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High pT physics at the LHC Lecture IV Searches

- Miriam Watson, Juraj Bracinik
- (University of Birmingham)
- Warwick Week, April 2011

- LHC machine
- High PT experiments Atlas and CMS
- Standard Model physics
- Searches

Introduction

- Topics I will cover today
- Higgs searches
- SUSY
- Extra Dimensions
- Inclusive searches
- I will not cover
- All the details of every search!
- I will concentrate on ATLAS and CMS

Why we think a Higgs field exists

- The SM is really two separate theories - QCD and

GSW electroweak - We know that the electroweak piece must be broken
- Separate EM and weak forces
- Unified electroweak theory involves massless

gauge bosons only - Short range of the weak interaction ? gauge

bosons mediating the weak force must be quite

massive - Something has to break the electroweak symmetry

and something has to give the W,Z mass - All the fermions that are massless
- ?Something has to give them mass as well

Electroweak Symmetry Breaking

- The gauge group for the GSW theory is SU(2)L?U(1)
- This must be a broken symmetry, but do not want

to destroy gauge invariance of theory (SM) - We want to add a new field to the SM that will

initially have SU(2)L?U(1) symmetry. When this

symmetry is broken, the massless bosons become

the massive W,Z and a massless photon - The addition of a single SU(2) doublet of complex

scalar fields satisfies these requirements

Higgs Potential

- Distance from the centre describes the strength

of the Higgs field - Height denotes the energy of a particular field

configuration. - The zero-field configuration (centre) is

unstable to small perturbations - system will fall into the lower energy state in

the moat - lowest energy state of space (the vacuum) is not

empty, but is permeated by the Higgs field - in the ground state there is no symmetry in the

radial direction - As the universe fell into the ground state

electroweak symmetry was spontaneously broken

Vacuum expectation value (vev) 246 GeV

Theoretical constraints on the Higgs Mass

- In order to confirm the existence of a Higgs

field and the Higgs mechanism, we need to find a

quantum of this field (Higgs boson) - Theoretical bounds on the allowed Higgs mass
- ? a chimney around 180 GeV extending to the

Planck scale - Additional constraints from fine tuning limits

? new physics O(TeV)

(non-perturbative)

? cut-off scale at which new physics becomes

important

Indirect limits from electroweak precision data

- W mass and top quark mass are fundamental

parameters of the Standard Model - There are well defined relationships between mW,

mt and mH

Karl Jakobs, 2010

W and top mass measurements

DMW/MW 3.10-4

Measurements up to July 2010

DMt/Mt 6.10-3

These measurements favour a light Higgs boson

MH89 35 -26 GeV (68 CL)

LEP2 direct search MH gt 114.4 GeV (95 CL)

Tevatron constraints on the Higgs Mass

- Recent CDF and D0 combination
- excludes 158 lt MH lt 173 GeV at 95 CL

Higgs processes at the LHC

- The Higgs will be produced through a variety of

processes at the LHC - Some dominate
- (gg fusion)
- Others are rare (ttH)
- If a Higgs exists, it will be produced at the LHC
- Finding it is another matter

SM Higgs production cross-sections

- Cross-sections O(100 pb) ?significant no. of

Higgs will be produced by the LHC in a very short

time (weeks/months) - It will take longer than that to claim a

discovery - We have seen the relative cross-sections of Higgs

and QCD/EW processes

Standard Model Higgs decays

- For mH lt 1 TeV, divide into low, intermediate

and high mass regions - Decay modes change as a function of mH since the

Higgs couples to mass and will decay to the

heaviest particle(s) - Low mass dominant decay mode (bb) is essentially

useless due to overwhelming QCD backgrounds - ? concentrate on H?gg

Low mass Higgs H?gg

- Low branching ratio, but take advantage of the

excellent photon resolution to see a narrow peak

above continuum background - Need at least 10 fb-1

Simulation

With good segmentation

Low mass Higgs vector boson fusion

- Tag two forward jets
- Select Higgs bosons in the channel H?tt (t?l or

t?had) - Decay products in central region, i.e. high pT
- Make a collinear approximation (assume neutrinos

