A search for free n → n oscillations at t he ESS π π ? n n π π π D. Milstead Stockholm University Why baryon number violation ? Why baryon number violation ? • Baryon number is not a ”sacred” quantum number – Approximate conservation of BN in SM • ”Accidental” global symmetry at perturbative level – Depends on specific matter content of the SM • BNV in SM by non-perturbative processes –Sphalerons – B-L conserved in SM, not B,L separately. – Generic BNV in BSM theories, eg, SUSY. – BNV a Sakharov condition for baryogenesis Why n→ n ? n→ n • Theory • Baryogenesis via BNV (Sakharov condition) • SM extensions from TeV mass scales scale-upwards • Complementarity with open questions in neutrino physics • Experiment • One of the few means of looking for pure BNV • Stringent limit on stability of matter Neutron oscillations – models • Back-of-envelope dimensional reasoning: cΛ6 6 q operator for ∆B =2, ∆ L = 0⇒ δ m= QCD ⇒ M ∼ 1000 TeV n→ n M 5 • R-parity violating supersymmetry • Unification models: M ∼ 1015 GeV • Extra dimensions models • Post-sp haleron baryogenesis • etc, etc: []arXiv:1410.1100 High precision n→ n search ⇒ Scan over wide range of phase space for generic BNV + ⇒ model constai nts. Extend sensitivity in RPV-SUSY ATLAS multijet ATLAS CMS dijet CMS ESS Arxiv:1602.04821 (hep-ph) Displaced jets RPV-SUSY – TeV-scale sensitivity Neutrino physics ⇔ neutron oscillations Neutrinoless 2β -decay n→ n Eg seesaw mechanism for light ν Eg Unification models ∆L =2, ∆ B = 0, ∆L =0, ∆ B = 2, 2 ∆()B − L = 2 ∆()B − L = Neutrinoless 2β -decay ⇔ nn → linked under BL - viol ation. Eg Left-right symmetric models. Neutron oscillations – an experimentalist’s view Hypothesis: baryon number is weakly violated. How do we look for BNV ? Single nucleon decay searches, eg, p→π 0 + e + ? ⇒ L-violation, another (likely weakly) violated quantity. Decays without leptons, eg, p → π+ π , impossible due to angular momentum conservation. Nature may well have chosen BNV albeit wit h few processes to observe it. n→ n and dinucleon decay searches sensitive to BNV -only processes. Free n → n searches ⇒ cleanest experimental and theoretical approach. Previous searches for BNV and nnbar@ESS Few searches for ∆B ≠0, ∆ L = 0 Limits on τ life from all searches ∼1030 − 10 34 yrs New experiment: ∆B ≠0, ∆ L = 0 35 τ life sensitivity∼10 yrs Discovery or new stringent limit on stab ility of matter. n → n mixing formalism n ? n ∂ nEn δ m n iℏ = ∂t nδ mE n n −29 δ m= n Heff n <10 MeV = nn mixing physics 2 δ m 2 P= sin () ∆× EtEEE ; ∆=− n→ n ∆E n n Two interesting cases: 2 • Free neutron oscillation: ∆Et × ≪ 1 ⇒ P ∼ ()δ mt× • Bound neutron oscillation: ∆E × t ≫ 1 Searching with bound neutrons Nuclear disintegration after neutron oscillation n→ n n+ N n n + + π’s 2 δ m 2 P= sin() ∆ E × t , n→ n ∆E ∆E ∼ 100 MeV . 2 δ m ⇒ Suppression: <10 −60 ∆E ⇒ 8 Best current limits (SuperKamiokande) τ free > 2.5× 10 s Irreducible bg's prevent large improvements. Model-dependent (nuclear interactions). Free neutron search at ILL Institute Laue-Langevin (Early 1990's). Cold neutron beam from 58MW reactor. ∼ 130µ m thick carbon target Signal of at least two tracks with E > 850 MeV 0 candidate events, 0 background. ⇒ 8 τ n→ n >0.86 × 10 s. The European Spallation Source High intensity spallation neutron source Multidisplinary research centre with 17 European nations participating. Lund, Sweden. Start operations in 2019 . 2 GeV protons (3ms long pulse, 14 Hz) hit rotating tungsten target. Cold neutrons after interaction with moderators. The European Spallation Source ∼ 22 instruments/experiments with capability for more. Overview of the Experiment 2 Sensitivity =( free neutron flux at target )×Pn( → n) ∝ Ntn • Cold neutrons (E <5 meV, v <1000ms −1) • Low neutron emission temperature (50-60 K) • Supermirror transmission and transit time • Large beam port opt ion, large solid angle to cold moderator. ∼ 3 Increase in sensitivity for Pnn 10 compared to previous experiment (ILL) • Neutron guiding, larger opening angle, higher flux, particle ID technologies, run ning time. (4) (3) (1) (2) Neutronics (1) cold cold side Tungsten view target cold cold ESS moderators will be of “butterfly” design • Increase cold yield • Convenient beam extraction H Additional challenge for nnbar which could 2 benefit from extracting neutrons from all four visible cold surfaces Top • Conventional point-to-point focusing of a view cold neutron beam using ellipsoidal H2O ambient mirrors inefficient. • Ongoing