Higgs Boson Production Via Double Pomeron Exchange at the LHC

Physics Letters B 550 (2002) 93–98 www.elsevier.com/locate/npe

Higgs boson production via double Pomeron exchange at the LHC

M. Boonekamp a,b,A.DeRoecka, R. Peschanski c,C.Royonb,d

a CERN, CH-1211, Geneva 23, Switzerland b CEA/DSM/DAPNIA/SPP, CE-Saclay, F-91191 Gif-sur-Yvette cedex, France c CEA/DSM/SPhT, Unité de recherche associée au CNRS, CE-Saclay, F-91191 Gif-sur-Yvette cedex, France d University of Texas at Arlington, Arlington, TX 76019, USA Received 2 September 2002; received in revised form 13 October 2002; accepted 14 October 2002 Editor: P.V. Landshoff

Abstract We study Higgs boson production via double Pomeron exchange allowing for the presence of Pomeron remnants. We estimate the number of events produced at the LHC collider, as a function of the Higgs boson mass and its decay channel. The model which successfully describes the high mass dijet spectrum observed at Tevatron run I is used to predict rates of events with tagged protons at the LHC. Sizeable cross-sections and encouraging event selection signals are found, demonstrating especially for smaller Higgs boson masses the importance to study the diffractive channels. Tagging of the Pomeron remnants can be exploited to achieve a good resolution on the Higgs mass for inclusive diffractive events, by optimizing an analysis between higher cross-sections of the inclusive mode (all Pomeron remnants) and cleaner signals of the exclusive mode (without Pomeron remnants).  2002 Elsevier Science B.V. All rights reserved.

1. Introduction observed high mass dijet production at the Tevatron (run I) [4], allowing to compare and normalize the pre- It has been suggested [1,2] that diffractive Higgs bo- dictions to the data using a (simplified) simulation of son production via Double Pomeron Exchange (DPE) the detector. One important difference with previous is an interesting channel to study the Higgs boson (purely exclusive) estimates lies on the consideration → + + + + at hadron colliders. The lack of a solid QCD based of inclusive production pp p X H Y p, framework for diffraction made a purely theoretical see Fig. 1, namely, with particles accompanying the study difficult. Recently [3], however, the possibil- Higgs/dijet production in the central region. We will ity of a better determination of the cross-sections and call X, Y the “Pomeron remnants” in the following. event rates was proposed, using a model allowing the The presence of the Pomeron remnants is vital for the joint description of Higgs boson production and of the good description of the dijet mass spectrum as dis- cussedin[3]. Further estimates using a different Pomeron model [5] gives different numbers, since important sources E-mail address: [email protected] (C. Royon). of uncertainties still remain, but confirm the viabil-

LHC can be expected in double proton tagged experiments, by using a new method based on cuts on Pomeron remnants. In Section 3, we derive the pre- dicted number of events. as a function of MH , depend- ing on experimental cuts and the Higgs decay channel, and in Section 4 the potentialities for Higgs mass reconstruction.

