center-of-mass energy from to 4.6 GeV Xiongfei Wang( 王雄飞 ) Xinchou Lou IHEP April 18th, 2018 Charmonium group meeting

Transcription

1 Measurement of σ(e $ e % ΛΛ() at the center-of-mass energy from to 4.6 GeV Xiongfei Wang( 王雄飞 ) Xinchou Lou IHEP April 18th, 2018 Charmonium group meeting 1

2 Outline n Motivation n Data sets n Event selection n Yield extraction (counting) n Detection efficiency n Born cross section &FFs calculation n Systematic uncertainty n Summary 2

3 Motivation n The study of the production of Charmonium(-like) Y states in e $ e % annihilations above open-charm threshold can provide a test for PQCD. nmany The anomalous states in experiment behavior differing are observed from the pqcd prediction at threshold is in observed. the final PRD 97, (2018) states of meson constituents, such as Y(4260), Y(4360), etc., What sbut thelittle situation evidence far away for these states are found from the in threshold, the final states which of baryon pairs with exception should of be oalsobservation an interesting of Y(4634) in the Λ * Λ( * final states. topic. What s the situation for otherbaryon pairs, such as ΛΛ(,etc.,? n The unexpected features of baryon pair production have driven many theoretical interests in Ref.[PRC79, (2009), PLB643, 29 (2006)], including scenarios that invoke BB, bound states or unobserved meson resonances. To test the hypotheses, a precision measurement of σ - (e $ e % ΛΛ() beyond the threshold is desired. n BESIII so far has collected large data sample for above open charm threshold, it should be a good chance to measure cross section and form factor for e $ e % ΛΛ( beyond threshold, which may provide us more insights into the nature above the open charm region and experimental evidence. 3

4 Data sets XYZ Data 16 energy points Exclusive MC (PHSP) 100k for each energy point e $ e % ΛΛ( pp π % π $ BOSS version: 703 4

5 Event selection n Initial selection: Ø Charged track : ü N $ = 2&N % = 2 ü n(charge) = 0 Ø PID: Yes! ü N p = 1&N p( = 1&N π I= 1&N π J = 1 Ø Λ reconstruction: ü Using secondary-vertex reconstruction strategy by looping pπ with combination of δ min = M pπ M Λ. Ø 4C kinematic fit (ΛΛ( hypothesis) n Further selection: Ø L Λ/Λ ( > 0 Ø χ 2 4C < 100 Data Exc. MC 5

6 Extraction of signal yields (counting) N wxy = N z 1 2 N N n The statistics significance for counting experiments can be calculated based on Poisson by 2 log TMath::Poisson(S,b) TMath::Poisson(S,S) ID N S N B N A N^_`. S(σ) N UP 3773 $il.k $k.n $p.k %ij.k 24.0 %j.m 2.0 $ik.o %i.o %il.n $j.k %o.j $l.p %j.i $j.m 1.0 %n.q %o.j $o.m 8.0 %p.q 1.0 %n.q $k.n 13.3 %j.k %n.q %n.q $o.p %p.n %n.q %n.q $j.k %o.j $o.p %p.n $p.m %i.k $o.j %p.p 1.0 %n.q 1.0 %n.q $o.q 3.8 %p.i $p.m $o.i 3.0 %i.k %i.j $p.q 1.0 %n.q %i.n $p.k 2.0 %i.o 0 $l.n 11.0 %o.l $o.k 1.0 %n.q %p.i $o.o 1.0 %n.q %p.i $p.k $o.m 2 %i.o %p.j $p.m $p.m %i.k %i.k %n.q %n.q $o.p %p.n %n.q $o.p %p.n %n.q $o.j 4.3 %p.n $o.k 1.0 %n.q %p.i $p.m $p.q 3.0 %i.k %i.p n Upper limit for counting experiment is estimated by statistics

