Tau g-2 and beyond Lydia Beresford HEP Seminar Birmingham, 29th January 2019
Our proposal 1908.05180 Lydia Beresford � 2
… October 2018 ~ One year on τ Lydia Beresford � 3
� What is g-2? How objects interact with a magnetic field Magnetic moment: Quantifies torque experienced in � field B τ = μ × B torque magnetic moment Possessed by e.g. bar magnet, loop of current etc. Lydia Beresford � 4
What is g-2? Charged particles with spin have an intrinsic magnetic moment μ = g q 2 m S For spin 1/2 particles: ℓ ℓ γ γ μ . B ℓ ℓ g = 2 + loop corrections Dirac, 1928 Lydia Beresford � 5
What is g-2? Anomalous magnetic moment a = ( g − 2) 2 Lydia Beresford � 6
Why is it interesting? Powerful probe of new physics New particles could be in the loop Example: SUSY Scalar Lepton Dark Matter Lydia Beresford � 7
� Why is it interesting? Powerful probe of new physics Sensitive to compositeness Historical examples: proton, neutron u Neutron g-2: -5.8 Composite! → d d Lydia Beresford � 8
Why is it interesting? Fundamental test of QED -2.5 � tension σ Electron g-2: 10 -8 precision 3-4 � tension Muon g-2: 10 -7 precision σ Electron: Odom et at PRL (2006) Bouchendira et al PRL (2011) Aoyama et al 1205.5368 Parker et al Science (2018) Muon: BNL E821 hep-ex/0602035 J-PARC 1901.03047 Davier et al 1908.00921 Keshavarzi, Nomura and Teubner 1802.02995 Lydia Beresford � 9
a time evolution Lusiani (2019) μ Lydia Beresford � 10
Muon g-2 experiment @ Fermilab ‘if I were to put my money on something that would signal new physics, it’s the [muon] g-2 experiment at Fermilab’ Brian Cox Lydia Beresford � 11
Muon g-2 experiment @ Fermilab Further details Basic idea: • Inject polarised muons eB • Spin precesses around B ω a = a μ at rate related to a mc μ • Measure precession rate via decay to positron New muon g-2 results coming soon - one to watch! Lydia Beresford � 12
What about tau g-2? Mass 0.511 MeV 106 MeV 1.7 GeV Lydia Beresford � 13
� � What about tau g-2? > � by order of magnitude! m τ m μ Composite? → 280x more sensitive than � a μ δ a τ ∝ m 2 l / m 2 m 2 τ / m 2 � and � μ = 280 SUSY Martin and Wells PRD (2001) Models for electron & muon g-2 could apply here too e.g. Z’, dark photon, 2HDM … Lydia Beresford � 14
� Problem: Lifetime ~ 10 -13 s τ ~ 10 -6 s μ Can’t use same technique! → Lydia Beresford � 15
Solution: Photon collisions Lydia Beresford � 16
Solution: Photon collisions Photons from electric field surrounding electrons collide to produce new particles e p e Lydia Beresford � 17
PDG value Delphi EPJC (2004) DELPHI 2004, LEP collider e e τ Cross section sensitive to τ moments e e Photo production of tau pairs Lydia Beresford � 18
Disgression Turning light into matter major goal in laser physics Lydia Beresford � 19
Disgression Breit Wheeler process Real photon, virtual @ collider Turning light into matter e major goal in laser physics e Proposal paper Lydia Beresford � 20
Disgression Lydia Beresford � 21
PDG value Delphi EPJC (2004) DELPHI 2004, LEP collider e e τ Cross section sensitive to τ moments e e Photo production of tau pairs Lydia Beresford � 22
� � � � a exp Delphi EPJC (2004) Sibling Rivalry τ a theory Eidelman, Passera hep-ph/0701260 τ Tensions seen for electron & muon, what about the tau? Beresford & Liu 1 � 2 � Existing measurement a e Harvard06 (error bar × 10 9 ) Theoretical prediction a µ BNL06 (error bar × 10 6 ) PbPb → Pb( �� → ⌧⌧ )Pb (this work) LHC √ s NN = 5 . 02 TeV a ⌧ DELPHI04 SM a pred (error bar × 10 4 ) ⌧ − ⌧ B ⌧ Λ = 140 GeV Λ = 250 GeV − 0 . 06 − 0 . 05 − 0 . 04 − 0 . 03 − 0 . 02 − 0 . 01 0 . 00 0 . 01 0 . 02 a ` = ( g ` − 2) / 2 a exp = − 0.018 (17) τ a theory = 0.00117721 (5) τ Lydia Beresford � 23
Can we beat it? Many interesting proposals for future LHeC/Fcc-he Belle II . Köksal 1809.01963 Eidelman et al 1601.07987 Gutiérrez-Rodríguez et al 1903.04135 Chen, Wu 1803.00501 Bent crystal CLIC/ILC/Fcc-ee Fomin et al 1810.06699 Koksal et al 1804.02373 Howard et al 1810.09570 Fu et al 1901.04003 HL-LHC Galon, Rajaraman and Tait 1610.01601 Lydia Beresford � 24