2. Formulation

Let us introduce the formulae for inclusive Higgs boson and dijet production cross-sections via DPE [3]: g g Fig. 1. Production scheme. xi ≡ 1 − ξi ,vi are the longitudinal and 2 2 incl x1 x2 s MH transverse 2-momenta of the diffracted (anti)proton (see formula (1) dσ = CH δ ξ1ξ2 − H M2 xgxgs and text for the other kinematical notations). H 1 2 g g 2 dξi 2α v2 × G x ,µ dx d v ξ i P i i i − i = 1 ξi ity of such processes for the LHC. Pioneering stud- i 1,2 ies which will be possible in the near future at the × exp −2v2λ , Tevatron [3,8], on the evaluation of experimental pos- i H sibilities using outgoing (anti)proton tagging, on the g g 2 incl x1 x2 s (2) resolution achievable with missing mass methods and dσ = CJJ FJJδ (vi + ki) JJ M2 with information on the Pomeron remnants, can be JJ i=1,2 used to pin down remaining theoretical uncertainties. 2 × g 2 2 2α vi It can upgrade diffractive production to a complemen- GP xi ,µ dξi dηi d vi d ki ξi i=1,2 tary tool for the analysis of the Higgs boson character- istics at the LHC. × − 2 exp 2vi λJJ , (1) Our aim is to give predictions for Higgs boson production at the LHC based on inclusive dijet produc- g g where x1 ,x2 define, on each side (see Fig. 1), the tion at colliders via DPE. Thus, we (i) normalize the fraction of the Pomeron’s momentum carried by the theoretical predictions to the observed dijet rate at the g √ gluons involved in the hard process and GP (x1,2,µ), Tevatron (pp¯ collisions with s = 1.8 TeV), and as- is, up to a normalization, the gluon structure function sume the obtained normalization√ factor is also valid in the Pomeron extracted [9] from HERA experiments; at the LHC (pp collisions at s = 14 TeV), (ii) ob- µ2 is the hard scale (for simplicity kept fixed at tain a prediction for the Higgs boson production cross- 75 GeV2, the highest value studied at HERA; we sections and event rates at the LHC and, (iii) discuss neglect the small [9] contribution of quark initiated how the experimental opportunities at the LHC can be processes in the Pomeron); η1,η2 are the rapidities of used for precision measurements. the two jets and are defined as a function of the other In Section 2, we use as a starting point our kinematical variables by model [3] based on the extension to inclusive dif- f η1 + f η2 = g fractive production of the Bialas–Landshoff exclusive mT 1e mT 2e ξ1x1 , model for Higgs boson and heavy flavor jet produc- f −η1 f −η2 g m e + m e = ξ2x , (2) tion [1]. We showed [3] that this is able to reproduce T 1 T 2 2 f the observed distributions, in particular the dijet mass where mTi are the transverse mass of the quark with fraction spectrum, the normalization being fixed from flavor f. experiment. The main lesson of our study is that an in- The formulae (1) are written for a Higgs boson teresting potential for the Higgs boson studies at the of mass MH and two jets (of total mass MJJ), re- M. Boonekamp et al. / Physics Letters B 550 (2002) 93–98 95 spectively. The Pomeron trajectory is α(t) = 1 + produced initially by the long-range, soft DPE in- −2 + α t ( ∼ 0.08,α ∼ 0.25 GeV ), ξ1,2 (< 0.1) teraction, we assume that, up to a normalization, are the Pomeron’s fraction of longitudinal momen- the inclusive cross-section is the convolution of the tum, v1,2, the 2-transverse momenta of the outgo- “hard” partons → Higgs boson, (or partons → jets) ing pp¯, k1,2 those of the outgoing quark jets, λH ∼ subprocesses by the Pomeron structure function into −2 −2 2GeV (respectively, λJJ ∼ 3GeV ) the slope of gluons, see Fig. 1. The expected factorization break- the non-perturbative coupling for the Higgs boson (re- ing of hadroproduction will appear in the normaliza- spectively, dijets), and the constants CH ,CJJ are nor- tion through a renormalization of the Pomeron fluxes, malizations including a non-perturbative gluon cou- which are not the same as in hard diffraction at HERA. pling [1], appreciably cancelled in the ratio CH /CJJ. Indeed, this ansatz remarkably reproduces the dijet The dijet cross-section σJJ depends on the gg → mass fraction seen in experiment, see Ref. [3]. This Q f Qf and gg → gg cross-sections [10]. This gives model has been successfully applied [3] to Tevatron for five quark flavors data on dijets and to predictions for Higgs production possibilities at the same accelerator [3]. = f + gg FJJ FQ f Qf ρ 54Fgg ρ , f f f 4m m 4p p f ≡ T 1 T 2 gg ≡ T 1 T 2 3. Predictions ρ 2 ,ρ 2 , M f f Mgg Q Q ρf ρf 9ρf F f f ≡ 1 − 1 − , We can now give predictions for the Higgs boson Q Q 2f 2f m m 2 16 production cross sections in DPE events at the LHC, T 1 T 2 1 ρgg 3 i.e., with ξ1,2 < 0.1, by scaling our results by the same Fgg ≡ 1 − . (3) factor used for the CDF measurement on dijet cross p2 p2 4 T 1 T 2 sections, which means increasing it by a factor 3.8 [3]. The colour factor 54 appears in the ratio of gluon jets The results are given in Table 1 (first column) and in vs. quark jets partonic cross-sections [10]. Fig. 2. We note the high values of the cross-sections. Note that the gg → Q f Qf cross-section depend Since the typical luminosity for the LHC will be of the on transverse and not on rest quark masses. Thus, all 5 order of 10–100 fb−1/yr, this leads to several thousand quark flavors sizeably contribute to the dijet cross- Higgs particles diffractively produced per year, even section. This is to be contrasted with the exclusive case at low luminosity. Hence, for the inclusive channel which is proportional to rest masses [1,3], and thus the cross-section is large, much larger than recent considerably smaller. calculations for the exclusive channel (see Ref. [6]). The physical origin of formulae (1) is the fol- In Ref. [5], similar conclusions are reached (note, lowing: since the overall partonic configuration is however, [7]).

