Measurement of e + e − →p p Cross Section Lei Xia, Xiaorong Zhou, Guangshun Huang, Zhengguo Zhao University of Science and Technology of China The BESIII Collaboration Winter Meeting 2015 Dec 14th 2015 Institute of High Energy Physics, Chinese Academy of Sciences

Measurement of e+e-→ppbar Cross Section Outline Introduction Data sets and event selection of e + e − →p p Boss version & Data sets Preliminary Selection Criteria Momentum window Cross section of e + e − →p p and effective Form Factor Cross section measurement Effective Form Factor Detection efficiency : Model dependence Extraction of electromagnetic Form Factor ratio Fit on the polar angular distribution of proton Preliminary results Summary December 14th 2015 Measurement of e+e-→ppbar Cross Section

Measurement of e+e-→ppbar Cross Section Introduction December 14th 2015 Measurement of e+e-→ppbar Cross Section

Motivation FIGURE I. The Feynman diagram of e + e − →p p . Improve the uncertainty of G E G M ratio. Research for electromagnetic Form Factor ratio of proton. FIGURE I. The Feynman diagram of e + e − →p p . December 14th 2015 Measurement of e+e-→ppbar Cross Section

Measurement of e+e-→ppbar Cross Section Motivation Reveal the structure around 2.25 GeV and 3.0 GeV observed by Babar. Especially at 2.0 GeV, where large disagreement is observed bet- ween BaBar results. FIGURE II. Cross Section around 2.25 GeV and 3.0 GeV observed by Babar. December 14th 2015 Measurement of e+e-→ppbar Cross Section

Data sets and event selection e + e − →p p Boss version & Data sets Preliminary Selection Criteria Momentum window December 14th 2015 Measurement of e+e-→ppbar Cross Section

Boss version & Data sets Boss version: Boss6.6.5.p01 Data sets: TABLE I. Data set December 14th 2015 Measurement of e+e-→ppbar Cross Section

Boss version & Data sets FIGURE III. Data set 500k p p , p p π 0 MC sample at each c.m. energy, generated with Conexc. 500k p p MC sample at each c.m. energy, generated with Phokhara. 500k bhabha MC sample at each c.m. energy, generated with Babayaga. 500k dimu MC sample at each c.m. energy, generated with Babayaga. December 14th 2015 Measurement of e+e-→ppbar Cross Section

Measurement of e+e-→ppbar Cross Section Preliminary Selection Criteria Good charged tracks V r <1.0cm, V z <10cm, cos θ <0.93 , p<2GeV Particle identification dE/dx+Tof @2.05~3.08GeV Prob(p)>Prob(K/ π) @2.05~3.08GeV For proton track, require 𝐸 𝑝 <0.5, cos θ <0.8 Charged tracks in a good event N charged =2 and N p =N p =1 December 14th 2015 Measurement of e+e-→ppbar Cross Section

Preliminary Selection Criteria FIGURE IV. For proton track, require 𝐸 𝑝 <0.5, cos θ <0.8 @3.08GeV December 14th 2015 Measurement of e+e-→ppbar Cross Section

Preliminary Selection Criteria Selection criteria in TOF tof p − tof p <4ns veto cosmic ray Tracks angle Angle p p >179° @ 2.6444~3.08 GeV >175° @ 2.05~2.5 GeV >170° @ 2.0 GeV FIGURE V. TOF cut and Angle cut December 14th 2015 Measurement of e+e-→ppbar Cross Section

Measurement of e+e-→ppbar Cross Section Momentum window Momentum window cut for proton and anti-proton Signal region: p− p exp <5σ FIGURE VII. Momentum window cut for proton and anti-proton at 2.6444 GeV 2.6444GeV December 14th 2015 Measurement of e+e-→ppbar Cross Section

