Precision Measurement of R Value in the continuum Region Zhen Gao①, Bingxin Zhang② ①:University of Science and Technology of China ②Institute of High Energy Physics On behalf of the R working group

Outline Introduction Data sets Event selection criteria Background analysis Determination of hadronic events and R-value Summary & next to do

Introduction(1) 𝑅= 1 𝜎 𝜇𝜇 0 ∙ 𝑁 ℎ𝑎𝑑 − 𝑁 𝑏𝑘𝑔 𝐿∙ 𝜀 ℎ𝑎𝑑 ∙ 𝜀 𝑡𝑟𝑖𝑔 ∙(1+𝛿) At lowest order, 𝑅≡ 𝜎( 𝑒 + 𝑒 − →ℎ𝑎𝑑𝑟𝑜𝑛𝑠) 𝜎( 𝑒 + 𝑒 − → 𝜇 + 𝜇 − ) = 𝑞 𝜎( 𝑒 + 𝑒 − →𝑞 𝑞 ) 𝜎( 𝑒 + 𝑒 − → 𝜇 + 𝜇 − ) In experiment, 𝑅= 1 𝜎 𝜇𝜇 0 ∙ 𝑁 ℎ𝑎𝑑 − 𝑁 𝑏𝑘𝑔 𝐿∙ 𝜀 ℎ𝑎𝑑 ∙ 𝜀 𝑡𝑟𝑖𝑔 ∙(1+𝛿) 𝜎 𝜇𝜇 0 : born-level cross section for 𝑒 + 𝑒 − → 𝜇 + 𝜇 − 𝑁 ℎ𝑎𝑑 : number of hadronic events 𝑁 𝑏𝑘𝑔 : number of background hadronic events 𝐿: integrated luminosity 𝜀 ℎ𝑎𝑑 : detection efficiency for hadrons 𝜀 𝑡𝑟𝑖𝑔 :𝑡𝑟𝑖𝑔𝑔𝑒𝑟 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 1+𝛿: radiative correction factor

Introduction(2) One of the most fundamental quantities in particle physics that directly reflect the flavor and the color of the quarks A necessary input for : 𝛼 𝑠 −𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑐 𝑟𝑢𝑛𝑛𝑖𝑛𝑔 𝑐𝑜𝑢𝑝𝑙𝑖𝑛𝑔 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑎 𝜇 −anomalous magnetic moment of the muon

Introduction(3) BESII measured the R-value between 2 and 5GeV , with an average precision of 6.6%. BESIII is goal to low the precision of R both in the charmonium region( 3% ) and continuum region.

Potential backgrounds QED process : Bhabha , 𝛾 𝛾𝛾 , 𝛾 𝜇 + 𝜇 − , 𝛾 𝜏 + 𝜏 − 𝑒 + 𝑒 − → 𝑒 + 𝑒 − + hadrons Cosmic ray Beam-associated backgrounds : beam-wall; beam-gas; Touschek effect

Data sets Collision data at : 2.2324,2.4,2.8,3.05,3.06,3.08,3.4,3.5,3.5424,3.5538,3.5611,3.6002,3.65,3.671GeV 14 energy points BOSS version: BOSS664p01 MC sample: (2.2324~3.671 GeV 13 energy points) 400000 e+e-→hadrons(LUNDARLW+exclusive ) 500000 e+e-→e+e- (babayaga) 500000 e+e-→μ+μ- (babayaga) 300000 e+e-→τ+τ- (KKMC) 500000 e+e-→γγ (babayaga) 500000 e+e-→e+ e- X (two photon process)

MC Generator Comparison of summed cross section for exclusive decays to the measured ones. Black : measured cross section Red : summed exclusive cross section

Event Selection(I) Veto Bhabha and 𝑒 + 𝑒 − →𝛾𝛾 (Only EMC) Two showers with maximum deposited energy 𝜃 1 + 𝜃 2 −180 <10° & 𝐸 > 0.65∙ 𝐸 𝑏𝑒𝑎𝑚 Good hadronic tracks (track level) |Vr|<1.0 cm, |cosθ|<0.93, no Vz cut. Momentum < 1.1· 𝐸 𝑏𝑒𝑎𝑚 𝑑𝐸 𝑑𝑥 𝑚𝑒𝑎. − 𝑑𝐸 𝑑𝑥 𝑝𝑟𝑜𝑡𝑜𝑛 𝜎 𝑝𝑟𝑜𝑡𝑜𝑛 <10 Veto large momentum electrons, if E/p > 0.8, p<0.65· 𝐸 𝑏𝑒𝑎𝑚 Veto gamma conversion, if M( 𝑒 + 𝑒 − )<0.1 and angle( 𝑒 + 𝑒 − )<15 °

