A SM-like Higgs boson can be produced in a variety of channels at the LHC. By
combining information from production via gluon fusion and weak boson fusion,
various partial widths and the total Higgs boson width can be extracted.
Expected accuracies for 200 fb^{-1} of data are in the 10% range.Comment: 6 pages, Revtex (singlespaced), uses epsfig.sty, 1 postscript figure.
  Talk given at the 35th Rencontres de Moriond, QCD and High Energy Hadronic
  Interactions, Les Arcs, France, March 18-25, 200

Zeppenfeld, D.

English

arXiv

A SM-like Higgs boson can be produced in a variety of channels at the LHC. By combining information from production via gluon fusion and weak boson fusion, various partial widths and the total Higgs boson width can be extracted. Expected accuracies for 200 fb^{-1} of data are in the 10% range

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MEASUREMENT OF HIGGS PROPERTIES AT THE LHC D. ZEPPENFELD Department of Physics, University of Wisconsin, Madison, WI 53106, USA and CERN, 121 1 Geneva 23, Switzerland A SM-like Higgs boson can be produced in a variety of channels at the LHC. By combining information from production via gluon fusion and weak boson fusion, various partial widths and the total Higgs boson width can be extracted. Expected accuracies for 200 fb- 1 of data arc in the 103 range. 1 Introduction One of the prime tasks of the LHC will be to probe the mechanism of electroweak gauge sym­metry breaking. Beyond observation of the various CP even and CP odd scalars which nature may have in store for us,1•2 this means the determination of the couplings of the Higgs boson to the known fermions and gauge bosons, i.e. the measurement of Htt, Hbb, Hrr and HWW, HZZ, H11 couplings, to the extent possible. Clearly this task very much depends on the expected Higgs boson mass. For mH > 200 GeV and within the SM, only the H -->  ZZ and H -->  WW channels are expected to be observable, and the two gauge boson modes are related by SU(2) . A much richer spectrum of decay modes is predicted for the intermediate mass range, i.e. if a SM-like Higgs boson has a mass between the reach of LEP2 (;S 110 GeV) and the Z-pair threshold. The main reasons for focusing on this range are present indications from electroweak precision data, which favor mH < 250 GeV� as well as expectations within the MSSM, which predicts the lightest Higgs boson to have a mass mh ;S 130 GeV. Recently, an analysis of Higgs coupling measurements at the LHC was completed for this intermediate mass range 4 and in this talk I sumarize the results. 165 Table 1: Number of events expected for qq -+ qqH, JI -+ W W I •) -+ µ± e"pr in 200 fb - 1  of data, and cor­responding backgrounds 9 The expected relative statistical error on the signal cross section is given in the last line. mH 120 130 140 150 160 170 180 190 Ns 136 332 592 908 1460 1436 1 172 832 Ns 136 160 188 216 240 288 300 324 !J.crH /crH 12 . 13 6.73 4.73 3 .73 2.83 2.93 3.33 4.13 2 S urvey of intermediate mass Higgs channels The total production cross section for a SM Higgs boson at the LHC is dominated by the gluon fusion process, gg ---+ H, which largely proceeds via a top-quark loop. Thus, inclusive Higgs searches will collectively be called "gluon fusion" channels in the following. Three inclusive channels are highly promising for the SM Higgs boson search,1'2 gg ---+ H ---+ 'TY ,  for mH ;S 150 GeV , ( 1 )  gg ---+ H ---+ ZZ* ---+ 4£ , for mH ,2: 130 GeV , (2) and gg ---+ H ---+ WW* ---+ fvlv , for mH ,2: 1 30 GeV . (3)  The H ---+ II signal can be observed as a narrow and high statistics II invariant mass peak, albeit on a very large di photon background. A few tens