A. Blondel EW measurements and neutrinos
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26.04.2023
Electroweak Precision Measurements
and Neutrinos
A. Blondel EW measurements and neutrinos
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26.04.2023
Motivation for the precision measurements *and* precision calculations
1. Given that the minimal SM is complete with the Higgs discovery, how do we find out: �-- if the Higgs boson is exactly what is foreseen by the standard model? (🡪 Higgs Factory)�-- where/what are the new physics phenomena that must be present to explain:
baryon asymmetry
dark matter,
neutrino masses (and other mysteries we don’t understand) (🡪 EW/top factory)��2. A powerful and broadly efficient method is to perform precision EW measurements�-- many observables contain sensitivity to new phenomena, either by loops, direct long distance propagator effects, or mixing with SM coupled particles.
🡺 are there any more weakly coupled particles?
The top quark effect at LEP was 10σ! (🡺 there is *not* another t-b quark system)
any custodial SU(2)-violating effect appears regardless of mass scale
-- is there mixing ? in particular active-sterile neutrino mixing
-- high mass SM-coupled and custodial SU(2)-respecting 🡪 (ex: Z’ or degenerate SuSy)
Emphasis on different observables depending on the question asked.
«T»
«S»
«ν»
not to forget:
QCD
Lepton-quark
lepton and quark family
Universality
Veltman: Δρ =ΔT.α=
α/π . (m2top-m2b)/m2W
Alain Blondel Groupe Neutrino Université de Genève
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NEUTRINOS
The fact that neutrinos have mass is THE observed BSM phenomenon there is in particle physics
It is intimately a Higgs Physics question:
🡺 what are the couplings of the H(125) (Nobel 2013) to massive neutrinos (Nobel 2015)?
A. Blondel EW measurements and neutrinos
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Neutrino masses occur via processes which are intimately related to the Higgs boson
🡺 what are the couplings of the H(125) to neutrinos?
Adding neutrino masses to the Standard model 'simply' by adding a Dirac mass 🡺 right-handed neutrino
mD is the neutrino Yukawa coupling (like for all known massive fermions). Then the right handed neutrinos are perfectly sterile, (except that they couple to both the Higgs boson and gravitation). *🡪
B. Kayser
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26.04.2023
L
L
L
I = 1/2
Q= -1
Q= 0
I = 0
R
R
R
R
R
R
Electroweak eigenstates
Right handed neutrinos
are singlets
no weak interaction
no EM interaction
no strong interaction
can’t produce them
can’t detect them
-- so why bother? –
Also called ‘sterile’
NB unlike for vL , nothing distinguishes the particle and antiparticle of vR which is a singlet (no ‘charge’)
🡪 naturally a Majorana particle
my SM training in 1976
A. Blondel EW measurements and neutrinos
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26.04.2023
Neutrino masses occur via processes which are intimately related to the Higgs boson
🡺 what are the couplings of the H(125) to neutrinos?
