Search for the Higgs Boson at DØ in the Trilepton Channel

James Kraus University of Mississippi

1 Outline

 The Standard Model and the Higgs Boson  Fermilab Tevatron and the DØ experiment  Higgs Production and Higgs Searches at DØ – Motivation for the trilepton Higgs search  Search for the Higgs in the trilepton + Missing Transverse Energy (MET) final state  Discussion of DØ and Tevatron combinations – Comparison with LHC results

 Describes 3 of the 4 Fundamental Forces – Strong Force • Force carrier is gluon • Binds quarks into protons, neutrons • Holds protons & neutrons in nucleus – Electromagnetic Force • Force carrier is photon • Acts on charged particles • Responsible for electricity, chemical bonds

3 Standard Model

 Describes 3 of the 4 Fundamental Forces – Strong Force – Electromagnetic Force – Weak Force νe ± 0 137 137 • Force carriers W , Z Cs Ba • Responsible for some radioactive 55 56 decays, nuclear fission e- – Gravity • Planetary motion, falling apples

4 Standard Model

included in 5 Standard Model Standard Model

 The Standard Model arranges 6 quark flavors and 6 lepton flavors into 3 generations of matter

Quarks experience The generations strong, weak, and are arranged by electromagnetic forces increasing mass, but are otherwise Leptons do not similar experience the strong force. Neutrinos only interact via the weak 6 force Electroweak Theory

 Salam, Glashow, & Weinberg develop Electroweak theory in 1967 (Nobel prize in 1979) – Describes EM and Weak forces with a single theory – Predicted existence of massive, neutral Z boson  Electroweak symmetry must be broken to give us the observed electromagnetic and weak forces  With electroweak symmetry unbroken, particles are massless...

7 A Massive Problem

u d s c b W Boson Z Boson 80 GeV 91 GeV

(Photon & Gluon = Massless)

Top Quark        e e 173 GeV

1 GeV ≈ mass of the proton 8 A Massive Solution

2010 Sakurai Prize ... for "elucidation of the properties of spontaneous symmetry breaking in four-dimensional relativistic gauge theory and of the mechanism for the consistent generation of vector boson masses."

Englert Brout Higgs Guralnik Hagen Kibble

PRL 13, 321-323 PRL 13 508-509 PRL 13, 585-587 9 (1964) (1964) (1964) The Higgs Boson in the Standard Model

 Higgs mechanism responsible for electroweak symmetry breaking  Interactions with Higgs field gives particles their masses – No prediction of the strength of this interaction  The Higgs is the last undiscovered particle predicted by the Standard Model

– Standard Model does not predict the mass of the Higgs 10 Fermilab

 Fermilab is a national lab located in Batavia, IL, outside Chicago − Is home to US-based high energy physics experiments, including Tevatron

To Chicago

11 The Fermilab Tevatron DØ  Run II of Fermilab Tevatron March 2001 to Sept. 2011 − p-p collisions with energy of

1.96 TeV = 1960 mp  One collision every 396 ns − >1.7 million collision/min  Technology − Superconducting magnets  Size − 4 mile circumference CDF 12 Fermilab Site  Home to a healthy herd of bison

13 Fermilab at Night

14 The DØ Experiment

η θ  A multipurpose Pseudorapidity = -ln(tan( /2)) particle detector  Innermost detectors are the trackers, followed by calorimetry and muon chambers

 Using information from all subdetectors, we can reconstruct the pp collision 15 Electrons, Photons

 Lighter particles (e, γ) are stopped in the first few layers of the the calorimeter  Hadrons make it farther into the detector

 Only charge particles leave tracks  Track matching allow us to distinguish e from γ 16 Hadronic Jets

 When a high momentum quark or gluon is produced in a collision, results in a jet of hadronic particles into the detector

17 Muons

 Muons can pass through our entire detector – µ are massive enough not to be stopped like e or γ in EM calorimeter – Don't feel strong force, so hard interactions are rare  Muons are charged, so they leave tracks

 Place trackers outside the calorimeter to detect

passage, and match to a 18 track in inner tracker Missing Transverse Energy  Neutrinos do not interact with our detector MET muons – Infer their presence through missing transverse energy jet – Transverse = ⊥ to beamline

 To conserve momentum,

 vector sum  pT = 0 – If non-zero, then either

• Energy Mis-measurement electron • Missed one or more particles (usually neutrinos)

19 Higgs Production at the Tevatron

 The Higgs at Tevatron produced by gluon fusion (gg→H), associated production (qq→W/Z+H), and vector boson fusion (qq→qqH)

20 Higgs Decay Modes

 The Higgs decays primarily to bottom quarks at low mass, WW at higher mass

 The searches for the Higgs at the Tevatron focus on H→bb at low mass, H→WW at high mass.

