Status of Higgs boson measurements at the LHC

Status of Higgs boson measurements
                        at the LHC

                                Kajari Mazumdar
                    Tata Institute of Fundamental Research
                                     Mumbai

       Disclaimer: not a full review of all the available public results .
       Could not avoid a bit of personal bias towards CMS!

       For more information visit:
       https://twiki.cern.ch/twiki/bin/view/AtlasPublic/HiggsPublicResults
       https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsHIG

DAE-BRNS symposium, IIT, Guwahati                                      December 8, 2014

Higgs boson is for the mass (of elementary particles),
                     by the mass (creations by artists inspired by science),
                     of the mass (science is part of human culture!)

                Higgs decay candidates to various final states

12/5/2014                                                                      2

Triumph of human intelect
Discovery of Higgs boson in 2012,
 conceived ~50 years earlier
     human understanding of Nature reaches new height

Elegance of the idea of unification and other hall
marks achieved in the field during the last century

Gigantic efforts in many fronts led to the success
at the LHC, in first phase of operation (2010-13).
-- 2012: “a Higgs like new particle”
-- 2013: “a Higgs boson”  Physics Nobel 

After the discovery: remarkable progress in understanding the Higgs sector
           • Is it the Higgs boson of standard model (SM)?
           • Is there any other Higgs particle?
           • Is the resonance a window to new physics?

   12/5/2014                                                            3

LHC played the perfect role for the discovery
2 major multipurpose experiments                     • Integrated luminosity (L ) delivered
• A Toroidal LHC Apparatus (ATLAS)                   by LHC machine during Run1 ~ 30/fb
• Compact Muon Solenoid (CMS)
                                                     • √s = 7 TeV, data used L ~5 /fb
High efficiency of experiments and                     √s = 8 TeV ..           L ~20 /fb
fast computing  quick and grand                      ~ 0.5 Million Higgs produced/expt
harvest                                                     ~ 500 per hour!

Higgs production at LHC: main modes

         gluon-gluon fusion   Vector boson fusion   Associated productions with W, Z, top-pair

                                                                   One of the rare modes of
                                                                   production being studied: tHq

Total ~ 22 pb
   12/5/2014                                                                                     4

Main decay channels of the Standard Model Higgs boson

Statistical analysis
• Background distribution mostly Gaussian
 stability of result expressed in terms of width s of the Gaussian.

•   Characterization of excess using test statistic

Significance                              where

•    Greater the significance (s)  minor the p-value  lower is the chance
    that the observed excess is due to background fluctuation.
     12/5/2014                                                           5

Higgs Hunt
Comprehensive Higgs pursuit : survey of each accessible production mechanism
                               for each primary decay mode
                              Categorization provided better sensitivity
 Example of categorization
 of production process

                                     Tagging a search channel via production mode
                                     Tagging done for decay mode as well .
 0, and 1 jet boosted

   12/5/2014                                                                   6

Individual analyses
Illustration
                                       sobs /sSM = m
                                       measure of signal strength
                                      compared to SM expectation for
                                      Higgs mass at the fitted value.

Significance of observation of 125 GeV Higgs boson: CMS summary

   12/5/2014                                                       7

Higgs  2 photons
Tiny peak above a falling background distribution
for invariant mass of 2 energetic, isolated g s.

 Branching ratio: 2*10-3

Huge SM background estimated by fitting
distributions on the side bands with polynomials.

Enormous effort to understand the detector
response to photons.
 Signal strength

   12/5/2014                                        8

H ZZ* 4 leptons
 Use matrix element calculations for signal and background processes to
 evaluate probability densities on event-by-event basis.
  Increased sensitivity to discriminate background against signal
  Used for discovery & determination of mass, width, spin, parity, couplings.

 Signal: 4 energetic isolated leptons                         Z 4l
 Discovery based on 25 events (18 expected)

                                                             H  4l
Backgrounds:
• Irreducible backgrounds: qq ZZ 4l
 + gg  ZZ 4l (at NNLO)                                              4l continuum

• Reducible background form Z+X, tt, Zbb

• Kinematic information utilized from MEM to construct
discriminant  improves significance by 20%.

 Interference of diagrams for off-shell resonance &
continuum background must be taken into account.
    12/5/2014                                                               9

H WW
                                         Decay leptons closer
                                         compared to background
                                         V = W/Z

2 (3) leptons + missing
energy+ 0/1/2 jets

CMS: measured rate
less than that expected
 from SM
  m = 0.72+20 -0.18

Recent from ATLAS

                              ggH production  mggF = 1.01+0.27 -0.25
   12/5/2014              VBF + VH production  mVBF = 1.27 +0.53 -0.45   10

Higgs decay to fermions
•     The fermionic decay channels of Higgs need to be observed to confirm the
      nature of coupling of H to fermions . Yukawa coupling   G ff : (√2GF /8p) mf2 mH

•     H gg : mixed coupling: both vector boson and fermions.

