Double Parton Scattering Background to Higgs Boson Production at the LHC

1. Double Parton Scattering Background to Higgs Boson Production at the LHC M Y Hussein Department of Physics University of Bahrain • ·Introduction • ·Double Scattering Mechanism • ·Cross Section Results • ·Summary

2. Introduction: • The Standard Model beautifully describes all the observed properties of elementary particles and their interactions: quarks, leptons and gauge bosons corresponding to the EM, Weak and Strong forces. • If nature had chosen the most elegant and symmetric mathematical construct for the SM, all fundamental particles would have zero mass. Experiments show that this is not the case: all particles have a small mass. • The Standard Model has no mechanism that would account for any of these masses, unless we supplement it by adding additional fields, of a type known as scalar fields. “scalar” means that these fields do not carry a sense of direction, unlike the electric and magnetic fields and the other fields of the Standard Model. • One of the greatest achievements of twentieth-century theoretical physics was the discovery of how to incorporate particle masses into the Standard Model theory without spoiling the symmetry and mathematical consistency. The most straightforward way to do that is through what is known as “ Higgs mechanism”.

3. An important by-product of this mass generating addition to the Standard Model is the prediction of the existance of a totally new type of particle-the as-yet- undiscovered “Higgs boson” • The discovery of the Higgs boson would solve the mystery of how particles get their masses. The spontaneous breakdown of the weak-electromagnetic gauge group can be accomplished in only one known way Higgs fields. • The Higgs is elusive and plays a negligible rule in low energy phenomenology Because it couples with particles according to their masses very weakly coupled to the lepton and quark constituents of ordinary matter.

4. There are a few production mechanisms which lead to detectable cross section at the LHC. Each of them makes use of the preference of the SM Higgs to couple the heavy particles either massive vector boson or massive quarks (especially t-quark). They are: • Gluon-gluon fusion, • WW, ZZ fusion, • Associated production with W, Z boson. • Associated production with Here we discuss the production of Higgs particle more relevant to an experimental search “association production of an intermediate vector boson W or Z and a Higgs particle”

5. The discovery of the SM Higgs boson is one of the most important goals of LHC. An important prerequisite of LHC Higgs phenomenology is a precise knowledge of the various production and decay branching ratios. • The channel is the most favorite Higgs boson decay mode when the Higgs mass is below the threshold, and since b quark jet are identified with high efficiency, final state with pairs have a great potential for discovering the Higgs boson at the LHC, if the Higgs mass is in the range

6. An effective way to reduce the huge QCD background is to look for pairs accompanied by isolated lepton resulting from the decay of a W boson. p V = W,Z p H V*

7. At the LHC, Higgs boson production in association with W and Z boson is the most process of interest for detecting the Higgs boson through the channel in the form: p+ p WH +X p+p ZH +X W (1) Z H , where

8. At high energies and due to large flux in particular at the LHC, another type of scattering mechanism contributes to the cross section besides to single scattering. Thus, in the case of W(Z)H production there would be two computing mechanisms: single parton scattering and double parton scattering featuring two Drell-Yan processes happening simultaneously. ( P. Landshof and J. Polkinghone;F. Halzen, D. Hoyer and W. stirling; CDF Collaboration;……)

9. The purpose of the present work is to point out that the same , final state can be produced also by double parton scattering collision process, which represents therefore a further background to be taken into account in addition to the other background processes. • The large rate of production of pairs expected at the LHC gives rise to a relatively large probability of production of a in the process underling the W production. • In fact as a result of the present analysis it is found that double parton scattering may represent a rather sizable source of background.

10. II. DOUBLE SCATTERING MECHANISM Multiple parton interaction processes, where different pairs of partons have hard scattering in the same hadronic collision, become experimentally important at high energies because of the growing flux of partons. Recently, the importance of double parton scattering at the Large Hadron Collider (LHC) has been redressed. ( D. Treleani, N Paver and A. Del Fabbro…….)

