study of muon pairs production process at panda and its backgrounds

Study of muon pairs production process at PANDA

and its backgrounds

A.N.Skachkova

(JINR, Dubna)

Obninsk 2010Obninsk 2010

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Quark-antiquark annihilation and gluon - quark scattering subprocess in antiproton-proton collision, in which the energetic electromagentic particles (e, muon, photon) are produced, planned to be the subject of our study at PANDA experiment.

The results of Monte Carlo simulation of processes with lepton pairs done by use of PYTHIA generator and PANDARoot fast simulaion tool,

are presented here. They allowed to work out the cuts needed for background separation and to make reasonable estimations of expected selection rates for these processes. In this talk we base on the results

obtained for a case of E beam = 14 GeV (i.e., E cm = 5.3 GeV).

The corrections for the final estimations are planned to be done in the nearest years on the basis of the existing detector simulation tools.

Anna Skachkova : : ““Muon pairs production at PANDA”,”, 12.12.2010 12.12.2010 , Obninsk

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The considered processes.

We present the results of study:

1. Production of lepton pairs with the continuum invariant mass.

2. “Resonance” production of lepton pair in J/Ψ decay:

q q bar q q bar c cbar c cbar J/ J/ΨΨ l l+ + ll- - + X+ X


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Production of lepton pair (continuum Minv (l+l-) case)

The process of lepton pair production q qbar / Z˚ l+l-

is of big physical interest because:

A. The spectrum of final state leptons (e and muons) obviously depends on the form of parton distributions inside colliding protons.

B. The transverse momentum of lepton pair PT (l+l-) provides the information about intrinsic transverse momentum < kT> of quark

inside the proton (Fermi motion).

C. The results of study of leptons angle and energy spectra distributions, based on Monte-Carlo simulation, was used for a proper “geometrical” design of such important component of PANDA detector as muon system.

Anna Skachkova : : ““Muon pairs production at PANDA”,”, 12.12.2010 12.12.2010 , ObninskAnna Skachkova : : ““Muon pairs production at PANDA”,”, 12.12.2010 12.12.2010 , Obninsk

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V.A. Matveev, R.M. Muradian, A.N. Tavkhelidze (MMT)( V.A. Matveev, R.M. Muradian, A.N Tavkhelidze, JINR P2-4543, JINR, Dubna, 1969; SLAC-TRANS-0098, JINR R2-4543, Jun 1069; 27p. )

process, called also as “Drell-Yan process”( S.D. Drell, T.M. Yan, SLAC-PUB-0755, Jun 1970,12p.; Phys.Rev.Lett. 25(1970)316-320, 1970 )

The dominant mechanism

of the l+l- production is

the perturbative QED/QCD

partonic 2 2 subprocess

qiqi / Z˚ l+l-

σ = 5.6 * 103 pb

PYTHIA 6 simulation for the E beam = 14 GeV ( i.e., E cm =5.3 GeV)without detector effects (“ideal detector” --> all particles are detected)

allows a proper account of the relativistic kinematics during the simulation


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Signal l± distributions

0 ≤ El ≤ 10 GeV,

<El> = 2.6 GeV,

Epeak = 0.4 GeV

0 ≤ PTl ≤ 2 GeV,

<PTl> = 0.7 GeV

<Θl> = 27.3 °

some Θl > 90° !!!


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Signal l± : (l = μ,e) FastSim

0 ≤ Pz ≤ 10 GeV,

< Pz > = 4.2 GeV

-2 ≤ Px, Py ≤ 2 GeV

l l -- - left- left, , l l ++ - right- right::


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0 ≤ P, El, ≤ 10 GeV,

< P, El > = 4.4 GeV

0 ≤ PTl ≤ 2 GeV,

< PTl > = 1.2 GeV

PTl peak = 1.4 GeV



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Signal l± : (l = μ) FastSim

< Θl > = 27.2 °

some Θl > 90°

0.9 < M inv < 2 GeV



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Fake muons distributions

in signal events

1. The part of signal events which include

fake muons is about 16%.

2. Up to 4 fake muons in the final state.

3. Fake muons production

vertices are distributed

within detector volume

Vertex position information will be useful

for Signal / Background separation


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Parents of fake leptons

The most probable parents of fake

electrons are neutral pions (Dalits decay)

muons are charged pions

The most probable grandparents of fake

electrons are string (Lund model),

ρ+,η, ω, Δ0 ,Δ+ ,Λ0

muons are string (Lund model),

ρ0, ρ+, ω, Δ+ ,Δ++ ,Λ0


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Applied cuts for signal events

1. We select the events with only 2 leptons with El > 0.2 GeV, PTl > 0.2 GeV

2. These 2 leptons must be of the opposite sign

3. The vertex of origin lies within the R < 15 mm

from the interaction point

Up to now we supposed the ideal muon system and EM calorimeter covering 180o


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Cuts influence on signal events Applying these cuts we have loss of the signal

events:

N of cuts e+e- production μ+μ- production

1 14.330 % 16.525 %

1 & 2 14.340 % 16.805 %

1 & 2 & 3 14.341 % 17.108 %

The rate of events with left fake leptons is negligible !

