Transcript Document 7520357

Current Issues and Challenges in Global Analysis of
Parton Distributions
DIS06
Tsukuba
Tung
Outline
• Challenges for Global QCD Analysis in the era of HERA II,
Tevatron Run II, and LHC
• New Experimental Input to Current Global Analysis of PDFs
• More Precise Theoretical Calculations needed to meet these
challenges
• Recent Results and Work in Progress (CTEQ):
 Implications of PDF uncertainties on Physical Applications;
 A new implementation of the general PQCD formalism with
quark mass effects. This is applied to a comprehensive
global analysis, including the full HERA I total and semiinclusive cross section data sets with correlated errors;
 Close examination of new fixed-target experimental data
(NuTev, E866) and large-x behavior of PDFs;
• Summary and Outlook
Challenges for Global QCD Analysis
—from here to LHC
In spite of steady progress in over 20 years of global
analysis of PDFs, it is surprising how much knowledge is
still missing on the parton structure of the nucleon !
• Gluon Distribution;
• Small-x and Large-x behavior of all
distributions;
• Strange distribution;
• Charm and bottom distributions;
• Quantifying uncertainties of all PDFs.
A successful LHC program depends on making
substantial improvement on most of these fronts.
Some recent “CTEQ” work (2005–)
• NLO PQCD is stable for Collider phenomenology
(Huston, Pumplin, Stump, wkt. JHEP 0506:080,2005);
• LHC phenomenology: Uncertainties of the inclusive
Higgs production cross section at the Tevatron and
the LHC (Belyaev, Pumplin, Yuan, wkt. JHEP 0601:069,2006);
(PDF uncertainties can be larger than commonly estimated
“theoretical uncertainties”, depending on the Higgs mass.)
• CTEQ6A,B series of PDFs: for physical applications
that are sensitive to as.
(Pumplin, Belyaev, Huston, Stump, wkt. JHEP 0602:032,2006 );
Uncertainties of the inclusive Higgs production
cross section at the Tevatron and the LHC
(Belyaev, Pumplin, Yuan, wkt. JHEP 0601:069,2006)
vs.
T
Enhanced bbA-coupling in MSSM
CTEQ6A,B PDFs for a series of as
(Pumplin, Belyaev, Huston, Stump, wkt: JHEP 0602:032,2006 )
Many applications. One example:
* Curves: as = 0.110 – 0.126 ; (Realistic range: between blue lines.)
* Shaded areas: uncertainty band based on CTEQ6 error analysis
for fixed as = 0.118 .
New Experimental Input to Current Global Analysis
• Extensive HERA I data sets (complete?) on
 total inclusive NC and CC cross sections, covering a
wide range of kinematic phase space;
 semi-inclusive (tagged heavy flavor) cross sections:
charm and bottom;
 semi-inclusive jet cross sections.
• (Note: out go the SFs, F1,2,3; in come the xSec’s!)
• Fixed-target Experiments (Last of the kind?)
 NuTeV n DIS S.F.s and cross sections;
 E866 DY pp and pd cross sections (finally?).
• New Tevatron Data on W/Z production, jet
production, … etc.
Available HERA Data Sets for Current Global Analysis
(by our reckoning)
• H1
 CCe+9497X
 CCe+9900X
 CCe-9899X
 (NCe+9497X ?)
 NCe+9697X
 NCe+9900X
 NCe-9899X
 NCe-9900X
 NCe+9697F2c
 NCe+9900Xc
 NCe+9900Xb
• ZEUS
 CCe+9497X
 CCe+9900X
 CCe-9899X
 NCe+9697X
 NCe+9900X
 NCe-9899X
 NCe+9697F2c
 NCe+9890F2c
Wow!
(both in coverage
and in accuracy.)
Just wait until
HERA II data
come along!
“New” Precision Global Analysis
(S. Belyaev, H.L. Lai, J. Pumplin, D. Stump, wkt, C.P. Yuan)
“New” Phenomenology work always trail frontier Theory and
Experimental advances (examplified by the plenary talks), by yeas!
