LHC Physics with CMScharm.physics.ucsb.edu/people/incandel/Incandela_Bad... · 2007. 9. 1. · 1. A new symmetry 2. New particles at the electroweak scale (M EWK ~ 0.1-1.0 TeV) •

LHC Physics with CMS: Part 2:

Potential for Early Discovery

Joe IncandelaUC Santa Barbara

August 27-28, 2007Physikzentrum, Bad- Honnef

LHC Lectures, Physikzentrum Bad Honnef, August 27-28, 2007 Joe Incandela

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There’s currently an interesting set of circumstances in two “fundamental” areas of Physics*:– Experimental Particle Physics

• Many precise results with no substantial discrepancies with the Standard Model (SM)

– Experimental Astrophysics and Cosmology• Abundant (literally) evidence for new physics

– Dark energy and non-baryonic dark matter– Neutrino oscillations– Cosmic matter-antimatter asymmetry– Cosmic density fluctuations consistent with inflation

*next few slides inspired byIan Low (UC Irvine)

Possible Implications• The division is itself a major clue and constrains theory• Viable models (that evade constraints of precision

measurements) often have common features:1. A new symmetry 2. New particles at the electroweak scale (MEWK ~ 0.1-1.0 TeV)

• The new symmetry allows the new particles to exist without contradicting existing measurements:– Minimal impact on corrections to SM parameters because the

new symmetry forces the new particles to be produced in pairs and thus enter at higher order than tree level (impact is reduced by a factor of 1/(16π2)

– The new particles cancel divergences in the Higgs self energy• Examples

– SUSY with R parity: • partners of SM particles have opposite spin statistics, Higgs is natural

– Little Higgs theories with T parity:• partners of SM particles have same spin statistics, Higgs is natural

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The common structure can produce similar phenomenology

We may see something that is not so easy to interpret

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chalk drawings by Julian Beever

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Nevertheless, the case for Supersymmetry (SUSY) is

compelling

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• Extension of known space-time symmetries– 10 generators of Poincare group:

• Lj, Kj, Pμ for rotations, boosts, translations– SUSY ⇒ fermionic operators Qα

• Arises naturally in String theory– Is the maximal possible extension of the Poincare group

⇒ Qα acting on any state produces a new state having the same quantum numbers - except spin which is shifted by ½

– Fermions ↔ Bosons are interchanged under group transformation.• Initial state a SM particle ⇒ final state its superpartner• No SM particle is the super-partner of another SM particle

– Supersymmetry is broken• Mass degenerate superpartners would have been discovered long

ago ⇒ there must be symmetry breaking contributions to the masses which are large and positive.

• Once broken…Superpartner mass scale is unconstrained but there is strong motivation for the weak scale

Supersymmetry*

* J. Feng hep-ph/0405215v2

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9SUSY and the weak scale

• SM Alone: – correction has quadratic divergence!

• Λ a cut-off scale – e.g. Planck scale

• Superpartners fix this:• Need same coupling λ• Need superpartners at the weak scale

– Otherwise the logarithmic term becomes too large, which would require more fine-tuning.

– Known as “soft” SUSY-breaking terms (others are possible)

Cancellation


10SUSY Spectrum of Neutrals*

• Need ¥ 2 Higgs doublets– Avoids triangle anomalies (divergent process involving a fermion

triangle loop with gauge bosons at the vertices)

• Elegant choice Hu and Hd– They give mass to up- and down-like fermions separately

• Helps evade large Flavor Changing Neutral Currents (FCNC)

• Neutral Spectrum: – Spin 0 sneutrinos, spin 3/2 gravitino, spin ½ Bino, Wino, and Higgsinos

Mass parameters


• The 4 spin ½ neutral SUSY partners only differ in their electroweak quantum nos.– With SUSY broken, they are free to mix to form

mass eigenstates• These are the neutralinos ck with k=1,2,3,4• These fermions are Majorana (particle=antiparticle)

• Beyond neutrals - spectrum as expected– Fermion (boson) superpartner for each SM boson (fermion)

Spectrum (cont.)


