having mass and oscillations. • ~1980 Grand Unified Theories predict that proton decay may exist at measurable rates. An industry of large underground detectors is born. • 1988 Proton decay is not discovered but the IMB experiment notes that they find fewer of their background  interactions than predicted. This is followed by Kamiokande and Soudan 2, but nobody believes that it is due to neutrino oscillations.

4. A Short History of the Neutrino • 1998 Super-K at last has unequivocal evidence for atmospheric neutrino oscillations by detecting the a difference between the rate of upward and downward going neutrinos, later confirmed by Soudan 2. • 2002 SNO detects the predicted rate of neutral current interactions of solar neutrinos, the solar standard model is correct and neutrinos are oscillating • Today Everybody believes that neutrinos have mass and oscillate. The first physics “beyond the standard model”

5. Neutrino Phenomenology • We assume that there are three neutrinos (if there are four and LSND is right, things are more complicated yet) • Neutrinos can be described as eigenstates of flavour (e,,) or of mass, they are not necessarily the same. • The flavour eigenstates (e,,) are a mixture of the mass eigenstates (1,2,3) • When they are produced neutrinos are eigenstates of flavour, e.g. • When neutrinos propagate they do so as the mass eigenstates • This is what produces neutrino oscillations

6. +e+  1 2 3 Time t Distance L later Why do  Oscillate? • Quantum mechanical phenomenon, not a special property of neutrinos • Initial state has pure flavor, e.g.  • But is a mixture of mass states • Each mass state has the same initial energy but different mass, therefore different velocity • After traveling some distance the particle wave packets will have changed phase • Now a different mixture of mass states • Therefore a different mixture of flavor states

7. Three Neutrino Phenomenology • The Matrix U can be decomposed into three submatrices with elements which are the sines and cosines of 3 angles 12, 13, 23 and a phase  (responsible for CP violation) MINOS NOA

8. Two neutrino oscillations • At short distances (~100s of kilometers) the atmospheric data says that to a good approximationoscillate to  as though there were just two mass states m2 and m3 • Quantum mechanics says after oscillation the probability of a remaining a  is • Where L is the distance traveled • E is the neutrino energy • m232 is the mass squared difference (m32-m22) • sin2223 defines the amplitude of the oscillations • The full formalism for three neutrinos is more complicated (see later)

9. What we know today Solar e oscillate to  ( SNO, Super-K, Kamland, GNO) Atmospheric  oscillate to  not to e (Super-K, K2K, CHOOZ) 3 is an approximately equal mixture of  and  with only a small, as yet unmeasured, amount of e the compositions of 1 and 2 are well determined from the solar data We know nothing of the sign of m2 or of 

10. THE fundamental particles Quarks Leptons Why do we want to know? • The standard model is very successful BUT • Why this set of fundamental particles? • Why do they have these masses? • Why are they mixed together in the way they are? • The best guess is that at very high energies they are governed by a fundamental symmetry which is broken at low energies. The patterns of the breaking may give clues to the underlying symmetry • CP violation in the weak interaction may be the origin of the matter-antimatter asymmetry of the universe. • Neutrinos may give the clue leading to the theory of everything!!

11. MINOS NOA The Experiments • Verification of the oscillation model (MINOS) • Better determination of parameters (MINOS) • Detection of →e (13) (MINOS, NOA) • Determination of sign of m2 (NOA) • Observation of CP violation () (NOA?)

12. Receipe for a Neutrino Experiment • First make your neutrino • Neutrinos are produced in the weak decay of particles, , , K, n • Neutrinos do not interact very often, therefore we need to make a lot of particles, particularly  • The Fermilab New Main Injector (NuMI). 120 GeV, high intensity injector for the collider complex. Currently the most powerful accelerator in the world in terms of the energy delivered to a target and thus the number of secondary particles produced.

