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Higgs Search Strategy at the LHC

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Title: Higgs Search Strategy at the LHC


1
Higgs Search Strategy at the LHC
  • 5.1 Cross Sections at the LHC
  • 5.2 Higgs Loop Decays and Direct Decays
  • 5.3 Higgs Production Rates
  • gg Fusion
  • WW Fusion and Tag Jets
  • Associated Production - HV, Htt
  • 5.4 Higgs Branching Ratios and Search Strategy ZZ
    --gt 4l, ??, bb, ???, 2l 2?, 2l 2J, WW
  • 5.5 Lower limit on Higgs mass
  • 5.6 Luminosity and Discovery Limits
  • 5.7 Others- Composites, Extra Dimensions

2
Cross Sections at the LHC
Resonances - narrow width approximation
e.g.
LHC Cross Sections
There is a factor gt 1010 between the Higgs cross
section and the total inelastic cross section.
There is also the final state branching fraction
to consider. This is why the LHC design
luminosity is so high.
3
Higgs Cross section vs Energy
CDF and D0 successfully found the top quark,
which has a cross section 10-10 the total cross
section. A 500 GeV Higgs has a cross section 1000
times smaller at the Tevatron A 500 GeV Higgs
has a cross section ratio of 10-11, which
requires great rejection power against
backgrounds and a high luminosity. Multiple
redundant measurements of the SM particles are
needed.
4
LHC Rates at 1033/(cm2sec)
low luminosity at or near LHC startup is such
that SUSY and low mass Higgs are still accessible.
5
Higgs - Loop Decays
  • There are decay modes that are not accessible
    through diagrams with a single coupling constant
    - zero mass states.
  • The coupling is to the heaviest quark in the
    loop. For a light Higgs, ?(H --gt ??)/MH
    ?2?W/?2(MH/MW)2(mt/MH)8
    sin-1(MH/2mt)4
  • ?(H --gt gg)/MH can be estimated by replacing ? by
    ?s. This diagram is the major production mode at
    the LHC, g g from p p fusing into a Higgs
    boson. The loop integral I is O(1).
  • For a light Higgs, ?(H --gt gg)/MH
    ?s2?W/72?2(MH/MW)2.


? ?
H
COMPHEP does not do loops
t
6
Higgs Decay Rates - Direct and Loop
Direct Quarks and Leptons
Gauge Bosons
Loop Decays - Gauge Bosons
Higgs couples to mass, with no direct H?? or Hgg
coupling
7
Higgs - Production via gg Fusion
  • The formation cross section is,
  • Using the expression for ?(H--gtgg) and
    normalizing the gluon distribution with a 6,
  • Note that the MH3 behavior of ? cancels the 1/
    MH3 behavior of d?/dy , leaving a roughly
    constant cross section,

d?/dy ?2?(H?gg)/(MH3)xg(x)x1xg(x)x2
d?/dy 49?2?(H ? gg)/(4MH3)(1 - MH/?s)12
49?2?(H ? gg)/(4MH3)
d?/dy 49I2?s2?W/288MW2.
8
Higgs Event Rate Estimate
Ignore I dependence on Higgs mass (n.b. peak at
twice the top mass). If MH 500 GeV then the
gluon source factor (1-MH/?s)12 is 0.65. The
falloff of the cross section with source function
at the LHC is small for low mass Higgs.
9
Higgs Production Modes
The Higgs cross section has as largest
contribution gg with an internal top loop. Note
that qqH is quite large, followed by associated
production modes including DY production of W, Z
with H bremmstrahlung and H with top pairs
10
Tevatron Limits Run II gg Fusion
Note that with 0.3 fb-1 integrated luminosity the
limit is 100 x above the SM expectation.
11
Higgs Production - Faked
Make a fake Hgg coupling and add to SM - Unitary
gauge
12
VBF Mechanism 2 Photon Physics
In pp collisions there are valence quarks. The
h-gt Z is a good control sample. VBF is similar to
2 photon physics which is larger than 1 photon
processes at sufficiently high energy. The 2
photon process is tagged by the observation of
2 small angle photons.
2 photon production means there is a quantum
number filter JPC 0, 2
13
Tag Jets in WW Fusion
Soft and collinear radiation --gt tag jets in WW
fusion. Distribution function of W calcuable in
perturbation theory
Heavy Higgs formed by WW fusion is a large
fraction of the total Higgs cross section
14
VBF at Tevatron and LHC
Over the Higgs mass range (100,1000) GeV VBF is a
substantial fraction of the Higgs production rate
and it offers a good signature 2 tag jets. The
tevatron cross section is down by gt 10 x. In
addition the rapidity plateau is wider at the LHC
which makes the tag jets, with lt?gt 3 more
distinct there than at the Tevatron.
15
H Production from WW
HF
HE
HB
Use the EW radiation of a W by a quark. The
effective W approximation analogous to the WW
approximation. Need good jet coverage to low PT
and small angles. Cross section depends only on
the Higgs coupling to W, Z isolate gHWW.
16
VBF is a Discovery Mode at the LHC
The continuum WW cross section is small w.r.t.
the Higgs contribution.
In a full study the main background is from top
pairs and WWJJ radiative DY background.
17
Associated production - Tevatron
Basically the D-Y process of W or Z production
has an accompanying, radiated H (or Z as a
control process)
There is a large background from W b pairs.
This is not a competitive strategy at the LHC.
18
Associated production Limits in Run II
With 0.4 fb-1 the limits set are 100x above
the rate expected for a SM Higgs. If the logged
luminosity rises to 4 fb-1, then
19
Higgs Top Pairs at the LHC
Top pair QCD production with a radiated
accompanying H.
A top pair is 0.35 TeV. This is too heavy for
the Tevatron to make a useful rate. It is a
useful mode at LHC for low mass Higgs with a good
ratio of
Take Higgs cross section and divide by - 2 sigma
in the mass resolution 20 GeV (recall Z) to get

