Photon decay modes of the intermediate mass Higgs ECFA Higgs workmg group C.Seez and T. Virdee L. DiLella, R. Kleiss, Z. Kunszt and W. J .Stirlin g Presented at the LHC Workshop, Aachen, 4 - 9 October 1990 by C. Seez, Imperial College, London. A report is given of studies of: (a) H —> yy (work done by C. Seez and T. Virdee) (b) W H -> yy (work done by L. DiLella, R. Kleiss, Z. Kunszt and W. J. Stirling) for Higgs bosons in the intermediate mass range (90< mH<l50 GeV/cz). The study of the two photon decay mode is described in detail. Introduction A Standard Model neutral Higgs boson having a mass above the highest reach of LEP II (around 90 GeV/cz) [1], and below about 2mZ will be difficult to detect at a hadron collider. The most promising channels for detection are H0- >yy, or, for muzi 30 GeV/c2, H0->ZZ*->e+e·e+e· [2]. As the decay width of the Higgs is about 5.5 MeV at mH=100 GeV/c2, and 8.3 MeV at 150 GeV/c2, the width of the reconstructed mass distribution, and hence the signal/background ratio, will be limited by the detector, and in particular by the energy resolution of the electromagnetic calorimeter. The decay channel H9 —> Zy also appears to be potentially attractive, but, after requiring that the Z decay into electrons or muons, the combined branching fraction times cross·section is very small. The intrinsic background (i.e. the background with the same final state as the signal) is large and rules out the possibility of detecting the Higgs boson in this channel. In this paper a detailed study of the possibility of detecting an intermediate mass Higgs boson in the di-photon channel is reported. Results from another study are also reported in which the same decay is considered but for a Higgs boson produced in association with an intermediate vector boson. 1. H0 -> Previous studies of H9 -> yy [3] concluded that it would probably be possible to fmd a Higgs of mass 150 GeV/cz at the SSC (\}s=40 TeV, and 104/pb) with an excellent calorimeter, but that it would probably not be possible to obtain a significant signal if the mass was 100 GeV/c2. The study described here differs from this previous work by: a) considering LHC energies and luminosities (\}s=l6TeV, and 105/pb) and the problems resulting from pileup, b) investigating a large range of calorimetric energy resolutions, c) including the prompt photon background from bremsstrahlung diagrams, and d) making a detailed study of the background from jet processes. 2. Cross-sections The Higgs production cross-section multiplied by the branching ratio to two photons (c$H.Brw) has recently been re-calculated by Kunszt and Stirling [4] for Higgs masses in the 474 OCR Output 475 intermediate range, and is shown in figure l. The background to the H0->*yy signal may be divided into three categories: i) prompt di-photon production from the quark annihilation and box diagrams which provides an intrinsic background, ii) prompt di-photon production from significant higher order diagrams (mainly bremsstrahlun g diagrams), iii) the background from jet processes where an electromagnetic energy deposit results from a quark or gluon jet rather than a prompt photon. The intrinsic background cross—sections have been calculated using ISAJET [5] as a framework with the matrix elements of Berger, Braaten and Field [6]. For this calculation the HMRSB structure functions [7] were inserted into ISAJET so as to be consistent with the calculation of the Higgs production cross-section. The bremsstrahlung contribution can be generated with ISA] ET. Its cross-section has been fixed by requiring the ratio bremsstrahlung/(quark annihilation+box diagrams) to be that calculated by the direct photon working group [8]. The calculated cross-section for Higgs boson production in the intermediate mass range is insensitive to the choice of top quark mass: for mH=l0O GeV/cz it varies by less than 10% for values of top mass ranging from 100 to 200 GeV/c2. However, choice of structure functions can vary the calculated cross-sections for both Higgs production and the background processes by i50%, even when the choice is restricted to sets fitting recent BCDMS data [9]. Tables 1 and 2 summarise the cross—sections used. For both signal and background processes Monte-Carlo events have been generated using ISAJ ET. mg (GeV/c2) 0.Br (H0—>vy) (fb) before cuts after cuts 80 36.4 9.6 100 46.5 19.0 150 26.5 13.5 Z. Kunszt and W. J. Stirling, HMRSB structure functions, m,0p=150 GeV/cz kinematic cuts: h]|£2, pTl>40, pT2>25GeV/c, pTl/(pTl+pT2)<0.7 Table I : The cross—sccti0n 0f the signal OCR Output 476 mw qq—>·Yy gg—>Yy Brems- Total (GeV/c2) strahlung 80 31.7 84.7 87.5 204. 