10 October 2002 Stefania Xella - RAL
The next linear colliderThe next linear collider
Stefania Xella Rutherford Appleton Laboratory
10 October 2002 Stefania Xella - RAL
SummarySummary
• Motivations for a new linear collider of electrons & positrons at high energy (500 GeV or more)
• Status of different projects• Physics potential of the new linear
collider: precision measurements of the Higgs boson
10 October 2002 Stefania Xella - RAL
These are times of great expectation for particle physicists. The LEP/SLD experiments have tested very precisely the Standard Model, and are clearly hinting for a Higgs boson just around the corner.
Most likely the physics scenario waiting for us at energies above 200 GeV includes a light Higgs boson and possibly supersymmetric particles
WHY?
10 October 2002 Stefania Xella - RAL
• LEP and SLD tell us that 114< Higgs (SM) <210GeV @95%
c.l.• Unification for different coupling
constants is possible (so far) only introducing SUperSYmmetry
• SUSY lightest particle is the most favoured candidated for dark matter
• No realistic theory model of electroweak interactions can avoid introducing a Higgs boson
10 October 2002 Stefania Xella - RAL
Running (now or soon) Running (now or soon) exp’sexp’s
Tevatron Run II (p-antip) running. energy : 2TeV, lumi: 2 fb-1 in 2 years• Higgs>115 GeV @95%c.l. with 2 fb-1
Higgs>200 GeV @95%c.l. with 50fb-1
but observed (3 ) with 20fb-1 up to 180 GeV
• SUSY: some coverage, rates <= SM
(see fig)
10 October 2002 Stefania Xella - RAL
Running (now or soon) Running (now or soon) exp’sexp’s
LHC (p-antip) start 2007.Energy: 14 TeV, lumi:300 fb-1 (6
years)• Higgs detection (5 ) up to 1 TeV with 30 fb-1 (3 years, p.e.) • SUSY particles detectable for
masses up to 2 TeV (g,q) and to 400 GeV (l)
~~~
(see fig)
10 October 2002 Stefania Xella - RAL
So why do we need more?So why do we need more?Hadronic machines are very good for
discovery because they can go “easily” very high up in energy
BUT
They are not as good in obtaining clean and precise measurements as e+e- machines are. WHY?
10 October 2002 Stefania Xella - RAL
pp vs e+e- (I)pp vs e+e- (I)e+ e- are fundamental particles, soe+
e-new particle
pp are not fundamental particles, so
Strong interactions
“easy” to gofrom final stateto new particle
difficult to gofrom final stateto new particle
10 October 2002 Stefania Xella - RAL
pp vs e+e- (II)pp vs e+e- (II)
in e+e- interactions the energy involved in the interaction between fundamental particles is known
=> 4-p conservation is important constraint (e.g. final states with )
with pp interactions one cannot use this
10 October 2002 Stefania Xella - RAL
pp vs e+e- (III)pp vs e+e- (III) Background in e+e- is EW: (signal)/(backgr) ~ 1 while in pp is mainly QCD -> high, and also high EW (many partons involved)
e.g. (ee->ZH) ~ 0.3 (pp->WH,W->l) ~10-4
(ee->ZZ) (pp->Wjj,W->l)
10 October 2002 Stefania Xella - RAL
pp vs e+e- (IV)pp vs e+e- (IV)
Theoretical predictions for signal and background are more
precise for e+e- interactions than for pp ones: e+e- ~ 0.1 - 10% pp ~ 10 – 100% (HO QCD,
structure functions, ... )
10 October 2002 Stefania Xella - RAL
pp vs e+e- (V)pp vs e+e- (V)
• In pp machines the frequency of events where something happens is
high -> lots of radiation on detectors -> limits the choice of detectors -> limits the physics potential e.g. CCD pixel detector only at e+e-
10 October 2002 Stefania Xella - RAL
for precise measurements e+e- is clearly better than pp
a high energy(> 500 GeV) e+e- machineis necessary now because:
• LHC/TevaTron find Higgs? then they cannot describe accurately its
properties (see following)• LHC/TevaTron find nothing? then precision measurements is our only chance to get a hint on what’s beyond e.g. LEP and top mass…and Higgs mass (?)
10 October 2002 Stefania Xella - RAL
Status of LC R&D projectsStatus of LC R&D projectse+e- machines : lots of advantages
BUT one main disadvantage: it is difficult and expensive to go
to high energy and high luminosity
to cover next energy frontier
interesting processeshave sometimes low
=> proposed machine: e+e- linear collider
10 October 2002 Stefania Xella - RAL
Energy/luminosity Energy/luminosity requiredrequired
Luminosity = Frep . Ne
2
4 xy
Energy 210-300 GeV for Higgs
350 GeV for tt production ?? for susy
<TeV new strong interactions
Beam vertical dispersion ~1/100
~100 more bunches,more particles
10 October 2002 Stefania Xella - RAL
Existing projects (phase 1)Existing projects (phase 1)
R&D work (machine/detector) very active, physics studies well advanced
(ECFA/DESY) TESLA @ DESY (T.D.R. 2001) NLC @ SLAC/FNAL (T.D.R. 2003)* start at 500GeV -> 800GeV-1 TeV * possible run at 91.2 GeV (GigaZ)* polarized beams (90% e-, 50% e+) to enhance signal vs background* start envisaged by 2014
(see fig)
10 October 2002 Stefania Xella - RAL
500 GeV TESLA NLC
RF cavity Niobium, superconducting
Copper, conventional
RF pulse 1.3 GHz 11.4 GHz
Gradient 23.4 MV/m 50 MV/m
Bunches/train 2820 192
Bunch spacing
337 ns 1.4 ns
Rep. rate 5 Hz 120 Hz
<bunch rate> 14 KHz 23 KHz
luminosity 3.4 1034 cm-1 s-1 2 1034 cm-1 s-1
Charge/bunch
2 1010 0.75 1010
length 33 Km 26 Km
10 October 2002 Stefania Xella - RAL
Existing projects (phase 2)Existing projects (phase 2)
Higher energy/lumi operation than TESLA/NLC requires big step ahead
R&D started: CLIC @ CERN -> long way to go to achieve:RF pulse 30 GHz, Gradient 150 MV/mEnergy 3-5 TeV, Luminosity 1034 cm-1 s-1
(see fig)
10 October 2002 Stefania Xella - RAL
LC Physics potential: HiggsLC Physics potential: HiggsThe LC can measure precisely SM-like H :• Mass • Coupling to gauge bosons and fermions• Total width • CP : phases, properties• Self coupling (-> Higgs potential)• JPC
LHC can discover the Higgs quickly, but only measure 1., and poorly some of 2.
