Precision EW measurements at Future
accelerators
‘Will redo te LEP program in a few minutes…. ’
15 July 2015
Alain Blondel Precision EW measurements
at future accelerators
1
1994-1999: top mass predicted (LEP, mostly Z mass&width)03/94
top quark discovered (Tevatron) 06/95
t’Hooft and Veltman get Nobel Prize 10/98
(c) Sfyrla
1997-2013
Higgs boson mass cornered (LEP H, MZ etc +Tevatron mt , MW)
Higgs Boson discovered (LHC)
Englert and Higgs get Nobel Prize
(c) Sfyrla
Is it the end?
Is it the end?
Certainly not!
-- Dark matter
-- Baryon Asymmetry in Universe
-- Neutrino masses
are experimental proofs that there is more
to understand.
We must continue our quest
HOW?
1. ELECTROWEAK PRECISION TESTS (EWPT)
Due to the non-abelian Gauge theory, Electroweak observables offer sensitivity to
electroweakly coupled new particles ...
-- if they are nearby in Energy scale
or
-- if they violate symmetries of the Standard Model (in which case, no «decoupling»)
Higgs boson and top-bottom mass splitting constiture such symmetry violations
2. TESTS OF ELECTROWEAK SYMMETRY BREAKING (EWSB)
Is the H(125) a Higgs boson?
 couplings proportional to mass?
if not could be more complicated EWSB e.g. more Higgses
 Higgs supposed to cancel WW scattering anomalies at TeV scale
does this work?
15 July 2015
Alain Blondel Precision EW measurements
at future accelerators
6
EWRCs
relations to the well measured
GF mZ aQED
at first order:
Dr = a /p (mtop/mZ)2
- a /4p log (mh/mZ)2
e3 = cos2qw a /9p log (mh/mZ)2
dnb =20/13 a /p (mtop/mZ)2
complete formulae at 2d order
including strong corrections
are available in fitting codes
e.g. ZFITTER , GFITTER
Alain Blondel
WIN 05 June 2005
The main players
Inputs:
GF = 1.1663787(6) × 10−5 /GeV2
MZ = 91.1876 ± 0.0021 GeV
α = 1/137.035999074(44)
from muon life time
Z line shape
electron g-2
6 10-7
2 10-5
3 10-10
EW observables sensitive to new physics:
MW = 80.385 ± 0.015
sin2qWeff = 0.23153 ± 0.00016
LEP, Tevatron
WA Z pole asymmetries
2 10-4
7 10-4
Nuisance paramenters:
a (MZ) =1/127.944(14)
hadronic corrections
1.1 10-4
to running alpha
aS (MZ) =0.1187(7)
strong coupling constant
7 10-3
mtop = 173.34 ± 0.76 GeV
from LHC+Tevatron
4 10-3
combination
mH
= ATLAS 125.36 ± 0.37 (stat) ± 0.18 (syst) GeV 125.17 ± 0.25 2 10-3
CMS 125.03 ± 0.26 (stat) ± 0.14 (syst) GeV
15 July 2015
Alain Blondel Precision EW measurements
at future accelerators
8
FUTURE ACCELERATORS
1. High Luminosity LHC (3000 fb-1 @ 14 TeV)  2035
An essentially approved program
2. ILC as GigaZ, MegaW, Higgs and top factory
A very ‘mature’ study of a new technique
3. Circular e+e- Z,W,H,top factories
A «young» study of a very mature technique
4. 100 TeV hadron collider
$$$$$$$$$$$
15 July 2015
Alain Blondel Precision EW measurements
at future accelerators
9
SNOWMASS report
References:
LEP Z peak paper arXiv:hep-ex/0509008
Phys.Rept.427:257-454,2006
LEP2 Electroweak paper arXiv:1302.3415 [hep-ex]
Phys. Rep.
