Juggernaut and Behemoth: Grappling with the Enormity of Large Hadron Collider Matt Strassler
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Juggernaut and Behemoth:
Grappling with the Enormity of
Large Hadron Collider
Matt Strassler
Indian-Israeli String Meeting
Matthew Strassler Rutgers
1
University
The LHC is Huge and Powerful
• English:
– Juggernaut – something powerful and unstoppable
• From Hindi Jagannāth, literally, lord of the world, title of Vishnu
First Known English Use: 1841
– Behemoth – something of monstrous size, power, or appearance
• From Hebrew bĕhēmōth, a mighty animal described in Job
• Middle English, from Late Latin; First Known Use: 14th century
Fair description, I think, of the Large Hadron Collider
• Produces vast amounts of data, information, knowledge
• And will change our field forever
– Indeed it already has – we just cannot entirely see how yet…
Accelerator in tunnel used
for LEP e+e- Collider
Four Collision Points with
Four Large Detectors
ATLAS, CMS, LHCb, ALICE
The Accelerator
– 1000s of superconducting magnets
• (1232 dipoles, peak field 8T)
– Largest cryogenic system
– (8 copies for 8 octants)
• 100s of kW
• 1.9 K
• 120 tons of liquid helium
• 4700 tons to cool per system
– Insulation vacuum 9000 m3
– Beampipe vacuum 10-13 atm
4
Energy and Collision Rate
• 14 TeV design
• Sept. 2008 startup followed by magnet quench (loss of superconductivity):
– spark, cascading failures, loss of vacuum
– boiling helium overpressure explosion; safety valves overpowered
• Investigation revealed second issue -- design flaw in quench protection
• Decision made to keep currents in magnets lower in initial phase until
repairs can be made
lower magnetic field
lower energy (10 TeV risky, so 7 in 2010-2011, perhaps 8 in 2012)
• Make up for lower energy with high collision rate sooner
– costs about 3-4 in most processes below ~ 1 TeV, including Higgs;
– cost is much worse at higher mass
– of course to go to highest masses you need 14 TeV
Beams
• Bunch structure,
– 1011 protons per bunch,
– 100's of bunches (up to 2800)
– bunches cross as often as every
• 25 ns (design) or
• 50 ns (current) or
• 75 ns (earlier 2010-2011)
• At each bunch crossing, typically multiple proton-proton collisions
– “pile-up”
--- but (up to a point!!) this isn't a problem.
[was also true of Tevatron]
Cross Section Process Rate (HZ)
(nanobarns) at 1033 cm-2 s-1
(Multiply by 3 – 7 in 2012)
109 Hz at 50 ns
bunch crossings
10 TeV
14 TeV
7 TeV
50 simultaneous
collisions
125 GeV Higgs Cross Section
~17 pb = 17000 fb
Cross Section Process Rate (HZ)
(nanobarns) at 1033 cm-2 s-1
(Multiply by 3 – 7 in 2012)
109 Hz at 50 ns
bunch crossings
10 TeV
14 TeV
7 TeV
50 simultaneous
Maximum Storage Rate
collisions
125 GeV Higgs Cross Section
~17 pb = 17000 fb
Cross Section Process Rate (HZ)
(nanobarns) at 1033 cm-2 s-1
(Multiply by 3 – 7 in 2012)
109 Hz at 50 ns
bunch crossings
10 TeV
14 TeV
7 TeV
50 simultaneous
Maximum Storage Rate
collisions
TRIGGER!!!
Discard (real-time)
99.9999% of data
125 GeV Higgs Cross Section without losing
~17 pb = 17000 fb
Higgs/SUSY/etc.
