Towards precision neutrino physics Patrick Huber Center for - PowerPoint PPT Presentation

Towards precision neutrino physics Patrick Huber Center for Neutrino Physics at Virginia Tech IPPP/NuSTEC topical meeting on neutrino-nucleus scattering Arpril 18–20, 2017, IPPP, Durham, UK P. Huber – VT-CNP – p. 1

A dangerous journey into uncharted waters. P. Huber – VT-CNP – p. 2

CP violation There are only very few parameters in the ν SM which can violate CP • CKM phase – measured to be γ ≃ 70 ◦ • θ of the QCD vacuum – measured to be < 10 − 10 • Dirac phase of neutrino mixing • Possibly: 2 Majorana phases of neutrinos At the same time we know that the CKM phase is not responsible for the Baryon Asymmetry of the Universe... P. Huber – VT-CNP – p. 3

What can we learn from that? – If we refute three flavor oscillation with significance, we have found new physics, but this requires great precision. – If we confirm three flavor oscillation with great precision, we need the context of specific models to learn anything about BSM physics. Corollary: Only if we do this precisely we really will learn something! P. Huber – VT-CNP – p. 4

The way forward Clearly, we are on Exps. Running 50% in neutrino mode 1600 the (slow) road to- T2K T2K II 1400 wards 3% measure- NOvA 2.8% T2K(II)+NOvA CD-R at our bf DUNE 1200 ments of the event Total signal events 1000 3.2% rates stat. error sin 2 θ 12 =0.304 sin 2 (2 θ 13 )=0.085 800 sin 2 θ 23 =0.452 Translating this into 3.8% δ CP =- π /2 600 ∆ m 2 21 =7.5x10 -5 eV 2 ∆ m 2 31 =2.457x10 -3 eV 2 a 3% measurements 400 5.0% of the oscillation 200 probability is very 10.0% GLoBES 2016 0 2016 2021 2026 2031 2036 difficult Note, T2HK would reach 1000 ν e signal events very quickly. P. Huber – VT-CNP – p. 5

The basic concept In order to measure CP violation we need to reconstruct one out of these P ( ν µ → ν e ) or P ( ν e → ν µ ) and one out of these P (¯ ν µ → ¯ ν e ) or P (¯ ν e → ¯ ν µ ) and we’d like to do that at the percent level accuracy P. Huber – VT-CNP – p. 6

The reality We do not measure probabilities, but event rates! � R α β ( E vis ) = N dE Φ α ( E ) σ β ( E, E vis ) ǫ β ( E ) P ( ν α → ν β , E ) In order the reconstruct P , we have to know • N – overall normalization (fiducial mass) • Φ α – flux of ν α • σ β – x-section for ν β • ǫ β – detection efficiency for ν β Note: σ β ǫ β always appears in that combination, hence we can define an effective cross section ˜ σ β := σ β ǫ β P. Huber – VT-CNP – p. 7

The problem Even if we ignore all energy dependencies of efficiencies, x-sections etc. , we generally can not expect to know any φ or any ˜ σ . Also, we won’t know any kind of ratio Φ α Φ α or Φ ¯ Φ β α nor σ α ˜ σ α ˜ or σ ¯ ˜ σ β ˜ α Note: Even if we may be able to know σ e /σ µ from theory, we won’t know the corresponding ratio of efficiencies ǫ e /ǫ µ P. Huber – VT-CNP – p. 8

The solution Measure the un-oscillated event rate at a near location and everything is fine, since all uncertainties will cancel, (provided the detectors are identical and have the same acceptance) R α α (far) L 2 α (near) = N far Φ α ˜ σ α P ( ν α → ν α ) R α N near Φ α ˜ σ α 1 R α α (far) L 2 α (near) = N far P ( ν α → ν α ) R α N near And the error on N far N near will cancel in the ν to ¯ ν comparison. Real world example: Daya Bay. P. Huber – VT-CNP – p. 9

Some practical issues • Same acceptance may require a not-so-near near detector • Near and far detector cannot be really identical • Energy dependencies will remain P. Huber – VT-CNP – p. 10

