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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. I. Antiproton beam with E beam < 15 GeV may provide an interesting information

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A.N.Skachkova, N.B.Skachkov (JINR, Dubna)

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  1. 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. I. Antiproton beam with Ebeam < 15 GeVmay provide an interesting information about quark dynamics inside the hadron andproton structurein the energy region where the perturbative QCD comes ininterplay 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-protoncollisionsallow to make measurements in the region of positive valuesof 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 ”, 22.11.08

  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. Theresults 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 ofexpected 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). Thecorrectionsfor the final estimations are planned to be donein the nearest yearson the basis of the existing detector simulation tools. Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”, 22.11.08

  4. 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 leptonpairs 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. 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)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=5.9 E- 06mb 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 ”, 22.11.08

  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 ”, 22.11.08

  9. In each signal l± event: El”fast” > El”slow” Left column 0 < El”slow” < 5 GeV 0 < Θl”slow” < 180° (mostly) But !!! Some ofless 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 ”, 22.11.08

  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, ωor 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 ”, 22.11.08

  11. Fake muon distributions 1.The part of signal events which include fakemuons is about16%. 2.Up to 4 fake muons in the final state. 3. Fake muonsproduction 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 ”, 22.11.08

  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 ”, 22.11.08

  13. Global variable forq qbar    l +l - process - Minvl+l- • Minvl+l-= √(Pl+ + Pl-)² = q2 Minvl +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 ”, 22.11.08

  14. 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 0.00005% of events with the σ=5.02 E- 06mb); So, initially we have 1 signal event among 2.000.000 of QCD background S/B≃ 5.5 * 10-6 Background QCD processes for the q q    l +l -one 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 ”, 22.11.08

  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 ”, 22.11.08

  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 ”, 22.11.08

  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 ”, 22.11.08

  18. 1. Low - PT scattering(gives 68% of events with the σ= 34.25 mb); 2.Single diffractive(gives 6% of events with the σ= 3.32 mb); and other 3. ¯q + q  l+ + l-(has0.000012%of events with theσ=5.9 E- 06mb); So, we have 1 signal event among 8.333.333 of Mini-bias bkgd  S/B ≃ 10-7 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: Anna Skachkova: “Lepton pair production at intermediate energies and the main backgrounds ”, 22.11.08

  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 ”, 22.11.08

  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 ”, 22.11.08

  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 7x106signal dilepton eventsper 1 year (10^7 sec.)

  23. 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. 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 Fermilaband 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. Theproton-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

  27. Our Physical goal: To estimate the possibility of getting theinformation 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: Newinformation 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” diagram • 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> = 0.67 GeV 0 ≤ Θγ ≤ 180 ° , <Θγ> = 29.6 ° some Θγ> 90° Anna Skachkova: EM group workshop, Ferrara, 15-16 October 2007

  30. In pbar + p  gamma + X process (chosing pbar beam direction as the z-axis) the role of the transfered momentumQ 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 Ptspect-rum, to estimate the range of kinematical x-Q^2 region, available for the measurement at PANDA . Anna Skachkova: EM group workshop, Ferrara, 15-16 October 2007

  32. Neutral pions distributions 0 ≤ E γ ≤ 7 GeV, <E γ > = 1.2 GeV 0 ≤ PT γ ≤ 1 GeV, <PT γ> = 0.29 GeV 0 ≤ Θγ ≤ 180 ° , <Θγ> = 23.5 ° some Θγ> 90° Up to7neutral 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, 15-16 October 2007

  33. Fake (decay) photons distributions 0 ≤ E γ ≤ 4 GeV, <E γ > = 0.6 GeV 0 ≤ PT γ ≤ 1 GeV, <PT γ> = 0.16 GeV 0 ≤ Θγ ≤ 180 ° , <Θγ> = 29.6 ° some Θγ> 90° Vx, Vy, Vz distributions show mostly zero value Up to 10background photons in a signal event (in some few events up to 14) Anna Skachkova: EM group workshop, Ferrara, 15-16 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 directphoton bottom plot fakephoton 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, 15-16 October 2007

  35. Conclusion. 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 ECALπo, 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:0912 5104 [hep-ex] 37

  38. Outback

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

  40. Conclusion The proposed cuts: • Events with only2leptons of theopposite sign and El > 0.2 GeV, PTl > 0.2 GeV 2.The vertex of origin lies within the distance fromthe 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 ”, 22.11.08

  41. 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

  42. 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.

  43. 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.

  44. Outback

  45. 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 ”, 22.11.08

  46. 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 ”, 22.11.08

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