1. A.N.Skachkova, N.B.Skachkov (JINR, Dubna)
QCD at intermediate energies: Multimuon production and double parton interactions at PANDA
A.N.Skachkova,
N.B.Skachkov
(JINR, Dubna)
2-nd PANDA Russia
workshop
26-28 April 2010, IHEP

2. about quark dynamics inside the hadron and proton structure in the
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 in interplay with a reach
resonance physics.
II. Different to electron beams, used for measurements of
proton structure functions in the region of
negative values of the square of transversed
momentum (q^2 < 0, “space-like” region),
antiproton-proton collisions allow to make measurements in the region of
positive values of the square of the transverse
momentum (q^2 > 0, “time-like”, region, less
studied ! ).
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

3. 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 pair and
prompt photon production, done by use of PYTHIA generator, 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 Ebeam = 14 GeV (i.e., Ecm = 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: “Lepton pair production at intermediate energies and the main backgrounds ”,

4. low Q**2 measuremens and combination with old (97-98) results ZEUS
New
measuremens
and combination
with old (97-98)
results at
low
Q**2
at H1 and
ZEUS

5. Nowdays
there are
new
measuremens at
low
Q**2
at JLab 

6. Production of lepton pair (continum 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. Double lepton pairs production rate gives the information about
multiple parton interactions (MPI), which rate depends on the
space distribution of partons, i.e. on space structure of proton.

7. V. A. Matveev, R. M. Muradian, A. N. Tavkhelidze (MMT) ( V. A
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 ( S.D. Drell, T.M. Yan, SLAC-PUB-0755, Jun 1970,12p.; Phys.Rev.Lett. 25(1970) , )
The dominant mechanism
of the l+l- production is
the perturbative QED/QCD
partonic 2  2 subprocess
̅qiqi   ̽ / Z˚ l+l-
σ = 5.9 E- 06 mb
PYTHIA 6 simulation for the E beam = 14 GeV ( i.e., Ecm =5.3 GeV).
without detector effects (“ideal detector” --> all particles are detected) allows a proper account of the relativistic kinematics during the simulation
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

8. Signal l± : 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°!!!
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

9. In each signal l± event: El ”fast” > El ”slow”
Left column
0 < El ”slow” < 5 GeV
0 < Θl ”slow” < 180° (mostly)
But !!!
Some of less energetic slow leptons have Θl ”slow” > 90°
Right column
0.4 < El ”fast” < 10 GeV
0 < Θl ”fast” < 60°
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

10. Fake electron distributions
In approximation when particles are allowed to decay in cylinder volume R=2500 x L=8000 mm
The number of events which include fake electrons is about 1 -2% of events
The most of electron pairs do appear as Dalitz pairs (πºe+e-) produced in decays of neutral mesons, which appear from η, ω or more heavy mesons or barions, produced in their turn as the resonance states according to the LUND fragmentation model.
The life time of these resonance states is rather short  the electron pairs are produced close to the interaction point  the vertex position information will not be efficient for the Signal / Background separation..
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

11. Fake muon distributions
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
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

12. 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
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

13. Global variable for q qbar    l +l - process - Minv l+l-
Minv l+l- = √(Pl+ + Pl-)² = q2
Minv l +l- min = Minv¯qq = 1 GeV – originates from the internal PYTHIA restriction
Minv¯qq = √(Pq + P¯q)² = m_hat
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

14. Background QCD processes for 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  l+ + l- (has % of events with the σ = 5.02 E- 06 mb);
So, initially we have 1 signal event among of QCD background  S/B ≃ 5.5 * 10-6
The simulation was done within approximation when particles are allowed to decay
in cylinder volume R=2500 x L=8000 mm
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

15. QCD Muon background
The shape of muon distributions, produced in these 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 QCD events (which are other than the signal one)
 fake pretty good the signal events
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

16. QCD background electrons
The shape of
QCD background electron’s
distributions are identical to the ones, surrounding the signal process.
The most of electron pairs do appear as Dalitz pairs (πºe+e-)
The electron pairs are
produced close to the
interaction point 
the vertex position information
will not be efficient for the
Signal / Background separation..
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

