coherent photoproduction of proton anti- proton pair on deuterium · the production has been...
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Coherent photoproduction of proton anti-proton pair on deuterium
Y. Ghandilyan (grad.student, YerPhI), S. Stepanyan (JLAB) CLAS Collaboration Meeting
February 24-26, 2016, Jefferson Lab, Newport News, VA
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Ø Purpose: study claims of several groups on existence of states at masses M ∼ 2.02 GeV and ∼ 2.2 GeV decaying to
Ø Data from CLAS/eg3 run – high luminosity, high energy, Eγ<5.7 GeV, photoproduction on deuterium (40 cm long LD2 target)
Ø 3-charged particle trigger for most of the run with Eγ>4.5 GeV
Ø Revers torus field (positives inward)
Ø Analysis include photoproduction reactions in fully exclusive final states:
(physics background)
Ø The production has been studied from CLAS/g6c and CLAS/g12 data stets, with much higher (x50) statistics. No evidence of resonances has been found. But, there are ambiguities in production mechanism in photo-production on the proton target
S. Stepanyan, CLAS collaboration meeting
This analysis
!d" pp #d
pp
!d" K +K # $d!d"#+#$ %d
pp!d" pp#$
3
Coherent production on deuterium n No ambiguities, production of
can happen only in t-channel, no s- or u-channel contributions
n Detection of the recoil deuteron filters out most of the physics backgrounds
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hh n There is a recoil and produced (decay) proton ambiguity in case of hydrogen target
4
Analysis steps Event selection
a) Event topology, 3 charged tracks (++-) only, no neutrals b) 3-momentum conservation cuts c) Tagged photon identification, re-determination of the event start time d) Loose PID cuts using TOF mass e) Deuteron ID, dE/dx and vertex time, and momentum corrections f) Selection based on energy conservation between initial and final states
Yield/cross section extraction a) Acceptance simulations, t-channel production model/GSIM/GPP/
RECSIS (eg3 setup) b) Coherent ρ0-photoproduction, cosθ* and t-dependences c) Invariant mass, cosθ* and t-dependences of cross sections in -
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!d" pp #d!d" K +K # $d!d"#+#$ %d
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3-momentum conservation: px and py
Events with only 3-tracks, (++-) For fully exclusive final state with 3 particles:
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pxi
i=1
3
! " 0 pyi
i=1
3
! " 0
pxi
i! < ±4"
pyi
i=1
3
! " 0pxi
i=1
3
! " 0
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3-momentum conservation: pz and Eγ
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For fully exclusive final state:
!pz = pzi
i=1
3
" #E$ % 0
±4σ cut is applied to select tagged photon candidate
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Matched tagger hits
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§ The number of 2-matched photons is 10% of 1-matched photon events
§ The number of events with 3- or 4-matched photons is <0.5% of 1- and 2-matched photon cases, and will be ignored
§ Only tiny fraction of 2-matched photons are in the triggered region, Eγ>4.5 GeV (Case#3) and will be ignored as well
§ Most of 1-matched photons are the ones selected during the initial data processing (Case #1), for new tagged photon cases (#2), the event start time has been re-calculated
Number of photons ±4σ cut Δpz
1
2
3
4
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Loose cuts on TOF-mass
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m2 =p2 !(1"#2 )
#2; # =
1c
Rt " tst
h+ h-
p – momentum from tracking R – path length to SC from tracking t – time measured by SC tst – event start time from tagger
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Deuteron identification after loose cut on m2
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Energy loss in TOF paddle Vertex time distribution:
dtv = t ! ttof ! tst ; ttof =R"c;
" =pE; E = p2 +md
2
Cut on low energy deposition
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Energy conservation
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!E = pzi
i=1
3
" +md # Eii=1
3
" $ 0
!d"#+#$ %d
Ep = p2 +mp2 ; EK = p2 +mK
2 ; E!= p2 +m
!2
E(pp–d) (GeV)
0
10
20
30
40
50
60
70
-0.1 -0.075 -0.05 -0.025 -0 0.025 0.05 0.075 0.1
E( + -d) (GeV)0
500
1000
1500
2000
2500
3000
3500
4000
-0.1 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08 0.1
!d" pp #d
E(K+K-d) (GeV)0
500
1000
1500
2000
2500
3000
-0.5 -0.4 -0.3 -0.2 -0.1 -0 0.1 0.2 0.3 0.4 0.5E(pp
–d) (GeV)
0
200
400
600
800
1000
1200
-0.5 -0.4 -0.3 -0.2 -0.1 -0 0.1 0.2 0.3 0.4 0.5
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Deuteron ID and the reaction
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!d" pp#$
!E = pzi
i=1
3
" +md # Eii=1
3
" $ 0
Ei = pi2 +mi
2 ; i = p, p, %#
After deuteron ID
!d" pp#$
!tv = ( t1 "R1c#1
)"( t2 "R2c#2
)
# =p
p2 +mp2
;
1" proton1, 2" proton2
p – momentum from tracking R – path length to SC from tracking t – time measured by SC mi – particle PDG mass
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Simulations
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M(pp–) (GeV))
0
2.5
5
7.5
10
12.5
15
17.5
20
22.5
1.8 2 2.2 2.4 2.6 2.8 3-t (GeV2))
0
10
20
30
40
50
60
70
80
0 0.5 1 1.5 2 2.5 3
• Physics model: t-channel production of hadron pairs.