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• Superpartners solve some and create other problems– Gauge hierarchy problem eliminated– But now protons decay too rapidly

• Superpartners mediate both L and B number violation

• Need a new symmetry: R parity conservationR = (-1)3(B-L)+2S where B,L,S=Baryon #, Lepton #, and Spin

R= +1 (-1) for all SM particles (SUSY partners)

• Consequence: Lightest SUSY Particle (LSP) stable– Cannot decay into SM particles– And as mentioned earlier, impact of SUSY spectrum on SM particles is

diminished

R-Parity


• What is the LSP?– Must understand how SUSY is broken

• specifies soft SUSY breaking terms & the mass spectrum

• SUSY breaking is a vast and technical subject!– Popular models assume a hidden sector is involved:

• Sounds ad-hoc, but there is a precedent: Electroweak Symmetry Breaking (EWSB) as discussed yesterday

SUSY breaking (SB)


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• EWSB divides SM interactions into 3 sectors1. “EWSB”: involving only the Higgs2. “Observable”: involving quarks and leptons3. “Mediation”: involving the interactions between

sectors 1 and 2 (i.e. Yukawa interactions)• Higgs obtains non-zero vacuum expectation value (vev)

and this occurrence is communicated to the SM fermions via some unknown mediator.

• Same concept applies to SUSY breaking1. “SUSY-Breaking”: Fields Z not in SM 2. “Observable”: SM particles and their superpartners3. “Mediation”: all interactions between SUSY-breaking

fields Z and observable fields

Mediation


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15SUSY Breaking• Simplest cases, one field Z has non-zero vev F

– Gravitino acquires mass m3/2 = F/(◊3 M*)• M* = (8pGN)-½ º 2.4 x 1018 GeV (reduced Planck mass)

– Mediation sector terms for Z interacting with superpartners become mass terms when Z →F

ḟ, λ=superpartners of SM fermions and gauge bosons


• Supergravity Models– Mediating interactions are gravitational: Mm ~ M*

• m3/2, mḟ, mλ ~ F/ M*

• ◊F ~ ◊(Mweak M*) á 1010 GeVflHigh Scale SUSY-Breaking

flAny superpartner OR the Gravitino could be the LSP

• Gauge-Mediated (GMSB) – Mediating fields are gauge fields: Mm á M*

• m3/2 = F/(M*◊3)á mḟ, mλ ~ F/ Mm

• ◊F ~ ◊(Mweak Mm) á 1010 GeVflLow-scale SUSY-breaking

fl Gravitino = LSP

Models


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• SUSY spectrum depends on the specific model of SUSY breaking– Infinite possibilities– Can narrow the field with several assumptions

• Assume weak-scale SUSY derives from something more fundamental (e.g. Grand Unified or String theories)

• The fundamental theory is highly structured

• Why highly structured? – Partly driven by aesthetics (simplicity) – Also find that the gauge couplings unify

• Occurs in SUSY models that are run up to higher energy via the renormalization group equations

SUSY Models


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Gauge Coupling Unification


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• Thus, the belief is that – the many (>100) parameters of weak-scale SUSY

should be derived from a minimal set of parameters at the unification scale.

• mSUGRA: the Canonical model– 5 main parameters

• mo , m1/2 , Ao , tan(β), and sign(μ)– mo , m1/2 are universal scalar and fermion masses

• Like the couplings, one assumes that the spectra also derive from a few fundamental masses

– m3/2 is a 6th free parameter• Gravitino could be LSP but in most of the literature it is

assumed to be very heavy and so can be ignored.

Minimal Supergravity (mSUGRA)


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20An example of renormalization group evolution of universal SUSY masses in mSUGRA

• Generally valid features:– Evolving from GUT scale

• Gauge couplings increase SUSY masses

• Yukawa couplings decrease them

– Thus• Colored particles are heavy ⇒ Not

LSP candidates• Bino is the lightest gaugino• Right-handed slepton the lightest

scalars (specifically stau ÌR)

– (mHu)2 is driven negative by the large top Yukawa coupling!