• Data: Full HERA I total-inclusive and semi-inclusive
heavy flavor cross sections with correlated errors + F.T.
DIS + DY + Tevatron Jet;
• Theoretical tool: New implementation of General Mass
Variable-Flavor-Number-Scheme factorization
formalism of Collins for consistency + recent SAcot,
Acot-c, … practical prescriptions for efficiency.
(cf. talk in the joint HQ/SF session, wkt).
———————————
• Sorry! DIS jet inclusive not yet implemented. ( Jon Pumplin)
• NLO for now. Extension to order as2 is “straightforward”, given
the perturbative approach.
Results
• Excellent fit to 32 sets of data—CTEQ6C0:
Details in the c2 analysis are dependent on: (i) using
the xSec data (vs. SFs); and (ii) using CorSysErr’s.
(representative plots.)
• Comparison of CTEQ6.1M and CTEQ6C0 PDFs;
(representative plots)
• Where do mass effects matter?
(H1NCe+9697X, ZeusNCe+9697X)
—low Q2 data.
H1
NC e+ 96-97 X
ZEUS
NC e+ 99-00 X
H1
Zeus
Pull Plots: NC e+ 99-00 X
• N.B. This is after correlated
SysErr’s have been taken into
account!
• The shifts for individual
SysErr’s are generally ~ 1.
Zeus
H1
Close to normal distribution.
Comparison to
CC data
Lines:
theory (fits);
Red points:
raw data points;
Blue points:
data points shifted
by optimal
correlated SysErr.
(usually within 1s)
H1 CC
e+ 99-00 X
ZEUS
CC e+ 99-00X
Where does the General Mass Formalism make
a difference? Compare with CTEQ6.1M (ZM)
Low Q2 bins, of course.
GM
H1 96-97
ZM
ZEUS 96-97 data show the same effects
Charm Production S.F. and xSec.
Only show comparison
with 1 (out of 4) data
set—will re-visit this
comparison in Talk in
joint SF & HQ session,
which concerns the
theoretical basis, and
heavy flavor physics.
Zeus NC
e+ 98-00 F2C
An interesting application of the GM formalism
and the precision global analysis:
The first phenomenological study of
“Is there room in the nucleon for intrinsic charm?”
or
“How well can available data constrain the charm
content of the nucleon?”
Talk in the joint HQ/SF session, wkt
Comparison of New PDF (CTEQ6C0) with
previous PDFs (CTEQ6.1 and its uncertainty band)
• Space only permit a very brief comparison: Gluon at two scales
• New PDFs lie within the previously estimated uncertainty bands.
• After a new round of careful study, the new uncertainty bands should
be narrower, due to improvements on both theory and experiment.
“New” Fixed Target Data for Global Analysis
• E866:
 Measurement of pp and pd DY cross sections;
 Interest: Preliminary results (hep-ex/0302019)
indicated discrepancies with existing PDFs;
• NuTeV:
 extensive n and anti-n F2,3 & cross section data;
 Interest:
• Experimental: extracted F2,3 data indicates
discrepancies with CCFR results at large x;
• Theoretical: new data pull the PDFs in the opposite
direction compared to the E866 results!
Study of Large-x Behavior of PDFs
(Kuhlmann, Morfin, Olness, Owens, Stump)
• E866 pp and pd cross sections
 Preliminary results (hep-ex/0302019)
 CTEQ studies (deuteron corr. + other effects)
 Final results ??
• NuTeV n and anti-n scattering data
 Comparison of F2 vs. theory
• NuTeV (Tzanov)
• CTEQ study (nuclear corr., quark mass effects)
 Comparison of cross section data vs. theory
• Petti & Kulagin (emphasizes nuclear correction)
• CTEQ study (nuclear corr., quark mass effects)
E866
• “ The results imply that the u quark distributions in CTEQ6 and
MRST2001 are overestimated as x 1.