13. 1000m Protons | p + + K +  +  |  +  |  The Fermilab Neutrino Beam • Need to make a beam directed at your detector • Neutrinos are neutral and don’t interact, they cannot be focused • Produce secondary particles (,K mesons) by the proton beam hitting a target • Focus the secondary particles into a beam, then when they decay the neutrinos will follow approximately the beam path. • Allow the secondary particles to decay • The decay length of a 10 GeV  is 560m. Need a long decay volume. • Finally need an absorber to get rid of everything in the beam except neutrinos.

14. Beam Components

15. The Fermilab Neutrino Beam • Focusing horns • Attempt to produce a parallel beam of secondary particles • Produce a very high magnetic field between inner and outer conductors • Arranged such that particles produced at a large angle see the most field and are thus bent most towards the beam axis • Very high currents ~200kA, therefore pulsed • Selects one sign of secondary and thus produces a mostly neutrino or anti-neutrino beam

16. Recipe for a Neutrino Experiment • Next catch your neutrino • Neutrinos don’t interact very often • Number of interactions is proportional to the number of nucleii in your detector. • Need a very massive detector to give enough interactions. • Want to detect and measure the directions and energies of the outgoing particles in a neutrino interaction. • Need a magnetic field to measure the outgoing particle momenta by curvature. • Need fine segmentation to give accurate determination of particle trajectories. • Fine segmentation and a lot of mass are very expensive • Need to have a detection system that collects information from a large volume cheaply.

17. Recipe for a Neutrino Experiment • Observe Oscillations • Measure the composition of the beam in a detector at Fermilab where the beam is produced (Near detector). • Beam is intense and narrow, the detector can be relatively small but must be able to distinguish interactions produced in the same beam burst. • Allow the beam to propagate to Minnesota where the composition is measured again (Far detector). • Beam is broad (~km) and weak, the detector must be as large as we can afford to give sufficient events • If the composition is different the neutrinos have oscillated • Observe oscillation structure in the energy distribution

18. U.K. Russia U.S.A. Greece France Brazil The MINOS Collaboration 175 physicists from 31 institutes in 5 countries Argonne – Athens – Brookhaven – Caltech – Cambridge – Campinas – Fermilab – College de France – Harvard – IIT – Indiana – ITEP Moscow – Lebedev – Livermore – Minnesota, Twin Cities – Minnesota, Duluth – Oxford – Pittsburgh – Protvino – Rutherford Appleton – Sao Paulo – South Carolina – Stanford – Sussex – Texas A&M – Texas-Austin – Tufts – UCL – Western Washington – William & Mary - Wisconsin Minos collaboration members at Fermilab with the NearDetector surface building in the background (right)

19. MINOS Timeline • The Soudan 2 collaboration had the first thoughts of a long baseline neutrino experiment at Soudan around 1989. The detector would be Soudan 2. • But the Main Injector was still years in the future • However it became obvious that Soudan 2 was too small and the MINOS collaboration formed in 1994 to design and construct a new bigger detector • Final approval was given in 1998 and construction started • The far detector was completed in 2002 and started collecting data on atmospheric neutrinos and cosmic ray muons • The near detector and the beam were completed at the end of 2004 • First data March 2005 • The experiment is now running smoothly, first results next year. • You need your health and strength and lots of patience

20. MINOS Technology • The MINOS active element is a solid scintillator strip 4.1x1x800 cm3 • Emits a flash of light when traversed by a particle • Photons are absorbed in a fiber glued to the strip • Photons are re-emitted at a different wavelength and propagated along the fiber by total internal reflection • Photons are detected by a multi-anode photomultiplier • 8 fibers go to each of 16 pixels, each photomultiplier reads out 40m3 of detector

21. MINOS Far Detector • 192 strips, making a 8mx8m hexagon of active detector are sandwiched between 2.5cm steel plates • The steel • acts as a target for the neutrinos • Has a toroidal magnetic field produced by a coil passing through the center to measure outgoing muon momenta • Total mass of the detector 5400 tons

22. MINOS Near Detector • The Near detector is as nearly identical as possible so that detection inefficiencies cancel between the two. But • the beam is much smaller • the rates are much higher • 1000 tons mass • Can do lots of conventional neutrino physics as well as oscillation physics • A much finer grained detector (MINERA) is planned to go in front.

