Electroweak physics at the Z and W
experimental capabilities
Paolo Azzurri – INFN Pisa
FCC week Berlin
May 30th 2017
EWK CDR content
R.Tenchini, F.Piccinini work in progress
• 1. Electroweak physics at the Z pole
1.a Experimental categorisation of Z decays
1.b The Z lineshape
1.c Measurement of the number of neutrino species
1.d Measurement of Rb and Rc
1.e Measurement of forward-backward asymmetries
• 2. Electroweak physics at the WW production threshold and above
2.a Measurement of the W mass and width at threshold
2.b Measurement of W decay branching fractions
2.c Direct measurement of W mass and width
2.d Single and double boson cross sections
2.e Constraints on gauge couplings
2.f Radiative Z events
FCC week Berlin 30/05/17
P. Azzurri -- FCee experimental EW Z and W capabilities
2
the Tera Z pole precision
L~2 1036 ~4 1012 Z decays
continuous ECM calibration (resonant depolarization)
Z mass and width : 10 KeV (stat) + 100 KeV (syst)
Radiation function calculated
up to O(a3) : 10-4 precision
relative precisions JHEP01(2014)164
Rl hadronic/leptonic width : 5 10-5
Rb Zbb partial width : 5 10-5
Invisible width : 10-3 Nν (Zγ)
αs(m2Z) : 2 10-3
FCC week Berlin 30/05/17
aQED(mZ2) : 3 10-5
P. Azzurri -- FCee experimental EW Z and W capabilities
3
Z pole acceptance
• @LEP acceptance effects at 10-4
OK for cross sections at 10-3 level.
Main effects were due to track
losses, angle mis-measurements
and knowledge of boundaries.
• @FCCee exploit a statistical
uncertainty at 10-5 !
Example from ALEPH, EPJC 14 (2000) 1
@LEP detectors inner edge (relevant boundary) was known at the level of up to 20 μm
The beam displacement (vertical and horizontal) becomes ineffective by choosing two
fiducial regions (loose and tight) and alternating them in the two sides
@FCCee can use similar methods for cross sections measurements (e.g. different and
alternating forward and backward fiducial regions), but still need to identify and know
well the relevant boundaries (~1μm level)
FCC week Berlin 30/05/17
P. Azzurri -- FCee experimental EW Z and W capabilities
4
couplings and Rb
couplings measurements require asymmetry and width ratios
s -s
AFB(b) = sF B
tot
( Z  bb )
Rb 
had
•
•
•
•
•
•
gVf
=
3
4
Ae Ab (LEP)
g Af
Ab = AFBpol (b) = 0.921 ± 0.021 (SLC)
Rb = 0.21646 ± 0.00065 (LEP +SLC)
(g ) + (g )
2
Af
2
Vf
Rb Very sensitive to rad. vertex corrections due to new particles
Important to sort out LEP b-couplings issue
Measurement exploits the presence of two b hadrons and b-tagging.
Independent from b-tagging efficiency, but not from hemisphere correlations
Higher b-tagging performance (vertex detectors) helps in reducing the correlation
Correlations sources should be identified and studied with data (done at LEP)
ΔRb =≈1 (5-20) 10-5 stat (syst)
FCC week Berlin 30/05/17
ΔRc =≈3 (50) 10-5 stat (syst)
P. Azzurri -- FCee experimental EW Z and W capabilities
5
Z asymmetries
• Z boson decay to ff : 3 observables from the direction and decay of
the outgoing fermion
AFB = ssF -s B
tot
Af 
2 gVf g Af
( gVf ) 2  ( g Af ) 2
sin 2  eff 
l
1
g 
1  Vl 
4
g Al 
=
3
4
Ae A f Can measure for e,m,t,c,b
s F,R + s B ,R - s F,L - s B,L
= -A f
s tot
Can measure with t’s
s - s B ,R - s F,L + s B ,L
Apol FB = F,R
= - 43 Ae
s tot
Apol =
• Additional asymmetries with polarization of initial state :
ALR  l  r
tot
AFB pol 
FCC week Berlin 30/05/17

Ae
 F ,l   B , l   F , r   B , r 3
4A
f
 tot
P. Azzurri -- FCee experimental EW Z and W capabilities
6
Z asymmetries
FCCee will sizably improve b asymmetry
• use semileptonic b decays
• use weighted charge of particles in the hemisphere
•different systematic effects [QCD corrections to be improved]
 ΔAb ≈2 (40) 10-5 stat (syst) ΔAc ≈3 (40) 10-5 stat (syst)
tau polarization A
Polarization vs the production angle
allows Ae to be separated from At :
Universality test and sin2W
 ΔAτ ≈4 (30) 10-5 stat (syst)
 ΔAe ≈5 (10) 10-5 stat (syst)
AFB(m+m-) and AFB(t+t-) can also be considerably improved.