Event Selection(II) Hadronic event candidates 𝑁 𝐺𝑜𝑜𝑑 ≥2 ;𝑉𝑖𝑠𝑖𝑏𝑙𝑒 𝑒𝑛𝑒𝑟𝑔𝑦(𝐸𝐸𝑀𝐶) >0.4∙ 𝐸 𝑏𝑒𝑎𝑚 If 𝑁 𝐺𝑜𝑜𝑑 = 2 : Veto: 𝜃 1 + 𝜃 2 −180 <15°& 𝜑 1 − 𝜑 2 −180 <10° Number of isolated photon ≥ 2 If 𝑁 𝐺𝑜𝑜𝑑 ≥3 : Veto: Angle of two largest energetic tracks, 𝜃 1𝑠𝑡 + 𝜃 2𝑛𝑑 −180 <15°& 𝜑 1𝑠𝑡 − 𝜑 2𝑛𝑑 −180 <10°

Veto Bhabha and 𝒆 + 𝒆 − →𝜸𝜸 @ 2.23GeV Veto : Signal survive ratio : > 99% Background veto ratio : ~70%

Vr @ 2.23GeV |Vr| < 1cm for each track is applied!

dE/dx @2.8 GeV @3.4 GeV cut： 𝑑𝐸/𝑑𝑥(𝑚𝑒𝑎𝑠.)−𝑑𝐸/𝑑𝑥(𝑝𝑟𝑜𝑡𝑜𝑛) 𝞼(𝑝𝑟𝑜𝑡𝑜𝑛) <10

Veto large momentum e±

Veto large momentum e±

Event selection : Veto: E/p > 0.8 && p > 0.65Ebeam After adding this cut :

Veto gam-conversion @2.23 GeV @3.08 GeV e± : E/p > 0.8 Gamma conversion : if M( 𝑒 + 𝑒 − )<0.1 and angle( 𝑒 + 𝑒 − )<15 °

Visible energy 𝐶𝑢𝑡 : 𝑉𝑖𝑠𝑖𝑏𝑙𝑒 𝑒𝑛𝑒𝑟𝑔𝑦 >0.4∙ 𝐸 𝑏𝑒𝑎𝑚 @ 3.4GeV Visible energy : all energy deposition in EMC both for charged tracks and neutral ones.

Isolated photon @ 2.23GeV @ 3.08GeV Only for NGood = 2, the minimum isolated photon requirement is added Isolated photon : Angletrack > 20 ° , Eγ > 0.1GeV and 0 < T < 15 Number of isolated photon >= 2 @ 2.23GeV @ 3.08GeV

Data&MC comparison nGood ① : 2.2324 ② : 2.4 ③ : 2.8 ④ : 3.05 ⑤ : 3.06 ① : 2.2324 ② : 2.4 ③ : 2.8 ④ : 3.05 ⑤ : 3.06 ⑥ : 3.08 ⑦ : 3.4 ⑧ : 3.5 ⑨ : 3.5424 ⑩ : 3.5538 ⑪ : 3.5611 ⑫ : 3.6002 ⑬ : 3.65 ⑭ : 3.671

Data&MC comparison cos𝞱 ① : 2.2324 ② : 2.4 ③ : 2.8 ④ : 3.05 ⑤ : 3.06 ① : 2.2324 ② : 2.4 ③ : 2.8 ④ : 3.05 ⑤ : 3.06 ⑥ : 3.08 ⑦ : 3.4 ⑧ : 3.5 ⑨ : 3.5424 ⑩ : 3.5538 ⑪ : 3.5611 ⑫ : 3.6002 ⑬ : 3.65 ⑭ : 3.671

Data&MC comparison nGam ① : 2.2324 ② : 2.4 ③ : 2.8 ④ : 3.05 ⑤ : 3.06 ① : 2.2324 ② : 2.4 ③ : 2.8 ④ : 3.05 ⑤ : 3.06 ⑥ : 3.08 ⑦ : 3.4 ⑧ : 3.5 ⑨ : 3.5424 ⑩ : 3.5538 ⑪ : 3.5611 ⑫ : 3.6002 ⑬ : 3.65 ⑭ : 3.671

