EF04 Topical Group Community Meeting

US/Eastern
Description

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Meeting ID: 955 4136 9778

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Mika Vesterinen - LHC W mass
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LEP legacy

observables: lepton pT (problem is influence by boson pT; best for LHC high-PU); mT (poor resolution; best at Tevatron, and in low-PU runs); MET (both problems)

Tevatron legacy: 11+-20 DO; 12+-15 CDF
stat ~ calibration >> theory

LHC: theory >> stat ~ calibration
issue about theory: dominated by valence+sea or valence+gluon subprocesses

ATLAS: 7+-11+-14
7TeV 2011 data; base simulation is POWHEG+PYTHIA, reweighed to PYTHIA W pT and fixed-order angular coefficients and rapidity
dominant uncertainty: PDF and transport of Z pT model to W

LHCb uses complementary rapidity range: partial anti-correlation of PDF uncertainties w.r.t. ATLAS and CMS measurements
key challenge: W pT modelling

LHCb: simultaneous fit of mW and W-specific tune of pT

LHCb: 23(stat)+-11(exp)+-17(theory)+-9(pdf)
base simulation: PYTHIA, reweighed to POWHEG and DYTurbo
use average of three PDF sets: NNPDF31, CT18, MSHT20

collaborative effort to combine with ATLAS (and CMS)
simple assumptions and BLUE method: PDF is somewhat in anti-correlated region, theory somewhat in correlated: expect combined average around 16MeV?

CMS: double-differential cross section measurement w/ full 2016 dataset
WR and WL disentangled
potential for precise W mass measurement, but hard to constraint the W theory uncertainties (that is the challenge)

HL-LHC
ATLAS, CMS: key enhancement is extended lepton eta coverage; add to it possibility of low-PU runs
LHCb: expect 500M W->munu events, and upgraded ECAL would allow to use electrons as well

ATLAS: estimate that additional eta acceptance (from 2.5 to 4) and 1/fb low-PU at 14TeV can get stat uncertainty ~4MeV; PDF uncertainty at 1-2MeV level, when including LHeC run

what at end of HL-LHC?
now 19/32 (ATLAS, LHCb)
goal per experiment is 9MeV; combination would be ~5MeV; dominant uncertainty: QCD, then PDF
in general, expecting reduction by factor 2-3 w.r.t. current 

LHCb upgrade will allow to increase data sample by factor 20

challenges

need momentum scale calibration at MeV level: Z (know mass, but need interpretation in EW fit, once mW reaches 5MeV), J/psi, Upsilon (do we know its mass well enough? >3sigma tension between PDG 
measurements)

theory uncertainty at MeV level
need simulation: N3LO? N3LL? how to evaluate scale uncertainties and the range of variation? what about correlations? some formal theory work is needed
EW vs QCD-EW corrections: unclear how to best add to full event simulation (link: https://arxiv.org/abs/1912.10951)

PDF uncertainties: correlations among different groups, in-situ profiling/ weighing?
LHeC would clearly help

conclusion: hard to say what legacy HL-LHC combination could be! (5MeV)
long way to go, some bottlenecks identified (e.g., prescriptions for scale uncertainties and PDF)

Tania: question about paper on EW-QCD corrections; have link to study (https://arxiv.org/abs/2103.02671)
understanding is that people claimed had result on calculations, but that result was not end of discussion

Graham: combination among measurements is tricky
MV: difficulty is figuring out correlations among PDF uncertainties

Ayres: mentioned importance of QCD corrections; what about photon emission? now superficially simulated
MV: certainly it is a concern (EWK corrections, ISR, FSR, IFI, PDF w/ photons...)
there is interest discussion across experiments about this

Paolo Azzurri - FCC-ee W mass
-----------------------------
LOI #166
FCC-ee can provide mW and GammaW with scan of WW threshold cross-section lineshape, and measurement from decay products kinematics

summary of talks in https://arxiv.org/abs/2107.04444

FCC-ee program: 4 energies: Z, WW, HZ, ttbar
expect to produce a total of 300M W decays

method: threshold lineshape: look at cross section vs sqrt(s), and fit its dependency on W mass and width
slope of cross section rise is related with width, intercept with W mass
could reach statistical uncertainty of order 0.5MeV

statistical sensitivity: maximal at sqrt(s)=2*mW+600MeV=161.4GeV
in general, sensitivity curves quite wide

plan: measure cross section at two energy points

what about systematics?
interest in taking more than 2 energy points, then study correlated systematics (background, luminosity...)
a (not realistic!) scan would have data points where derivative factors are equal (on the two sides of each minimum), then cancel correlated uncertainty

impact of beam energy spread
at level of statistical uncertainty for W mass
beam spread changes the lineshape; effects related with second derivative of cross section w.r.t. energy; crossing point close to where dsigma/dGamma=0
need beam RMS < 50% to avoid additional systematic

interlude on ZH threshold: can measure Higgs mass with 5MeV statistical uncertainty, using 5/ab
precise Higgs mass needed to measure its Yukawa (to electron)
best energy point is 217GeV
https://arxiv/org/abs/2106.15438

W kinematic reconstruction different from LHC...
here 4-momentum conservation is very important (and useful)
mass of W essentially derived from angular opening; momentum scales fixed by conservation (can write Z mass as function of beta's and angles only...)