Adding neutrino masses to the Standard model 'simply' by adding a Dirac mass 🡺 right-handed neutrino
mD is the neutrino Yukawa coupling (like for all known massive fermions). Then the right handed neutrinos are perfectly sterile, (except that they couple to both the Higgs boson and gravitation). *🡪
Then things become more interesting: a Majorana mass term arises. So-called Weinberg Operator (only Dim5 operator in EFT) and involves the Higgs boson and the neutrino Yukawa coupling
Pilar Hernandez,
Granada 2019-05
Majorana mass term is extremely interesting as this is the
particle-to-antiparticle transition that we want
in order to explain
the Baryon asymmetry of the Universe
(+ CP violation in e.g. neutrinos)
MR
B. Kayser
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See-saw type I :
MR ≠ 0
mD ≠ 0
Dirac + Majorana
mass terms
MR = 0
mD ≠ 0
Dirac only, (like e- vs e+):
νL νR νL νR
½ 0 ½ 0
4 states of equal masses
m
Iweak=
Some have I=1/2 (active)
Some have I=0 (sterile)
MR ≠ 0
mD = 0
Majorana only
νL νR
½ ½
2 states of equal masses
m
Iweak=
All have I=1/2 (active)
MR > mD ≠ 0
Dirac + Majorana
ν N ν N
½ 0 ½ 0
4 states , 2 mass levels
m
Iweak=
m1 have ~I=1/2 (~active)
m2 have ~I=0 (~sterile)
see-saw
Mass eigenstates
dominantly:
A. Blondel EW measurements and neutrinos
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Manifestations of right handed neutrinos (HNL)
what is produced in W, Z decays is:
-- mixing with active neutrinos leads to various observable consequences in High Energy experiments
If N is not too heavy it will decay in the detector 🡺 POTENTIAL FOR DIRECT DISCOVERY
If it is too heavy to be produced
🡺 PMNS matrix unitarity violation and deficit in Z «invisible» width
🡺 violation of unitarity and lepton universality in Z, W or τ decays
🡺 EW observables that contain neutrinos are modified, GF in particular μ→ e νe νμ
🡺 affect prediction of both {α GF mZ} 🡪 sin2θWeff, mW and all that follows
🡺 tau life time � 🡺 Nv < 3
-- missing neutrino rate is proportional to sin2θmix regardless of HN mass.
Antusch, vdBeij...
A. Blondel EW measurements and neutrinos
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F. Gianotti, �P5 meeting
2023-04-15
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NEUTRINOS -- comments
The impact of the neutrinos having masses is (in the simplest see-saw type I) is either or both
-- a very rare process : e.g. if light neutrino masses are ~50 meV, a HNL of 50 GeV has a mixing of 10-12
-- or small effects: the experiments today are sensitive at the 10-3level. With the improved measurements at FCC the limits will be improved to 10-5 for νe νμ and, thanks to the tau lifetime measurement, ντ
Mixing angles can be somewhat larger, see S. Antuschs talk, and statistics of up to 106 HNLs cannot be excluded.
Other more complicated schemes can be built to increase the chances of observation (but less ‘natural’)
-- the neutrinos are the only fermions for which the Yukawa coupling is not the mass �The Dirac mass in the case of a 50 GeV HNL is typically 1/10 of the electron mass.
This also implies that the process H🡪vN which proceeds by the Yukawa coupling, is very small
The possibility – albeit small – of such a discovery is an exciting motivation for the TeraZ run of FCC.
A. Blondel Low angle cuts for the dilepton and diphoton selections
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« The Standard Model is complete »
This statement is correct in the following sense, which allows to separate ‘SM’ from ‘BSM’
we should distinguish
A. ‘Theory of particle physics’
based on Quantum Field Theory, relativity, quantum mechanics, principles of Gauge invariance etc... using in particular the SU(3)_color ⊗ SU(2)_L ⊗ U(1) gauge groups or extensions thereof. �In itself it is not necessarily predictive, but provides a wide toolset to include further discoveries.
** definitely not complete**, but completeness is not the point here
and
B. « the Standard Model » which is one possible model witihin the above, with a specific set of constituants (fermions and gauge bosons), their couplings, chiralities and their masses, which are all extracted from experiment
It was created (and named in ~1976) after the discovery of the Neutral Currents (1973), Charm (74/76) and the tau and ντ (77-86)
With the discovery of the Higgs boson, the Standard Model is complete and forms a predictive and quantitative tool.
-- assumes neutrinos are massless
-- comprises 3 families of quarks and leptons and a single, elementary Higgs boson, and as such
contains no free parameter (only parametric uncertainties) ANY DEVIATION from SM is BSM DISCOVERY
-- does not explain in a unique way the neutrino masses or the Baryon Asymetry of the Universe, does not comprise a candidate DM particle, etc... for which we know for sure that BSM is needed.