Measurement of e+e-→ppbar Cross Section Momentum window Signal region: p− p exp <5σ Sideband region: 6σ< p− p exp <11σ FIGURE VII. Momentum window cut for proton and anti-proton at 2.396 GeV 2.396GeV December 14th 2015 Measurement of e+e-→ppbar Cross Section

Cross section of e + e − →p p and effective Form Factor Cross section measurement Effective Form Factor (widely adopted by previous experiments) Detection efficiency : Model dependence December 14th 2015 Measurement of e+e-→ppbar Cross Section

Cross section measurement σ Born = N obs − N bkg L∙ε∙ 1+δ σ obs = N obs L N obs : The observed number of signal in data. N bkg : The number of background evaluated from sideband. L: The integrated luminosity ε: Detection efficiency by MC sample generated by Conexc. 1+δ : Radiative correction factor Effective Form Factor (widely adopted by previous experiments): σ p p s = 4π α 2 βC 3s G M s 2 + 2 m p 2 s G E s 2 Assuming G E = G M (which holds at pp mass threshold only) 𝐺 = σ Born 86.83∙ 𝛽 𝑠 1+ 2 m p 2 s December 14th 2015 Measurement of e+e-→ppbar Cross Section

Cross section of e + e − →p p and effective Form Factor TABLE II. Cross section of e + e − →p p and effective Form Factor December 14th 2015 Measurement of e+e-→ppbar Cross Section

Detection efficiency : Model dependence d σ p p s dΩ = α 2 βC 4s G M s 2 1+ cos 2 θ p + 4 m p 2 s G E s 2 sin 2 θ p G E =0: angular distribution of proton: 1+ cos 2 θ p . G M =0: angular distribution of proton: 1− cos 2 θ p . Two methods to get the nominal detection efficiency: 1. Two sets of MC samples are generated, with G E =0 and G M =0 hypothesis. – The mean detection efficiency is the average of them. 2. Another set of MC samples is generated, with G E = G M hypothesis. December 14th 2015 Measurement of e+e-→ppbar Cross Section

Detection efficiency : Model dependence TABLE III. Detection efficiency : Model dependence December 14th 2015 Measurement of e+e-→ppbar Cross Section

Cross section of e + e − →p p and effective Form Factor TABLE IV. Cross section of e + e − →p p and effective Form Factor Detection efficiency : Average of G E =0, G M =0 December 14th 2015 Measurement of e+e-→ppbar Cross Section

Cross section of e + e − →p p and effective Form Factor FIGURE VIII. Cross section of e + e − →p p and effective Form Factor Detection efficiency : Average of G E =0, G M =0 December 14th 2015 Measurement of e+e-→ppbar Cross Section

Cross section of e + e − →p p and effective Form Factor TABLE V. Cross section of e + e − →p p and effective Form Factor Detection efficiency : | G E |=| G M | December 14th 2015 Measurement of e+e-→ppbar Cross Section

Cross section of e + e − →p p and effective Form Factor FIGURE IX. Cross section of e + e − →p p and effective Form Factor Detection efficiency : | G E |=| G M | December 14th 2015 Measurement of e+e-→ppbar Cross Section

Cross section of e + e − →p p and effective Form Factor TABLE VI. Cross section of e + e − →p p and effective Form Factor Generator : Phokhara December 14th 2015 Measurement of e+e-→ppbar Cross Section

Cross section of e + e − →p p and effective Form Factor FIGURE X. Cross section of e + e − →p p and effective Form Factor Generator : Phokhara December 14th 2015 Measurement of e+e-→ppbar Cross Section

Cross section of e + e − →p p and effective Form Factor FIGURE XI. Cross section of e + e − →p p and effective Form Factor Compare December 14th 2015 Measurement of e+e-→ppbar Cross Section

Extraction of electromagnetic Form Factor ratio Fit on the polar angular distribution of proton Preliminary results December 14th 2015 Measurement of e+e-→ppbar Cross Section