What can we do right now? Lydia Beresford � 25
The LHC is also a photon collider Lydia Beresford � 26
The LHC is also a photon collider p p Pb p p Pb Pb ATLAS 13 TeV 8.16 TeV 5.02 TeV s ~140 fb -1 ~170 nb -1 ~2 nb -1 ℒ ∝ Z 2 ∝ Z 4 σ - Z = 82 for Pb Lydia Beresford � 27
The LHC is also a photon collider p p Pb p p Pb Pb ATLAS 5.02 TeV 13 TeV 8.16 TeV s ~140 fb -1 ~170 nb -1 ~2 nb -1 ℒ ∝ Z 2 σ - Z 4 ∝ Z = 82 for Pb Lydia Beresford � 28
Head on PbPb collision Lydia Beresford � 29
Ultra Peripheral PbPb collision electron muon Pb+Pb, 5.02 TeV Run: 365914 Event: 562492194 2018-11-14 18:05:31 CEST All calo cells with E T > 500 MeV shown Lydia Beresford � 30
� Ultra Peripheral PbPb collision Ultra Peripheral PbPb collisions Super clean with ~ 0 pile-up One month to gather dataset Low trigger thresholds � Trigger on soft taus! → Quantify potential using MC → MG with modified photon flux + Pythia + Delphes (ATLAS) Lydia Beresford � 31
� Aguila, Cornet and Illana PLB (1991) Di-tau Production Beresford, Liu 1908.05180 LHC PbPb Pb Pb Ze ⇡ ± ⌧ ⇡ 0 � τ ⌫ ⌧ ⌧ ⌫ ⌧ � τ ⌫ ` � a ⌧ ` Ze Pb Pb Photo production of tau pairs Not yet observed @ LHC Lydia Beresford � 32
� � � Tau decays 46% 35% 19% 1 prong Leptonic 3 prong τ ± → π ± ν τ τ ± → l ± ν l ν τ τ ± → π ± π ∓ π ± ν τ + neutral � ’s π + neutral � ’s π Tau is only lepton that can decay into hadrons Lydia Beresford � 33
Tau decays Di-tau pair H1 detector @ HERA Lydia Beresford � 34
Backgrounds Generated: Pb Pb Ze ⇡ ± ⌧ ⇡ 0 � ℓ , q ⌫ ⌧ ⌧ ⌫ ⌧ � ℓ , q ⌫ ` � a ⌧ ` Ze Pb Pb Lydia Beresford � 35
Signal Regions Need low p T : e, mu, track > 4.5, 3, 0.5 GeV Signal Regions (SRs) 1 � + 1 track ℓ 1 � + 2 track ℓ 1 � + 3 track ℓ Lydia Beresford � 36
Background mitigation 1 � + 1 track SR Keep ℓ And veto � & � masses J / ψ Υ Lydia Beresford � 37
Setting constraints SM effective field theory for modified moments (& SM signal) 10 1 − PbPb Pb( )Pb, s = 5.02 TeV, 2.0 nb → γ γ → τ τ LB JL SR1 l 1T 1 Sample (Yield) , a =0, d =0 (1.3e+03) τ τ δ δ τ τ , a =0.02, d =0 (1.8e+03) τ τ δ δ τ τ , a =-0.02, d =0 (1.3e+03) τ τ δ δ Entries τ τ , a =0.0, d =0.015 (2.9e+03) τ τ δ δ τ τ − 1 10 , a =0.0, d =-0.015 (2.9e+03) τ τ δ δ τ τ Unit normalised 2 − 10 3 − 10 0 2 4 6 8 10 12 14 16 18 20 p (l ) [GeV] 1 T Modifying moments alters shape of lepton p T Lydia Beresford � 38
� Putting it all together: a τ χ 2 Assume observe SM & quantify constraint using � 1 − s = 5.02 TeV, 2 nb , δ d = 0.0 , ζ = ζ = 0.1 τ s b 10 LB JL 8 SR1 l 1T, SR1 l 2T Assuming 10% and SR1 l 3T combined systematic SR1 l 1T p [ 6],[>6] GeV, ∈ ≤ 6 l T SR1 l 2T and SR1 l 3Tcombined 2 χ Σ χ 2 for all SRs 95 % CL 4 (orthogonal) 2 68 % CL 0 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 0.01 0.02 − − − − − − − a δ τ Shape analysis strengthens constraints :) Split 1 � + 1 track SR @ 6 GeV � Coarse shape analysis ℓ → Lydia Beresford � 39
Putting it all together: a τ Beresford & Liu 1 � 2 � Existing measurement a e Harvard06 (error bar × 10 9 ) Theoretical prediction PbPb → Pb( �� → ⌧⌧ )Pb (this work) a µ BNL06 (error bar × 10 6 ) LHC √ s NN = 5 . 02 TeV PDG a ⌧ DELPHI04 a ⌧ 2 nb − 1 , 10% syst LHC PbPb a ⌧ 2 nb − 1 , 5% syst potential a ⌧ 20 nb − 1 , 5% syst SM a pred (error bar × 10 4 ) ⌧ Λ = 140 GeV Λ = 250 GeV SMEFT a pred , C ⌧ B = − 1 ⌧ − 0 . 06 − 0 . 05 − 0 . 04 − 0 . 03 − 0 . 02 − 0 . 01 0 . 00 0 . 01 0 . 02 a ` = ( g ` − 2) / 2 Surpass DELPHI … or discover tension! Lydia Beresford � 40
� Also sensitive to tau EDM How objects interact with an electric field EDM = Electric Dipole Moment τ = d × E torque electric dipole moment Possessed by e.g. water (polarised molecule) d = q x EDM tells us - about charge d x distribution + Lydia Beresford � 41
Why are EDMs interesting? Further details Non-zero EDM � CP violation! → assuming CPT conserved d d Reverse time S S EDM tiny in SM, observation = New Physics! Lydia Beresford � 42
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