9512.net
甜梦文库
当前位置:首页 >> >>

Higgs Boson Properties and BSM Higgs Boson Searches at LHC

Hadron Collider Physics Symposium (HCP2008), Galena, Illinois, USA

Higgs Boson Properties and BSM Higgs Boson Searches at LHC
W. F. Mader (on behalf of the ATLAS and CMS collaborations)
Technical University Dresden, 01062 Dresden, Germany

arXiv:0809.4154v1 [hep-ex] 24 Sep 2008

At the end of 2008, the Large Hadron Collider (LHC) will come into operation and the two experiments ATLAS and √ CMS will start taking data from proton-proton collisions at a center-of-mass energy of s = 14 TeV. In preparation for the data taking period, the discovery potential for Higgs bosons beyond the Standard Model has been updated by both experiments and is reviewed here. In addition, the prospects for measuring the properties of a Higgs boson like its mass and width, its CP eigenvalues and its couplings to fermions and gauge bosons are discussed.

1. MSSM HIGGS BOSON SEARCHES
In the Minimal Supersymmetric Standard Model (MSSM), the minimal extension of the Standard Model (SM), two Higgs doublets are required, resulting in ?ve observable Higgs bosons. Three of them are electrically neutral (h, H, and A) while two of them are charged (H ± ). At tree level their properties like masses, widths and branching fractions can be predicted in terms of only two parameters, typically chosen to be the mass of the CP-odd Higgs boson, mA , and the ratio of the vacuum expectation values of the two Higgs doublets, tan β. In the MSSM the couplings of the Higgs bosons to fermions and bosons are di?erent from those in the Standard Model resulting in di?erent production cross-sections and decay rates. The coupling of the Higgs bosons to third generation fermions is strongly enhanced for large regions of the parameter space which determines the search strategies for such Higgs bosons. In the following, searches for the neutral and charged Higgs bosons in the MSSM in the two experiments ATLAS and CMS at the LHC are described. Unless indicated otherwise, all results will be given in the mmax scenario [1]. A h detailed description of the ATLAS and CMS detectors can be found elsewhere [2, 3]

1.1. Higgs Boson Searches in h/H/A → ??
In the SM the discovery potential for the Higgs boson in the dimuon ?nal state is limited due to its small branching fraction and the high backgrounds expected from several SM processes. However, in the MSSM the decay of the three neutral Higgs bosons h, H, and A into a dimuon ?nal state can be strongly enhanced depending on the value of tan β. This channel can therefore serve as a discovery channel for high values of tan β, or as a tool to exclude large parts of the mA vs. tan β plane. In addition, in the intense coupling region around mA = 130 GeV where all the neutral Higgs bosons have a comparable mass, the excellent invariant mass resolution of the dimuon ?nal state o?ers the potential to observe these states individually at the same time. The event selection is optimized separately for the cases where zero r at least one b jet are identi?ed in the ?nal state. In the ?rst case Drell-Yan Z boson production is the dominant background, while in the second case the ? dominant contribution is approximately equally shared between the Z and tt processes. At the preselection step two isolated muons of opposite charge are required inside the acceptance region with a pT > 20 GeV. Due to the high momenta of the muons from the Higgs boson decays this ?nal state can be e?ciently triggered by a high-pT singlemuon trigger. Further selection criteria include an upper cut on the amount of missing energy found in the event, requirements on the jet identi?ed as coming from a b quark (or a b-jet veto in the case of the zero b-jet analysis), on the accoplanarity of the dimuon system and the sum of pT of all jets in the event (in the case of the one b-jet analysis).

Entries / (4 GeV)

105 mA =150 GeV 104 103 102 10 1

bbA Z+l jets preliminary Z+b jets

ATLAS

Entries / (4 GeV)

105 104 103 102 10

WW tt ZZ

mA =200 GeV mA =300 GeV

bbA preliminary Z+l jets Z+b jets WW tt ZZ

ATLAS

mA =150 GeV mA =200 GeV mA =300 GeV

×10 150 200 250 300 350 m?? (GeV)

3

1

×10 150 200 250 300 350 m?? (GeV)

3

Figure 1: Invariant dimuon mass distribution of the main backgrounds and the A boson signal at masses mA = 150, 200, and 300 GeV and tan β = 30, obtained for an integrated luminosity of 30 fb?1 in ATLAS. B-tagging has been applied for the event selection. The production rates of H and A bosons have been added. Zero or at least one b-jet have been required in the left-hand side and the right-hand side plot, respectively.

