Dear All,
please find attached the updated list of bulletin points for the EW
study group.
Cheers,
Doreen and Ashutosh
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Doreen Wackeroth
Professor of Physics
Department of Physics
239 Fronczak Hall
University at Buffalo - The State University of New York
Buffalo, NY 14260-1500, USA
Phone: (716) 645-5445
Fax: (716) 645-2507
Homepage: http://ubpheno.physics.buffalo.edu/~dow
E-mail: [log in to unmask]
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Study of Electroweak Interactions at the Energy Frontier
=========================================================
Findings:
==========
I. Electroweak Precision Physics
=================================
* The knowledge of the Higgs mass has sharpened the predictions of
these EWPOs such that the predictions are a factor of 2-4 more
precise than the experimental measurements.
* In extensions of the SM, which are associated with the electroweak
symmetry-breaking sector, these EWPOs usually receive corrections
due to quantum loops (due to e.g. supersymmetric particles or
techni-fermions), or due to effective operators (induced for example
in strongly-interacting light Higgs models), or due to Kaluza-Klein
modes in extra-dimensional models. Typical deviations are comparable
to current experimental errors.
* M_W and sin^2\theta_eff^l typically have different sensitivities to
the sources of new physics. This may be demonstrated by the
parametrization of new physics in the gauge boson self-energies in
terms of the S, T and U "oblique" corrections. Fixed values of M_W
and sin^2\theta_eff^l correspond to lines in the S-T plane with
different slopes. Thus, improved measurements of both EWPOs can
constrain all of the above sources of new physics in a relatively
model-independent fashion.
* The current world average M_W has a precision of 15 MeV, dominated
by the combined Tevatron measurement, which has a precision of 16
MeV based on the analysis of partial datasets. CDF and DO have
projected that analysis of the full Tevatron statistics can yield a
10 MeV measurement, assuming a factor of two improvement in the
uncertainty due to parton distribution functions, improvement in the
calculation of radiative corrections and improved understanding of
the trackers and calorimeters.
* First studies based on pseudo-data have demonstrated that
measurements of W and Z boson distributions with the 2011-2012 LHC data may
be able to improve the PDFs relevant for the Tevatron M_W measurement by a
factor of two in the near future, enabling the Tevatron potential
for M_W to be realized. Further studies are ongoing.
* Enormous statistics of W bosons and control samples at the LHC offer
the prospect of higher M_W precision. Studies based on pseudo-data
have shown that the PDF uncertainty in M_W is about twice as big at
the LHC as the Tevatron, due mainly to the larger fraction of sea
quark-initiated production (including strange quark). Improvement
by a factor of about 7 over the current PDF uncertainty will be
required. PDF sensitivity at the LHC is a challenging issue and
needs a dedicated program of precision measurements of relevant
observables.
* Additional improvements in the QED radiative correction
calculations and NNLO+NLL matched parton shower generators for W and Z bosons will
also be required.
* Considering the 15-year time scale
for the ultimate M_W measurement from the LHC, we consider a
target precision of 5 MeV to be appropriate for the LHC.
* Studies of the M_W measurement at the ILC using the threshold scan
and final-state reconstruction have been updated. The ILC TDR
assuming 100 fb-1 quotes an uncertainty of 6-7 MeV for M_W measured
at the WW threshold. The updated study projects that the ILC will
be able to perform the M_W measurement with a precision of 4 and
2.5 MeV for 100 fb-1 and 480 fb-1 respectively. These estimates
assume an electron(positron) polarization of 90%(60%) and beam
energy calibration using final state reconstruction of Z->mu mu and
J/Psi etc.
* The circular electron-positron TLEP machine, running at the WW
threshold, can produce very high statistics for the M_W measurement,
and is likely to achieve energy calibration at the level of 0.1 MeV
using the resonant depolarization technique. This potential
motivates further studies of other systematics achievable at
TLEP. Given an integrated luminosity that can enable a statistical
precision of about 0.3 MeV with four detectors, further
investigations of related issues are warranted. Assuming a
theoretical uncertainty of 1 MeV in the threshold shape calculation
the target uncertainty is 1.2 MeV.