Table 1 − Number of Higgs boson events for 10 fb 1. The ﬁrst column gives the number of events at the generator level (all decay channels included), and the other columns take into account the roman pot acceptance and give the number of events for different Higgs boson decay channels + − ((2): bb,¯ (3): τ τ , (4): WW, (5): ZZ, (6): tt¯ )

MHiggs boson (1) (2) (3) (4) (5) (6) 120 3219 2043 228 447 48 0 150 2637 417 48 1827 222 0 200 1995 3 0 1470 522 0 300 1419 0 0 984 438 0 375 1674 0 0 1047 483 138 500 813 0 0 441 213 156 700 126 0 0 72 33 18 1000 4 0 0 2 1 0 96 M. Boonekamp et al. / Physics Letters B 550 (2002) 93–98

−1 Fig. 2. Diffractive Higgs boson production cross section. Upper plot: number of Higgs boson events for 10 fb as a function of MH obtained for different roman pot conﬁgurations (see text). Bottom plots: diffractive Higgs boson production cross section as a function of MH for the LHC (and the Tevatron). The standard inclusive Higgs boson production cross section is also shown for comparison.

In Fig. 2, we show the effects of the acceptance demonstrates that such detectors are needed to obtain a of the possible roman pots detectors at the LHC, good acceptance for low mass Higgs production. Con- which are used in conjunction with a central detec- fig. 4 corresponds to the full system using all detectors. tor. As an example, following ideas presently dis- In Table 1, the acceptance of the roman pot detectors cussed in a common study group of the central detector in the case of Config. 4 is taken into account, and we CMS [11] and the elastic/soft diffraction experiment give the number of events for 10 fb−1 in the different TOTEM [12], which both will use the same interaction Higgs decay modes. region at the LHC, we choose four possible configu- rations for roman pot detectors to measure the scattered protons. The used acceptance numbers are based 4. Higgs boson mass reconstruction on [13]. The first one (see, Config. 1 on Fig. 2) has roman pot detectors located in the warm region of the The advantage of DPE events with respect to LHC, respectively, at 140–180 and 240 m and assumes standard Higgs boson production lies in offering a a good acceptance for protons with |t| < 2GeV2,and potential to reconstruct the Higgs particle parameters ξ>0.01. The second one (Config. 2) considers only more precisely. For example, one can hope to obtain roman pots at 140–180 m [12] and gives a good ac- a very precise Higgs mass reconstruction if one can ceptance only for |t| < 1.5GeV2,andξ>0.02. Con- tag and measure both the protons in the roman pot fig. 3 assumes the presence of roman pot detectors in detectors as well as the Pomeron remnants. the cold region of the LHC at about 425 m and gives In Fig. 3 (upper part), we show the distribution a good acceptance for |t| < 2GeV2,and0.002 <ξ< in pseudo-rapidity of the tagged proton (highest |η|), 0.02. Certainly the latter will be challenging both from the Pomeron remnants at the parton level (medium the machine and experimental point of view, but Fig. 2 |η|) and the Higgs decay products for a Higgs mass M. Boonekamp et al. / Physics Letters B 550 (2002) 93–98 97