What is this about?


21Minimization of the EWK potential for Electroweak Symmetry Breaking

• At tree level this requires

– True for all but lowest values of tan(β) (which are disfavored anyway)

– Can only be satisfied if (mHu)2 <0

– No other mass parameters are so significantly affected by the large top Yukawa coupling

A natural explanation of why SU(2) is the only SM symmetry that is broken

A large top mass then has a significant role in SM phenomenology through SUSY


• Minimal Case of 2 doublets: – After W,Z masses, 5 remaining d.o.f.

• 5 physical Higgs bosons ho, Ho, Ao, H±

• Scalar potential has one free parameter–masses are expressed in terms of mA and tanβtanβ = v2/v1 and v12+ v22 = v2

Where v1 (v2) are the vev’s for the Hd (Hu)– Large radiative corrections (at one-loop)

• Mh2 < MZ

2 + (3GF/(21/2π2)) Mt4 ln(1+m2/Mt

2)• Mh ƒ 130 GeV

ƒ 150 GeV (if there are also Higgs singlet(s))

• Important feature• Couplings to vector bosons now shared

ghoVV

2 + gHoVV2 = gHVV

2 (SM)

Minimal SUSY Higgs Sector


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23Back to the LSP• Scan parameter space

for LSP possibilities– One slice through

mSUGRA shown here

• The LSP in mSUGRA– Lightest neutralino χ1 or the RH stau ÌR

• Many other models exist– But mSUGRA contains a very wide variety of

phenomenological possibilities and LSP candidates⇒ Useful for studying a broad array of signatures. This is

what is done in CMS.


Dark Matter

Another motive for R-Parity-Conserving SUSY


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LHC Lectures, Bad-Hoffen Germany, August 27-28, 2007 Joe Incandela

R. Kolb at SUSY07 Karlsruhe

• Matter is only 5% of the energy in the universe.– Cosmology-Astrophysics evidence for physics beyond the

Standard Model (BSM) is overwhelming• Yet provides very little by way of constraint • Particle physics is required

• Relic Density for non-baryonic dark matter:– 0.094 < ΩDM h2 < 0.129 (95% CL), h = 0.71 (km/s)/Mpc

(Hubble expansion)• Weak scale SUSY with R-Parity conservation is the

best-motivated framework– Provides a natural dark matter candidate (neutralino)– Leads to remarkable gauge coupling unification– Can provide an explanation for why SU(2) is broken– Solves the gauge hierarchy problem

The Dark Side 26

27Thermal Relic Density

• As universe expands– Interactions/annihilations

cease at a time that depends on annihilation cross section σA times mean velocity v

– Freeze out condition:• Neq ~ ‚σAvÚ∼Τ2/Μ∗

• Weakly Interacting Massive Particles (WIMPs)– Mass and annihilation

xsec set by weak scalem2 ~ ‚σA vÚ-1 ~ Mwk

2

– Thus a 300 GeV WIMP freezes out at T ~ 10 GeVand t~10-8 s

– The freeze-out density is:Wc~ 10-10 GeV-2/ ‚σA vÚ– Typical weak xsec: ‚σA vÚ∼α2/Mwk

2~10-9 GeV-2

⇒ Wch2 ~0.1 !!


• Though SUSY looks very compelling, theorists have proposed many alternatives and we do not know which if any is the right one until we get data…– Strong dynamics – Grand Unified theories– Little Higgs– String-theory motivated models

• ADD Large extra dimension• Randall Sundrum warped extra dimension

Beyond SUSY


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So back to the LHC


Early Discoveries

• Several categories – Self-calibrating (Mass peaks)– No need to calibrate

• Event count >> SM prediction,• A distribution of some kinematical quantity that is

overtly inconsistent with the SM • An all new topology…

• In the absence of such things, the job is difficult and may be slow

Lectures on LHC Physics; Bad-Honnef ; August 27-28, 2007 Joe Incandela

31Good stuff comes early…and late.• SPS

– CM = 683 GeV and ~100 GeVmean parton interaction

• Tevatron I– CM=1800 GeV and ~270 GeV

mean parton interaction• SPS & Tevatron Discoveries

– SPS turn-on led to quick major discoveries

– Not true at the Tevatron• SPS had a lot of data

– Already probed quite a bit higher than the mean constituent CM energy (~100 GeV)

– Tevatron needed to ~match SPS integrated luminosity in order to probe a “new” energy domain

• And then discovered top!