AFB(e+e-) more difficult because of t-channel.
 ΔAFB(e) ≈? (??) 10-5 stat (syst)
 ΔAFB(μ) ≈? (??) 10-5 stat (syst)
S-matrix approach is desirable: trade statistical
 ΔAFB(τ) ≈? (??) 10-5 stat (syst)
power for reduced theoretical assumptions
FCC week Berlin 30/05/17
P. Azzurri -- FCee experimental EW Z and W capabilities
7
Z pole summary
Present
precision
TLEP stat
Syst Precision
X
Physics
MZ
MeV
Input
91187.5
2.1
Z Line shape
scan
Z
MeV
 (T)
(no a!)
2495.2
2.3
Rl
as , b
N
Stolen from A. Blondel
TLEP key
Challenge
0.005 MeV
<0.1 MeV
ECM
QED
corrections
Z Line shape
scan
0.008 MeV
<0.1 MeV
ECM
QED
corrections
20.767
 0.025
Z Peak
0.0001
 0.002
Statistics
QED
corrections
Unitarity of
PMNS,
sterile ’s
2.984
0.008
Z Peak
0.00008
0.004
0.001
->lumi
QED Bhabha
corrections
Rb
b
0.21629
0.00066
Z Peak
0.000003
0.000020 - 60
Statistics, small
IP
AFB
, 3 ,a
(T, S )
0.0171
0.0010
Z peak
0.000003
0.00001
Z+(161 GeV)
FCC week Berlin 30/05/17
Statistics
P. Azzurri -- FCee experimental EW Z and W capabilities
Hemisphere
correlations
8
Direct measurement of aQED(mZ2)
arXiv:1512:05544, JHEP 2016(2) 1
g, Z
m-
High precision of FCCee will require higher order perturbative
calculations : a bottleneck will be represented by the
hadronic contributions to the vacuum polarization
e-
Patrick Janot:
Rely of a self-normalizing quantity, the forward-backward asymmetry
(a)/a plot, for a year of running at any √s
Optimal centre-of-mass energies for a 3×10-5
uncertainty on aQED :
√s = 87.9 GeV and √s = 94.3 GeV
FCC week Berlin 30/05/17
P. Azzurri -- FCee experimental EW Z and W capabilities
9
Determination of aQED(mZ2)
Two measurements :
Solve for a0 = aQED(mZ2)
+
De term ina tio n of ∆ α had
9/11
+
f
e
1 e µ
e µ′
e ′ µ IFI at better than 1% to reach the
S=
gγ gγ + gγ gZ + gZ gγ + Sb ox
M Z2
2
g
-
required precision eon aQED(mZ ) :f
work in progress +
f
e
Current status: full 1-loop for gγℓ , gZℓ ′ , Sb ox
Bardin et al. ’99 (ZFITTER 6.21)
µµ
A F B ( s2 )
e
-
Z
Box + Vertex
EW correction
e
capabilities
e-
µµ
A F B ( s1 )
Relative impact on
−
√
√
FCC week Berlin 30/05/17
Azzurri
-- FCee
Z and
for s1 = P.88
GeV
, experimental
s2 = 95 EW
GeV
: W
+
W
W
f
f
f
10
the OkuW
√s=161 : L~4 1035 collect 8/ab ~30 106 WW decays
√s=240 : L~0.9 1035 collect 5/ab ~80 106 WW decays
FCC week Berlin 30/05/17
P. Azzurri -- FCee experimental EW Z and W capabilities
11
WW threshold
At LEP2 √s=161 GeV σ=4pb
ε=0.75, σB=300 fb
p=0.9 : εp≈0.68 (@161)
 mW=80.40±0.21 GeV
with 11/pb @ECM=161 GeV
-1
æ ds ö
DmW = ç
÷ Ds
è dmW ø
FCC week Berlin 30/05/17
P. Azzurri -- FCee experimental EW Z and W capabilities
12
mW from σWW: sensitivity vs ECM
σWW with YFSWW3 1.18
statistical precision
with L = 11/pb  Δmw≈350MeV
with L= 8/ab  Δmw≈0.40 MeV
need syst control on :
• ΔE(beam)<0.40 MeV (5x10-6)
• Δε/ε, ΔL/L < 10-4
• ΔσB<0.7 fb (2 10-3)
Max stat sensitivity at √s~2mW+600 MeV
√εp with fixed : ε=0.75 and σB=0.3pb
FCC week Berlin 30/05/17
P. Azzurri -- FCee experimental EW Z and W capabilities
13
ΓW from σWW
Measure σww in two energy points E1, E2
with a fraction f of lumi in E1
 determine both mW & ΓW
Determine f, E1, E2 such to mimimise (ΔΓW,
ΔmW) with some target
Evaluate loss of ΔmW precision in the single