The invariant dimuon mass after all selection criteria is displayed for the cases of zero and at least one b-jet on the ? left-hand side and the right-hand side of Figure 1, respectively. The background from Z and tt events is estimated from data. The Z background is estimated from data by analyzing the ee ?nal state which is signal-free. In the case ? ? of tt events, the e? ?nal state or a tt enriched sample obtained by requiring large missing ET in the event can be used. The discovery potential as obtained by CMS and ATLAS is displayed on the left-hand side and the right-hand side of Figure 2, respectively [4, 5]. The theoretical uncertainties include the uncertainty on the parton density functions and on the renormalization and factorization scales. The dominant experimental uncertainties (of about 10% in total) include the reconstruction e?ciency, momentum resolution and momentum scale for the muons, the jet energy scale and resolution as well as the uncertainty on the b tagging e?ciency and the light-jet rejection rate.

tanβ

tan β for 5 σ discovery

50

60 50 40 30
Combined Analysis

40

CMS, 30 fb-1

5σ discovery contour

mmax ? scenario h
L=10 fb
?1

30
MSUSY = 1 TeV/c2 ? = 200 GeV/c2 M2 = 200 GeV/c2

L=30 fb

?1

20

20 10 0 50 ATLAS
preliminary

Without Systematics With Experimental Systematics Theoretical Uncertainty

10

Xt = 6 MSUSY

100

150

200

250

300

350

MA (GeV/c2)

400

100

150

200

250

300

350

400 450 mA (GeV)

Figure 2: Discovery potential in the h/H/A → ?? channel as a function of mA and tan β. Left: Discovery potential from CMS for an integrated luminosity of 30fb?1 . In the shaded area > 5 σ signi?cance can be obtained; the dashed line corresponds to the 5 σ contour without systematic uncertainties included. Right: The 5 σ discovery contours from ATLAS corresponding to integrated luminosities of 10 and 30 fb?1 with (dashed line) and without (dotted line) systematic uncertainties taken into account. The theoretical uncertainties are illustrated by the shaded bands.

Cross Section (fb / 15 GeV)

Cross Section (fb / 62 GeV)

→ 60 50 40 30 20 10 0 0 50 100



H→ττ Z→ττ Z→? ? Z→ee ttbar W+jets
preliminary

30 25 20 15 10 5 0





H→ττ Z→ττ Z→ ?? Z→ee ttbar W+jets
preliminary

ATLAS

ATLAS

mA =110 GeV tanβ=20

tanβ=20 mA =200 GeV 50 100 150 200 250 300 350 400 450 mττ [GeV]

150

200

250 300 mττ [GeV]

Figure 3: Invariant mτ τ distribution for signal and background events. The distributions are shown after all selection cuts for Higgs bosons masses and tan β values as indicated in the plots. The vertical lines indicate the mass window used for calculating the signal signi?cance.

1.2. Higgs Boson Searches in h/H/A → τ τ
Compared to the dimuon ?nal state, h/H/A → τ τ decays have a substantially larger branching fraction which scales as (mτ /m? )2 for a given tan β. However, since the τ leptons can decay both, leptonically and hadronically, the signatures observed in the detector are very di?erent and therefore have to be treated individually.
1.2.1. Higgs Boson Searches in h/H/A → τ τ → ??4ν

