Experimental proposal for MDM/EDM of strange baryons in LHCb
Transcript of Experimental proposal for MDM/EDM of strange baryons in LHCb
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
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• 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
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• 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
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• 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
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• 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
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• 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
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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
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!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
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UT
!x!z
!y!z
x=0,y=0
LHCb charged tracks
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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
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• 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
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• ! 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
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• 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
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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
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• 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
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• ! , !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
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• 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
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• spin-polarisation can be analysed through angular distribution of !Λ → pπ−