Probing Higgs Yukawa Couplings with Rare Decays Birmingham HEP - PowerPoint PPT Presentation

Probing Higgs Yukawa Couplings with Rare Decays Birmingham HEP Seminar Andy Chisholm University of Birmingham 13th May 2015 This project has received funding from the European Union’s 7th Framework Programme for research, technological development and demonstration under grant agreement number 334034 (EWSB) Probing Higgs Yukawa Couplings with Rare Decays 1 / 39

Introduction - Overview Higgs Boson Yukawa Couplings ◮ What is the Higgs boson and Yukawa coupling? ◮ How can we study them through rare Higgs decays? Experimental Investigations ◮ How can we study these rare decays at the LHC? ◮ First search: Phys. Rev. Lett. 114 (2015) 121801 (arXiv:1501.03276) Discussion ◮ What have we learnt from this search? ◮ What can we expect from future studies? Probing Higgs Yukawa Couplings with Rare Decays 2 / 39

Introduction - The “BEH” Mechanism Figure from Philip Tanedo ◮ Complex scalar SU(2) doublet φ introduced to SM, “The Higgs field” (4 real d.o.f.) ◮ Then consider the symmetry spontaneously broken ◮ Potential of the field has non-zero VEV, 3 d.o.f. become Goldstone bosons ◮ Three Goldstone bosons mix with W ± , Z fields ◮ Provides gauge invariant mass terms (and longitudinal pol) to the W ± and Z ◮ The fourth d.o.f. is a scalar “Higgs” boson! Provides masses to the W ± and Z bosons! Probing Higgs Yukawa Couplings with Rare Decays 3 / 39

Introduction - Yukawa Couplings Now we have a Higgs field, “Yukawa” couplings between the Higgs and Fermion fields are possible: � ¯ � ψ L φψ R + ¯ ψ R ¯ L fermion = − y f · φψ L If φ has a non-zero VEV, expansion leads to: L fermion = − y f v y f · ¯ · h ¯ √ ψψ − √ ψψ 2 2 � �� � � �� � mass term Yukawa coupling term where h is the physical Higgs boson field... f The End Result: H ◮ Gauge invariant Fermion mass terms � g Hf ¯ f ◮ Higgs-Fermion coupling proportional to the Fermion mass ( g Hf ¯ f = m f / v ) � ¯ f While y f are still free parameters in the model, v ≈ 246 GeV is known from Electroweak measurements and we know the fermion masses... We can predict the couplings in the SM! Probing Higgs Yukawa Couplings with Rare Decays 4 / 39

σ ATLAS Preliminary Total uncertainty (obs.) ± σ µ m = 125.36 GeV 1 on H σ (exp.) In 2012, the ATLAS and CMS → γ γ H µ 0.28 = 1.17 + 0.26 obs - experiments discovered a new boson, µ 0.25 = 1.00 + 0.23 exp - with a mass of around 125 GeV → µ 0.40 H ZZ* = 1.46 + 0.34 obs - µ + 0.31 = 0.99 0.26 exp - 3 ) µ → γ γ → µ 0.24 ATLAS H WW* = 1.18 + H Signal strength ( ATLAS and CMS 0.21 → → obs - ATLAS H ZZ 4 l LHC Run 1 → γ γ CMS H µ 0.21 = 1.00 + 2.5 → → CMS H ZZ 4 l 0.19 exp - All combined → µ 0.39 + H b b = 0.63 0.37 - obs Best fit 2 68% CL µ 0.41 = 1.00 + 0.38 exp - → τ τ µ 0.42 H + = 1.44 0.37 - obs 1.5 µ 0.36 = 1.00 + exp - 0.32 → µ µ H µ 3.7 + = -0.7 - 3.7 1 obs µ 3.4 + = 1.0 exp - 3.5 → γ H Z µ 4.6 + 0.5 = 2.7 4.5 - 124 124.5 125 125.5 126 126.5 127 obs µ 4.2 = 1.0 + [GeV] m 4.2 exp - H µ 0.15 Combined + = 1.18 - 0.14 obs All subsequent measurements suggest µ 0.13 = 1.00 + 0.12 exp - compatibility with the Higgs boson of the − 1 0 1 2 3 Standard Model... -1 s = 7 TeV, 4.5-4.7 fb µ Signal strength ( ) -1 s = 8 TeV, 20.3 fb Probing Higgs Yukawa Couplings with Rare Decays 5 / 39

Higgs Yukawa Couplings - Experimental Status What do we know about Higgs couplings to: Evidence for Higgs Yukawa couplings ( H → ττ ) from the LHC! ◮ t quark: No firm evidence for t ¯ tH JHEP 04 (2015) 117 (arXiv:1501.04943) production from LHC experiments Events / bin 4 10 ◮ b quark: No firm evidence for H → b ¯ b Data µ Background ( =1.4) decays from LHC experiments, µ Background ( =0) 3 only 1 − 2 σ excesses 10 → τ τ µ H (125) ( =1.4) → τ τ µ H (125) ( =1) ◮ c quark: No direct evidence, only loose bounds from H → b ¯ 2 10 b searches ◮ u , d , s quarks: Nothing! → τ τ H 10 ◮ τ lepton: Evidence for H (125) → ττ ATLAS -1 s = 8 TeV , 20.3 fb decays from ATLAS and CMS! 1 -1 = 7 TeV , 4.5 fb s ◮ e , µ leptons: No evidence, but that -4 -3 -2 -1 0 1 suggests lepton coupling isn’t universal! log (S / B) 10 Data suggest lepton Yukawa couplings are present and non-universal... But not too much else! Probing Higgs Yukawa Couplings with Rare Decays 6 / 39