Fit on the polar angular distribution of proton We need fit on the polar angular distribution of proton, dσ dΩ q 2 ，θ = α 2 βC 4s G M s 2 1+ cos 2 θ p + R em 2 1 τ sin 2 θ p τ= 4 m 2 q 2 R em = G E q 2 G M q 2 FIGURE XII. Fit on the polar angular distribution of proton 3.08GeV 2.9GeV 2.6464GeV December 14th 2015 Measurement of e+e-→ppbar Cross Section

Fit on the polar angular distribution of proton FIGURE XII. Fit on the polar angular distribution of proton 2.6444GeV 2.396GeV 2.3864GeV 2.3094GeV 2.2324GeV 2.2GeV December 14th 2015 Measurement of e+e-→ppbar Cross Section

Fit on the polar angular distribution of proton FIGURE XII. Fit on the polar angular distribution of proton 2.175GeV 2.15GeV 2.1GeV 2.05GeV 2.0GeV The distributions of cos θ have been corrected with detection efficiencies by MC simulation samples. December 14th 2015 Measurement of e+e-→ppbar Cross Section

Preliminary results Preliminary results of fit on cos θ TABLE VII. Preliminary results of fit on cos θ The range of relative statistical error of G E G M ratio is between 7.2% and 69.3%. December 14th 2015 Measurement of e+e-→ppbar Cross Section

Preliminary results FIGURE XIII. G E G M of proton, Comparation with BaBar, PS170 December 14th 2015 Measurement of e+e-→ppbar Cross Section

Measurement of e+e-→ppbar Cross Section Summary December 14th 2015 Measurement of e+e-→ppbar Cross Section

Measurement of e+e-→ppbar Cross Section Summary Using 21 data sets from 2.0 to 3.08 GeV, the p p cross section is measured. No indication for structure around 2.25 and 3.0 GeV. The cross section at 2.0 GeV is smaller than Babar result. December 14th 2015 Measurement of e+e-→ppbar Cross Section

Thanks for Your Attention! December 14th 2015 Measurement of e+e-→ppbar Cross Section

Measurement of e+e-→ppbar Cross Section Backups December 14th 2015 Measurement of e+e-→ppbar Cross Section

Preliminary Selection Criteria FIGURE IV. For proton track, require 𝐸 𝑝 <0.5 3.08GeV 2.9GeV 2.6444GeV 2.396GeV 2.05GeV 2.0GeV December 14th 2015 Measurement of e+e-→ppbar Cross Section

Preliminary Selection Criteria FIGURE IV. For proton track, require cos θ <0.8 3.08GeV 2.9GeV 2.6444GeV 2.396GeV 2.05GeV 2.0GeV December 14th 2015 Measurement of e+e-→ppbar Cross Section

Measurement of e+e-→ppbar Cross Section Preliminary Selection Criteria FIGURE V. Selection criteria in TOF tof p − tof p <4ns 3.08GeV 2.9GeV 2.6444GeV 3.08GeV 2.396GeV 2.2324GeV 2.0GeV December 14th 2015 Measurement of e+e-→ppbar Cross Section

Preliminary Selection Criteria FIGURE VI. Tracks angle cut at different energy point 3.08GeV 2.9GeV 2.6444GeV 2.396GeV 2.05GeV 2.0GeV December 14th 2015 Measurement of e+e-→ppbar Cross Section

Measurement of e+e-→ppbar Cross Section Momentum window FIGURE VII. Momentum window cut for proton and anti-proton at energy points 2.9GeV 2.6464GeV 2.3094GeV 2.2GeV 2.05GeV 2.0GeV December 15th 2015 Measurement of e+e-→ppbar Cross Section

Comparation with HIM Group @2.0 GeV Particle identification @2.0 GeV Preliminary results December 14th 2015 Measurement of e+e-→ppbar Cross Section