—hep-ex/0302019
• These .. discrepancies .. imply that future PDF fits will see a
substantial correction to the u and d quark distributions at large x.”
•CTEQ group have studied possible sources for this discrepancy,
particularly deuteron target corrections. Results were inclusive.
Reports by E866 in subsequent conferences indicate that improved
radiative corrections seem to reduce the discrepancy, but it has not
completely gone away.
… stay tuned.
Tzanov, DIS2005
Notable NuTeV Results
F2 Measurement
• Isoscalar ν-Fe F2
• NuTeV F2 is compared with CCFR and
CDHSW results
- the line is a fit to NuTeV data
• All systematic uncertainties are included
• All data sets agree for x<0.4.
• At x>0.4 NuTeV agrees with CDHSW
• At x>0.4 NuTeV is systematically above CCFR
Our Attempts to Incorporate the NuTeV Structure
Function, F2,3, data in the Global Analysis
Find this data set “incompatible” with the other data
sets in the global analysis—with our usual assumptions;
• Total c2 of the NuTeV data set unacceptably high;
shape discrepancy clearly seen;
• Tried to: (i) vary nuclear target corrections; (ii)
include target and heavy quark mass effects, …
• None of these help; some make things worse.
However, it is more preferable to directly compare
theory with the cross section data !
Kulagin and Petti
Caution: 2, out of many, energy bins (total
# data points > 1000); details impossible
to see in these plots.
NuTeV xSec. data
compared to a
typical fit:
•This is only an impression
plot;
•Data with different E are
superimposed;
•Error bars are shown only
for two bins;
•Overall c2 is
1140 / 1170 pts.
•However,
•CorSysErr not yet
included;
•the devil may be in the
details.
Can Nuclear Corrections Help?
•For each x, data are
combined and errors
are weighted;
•See a systematic xdependent deviation
that cannot be
reduced
substantially by
nuclear correction
models.
Our Attempts to Incorporate the Full NuTeV
Cross section data in the Global Analysis
The situation improves!
• Total c2 of the NuTeV data set becomes acceptable—
not too different from 1 /dof ;
However, upon closer examination, a clear shape
discrepancy persists;
• Tried to: (i) vary nuclear target corrections; (ii)
include target and heavy quark mass effects, …
• None of these help; some make things worse.
Therefore, we believe the compatibility of this
measurement with the other data sets in the global
analysis within the conventional PQCD framework
remains an open question.
Summary and Outlook
• The impressive consistency between the improved theoretical
calculation and much improved HERA input on DIS NC, CC &
heavy flavor production (and other F.T. and hadron collider
processes) provides a new basis for performing precision
phenomenology within and beyond the SM.
• A lot of work remains to be done to pin down the full parton
structure of the nucleon (particularly gluon, s, c, b);
• HERA II and Tevatron Run II data can contribute
substantially to fill the gaps. More specifically,
• With more accurate data on CC cross sections, we gain
additional (clean) handles for differentiating up and down
types of quarks;
• Direct FLong measurement in the cards?
• With W/Z/g + tagged heavy flavor events at the hadron
colliders, we can get direct information on s/c/b quark
distributions;
• ….
LHC physics is waiting for these advances …
?
Reserve Slides
Comments on NNLO
• In the perturbative approach, for the total inclusive S.F.s
and cross sections, once a comprehensive NLO calculation
is in place, it is “straightforward” to include known NNLO
corrections additively.
• Extending global analysis to NNLO is certainly desirable,
but not necessarily urgent for current applications, since
experimental errors for most measured quantities, as well
as other sources of uncertainties (such as
parametrization, power-law corrections …), largely
outweigh the NNLO corrections.
• Additional note: unlike total inclusive F2,L, quantities such
as ”F2c” are not well defined theoretically at NNLO and
beyond. (It is not infra-red safe!) It is rather misleading
to talk about a true “NNLO theory” of F2c (except within
the 3-flv scheme, which has a very limited range of
applicability).