Searches for Beyond the Standard Model Physics at the LHC

Andy ParkerCavendish Lab

The Cambridge SUSY Working Group

• Formed in 1998 with Bryan and Ben Allanach: ideal combination of youth and experience!

Theorists and experimentalists work together with weekly discussions and joint papers.

• Bryan has been mainstay of group: many analyses have relied on Herwig, suitably modified, on new processes being simulated, or on understanding of QCD.

• A great success: still going after 12 years, 58 papers on website posted by group members

• Many students and post-docs who worked in the group have gone on to successful careers

• Will pick a few themes from this work.

Some big themes

• Measuring SUSY• Finding extra

dimensions• Things may not be

as they seem…

Measuring SUSY

• In 2000: many models, studies showing that particular model could be seen with particular cuts.

• Measuring sparticle masses in non-universal string inspired models at the LHC.

• JHEP 0009:004,2000 (Allanach, Lester, Webber, MAP)

String models from BA, event generation BRW, reconstruction CL/MAP, analysis by all.

155 citations (not even in BRW top ten!)

• SUSY points under study assumed universal scalar and fermion masses and couplings

• These were not compatible with weakly coupled string theory scenarios: generate charge/colour breaking minima (CCB) or infinitely negative potentials (UFB).

• -> construct an “optimised” non-universal string model without CCB and UFB problems.

Dr Benjamin Allanach, a research associate at Cern, the European particle laboratory, said that a chance fluctuation of the "vacuum universe" would disintegrate all atoms. He said: "The universe is perched on a terrible precipice. It could catastrophically tunnel to a new state, disintegrating every atom.”

Killer plasma ready to devour the Earth! Daily Telegraph website Sep 2001

And if that does not wipe out all known life anywhere in the universe, Dr Allanach said so-called killer strangelets could "eat up the Earth from the inside out".

• Can one distinguish this from the most similar mSUGRA model (S5)?

• Herwig adapted to study observables: models show different slepton rates

• Use kinematic constraints from long decay chains

• Kinematic edges then simulated and likely errors determined.

• Use MT2 to deal with neutralino emission

• Same analysis for both models

• Particle masses extracted successfully, and models distinguished from each other.

First steps towards model independent analysis of SUSY signals

10 years on…• Measuring supersymmetric particle masses at the LHC in scenarios with

baryon-number R-parity violating couplingsB.C.Allanach, A.J.Barr, L.Drage, C.G.Lester, D.Morgan, M.A.Parker, P.Richardson, B.R.WebberJHEP 0103:048,2001

• Extracting the flavor structure of a baryon number R-parity violating coupling at the LHCB.C. Allanach, A.J. Barr, M.A. Parker, P. Richardson, B.R. WebberJHEP 0109:021,2001

• Detecting exotic heavy leptons at the Large Hadron ColliderB.C. Allanach, C.M. Harris, M.A. Parker, P. Richardson, B.R. WebberJHEP 0108:051,2001

• and a few papers later…

…Mass determination in sequential particle decay chains

• B.R. Webber JHEP 09 (2009) 124• Abstract: A simple method is proposed for determining

the masses of new particles in collider events containing a pair of decay chains (not necessarily identical) of the form

• Z → Y + 1, Y → X + 2, X → N + 3, • where 1,2 and 3 are visible but N is not. Initial study of a

possible supersymmetric case suggests that the method can determine the four unknown masses in effectively identical chains with good accuracy from samples of a few tens of events.

Make use of correlated masses in two decay chains in same event.Do not require chains to be identical.

Applicable to SUSY

or other long chain processes

3 constraints x 2 chains + 2 missing pT constraints = 8 constraints total

Elegant matrix solution which generates c2-like variable to identify most likely mass assignments

Likely to be as influential as the 2000 paper.

Extra Dimensions

• ADD model with LED produces mono-jet signature with missing energy: not very appealing experimentally

• But RSS/UED models give more interesting phenomenology -> SUSY working group became active in this area

Charybdis• Charybdis: A black hole event generatorC.M. Harris, P.

Richardson, B.R. WebberJHEP 0308:033,2003

• Charybis2: James A. Frost, Jonathan R. Gaunt, Marco O.P. Sampaio, Marc Casals, Sam R. Dolan, M. Andrew Parker, Bryan R. Webber, arXiv:0904.0979v1

A full event generator for black hole events, including Hawking radiation with correct grey-body factors, spin down, and treatment of hadronization and final decay of remnant.

