Prospects for NUMI Off-axis Initiative

1. Prospects for NUMI Off-axis Initiative Kwong Lau University of Houston November 28, 2003

2. Kwong Lau Outline • Introductory Comments • Advantages of an Off-axis Beam • Important Physics Issues • NuMI Capabilities • Detector issues • Present schedule

3. Kwong Lau Introductory Comments The current generation of long and medium baseline terrestial n oscillation experiments is designed to: • Confirm SuperK results with accelerator n’s (K2K) • Demonstrate oscillatory behavior of nm’s (MINOS) • Make precise measurement of oscillation parameters (MINOS) • Improve limits on nmne subdominant oscillation • Demonstrate explicitly nmnt oscillation mode by • detecting nt’s (OPERA, ICARUS) mode, or detect it (MINOS, ICARUS) • Resolve the LSND puzzle (MiniBooNE) • Confirm indications of LMA solution (KamLAND) Many issues in neutrino physics will then still remain unresolved. Next generation experiments will try to address them.

4. Kwong Lau The Physics Goals • Observation of the transition nmne • Measurement of q13 • Determination of mass hierarchy (sign of Dm23) • Search for CP violation in neutrino sector • Measurement of CP violation parameters • Testing CPT with high precision

5. Kwong Lau Kinematics of p Decay Compare En spectra from10,15, and 20 GeV p’s • Lab energy given by length of vector from origin to contour • Lab angle by angle wrt vertical • Energy of n is relatively independent of p energy • Both higher and lower p energies give n’s of somewhat lower energy • There will be a sharp edge at the high end of the resultant n spectrum • Energy varies linearly with angle • Main energy spread is due to beam divergence EnLAB qLAB

6. Kwong Lau Off-axis ‘magic’( D.Beavis at al. BNL Proposal E-889) NuMI beam can produce 1-3 GeV intense beams with well defined energyin a cone around the nominal beam direction

7. Kwong Lau The Off-axis Advantage • The dominant oscillation parameters are known reasonably well • One wants to maximize flux at the desired energy (near oscillation maximum) • One wants to minimize flux at other energies • One wants to have narrow energy spectrum

8. Kwong Lau Optimization of off-axis beam • Choose optimum En (from L and Dm232) • This will determine mean Ep and qLAB from the 90o CM decay condition • Tune the optical system (target position, horns) so as to accept maximum p meson flux around the desired mean Ep

9. Kwong Lau nm ne transition equation P (nm ne) = P1 + P2 + P3 + P4 A. Cervera et al., Nuclear Physics B 579 (2000) 17 – 55, expansion to second order in

10. Kwong Lau Several Observations • First 2 terms are independent of the CP violating parameter d • The last term changes sign between n and n • If q13 is very small (≤ 1o) the second term (subdominant oscillation) competes with 1st • For small q13, the CP terms are proportional to q13; the first (non-CP term) to q132 • The CP violating terms grow with decreasing En (for a given L) • There is a strong correlation between different parameters • CP violation is observable only if all angles ≠ 0

11. Kwong Lau q13 Issue • The measurement of q13 is made complicated by the fact that oscillation probability is affected by matter effects and possible CP violation • Because of this, there is not a unique mathematical relationship between oscillation probability and q13 • Especially for low values of q13, sensitivity of an experiment to seeing nmnedepends very much on d • Several experiments with different conditions and with both n and n will be necessary to disentangle these effects • The focus of next generation oscillation experiments is to observenmne transition • q13 needs to be sufficiently large if one is to have a chance to investigate CP violation in n sector

12. Kwong Lau Matter Effects • The experiments looking at nm disappearance measure Dm232 • Thus they cannot measuresign of that quantity ie determine mass hierarchy • The sign can be measured by looking at the rate for nmne for both nmand nm. • The rates will be different by virtue of different ne-e- CC interaction in matter, independent of whether CP is violated or not • At L = 750km and oscillation maximum, the size of the effect is given by A = 2√2 GF ne En / Dm232 ~ 0.15

13. Kwong Lau Source of Matter Effects

14. Kwong Lau CP and Matter Effects

15. Kwong Lau Experimental Challenge

16. Kwong Lau ne Appearance: Experimental challenges • Need to know the expected flux • Need to know the beam contamination • Need to know the NC background* rejection power (Note: need to beat it down to the level of ne component of the beam only) • Need to know the electron ID efficiency

