Marta Łuszczak Department of Theoretical Physics, University of Rzeszów, PL Rzeszów, Poland

Transcription

1 From deepineastic structure functions to twophoton diepton production in protonproton coisions Institute of Nucear Physics, Poish Academy of Sciences, Radzikowskiego 5, PL334 Kraków, Poand Emai: Marta Łuszczak Department of Theoretica Physics, University of Rzeszów, PL35959 Rzeszów, Poand Emai: Wofgang Schäfer Institute of Nucear Physics, Poish Academy of Sciences, Radzikowskiego 5, PL334 Kraków, Poand Emai: We compare two different approaches used for cacuating cross sections for the twophoton pp X process. In one of the approaches photon is treated as a coinear parton in the proton. In the second approach a recenty proposed k T factorization method is used. In this presentation we discuss sensitivity of the resuts to the choice of structure function parametrization and experimenta cuts in the k T factorization approach. We compare resuts of our cacuations with recent experimenta data for diepton production and find that in most cases the contribution of the photonphoton mechanism is rather sma. We discuss how to enhance the photonphoton contribution. We aso compare our resuts to those of recent measurements of excusive and semiexcusive e e pair production with certain experimenta data by the CMS coaboration. PoS(DIS06)04 XXIV Internationa Workshop on DeepIneastic Scattering and Reated Subjects 5 Apri, 06 DE Hamburg, Germany Speaker. The work has been supported by the Poish Nationa Science Center grant DEC04/5/B/ST/058. c Copyright owned by the author(s) under the terms of the Creative Commons AttributionNonCommerciaNoDerivatives 4.0 Internationa License (CC BYNCND 4.0).

2 From deepineastic structure functions. Introduction In this presentation we show our recent resuts pubished in [] where we have considered pp pp (excusive) and pp (semiexcusive, with proton dissociation) doube photon fusion processes in the proposed somewhat earier k t factorization approach []. In our presentation at DIS06 we have focussed on the reation of the cross section for the charged epton pair production with the dependence of the deepineastic structure functions F, that are the input for our approach, on x and Q. The main mechanisms of the diepton production considered in [] are shown in Fig.. The considered mechanism has the same fina state as the dominant DreYan mechanism. For k t factorization approach to the DreYan see [3, 4, 5, 6]. In our recent paper [] we discussed in detai the photonphoton mechanism. p p f e e f p p p p f ine f ine Figure : Different mechanisms of twophoton production of dieptons incuded in [].. Basic formuae X X p p f e ine f p X p p ine f e f X p PoS(DIS06)04 In coinear approximation the cross sections are cacuated as: dσ γ inγ in dy dy d p t = 6π ŝ x γ i (x, ) x γ j (x, ) M γγ, (.) where i,j=e,in and f i are photon PDFs. The eastic photon fuxes are cacuated using the Drees Zeppenfed parametrization, where a simpe parametrization of nuceon eectromagnetic form factors was used. In the k t factorization approach the differentia cross section can be written as: dσ (i, j) d q dy dy d p d = p πq d q πq F (i) γ /A (x,q )F ( j) γ /B (x,q ) dσ (p, p ;q,q ) dy dy d p d, p (.) where i,j=e,ine and F k are unintegrated fuxes of photons. As shown in [] the unintegrated fuxes can be expressed in terms of the (deepineastic) structure functions F (x,q ).

3 From deepineastic structure functions 3. Numerica resuts In our studies in [] we have used a few different parametrizations of the proton structure function F taken from the iterature: [8, 9]. This parametrization gives a very good fit to F in most of the measured region. FJLLM []. This parametrization expicity incudes the nuceon resonances and gives an exceent fit of the CLAS data. []. This parametrization concentrates on the owx, or high mass region. It features a Froissartike behaviour at very sma x. []. This paramerization of Suri and Yennie from the eary 970 s does not incude QCDDGLAP evoution. It is sti today often used as one of the defauts in the LPAIR event generator. [3]. A parametrization which concentrates to give a good description at rather sma and intermediate Q at not too sma x. We aso show F cacuated from the CTEQ6L parametrization [4]. In Fig. we show ony two exampes of the proton structure function F (x,q ) obtained from the various parametrizations at Q =.5,4.5GeV as a function of Bjorkenx. It is surprising that the od SuriYennie [] fit, sti gives a reasonabe description of F except of very sma x. For expicit account of resonances we recommend to use the Fiore et a. [], but care has to be taken to stay within the resonance region, as the quaity of the fit beyond this region quicky deteriorates. The overa best description appears to be given by the [8, 9] fit CLAS data NMC data CTEQ6L CLAS data NMC data CTEQ6L PoS(DIS06) x x Figure : The proton structure function F (x,q ) as a function of x for Q =.5GeV (eft), and Q = 4.5GeV (right). Shown are resuts for different parmetrizations avaiabe in the iterature. In Ref.[] we have compared our cacuations with measured diepton data [5, 6, 7, 8, 9]. Here we show ony a few exampes. Most of the experiments for the diepton production concentrate on determination of diepton invariant mass distributions. In Fig.3 we show invariant mass distributions of diepton pairs produced in the photonphoton ineasticineastic mechanism for kinematica conditions reevant for

