Test of LFU with charm semileptonic decays at the LHCb experiment

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Test of LFU with charm semileptonic decays at the LHCb experiment

Alessandra Gioventù

XI CPAN Days

Oviedo, 21 st October 2019

1

IGFAE - USC

[email protected]

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‣ Introduction

‣ The LHCb detector

‣ Lepton Flavour Universality

‣ LFU in charm decays

‣ Test of LFU in

‣ Analysis strategy

‣ Electrons in LHCb

‣ Neutrino reconstruction

‣ Preliminary results

‣ Conclusions and future prospects

D

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0 ! K ` + ⌫ `

Outline

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A. Gioventù Test of LFU with charm semileptonic decays at the LHCb experiment XI CPAN Days

The LHCb detector

3

[JINST 3 S080005 (2008)]

[Int. J. Mod Phys. A30 1530022 (2015)]

[PRL 118 052002 (2017)]

[JHEP 05 (2017) 074]

‣ Forward spectrometer for b- and c-hadron decays (2< η <5)

‣ Good vertex and impact parameter resolution ( σ (IP)~20 µm)

‣ Excellent momentum resolution ( δ p/p = [0.5 - 1] % p<200 GeV)

‣ Excellent charged particle identification (μ ID 97% for (π → μ) misID of 1-3%)

‣ Capability for neutral identification

‣ High production cross section of charmed mesons in LHCb

‣ At 13 TeV: σ (pp → D* + X)~784 μb and σ (pp → D 0 X)~2072 μb

2015 - 2016 data sample √s = 13 TeV, 1.9 fb -1

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‣ The SM predicts equal couplings between gauge bosons and the three lepton families. This is called Lepton Flavour

Universality (LFU)

‣ Observation of LFU violation → sign of New Physics (NP)

‣ B semileptonic decays show tensions between SM expectations and experimental results:

‣ b → c transitions: R(D*), R(D) and R(J/ ψ )

‣ b → s transitions: R(K*) and R(K)

‣ Tackle SM using similar observables as B decays for c → s and c → d transitions

Lepton Flavour Universality

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where contains a s or a d quark, and q 2 is the transfer 4-momentum

LFU in charm decays

5

[BESIII Beauty 2018]

Adapted by L. Capriotti from [1703.10695]

R µ/e

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R µ/e (q 2 ) = d (D 0 ! H µ + ⌫ µ )/dq 2 d (D 0 ! H e + ⌫ e )/dq 2

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‣ New results from BESIII compatible with SM expectation within 2 σ H

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‣ Study of the decay

‣ Deviations of from the SM up to 10 − 20%, depending on the q 2 , cannot be discarded

LFU in charm decays

0 .2 0.4 0.6 0.8 1.0 1 .2 1 .4

0 .8 0 0 .8 5 0 .9 0 0 .9 5 1 .0 0 1 .0 5 1 .1 0 1 .1 5 1 .2 0

q 2 @ GeV 2 D

R m ê e H q 2 L

[PRD 91 (2015) 094009]

Red: SM prediction on R μ/e

Grey: allowed NP region from constraints on the scalar Wilson coefficients

D

0 ! K ` + ⌫ ` R µ/e

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A. Gioventù Test of LFU with charm semileptonic decays at the LHCb experiment XI CPAN Days

Analysis strategy

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‣ Theoretically clean observable

‣ D 0 from prompt

‣ Measurement in bins of q 2

‣ Two main challenges:

‣ Missing neutrino momentum

‣ Electrons reconstruction

D ⇤ + ! D 0 ⇡ s +

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R e/µ = B (D 0 ! K e + ⌫ e ) B (D 0 ! K µ + ⌫ µ )

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✏ tot (X ) = ✏ acc · ✏ rec | acc · ✏ sel | rec · ✏ P ID | sel

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R e/µ = N (D 0 ! K e + ⌫ e )

N (D 0 ! K µ + ⌫ µ ) ⇥ ✏ tot (µ)

