Search Potential for True Muonium at LHCb
Xabier Cid Vidal, Philip Ilten, Jonathan Plews, Brian Shuve, Yotem Soreq
Hadronic Contributions to New Physics Searches 2019
Paper: Discovering True Muonium at LHCb, arXiv:1904.08458
Jonathan Plews True Muonium at LHCb - HC2NP 2019 26th September 2019 1 / 11 True Muonium
∗ SM predicted QED Mixing with γ gives similar bound state of µ+µ− phenomenology to the dark photon 3 , S1 is most 2 TM 10− promising state Belle II FASER + − HPS SeaQuest e e , 98% 10 3 + TM → − LHCb µ µ− SHiP 0 LHCb D∗
4 10−
properties predicted [unitless] byTM QED [1]: ε TM 5 10− mTM = 2mµ BE 211 MeV 6 − ≈ 10− 2 −5 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 TM = α /2 2.66 10 ≈ × m [GeV] 5 τTM 6/α mµ 1.8 ps ≈ ≈ Dark photon parameter space. (Grey) excluded, (coloured) current and future analyses [1]
Jonathan Plews True Muonium at LHCb - HC2NP 2019 26th September 2019 2 / 11 Motivation
Deviations from SM predictions could indicate New Physics coupled to muons:
Dark photons, Lµ Lτ gauge bosons, scalars or ALPs − Can be tested by precision measurements: mass (via BE), lifetime, decay rates and spectroscopy TM Motivated by other experimental discrepancies in the muon sector; namely (g 2)µ[2] and proton charge radius [3] −
Jonathan Plews True Muonium at LHCb - HC2NP 2019 26th September 2019 3 / 11 Dissociation
Major difference from dark photon: can dissociate into constituent muons in material interactions TM σ( µµ) 13Z 2 b TM → ≈
100 LHCb VELO (Run-III) [4]: signal signal w/out 250 µm thick Al RF-foil Pdis background 1 200 µm thick Si sensors 10− ] 1 − Inner radius 5.1 mm [mm dr dσ 1 σ 2 10− Each material interaction has dis 90% P & 3 10− 0 2 4 6 8 10 PYTHIA 8 simulation with r (radial flight distance) [mm] material map gives 50% of + − Radial flight distance for e e signal and B expected to dissociate hadron background [1] TM Jonathan Plews True Muonium at LHCb - HC2NP 2019 26th September 2019 4 / 11 Proposed LHCb Search: Production
behaves like a dark photon, so signal rate calculated from off-shell TMphoton rate
Dominant γ∗ source: η γγ∗ → Signal cross-section estimated using values from PYTHIA 8 simulation
tot σpp = 100 mb fid Nη = 0.83 per event
Using BR(η γ ) = 4.8 10−10 [5], cross-section in fiducial region: → TM × σfid = σtot Nfid BR(η γ ) 40 pb TM pp η → TM ≈
Jonathan Plews True Muonium at LHCb - HC2NP 2019 26th September 2019 5 / 11 Proposed LHCb Search: Selection
Two proposed search strategies for Run-III (2021 - 24):
η γ γe+e− → TM→ Inclusive: Select e+e− pair with a good vertex
Reconstruct in 2σm mass window TM ee Exclusive: Same as for inclusive Also select photon
Reconstruct η meson in 2σmγee mass window
Aside: Muons from dissociation cannot be used to reconstruct produced colinearly and indistinguishable from photon conversionsTM −
Jonathan Plews True Muonium at LHCb - HC2NP 2019 26th September 2019 6 / 11 Proposed LHCb Search: Backgrounds
Potential sources of e+e− background: Heavy quark decays - B hadrons dominate, D hadrons decay outside of kinematic region Photon conversions, pair production in material - reduced by VELO material veto η e+e−γ decays - vetoed by a displacement cut → B decay backgrounds can be reduced with the following techniques: candidate is isolated, require large distance of closest approach (DOCA)TM between electrons and other tracks in the event Require opening angle, θ, between flight vector and momentum vector is consistent with zero TM