Even though the branching fraction of the two τ leptons into a fully leptonic ?nal state is only 12%, it contributes signi?cantly to the discovery potential in particular for Higgs boson masses in the range 110 < mA < 300 GeV. These events can be triggered on with high e?ciency using an electron and/or muon trigger. The event topology consists of two leptons, missing ET , and jets. The main background is coming from Drell? Yan τ τ production, tt processes, and W +jets topologies. The CMS analysis only studies the e? ?nal state [6] while ATLAS exploits all leptonic ?nal states [5]. The invariant di-τ mass is reconstructed using the collinear approximation technique [7]. In Figure 3 the invariant di-τ mass distribution as obtained in ATLAS is shown for Higgs boson masses of mA = 110 GeV, and mA = 200 GeV, tan β = 20 and an integrated luminosity of 30 fb?1 . For low masses, the irreducible ? Z → τ τ background dominates over that from tt processes, while for high masses the situation is reversed. The dominant experimental systematic uncertainties come from the jet-energy scale and resolution uncertainty and from the uncertainty on the b-tagging e?ciency. The theoretical uncertainties come from the uncertainties on the parton ? density functions, on the factorization and renormalization scale for signal and tt background. The discovery potential for this channel is shown in Figure 4 for ATLAS (left) and CMS (right). For low Higgs boson masses of the order of 130 GeV, a discovery with at least 5 σ signi?cance will be possible for tan β values of 15 or larger, while for higher Higgs boson masses like mA = 200 GeV tan β of the order of 30 would be necessary.
1.2.2. Higgs Boson Searches in h/H/A → τ τ → ?h3ν

The lepton-hadron ?nal state consists of one electron or muon plus jets, one of which is identi?ed as coming from a hadronically decaying τ lepton, and missing energy in the event. A single electron or muon trigger, either standalone or combined with a τ trigger, is used to preselect the events. One jet in the event is required to be identi?ed as coming from a b quark in order to suppress backgrounds from Drell-Yan τ τ production, from QCD multi-jet events and from W +jet backgrounds. Details of the analyses can be found in [8, 9].

tanβ

ATLAS

50 bb h/H/A →ττ→ 2 l + 4 ν 40 5 σ Discovery 30 20 10 0 150 200 250
mmax scenario h
Exp. Systematics only +10% σ(tt) Uncertainty Theoretical Uncertainty

preliminary

s=14 TeV, 30 fb?1

tanβ

60

50
CMS, 30 fb
-1

40

H/A→ττ→e? 30
mmax scenario h ? = 200 GeV/c2

20

Full simulation, with systematic uncertainties Full simulation

10
Excluded by LEP

300

350

400

450

mA / GeV

100 150 200 250 300 350 400 450 500

mA (GeV/c )

2

Figure 4: The 5 σ discovery contours in the mA vs. tan β plane for the fully leptonic ?nal state and for an integrated luminosity of 30 fb?1 with systematic uncertainties taken into account for ATLAS (left) and CMS (right).

The invariant τ τ mass is reconstructed using the collinear approximation technique, and is displayed in Figure 5 for the ? + τ -jet (left) and e + τ -jet (right) ?nal state, respectively. For the ? + τ -jet ?nal state the main background after all selection cuts is represented by τ τ b? Drell-Yan Z → τ τ , b, ? ? and tt processes. The tt background is estimated from data by inversion of the electron veto cut and has a systematic uncertainty of 12.4%. The Drell-Yan τ τ prediction is taken from a high precision measurement assumed to be done at the time of this analysis and a total systematic uncertainty of 8% is assigned. Finally, the b? τ background is bτ assumed to be know with a systematic uncertainty of 15%, derived from the uncertainty on a ??b? cross section b measurement and from the jet-energy scale uncertainty. ? For the e + τ -jet analysis the main background comes from (b? b)Z/γ ? → ee/τ τ ?nal states and from tt processes. The total systematic uncertainty was calculated from the background uncertainties (either measured or predicted from theory) and the experimental uncertainties of the event selection, like electron and τ identi?cation, calorimetric energy scale and b tagging e?ciency. The discovery potential for the ? + τ -jet and the e + τ -jet analysis is displayed in Figure 6 on the left-hand side and the right-hand side, respectively. The discovery of a Higgs boson with a mass of the order of mA = 200 GeV would be possible even for low tan β values, while for high masses such a discovery would be challenging.