✏ tot (e)

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2015 - 2016 data sample √s = 13 TeV, 1.9 fb -1

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p T (K `)

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D

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⁰

D ⇤ +

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⇡

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s +

⌫

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`

IP PV

(K ` + )

<latexit sha1_base64="zqE62evh0SLl+VzDIiWnaEplVLI=">AAAB8nicdVDLSsNAFJ3UV62vqks3g0WoiCFpS9PuCm4ENxXsA9K0TKaTduhkEmYmQgn9DDcuFHHr17jzb5w+BBU9cOFwzr3ce48fMyqVZX0YmbX1jc2t7HZuZ3dv/yB/eNSWUSIwaeGIRaLrI0kY5aSlqGKkGwuCQp+Rjj+5mvudeyIkjfidmsbEC9GI04BipLTkFm/6lz3CWP/ifJAvWGbZqVfqVahJzSlVrCWxnDq0TWuBAlihOci/94YRTkLCFWZISte2YuWlSCiKGZnleokkMcITNCKuphyFRHrp4uQZPNPKEAaR0MUVXKjfJ1IUSjkNfd0ZIjWWv725+JfnJiqoeSnlcaIIx8tFQcKgiuD8fzikgmDFppogLKi+FeIxEggrnVJOh/D1KfyftEumXTZLt5VCo7qKIwtOwCkoAhs4oAGuQRO0AAYReABP4NlQxqPxYrwuWzPGauYY/IDx9gk1xpCJ</latexit>

(8)

‣ Extract from two different fit frameworks:

1. Bidimensional template fit to ∆M vis and M corr

‣

‣ p T : transverse momentum of the neutrino with respect to the D 0 direction of flight

2. Bidimensional template fit to and

‣ obtained using the “Global fit” algorithm (GF)

‣ Both methods minimise the Barlow-Beeston log-likelihood, to take into account the limited size of the MC samples

‣ The value of is blinded in the fits

Analysis strategy

M corr =

q

m(K ` + ) 2 + (p 0 T ) 2 + (p 0 T )

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M vis = m(K ` + ⇡ s ) m(K ` + )

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M vis = m(K ` + ⇡ s ) m(K ` + )

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m(D ⇤ )

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m(D ⇤ )

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R e/µ

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N (D 0 ! K ` + ⌫ ` )

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(9)

A. Gioventù Test of LFU with charm semileptonic decays at the LHCb experiment XI CPAN Days

Electron reconstruction in LHCb

9

[LHCb-PAPER-2019-19]

‣ Electrons lose a large fraction of their

energy through Bremsstrahlung radiation

‣ Improve electron energy resolution by using a recovery procedure:

‣ Look for photon clusters in the

calorimeter compatible with electron direction before magnet

19/04/16 M. Borsato - USC 6

● Large emi ssi on of bremsstrahlung radi ati on ( FSR i ncl uded i n MC usi ng PHOTOS )

● Emi ssi on before the magnet needs recovery

● Di < cul t 0 mass tai l to l ower masses

● Correcti on depends on ECAL resol uti on 0 addi ti onal term to mass resol uti on:

ECAL = E > = p from tracki ng

● Si gnal gets mi xed wi th background:

● Pure combi natori al

● Semi l eptoni c cascade such as

● Parti al l y reconstructed (mai nl y an addi ti onal ? )

Backgrounds and mass shape

LHCb: JHEP 1504 (2015) 064

ups trea m

dow nstre am

ψ(2S) B +

Part. reco

J/ψ(1S)

(10)

‣ Calculating q 2 is not trivial

‣ Cone closure method: full constraint of the system using the D* + mass constraint

‣ Boost to rest frame where holds

‣ Set and to nominal value

‣ lies on the surface of a cone around the pion direction

‣ The polar angle is given by kinematics

‣ Sample 1000 points along the cone and take the azimuthal angle that best aligns the direction to the total final-state momentum