0.008 % 0.001 %


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Background QCD processes to the q q l +l - one

The generation was done with the use of more than 20 QCD subprocesses existed in PYTHIA (including the signal one ¯q q l +l - ).

The main contributions come from the following partonic subprocesses:

q + g q + g (gives 50% of events with the σ = 4.88 mb);

g + g g + g (gives 30% of events with the σ = 2.96 mb);

q + q’ q + q’ (gives 18% of events with the σ = 1,75 mb);

q + q bar g + g (gives 0.6% of events with the σ = 5.89 E- 02 mb);

q + q bar l+ + l- (signal process gives 0.00005% of bkgd events, due to σ = 5.02 E- 06 mb);

So, initially there is 1 signal event among 2.000.000 of QCD background

i.e., initially S/B ≃ 5.5 * 10-6

The simulation was done with approximation when particles are allowed to decay

within cylinder volume of R=2500 and L=8000 mmAnna Skachkova : : ““Muon pairs production at PANDA”,”, 12.12.2010 12.12.2010 , Obninsk

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The main source of background for q qbar l +l – are the Minimum-Bias processes:

Some examples:

Low - PT scattering (gives 68% of events with the σ = 34.25 mb);

Single diffractive (gives 6% of events with the σ = 3.32 mb);

qbar + q l+ + l- (gives 0.000012% of events, σ = 5.9 E- 06 mb);

So, we have 1 signal event aginst 8.333.333 of Mini-bias bkgd S/B ≃ 10-7

Mini-bias background is 5 times harder than QCD background


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Muon’s distributions from background events

The shape of muon distributions, produced in background events do not differ from those of fake “decay” muons, produced in the signal process

a rather high probability of appearing the muon pair with the different signs of their charges in Minimum-bias events (which are other than the signal one)

fake pretty well the signal events


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Parents of background leptons

The most probable parents of fakeelectrons are neutral pions (πºe+e-)

muons are charged pions

The most probable grandparents of fakeelectrons are strings (Lund model),

ρ+,η, ω, Δ0 ,Δ+

muons are strings (Lund model),ρ0, ρ+, ω


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Cuts for mini-bias and QCD processes (including the signal one)

The following cuts were applied to the minimum bias and QCD sample:

1. selection of events with the only 2 leptons, having El > 0.2 GeV, PTl > 0.2 GeV;

2. these 2 leptons have charges of the opposite charge;

3. the vertex of lepton origin lies within the R< 15mm from the interaction point;

4. Minv (l +l-) ≥ 0.9 GeV;

5. leptons have to satisfy the isolation criteria: the summed energy of particles E sum < 0.5 GeV within the cone of R isolation = √ Δη

2+Δφ2 = 0.2.


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The plots show the distributions over summarized energy of the final state

particles in the cones of radius R isolation = √ η2+φ2 respect to the

(η – pseudorapidity)

upper plot signal eventssignal events

bottom plot Mini-bias background

Isolation criteria (R R isolationisolation = 0.2 ) = 0.2 )E (of particles) = 0.5 GeV

allows to separate 100% of Mini-bias bkg leptons

with the loss of 8% of signal events

(after applied 3 cuts discussed above + cut M inv (l+,l-) > 0.9)

Lepton (µ) isolation criteria


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Efficiency of cuts in the presence of mini-bias and QCD bkgd & S/B ratio

The total loss of signal events after application of all five cuts is expected to be about 20%.

So, we can expect to gain a huge sample of about 7x106 signal dilepton events per 1 year (107 sec.)