•     H  tt : evidence  3.2 s significant excess above background.

•     H bb: 2.1 s observation, m = 1.0 ± 0.5
    Search in W( ln) H, Z ( ll)H , (Z nn) H modes

•     ttH : excess , but presently large error

Higgs decay to fermions established by combining
results from direct decay channels WH, ZH, with
H bb & H tt

Combined significance obs.(exp.) = 3.8 (4.4) s
m = 0.83 ±.24
       12/5/2014                                                                  11

H tt
3 production modes, combined with different decays t  e/m/ hadrons
                                          •Invariably missing energy in final state
                                          • event by event estimator for di –t mass
                                          likelihood

background only hypothesis includes H(125GeV)  WW for each value of mH

ATLAS: m = 1.42 +0.44 -0.38 significance obs.(exp.) = 4.5(3.5) s , mass = 125 GeV
CMS: m = 0.78 ± 0.27 , mH = 125 GeV , significance obs.(exp.) = 3.2(3.7) s
             best fit for mH = 122 ±7 GeV
   12/5/2014                                        Results consistent with SM      12

H mm, ee
Probes 2nd and 3rd generation couplings, lepton non-universality of H decay.
Similar to H  gg search, but even rarer, though clean.
Br(H mm) = 2.2 *10-4, Br(Hee) = 5 *10-9

Huge background due to Drell-Yan
Don’t expect observation of SM signal in current data

CMS, mm channel
Observed significance < 1s
signal strength = 0.8 ± 3.5            s/sSM                          Br (H ee)

                        •     Br (H  mm) upper limit = 7.4 * SM
                        •     Br (H ee) upper limit = 3.7*105 SM
   12/5/2014
                        •     Measured Br (H tt) = 0.0632 ± 0.0036       13

Higgs and the top quark
•   Higgs coupling to top quark appears crucially in multiple aspects.
•   ttH coupling has been probed by searching in ttH production with H  bb
                                            Direct probe for Yukawa coupling

                                             Search in events with 2 b-tagged jets
Note top quark in the loop can be            and H  hadrons, leptons, photons
replaced by new particles suitably

Production rate too low to
be observed.

Best fit for signal strength
m = 2.8 ± 1.0 at 68% CL

    12/5/2014                                                                  14

Rare and Exotic: H  Z g
• One-loop decay
  probing couplings
  complementary to
  Hgg and H gg
 • SM Branching ratio ~0.1%
 • In composite Higgs model, H  Z g rate can be large, though H  gg & H gg remain small

 • Experimental advantage:
   highest resolution photons give usable S/B

    Run1: limited sensitivity in the 5-20X range/expt.
                                                                                     15
    No evidence ~10X SM limit observed/expt.

Compatibility of Higgs signal with standard model

                                   Production &
                                   decay tagged

ATLAS: m = 1.3 ± 0.20           CMS: m = 1.0 ± 0.09(stat. )+.08-.07 (theo.)± 0.07(syst)
                                             m (VBF+VH)/ m(ggH+ttH) = 1.25 +0.63 -0.49
NNLO calculation of Higgs production
 test of theory framework                         Theo. Includes QCD scales, PDF + as
For LO, m = 2.6 ± 0.4, NLO , m = 1.9 ± 0.3        UEPS, BR
   12/5/2014                                                                       16

Measurement of mass
•   Fundamental property, not predicted by theory
      Once measured, SM predictions are completely determined

•   Use resonance structure in high resolution channels H gg, H 4leptons

         CMS : 125.03 ± 0.26 (stat.) ± 0.14 (syst.) GeV     CMS and ATLAS results
         ATLAS: 125.36 ± 0.37 (stat.) ± 0.18 (syst.) GeV   compatible within errors

Off-shell Higgs production diagrams interfere with the diagrams for continuum
  backgrounds  cause a mass shift, significant effect for 2 photon mode.