Beguin's thesis: https://cds.cern.ch/record/2710098
can get statistical precision ~0.5MeV (same as threshold measurement)
preliminary studies (on color reconnection) for hadronic channel; precision degraded by jet reconstruction w/ cone and momentum cuts (degradation by 5-15%, depending on energy)

what about other systematics?
c.o.m. important ingredient: could achieve 1MeV uncertainty on energy at 240-365GeV using radiative Z-returns and ZZ events
table of uncertainties: jet boost and non-perturbative QCD effects (fragmentation) dominate W mass (and GammaW, in the opposite order)

Mika: beam energy calibration: will it be done in-situ? will it be possible to calibrate?
PA: differently from LEP, beam energy will be calibrated in-situ continuously; spread can be measured with di-muon events, look at angular spread (di-muon angular spread present if there is beam energy 
spread)
Graham: https://arxiv.org/abs/1909.12245 (pilot bunches for RDP calibration)
PA: paper is also reference for FCCee beam energy spread and asymmetries monitoring with dimuon events

Ayres: intrigued by Higgs mass w/ threshold; how does it compare with mass measurements using kinematic reconstruction? remember kinematic reco around few tens MeV
PA: projections for kinematic reco are ~10MeV; measurement cleaner, safe to pursue (different systematics too?)

Alan Price - QED ISR/FSR with YFS resummation
---------------------------------------------
motivation: QED corrections needed for FCCee, so far adapted from EPJC 79(2019)
emission of soft/collinear photons cause large logs; LEP calculations not enough (FCC uncertainties 1-2 orders of magnitude smaller than LEP)

treatments of ISR
collinear resummation: calculated at NLL (hard to do better), including photon PDF; need match to PS
soft resummation: soft photons can be resummed at all orders, can do systematic improvement of calculations order by order (truncated expression used to include collinear logs)

resummation YFS (Yennie, Frauschi, Suura): hard coded in SHERPA, but one can get them from other tools (Recola, COMIX)

SHERPA status:

  for ISR, one can use YFS algorithm, corrections up to alpha^3L^3

  recently FSR added, implemented for cays in PHOTONS++, well validated for e+e- to ffbar (testing ongoing for WW, ZZ, ZH)

  IFI not yet included, one can do by hand for e+e-, but difficult to automate


e+e- -> ffbar
state of the art is KKMC (Comp.Phys.Comm 130(2000) 260-325)
includes ISR, FSR, and IFI

e+e- -> W+W-
dedicated codes in LEP era: YSFWW, KoralW, RacoonWW
with ISR corrections and O(alpha) corrections, option of FSR with PHOTOS, Coulomb corrections

SHERPA: ISR and FSR via YFS (FSR also available via PS), Coulomb, soft-photons resummed for W, complete O(alpha) with EW loops from RECOLA

e+e- -> ZH
ISR only via electron PDF before, now SHERPA can use YFS for ISR+FSR
QED emission of final-state leptons can be resummed
from Born to ISR+FSR, cross section changes from 7 to ~6fb
plot of O(alpha^3L^3) SHERPA vs Born

conclusion
SHERPA traditionally focused on hadron-hadron collisions
IFI to be implemented
more loops needed: full 1-loop EW with RECOLA, framework exists to add 2-loop
linear colliders: interface to LCIO and CIRCE (to be added to Sherpa 3.x)

Tania: understanding that some is implemented in other tool, what about WHIZARD?
AP: WHIZARD can create photons, but only 2, quite ad-hoc approach (not physically correct); plan to apply QED PS, with correct Sudakov
agree on things like total cross section with WHIZARD, disagree in pT distributions because they have only 1 photon per bin, while in YFS you get that correct for free

Graham: on WW slide, encourage to look at higher mass; in that plot, only some of the diagrams?
AP: neglected because Sherpa can do 4-fermion, but other tools (which are used for comparison here) cannot
Graham: few years ago used WHIZARD to study sensitivity to W mass from kinematics at high c.o.m., but ended up being very scheme dependent; would be good to check
AP: most of plots are constrained by what other tools can do; SHERPA can do more, used with fewer capabilities
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