Jonathan Plews True Muonium at LHCb - HC2NP 2019 26th September 2019 7 / 11 Proposed LHCb Search: Expectation
Table 1: Expected signal and background yields in 15fb−1 of Run-III data for the ee (eeγ) final state label asi(ii), assuming 100 % final state reconstruction efficiency
(i) (i) (ii) (ii) requirement STM Btot STM Btot base 3.4 × 103 3.2 × 107 1.6 × 103 5.4 × 106 DOCA 3.0 × 103 8.5 × 106 1.3 × 103 1.1 × 106 θ 1.5 × 103 1.8 × 104 6.4 × 102 1.9 × 103 efficiency 4.4 × 10−1 5.6 × 10−4 4.0 × 10−1 3.5 × 10−4
Also have to consider efficiency of reconstructing the final state, f , currently unknown for Run-III
Make estimates based on previous analyses: ∗0 + − > 10% - from B J/ψK analysis [6] e e → + − 0.3 + − > 3% - from ratio of χc to J/ψ analysis [7] γe e ≈ e e Jonathan Plews True Muonium at LHCb - HC2NP 2019 26th September 2019 8 / 11 Proposed LHCb Search: Expectation
Significance limited by statistical uncertainty, given as r STM f σstat L−1 ≈ √Btot 15 fb 30 f is left as an unknown 25 15 fb−1 expected in 20 LRun-III ≥ ] 1 − 15 [fb
L A 5σ discovery of can 10 be made with: TM + 5 (i) e e− + f > 20% for inclusive (ii) e e−γ 0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 f > 12% for exclusive εf [unitless] Realistic improvement to Integrated luminosity for 5σ discovery as a efficiencies with triggerless function of final reconstruction efficiency, f [1] readout in Run-III
Jonathan Plews True Muonium at LHCb - HC2NP 2019 26th September 2019 9 / 11 Conclusions
30
25
20 ] 1 − 15 [fb L 10
+ 5 (i) e e− + (ii) e e−γ 0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 εf [unitless] Measurements of properties allow precise tests of SM predictions TM Deviations could be indications of new physics coupling to muons LHCb can make a5 σ discovery of during Run-III TM Paper: Discovering True Muonium at LHCb, arXiv:1904.08458
Jonathan Plews True Muonium at LHCb - HC2NP 2019 26th September 2019 10 / 11 References
[1] X. C. Vidal et al. “Discovering true muonium at LHCb”. In: Phys. Rev. D 100 (5 Sept. 2019), p. 053003. doi: 10.1103/PhysRevD.100.053003. [2] G. W. Bennett et al. “Final Report of the Muon E821 Anomalous Magnetic Moment Measurement at BNL”. In: Phys. Rev. D73 (2006), p. 072003. doi: 10.1103/PhysRevD.73.072003. arXiv: hep-ex/0602035 [hep-ex]. [3] C. E. Carlson. “The Proton Radius Puzzle”. In: Prog. Part. Nucl. Phys. 82 (2015), pp. 59–77. doi: 10.1016/j.ppnp.2015.01.002. arXiv: 1502.05314 [hep-ph]. [4] L. Collaboration. LHCb VELO Upgrade Technical Design Report. Tech. rep. CERN-LHCC-2013-021. LHCB-TDR-013. Nov. 2013. [5] Y. Ji and H. Lamm. “Scouring meson decays for true muonium”. In: Phys. Rev. D99.3 (2019), p. 033008. doi: 10.1103/PhysRevD.99.033008. arXiv: 1810.00233 [hep-ph]. [6] M. Nicol. “Analysis of the rare decay B0 → K ∗0e+e− at LHCb”. PhD thesis. Orsay, LAL, 2012.
[7] R. Aaij et al.√ “Measurement of the ratio of prompt χc to J/ψ production in pp collisions at s = 7 TeV”. In: Phys. Lett. B718 (2012), pp. 431–440. doi: 10.1016/j.physletb.2012.10.068. arXiv: 1204.1462 [hep-ex].
Jonathan Plews True Muonium at LHCb - HC2NP 2019 26th September 2019 11 / 11