This note summarizes the discussion session on diffractive Higgs production
at the DIS2002 workshop.Comment: Discussion session at DIS 2002, Cracow, May 200

De Roeck, A.

Royon, C.

English

arXiv

This note summarizes the discussion session on diffractive Higgs production at the DIS2002 workshop.This note summarizes the discussion session on diffractive Higgs production at the DIS2002 workshop.This note summarizes the discussion session on diffractive Higgs production at the DIS2002 workshop.This note summarizes the discussion session on diffractive Higgs production at the DIS2002 workshop.This note summarizes the discussion session on diffractive Higgs production at the DIS2002 workshop

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arXiv:hep-ph/0209171v1  16 Sep 2002 Discussion session on diffractive Higgs production∗A. De RoeckCERN 1211 Geneva 23 SwitzerlandandC. RoyonCEA, SPP, DAPNIA, CE-Saclay, F-91191 Gif-sur-Yvette Cedex, France and SPPand Texas U at Arlington (USA)This note summarizes the discussion session on diffractive Higgs pro-duction at the DIS2002 workshop.1. IntroductionOne of the main goals of the Large Hadron Collider (LHC) under con-struction at CERN is the search for, discovery and measurement of the Higgsboson, the particle associated with the field that can provide a mechanismfor electroweak symmetry breaking in the Standard Model. The mass of theHiggs boson is unknown but precise measurements of electroweak processeshint towards a value below 196 GeV (95% C.L.). Direct searches exclude aHiggs with a mass smaller than 114.1 GeV (95% C.L.). If Supersymmetrywill turn out to be the mechanism that stabelizes the Standard Model athigh energies, then the theoretically preferred region for the (lightest) Higgsmass is below 135 GeV.Measuring a light Higgs at the LHC will not be an easy task [1], butrather a delicate trade-off between signal and background. E.g. inclusiveHiggs production with the Higgs decaying into its most favourable mode,bb, cannot be used to discover the Higgs due to the too high backgroundof bb production. It is therefore important to explore more, in particularclean, processes which would allow to discover the Higgs boson.Recently, renewed attention has been drawn to diffractive Higgs produc-tion [2], being first discussed –as well as heavy quark production – in [3].∗Presented at DIS2002(1)2 crac23 printed on February 11, 2014Since then, several groups have studied the processes but there are sub-stantial differences in the approaches used and results obtained. At theDIS02 meeting the most recent approaches were confronted in a discussionsession. For simplicity we will distinguish here only two main categories:exclusive and inclusive production, see Fig. 1. For each of these there areseveral different models discussed. Furthermore we will only discuss thosediffractive channels where the incident protons survive the interaction andcan be detected in e.g. roman pot detectors, as these constitute the mostinteresting event classes. But all these processes do have contributions fromchannels where the protons dissociate, and which can be used in case one isonly interested in the presence of a gap and not in measuring the scatteredproton.2. Exclusive productionIn the case of exclusive production (Fig. 1a), the final state is simplepp → p + H + p. In such a configuration one can benefit from strongspin JZ = 0 selection rules which reduces the LO order QCD backgroundproduction such as bb production by about two orders of magnitude.ppQtHx2x1x1′x2′pggp_HPIPIFig. 1. Diagrams for (left) exclusive and (right) inclusive diffractive Higgs produc-tionThe light Higgs production cross sections for exclusive production bythe different calculations range from approximately 100 fb [3, 4] to 3 fb [5].Much of the difference between the results comes from whether and how a socalled gap survival probability is included in the calculation. The Tevatrondiffractive data imposes the need of a gap survival probability of order 0.1for most calculations of diffractive hard scattering processes.The most detailed recent analysis of the exclusive channels is performedin [5] and they find a cross section of the order of 3 fb. They also estimate–within their approach– an uncertainty of a factor of two on this result [1].crac23 printed on February 11, 2014 3In other words the exclusive Higgs production cross section could well berather small, but still detectable at the LHC.3. Inclusive