17. Parents of QCD background leptons
The most probable parents of fake
electrons  are neutral pions (πºe+e-)
muons  are charged pions
The most probable grandparents of fake
electrons  are strings (Lund model),
ρ+,η, ω, Δ0 ,Δ+
muons  are strings (Lund model),
ρ0, ρ+, ω
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

18. This source of background is 5 times harder !!! than QCD background
The main source of background for the signal q qbar     l +l – are the Minimum-Bias processes:
1. Low - PT scattering (gives 68% of events with the σ = mb);
2. Single diffractive (gives 6% of events with the σ = 3.32 mb); and other
3. ¯q + q  l+ + l- (has % of events with the σ = 5.9 E- 06 mb);
So, we have 1 signal event among of Mini-bias bkgd 
S/B ≃ 10-7
This source of background is 5 times harder !!! than QCD background
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

19. Minimun-bias background electrons
The shape of
QCD background electron’s
distributions are identical to the ones, surrounding the signal process.
The most of electron pairs do appear as Dalitz pairs (πºe+e-)
The electron pairs are
produced close to the
interaction point 
the vertex position information
will not be efficient for the
Signal / Background separation..
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

20. Lepton (µ) isolation criteria
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 events
bottom plot  Mini-bias background
Isolation criteria (R isolation = 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)
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

21. Cuts used for separation of single lepton pairs from Minimum bias and QCD background
1. selection of events with the only 2 leptons, having;
2. these 2 leptons must have opposite charges;
3. the vertex of lepton origin lies within the R< 15mm from the interaction point;
4. Minv ( 2 leptons) ≥ 0.9 GeV;
5. leptons do satisfy the isolation criteria: the summed energy ( Esum ) of all particles within the cone of
R(in eta-phi space) =0.2 is less than 0.5 GeV, i.e.,
Esum < 0.5 GeV.

22. Efficiency of cuts and S/B ratio
The total loss of signal events after application of all five cuts is expected to be about 20%.
So, one can expect to gain a huge sample of about 7x106 signal dilepton events per 1 year (10^7 sec.)

23. A.N.Skachkova (JINR, Dubna)
Direct photon production in ̅pp collisions at PANDA energies (E beam = 14 GeV)
A.N.Skachkova
(JINR, Dubna)
PANDA EM group workshop, Ferrara 2007

24. N. Skachkov: EM group. ITEP PANDA workshop, 22 October 2007
Previous experiments
First (pp -> g + X) experiments were done at CERN:
at ISR ( pp - collider, Ecm = 31, 45, 53 GeV ):
R412 experiment (Darriulat et. al., 1976);
R107 (Amaldi et. al., 1978);
WA98 (Aggarwal et. al., 2000); and many others at higher E.
Also at Fermilab and BNL:
E557 (Baltrusaitis et. al. 1979, pBe, Ecm = 19.4, 23.7 GeV)
E629 (McLaughlin et. al. 1983, pC, Ecm = 19.4 GeV)
N. Skachkov: EM group. ITEP PANDA workshop, 22 October 2007

25. The proton-antiproton cross section ( )
in quark-parton model is expressed as follows: .
Here f( x, Q**2) are the structure functions, is
the 22 parton level ( ) cross section ;
= is the square of transfered momentum,
is the energy fraction carried by a quark in a proton.
In our case, ,( or ) and

26. Nikolay Skachkov: ISHEPP XIX, Dubna, Sept.29-Oct.4, 2008
There is also another diagram that
describes fragmentation into a photon.
Its contribution is supressed by
photon isolation criteria
and drops with growth
Nikolay Skachkov: ISHEPP XIX, Dubna, Sept.29-Oct.4, 2008

27. Our Physical goal: To estimate the possibility of
getting the information about
proton structure functions
f(x, Q**2) .
Main interest: To estimate the size of the
x-Q**2 kinematical region
in which
the structure functions
can be measured.
Physical goal: New information about proton structure functions
(gluon-distrib.) in new x-Q**2 kinematical region.