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POTENTIAL FOR DIRECT DISCOVERY of HEAVY NEUTRINOS
Heavy Neutrinos at the FCC
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RH neutrino production in Z decays
multiply by 2 for antineutrino and add contributions of 3 neutrino species (🡺 Σλ=e,μ,τ |Uλ |2 )
Production:
Decay
Decay length:
cm
Backgrounds : four fermion: e+e- 🡪 W*+ W*- e+e- 🡪 Z*(vv) + (Z/γ)*
NB CC decay always leads to
≥ 2 charged tracks
Long life time 🡪 detached vertex for ~<MZ
HNL
Heavy Neutrinos at the FCC
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Production of HNL in Z decays
We begin experimentally by assuming HNL production one at a time.
This is an approximation of the more favored situation where two or three almost degenerate HNLs are produced,
possibly generating a phenomenology akin to e.g. KL and KS or (K <->K) system with oscillations and other lifetime effects.
This is extraordinarily interesting ... see S. Antusch
In the simplified, one-N-at-a-time assumption the particle has one mass, one cross-section and one decay width/life time.
and four decay modes N🡪 eW*, μW*, τW* (CC decays) and N🡪 vZ* (NC decay)-- two or more charged tracks except N->vvv
N🡪 λ W*🡪 qq
N🡪 v Z* 🡪 qq
N🡪 λ+λ(‘)- v
N🡪 λ W*🡪λ’v and
N🡪 v Z*🡪 λ λ)
N🡪 ν νν
0.6
0.5
0.4
0.3
0.2
0.1
0.0
BRANCHING
RATIO
10 20 30 40 50 60 70 80
HNL Mass (GeV/c2)
Tanishq Sharma
Heavy Neutrinos at the FCC
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50 GeV HNL with a mixing angle of 10-10, decaying 50cm from the collision point into
N🡪 e- {W*🡪 hadrons}.
Heavy Neutrinos at the FCC
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This is the FCC CDR plot – as anticipated the limit stops at the W mass,
when life times get very short due to on-shell W decay.
The horizontal line corresponds to GF effect on EWPOs
Plot by O. Fischer for the FCC-ee CDR with 1012Z
Heavy Neutrinos at the FCC
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This picture from the ESPP BB is relevant to Neutrino, Dark sectors and High Energy Frontiers. � FCC-ee (Z) compared to the other machines for right-handed (sterile) neutrinos
How close can we get to the ‘see-saw limit’?
-- the purple line shows the 95% CL limit if no HNL is observed. (here for 1012 Z),
-- the horizontal line represents the sensitivity to mixing of neutrinos to the dark sector,
using EWPOs (GF vs sin2θWeff and mZ, mW, tau decays) which extends sensitivity from 10-3 (now)
to 10-5 (FCC) mixing all the way to very high HNL masses (500-1000 TeV at least). arxiv:2011.04725
Heavy Neutrinos at the FCC
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Dirac vs Majorana(I)
It has been emphasized by Matthew Mccullough that there exist ways to create models of Dirac HNL-like particles
and that the discovery of a RHv requires the observation of the Majorana nature of the particle.
In any case Fermion number violation is of the greatest interest.
At a hadron or ep collider the HNL can appear in W decay W(*)🡪 λ+N 🡪 λ’± W*+ . The charge of the final vs initial lepton is a test of fermion number violation which arises if HNL are Majorana particles.
At the Z the process Z🡪 Nv does not make it very easy.
Several methods
🡺 uses N🡪 λqq decay and requires lepton charge reconstruction.
🡺 uses N🡪 λqq and requires lepton momentum reconstruction (but not the charge)
NB analysis sensitive to detail W* mass distribution, esp. for small masse W* (in tau & D mass region and below)
These methods work for the prompt analysis as well as for the LLP analysis within presumably a smaller radius.
Heavy Neutrinos at the FCC
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Drewes, ICHEP
Heavy Neutrinos at the FCC
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Heavy Neutrinos at the FCC
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Dirac vs Majorana(II)
in addition the lifetime is reduced by a factor 2 for a Majorana vs Dirac particle
CMD= 1(Dirac), 2(Majorana)
CAVEATs!!