of H ---+ Z Z* --+ 4£ events are expected to be visible in 100 fb-1 of data, with excellent signal to background ratios (S/B) ,  ranging between 1 : 1  and 6 : 1 ,  in a narrow four-lepton invariant mass peak. Finally, the H ----> WW* ---+ fvlv mode is visible as a broad enhancement of event rate in a 4-lepton transverse mass distribution, with S/B between 1 :4 and 1 : 1  (for favorable values of the Higgs mass, around 1 70 GeV) . Additional, and, as we shall see, crucial information on the Higgs boson can be obtained by isolating Higgs production in weak boson fusion (WBF) ,  i .e. by separately observing qq ---+ qqH and crossing related processes, in which the Higgs is radiated off a t-channel W or Z.  Specifically, i t  was recently shown in parton level analyses that the weak boson fusion channels, with subsequent Higgs decay into photon pairs,5'6 qq ---+ qqH, H ---+ 11 , qq ---+ qqH, H ---+ TT ' or into W pairs 6•9 for mH ;S 150 GeV , for mH ;S 140 GeV , qq ---+ qqH, H ---+ ww<•l ---+ e±µ�pr , for mH 2: 120 GeV , ( 4) (5) (6) can be  isolated at the LHC. The weak boson fusion channels utilize the significant back­ground reductions which are expected from double forward jet tagging 10 and central jet vetoing techniques,11 and promise low background environments in which Higgs decays can be studied in detail. An example of expected events rates (after cuts and including efficiency factors) are sum­marized in Table 1 for the qq ---+ qqH, H ---+ ww<•l ---+ e± µ�pr signal. The rates and ensuing statistical errors of the signal cross section are given for 100 fb-l of data collcted in both the ATLAS and the CMS detector. The statistical accuracy with which the signal cross sections of the processes in Eqs. ( 1-6) can be determined is shown in Fig. la) . 166 30 :11 b 20 " :11 b <I 10 \ gg -+ H -+ Wlr  Jl -+ rr ·. I \ ' - . · - . ' \ - -'.-\ VV -+ H -+ � ' ' , , "· ; \ . \ a) gg -+ H -+ ZZ / / /.. I o ._._...__......._ ....... _._ ....... ..._._._...__......._ ..._. ...._ ....... .._ ....... ...._. 100 120 140 160 160 200 mH (GeV) >. 30 0 tO s 0 0 tO 20 "C Q) +> 0 Q) p.. � 10 b) r( 1-e)2 rll'(l-e) - - - - - - - - -o ._.__._._._...__. ......... ........ _._ ......... _._._... .__.._._....._._._ .......... ........ 100 120 140 160 160 200 mH (GeV) Figure l: Relative accuracy expected at the LHC with 200 fb-1 of data. a) Cross section times branching fraction for several inclusive modes (dotted and dash-dotted lines) and WBF channels (dashed and solid lines). b) Extracted total width (solid line) and H ...... WW partial width (dashed line).  3 Measurement of Higgs properties In order to translate the cross section measurements of the various Higgs production and decay channels into measurements of Higgs boson properties, in particular into measurements of the various Higgs boson couplings to gauge fields and fermions, it is convenient to rewrite them in terms of partial widths of various Higgs boson decay channels. The Higgs-fermion couplings 9Hff> for example, which in the SM are given by the fermion masses, 9HJJ = m1(mH)/v, can be traded for r f = r(H ---+ f !).  Similarly the square of the HWW coupling (9HWW = gmw in the SM) or the HZZ coupling is proportional to the partial widths fw = f(H ---+ WW*) or fz = r(H ---+ ZZ*). r 1 = f(H ---+ yy) and r 9 = r(H ---+ gg) determine the squares of the effective H11 and Hgg couplings. The Higgs production cross sections are governed by the same squares of couplings, hence, a(VV ---+ H) � fv (for V = g, W, Z) . Combined with the branching fractions B(H ---+ ii) = r;jr the various signal cross sections measure different combinations of Higgs boson partial and total widths, r;rj/r. The