coherent photoproduction of proton anti- proton pair on deuterium · the production has been...

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

2

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


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 -


!d" pp #d!d" K +K # $d!d"#+#$ %d

5

3-momentum conservation: px and py

Events with only 3-tracks, (++-) For fully exclusive final state with 3 particles:


pxi

i=1

3

! " 0 pyi

i=1

3

! " 0

pxi

i! < ±4"

pyi

i=1

3

! " 0pxi

i=1

3

! " 0

6

3-momentum conservation: pz and Eγ


For fully exclusive final state:

!pz = pzi

i=1

3

" #E$ % 0

±4σ cut is applied to select tagged photon candidate

7

Matched tagger hits


§  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

8

Loose cuts on TOF-mass


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

9

Deuteron identification after loose cut on m2


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

10

Energy conservation


!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

11

Deuteron ID and the reaction


!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

12

Simulations


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

13

Acceptance calculations


-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

14

Cross section of coherent ρ production


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

15

Invariant mass and t-dependence


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

16

h+ angular distribution in the helicity frame


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)


!+!" ppK +K !

!+!"

pp

pp

18 Proton-Antiproton Pair Production in Two-Photon Collisions at LEP


L3 collaboration, arXiv:hep-ex/030601

qq–

qq–

qdi-q

di-q–q–

coherent photoproduction of proton anti- proton pair on deuterium · the production has been...

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