The other methods (polarization and asymmetry) can be always used – 400 events are needed for 2σ separation
Heavy Neutrinos at the FCC
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Increasing the detection efficiency with large detectors
Heavy Neutrinos at the FCC
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12m high
5m radius,
assume some
muon chambers outside
CLD detector
Heavy Neutrinos at the FCC
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HECATE DETECTOR TO FILL THE WHOLE CAVERN
with e.g. RPC or Scintillator modules.
Heavy Neutrinos at the FCC
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4 event lines for LLP signature (Exclusion if you do the search and find nothing!) 5 1012 events
Heavy Neutrinos at the FCC
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Full legend of previous plot
see Alimena et al,
arXiv:2203.05502v3
Heavy Neutrinos at the FCC
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A. Blondel EW measurements and neutrinos
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Electroweak precision measurements and neutrinos
A. Blondel EW measurements and neutrinos
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WW at & above threshold
Z scan
αQED, mZ, GF
sin2θeffW
ΔαQED
mtop
S,T,ν, NP
ratios of partial Z widths
Rlept (e,μ,τ)
Rb
Rc
σhad (mZ)
ΓZ
mtop
T,ν, NP
αs
δvb
partial widths, (gA2 +gV2)(f), Nv
asymmetries
from Parity violating couplings
AFBf
P(τ), AFBpol(f)
(initial of final state)
ALR
mW
mtop
S,T, NP
(gV/gA)(f)
ΓW
part. widths
αs Vcs, Vcb
τ
(mass)�BR,
lifetime
αs
Overview of EW relationships and examples of new physics effects
ν
ILC GigaZ long. Pol.�FCC 21011taus
FCC TeraZ
statistics,
precise ECM
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NP Sensitivity by oblique/vertex loops or mixing
the 91km circumference is broadly optimal for this
aim at reducing to the level of statistics
Precision EW measurements @ FCC is the SM complete?
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ILC GigaZ and W runs
strong point is the availability of longitudinal polarization
for both e- and e+!
7 points Z scan ... No Nv measurement.
Energy calibration only at point-to-point level �to be sure that the peak is really the Z peak
measure the Z width, check that expt is running on Z pole.
~10-4 mixing limits on neutrino mixing for e, mu neutrinos
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The Jewel on the ILC crown
Measuring sin2θWeff (mZ)
sin2θWeff ≡ ¼ (1- gV/gA)
gV = gL + gR
gA = gL - gR
Alain Blondel
WIN 05 June 2005
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A. Blondel EW measurements and neutrinos
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Here the uncertainty is clearly
detector dependent. Detectors with
highly granular EM calorimeter
and efficient tracker (TPCs)
(ALEPH and DELPHI) fared better that
drift chamber + cristal/leadglass blocks.
and should have heavy impact on detector
design especially the EM calorimeter
(granular rather than high energy resolution.
Δsin2θeff 🡪 ±2 10-6 (abs)
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FCC scan points for mZ and mW
90 ab-1
30 ab-1
30 ab-1
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arXiv:2106.13885
Electroweak measurements:
preliminary systematics (book-keeping) have been given
NEXT: aim at reducing systematics to the level of statistics
FCC Ongoing:
-- dedicated EPOL group to improve ECM calibration using resonant depolarization: � targets: -- reduction of point to point uncertainties 40🡪 5 keV 🡺 4 keV Z width precision
-- reduction of uncertainty at W mass
-- understand fundamental errors between spin resonance frequency and beam energy.� -- improve running scheme accordingly.
big steps achieved already.
-- assess possibility to reduce systematics on dilepton and diphoton selections to statistical level
-- requires uncertainty on the cut on low angle acceptance at the level of 2.5 microradians (6 microns)
🡪 two methods proposed (complementary): detector construction and in-situ calibration
-- target reduce error on Rlept = Γhad/Γlept relative: from (LEP 1.2 10-3) 🡪 CDR 5 10-5 🡪 stat 2.10-6
reduce luminosity determination uncertainty using ee🡪 γγ events to 10-5 absolute
-- improve flavour tagging and uncertainties to reduce Rb= = Γb/Γhad uncertainty
(LEP 0.2163±0.000600) 🡪 CDR 0.000060 🡪 stat 0.0000003 (reduction wrt LEP by factor 2000! )
-- τ lifetime (discussing 2 10-5 on Gτ see M. Dam for the challenge)
-- include all leptons in determination of αQED (mZ) 3 10-5 🡪 anther factor 2?
