EF04 Topical Group Conversation

US/Eastern
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Jakob Beyer / Jenny List - Beam polarization for EWK precision
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two advantages:
direct access to chiral interactions (shown for future colliders)
isolating systematic effects (still open question)

looking at EWK fit in different scenarios:
both beam polarized (ILC-like)
only e- polarized (CLIC-like)
no-polarized scenario not yet investigated

2/ab and 10/ab scenarios
in fit, polarization and luminosity (the uncertainties?) are free parameters

look at e+e- -> Z/G* -> mu+mu-
return-to-Z events, and high sqrt(s)
polarization-weighed distributions -> toy-fluctuated distributions -> combined log-likelihood fit

6 fit parameters
physical parameters (3):
total chiral cross section; initial/final fermion chiral asymmetry
correction parameters (3):
interference correction
radiative correction factors (L and R separately)

fit results
uncertainty drops by factor 2 in high-lumi case
about 20% reduction when adding e+ polarization?

Ae (the electron - i.e., initial - chiral asymmetry) decreases a lot (factor 4?) when positron beam polarized
results on polarization (nuisance parameter) and Ae shows importance of redundancy offered by knowing e+ polarization

systematic effects
flipping polarization, get different physics, in same detector -> unique global signatures of systematic effects, can be decoupled and reduce their effect

looked at some of them: first tested (looked at LEP papers) is muon acceptance
(ALEPH, L3, OPAL: muon acceptance was among dominant)
used two parameters to describe acceptance: dC and dW (C is acceptance, W is cos\theta\*)
acceptance modeled as a box (center, width - C and W)

ILD: could be able to ID muons up to 7deg from beamline

in fit, dC and dW (size of box) are determined very well
beam polarization: uncertainty on C and W halved
for this setup, only two parameters affected by letting dC and dW free: KL and KR
see that effect is very small, and there is no additional advantage from e+ polarization (mu acceptance is well determined)

question: is model too simplified? what is case with unpolarized beams?

direct access to chiral part of interactions makes EWK measurements much easier: decouples them from other physical or systematic effects that do not depend on 
polarization

Ayres: Ae determined much less precisely than Af; Af is from AFB, which, at Z pole, is proportional to product of Ae and Af: why one much more precisely than the other?
JB: we have access to polarized AFB, and that is directly proportional to Af
Ayres: then, would not you need to know polarization well? 
JB: shall check interpretation of results; all improvement in Ae, Pe-, Pe+ is from redundancy, need to think about that

Ayres: what about e+e- -> e+e-? Af becomes Ae, one could do combined fit of the two processes... would that make advantage of positron polarization less important? (one 
can get Ae from final state)
JB: issue is extra term from t-channel, interpretation less direct (but can be done)

Ayres: all quantities improve, but need also to keep in mind that polarization comes at price of luminosity

in parameterization (slide 5), it is linear in cos\theta; summing the two to get unpolarized cross section, get term proportional to Ae*Af

Junping: this is single channel w/ muon, Ae should be better at end if looking at all fermion pairs
JB: indeed

question is why Ae precision much worse than Af; then, adding all channels, difference goes away?

Alberto: unpolarized scenarios?
JB: parameter like Ae cannot be determined, fit does not converge, cannot get uncertainty
planning to add constraints to parameters that cannot be determined

Junping: Ae and Af can be decoupled if one integrates out cos\theta distribution (Af disappears), and Ae is just count of events; is that why Ae limited by knowledge of 
polarization, while Af is from asymmetry?
Ayres: one could go to double-AFB (initial and forward); this depends on Af, but again affected by polarization uncertainties

Junping: on systematic uncertainties; how are C and W fixed from data? cut on angular distribution in cos\theta?
JB: determining w/ MC what is the influence of cut on C and W (and variations of these cuts) on the differential distributions, then parameterize the bin variations (when 
varying the cuts on C and W)
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    • 1
      Open discussion
      Speakers: Adam Martin (University of Notre Dame), Andre de Gouvea (Northwestern University), Aram Apyan (Fermi National Accelerator Laboratory), Brigitte Vachon (McGill University), Cen Zhang (Institute of High Energy Physics, Chinese Academy Sciences), Daniel Britzger, Daniel Stolarski, Daniel Wiegand (Northwestern University/Argonne National Lab), Emanuele Mereghetti (Los Alamos National Laboratory), Graham Wilson (University of Kansas), Ian Lewis (University of Kansas), Jakob Beyer (DESY), Jeffrey Berryhill (Fermilab), Jenny List (DESY), Jiayin Gu (JGU Mainz), John Michael Roney (University of Victoria), Juan Alcaraz Maestre (CIEMAT - Madrid), K Yumino, Keping Xie (University of Pittsburgh), Li Huang, Maximilian Swiatlowski (TRIUMF), Mogens Dam (Niels Bohr Institute, Copenhagen University), Paolo Azzurri, Radja Boughezal (Argonne National Laboratory), Rajan Gupta (Los Alamos National Lab), Roberto Petti (University of South Carolina), Samuel Lane, Saptaparna Bhattacharya (Northwestern University), Sergei Chekanov (ANL), Siqi Yang, Sven Heinemeyer (IFT (CSIC, Madrid)), Swagato Banerjee (University of Louisville), Taikan Suehara (Kyushu University), Takahiro Mizuno, Vladimir Litvinenko (Stonybrook University), William Shepherd (Sam Houston State University), Yang Ma (University of Pittsburgh), Yongcheng Wu (ycwu@physics.carleton.ca), bo li