Gfitter Group arXiv:1209.2716v2
The Electroweak Fit of the Standard Model after the
Discovery of a New Boson at the LHC
J. Erler and P. Langacker
ELECTROWEAK MODEL AND CONSTRAINTS ON NEW
PHYSICS PDG dec 2011 «and references therein»
15 July 2015
Alain Blondel Precision EW measurements
at future accelerators
10
15 July 2015
Alain Blondel Precision EW measurements
at future accelerators
11
15 July 2015
Alain Blondel Precision EW measurements
at future accelerators
12
NB (AB): time scale (2030++) is typical of any new machine @ CERN or with CERN contribution;
no real funding until HL-LHC upgrade is complete.
15 July 2015
Alain Blondel Precision EW measurements
at future accelerators
13
http://cern.ch/fcc and
http://cern.ch/fcc-ee
first
15 July 2015
AlainNB
Blondel
Precision
EWscale
measurements
(AB):
time
for FCC-ee
at future accelerators
similar to CLIC (2030++)
14
15 July 2015
Alain Blondel Precision EW measurements
at future accelerators
15
Goal performance of e+ e- colliders
FCC-ee as Z factory: 1012 Z
(possibly 1013 with crab-waist)
possible
upgrade
complementarity
ww
NB: ideas for lumi upgrades:
-- ILC arxiv:1308.3726 (not in TDR).
Upgrade at 250GeV by reconfiguration after 500 GeV running; under discussion)
-- FCC-ee (crab waist)
16
At the end of LEP:
Phys.Rept.427:257-454,2006
Nn = 2.984 0.008
- 2  :^) !!
This is determined from the Z line shape scan
and dominated by the measurement of the
hadronic cross-section at the Z peak maximum 
The dominant systematic error is the theoretical
uncertainty on the Bhabha cross-section (0.06%)
which represents an error of 0.0046 on Nn
Improving on Nn by more than a factor 2 would require a large effort
to improve on the Bhabha cross-section calculation!
Neutrino counting at TLEP
given the very high luminosity, the following measurement can be performed
Beam polarization and E-calibration @ TLEP
Precise meast of Ebeam by resonant depolarization
~100 keV each time the meast is made
At LEP transverse polarization was achieved routinely at Z peak.
instrumental in 10-3 measurement of the Z width in 1993
led to prediction of top quark mass (179+- 20 GeV) in March 1994
Polarization in collisions was observed (40% at BBTS = 0.04)
At LEP beam energy spread destroyed polarization above 60 GeV
E  E2/r  At TLEP transverse polarization up to at least 80 GeV
to go to higher energies requires spin rotators and siberian snake
TLEP: use ‘single’ bunches to measure the beam energy continuously
no interpolation errors due to tides, ground motion or trains etc…
<< 100 keV beam energy calibration around Z peak and W pair threshold.
DmZ ~0.1 MeV, DZ ~0.1 MeV, DmW ~ 0.5 MeV
350 GeV: the top mass
• Advantage of a very low level of beamstrahlung
• Could potentially reach 10 MeV uncertainty (stat) on mtop
From Frank Simon, presented at 7th TLEP-FCC-ee workshop, CERN, June 2014
A Sample of Essential Quantities:
TLEP stat
Syst Precision
X
Physics
Present
precision
MZ
Input
91187.5
2.1
Z Line shape
scan
Z
Dr (T)
(no Da!)
2495.2
2.3
Rl
as , db
Nn
TLEP key
Challenge
0.005 MeV
<0.1 MeV
E_cal
QED
corrections
Z Line shape
scan
0.008 MeV
<0.1 MeV
E_cal
QED
corrections
20.767
 0.025
Z Peak
0.0001
 0.002
- 0.0002
Statistics
QED
corrections
Unitarity of
PMNS,
sterile n’s
2.984
0.008
Z Peak
0.00008
0.004
0.001
->lumi meast
QED
corrections to
Bhabha scat.