7 Signal yields extraction

8 Detection KKMC n Iteration method n Initial state radiation correction n To be stable (a) The curve of 1 + σ ε, (b) and (c) the Born cross section for last two iterations fitted by second polynomial. detected The efficiency is defined by: ε = N ΛΛ( generated N ΛΛ ( 8

9 Signal MC

10 Calculation of Born cross sections and FFs n Experimentally, Born cross sections of e $ e % ΛΛ( are calculated by: σ B = N obs L 1$δ 1$Π εb 2 (Λ pπ), where N obs number of observed events, L luminosity, 1 + σ ISR factor, 1 + Π vacuum polarization factor, B the branching fraction. n Theoretically, Born cross section can be expressed as: σ B = 4πα2 Cβ [ G 3s M 2 + 2m Λ 2 s The effective form factor defined by G eff (s) = G E 2 ]. G M 2 $ 2m 2 Λ G s E 2 1$2m2 Λ/s G M/E : electric/magnetic FF is proportional to the square root of the baryon pair production cross section, to be β = 1 4m Λ 2 : velosity s α = 1 : fine structure constant 137 s: the square of CM energy Coulomb factor C =1 for neutral mode G eff (s) = 3sσ B 4πα 2 β(1$ 2m Λ 2. s ) 10

11 Numerical results ID s (MeV) L(pb %1 ) 1 + Π δ ε(%) S(σ) N obs N UP σ B (pb 10 %3 ) G eff s 10 %3 $il.p $jk.n ± %ij.p %jo.n $o.m $kk.m ± %p.m %jm. $j.j $ip.p ± %o.n %q.o ± %n.q $oq. %io.l (< 65.6) $j.j ± %o.n -- $ j.n 63.9 %ln.l ± %n.l $pp.q %q.q (<45.7) ± %n.o $pp.j %l.p (< 67.2) $o.j $pq.o ± %p.o %im.i $p.j ± %i.j -- $jp.o 61.7 %iq.l $p.i ± %i.i $ol.o %iq.l (<112.8) $o.p ± %p.n $oj.k %pi.k (<86.6) $p.m ± %i.k $ln.i %p. (<115.9) ± %n.q $ipp.n %jp.j (<206.8) ± $p.m 4.3 %i.l -- $ln.oo 74.6 %pk.n $p.j $pp.k ± %i.j %io.p ± %n.o $io.m %o.p (< 24.6) $p.q 10.3 %p. $o.k 6.2 %o.i $i.k 2.8 %i.o 1.9 $p.q %i. (<3.7) $o.m 3.7 %o.o 1.4 $p.p %i.j (<3.7) 1.3 $p.p %i.n (<3.1) $p.j 4.4 %p.n $o.n 3.6 %p.o 3.0 $p. %p.n (< 4.9) 3.1 $p. %p.p (<4.3) 3.3 $o.o %p.j (< 5.0) 3.4 $l.i %o.n (<6.7) $o.j 4.1 %p.j 2.8 %i. 1.2 $i.m %n.m (< 2.5) σ - (e $ e % ΛΛ() = N wxy L ε 1 + Π p 1 + σ B p Λ pπ 11

12 Systematic uncertainty n Luminosity: the published paper and BESIII preliminary measurement. n Λ reconstruction: track, PID are proposed to be combined, quoted by published paper Phys. Rev. D 87, (2013). n Branching fraction for Λ pπ: 1.1%. n ISR factor: the difference forlasttwo iteration. n Λ decay length: the difference between the data and MC with and without requirement of Λ decay lengthbased on the all data. n Angular distribution: the difference between the data and PHSP MC n 4C kinematic fit: Done with control sample ψ 3686 ΛΛ( Source Value(%) Luminosity 1.0 Branching fractionofofλ pπ 1.1 ISR factor 1.8 Λ and Λ( reconstruction 5.4 Λ and Λ( decay length 1.8 4C kinematic fit 1.0 Angular distribution 6.3 Total

13 Systematic uncertainty (σ - ) for ISR factor The relative difference of ε 1 + σ for each energy point Conservatively, we take the largest one 1.8% as the systematic uncertainty on the ISR factor for each energy point. 13