* The measurement of sin^2 \theta_eff^l from LEP and SLC have
averaged to a precision of 16 x 10^{-5}, albeit with a
~3 \sigma difference between them. Additional, especially improved,
measurements will be valuable to shed light on this difference.
* A measurement of sin^2\theta_\eff^l using the full Tevatron dataset
is projected with a precision of 41 x 10^{-5}. This
measurement will be interesting to compare with LEP and SLC.
* Compared to the Tevatron, measurement of sin^2 \theta_\eff^l at the
LHC is handicapped by a larger sensitivity to PDFs due to the
dilution of the quark and antiquark directions. As with the M_W
measurement, considerable control of the experimental and production
model uncertainties will be required. Under the condition that a
factor of about 7 improvement on PDFs is achieved (a condition also
required for the M_W target for the LHC), a projected uncertainty
on \sin^2\theta_\eff^l of 21 x 10^{-5} is obtained. This
precision is similar to the current LEP and SLC measurements and is
valuable before the advent of future lepton colliders.
* Considerably more precise measurements of \sin^2 \theta_\eff^l are
highly desirable for taking the stringency of the SM tests to the
next order of magnitude. Such measurements are possible at future
lepton colliders running on the Z-pole such as ILC/GigaZ and TLEP.
* The ILC/GigaZ projection for the precision on sin^2 \theta_eff^l is
0.5 x 10^{-5} statistical and total 1.3 x 10^{-5} without theory systematics.
This is a factor of 10 improvement on the current world average.
* TLEP may have the potential to go beyond ILC/GigaZ in the precision
on sin^2\theta_eff^l, which warrants a detailed study. The target
precision on sin^2\theta_eff^l is currently estimated to be 0.3 x 10^{-5} when extracted from
A_LR at the Z peak without including theory
systematics.
* The anticipated precision at the ILC and TLEP will require that all
aspects of EWPOs, both theoretical and experimental, need to be
revisited. For instance, present predictions for Z pole observables
are based on a framework designed for NLO and rely on certain
assumptions and approximations to include dominant NNLO
corrections. A framework needs to be constructed that treats the
radiative corrections to Z-pole physics systematically and
consistently at the NNLO level and beyond.
* These improved measurements and predictions of EWPOs will enable
stringent tests of the SM and of BSM scenarios. Once a discovery is
made, EWPOs will help to test and predict other aspects of the BSM
model.
* Measurements of M_W at the few MeV level, and sin^2\theta_eff^l at
the level of 10^{-5}, require that the predictions for these
observables within the SM are at least at the same level
of precision. This requires that the parametric uncertainties from m_top, M_Z and
alpha_{had} (the contribution to the running of alpha_{EM} from
hadronic loops) as well as uncertainties from the missing higher
order corrections be addressed:
* With a dedicated effort by the
theory community, it is anticipated that calculations in the coming
years will reduce the effect of missing higher-order corrections to
a sub-dominant level.
* Current parametric uncertainties from m_{top}, \alpha_{had}, and M_Z
affect the uncertainties in the SM predictions of M_W as ollows:
\Delta m_t=0.9 GeV: \Delta M_W= 5.4 MeV -> current
\Delta m_t=0.5 GeV: \Delta M_W= 3.0 MeV -> needed for the LHC
\Delta m_t=0.1 GeV: \Delta M_W= 0.6 MeV -> needed for a few MeV measurement at ILC and TLEP
\Delta (\Delta \alpha_{\rm had})=1.38 x 10^{-4}: \Delta M_W= 2.5 MeV -> current
\Delta (\Delta \alpha_{\rm had})=0.5 x 10^{-4}: \Delta M_W= 1.0 MeV -> needed for ILC/TLEP
\Delta M_Z=2.1 MeV: \Delta M_W= 5.4 MeV -> current (needs to be improved!)