100 30.8 60.8 61.4 153. 150 12.0 16.1 17.0 45. do/dmw (fb/GeV) of di-photon backgrounds kinematic cuts: |n|S2, pT1>40, pT2>25GeV/c, pTl/(pT1+pT2)<0.7 qq->yy, gg->*y;a ISA] ET, using matrix elements of Berger, Braaten and Field, HMRSB structure functions Bremsstrahlung: ISAJET, after isolation cut, normalized to analytic calculation (see text) £1b!Q.·I@ crpas-sections ofthe prompt di-photon backgrounds 3. The calorimeter, acceptance and cuts If quantitative and realistic results are to be produced it is necessary to define the properties and characteristics of a detector whose performance can then be simulated. The model calorimeter used in this study covers a pseudo—rapidity range -2 S 1] S 2, and has an inner radius of 2 metres at 1]:0, tapering to 1.5 metres at 1]:12. It is segmented into fully pointing towers, and it is assumed that sufficient lateral shower containment is obtained by reading out an area of 20 x 20 cm2, i.e. photons are reconstructed in an area of A¢xAn=0.1x0.1. It is assumed that only the energy deposited by the interactions occuring in a single bunch crossing is seen. The cuts placed on the photon transverse momenta are sufficiently high that triggering would pose no problem: pTl > 40 and py? > 25 GeV/c (the photons being ordered by Pr) The photon pairs from bremsstrahlung diagrams tend to be asymmetric in the amount of transverse momentum they carry. A cut on the pT balance of the photon pair, pTl/(pTl+pT2)<0.7 (where pTl>p-Y2), is found to remove about 20% of the bremsstrahlung background remaining after the 11 and PT cuts at mflllo GeV/c2 while removing only 6.2% of the signal. Since this cut rejects very little of the background contributions from the annihilation and box diagrams the gain in signal significance (measured by NS/VNS) is, unfortunately, small. As is described below, an isolation cut is applied to the photons to help reject the jet background. This isolation cut, though only over a small area (AnxA¢=0.1x0.1), should significantly reduce the remaining hard bremsstrahlung even after the application of the balance cut, as is shown by a semi-analytic calculation [8]. However, the algorithm used in ISAJET to generate photon radiation from quarks during their 'evo1ution' results in a distribution of the radial separation between radiating quark and photon that is essentially flat, rather than being strongly peaked around AR=0. Consequently the isolation cut has little effect on the OCR Output bremsstrahlung events generated with ISAJET. We assume that this is incorrect and have corrected the level of the bremsstrahlung background assuming that our isolation cut over an area of AnxA¢=0.2x0.2, after the PT balance cut, should further reduce it by 50%, as is suggested by the analytic calculation. The cuts against the bremsstrahlun g background could be further optimised. A fairly loose cut on the EET within a somewhat larger isolation area about the photons, e.g. a circle of radius AR=0.3, should be very effective in reducing the bremsstrahlung background. Since the photons failing the balance cut are, in general, accompanied by harder jets than those passing it, an efficient isolation cut should make the py balance cut unnecessary. This deserves to be studied with a realistic simulation taking account of the energy deposited in the isolation cone by pileup. The limited rapidity range results in a geometric acceptance of around one half (47.1% at mH=80 GeV/c2, 48.2% at mH=10O GeV/02, and 52.6% at mH=150 GeV/cz). Figure 2 shows the distribution of the absolute rapidity of the photon, in each event, at the highest absolute rapidity, for photon pairs from (a) a Higgs of mass 100 GeV/cz, (b) the two prompt di-photon background diagrams. In all cases a requirement of pTl>40 and pT2>25 GeV/c has been made on the photons. For the background processes a mass cut of 105 2 mw2 95 GeV/c2 has also been made. The cuts on the transverse momenta of the photons result in very little additional loss of the signal for mH>100 GeV/cz and thus the final acceptance is 40.9% for mH=100 GeV/cz, and 50.8% for mH=l50 GeV/c2. At mH=80 GeV/c2 the transverse momentum cuts significantly deplete the signal (the final acceptance is 26.4%), however, because the pq- spectra of the intrinsic background is so steep at low pT, reducing the pT cuts gives only a small gain in the ratio signal/N/background.