Semileptonic Hyperon Decays

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Semileptonic Hyperon Decays

A. Brea Rodríguez, X. Cid Vidal, D. Martínez Santos CPAN 22/10/2019, Oviedo

at LHCb

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Hyperons

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SEMILEPTONIC

HYPERON DECAYS

A hyperon is any baryon containing one or more strange quarks, but no charm, bottom, or top quark.

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Outline

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• LHCb capacity to study SHD.

• Theoretical relevance.

• Current status.

• Prospects and expected yield.

• Next steps.

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LHCb Experiment at CERN

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Acceptance 2 < η < 5 Upgrade Hyperons copiously

produced at the LHC.

~ 1 Hyperon per event

LHCb collaboration, R. Aaij et al., LHCb detector performance, Int. J. Mod. Phys. A30 (2015) 1530022, arXiv:1412.6352

CERN-LHCC-2014-001 ; LHCB-TDR-015.

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LHCb Experiment at CERN

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LONG

DOWNSTREAM

The LHCb Collaboration, R. Aaijet al., “PromptKS0production in ppcollisions at √s = 0.9 TeV”, Phys.

Lett. B 693 (2010) 69, arXiv:1008.3105 [hep-ex].

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The LHCb experiment has shown its capability to obtain leading strange physics measurements, particularly searching for their rare decays. In fact, the LHCb collaboration has published the world’s most precise measurements in K

_S⁰

→ µµ and Σ

⁺

→ p

⁺

µµ .

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Strange Physics at LHCb

S - TRANSITIONS

The latest results from semileptonic b → c transitions suggest the possibility of BSM contributions in charged-current quark decays breaking Lepton Flavour Universality (LFU). Hence, it is natural to

investigate if similar patterns can be found in s → u transitions.

https://indico.cern.ch/event/769729/contributions/3510936/ (K_S⁰→ µµ ) https://indico.cern.ch/event/769729/contributions/3511079/ (Σ⁺→ p⁺µµ )

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It has been shown that semileptonic decay of hyperons can be sensitive to BSM dynamics that break lepton universality.

These decays are controlled by a small SU(3) flavour breaking parameter that allows for systematic expansions and accurate predictions in terms of a reduced dependence on hadronic form factors.

Muonic modes are very sensitive to nonstandard scalar and tensor contributions and these could provide a powerful synergy with direct searches of new physics at the LHC.

H.-M. Chang, M. González-Alonso, and J. Martin Camalich, “Nonstandard Semileptonic Hyperon Decays”, Phys.

Rev. Lett. 114 no. 16, (2015) 161802,arXiv:1412.8484 [hep-ph].

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Relevance

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Old Measurements !

Measurement of the Λ⁰ → pµν branching ratio, M. Baggett et al. (1972) 8

M. Tanabashi et al. (Particle Data Group), Phys. Rev. D 98, 030001 (2018) and 2019 update.

Update needed

The branching fractions of SHD show uncertainties at the 20% - 100% level, leaving vast room for progress. For example !(Ξ^- → Λ⁰µ^- ν̅ ) = #. %_&'.'^(#.% x 10 ^-4and !(Ξ^- → Σ⁰ µ^- ν̅ ) < 8 x 10 ^-4at 90% CL. Improved measurements directly translate into tighter bounds on LFU, since the electron modes have already been measured very precisely.

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Update needed

While a lot of attention has recently been justly devoted to the higher mass sector of the CKM matrix, it is the low mass sector, in particular V_ud and V_us where the highest precision can be attained, and which can provide the most sensitive test of the unitary of the CKM matrix through the relation | V_ud | ² + | V_us | ² + | V_ub | ² = 1. Given the fact that the | V_ub | ² factor is totally negligible, the unitary test reduces to the consistency of cos θ_C determined from nuclear beta decay and of sin θ_C determined from strangeness changing semileptonic decays.

SEMILEPTONIC HYPERON DECAYS

Nicola Cabibbo, Earl C. Swallow, and Roland Winston

Annual Review of Nuclear and Particle Science 2003 53:1, 39-75

Cabibbo

We indicate the need for more experimental and theoretical work, both on hyperon beta decays and on K_l3 decays.

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Prospects for measurements with Strange Hadrons, arXiv:1808.03477, JHEP 05 (2019) 048

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Prospects for measurements with strange hadrons

Fast simulation of the LHCb upgrade tracking system

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Λ

⁰

→p

⁺

µ

^-

! ν Signal vs Λ

⁰

→p

⁺

π

^-

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Prospects for measurements with Strange Hadrons, arXiv:1808.03477, JHEP 05 (2019) 048

ν

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Ξ

^-

→ Λ

⁰

µ

^-

! ν Signal vs Ξ

^-

→Λ

⁰

π

^-

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Prospects for measurements with Strange Hadrons, arXiv:1808.03477, JHEP 05 (2019) 048

Decay in flight of the pion and final state radiation in the Λ⁰ not included!

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THEORETICAL MODEL

Kinematic Distributions for B

¹

B

²

lepton ν

We start with angular coefficients of the 3 body decay (I

^1,2,3

)

Where q² is the invariant mass squared of the dilepton pair, f

¹

(0) the vector coupling, Δ = M

¹

– M

²

, with M

¹

(M

²

) the parent (daughter) baryon mass and δ = (M

¹

–M

²

)/M

¹

.

First order SU(3)

^F

expansion. Estimate error of 5%.

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H.-M. Chang, M. González-Alonso, and J. Martin Camalich, “Nonstandard Semileptonic Hyperon Decays”, Phys. Rev. Lett. 114 no. 16, (2015) 161802,arXiv:1412.8484 [hep-ph].

