Oscillating Features in the Neutron Electromagnetic Structure

Speaker：Jifeng Hu (on behalf of BESIII Collaboration)

Affiliation：Institute of quantum matter, South China Normal University

Motivation

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Before Discovery of the Neutron

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Discovery of the Neutron (1932)

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Electron Scattering (1950s)

Hofstadter R. etc., “electron scattering and nuclear structure” Rev. Mod. Phys. 28, 214–254 (1956)

Robert Hofstadter - Biographical (nobelprize.org)

Over a period of time lasting at least two thousand years, Man has puzzled over and sought an understanding of the composition of matter. … Indeed this structure may be quite complex, so that the elegant idea of elementarity must be abandoned.

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Electromagnetic Form Factor

• A parameterized function depending on the momentum transfer q².
• Imaging the nucleon structure using the electromagnetic probe q².

Point-like scattered by point-like

The structured scattered by point-like

|F(q)|²≡1

Spin=0

Spin=½

|F(q)|²

^dσ/dΩ_Mott

Hofstadter R. etc., Rev. Mod. Phys. 28, 214–254 (1956)

Prog.Part.Nucl.Phys.59:694-764,2007

Simplest observable

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Developments

Laboratoire de l’Acc´elerateur Lin´eaire in Orsay (1956)

Cambridge Electron Accelerator (1962)

Electron-Synchrotron at Bonn (1963)

Stanford Linear Accelerator Center (1962) BaBar

Deutsches Elektronen-Synchrotron (1959)

Electron accelerators at CEA-Saclay (1945)

Nationaal Instituut voor Kernfysica en Hoge Energie Fysica (1975)

Continuous Electron Beam Accelerator Facility (CEBAF) of the Jefferson Lab

DAΦNE, KLOE, FENICE

Cornell High Energy Synchrotron Source (1999) CESR (1979)

CLEO

Beijing Electron-positron Collider (1988)

Japan Proton Accelerator Research Complex 

High Energy Accelerator Research Organization (1997) in Tsukuba

The Mainz Microtron MAMI

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Experiments

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Space-like versus Time-like

Borrowed from Prof. Olsen’s talk.

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The γ*-Nucleon Coupling Puzzle (1998)

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Predictions before FENICE

http://personalpages.to.infn.it/~feliciel/pub/mySlides/HS96.pdf

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Precision Era

Borrowed from Prof. Olsen’s talk.

does BESIII address something new ?

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The BEPCII Collider

BEPCII is a high luminosity collider.

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The BESIII Experiment

Nucl. Instrum. Meth. A 614, 345 (2010)

A precision frontier experiment under running in tau-charm energy region.

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China (48/367)

Institute of High Energy Physics (146), other units(221): Beijing Institute of Petro-chemical Technology, Beihang University,

China Center of Advanced Science and Technology, Fudan University,

Guangxi Normal University, Guangxi University,

Hangzhou Normal University, Henan Normal University,

Henan University of Science and Technology,

Huazhong Normal University, Huangshan College, Hunan University,

Hunan Normal University, Henan University of Technology

Institute of modern physics, Jilin University, Lanzhou University, Liaoning Normal University, Liaoning University, Nanjing Normal University, Nanjing University, Nankai University, North China Electric Power University,

Peking University, Qufu normal university, Shanxi University,

Shanxi Normal University, Sichuan University, Shandong Normal University,

Shandong University, Shanghai Jiaotong Univeristy, Soochow University,

South China Normal University, Southeast University, Sun Yat-sen University,

Tsinghua University, University of Chinese Academy of Sciences, University of Jinan, University of Science and Technology of China,

University of Science and Technology Liaoning,

University of South China, Wuhan University, Xinyang Normal University,

Zhejiang University, Zhengzhou University,YunNan University , China University of Geosciences

Europe (17/115)

Germany (6): Bochum University,

GSI Darmstadt, Helmholtz Institute Mainz, Johannes Gutenberg University of Mainz, Universitaet Giessen,University of Münster �Italy (3): Ferrara University, INFN, University of Torino�Netherlands (1):KVI/University of Groningen

Russia (2): Budker Institute of Nuclear Physics, Dubna JINR

Sweden (1):Uppsala University

Turkey (1):Turkish Accelerator Center Particle Factory Group�UK (2): University of Manchester, University of Oxford

Poland (1)National Centre for Nuclear Research

​

USA(4/8)�Carnegie Mellon University�Indiana University�University of Hawaii�University of Minnesota

~500 members

From 76 institutions in 16 countries

Asia (6/10)

Pakistan (2): COMSATS Institute of Information Technology

University of the Punjab, University of Lahore

Mongolia (1): Institute of Physics and Technology

Korea (1): Chung-Ang University

India (1): Indian Institute of Technology madras

Thailand (1): Suranaree University of Technology

South America (1/1)� Chile: University of Tarapaca

BESIII Collaboration

Borrowed from Prof. Li Hai-bo’s talk.