The LHC plans to
achieve \delta m_{top} ~ 0.5 GeV (see Top WG report) but further progress at the
LHC will likely be limited by theoretical uncertainties in the
non-perturbative QCD effects associated with translating the
kinematically-reconstructed m_{top} to the pole mass.
Further considerable improvement in parametric and theory uncertainties is
needed for the target uncertainties at ILC and TLEP.
II. Multi-boson processes
==========================
* The EFT formulation is not limited to specific models; any high
energy theory can be reduced to a low-energy EFT and the former will
specify the values of operator coefficients in the latter. Therefore,
EFT operators provide a general method of parametrizing the effects
of new physics at a high scale.
* The estimates errors in the measurement of CP conserving trilinear gauge couplings (TGC)
describing the WWZ and WWgamma vertices at the LHC 3000 fb-1 are
about 5 x 10^-3 - 5 x 10^-4 (apart from lambda_gamma where it is 2 x 10^-4)
and about 4 x 10^-4 at the ILC 500 and (1-2) x 10^-4 at the ILC 1000.
* Studies of vector boson scattering and triboson production have
become possible, for the first time, at the LHC. These processes constrain
quartic gauge couplings.
* For the next decade, the LHC will continue to be the facility to
explore trilinear and quartic couplings at higher levels of precision.
Precision on TGCs will improve by about an order of magnitude over LEP.
* The definite proof of unitarization via the Higgs will be difficult
at an integrated luminosity of 300 fb$^{-1}$. The HL-LHC, however,
will be able to demonstrate that the Higgs couplings to the
electroweak vector bosons is an essential component of the
unitarization mechanism for vector boson scattering.
* The sensitivity to higher-dimension operators improves by a factor
of 2-3 with the HL-LHC, in comparison with the 300 fb^{-1} at the
LHC.
* Triboson production is also a sensitive probe of higher-dimension
operators, complementary to vector boson scattering. This process
becomes rapidly more sensitive with increasing beam energy.
* Based on first, preliminary comparisons of ILC 1000 and LHC
sensitivities, the LHC is more sensitive to quartic anomalous
couplings. Further studies will be performed to follow up on this
conclusion.
* We expect anomalous quartic couplings induced by dimension eight operators
to be probed with rapidly increasing sensitivity for higher energy colliders.
Key Bullet Conclusions:
=======================
A) The W mass measurement at the LHC is limited by the PDF
uncertainty. If the PDF uncertainty can be controlled at the few MeV
level, ie a factor of about 7 improvement over pre-LHC knowledge, a final
W mass measurement of 5 MeV is a reasonable target. This will need a
dedicated program of precision measurements of relevant PDF observables.
The 100 fb-1 ILC projects a measurement of about 4 MeV precision,
which can be further improved up to about 2.5 MeV with 480 fb-1.
B) For all other EWPOs, lepton colliders will make significantly more
precise measurements of EWPOs than the LHC. This is especially the
case for sin^2theta_eff^l, where ILC will improve upon LEP/SLC by an
order of magnitude. Preliminary estimates of the TLEP potential find
that the current LEP measurement of M_Z improves by a factor of 10.
Contingent on further studies, TLEP may be able to improve on M_W and
sin^2theta_eff^l by a factor of about 2 and 4, respectively, compared
to ILC projections.
C) The sensitivity for TGCs can be improved by an order of magnitude
over LEP by the HL-LHC. ILC 1000 can improve TGC precision by an order
of magnitude over HL-LHC.
D) Based on first, preliminary comparisons of ILC 1000 and LHC
sensitivities, the LHC is more sensitive to quartic anomalous
couplings. Further studies will be performed to follow up on this
conclusion.
E) The sensitivity to higher-dimension operators improves by a factor
of 2-3 with the HL-LHC, in comparison with the 300 fb^{-1} LHC.
F) A dedicated effort by the theory community will be needed to
provide the necessary higher-order calculations at the 2-loop level
and beyond for predictions of EWPOs and for observables from which
EWPOs are extracted.
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