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THEORETICAL MODEL

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¹

B

²

lepton ν

Channel Δ (GeV) δ (GeV) f₁(0) g₁(0)/f₁(0) f₂(0)/f₁(0) ! (s)

Λ⁰ → p⁺µ^-ν̅ 0.175 0.156951

- ^#_$ 0.718 1.066 2.632 x 10^-10

Ξ^-→ Λ⁰µ^-ν̅ 0.20671 0.156396

+ ^#_$ 0.25 0.25 1.639 x 10^-10

Ξ^- → Σ⁰µ^-ν̅ 0.129068 0.0976523

+ ^%_$ 1.21 1.21 1.639 x 10^-10

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EVTGEN MODEL

Significantly different from Phase Space!

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MASS

LHCb-FIGURE-2019-006

1020 1040 1060 1080 1100 1120

2) ) (MeV/c

p

µ M(

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

)2 Normalised Entries/(2 MeV/c

LHCb Simulation n µ-

p+

® L0

p-

p+

® L0

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MISSING TRANSVERSE MOMENTUM

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0 50 100

(MeV/c) pT

1040 1050 1060 1070 1080 1090 1100 1110 1120 1130

2 )) (MeV/cµ

p

M( 1140

n µ-

p+

® L0

p-

p+

®

L0 LHCb Simulation

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MISSING TRANSVERSE MOMENTUM

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0 50 100

(MeV/c) pT

1040 1050 1060 1070 1080 1090 1100 1110 1120 1130

2 )) (MeV/cµ

p

M( 1140

n µ-

p+

® L0

p-

p+

®

L0 LHCb Simulation

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SELECTION CUTS

We confirmed the Strange Prospects results using the LHCb software. So, some selection cuts can be applied on this planes to separate Signal and Λ→pπ Background.

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Corrected Mass could also be useful to discriminate against combinatorial background

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ARMENTEROS-PODOLANSKI

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D. Martínez Santos, Study of the very rare decay Bs0→ µµ in LHCb, PhD thesis, Universidade de Santiago de Compostela, Santiago de Compostela, 2010, CERN-THESIS-2010-068.

! Λ Λ

K_S⁰

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Λ

⁰

→p

⁺

π

^-

yield at LCHb

• Λ⁰→p⁺π^–normalization cannel

• Yield 107.3 x 10⁶ candidates

• Luminosity 1.6 fb^-1

• Rate = 65.2 x 10⁶ per fb^-1

1105 1110 1115 1120 1125

2) ) (MeV/c

p

p M(

0 500 1000 1500 2000 2500 3000 3500

) 2 Entries/(0.1 MeV/c

LHCb Preliminary data = 13 TeV s

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EXPECTED YIELD

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Λ → p⁺ π^-Yield = 65.2 x 10⁶ events/fb Λ⁰ → pπ Branching Ratio = 63.9 % Λ⁰ → pµν Branching Ratio = 1.57 x 10^-4

Estimated Eff_Selection = 58.66 % Prospects for measurements with Strange Hadrons

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EXPECTED YIELD

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Λ → p⁺ π^-Yield = 65.2 x 10⁶ events/fb Λ⁰ → pπ Branching Ratio = 63.9 % Λ⁰ → pµν Branching Ratio = 1.57 x 10^-4

Estimated Eff_Selection = 58.66 % Prospects for measurements with Strange Hadrons

Λ⁰ → pµν Expected Yield ~ 65.2 x 10⁶ * (1.57 x 10^-4/0.639) * (0.32/0.41) * 0.5866 ~ 7330 reco+sel /fb

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EXPECTED YIELD

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Λ

⁰

→ pµν Expected Yield ~ 7330 reco+selected /fb Run II ~ 6 fb

^-1

We expect ~ 44K reco+selected Λ

⁰

→ pµν

With this yield, measurement should be dominated by systematics. The statistical uncertainty should be at the level of 1%

We should be able to improve the current experimental uncertainty on the BR, which is ~23%

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K

_S⁰

→ π

⁺

µ

^-

! ν

A search for K_S⁰→ π⁺µ^- ^!ν , which is as yet unobserved experimentally, could be performed at LHCb. This would be useful as a measurement of Lepton Flavour Universality when comparing to the well known decay K_S⁰→ π⁺e^-^!ν .

Depending on the precision achieved, the measurement of this branching fraction could also be useful in costraining the CKM matrix element |V_US|.

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K

_S⁰

→ π

⁺

µ

^-

! ν

A search for K_S⁰→ π⁺µ^- ^!ν , which is as yet unobserved experimentally, could be performed at LHCb. This would be useful as a measurement of Lepton Flavour Universality when comparing to the well known decay K_S⁰→ π⁺e^-^!ν .

Depending on the precision achieved, the measurement of this branching fraction could also be useful in costraining the CKM matrix element |V_US|.

https://indico.cern.ch/event/769729/contributions/3510960/

(Flavour Physics and CP Violation at KLOE-2)

KLOE-2

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NEXT STEPS

• Study combinatorial background.

• Measurement of the Λ

⁰

→ pµν branching ratio at LHCb (current value should be improved)

• Explore other Semileptonic Hyperon Decays (Ξ

^-

→ Λ

⁰

µ

^-

ν ̅ , Ξ

^-

→ Σ

⁰

µ

^-

ν ̅ )

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Thank you

Questions?

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THEORETICAL MODEL

¹

B

²

lepton nu

We start with angular coefficients of the 3 body decay (I

^1,2,3

)

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THEORETICAL MODEL

¹

B

²

lepton ν

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EVTGEN MODEL

Good Behaviour of the Model!