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Mu2e Experiment at Fermilab: Finding a Needle in a Trillion Haystacks

This seminar provides an overview of the Mu2e experiment at Fermilab, which aims to study muon-to-electron conversion. This charged lepton flavor violating process is strongly suppressed in the Standard Model, but could be enhanced in new physics scenarios. The seminar discusses the experimental signature, Fermilab's accelerator complex, and the future Project X. It concludes with the importance of muon-to-electron conversion as a powerful probe of new physics.

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Mu2e Experiment at Fermilab: Finding a Needle in a Trillion Haystacks

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  1. The Mu2e experiment at Fermilab Finding a needle in a trillion haystacks Bertrand Echenard California Institute of Technology HEP seminars – April 2013

  2. Overview The physicsCharged lepton flavor violation andmuon-to-electron conversion The signalExperimental signature The experimentFermilab Accelerator complex and the Mu2e experiment The futureProject X Conclusion CDR PICTURE http://mu2e.fnal.gov/

  3. Mu2e and charged lepton flavor violation • The Mu2e experiment will study muon-to-electron conversion in the coulomb field of a nucleus: mN→ eN • Muon-to-electron conversion is a charged lepton flavor violating process (CLFV). • These reactions are strongly suppressed in the Standard Model. For example, BR(m→eg) ~10-52 in the SM, effectively zero!!! g W m e nm ne

  4. Mu2e and charged lepton flavor violation • The Mu2e experiment will study muon-to-electron conversion in the coulomb field of a nucleus: mN → eN • Muon-to-electron conversion is a charged lepton flavor violating process (CLFV). • These reactions are strongly suppressed in the Standard Model. For example, BR(m → eg) ~ 10-52 in the SM, effectively zero!!! • New Physics could enhance CLFV rates to observable values ~ li ~ lj Observation of CLFV is New Physics e m g q q c0 ~

  5. CLFV Landscape Conversion mN→eN Lepton decays t→ lg, t→ lll,t→ lh m → eg, m→ eee Fixed target eN → mN eN → tN CLFV Meson decays K, f, …, B → ll’ Collider (LHC) Z,Z’ → ll’ SUSY, Exotic,… CLFV processes are a powerful probe of New Physics, complementary to direct searches at colliders

  6. Already a long history… Many people have searched for CLFV in muon decays Muon an independent lepton, no m→eg m→eg ~ 10-4/10-5 or two nFeinberg (1958) No m→eg Two neutrinos!

  7. Already a long history… Many people have searched for CLFV in muon decays Muon an independent lepton, no m→eg m→eg ~ 10-4/10-5 or two nFeinberg (1958) No m→eg Two neutrinos! Mu2e goal

  8. Possible contributions to CLFV BSM scenarios often induce CLFV, either through loops or exchange of heavy intermediate particles Marciano, Mori, and Roney, Ann. Rev. Nucl. Sci. 58, See also Flavour physics of leptons and dipole moments, arXiv:0801.1826

  9. CLFV and SUSY SUSY models Probe SUSY through loops If SUSY seen at LHC → rate~10-15 Implies dozens of signal events with negligible background in Mu2e for many SUSY models. ~ li ~ lj m e g q q ~ Altmannshofer, Buras, et al, Nucl.Phys.B830:17-94, 2010 c0

  10. SUSY model diagnosis L. Calibbi et al., hep-ph/0605139 SUSY GUT in an SO(10) framework mN→eN(tanb = 10) t → mg(tanb = 10) current current SuperKEKB CR(m→ e) x 1012 BR(t → mg) 107 Mu2e Project X M1/2 (GeV) M1/2 (GeV) Neutrino-Matrix Like (PMNS) Minimal Flavor Violation (CKM) Complementary with the LHC experiments and provide model discrimination

  11. SUSY model diagnosis L. Calibbi et al., hep-ph/0605139 SUSY GUT in an SO(10) framework mN→eN(tanb = 40) t → mg(tanb = 40) current current SuperKEKB CR(m→ e) x 1012 BR(t → mg) 107 Mu2e M1/2 (GeV) M1/2 (GeV) Neutrino-Matrix Like (PMNS) Minimal Flavor Violation (CKM) Complementary with the LHC experiments and provide model discrimination