28. The diagrams contribution
The contributions
of both diagrams
to the total
cross section
“Annihilation”
diagram 
“QCD Compton”
Cross Sections
Total:  = 2.9E-03 mb;
q + qbar  gluon + photon (62%):
 = 1.78E-03 mb;
gluon + q  q + photon (38%):
 = 1.10E-03 mb
N. Skachkov: EM group. ITEP PANDA workshop, 22 October 2007

29. Signal photons distributions
0 ≤ E γ ≤ 10 GeV, <E γ > = 2.6 GeV
0 ≤ PT γ ≤ 2 GeV, <PT γ > = GeV
0 ≤ Θ γ ≤ 180 ° , <Θ γ > = °
some Θ γ > 90°
Anna Skachkova: EM group workshop, Ferrara, October 2007

30. In pbar + p  gamma + X process
(chosing pbar beam direction as the z-axis)
the role of the transfered momentum Q
plays the photon transverse momentum, i.e.
As it shall be shown below, at E beam = 14 GeV
we can expect < 2 GeV, i.e,
This region is under study now at HERA and JLab but in “space-
like” region of the square of the momentum transferred q^2 <0.

31. Structure functions distributions
0.05 < x < 0.7
0.05 < x < 0.7
The x-distribution, given by the generated structure
functions, allows, together
with the photon Pt spect-rum, to estimate the range of kinematical x-Q^2 region, available for the
measurement at PANDA .
Anna Skachkova: EM group workshop, Ferrara, October 2007

32. Neutral pions distributions
0 ≤ E γ ≤ 7 GeV, <E γ > = 1.2 GeV
0 ≤ PT γ ≤ 1 GeV, <PT γ > = GeV
0 ≤ Θ γ ≤ 180 ° , <Θ γ > = °
some Θ γ > 90°
Up to 7 neutral photons in signal event (i.e. ≤ 14 photons/event).
Distribution of N_ πº / event well explains the distribut. of N_ γ / event
(due to πº  2 γ).
The most probable parents of πº are
η, ρ+, strings and Δ0
Anna Skachkova: EM group workshop, Ferrara, October 2007

33. Fake (decay) photons distributions
0 ≤ E γ ≤ 4 GeV, <E γ > = 0.6 GeV
0 ≤ PT γ ≤ 1 GeV, <PT γ > = GeV
0 ≤ Θ γ ≤ 180 ° , <Θ γ > = °
some Θ γ > 90°
Vx, Vy, Vz distributions show
mostly zero value
Up to 10 background photons in a
signal event (in some few
events up to 14)
Anna Skachkova: EM group workshop, Ferrara, October 2007

34. Gamma isolation criteria
The plots show the distributions over summed energy of stable particles in the cones of radius
R = √ η2+φ2 respect to the
upper plot  direct photon
bottom plot  fake photon
Isolation criteria E (of stable particles) =0 in the R=0.3
allows to separate 100% of fake
photons with loss of 4.2% of signal events
Anna Skachkova: EM group workshop, Ferrara, October 2007

35. Conclusion. 0.05 < x < 0.7;
The simulation with PYTHIA ( used as the first step
approximation ) has shown that with E beam = 14 GeV :
1. PANDA can perform the measurement of proton structure
functions in the time-like !!! (q2 >0) x-Q2 kinematical region:
0.05 < x < 0.7;
2. This measurement will have the advantage over DIS because
it shall be more sensitive to gluon distribution.
3. To separate the contribution of fake photons, which come from
the decays of pions and other neutral fake mesons in the signal
and mini-bias events, a lot of work with full GEANT simulation is
needed in nearest years to find a way to identify/separate by
ECAL πo, η, ω and other mesons, produced in QCD and mini-
bias background events (like it was done at ISR, SppS,WA98, PHENIX,
and other collaborations …). .
For this region PYTHIA predicts about 5 x109 !!! signal events per year.

36. Double parton (DP) interaction measurements at PANDA
1.The inelastic scattering of nucleons need not to occur only through a single parton-parton interaction (A) and the contribution from double parton collisions (A and B) can be significant.
2.The probability of the second parton-parton collision (B) will be larger or smaller depending on the parton spatial distribution If partons were concentrated within small region inside the proton then the second collision (B) would be more likely to occur for a given A collision By contrast, in a case of uniform parton density throughout the proton, the presence of A would not increase the probability of B.