in tau decays are in same direction as visible

decay products) - Reconstruct Higgs mass ? excess if sufficient

luminosity

Simulation

High mass Higgs H? 4 leptons

- Finding a high mass Higgs is much easier
- Both H?WW?lnln, and H?ZZ?4l are viable search

modes (l e, m) - Multi-lepton signatures are relatively easy to

discern above background - Both are easier if bosons are on-shell (WW mH gt

160 GeV, - ZZ mH gt 180 GeV)
- H?ZZ?4l is considered to be the golden mode for

Higgs searches - Low backgrounds (ZZ,Zbb,tt)

CMS simulation

What has the LHC found so far?

Close to SM sensitivity in H?WW?lnln (1.2 x SM)

with 35 pb-1

H?WW?lnln

H?WW?lnln

Note different mH ranges on plots

H?ZZ?llqq/llnn

H?gg

Prospects for SM Higgs in 2011-12

Indicates contributions from different channels

Could exclude down to LEP limit with lt4fb-1 !

(possibly)

Higgs boson properties

- If the Higgs boson is discovered, want to measure

its properties - mass, width
- spin, CP (SM predicts 0)
- coupling to other bosons and to fermions
- self-coupling
- and check whether it is a SM Higgs, or if it is

compatible with theories beyond the SM (e.g.

SUSY) - in principle there could be more than one Higgs

boson - perform direct searches for extra Higgs bosons

MH measurement dominated by ZZ?4l and H?gg

modes Eventual precision 0.1 over large mass

range

Need for a theory beyond the Standard Model

- Gravity is not included in the Standard Model
- Hierarchy problem
- In order to avoid the significant fine-tuning

required to cancel quadratic divergences of the

Higgs mass, some new physics is required (below

10 TeV) - Unification
- of gauge
- coupling
- constants

SM appears to be a low-energy approximation of a

fundamental theory

De Santo, 2007

Supersymmetry

- One favoured idea to solve the hierarchy problem

is supersymmetry (SUSY) - Space-time symmetry between fermions and bosons
- To make the SM lagrangian supersymmetric requires

each bosonic particle to have a fermionic

superpartner and vice-versa - These contribute with opposite sign to the loop

corrections to the Higgs mass providing

cancellation of the divergent terms!

Spin differs by ½ Identical gauge

numbers Identical couplings

Supersymmetric particles

Now have unification of gauge couplings

- Superpartners have not been observed!
- Minimal Supersymmetric SM (MSSM)
- Gauginos and higgsinos mix
- ? 2 charginos, 4 neutralinos
- Two Higgs doublets
- ? 5 Higgs bosons (h,H A, H)

R-parity

- SUSY allows for proton decay to occur via p ?

ep0 - But proton decay experiments have established

that tp gt 1.6 x 1033 yrs - This can be prevented by introducing a new

symmetry in the theory, called R-parity - All SM particles have even R-parity (R 1)
- All SUSY particles have odd R-parity (R -1)
- R-parity conservation ? proton cannot decay
- Two consequences
- Lightest SUSY particle (LSP) is stable
- Sparticles can only be pair-produced

The LSP and Dark Matter

- The LSP would make a very good dark matter

candidate - Stable
- Electrically neutral
- Non-strongly interacting (weak and gravitational

interactions only) - This is why many models are popular in which the

LSP is the lightest neutralino, - Whenever SUSY particles are produced they always

cascade down to the massive but stable LSP - ? Missing energy is the canonical SUSY signature

SUSY Phenomenology

- There are a very large (gt100) number of free

parameters in the MSSM! - e.g. none of the masses are predicted
- Impossible to make any phenomenological

predictions without making further assumptions

- Some possible constraints
- Impose boundary conditions at higher energy scale

and evolve down to the weak scale via

Renormalisation Group Equations (mSUGRA) - Constraints related to the way SUSY is broken