parameter (mW) determination
wrt scenario of running only at an optimal
E0=161 point
dσWW/dΓW =0
at ECM~162.3 GeV
~2mW + 1.5 GeV
FCC week Berlin 30/05/17
P. Azzurri -- FCee experimental EW Z and W capabilities
14
mW &ΓW from σWW
D mW , D G W (MeV)
157.1 GeV
7
6
5
4
3
2
1
0.1 0.2
0.3
0 (D mW, DGW) correlation
-1
0
ΔΓW
0.4
8/ab
0.5
0.6
0.7 0.8 0.9 1
luminosity fraction
ΔmW
ΔmW
162.3 GeV
scan of lumi fraction at higher energy
ΔmW , ΔΓW: error on W mass and width from fitting both
ΔmW : error on W mass from fitting only mW
with E1=157.1 GeV E2=162.3 GeV f=0.4
ΔmW=0.62 ΔΓW=1.5 ΔmW=0.56 (MeV)
FCC week Berlin 30/05/17
ΔαS≈(3 π/2)ΔΓ/Γ≈ 0.003
P. Azzurri -- FCee experimental EW Z and W capabilities
15
acceptance
how do we control acceptance at the 10-4 level (0.01%) ?
 aim for the highest possible acceptance and efficiency WP
• lepton tracking reco efficiency (was controlled at the 10-3 level at LEP2)
• lepton identification performances
• @LEP2 10-3 level: (T&P with Z): effects on total Δσ mitigated down to
the 2-3 10-4 level thanks to te,u channel migrations recoveries
• would need lepton-id at 10-4 level for max BR precision
• jet reconstruction and energy calibration
• @LEP2 1-2% level  0.1% on Δε:
• FCCee would need calibration at 0.1% level (10x better) with control
data ; best possible jet energy resolution helps
• missing momentum scale/resolution : similar to jet energy for qqlv
• lepton isolation
• @LEP2 control at the Δε~2 10-3 level: need to do 10x better
• jet modeling (signal & bkg)
• was important syst on σWW@LEP2 (at the 2 10-3 level)
impact of theoretical uncertainties will hopefully not be limiting
but work is needed to reach the target 0.5 10-3 precision level
FCC week Berlin 30/05/17
P. Azzurri -- FCee experimental EW Z and W capabilities
16
background control
decay
efficiency
purity
bkg
70-80%
80-90%
50fb (tt,tt,Z*ll)
eqq
85%
~90%
30fb (qq, Zee, Z*) -10fb (We)
mqq
90%
~95%
10fb (Z*,qq)
tqq
50%
80-85%
50fb (qq, Z*)
qqqq
90%
~90%
~200fb (qq (qqqq,qqgg))
2-fermion : tt, qq
ll
4-fermion : tt,ll, Zee, We
some 4f bkg is identical to the
signal final state  CC03-4f
interferences
measure directly the backgrounds with very
different S/B levels at different ECM points
measure forward electrons (θ≤0.1 rad) for
Zee Wev : determine forward pole
dσ/dθ and WW interference effects
acceptance down to θ=0.1 [cosθ= 0.995]
would also cover forward jets
FCC week Berlin 30/05/17
[LEP1996]
concern is mostly on the
four-jet background
limiting correlated systs
can cancel out taking data at more
ECM points where
-1
æ ds ö
ç
÷
è dGW ø
-1
æ ds ö
ç
÷
è dmW ø
-1
-1
æ ds ö
æ ds ö
÷ s
ç
÷ sç
è dGW ø
è dmW ø
differential factors are equal
P. Azzurri -- FCee experimental EW Z and W capabilities
17
W BR
Winter 2005 - LEP Preliminary
Winter 2005 - LEP Preliminary
W Leptonic Branching Ratios
W Hadronic Branching Ratio
23/02/2005
10.78 ±
10.55 ±
10.78 ±
10.40 ±
ALEPH
DELPHI
L3
OPAL
23/02/2005
0.29
0.34
0.32
0.35
10.65 ± 0.17
LEP W®en
10.87 ±
10.65 ±
10.03 ±
10.61 ±
ALEPH
DELPHI
L3
OPAL
0.26
0.27
0.31
0.35
ALEPH
67.13 ± 0.40
DELPHI
67.45 ± 0.48
L3
67.50 ± 0.52
OPAL
67.91 ± 0.61
10.59 ± 0.15
LEP W®mn
11.25 ±
11.46 ±
11.89 ±
11.18 ±
ALEPH
DELPHI
L3
OPAL
8/ab@160GeV + 5/ab@240GeV
