Recent CLFV measurements and New Physics searches at BESIII Tao Luo - - PowerPoint PPT Presentation

Recent CLFV measurements and New Physics searches at BESIII

Tao Luo (luot@fudan.edu.cn) (for the BESIII Collaboration) Fudan University CLFV2019 @ Fukuoka, Japan, 17-19 June 2019

Beijing Electron Positron Collider (BEPCII)

• Working energy range: 2.0 ~ 4.6 GeV

2 ~ 4.9 GeV from 2019

• Double-ring structure
• First collision in 2008, physics run

started in 2009

• Design luminosity: 1 x 1033 cm-2s-1 @

1.89GeV

• Achieved luminosity: 1 x 1033 cm-2s-1

@ 1.89GeV (Apr. 5, 2016)

BESIII detector

• TOF
• Int. J. Mod. Phys. A24, 377 (2009); NIM A614, 345 (2010)

Magnet Yoke Be beam pipe MDC MUC-µID

1 m

EMC 1 T SC magnet

Position resolution: 120 µm Momentum resolution: 0.5%@1.0GeV/c

TOF

Barrel time resolution: 70 ns (Barrel) PID: 3s K/p separation for P<0.7GeV/c@90 (Barrel)

Energy resolution: 2.5% (Barrel) @ 1.0GeV/c Position resolution: 8mm (Barrel)

Position resolution: 2cm

CGEM-IT is being built & placed here

All subdetectors are in very good status!

• BESIII Collaboration
• 14 countries

67 institutions ~500 members

BESIII Dataset

Outline

• Charged lepton flavor violation (CLFV) and

prospects at BESIII

• Other New Physics searches at BESIII
• Highly Suppressed EM process
• Suppressed FCNC processes
• Invisible Decays
• Dark Photon searches
• Baryon Number Violation
• Lepton Number Violation
• Summary

CLFV in charmonium decay

• CLFV processes are highly suppressed in the SM
• Possible enhancement by NP models

ü SUSY ü Heavy Neutrinos ü Extended higgs models ü ……

ℳ~#(10'())

CLFV measurement at BESIII (I)

• yàµ

report on CLFV2016

• Select events with back-to-back charged tracks and no obvious extra

EMC response

• Dominant backgrounds: !/" → #+#−, !/" → $+$−, !" → %+%−, !" →

&+&−, #+#− → ' #+#−, #+#− → ' $+$−

()*+ = 4.75 ± 1.09

CLFV measurement at BESIII (II)

• ! " "

CLFV prospects at BESIII (III)

• The sensitivity to cc0à µ t and cc0à e t could reach (1-3) ×10-7.
• The sensitivity to hcà µ t and hcà e t could reach (2-5) ×10-7.

Highly Suppressed EM processes

• yàfe+e-
• Within the SM, the partial widths from the leading EM

and mixed loop processes are predicted to be at a level

• f 10−6 and 10−9 keV, respectively, corresponding to

branching fractions at the order of 10−8 and 10−11.

• If there is a new particle involved in the intermediate

process, such as a dark photon with amass of several MeV/c2 or a glueball with certain quantum numbers, the contribution from Fig. 1(b) may be enhanced to an

• bservable level.
• Any deviations from the predictions à NP BSM

Highly Suppressed EM processes

• yàfe+e-

Suppressed FCNC processes (I)

• y/y’àe+e-
• FCNCs are forbidden at the tree level due to the Glashow-Iliopoulos-Maiani (GIM)

mechanism, sensitive to NP.

• The decay branching fraction for this kind of rare process is expected to be of order 10−10

to 10−13 in the SM.

• Many new physics models, such as the topcolor models, the minimal supersymmetric

standard model with R-parity violation, and the two Higgs doublet model, predict the decay branching fractions can be enhanced by 2 or 3 orders of magnitude.

• Search for FCNC à ideal opportunity to study nonperturbative QCD effects and search for

NP

Suppressed FCNC processes (I)

• y/y’àe+e-
• y/y’àe+e-
• No evidence of NP in this decay channels. Will be updated using

larger data samples

Suppressed FCNC processes (II)

• y'àΛ"

# ̅

%e+e-

• y'àΛ"

# ̅

% e+e-

• FCNCs are forbidden at the tree level due to the Glashow-Iliopoulos-Maiani (GIM)

mechanism, sensitive to NP.

