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High pT physics at the LHC Lecture IV Searches

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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 ... – PowerPoint PPT presentation

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Title: High pT physics at the LHC Lecture IV Searches


1
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

2
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

3
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

4
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

5
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
6
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
7
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
8
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)
9
Tevatron constraints on the Higgs Mass
  • Recent CDF and D0 combination
  • excludes 158 lt MH lt 173 GeV at 95 CL

10
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

11
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

12
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

13
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
14
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
15
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
16
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
17
Prospects for SM Higgs in 2011-12
Indicates contributions from different channels
Could exclude down to LEP limit with lt4fb-1 !
(possibly)
18
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
19
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
20
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
21
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)

22
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

23
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

24
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

25
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)
26
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

27
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

28
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
29
Examples of results
Jets MET b tagging
3 leptons jets
  • Some LHC SUSY limits are already similar to or
    better than TEVATRON

30
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
31
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
32
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)

33
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
34
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
35
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

36
Extra Dimensions in the gg channel
R compactification radius, k
curvature, coupling defined by k/MPL
37
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

38
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
39
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....

40
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)
41
Inclusive searches dileptons
  • Study invariant mass spectrum to look for
    dilepton resonances (Z')
  • Also
  • String-theory-inspired E6 models
  • ADD extra dimensions

42
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

43
Leptoquarks
  • Leptoquarks possess both lepton and quark quantum
    numbers
  • Pair produced search for qqll or qql? daughters
  • Look at sum of transverse energy

44
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...

45
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!

46
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
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