(e.g. GMSB) - we know it must be broken, because

there are no sparticles with same mass as

particles

mSUGRA

- Only five parameters
- m0 universal scalar mass
- m1/2 universal gaugino mass
- A0 soft breaking parameter
- tanß ratio of Higgs vevs
- sgn(µ) sign of SUSY mH term
- Highly predictive masses determined mainly by

m0 and m1/2 - Useful framework to provide benchmark scenarios

LHC experiments have agreed to examine 13 points

in mSUGRA space 9 at low mass (LM1-gtLM9)

4 at high mass (HM1-gtHM4)

Searches for SUSY

- Signatures for SUSY
- Several high-pT jets
- High missing ET (R-conservation)
- Possibly leptons and/or b-jets
- LEP and the Tevatron have set the most stringent

limits to date on sparticle masses. Roughly

speaking these are - m_sleptons/charginos gt 95 GeV
- m_LSP(neutralino) gt 45 GeV
- m_gluino gt 290 GeV
- m_squark gt 375 GeV

Searching for SUSY at the LHC

Expected limits with 100 pb-1 1 fb-1

- If any of the more common variants of SUSY do

exist, the LHC will find it - Should be found relatively quickly in one or more

modes - Plot is for multi-jets missing ET

Example LHC Search Mode - Squark/ Gluino

Production

- These particles are strongly produced and thus

have cross-sections comparable to QCD processes

(at the same mass scale) - Will produce an experimental signature of

multi-jets leptons missing ET - A useful variable is the effective mass
- Typical selection
- njets 4, ET gt 100,50,50,50 GeV
- 2 leptons ET gt 20 GeV,
- MET gt100 GeV

De Santo

Examples of results

Jets MET b tagging

3 leptons jets

- Some LHC SUSY limits are already similar to or

better than TEVATRON

Measuring SUSY masses

- If SUSY is found, how can the underlying model be

disentangled? - Aim to map out the SUSY mass spectrum
- One strategy is to measure the endpoint of

cascade decays - Make as many such measurements as possible
- Other combinations within this chain m(lq),

m(llq) - Different decay chains

m(ll) / GeV

MSSM Higgs searches

- There are five Higgs bosons in the MSSM h0, H0,

H, A0 - In nearly all models, the lightest neutral SUSY

Higgs needs to be light (mh lt 130 GeV) - The phenomenology is sensitive to SUSY

parameters, e.g. tanß - If tanß is large, couplings to down-type fermions

are enhanced and the role of b jets and t leptons

become increasingly important - Production cross-sections are enhanced by (tanß)2
- Event rates can be large

Mtt

An alternative to SUSY Extra Dimensions

- The hierarchy problem
- the weak force is much stronger than gravity

(1/MPlanck1/MEW 10-17) - Supersymmetry gives one solution to this problem
- Can also be addressed as a geometrical space-time

phenomenon - Our 3D space could be a 3D membrane embedded in

a much larger extra dimensional space - Two examples of models
- ADD (Arkani-Hamed, Dimopoulos, Dvali)
- RS (Randall-Sundrum)

Large Extra-Dimensions (ADD)

- Electroweak interactions have been probed down to

1/MEW O(10-15 m) - Gravitational interactions had only been studied

to 1 mm - Gravity may diverge from Newtons Law at small

distances - For r ltlt R, gravity behaves as if it were 4n

dimensonal (field lines spread out uniformly

throughout the bulk) and is stronger - For r R gravitational field lines are deformed

since they are confined to the 4 dimensions

(represented by a 3-D cylinder in the picture)

MPl is a smaller number in ADD Hierarchy

problem is solved

Detecting ADD extra dimensions

- Gravitons can escape into the extra dimensions

and appear as missing energy at the LHC - ? Search for an overall excess of ETmiss
- Or an excess of monojet ETmiss events

De Santo

Missing transverse energy plus single jet

Dedicated experiments have also measured

consistency with Newtonian gravity to scales lt

10-100 µm

n MDgt TeV

2 2.37

3 1.98

4 1.77

Warped Extra Dimensions (RS Model)

