Broken Symmetry:W, Z and Higgs Bosons at D0

The Collider Experiments High Energy Group

Outline

• Physics introduction• W mass measurement• Higgs boson searches• Our group• What do grad students do?

– Atlas or D0. Which is right for you?

What’s the Physics Motivation?The goal of particle physics is to understand the universe at the smallest distances and, equivalently, the highest energies.

And now a (partial) picture, the standard model… including an H (not shown)

What’s the Physics Motivation?Many talks begin with “The standard model is extremely successful”, and it is. Interestingly,

the SM is a gauge theory and therefore has matter particles (fermions) interacting via force carrier particles (gauge bosons: , W, Z).

and minimal gauge theories require the bosons to be massless, e.g. m = MW = MZ = 0.

but we know from experiment, m = 0 MW = 80.399 +- 0.025 GeV, MZ = 91.1876 +- 0.0021 GeV

electroweak symmetry is broken (EWSB)

How is this dealt with?Introduce a new scalar field (particle) into the theory. When put in properly, gauge invariance is maintained, and (some) bosons must get mass.

This is the Higgs mechanism

nice theory, but the Higgs boson has not been found! Is it right?

We are working on two areas directly related to understanding this symmetry breaking and the Higgs mechanism:

1. Measuring the W boson mass, 2. Searching for the Higgs

W mass measurements• In the absence of Higgs discovery

– Use internal consistency of the S.M. to constrain mH. Biggest uncertainty from Mt, MW…

W Wt

b

W WH

W

In SM, MW/MZ = cosW

sinW = f(GF,aEM,MZ,R) R = c1(Mt/MZ)2 + c2ln(MH/MW)

And these constraints give

• Experimental issues: W-> ev

– It’s true that, M2 = (Ee+Ev)2 – (pe+pv)2, but..

Electron

MET

Measuring the W mass

Measuring the W mass

– We cannot measure neutrino. Infer from momentum conservation, but

– We cannot constrain pv along beamProtons are bags of quarks, not

fundamental…Instead measure pT

e, pTv, mT and infer MW

– We cannot analytically predict these distributions, need a simulation

Much of the work goes into developing simulation

Use data control samples to calibrate“T” subscript means perp. to beam

Luckily, there’s the Z boson

• Z->ee, so can measure mass directly– Use this to calibrate our simulation

But many other effects

• Trigger…• e identification

• What happens with W pT? (“recoil”)

• Beam luminosity increasing…• W (and Z) production details…• Bremstrahlung…• …

This is a measurement at roughly 1 part in 5000. Everything must be done extremely thoroughly

Does the W pT matter?

g

The recoil resolution and model affects the pT

v and mT variables!

We can test our simulation using Z’s…

The Z pT is very well measured from the ee pair. For W, we cannot do this and must use “rest” of the detector. This plot compares the “rest” in Z events to the well measured ee pair in Z events…

Measuring the W mass • Having tuned up the simulation, what

do we see?

http://www-d0.fnal.gov/Run2Physics/ WWW/results/prelim/EW/E27/

Measuring the Mass

• Our brand new result:

• Uncertainties:

MW = 80.401 +- 0.044 GeV

Limited by stats in control samples!

The world’s best measurement

Measuring the Mass

• Where to now?– Beginning next round with 4x the data.

Finish in <1 yr if all goes well.

– Then on to the final version with full data, at least another 2x improvementFinish in 2012/13?

• Expect to share the world’s best measurement for 10 years (or forever?)– Room for a student to work on this. A

great thesis topic!

But, really, just Find the Higgs!

• or whatever is responsible for EWSB– This is a major component of the current

D0 research program– Many people, but still many

opportunities

• Unlike W mass, Higgs is very low S/B– The whole issue is needles in haystacks– Better look in all the possible haystacks

• What is already known?

What do we know?

Reminder from earlier mH > 114 GeV, butalso probably mH < 160 GeV (or so)

At D0: Lots of stuff in the way…

BF

Rates for different processes

And then the HiggsWH: e/ bb

bbqq’ e/ W(e/)W(e/)

ZH: ee/ bb bb bbqq

ttH: lb qq’b bb

gg→H: W(e/)W(e/)gg (+ 2 jets)

WW →H: (+ 2 jets)

Two regimes: mH < 135 GeV: H->bb, needs additional info mH > 135 GeV: H->WW/ZZ, stands alone

Low Higgs Mass, mH < 135 GeV

• Cannot get sufficient S/B with only Higgs, so add something else at a price in rate

EVENT DISPLAYq

q

W/Z

W/Z

H bb

l, l, , l,

time

Low Mass (con’t)

• Use the W or Z as a “tag” to reject background:– W->lv or Z->ll, vv– Look for lepton, or missing energy or

both,

• and then also for the Higgs decay– But don’t see bb quarks. – See “jets”, or streams, of particles– Reconstruct Higgs mass (limited res.)

Mass Reconstruction (con’t)

Do you see the Higgs? I don’t…

Low Mass• Use advanced computing techniques

– H matrix, neural nets, boosted decision trees, …

• As well as physics insight– Better resolution (Strauss…)

High Mass, mH > 135 GeV

• Here, H->WW, then Wlv with l=e,– Very low backgrounds

primarily straight WW from SMDifferent spin structure, so use

angles, pT’s

– Very low rate, so need efficiency!

q

q

t

t

HtW

W

l

l’,

High Mass (con’t)

But it’s still difficult!

High Mass (con’t)

So, again, be smarter

And where we stand now

What next?

• Will continue to add data. 3x more?• And improve analysis techniques

We are doing much more than simply adding data. Getting smarter all the time

An aside about Fermilab & D0

•Fermilab Tevatron–ppbar accelerator

– ECM = 1.96 TeV

– 60% through running– Near Chicago

•D0– detector at FNAL– broad purpose HEP program; 600 collabs.

Both Tevatron and D0 are running very well…

1/5 of “official” D0

Our SBU D0 Group

• People– 3+ faculty

Grannis, Hobbs, McCarthy, Rijssenbeek

– 2+ post docs (long term)– 2-4 graduate students

Not shown: RM, MR

Students?

• Typically, reside at FNAL– After 2 years for classes– +3 years for thesis

• Technical work – shifts, computing, detector hardware

• Thesis analysis– Start with a small, self-contained study– and apply it to an analysis and

complete the full analysis on a given data set

Students?

• Analysis groups include collaborators at other institutions (e.g. W mass)

• so although D0 is big, really work with 4-10 people on a daily basis.

About eight of these folks are finishing working on this topic (graduating, new job, other exp.)

Recent History

• We’ve had or are about to have– 6 theses on Higgs (or related topics)

Zdrazil, Mutaf, Dong, Desai, Herner, Strauss

– 2 theses on W massGuo, Guo

• And both topics are going strong at D0 for another 3-4 yrs.

SBU: Atlas or D0

• Both are very interesting, and faculty in both. Which one?– Personal choice

Why D0? long-term important science running very well! “guaranteed” timeline interested in detailed work at a mature exp. stay in U.S.

Why Atlas (see earlier talk)? the up-and-coming thing; brand new, so learn how detector really works highest energies ever, so good discovery potential. live in France/Switzerland

Summary

• The D0 experiment is doing fundamental work, and SBU towards EWSB study– W boson mass– Higgs search

• to illuminate basic issues at the interface of theory and experiment– Do gauge theories really work as we think?– What is the structure of matter and the

interactions that govern it?