  12. More SUSY model diagnosis Yaguna, hep-ph/0502014 Antusch et al.,hep-ph/0610439 CMSSM - seesaw MSSM - mSUGRA MEG m>0 m<0 BABAR Belle BR(m → eg) BR(t → mg) Several CLFV measurements or evidence from LHC allows to discriminate between different models of New Physics

  13. Combining with other CLFV channels M. Kakizaki et al., PLB566 (2003) 210 Normal hierarchy Inverted hierarchy Higgs triplet model MEG m → eee mN→eN BR Dependence on neutrino mass hierarchy and θ13 m → eg Mu3e Mu2e Ue3 Ue3 Mu2e MEG Rme in Tl Littlest Higgs with T-parity BR(m →eg) • M. Blanke et al., ActaPhys.Polon.B41:657,2010

  14. “Model independent” Lagrangian “Loop” “Contact” k = relative strength of the two terms L (TeV) • k << 1 magnetic moment type operator • m → egrate ~ 300X mN→ eNrate • k >> 1 contact interaction • mN→ eNrate >> m → egrate k A. De Gouvea

  15. Why m→ e conversion • Large potential to discover CLFVor to set very stringent bounds on New Physics • Muons are plentiful and offer the best combination of new physics reach and experimental sensitivity • Complementary to CLFV searches in m → eg, m→ eee, quark flavor and the LHC. • Can probe mass scales way beyond direct reach of LHC L (TeV) k A. De Gouvea

  16. Overview The physicsCharged lepton flavor violation andmuon-to-electron conversion The signalExperimental signature The experimentFermilab Accelerator complex and the Mu2e experiment The futureProject X Conclusion

  17. Muon-to-electron conversion m Neutrinolessmuon to electron conversion in the coulomb field of a nucleus m- N → e- N e Experimental signature: One single mono-energetic electron For N=Al, electron energy = 105 MeV Coherent process, the nucleus remains intact

  18. The conversion process m e • Muon captured by nucleus • Cascade to 1s (~ps) emitting X rays →normalization • Bohr radius ~ 20fm → muon sees whole nucleus • Conversion electron ~105 MeV (coherent process) • Measure electron energy in the detector Time scale for entire process ≤ 1 μs BUT WHAT YOU EXPECT MOST OF THE TIME…

  19. Muon decay in orbit • Muonm→ enndecay: end point of Michel spectrum is at well below the signal energy Free m→ enndecay Emax = 52.8 MeV Ee (MeV)

  20. Main background : decay in orbit m • Muonm→ enndecay: end point of Michel spectrum is at well below the signal energy • However, in the nucleus, the DIO electron can be exactly at conversion energy (up to neutrino mass) if the neutrinos are at rest. (Econv - Ee)5 e Ee (MeV) Czarnecki et al., Phys. Rev. D 84, 013006 (2011)arXiv:1106.4756v2

  21. And other backgrounds… • Radiative pion capture (RPC) p-N →g N’, g →e+e-and p-N →e+e- N’ • Radiative muon capture (RMC) μ- + A→νμ+ A’ + an+ bp+ cgwhere the photon can convert to an e+e- pair • Pion/muon decay in flight • Antiprotons: produce pions whentheyannihilate in the target • Electrons from beam • Cosmic rays Photon energy spectrum from radiative pion capture in Mg J.A. Bistirlich et al., Phys Rev C5, 1867 (1972)

  22. Current best result: Sindrum II W. Bertl et al., Eur. Phys. J. C 47, 337–346 (2006) “prompt removed” SINDRUM II at PSI Final results on Au: Rme < 7x10-13 @ 90% CL One candidate event past the end of the spectrum. Pion capture, cosmic ray? Timing cut shows the contribution of prompt background (0.3 ns muon pulse separated by 20 ns) “prompt enriched” Little time separation between signal and prompt background, this becomes problematic at higher rate.

