Study of low_Pt jets and low-x physics at CMS. Ver 2_02
Last update: 3 mar 2008
Table of contents:
- Introduction and goal of this study.
- HCAL (HBHE, HF).
- Jets reconstruction algorithms.
- Step by step analysis, plots.
- Links.
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1. Introduction and goal of this study.
What is x?
It is well known that proton is not just the uud composition. Proton also contains all
other quarks called sea-quarks. Assume that the proton is propogating with the momentum P.
Then each quark has the partial momentum pf and x = pf/P.
Now lets P = 100 GeV and lets proton to have the s-quark with the momentum ps = 1 GeV.
Then xs = 10-2 for this parton.
If P = 7000 GeV as at the LHC then xs = 1.4*10-4 for the same 1 GeV energy of the parton.
So at the LHC we will be more sensitive to the low x values then all previous experiments. For prepearing the future
results we need to study the probability that the parton carry the fraction x of the P.
That is called structure functions.
Protons Structure Functions (PDF's)
The theory and fenomenology of PDF's one can learn from
the article on
PDG.
ZEUS and HERA experiments do the presize mesurement of
the partons (quarks, gluons) momentum distribution in the proton. More recent results about this topic
can be obtained at
HEPDATA.
There are some groups work on this field. They do a slightly different parametrisation of those PDF's functions.
Most popular are
MRST and
CTEQ.
One may plot their PDF's and see the difference
here.
Evolution of the PDF's is also an impotant issue. Here you can see some evolution animation:
See BFKL theory is an example where this evolution is under study.
The idea is to study the physical distributions dependaning on different PDF sets.
So it is possible now to generate Pythia samples with different PDF's. The package for that is called
LHAPDF.
2. Hadronic Calorimeter at CMS.
HF
In the CMS experiment, very forward jets can be identified using the two HF calorimeters (3<|η|<5). The HF, located 11.2 meters away
on both sides of the interaction point (IP), is a steel plus quartz-fiber Cherenkov calorimeter segmented into 1200 towers of
Δη×Δφ∼0.175×0.175. It has an interaction length of 10.3λ and is sensitive to deposited
electromagnetic (EM) and hadronic (HAD) energy. The two HFs have been specifically designed for forward jet and missing-energy
measurements. In particular, the HF plays a prominent role in forward jet tagging for the vector-boson-fusion (VBF, qq→ qqH) Higgs
production channel. The HFs have an energy (position) resolution of ∼20%(∼10%) for typical jets with ET ∼ 40 GeV
(i.e. E = ETcoshη ≈ 1 TeV at η=4).
Events for this analysis can be selected online with a L1 trigger requirement of a jet candidate
with a transverse energy threshold of ET≈10 GeV since the default CMS jet L1 algorithm includes
as primitives the 2×72 trigger towers in HF+/- (each tower has an η−φ segmentation
of about 0.5×0.35). In addition, an HLT trigger currently exists (although with a higher ET =
30 GeV threshold) for the fast jet reconstruction and tagging of forward-backward jets in HF
emitted in the Higgs VBF channel.
-- new regions in Q-x plot
-- test resolution of HF calorimeter
Material about "Low-x QCD physics" can be found in CMS Note-2007/002
and article Forward Physics at the LHC
See HF Layout and Design and CMS NOTE
2006/044: "Design, Performance, and Calibration of CMS Forward Calorimeter Wedges".
3. Jets reconstruction algorithms.
Tevatron-for-LHC Report of the QCD Working Group
4. Step by step analysis, plots.