 Also use H → ττ, H → γγ due to low backgrounds

21 Higgs Backgrounds Or, You've been looking for over 10 years, why haven't you found it already?

 The rate of Higgs production is small compared to backgrounds − Bottom decays swamped by QCD background except for associated ZH/WH production − WW cleaner, but still large backgrounds

22 Trilepton Higgs Search

 New search channel at DØ – not in July 2011 combination

 H0 ZH and WH production followed by H → WW decay

 For WH → WWW → lνlνlν, where l = e, µ – Ratio of eee:eeµ:µµe:µµµ = 1:3:3:1  For ZH → ZWW → lllνX, ratio is 1:2:2:1  Currently, only considering the eeµ and µµe final states, with others to be added in future 23 Higgs Decay Modes

 The expected Higgs signal from VH→VWW is between one tenth and one hundreth that from H→WW

 However, eliminates almost all W, Z, Wγ, and WW backgrounds

 So there is a gain in the signal over background in the three lepton channels compared to the 2 lepton channels

24 Trilepton Backgrounds

 Principle backgrounds for the trilepton search – WZ → lνll – ZZ → llll  Also have backgrounds with 2 real leptons and a jet or photon fake – Z + jets – Z + γ ν ν – tt→l bl b 25 Z + jets

q Z/γ*  Z→l+l- + jets cross section is the largest of all the backgrounds Fakes lepton  However, it is suppressed by low q g j→l fake rates – Z + jet fake rate is estimated by passing simulated Z+jet events through our MC detector model  Z + jet events have no real missing energy – Variables that are sensitive to the missing energy separate this background from our signal

26 Z + γ

 Z + γ significant background for µµe final state  Only difference between electrons and photons in our detector is a track

27 Z + γ

 Z + γ significant background for µµe final state  Only difference between electrons and photons in e + - our detector is a track γ e  Sometimes the photon undergoes interaction with the tracker, leading to γ→ee – If one electron gets most of the photon energy, will

be reconstructed as an electron 28 Estimating the Z + γ Background

 Correctly estimating the γ→e fake rate in MC difficult

 We use a data-driven method to estimate this background – Calculate the γ→e fake ratio from a photon sample in data – Reconstruct µµγ events, where the photon is not matched to a track – Apply the fake ratio to the data events to get the background estimate 29 Top Background

 Like Z + jets, tt requires jet to fake lepton – tt events have real missing energy – b-quark jets have a higher fake rate than light quark jets – The tt background is the smallest SM background, but tends to be more signal-like, so it is significant

Fakes lepton

30 WZ and ZZ Backgrounds

 WZ → lνll and ZZ → llll are estimated from MC – Currently, there is no dedicated 4l Higgs search at DØ, so these events are included in our search – Of all backgrounds, WZ most closely mimics signal

31 Trilepton Selection Cuts

 We have requirements on both eeµ and µµe channels – 3 good leptons must be found – M(ee) or M(µµ) > 15 GeV

– One lepton with pT > 15 GeV,

all with pT > 10 GeV – All leptons from same pp collision – Δ R = √ Δ η 2 + Δ ϕ 2 > 0.3 between any two leptons

32 Reducing Backgrounds in µµe Channel

 The µµe channel has larger backgrounds than eeµ – From Z backgrounds – jet → e fake rate higher than jet→µ fake rate, γ fake e but not µ  Have additional cuts on µµe to reduce backgrounds – Remove final state radiation (FSR) – Photon is radiated off a muon after Z decay, so µµγ will have Z mass – Also, no real missing energy, so can keep most signal in the Z mass range by cutting only low MET events 33 Reducing Backgrounds in µµe Channel

34 Reducing Backgrounds in µµe

 To reduce background more, turn to variables related to MET  Special MET – If ∆φ between MET and nearest object < 90°. specialMET = MET×sin∆φ, otherwise = MET  MET Significance – Based on the uncertainty on the energy measured in the 35 detector Reducing Backgrounds in µµe

 We require either – Special MET > 15 GeV

 or – MET Significance > 3  Removes most Z + jets and Zγ, leaves most signal

36 Event Yields

 After all cuts, have 175 trilepton events

 Less than 3 expected signal events  S/B ~ 0.15

Errors reflect uncertainty due to sample size. With all statistical and systematic errors, expected 37 and observed agree within 1σ Signal vs. Background

 Once we have a good background model, we look for differences between signal and background  Example – ∆φ between leptons  The Higgs is a spin-0 particle; W is spin-1 ⇒ small opening angle in leptons from the W decays  Leptons from Z tend to be back-to-back 38 Signal vs. Background

 Once we have a good background model, we look for differences between signal and background  Example – ∆φ between leptons

39 Signal vs. Background

 Once we have a good background model, we look for differences between signal and background  Example – ∆φ between leptons  The signal to background ratio is so small that any one variable will not be sufficient to isolate the Higgs  Instead, we look at many variables associated with each event at once – multivariate analysis