DO NOT MAKE ANY
MISTAKES WITH THE
TRIGGER
Timeline • 2008: 1 week and then accident • 2009: two months at 0.9-2.2 TeV • 2010: pilot run at 7 TeV (35 pb-1 = 0.035 fb-1) • 2011 the real deal (5000 pb-1 = 5 fb-1 – larger than promised)
Timeline
• 2008: 1 week and then accident
• 2009: two months at 0.9-2.2 TeV
• 2010: pilot run at 7 TeV (35 pb-1 = 0.035 fb-1)
• 2011 the real deal (5000 pb-1 = 5 fb-1 – larger than promised)
– 500,000,000,000,000 collisions
– 100 ,000,000 W bosons
» 90,000 SM 125 GeV Higgs bosons!Timeline
• 2008: 1 week and then accident
• 2009: two months at 0.9-2.2 TeV
• 2010: pilot run at 7 TeV (35 pb-1 = 0.035 fb-1)
• 2011 the real deal (5000 pb-1 = 5 fb-1 – larger than promised)
– 500,000,000,000,000 collisions
– 100 ,000,000 W bosons
» 90,000 SM 125 GeV Higgs bosons!
• 2012 plans: 20 fb-1 (?) at 8 TeV (?)
• 2013-2014 shut down for repair/retrofit/upgrade
– but data analysis and results will continue to pour out because there
are so many questions to ask of all this data!!
• 2015 and beyond 14 TeV? 13? 12.5?
– At 14 get access to 3-4 TeV gluinos, resonances.
– Getting to 100 fb-1 of new data by 2017?How 2011 and 2012 differ from expectation
• Very good total data -- numbers better than most of us dared hope
– On the whole, the accelerator very reliable and clean
– The detectors work very, very well, with challenges
– Many novel techniques work wonderfully!
• But very difficult running conditions
– To make up for lower energy more collisions are desired.
– But!
• Plans were to run machine at 25 ns between crossings
• Rate is at 50 ns and so bunches are made tighter
– (and harder for detector electronics)
– Number of proton-proton collisions per crossing ~ 15 30
• This was not expected to happen in the 1st and 2nd year of full running!!
– Major challenges for the trigger and some for data analysis too
– Worrying... are the right compromises being made?Things to Know
• What can be measured?
– Momentum and energy of electrons, muons, photons
• But: electron minus its track = photon, photon + stray track = electron
• Photons convert in material to e+e- pairs
– Approximate momentum and energy of jets (collimated sprays of hadrons)
• Imperfect surrogates for quarks (except t) and gluons
• Detect both charged and neutral hadrons
– Momentum conservation perpendicular to the beam
– Any new charged particle
• What cannot be measured
– Difference between quarks and gluons (only statistically)
• Exception: b quarks, due to long lifetime and large mass of B hadrons
– Weakly interacting neutral particles, including neutrinos, dark matter
– Conservation of energy and of momentum along the beamThings to Know
• The proton’s structure is something we measure, not calculate
– Huge numbers of gluons and many quark-antiquark pairs
– Three extra quarks (u u d )
– But the extra quarks carry a larger than average fraction of energy
• A quark or gluon flying out from the collision point cannot remain uncloaked
– Color “confinement”
– Perturbative radiation forms a jet of gluons around a fast quark
– Non-perturbative confinement causes hadronization, turns this jet of
mostly gluons into a jet of hadrons
– NOTE JETS HAVE NOTHING TO DO WITH CONFINEMENT!
• In fact, they exist DESPITE confinement!
– Instead properties of jets can be computed in resummed perturbation thy!Developments in Theory Major theoretical developments in last five years • Improvements in definition of “jet” thanks to Cacciari, Salam, Soyez – Computational issues resolved – Consistent with QCD theoretical requirements (calculationally stable) – Can be used for novel measurements – REALLY WORKS IN EXPERIMENTS! • BIG improvements in SM background calculations – 1-loop breakthroughs – MAY prove essential but still need to be tested in data – Experiments use mix of data and theory for predicting backgrounds
Developments in Theory
Major theoretical developments in last five years
• Much improved simulation tools
• LHC physics far too complicated for complete analytic calculation
– Proton is complicated
– Proton remnants after collision are complicated
• LHC detectors far too complicated to compute “acceptance”
– Incorporating the latest theoretical and experimental knowledge
– Needs more testing
Still, most searches up to now are low-precision, looking for high-rate processes
– Can afford large systematic errors from background uncertainties
– Will change in 2012What We Learned in 2011
QFT Still Works Up to 3 TeV
• 2 2 q g, q q scattering
– QCD prediction works
– New range 1-3 TeV
– No excess
– No depletion
– No resonances
– No thresholds
QCD 2 2 Prediction
– No irrelevant operators
– No compositenessQFT Still Works Up to 3 TeV
Fit low-energy data
and extrapolate
• No High Muliplicity
High Energy States
– No Black Holes
– No FireballsQFT Still Works Up to 3 TeV
No sign of 1—2 TeV scale
– Extra dimensions,
– Strings,
– Little strings,
– Black holes,
– Noncommutative QFT,
– Composite gluons a la Seiberg,
– Anything dramatic
• Idea that string theory would miraculously become LHC experimental subject
seems unlikely now
– could anything still hide? [maybe...]