Into the black hole…• Searching for narrow graviton resonances with the ATLAS detector at

the Large Hadron Collider.B.C. Allanach, K. Odagiri, M.A. Parker, B.R. WebberJHEP 0009:019,2000

• Exploring small extra dimensions at the Large Hadron ColliderB.C. Allanach, K. Odagiri, M.J. Palmer, M.A. Parker, A. Sabetfakhri, B.R. WebberJHEP 0212:039,2002.

• Exploring Higher Dimensional Black Holes at the Large Hadron ColliderC. M. Harris, M. J. Palmer, M. A. Parker, P. Richardson, A. Sabetfakhri, B. R. WebberJHEP 0505 (2005) 053

Black Hole production cross-section

•Classical approximation to cross-section•Controversial…see review by Gingrich hep-ph/0609055•Very large rates for n=2-6 See hep-ph/0111230•Almost independent of n

€

σBH ~ π rh2

Black Hole DecayDecay occurs by Hawking radiation, modified by “grey

body” factors

Hawking Temperature TH

Black Hole radius rh

Use observed final state energy spectrum to measure TH and hence n?

But we have no model of decay near Planck mass

€

TH = (n +1) /4π rh

rh ~ hMPlc

mBHMPl

⎛ ⎝ ⎜

⎞ ⎠ ⎟1n+1

Modelling BH eventsModelling performed using Charybdis, TrueNoir or CatFish

BH loses hair, spins down, then decays - only decay phase is modelled in C1, spin also in C2.

Theory doesn’t cover phase when MBH~MPl and final state decay is forced to 2 or 4 bodies.

BH remnant decay

n=2 n=4

2 body

4 body

Look at photon energy as example of primary emission - spectrum strongly dependent on assumption on remnant decay.

BH shape parameters

CMS

Black hole generators gives spherical events, very different from high mass QCD and tt events - but beware of modelling systematics

“The thermal nature of Hawking radiation requires the distribution of BH final state particles to be spherical”

Not true if BH is hairy, or spinning, or if remnant decay has dynamics

BH Temperature variation

Effect of varying temperature as BH decays. True n=2 Fit TH against Black Hole mass:

fixed T n=1.7±0.3varying T = 3.8 ± 1.0

Cannot measure T with sufficient accuracy - about 10 GeV. Also black hole is boosted by each decay, changing successive energy distributions

Black Hole SearchesCan measure characteristic T at average mass -> combine this with cross section data to extract n. Assume 20% error on σ

Big Bang machine could destroy the planet – telegraph.co.uk 2008

Doomsday!

BRW reviews LSAG report for CERN SPC – planet is safe!

Who you gonna call?

The snark was a boojum, you see*• How can we tell SUSY from ED models, or any

other scenario involving a spectrum of TeV-scale new particles?

• Masses and charges are not enough: need spin as well.

* In the midst of the word he was trying to sayIn the midst of his laughter and glee

He had softly and suddenly vanished awayFor the Snark was a Boojum, you see.

Lewis Carroll, The Hunting of the Snark, 1874

ATLAS status report LHCC todaySUSY search 2j+Etmiss:

almost into signal region

q* search already beyond TevatronSet limit at m(q*) > 1.26 TeV

• Distinguishing Spins in Supersymmetric and Universal Extra Dimension Models at the Large Hadron ColliderJennifer M. Smillie, Bryan R. WebberJHEP 0510 (2005) 069

• Distinguishing Spins in Decay Chains at the Large Hadron ColliderChristiana Athanasiou, Christopher G. Lester, Jennifer M. Smillie, Bryan R. WebberJHEP 08 (2006) 055

Spin-doctoring

• SUSY predicts TeV-scale partners to SM particles, with same gauge couplings, differing in spin by ½ unit

• UED predicts TeV-scale partners to SM particles, with same gauge couplings, with the same spin.

• BRW: “the number of unknowns and arbitrary assumptions in each case is so great that the exclusion of either class of model in favour of the other would only be truly convincing if their spin structures could be distinguished.”

• Barr: spin correlations produced in pp collisions – more squarks than antisquarks (Phys Lett B. 596 205-212, 2004)

• Method extended to UED

Some discrimination possible if SUSY spectrum is hierarchical as expected

Standing on the shoulders of Giants

• "Bernard of Chartres used to say that we are like dwarfs on the shoulders of giants, so that we can see more than they, and things at a greater distance, not by virtue of any sharpness of sight on our part, or any physical distinction, but because we are carried high and raised up by their giant size.”

• The Metalogicon of John Salisbury 1159