• No evidence of NP in this decay channel.

Suppressed FCNC processes (III)

• Dà'e+e-
• No evidence of NP in these decay channels.

Invisible decay search

• w f
• There is strong evidence from many astrophysical observations for the existence of dark

matter.

• Dark matter is invisible in the entire electromagnetic spectrum, and its existence is inferred

via gravitational effects only.

• Any information about its interactions with a SM particle would shed light on the nature of

dark matter.

• Quarkonium states contains a quark and its own antiquark, are expected to annihilate into

a neutrino pair ! ̅ ! via a virtual Z0 boson.

• The branching fraction of the invisible decays might be enhanced by several orders of

magnitude in the presence of light dark matter (LDM) particles c.

Invisible decay search

• w f

Invisible decay search

• w f yà w/f h
• First attempt on BESIII.

Light Dark Photon searches at BESIII

• We still know very little about the constituents and interactions of dark matter.
• Many models beyond the SM of particle physics have proposed the existence of a dark

sector, which is being searched for with efforts from different types of experiments

• The simple realizations of these models usually consist of an extra U(1) gauge group, with

a corresponding massive vector boson force carrier, called a dark photon (γ’), which is neutral under the SM gauge symmetries, but couples to the SM photon via kinetic mixing and decays into SM particles. (B. Holdom, Phys. Lett. 166B, 196 (1986).)

• Such models provide a natural scenario for dark matter interactions.
• Low-energy electron-positron colliders, such as BESIII, offer an ideal environment to test

these low-mass dark sector models

Dark Photon search in J/yàh(')e+e- (I)

• g

Dark Photon search in J/yàh(')e+e- (II)

• J/yàh'g'

Dark Photon search with ISR process

• y
• Comparable to other experiments

Dark Photon search results from different experiments

• BESIII results are cited into Darkcast

a global fit to constrain models with experimental results

Baryon Number Violation

• J/yàΛ"

# e-

• J/yàΛ"

# e-

• Many proposals predict BN violation

within the extended SM and beyond

• This is the first BNV searching in

quarkonium decay

Lepton Number Violation (I)

• DàKp e+e+
• Neutrino oscillation à neutrinos have a tiny mass.
• Dirac or Majorana neutrino?
• Majorana neutrino: Lepton number violation (LNV) by two units (Δ"=2)
• 0$%% decay: ,, . → ,, . + 2 + 223, most promising
• Three body or four body decays of 4, 5, 6, 7.

BESIII

Lepton Number Violation (II)

• Fit function: MC simulation Ä a Gaussian + ARGUS
• Unbinned maximum likelihood fit

Upper limit: Take the systematic uncertainty into account First measurement Best UL Best UL a) #$ → &'()*)*) b) #) → &,

$('*)*)

c) #) → &'($*)*)

Lepton Number Violation (III)

• DàKp e+e+
• " #, %, & ', (, ) = %# + & ,

'! %.(0123)× 1 28 90 %

. :.0 ; <=>

;

× ?& @ (! %.A3 The number of events in the BCD signal region: E ∼ GHIJ %# + & , The number of events in the BCD sideband regions: K ∼ GHIJ ? & , The efficiency: L ∼ MNOJ %, 90 p Based on TROLKE program [1, 2], assume: p Likelihood function: ~39

R2 → T

U V%2WX(8.%2)

MC Data

signal Sideband Sideband RV → T.e2WX(8.%2) R2 → T

U V%2WX(8.%2)

BZ[ signal region: [1.865, 1.875] GeV/c2; Sideband region: [1.842, 1.854] ⋃[1.878, 1.886] GeV/c2

10.^ ~ 10._

Summary

Ø BESIII has performed wide range of searches of BSM new physics, including CLFV, etc; Not all topics are covered in this talk. Ø World largest J/y, y(2S), y(3770), y(4160), y(4260) , …… samples have been collected at BESIII, a unique place to search for new physics. Ø BEPCII beam energy is upgraded from 2.3 to 2.45 GeV from 2019; topup injection increases luminosity by 30%; peak luminosity upgrade at high energy is under discussion. Ø BESIII inner detector upgrade in progress. Ø More inspiring results are on the way.