• Will QFT survive the LHC?
– we get one more chance as 7/8 TeV jumps to 13/14 in 2015; we'll know by
late 2015
– But not looking as though nature will surprise us hereNo Big Resonances
• No dijet resonances
– Not in q q
– Not in q g
– Limit a bit less
strong in q - qbar
QCD 2 2 PredictionNo Big Resonances • No W’ or Z’ – W’: M > 2.2 TeV for W partner – Z’ limit approaching 2 TeV
No Big Resonances • No high-mass diphoton bump • No photon-lepton bumps • No jet-photon bumps – Reminder q,g jet – b quark jets special – t quark special • No jet-lepton bumps
No Big Resonances
• No WW, ZZ, WZ bumps
• No top/antitop bumps
• No top/antibottom W’
No Resonances, and
No Enhancements in the UV either
(i.e. no higher dimension ops)Many Colored Particles Decaying to Dark Matter?
• Example: Classic TeV-Scale Supersymmetry
– Make squarks/antisquarks and gluinos
– They decay to quarks/antiquarks and dark matter
• jets + “missing energy” (“MET”)
• Sorry folks – it’s not there.
• Either
– The physics isn’t there
– The jets aren’t there
– Or the missing energy isn’t there
• But there could be many colored particles not decaying to dark matter
• Or there could be one type of colored particle decaying to dark matterMany Colored Particles Decaying to Dark Matter?
• Example: Gauge Mediated Supersymmetry
– Make squarks/antisquarks and gluinos
– They decay to quarks/antiquarks and neutralinos
– Neutralino decays to photon + gravitino
• jets + two photons + “missing energy”
• Sorry folks – it’s not there.SUSY Excluded • Up to TeV scale masses for gluinos/squarks
• Yes, but…
Top Partners: Long Way to Go
• t’ obviously motivated by many types of models that tie top and Higgs
together to solve hierarchy problem
– Supersymmetry
– Little Higgs
– Extra dimensions
• (almost any type)
– Top-color?
• Many final states
• t’ bW, tZ, t gamma
t+invisble, t+jj, cZ
• Top squark t + invisible,
t + neutralino, b+chargino,…New Non-Resonant Electroweak Particles
Particles that
– Have no strong interactions
– Cannot be produced resonantly
– Must be produced in pairs
have very small cross-sections and very large backgrounds.
Mostly these have not yet been searched for and so limits are very weak
– 2012 will see first real results
• But LHC will not reach the TeV scale; only a few hundred GeV
• And triggering is an issue!
– Low cross-section means trigger needs to be efficient
– Low mass Low energy Huge backgrounds)Top Partners • t’ W b – M > 450 • Similar limits for other final states (400-500) • Much harder to find than SUSY because top background is comparable to signal! • Stops even harder; ~6 times smaller than t’ signal • Much will happen in 2012
No New Long-Lived Particles New Long-Lived Particles are a prediction or a possibility in many models But • No stable colored particles making exotic hadrons (up to ~TeV) • No stable charged particles (up to few hundred GeV) • No colored particles stopping in the detector material and decaying late • No heavy particles decaying in flight – But only certain mass ranges, lifetimes and final states
Far Too Early for Pessimism
• Most searches are only reported so far through summer (1 to 2 fb-1)
– Many of these will be updated for the fall data between now and June
• Many searches have not yet been done
– Many of these will appear for the first time between now and June
So the next few months will have plenty of potential.
