Rare Charm Decays at Lhcb

Rare charm decays at LHCb

Chris Burr on behalf of the LHCb collaboration 28th July 2020 - ICHEP 2020

➤ Search for 25(!) decays of the form + ± + ′∓ D(s) → h ℓ ℓ ➤ h is a charged kaon or pion ➤ ℓ is an electron or muon ➤ Includes LFV and LNV decays

➤ Topologically similar but underlying processes vary

+ + + − + + + − + + + − + + + − + + + − D → π μ μ D → π e e D → K e e Ds → π e μ Ds → K μ e + − + + + − + + + + + − + + + − + − + + D → π μ μ D → π e e Ds → π μ μ Ds → π e e Ds → K μ e + + + − + + + − + − + + + − + + + + + − D → π μ e D → K μ μ Ds → π μ μ Ds → π e e Ds → K e μ + − + + + + + − + + + − + + + − + + + − D → π μ e D → K μ e Ds → π μ e Ds → K μ μ Ds → K e e + + + − + + + − + − + + + − + + + − + + D → π e μ D → K e μ Ds → π μ e Ds → K μ μ Ds → K e e

Allowed in the standard model Effectively forbidden in the standard model [email protected] ○ ICHEP 2020 ○ Rare charm decays at LHCb 2 Why study rare charm decays?

➤ Standard Model allowed decays involve FCNC or Weak Annihilation ➤ Forbidden at tree level in the Standard Model and CKM suppressed ➤ Additionally suppressed in charm by the GIM mechanism ➤ Expected to be dominated by long distance tree-level contributions ➤ Scope for a wide range of null tests of the Standard Model†

Standard Model prediction for D+ → π+μ+μ− Stefan de Boer and Gudrun Hiller ○ Phys. Rev. D 93, 074001 † Eur. Phys. J. C80(2020) 65 [email protected] ○ ICHEP 2020 ○ Rare charm decays at LHCb 3 Why study rare charm decays?

➤ Potential for enhancements from BSM physics (leptoquarks, MSSM, …)

➤ Complimentary to other measurements

Mediated by a Majorana neutrino Mediated by a oscillating Standard Model neutrino (far beyond the reach of any planned experiment) [email protected] ○ ICHEP 2020 ○ Rare charm decays at LHCb 4 Anomalies

➤ Several results involving b → sℓ+ℓ− are in tension with the Standard Model ➤ Angular distributions in B0 → (K*0 → K+π−) μ+μ− decays ' 5

P 1 LHCb Run 1 + 2016 SM from DHMV 0.5 LHCb-PAPER-2020-002 0 See: Lepton Flavour Violation at LHCb #462

−0.5 (1S) ψ (2S) / J ψ −1 0 5 10 15 q2 [GeV2/c4] ➤ Lower than expected branching fractions at low m (ℓ+ℓ−) for many decays involving b → sμ+μ−

LCSR Lattice Data ] 4 ] 9 c 2

-2 LHCb 5 B+→ K+µ+µ− 8 7 SM pred. GeV

/GeV 4 LHCb -8 4 6 Data c

[10

× 5

3 2

q -8 4 2 )/d µ 3

[10 µ 2 φ 2 q 1 → /d 0 s 1 B B

d 0 0 0 5 10 15 20 5 10 15 q2 [GeV2/c4] dB( q2 [GeV2/c4] LHCb-PAPER-2014-006 Phys. Rev. D 93, 074501 LHCb-PAPER-2015-023 [email protected] ○ ICHEP 2020 ○ Rare charm decays at LHCb 5 Why study rare charm decays at LHCb?

➤ LHCb has the world’s largest sample of charm decays ➤ Excellent particle identification and momentum/vertex resolution

➤ Enables extremely precise measurements ➤ Observation of D0 → h+h−ℓ+ℓ− and subsequent angular constraints (LHCb-PAPER-2017-019 and LHCb-PAPER-2018-020) ➤ World’s most precise constraints on D0 → μ+μ− and D0 → μ+e− (LHCb-PAPER-2013-013 and LHCb-PAPER-2015-048) ➤ World’s most precise constraints on + ± + ∓ (LHCb-PAPER-2012-051) D(s) → π μ μ ) 2 60 50 S 1.0 2 c c