But ... This all works only for disappearance measurements! R α β (far) L 2 β (near) = N far Φ α ˜ σ β P ( ν α → ν β ) R α N near Φ α ˜ σ α 1 R α β (far) L 2 β (near) = N far ˜ σ β P ( ν α → ν β ) R α N near ˜ σ α 1 Since ˜ σ will be different for ν and ¯ ν , this is a serious problem. And we can not measure ˜ σ β in a beam of ν α . NB: Using many different event samples to constrain the interaction model requires that we have a reliable cross section model. P. Huber – VT-CNP – p. 11

Neutrino cross sections 0.5 Using current cross T2HK CPV at 3 σ section uncertainties and ∼ ∼ constraint on σ e / σ µ 0.4 ∼ σ µ @ 1% a perfect near detector. ∼ σ e @ 1% 0.3 all systematics @ default δ CP / π Appearance experiments using a (nearly) flavor 0.2 pure beam can not rely % 5 on a near detector to 0.1 2% statistics only predict the signal at the 1% GLoBES 2007 0 far site! -3 -2 -1 10 10 10 2 2 θ 13 sin PH, Mezzetto, Schwetz, 2007 Differences between ν e and ν µ are significant below 1 GeV, see e.g. Day, McFarland, 2012 P. Huber – VT-CNP – p. 12

Nuclear effects – example 140 In elastic scattering 120 a certain number of Perfect Rec., Cal. Χ 2 � dof � 0.4 � 52 80 � E miss 100 neutrons is made 50 � E miss Χ 2 � dof � 2.6 � 52 ∆ � ° � 20 � E miss Χ 2 � dof � 7.5 � 52 80 � Neutrons will be largely invisible even 60 in a liquid argon TPC 1 Σ contours � 2 d.o.f. � 40 Wide Band, L � 1300 km ⇒ missing energy 20 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 Θ 13 � ° � Ankowski et al. , 2015 We can correct for the missing energy IF we know the mean neutron number and energy made in the event... P. Huber – VT-CNP – p. 13

Theory and cross sections Theory is cheap, but multi-nucleon systems and their dynamic response are a hard problem and there is not a huge number of people with expertise working on this... Any result will contain as- sumptions, which are not based on controlled approxi- mations. P. Huber – VT-CNP – p. 14

Generators Many talks on this topic, key issues • Tremendous progress in the past years • Most of them implement very similar physics (exception GiBUU) • Tuning is a central part in this game • Once tuned, different physics models often yield same result • Tuning has to be repeated with each new data set P. Huber – VT-CNP – p. 15

Corollary: Without data generators are not reliable, ever. P. Huber – VT-CNP – p. 16

Give me a lever long enough and a fulcrum on which to place it, and I shall move the world. Archimedes, ca. 250BC P. Huber – VT-CNP – p. 17

Towards precise data Needs better neutrino sources • Sub-percent beam flux normalization • Very high statistics needed to map phase space • Neutrinos and antineutrinos • ν µ and ν e One (the only?) source which can deliver all that is a muon storage ring, aka nuSTORM. P. Huber – VT-CNP – p. 18

nuSTORM in numbers Beam flux known to better than 1% µ + µ − Channel N evts Channel N evts ν µ NC 1,174,710 ν e NC 1,002,240 ¯ ¯ ν e NC 1,817,810 ν µ NC 2,074,930 ν µ CC 3,030,510 ν e CC 2,519,840 ¯ ¯ ν e CC 5,188,050 ν µ CC 6,060,580 π + π − ν µ NC 14,384,192 ν µ NC 6,986,343 ¯ ν µ CC 41,053,300 ν µ CC 19,939,704 ¯ nuSTORM collab. 2013 Approximately 3-5 years running for each polarity with a 100 t near detector at 50 m from the storage ring P. Huber – VT-CNP – p. 19

Outlook Neutrino oscillation is solid evidence for new physics • Precision measurements have the best potential to uncover even “newer” physics – either by finding discrepancies or correlations among results • This will require unprecedented levels of accuracy in our understanding of neutrino-nucleus interactions. Are near detectors alone enough? P. Huber – VT-CNP – p. 20