• Early discoveries have been followed by other important results at hadron colliders – but these have generally come late


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LHC will startup in new territory

– At 1 TeV constituent com energy • gg: 1 fb-1 at Tevatron is like 1 nb-1 at LHC• qq: 1 fb-1 at Tevatron is like 1 pb-1 at LHC

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gg luminosity @ LHCqq luminosity @ LHCgg luminosity @ Tevatronqq luminosity @ Tevatron




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Early and Late

• Parton Luminosity falls steeply– In multi-TeV region, ~ by

factor 10 every 600 GeV• New states produced near

threshold– Suppose you have a limit on

some pair-produced object, M > 1 TeV. How does your sensitivity improve with more data?

• By ~ (600/2)=300 GeV = 30% for 10 times more integrated luminosity

Improving sensitivity is tough....but you can turn evidence into an observation


Mainly jets!~ 10 μb/GeV @ 100 GeV~ 0.1 pb/GeV @ 1 TeV

But also: bb ,W, Z, tŧ• σ(bb, high PT) ~ 1 μb• σ(W lν) ~ 60 nb• σ(WW) ~ 200 pb• σ(tŧ) ~ 1 nb

LHCTevatron

Jet cross section

What we know we’ll see at 14 TeV


Promising areas for early searches and examples of important physics objects

New Physics Searches with CMS• Source: Physics Technical Design Report Vol. II

– J. Phys. G. Nucl. Part. Phys. 34 (2007) 995-1579• A huge amount of good work • General Focus: low luminosity (2 x 1033) operation and

integrated luminosities up to 30-60 fb-1

• Many studies also considered very early data– From a few pb-1 to a few fb-1

• Will draw on this work for this talk• Cannot cover everything (fortunately for you) • Not an expert on these analyses (except tŧH, H→bb which

we seem to have killed…)– Highlight a few of the areas where it appeared that

new physics could reveal itself in < 1 fb-1

• NB: We are now in the process of doing a dedicated exercise: 10 pb-1, 100 pb-1, 1 fb-1

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Next six months

• Approach “1fb-1” by successive approximations.– 10pb-1 and 100 pb-1 stages as well. – What will we get done with these amounts of data?

• June: a preliminary look at the 100pb-1 studies.• October: Results for 10,100 pb-1 and 1fb-1

– Brief note (30 pages) on the “CMS plan for X pb-1” summarizing each of these stages (10, 100, 1000 pb-1) across all groups

• NB: Detector Groups are to provide new guidance on expectations and requirements for these startup datasets

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Higgs

Close, maybe even a cigar, If ATLAS helps…

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39Photons: H → γγ

• 1 fb-1

– Above:Signalx10 and backgrounds

– Signal efficiency is of order 20-30%

• Need ~ 10 fb-1

– At right: 120 GeV Higgs in 7.7 fb-

1

Generation: PYTHIA + k-factors

Full simulation

Resolution: 0.3% (EB) to 1% (EE)

Isolation Tracks: none with pt>1.5 in ΔR<0.3Calo: Barrel (Endcap)<6(3) GeV in 0.06<ΔR<0.35

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40Electrons: H→ZZ(*) →eeee

• 30 fb-1 shown: 1 fb-1 signal is too small– σ⋅B ~1 - 4 fb (NLO)

• Backgrounds (Direct ZZ(*), Zbb, tŧ)~20,120,200 fb

Pre-selection Final selection

Final selection:

30 fb-1 two pseudo experiments (lucky and not so lucky)


Mystery of dark matter in the universe solved:it’s in front of CMS/ATLAS ECAL…

Affects electrons and photons: energy loss, conversions

18From P. De Jong - Moriond 2007

41


42Electrons (below 50 GeV)

• substantial bremstrahlung at low energies– Use an energy loss model in tracking to take Brem into account.