Data&MC comparison Phi ① : 2.2324 ② : 2.4 ③ : 2.8 ④ : 3.05 ⑤ : 3.06 ① : 2.2324 ② : 2.4 ③ : 2.8 ④ : 3.05 ⑤ : 3.06 ⑥ : 3.08 ⑦ : 3.4 ⑧ : 3.5 ⑨ : 3.5424 ⑩ : 3.5538 ⑪ : 3.5611 ⑫ : 3.6002 ⑬ : 3.65 ⑭ : 3.671

Data&MC comparison M(γγ) ① : 2.2324 ② : 2.4 ③ : 2.8 ④ : 3.05 ⑤ : 3.06 ① : 2.2324 ② : 2.4 ③ : 2.8 ④ : 3.05 ⑤ : 3.06 ⑥ : 3.08 ⑦ : 3.4 ⑧ : 3.5 ⑨ : 3.5424 ⑩ : 3.5538 ⑪ : 3.5611 ⑫ : 3.6002 ⑬ : 3.65 ⑭ : 3.671

Data&MC comparison M(pipi) ① : 2.2324 ② : 2.4 ③ : 2.8 ④ : 3.05 ⑤ : 3.06 ⑥ : 3.08 ⑦ : 3.4 ⑧ : 3.5 ⑨ : 3.5424 ⑩ : 3.5538 ⑪ : 3.5611 ⑫ : 3.6002 ⑬ : 3.65 ⑭ : 3.671

Data&MC comparison M(KK) ① : 2.2324 ② : 2.4 ③ : 2.8 ④ : 3.05 ⑤ : 3.06 ① : 2.2324 ② : 2.4 ③ : 2.8 ④ : 3.05 ⑤ : 3.06 ⑥ : 3.08 ⑦ : 3.4 ⑧ : 3.5 ⑨ : 3.5424 ⑩ : 3.5538 ⑪ : 3.5611 ⑫ : 3.6002 ⑬ : 3.65 ⑭ : 3.671

Fitting the average Vz @ 2.23GeV 𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑉𝑧= 1 𝑁 𝐺𝑜𝑜𝑑 𝑉𝑧(𝑖) 𝑁 𝐺𝑜𝑜𝑑 Black dots : data Green : MC shape Red : fitting background Blue : total fitting result @ 2.23GeV Signal : MC shape convoluted Gaussian Bkg : 3rd polynominal N(signal) : 90463 ± 312 N(bkg) : 2364 ± 95 𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑉𝑧= 1 𝑁 𝐺𝑜𝑜𝑑 𝑉𝑧(𝑖) 𝑁 𝐺𝑜𝑜𝑑

Cross check: Fitting result @ 2.6444 GeV Black dots : data Green : MC shape Red : fitting background Yellow : Separated beam data Blue : total fitting result ChiSquare = 5.48 Signal : MC shape Bkg : 2nd polynominal N(signal) : 286119 ± 566 N(bkg) : 26974 ± 248

QED background estimation @ 2.23GeV Bhabha digam dimu eeX Total 500000 nGood == 2 145 289 527 1431 nGood >= 3 185 41 265 332 Remain 330 792 1763 Survive ratio 0.066% 0.15% 0.35% Total background after scaled : 3.08 % @ 3.08GeV Bhabha digam dimu eeX Total 500000 nGood == 2 106 212 374 1551 nGood >= 3 167 28 190 549 Remain 273 240 564 2100 Survive ratio 0.054% 0.048% 0.11% 0.42% Total background after scaled : 2.49 %

QED background estimation Scaled Number!

Efficiency

Difference between two requirements ① . Veto large momentum electrons, if E/p > 0.8, p<0.65· 𝐸 𝑏𝑒𝑎𝑚 ② . Veto all electrons, E/p > 0.8 Data① Data② MC① MC② Diff 2.23 87675 75014 71.58% 62.36% -1.79% 2.4 101614 87593 73.58% 65.06% -2.51% 3.4 32667 29030 82.67% 76.36% -3.79%

Costheta distribution using cut2 Costheta of all the hadronic tracks ② . Veto all electrons, E/p > 0.8

Summary & next to do Preliminary result of the R-value using 14 energy points in the continuum region; selection criteria needs to be studied further; More work on MC tuning needs to be done; Systematic uncertainty.

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Fitting result @2.2324, 2.4 GeV N(signal) : 90463 ± 312 N(bkg) : 2364 ± 95 N(signal) : 104409 ± 336 N(bkg) : 3348 ± 108