LHCb Data 45 CL 0.9 Observed CLs 50 LHCb (b) Fit 40 0.8 Expected CLs - Median 40 D0→K +K −µ +µ − 35 0.7 Expected CLs ± 1 0 Expected CLs ± 2 D →K +K −π +π − 30 0.6

30 Comb. backg. 25 0.5 20 0.4 + 20 Previous LHCb Ds results 15 0.3 10 0.2 Candidates per 5 MeV/ 10 Candidates / (10 MeV/ 5 0.1

0 0 0.0 -8 1850 1900 1800 1850 1900 1950 2000 0 2 4 6 ◊± 10 + − + − 2 2 0 ± m(K K µ µ ) [MeV/c ] mµ +µ − [MeV/c ] B(D ⇥ e µ ) LHCb-PAPER-2018-020 LHCb-PAPER-2013-013 LHCb-PAPER-2015-048 [email protected] ○ ICHEP 2020 ○ Rare charm decays at LHCb 6 Datasets LHCb-PAPER-2020-007 (in preparation)

➤ Analysis performed on LHCb’s 2016 dataset (1.7 fb−1)

➤ + + − + Normalised to D(s) → (ϕ → ℓ ℓ ) π ➤ Regions dominated by resonances are vetoed when fitting for the signal

5000 + + D LHCb Preliminary D Combined + 500 + + Ds Ds D →⇡⇡⇡ + 4000 Non-peaking Non-peaking Ds →⇡⇡⇡ Combined 400 LHCb Preliminary D+→⇡⇡⇡ 3000 + Ds →⇡⇡⇡ 300

2000 200 Candidates per 2 MeV Candidates per 4 MeV 1000 100

0 0 1850 1900 1950 2000 2050 1850 1900 1950 2000 2050 + + + + m (⇡ µ µ) [MeV] m (⇡ e e) [MeV] [email protected] ○ ICHEP 2020 ○ Rare charm decays at LHCb 7 Challenges with electrons

➤ Problem: Electrons loose momentum to bremsstrahlung radiation 8.5. ELECTRON IDENTIFICATION 77 ➤ Solution: Apply corrections based on calorimeter energy deposits

➤ But: Deposit location varies depending on when the photon was emitted 0.05 ECAL Magnet electrons 0.04 hadrons γ E1 0.03 Downstream bremsstrahlung p 0.02 γ e Upstream 0.01 E0 bremsstrahlung E2 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Figure 8.16: Schematic illustration of E / p ➤ Correctionscluster track are appliedBremsstrahlung but the resolution correction. Anis still electron affected may Figure 8.14: The ratio of uncorrected energy of the radiate photons when passing through material charged [email protected] in ECAL to ○ the ICHEP momentum 2020 ○ Rare charm of decays re- at LHCbbefore or after the magnet: in the first case, a well 8 constructed tracks for electrons (open histogram) defined cluster is seen in the ECAL, with energy and hadrons (shaded histogram). E1,whilstinthesecondcasetheBremsstrahlung energy forms part of the electron cluster with energy E2;forelectronidentificationE2 = p, the momentum measured in the spectrometer, while the energy of the electron at the origin,

0.1 E0 = E1+ E2. (a) (b) 0.03 electrons 0.08 hadrons (ghosts are ignored). After normalisation the his- 0.06 0.02 tograms provide the likelihood distributions for 0.04 electrons and background. For a given track, the 0.01 difference of log-likelihood for the electron and non- 0.02 electron hypotheses are computed, and summed 0 0 0 100 200 300 400 0 100 200 300 400 for the different variables. Finally, the Calorime- 2 χ2 χ e brem ter information is combined with the RICH and Muon detectors, as described Sect. 8.3, significantly (c) (d) 0.03 improving the electron identification performance.