Momentum at innermost layer pin > pout at outermost layer and difference is correlated with Radiated energy

– ECAL superclusters incorporate radiated energy


fbrem =pin -pout used to estimate material budget

η η

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Classifying Low E Electrons

• Four Classes – Different

corrections

• Esc/Pin>0.9– Golden

• Fbrem<0.2• Δφ < 0.15

– Big Brem• Fbrem>0.5• Δφ < 0.15

– Narrow• Complement of

the other two

• The rest is junk– Called

“Showering”

Barrel

Endcaps

Electrons5-50 GeV

Jets25-50 GeV

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ECAL versus Tracker: E Resolution 45


46Results for H→ ZZ*

• 4μ channel has some potential for 95% CL exclusion at a few fb-1

• 10’s of fb-1 for 5 σ discovery


muons

Muon in Silicon Tracker

Standalone Muon Track

Hits and Track Segments

Global Muon Track

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H →WW(*)

• H WW lν lνgg→H, qq →V V q’q’→Hq’q’

• A counting experiment– Must understand backgrounds by direct

measurement of SM and fakes. • Backgrounds:

qq WW, gg WW, tt WWbb, tWb WWb(b), ZW lllν, ZZ llνν etc.

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49Cross sections: H →WW(*)

• All processes LO– Signal and W-pair

background phase-space dependent NLO k-factor reweightings

• Match PYTHIA ptdistributions of the H and WW systems respectively to those predicted by MC@NLO


50H →WW(*) →2μ2ν Selection

• Selection– Trigger

• Single μ 97% eff.

– Optimize separately• Isolation variables

– To kill bb

• Jet and MET thresholds– Central jet veto kills tŧ

» ET>15– MET Kills DY

» Net efficiency a few per 10 million!


51H →WW(*) and bkd after selections


Background Estimates

• Use Data control Samples– E.g. back off central jet veto and add b tagging to get

a tŧ enriched sample– S=[NMC(s_region)/NMC(cntrl_region)]*NData(cntrl_region)– WW region harder to isolate – have to estimate and

subtract other processes• E.G. normalize DY by selecting in Z peak• Normalize tŧ by requiring two b tags etc…

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Results

H→WW→2μ2ν

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SM Higgs 54


Efficiencies from data

Z’

High mass dimuons:

Tracking: alignment and propagation muons ↔ tracker importantAs noted yesterday: Mass resolution (and so discovery potential) not too strongly affected by tracker alignment scenario

Z’, graviton resonances, large extra dimensions…

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Massive Z’s

• Generation and reconstruction– Pythia with 3-way interference terms

• KQCD(NNLO)=1.35 applied• CTEQ6L – LHAPDF set

– Background Pythia with K=1.35 also• DY mainly• V V, tŧ at percent of DY• Dijets, cosmics, W+j, bb, punch-through not studied yet

– Reconstruction• Includes search & recovery for photons in ΔR<0.1

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*LM9

*LM8

SUSY Benchmark Points from PTDR• Selection of 13 Points

– Low mass LM1→LM9– High mass HM1→HM4

• Important: different topologies/decay modes, i.e. on different signatures– LM1,2,6,9 are also close

to WMAP benchmarks

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Signature based analyses

• A Variety of inclusive analyses @ a specific benchmark point then extended to the m1/2-moplane using FAMOS– MET + jets @ LM1: MET>200– Muons + MET + jets @ LM1: MET>130 – Same sign di-muons @ LM1: MET>200– Opposite sign dileptons @ LM1:MET>200– Di-taus @ LM2 : decays 95% to ττ: MET>150– Inclusive analysis with Higgs @LM5:MET>200– Inclusive Zo @LM4:MET>230– Inclusive top @ LM1: Top plus leptons:MET>150