14 Systematic uncertainty (σ - ) for angular distribution All data samples n n n The efficiency for each cosθinterval is defined by ε i = N i obs, where the subscript i runs overevery cosθ interval, N obs i number of events for each cosθ interval, N obs all number of events for whole cosθinterval. The efficiency difference is defined by σ i = 1 ε Data ε MC. Nobs all The weightedaverage difference is takenas the systematic uncertainty δ Λ = σ i 2 i /N=5.1%, δ Λ ( = σ i 2 i /N=6.3%, N is the number ofcosθ intervals. Conservatively, 6.3%is taken as the systematic uncertainty for angulardistribution. 14

15 Line shape for Born cross section & FFs n The cross section and FFs for e $ e % Ξ % Ξ($ are measured for the first time between and 4.6GeV n Dueto the limited statistics andenergy points, we just perform a smooth fit to the dressed cross section (including the vacuum polarization effect). 15

16 Summary n Summary ü Using XYZ data for 16 energy points, Born cross section & FFs for e $ e % ΛΛ( are measured for the first time in the center-of-mass energy between and 4.6GeV/c p. üa fit to the dressed cross section is performed by exponent function without consideration of Charmonium(-like)Y states due to the limited statistics and energy points. nnext to do ü Improve the details ü Prepare the memo 16

17 Backup 17

18 Error calculation

19 Numerical results ID s (MeV) L(pb %1 ) 1 + Π δ N obs σ ε(%) σ B (pb) $il.p %ij.p ± ±0.04 $o.m %p.m ±0.17 $j.j %o.n ± ± %n.q ±0.16 $j.j %o.n ± %n.l ± %n.o ±0.15 $o.j %p.o ± ±0.02 $p.j %i.j ±0.15 $p.i %i.i ±0.15 $o.p %p.n ± ±0.03 $p.m %i.k ± %n.q ± $p.m 4.3 %i.l ±0.14 $p.j %i.j ± ± %n.o ±0.12 The statistical significance is calculated to be σ = s/ s + b σ - (e $ e % Ξ % Ξ, $ ) = N wxy 2 L ε 1 + Π p 1 + σ B Ξ πλ B Λ pπ 19

20 Numerical results ID s (MeV) L(pb %1 ) 1 + Π δ N obs σ ε(%) σ B (pb) $il.p %ij.p ± ±0.04 $o.m %p.m 24.26±0.16 $j.j %o.n ± ± %n.q 17.60±0.13 $j.j %o.n 17.61± %n.l 16.88± %n.o 16.75±0.13 $o.j %p.o ± ±0.02 $p.j %i.j 16.06±0.13 $p.i %i.i ±0.13 $o.p %p.n ± ±0.03 $p.m %i.k 16.90± %n.q 14.47± $p.m 4.3 %i.l ±0.11 $p.j %i.j ± ± %n.o 8.52 ±0.10 The statistical significance is calculated to be σ = s/ s + b σ - (e $ e % Ξ % Ξ, $ ) = N wxy 2 L ε 1 + Π p 1 + σ B Ξ πλ B Λ pπ 20

21 Numerical results ID s (MeV) L(pb %1 ) 1 + Π δ ε(%) S(σ) N obs N UP σ B (pb) $il.p ± %ij.p ±0.04 $o.m ± %p.m -- $j.j ± %o.n ± ± %n.q 3.89 $j.j ± %o.n ± %n.l ± %n.o 3.89 $o.j ± %p.o ±0.02 $p.j ± %i.j -- $p.i ± %i.i 6.68 $o.p ± %p.n ±0.03 $p.m ± %i.k ± %n.q ± $p.m 4.3 %i.l -- $p.j ± %i.j ± ± %n.o 2.30 σ - (e $ e % ΛΛ() = N wxy L ε 1 + Π p 1 + σ B p Λ pπ 21

22 Single tag takes large background