• Also, most searches (with some exceptions) have been easy
– Hard searches will begin only when easy ones show nothing
• Example: S/B ~ 1:1 or 1:3 instead of 10:1 or 3:1
• Example: Novel specialized techniques that have to be tested first
– Low-rate processes simply haven’t had enough data yet
– To model backgrounds better sometimes requires high statistics
• Many searches will debut in 2012, 2013, 2014 even!Unconfirmed Oddities at the Tevatron
• Multimuons [only CDF]
• Wjj Excess [only CDF]
• Top-antiTop Forward-Backward asymmetry [BOTH experiments]
• Top goes in proton direction more often than antiproton direction
– Expect 8% asym in SM, find >20%, (2-3 sigma both expts)
– Is it a subtlety with the measurement and the Standard Model prediction?
– Expecting very interesting measurements in coming months
• So far LHC has seen no sign of anything related to these discrepancies
• But we expect interesting results soon from CMS and ATLAS looking for
consequences of models that give a FB asymmetry to tops
• And possibly first detection of the asymmetry itself (tricky at LHC)Flavor Physics What’s up with Charm decays at LHCb? • Do D mesons (c ubar and cbar u) violate CP in their decays? • Study ACP(D0 → K+ K−), ACP(D0 → π+ π−) – (actually the difference of the two which is more robust) – Find something ~ 1% (should be much closer to 0) – 3.5 sigma • Is theory prediction correct? • Is it a very big fluctuation? • Is it connected with top physics somehow?
The Search for the Higgs Boson(s)
gg h
tth
Vh
Wh & Zh
or associated production
qq qqh
or vector boson fusion (VBF)gg h
tth
Vh
Wh & Zh
or associated production
qq qqh
or vector boson fusion (VBF)Other Data and Meta-Data • CMS results includes minor (! Sigma) excesses at CMS in H b’s, tau’s, WW • ATLAS includes some fraction of its WW data – all consistent w/ SM Higgs • Widespread rumors: bump at CMS in Vector Boson Fusion kinematic region – Two photons, and – Two jets, one forward one backward (if true this bump would be very small, a few events above a small bgkd) • Calibration issues on the locations of the bumps – Remember these are 1% measurements! – Widespread rumors that things are shifting around – ATLAS calibration was not yet stable in December My view: • until the calibration is settled, or statistics gets a lot better, it could still go either way
Pentaquarks
Humans, Patterns and Statistics
Humans are designed to recognize patterns
• We notice patterns even in random distributions
– We have a bias to see things thare are not there
– Discount your eyes and intuition by at least a couple of sigma
– Psychological research backs this up
• Train your eyes on random distributions
– Watch the peaks come and go
– Weird things happen all the time
• Ask yourself probability questions very carefully
– The probability of a particular weird effect is very small
– The probability of some kind of weird effect happening is much larger
– Systematic errors are not Gaussian or even random (correlations!)
• e.g. photon resolution affects each event separately while
calibration affects all the events together
– Do not take probability plots and numbers of sigmas at face valueImplications of a 125 GeV Higgs
• Suppose it is there: what is to be done next?
– Measure Production Mechanisms and Branching Fractions
• Production: gg h , VBF (qq qqh) , Wh, Zh, tth
• Decay: bb, WW*, tau tau, Z Z*, two photons, mu mu
» Also two jets (gluons, cc, ss…)
• However, this is not so easy; LHC will have decent but not excellent precision:
– Only Production x Branching Fraction can ever be measured
– Some final states cannot be measured at all
– SM predictions cannot to easily be tested
• gg h only predicted to ~15-20%
• VBF well predicted, but contribution of gg h + 2 jets in
experimentally relevant regions is not!
• tth is rather small and quite challenging
• Charm fraction very tough, muons may be (but maybe)Rare Decays
• If the 125 GeV Higgs is there (and also if it isn’t) we need to remember the
possibility of exotic decays
– Hidden sectors or new electroweak particles do allow for
• H invisible
• H X X pairs of leptons or jets or photons
• H X X YYYY - two weird-looking jets
• H X X two photons + invisible
– Branching fractions could be high
• If 125 GeV Higgs a mirage, then could be ~100%
• If 125 GeV hints are real, then could still be ~10-20%, or 0.01%
– But remember we have made 90,000 Higgses so far!!
