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High pT physics at the LHC Lecture IV Searches


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 ... – PowerPoint PPT presentation

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Title: High pT physics at the LHC Lecture IV Searches

High pT physics at the LHC Lecture IV Searches
  • Miriam Watson, Juraj Bracinik
  • (University of Birmingham)
  • Warwick Week, April 2011
  1. LHC machine
  2. High PT experiments Atlas and CMS
  3. Standard Model physics
  4. Searches

  • 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
  • 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
  • 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)

? cut-off scale at which new physics becomes
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
  • 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

With good segmentation
Low mass Higgs vector boson fusion
  • Tag two forward jets
  • Select Higgs bosons in the channel H?tt (t?l or
  • 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

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
Note different mH ranges on plots
Prospects for SM Higgs in 2011-12
Indicates contributions from different channels
Could exclude down to LEP limit with lt4fb-1 !
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.
  • in principle there could be more than one Higgs
  • perform direct searches for extra Higgs bosons

MH measurement dominated by ZZ?4l and H?gg
modes Eventual precision 0.1 over large mass
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
  • 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)

  • SUSY allows for proton decay to occur via p ?
  • 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
  • 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

  • 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
  • Plot is for multi-jets missing ET

Example LHC Search Mode - Squark/ Gluino
  • 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
  • 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),
  • 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

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
  • 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
  • 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

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
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

Excludes quark compositeness for ?lt4.0TeV
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 possess both lepton and quark quantum
  • 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...

  • 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
  • CERN Academic Training lectures (Sphicas and
  • http//indico.cern.ch/conferenceDisplay.py?confId
  • http//indico.cern.ch/conferenceDisplay.py?confId
  • London lectures (de Santo et al.)
  • http//www.hep.ucl.ac.uk/mw/Post_Grads/2007-8/Wel
  • ATLAS and CMS public results
  • https//twiki.cern.ch/twiki/bin/view/CMSPublic/Phy
  • https//twiki.cern.ch/twiki/bin/view/AtlasPublic/W
  • Moriond Electroweak and QCD
  • http//indico.in2p3.fr/conferenceOtherViews.py?vie
  • http//moriond.in2p3.fr/QCD/2011/MorQCD11Prog.html
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