K ` +

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p(D 0 ) = p(⌫ )

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m(D

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⇤ ) m(D

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0 ) p(⌫ )

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D

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⁰

Neutrino reconstruction: cone closure method

(11)

‣ Obtain the neutrino momentum constraining the kinematic of the whole decay

Neutrino reconstruction: the GF algorithm

11

‣ Write a single χ 2 function

using the measured quantities

‣ Add kinematical constraints

‣ The function to minimise is the sum of all the χ 2

‣ Charged particles tracks

‣ Position of the two decay vertices

` +

p T (K `)

p T (⌫ )

K

D

⁰

D ⇤ +

⇡ s +

⌫

`

IP PV

(K

` + )

(12)

‣ Obtain the neutrino momentum constraining the kinematic of the whole decay

‣ Add kinematical constraints

‣ Invariant mass of the D 0

‣ The D* decay vertex must coincide with the primary vertex (PV)

‣ The D 0 flight direction must coincide with the direction from the PV to the D 0 decay vertex

‣ and tracks must come from the secondary vertex D

0 ! D

K 0 ! ` K + ⌫ ` ` + ⌫ `

` +

p T (K `)

p T (⌫ )

K

D

⁰

D ⇤ +

⇡ s +

⌫

`

IP PV

(K

` + )

Neutrino reconstruction: the GF algorithm

(13)

A. Gioventù Test of LFU with charm semileptonic decays at the LHCb experiment XI CPAN Days 13

‣ Obtain the neutrino momentum constraining the kinematic of the whole decay

‣ Add kinematical constraints

‣

‣ 16 free parameters:

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p ~ ⇡ , ~ p ` , ~ p K , ~ p ⌫ , ~ x D , z PV

` +

p T (K `)

p T (⌫ )

K

D

⁰

D ⇤ +

⇡ s +

⌫

`

IP PV

(K

` + )

Neutrino reconstruction

Neutrino reconstruction: the GF algorithm

(14)

LHCb UNOFFICIAL

LHCb UNOFFICIAL LHCb UNOFFICIAL

LHCb UNOFFICIAL

LHCb UNOFFICIAL LHCb UNOFFICIAL 14

Preliminary fit results - UNOFFICIAL

(15)

LHCb UNOFFICIAL

LHCb UNOFFICIAL 15

A. Gioventù Test of LFU with charm semileptonic decays at the LHCb experiment

Preliminary fit results - UNOFFICIAL

(16)

‣ LFU in charm decays is a potential road to new physics, still not so explored

‣ The analysis is ongoing and in a good shape

‣ The blinding procedure is implemented

‣ Fitting framework for the template fits is in phase of testing

‣ We expect the precision to be limited by systematic uncertainties

Conclusions and perspectives

STAY TUNED!

(17)

BACKUP

17

(18)

BESIII result [BESIII Beauty 2018]

R = 0.978 ± 0.007 ± 0.012

(19)

‣ In LHCb tracks are parametrised as

‣ It is possible to write the χ 2 function for every track as

‣ Where the expected values are calculated as

‣ While for the D 0 mass

‣ m(D 0 ) = 1864.83 ± 0.05 MeV/c 2

‣ σ 2 = 1 MeV 2 /c 4

Global fit algorithm: implementation of the constraints

19

2 D 0 = (m D 2 0 2 m ˆ D 0 ) 2 m D 0

( x ˆ K/` = x D 0 + (z Ref z D 0 )ˆ t K/` x ˆ

y K/` = y D 0 + (z Ref z D 0 )ˆ t K/` y

2 = (~ ↵ ↵)C ~ ˆ 1 (~ ↵ ↵) ~ ˆ T

~

↵ = ⇣

x, y, t x , t y , p q ⌘

z Ref

( x P V = x D 0 + (z P V z D 0 )t D x 0

y P V = y D 0 + (z P V z D 0 )t D y 0