10 October 2002Stefania Xella - RAL The next linear collider Stefania Xella Rutherford Appleton...

10 October 2002 Stefania Xella - RAL

The next linear colliderThe next linear collider

Stefania Xella Rutherford Appleton Laboratory


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


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?


• 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


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)


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)


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?


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


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


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)


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, ... )


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-


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 (?)


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


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


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)


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


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)


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.


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


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

~ ~ ~~


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-


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


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)


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)


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)


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%


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)


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)


ConclusionsConclusions

The next linear collider isan essential and unavoidable step in the understanding of physics at

energies >200 GeV

LET’S BUILD IT ASAP !

10 October 2002Stefania Xella - RAL The next linear collider Stefania Xella Rutherford Appleton...

Documents

Transcript of 10 October 2002Stefania Xella - RAL The next linear collider Stefania Xella Rutherford Appleton...