37. History of Measurements of Double Parton scatterings
A new measurement was done by our group at D0:
Phys. Rev. D 81:052012, 2010; arXiv: [hep-ex] 37

39. Outback

42. DP: Signal Variables
Calculated for the pair that gives the minimum value of S: 42 42

43. Conclusion The proposed cuts:
Events with only 2 leptons of the opposite sign
and El > 0.2 GeV, PTl > 0.2 GeV
2. The vertex of origin lies within the
distance from the interaction point < 15 mm
3. Minv (l +,l -) > 0.9 GeV
4. Isolation criteria E (R isolation = 0.2) = 0.5 GeV
Allow to suppress QCD & Mini-bias bkgd to:
completely for muons; S/B = 9 for electrons
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

45. Double parton (DP) events at PANDA
Signal process
Backgroud process
PT-balanced muon pairs
PT- inbalanced lepton pairs
Preliminary PYTHIA estimations (model dependent) has shown a moderate reduction
of the total cross section as comparing to lepton-pair production (by ~10-2 ). 45

46. Double parton (DP) events at PANDA
To perform the analogous measurement we need only to substitute jets and photon by leptons: e/muons;
The electrons and muons with E_l > 0.8 can be used for such kind of measurement;
The measurement with leptons (which are much more better measured: Energy, Vertex position, no need of jet energy calibration) can provide a much higher preci-sion.

47. Conclusions:
1. PANDA energy range is quite promising for studying of a large
number of very interesting Deep Inelastic QCD processes.
2. The study of single lepton pair production can provide the informa-
tion about valence quark distribution in a new kinematical region
of the intermediate “time-like” momentum transferred q2 > 0.
3. Direct photon production shall allow to measure the gluon
distribution component of proton structure function at q2 > 0 .
4. Double parton interactions can bring the information,
may be the most clean one (jets are substituted by e/muons)
about the spatial distribution of partons inside the proton.

48. Outback

49. Θl ”slow” / Θl ”fast” correlation
Muon system geometry cuts :
Θl_”slow” & Θl_”fast” ≤ 20º
 14% events saved
Θl_”slow” & Θl_”fast” ≤ 40º
 50% events saved
Θl_”slow” & Θl_”fast” ≤ 60º
 72% events saved
Θl_”slow” & Θl_”fast” ≤ 90º
 94% events saved
( 6% of events loss )
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

50. El ”slow” / El ”fast” correlation
Applying the Energy cuts:
El_”fast” & El_”slow” > 0.5 GeV
 loss of ~25% of signal events
El_”fast” & E l_”slow” > 1 GeV
 loss of ~45% of signal events
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

51. Θl/El correlation for signal l±
Θl ”slow” / E l ”slow” Θl ”fast” / E l ”fast”
1. for El > 0.5 GeV
“slow “ “fast”
Nevent ≈ 70% Nevent ≈ 98%
2. for El > 4 GeV
Nevent ≈ 2% Nevent ≈ 40%
3. for Θl < 60°
Nevent ≈ 78% Nevent ≈ 99.96%
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

52. Some first estimations were
Published (2006) as PANDA
Note PHY-003:
and hep-ph/
A.N.Skachkova, N.B.Skachkov
“Monte-Carlo simulation of lepton pair production in “ppbar  l+l - + X” events at Ebeam = 14 GeV”
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

53. Cuts efficiency for Minimum-Bias background events
e+e- production
N of cuts
S/B ratio
Efficiency
1 (exactly 2 leptons
with El > 0.2 GeV, PTl > 0.2 GeV )
5.3 *
1.78 *
2 (2 leptons are of the opposite sign)
5.4 *
0.98
3 (The vertex of origin lies within
the R < 15 mm)
5.5 *
4 (Minv(l1,l2) > 0.9)
0.09
0.006
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

54. Efficiency of M inv (l+,l-) cut
e+e- case
Minv(l+,l-)>
S / B
Efficiency
The rest of signal events
0.9
0.09
0.0057
84 %
1.0
0.15
0.0034
81 %
1.03
0.38
0.0013
73 %
1.05
0.40
0.0012
69 %
1.1
0.83
0.0005
58 %
1.2
2.00
0.0002
42 %
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

55. 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 % %
1 & 2 % %
1 & 2 & 3 % %
The rate of events with left fake leptons is negligible !
0.008 %
0.001 %
Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”,

56. 56