-1 10 The log-likelihood diﬀerence ∆ln eπ is shown in Fig. 8.17, for tracks that have informationL avail- 0.02 able from the Calorimeter system. -2 10 0.01 To illustrate the performance of electron iden- tiﬁcation, the J/ψ mass plot is shown as the open

0 points in Fig. 8.13(b). The signal is fit with a func- 0 20 40 60 80 100 120 140 0 2 4 6 8 10 tion including a radiative tail, to account for the E (MeV) E (GeV) PS HCAL imperfect correction of Bremsstrahlung. The back- Figure 8.15: Electron identification estimators: ground is larger than in the muon channel, and 2 (a) the value for the χe estimator for track- is either due to real (secondary) electrons, or due cluster energy/position matching procedure for re- to one of the pair of tracks being a ghost track; constructed tracks and charged clusters in ECAL, the contribution from misidentified hadrons is very 2 (b) the value of the χbrem estimator, (c) the energy small. These background tracks are dominantly of deposited in the Preshower, and (d) the deposi- low pT,andcanbeefficientlyrejectedbyapply- tion of the energy along the extrapolated particle ing the requirement pT > 0.5GeV/c for the elec- trajectory in the hadron calorimeter. The track tron candidates, as shown by the solid points in sample for these plots was taken from a selection Fig. 8.13 (b). of B-decay channels, and the shaded component la- The average efficiency to identify electrons in + belled “hadrons” also includes the muons from that the calorimeter acceptance from J/ψ e e− sample; the electron and hadron distributions are 0 0 → decays in B J/ψ KS events is 95%, for a normalised (including overflows). pion misidentification→ rate of 0.7%, as shown in Signal samples LHCb-PAPER-2020-007 (in preparation) ➤ PID used to suppress hadronic backgrounds

➤ Signal is extracted using a fit to the three body invariant mass

➤ Peaking background modelled using fast simulation generated with RapidSim†

➤ Variation in the sources of background in different final states 300 Non-peaking Non-peaking LHCb Preliminary D+→⇡⇡⇡ D+→⇡⇡⇡ LHCb Preliminary 50 + 250 + Ds →⇡⇡⇡ Ds →⇡⇡⇡ 60 Combined Combined 40 200 40 30 150

20 100 Non-peaking 20 D+→K⇡⇡ + 10 50 Ds →K⇡⇡ Candidates per 4 MeV Candidates per 4 MeV LHCb Preliminary Candidates per 4 MeV Combined 0 0 0 1850 1900 1950 2000 2050 1850 1900 1950 2000 2050 1850 1900 1950 2000 2050 m (⇡⌥µ±µ±) [MeV] m (⇡⌥µ±e±) [MeV] m (K±e±e⌥) [MeV] † Computer Physics Communications 214C (2017) pp. 239-246 [email protected] ○ ICHEP 2020 ○ Rare charm decays at LHCb 9 Results LHCb-PAPER-2020-007 (in preparation) ➤ All results consistent with background only hypothesis

+ + + D ⇡µ µ ! + + + D ⇡ µ µ ! + + + D K µ µ ! + + + D ⇡µ e ! + + + D ⇡ e µ ! + + + D ⇡ µ e ! + + + D K e µ ! + + + D K µ e ! + + + D ⇡e e ! + + + D ⇡ e e ! + + + LHCb Preliminary D K e e ! 8 7 6 5 10 10 10 10 2016 limit at 90% conﬁdence

+ + + Ds ⇡µ µ + ! + + Ds ⇡ µ µ + ! + + Ds Kµ µ + ! + + Ds K µ µ +! + + Ds ⇡µ e + ! + + Ds ⇡ e µ + ! + + Ds ⇡ µ e + ! + + Ds Kµ e + ! + + Ds K e µ + ! + + Ds K µ e +! + + Ds ⇡e e + ! + + Ds ⇡ e e + ! + + D Ke e s ! PRELIMINARY! + + + LHCb Preliminary D K e e s !

8 7 6 5 10 10 10 10 2016 limit at 90% conﬁdence | expected median with ±1σ, ±2σ intervals ○ x observed limit ○ | previous world's best limit (BaBar, CLEO, LHCb) [email protected] ○ ICHEP 2020 ○ Rare charm decays at LHCb 10 Conclusions

➤ 25 rare/forbidden charm decays have been searched for ➤ No significant deviation from the background only hypothesis ➤ Results improve upon the prior world’s best constraints by up to a factor of 500

➤ Many more analyses of rare charm decays are in progress and coming soon!