- ONE small, highly curved (warped) extra

dimension connects the SM brane at O(TeV) to the

Planck scale brane - Gravity is weak on the weak brane where SM

fields are confined but increases in strength

exponentially in the extra dimension (since

space-time is accordingly warped) - Signature a series of narrow, high-mass

resonances

Extra Dimensions in the gg channel

R compactification radius, k

curvature, coupling defined by k/MPL

Micro Black Holes

- MPl is the energy scale at which gravitational

interactions become important - We normally assume this scale is 1019 GeV and we

completely ignore the gravitational interaction

of the colliding particles - But if, due to extra-dimensions, MPl MEW then

gravitational interactions will be important - In fact, at length scales below 1/MPl, gravity

will dominate, and a micro-black hole will form

Micro Black Hole signature

- These micro black holes will rapidly evaporate

via Hawking radiation and will radiate like a

black body - Democratic decays to all sorts of particle at the

same time

ST is the scalar sum of the ET of the N

individual objects (jets, electrons, photons, and

muons)

Excludes the production of black holes with

minimum mass of 3.5 -4.5 TeV

Inclusive searches di-jets

- Very early search for numerous non-SM resonances

string resonance, excited quarks, axi-gluons,

colorons, E6 diquarks, W Z, RS gravitons....

Di-jet centrality and angular distributions

- Di-jet centrality ratio evts with two leading

jets in ?lt0.7 compared to events with both

leading jets in 0.7lt?lt1.3 - Sensitive to deviations from the SM due to quark

sub-structure, i.e. Compositeness - Angular distribution sensitive to contact

interactions

Excludes quark compositeness for ?lt4.0TeV

(95CL)

Lower limit on scale of contact interaction ?5.6

TeV (95 CL)

Inclusive searches dileptons

- Study invariant mass spectrum to look for

dilepton resonances (Z') - Also
- String-theory-inspired E6 models
- ADD extra dimensions

Inclusive searches leptonsMET

- Example W search
- W has W-like fermionic couplings
- W does not couple to other gauge bosons
- Tevatron limits mW gt 1.1TeV

Leptoquarks

- Leptoquarks possess both lepton and quark quantum

numbers - Pair produced search for qqll or qql? daughters
- Look at sum of transverse energy

Other models

- There are many other exotic possibilities...
- Stopped gluinos
- Split SUSY models
- Hidden sectors
- .....
- It would be impossible to cover all of these in

one lecture (and too confusing!) - ? Please go and find out more!
- ? Or, better still, find a particle...

Summary

- With 40 pb-1 the LHC experiments have begun

detailed measurements of Standard Model physics - The SM processes give a solid basis for

understanding the detectors and the background

to searches at higher mass and high ET - Numerous analyses are in place for searches
- With 1-5 fb-1 in 2011-12 we could have
- A firm discovery of the Higgs
- Indications of SUSY
- New resonances
- Other new physics
- And we could find something completely unexpected!

Additional material (and acknowledgements)

- Last years lectures
- http//www2.warwick.ac.uk/fac/sci/physics/staff/ac

ademic/gershon/gradteaching/warwickweek/material/l

hcphysics - CERN Academic Training lectures (Sphicas and

Jakobs) - http//indico.cern.ch/conferenceDisplay.py?confId

124047 - http//indico.cern.ch/conferenceDisplay.py?confId

77835 - London lectures (de Santo et al.)
- http//www.hep.ucl.ac.uk/mw/Post_Grads/2007-8/Wel

come.html - ATLAS and CMS public results
- https//twiki.cern.ch/twiki/bin/view/CMSPublic/Phy

sicsResults - https//twiki.cern.ch/twiki/bin/view/AtlasPublic/W

ebHome - Moriond Electroweak and QCD
- http//indico.in2p3.fr/conferenceOtherViews.py?vie

wstandardconfId4403 - http//moriond.in2p3.fr/QCD/2011/MorQCD11Prog.html