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Conclusion

The proposed cuts:

1. Events with only 2 leptons having El > 0.2 GeV, PTl > 0.2 GeV; 2. The charges of these 2 leptons have opposite sign;3. The vertex of origin lies within the distance < 15 mm

from the interaction point

4. M inv (l +,l -) > 0.9 GeV

5. Isolation criteria E (R isolation = 0.2) = 0.5 GeV

allow:

I. To gain about 7x106 signal dilepton events per 1 year; II. To suppress QCD & Mini-bias bkgd:

completely for muons and to S/B = 9 for electrons;

III. Next step: full detector MC simulation.


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J/Ψ production processes (benchmark process)

421) g g cc¯ [3S 1 (1) ] g ll + X

422) g g cc¯ [3S 1 (8) ] g ll + X

423) g g cc¯ [3S 0 (8) ] g ll + X

424) g g cc¯ [3P J (8) ] g ll + X

425) g q cc¯ [3S 1 (8) ] q ll + X

426) g q cc¯ [3P J (8) ] q ll + X

427) g g cc¯ [3S 1 (1) ] q ll + X

428) q q¯ cc¯ [3S 1 (8) ] g ll + X

429) q q¯ cc¯ [1S 0 (8) ] g ll + X

430) q q¯ cc¯ [3P J (8) ] g ll + X

1) q1) q i i q q i i¯ c c c c¯ ¯ J/ J/ΨΨ l l ++ll - - + X+ X

86 ) g g 86 ) g g J/ J/ΨΨ + g + g l l ++ll - - + X + X R.Baier and R.Rücke, Z.Phys. C19R.Baier and R.Rücke, Z.Phys. C19 (1983) (1983) 251251

106) g g 106) g g J/ J/ΨΨ + + l l ++ll - - + X + X M.Drees and C.S.Kim, Z.Phys. C53 (1991) M.Drees and C.S.Kim, Z.Phys. C53 (1991) 673673 431) g g 431) g g cc¯ [cc¯ [33P P 00 ( (11) ] g) ] g ll ll + X + X

432) g g 432) g g cc¯ [cc¯ [33P P 11 ( (11) ] g) ] g llll + X + X

433) g g 433) g g cc¯ [cc¯ [33PP 2 2 ( (11) ] g) ] g ll ll + X+ X

434) g q 434) g q cc¯ [cc¯ [33P P 00 ( (11) ] q) ] q ll ll + X + X

435) g q 435) g q cc¯ [cc¯ [33P P 11 ( (11) ] q) ] q llll + X + X

436) g q 436) g q cc¯ [cc¯ [33PP 2 2 ( (11) ] q) ] q llll + X + X

437) q q 437) q q cc¯ [cc¯ [33P P 00 ( (11) ] g) ] g ll ll + X + X

438) q q¯ 438) q q¯ cc¯ [cc¯ [33P P 11 ( (11) ] g) ] g llll + X + X

439) q q¯ 439) q q¯ cc¯ [cc¯ [33PP 2 2 ( (11) ] g) ] g llll + X + X

G.T.Badwin, E.Braten and G.P.Lepage, Phys.Rev. D51 (1995) 1125];

M.Beneke, MKrämer and M.Vänttinen, Phys.Rev.D57 (1998) 4258;

B.A.Kniehl and J.Lee, Phys.Rev. D62 (2000) 114027


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The maximum cross section value (obtained by PYTHIA 6.4

simulation) is

σ = 20.75 pb that corresponds to 358.5 events / day

for the E beam = 14 GeV and Luminosity = 2*105

1/mb*sec (= 2*1032 cm-2 sec-1)

J/Ψ production The main contributions to the cross section give the

following processes:

1) qi qi¯ c c¯ J/Ψ l+ l - + X

428) q q¯ cc¯ [3S1(8)] g l+ l- + X

430) q q¯ cc¯ [3PJ(8)] g l+ l- + X


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Signal l± : (l = μ,e)

0 ≤ El ≤ 10 GeV, < El > = 4.4 GeV 0 ≤ PTl ≤ 2 GeV, < PTl > = 1.2 GeV PTl

peak = 1.4 GeV

< Θl > = 27.2 ° some Θl > 90° Important: The same range

of angle distributions



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0 ≤ Pz ≤ 10 GeV,

< Pz > = 4.2 GeV

-2 ≤ Px, Py ≤ 2 GeV



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0 ≤ P, El, ≤ 10 GeV,

< P, El > = 4.4 GeV

0 ≤ PTl ≤ 2 GeV,

< PTl > = 1.2 GeV

PTl peak = 1.4 GeV



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Signal l± Fastsim

< Θl > = 27.2 °

some Θl > 90°

Important: The same range of angle distributions



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Outline

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I. Antiproton beam with Ebeam < 15 GeV may provide an interesting information about quark dynamics inside the hadron and proton structure in the energy region where the Perturbative QCD comes into interplay with a reach resonance ( i.e. Nonperturbative ) physics.

II. Different to electron beams, used for measurements of proton structure functions in the region of !negative! values of the square of transferred momentum (q2 < 0, “space-like” region), antiproton-proton collisions allow to make measurements of proton structure functions in the region of !positive! values of the square of the transferred

momentum (q2 > 0, “time-like”, region, which is less studied ! ).


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