➤ Will soon enter the LHCb upgrade era ➤ Significantly higher luminosities ➤ Fully software trigger for improved efficiency and output bandwidth utilisation ➤ Many talks at ICHEP covering various aspects ➤ See: “The LHCb Upgrade Programme for Run 3 and Run 4” #528 ➤ Should enable limits in D+ → hℓ+ℓ− to improve to ∼ 10−8

[email protected] ○ ICHEP 2020 ○ Rare charm decays at LHCb 11 Questions? Mass distributions LHCb-PAPER-2020-007 (in preparation) Non-peaking 60 80 D+→K⇡⇡ LHCb Preliminary + Ds →K⇡⇡ 50 Combined 60 40

30 40 20 20 10 Non-peaking Candidates per 4 MeV LHCb Preliminary Candidates per 4 MeV Combined 0 0 1850 1900 1950 2000 2050 1850 1900 1950 2000 2050 m (K±µ±µ⌥) [MeV] m (K⌥µ±µ±) [MeV]

Non-peaking Non-peaking 125 D+→⇡⇡⇡ LHCb Preliminary D+→K⇡⇡ 150 + + Ds →⇡⇡⇡ 80 Ds →K⇡⇡ 125 100 Combined Combined 60 100 75 LHCb Preliminary 75 40 50 50 Non-peaking + 20 D →K⇡⇡ 25 + 25 Ds →K⇡⇡ Candidates per 4 MeV Candidates per 4 MeV LHCb Preliminary Candidates per 4 MeV Combined 0 0 0 1850 1900 1950 2000 2050 1850 1900 1950 2000 2050 1850 1900 1950 2000 2050 m (⇡±e±µ⌥) [MeV] m (K±e±µ⌥) [MeV] m (K±µ±e⌥) [MeV] [email protected] ○ ICHEP 2020 ○ Rare charm decays at LHCb 13 Calibration and normalisation channels LHCb-PAPER-2020-007 (in preparation) Non-peaking Non-peaking D+→⇡⇡⇡ 125 D+→⇡⇡⇡ + + 150 Ds →⇡⇡⇡ Ds →⇡⇡⇡ Combined 100 Combined

100 75

50 50 25 Candidates per 4 MeV LHCb Preliminary Candidates per 4 MeV LHCb Preliminary 0 0 1850 1900 1950 2000 2050 1850 1900 1950 2000 2050 m (⇡±e±e⌥) [MeV] m (⇡⌥e±e±) [MeV] 300 250 Non-peaking D+→⇡⇡⇡ 150 250 D+ ⇡⇡⇡ 200 s → Combined 200 150 100 LHCb Preliminary 150 LHCb Preliminary 100 100 Non-peaking 50 Non-peaking D+→⇡⇡⇡ D+→⇡⇡⇡ 50 + + Ds →⇡⇡⇡ 50 Ds →⇡⇡⇡ Candidates per 4 MeV Candidates per 4 MeV LHCb Preliminary Candidates per 4 MeV Combined Combined 0 0 0 1850 1900 1950 2000 2050 1850 1900 1950 2000 2050 1850 1900 1950 2000 2050 m (⇡±µ±e⌥) [MeV] m (⇡⌥µ±e±) [MeV] m (⇡±µ±µ⌥) [MeV] [email protected] ○ ICHEP 2020 ○ Rare charm decays at LHCb 14 Calibration and normalisation channels LHCb-PAPER-2020-007 (in preparation) 250 LHCb Preliminary 200

150

100

50 Non-peaking Candidates per 4 MeV Combined 0 1850 1900 1950 2000 2050 m (K⌥µ±e±) [MeV]

Non-peaking LHCb Preliminary D+→⇡⇡⇡ LHCb Preliminary LHCb Preliminary 50 + Ds →⇡⇡⇡ 150 60 Combined 40

100 30 40

20 Non-peaking 50 20 D+→K⇡⇡ + 10 Non-peaking Ds →K⇡⇡ Candidates per 4 MeV Candidates per 4 MeV Candidates per 4 MeV Combined Combined 0 0 0 1850 1900 1950 2000 2050 1850 1900 1950 2000 2050 1850 1900 1950 2000 2050 m (⇡⌥µ±µ±) [MeV] m (K⌥e±e±) [MeV] m (K±e±e⌥) [MeV] [email protected] ○ ICHEP 2020 ○ Rare charm decays at LHCb 15