processesSeveral groups have also studied the inclusive production cross section(Fig. 1b). Here we distinguish a so called factorizable, non-factorizable andsoft color interaction model. A compilation of different recent calculationsfor the cross sections is given in Table 1.In the factorizable model [6], two pomerons are emitted with a structurefunction and flux factor as measured in deep inelastic data at HERA. Thecross sections for both diffractive di-jet and Higgs production are calculatedfor Tevatron and LHC energies. Diffractive di-jets have been measured atCDF, so the prediction can be compared to the data. The authors find thisprediction a factor 10 too large, and rescale the Higgs cross section withthis gap survival probability accordingly. The cross sections for LHC intheir paper are not rescaled. In [5] similar cross sections are calculated, andresults similar results obtained, be it using actually different diagrams asexplained in [1]In the non-factorizable model [7, 8], two pomerons are emitted witha structure function as measured in deep inelastic data at HERA, but asoft flux factor (ǫ = 0.08 instead of ∼ 0.2) is used, meant to absorb thefactorization breaking seen between HERA and Tevatron hard diffractivemeasurements. Similarly the cross sections for both diffractive di-jet andHiggs production are calculated for Tevatron and LHC energies. The di-jetcross section is found to be a factor 3.8 too small at the Tevatron when calcu-lated with conventional parameters, and the Higgs cross section is rescaledaccordingly, also for the LHC results. The resulting numbers are similarto [6], but for the latter no gap suppression factor has been applied for theLHC predictions. If this is applied the result in [6] would be a factor 10-20smaller than these results.In [9] the authors use the soft colour interaction (and also the generalarea law) model to predict Higgs production cross sections. This model candescribe a variety of diffractive data at the Tevatron and HERA. It predictsa small cross section for diffractive Higgs production at the Tevatron.4. DiscussionThe following discussion developed at the meeting.• Comparison between the different calculations, especially the exclusivechannels: what is the origin of the differences in the models? How canwe control the all important gap survival probability experimentally?4 crac23 printed on February 11, 2014σH(fb) Normalization Ref.320 x 3.8 [8]260-390 no rescaling [6]0.19-0.16 [9]Table 1. Cross sections for inclusive Higgs productionET* = (ET1+ET2)/2  (GeV)1 / NTOT (dN / dE T*)DPESDND(a)η* = (η1+η2)/21 / NTOT (dN / dη*)(b)φ = |φ1-φ2|  (rad)1 / NTOT (dN / dφ) (c)Dijet Mass Fraction1 / NTOT (dN / dR jj)(d)10-210-115 10 15 20 2500.20.40.6-4 -2 0 2 400.510 1 2 31100 0.2 0.4 0.6 0.8 1Fig. 2. Di-jet quantities DPE events as measured by CDF. The bottom-right plotshows the dijet mass fractionFor the pomeron processes, the difference in predictions is a factor 10for those models that ’use’ emition of pomerons. It appears that thisfactor can be mostly explained by the different value ǫ used for theflux [10].• What are the uncertainties on the calculations? By varying the param-eters in the models for the cross section calculations (input structurefunctions etc.) one could get a handle on the spread within a givenmodel. One author of [3] reported that the uncertainty can be easilyas large as a factor 10. Recently in [1] the uncertainty was evaluatedto be a factor two only for the calculations first presented in [5]• How can one test these models? While now generally accepted that thediffractive Higgs production rates for the Tevatron are probably toosmall to be of use, there are several processes which can be exploited atthis collider in the next years, e.g. diffractive di-jet, di-photon, χc andpossibly χb production. Di-photon production should be measurablecrac23 printed on February 11, 2014 5in run-II according to the predictions in [6]. Perhaps also vector mesonproduction at HERA can play a role in discriminating the models.In particular diffractive di-jet production is very interesting. CDF [11]has already measured double pomeron exchange (DPE) dijet produc-tion. Fig. 2 shows –among other distributions– the fraction of theenergy of the dijets compared to the total energy in the central sys-tem. Clearly every inclusive model for Higgs production, when appliedto predict Higgs and di-jet production rates should describe the shapeand normalization of this distribution. Note however that the CDFdata are not corrected for detector smearing, and thus to reproducethese signals one needs an event generator and detector simulationwhich has the proper energy smearing.CDF also sets a limit on the cross section for exclusive di-jet produc-tion i.e. where all visible energy enters in the two jets. They find thata most 5.1 events ( 3.7 nb) are compatible with this hypothesis. E.g.any model predicting a LARGER cross section than this can alreadybe excluded.