4 From deepineastic structure functions different experiments. We show resuts obtained with the different parametrizations of the structure functions known from the iterature. Surprisingy the different structure functions give quite different resuts. For competeness in some cases we aso show the resut obtained in the coinear approach with the MRST004(QED) photon distribution [7] with (soid back ine) and simiar one when ignoring the initia input (ongdashed back ine). The resut obtained within the coinear approach with the MRST004(QED) distribution is much above the resuts obtained within the k t factorization approach. In our opinion this is mainy reated to the arge input photon distribution at the initia scae Q 0 = GeV. If the input is discarded (ongdashed back ine) the coinear resut is simiar to the resuts obtained within the k T factorization. The ineasticineastic contribution gives ony a sma fraction of the measured cross section for most experimenta conditions (ATLAS,LHCb). dσ/dm X ineasticineastic p p coinear MRST04 QED coinear (without input) M ATLAS data.5 y.5 ± p 5 GeV dσ/dm 3 p p.0 y 4.5 ± p 3.0 GeV X M LHCb data ineasticineastic coinear MRST04 QED coinear (without input) Figure 3: The ineasticineastic contribution to diepton invariant mass distributions for ATLAS (eft) and LHCb (right) experiments for different structure functions. In Fig.4 we show diepton invariant mass distributions for easticineastic and ineasticeastic (added together) contributions. As for ineasticineastic contribution the resuts strongy depend on the parametrization of the structure functions used. The spread of resuts for different F from the iterature is now somewhat smaer than in the case of ineasticineastic contributions where the structure functions enter twice. As for the doube ineastic case we aso show a resut for the coinear approach. The mixed components give simiar contribution to the diepton invariant mass distributions as the ineasticineastic one. In most of the cases considered so far DreYan processes dominate [4, 5, 6]. The twophoton processes are interesting by themseves. Can they be measured? In order to reduce the DreYan contribution and reativey enhance the twophoton contribution one can impose an extra condition on epton isoation. First trias have been done by the CMS coaboration [0]. In their anaysis an extra epton isoation cuts were imposed in order to eiminate the dominating DreYan component. In Figs. 5,6,7 we show our resuts for two different ( and ) parametrizations of the structure functions for distributions in dimuon invariant mass, in transverse momentum of the pair and in reative azimutha ange between. and parametrizations give amost the same contributions to a the distributions considered. In the first evauation we have taken into account integrated uminosity of the experiment (L = 63. pb ) as we as experimenta acceptances given in Ref.[0]. Rather good agreement with the ow statistics CMS experimenta data is achieved without incuding any extra corrections due to absorption effects. It may mean that PoS(DIS06)04 3

5 From deepineastic structure functions dσ/dm X easticineastic ineasticeastic p p coinear MRST04 QED coinear (without input) M ATLAS data.5 y.5 ± p 5 GeV dσ/dm 3.0 y 4.5 ± p 3.0 GeV p p X M LHCb data easticineastic ineasticeastic coinear MRST04 QED coinear (without input) Figure 4: The (easticineastic)(ineasticeastic) contribution to diepton invariant mass distributions for ATLAS (eft) and LHCb (right) experiments for different structure functions. the absorption effects are sma or aternativey that a contamination of the DreYan contribution is sti not competey removed. Both effects shoud be therefore studied in detai in a future. Events / GeV.5 y.5 e ± ineasticineastic easticineastic ineasticeastic easticeastic M e e F Events / GeV.5 y.5 e ± ineasticineastic easticineastic ineasticeastic easticeastic M e e F Figure 5: Number of events per invariant mass interva for the CMS experimenta cuts for (eft) and (right) structure functions. The experimenta data points are from Ref.[0]. PoS(DIS06)04 Events / GeV.5 y.5 e ± ineasticineastic easticineastic ineasticeastic easticeastic F Events / GeV.5 y.5 e ± ineasticineastic easticineastic ineasticeastic easticeastic F p T(e e ) p T(e e ) Figure 6: Number of events per pair transverse momentum interva for the CMS experimenta cuts for (eft) and (right) structure functions. The experimenta data points are from Ref.[0]. 4

