WH/ZH cross section group kickoff meeting

1/1-025 (CERN)



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Giacinto Piacquadio (Conseil Europeen Recherche Nucl. (CERN)), Jim Olsen (Princeton University), Robert Harlander (Bergische Universitaet Wuppertal), Stefan Dittmaier (MPI fuer Physik)

-> Stefan Dittmaier (theory)
-> Robert Harlander (theory)
-> Jim Olsen (CMS)
-> Giacinto Piacquadio (ATLAS)
-> Clara Matteuzzi (LHC-B)
-> Reisaburo Tanaka (ATLAS)
-> many others

*** Assessment of state of the art in predicting WH/ZH cross sections:
overview of available tools / programs

The state of the art is represented by:
--> NNLO QCD corrections [ Brein, Djouadi, Harlander ]
--> NLO EW+QCD corrections [ Ciccolini, Dittmaier, Kramer ]
No public code available, for both calculations.

Both calculations have been performed only by a single group. However in
addition the following
NLO QCD calculations are available in public codes:
--> V2HV [Spira] calculation by [Han, Willenbrock]
--> MCFM [Campbell, Ellis]

Plans to make the NNLO QCD and NLO EW codes public:

--> NNLO QCD (Robert Harlander)
The plan is to make the code public in few months / max half a year.

--> NLO EW+QCD (Stefan Dittmaier)
The plan is to make the code public in the following version (v.2) of HAWK
(http://omnibus.uni-freiburg.de/~sd565/programs/hawk/hawk.html), the
is again few months / max half a year

An important aim of the cross section group is to make all code used by
the working groups
public and shared among the community. The proposal for the WH/ZH
working group would be to start
the work by using the private code and then hopefully have the code
public as soon as possible.

*** Organisation of concrete numerical evaluations of the
state-of-the-art cross sections

Assessment of inclusive cross sections

--> Calculate inclusive cross sections for WH and ZH, for center of mass
energies sqrt(s)=7,10,14 TeV
   (on the line of what already done in the ATLAS study) using whenever
   the parameters suggested in


   + evaluating scale variation, PDF and alpha_s uncertainty

--> Cross-check the NLO QCD result with V2HV and MCFM

   In the ATLAS result the cross section by V2HV is approx. 3% higher
(see pag. 5 of
   this shouldn't be the case, so cross-checking the NLO calculation
with V2HV and also with
   MCFM is very important (make sure same input parameters are used)

Assessment of differential cross sections

No work has been done so far to assess WH/ZH differential cross
sections. Distributions of interest:
a) pT(H)
b) pT(W)
c) eta(H)
d) eta(W)
e) pT(additional hardest jet)
f) eta(additional hardest jet)
g) deltaPhi(W,H)
h) deltaEta(W,H)
i) m(WH)
Please suggest further distributions if you think they can be useful...

We need to define what jet clustering algorithm to use. Would be good to
contact the people
doing the inclusive WH analysis in ATLAS and CMS to ask them.

Estimate uncertainties (scale, PDF, alpha_s) on these distributions ?
(e.g. at least on pT(H), pT(W) and pT(add. hard. jet) ?)

This can be evaluated at:
1) NLO EW (private code by Stefan et al.)
2) NLO QCD (MCFM, V2HV, ...)

Robert pointed out that it is not possible to obtain fully differential
predictions by using their
NNLO code. Stefan suggested to cross-check the NNLO effect on the
distributions by using the NNLO
QCD computation for the Drell Yan process only.

3) NNLO Drell Yan only (what code? how to trat the virtual W?)

*** Comparison with parton shower Monte Carlo

The following codes contain the WH and ZH processes:
* MC@NLO [Frixione, Webber]
* POWHEG (all Higgs productino processes implemented by Hamilton,
Richardson, Tully,
         see also arXiv:0903.4345 [hep-ph] )

Plan: compare the differential distributions of the parton level NLO
codes with
the parton shower Monte Carlo based distributions (obtained by running
jet clustering on
the partons after parton shower, switching underlying event off). Study
possible effects
of LL showering which are beyond NLO.

*** Identification of loose ends, i.e. we should work out where
theorists still have some homework to do

Jim suggested to check for interferences between the WH signal and the
WW and WZ backgrounds. This could be achieved by considering all
relevant LO diagrams with general purpose LO codes (like MadGraph).
Nobody volunteered for doing this check.

Jim proposed also to have a look at the di-boson background. While
backgrounds are not directly a part of this joint effort, we may want to
give a look at them (in particular for those where a parton shower based
matched NLO Monte Carlo is not available) and need to coordinate this
with the relevant SM Monte Carlo groups in ATLAS and CMS.

*** Specific analysis-driven scenarios, e.g. Higgs at large pT

There is strong interest both from ATLAS and CMS to have specific theory
predictions for the phase space region where the Higgs boson is highly
boosted (e.g. pT(H)>200 GeV), due to the recent analysis efforts in this

Proposal. Obtain the same differential distributions as proposed for the
inclusive case, but after having applied the following cuts:
1) pT(W,Z)>200 GeV, |eta(W,Z)|<2.5
2) pT(bb)>200 GeV
3) pT(b)>20 GeV, |eta|(b)<2.5
4) DeltaR(bb)<1.2
5) pT(non b-jet)<60 GeV, |eta(non b-jet)|<5 (if present in the event)
and, using an inclusive kT algorithm with size 0.3 and E-invariant
recombination scheme.

If, not possible, we should cut on the Higgs eta directly rather than on
the pT and eta of
the the b-quarks.

This set of cut is only a proposal and we should discuss within ATLAS
and CMS to agree on this or a similar set of cuts. We may also go a bit
lower than the 200 GeV cut (in particular for the ZH analysis this turns
out to be better for the analysis). Should we go to 150 GeV for the ZH
based analysis. ATLAS did not do dedicated studies on going lower with
pT, did CMS do?

Stefan pointed out that the EW corrections can become very important at
high pT(H), so it will be very important to estimate their effect in the
high pT kinematic region.

*** LHCb?

The inclusive cross section could be estimated also in the region of
phase space which is more
relevant for LHCb. Clara Matteuzzi already sent an e-mail to the
lhc-higgs list with details about the pseudo-rapidity coverage of the
LHCb experiment and their Higgs boson identification strategy:
- LHCb covers the region 15-300 mrad in polar angle (with full
 instrumentation for charged and neutral particles)
- the eta acceptance is 1.9 - 4.9 
- we look for HZ,HW triggering on a lepton with pt>12-15 GeV
 with the H decaying to bbbar
- we identify b-jets with a very good efficiency
- the accepted b-jets have a pt peaking around 35 geV
- we have acceptance for Higgs of mass not greater than about 135 GeV
- we are using PYTHIA to simulate the HZ/HW events

NLO QCD (+ EW ?) cross section for WH and ZH could be obtained in this
kinematic region + the PDF,
scale variation and alpha_s uncertainty evaluated properly.

*** Are there specific requirements from the theorists?

Stefan pointed out that, for any of the calculations we are going to
the input parameters, cuts which need to be applied, jet clustering
algorithm and differential distributions need to be very well defined,
so that we can properly interpret the results we obtain.

*** Call for contributions

We would like to ask people in ATLAS, CMS, LHC-b or in the theory community if
they would like to work on any of the points made above, so that we can understand where
most of the interest lies and concentrate the effort on what is really doable given the number of
interested people. Please contact us via e-mail: we will report about the feedback obtained
at the next meeting. Any additional suggestions about the work plan,
including additional uncovered points, are welcome.


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    • 1
      WH/ZH cross section studies in ATLAS and discussion points from Stefan Evo only

      Evo only