Events for 20 fb-1/ 20 [GeV/c2]

Events/25 GeV/c2 for 30 fb-1

70 60 50 40 30 20 10 0 0 200

45 40 35 30 25
X t = 2 TeV/c , MSUSY = 1 TeV/c
2 2

Background Signal+Bkg. for tan(β)=20, mA =200 GeV/c2 Signal+Bkg. for tan(β)=30, mA =500 GeV/c2

CMS
H/A→ττ→e+τ-jet + X mA = 300 GeV/c2 tanβ = 25

20 15 10 5 Backgr

? = 200 GeV/C , M2 = 200 GeV/c

2

2

Signal
Z/γ *→ee

400

600

800

1000

0 0

100 200 300 400 500 600 700 800

mτ τ [GeV/c2]

mττ (GeV/c2)

Figure 5: Invariant mτ τ distribution for the ? + τ jet + X (left) and the e + τ jet + X (right) ?nal state for Higgs boson masses, tan β values as indicated in the plots, and for an integrated luminosity of 30 fb?1 . In the left plot, the mass windows in which the numbers of events are counted for the signi?cance calculation are indicated by the vertical arrows.

tan(β)

tanβ

60 50 40 30 20 10
max mH = 1000 GeV/c2 = 2449 GeV/c2 = 200 GeV/c2 = 200 GeV/c2

70 60
CMS, 30 fb-1
HSUSY →ττ→e + τ-jet + X
X t = 2 TeV/c , MSUSY = 1 TeV/c
2 2 2

Discovery area for 30 fb-1

50 40 30 20 10

? = 200 GeV/C , M2 = 200 GeV/c mtop = 175 GeV/c
2

2

mSUSY MS Xt ? M2

Full simulation, with syst. uncertainties Full simulation

200 250

300

350 400 450

500 550

600
2

Excluded by LEP

mA [GeV/c ]

100 200 300 400 500 600 700 800

mA (GeV/c2)

Figure 6: The 5 σ discovery contours in the mA vs. tan β plane and for an integrated luminosity of 30fb?1 . Left: Discovery contour for the ? + τ jet + X ?nal state, with (dashed) and without (solid) the systematic uncertainties on the background taken into account. Right: Discovery contour for the e + τ jet + X ?nal state, with and without the systematic uncertainties taken into account as indicated in the plot. The exclusion limit as obtained from LEP is indicated by the shaded area.

1.2.3. Higgs Boson Searches in h/H/A → τ τ → hh2ν

This analysis has been performed for Higgs bosons of mass 200, 500, and 800 GeV. The observed ?nal state consists of two τ -like jets identi?ed by their high transverse energy and a pT > 30 GeV for the leading track inside the τ -jet. Furthermore, exactly one additional jet with ET > 20 GeV was allowed. This jet had to pass the tagging criteria for a b-jet based on 3D-impact parameters. Details of the analysis can be found in [10]. After all selection criteria, the dominant background comes from QCD multi-jet events. In order to estimate this background from Monte Carlo, the selection has been factorized into three categories: Trigger and o?ine calorimetric reconstruction, τ identi?cation, and ?nally jet reconstruction, b-tagging and mτ τ mass reconstruction. The invariant mτ τ mass distribution for two Higgs boson masses (mA = 200 and 600 GeV) and for an integrated miss luminosity of 60 fb?1 are displayed in Figure 7. The systematic uncertainties considered are from the ET and jet energy scale (3 ? 10%), tracker misalignment (? 10%), and the measurement of the QCD background from data (5 ? 20%). Including all systematic uncertainties, a 5 σ discovery for a Higgs boson of mass mA = 200/500/800 GeV can be achieved for a tan β value of 21/34/49.

1.3. Searches for the Charged Higgs Boson
The discovery of a charged Higgs boson would be a de?nite signal for the existence of new physics beyond the SM. It is predicted in many non-minimal Higgs scenarios like Two Higgs Doublet Models (2HDM) or models with Higgs triplets. The search strategies for a charged Higgs boson depend on its mass which determines the production rate as well as the available decay modes. Below the top quark mass, the main production mode is through top quark decays (t → H + b) and the decay of the charged Higgs boson proceeds predominantly via the H + → τ ν process. Above the top quark threshold, the production processes are gg → tbH + and gb → tH + , where the latter dominates. The decay proceeds predominantly via the tb ?nal state. Charged Higgs boson searches involve several higher level reconstructed physics objects such as electrons, muons, jets, jets tagged as b jets and jets identi?ed as τ jets. The trigger to select the relevant event topologies consists of a miss combination of τ triggers, ET triggers, and jet triggers. Details on the analyses can be found in References [5? ].