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About Precision Measurements...
Recent CDF: mW (MeV)= 80’433.5 ± 6.4 stat ± 6.9syst (10-4 precision)
-- « could hint at new physics » and surely created a buzz!
-- precision measurements as broad exploration of new physics in quantum corrections, or mixing (SUSY, Heavy neutrinos, etc..)
(-- questions because inconsistent with previous measurements)
CDF measurement is remarkable in several ways:
systematic errors similar to statistical precision
all measured in e+e- colliders...
using resonant depolarization!
3. emphasizes the need for more than one experiment
Resonant depolarization is the cornerstone of the precision programme of FCC-ee
🡺 Improvement by factor 10-1000 on a long list of precision measurements.
e.g. W mass down to ±250 keV, Z mass and width ±4 keV, sin2θW eff ± 2.10-6 etc..
🡺 explore new physics at 10-100 TeV scale, or 10-5 mixing with known particles.
~40 times more
precise than CDF
factor 500 more
precise than LEP
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FCC-ee beam polarization and
centre-of-mass energy calibration
arXiv:1909.12245
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Beam Polarization can provide two main ingredients to Physics Measurements
by resonant depolarization
🡪 low level of polarization is required (~10% is sufficient)
🡪 at Z & W pair threshold comes naturally σE ∝ E2/√ρ
🡪 at Z use of asymmetric wigglers at beginning of fills � since polarization time is otherwise very long (250h🡪 ~1h)
🡪 should be used also at ee → H(126)
🡪 use ‘single’ non-colliding bunches and calibrate continuously
during physics fills to avoid issues encountered at LEP
🡪 Compton polarimeter for both e+ and e-
🡪 should calibrate at energies corresponding to half-integer spin tune
🡪 must be complemented by analysis of «average E_beam-to-E_CM» relationship
For beam energies higher than ~90 GeV can use ee → Z γ or ee → WW events
to calibrate ECM at ±1-5 MeV level: mH (5 MeV) and mtop (20 MeV) measts
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Beam Polarization can provide two main ingredients to Physics Measurements
2. Longitudinal beam polarization provides chiral e+e- system
-- High level of polarization is required (>40% )
-- Must compare with natural e+e- polarization due to chiral couplings of electrons (15%)
or with final state polarization analysis for CC weak decays (100% polarized) (tau and top)
-- Physics case for Z peak is very well studied and motivated: � ALR = Ae , AFBPol(f) etc… (CERN Y.R. 88-06)
figure of merit is L.P2 --> must not lose more than a factor ~10 in lumi.
self calibrating polarization measurement requires controlled e+ and e- polarization
at high statistics AFBPol = Ae plays the role of ALR (Tenchini)
-- enhance Higgs cross section (by up to ~30%)
top quark couplings? final state analysis does as well (Janot arXiv:1503.01325)
enhance signal, subtract/monitor backgrounds, for ee→WW , ee →H
-- requires High polarization level and often both e- and e+ polarization
🡺 not interesting If loss of luminosity is too high
-- Obtaining high level of polarization in high luminosity collisions is delicate in top-up mode
DECIDED to FOCUS ON TRANSERSE POLARIZATION FOR ENERGY CALIBRATION
As far as we could check, there is no physics that can be done with longitudinal polarization
that cannot be done without, given enough luminosity
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spin precession (ν is the spin tune)
δθspin = (g-2)/2 . E/m δθtrajectory
= ν . δθtrajectory
ν = Ebeam / 0.4406486
= 103.5 at the Z peak
RESONANT DEPOLARIZATION
Once the beams are polarized,
an RF kicker at the spin precession frequencv
will provoke a spin flip and complete
depolarization
Simulation of FCC-ee by I. Kopp:
A. Blondel FCC-ee EPOL session FCC week 2022
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01/06/2022
4
4
2
3
mW(MeV) 0.250 -- 0.300 --
First set of results obtained in the FCC Design Study:
Polarization and Centre-of-mass Energy Calibration at FCC-ee, arXiv:1909.12245
Next challenges for the feasibility study:
-- Ascertain the above with integrated simulations (simulation of polarization and depolarization on real machine)
-- Match systematic errors with statistics.
