Experimental proposal for MDM/EDM of strange baryons in LHCb

Jinlin Fu University of Milano

on behalf of the SELDOM Collaboration

Workshop on electromagnetic dipole moments of unstable particles October 3-4, 2019, Milano

Introduction

2

• Electromagnetic dipole moments are static properties of particles

• EDM violates P and T, thus violates CP. EDM searches are sensitive to new physics

EDM: ! ⃗δ = dμB ⃗s /2

MDM: ! ⃗μ = gμB ⃗s /2

• MDM measurement of particle and anti-particle allows to test CPT

Gaussian units !μB = eℏ/(2mc)

How to access EDM/MDM

3

• Spin precession induced by interaction of its EDM and MDM with external EM field

dsdt = s⇥⌦

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⌦ = ⌦MDM +⌦EDM +⌦TH<latexit sha1_base64="rPf5ZW9/NULqSPUvLJYvAL1mkwY=">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</latexit>

⌦MDM = gµB

~

⇣B� �

�+1 (� ·B)� � � ⇥E⌘

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⌦EDM = dµB

~

⇣E� �

�+1 (� ·E)� � � ⇥B⌘

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• Known polarised sample, intense E and B inducing sizeable precision angle

T-BMT equation:

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Experimental status

4

• Last direct measurement in 70’s,80’s from E761@Fermilab

< 1.5 x 10-16 e cm @95% C.L.dΛ

Phys. Rev. Lett. 41 (1978) 1348

• Prediction using experimental upper limit of neutron EDM

< 4.4 x 10-26 e cm @95% C.L.dΛJHEP 12 (2012) 097PLB291 (1992) 293Nucl. Phys. B367(1991) 313Phys.Rev.D61 (2000) 114017

✓ Fixed target experiment with 300GeV proton beam on Be✓ Reconstructed 3x106 decaysΛ → pπ−

✓ No measurement for ! , hence no CPT test via MDMΛ

! =(–0.613±0.004) !μΛ μN

Phys. Rev. D23 (1981) 814

✓ Small transverse polarisation ~8%

plenty of scopy to improve direct measurement

✓ Magnet: 5m,±15Tm

✓ Momentum direction fixed to be perpendicular to ! ⃗B

Advantages in LHCb

5

• Production

• Large initial longitudinal polarisation

• Magnet

Perfect tracking, vertex determination, particle identification

✓ from weak decays of c and b baryons

✓ a tracking dipole magnet providing ! Tm over 10 metersDy ≈ ± 4

• Single arm forward detector, optimised for c and b hadron physics with large pseudorapidity (2< ! <5)η

✓ allow sizeable ! spin precessionΛ

✓ compatible ! and !Λ Λ

Challenges in LHCb

6

• LHCb Upgrade for Run3 (2021~2023):

• Details of trigger and reconstructions, see talks from Salvatore and Louis

• Significant backgrounds and limited resolution on measurement of ! momentum and decay pointΛ

✓ 40 MHz readout

✓ Real-time alignment and calibration

✓ Offline quality reconstruction at the online level

✓ Only software trigger

✓ (14TeV) 5 times than Run2 (13TeV), 50fb-1 end Run32 × 1033cm−2s−1

LHCb upgraded detector

7

Detectors for Run3

!y!z

New pixelVELO detector

New silicon Upstream Tracker (UT) New scintillating fibre tracker (SciFi)

New RICH optics and photodetectors

New electronics for calorimeter and muon system

LHCb Magnet

8

!y!z

• Measured B field values stable within ! =±0.07gaussσ

• ! 4Tm ∫ Bdl =

• Acceptance: hor. ±300 mrad vert. ±250 mrad

LHCb-INT-2015-034

x=0,y=0

! procession in LHCbΛ

9

UT

!x!z

!y!z

x=0,y=0

LHCb charged tracks

10

VELO

UT B

upstream track

velo track

long track

Downstream track

T track

• Long track: reconstructed in VELO and T-stations. Perfect reconstruction

• Downstream track: UT + T-stations. Good reconstruction• T track: T-stations. Worse reconstructed, not used in physics analysis

yet. Need to improve. (details see Salvatore's talk)Long + Downstream tracks to measure polarisation before MagnetT tracks to measure polarisation after Magnet

!x!z

T–BMT for ! in LHCbΛ

11

• Negligible field gradient effects, , E=0, q=0

• Specific case: ! and heavy baryon flying along z axis in lab systemΛ

• Main precession (MDM) in ! planexz• Non zero ! indicates EDMsy

! ≈ ± π/4

! Tm≈ 4

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• Sizeable spin precession achieved

Initial ! polarisation in LHCbΛ

12

• ! directly produced from pp collisions via strong interactions:Λ

✓ Initial transversal polarisation perpendicular to production plane, ! ⃗p beam × ⃗p Λ

✓ polarisation increase with !PtΛ

๏ not within LHCb angular acceptance

• ! produced from heavy baryon weak decays:Λ

✓ large longitudinal polarisation: ~90% in !Λ+c → Λπ+

✓ polarisation measured via analysis angular distribution of ! decay

Λ → pπ−

Source and production of !Λ

13

• Consider c baryon decays and charged final particles• At least one particle from heavy baryon decay vertex

! : c,b cross section pp@14TeVσqq̄

! :fragmentation fraction into heavy baryonf(q → H)

!1.5 × 1011

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!3.8 × 1011short-lived (SL) long-lived (LL)

Geometry efficiency

14

Experimental yield: Geometry eff. for SL topology estimated from simulation

R1: measure initial polarisationR2: measure polarisation as decay length in ! ⃗BM3: most sensitivity to MDM/EDM

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in %

Sensitivity

15

• Pseudo-experiments using simplified detector geometry

• ! -factor uncertainty:d

• With 50 fb-1 (end Run3) :

! =1MNrecoΛ

✓ ! , ! e cm, 2 orders of magnitude improveσd ≈ 3 × 10−4 δ(Λ) ≲ 1.3 × 10−18

✓ First ! MDM measurement at similar precisionΛ✓ First CPT test via ! MDM at ! levelΛ 10−3

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Polarisation measurement before magnet

16

• ! , !Ξ0c → Λh±h∓ Λ+

c , Ξ+c → Λh±h±h∓

• ! reconstructed using long and downstream tracksΛ• Program starting with Run 2 (6 fb-1)

2400 2450 2500 25502) MeV/c+π-KΛm(

0200400600800

1000120014001600

small fraction of data, not optimal selection

LHCb unofficial LHCb unofficial

• Still in tuning

LHCb unofficial

Long Downstream Downstream

Summary

17

• Proposed ! EDM/MDM measurements in LHCbΛ

• With 50 fb-1 in LHCb, expect 100 improvement of EDM, first MDM measurement with similar precision as EDM ~10-4, and first CPT test via MDM at 10-3 level

Λ

• Measurement of polarisation before Magnet starting from Run2 data

• Hard work needed to improve ! reconstructions: trigger, T-track reconstruction…

Λ

Probability Distribution Function

18

• spin-polarisation can be analysed through angular distribution of !Λ → pπ−