Measurement of e+e-→ppbar Cross Section Particle identification @2.0 GeV Good charged tracks V r <1.0cm, V z <10cm, cos θ <0.93 , p<2GeV Vertex fit 𝜒 2 𝑣𝑡𝑥 <100 Tracks angle Angle p p >178° @ 2.3864~3.08 GeV >175° @ 2.05~2.3094 GeV >170° @ 2.0 GeV Veto cosmic rays tof p − tof p <4ns If p or p does not have hit TOF, event is kept at (2.0~2.3094) GeV, but discarded at (2.3864~3.08)GeV. December 14th 2015 Measurement of e+e-→ppbar Cross Section

Measurement of e+e-→ppbar Cross Section Particle identification @2.0 GeV Particle identification Probability of p and p larger than that of electron, pion, kaon and 𝐸 𝑝 <0.5 @ (2.3864 ~ 3.08) GeV Probability of p and p larger than that of pion, kaon and 𝐸 𝑝 < 0.5 @ (2.15 ~ 2.3094) GeV (tracks with no EMC info are accepted) normPH>CUT of p and p CUT=(3.0, 2.5, 2.0, 1.7, 1.7, 1.5, 1.5, 1.4, 1.265) for (2.0, 2.05, 2.1 2.15, 2.175, 2.2, 2.2324, 2.3094, 2.3864, 2.396) GeV Charged tracks in a good event N charged =2 and N p =N p =1 December 14th 2015 Measurement of e+e-→ppbar Cross Section

Measurement of e+e-→ppbar Cross Section Particle identification @2.0 GeV Momentum window cut for proton and anti-proton Signal region: p−4σ< p exp <p+3σ December 14th 2015 Measurement of e+e-→ppbar Cross Section

Measurement of e+e-→ppbar Cross Section Preliminary results Preliminary results p and p events comparation: Mine:4857 Vs Yadi:4814 Efficiency comparation Mine:62.1%7 Vs Yadi:59.6% December 14th 2015 Measurement of e+e-→ppbar Cross Section

Measurement of e+e-→ppbar Cross Section Tracking Efficiency Tracking Efficiency with p Tracking Efficiency with p December 14th 2015 Measurement of e+e-→ppbar Cross Section

Measurement of e+e-→ppbar Cross Section Events 3.08 4472 2.9 4452 2.6464 821 2.6444 826 2.396 309 December 14th 2015 Measurement of e+e-→ppbar Cross Section

Tracking Efficiency with p FIGURE VIII. Tracking Efficiency with p @3.08 GeV December 15th 2015 Measurement of e+e-→ppbar Cross Section

Tracking Efficiency with p FIGURE VIII. Tracking Efficiency with p @2.9 GeV December 15th 2015 Measurement of e+e-→ppbar Cross Section

Tracking Efficiency with p FIGURE VIII. Tracking Efficiency with p @2.6464 GeV December 15th 2015 Measurement of e+e-→ppbar Cross Section

Tracking Efficiency with p FIGURE VIII. Tracking Efficiency with p @2.6444 GeV December 15th 2015 Measurement of e+e-→ppbar Cross Section

Tracking Efficiency with p FIGURE VIII. Tracking Efficiency with p @2.396 GeV December 15th 2015 Measurement of e+e-→ppbar Cross Section

Tracking Efficiency with p FIGURE IX. Tracking Efficiency with p @3.08 GeV December 15th 2015 Measurement of e+e-→ppbar Cross Section

Tracking Efficiency with p FIGURE IX. Tracking Efficiency with p @2.9 GeV December 15th 2015 Measurement of e+e-→ppbar Cross Section

Tracking Efficiency with p FIGURE IX. Tracking Efficiency with p @2.6464 GeV December 15th 2015 Measurement of e+e-→ppbar Cross Section

Tracking Efficiency with p FIGURE IX. Tracking Efficiency with p @2.6444 GeV December 15th 2015 Measurement of e+e-→ppbar Cross Section

Tracking Efficiency with p FIGURE IX. Tracking Efficiency with p @2.396 GeV December 15th 2015 Measurement of e+e-→ppbar Cross Section