production rate for WBF is a mixture of Z Z ---+ H and WW ---+ H processes, and we cannot distinguish between the two experimentally. In a large class of models the ratio of HWW and HZZ couplings is identical to the one in the SM, however, and this includes the MSSM. We therefore assume that 1) the H ---+ Z Z* and H ---+ WW* partial widths are related by SU(2) as in the SM, i.e. their ratio, z, is given by the SM value, z = fz/I'w = ZSM · Note that this assumption can be tested, at the 15-203 level for mH > 130 GeV, by forming the ratio Ba(gg ---+ H ---+ ZZ*)/Ba(gg ---+ H ---+ WW*) .  With W, Z-universality, the three weak boson fusion cross sections give us direct measure-167 ments of three combinations of (partial) widths, X - rwr,, from qq -> qqH, H ->  11 , ,, r from qq -> qqH, H -> TT , from qq -> qqH, H ->  ww<•l , In addition the three gluon fusion channels provide measurements of Y, - I'gI'-y from gg -> H ->  11 , ,, r Yz = I'gI'z from gg -> H ->  zz<•l , r Yw = I'gI'w r from gg -> H _, ww<•l . (7) (8) (9) (10) (11) (12) When extracting Higgs couplings, the QCD uncertainties of production cross sections enter. These can be estimated via the residual scale dependence of the NLO predictions and are small (of order 53) for the WBF case,12 while larger uncertainties of about 203 are found for gluon fusion.13 A first test of the Higgs sector is provided by taking ratios of the X; 's and ratios of the Y;'s. QCD uncertainties, and all other uncertainties related to the initial state, like luminosity and pdf errors, cancel in these ratios. They test W, Z-universality, and compare the TT H Yukawa coupling with the HWW coupling. Typical errors on these cross section ratios are expected to be in the 15 to 203 range. Accepting an additional systematic error of about 203, a measurement of the ratio I'9/I'w, which determines the Htt to HWW coupling ratio, can be performed, by measuring the cross section ratios Bc;(gg -> H ->  11)/c;(qq -> qqH)B(H -> 11) and Ba(gg _, H _, WW*)/a(qq _, qqH)B(H _, WW*) .  Beyond the measurement of  coupling ratios, minimal additional assumptions allow an indi­rect measurement of the total Higgs width. First of all, the T partial width, properly normalized, is measurable with an accuracy of order 103. The T is a third generation fermion with isospin - � , just like the b-quark. In many models, the ratio of their coupling to the Higgs is given by the T to b mass ratio. In addition to W, Z-universality we thus assume that (2) y = rb/r 7 = YsM and, finally, (3) the branching ratio for unexpected channels is small, i.e. € = 1 - (B(H -> bb) + B(H _, TT) + B(H _, ww<•l ) + B(H -> zz<•) ) + B(H _, gg) + B(H _, 11) « 1 .  With these three assumptions consider the observable f'w X7(1 + y) + Xw (l + z) + X., + Yw (rr + rb + rw + rz + r,, + r9)r; = (1 - t)I'w . (13) f'w provides a lower bound on I'(H _, ww<•l ) = I'w. Provided t is small (within the SM and for mH > 110 GeV, t < 0.04 and it is dominated by B(H -> cc)) ,  the determination of f'w provides a direct measurement of the H _, ww<•l partial width. Once I'w has been determined, the total width of the Higgs boson is given by rw 1 ( - ) 2 1 I' =  Xw = Xw Xr(l + y) + Xw(l + z) + X,, + X9 (l - t)2 •  (14) The extraction of the total Higgs width, via Eq. (14), requires a measurement of the qq _, qqH, H :..... WW(•) cross section, which is expected to be available for mH i::: 115 GeV.9 Consequently, errors are large for Higgs masses close to this lower limit but decrease to about 103 for Higgs boson masses around the WW threshold. Results are shown in Fig. lb) and look highly promising. 