                                                           CMS observes opposite
                                                             trend of ATLAS!
                                                           DmH = -0.87+0.54-0.59 GeV

                                                           ATLAS DmH =1.47 ± 0.72 GeV

    12/5/2014                                                                          17

Higgs couplings
• Cross section measurement mixes the couplings
• In Narrow width approximation

                                           Higgs Coupling to P at production

                                                        Higgs coupling to X for decay

If actual couplings are modified compared to SM

                                     SM  K I =1                    V = vev =246 GeV

                        Eg. for gg H  gg , assuming SM particles in the loop
                        only, the rate can be modified.
                        • Or there could be new particles in the loop.

          Disentangle the couplings using multiple final states

Framework for fitting couplings
Leading order tree-level motivated: KW, KZ, Kt, Kb, Kt,…
Effective couplings for the loop-induced processes : Kg, Kg
   ie., H  gg, H Zg, gg H

Cross section rescaled using couplings
 Quantify possible small deviations

Assumptions:
• Single resonance of mass 125 GeV
• Narrow-width approximation
• SM tensor structure of the Lagrangian (spin0, CP even)

•    Current measurements insufficient to simultaneously fit all couplings
     with precision  assume universality of vector and fermion couplings
     though actual couplings are still different.
    KW = KZ = KV , and Kt = Kb = Kt = Kf

       Invisible Higgs decay inferred from

     12/5/2014                                                               19

Measurement of couplings

    Bosonic couplings with
    custodial symmetry                            Top coupling from
                                                   ttH measurement
     fermionic couplings
                                                  Effective couplings
            Loop-induced
             couplings                               Contribution from
                                                      new physics

    Regge Plot: scale couplings according to mass

    For SM, vev = M= 246 GeV, e = 0

•    consistent with minimal scalar sector (only 1 Higgs ).

•    Current precision of some couplings (W/Z, g, g) ~ 10- 20 %
       12/5/2014                                                         20

Global measurement of Higgs width
• Availability of decay channels for Higgs depend on its mass
• Fortuitous value of mH!  fermionic decays are possible to observe.
  chance to study the Yukawa couplings.

• At 125 GeV, total decay width GH ~ 4.2 MeV
• Experimental resolution for 2 photon and 4 leptonic channels ~ 1-3 GeV!
• A direct measurement of width can indicate a wider resonance or mix of 2.

Given the mass, use global fit to estimate total decay
width, fixing unmeasured modes to SM predictions.

• GHtotal = Si GiH + GH   BSM

•    Even if Higgs couples to any new massive particles
    total decay width may or may not be affected.

•    Direct measurement of Higgs width
     H ZZ 4l and H gg combined.

      12/5/2014       GH obs.(exp.) < 1.7 (2.3) GeV.                     21

Measurement of width using interferometry
                                       qq  ZZ and gg  ZZ (continuum)
                                       gg  H ZZ (resonance production)
                                        interference gg initiated processes.

•   Analysis strategy: larger GH produces more events for large mZZ >> mH

                                    Simultaneous fit to mass distribution of
            Off-shell               on-shell and off-shell regions .
                                    Use H  ZZ 4l and 2l 2n

                                    CMS: GH < 22 MeV ~ 5GH SM
                                   ATLAS: GH < (4.8 – 7.7)GH SM

                                                   More details in
    12/5/2014                                                                  22
                                                   parallel session

Determination of Spin-Parity (JP) state
•   Higgs is the only particle known so far to have spin = 0  scalar boson
•   Theoretical arguments forbids it to have spin =1, since it decays to 2g
•   It could have spin = 2 !

• Spin-parity and tensor structure probed with kinematic information
•  no real measurement .
   experiments have tested alternative hypotheses utilizing kinematics of
decay products: 0+ or something different.

          All exotic states excluded at 99.9% CL. JP hypothesis of 0+ favoured
    12/5/2014                                                                    23

Are there 2 degenerate states?
Non-zero spin state scattering amplitude contains large no. of free parameters

There may be 2 Higgs particles, of slightly different other properties, eg, JP
eg: 2m+ : Kaluza Klein graviton like with minimum coupling
     2b+ : Kaluza Klein graviton like with standard model in the bulk
     2h+ : BSM tensor with higher dimension operators.
     2h- : BSM pseudo-tensor with higher dimension operators.

 Not possible to determine with H ZZ4l mass
 But presence of a 2nd state would modify the decay kinematics.
 The fraction of the 2nd state determined using

Assume: widths of SM or
alternative states are all
much smaller than
experimental mass
resolution.

