Amendments to the paper
“A Search of a New Boson W1 in the Minimal
Higgsless Model at the LHC”
Mingshui Chen
CMS-IHEP 2009.02.27
Key points
• 1. need to add more backgrounds
– Beam-related backgrounds, cosmic ray, pile-up
– jets faking leptons
• 2.systematic uncertainties
– lepton ID uncertainty? isolation modeling?
Jet energy scale and resolution (affecting the dijet
mass window cut's efficiency). ISR? FSR?
– How do you justify the uncertainties on the
background? 100% may not be enough. Or is it? No
support is given for the 100% error.
Most serious complaint
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Perhaps the most serious complaint is that the relevance it provides to the scientific community is
not obvious.
Ref. 2 already describes the model to be sought in more detail, although the simulation of the
search is performed in less detail (parton-level calculations only, while the current paper has
hadron-level calculations).
The current paper still falls far short of the realism that will come when the LHC experiments take
enough data to start testing this model.
In particular, while this paper includes more background sources than ref. 2, some sources remain
unexplored, and some will require collider data to even estimate the order of magnitude of them.
Specifically, jets faking leptons is of concern, and multiple-event pileup is another.
This paper is at best an intermediate step, between the paper that describes the model to be
tested, and the desired one in which it is tested with collider data.
Instead, this is a partial refinement of ref. 2's simulation of a search, and one can imagine several
additional steps along the way in realism.
One is to include the detector simulations already available from the LHC collaborations, and
others to include beam-related backgrounds, cosmic rays, pileup, and an estimation of fakes.
Some kind of simulation of control samples is also a step towards realism -- what kinds of things
can be done by the experiment to give confidence that the background is properly estimated?
Most serious complaint (cont.)
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A real search will probably not look exactly like the one here.
Additional modes, such as W->lv,Z->jjZ->ll would probably be added.
The energy resolutions are probably too optimistic for the jets.
Demanding four leptons may reduce acceptance -- can three be
enough?
• How about Missing-Et? Can that be used to veto backgrouds,
particularly those containing leptonically decaying W's.
• I'd expect papers that describe searches for these signals when the
data are available may look rather different from this one.
• ArXiv:hep-ph may be the right place to submit this article, it isn't
hep-ex without an experiment.
About the title
• Here's a suggestion:
A Search for a New Boson W_1^\pm in the Minimal Higgsless Model
at the LHC
reads better than the proposed title (and put the W_1^\pm in
boldmath), but still has problems.
• One is that there is no search -- only Monte Carlo calculations to see
if such a search is likely to be sensitive enough to carry forwards
with. The Minimal Higgsless model is not the only such model one
can test here.
Resolution of jet and lepton
• Eq's 2 and 3 effectively assume the same energy resolution for
leptons and jets -- typically leptons have much better resolution.
– Ml+l-=91±15 GeV
– Mjj=80±15 GeV
Pt balance
• Fig. 4 actually looks like a cut on pt balance might be a good idea.
At low pt balance, the dominant background is a factor of 2 under
the signal, while at high pt balance, it looks more like a factor of 10.
If you don't want to cut on it, maybe it can go into a neural network
or something.
Lepton-ID efficiency
• The four-lepton efficiency of 97% seems awfully high. Granted the
events are only generated inside of the lepton acceptance, so many
events that would be produced by this signal fail the four-lepton
requirement, but even so, such a high lepton-ID efficiency within the
acceptance of the detector is unrealistic.
Systematic errors
• PDF's and MC statistics are usually the smallest of the systematic
uncertainties an experiment has to deal with.
• How about lepton ID uncertainty? isolation modeling? Jet energy
scale and resolution (affecting the dijet mass window cut's
efficiency). ISR? FSR?
• How do you justify the uncertainties on the background? 100% may
not be enough. Or is it? No support is given for the 100% error.
• There may be overlaps between the ZZjj and ZZj backgrounds for
example, if Pythia is allowed to make extra jets -- you need some
matching procedure so as not to double-count predictions.
Systematic errors (cont.)
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The systematic errors on the background appear to have been propagated
incorrectly to the sensitivities in Table IV. It looks as if the upward-fluctuated
backgrounds have been simply substituted into eq. 5, which itself does not take
background systematics into account. What this means is that while the background
may be larger than predicted here, it will be known with zero uncertainty when the
experiment is done, which is never the case.
In particular, if one desires 5 sigma sensitivity, the signal has to be at least 5 times
bigger than the error on the background. And that assumes one has an infinite data
sample, the systematics can be a brick wall. At 800 GeV, the signal is only three
times bigger than the uncertainty on the background, meaning that at most 3 sigma
significance can be had, dominated by systematics. Of course as more data are
collected, backgrounds can be constrained, but none of this is studied or mentioned
in this paper.