studies on neutron optics (4) (3) (1) (2) Neutron supermirror Smooth surface Supermirror θc =Critical angle for total internal reflection θ→ m θ Ni c C m =1 m >1 Need efficient focusing and minimal inte ractions (each interaction "resets the n-clock") Commercial supermirrors Commercial supermirrors with m ∼ 7 Acceptance for straight guide ∝ m2 ILL experiment used m ∼1 neutron optics. Increase from use of focusing reflector and optimised mirror arrays. Crucial contribution to incr ease of sensitivity wrt ILL. (4) (3) (1) (2) The need for magnetic shielding n(µ ↓) nµ↓ n µ ↑ ( ) ( ) 2µ • B E B ∼ 0 n (µ ↑) Degeneracy of n, n broken in B-field due to dipole interactions: ∆EB =2µ • Flight time ≤ 1s For quasi-free condition ∆E × t ≪1 ⇒ B ≤5nT and vacuum ≤ 10−5 Pa. Shielding Magnetic shielding for flight volume • B < 5nT, P ∼ 10−5 mbar • Aluminium vacuum chamber • Passive magnetic shield from magnetizab le alloy • External coils for active compensation • Background studied by tu rning on/off B-field. Maybe shielding isn’t needed Interesting discussion in the literature. Overview of the Experiment (4) (3) (1) (2) (4) Detector Expect n+ N →∼ 5π at s ∼ 2 GeV. Cosmic Detector design for high efficiency ()ε > 0.5 veto Calorimeter and low bg ()∼ 0 . Tracker TOF • Annihilation target - carbon sheet • Tracker - vertex reconstruction Neutron Target • Time-of-flight system beam membrane - scintillators aro und tracker. Vacuum • Calorimeter - lead + scintillating and clear fibre. • Cosmic veto - plastic scintillator pads • Trigger - Track and cluster algorith ms GENIE: NNBar Final State Primaries Preliminary Final state list prepared by R. W. Pattie GENIE-2.0.0: intranculear propagation based on INTRANUKE C.Andreopoulos et al., The GENIE Neutrino Monte Carlo Generator, Nucl.Instrum.Meth.A614:87-104,2010. Final State Pionic Mode Nevents % Total π+π-2π0 530 10.60% 2π+π-π0 486 9.72% π+π-π0 417 8.34% 2π+π-2π0 409 8.18% π+π-3π0 329 6.58% 2π+2π-π0 315 6.30% π+2π0 290 5.80% π+3π0 219 4.38% π+π-ω 145 2.90% π+π0 137 2.74% π+2π-π0 132 2.64% 2π+2π- 124 2.48% 6/13/1428 A. R. Young, D. G. Phillips II, R. W. Pattie Jr. Energy Threshold Acceptance (Signal) ILL Trig. Thresh. 6/13/1429 A. R. Young, D. G. Phillips II, R. W. Pattie Jr. Annihilation event Collaboration and approximate timescales Several workshops (CERN, Lund, Gothenburg) Collaboration formed – interim spokesperson G. Broojimans Expression of Interest submitted to ESS. Signatories from 26 institutes , 8 countries. Sweden: Stockholm, Uppsala, Lund, Chalmers. More collaborators are welcome! ESS nn 2019 Commissioning, Construction, Intensity ramp, commissioning, early experiments early data-taking 2023 Initial user program Physics runs 2026 Routine operations End run, complete analysis Particle Physics Strategy Consensus in the field is to pursue experiments with unique capabilities and physics reach. Summary • The search for neutron-antineutron oscillations addresses open questions in modern physics. • An experiment at the ESS offers a new opportunity to extend sensitivity to neutron oscillation probability by several orders of magnitude and set a new limit on the stability of matter. • Collaboration formed and EOI submitted • Provisional schedule made. Potential gains Factor Gain wrt ILL Brightness ≥1 Moderator temperature ≥1 Moderator area 2 Angular 40 acceptance/neutron transmission Length 5 Run time 3 Total ≥1000 Baryon number violation searches Decay mode Partial mean life (x 10 30 yrs) Few searches for ∆B ≠0, ∆ L = 0 (RPP) τ limits ∼1030 − 10 34 yrs τ limit from new experiment ∼10 35 yrs ∆B ≠0, ∆ L ≠ 0 ∆B ≠0, ∆ L = 0 BNV searches RPP L and B violated B violated Poor experimental coverage of ”pure” B violation tests Complementary searches for BNV and LNV Single nucleon decay Neutrinoless 2β -decay n→ n Eg unification models Eg seesaw mechanism for light ν Eg Unification models ∆L =1, ∆ B = 1, ∆L =2, ∆ B = 0, ∆L =0, ∆ B = 2, 2 ∆()B − L = 0 ∆()B − L = 2 ∆()B − L = Each search tests complementary conserva tion laws. Neutrinoless double β -decay ⇔ nn → linked unde r BL- violation. Eg Left-right symmetric models. n→ n in a SUSY framework Reduced particle content: ud, + gdsbɶ ,,,ɶ ɶ ɶ '' 2 2 λ11 k m ɶɶ ɶ 1 16 g dR s R, b R τ = ; C = s () nn O 2 2 C n|1 | n 3 mgɶ mɶ mɶ ɶ dR s, b R ⇒ '' Yukawa coupling: λ11 k . '' ''⇒ '' λijk= − λ ikj λ 111 = 0 '' '' ⇒ nn steered by λ112, λ 113 . ɶ ɶ ɶ ɶ ⇒ Flavour mixin g s→ db, → d m2 dɶ sɶ , b ɶ ⇒ Mixing parameters: eg δ d =R() R R ≠ 0 ()RR 1k 2 m ɶ dR.