23. Hadron shower m n Simulated events e e What do we expect to see? • A  interacting produces • Either a  meson, a non-interacting track, plus hadrons, which make a shower of hits in MINOS (charged current interaction) • Or a , which we don’t see, plus a hadron shower (neutral current interaction) • A e interacting produces • Either an electron, which produces a denser shower of hits, plus hadrons • Or a e plus a hadron shower • There can be 0→ in the hadron shower in neutral current events which produce electron showers and can be misinterpreted as e cc events

24. We do have real evemts • The very first Fermilab neutrino seen at Soudan • A  interacted in the rock upstream of the detector and sent a  into the detector

25. Events in the near detector • Lots of neutrinos interact every beam pulse in the near detector • The software programs have to sort them out • Already > 100,000 events obtained • They are used to understand the beam composition and properties and the analysis programs

26. What do we expect to find? • We are doing a “blind analysis” so we cannot show yet far detector data. • I showsimulated data that we expect at the end of the experiment • We will measure the ratio of the energy spectra measured at Fermilab to that measured at Soudan • If the neutrinos have oscillated there will be a deficit at Soudan which peaks at the oscillation maximum (m2) • The depth of the deficit gives sin2223

27. 3 limits for various exposures What do we expect to find? • We will search for →e, signaled by an excess of events with an electron in the final state, compared with the small (~0.5%) background of e in the beam. • MINOS is not designed for electron detection so the sensitivity only improves by approximately a factor of 2 on the current limit from reactor experiments , we need the follow-up NOA experiment…. m2=0.0025eV2 sin2213=0.067 25 x 1020 protons on target

28. 132m 1984 planes 132 m, 1984 planes Magnification of ~ 30 x Magnification of ~ 30 x Magnification of ~ 30 x Magnification of ~ 30 x 8 planes 8 planes 15.7 m, 15.7 m, 384 cells 384 cells 8 planes, 8 planes, each with 8 cells each with 8 cells 15.7 m, 15.7 m, 384 cells 384 cells What Next? • MINOS will confirm (or otherwise) that the phenomenon observed in atmospheric neutrinos is neutrino oscillations and improve the parameter measurements. • Next we want to investigate the rest of the neutrino parameters by studying in detail →eNOA • Competition: T2K experiment in the Super-K detector in Japan

29. Probability of  oscillating to e in vacuum: • P=P1+P2+P3+P4 • P1=sin2θ23sin22θ13sin2(1.27m132L/E) “atmospheric” • P2=cos2θ23sin22θ12sin2(1.27m122L/E) “solar” • P3= Jsinδsin(1.27m122L/E) • P4=Jcosδcos(1.27m122L/E) • J=cosθ13sin2θ12sin2θ13sin2θ23sin(1.27m122L/E)sin(1.27m132L/E) “atmospheric- solar interference” Formalism →e Oscillations • The P1 term involves the atmospheric oscillation length (m132) and is the dominant term • One probability, two unknowns, therefore ambiguities, need extra information to solve for all the parameters.

30. n n n e x x e Z W n e e e e Matter effects • Matter is not CP invariant, it is made up of matter, not antimatter. • Neutrinos passing through matter interact differently than anti-neutrinos • All flavours of neutrinos can interact with electrons via Z exchange, only e interact via W exchange • In matter at oscillation maximum, P1 will be approximately multiplied by (1 ± 2E/ER) where the + sign is for neutrinos with normal mass hierarchy and antineutrinos with inverted mass hierarchy. ER11GeV for the earths crust • About a ±30% effect for NuMI, but only a ±11% effect for T2K . • If sin2213 is big enough we can determine the sign of m2

31. 15.7 m 3.9 cm 6 cm How to study →e • NOA needs to detect final state electrons from interactions of oscillated e and separate them from neutral current events producing a 0→ • Needs to be less dense and more active then MINOS, so event details are revealed • Needs to have long radiation length so that  conversions are separated from the event vertex. • Get rid of the iron in MINOS and make a detector out of liquid scintillator in extruded plastic cells • Read out the cells with an APD and a looped fiber • Cheaper and more sensitive than MINOS