Nev / 40 GeV/c 2

Nev / 40 GeV/c 2

60 CMS, 60fb . L=2× 1033cm-2s-1 50 40 30 20 10 0 0 scenario MA=200 GeV/c2 tanβ=20 mh max
-1

14 12 10 8 6 4 2 0 0

CMS, 60fb . L=2× 1033cm-2s-1 mh scenario max MA=500 GeV/c2 tanβ=30

-1

signal QCD EW sum
200 400 600 800 1000 1200 1400
2

signal QCD EW sum
200 400 600 800 1000 1200 1400

Mτ τ, GeV/c

Mτ τ, GeV/c

2

Figure 7: The mτ τ distributions for a signal of mA = 200 GeV with tan β = 20 (left), for mA = 500 GeV with tan β = 30 (right), and for the background in 60 fb?1 of data. The solid histogram represents the distribution expected from all candidates which is composed of the signal (thick solid line), QCD background (dashed line), and irreducible background (thick dasheddotted line).

1.3.1. Light Charged Higgs Boson Searches: mH ± < mt

If the charged Higgs boson is light, the branching fraction of the top quark into a bW ?nal state might be less than ? that predicted by the SM. In that case the expected background from SM-like tt decays is reduced which has to be taken into account. Three di?erent channels have been analyzed, classi?ed according to the ?nal state of the τ and W decays: ? bτ (had)ν bW (had): This channel has a high branching fraction which makes it a priori one of the most important discovery channels. However, the absence of leptons and the high hadronic activity makes this channel particularly challenging. ? bτ (lep)ν bW (had): This channel is characterized by a single isolated lepton and large missing ET due to the neutrinos in the ?nal state which make a full kinematic reconstruct of the event impossible. The presence of a signal could be detected via the excess of kinematically τ -like events or it could be inferred from an analysis of the ‘generalized transverse mass’ spectrum [11]. ? bτ (had)ν bW (lep): As above, due to the large number of neutrinos in the ?nal state, a full kinematic reconstruction of the event will not be possible and a potential signal is again recognized by an excess of kinematically τ like events. The most sensitive channel for a discovery of a light charged Higgs boson is that where both the τ and the W decay hadronically.
1.3.2. Heavy Charged Higgs Boson Searches: mH ± > mt

For the search for a heavy charged Higgs boson, two channels are considered: ? bqq[b]τ (had)ν: This channel is characterized by one hard τ jet from the decay of the charged Higgs boson, large missing transverse momentum, one or two b jets, and two light jets. ? b?ν[b]bqqb: Here, the charged Higgs boson decays into a tb ?nal state. In addition 3 ? 4 b-jets are expected in the event (depending on the production mechanism), 2 light quark jets, and one high pT lepton. Of these two channels, only the ?rst one shows a sensitivity to charged Higgs boson production.

tanβ

on

80 70 60 50 40 30

60

by T

gg → tbH , H → τνν
CMS, 30 fb-1
Full simulation with systematic uncertainties Full simulation

±

±

55 50 45 40 35

CDF Run II Excluded 95% CL

Exclu

ded

evatr

tanβ

30 25

Maximal mixing scenario

20 15 10 5

30 fb 10 fb 1 fb

?1 ?1

20 10
Excluded by LEP

? = 200 GeV/c2 , M2 = 200 GeV/c2 Xt = 2 TeV/c , MSUSY = 1 TeV/c
2 2

?1

ATLAS
preliminary
110 130 150 170 200 250
+

100

200

300

400

500

600

mA (GeV/c2)

90

400

600

m

H

[GeV]

Figure 8: Left: Discovery potential for a charged Higgs boson as a function of mA and tan β for an integrated luminosity of 30 fb?1 (CMS). Right: Discovery potential for a charged Higgs boson as a function of mH ± and tan β for integrated luminosities between (1 ? 30) fb?1 (ATLAS). In addition, the 95% exclusion limit from CDF (Run-II) is shown.