10 October 2002 Stefania Xella - RAL
LC Physics potential:LC Physics potential:SUSY extension of SM (I)SUSY extension of SM (I)
1. MSSM has about 105 free parameters in addition to the SM ones ! => precise measurements are needed
2. LC one can do energy scan at different prod. thresholds of SUSY particle pairs
3. Polarized beams greatly improves sensitivity
4. CP, mixing, q.n. SUSY particles can be studied precisely
10 October 2002 Stefania Xella - RAL
LC Physics potential:LC Physics potential:SUSY extension of SM (II)SUSY extension of SM (II)
5. Masses of SUSY particles measuredindipendently of modele.g. g -> qq -> qq 2
0 -> qqll 10
6. sneutrinos can be measured
LHC cannot cover points 2. - 6., and cannot avoid using model in interpretation of the results: kinematic is not clean enough
~ ~ ~~
10 October 2002 Stefania Xella - RAL
Higgs at the LCHiggs at the LCSM MSSM H h0,H0,A,H±
production
e
e
e
e
ZZ*
H
H
Low E
High E
Same as SMH<-> h0,H0
+
ee->A h0,H0
ee->H+ H-
10 October 2002 Stefania Xella - RAL
Higgs at the LCHiggs at the LCdecay
bb m<140GeV(,gg,cc)
WW m>140GeVZZ
tt m>300GeV
Depends a loton value of tg=v2/v1
e.g.Tg H<->
h0,H0
10 October 2002 Stefania Xella - RAL
SM HiggsSM Higgs1. Mass not predicted=> important to measure it well ee -> Z H m<130 GeV qq bb MH
50MeV ll bb 110 ll qq 70 m>130 GeV qq,ll WW 130 qq,ll “recoil 90 technique” more precise
important if H->invisible
dominantindipendentof H nature
500 fb-1
(see fig)
10 October 2002 Stefania Xella - RAL
SM HiggsSM Higgs2. Coupling to gauge bosons (W,Z)
measured through production Xsection.
important also for gHff and tot (ee->HZ) ~ gHZZ
#
(ee->H) ~ gHWW
Br(H->WW*) ~ gHWW
# recoil mass technique used ->
result independent of model and decay (see fig)
10 October 2002 Stefania Xella - RAL
SM HiggsSM Higgs3. Coupling to fermions (f) gHff ~ mf / v => measurement can tell if H is SM or not
Br(H->ff) ~ gHff Br/Br 2.4% (bb) 8%(cc) 5%(gg) 5% () fundamental: optimal flavour tagging ->
VXD (see fig)
Uses (ee->HZ)& (ee->H)
10 October 2002 Stefania Xella - RAL
SM HiggsSM Higgs4. Total decay width mH>200 GeV: from H lineshape mH<200 GeV: indirect, from tot = x
Br(H->x) need indep. meas. e.g. x=WW : x from (ee->H) Br => tot / tot ~ 4%
10 October 2002 Stefania Xella - RAL
SM HiggsSM Higgs5. Higgs potential gHff -> nature of H, but potential needed
too V= (||2 –1/2 v2)2 v2=1/(2GF
2)
measure from physical H potential V= v2 H2 + v H3 + H4 Difficult
For LC from MH e
e
Z
HH
Difficult backgrounds, reco, tagging => need optimal vxd HHH/HHH ~ 22%
(see fig)
10 October 2002 Stefania Xella - RAL
MH
(GeV)
X/X LHC2x300 fb-1
X/X LC500 fb-1
MH
MH
tot
120 160 120-140
9 104
10 104
-
3 104
4 104
0.04-0.06
gHuu
gHdd
gHWW
120-140120-140120-140
- - -
0.02-0.040.01-0.020.01-0.03
gHuu/gHdd
gHbb/gHWW
gHtt/gHWW
gHZZ/gHWW
120-140120-140 120 160
- - 0.070 0.050
0.023-0.0520.012-0.022 0.023 0.022
CPHHH
120 120
- -
0.03 0.22(see fig)
10 October 2002 Stefania Xella - RAL
ConclusionsConclusions
The next linear collider isan essential and unavoidable step in the understanding of physics at
energies >200 GeV
LET’S BUILD IT ASAP !
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