20
Z Control Sample
In many cases the H can be replaced by a Z and
the process will have a similar cross section and
other dynamics. Then the dilepton decays of the Z
can be used as a clean signature and the process
can be validated prior to looking for the H
itself.
21
Quartic gauge Couplings D-Y, VBF
Beyond the range of Run II at present. Seen at
LEP -II
High WW or ZZ mass VBF processes will let us
explore quartic couplings at the LHC.
22
Higgs Pair Production at SLHC
Higgs potential vev at potential minimum Expand
about the minimum
There are 2 parameters in the potential. One is
fixed by G. Once the Higgs mass is known the SM
will be completely specified. Verify the SM by
exploring the triple and quartic Higgs
self-couplings?
23
Higgs Self-couplings at the SLHC
Cross section is 20 fb _at_ 160 GeV. If SLHC is
1000 fb-1/yr, then 20,000 HH produced/yr. There
are 400 events with 2 leptons, missing energy
and 4 jets (2 W mass peaks).
24
Higgs Particle Production at the LHC
  • The coupling of the Higgs field to the fermions
    is proportional to the fermion mass. Thus the
    Higgs field couples only weakly to ordinary
    matter, u,d quarks and gluons. Therefore,
    production cross sections are rather small,
    making discovery difficult.
  • The Higgs mass is unknown. For low masses bb and
    tt modes are favored. When energetically allowed
    , WW and ZZ modes dominate.
  • The large top mass makes the tt mode substantial,
    gt 10. The backgrounds are overwhelming, however.
  • The CMS detector is designed to discover the
    Higgs for all masses lt 1 TeV in 1 year of full
    luminosity operation.

25
Higgs Branching ratios
Note that q,l width M while W,Z width M3.
Hence bb dominates below WW threshold. ?? is
down by 9 due to coupling to mass, and 1/3
color factor.
Similarly for WW
26
Higgs Width to q and l M
27
Higgs Width into V V M3
Below WW threshold there is a Wlv mode with an
off shell W. Note that the Lorentzian width, ?W
2.2 GeV takes the rate down by a factor
(?/2)/(M-Mo)2 w.r.t. the WW rate. Going from
WW at 160 GeV to 140 GeV WW, expect width 1.2
MeV the bb width.
28
Higgs Strategy and BR
LEP-II
H--gt?? is a clean decay mode for low mass Higgs.
The ZZ --gt 4l mode is clean when it is above
threshold at 150 GeV. The dip in ZZ is due to
WW rise above threshold at 160 GeV. The WW
decay mode does not have a mass peak and is
unused save just at threshold, except in the VBF
mode where it is a discovery mode.
VBF, H -gtWW -gt 2l
29
bb QCD Backgrounds


Assume calor reconstruction of bb at 100 GeV has
?M 20 GeV. ? /?M 2pb/GeV. Thus Higgs is
buried by 1000x
30
Tau Pairs Higgs and D-Y Background
Although the S/?B is much more favorable.
However, must identify hadronic tau decays in
order not to be swamped by QC jets.
Use the BR given above. For M 100 GeV, ? B/?M
0.2 pb/GeV S/B is better due to reduced coupling
and smaller source, g --gt u
31
Tau ID and QCD J Rejection at LHC
Leptonic decay small BR
Hadronic decay large BR with low multiplicity J
and missing Et
32
?? Background
Recall Chpt. 4, uu --gt ? ?