  23. Overview The physicsCharged lepton flavor violation andmuon-to-electron conversion The signalExperimental signature The experimentFermilab Accelerator complex and the Mu2e experiment The futureProject X Conclusion

  24. How can we do better ? • Need more, more and more muons • Pulsed beam with wait period between bunches to get rid of prompt background like Radiative Pion Capture The Mu2e experiment at Fermilab

  25. The Mu2e collaboration Boston University Brookhaven National Laboratory University of California, Berkeley University of California, Irvine California Institute of Technology City University of New York Duke University Fermilab University of Houston University of Illinois, Urbana-Champaign University of Massachusetts, Amherst Lawrence Berkeley National Laboratory Lewis University Los Alamos National Laboratory Northern Illinois University Northwestern University Rice University University of Virginia University of Washington, Seattle Istituto G. Marconi Roma LaboratoriNazionale di Frascati Università di Pisa, Pisa INFN Lecce and Università del Salento GruppoCollegato di Udine Institute for Nuclear Research, Moscow, Russia JINR, Dubna, Russia ~130 collaborators and growing…

  26. Pulsed beam and prompt background target Stopped pionsvs time p,m,e 1011 reduction e- Prompt bkglike RPC e+ t (ns) p-N →g N’, g→e+e- Must wait 700-800 ns until the prompt background level is sufficiently low e- conversion

  27. Pulsed beam and prompt background Stuff arriving at stopping target Mainly Decay In Orbit (DIO) background Beam hits target Prompt background like RPC Next bunch Need a pulsed beam that let us wait this long! Fermilabhas exactly the beam we need!

  28. Fermilabmuon campus Mu2e building g-2 Mu2e • Small changes to existing accelerator complex, reuse as much as possible • Mu2e and g-2 can run this decade • Run in parallel with Nova, no interference with neutrino program • And you’re close to the cafeteria, can’t hurt in winter…

  29. Fermilab accelerator complex Fermilab ideally suited for Mu2e • Booster: batch of 4×1012protons every 1/15th second • Booster “batch” is injected into the Recycler ring • Batch is rebunched into 4 bunches • These are extracted one at a time to the Debuncher ring • As a bunch circulates, protons are extracted to produce the desired beam structure • Produces bunches of ~3x107 protons each, separated by 1.7ms (debuncher ring period) Pulse structure: width << tm(Al) with separation ~ 2 x tm(Al)

  30. Mu2e and Nova Beam structure delivered by the booster Mu2e and Nova can coexist in peace

  31. Extinction monitor • Need to have very few protons between bunches. • Requires an extinction < 10-10 (based on simulation) • - Extinction = # protons out of pulse / # protons in pulse • Extinction reached using beam formation (debuncher, momentum scraping) and AC dipole • Measure with • Thin foils in the debuncher→ Mu2e production target transport line (fast feedback) • Off-axis telescope looking at the production target (slow feedback – timescale of hours)

  32. Extinction monitor Monitor Spectrometer based on ATLAS pixel detector Reach a 10-10 extinction sensitivity in an hour or so Exit collimator Silicon sensors

  33. Mu2e overview V. Lobashev, MELC 1992 • Production Target / Solenoid (PS) • Proton beam strikes target, producing mostly pions • Graded magnetic field contains backwards pions/muons and reflects slow forward pions/muons 2.5T protons 1T 4.6T 2T • Target, Detector and Solenoid (DS) • Capture muons on Al target • Measure momentum in tracker and energy in calorimeter • Graded field “reflects” downstream conversion electrons emitted upstream (isotropic process) • Transport Solenoid (TS) • Selects low momentum, negative muons • Antiproton absorber in the mid-section

  34. Production Target Shielding Proton Target (W rods) Protons enter here (opposite to outgoing muons) Protons exit through thin window; extinction measurement follows Muons leave to transport solenoid Pions (captured in graded B field)

  35. Transport • The S shape eliminates photons and neutrons • Curvature and collimators select momentum and sign of particles • Antiproton absorber foil reduces the antiproton flux collimator Antiproton absorber Positive Negative muons

  36. Detector muons Proton Absorber Muon Absorber 1T 2T muonstoppingtarget EM Calorimeter Tracker Graded field “reflects” downstream a fraction of conversion electrons emitted upstream (isotropic process)

  37. Target • Lifetime is shorter for higher Z • RMC: E Photon < E conversion Target made of17 aluminium foils

  38. Target V. Cirigliano et al., PRD 80 (2009) 013002 • Lifetime is shorter for higher Z • RMC: E Photon < E conversion • Measure the Z dependence → underlying physics VZ Rme(Z) / Rme(Al) Vg D Ti S Pb Al Z