168 4 Summary With an integrated luminosity of 100 fb-1 per experiment , the LHC can measure various ratios of Higgs partial widths, with accuracies of order 10 to 203. This translates into 5 to 103 measurements of various ratios of coupling constants. The ratio r 7 /rw measures the coupling of down-type fermions relative to the Higgs couplings to gauge bosons. To the extent that the H II triangle diagrams are dominated by the W loop, the width ratio r 7 /r..,, probes the same relationship. The fermion triangles leading to an effective H gg coupling are expected to be dominated by the top-quark, thus, r g/rw probes the coupling of up-type fermions relative to the HWW coupling. Finally, for Higgs boson masses above >::::< 1 20 GeV, the absolute normalization of the HWW coupling is accessible via the extraction of the H _, WW(•) partial width in weak boson fusion. These measurements test the crucial aspects of the Higgs sector. The HWW coupling, being linear in the Higgs field, identifies the observed Higgs boson as the scalar responsible for the spontaneous breaking of SU(2) x U(l ) :  a scalar without a vacuum expectation value does not exhibit such a trilinear coupling at tree level. The measurement of the ratios of 9Htt/ 9HWW and 9HTT/9HWW then probes the mass generation of both up and down type fermions. Thus the LHC can do much more than mereley discover the Higgs: it can give us detailed and reasonably precise information on the dynamics of electroweak symmetry breaking. Acknowledgments I would like to thank R. Kinnunen, A. Nikitenko and E. Richter-Was for a most enjoyable collab­oration leading to the results summarized here, and the CERN theory group for its hospitality. This work was supported in part by the University of Wisconsin Research Committee with funds granted by the Wisconsin Alumni Research Foundation and in part by the U. S .  Department of Energy under Contract No. DE--FG02-95ER40896. References 1. ATLAS Collaboration, ATLAS TDR, report CERN/LHCC/99-15 ( 1999) . 2. G. L. Bayatian et al. , CMS Technical Proposal, report CERN/LHCC/94-38 ( 1994) ; R. Kinnunen and D. Denegri, CMS NOTE 1997 /057; R. Kinnunen and A. Nikitenko, CMS TN/97-106; R.Kinnunen and D. Deneg:i, hep--ph/9907291 . 3. LEP electroweak working group, http://lepewwg.web.cern.ch/LEPEWWG/ .  4 .  D .  Zeppenfeld, R .  Kinnunen, A. Nikitenko and E .  Richter-Was, hep--ph/0002036. 5. D. Rainwater and D. Zeppenfeld, Journal of High Energy Physics 12 ,  005 ( 1997) .  6. D. Rainwater, PhD thesis, hep-ph/9908378. 7. D. Rainwater, D. Zeppenfeld and K. Hagiwara, Phys. Rev. D59, 014037 ( 1999) . 8. T. Plehn, D. Rainwater and D. Zeppenfeld, Phys. Rev. D61, 093005 (2000) .  9. D .  Rainwater and D. Zeppenfeld, Phys. Rev. D60, 1 13004 ( 1999) ,  Erratum-ibid. D61,  099901 (2000) ,  [hep--ph/9906218] . 10. R. N. Cahn et al. ,  Phys. Rev. D35, 1626 ( 1987); V. Barger, T. Han, and R. J .  N. Phillips, Phys. Rev. D37, 2005 ( 1988) ;  R. Kleiss and W. J. Stirling, Phys. Lett. 200B, 193 ( 1988) .  1 1 .  Y. L. Dokshitzer, V. A. Khoze, and S .  Troian, in Proceedings of the 6th International Conference on Physics in Collisions, ( 1986) ed. M. Derrick (World Scientific, 1987) p.365; J .  D. Bjorken, Int. J. Mod. Phys. A7, 4189 ( 1992) ;  Phys. Rev. D47, 101 ( 1993) .  12. T.  Han, G. Valencia and S .  Willenbrock, Phys. Rev. Lett. 69 ,  3274 (1992) . 13. A. Djouadi, N. Spira and P. Zerwas, Phys. Lett .  B 264, 440 ( 1991) .  M. Spira, A. Djouadi, D. Graudenz and P.M. Zerwas, Nucl. Phys. B453, 17 ( 1995) .  169 

Measurement of Higgs Properties at the LHC

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Measurement of Higgs Properties at the LHC

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