Access to Time-like Form Factor

Energy scan

Initial-state-radiation

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Taking Data

688 pb^-1 R-scan data and 10 billion J/Ψ data.

​

​

Borrowed from Prof. Li Hai-bo’s talk.

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Taking Data

[GeV]

2011 tau mass scan, 2012 R scan, 2013-2014 R scan phase 2, 2014-2015 R scan phase 1.

[亿]

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How to Measure ?

Hunt …

Assumption

This talk demonstrates effective form factor |G|.

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Signal features

Main Drift Chamber (MDC)

Time-of-Flight Detector (TOF)

Electromagnetic calorimeter (EMC)

Muon Chamber (MUC)

The analysis team developed new reconstruction packages for neutral events, making use of EMC+TOF+MUC.

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1. Optimization: three categories

Use TOF reconstruction

Use MUC rejection

1. mutually exclusive categories 2. maximize the signal yields 3. cross-check with each measurement

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2. Verification of Selection Logic

e⁺e^- 🡪 γγ@2.125 GeV

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2. Verification of Selection Logic

Time of flight

crossing angle

Using BDT

crossing angle

PDG

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3. Efficiency & Radiation Correction

efficiencies are checked with two ways:

1. an exclusive MC simulation sample,
2. weighted by a control sample.

Sample1: MC simulation with ISR photons

(NLO)

Sample 2: MC simulation only the leading order

(LO)

Event selections for category A

Suppose we observed the number of events :N^obs

The selection efficiency is defined to be:

Thus the BORN cross-section is obtained:

The radiation function:

PDF(E) must be already known.

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3. Efficiency Calibration

The precision reaches down to <2% for the efficiency calibration of both antineutron and neutron.

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4. Extraction of Signal Yields

Time of flight

crossing angle

Using BDT

crossing angle

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Results of Three Categories

We combine 3 categories to get the best measurement.

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Cross-sections and Form Factors

• 1^st highlight of the results: most precise in the time-like region.
• The precision improved > a factor of ~30 over previous results.

(a) Cross-section (b) Effective Form Factor

Nature Physics volume 17, p1200–1204 (2021)

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Cross-section Ratios

• 2^nd highlight of the results: precise ratios in 2.0~3.08 GeV.
• Clearly clarifying the photo-nucleon interaction puzzle.

PRL. 124 (2020) no.4, 042001 

[8]: Phys. Rev. D 99, no. 1, 014510 (2019); [10]: Z. Phys. C 52, 631 (1991).; [11]: Phys. Rept. 112, 173420 (1984)

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Oscillation in the Form Factors

• 3^rd highlight of the results: oscillation around the dipole law.
• A phase shift around (125±12)° is observed.

PRC 93, 035201 (2016).

PRL 114, 232301 (2015).

PRD 92, 198 034018 (2015).

Nobody predicted this

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Understanding the Form Factor

Physics Reports 550–551 (2015) 1–103

proton

Semi-classical:

Generalized:

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Understanding the Form Factor

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Understanding the Form Factor

N = p, n, Λ, Σ ...

Leading order

Unique access to non-stable baryons in the TL region at BESIII.

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A Story of the Nucleon Picture

BESIII achieved a mile stone in understanding the neutron.

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From Idea to Fruit

Carrying out a typical research @BESIII

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Summary

For the neutron time-like form factor, BESIII collaboration

1. released novel results with the best precision so far.
2. clarified the interaction puzzle persistent for over 20 years.
3. observed an oscillating behavior similar to the one in the proton.

Thanks for your attention

More work needed to dig out the implication from our data

Acknowledgement to the owners because some pictures are borrowed from the internet without any contact.

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Appendix

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The Proton Time-like Form Factor

PRL 124, 042001 (2020)

PRD 99, 092002 (2019)

PRD 91, 112004 (2015)

Most precise measurements in the time-like region

The Proton

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1^st Precise Separation G_E from G_M

Precision: |G_E| 2-94%, |G_M| 1.5-9%

the precision comparable to the one in spacelike region

PRL 124, 042001 (2020)

The Proton

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Confirmation

Oscillation exists and needs more understanding.

G_osc

The Proton

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Periodic Interference Structures

PRL 114 (2015) no.23, 232301

Nobody predicted this

BaBar Data

The Proton

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