  39. Tracker • Tracker is made of arrays of straw drift tubes (red/blue stripes in tracker stations) • 21600 tubes (and a lot of screws) arranged in planes on stations, the tracker has 18 stations • Tracking at high radius only ensures operability (beam flash produces a lot of low momentum particles + DIO background) Stations are rotated Tracker station Tracker

  40. Straw tubes Straw tube • Characteristics: • 5mm diameter and 334-1174 mm length • 25 mm W sense wire (gold plated) at the center • 15 microns Mylar wall • Must operate in vacuum Straw tubes • Proven technology • Low mass → minimize scattering (track typically sees ~ 0.25 X0) • Modular, connections outside tracking volume • Challenge: straw wall thickness (15 mm) has never been done before

  41. Calorimeter Calorimeter composed of LYSO crystals • Provide independent measurement of energy / time / position • Particle identification • Alternative tracking, seed for track finding algorithm • Independent trigger Original proposal: four vanes, electrons hit flat faces Current proposal: two disks separated by ½ wavelength Four vanes Two disks separated by ½ wavelength Significant contribution to the calorimeter design from our group

  42. Calorimeter • Disks • Charge symmetric, can measure m- N→e+ N • Allow pion p+ → e+necalibration in situ • Use e+ from RPC to estimate background from e- in signal region without opening “the box” • Can detect photons • Better for mechanics and photo-sensors • But are • more sensitive to accidentals (neutron / photon pile-up) • The m- N→e+ N’channel can probe heavy neutrinos and leptoquarks models. Mu2e will greatly improve the existing limits!

  43. Hexagonal crystals Hexagonal cross section is a good match to an annular disk - May use boule volume more efficiently than rectangular or trapezoidal shape - Fewer crystals on average in a cluster - More light and improved uniformity, due to more efficient photon propagation Some artwork, nothing to do with calorimeter

  44. Calorimeter Full Geant4 simulation to determine the energy resolution for disks and vanes including all the known backgrounds (still work in progress) Disk Vane s= 1.2 ± 0.1 MeV FWHM/2.35 = 2.0 ± 0.1 MeV s= 1.5 ± 0.1 MeV FWHM/2.35 = 1.7 ± 0.1 MeV Crystal Ball Crystal Ball Egen-Ecluster(MeV) Egen-Ecluster (MeV) Results quite similar, accidental pile-up has small impact → disks were chosen because of all the other advantages

  45. Calorimeter Full Geant4 simulation to determine the energy resolution for disks and vanes including all the known backgrounds (still work in progress) Disk Vane s= 1.2 ± 0.1 MeV FWHM/2.35 = 2.0 ± 0.1 MeV s= 1.5 ± 0.1 MeV FWHM/2.35 = 1.7 ± 0.1 MeV Crystal Ball Crystal Ball Egen-Ecluster(MeV) Egen-Ecluster (MeV) Our group is active in the simulation of the calorimeter, reconstruction software, crystal calibration system, calorimeter construction,…

  46. Cosmic ray veto Cosmic muons can fake a conversion electron!

  47. Cosmic ray veto Cosmic muons can easily fake a conversion electron! • Solution: cosmic ray veto • Hermetic around the detector • Scintillating counters read by silicon photomultipliers • 99.99% efficiency • Excellent time resolution (< 5 ns) • Resistant to neutrons Scintillating counters read by silicon photo-multipliers

  48. A typical event (single proton pulse) 500 – 1695 ns window ± 50 ns around conversion electron Find the conversion electron… need good pattern recognition algorithm!!!

  49. Pattern recognition Pattern Recognition based on BABARKalmanFilter algorithm No significant contribution of mis-reconstructed background Momentum resolution core s~100 keV tail s~150 keV (1.1%) x-y view Fit: Crystal Ball + exponential r-z view

  50. Expected sensitivity 3 years running period with 1.2x1020 protons on target per year Reconstructed momentum assuming conversion rate 10-15 p MeV/c • Bottom line: • Single event sensitivity: Rme=2x10-17 • 90% C.L. (if no signal) : Rme<6x10-17 • Typical SUSY Signal: ~50 events or more for rate 10-15

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