χ02

~ ~


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59LM1: MET and ≥3 jets

• Cleanup – Instrumental bkds,

halo, cosmics, etc. – Require a primary

vertex – And total EM fraction

Fem>0.175 • Fem = ET weighted EM

fraction in |η|<3

– and event charged fraction Fch>0.1

• Fch = PT of charged tracks associated to jets over calorimeter jet ET in |η|<1.7


60MET in QCD events

• MET in QCD (left)– QCD MET tends to be

along leading or 2nd

leading jet directions– SUSY populates a

distinct region


61Veto Electrons (Indirectly)

• To eliminate W, Z +jets, tŧ etc.– Require the two leading jets to be non-EM

• EM/(Had+EM)<0.9


Calibrate Z→νν + jets with Z →μμ +jets

• MET+ ≥3 jets– Expected from

• Z→νν + ≥ 3jets • W →τν + ≥ 2jets, 3rd jet the τ hadronic decay• Possible residual contrib. from W →eν,μν + ≥ 3jets

– MC prediction for • Z→μμ + ≥ 3jets with PT

Z > 200• Ditto for W decays

– Normalized to data (for higher stats)• Z→μμ + ≥ 2jets with PT

Z > 200– Assume these ratios are correctly calculated in MC

e.g. Alpgen

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63Z Candle

Selected Z→μμ + ≥ 2jets with PT(Z) > 200Muons included

Selected Z→μμ + ≥ 2jets with PT(Z) > 200Muons excluded

Selected Z→νν+ ≥ 2jets

Concern:

Modeling the tails of the MET distribution.

A somewhat ad-hoc enhancement was done (up-weighting events for which the jets are poorly reconstructed as determined by comparison of reconstructed ET with initial parton ET).

Even very substantial tail enhancement does not qualitatively alter the result


Jets + Missing ET

ETmiss

@ LM1Normalizing Z→νν ET

miss

to Z→μμusing data

Low mass SUSY

64


65Final event counts

• Final Cuts on ET of j1,j2,HT > 180,110,500 GeV• Global signal efficiency 13%, S/B~26


66Inclusive SUSY searches

• Low-mass SUSY (Msp~500GeV) accessible with O(10-1) fb-1. Δt to discovery determined by:– Time to understand detector performance: ET

miss tails, jet performance and energy scale, lepton id

– Time to collect control samples -- e.g. W+jets, Z+jets, WW, top..


67HM1

• Prior to data, backgrounds are an open issue….– And are more of a relevant issue for High Mass (HM) points


68SUSY signals (cascades)

02χ%

%l

l

g~q~

q q

02χ

0hM(bb)

Can be discovery channel for the Higgs

1 fb-1

missTE⇒0

1χ missTE⇒0

1χ

hb

l02χ% b

l


1 fb-1 is well into new territory:Jets up to ~3-3.5 TeVDi-jet masses up to ~5-6 TeV

Challenges: Jet energy scale,Parton density functions (PDF),underlying event, trigger, jet

definition

Deviation from SM

CDF

Anomalous jets, dijet cross-sections

Substructure, contact interactions, high mass resonances

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Dijet xsec ratio and new Physics


Maybe nature has some REAL SURPRISES in store…

sphericity

Large extra dimensions,Planck scale ~ EW scale

Possible micro black holeproduction; decay viaHawking radiation intophotons, leptons, jets…

CMS and ATLAS might seethis with 1-100 pb-1 !

From P. DeJong Moriond 2007

71


Summary

• There are good reasons to believe that there is something new at the energy scales accessible to the LHC that could appear early.

• Indications are that CMS will be ready to exploit this opportunity.

• Many studies documented in PTDR• Much achieved, but much more to learn

– Focus on the first data (0.01 to 1.0 fb-1) from now until first collisions

– Many improvements in tools and our understanding of our capabilities are expected

• Initial detector performance and speed of optimization will be crucial

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Documents

LHC Physics with CMScharm.physics.ucsb.edu/people/incandel/Incandela_Bad... · 2007. 9. 1. · 1. A new symmetry 2. New particles at the electroweak scale (M EWK ~ 0.1-1.0 TeV) •