   12/5/2014   All fractions consistent with JP = 0+                       24

Heavy Higgs in H  W(ln) W(jj) final state
                                • Use angular correlation among final state
                                  particles as discriminant.
                                • standard model-like Higgs boson excluded for
                     l = e, m
                                  mass ranges 170–180 GeV and 230–545 GeV
                                  at 95% CL.
                                • No possibility of 2 Higgs-like particles jointly
                                  ensuring unitarization of WW scattering:
                                  a light (125 GeV) and a second heavy Higgs

Invariant mass distributions for various mass hypothesis in m final state

   12/5/2014                                                                  25

Higgs production in association with a single top quark
                       Probes tHq coupling
                       destructive interference in SM
                      sensitive to relative sign of Higgs boson coupling
                      to fermions & bosons.

                        Assume Yukawa coupling of Higgs boson to top
                         quark Yt = -1
                         rate increases by a factor of ~13 ~ 234 pb.

                         Final state studied
                         4b quarks, 1 lepton, 1 light jet

• Multivariate analysis technique
• Observation agrees with b only and s+ b hypothesis.
• Exclude production rate larger than 1.77 pb at 95% CL.
12/5/2014                                                           26

Higgs self-coupling
 Higgs self-coupling: key aspect of electroweak symmetry breaking mechanism
• Observing two Higgs boson in the event is the only way to probe it.
• Accurate measurement may indicate the extension of Higgs sector, if any.

Higgs potential

• SM production rate of double Higgs is small,
   signal interfere with background destructively.
 Enhancement possible through resonant production of H  hh
  in MSSM, NMSSM, 2HDM, Higgs portal model etc.

 Very good prospect for HL-LHC, with L=3000 fb-1, ~120000 HH events
• bbgg : small rate but relatively clean signature
• bbWW: large rate but large background as well
    12/5/2014                                                          27

Di-Higgs production via narrow resonance and decay to 4b final state

                                                            Spin0 radion

8 TeV data after b-tagging and
kinematic selections

                                                        Spin2 KK graviton

   12/5/2014                                                        28

Invisible decay of Higgs boson
•   Higgs can decay “invisibly” in SM via process  Br(H→ZZ*→νννν) ~0.1%
•   Current LHC results do not exclude the possibility of a sizeable decay
    branching ratio to invisible particles of the discovered Higgs boson.

eg.1.: H  to stable or long-lived particle(s) which can’t be detected in expt.
   2: H decays to a pair of dark matter candidate particles which have very low
interaction probability with SM particles.

Search performed in vector boson fusion
and associated production modes

                               Z (bb) H(inv.) overlaps
                               with Z(nn) H(bb) !

                                Combined Br(H invisible) < 58 (44)% obs.(exp.)
     12/5/2014                                                                    29
                                  Details in Parallel session

Search for Higgs bosons invoked in SUSY
General 2Higgs doublet model with MSSM as the most popular scenario.
• 5 Higgs bosons: h, H, A, H± , all can have massed at or below TeV
  access at LHC

•    Various possibilities for flavour conserving couplings for the lightest neutral, h
•   For high masses of H, A, H± (decoupling), h is essentially SM Higgs
•   For light H, A, H± , cos (b-a)  0  little sensitivity to tanb

• Search for the H is extension of SM search in high mass region
  measurement of cross section complements limit on couplings.
• Additional scalar singlet is motivated by dark matter, may mix with SM Higgs
 various implications, not covered here.

Due to lack of time, can hardly discuss any result. Several talks in parallel sessions
• BSM Higgs
• Invisible Higgs
• Search for Neutral MSSM Higgs Bosons in the Di-Tau channel
• Search for a low mass charged Higgs boson decaying into cs quarks.
    12/5/2014                                                                         30
                                                   2 CMS results, just as a sample.

Search for H hh and A Zh

1. Gluon fusion to produce heavy H, decay to 2 SM like h
• Search for decays      WWWW, WWZZ, WWtt, ZZZZ, ZZtt, ZZbb, tttt, ggWW,
    ggZZ ,ggtt

2. A Zh in ll WW, ll ZZ, ll tt, ll gg (can probe upto 1.5 pb)
Observed and expected limits with 1 and 2 s bands in mulyileptoon and gg channels

  No contribution from A  Zh                             No contribution from H  hh
                                95% CL Upper limit
                                s (gg H) < 7 pb

                                     95% CL Upper limit
                                     s* Br(Ahh) < 2 pb

  12/5/2014   Constrain on tanb vs. cos(b-a) plane, in the context of 2HDM      31

Pseudoscalar boson A  Zh  llbb

                Narrow width approx.           GA = 30 GeV

12/5/2014                                             32

Future

12/5/2014            33

Projections for measurements of Higgs in high luminosity LHC

                                Higgs couplings can be measured with
                                uncertainties ~ 5% with 300/fb. @ HL-LHC

                                Expected deviation from SM O(v2/TeV2)
                                 k parameters get small contribution
                                 but pattern may indicate specific physics.