40 Multivariate Analysis

 Take multiple variables, each of which has some separating power  Combine them into one variable that separates the signal and background

41 Limit Setting

 Multivariate discriminants are used to hunt for signal  If there is a significant amount of signal, will see pile-up of events near one, in excess of background

42 How We Set Limits

 Based on log-likelihood ratio

– L is the Poisson likelihood that the s+b (b) correctly models the data – θ represents the systematic uncertainties on the measurements (luminosity, energy scale, etc.)  Calculate probability density of LLR based on models of signal and background

43 How We Set Limits

 Based on log-likelihood ratio

 CLb and CLs+b are given by the integrals of the b and s+b LLR distributions above observed LLR – Represents how well the given model agrees with data  We vary the signal content of the s+b model to find

where CLs = CLs+b/CLb < 0.05 – We exclude those points at the 95% Confidence Level (CL)44 Limit Setting vs Mass

 No prediction of Higgs mass, so calculate LLR at multiple mass points – Set limits at 5 GeV intervals between 100 and 200 GeV – Solid black observed, dashed black b, dashed red s+b for signal set to SM expectation

45 Limit Setting vs Mass

 Data consistent with background, so set limits – 95% CL limits shown for each mass point, adjusted for expected Higgs cross section at that mass – If observed limit falls below 1, we exclude SM Higgs at that mass point

46 Trilepton Higgs Limits

47 We expect a 95% CL limit of 12x SM expectation at mH = 125 GeV, measure a limit of 18x SM expectation DØ Combination

 Trileptons only one channel out of many

48 DØ Combination

 Trileptons only one channel out of many – Others include ZH→llbb,

49 DØ Combination

 Trileptons only one channel out of many – Others include ZH→llbb, WH→lνbb,

50 DØ Combination

 Trileptons only one channel out of many – Others include ZH→llbb, WH→lνbb, ZH → ννbb,

51 DØ Combination

 Trileptons only one channel out of many – Others include ZH→llbb, WH→lνbb, ZH → ννbb, H→WW→lνlν,

52 DØ Combination

 Trileptons only one channel out of many – Others include ZH→llbb, WH→lνbb, ZH → ννbb, H→WW→lνlν, VH→l+l+X,

53 DØ Combination

 Trileptons only one channel out of many – Others include ZH→llbb, WH→lνbb, ZH → ννbb, H→WW→lνlν, VH→l+l+X, H→ττ,

54 DØ Combination

 Trileptons only one channel out of many – Others include ZH→llbb, WH→lνbb, ZH → ννbb, H→WW→lνlν, VH→l+l+X, H→ττ, H → γγ, and more

55 DØ Combination

 Trileptons only one channel out of many – Others include ZH→llbb, WH→lνbb, ZH → ννbb, H→WW→lνlν, VH→l+l+X, H→ττ, H→γγ, and more – Below 145 GeV, H→bb searches dominate, but H→WW still contributes significantly

56 DØ Combination

Exclude 159 to 166 GeV

57 We expect a 95% CL limit of 1.85x SM expectation at mH = 125 GeV, measure a limit of 2.53x SM expectation Previous Higgs Search

 The Tevatron Higgs search limits from July `11

156–177 GeV

58 Current Large Hadron Collider Results

 The highest energy collider in the world is now the LHC at CERN

– 122.5 < mH < 127.5 allowed at 95% CL

59 Tevatron Combination

 The CDF experiment does similar Higgs searches – CDF sees larger excess at low mass than DØ

149–178 GeV

60 Tevatron Combination

147 – 179 GeV

61 We expect a 95% CL limit of 1.1x SM expectation at mH = 125 GeV, measure a limit of 2.2x SM expectation Tevatron and LHC results

62 Magnitude of Excess  Most significant local excess – Tevatron: 2.7σ at 120 GeV – CMS: 2.8σ at 125GeV – ATLAS: 2.6σ at 126 GeV • H→γγ+H→ZZ channels 3.4σ

63 Global Excesses

 However, need to consider look-elsewhere effect – The more independent mass points examined, the more likely it is that at least one of them will be found to be high due to a statistical fluctuation – The look-elsewhere factor scales the probability for the number of independent mass points  After application of look-elsewhere factor – Tevatron global excess of 2.1σ over 100 GeV – 200 GeV – ATLAS global excess of 2.2σ over 110 GeV – 146 GeV σ – CMS global excess of 2.1 over 110 GeV – 145 GeV 64 Plans for Improving Results

 Tevatron no longer taking data, but we are still refining our search to improve our limits  For the trilepton search, we are looking into – Improving our multivariate analysis by including jet information – Adding eee and µµµ channels – Improving jet modeling  More is still to come – Tevatron is close to having the expected limits less than the SM expectation between 100 and 180 GeV 65 Conclusions