• Pair production according to phase space, isotropic in CM
!d" pp #d!d" pp #dData
MC reconstruction
Y ! e3t
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Acceptance calculations
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-t (GeV2))
Acc
epta
nce
0
0.005
0.01
0.015
0.02
0.025
0 0.5 1 1.5 2 2.5 3
!d" pp #d
• Much smaller acceptances compared to CLAS proton data are due 3-particle final state and reverse torus field.
• Track reconstruction efficiency of 95% was applied in addition
cos( *)
Acceptance
0
0.0025
0.005
0.0075
0.01
0.0125
0.015
0.0175
0.02
0.0225
-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1
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Cross section of coherent ρ production
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0
5
10
15
20
25
-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1cos( CM)
d/d
cos(
CM
)
~ 1-cos2( CM)
-t (GeV2)
d/d
t (nb
/GeV
2)
10-2
10-1
1
10
10 2
10 3
10 4
0 0.5 1 1.5 2 2.5 3
SLAC
CLAS/eg3
Normalization: gflux, number target nuclei, trigger correction (large for eg3)
Acceptance is < 20% of max
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Invariant mass and t-dependence
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10-4
10-3
10-2
10-1
0 0.5 1 1.5 2 2.5 3M(h+h-) (GeV)
d/d
M (n
b/G
eV)
!+!"
pp
K +K !
10-4
10-3
10-2
10-1
0 0.5 1 1.5 2 2.5 3-t (GeV2)
d/d
t (µ
b/G
eV2 )
0.52e(1.74t)
!+!"
pp
M>2 GeV
!d" h+h# $d
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h+ angular distribution in the helicity frame
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1.9 GeV M(h+h-) 2.5 GeV
10-2
10-1
1
d/dcos(
CM)
a+b*cos( )+c*sin( )cos( )+d*cos2( )
a=0.0573b=0.1052c=0.0232d=0.1384
10-2
10-1
-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1cos( CM)
d/dcos(
CM)
a=0.0158b=-0.0439c=0.0604d=0.1186
pp pp
!+!"
2.1 GeV M(h+h-) 2.3 GeV
10-2
10-1
1a+b*cos( )+c*sin( )cos( )+d*cos2( )
d/dcos(
CM)
a=0.0224b=0.0753c=0.0118d=0.1204
10-2
10-1
-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1
a=0.0094b=-0.0333c=0.0464d=0.0517
cos( CM)
d/dcos(
CM)!+!"
Acceptance is < 20% of max
!E = pzi
i=1
3
" +md # Eii=1
3
" $ 0
Ei = pi2 +mi
2 ; i = p, p, %#
17 Summary n Coherent photoproduction of hadron pairs, , , and , has
been studied using CLAS /eg3 data. The cross sections as a function of pair invariant mass, transferred momentum squared, and pair CM angle have been extracted
n No resonance structures have been found in the invariant mass of
n In the region of the pair invariant masses >2 GeV, extracted yields for and came out to be comparable – need an explanation or a model (di-quarks?)
n Next steps:
q redo simulations after refining the model using input from extracted yields (corrected enery, M(h+h-), t, and cos(θ*) dependences)
q study run dependence (before and after 2004 Christmas breaks)
q study track reconstruction efficiency, ρ cross section came out lower than previous SLAC results
q refine statistical uncertainties (dominated by acceptance and trigger efficiency estimate)
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!+!" ppK +K !
!+!"
pp
pp
18 Proton-Antiproton Pair Production in Two-Photon Collisions at LEP
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L3 collaboration, arXiv:hep-ex/030601
qq–
qq–
qdi-q
di-q–q–