11. The multiple parton scattering occurs when two or more different pairs of parton scatter independently in the same hadronic collision. Fig.(1) Double parton scattering

12. These parton distribution depends on the fractional momenta of all the interacting partons and, additionally, on their distance in transverse space which has to be the same for both the target and the projectile partons in order for the collisions to occur. The inclusive cross-section of a double parton scattering can be expressed, as: (2)

13. Where, , are two distinguishable partonic cross sections and, represents the two distribution function with frictional momenta and transverse relative distance with in the hadron . Multiple parton distribution functions, which contains in principle a new, more detailed information on the hadronic structure and probes correlations between partons in the hadron in the transverse structure.

14. These correlations can be assumed to be negligible if a scattering event is characterized by high center-of-mass energy. The two-body parton distribution functions factorize , where is the parton distribution function and describes the distribution of parton in the transverse plane.With these assumptions the cross-section for a double collision leads in the case of two distinguishable parton interactions to the simplest factorized expression:

15. (3) If the two interactions are indistinguishable, double counting is a voided by replacing (4) Here represents the single scattering cross section (5) With is the standard parton distribution and represents the sub-process cross section.

16. The parameter, is the effective cross section and it enters as a simple proportionality factor in the integrated inclusive cross section for a double parton scattering . The experimental value measured by CDF yields . It is believed that is largely independent of the center-of-mass energy of the collision and on the nature of the partonic interactions.

17. III. Cross-Sections Results a) Single parton Scattering Process For the production of Higgs boson we consider the simplest spontaneous broken gauge boson involving exactly one physical Higgs boson. The elementary cross section for is convoluted with integrated over all appropriate quark-antiquak distribution function and integrated over all appropriate final state: (6)

18. Where is the center of mass energy Where and is weak mixing angle

19. In Figure 1 we show total cross section for the processes p + p W H + X p + p Z H + X As function of Higgs mass at the LHC energy. In our calculation the integration was performed by VEGAS with MRST parton distribution. In Figure 2 we show total cross section for WH and ZH production times the branching ratio for the decay modes, as function of the Higgs mass, at LHC energy. The cross section for Higgs production in association with W and Z bosons are not large, but may be useful if high luminosity is available, since the Higgs can be “tagged” by triggering on the weak boson.

20. Fig. (2) Total cross section for WH and ZH production at the LHC ( ), as function of Higgs mass MH.

21. Fig. (3)Branching ratios of the SM Higgs boson in the mass range , for the decay modes. and ; and

22. Fig. (4) Total cross section for WH and ZH production times the branching ratios of the decay modes at the LHC( ),, as function of the Higgs mass MH.

23. b) Double parton scattering background process: ِA Background to the process p + p W(Z)H+X with W and H is represented by the double scattering interactions. The corresponding integrated rate is evaluated by combining the expected cross section for W and production at LHC energy. Using equation (4) one obtain the cross section for a double parton collision production.

24. Table 1: Shows the total cross section for W(Z)H production at the LHC corresponding to single and double scattering mechanism.It is clear that one obtains a cross section in double scattering background three orders of magnitude larger that one expected signals from Higgs decay.

25. The large rate of pairs expected at the LHC gives rise to a relatively large probability of production of pair in the process underlying the W productionA very promising channel to detect Higgs namely the final state with a and with isolated lepton. Figure summarize our results , the double parton scattering process remains a rather substantial component of the background to Higgs production.

26. Fig. (5) Double parton scattering background to the Higgs boson production as a function of the bb invariant mass compared to the expected signal for three possible value of the Higgs mass.

27. IV. Summary In this work we have computed the most promising signatures which should allow for a SM Higgs particle at the LHC with a mass below about 135 GeV, where Higgs decays into final states are dominant. The production of the Higgs boson, in the intermediate mass range, namely the final state with a pair and with isolated lepton, is affected by a sizable background due to double parton collision process. Although the double parton collision cross section is a decreasing function of the invariant masses of the pair , the relatively large value of the invariant mass required to the pair to be assigned to the Higgs decay is not large enough, at the LHC energies, to allow one neglect the double parton scattering background.

28. However, in the hadron collider environment, the large QCD backgrounds may cause the observation impossible for those events if one or more of the gauge bosons decay hadronically. Individual channels with hadronic decays should be studied on case by case. It is useful to point some of the inherent uncertainties which affect the final results. The most significant are: (i) the lake of precise knowledge of the parton distribution at small x, which is important for the intermediate mass Higgs and (ii) the effect of unknown higher order perturbation QCD corrections

29. Thank You