32. A Gigantic Experiment • It also has to be big since we know that sin2213 is small and thus the event rate low • Needs to be 30,000 tons, 6 times bigger than MINOS and much less dense • 15m x 15m x 132m • 761,856 of 3.9 x 6 x 1500cm cells, 80% active • Held together by plastic and epoxy BaBar CDF DZero CMS ATLAS at about the same scale

33. The Off-axis Beam • MINOS will tell us an approximate value of m2 in the next few months. • The NuMI beam was designed to have a broad energy spread since we did not know m2 and thus the oscillation energy, and we want to map out the oscillation spectrum as a function of energy. • Now we would like to tune the beam to the oscillation energy (~2GeV at 750-800km distances). • Maximizes the appearance event rate • Minimizes the background from high energy neutral current events • We want to be as far from Fermilab as possible to maximize matter effects and sensitivity to the sign of m2. • The NuMI beam points directly at Soudan • WE CAN USE THE SAME BEAM FOR NOA IF WE PLACE IT OFF THE MAIN AXIS OF THE BEAM!

34. The Off-axis Beam • Its all down to kinematics In the pion rest frame • and  energies determined by energy conservation In the lab frame  energy depends on the boost and the angle between the  and 

35. The Off-Axis beam • At 10 km off axis at ~800km compared to the zero degree beam we have; • ~5 times the event rate around the oscillation maximum • Very much smaller high energy component, very much reduced neutral current background • A proton driver (new high intensity injector for the Main Injector) at Fermilab could increase the rates by at least a factor of 5

36. Orr-Buyck Soudan Where should we put it? • It should be as far from Fermilab as possible • The Ash River site in Northern Minnesota is 810 km from Fermilab and 12km from the center of the beam • Also there will be a much smaller near detector on the Fermilab site

37. e  p p e-  p   - + o  + Typical events

38. 3 sensitivity to sin2213 2.5 years each of and What do we expect to find? • At the current limits on sin2213 we should obtain ~100 electron events from a three year  beam run • The limits on sin2213 are dependent on sign(m2) and  • We will be sensitive to sin2213 if it is greater than 0.01-0.02 with the current beam and 0.003-0.01 with a proton driver • Limits are comparable or better than those of the competition, the Japanese experiment, T2K

39. 95% confidence limit on sign(m2) Sign of m2 • To measure sign m2 we have to run with both neutrinos and anti-neutrinos. • Matter effects on passing through the earth are of opposite sign depending on the sign of m2 • Measure the asymmetry between the two cases. • Again the sensitivity depends on  • T2K has a much shorter baseline (295km) and thus is not very sensitive to this sign

40. CP violation () • CP violation is the weakest of the effects in neutrino oscillations • The matter effect produces an apparent CP violation • Need at least two measurements to resolve the two effects • NOA + T2K, different matter effects • Only sensitive to  if we are very lucky with the parameters AND both experiments have proton drivers • Opportunity for a SUPERNOA experiment, 100ktons of liquid argon at the second oscillation maximum?

41. 3 sensitivity to sin2213 NOA Timeline • Now, recommended approval by the PAC and great support from the Fermilab management • Now, Detector R&D ongoing • Autumn 2006, project approval • Summer 2009, start detector construction • Spring 2010, 5ktons (MINOS size) operational • Summer 2011, detector completed

42. Summary • A long term program of neutrino physics is under way in Minnesota • Soudan 2 • Confirmation of atmospheric neutrino anomaly, probably oscillations • MINOS • Measurement of oscillation parameters in → oscillations • Rule out alternative explanations of atmospheric neutrino effect • NOA • Observe →e oscillations • Measure sin2213, sign(m2), CP violation parameter  • First physics beyond the standard model, hopefully a window on the ultimate theory of everything • Minnesota is at the cutting edge of particle physics today