 30M+ 80M W-pairs
67.48 ± 0.28
LEP
0.38
0.43
0.45
0.48
 ΔBR(qq) (stat) =[1.5] 10-4 (rel)
 ΔαS≈(9 π/2)ΔBR≈2 10-3
 ΔBR(lv)(stat)=[3]10-3 (rel)
c /ndf = 15.4 / 11
2
11.44 ± 0.22
LEP W®tn
c /ndf = 6.3 / 9
2
66
10.84 ± 0.09
LEP W®ln
68
70
Br(W®hadrons) [%]
c2/ndf = 15.4 / 11
10
11
q/ l universality at 0.6%
 FCCee @ 10-4 level
12
Br(W®ln) [%]
Lept universality test at 1% level
tau BR ~2.7 σ larger than e/mu
 FCCee @ 2 10-4 level
will need much better control of lepton id
i.e. cross contaminations in signal channels
( τe,μ in the e,μ channels and v.v. )
Flavor tagging would also allow to measure coupling to c & b-quarks (Vcs, Vcb,.. )
FCC week Berlin 30/05/17
P. Azzurri -- FCee experimental EW Z and W capabilities
18
reco mass of the W boson
250
mnqq channel
WW
200
qq
ZZ
150
ALEPH
Number of events per GeV/c2
ALEPH
Number of events per GeV/c2
Number of events per GeV/c2
full mW reco with kinematic fit. main ingredients :
ECM – jet/lepton angles – (jet boost )
225
200
enqq channel
WW
175
qq
150
ZZ
125
100
ALEPH
700
600
4q channel
WW
qq
500
ZZ
400
300
100
75
200
50
50
100
25
0
30
40
50
60
70
80
90
100 110 120
2
2C Mass (GeV/c )
0
30
40
50
60
70
80
90
100 110 120
0
30
40
50
60
70
2
2C Mass (GeV/c )
80
90
100 110 120
2
5C Mass (GeV/c )
ALEPH Eur.Phys.J.C47:309 (2006) : 683 /pb ~10k WW events
ignoring low energy particles in the qqqq channel
mW = 80440±43(stat.)±24(syst.)±9(FSI)±9(LEP) MeV
ΓW = 2140 ±90(stat.) ±45(syst.) ±46(FSI) ± 7(LEP) MeV
FCC week Berlin 30/05/17
P. Azzurri -- FCee experimental EW Z and W capabilities
19
reco mass of the W boson
8/ab@160GeV + 5/ab@240GeV
 30M+ 80M W-pairs
 ΔmW (stat)= 0.5 MeV
 ΔmW (syst) ≤ 1 MeV ?
Is ΔEbeam<1MeV at
ECM=240 GeV possible ?
With Zγ events ?
Do WW events produced
near threshold carry kin
information on mW ?
jet boost
lepton and jet uncertainties
from (Z) calibration data
FCC week Berlin 30/05/17
P. Azzurri -- FCee experimental EW Z and W capabilities
20
TGCs at threshold
We
SU(2)U(1) Gauge Cancellations
β
WWZ
β3
WW
without TGCs
σWW +40% @157GeV +25%@162GeV
while Δσ/σ ≈ 0.5
FCC week Berlin 30/05/17
10-3
add here some rough
extrapolations from LEP2 limts ?
P. Azzurri -- FCee experimental EW Z and W capabilities
21
Conclusions
•
•
•
FCC ee is a total game-changer for EW W/Z physics measurements
No “a priori” walls on the road map to achieve the FCC goals for EW precision
measurements but a lot of work, firstly on the theoretical calculations side
At the Z, off peak data will play an important role (more than at LEP times)
–
•
The WW threshold lineshape is a great opportunity to measure both mW and ΓW:
–
•
•
optimal points to take data are √s=2mw+1.5 GeV (Γ-insensitive) and √s=2mw-2-3 GeV (-Γoff shell)
Huge potential for other W physics measurements including higher energy data still
need to be explored with attention
–
•
can deliver aQED(mZ2) to 3 × 10-5
direct mW , W BRs, TGCs
Work from experimentalist needed to evaluate with care limiting systematics, study
ways to overcome them, and reflect on the detector design consequences:
opportunities to contribute
The potential of FCCee data for EW Z/W measurements needs to be fully unraveled
FCC week Berlin 30/05/17
P. Azzurri -- FCee experimental EW Z and W capabilities
22