1.3.3. Overall Discovery Potential for Charged Higgs Bosons

In Figure 8 the overall discovery potential is displayed for CMS (left) [4] and ATLAS1 [5] (right). The range of low charged Higgs boson masses up to the top quark mass is well covered, and a charged Higgs boson can be discovered with a data set corresponding to an integrated luminosity of 30 fb?1 . For charged Higgs boson masses above the top quark mass, high values of tan β would be necessary for a 5 σ discovery.

2. MEASUREMENT OF HIGGS BOSON PROPERTIES
Once the Higgs boson has been discovered at the LHC, the measurement of its properties like mass, spin, CP eigenvalue and its couplings to fermions and gauge bosons would have to be measured in order to obtain further insight into the mass generation mechanism realized in nature.

2.1. The Higgs Boson Width
In addition to a potential Higgs boson discovery in the h/H/A → ?? channel, due to its excellent mass resolution, the width of the Higgs boson and therefore tan β can be directly measured [4]. In Figure 9 (left) the intrinsic width of the Higgs boson (circles) and that measured (solid triangles) for mA = 150 GeV is shown. In such an analysis it has to be taken into account that the mass degeneracy of the neutral Higgs bosons A and H is not exact which is illustrated by the open triangles. This is particularly evident for mA = 150 GeV and low tan β where the mass di?erence is larger than the intrinsic width. In Figure 9 (right) the expected uncertainty (including a theoretical uncertainty of 15%) of the tan β measurement is shown as a function of mA and tan β. The measurement of tan β can be further constrained by cross section measurements since σ × BR ? tan βe? .

2.2. The Higgs Boson Mass
In the SM, the mass of the Higgs boson can be measured with the highest precision in the decay channels H → ZZ (?) → 4?, in H → γγ, and in H → b? [12]. However, recent studies indicate that a discovery of the Higgs b

1 Here,

a statistical uncertainty corresponding to that expected for the integrated luminosities given is assumed.

widths (GeV/c2)

16 14 12 10 8

tanβ
MA = 150 GeV/c
2

MA - MH Intrinsic width (ΓA ) ΓA + ? MH-A Measured (hard b) Measured (soft b)

60 50 40
mmax scenario h

CMS, 30 fb

-1

6 4

30 20

MSUSY = 1 TeV/c2 M2 = 200 GeV/c 2 ? = 200 GeV/c2 mgluino = 800 GeV/c 2 Stop mix: Xt = 2 MSUSY

2 0 15 20 25 30 35 40 45 50 55

160

180

200

220

240

tanβ

MA (GeV/c )

2

Figure 9: Left: The comparison between the expected Higgs boson width and the measured width as a function of tan β for mA = 150 GeV. Right: Uncertainty on the tan β measurement obtained from the Higgs boson width measurement with an integrated luminosity of 30 fb?1 .

boson in the b? and therefore a measurement of its mass might be challenging [5]. Assuming 300 fb?1 of data, a b relative precision of such a measurement of 0.1% in the range 100 < mH < 400 GeV can be achieved as displayed in Figure 10 (left). The anticipated precision degrades down to 1% for Higgs boson masses of mH = 700 GeV. The systematic uncertainties are dominated by the knowledge of the absolute calorimetric energy scale which, for leptons and photons, was assumed to be 0.1%. However, a ?nal value of 0.02% for that uncertainty is anticipated driven by the objective to measure the mass of the W boson with a precision of 15 MeV. An uncertainty of 1% on the jet energy scale was assumed. In models beyond the SM (like the MSSM), the coupling of the Higgs boson to gauge bosons might be suppressed or even absent. In that case the mass of the Higgs boson can be measured, e.g. in the H → b? H → ?? or H → τ τ b, channel. In the latter case, due to the neutrinos from the τ decays, one would have to use techniques like the collinear approximation to reconstruct the mass of the Higgs boson. In all cases, a precision on the % level or better can be expected.
-1