?? mode is favorable w.r.t. bb as g --gt u and
strong --gt EM coupling. ECAL mass resolution for
?? is 10x better than bb. Still buried by 50x
in the continuum background
33
Low Mass Higgs
  • H?gg decay is rare (B10-3)
  • But with good resolution, one gets a mass peak
  • Motivation for PbWO4
  • calorimeter
  • CMS at 100 GeV, s ? 1GeV
  • S/B ? 120

34
Higgs Search - Four Leptons
Z
ZZ gives 1 resonant Z mass and 4 isolated
leptons as a signature.
Z
35
ZZ Background
For Higgs mass 350, 700 GeV the cross section
is 10 and 1 pb. The ZZ branching fraction is 1/3.
The mass window, ?M is 70, 200 GeV. The natural
width dominates at high mass.
ZZ continuum background is small w.r.t. Higgs
signal gold plated mode

36
Full Monte Carlo Study, H-gtZZ-gt4l
37
Higgs --gt ZZ --gt2l 2 Jets
  • For masses gt 600 GeV, larger branching ratio
    decay modes are needed due to rate limitations..

J J
Note HCAL granularity Chpt.2
e e
38
WW Mode -tt QCD Background
tt has the same Feynman diagrams and hence the
same cross section as bb above threshold effects,
WW background from t --gt Wb

39
WW Backgrounds - 1/200 tt
D-Y Production of WW is an irreducible background

Can clean up top pairs using jet veto of the b
quarks. However, there is no resonant mass
bump. In VBF top pairs are a major background,
but a mass bump can be formed using the tag jets.
t pairs/100
40
Higgs Mass - Upper Limit
  • The couplings are a function of the mass scale at
    which they are probed. We require that ?(Q) is
    well behaved from 174 GeV up to a
    scale ?, with 1/ ?(?) 0 (strong coupling at ?),
    the running of ? includes loops with H and t -
    with opposite sign.

41
Higgs Mass - Upper Limit
If there is no new physics up to the GUT scale,
then the Higgs mass must be lt 160 GeV. Again a
low mass Higgs is favored.
H H,W,Z,t H
42
Higgs Discovery Limits
Limits from ATLAS for 30 fb-1, S/?B
LEP-II
43
Statistics and Significance
44
Higgs Quantum Numbers
If the Higgs is seen in the 2 photon decay mode ,
it cannot be a J1 state (Yangs theorem). Recall
that the 2 photon state is a quantum number
filter.
Suppose the Higgs is found in the WW decay mode.
Look at the spin correlations expected for a 0
state. The emission of the 2 leptons is then
preferentially in the same direction.
45
H --gt ZZ --gt 4l, Spin and Parity
  • Recall the classical pion parity in
  • for J 0 into ZZ, CP requires that S 0 for the
    ZZ, with a longitudinal and transverse Z
    polarization

46
Full Monte Carlo results
0- - decay planes are perpendicular 0 - decay
planes are parallel.
47
Higgs Total Width Measurement
Resonant mass of Higgs will be well measured At
low mass the natural width is dominated by
detector resolutions. At high mass the natural
width dominates and can be well measured at the
5 level.
48
Extraction of Partial Widths
For a light Higgs several partial widths can be
determined at the 10-20 level. If the VBF
method is successful with the WW and ZZ final
state, then gHWW and gHZZ can be determined
unambiguously.
49
Early Physics Reach q
If the calorimetry is understood, resonances up
to a few TeV in mass are accessible early in the
LHC run.
50
Composites - DY

Search for lepton composites in D-Y production of
dilepton pairs. At masses above the Z there is no
known resonant state. Reach is 20 TeV. Early
reach is 5 TeV for 10 fb-1.

51
TeV Scale Extra Dimension
Black hole production ? Democratic Hawking
evaporation ? copious Higgs production. Study
with full CMS simulation.
KK excitations of the ?, Z in D-Y LHC at 600
fb-1 has a reach to 6 TeV. SLHC would push
out 30 further.
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