Precision Neutrino Oscillation Measurements & the Neutrino Factory Scoping Study for a Future Accelerator Neutrino Complex – Discussion Meeting Steve Geer, 6 May 2005

2 Introduction Neutrino physics is exciting The main accelerator-based neutrino physics questions we want to address are: What is the rough value of sin 2 2   ? Is the Mass Hierarchy Normal or Inverted ? What is the value of  (Is there CPV in the sector) ? What is the precise value of   (Is sin   =1) ? Is three-flavor mixing the whole story ? Answering these questions will be challenging

3 Neutrino Factory Beam Properties Precisely known neutrino fluxes & spectra Precise comparison between neutrino & antineutrino properties possible e and  beams (a wealth of measurements)    e  e   50% e, 50%   -  e - e   50% e, 50%  _ _ _ _ Muon decay kinematics very well known:

4 Low Backgrounds Neutrino factories provide electron neutrinos (antineutrinos) in addition to muon anti-neutrinos (neutrinos). Backgrounds to the detection of a wrong-sign muon are expected to be at the level  background-free e   oscillations with amplitudes as small as O(10 -4 ) can be measured ! e   oscillations at a neutrino factory result in the appearance of a “wrong-sign” muon … one with opposite charge to those stored in the ring:    e  e     CC     - CC

5 Rates Many groups have calculated signal & background rates. Example: Hubner, Lindner & Winter; hep-ph/ JPARC-SK: Beam = 0.75 MW, M fid = 22.5 kt, T = 5 yrs JPARC-HK: Beam = 4 MW, M fid = 1000 kt, T = 8 yrs NUFACT: Beam = 2.6  decays/yr, M fid = 100 kt, T = 8 yrs  m 32 2 = eV 2,  m 21 2 = 3.7  eV 2, sin 2 2  23 = 1, sin 2 2  13 = 0.1, sin 2 2  12 = 0.8,  = 0 Superbeams Neutrino Factory JPARC-SK JPARC-HK Signal Background S/B 6 360

6 Measurements There is a wealth of information that can be used at a neutrino factory. Oscillation parameters can be extracted using events tagged by: a)right-sign muons b)wrong-sign muons c)electrons/positrons d)positive  -leptons e)negative  -leptons f)no leptons  2 (  + stored and  - stored) Bueno, Campanelli, Rubbia; hep-ph/ kt LAr detector, L = 7400 km, 30 GeV nu-factory with  + decays.

7 The Challenge

8 Neutrino Factory Sensitivity The Neutrino Factory provides hope that the full program (measuring  13, determining the mass hierarchy, & searching for CPV) can be accomplished if sin 2 2  13 > O(10 -4 ) ! Huber, Winter; Phys. Rev. D68, sin 2 2  km km sin 2 2  13 Mass Hierarchy CP Violation As  13  0, P( e   )  0 If sin 2 2  13 < O(10 -4 ) a NF would make the first observation of e   appearance  important test of three-flavor mixing.

9 e   and Other Channels e   and Other Channels If sin 2 2  13 > O(10 -3 ) the e   channel enables elimination of false solutions for combined NF + SB scenarios (Donini et al; hep-ph/020940, and others). The e   channel is unique to NFs  the only direct test of e   mixing. To analyze the full NF potential the e   channel must be analyzed together with right-sign & wrong-sign muons, and the two NC rates or the two e rates.

10 “Short-Term” Program YEAR sin 2 2  13 Reach (3  ) Ability to observe non-zero  13 versus time Calculations of W. Winter Within bands  varies Substantial uncertainties on time-axis … but the trend is clear

11 “Longer-Term” Program ? Calculations of W. Winter Even greater uncertainties on time-axis Need to develop a clearer picture: NF timescale ? Branching point(s) ? Alternative paths and basis for choice ? YEAR sin 2 2  13 Reach (3  )

12 Mass Hierarchy Sensitivity Figures like this can help us develop the neutrino physics “road map” It would be good to develop an agreed on list of figures and experiments to be plotted, and timelines to be used. YEAR sin 2 2  13

13 Wish-List for Study There have been a series of neutrino physics studies in Europe, Japan and the US, aimed at understanding future needs and options. Lots has been done, but there are still some questions to be nailed … for example: Is a NF needed if sin 2 2   is large ? What is the minimum NF energy that will deliver the physics (cost issue) ? How do we best test the three-flavor framework and how do we quantify the test  ? How can we best articulate the physics case for precision measurements of the neutrino parameters if sin 2 2   > O(0.01), & continuing the program if sin 2 2   < O(0.01) ?

14 Final Remarks Neutrino Factories offer great physics potential If, within our lifetimes, we want answers to the basic neutrino questions then, beyond the foreseen program we will need to make a big step in detectors and facilities. We must continue to work on articulating the physics case for this big step  plots, numbers, road-map. We also need to work on consensus within the neutrino community (proton drivers, beta beams, neutrino factories). The physics drives us (not the facility). Cost is an issue for all our desired future facilities. For NFs muon acceleration is a cost driver. The NF energy needed must be considered carefully.