6 From deepineastic structure functions Events / 0.05 rad.5 y.5 e ± F ineasticineastic easticineastic ineasticeastic easticeastic Events / 0.05 rad.5 y.5 e ± F ineasticineastic easticineastic ineasticeastic easticeastic φ (rad) e e φ (rad) e e Figure 7: Number of events per pair reative azimutha ange interva for the CMS experimenta cut for (eft) and (right) structure functions. The experimenta data points are from Ref.[0]. 4. Concusions We marize our studies in [] as foows: Two different approaches (coinear and k t factorization) for γγ processes were discussed and compared. Strong dependence on the structure function input in the k t factorization approach were found. Semiexcusive contributions with proton dissociation is arge (this may be interesting esson for other processes such as e.g. the pp ppj/ψ reaction). Photonphoton contribution is rather sma compared to DreYan contribution but is important in precision cacuations. Reasonabe description of the with isoated eectrons was achieved (recenty aso ATLAS obtained simiar resut). PoS(DIS06)04 The regions of the arguments of the structure function F important for the discussed γγ process was identified. So far ony coinear approach was appied to pp (γγ) W W XY processes which is important in searches for Beyond Standard Mode effects. References [] M. Luszczak, W. Schäfer and A. Szczurek, Twophoton diepton production in protonproton coisions: Two aternative approaches, Phys. Rev. D93 (06) [] G. G. da Siveira, L. Forthomme, K. Piotrzkowski, W. Schäfer and A. Szczurek, Centra production via photonphoton fusion in protonproton coisions with proton dissociation, JHEP 50, 59 (05) [arxiv: [hepph]]. [3] A. Szczurek and G. Sipek, Parton transverse momenta and DreYan diepton production, Phys. Rev. D 78 (008) 4007 [arxiv: [hepph]]. 5

7 From deepineastic structure functions [4] M. A. Nefedov, N. N. Nikoaev and V. A. Saeev, DreYan epton pair production at high energies in the Parton Reggeization Approach, Phys. Rev. D 87 (03), 040 [arxiv:.5539 [hepph]]. [5] S. P. Baranov, A. V. Lipatov and N. P. Zotov, DreYan epton pair production at the LHC and transverse momentum dependent quark densities of the proton, Phys. Rev. D 89 (04) 9, [arxiv: [hepph]]. [6] W. Schäfer and A. Szczurek, Low mass DreYan production of epton pairs at forward directions at the LHC: A hybrid approach, Phys. Rev. D93 (06) [7] A. D. Martin, R. G. Roberts, W. J. Stiring and R. S. Thorne, Parton distributions incorporating QED contributions, Eur. Phys. J. C 39, 55 (005) [hepph/0440]. [8] H. Abramowicz, E. M. Levin, A. Levy and U. Maor, A Parametrization of sigmat (gamma* p) above the resonance region Q** >= 0, Phys. Lett. B 69 (99) 465. [9] H. Abramowicz and A. Levy, The parameterization of sigma(tot)(gamma* p): An Update, hepph/9745. [] R. Fiore, A. Fachi, L. L. Jenkovszky, A. I. Lengye and V. K. Magas, Expicit mode reaizing parton hadron duaity, Eur. Phys. J. A 5, 505 (00) [hepph/00607]. [] M. M. Bock, L. Durand and P. Ha, Connection of the virtua γ p cross section of ep deep ineastic scattering to rea γ p scattering, and the impications for νn and ep tota cross sections, Phys. Rev. D 89, no. 9, (04) [arxiv: [hepph]]. [] A. Suri and D. R. Yennie, The Spacetime Phenomenoogy Of Photon Absorption And Ineastic Eectron Scattering, Annas Phys. 7, 43 (97). [3] A. Szczurek and V. Ueshchenko, Nonpartonic components in the nuceon structure functions at sma Q** in the broad range of x, Eur. Phys. J. C, 663 (000) [hepph/990488]. [4] J. Pumpin, D. R. Stump, J. Huston, H. L. Lai, P. M. Nadosky and W. K. Tung, New generation of parton distributions with uncertainties from goba QCD anaysis, JHEP 007 (00) 0 [hepph/0095]. [5] A. Adare et a. [PHENIX Coaboration], Diepton mass spectra in pp coisions at s**(/) = 00GeV and the contribution from open charm,, Phys. Lett. B 670 (009) 33 [arxiv: [hepex]]. [6] [The LHCb coaboration], Incusive ow mass DreYan production in the forward region at sqrt(s)=7 TeV, LHCbCONF003; Conference report prepared for XX Internationa Workshop on DeepIneastic Scattering and Reated Subjects, 630, March 0, Bonn, Germany. [7] G. Aad et a. [ATLAS Coaboration], Measurement of the owmass DreYan differentia cross section at using the ATLAS detector, JHEP 406, (04) [arxiv:404. [hepex]]. [8] G. Aad et a. [ATLAS Coaboration], Measurement of the highmass Dre Yan differentia crosssection in pp coisions at sqrt(s)=7 TeV with the ATLAS detector, Phys. Lett. B 75, 3 (03) [arxiv: [hepex]]. [9] C. Kourkoumeis, L. K. Resvanis, T. A. Fiippas, E. Fokitis, A. M. Cnops, J. H. Cobb, R. Hogue and S. Iwata et a., Study of Massive Eectron Pair Production at the CERN Intersecting Storage Rings, Phys. Lett. B 9 (980) 475. [0] S. Chatrchyan et a. [CMS Coaboration], Search for excusive or semiexcusive photon pair production and observation of excusive and semiexcusive eectron pair production in pp coisions at, JHEP, 080 (0) [arxiv: [hepex]]. PoS(DIS06)04 6