Fig. 3. Pseudo-rapidity distributions. Upper plot: pseudo-rapidity Fig. 4. Higgs mass resolution. The resolution on the Higgs boson distributions of the tagged protons, Pomeron remnants (parton level) mass is shown after four different cuts on the Pomeron remnant −1 and Higgs boson decay products for MH = 120 GeV; Medium energies at 20, 50, 100 and 500 GeV for a luminosity of 30 fb . plots from left to right: rapidity gap distribution between the tagged proton and the Pomeron remnant for two cases MH = 120 GeV, MH = 700 GeV; Bottom plots: distance between the Pomeron momentum conservation to all particles in the final remnant (parton level) and the nearest jet from the Higgs boson, state, namely, the Higgs boson, the scattered protons for M = 120 GeV, M = 700 GeV. H H in the roman pot detectors,√ and the Pomeron remnants. The energy E = ξ1ξ2s is used to produce the Higgs of 120 GeV in our model. In Fig. 3 we also give the boson and the Pomeron remnants. Detecting these size of the rapidity gap between the tagged proton and remnants requires the presence of detectors with an the Pomeron remnant in the case of a Higgs mass acceptance at high pseudo-rapidity, ideally up to |η|= of 120 GeV (middle left), 700 GeV (middle right). 10 (see Fig. 3), but already taggers up to a rapidity In the last row of this figure we show the distance of 7.5–8 [11] give a good acceptance for a Higgs between the Pomeron remnant and the nearest jet from boson with a mass of 120 GeV. With such taggers, it the Higgs, for the two Higgs masses. The rapidity is possible to reconstruct Higgs boson masses2 up to distance is large at the parton level. This region will about 600 GeV. be, however, mostly filled with soft particles from The quality of the reconstruction of the remnants QCD radiation and from hadronization of the partons. needs to be demonstrated after including hadroniza- We nevertheless expect that the remnants will remain tion and detector effects, and are subject to a future de- 1 visible after a cut on soft activity (e.g., particles tailed study. Meanwhile√ we assume it can be done√ with below 1 GeV). a resolution of 100%/ E (respectively, 300%/ E) In Fig. 4, we describe the results of the Higgs boson in an optimistic (respectively, more pessimistic) sce- mass reconstruction assuming one is able to select nario. The mass distribution is then determined from and measure the Pomeron remnants. Then the Higgs the missing mass measurement from the scattered pro- boson mass can be reconstructed by applying quadri-

2 The precision obtained on the Higgs boson mass reconstruction 1 Such a cut may anyway be required in the analysis to reduce using standard events is quite high for high Higgs boson masses. the effects of soft inelastic overlaid pile-up events due to multiple Our method is especially useful at low Higgs masses where the interactions per bunch crossing at the LHC. measurement is harder for standard events. 98 M. Boonekamp et al. / Physics Letters B 550 (2002) 93–98 tons, and the remnants are subtracted. The smearing To summarize, we have shown that the diffractive on the resulting Higgs mass distribution is mainly due inclusive Higgs production leads to large event rates to the Pomeron remnant measurements, hence a good at the LHC. A Higgs mass reconstruction with good resolution can be obtained when the energy of the precision is possible if both protons in the final state Pomeron remnants is small, i.e., a configuration nearer can be tagged with roman pot detectors and if the to the exclusive case. Pomeron remnants can be measured in the forward In Fig. 4, we display the resolution (respectively, region with sufficient resolution. This channel and 2.1, 4.0, 4.6 and 6.6 GeV) on the Higgs boson method will be especially useful in the low mass mass reconstruction for four different cuts on the Higgs region where the standard methods for Higgs Pomeron remnant energy (respectively, 20, 50, 100 measurements at the LHC are challenging. and 500 GeV) and for the optimistic scenario. This does not take into account the additional resolution smearing of 1–2 GeV expected from the missing mass References analysis of the scattered protons [13]. The plot with the best resolution (2.1 GeV) is shown for a luminosity −1 [1] A. 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