Set to explore new territory!

 12/5/2014                                                            34

Scattering of longitudinal vector bosons
  Fundamental probe to test the nature of Higgs boson and its role in EWSB
                                        Each diagram ~ s2
                                         s(ppWW) > s(pp  anything)!

                 Unitarity restored by scalar Higgs
                 Cancellation also requires Higgs < 800 GeV

• Taming the rate could be managed by alternative EWSB mechanism
 Search for possible resonances         VV Scattering spectrum, σ(VVVV) vs M(VV)
                                                               SB < 1TeV       SB > 1TeV
 Eg.,sensitivity to anomalous WW                               SB sector        SB sector

                                                   ds/dM(VV)
 resonances in Vector boson scattering                         weakly coupled   strongly coupled

     12/5/2014                                                                            35

Summing it up
LHC experiments have moved from discovery to precision physics era in Higgs sector.

•   125 GeV Higgs has been extensively characterized with Run1 data.
•   Current measurements are in agreement with minimal Higgs mechanism.
•   No exotic discovery as yet

•   Established : Origin of mass (scalar field BEH mechanism) of particles in a
    quantum field theory with local (point-like) gauge interaction.

•   Starting from a reductionism strategy: question of structure of matter evolved
    into the question of origin of interactions (local gauge symmetries) and matter
    (interaction with Higgs field)

•   The rise in centre of mass energy at LHC in next run, gives access to new territory
    for the search of the unexpected .
    Actually miles to go before we sleep!
                                                                   Stay tuned!
•   However , we shall always manage to know only a drop of the ocean!
    12/5/2014                                                                     36

Backup

12/5/2014            37

Precision for determination of signal strength

12/5/2014                                                    38

Diboson production
                                      CMS Preliminary (Aug.2014)

                                      measurement of WW
                                      Cross section being finalized

                                 Latest from ATLAS
12/5/2014                                                     39

Upper limit on spin-independent DM-nucleon cross section

 Note: 90% CL Upper Limit is used for all experimental measurements.
 CMS limit: < 95% CL
12/5/2014                                                              40

Foundation for Higgs search: stairways to heaven
        Yesterday’s discovery is today’s background AND signal as well!

12/5/2014                                                                 41

Run III

Run I                      Run II                                                    Run IV

0.75 1034 cm-2s-1                1.5 1034 cm-2s-1     1.7-2.2 1034 cm-2s-1   ~5(7.5!) 1034 cm-2s-1
  50 ns bunch                     25 ns bunch            25 ns bunch             25 ns bunch
high pile up 40                   pile up 40            pile up 60         pile up 140 - 200

                    50  25 ns

   12/5/2014                                                                                  42

Precision on Higgs coupling

   12/5/2014                  43

Lepton flavour violation in Higgs decays?
If SM fermions mix with vector like fermions
 not all mass originate from Higgs mechanism .
 Higgs dceay can have lepton flavour violation

Search for H mt in electron and hadronic decays of t
• Has to compete with H  tt mode
• Transverse momentum distribution of m in LFV decay will
have a Jacobian peak at 0.5* mH
• Expect small peak in visible mass distribution beyond
  Z  tt decay

                        95% CL UL on B(H mt)

Charged Higgs search in fully hadronic final state
Parallel talk
                                              Heavy Higgs (m = 180 -600 GeV)
Light Higgs(m = 80 -160 GeV)
                                              4-flavour scheme
                                              5-flavout scheme

                                              t(b) H ± H ± t n

  t b H ± H ± t n
                                                        In (mH± , tanb) plane
                                                        large regions excluded
                                                        for light Higgs, except
                                                         tanb  9 , mH± > 150 GeV

                                                        Heavy Higgs has also been
                                                        searched in H ± tb mode
                                                        via inclusive dilepton and
                                                         single lepton final state.
     No luck till now, score cards 0 for BSM Higgs
    12/5/2014                                                                  45

MSSM Higgs to tau pair

12/5/2014                            46

Improvement in searches for future

                                     MSSM Higgs
                                       h, H, A

                                                             large tan b
                                          NOW
Small, moderate tan b

                                     Future LHC
                                      13 TeV
                                       300/fb

                               Cross sections scaled
                               by expected parton
                               luminosity

    12/5/2014                                                          47

ATLAS coupling

                 4 Independent signal strengths
                 + SM decay branchings

12/5/2014                                48

Experiments almost ready

12/5/2014                              49