Introduction - Charm Quark Yukawa Coupling The “traditional” approach is to search 5fb � 1 � 7TeV � � 20fb � 1 � 8TeV � 3 for inclusive H → c ¯ c decays Stat. � Monte Carlo Error 2 ◮ Direct searches suffer from very large � a � backgrounds from inclusive jet 1 production Μ b 0 ◮ Recent dedicated efforts to develop 68.3 � 95 � charm tagging! (ATL-PHYS-PUB-2015-001) � 1 � e � � c � � f � ◮ Not yet applied to H → c ¯ c searches... � 2 � b � � 200 � 100 0 100 200 300 400 500 Μ c 1 Fraction of jets ATLAS Preliminary b jets † See arXiv:1503.00290 for details t simulation, p s = 8 TeV t ¯ c jets T > 20 GeV, | η jet | < 2.5 p jet Light jets JetFitterCharm ◮ Existing H → b ¯ 10 -1 b searches an be reinterpreted to include the possibility 10 -2 of anomalous H → c ¯ c production ◮ Exploit the non-zero rate of charm 10 -3 quarks mistagged as bottom ◮ ATLAS and CMS data provide 10 -4 6 4 2 0 2 κ c < 234 at 95% CL upper bound † log( P c /P b ) Probing Higgs Yukawa Couplings with Rare Decays 7 / 39

Introduction - H → Q γ H → Q γ decays could provide a clean probe of the charm (and bottom) Yukawa couplings ◮ Q is a vector ( J PC = 1 −− ) quarkonium state ◮ Interference between direct (top) and indirect (bottom) contributions ◮ Indirect (bottom) amplitude provides dominate rate contribution ◮ Direct (top) amplitude provides sensitivity to Hc ¯ c and Hb ¯ b couplings ◮ Very rare SM decay (c.f. B ( H → γγ ) ≈ 2 × 10 − 3 ) ◮ Will need a HL-LHC with (at least) 3000 fb − 1 to approach observation B ( H → J /ψ γ ) = 2 . 8 × 10 − 6 † B ( H → Υ(1 S , 2 S , 3 S ) γ ) = { 0 . 6 , 2 . 0 , 2 . 4 } × 10 − 9 † More details: Phys. Rev. D 88 , 053003 (2013) (arXiv:1306.5770) and † Phys. Rev. D 90 , 113010 (2014) (arXiv:1407.6695) Probing Higgs Yukawa Couplings with Rare Decays 8 / 39

Introduction - Z → Q γ Z → Q γ decays could provide a stepping stone towards the observation of the Higgs decays at the LHC J/ ψ ◮ Analogous to Higgs decay, could provide useful control channel Z ◮ Similar interference between direct (top) and indirect (bottom) contributions γ ◮ Indirect amplitude suppressed w.r.t. Higgs case ◮ While a rarer decay in the J /ψ case, Z bosons much J/ ψ more copiously produced than Higgs at the LHC, better prospects for observation Z B ( Z → J /ψ γ ) = 1 . 0 × 10 − 7 † B ( Z → Υ(1 S ) γ ) = 4 . 9 × 10 − 8 † γ More details: † (arXiv:1411.5924) Further work: Nucl. Phys. B 174, 317 (1980), Theor. Math. Phys. 170, 39 (2012), arXiv:1501.06569 Probing Higgs Yukawa Couplings with Rare Decays 9 / 39

H / Z → Q γ Decays - Experimental Status Experimental limits on Z → Q γ decays ◮ Only information from LEP measurements of inclusive Z → Q X decays ◮ LEP “only” produced around 17 million Z bosons... ◮ Can expect only around one Z → J /ψ γ decay in the dataset! ◮ Existing knowledge on these exclusive decays is in the form of upper bounds from inclusive Z → Q X measurements/limits Combined (PDG) LEP Measurements: � � × 10 − 3 3 . 5 +0 . 23 B ( Z → J /ψ X ) = − 0 . 25 B ( Z → Υ( nS ) γ ) = (1 . 0 ± 0 . 5) × 10 − 4 Nearly 4 orders of magnitude away from SM branching fraction! Experimental limits on H → Q γ decays ◮ Nothing known, until now... Probing Higgs Yukawa Couplings with Rare Decays 10 / 39

Analysis - The ATLAS Analysis (arXiv:1501.03276) The first experimental information on H / Z → Q γ decays, from the ATLAS experiment! week ending P H Y S I C A L R E V I E W L E T T E R S PRL 114, 121801 (2015) 27 MARCH 2015 Search for Higgs and Z Boson Decays to J = ψγ and ϒ ð nS Þ γ with the ATLAS Detector G. Aad et al. * (ATLAS Collaboration) (Received 15 January 2015; published 26 March 2015) A search for the decays of the Higgs and Z bosons to J= ψγ and ϒ ð nS Þ γ ( n ¼ 1 ; 2 ; 3 ) is performed with pp collision data samples corresponding to integrated luminosities of up to 20 . 3 fb − 1 collected at p ¼ 8 TeV with the ATLAS detector at the CERN Large Hadron Collider. No significant excess of events ffiffi s ffi is observed above expected backgrounds and 95% C.L. upper limits are placed on the branching fractions. In the J= ψγ final state the limits are 1 . 5 × 10 − 3 and 2 . 6 × 10 − 6 for the Higgs and Z boson decays, respectively, while in the ϒ ð 1 S; 2 S; 3 S Þ γ final states the limits are ð 1 . 3 ; 1 . 9 ; 1 . 3 Þ × 10 − 3 and ð 3 . 4 ; 6 . 5 ; 5 . 4 Þ × 10 − 6 , respectively. DOI: 10.1103/PhysRevLett.114.121801 PACS numbers: 14.80.Bn, 13.38.Dg, 14.70.Hp, 14.80.Ec Probing Higgs Yukawa Couplings with Rare Decays 11 / 39