– Rare decays of the Higgs could potentially be the first or ONLY sign of
new physics at the LHC , and could be seen in 2011 data or 2012 data
• So enormously important!!!Will We Trigger On Rare Decays
• But Higgs is not easy to trigger on
– Low mass Low energy jets, leptons, photons, taus as decay products
– At low energy, backgrounds are largest and trigger is most problematic
– This gets much worse at high pile-up
• Only guarantee: some fraction of Wh and Zh will trigger on W/Z decays
• 14 TeV may not help
– Higgs cross-sections increase by ~3
– But not Wh/Zh (and triggering on W/Z decays will be tough)
– And all trigger thresholds will go up because all backgrounds go up
It is possible that 2012 will be the best year for exotic decays
or at least that 2015-2018 will not increase the data sample much!Implications of a 125 GeV Higgs
Suppose it is there, what does it mean for our deeper understanding of nature?
• Unfortunately, not that much, at least not yet
• Can’t say nature is fine tuned or not
– The Standard Model is ok [extreme fine tuning]
• Perhaps metastable, but we are no longer bothered by this
• And perhaps not, since it’s borderline
– little adjustments at high energy could change the answer
– Supersymmetry Is newly constrained but it isn’t dead
• Moderately Split SUSY [major fine tuning]
• Non-minimal SUSY [maybe not as much fine tuning]
– And since you’re now willing to allow some fine tuning…
• Maybe even some extended versions of technicolor [why not?]
• What guiding principles remain if we accept fine tuning??
– Need to go back and dig up old models rejected because they were tunedWider Implications
Huge, but Unknown
Given nothing (except hint of SM Higgs) has been found as of yet, the Standard
Model still remains the best theory in play
A theory (such as Split SUSY) with considerable fine tuning does not address the
big issues, and experimentally may be indistinguishable for the near future.
But there may still be lots of new physics hiding in the data
• At higher mass than expected (rare now, need 14 TeV)
• With only Electroweak Particles (and thus at lower rates; need 2012 data)
[but what about top and naturalness?]
• From Hidden Sectors that have visible effects (typically hidden valleys)
Many tricky measurements yet to be done
Many sectors with difficult-to-predict phenomenology
Few examples among favorite theories, but not studied muchWider Implications
The big question: Naturalness in Danger
• Our field has relied on naturalness as a principle for about 40 years
– It works very well in our best analogue, condensed matter physics
– It has worked very well in the SM (e.g. QCD, higher-dimension operators)
• We’ve certainly considered relinquishing it *long before the “landscape”+
• But no reliable principles if nature determined by selection effect
– Counting vacua blindly is fine, but as likely to be wrong as right
– Different methods of doing statistics will give different answers
– No known way to verify via experiment that this is correct approach
• Without experiment, does the field return to pre-Baconian times?The Limits of Statistical Arguments
• You cannot do biology with statistics
– The system has too much structure and dynmaics
– Statistics of all possible organisms will only go so far (perhaps nowhere)
– You must know the biochemistry and biophysics of DNA, other
biochemicals, membranes which form CONSTRAINTS on statistics
• These are highly non-linear, but also powerful and essential
• The landscape of possibilities in our universe may also be highly structured
– Dynamics may be as important or more important than statistics
– Better said: even if a selection effect is in operation, without an
understanding of the dynamics, a pure statistical conclusion may be
completely wrong
• Without dynamics, and experiments to check our understanding of them,
statistical arguments will remain unfalsifiable and at the edge of scienceGrappling with the Monster
• The enormity of the issues facing our field is daunting
– This is a one-of-a-kind machine; it must not fail, in any way
– We have so many questions, so much data, and so few people
– The implications of LHC data are obviously historic
– The field’s future is being determined as we speak
• Things that must NOT happen:
– Data is imperfectly corrected because the triggers were not wisely set
• Adjustable before the fact, to a degree
• Not adjustable after you’ve taken the data! (i.e. urgent for 2012!!!)
– Widest range of questions not asked because
• Experimenters and theorists are not sufficiently inventive
• People get unreasonably or prematurely discouraged
– Questions not answered well because theorists diidn’t do their job
• Helping both predict backgrounds
• Suggesting ways to avoid having to predict themAre We Making History,
or
Are We Becoming History?You can also read
