The physics of Mu2e Bertrand Echenard California Institute of Technology Mu2e computing review doc-db XXXXX

Bertrand Echenard - The physics of Mu2e – 5 Mar p.2 The Standard Model (SM) of particle physics incorporates our current understanding of particles and forces (besides gravity). Bosons / fermions acquire a mass via the Higgs mechanism, photon and neutrinos remain massless. Lepton number of each generation (+1/-1 for fermion/anti-fermion) is conserved, e.g.  - → e - e  +1  +1 e -1 e +1  The SM is very successful at describing a wide range of observations, but it doesn’t account for neutrino mixing, which requires massive neutrinos, and doesn’t explain other phenomena, such as dark matter or the prevalence of matter over antimatter. The Standard Model

Bertrand Echenard - The physics of Mu2e – 5 Mar p.3 Neutral lepton flavor violation (i.e. neutrino mixing) implies charged lepton flavor violation (CLFV) through neutrino mixing. However, CLFV processes are strongly suppressed in the Standard Model. For example, BR(  → e  ) < in the SM, effectively zero!!! New Physics can enhance CLFV rates to observable values. Observation of CLFV is an unambiguous sign of New Physics Charged Lepton flavor violation

Bertrand Echenard - The physics of Mu2e – 5 Mar p.4 Mu2e goal The Mu2e experiment will search for muon-to-electron conversion in the coulomb field of a nucleus  → e  with a sensitivity  better than the current world’s best limit Mu2e Goal: R μe < 6×10 CL Already a long history… Many people have searched for CLFV in muon decays Muon an independent lepton, no  → e   → e  ~ /10 -5 or two Feinberg (1958) No  → e   Two neutrinos! R.H. Bernstein, P.S. Cooper, Phys Rept. 532(2013)27

Bertrand Echenard - The physics of Mu2e – 5 Mar p.5 A wide array of New Physics scenarios can induce CLFV, either through loops or exchange of heavy intermediate particles, e.g. See for example Marciano, Mori, and Roney, Ann. Rev. Nucl. Sci. 58 M. Raidal et al, Eur.Phys.J.C57:13-182,2008 A. de Gouvêa, P. Vogel, arXiv: Supersymmetry Heavy neutrinoTwo Higgs doublet LeptoquarksCompositness New heavy bosons / anomalous coupling Loops Contact interaction Possible contributions to CLFV

Bertrand Echenard - The physics of Mu2e – 5 Mar p.6 Muon-to-electron conversion : experimental concept Mu2e will measure the ratio R  e of  → eN conversion to the number of muon captures: To do so, we need to produce muons, stop them in matter, measure the number of stopped muons and the conversion rate. Here is how we plan to do so…

Bertrand Echenard - The physics of Mu2e – 5 Mar p.7 p    …  Muon-to-electron conversion : experimental concept Experimental concept to search for muon-to-electron conversion Produce muons via protons hitting a fixed target: p + nucleus →   →     (see D. Brown’s talk)

Bertrand Echenard - The physics of Mu2e – 5 Mar p.8 Experimental concept to search for muon-to-electron conversion Produce muons via protons hitting a fixed target: p + nucleus →   →     (see D. Brown’s talk) Collect and stop low momentum muons in atoms Aluminum target for Mu2e  Muon-to-electron conversion : experimental concept

Bertrand Echenard - The physics of Mu2e – 5 Mar p.9 Experimental concept to search for muon-to-electron conversion Produce muons via protons hitting a fixed target: p + nucleus →   →     (see D. Brown’s talk) Collect and stop low momentum muons in atoms Aluminum target for Mu2e Muon cascade to K shell (~ps) firing off X rays one way to estimate the number of captures is to measure the X-ray spectrum Wait for muon to convert into electron for Al,   Al = 864 ns Signal is a mono-energetic electron (recoiling nucleus not observed)  e This sounds simple, but what happens most of the time… Muon-to-electron conversion : experimental concept

Bertrand Echenard - The physics of Mu2e – 5 Mar p.10 Muonic atom decay Nuclear capture (~61% for Al)  N →  N’* Muon decay in orbit (~39% for Al)  → e  e E e (MeV) (E conv - E e ) 5 See e.g. Czarnecki et al., Phys. Rev. D 84, (2011) 27 Al 27 Mg* p n  The Michel spectrum is distorted by the presence of the nucleus and the electron can be at the conversion energy if the neutrinos are at rest ~~XX n, XX p, XX  per capture  e  e

Bertrand Echenard - The physics of Mu2e – 5 Mar p.11 J.A. Bistirlich et al., PRC 5, 1867 (1972) Photon energy spectrum from radiative pion capture in Mg Prompt background Particles produced in primary target (pions, neutrons, antiprotons) which interact with the stopping target just after reaching it. Radiative pion capture (RPC)   N →  N’,  → e + e -   N → e + e - N’ Pion/muon decays in flight Other background Antiprotons producing pions when annihilating in the target Beam flash Cosmic rays induced,... Produce electron with energy varying slowly between 100 – 110 MeV R. Bernstein will discuss these contributions in detail later Prompt background

Bertrand Echenard - The physics of Mu2e – 5 Mar p.12 Ideal Realistic Excellent momentum resolution (~100 keV) is needed to separate signal from DIO background Momentum resolution From an ideal to a realistic experiment

Bertrand Echenard - The physics of Mu2e – 5 Mar p.13 Momentum resolution From an ideal to a realistic experiment: expected electron momentum p MeV/c Reconstructed e - momentum Tracker resolution + Energy loss in target Bremstrahlung DIO

Bertrand Echenard - The physics of Mu2e – 5 Mar p.14 Proton beam hits target Prompt background like RPC decreases rapidly Use a pulsed muon beam with a long interval between bunches to suppress prompt background (no protons between bunches.) Particles hits the stopping target , e,p Mostly DIO background Muonic Al has “long” lifetime  = 864 ns, Search for conversion Next bunch after 1700 ns Pulsed beam Must achieve an extinction = #protons out of bunches/#proton in bunch ~ or better, and must demonstrate it

Bertrand Echenard - The physics of Mu2e – 5 Mar p.15 Summary – experimental concept p     1. Produce muons  2. Collect and stop in thin Al foils e 3. Measure energy of outgoing electron 4. Look for an excess at endpoint

Bertrand Echenard - The physics of Mu2e – 5 Mar p.16 3 years running period with 1.2x10 20 protons on target per year Bottom line: Almost background free:background < 0.5 event Single event sensitivity: R  e = 2.9x (goal is 2.4x ) Typical SUSY Signal:~50 events or more for rate p MeV/c Reconstructed e- momentum Estimated background Expected sensitivity

Bertrand Echenard - The physics of Mu2e – 5 Mar p.17 Mu2e will measure muon-to-electron conversion with a single event sensitivity R  e ~ 2.5 x Mu2e is complementary to the LHC, and will either provide unambiguous evidence of Physics beyond the Standard Model, and help elucidate the nature of New Physics OR improve the current limit on R  e by four orders of magnitude, probing New Physics up to 10 4 TeV. Conclusion