17. Kwong Lau Detector(s) Challenge • Surface (or light overburden) • High rate of cosmic m’s • Cosmic-induced neutrons • But: • Duty cycle 0.5x10-5 • Known direction • Observed energy > 1 GeV • Principal focus: electron neutrinos identification • Good sampling (in terms of radiation/Moliere length) • Large mass: • maximize mass/radiation length • cheap

18. Kwong Lau Receipe for a Better ne Appearance Experiment • More neutrinos in a signal region • Less background • Better detector (improved efficiency, improved rejection against background) • Bigger detector • Lucky coincidences: • distance to Soudan = 735 km, Dm2=0.025-0.035 eV2 • Below the tau threshold! (BR(t->e)=17%)

19. Kwong Lau NuMI Off-axis Detector • The goal is an eventual 50 kt fiducial volume detector • Liquid scintillator strips readout by APDs with particle board absorber is the baseline design • Backup design is glass RPCs • Location is 810 km baseline, 12 km off-axis (Ash River, MN) • Present cost is about 150 M$

20. Kwong Lau The off-axis detector: Stacks 28.8 m

21. Kwong Lau The absorbers

22. Kwong Lau The active detectors: scintillators

23. Kwong Lau The active detectors: WLS fibers

24. Kwong Lau The DAQ system

25. Kwong Lau CC ne vs NC events in a tracking calorimeter: analysis example • Electron candidate: • Long track • ‘showering’ I.e. multiple hits in a road around the track • Large fraction of the event energy • ‘Small’ angle w.r.t. beam • NC background sample reduced to 0.3% of the final electron neutrino sample (for 100% oscillation probability) • 35% efficiency for detection/identification of electron neutrinos

26. Kwong Lau A ‘typical’ signal event Fuzzy track = electron

27. Kwong Lau A ‘typical’ background event

28. Kwong Lau Sources of the ne background All ne/nm ~0.5% At low energies the dominant background is from m+e++ne+nm decay, hence • K production spectrum is not a major source of systematics • ne background directly related to the nmspectrum at the near detector K decays

29. Kwong Lau Sensitivity dependence on neefficiency and NC rejection Major improvement of sensitivity by improving ID efficiency up to ~50% Factor of ~ 100 rejection (attainable) power against NC sufficient NC background not a major source of the error, but a near detector probably desirable to measure it

30. Kwong Lau Sensitivity dependence on ne efficiency and NC rejection Major improvement of sensitivity by improving ID efficiency up to ~50% Factor of ~ 100 rejection power against NC sufficient NC background not a major source of the error, but a near detector probably desirable to measure it Sensitivity to ‘nominal’ |Ue3|2 at the level 0.001 (phase I) and 0.0001 (phase II)

31. Kwong Lau Off-axis potential

32. Kwong Lau Numerology: My perspective • A 20-kton detector 712 km from Fermilab, 9 km off axis will have order NCC =10,000 muon-type charged-current interactions in 5 years of running • There will be order NNC =4,000 neutral current interactions in the same exposure. • Software will suppress neutral current interactions with a rejection factor of order R=500 and an efficiency of order 30%. • There will be order 3% x NCC=300 genuine electron-type CC interactions. These events will be reduced to same order as NC fakes due to its broad energy spectrum. • Cosmic background is negligible. • The signal to noise (background fluctuation) for P=0.001

33. Kwong Lau Letter of Intent (LOI) • A Letter of Intent has been submitted to Fermilab in June expressing interest in a new n effort using off-axis detector in the NuMI beam • This would most likely be a ~15 year long, 2 phase effort, culminating in study of CP violation • The LOI was considered by the Fermilab PAC at its Aspen July, 2002, meeting

34. Kwong Lau Fermilab Official Reaction Given the exciting recent results, the eagerly anticipated results from the present and near future program, and the worldwide interest in future experiments, it is clear that the field of neutrino physics is rapidly evolving. Fermilab is already well positioned to contribute through its investment in MiniBooNE and NuMI/MINOS. Beyond this, the significant investment made by the Laboratory and NuMI could be further exploited to play an important role in the elucidation of q13 and the exciting possibility of observing CP violation in the neutrino sector. ( June 2002, PAC Recommendation) We will encourage a series of workshops and discussions, designed to help convergence on strong proposals within the next few years. These should involve as broad a community as possible so that we can accurately guage the interest and chart our course. Understanding the demands on the accelerator complex and the need for possible modest improvements is also a goal. Potentially, an extension of the neutrino program could be a strong addition to the Fermilab program in the medium term. We hope to get started on this early in 2003. Michael Witherell