Significance for exclusion of Spin 1 CP +1 - 100fb

σ 10
8 6 4 2 0

Combination Polarisation

Plane Angle

200

200

220

240

250

260

280

300

300

320

340

m H [GeV]
360

Significance for exclusion of Spin 1 CP -1 - 100fb-1

σ 10
8 6 4 2 0

Combination Polarisation

Plane Angle

200

200

220

240

250

260

280

300
-1

300

320

340

m H [GeV]
360

Significance for exclusion of Spin 0 CP -1 - 100fb

σ 30
25 20 15 10 5 0

35

Combination Polarisation

Plane Angle

200
200

220

240

250

260

280

300
300

320

340

m H [GeV]
360

Figure 10: Left: The anticipated precision of a measurement of the Higgs boson mass in the decay channels H → γγ (open circles), H → b? (open triangles), and H → ZZ (?) → 4? (rectangles). A precision of the order of 0.1% can be achieved for b Higgs boson masses of 100 < mH < 400 GeV, while for mH = 700 GeV 1% is expected in 300 fb?1 of data. Right: The signi?cance for the exclusion of a Higgs boson with non-SM like spin and CP eigenvalues as a function of its mass.

2.3. Spin and CP of the Higgs Boson
After the discovery of the Higgs boson, the highest priority is to establish its spin and CP eigenvalue. This can be determined from a study of the angular distributions and correlations in the H → ZZ(?) → 4? channel [13, 14]. The angular distributions investigated are the polar angle of the leptons relative to the direction of ?ight of the Z boson, and the angle between the decay planes of the two Z bosons, both calculated in the Higgs boson rest frame. The result of this study is shown in Figure 10 (right) for non-SM like spin and CP con?gurations for the Higgs boson. For masses larger than 230 GeV a spin-1 hypotheses with either even or odd CP eigenvalue can be ruled out with a data set of 100 fb?1 . For masses as low as 200 GeV and a luminosity of 300 fb?1 , a Higgs boson with (spin, CP)=(1,+) can be ruled out at the 6.4 σ level, while for (spin, CP)=(1,–) a signi?cance of only 3.9 σ can be obtained. A Higgs boson with (spin, CP)=(0,–) can be ruled out with less than 100 fb?1 over the whole mass range considered here.

2.4. The Higgs Boson Couplings
The coupling of the Higgs boson to gauge bosons and fermions determines its production cross section and branching fractions. Measuring the rates in multiple channels allows for a determination of various combinations of couplings at the LHC [15]. However, a model independent measurement of the (partial) decay width(s) of the Higgs boson will not be possible at the LHC. The reason being that on the one hand a measurement of the missing mass spectrum like at e+ e? colliders is not possible, and that on the other hand some of the decay channels of the Higgs boson are either hard (like H → b? or even impossible (like H → gg) to detect due to the overwhelming background b) from QCD processes. An absolute measurement of the above quantities will only be possible if additional theoretical assumptions are being made. Introducing moderate theoretical constraints as detailed below, allows to overcome the experimental di?culties described above and to measure the couplings of the Higgs boson. In a ?rst approach, only the couplings of the Higgs boson to the gauge bosons is constrained to less than 1.05 times its value in the SM2 , which is justi?ed in any model with an arbitrary number of Higgs doublets. Furthermore,
? g2(H,X) g2(H,X) ? g2(H,X) g2(H,X) 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 110 120 130 140 150 160 170 180 190 mH [GeV]

g2(H,Z) g2(H,W) g2(H,τ) g2(H,b) g2(H,t) ΓH
without Syst. uncertainty

1 0.9 0.8 0.7 0.6

g2(H,τ) g2(H,b) g2(H,t) ΓH
without Syst. uncertainty

2 Experiments

∫ L dt=2*300 fb
WBF: 2*100 fb

2 Experiments

-1 -1

0.5 0.4 0.3 0.2 0.1 0

∫ L dt=2*300 fb
WBF: 2*100 fb

-1 -1

110 120 130 140 150 160 170 180 190 mH [GeV]

Figure 11: Relative precision of a measurement of the Higgs boson couplings squared as a function of mH for an integrated luminosity of 300 fb?1 per experiment with moderate (left) and more restrictive (right) theoretical assumptions as explained in the text.