• Concerning the debate of exclusive and inclusive production: Whatcan one finally gain from the inclusive diffractive Higgs productionwith respect to inclusive Higgs production? In this case there will beno JZ selection rule to suppress the background and one cannot use therelation Mpp = MH , since there are always remnants around. Someinitial ideas have been proposed in [8], but need to be substantiatedwith real hadronization and detector simulation. In particular inclu-sive production studies should make a full background calculations toshow that the signal will be visible at the end.There were concerns expressed whether exclusive events at such largescales really happen in nature. Will there not always be some softgluons around which spoil the exclusiveness? Di-photon productionat the Tevatron would be a good testing ground to confirm that theseevents are produced at high energies.5. Suggested homework• The new Tevatron run-II data will be of pivotal importance to set-tle some of these questions. We suggest that the following data becollected.Measure the DPE dijet spectra, preferably with double proton taggingto really constrain the Mdijet, and measure it for different ET scales(such that one can test the ǫ value of the flux).6 crac23 printed on February 11, 2014Try to measure the exclusive di-photon or a χ states. These havethe advantage over the di-jets that it is easier to determine their ’ex-clusiveness’. The cross sections are however much lower, so here theRun-II luminosity will be needed.• For the different models it would be useful to have the comparisonsof the predictions with Fig.2. Predictions for higher jet ET cuts, suchas 10 and 15 GeV would be useful for future comparisons with dataand to demonstrate the cross section behaviour with the scale in themodel.• For the different models it would be useful to have predictions fordi-photon production rates, e.g. for photons with ET > 7 GeV, asin [6].• A Monte Carlo generator for all these processes would be useful, tocompare with experimental data, eg. the di-jet mass fraction.The goal is to have these model numbers available by the Low-x meetingin September 2002 (Antwerpen/Belgium).We thank all contributers to this session for their presentation and alively discussion. Furthermore we thank R. Peschanski and V. Khoze for acritical reading of the paper.REFERENCES[1] A. De Roeck et al, hep-ph/0207042.[2] M.G. Albrow and A. Rostovtsev, hep-ph/0009336.[3] A. Bialas and P.V. Landshoff, Phys. lett. B256 (1990) 540, A. Bialas and W.Szeremeta, Phys. lett. B296 (1992) 191, A. Bialas and R. Janik, Zeit. fu¨r.Phys. C62 (1994) 487.[4] A. Schafer, O. Nachtmann and R. Scho¨pf, Phys. lett. B249 (1990) 331, J.D.Bjorken, Phys.Rev. D47 (1993), J-R Cudell and O.F. Hernandez, Nucl.Phys. B471 (1996) 471; H.J. Lu, J. Milana Phys.Rev. D51 (1995) 6107; D.Graudenz, G. Veneziano Phys. lett. B365 (1996) 302; M. Heyssler, Z. Kunszt,W.J. Stirling, Phys. lett. B406 (1997) 95; E.M. Levin hep-ph/9912403 andreferences therein.[5] V.A. Khoze, A.D. Martin and M.G. Ryskin, Eur.Phys.J. C14 (2000) 525, id.C19 (2001) 477, hep-ph/0006005; V.A. Khoze, hep-ph/0105224.[6] Brian Cox, Jeff Forshaw, Beate Heinemann, hep-ph/0110173.[7] M. Boonekamp, R. Peschanski and C. Royon, Phys. Rev. Lett. 87 (2001)251806. Some misprints are corrected in formulae (1) and (3) of Ref. [8].[8] M. Boonekamp, A. De Roeck, R. Peschanski and C. Royon, hep-ph/0205332.crac23 printed on February 11, 2014 7[9] R. Enberg, G. Ingelman, A. Kissavos, N. Timneanu, hep-ph/0203267.[10] M. Boonekamp, talk at this conference.[11] T. Affolder et al., CDF Coll. , Phys. Rev. Lett. 85 (2000) 4215.

Discussion session on diffractive Higgs production

Discussion session at DIS 2002, Cracow, May 2002This note summarizes the discussion session on diffractive Higgs production at the DIS2002 workshop

Discussion session on diffractive Higgs production

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