hep-ph/ Nov 1995

Transcription

1 D Note: 78 W+jets production at Tevatron { VECBOS and CompHEP comparison November 13, 1995 A. Belyaev 1, E. Boos, L. Dudko 3, A. Pukhov Institute of Nuclear Physics, Moscow State University, Moscow, ussia Abstract hep-ph/ Nov 1995 esults of calculation of all subprocesses in proton-antiproton collisions which contribute to the W+jets nal state are presented at Tevatron energy. The calculation has been carried out by means of the CompHEP software package. A detail comparison with VECBOS generator results for cross sections and various distributions shows an agreement at the level of Monte-Carlo accuracy. Therefore the additional independent check of VECBOS generator has been done. In complement to the VECBOS generator a new generator based on CompHEP allows to study individual subprocesses like W b b or W cc. The last point is important, for instance, for study W b b part of the background for single top or Standard Model Higgs signal at Tevatron. 1 Introduction eactions with jets production at Tevatron provide a very important part of physical backgrounds to a dierent signal processes, like strong top pair production [1, ], electroweak bosons pair production [3, ], electroweak single top production [5, 6, 7, 8, 9, 1, 11, 1, 13, 1, 15, 16], Standard Model Higgs production [17, 18]. In various searches VECBOS generator [19] has been used for such a background simulation. The processes with ; 3; jets have been calculated in the past []. The calculations of the W b b processes including b-quark mass have been carried out in [1]. In this paper we present additional detail comparison of the rates and distributions between VECBOS and CompHEP for the simplest ( from calculation point of view) case of W+jets production taking into account all nontrivial masses. Also contributions from different parton subreactions are presented separately. CompHEP [, 3] is known software package for automatic calculation of cross sections and distributions for particle processes in Standard Model or any model which can be easily inserted by user ( e.g. composite models, supersymmetry, model with leptoquarks). For a comparison we did not include sea s - and c - quarks in the initial states. In fact this contribution is about 5 % from the total W+jets rate. We also did not include any 1 belyaev@sgi.npi.msu.su boos@theory.npi.msu.su 3 dudko@sgi.npi.msu.su pukhov@sasha.npi.msu.su 1

2 A) B) u u -u -u u- -u? - d d d - d diagr.1 diagr. diagr.3 diagr. diagr.5 U u- -d d diagr.6 D - diagr.1 diagr. D I U u u d - d diagr.7 diagr.8 U U - D - D diagr.3 diagr. u U diagr.5 D C) u - I U D u diagr D - D diagr.1 diagr. D d d6 D d u 6 u- diagr.7 diagr.3 I U D D diagr.8 diagr. u - - u- D diagr.5 Figure 1: diagrams for the most important subreactions for W+jets background process: A) ug! dg, B) g d! ug and C) u d! gg fragmentation of partons to jets. So we considered nal states just on a parton level for both VECBOS and CompHEP. There are many electroweak diagrams in the Standard Model which contribute to the W+jets nal state. The complete set of Standard Model diagrams one can easily get using the program CompHEP. However like in VECBOS we have taken into account only the strong diagrams which provide the dominate contribution. An example of such diagrams for the most important subreactions is presented in Fig.1. In Fig.1 the CompHEP notation for particles are shown. Namely, small letters `u' and `d' correspond to the quark line and capital letters `U' and `D' correspond to the anti-quark line. Capital `' denotes the gluon line, `W+' corresponds to the -boson. In the calculations we have used for both VECBOS and CompHEP the CTEQpMS set of structure functions [, 5] with QCD scale chosen equal W-boson mass and QCD = :135 ev. For all partonic jets in the nal states the P T j > ev cut has been used and we have used jj > :5 cut for jet separation. ates and distributions With assumptions mentioned above we have got the following total rate for the process pp! jet at the 1:8 TeV Tevatron energy:

3 Table 1: Contributions from dierent subreactions for W+jets background process CompHEP VECBOS [pb] % from total rate % from total rate gg! sc 1.97 gg! du 1.96 gg! jet % 1.7% gu! dg.7 g d! ug 16.6 gq + gq! jet % 1.5% ug! dg.631 dg! ug 3.9 qg + qg! jet % 3.8% uu! du 1.33 d d! du.789 u d! uu.59 du! uu 1.16 uu! du 1.33 qq + q q! jet % 3.7% u d! b b u d! ss 1.75 u d! cc 1.61 u d! d d.898 u d! uu.91 uu! du 8. uu! sc.97 u d! gg 33.3 d d! du.99 d d! sc. du! dd 3.1 du! b b.36 du! ss.39 du! cc.39 du! d d.13 du! uu.13 du! gg.936 d d! du 1.91 d d! sc.1 uu! sc.1 uu! du.37 qq! jet % 6.3% 3