2 The additional 5% account for theoretical uncertainties in the translation between the couplings squared and the partial widths, and also for small possible admixtures of exotic Higgs states.

additional particles running in loops in the H → γγ and gg → H processes are allowed. The relative precision of the measured couplings as a function of the Higgs boson mass is illustrated in Figure 11 (left) assuming an integrated luminosity of 300 fb?1 per experiment. The couplings of the Higgs boson to the W and Z bosons, to the top quark, and to the τ lepton can be measured with a precision between (10 ? 40)% depending on mH . If in addition, the ratio of the W and Z couplings to the Higgs boson are constrained to within 1% of that in the SM, the absolute value of the W coupling to within 5%, and no new non-SM particles are allowed in the loops, a higher precision can be obtained as illustrated in Figure 11 (right). In that case a precision of the measurement of the couplings squared can be achieved which is about a factor of two higher.

3. SUMMARY
In this note, the discovery potential for neutral (h/H/A) and charged (H ± ) Higgs bosons in the MSSM in ATLAS and CMS at the LHC has been reviewed. After a potential discovery of a Higgs boson, its properties have to be measured in order to gain insight into the mass generation mechanism realized in nature. Possible measurements of the Higgs boson mass and width, its spin and CP eigenvalue, and its couplings to fermions and gauge bosons have been discussed.

Acknowledgments
The author wishes to thank the organizers of the conference for the invitation and everybody in the ATLAS and CMS Higgs working groups that helped in preparing the talk.

References
[1] M. S. Carena, S. Heinemeyer, C. E. M. Wagner and G. Weiglein, Eur. Phys. J. C 26 (2003) 601. [2] The ATLAS Collaboration, G. Aad et al., “The ATLAS Experiment at the CERN Large Hadron Collider”, JINST 3 (2008) S08003. [3] The CMS Collaboration, S. Chatrchyan et al, “The CMS experiment at the CERN LHC”, JINST 3 (2008) S08004. [4] The CMS Collaboration, G. L. Bayatian et al., J. Phys. G 34 (2007) 995. [5] ATLAS Collaboration, Expected Performance of the ATLAS Experiment, Detector, Trigger and Physics, CERNOPEN-2008-020, Geneva, 2008, to appear. [6] S. Lehti, “Study of H/A → τ τ → e? + X in CMS”, CMS Note 2006/101 (2006). [7] R. K. Ellis, I. Hinchli?e, M. Soldate and J. J. van der Bij, Nucl. Phys. B 297 (1988) 221. [8] R. Kunnunen, and S. Lehti, “Search for the Heavy Neutral MSSM Higgs Boson with the H/A → τ τ → Electron plus Jet Decay Mode”, CMS Note 2006/075 (2006). [9] A. Kalinowski, M. Konecki, and D. Kotlinski, “Search for MSSM heavy neutral Higgs boson in τ + τ → ? + jet Decay Mode”, CMS Note 2006/105 (2006). [10] S. Genai, A. Nikitenko, and L. Wendland, “Search for MSSM Heavy Neutral Higgs Boson in τ τ → two Jet Decay Mode”, CMS Note 2006/126 (2006). [11] E. Gross and O. Vitells, arXiv:0801.1459 [physics.data-an]. [12] ATLAS Collaboration, “Detector and Physics Performance Technical Design Report”, CERN-LHCC/99-14/15 (1999). [13] C. P. Buszello, I. Fleck, P. Marquard and J. J. van der Bij, Eur. Phys. J. C 32, 209 (2004). [14] M. Bluj, “A Study of Angular Correlations in H → ZZ → 2e2?”, CMS Note 2006/094. [15] M. Duhrssen, S. Heinemeyer, H. Logan, D. Rainwater, G. Weiglein and D. Zeppenfeld, Phys. Rev. D 70 (2004) 113009.



学霸百科 | 新词新语

All rights reserved Powered by 甜梦文库 9512.net

copyright ©right 2010-2021。
甜梦文库内容来自网络,如有侵犯请联系客服。zhit325@126.com|网站地图