Rb
db
0.21629
0.00066
Z Peak
0.000003
Statistics,
0.000020 - 60 small IP
Hemisphere
correlations
ALR
Dr, e3 ,Da
(T, S )
0.1514
0.0022
Z peak,
polarized
0.000015
4 bunch
scheme
Design
experiment
MW
Dr, e3 , e2, Da 80385
(T, S, U)
± 15
Threshold
(161 GeV)
0.3 MeV
<1 MeV
E_cal &
Statistics
QED
corections
mtop
Input
Threshold
scan
10 MeV
E_cal &
Statistics
Theory limit
at 100 MeV?
MeV/c2
MeV/c2
MeV/c2
MeV/c2
Z+(161 GeV)
173200
± 900
Statistics
Theoretical limitations
FCC-ee
R. Kogler, Moriond EW 2013
SM predictions (using other input)
0.0005
- 0.001
0.000002
0.0005
0.0001
0.0005?
0.0000
0.000003
0.000001
0.000001?
0.000000
0.0005?
0.000003?
Experimental errors at FCC-ee will be 20-100 times smaller than the present errors.
BUT can be typically 10 -30 times smaller than present level of theory errors
Alain Blondel Precision EW measurements
Will require significant theoretical
effort
and additional measurements!
at future accelerators
22
The Higgs
15 July 2015
Alain Blondel Precision EW measurements
at future accelerators
23
H
Full HL-LHC
W
b


Z
t
Higgs Production Mechanism in e+ e- collisions
Light Higgs is produced by “Higgstrahlung” process close to threshold
Production xsection has a maximum of ~200 fb
TLEP: 2. 1035/cm2/s  400’000 HZ events per year (2 million Higgses in 5 years)
Z – tagging by missing mass
e-
H
Z*
e+
Z
For a Higgs of 125GeV, a centre of mass energy of 240GeV is sufficient
 kinematical constraint near threshold for high precision in mass, width, selection purity
ILC
Z – tagging
by missing mass
total rate  gHZZ2
ZZZ final state  gHZZ4/ H
 measure total width H
empty recoil = invisible width
‘funny recoil’ = exotic Higgs decay
easy control below theshold
e-
H
Z*
e+
Z
the
8B$ ILC
This will remain the reserved domain of the hadron colliders with HL-LHC and FCC-hh!
15 July 2015
Alain Blondel Precision EW measurements
at future accelerators
31
Outlook
Future colliders will improve the precision on Electroweak Precision Tests by one to two
orders of magnitude, providing inclusive probe of the existence new, weakly coupled, physics.
HL LHC will contribute to map the relative Higgs couplings
including ttH (4%) and HHH (30%/exp?)
Further improvements can be expected (Tevatron, LHC) for mW (5 MeV?) and mtop (500 MeV?)
e+e- colliders provide
-- invisible Higgs width and absolute coupling normalization at the ZH thr,
-- top mass with <100 MeV precision.
-- W mass at threshold and sin2 qWeff
Circular collider can improve Z mass and width (<0.1 MeV) and mW (beam energy calibration)
and generally provide higher statistics  invisible widths of Higgs and Z bosons.
 another order of magnitude
HHH coupling will remain above 10% level until the 100 TeV collider.
WW scattering is best done at hadron colliders
More theoretical work and dedicated measurements will be required to match improving
experimental
errors!
Alain Blondel Precision EW measurements
15 July 2015
32
at future accelerators
Status of Tevatron W mass
PRL 108 (2012) 151803 PRD 89 (2014) 072003
• CDF and DØ have world’s most precise
measurements based on 20% and 50%
of their data → 1.1M and 1.7M Ws,
resp.
• MT is the most sensitive single
variable, lepton PT and MET used also
• Precision lepton response (0.01%) and
recoil models (1%) built up from Z
dileptons, Z mass reproduced to 6X
LEP precision
• MW precision:
• CDF 19 MeV,
• DØ 23 MeV,
• LEP2 33 MeV
• 2012 world average: 15 MeV
33
Prospects for Tevatron W mass
arxiv:1310.6708
projected
• Largest single uncertainties are
stat. and PDF syst.