4 CompHEP : pp! jet VECBOS : pp! jet 13. pb 17.3 pb Therefore the dierence is of order of % and it means the results are in an agreement within an accuracy of Monte-Carlo calculation. Contributions from dierent subreactions are presented in Table 1. Table : Main processes for W+jets background process cross section ug! dg.6 pb ( 31.% ) g d! ug 16. pb ( 1.3% ) u d! gg 33. pb ( 5.% ) dσ/dp T [pb/ev] VECBOS CompHEP dσ/dp T [pb/ev] P T of jet with maximum P T [ev] P T of jet with minimum P T [ev] dσ/dp T [pb/ev] P T of W-boson [ev] Figure : P T distribution of a jet with a maximum and minimum P T In all cases the integration over initial parton distributions has been performed. In the Table 1 the rst initial parton is in the proton and the second one is in the antiproton. It is interesting to point out that about 68.9 % from the total rate is coming from the three dominating subprocesses (see Fig.1) listed in Table once more. In Table 1 the comparison between CompHEP and VECBOS is presented for dierent sets of subprocesses combined according to the initial parton states conguration. These sets are used in VECBOS. One can see a reasonable agreement for all cases.

5 dσ/η [pb] VECBOS CompHEP dσ/η [pb] η of jet with maximum p T η of jet with minimum p T dσ/η [pb] VECBOS CompHEP η W-boson Figure 3: Pseudorapidity distribution of a jet with a maximum and minimum P T In Fig. the P T distribution of a jet with a maximum and minimum P T is shown. The CompHEP P T distribution is a little bit harder at high P T. However this dierence is of order of statistical uctuations. In Fig.3 pseudorapidity distributions are demonstrated. One can see that the dierence between CompHEP and VECBOS for the same jets with a maximum and minimum P T is rather small like in previous case. For various physical cases it is very important to know invariant mass distributions. The invariant mass distributions of two jets and W and jet are shown in Fig.. One can see a very good agreement for VECBOS and CompHEP results. 3 Conclusions We have calculated contributions from all subprocesses to the W+jets production process at Tevatron using CompHEP program. For calculation of the total rates CompHEP program has been used itself while for event generation and analysis of the various distributions special event generator on the base of CompHEP package has been created. The only QCD part from a complete set of tree level diagrams has been taken into account. For a total rate and basic distributions an agreement between CompHEP and VECBOS results at the level of statistical uctuations has been found. It means an additional independent check of a VECBOS generator has been done. This is important because the VECBOS has been used in particular for a background simulation in the top-quark analysis and discovery [1, ]. One can stress, however, that in the VECBOS case user can not get an information for an individual subreactions listed in the Table 1. But in some cases it can be very useful. For instance, the process with W b b production provides an important background 5

6 dσ/dm jj [pb/ev] VECBOS CompHEP invariant mass of two jets [ev] dσ/dm wj [pb/ev] invariant mass of W+jet [ev] Figure : Invariant mass distributions of two jets and W and jet are shown for searches for single top (see [16] and references therein) or SM Higgs [17, 18] in case if an eective b-tagging procedure is used. With the help of CompHEP one can separately calculate and analyse each of the subreaction, in particular, with W b b or W cc production taking into account fermions masses. Complete tree level calculations in Standard Model for the reactions W b b and W b b+ jet including all nontrivial masses have been done in [6] using COmpHEP package. Acknowledgements We would like to thank Pavel Ermolov, Boaz Klima, Ann Heinson and Slava Ilyin for useful discussions. We thank the D Collaboration for their kind hospitality during our stay at Fermilab. We acknowledge the nancial support of the U.S. Department of Energy and the Ministry of Science and Technology Policy in ussia. This work has been supported in part by grants #M9B3, #a1-f from the International Science Foundation and grand and grant #93-9 from ICFPM&INTAS. 6

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