most relevant targets : the point-to-point systematics, improve the calibration at WW energy
– these are effects that would lead to a deviation from relation between
-- the spin tune as measured by resonant depolarization
-- and the center-of-mass energy.
-- examples: 1. interference between depolarizing resonances and the induced depolarizing resonance
because the spin tune varies with energy.
2. effects due to collision offsets folded by opposite sign dispersion
-- designevaluate performance and cost the polarimeter at conceptual level
-- finalize implementation in the realistic machine, study operational aspects
stat/present
500
400
75
15 (qualitiative!)
40
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Progress in FCC-ee energy calibration
From FCC week 2022 and FCC EPOL workshop:
Z pole
WW threshold
-- Resonant depolarization measures energy every 15 minutes
at < 50 keV/beam level at Z, 100 keV/beam at W
-- Only one RF station around the ring, + the energy losses of the
two beams are strongly constrained from the direct measurement
of boost at the IPs O(5 keV level) every 8hrs shift in 2/4 experiments
-- beam-beam deflection measurement is extremely sensitive to
beam beam offset and local opposite-sign dispersion �(previously large point-to-point error):�still lots to do but O(20 keV) per measurement every 3second
🡺 targeting to match or go below statistical errors
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2. offset by δy = 0.1σy (=3.5nm)
(Shatilov) in opposite directions for e+ and e-
± 2 μrad x 2.1m = ±4.2 μm
(x1000 demagnification due to optics)
with a very specific pattern of movements
Vertical beam size at the IP: ~35 nm (at Z pole). Vertical offset of 0.1σy leads to additional orbit angles about ±2 μrad for the nominal bunch population 2.5E+11. (D. Shatilov, simulation)
4.2 μm
COLLISION OFFSET
4μrad
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EW Precision - Conclusions
The Electroweak programme at the Future Higgs/Electroweak/top factories is stunningly precise and effective search for
the existence of new particles with SM couplings or mixing with SM (esp neutrinos) at the level of 10-5
any new significant signal will be discovery.
Precision = discovery potential! �
-- from 1 to 3 orders of magnitude improvements in experimental precision, over a large number of observables.
-- the full exploitation of these capabilities will require creativity for detector design and accelerator operations,
and a coordinated campaign of measurements (noise parameters, ancillary measaurements)
-- Matching the mind-boggling statistical precision with experimental systematics and theoretical accuracy will require
a large number of clever tricks and careful methodology
-- a great opportunity for early carrier scientists (acc. exp. and th. alike) to
-- have fun! � -- make a name for themselves and produde or contribute to original contributions.
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NEUTRINOS -- conclusions
The impact of the neutrinos having masses is (in the simplest see-saw type I) is either or both
-- a very rare process : e.g. if light neutrino masses are ~50 meV, a HNL of 50 GeV has a mixing of 10-12
-- or small effects: the experiments today are sensitive at the 10-3level. With the improved measurements at FCC the limits will be improved to 10-5 for νe νμ and, thanks to the tau lifetime measurement, ντ
Mixing angles can be somewhat larger, see S. Antuschs talk, and statistics of up to 106 HNLs cannot be excluded.
Other more complicated schemes can be built to increase the chances of observation (but less ‘natural’)
-- the neutrinos are the only fermions for which the Yukawa coupling is not the mass �The Dirac mass in the case of a 50 GeV HNL is typically 1/10 of the electron mass.
This also implies that the process H🡪vN which proceeds by the Yukawa coupling, is very small
The possibility – albeit small – of such a discovery is an exciting motivation for the TeraZ run of FCC.