• 2X PDF improvement and
incremental improvement
elsewhere results in 9 MeV
projected final Tevatron
precision
• <10 MeV precision is well
motivated to further confront
indirect precision (11 MeV)
34
Prospects for LHC W mass
Phys.Rev.D83:
113008,2011
• The LHC has excellent detectors and
semi-infinite statistics and thus has a
good a priori prospect for a <10-MeV
measurement
• Biggest three obstacles to surmount:
• PDFs: sea quarks play a much
stronger role than the Tevatron.
Need at least 2X better PDFs.
• Momentum scale
• Recoil model/MET
arxiv:1310.6708
35
Higgs factory performances
Precision on couplings, cross sections, mass, width, Summary of the ICFA
HF2012 workshop (FNAL, Nov. 2012) arxiv1302:3318 (as available at the time)
Coupling measurements @HL-LHC
precision 1-4% with 3000 fb-1
LC adds Inv + total
widths at % level
Circular Higgs Factory
precision at few permil level.
NB without TLEP the SM line would have a 2.2 MeV width
in other words .... D(Dr)=  610-6 +several tests of same precision
The LHC is a Higgs Factory !
1M Higgs already produced – more than most other Higgs factory projects.
15 Higgs bosons / minute – and more to come (gain factor 3 going to 13 TeV)
Difficulties: several production mechanisms to disentangle and
significant systematics in the production cross-sections prod .
Challenge will be to reduce systematics by measuring related processes.
if
observed
 prod (gHi )2(gHf)2 extract couplings to anything you can see or produce from
H
if i=f as in WZ with H ZZ  absoulte normalization
Example
(from Langacker, Erler PDG 2011)
Dρ =e1=a(MZ) . T
e3=4 sin2θW a(MZ) . S
From the EW fit
Dρ = 0. 0004+0.0003−0.0004
-- is consistent with 0 at 1 (0= SM)
-- is sensitive to non conventional Higgs bosons (e.g. in SU(2) triplet with ‘funny v.e.v.s)
-- is sensitive to Isospin violation such as mt  mb
Measurement implies
15 July 2015
Alain Blondel Precision EW measurements
at future accelerators
39
Similarly
Would be sensitive to a doublet of new fermions where Left and Right have different
masses etc… (neutrinos are already included)
Note that often EW radiative corrections do not decouple with
mass => a very powerful tool of investigation
Dr = a /p (mtop/mZ)2 - a /4p log (mh/mZ)2
e3 = cos2qw a /9p log (mh/mZ)2
dnb =20/13 a /p (mtop/mZ)2
15 July 2015
Alain Blondel Precision EW measurements
at future accelerators
40
Back to the future
30 years later and with experience gained on LEP, LEP2 and the B factories
we can propose a Z,W,H,t factory of many times the luminosity of LEP, ILC, CLIC
CERN is launching a 5 years international design study of Circular Colliders
100 TeV pp collider (FCC-hh) and high luminosity e+e- collider (FCC-ee)
IHEP in China is studying CEPC a 50-70 km ring, e+e- Higgs factory followed by HE pp.
NB QED!
@ Z pole
amount for
0.3.MeV on mZ
LHC  5 MeV
(0.1)
0.15
0.1
15 July 2015
Alain Blondel Precision EW measurements
at future accelerators
1.5
1.8
42
Higgs Physics with e+e- colliders above 350 GeV?
1. Similar precisions to the 250/350 GeV Higgs factory for W,Z,b,g,tau,charm,
gamma and total width. Invisible width best done at 240-250 GeV.
2. ttH coupling possible with similar precision (2% full ILC) as HL-LHC (4%)
3. Higgs self coupling also very difficult… precision
~20% at 1 TeV similar to HL-LHC prelim. estimates (30% each exp)
10-20% at 3 TeV (CLIC)
 percent-level precision needs 100 TeV pp machine
 For the study of H(126) alone, and given the existence of HL-LHC, an e+ecollider with energy above 350 GeV is not compelling w.r.t. one working in
the 240 GeV – 350 geV energy range.
 The stronger motivation for a high energy e+e- collider will exist if new
particle found (or inferrred) at LHC, for which e+e- collisions would bring
substantial new information