35. Kwong Lau The Next Steps/Schedule • Workshop on detector technology issues planned for January, 2003 (done) • Proposal to DOE/NSF in early 2003 for support of R&D (done) and subsequent construction of a Near Detector in NuMI beam to be taking data by early 2005 • Proposal for construction of a 25 kt detector in late 2004 • Site selection, experiment approval, and start of construction in late 2005 • Start of data taking in the Far Detector in late 2007 • Formation of an international collaboration to construct a 50 kton detector

36. Kwong Lau Concluding Remarks • Neutrino Physics appears to be an exciting field for many years to come • Most likely several experiments with different running conditions will be required • Off-axis detectors offer a promising avenue to pursue this physics • NuMI beam is excellently matched to this physics in terms of beam intensity, flexibility, beam energy, and potential source-to-detector distances that could be available

37. Kwong Lau Scaling Laws (2) • If q13 is small, eg sin22q13 < 0.02, then CP violation effects obscure matter effects • Hence, performing the experiment at 2nd maximum (n=3) might be a best way of resolving the ambiguity • Good knowledge of Dm232 becomes then critical • Several locations (and energies) are required to determine all the parameters

38. Kwong Lau Important Reminder • Oscillation Probability (or sin22qme) is not unambigously related to fundamental parameters, q13 or Ue32 • At low values of sin22q13 (~0.01), the uncertainty could be as much as a factor of 4 due to matter and CP effects • Measurement precision of fundamental parameters can be optimized by a judicious choice of running time between n and n

39. Kwong Lau Sensitivity for Phases I and II (for different run scenarios) We take the Phase II to have 25 times higher POT x Detector mass Neutrino energy and detector distance remain the same

40. Kwong Lau An example of a possible detector Low Z tracking calorimeter Issues: • absorber material (plastic? Water? Particle board?) • longitudinal sampling (DX0)? • What is the detector technology (RPC? Scintillator? Drift tubes?) • Transverse segmentation (e/p0) • Surface detector: cosmic ray background? time resolution? NuMI off-axis detector workshop: January 2003

41. Kwong Lau Background rejection: beam + detector issue n spectrum NC (visible energy), no rejection Spectrum mismatch: These neutrinos contribute to background, but no signal ne background ne (|Ue3|2 = 0.01) NuMI low energy beam NuMI off-axis beam These neutrinos contribute to background, but not to the signal

42. Kwong Lau Fighting NC background:the Energy Resolution M. Messier, Harvard U. Cut around the expected signal region to improve signal/background ratio

43. Kwong Lau Scaling Laws (CP and Matter) • Both matter and CP violation effects can be best investigated if the dominant oscillation phase f is maximum, ie f = np/2, n odd (1,3,…) • Thus Ena L / n • For practical reasons (flux, cross section) relevant values of n are 1 and 3 • Matter effects scale as q132En or q132 L/n • CP violation effects scale as q13Dm122 n

44. Kwong Lau CP/mass hierarchy/q13ambiguity Neutrinos only, L=712 km, En=1.6 GeV, Dm232 = 2.5

45. Kwong Lau Kinematics Quantitatively

46. Kwong Lau Antineutrinos help greatly Antineutrinos are crucial to understanding: • Mass hierarchy • CP violation • CPT violation High energy experience: antineutrinos are ‘expensive’. Ingredients: s(p+)~3s(p-) (large x) For the same number of POT NuMI ME beam energies: s(p+)~1.15s(p-) (charge conservation!) Neutrino/antineutrino events/proton ~ 3 (no Pauli exclusion)

47. Kwong Lau 2 Mass Hierarchy Possibilities

48. Kwong Lau Two Most Attractive Sites • Closer site, in Minnesota • About 711 km from Fermilab • Close to Soudan Laboratory • Unused former mine • Utilities available • Flexible regarding exact location • Further site, in Canada, along Trans-Canada highway • About 985 km from Fermilab • There are two possibilities: • About 3 km to the west, south of Stewart Lodge • About 2 km to the east, at the gravel pit site, near compressor station

49. Kwong Lau Location of Canadian Sites Stewart Lodge Beam Gravel Pit

50. Kwong Lau Medium Energy Beam A. Para, M. Szleper, hep- ex/0110032 Neutrino event spectra at putative detectors located at different transverse locations More flux than low energy on-axis (broader spectrum of pions contributing) Neutrinos from K decays