 Have set limits on the Higgs in the trilepton channel at DØ – First time this has been done at DØ was in Feb. 2012

 Tevatron excludes Higgs masses 147 – 179 GeV @ 95% CL – 110 – 122.5 GeV, 127 – 600 GeV excluded by LHC experiments

 The global excesses are – Tevatron: 2.1σ for 100–200 GeV – σ ATLAS: 2.2 for 110 – 146 GeV 66 – CMS: 2.1σ for 110 – 145 GeV Backup Slides

67 Feynman Diagrams

 A pictorial way to depict particle interactions  = quark, lepton, or hadron  = gamma, W Z  = gluon

e+ e+ e- becomes e m

i e- t photons time 68 The Higgs Field

 The Higgs field is everywhere  The Higgs field impeded the progress of a particle, slowing it from speed of light – Effectively giving it mass  The Higgs boson is the quanta of the Higgs field

69 Indirect Measurements

Because of quantum mechanics, the Higgs bosons can influence other particles without being directly detected

More mass ⇒ more interactions with virtual Higgs bosons

Measuring the mass of the W boson and top quark can tell us about the Higgs boson

t t 70 Indirect Measurements

Fit to the precision EW data Prefers a low mass Higgs

⇒ MH ≈ 94 GeV

⇒ MH ≤ 152 GeV at 95% CL

t t 71 Look-Elsewhere Effect

 Consider a simple case – flipping a coin 100 times – Expect it to come up heads about 50 times – Not exactly 50 each time – probability distribution of number of heads – If we perform this task 1000 times (100, 000 coin flips) we may have a case where we find 65 heads, and not 50 – Does not mean coin is broken – The probability of getting 65 heads is 0.00087 – but if you perform the experiment 1000 times, there is a 87% chance72 this will happen at least once – “look elsewhere effect” eeμ/μμe Systematics Uncertainties

• Apply both flat and shape systematics; shape systematics include Z-pT smearing, electron pT smearing, and muon pT smearing

73 The Standard Model of Particle Physics

 The Standard Model describes the strong, electromagnetic, and weak forces – Strong force – holds quarks together in protons and neutrons, protons and neutrons in atomic nuclei – Electromagnetic force – runs your iPod, etc.

– Weak force – responsible for some nuclear decays 74 The Standard Model of Particle Physics

 The Standard Model (SM) describes the electromagnetic, weak, and strong forces  Only quarks feel the strong force  Force carrier is the gluon

75 The Standard Model of Particle Physics

 The Standard Model (SM) describes the electromagnetic, weak, and strong forces  All charged particles feel the electromagnetic force  Force carrier is the photon

76 Searching For The Higgs at DØ

 In all decay modes, there are at least 100 background events for every expected signal event  There is no one variable, such as dilepton mass or missing transverse energy, that can separate the Higgs signal from background  However, by looking at multiple variables, we can separate the signal and background sufficiently to possibly find the Higgs

77 The Weak Force

 Henri Becquerel discovered radioactivity in 1896 “missing” energy (neutrino) 137 137 Cs Ba 55 56

β particle (electron)  Enrico Fermi proposes model for beta decay in 1934 p

– Point interaction with strength GF -5 -2 n GF = 1.166x10 GeV 78 ν e- The Weak Force

 Fermi's point interaction fails at high energies – Weak force is short range: Massive force carrier – EM is long range: photon massless  - -  p e e e n W -  W ( q ) ( q )  α2 W ν  W p p q 2+M 2 e W e-

α2 α2 α2 ≪ W ≃ e e G = If q MW ⇒ 2 F 2+ 2 2 MW 79 q M γ q

Assume αe = αW ⇒ MW ≈ 100 GeV The Weak Force

 Henri Becquerel discovered radioactivity in 1896 “missing” energy (neutrino) 137 137 Cs Ba 55 56

β particle (electron)  Enrico Fermi proposes model for beta decay in 1934 p

– Point interaction with strength GF -5 -2 n GF = 1.166x10 GeV 80 ν e- The Weak Force

α2 α2 α2 ≪ W ≃ e e G = If q MW ⇒ 2 F 2+ 2 2 MW 81 q M γ q

Assume αe = αW ⇒ MW ≈ 100 GeV The Standard Model of Particle Physics

 The Standard Model describes the electromagnetic, weak, and strong forces  However, we have not yet found the last particle predicted by the Standard model, the Higgs Boson

82 WZ, ZZ, and top Backgrounds

 WZ → lνll and ZZ → llll are estimated from MC – Currently, there is no dedicated 4l Higgs search at DØ, so these events are included in our search  Like Z + jets, tt requires jet to fake lepton – tt events have real missing energy – b-quark jets have a higher fake rate than light quark jets – The tt background is the smallest SM background, but tends to be more signal-like, so it is significant